BASE AND PRECIOUS METAL MINERALIZATION RELATED TO LOW-ANGLE TECTONIC FEATURES Ir~ THE WHIPPLE MOUNTAINS, CALIFORNIA AND BUCKSKIN MOUNTAINS, ARIZONA

Joe Wilkins, Jr. Tom L. Heidrick Gulf Mineral Resources Co. Gulf Mineral Resources Co. 2015 North Forbes Blvd. Ste. #105 1720 So. Bellaire Tucson, Arizona 85745 Denver, Colorado 80222

ABSTRACT structure (Spude, 1976). Early ore shipments from the Planet mine in 1862 and from the Sue and Lion Specular hematite mineralization with varying Hill mines (Keith, 1978) were followed by a period Cu, Au, and Mn contents commonly associated with of promotional development at Planet in 1907 and at quartz, barite, or fluorite is present in a vari­ Swansea in 1908 (Spude, 1976). Significant ore ship­ ety of lithologic settings immediately above, along, ments began with the completion of the Parker­ and below the Whipple-Buckskin detachment fault. Wickenburg and Bouse-Swansea rail lines in 1914 The major deposits, Copper Basin, Planet-Mineral (Keith, 1978). Intermittent shipments and mine Hill, and Swansea-Copper Penny are localized along closures, corresponding to the rise and fall of ENE- to NE-trending synformal and anti formal mega­ copper prices, continued until 1923 when the Planet grooves on the fault surface associated with a mine closed and until 1937 when Swansea ceased thick sequence of non-mylonitic, but cataclastically production. The Mineral Hill mine, operated ini­ deformed, metamorphosed middle-plate rocks. Exten­ tially in the 1900's, was an active producer from sional tectonics related to NE- to ENE-directed 1964 to 1970. In total, 63.0 million pounds of movement in late Oligocene to mid-Miocene time copper, 11,900 ounces of gold, and about 1,000 tons created tectonic crush breccias, open to overturned of manganese were produced from mines along the folds, synthetic and antithetic listric faults, and detachment surface (Jones, 1919; Keith, 1978; Spude, gash veins that, along with the detachment surface 1976). Production from the major mines is listed and its underlying shatter breccia, served as loci in Table 1. for deposition of the metals, gangue minerals, and alteration aureoles. Table 1. Copper and Gold Production from the Buckskin Mountains, Arizona A detachment fault mineralization model gener­ ~li Ore Cu Cu Au ated from detailed mapping, drill core analyses, ne 3 6 3 and geochemical determinations at the Copper Penny (10 tons) % (10 1bs) (10 oz) prospect is characterized by: 1) specularite­ Planet 50 10.0 10.0 0.2 chalcopyrite ± Ba, Mn, F, Au concentrated along and Mineral Hi 11 1,000 0.65 13.0 decreasing away from the fault surface, 2) chlorite­ Swansea 490 4.0 YI.2 dominated alteration envelopes coplanar with the Empi re-Sue- fault, 4) deposition in extension-related struc­ Lion Hill 20 20.0 0.8 11. I tures, and 5) localization along megagrooves asso­ Totals ~ z:o 63.0 ITg ciated with thick upper-middle plate sequences. The (avg. ) consistency of the model holds when extrapolated to other Whipple-Buckskin deposits with variations Previous Investigations only in concentration intensities despite widely varying upper-middle plate lithologies. The iron deposits in the Buckskin Mountains bri efly menti oned by Bl ake (1865, 1898) and by r~cCarn INTRODUCTION (1904) were described at the Planet by Upham (1911) who ascribed their origin to derivation" .... from The Whipple-Buckskin Mountains detachment sur­ alteration of the ferromagnesian minerals in the face constitutes a small portion of a mid-Tertiary, adjoining rocks". Bancroft (1911) noted the marked low-angle tectonic phenomena that is associated lithologic and mineralogic similarity between many with metamorphic core complexes which extend the of the deposits and assigned a Precambrian age to 1ength of the North Ameri can Cordi 11 era (Coney, them. He commented on the intense structural defor­ 1979, 1980; Rehrig and Reynolds, 1977, 1980; Davis mation within the orebodies but apparently did not and others, 1980). The detachment fault tectoni­ recognize their tectonic setting. Blanchard (1913) cally juxtaposes Precambrian, Paleozoic(?), Mesozoic, mapped the detachment fault as an erosional surface and Tertiary units onto a mylonitically deformed, between Precambrian units and suggested that the lower-plate sequence (Davis and others, 1977, 1980; deposition of cupriferous specularite was due to hot Lingrey and others, 1977; Shackelford, 1977). Mines springs activity associated with Tertiary volcanism. and prospects with specular hematite containing Jones and Ransome (1919) and Jones (1919) described variable amounts of copper, gold, and manganese are the occurence of manganese with specularite and consistently located immediately above, along, and barite in brecciated rocks adjacent to low-angle below the detachment surface (Fig. 1) demonstrating faults in the Buckskin and Whipple Mountains. Wilson a spatial and suggesting a tectogenetic dependency. and Butler (in Cummings, 1946) mapped the Planet mine in detall and interpreted the detachment surface Past Production as a thrust fault. Wilson's subsequent mapping for the Geologic map of Yuma County (1960) expanded the The history of production in the Buckskin and extent of Laramide thrusting (Wilson, 1962) through­ Whipple Mountains is dominated by entrepreneurial out the Buckskin Mountains. promotion, bankruptcy, and copper's cyclical price

182 LITHOLOGY EXPLANATION STRUCTURE

B OSBORNE WASH FH. BASIN & RAr1GE FAULT --:=:~~~~~~~~~- POST -OETACHMENT I22l GENE CANYON-COPPER BASIN FM. t ~ METASEO-METAVOLC ROCKS UPPER PLATE LISTRIC NORMAL FAULT E.':J CRYSTALLINE ROCKS ~ o MYLONITIC ROCKS t LOWER PLATE ~ NON MYLONITIC ROCKS t

III Mn-Fe I:,,: I"r f~I~'J ; \;.1 .. I'·: I.

o 1 2 3 b---=--J b---=--J

Figure 1. Regional geologic map of the Whipple Mountains, California and Buckskin Mountains, Arizona as modified from compilation~ by Stone and Howard (1979), and Davis and others (1980). Prospects and mines designated by initials are: BC = BCC mine; C = Clara; E = Empire; LH = Lion Hill; M= Mamp;on; r~H = r~ineral Hill; P = Pride; RV = Riverview. The large open circles are mines and deposits discussed in the text. Even though the low-angle faults in the Buckskin structures and do not represent a folded detachment and Whipple Mountains were mapped by a number of surface. geologists (Ransome, 1931; Wilson, 1960; and Terry, 1972) the magnitude of the involvement of the Ter­ The upper plate moved NE to ENE relative to the tiary units was not recognized prior to work by lower plate during the Oligocene(?) to mid-Miocene Shackelford (1976, 1977) in the Rawhide Mountains and extensional tectonic event (Anderson, 1971; Shackel­ by Lingrey and others (1977) and Davis and others ford, 1976; Davis and others, 1977, 1980; and Dokka (1977) in the Whipple Mountains. Recognition of and Lingrey, 1979). This movement created tectonic the metamorphic core complex setting was virtually crush brecci as, open to overturned folds, syntheti c simultaneous (Davis and others, 1977, 1980; Lingrey and antithetic listric normal faults, and gash veins and others, 1977; Anderson and others, 1979). The (Davis and others, 1977, 1980; Frost, 1981) that, delineation of the lithology, structure and age of along with the detachment fault and its underlying movement in the Buckskin and Whipple Mountains es= shatter breccia, served as loci for deposition of tablished a critical data base for clarifying the the metals, gangue minerals and alteration aureoles. geology of the enigmatic mineral deposits in these All of these features are found at the Copper Penny ranges. prospect in varying degrees of intensity. The recently completed investigation of this deposit Mineral Deposits - Their Regional Setting (1979) provides a data base for interpreting the complex geology of other, less clearly defined base, From Figure 1, it is apparent that all of the precious and ferroalloy mineral occurrences in the major mines and the larger prospects are spatially Whipple and Buckskin Mountains. related to the detachment surface. Although miner­ alization along the surface is widespread - almost GEOLOGY OF THE COPPER PENNY AREA ubiquitous - the more notable concentrations are positioned within thick accumulations of upper-plate Between 1977 and 1979, a detailed geologic map rocks. At Swansea, Planet, Empire-Lion Hill, Copper (Fig. 2) of the Copper Penny prospect was prepared, Basin, and Copper Penny, the upper-plate rocks lie the deposit was geochemically sampled and 8 diamond within NE- to ENE-trending troughs or megagrooves on drill holes (Figs. 2 and 3) plus a detailed core the detachment surface that are subparallel to analysis were completed. The results of these inves­ antiformal arches in the adjacent mylonitic terrain tigations conclusively demonstrate that the sulfide (Rehrig and Reynolds, 1980). As shown by Woodward and oxide metallization in the upper, middle, and and Osborne (1980), the megagrooves are primary lower plates are coplanar with, and tectogenetically

183 o (> Qpg (> 'vJ 2

Figure 2. Geologic map of the Copper Penny prospect, Santa Maria mining district, Yuma County, Arizona, showing Geology by Tom L. Heidrick and Joe Wilkins, 1977-1979.

184 EXPLANATION LITHOLOGY

L,9.ai:'1 ALLUVIUM, SHOWING BOULDER ACCUMULATION 1·.

STRUCTURE

DIP AND STRIKE OF BEDDING

DIP AND STRIKE OF FOLIATION, SHOWING BEARING AND PLUNGE OF LINEATION

VERTICAL FOLIATION, SHOWING HORIZONTAL LINEATION 72 ~­ JOINT SET, SHOWING CONTINUOUS- PLANAR SETS WITH FREQUENCY AND DIP

CONTACT, SHOWING DIP,L DASHED" WHERE APPROXIMATELY LOCATED, DOTTED WHtRE INFERRED

HIGH -ANGLE FAULT, DASHED WHERE APPROXIMATELY LOCATED, PATTERNED WHERE SCHISTOSE AND BRECCIATED

LOW -ANGLE DECOLLEMENT SURFACE, SHOWING DIRECTION AND AMOUNT OF DIP

LOW -ANGLE DECOLLEMENT SURFACE, SHOWING BEARING AND PLUNGE OF SLICKENSIDES

ANTICLINE, SHOWING TRACE OF AXIAL PLANE AND DIRECTION AND AMOUNT OF AXIAL PLUNGE

SYNCLINE, SHOWING TRACE OF AXIAL PLANE AND DIRECTION OF PLUNGE

M/SCELL ANEOUS

HEMATITIC JASPER, EARTHY TO APHANIC CATACLASITE

LIMONITIC JASPER, MASSIVE GOETHITIC -JAROSITIC REPLACEMENTS OF CATACLASITE

~ GEOCHEMICAL SAMPLE LOCATION, SEMI-CONTINUOUS ()] ROCK-CHIP (TOP) AND SPOT (BOTTOM) • DRILL HOLE LOCATION * K-Ar AGE DATE SITE

the location of drill holes 1 through 8, rock-chip samples collected for geochemical analyses, and K-Ar age dates.

185 -! z o A c A'

1500fu=~r:" D~ -~ 1500' <_ k "_ 0

1000' 1000'

500' 500'

S"o Level o+--.,-----~~---~~-'''-----,----... -~ __. .. _''''''______.. ,_, , ~'''_ --- - ~~ --'" ,,~"--,--,.. _t__O Sea Level

Cross Section A-A' (Looking N 28° W)

~ co Ol -z

B B' ...... DDH-~2 TOP----_", - .... . '\ --- ...... '--- ::-----''::::- ~ '-.. 4--' _ .... _ 1500' 1500' \lE~.,;y • ,,- -~-=::-~;-;--=_~ :::;'~~~::v ",~, ,-'-,:::~_-_ --:------'--0" > , I, , '"', "' ~-- ~ '. <~~ ~ ""'''''--~...... ~~ 1000' 1000'

500' 500'

Seo~vaIO~ ~ _ ------,. -~-~__ ------~--~---.-~---".----_t__oSeo levul

500' I TLH/JW~791 500' Cross Section B-S' (Looking N500E) 1000 a 1000 ..2000

Figure 3. Geologic cross-sections A-A' and B-B', Copper Penny prospect, Yuma County, Arizona. The location of each respective cross-section along with the explanation for structural elements and lithologic symbology are provided on Figure 2. related to, the detachment fault. Penny area, will be discussed in succeeding sections. The mylonitic fabric is always deformed by a shatter Lithol ogy breccia at the base of the detachment surface. The flinty microbrecciated surface grades downward to Lower-Plate Rocks various combinations of shatter, crush, and crackle breccia; a relationship consistently found by Davis The lower-plate rocks are coarse to fine grained (1980), Rehrig and Reynolds (1980), and Davis and quartzo-feldspathic, augen to compositionally banded others (1980). Ch 1ori te, epi dote, pyri te, cha 1co­ mylonitic gneiss (Fig. 4). The gneiss possesses a pyrite, calcite, and rare fluorite accompanies the well developed, gently dipping foliation, elongate shatter breccia. At Copper Penny, the chlorite quartz rods, and flattened K-feldspar porphyro­ breccia gradually decreases in intensity and disap­ blasts typical of metamorphic core complex terrains pears 300 to 600 feet below the detachment surface (Davis, 1980; Rehrig and Reynolds, 1980; and Davis but occasionally continues uninterrupted to depths and others, 1980). The penetrative lineation direc­ greater than 1200 feet. tion in the gneiss, present throughout the Copper A K-Ar age derived from feldspar at the bottom of DH-8 (Fig. 2), gave an age of 17.7 ± .7 m.y., suggesting a reset minimum age for the time of move­ ment. Detachment Surface The detachment surface is a dense, compact, flinty, comminuted microbreccia that weathers dark reddish-brown to patina. At Copper Penny, the sole of the Buckskin fault is a readily mappable, resis­ tant, ledge-forming unit, identical to the surface found in the vihippl€'s (Davis and others, 1977, 1980). The detachment surface is overlain by diverse sets of lithologies which include andesitic and clastic rocks of the Gene Canyon Formation, redbed sedimen­ tary rocks of the Copper Basin Formation, Mesozoic(?) metamorphic rocks, and limestone and fanglomerate of the Arti llery Formation. The results of several whole rock analyses of microbreccias summarized on Table 2, were col­ lected from sites overlain by a variety of upper­ plate lithologies. Also included, for comparision, are average compositions of Laramide plutonic rocks and several lower-plate mylonitic gneisses. Figure 5 recasts these data on K20+Na 20-Si0 2 and K20­ Na 2 0-CaO diagrams. These data demonstrate that major element compositionof the microbreccia is not influenced by the composition of the hangingwall rocks. It is also apparent that losses in A1 20 3 and Na 20 are balanced by gains in K20 and FeO + Fe203' The Whipple-Buckskin detachment surface, once conceived, acted much like a conveyor belt, paying little attention to what was being moved across its surface. Middle-Plate Rocks A sequence of moderate to highly metamorphosed, weak to intensely fol iated but non-l ineated rocks occurs above the detachment surface and below the upper-plate rocks. The upper contact is an undu­ lating, low-angle zone of intense brecciation that is subparallel to the detachment surface. Identi­ fiable middle-plate rocks include: granite breccia (Fig. 6A) composed of rounded to subangular frag­ ments of Precambri an (?) grani te, i ntensJ; ly brecci ated dolomitic to limey marble breccia (Fig. 'bB), and cataclastic schist (Fig. 6C). In drill holes 3 and 4, these units are not present in the middle plate. Instead, a sequence of brecciated, bleached, and slightly chloritized arkosic mudstone, siltstone and shale is present showing the affects of weak to moderate dynamothermal metamorphism. The affects Figure 4. Polished slab of augen and compositionally and intensity of the metamorphism are midway between banded mylonitic gneiss from 225' in DH-8. unmetamorphosed upper-plate rocks, and the intensely The fractures are filled with black greasy deformed marble and schist. Hence, the middle chlorite. plate is more a metam00phic zone than a distinct

187 Table 2. Average and Comparative Major Element Analyses of Flinty Tectonic Microbreccia from the Whipple and Buckskin Mountains Detachment Faults, California and Arizona. Data from Heidrick and Wilkins (1979). SAMPLE DESCRIPTION SiO Avg. of 9 flinty Avg. of 75 fresh Laramide tectonic breccias, (75-50 m.y.b.p.) plutonic 67.5% 7.2% 1. 0% AZ and CA rocks SiO z 65.8 % Avg. mylonitized augen Al z03 12.78 gneiss (Swansea) and MgO 2.00 hornblende biotite 68.3 7.2 1.7 CaO 2.72 quartz diorite (Davis et MazO 1. 39 al.,1979) KzO 4.19 FezO 3 3.48 Avg. of 5 analyses of FeO 3.94 tectonic microbreccia L. 0.1. 2.66 from AGS Stop 2 and 3, 65.8 4.2 2.0 Sr .02 Whipple Mts., AZ Rb .01 TiO z .49 Avg. of 3 analyses of P205 .25 tectonic microbreccia MnO .10 from AGS Stop 11, Buck­ 65.5 6.9 3.9 F .09 ski n Mts., AZ S .05

EXPLANATION N'IOO 20 lithologic packet.

~oJ """'O"~"'" H" The granite breccia is not exposed in the map :•. "1<0" area but does outcrop near Swansea and was encoun­ " ~ A"ol,-U.." 'e1" tered in drill holes 5 and 6 in the Copper Penny area. The granite is porphyritic with large pink feldspar phenocrysts set in a crystalline matrix; all ferro­ magnesian minerals are altered to bright green chlo­ rite. This unit appears to correlate with Precam­ brian(?) granite porphyry described by Anderson and strong calcIc others (1979) in the Whipple Mountains. Along the northeast side of the middle plate (NoZO + K O)2. l margin (the SE~ Sec. 25 and NE~ Sec. 36, Fig. 2), S·--~ 5'02 - 43 outcrops of marble breccia show decreasing meta­ 04';:-0""""::;-'""";':SO-~5Sf--""'";6-:0 ~6::':S-';;70--:-:-""",",,:L::;.L...L-R:-:,,:-,mo-""-=-S":-ite-::7-YP'...... J morphic affects along strike away from the plate Weight Percent, SiO, margin. Several outcropping beds, traced to the K2 0- NOZO- Coo DIAGRAM )(20 northwest along strike, show a continuous metamorphic FRES HI G ~J E0 U S FlO CK 5 0 FAR I Z 0 N A spectrum ranging from thoroughly recrystallized marble breccia at the detachment to weakly recrystal­ o Vol,on" non 0' pl"~ "[.I'"I..e >clcan,cro,ks lized, bleached, and slightly brecciated lacustrine a O;ktor I"l ood limestone away from the fault. The low ragged hill

• Aplll< dlk. ENE of DH-6 (Fig. 2), is composed of a brown-grey, rough-weathering, brecciated marble breccia forming an ENE-trending arch and trough pair resting on the detachment surface. To the northwest, the marble breccia is tectonically kneaded into poorly exposed quartz mica schist. The middle-plate marble breccia unit is abruptly truncated to the east by a NNW­ trending Basin and Range fault (Fig. 2) showing 3500 feet of right-lateral separation. The marble breccia hosting the high-grade copper mineralization at Swansea represents the faulted-off extension. In detail, the marble breccia is a brecciated breccia, with subrounded to subangular clasts in a Figure 5. KzO+NazO-SiO z diagram (A) with rocks mi crobrecci a matri x. Vugs up to 5 em. in di ameter are classified according to the Pittman suite common and clasts of schist, chert, and arkose index and KzO-NazO-CaO ternary plot (B) (Artillery Formation) are present in the marble of 100 fresh Laramide (75-50 m.y.) plu­ breccia. A penetrative low-dipping foliation marked tonic and volcanic rocks of Arizona. by chlorite (or hematite where oxidized) veinlets that occurs throughout the unit could easily be Analyses from Table 1 include: m, aver­ age mylonitized augen gneiss;., tecton­ mistaken for bedding planes. Workers in the past ic microbreccia from the Whipple Mts., have consistently mapped this unit as Paleozoic(?), CA; and *, tectonic microbreccia from (l4ilson, 1960; Shackelford, 1976; and Woodward and the Buckskin Mts., AZ. Osborne, 1980) but it may also represent an intensely

188 o 2 3cm LI_-,-_-,I__J

Figure 6. Polished slabs of middle-plate lithologies. A. Granite breccia; rounded fragments of granite porphyry in a chlorite matrix. B. Marble breccia; clasts set in a matrix of supergene hematite developed from oxidized pyrite and chalcopyrite. C. Cataclastic schist; grey-green schist derived from Artillery Formation. metamorphosed limestone sequence of the Artillery incompletely to weakly metamorphosed Artillery arkose Formation similar to that found along the northeast clasts within marble breccia increases structurally margin of the middle plate. upward, and 4) the dynamothermal metamorphic affects on middle-plate Artillery lithologies increase The middle-plate cataclastic schist cut in drill markedly in moving laterally from drill holes 3 and 4 holes 2, 4, 5, and 6 (Fig. 2) forms a fault-bounded to holes 2, 5, and 6 (Fig. 2). The data indicate sandwich between the lower marble breccia and the that virtually all of the middle-plate lithologies upper plate. At the base, it appears as a grey-green, at Copper Penny were derived from the Artillery impure quartz sericite schist but as one moves struc­ Formation. turally upward, Artillery shale, mudstone, and siltstone fragments gradually increase toa point Upper-Plate Rocks where they comprise 100% of the unit. The precise stratigraphic succession of the sed­ Data compiled from our investigation of middle­ imentary and volcanic rocks in the upper plate is plate rocks help define the presence of strong to difficult to ascertain since most contacts are very strong vertical and lateral dynamothermal gra­ structural in character and monotonously repeated or dients within this structural unit. Supporting truncated by listric normal faults. The apparent evidence include: 1) variable lithologies of the section, estimated at 1600 feet, consists of a basal Artillery Formation are progressively transformed sequence of arkosic sandstone and siltstone with with increasing depth to cataclastic quartz seri­ shale partings, a middle sequence of fetid lacustrine cite schist, 2) slightly brecciated and bleached limestone beds intercalated with limey mudstone, lacustrine limestone of the Artillery Formation siltstone, and shale, and an upper fanglomerate and grades laterally into thoroughly recrystallized maroon arkose unit. Rhyolite vitric tuff is inter­ and brecciated marble breccia, 3) the abundance of bedded with the lower clastic unit and trachybasalt

189 flovi caps the fanglomerate sequence. The basal and overturned folds, tectonic crush breccias, bedding middle clastic lacustrine limestone sequence is fine plane slips, gash veins, and sandstone, shale, and grained and weathers grey or grey black to black. pebble dikes, ANW-trending high-angle fault, showing It contains a high organic content with carbonized 3500 feet of right-lateral separation, offsets both plant debris; syngenetic pyrite is rare. The upper the detachment surface and the plates above and be low. fanglomerate unit contains well-rounded, cobble-sized clasts of lineated quartz mica schist, granite por­ Low-Angle Faults, Slickensides, and Quartz Lineation phyry, upper-plate mudstone and limestone, and mylonitic gneiss in a matrix of chloritic to arkosic The spatial distribution of NE-dipping listric mud. normal faults, the Buckskin Mountains detachment surface, and the middle-plate fault, is shown on K-Ar age dates obtained on fresh biotite from Figure 2 and corresponding cross-sections A-A' and the rhyolite tuff unit and on hornblende from the B-B' on Figure 3. The detachment surface, a strand capping trachybasalt were 27.3 ± 1.1 m.y. and 18.6 ± of the Whipple-Buckskin-Rawhide detachment surface 1.5 m.y. Although some confusion exists regarding (Shackelford, 1976; Davis and others, 1980), forms the age of the Artillery Formation (Gassaway, 1972, a slightly asymmetrical trough elongate ENE with low 0 1977; Shackelford, 1976), the late Oligocene to (10-30 ) dips along the northwest margin and moderate 0 early Miocene K-Ar ages identify these rocks as (40-50 ) dips on the southeast. Brecciation on both belonging to the Artillery Formation (Lasky and sides of the fault surface is intense, 20 to 50 feet Weber, 1949; Otton, 1981). of gouge breccia occurs above, and 10 to 50 feet below the fl inty microbreccia surface. Structure - Tectonics In order to better define the geometrical dis­ The Copper Penny area is dominated by low-angle, tribution of linear kinematic indicators in the map extensional tectonic features with all units showing area, we compiled lower-plate mylonitic quartz lin­ some degree of cataclasis. Lithologic disruption ea ti on and s1i ckens i ded stri ae from the fl i nty mi cro­ and deformation in the upper plate increases dOvln­ breccia zone. As shown on Figure 7, the mylonitic ward culminating in total brecciation in the middle quartz lineation in lower-plate rocks, north of the plate and along the detachment fault. The intense COPBer Penny prospect, form a double-maxima: cataclasis immediately below the fault (sharply at N70 E±5° ard N55 0 E±5°. On the other hand, stri ae first, then gradually) decreases downward. The net from the detachment surface in the northesn pgrtion effect is extreme distension creating countless voids of the map area are unidirectional: N40 E±5 (Fig. that inflate the section to give it a megabreccia 70). Comparable analyses were made in the southern appearance. The primary structural features are low­ portion of the area. As indicated on Figuse 78 and angle faults at the tops of the lower and middle 7E, both elements are unid~rec6ional: N55 E±lOo plates. sympathetic listric normal faults, open to (quartz lineation) and N60 W±5 (striae). The

N c::::=J 0-2'\, N c::::=J 0-2'\, N c::::=J 0·2 .,~ ~ 2 ~5 f""=J 2-10 ~ 2~ 5 IIlIIIIIIJl 5-10 IIlIIIIIIJl 10·20 IIlIIIIIIJl 5-10

~ 10-15 ~ 20-30 1/;:';1 10-15

'15 ' 30 >15 W - W E- W -

No 121 N I1B No 239

A B C

N c::::=J 0-2 uri N c::::=J 0-2 ~, N c::::=J 0_2°, E3 2~5 ~ 25 g--~=1 2~4 IIlIIIIIIJl 5-10 IIlIIIIIIJl 5-10 IIlIIIIIIJl 4 6 ~ 10-15 1<%1 ~10 1/1 6-8

'15 >8 W - W W E-

N~ 120 N B2 N" 202

0 E F Figure 7. Lower hemisphere Schmidt equal-area net and corresponding strike frequency rosettes (all in percent) for penetrative lineation in lower-plate mylonite (A, B, and C) and striae, slickensides, and grooves from the prominen~ smooth, polished Buckskin detachment surface (0, E, and F). Data sets A and 0 were collected along or north of the detachment surface while Band E are from the southern portion of Figure 2. Plots C and F are corresponding summations. N equals the number of linear elements plotted.

190 summation of lower-plate lineations and striae from the greatest concentration of metallic mineralization the Buckskin detachment surface are given on Figures in the upper and lower plates. A similar fold pair 7C and 7F respectivelY6 A common direction is shared in bhe trach~basalt (NEl;" Sec. 31, Fig. 2) plunges by both elements: N40 E. Statistically significant N35 W, 20-30 . secondary trends, however, are clearly evident on these synoptic plots. The e data, when considered Throughout the upper and middle plates, a diverse in concert with the N40-506W-trend of major upper­ assortment of small scale mesoscopic fold styles were plate fold axes (Fig. 2), sugge t that upger-plate noted. Although many trends are present, a statis­ rocks were transported in a N406E-540 0 W±5 direction. tical analysis clearly detines a marked preference Similarly, it is apparent that the lower-plate for axes to plunge 520-50 E, 40-45 ; a direction mylonites do not possess the single unidirectional coaxial to the megascopic, upright, open folds of the lineation cited in Davis and others (1977, 1980). mapped area. At least three domains are present in the area investigated Vlith each being pervaded by either NE­ A doubly pl~nging, N65 0 E-trending anticline or ENE-trending mylonitic quartz lineations. flanked by a 540 W-plunging syncline in marble brec­ cia is shown in Section 30 (Fig. 2). The arch and The middle-plate fault actually represents the trough are not defined by bedding or foliation, but top of a zone of intense distributed shear which by the overall form of the outcrop. They represent extends upward from the detachment surface. As a megaboudin elongated into the direction of trans­ illustrated on Figure 3, it is subparallel to the port. Identical forms have been described by Davis detachment surface and marks the boundary between (1980) in the Catalina-Rincon complex. unmetamorphosed and metamorphosed rocks of the Artillery Formation. The fault often merges with Listric Normal Faults the detachment surface. particularly in the south­ western half of the map area (Fig. 2). Although only a few NE-dipping listric normal faults are shown on Figures 2 and 3, the extent of Folds this style of faulting is considerably greater. As seen in the drill core, faults with gouge and crush The 545 0 E, 40 0-plunging axis of an open, upright breccias occur at intervals of tens of feet suggest­ antic6ine intheArtillery Formation is flanked by ing several orders of magnitude more faulting than a 545 E-trending syncline in the center of the map were actually mapped. None of the listric faults area (Fig. 2). The anticline is coincident with cut the detachment surface; moreover, they flatten

Figure 8. Polished slabs of organic-rich, arkosic siltstones from the upper-plate Artillery Formation showing gash veins with a variety of sizes and intensities. Note the sphalerite grain (sp) in D.

191 into it or into the middle-plate fault, comparable and fault-vei n occurrences are the mos t important whi 1e to geometries described by Shackelford (1976) in the the most spectacular and highest metal concentrations Rawhide and by Davis and others (1980) and Frost occur along the detachment fault and in listric fault (1981) in the Whipple Mountains. zones. Figure 9 shows the distribution of copper and fluorine in the four drill holes along cross-section Small Scale Features B-B' plotted adjacent to a generalized geologic log of the holes. Note the 6 feet of significant copper Small scale structures include sandstone, mineralization localized along the detachment surface shale, and pebble dikes, bedding plane slips and in DH-2 and the 40 feet of >0.1% Cu in DH-4 related gash veins which are volumetrically the dominant to a listric normal fault. The fluorine distribution style of deformation in the upper and middle plates. mimics that of the copper, except in the lower-plate Clastic dikes and dikelets commonly follow bedding mylonites where the total fluorine rapidly decreases planes or occupy gash veins. Clasts vary in size to a background level. from 1 to 2 inches in diameter to silt-sized part­ icles, or smaller, set in a fine-grained matrix of In the chlorite shatter breccia, pyrite, chalco­ hematitic or chloritic mud. Virtually all bedding pyrite and specularite plus rare fluorite occur as planes show evidence of slippage with slickensides disseminated grains and in microveinlets that cut and contain a lustrous schistose sheen, especially fragmental mylonitic gneiss. Individual sulfide and on the parting shales. Gash veins, usually filled oxide grains are euhedral, subhedral, and anhedral with gangue or ore minerals, form in brittle units displaying variable degrees of cataclastic textures. sandwiched between plastically deformed units (Fig. Sulfide concentrations seldom exceed 2% and will 8). The frequency of gash veins increases downward average 0.2 to 0.5% and contain pyrite/chalcopyrite in the upper plate. ratios of 0.1 to 0.3. Specularite averages 0.2 to 0.5% with up to 4% in places. Basin and Range Faults Finely comminuted and abraded sulfides and spec­ A NW-trending, sub-vertical fault that cuts ularite set in a matrix of sulfide-hematite (probably and displaces the detachment surface is present in derived from specularite) mud was encountered at each the northeast corner of the map area (Figs. 2 and 3). drill hole penetrating the detachment surface (Fig. NW-trending faults with lateral movement are rela­ lOA). In DH-2, this zone was 6 feet thick and assayed tively common in the Buckskin Mountains (Fig. 1). 1.48% Cu (Fig. 9). The significant copper zone cut At Mineral Hill, the Norma-Continental fault system in DH-2 involves part of the flinty microbreccia and (Harrer, 1964) displays similar characteristics but the gouge-breccia above the surface with well-rounded also cuts and displaces the Osborne Wash volcanics. chalcopyrite and specularite grains (up to 0.2 in. A post-10 m.y., Basin and Range age is indicated by diameter) set in a foliated matrix of finely abraded comparative features of both fault systems. specularite and sulfides (Figs. lOB and laC). Al­ though the limits of this zone are unknown (it was Mineralization missed in DH-7, -4, and -5) the up-dip continuation of it probably correlates with the copper-rich hem­ Sulfide-oxide metallization consisting of atitic jasper mapped to the south along the Swansea pyrite, specularite and chalcopyrite with minor road (Fig. 2). sphalerite, galena, and manganese oxides is present in varying amounts throughout the lower, middle, Oxide and sulfide replacement of marble breccia and upper plates. The metallic minerals typically was economically important at Planet and Swansea fill an assortment of open spaces created by thin­ (Bancroft, 1911; Cummings, 1946) but rarely occurs at skinned,brittle deformation of extensional type with the Copper Penny. Traces of sulfides replacing minor but important replacements in brecciated­ marble breccia were encountered in DH-6 (Fig. 2) where sheared reactive units. The overall distribution remnant chalcopyrite, superficially altered to chalco­ of mineralization is coplanar with the detachment cite and earthy red hematite, replaces marble breccia surface, decreasing both above and below the fault. in the vicintiy of DH-3 (Figs. 2 and 9) and contains Mineral fabrics and textures indicate deposition numerous veinlets, microveinlets and irregular clots during and after final movement of the allochthonous of specularite and malachite-azurite-chrysocolla units suggesting a tectogenesis related to the low­ indicating partial replacement. At Swansea, Bancroft angle process. (1911) mapped large, irregular masses of cataclastic­ ally deformed chalcopyrite that are rimmed by massive Mode and Distribution of Mineralization specularite which replaces the same marble breccia. The ore minerals, in decreasing order of abund­ In the upper plate, chalcopyrite with gypsum ance, are: chalcopyrite, specularite, pyrite, (Fi g. 100), fl uorite, and/or calcite is found in sphalerite, manganese oxides, and galena. Assoc­ breccias associated with listric faults. The crush iated gangue minerals are: calcite, gypsum, fluo­ breccias range from 0.2 to 6.0 inches thick and rite, barite, and quartz, opal or jasper. Mineral­ repetitiously offset beds in the Artillery Formation. ization is distributed between five separate but Chalcopyrite, up to 0.5 in. diameter, is the dominant structure-tectonically related settings: 1) commi­ ore mi nera1 with occas i ona 1 traces of honey- co 1ored nuted disseminations within the flinty chlorite sphalerite and pyrite. Cataclastic textures in the microbreccia, 2) massive sulfide-oxide replacements sulfideS are always present; usually the exploded after reactive brecciated marble, 3) open space texture illustrated in Figure 100. The gangue filling of shoots and sheets of tectonic crush minerals are similarly shattered, sheared, rotated breccia at the intersection of synthetic listric and mixed with rounded wall rock fragments giving the normal faults and the detachment surface, 4) vein a fluidized microbreccia appearance. antithetic and synthetic gash- and fault-veins, and 5) longitudinal fissure fillings along the crest or The gash veins, generated during movement in trough of megascopic folds. Volumetrically, gash- brittle units, bounded on either side by ducti lly deforming shales, are common to abundant in the upper

192 DH 8 DH 1 DH 4 F. F

UPPER PLATE

DOMINANT LITHOLOGY UPPER PLATE MIDDLE PLATE [!TI Basalt flow ~Cataclastic schist t:;t;;;:i] Cong lome rate I2Qj Marb 1e brecci a li:1 Sands tone-arkose I;·; 1 Grani te-brecci a ~~ Sha 1e-muds tone LOWER PLATE ~ Limestone ~ Equi granul ar-grani ti c [El Rhyo 1i te tuff [:":-;:i Augen-banded ~ Flinty crushed breccia

%.4 .2 .15 .I .05 0 40 80 150 250 500 IOOOppm I I I !! t ! ! ! !! ! Fluorine --- -+- Copper [0 LOWER PLATE

[100Feet

Figure 9. Cross-section 8-B' detailing lithologic, structural, and copper-fluorine gradients in four drill holes. Copper and fluorine histograms are plotted to the right and left respectively of each schematic log. Copper assays were derived from 10' split intervals while fluorine assays are from 10' intervals in DH-l and 50' composites from DH-2, -4, and -3. plate. Figure 11 shows the intensity of calcite­ shows the distribution of chlorite-,epidote-, calcite-, filled gash veins plotted in veinlets/foot (actual and gypsum-filled fractures in veinlets/foot encoun­ count during core logging) for the 4 drill holes tered in the 4 drill holes shown on cross-section shown on Figure 9. The remarkable correlation B-B'. As illustrated in this figure, the chlorite between the ca 1cite i ntens ity (Fi g. 11) and fl uori ne is asymmetri ca lly centered on the detachment assays (Fig. 9) suggest a correspondence between surface decreasing above and below the fault. Calcite calcite gash veins and fluorite content. A series and epidote appear to decrease with increasing of gash veins cutting Artillery arkose and siltstone chlorite. Epidote occurs only in the lower plate is illustrated in Figure 3..The veins are invariably while the apices of the calcite curve, though confined to the brittle unit and seldom propagated strongest in the upper plates, flank the chlorite through the bounding shale. The veins range in zone. thickness from hairline cracks to 0.5 inches. Fill­ ings consist of calcite or gypsum with subordinate Chlorite and Epidote fluorite, barite, chalcopyrite, sphalerite (Fig. 3D) and rare galena. The mineral grains in the veins The chlorite is a black-green, greasy variety in usually have euhedral to subhedral outlines and the mylonite and bright to apple green in the upper almost never have cataclastic textures. plates. Chlorite pervasively replaces biotite and hornblende within the shatter breccia; an observation Alteration not shown by the mineral distribution in Figure 11. However, total veinlet chlorite greatly exceeds rock The dominant wall-rock alteration of Copper chlorite after mafics. The intensity of chlorite­ Penny is propylitic and characterized by the filled veinlets is a direct function of the degree development of chlorite and calcite with subordinate of brecci ati on found along the detachment surface. amounts of epidote, gypsum and clay. Figure 11 The increasing intensity of chlorite in DH-4 (Fig. 11)

193 Figure 10. Photomicrographs (X30) of polished core showing A) detachment fault (OF) separating hematite-rich flinty microbreccia from hangingwall marble breccia (OH-4); B-C) abrasively rounded chalcopyrite (cp)and marble (m) clasts set in a finely comminuted specularite microbreccia matrix (OH-2); and 0) chalcopyrite breccia cemented by shattered gypsum (g) from a listric normal fault zone (OH-4) . is due to chlorite selvages along gypsum veinlets. and chalcopyrite. Other minerals commonly ~ssociated with chlorite are pyrite, chalcopyrite, specularite, and fluorite. Clay and Gypsum Epidote (probably clinozoisite) is a pea-green Gypsum-filled veinlets are found only in the variety occurring in shatter breccia veinlets, re­ middle plate where they are associated with the placing mafic minerals and feldspars, and occasion­ weakly metamorphosed Artillery Formation (OH-4 on ally flooding the footwall mylonitic gneiss. Al­ Fi g. 8 and in OH-7). The gyps urn fi 11 s gash vei ns though the chlorite-epidote relatioship is antithetic, which cut calcite veins indicating a late stage of the total epidote rarely exceeds the chlorite content. formation. Veins up to .5 in. thick are filled with Outside the Copper Penny area, widespread epidote cryptocrystalline and cross-fiber gypsum associated mineralization occurs in areas well below the detach­ with chlorite, pyrite and chalcopyrite. The white ment suggesting a regional metamorphic origin. swelling variety of clay was recognized only in lower-plate rocks filling thin hairline cracks which Calcite cut all other veinlets. Clay alteration may be more widespread in the upper plates but it was not Calcite is commonly crypto-crystalline (white, recognized in the drill'core. The clay, where sub­ pink, or green) with a modest percentage of euhedral stantiated as non-supergene, is mineral destructive; to subhedral crystals or crystalline aggregates. pyrite was altered to earthy hematite. Calcite is found in gash veinlets (Fig. 8), breccia matrices and veins. Minor amounts of calcite, in Summary the lower plate, increase with decreasing chlorite below the detachment fault. Calcite-filled vein The principal geologic characteristics of the intensity decreases in intervals where gypsum-filled Copper Penny deposit are summarized as follows: veins are present. Minerals associated with calcite 1. Mineral deposition occurs within and below are fluorite, barite, pyrite, sphalerite, galena, a pile of allochthonous rocks preserved in

194 DH 8 DH 1 DH 4 DH 2

UPPER PLATE

DOMINANT LITHOLOGY UPPER PLATE MIDDLE PLATE ~ Basalt flow ~ Cataclastic schist 1: ••1Conglomerate ~ Marble breccia 1/.;[ Sandstone-arkose I1ill Grani te-brecci a Ij:~ Sha 1e-muds tone LOWER PLATE m Limestone I~~I Equi granul ar-graniti c IX I Rhyo 1i te tuff 1"=,=-:1 Augen-banded ~ Flinty crushed breccia

ax 70 30 10 a 10 50 100 ax Vnlets/Fl

Calcite Gypsum [' '"";;":0>'

l50 LOWER PLATE

100leet

Fi gure 11. Cross-section B-B' detailing litholgic, structural, and chlorite, epidote, calcite, and gypsum distributions and intensities in four drill holes. Chlorite (solid line) and epidote (dashed line) histograms of the number of veinlets per foot cutting the rocks are plotted to the right of the schematic log. Calcite (solid line) and gypsum (dashed line) histograms are plotted to the 1eft.

an ENE-trending megagroove within meta­ 4. The movement-generated open spaces were morphic core complex terrain. filled with varying concentrations of ore, 2. The allochthonous sequence consists of gangue, and alteration minerals during and gently folded, penetratively faulted, and following final movement of the allochthonous intensely brecciated but unmetamorphosed blocks. The rounded finely abraded chalco­ units of the Artillery Formation in the pyrite and specularite along the detachment upper plate and cataclastically deformed, fault and ubiquitous cataclastic ore metamorphosed, but non-lineated rocks of textures along listric-faults indicate the Artillery Formation in the middle deposition contemporaneous and after move­ plate. Both sequences overlie the Whipple­ ment. Conversely, footwall shatter breccia Buckskin Mountains detachment fault and the and gash vein oxide and sulfide mineralization footwall foliated and lineated mylonitic evidence exponentially decreasing cataclasis gneiss. with increasing depth beneath the detachment 3. The structure-tectonic setting is dominated surface. by extreme thin-skinned distension of 5. A rudimentary alteration-mineralization upper- and middle-plate lithologies due zoning pattern is recognized centered about to ENE-directed movement of hangingwall the detachment surface. The quantified rocks along the detachment fault. Tectonic changes in alteration intensity are crush breccias, synthetic and antithetic accompanied by systematic variations in ore gash veins, and high-to low-angle faults minerals, gangue, and alteration products that formed in response to this movement above and below the fault. created innumerable voids above and below 6. Assuming the K-feldspar K-Ar age data from the detachment fault. GH-8 accurately records the annealling-

195 cooling age of the footwall mylonitic denoted by the widespread calcite distribution (C0 ), shatter breccia, late stage mineralization the low sulfide to specularite ratio. (0 and S), 2 continued until 17.7 ± .7 m.y. This age and the presence of fluorite (F). Inferfed cationic date is congruent with the 18.6 ± 1.5 m.y. concentrations are suggested by coexisting chlorite K-Ar date for the mineralized trachybasalt compositions (7 hand-picked chlorite samples averaged and the 15.9 m.y. upper limits for detach­ 9.9% Fe and 5.6% Mg), the ubiquitous occurrence of ment faulting established by Martin and pyrite and specularite, and drill core assays for others (1980). Cu, Mo, Pb, Zn, Au, and Ag. 7. NW-trending Basin and Range faults have appreciable right lateral oblique slip We conclude that isothermal surfaces were cen­ components and offset' portions of the tered on, and subparallel to, the detachment surfaces mineralized systems. and that very close-spaced isotherms occurred above and below the fault. An internal maximum tempera­ Physical and Chemical Constraints on Mineral Depositj~ ture of 400 0 C is probably high due to substantial argon loss through mechanical annealling and cataclasis. A preliminary synthesis of assembled geologic We suspect that the ore-forming fluids actua16Y and geochemical data allows one to place qualitative attained temperatures ranging from 100 to 150 C at constraints on the temperature of deposition, fluid the detachment surface. flow paths, and determine the rudimentary chemistry of the mineralizing solutions. Genesis of the Copper Penny Deposit The maximum temperature range was estimated from The genesis of this deposit is indirectly re­ the thermal resetting of the K-Ar age dates. Mylon­ lated to the development of Laramide metamorphic itization ages in the Whipple and Rawhide Mountains core complexes and intimately related to detachment range from 78.5 m.y. (Martin and others, 1980) to fault tectonics. A variety of models have been 52.3 m.y. (Shackelford, 1977). However, a pronounced proposed to explain the features that, in aggregate, younging-trend, upward to the detachment fault, is define a metamorphic core complex (Coney, 1980; consistently present throughout the complex (Dokka Davis, 1980; Rehrig and Reynolds, 1980; Davis and and Lingrey, 1979; Davis and others, 1980). Martin others, 1980). Although many details of each model and others (1980) found that age dates for synkine­ differ, all agree that detachment faulting was the matic sills, dikes, and anastomosing apophyses deep terminal event and that it disrupted and deformed ;n the complex decrease structurally upward in a system­ upper- and lower-plate rocks. atic manner from 25.5 to about 15.3 m.y. at or immed­ Our investigations demonstrate that mineraliz­ i~tely above the flat detachment fault, implying ation is spatially related to the detachment fault a therma 1 and/or phys i ca 1 resetti ng of the K-Ar ages and we strongly imply a tectogenetic relationship of by detachment faulti ng. Damon (1968) documented it to the fault process. We propose that the flow similar conditions in the Catalina-Rincon complex and of mineralizing fluids at elevated temperatures suggested that the n~lonites ~ust have sustained were channeled into dilatant settings along: 1) the temperatures in excess of 400 C to completely reset detachment surface, 2) listric normal faults, 3) the K-Ar ages. The 17.7 m.y. K-feldspar K-Ar age crush/shatter breccias, and 4) axial plane joint sets; obtained from mylonitic shatter breccia gneiss in however, the origin of the fluid and heat sources DH-8 represents an annealling-cooling date of older responsible for fluid circulation are not readily 52-78 m.y. core complex mylonites. apparent in the Copper Penny-Swansea area. Quantity and distribution of alteration, ore, Potential sour.ces of heat include that released and gangue minerals in concert with the distribution by friction drag during movement of the allochthonous of breccia and open space development are useful blocks and intrusion of syn- or post-kinematic dikes, fluid flow indicators. That the detachment fault was sills and necks. Post-kinematic dikes, similar to the primary path for mineralizing solutions is . the microdiorite dikes described by Rehrig and indicated by: 1) the asymmetrically centered dis­ Reynolds (1980) in the nearby Harcuvar Mountains, tribution and intensity of chlorite, 2) the highest were not identified in the Copper Penny area. concentrations of sulfide and oxide mineralization, and 3) the inverse relationship of chlorite and copper A contribution to the overall heat budget to lower-temperature phases such as calcite, epidote, probably involved heat produced by movement of the clay and gypsum (Figs. 9 and 11). Cataclastic deform­ upper plates along the detachment fault. This could ation in middle-plate rocks permitted widespread constitute a significant contribution of heat accord­ circulation of the fluids through these units as ing to the stick-slip theory proposed by Brace and evi denced by pervas i ve but low concentrations of Byerly (1966). chlorite, chalcopyrite, gypsum, calcite, and minor specularite mineralization. Upward migration of the We propose that the observed lithologic, struc­ fluids was largely restricted to listric fault zones tural, alteration-mineralization, and geochemical because chalcopyrite, gypsum, chlorite, and fluorite data suggest a tectogenetic relationship between only occur in crush breccias along these faults. Wide­ the detachment fault process and ore 1oca 1i zati on. spread, low-temperature fluid flow in the upper plate Movement of the upper plates generated widespread is indicated by the appreciable volume of sphalerite­ shatter/crush breccias and gash veins that provided bearing gash veins that occur in unaltered, organic­ paths for metal-bearing fluid flow at elevated rich arkose and fetid limestone. temperatures. The oxidizing, carbonate- and fluorine-bearing solutions leached-out pre-existing The mineralizing fluids were highly oxidizing unstable mineral phases prior to and during are with high CO , moderate to low fluorine and low deposition. Ore and gangue minerals were deposited sulfur conce~trations. The percentage of iron, from the solution in the dilatant movement-generated magnesium, and manganese was high, copper and zinc voids or replaced reactive marble breccia units in were moderate to low, and lead, molybdenum, gold the upper, middle, and lower plates. Mineral and silver were very low. Species of anions are

196 deposition, alteration, and movement along the ing orebody distribution (Fig. 3) were constructed detachment surface were coeval; they ceased prior from drill hole data supplied by one of the property to the outpouring of the Osborne Wash volcanics. owners (J. Challinor), from Bancroft (1911). and unpublished mapping by the authors. Following OTHER DEPOSITS IN THE WHIPPLE AND BUCKSKIN MOUNTAINS cessation of operations in 1937 (Keith, 1978), Swansea lay dormant until 1959 when it was acquired by the Most important mines and prospects in the Buck­ current owners. A succession of mining groups since skin and Whipple Mountains (Fig. 1) have many geo­ 1979 have explored the property, completing several logic features in common with the Copper Penny geophysical and geochemical surveys plus about 50 prospect. The dominating characteristics shared by drill holes. Exploratory drilling was confined to the the deposits are as follows: immediate mine area resulting in the data void north­ 1. All mines and prospects occur immediately west of the mine (Fig. 13). above, below, or along the Whipple-Buck­ skin detachment fault or along tectogeneti­ The lithologic sequence and structural setting cally related structures. at Swansea is identical, in many aspects to Copper 2. All are positioned in upper- and middle­ Penny. The lower plate consists of mylonitic gneiss plate rocks along ENE-trending megagrooves with a gently dipping foliation and a penetrative and warps in the detachment surface. lineation. The detachment surface, a faulted offset 3. A district-wide mineral suite is dominated continuation of the fault at Copper Penny, is gently by specularite and chalcopyrite. and steeply dipping along the NW and SE margins respect­ 4. The mineral fabrics and textures consist­ ively. Details of the middle plate are lacking in the ently denote syn- and post-kinematic ore mine area for want of outcrops. At the main workings, deposition. a megaboudin of marble breccia overlies the detachment 5. Wall-rock alteration is always propylitic fault and interfingers with cata81astic schist. In (chlorite-dominant) and asymmetrically outcrop, the marble forms a N65 E-trending, doubly centered about the detachment fault. plunging anticline that is an offset continuation of 6. The geologic relationships indicate a post­ the middle~plate Copper Penny marble breccia. The Artillery Formation and pre-Osborne Wash cataclastic schist, poorly exposed in outcrop, is Formation period of oxide-sulfide deposi­ believed correlative with the schist at the Copper tion. Penny prospect. The three major producers (Swansea, Planet, The granite breccia, identical to the Copper and Mineral Hill) and one fully explored deposit Penny unit, outcrops NW and WNW of Swansea but the of major proportions (Copper Basin) were re-examined geologic relationship between it and the schist is within the context of the data gleaned from the Copper uncertain. Chapin Wash redbeds, consisting of COilrse­ Penny occurrences. grained clastics and interbedded lacustrine limestone showing camel tracks, outcrop 1 mile NE-ENE of Swansea Figure 12, a schematic cross-section depicting (Gast>away, 1972). This homoclinal dipping sequence the mode and distribution of metallization at Copper (N20 W, 45 0 SW) of Chapin Wash Formation represents Penny, illustrates the format used in describing the remaining remnants of the upper plates at Swansea. 'ore localized at each subsequent deposit. This cross-section was generalized from section B-B' The orebodies at Swansea occurred in two separate (Fig. 3) and shows our interpretation of the data but interrelated modes: the "footwall orebody" and compiled. Important settings depicted include the the "replacement orebodies" (Challinor, pers. comm.). massive chalcopyrite-specularite metallization along The ore zones were described by Bancroft (1911) as and immediately above the detachment surface (hachured), specularite with chalcopyrite and minor pyrite marble breccia replacements (solid), and metallization replacing marble breccia with a quartz-epidote gangue controlled by veinlets in footwall chlorite shatter and "... large limestone boulders ". He also noted breccia, open space filling in listric fault "Evidences of extreme movement " and "... great breccias and gash veins (stippled pattern). slickensided surfaces and seVere contortions ... " in the underground workings. He stated that the "move­ SWANSEA ment appears to be more recent than Precambrian". Bancroft's observations on the structural deformation The geologic cross-section and schematic show­ confirms the post-tectonic nature of ore deposition S400E N400W

DDH-GI DDH-G4

MINERALIZATION

Disseminated and microveinlet sulfides Sulfide on listric fault I ~ I Massive sulfide~specularite rep 1acements Gash vein sulfides Iii,i1"i. I Specularite with sulfides Figure 12. Schematic representation of cross section B-B', Copper Penny prospect (Fig. 3), showing five (5) principal structure-tectonic settings supporting significant Cu-Fe metallization.

197 LITHOLOGY 1500 ~ Marble Bx ~CataclasticsChist 1000 12J Grani te 8x ~ Detachrrent fault ~ Mylonitic gneiss 500

1000 2000 It MINERA.LIZATION oLI l , '

~ Massive Chalcopyrite + specularite

Di ssemi na ted-rni crovei n let suI fi de-specul ari te

Figure 13. Generalized geologic cross-section at the Swansea mine (top) and schematic representation of the principal structural-tectonic setting hosting Cu-Fe orebodies and other metallization (bottom). DF, detachment fault ore; and m, marble breccia replacement ore.

as documented at Copper Penny. The footwa 11 ore is is tentatively identified as Mesozoic(?) based on comparable (except in tonnage and grade) to the its physical resemblance to Mesozoic units in the detachment fault mineralization encountered in DH-2. Dome Rock Mountains (Marshak, 1980), and the Plomosa Mountains (Harding, 1980). The Chapin Wash The extent of the metallization at Swansea was Formation outcrops along a WNW-trending belt about expanded by the post-1959 drilling program. A zone 1.5 miles southwest of the Planet mine. Here the of microveinlet to disseminated sulfides (including ChapiB Wash Formation consists of a NW- to WNW-striking chalcopyrite, pyrite and bornite), specularite and 40-50 SW-dipping sequence of redbed conglomerates native copper was found in the hangingwall cataclastic and pebbly sandstone with intercalated basalt flows. schists. Pyrite and specularite with minor chalco­ An anastomosing dike-like body of breccia with clasts pyrite were encountered in the footwall chlorite ranging from less than 0.1 em. to 25-30 feet across shatter breccia immediately beneath the detachment crops out along a WNW tr'end, 4000 feet southwes t of surface (Challinor, pers. comm.). the Planet mine. This tectonic or intrusion breccia is shown in the center of the crgss-section (Fig. 14). The geologic features of the Swansea orebody The dike-like body dips about 50 NNE and cuts, deforms, closely resemble the features noted at Copper Penny. and displaces the intruded metatuff unit. It was The lithologicand structural characteristics, initially mapped as a low-angle fault breccia, but, excepting the well-defined upper plate, are identical upon completion of the structure contour map of the overall but differ in detail. detachment surface, the dike appears to upwarp and displace the detachment fault. The breccia matrix Planet - Mineral Hill is stained brick-red by finely comminuted hematite particles surrounding large clasts (±20 feet diameter The Mineral Hill to Planet cross-section and of marble breccia partially replaced by specularite corresponding mineralization schematic was prepared and chalcopyrite. A post-detachment fault and post­ from Harrer (1964), unpublished geologic maps by J. mineral origin for the breccia body is indicated. Wilkins (1969) and F. Bergwell (1978) and from drill hole data supplied by one of the property owners (E, Extensive drilling along the Mineral Hill-Planet R. Alcott). The Planet mine was explored for its iron section (Fig. 14) demonstrates convincingly that potential by the U. S. Bureau of Mines who completed the NE-dipping listric-normal faults flatten progress­ 12 churn and 10 diamond drill holes in the inmlediate ively with depth as shown. At Planet and Mineral Hill, vicinity of the mine. The results of a 15 hole -1979­ the detachment fault is repeatedly cut and offset drilling program that explored the interval between by a series of NW- to NNW-trending high-angle normal Planet and Mineral Hill were provided by Alcott. faults possessing significant oblique-slip. North A structure contour map on the top of the detachment of Mineral Hi 11, these same faults cut and displace surface provides the data base for preparing the 8 to 13 m.y. old volcanics of the Osborne Wash geologic cross-sections shown on Figure 14. Formation. The lower-plate mylonite is foliated, lineated, Mineralization at Planet and Mineral Hill is augen to compositionally banded gneiss inherent to dominated by specularite, chrysocolla, ± malachite most core complexes. The detachment surface is a derived from the supergene oxidation of chalcopyrite. flinty microbreccia overlying an extensive chlorite Specularite occurs predominately along the detachment shatter breccia. A thin wedge of marble breccia fault partially replacing superjacent marble breccia lies paraconformablyon the detachment surface ~f as well as shatter veinlets and microveinlets above Planet, Mineral Hill, and in several drill holes and below the fault (Fig. 14). Relict SUlfides found located midway between the mines (Fig. 14). The on the dump at Pl anet are pyrite and chalcopyrite marble breccia represents a middle-plate tectonite. showing cataclastic textures. Drill holes penetrating the detachment fault encountered chlorite shatter The Planet - Mineral Hill upper plat~ constitutes breccia with microveinlet and disseminated specularite, a homoclinal series of NW-striking, 30-50 SW-dipping, pyrite, chalcopyrite, and in some instances, native weakly fol iated and 1ineated metasedimentary and copper. Meta 11 i zati on along 1istri c faults i s \~i de­ metavolcanic rocks. This ±2200 foot-thick sequence spread consisting of pyrite-chalcopyrite ± native gold

198 S70W N70E

MINERAL HILL MINE PLANET ~ MINE

1000

500

2000 40'00 If.

\ \ \ \

LITHOLOGY MINERALIZATION

~ Breccia-Intrusive _ Phyllite ~ or Tectonic c=J Disseminated + microveinlet ~Metamudstone sulfide and speculari te ~ Metatuff ~ Massive Hematite-specularite ~ HarbleBx ~QuartziteBx with massive to diss. sulfides ~ Detachllx:'nt fault ~ Metaconglomerate M Hylonitic gneiss

Figure 14. Generalized geologic cross-section through the Planet and Mineral Hill mines (top) and schematic representation of the principal structure-tectonic settings hosting Cu-Fe orebodies and significant sulfide-oxide metallization (bottom). and their supergene oxidation products (earthy hematite, and schist intruded by diorite to granodiorite and malachite, and chrysocolla). Gangue minerals include andesite to dacite plutons, sills and dikes. The non­ quartz and cryptocrystalline silica pervading marble mylonitic crystalline rocks are paraconformably over­ breccia at the Planet mine with barite-calcite in lain by the Copper Basin Formation: a basal basalt attendant listric faults. flow (K-Ar age dated at 18 m.y.) and a succession of redbed conglomerates, fanglomerates, and pebbly Copper Basin to medium-grained sandstone (Frost, 1981). The Copper Basin cross-section (Fig. 15) was The cataclastically deformed upper-plate rocks constructed by T. L. Heidrick and E. G. Frost for are pervasively altered by chlorite and epidote. the 1980 Arizona Geological Society Road Lng (Heidrick Greasy black chlorite and to a lesser extent epidote and others, 1980) and subsequently discussed in detail replace mafic minerals and occur in veins, veinlets, by Frost (1981). The cross-section was prepared and microveinlets cutting upper-plate crystalline utilizing a geologic map completed by Lingrey and rocks. others (1977). Metallization details were derived from unpublished maps and reports provided by Mr. Although a minor amount of sulfide mineralization R. Odien of the Louisiana Land and Exploration Company. occurs below the detachment fault in the chlorite shatter breccia, the bulk of the metallic minerals Augen to compositionally banded mylonitic gneiss, lie within the upper plate. Three significant inherent to metamorphic core complexes, comprise the concentrations of copper mineralization were lower-plate sequence (Fig. 15). The detachment fault delineated during the drilling program. Each is a curviplanar surface composed of microbreccia concentration has a somewhat triangular outline in that dips 9 SE and overlies a variable thickness (10 plan with their bases striking NW and their apices to 300 feet) of sheared, shattered, and chlorite­ dipping gently to the NE; an orientation roughly flooded breccia. At Copper Basin, the detachment coplanar with mapped sympathetic listric faults. surface is warped into a series of low amplitude (200 The hypogene minerals consist of intimately inter­ to 300 feet) synformal and sntiformal megagrooves with grown pyrite-chalcopyrite-specularite and minor axial trends of N50-60 0 E, 6 (Heidrick and others, amounts of bornite and molybdenite. Both sulfides 1980; and Frost, 1981). The metallization at Copper and oxides occur predominantly in short discontinuous Penny is likewise concentrated along the flanks of veins, veinlets, and microveinlets of quartz and anti forms. epidote or structureless green chlorite + quartz. Mineralization in the Copper Basin Formation occurs The upper-plate crystalline rocks consist of as pyrite-chalcopyrite - specularite plus their cataclastically deformed but non-mylonitic gneiss oxidation products in veinlets and micro-

199 COPPER BASIN MINERAL DEPOSIT

2000

1000

~IOOO

loaD 2000 4000 fl

,,

LITHOLOGY MINERALIZATION

Copper Basin Fm. ~ Upper plate intrusive rock Oi ssemi nated~Mi crovei nlet ~ Detachment fault 1Z:1J Upper plate crystalline sulfides + specularite rocks ~ Myloniticgnelss

Figure 15. Generalized geologic cross-section through the Copper Basin deposit (top) adapted from Heidrick and others (1980) and a schematic representation of the structural-tectonic setting hosting significant Cu-Fe mineralization (bottom) veinlets near the base of the section and throughout are structure-dominated and without exception they listric fault breccias (Fig. 15). are extensional and dilatent in character. Figure 16 schematically portrays the spatial distribution CONCLUS IONS of each respective locus within upper- and lower­ plate rocks including, in order of their economic Detailed geologic field studies augmented by significance, the: subs tanti a1 di amond dri 11 i ng, geochemi ca 1, petro­ chemical, and petrofabric data, K-Ar age dating, 1. Granulated, brecciated, smeared-out, dissem­ and previ ous underground mi ni ng acti vity provi de inated to massive, subhorizontal orebodies a unique 3-dimensional data base depicting the within and immediately above the flinty, geology and ore deposits of the Copper Penny-Swansea chlorite, microbrecciated detachment surface. mi ne area. Our st ructure-tectoni c synthesis of the 2. Massive replacement and stringer orebodies area indicates that upper-plate Artillery Formation localized in silicified and brecciated rocks were actively transported in an N40-45 0 E marble that was juxtaposed onto the detach­ direction across the gently dipping Buckskin detach­ ment surface. ment fault during late-Oligocene to middle-Miocene 3. Open space filling ores forming moderately time. Detachment faulting was protracted, thin­ dipping sheets and gently raking shoots skinned, and distensional in nature, producing within dilatant crush breccia that are profound faulting, brecciation, and shattering spatially restricted to both synthetic and throughout upper- and middle-plate rocks and in anti theti c 1i stri c norma 1 faul ts. lower-plate mylonites immediately beneath the de­ 4. Fissure filling mineralization restricted tachment surface. Low-grade hydro-dynamothermal to moderately dipping conjugate systems of alteration and attendant base-precious metal mineral­ antithetic and synthetic gash veins and to ization occurred intermittently throughout the joint planes adjoining listric normal structurally disturbed sequence, but was restricted faul ts. to the period of detachment-related tectonism. 5. Fissure filling mineralization localized longitudinally along the dilatant crests Our interpretation of the data assembled and troughs of upright-open to overturned­ clearly defines six (6) principal structural tec­ closed, megascopic folds. tonic loci proven favorable for the localization of 6. Veinlet to microveinlet controlled mineral­ significant metallization. All recognized settings ization restricted to the footwall chlorite

200 greatly expanded during field trips with Eric Frost and in-depth discussions with him, Greg Davis, and 5 members of the Arizona Geological Society. Many details of mine geology were kindly provided by John Challinor, Earl Alcott, John Livermore of Cordex Exploration, and RobertOdien of Louisiana Land and Exploration Co.

Upper We gratefully acknowledge Gulf Mineral Resources Plate Co. for their continuing support and permission to publish these data. We wish to thank Parry Willard, Grant Cummings, and Bill Hardtke for editing the manuscript and Robert White-Harvey for drafting the figures. Norman Lehman critically reviewed early drafts and suggested changes which greatly promoted text clarity.

~ I - "I I II And finally, we are forever indebted to Georgia Recch i 0 who so ably typed the fi na 1 man uS cri pt. Her \ / cheerful and attentive attitude was instrumental in our completing the report on schedule. Fi gure 1G. Structural-tectonic model of mineraliza­ REFERENCES CITED tion loci related to the Whipple-Buckskin detachment fault. Numbered loci 1-6 Anderson, R. E., 1971, Thin-skinned distension in are keyed to text. Tertiary rocks of southeastern Nevada: Geolog­ ical Society of America Bulletin, v. 82, p. 43­ shatter breccia zone found immediately 58. beneath the detachment fault and occasionally Anderson, J. L., Podruski, cl. A., and RO\vley, M. C., extending downward for several hundred 1979, Petrological studies in the "Suprastruc­ feet into the footwall mylonite. tura1" and i nfrastructura1: crys ta11 i nerocks of the Whipple Mountains of southeastern Although each tectonic-mineralization setting California: Geological Society of America (1 through 6, Fig. 16) is present at all of the Abs tracts with Programs, v. 11, no. 3, p. 66. major deposits in the Whipple-Buckskin Mountains, Bancroft, H., 1911, Reconnaissance of the ore depos­ economically significant concentrations are dominated its in northern Yuma County: U. S. Geological by one (or more) settings. The principal loci for Survey Bull. 451, p. 47-67. ore-grade metallization at each mine or prospe~t Blake, W. P., 1965, Iron regions of Arizona: American are, in their order of importance, (keyed to Flg. 16) Journal of Science, 2nd ser., v. 40, p. 388. as follows: , 1898, Report of the Territorial -- Geo 1ogi st, in Report of the Governor of Ari zona: Copper Penny: 1, 3 Dept. of the-Interior report for fiscal year Swansea: 2, 1 ended June 30, 1898, Misc. Reports, Washington. Planet-Mineral Hill: 2, 1, 3 Blanchard, R., 1913, The geology of the western Copper Basin: 3, 4 Buckskin Mountains, Yuma County, Arizona: unpublished Ph.D. thesis, Columbia University, The existence of base or precious metals ore­ 80 p. bodies in settings 5 and 6 (or combination of these Brace, W. F., and Byerlee, J. D., 1966, Slick-slip as a and the others) have never been fully explored and mechanism for earthquakes: Science, v. 153, are distinct and viable targets. p. 990-992. Coney, P. J., 1979, Tertiary evolution of Cordilleran This discussion of the detachment fault-related metamorphic core complexes, ~ Armentrout, mineralization is a progress report representing the J. W., and others, eds., Cenozoic Paleogeography work completed to date in our ongoing efforts to of Western United States: Society of Economic clarify the nature of these deposits. Investigations Paleontologists and Mineralogist, Pacific using fluid inclusions, ore microscopy, and petro­ Section Symposium III, p. 15-28. fabric studies were undertaken in order to further , 1979, Cordilleran metamorphic core our understanding of the ore-forming process. -----c-o-m-p"le-x--es: An overview: Geological Society of America, Memoir 153, p. 7-31. lhe detachment-listric fault structure-tectonic Damon, P. E., 1968, Application of the potassium­ setting in the Mohave and adjacent terranes are argon method to the dating of igneous and meta­ current targets for additional exploration. Our morphic rock within the Basin Ranges of the reconnaissance of many of these areas demonstrate Southwest: Arizona Geological Society Guide­ that the tectonic-mineralization relationships, book III, p. 7-20 shO\',il in Figure 16, are present. We fi rmly bel ieve Davis, G. A., Evans, K. V., Frost, E. G., Lingrey, that concentrated efforts by the exploration industry S. H., and Shackelford, T. J., 1977, Enigmatic wi 11 locate orebodi es in one, or combi nati ons of the Miocene low-angle faulting, southwestern loci sllown on Figure 16. California and west-central Arizona: Geological Society of America Abstracts with Programs: ACKNOWLEDGEMENTS v. 9, no. 7, p. 943-944. Davis, G. A., Anderson, J. L., Frost, E. G., and Our knowledge about and understanding of the Shackelford, T. J., 1980, Mylonitization and nature and magnitude of thin-skinned detachment detachment faulting in the Whipple-Buckskin­ faulting in the Whipple-Buckskin Mountains was Rawhide Mountains terrane, southeastern

201 California and western Arizona: Geological Mountains: Unpub. consulting report to the Society of America, Memoir 153, p. 79-129. Metropolitan District, Los Angeles, Calif., Davis, G. H., 1980, Structural characteristics of p. 10. metamorphic core complexes, southern Arizona: Rehrig, W. A., and Reynolds, S. J., 1977, A Geological Society of America, Memoir 153, p. 35-77. northwest zone of metamorphic core complexes Dokka, R. K., and Lingrey, S. H., 1979, Fission track in Arizona: Geological Society of Americ~ evidence for a Miocene cooling event, Whipple Abstracts with Programs, v. 9, no. 7, p. 1139. ~ountains, southeastern California: Pacific ______-, , and Reynolds, S. J., 1980, Geolo~ic Section, Society of Economic Paleontologists and geochronologic reconnaissance of a north· and Mineralogists (Cenozoic paleogeography of west-trending zone of metamorphic complexes the western United States), p, 141-145. in southern Arizona: Geological Society of Frost, E. G., 1981, Mid-Tertiary detachment faulting America Memoir 153, p. 131-157. in the Whipple Mtns., Calif., and Buckskin Mtns., Shackelford, T. J., 1975, Late Tertiary gravity Ariz., and its relationship to the development sliding in the Rawhide Mount~ins, western of major antiforms and synforms: Geological Arizona: Geological Society of America Soci ety of Ameri ca Abstracts wi th Programs, Abstracts with Programs, v. 7, no. 3, p. 312-373. v. 13, no. 2, p. 57. ____~--=-~~-..' 1976, Structural geology of Gassaway, J. S., 1972, Geology of the Lincoln Ranch the Rawhide Mountains, Mojave County, Arizona: basin, Buckskin Mountains, Yuma County, Arizona: Unpub. Ph.D. dissertation, University of unpublished undergraduate thesis, San Diego Southern California, Los Angeles, California, State University, 64 p. p. 175. , 1977, A reconnaissance study of , 1977, Late Tertiary tectonic Cenozoic geology in west-central Arizona (M.S. denuda~t~i~o-n--o-;:f Mesozoic(?) gn-2iss complr:;: thesis): San Diego, California, San Diego Rawhide Mountains, Arizona: Geological State University, 120 p. Society of America with Programs, v. 9, Harding, L. E., 1980, Petrology and tectonic setting no. 7, p. 1169. of the Livingston Hills Formation, Yuma County, Spude, R. L., 1976, Swansea, Ari zona, the fOI'tunes ;,rizona: Arizona Geological Society Digest, and misfortunes of a copper camp: Journal of v. 12, p. 135-145. Arizona History, v. 17, no. 4, p. 375-306. Hei,'r'ic::, T. L., Davis, G. A., Anderson, J. L., Frost, Stone, P., and Howard, K. A., 1979, Co~pila6ion of ie. G., and Wilkins, J., 1980, Mylonitization, geologic mapping in the Needles 1 x 2 sheet, Jetachment faulting, and associated mineraliz­ California and Arizona; U.S.G.S. Open-~ile ation, Whipple Mountains, California, and Report 79-388. 0uckskin Mountains, Arizona: Arizona Geological Terry, A. H., 1972, The geology of the Whipple Society, 1980 Spring Field Trip Road Log, 55 p. Mountains thrust fault, southeastern California: Jones, E. L., 1919, Deposits of manganese ore in unpublished M. S. thesis, California State southeastern California: U. S. Geological University, San Diego, 90 p. Survey, Bill. 710, p. 185-209. Upham, W. E., 1911, Specular hematite deposits, Jones, E. L., and Ransome, R. L., 1919, Deposits of Planet, Arizona: Mining Science Press, v. manganese ore in Arizona: U. S. Geological 102, p. 521-523. Survey, Bull. 710, p. 93-184. Wilson, E. D., 1960, Geologic map of Yuma County, Keith, S. B., 1978, Index of mining properties in Arizona: Arizona Bureau of Mines. Yuma County, Arizona: Arizona Bureau of , 1962, A resume of the geology of Geology and Mineral Technology, Bull. 192, Ari zona: Ari zona Bureau of Mi nes Bull. 171, 185 p. 140 p. Las ky, S. G., and Webber, B. N., 1949, Manganese re­ Woodward, Robert J., Osborne, G. M., 1980, Low-angle sources of the Artillery Mountain region, Mohave detachment faulting and multiple deformation County, Arizona: U. S. Geological Survey Bull. of the central Buckskin Mountains, Yuma County, 961, 86 p. Arizona: Geo 1ogi ca 1 Soci ety of Ameri ca Ab­ Lingrey, S. H., Evans, K. V., and Davis, G. A., 1977, stracts with Programs: v. 12, no. 3., p. 245. Tertiary denudational faulting, Whipple Mountains, southeastern San Bernardino County, California: Geological Society of America Abstracts with Programs, v. 9, no. 4, p. 454-455. McCarn, H. L., 1904, The Planet copper mines: Engineering and Mining Journal, v. 78, p. 26-27. Marshak, S., 1980, A preliminary study of the Mesozoic geology in the southern Dome Rock Mountains, southwestern Arizona: Arizona Geological Society Digest, v. 12, p. 123-133. Martin, D. L., Barry, W. L., Krummenacher, D., and Frost, E. G., 1980, K-Ar dating of mylonitization and detachment faulting in the Whipple Mountains, San Bernadino County, California and the Buck­ skin Mountains, Yuma County, Arizona: Geological Society of America Abstracts with Programs, v. 12, no. 3, p. li8. Otton, J. K., 1981, Geology and genesis of the Ander­ son mine, a carbonaceous lacustrine uranium deposit, western Arizona: A summary report: U. S. Geological Survey, Open-file Report 81-780, 24 p. Ransome, F. L., 1931, Geological reconnaissance of the revised Parker route through the Whipple

202 APPENDIX This appendix presents the results of three new K-Ar ages from the Whipple-Buckskin detachment fault area, Yuma County, Arizona. Constants used 10 1 in the calculations are A = 0.575 x 10- yr- , 10 1 e As = 4.905 X 10- yr- , and 40 K/K total = 1.18 x 10- 4 mol/mol. Analysts: Geochron Labs., Cambridge, MA. Sample Descriptions NS-ll K-Ar Rhyglite vitric tuff, Artillery Formation (34 0 09'49''N, 113 53'44"W; Yuma County, Al). Quartz, orthoclase plagioclase and biotite noted in compacted, devitri­ fied, and slightly welded tuff. Sinuous veinlets of calcite, gypsum and quartz with fluorite, barite adn pyrite are present. Sample is about 500 feet structurally above the Whipple-Buckskin detachment fault. Analytical data: (biotite) K = 5.817%, 5.731%; - 4o Ar = 0.01114 ppm * 4°Ar/LAr 4o = 45.4%, 49.6%. Collected by: T. L. Heidrick (biotite) 27.3 ± 1.1 m.y. NS-6-95 K-Ar TrachYbasalt flow, at the top of the Arti llery Formation (34 0 09'49''N, ll30 52'20"W; Yuma County, Al). Plagioclase (56%), olivine (22%), hornblende (18%), and magnetite (4%) with a faintly diabasic texture. Olivine is replaced by fibrous dellessite and bowlingite. Sample is from drill core 100 feet above the Whipple-Buckskin detachment fault. Analytical data: (hornblende) K = 0.526%, 0.536%; * 4 °Ar - 0.00696 ppm * 4 °Ar/LAr 40 = 17.8%, 14.8%, 15.5%. Collected by: J. Wilkins (hornblende) 18.6 ± 1.5 m.y. NS-S-262 K-Ar Quartzo-feldspathic gnei s, Buckskin Mountains meta­ morphic core complex (34 610'12"N, ll30 53'49"W; Yuma County, Al). Mylonitic augen gneiss with a grano­ diorite porphyry composition. The fabric is laced with pennine veinlets with quartz, epidote, pyrite and fluorite. Sample is from drill core 240 feet below the Whipple~Buckskin detachment fault. Analytical data: (feldspar) K = 9.9l1%, 9.888%, 4o 4 40 10.041%; * Ar = 0.01240 ppm * °Ar/LAr = 50.3%, 39.4%. Collected by: T. L. Heidrick (feldspar) 17.7 ± .7 m.y.

203

THE TEUTONIA BATHOLITH: A LARGE INTRUSIVE COMPLEX OF JURASSIC AND CRETACEOUS AGE IN THE EASTERN MOJAVE DESERT, CALIFORNIA!

Gary 11. Beckerman 2, Jami e p. Robi nson 3, J. Lawford Anderson Department of Geological Sciences University of Southern California Los Angeles, California 90007

ABSTRACT that occur in the Clark Mountains, 20 km northeast of the batholith (Burchfie1 and Davis, 1975) and in Forming one of the larger intrusive complexes the New York Mountains along its eastern margin in the eastern Mojave Desert, the Teutonia batholith (Burchfie1 and Davis, 1977) largely predate the is comprised of Jurassic to Cretaceous metaluminous batholithic intrusions. An exception is the late to weakly peraluminous, magnetite-series granitic Cretaceous Morning Star thrust of the central plutons. The intrusives generally post-date, with Ivanpah Mountains (Burchfie1 and Davis, 1971; one exception, most thrust faults in the area and Weisenberg, 1973). have largely escaped the effects of Mesozoic and younger metamorphism, mylonitization, and detachment Occupying over 3000 km2, the Teutonia bathol ith faulting that characterize many of the nearby mountain is epizona1 in character and is composed of seven ranges immediately to the east and southeast. Mesozoic plutons: The Rock Spring monzodiorite and the Ivanpah granite are inferred to be of Jurassic Studies in progress show that the composite age, while the Mid Hills adamellite, the Teutonia nature of the batholith is due to emplacement of a adamellite, the Kessler Springs adamellite, the Live minimum of six non-comagmatic suites. The Jurassic(?) Oak Canyon granodiorite, and the Black Canyon horn­ Rock Spring monzodiorite appears to have generated blende gabbro are Cretaceous in age. As recognized from a silica-deficient source and evolved into a by Kistler (1974), Armstrong and Suppe (1973), and range of composition through fractionational Silver and Anderson (1978), a majority of post­ crystallization of plagioclase, biotite, hornblende, Triassic magmatic activity in the western U.S. clinopyroxene, and Fe-Ti oxides. Numerous mafic defines two non-parallel magmatic arcs (Fig. 1). inclusions are shown to be part of this fractionation The Jurassic arc contains anomalous volumes of sequence as cognate inclusions, rather than xenoliths silica-poor monzonite and syenite (Miller, 1978) or restite material. as well as alkali feldspar-megacrystic granitic plutons (John, 1981). The Cretaceous arc is All of the granitic plutons appear to have a primarily comprised of magnetite-series, I-type lower crustal origin (27-38 km) involving partial granitoids (terms coined by Ishihara, 1977 and melting of differing quartzofe1dspathic sources. Chappell and White, 1974, respectively), yet The unusually potassic composition of the Ivanpah Cretaceous and younger plutons are known to continue granite, must have been generated under different far inboard of the magmatic arc represented by conditions of crustal fusion: either drier or a scattered peraluminous (biotite + muscovite) (Miller lesser degree of partial melting of a more calcic and Bradfish, 1980) and marginally metaluminous source. These magmas were emplaced into the upper (biotite ± hornblende) granitoids. Coaxial with the crust (~7-8 km) most undergoing modest plagioc1ase­ Jurassic arc, the Teutonia batholith is 250 km east dominated fractional crystallization at high levels of of the main axis of the Cretaceous arc. oxygen fugacity (>Ni-NiO) near wet solidus tempera­ tures. The Ivanpah granite remains an exception with PREVIOUS WORK fractionation dominated by alkali feldspar and crystallization occurring at an order of magnitude The first detailed geological investigation of lower oxygen fugacity. the region was initiated by D. F. Hewett in 1921 which, as reported by Hewett (1956), dealt with the INTRODUCTION AND SETTING geology and mineral resources of the Ivanpah Quad­ rangle. In his classic study, Hewett proposed the The Teutonia batholith, one of the larger name Teutonia quartz monzonite (adamellite) for rocks granitic complexes of the eastern Mojave Desert, forming Teutonia Peak along the eastern side of Cima comprises much of the New York Mountains, Ivanpah Dome and extended the name for all similar granitic Mountains, Mid Hills, and Cima Dome region of rocks within the confines of this 1° quadrangle southern California. Marginal to the Colorado River sheet. Sutter (1968) completed a regional K-Ar trough, the subject of this dedicated volume, the geochronologic study of the southern Great Basin and the batholith has largely escaped the effects of dated specimens from what is now known to be the late Mesozoic or younger metamorphism, mylonitization, Kessler Springs adamellite and the Ivanpah granite. and Miocene detachment faulting that is so widespread Building on Sutter's work, Weisenberg (1973) in the nearby mountain ranges immediately to the attempted to constrain the timing of thrust faulting east and southeast (see, for example, Davis, et a1., in this general region, providing the first detailed 1980; Anderson, 1981; r·1iller et al., this volume, study of igneous rocks in the area. He named the Howard et a1., this volume). The east-directed Kessler Springs quartz monzonite (adamellite) for thrusts of the Mesozoic foreland fold and thrust belt porphyritic rocks exposed along Cima Road.

!Sequence of authors determined by height, shortest first; reprint request to J. L. Anderson. 2Present address: UniJn Oil, Northern District Exploration, Box 6176, Ventura, CA 93006 3Present address: Phill ips Petroleum Co., Englewood, CO. 80111

205 for trace elements less than 1000 ppm. Loss on igni­ tion (L.O.I.) is wt. % volatile loss at 1000 0 C. Electron microprobe analyses of plagioclase, -7-- -- alkali feldspar, biotite, hornblende, clinopyroxene, I I ilmenite, and magnetite were carried out by one of I I us (J.L.A.) on an automated 3-channel M.A.C. micro­ I probe at the California Institute of Technology. I Nevada I Simple oxides and well characterized silicate I I minerals were used as standards. The scheme of Bence ,I Utah and Albee (1968) was used for data reduction with I the afactors of Albee and Ray (1970). Operating con­ I ditions included a 15 kVaccelerating potential with ,I a 0.05 wA beam current set on brass and a 3-30 wm I I beam diameter. Additional determinations of plagio­ ,,---- clase composition have been done with a 4-axis uni­ versal stage. l"") Figure 2 Modal analyses were done by point counting of thin sections and stained slabs (1000 counts minimum). tv Arizona K-Ar determinations of 11 mineral separates were completed at the K-Ar isotopic laboratory of San Diego State University by one of us (G.M.B.) and Donna Martin under the direction of Daniel Krummen­ ! acher. Field mapping and sample collecting was done at a scale of 1:24,000 during 1977-1978 (J.P.R.) and 1979-1980 (G.M.B), utilizing expanded 15 minute topo­ graphic quadrangles (Mid Hills and Ivanpah) as a base. E8l LOCUS OF CRETACEOUS MAGMATIC ACTIVITY o LOCUS OF JURASSIC MAGMATIC ACTIVITY BORDER LITHOLOGIES OF THE BATHOLITH D PLUTONIC ROCKS Precambrian Metamorphic Rocks

Figure 1. Axial positions of the Jurassic and Cre­ Amphibolite-grade Precambrian crystalline rocks taceous magmatic arcs for the southwestern U.S. border the batholith on most margins (Fig. 2). After Kistler (1974). Generally having a steeply dipping, north-trending foliation, these rocks are comprised of granitic gneisses, rare pelitic and other metasedimentary This paper represents the first overview of a gneisses, and amphibolite. The granitic gneisses are major ongoing batholithic investigation that began by far the most common and include both banded and in 1977. Robinson (1979) and Robinson and Anderson augen gneiss varieties. Grey to reddish brown in (1979), in characterizing the plutonic rocks of Cima color, these gneisses contain subequal proportions Dome and the Ivanpah Mountains, named the Ivanpah of quartz, alkali feldspar and oligoclase (An17-27) granite after the mountain range which is largely with lesser amounts of biotite ± muscovite or horn­ formed out this major pluton and restricted the blende. Accessory minerals include allanite, Teutonia adamellite to the plutonic mass that under­ sphene, apatite, Fe-Ti oxides, and zircon. lies much of Cima Dome. Beckerman and Anderson (1981) and Beckerman (1982), extendi ng the study Metasedimentary gneisses occur only on the southeastward across the Ivanpah Valley into the northwest side of Cima Dome. These rocks contain up New York Mountains and Mid Hills, mapped and to 72% quartz with lesser amounts of alkali feld­ characterized the remaining plutons (three previously spar, muscovite, biotite, and sillimanite. Plagio­ termed Teutonia), and named the intrusive complex, clase is notably absent. the Teutonia batholith. AnJphibolite occurs as dikes in the above METHODOLOGY gneisses and as xenoliths in the batholith plutons. The rock is composed primarily of hornblende and Analyses of nine major elements, Rb, Sr, and Ba plagioclase ± minor amounts of quartz, biotite, and were done by atomic absorption and UV-VIS spectro­ sphene. photometry (SiO L only) on 38 rock samples from all intrusive units. Acid dissolution was done in a Phanerozoic Supracrustal Rocks teflon-lined bomb by a method modified after Bernas (1968). Synthetic multielement standards were Paleozoic and Mesozoic Sedimentary Rocks used in conjunction with three internal standards (G-2, SY-2, W-1) which were included with each run A thick succession (>2000 m) of regionally to monitor for any systematic errors. Replicate metamorphosed Paleozoic and Mesozoic strata have been analyses of these internal standards by our labora­ intruded by the batholith along its eastern and tory show an uncertainty of less than 0.6% (relative) southwestern margins. As mapped and subdivided by for concentrations greater than 1.0 wt. %, an Burchfiel and Davis (1977), a majority of these uncertainty of 0.6-2.3% (relative) for concentrations rocks are Paleozoic sandstones, shales, and car­ in the range of 1000 ppm to 1.0%, and 1.5% (relative) bonates correlative with platform units east of the

206 Qal

HWY,68 N,

Qal

Kt Qal

GEOLOGIC LEGEND MILES o 5 10 o QUATERNARY ALLUVIUM (Qol) I I oI 10 15 ~ CENOZOIC VOLCANICS (Cv) KILOMETERS TEUTONIA BATHOLITH BLACK CANYON HORBLENDE GABBRO(Kbc) 1:::1t~ ~ LIVE OAK CANYON GRANODIORITE(Klo) KESSLER SPRINGS ADAMELLITE (Kks)

~;>~~J MID HILLS ADAMELLITE (Kmh) c:=:J TEUTONIA ADAMELLITE (Kt) Figure 2. Geologic map of the L'~>I IVANPAH GRANITE (Jig) Teutonia batholith. Surroundin~ ~ geology from Burchfiel and ROCK SPRING MONZODIORITE (Jrs) Davis (1977). UNDIFFERENTIATED GRANITIC ROCKS OF THE BATHOLITH (Umg)

6--::j PALEOZOIC SEDIMENTARY ROCKS (pz) I"" "-' I PRECAMBRIAN BANDED AND QUARTZOFELDSPATHIC GNEISSES(P8)

207 Cordilleran geosyncline. Mesozoic strata are pre­ TABLE 2. K-Ar Analytical Datal and Ages for the dominately composed of intermixed tremolite- and Teutonia Batholith, S.E. California

diopside-bearing carbonates, quartzite, and meta­ Sample Rock Component , Ar~o volcanic rocks. No. Unit2 Analyzed K(wt.%) Ar~o/K~o (atm. X) Age (m.y.) MH-4 Jr, hornblende 0.855 0.01025 76 168.4±11,8 MH-4 Jrs biotite 7.820 0.00568 49 95.2±2.9 Cenozoic Volcanic Rocks MH-7 Jrs hornblende 1.515 0.00342 78 57.9±4.3 MH-172 Jrsq hornblende 1.325 0.00521 62 87.6±3.1 MH-172 Jrsq biotite 5.940 0.00318 58 53.9±1.6 Scattered erosional remnants of flat-lying MH-IOO Kbc hornblende 0.665 0.00642 72 107 .2±8. 0 MH-9 Kmhe hornbl ende 1 3.000 0.00625 30 104.5±3.7 Miocene pyroclastic strata form Pinto Mountain and MH-9 Kmhe biotite 6.785 0.00528 50 88. 7±2. 7 MH-27 Kmhp biotite 6.636 0.00496 58 B3.S±S.8 Table Mountain south of the Mid Hills. Once over­ MH-75 Kmhp(myl) biotite4 2.505 0.00435 71 73.4±4.4 I-6d Klo biotite 7.410 0.00475 61 79.9±2.4 lying much of the southern half of the batholith, 5 TP Kk' whole rock 4.58 n.r. 82 92.9±1.3 these volcanic rocks correlate to silicic pyroclastic TP Kk, hornblende 1.40 n.r. 74 92.1±O.5 volcanism originating in the Woods and Hackberry IP Jig biotite 6.39 n.r. 85 136.9±1.3 Mountains south of the batholith (McMurray, 1980). lOetennination by the Isotopic Laboratory at San Diego State University (SDSU) by Donna Martin and Gary Beckennan. except samples TP and IP which are reported in Sutter (l968). Constants used in age calculations are K~o '" 0.01167 atom percent. Quaternary alkali olivine basalts overly the AS'" 4.962 x 1O-10yr.-l, A '" 0.581 x 1O-IOyr.-1. 2Rock unit notations same a~ in Table 1 batholithic rocks west and northwest of Cima Dome. 3Mixture of approximately 65% hornblende and 35% biotite. Age dOOlinated by biotite as it contains more K. In the form of flows and eroded cinder cones, these ~Biotite, partly altered to chlorite; sampled from a mylonitic zone within the ~lid Hills rocks and their ultramafic xenoliths are presently adamellite in the western New York Mountains. being studied by A. L. Boetcher (this volume). sn. r. '" not reported. DESCRIPTION OF BATHOLITH PLUTONS Q

A. ROCK SPRING MONZODIORITE Jurassic(?) Intrusives LIVE OAK CANYON GRANODIORITE KESSLER SPRINGS ADAMELLITE MID HILLS ADAMELLITE Rock Spring Monzodiorite TEUTONIA ADAMELLITE 41} IVANPAH GRANITE The Rock Spring monzodiorite is a compositionally zoned pluton that occupies much of the eastern Mid Hills. The body is intrusive into Precambrian gneisses and is in turn cut by numerous dikes of the Mid Hills adamellite which borders the pluton on the west and north. K-Ar dates (Table 2) range widely, reflecting a complex thermal history (largely due to intrusion of the above adamellite). A minimum age of 168.4 ± 11.8 m.y. on hornblende suggests that the pluton may be Jurassic. Moreover, the pluton has a compositional affinity (see below section on composi­ tional characteristics) to the 170-230 m.y. monzoni­ tic suite described by Miller (1978). Dark grey to brown in color, the rock is medium to coarse grained and equigranular. The modal composition of the pluton, given in Table 1 and Figure 3. Modal composition of batholith plutons. Figure 3, varies from a biotite-clinopyroxene diorite Rock nomenclature follows that of Streckeisen along the northern edge to a mafic hornblende­ (1976) with the single modification of dividing biotite quartz monzodiorite and granodiorite in the granite field into granite (syenogranite) the southern portions of the intrusive. Reddish­ and adamellite (monzogranite). brown titaniferous biotite is the dominant ferro­ magnesian mineral throughout this compositional

TABLE 1. MODAL COMPOSITION OF BATHOLITH PLUTONS'

P1uton 2 Kbc K10 Kks Kmhe Kmho K+ Jiq Jrs Jrsq No. of Sarno 1es 3 5 5 5 15 7 8 4 7 Quartz --- 25.4±1.9 20.6±1.5 30.6±3.4 28.D±2.4 28.6±2.3 28.1±4.4 3.5±1.1 15.9±6.3 Al kal i Fel dspar --- 19.1±3.2 23.1±1.5 32.4±3.7 29.9±6.1 40.3±5.B 43.7±8.2 11.0±4.1 12.3±4.4 Plagioclase 44.2±16.2 44.0±3.4 41.1±3.2 30.6±5.4 37.6±4.2 24.6±5.2 19.2±5.7 57.h2.8 53.1±4.1 Muscovi te 3 ------0.8±1.6 O. 5±0. 9 ------Bioti te'l 1.2±1.2 9.5±4.0 7. 7±1. 5 4.5±1.3 3.1±1.8 3.1±2.6 4.5±2.3 14.6±4.8 10.7±3.6 Hornb 1ende 44.6±17.3 0.1±0.1 2.2±1. 7 0.0±0.1 0.6±1.1 --- 0.4±0.B 7.9±4.B 5.8±3.4 C1 i nopyroxene 3. 0±3. 9 ------1.5±2.3 --- Opaques 5 6.5±3.2 1.2±0.5 1.5±1.0 1.6±1.0 O. 5±0. 3 1.3±1.1 1. O±O. 2 1. 8±1. 2 1.0±0.4 Apatite 0.2±0.1 0.3±0.2 0.5±0.3 0.1±0.1 0.2±0.1 0.2±0.2 0.5±0.4 O. 9±0. 5 0.5±0.2 Sphene O. 3±0. 4 0.3±0.2 1.7±0.5 0.1±0.2 0.1±0.2 0.0±0.1 0.8±1.3 O. 9±0. 3 0.7±0.3 Allanite --- 0.l±0.1 1. 5±1.1 O.O±O. I 0.0±0.1 1.1±0.8 1.2±1.5 --- O.O±O.I Ii rcon --- 0.1±0.1 0.1±0.1 0.1±0.0 0.0±0.1 0.1±0.1 0.1±0.1 0.1±0.1 0.0±0.1 TOTAL IOO.O 100.1 100.0 100.0 100.0 100.1 100.0 99.9 100.0

Color Index 55.6±16.2 11.0±4.3 14.6±2.7 6.3±2.1 4.2±2.9 5. 6±3. 5 7.8±2.6 26.6±4.8 18. 2±6. 6 Anp1 ag 51-70 22-33 23-41 20-24 20-34 23-30 24-30 29-37 22-38

lModa1 composition based on a minimum of 1000 counts of both thin section and stained slab 2Pl uton Symbol s: Kbc"'Bl ack Canyon hornblende gabbro; Klo"'Live Oak Canyon grandiorite. Kks=Kess 1er Spri ngs adamell ite, Kmhe=equi granu1 ar phase of the Mi d Hill s adamellite, Kmhp=porphyriti c phase of the Mid Hill s adamellite, K+"'Teutonia adamellite. Jig=Ivanpah granite. Jrs=Rock Spring monzodiorite. Jrsq=quartz-richer phase of the Rock Spring monzodiorite 3Exc1usive of sericite 4Includes derived chlorite 5Primarily magnetite, ilmenite being rare to nonexistent

208 series followed in abundance by either calcic clino­ defined by alignment of feldspars and biotite. pyroxene (mafic variations) or hornblende (silica­ Although commonly seriate (inequigranular), the tex­ richer variations). Textural relations indicate that ture of the rock varies to being porphyritic with as apatite, oscillatory zoned plagioclase (An22-38), much as 50 to 60% phenocrysts (1.5-4.0 cm) of sub­ and magnetite are the liquidus or near liquidus hedral alkali feldspar. Modally, the rock has a high phases. In mafic versions, this is followed by the proportion of alkali feldspar to plagioclase (Fig. 3). crystallization of biotite and late quartz and alkali As noted below, the pluton varies from being feldspar. Hornblende occurs largely as a secondary marginally metaluminous to marginally peraluminous phase, forming reaction rims between biotite and Biotite is usually the sole mafic phase, but minor clinopyroxene. With increasing silica saturation, amounts of hornblende or muscovite occur in specimens hornblende becomes a late magmatic phase at the representative of the two extremes, respectively. expense of the pyroxene and occurs as large poikili­ Despite the large size and abundance of alkali feld­ tic crystals with relict cores of pyroxene and spar, this mineral formed late in the crystallization scattered inclusions of biotite, plagioclase and sequence as evidenced by the presence of poikilitic magnetite. inclusions of the other minerals. Apatite, zircon, and magnetite are the liquidus phases, followed by A ubiquitous feature of the monzodiorite is the plagioclase (Anz4-3o) and, in sequence, sphene, presence of oriented and elongate mafic inclusions allanite, biotite, alkali feldspar, and quartz. (Fig. 4). Up to a meter in length, the inclusions are fine grained, have a color index between 35 and Cretaceous Intrusives 50, and are dioritic in composition. Texturally and mineralogically, they show a marked affinity to the Mid Hills Adamellite pluton itself, and, as will be shown below, are interpreted to be aut0liths (cognate inclusions) The Mid Hills adamellite comprises a large area rather than foreign xenoliths or restite material. (300 sq. km) that extends from the eastern New York Mountains to the southern Mid Hills. The pluton Ivanpah Granite is intrusive into Precambrian gneisses along its southern margin and both Paleozoic and Mesozoic The Ivanpah granite, a coarse grained biotite metasedimentary rocks along its eastern margin. As granite, forms much of the Ivanpah Mountains where given in Table 2, K-Ar dates vary, the oldest being it is reported to intrude Paleozoic metasedimentary 104.5 m.y. on a hornblende-biotite mixture. Although rocks (G. A. Da vi s, personal COmin., 1981; Burchfi e1 regionally undeformed, the pluton is crosscut by a and Davis, 1971). The pluton is intruded by the major 400 m wide mylonite zone which may correlate to Teutonia adamellite along the eastern side of Cima the Morning Star thrust in the southern Ivanpah Dome (Teutonia Peak) and by the Kessler Springs Mountains. The youngest date obtained from within adamellite. A large xenolith of Ivanpah granite the pluton (73.4 ± 4.4 m.y.) is a mylonitic and occurs in Teutonia adamellite on the south side of lineated specimen from this zone. Although not Cima Dome (forms Wildcat Butte). A minimum K-Ar abundant, dikes of aplite and pegmatite are common (biotite) age of 137 m.y. (Sutter, 1968) suggests in this pluton. that the pluton may be Jurassic in age. Texturally, this medium to coarse grained Reddish to light tan in color, the rock has a biotite adamellite varies from being equigranular in poorly developed foliation of magmatic origin the central portions (northern Mid Hills, north of Old Government Road) to porphyritic in the northern (New York Mountains) and southern (central and southern Mid Hills regions. The equigranular phase is megascopically similar to the Teutonia adamellite (described below) but differs in that it occasionally contains some hornblende, no muscovite, and has a higher proportion of plagioclase to alkali feldspar (Table 1). The porphyritic phase contains 10-20% poikilitically zoned alkali feldspar phenocrysts (1.0-7.5 cm) and tends to have more plagioclase and less quartz than the equigranular phase. Both phases are light tan in color and are leucocratic (color index averaging 4.2-6.3). Textural relations indicate that most minerals crystallized simultaneously over a period of time yet a crude sequence of initial precipitation can be deduced. The accessory minerals all initially crystallized early (in sequence, apatite, zircon, magnetite, sphene, allanite) followed by oscillatory zoned plagioclase (Anzo-34) and, when present, horn­ blende. Biotite and quartz, both anhedral and inter­ stitial, crystallized next. Alkali feldspar, despite its euhedral and large size in the porphyritic phase, was one of the last minerals to crystallize. This crystallization sequence is nearly identical to that inferred for the other Cretaceous plutons described below except minor differences involving allanite and sphene. Details are reported in Robinson (1979) Figure 4. Oriented mafic inclusions (autoliths) and Beckerman (1982) and will be omitted here for within the Rock Spring monzodiorite. Location the sake of brevity. near Rock Spring.

209 Its external contact is not exposed, but its shape Teutonia Adamellite suggests that it has intruded the adamellite as a plug. The body is devoid of dike intrusion except A massive, texturally uniform rock, the Teutonia for a few quartz veins. A minimum age of 107.2 ± 8.0 adamellite underlies all of Cima Dome. Intrusive m.y. based on K-Ar (hornblende) is similar to the into Precambrian gneisses and the Ivanpah granite, 104.5 minimum age on the Mid Hills indicating that the pluton has not yet been dated. Its relative the two may be roughly coeval. timing and compositional affinity to the other Cretaceous plutons, however, suggests that it should Despite its small size (cuI sq. km), the pluton be placed within this age group. The pluton is cut is extremely variable in texture and modal composi­ only by a few aplite and pegmatite dikes and a tion. Much of the body is medium grained, equigranu­ marked abundance of quartz veins. lar with subequal proportions of hornblende and plagioclase and lesser amounts of clinopyroxene. A Light tan in color, the rock is medium to coarse hornblende-rich phase contains 60% phenocrysts of grained, equigranular, and leucocratic (average color pyroxene and hornblende while a plagioclase-rich index = 5.6). Alkali feldspar is predominate over phase contains less than 5% hornblende phenocrysts, plagioclase (Anz3_3o) and modally the rock ranges no pyroxene, and up to 4% biotite. The color index from adamellite to granite in composition (Fig. 3). varies between 40.4 to 72.7. Biotite is the sole mafic phase and minor amounts of late magmatic muscovite occasionally occur. For all phases, plagioclase (AnSI_70) was the Both ilmenite and magnetite occur as Fe-Ti oxides; earliest mineral to crystallize followed by apatite, this is the only pluton in the batholith that con­ magnetite and clinopyroxene, the latter being mantled tains significant amounts of ilmenite. Sphene, a by hornblende exhibiting various degrees of magmatic common primary phase in most batholith plutons, replacement. Only in the plagioclase-rich phase has occurs only as a secondary phase after Fe-Ti oxides all of the clinopyroxene disappeared and hornblende (magnetite and ilmenite). exhibits late magmatic to subsolidus replacement by bi oti te. Kessler Springs Adamellite MAGMA EVOLUTIONARY TRENDS The Kessler Sprin1S adamellite, distinctive in having large (to 6 cm) alkali feldspar phenocrysts, General Compositional Characteristics is intrusive into the Ivanpah granite (along Cima Rd.) and Precambrian gneisses (northwest of Cima Complete compositional data for the batholith Dome). The pluton has a minimum age of 92.1 m.y. are given in Table 3. As shown in Figures 5 and 6, (K-Ar, hornblende). The rock is porphyritic with all plutons are subalkalic and calc-alkaline. In 10-30% rectangular and zoned phenocrysts of pink terms of the molecular proportions of Al z0 3 relative alkali feldspar with lesser amounts of white plagio­ to CaO, NazO, and K2 0 ("Al" value of Table 3), all clase set in a medium grained, grey matrix. Plagio­ of the granitic plutons effectively straddle the clase is predominate over alkali feldspar in the metaluminous-peraluminous boundary. Only the Rock matrix resulting in a modal composition that Spring monzodiorite is primarily metaluminous, yet straddles the adamellite-granodiorite boundary this compositional series exhibits a marked trend (Fig. 3). Biotite with minor amounts of hornblende, toward alumina saturation with increasing silica. magnetite, and sphene constitute the dark minerals. Similar but less well defined trends exist for the Ivanpah granite and the Mid Hills adamellite which Live Oak Canyon Granodiorite could be explained by fractionation of some meta­ luminous phase, such as apatite, sphene, hornblende, Located in the eastern New York Mountains, or clinopyroxene (see below section). The noteable the Live Oak Canyon granodiorite intrudes Paleozoic lack of strongly peraluminous granitoids (Miller and and Mesozoic strata and is partly truncated along its Bradfish, 1980) is characteristic of the batholith western margin by the Slaughterhouse fault of Burch­ and sets it apart from nearby Mesozoic granitic fiel and Davis (1977). Its northern contact with the terrains elsewhere in the Mojave, including those of Mid Hills adamellite is not exposed. A biotite the Old Woman Mountains (Miller and Stoddard, 1981; mineral separate from the pluton yielded a r~iller, et al., this volume), the Iron Mountains and 79.9 ± 2.4 m.y. minimum age (Table 1) which is other portions of the Cadiz Valley batholith (Miller similar to a 71.7 ± 0.8 m.y. date reported by Burch­ et al., 1981; John, 1981), the Whipple Mountains fiel and Davis (1977) on an impure mineral separate (Anderson and Rowley, 1981; Davis et al., 1980) and from a granodiorite dike presumably derived from the Joshua Tree Monument area (Brand and Anderson, this pluton. 1982) . Medium to coarse grained and light grey in color, The marginally metaluminous and calc-alkaline the rock is generally massive and equigranular except character of the batholith allows direct comparison near contacts with older rocks where it becomes to the Sierra Nevada batholith (Bateman and Chappell, porphyritic. The rock has a modal composition simi­ 1979; Bateman and Dodge, 1971) which shares these lar to the Kessler Spring adamellite except for same characteristics. Figures 5-7 depict the trend slightly higher proportions of plagioclase (oscil­ of the Teutonia batholith relative to that of the latory zoned, Anzs_3z) and quartz. Biotite, sphene, Tuolumne series of the Sierra Nevada. The data are and magnetite are the dominant dark minerals result­ similar although there are small shifts to lower ing in an average color index of 11.0; hornblende values for CaO, Al z03, FeO, and NazO and higher occurs but is rare. values for KzO. This is predictable for the east­ ward position of the batholith (Kistler and Peterman, Black Canyon Hornblende Gabbro 1973) . The Black Canyon hornblende gabbro forms a Of the granitic plutons, two are somewhat circular body surrounded by the Mid Hills adamellite. different from the norm for Mesozoic granitoids. The

210 TABLE 3. GEOCHEMICAL DATAl FOR THE TEUTONIA BATHOLITH Live Oak Canyon Black Canyon Hornblende Gabbro Kessler Sprin9s Adamellite Granodiroite Mid Hills Adamellite

Samole MH-108 t1H-98 MH-lOO 1-123 1-116 1 ~ ISS 1-12 1-48 I-6d 1-5 l~H -27 HH-73

SiO z 43.14 44.20 48.86 68.13 68.55 68.56 70.18 72.32 68.71 69,26 71.05 71.30 TiO z .89 .72 .76 .17 .30 .38 .14 .15 .34 .33 .14 .26 AI,O, 18,85 24,72 11.55 16.00 15,47 14.96 14.67 14.62 15,52 16.01 15.08 14.50 z FeO ' 11.75 10,00 10,20 1.91 2.49 2.09 1.78 1.49 2.37 2.36 1.23 1.48 HgO 6.72 2.69 10.07 .68 1.29 1.30 .60 .57 .95 .93 .49 .59 HcO .143 .094 ,241 .039 ,050 .038 .039 .027 .064 .046 I .031 .042 C,O 13.62 12.89 13,38 2.31 2.79 2.70 2.22 1.81 2.71 3.11 1.98 1.80 HazO 1.46 2.07 1.52 3.85 3.71 3.25 3.94 3.46 3.79 3.89 3.95 4.02 K,O .45 .47 .62 5.71 3.47 4.08 4.60 4.44 3·64 3.29 4.78 4.18 l.0. I. 1 .87 .71 1.04 ------~- ~--- .54 .44 .33 .80 TOTAL 97.89 98.56 98.24 98.90 98.12 97.36 98.27 98.89 98.55 99.67 99.16 98.97 K O+Na O 1.81 z 2 2.54 2.14 9.56 7.18 7.33 8.54 7.90 6.50 7.10 8.73 FeO/FeO+tlgO .636 8.20 .788 .503 .737 .659 .617 .748 .723 .714 .717 .715 .715 "Al" ~ .686 .902 .420 .957 1.036 1.020 .947 1.060 1.042 1.025 .987 1.006 K,O/Na O .331 2 .227 .408 1.48 .935 1.26 1.17 1.28 .960 .846 1.21 1.04 103 B' 133 155 1224 938 B30 1155 1056 1005 1046 030 899 Rb 13.8 14.5 14.0 170 99.6 117 140 114 116 98.8 192 203 5, 654 870 379 624 769 726 634 604 564 673 492 378

Ba/Sr .157 .153 .409 1.96 1.22 1.14 1.82 1.75 1.78 1.55 1.89 Sa/Rb, 2.38 7.46 9.17 11.07 7.20 9.42 7.09 8.25 8.52 8.66 10.59 4.84 Rb/Sr 4.43 .0211 .0167 .0369 .2724 1295 .1612 .2208 .2053 .2057 .1468 .3902 .5370 K/Rb 270 271 369 279 289 289 273 297 261 276 i07 I71 QZ 0.00 0.00 0.00 15.79 22.61 23.73 21.85 27.63 22.47 22.67 22.54 24.67 OR 2.77 2.86 3.77 34.02 20.95 24.88 27 .69 26.69 21.99 19.64 28.58 25.20 AS 6.00 13.33 14.05 34.86 34.04 30.12 36.05 26.69 34.79 35.29 35.90 36.84 AN 45.79 58.54 23.55 9.60 14.15 13.83 8.93 9.14 13.75 15.59 9.42 9.12 CO 0.00 0.00 0.00 0.00 0.60 0.33 0.00 0.93 0.47 0.44 0.00 liE 0.10 4.02 3.50 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 01 20.53 5.91 35.85 1.56 0.00 0.00 1.83 0.00 0.00 0.00 HY 0.42 0.00 0.00 0.00 7.96 3.78 7.23 6,56 3.30 3.79 6.05 5.90 2.80 OL 3.70 20.53 14.83 13.73 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 IL 1.29 1.03 1.09 0.38 0.43 0.55 0.34 0.21 0.48 0.46 0.34 0.37 An/(Ab+An) .8841 .8146 .6263 .2160 .2936 .3146 .1986 .2243 .2832 ,3064 .2U79 .1984 ------Mid Hills Adamellite (con.) Teutonia Adamellite nock Spring Monzodiorite Pl ton Samnle MH-9 MH~ 122 1-26-2 1-28-2 MH-152 1-38 1-5b 1-11 1-34 1-122 I ~lH-4 " t1H-48

SiO z 71.33 71.42 71.44 71.93 72.66 72.62 74.16 74.87 75.72 76.25 55.48 '19.25 TiO z .16 .13 .23 .24 .26 .15 .11 .094 .085 .094 1.16 .93 AhGl 14.71 14.48 14,67 14.37 14.57 13.38 15.41 13.91 12,94 12.77 17.07 16.91 Fe0 2 1.47 1.82 1.17 1.37 1.67 1.61 .44 .47 .47 .59 7.10 5.86 H9 0 .46 .52 .36 .52 .50 .55 .026 .059 .063 .053 3.67 2.67 HoO .044 .089 .024 .037 .081 . lID .021 .014 .055 .042 ,129 .125 C'O 1.47 2.27 1.89 1.59 2.24 1.07 .71 .74 .47 .070 6.29 5.30 3.77 Na 2 0 4.16 3.85 3.94 3.73 3.85 4.69 3.66 3.97 2.83 3.86 3.62 K,O , 4.82 3.58 3.91 4.48 3.60 4.72 4.49 5.26 4.49 5.24 2.47 2.59 L.G.I. .39 .55 .45 .59 .33 ---- -~------.84 .61 TOTAL 99.11 98.71 98.08 98.86 99.76 98.90 99.02 99.39 97.13 99.35 9/.83 98.08 9.10 6.09 6.36 K,0+Na 2 0 8.98 7.43 7.85 8.21 7.45 9.41 8.15 9.23 7.32 FeO/FeO+HgO .762 .778 .765 .725 .770 .745 .944 .888 .881 .918 .659 .687 "A l"~ .998 1.010 1.037 1.035 1.019 .906 1.266 .989 1.248 1.051 .851 .907 .682 .687 K2O/Na 2 O 1.04 .930 .992 1.32 .935 1.01 1.23 1.33 1.59 1.36 B, 761 857 916 926 869 331 189 354 114 43B 679 587 Rb 139 lOB 128 197 109 186 253 99.5 229 130 86.9 90.2 5, 257 265 515 362 261 92.8 74.4 74.1 26.5 83.0 600 570

Ba/Sr 2.96 3.23 1. 78 2.56 3.33 3.57 2.54 4,78 4.30 5,28 1.13 1.03 Ba/Rb 4.43 7.94 7.16 4.70 7.97 1.78 .75 3.56 .498 5,28 7.81 6.51 Rb/Sr .5370 .4075 .2485 .5442 .4176 2.004 1,339 1.343 8.642 1.566 .1448 1582 K/Rb 171 l'75 254 20B 274 211 147 '" 163 335 2J5 238 QZ 22.31 26,73 26.78 26.41 27.81 22.10 31.78 27.69 39.31 31.12 2.49 8.35 DR 28.84 21.66 23.73 27.05 21.52 28.13 26.91 31.35 27.73 31.47 15.02 15.71 AB 37.83 35.40 36.35 34.23 34.98 42.48 33.34 35.96 26.56 35.23 33.45 34.75 All 7,33 11.54 9.64 8.06 11.25 1.54 3.57 3.70 2.44 0.35 23.72 22.16 CO 0,00 0.16 0.59 0,55 0.30 0.00 3.59 0.38 2.94 0.69 0.00 0.00 tiE 0.00 0.00 0.00 0.00 0,00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 01 0.04 0.00 0.00 0.00 0.00 3_05 0.00 0.00 0.00 0.00 6.72 3.87 HY 3.27 4.32 2.59 3.36 3.78 2.48 0.65 0.80 0.91 1.01 16.93 13,83 OL 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0,00 IL 0.37 0.19 0.33 0.34 0.37 0.21 0.16 0.13 0.12 0.13 1.66 1.33 An/(Ab+An) .1624 .2458 .2095 .1907 .2433 .0349 .0968 .0934 .0841 .0099 .4149 .3894

Rock Spring Monzodiorite (con.) IvanpahGranite Pluton I'lclusion2 Sample MH-174 MH-170 HH-l72 MH-132 MH-131 MH-147 MH-150 1-112 1-4 1-147 1-144 1-154

Si02 59,15 62.69 62.68 63.66 64.05 53.77 56.30 71.13 72.42 73.45 73.98 74.56 Ti0 2 .92 .40 .42 .36 .40 .B3 .64 .28 .IB .18 .15 .13 A1 2 O] 16.62 16.71 16.32 16.46 16.10 17.56 17.44 14.64 14.23 13,04 13.75 2 13.47 Fe0 5,99 4.28 4,83 4.35 4.31 7.93 6.30 1.07 1.19 1.41 .98 .73 H90 3.16 1.79 2.24 1.68 1.59 4.31 3.24 .24 .28 .27 .17 .12 HcO .131 .lD3 ,106 .111 ,110 .159 .114 C,O 6.04 4.68 4,84 4.14 4.03 7.19 6.16 1.15 .94 .66 .70 .58 tja~O 3.24 3.69 3.33 3.81 3.47 3.89 3.90 2.16 ~. 56 2.40 2.81 3.12 K,O 2.63 2.72 J.UO 2.92 2.83 1. 70 2.30 8.99 7.02 5.57 4.96 6.63 L.O.I .91 .68 .96 .75 .86 .85 .88 TOTAL 98.71 97.74 98.73 98.24 .97,75 98.19 97.27 99.70 98.85 96.98 97.24 99.65

K2 O+Na 2 O 5.87 6.41 6.33 6.73 6.30 5,59 6.20 11.15 9.58 7.97 7.77 9.75 FeO/FeO+HgO .655 .705 .683 .721 .731 .643 .660 .817 .810 .839 .852 .859 HAl"" .867 .953 .931 .971 1.000 ,824 .867 .952 1.052 1.166 1.196 1.029 K2 O/Ha 2 O .812 .737 .901 .766 .816 .437 .590 4.16 2.74 2.32 1.77 2.13 8, 643 839 897 972 1026 326 B39 841 640 502 234 348 Rb 80.1 87.7 91.5 85.1 90.1 59.9 80.2 242 274 209 365 463 5, 506 494 479 464 454 557 697 159 101 79.9 47,1 527 Ba/Sr 1.27 1.70 1.87 2.09 2.26 .585 1.20 5.29 6.34 6.28 4.97 .66 Ba/Rb 8.03 9,57 9.80 11,42 11.39 5.44 10,46 3.48 2.34 2.40 .64 .75 Rb/Sr .1583 .1775 .1910 .1834 1985 1075 1151 1.522 2.713 2.616 7.749 .8786 K/Rb 272 257 272 285 260 236 238 308 213 221 113 119

QZ 9.16 14.55 14.27 15.01 17.90 0,00 3.32 19.75 25,90 34.28 34.94 27.25 OR 15.92 16.58 18.19 17.72 17.35 10,26 14.02 53.72 42.38 34,49 30.55 39.60 AB 29.81 34.18 30.69 35.14 32.33 35.68 36.14 19.62 23.49 22.59 26.30 28.33 All 23,62 21.68 21.29 19.72 20.75 25.99 24.05 3.75 4.77 3.43 3.62 2,91 CO 0.00 0.00 0,00 0.00 0.00 0.00 0.00 0.00 0.80 2.13 2,51 0.43 tiE 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 01 5.67 1.82 2.69 1.10 0.00 8,36 6.00 1.62 0.00 0.00 0.00 0.00 tlY 14.51 10.61 12,26 10.80 11.08 15.32 15.56 1.14 2.42 2.81 1.85 1.30 OL 0,00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 IL 1.31 0.57 0.60 0.52 0.58 1.18 0.92 0.39 0,26 0.26 0.22 0.18

An/(Ab+An) .4421 .3881 .4095 ,3595 .3909 .4215 .3996 .1605 .1687 .1319 .1210 .0932

I '" Analyses done at the USC Petrochemistry L.~boratory by G. M. Beckerman, J. p. Robinson, and Whitney A. Hoore; major elements as wt. %; Ba, Sr, and Rb as ppm. '" All Fe analyzed as FeO. ~ ~A~~ ~n~~~~~1~~'proportions of A1 2 Ol /(CaQ+Na 2 O+K 1 0).

211 10 9,6 9 #.'" 4 8

0 7 OJ 0 -i56.------.c5B,-~6;i;0,-~6102.---<6!:;4-<6!o6--;!6~8 -~7D;--7of.2,-~7"'4--,7~6­ z °5:O2--;!5"4 + 6 %Si02 0 OJ MID HILLS ~ ROCK SPRING ROCK SPRING ADAMELLITE "" ~6 MAFIC INCLUSIONS MONZODIORITE TUOLUMNE SERIES IVEOAKCAN'r'ON 4 z GRANODIORITE cf? 4 " ..... """ --A.-t- ~"'... \:I::D""-t-~~ 3 BLACK CANYON SUBALKALIC ~ ~HORNBLENDE GABBRO 2 °5'=2--=5':-4-:':56.----f.58O--~6:';D.---~6"2.----;6"4--=6';:-6-::'68O----!,70.----;7,;;2.----;7;:;4-C;7"6­ I OfcSi02 0 40 45 50 55 60 65 70 75 80 %S102

Figure 5. Composition of batholith plutons in tenlls of silica and total alkalies (wt. %). Sample 1­ 20 ROCK SPRING 112 of the Ivanpah granite is off scale (K 2 0 + MAFIC INCLUSIONS Na 2 0 = 11.2%, Si0 2 = 71.1%). Alkalic subalkalic 18 division is from Irvine and Baragar (1971). Fields for the Tuolumne series and California mon­ zonites shown here and on other figures is based on data of Bateman and Chappell (1979) and Miller 14 (1976), respectively. 1~"\;2---;5"'4.---~5t6;--,5"'B;--;f60;--;f62;--;;6';:-4-06"'6-<6!oB--,7t;O--of;..----,t;---O!',----- TUOLUMNE SERIES "10 Si02 TREND ,,~ . ""to. LIVE OAK CANYON GRANODIORITE

... KESSLER SPRINGS 80 ADAMELLITE o LIVE OAK CANYON GRANODIORITE ROCK SPRING IIiII KESSL ER SPRINGS ADAMELLITE MAFIC INCLUSIONS • MID HILLS ADAMELLITE o TEUTONIA ADAMELLITE 75 o BLACK CANYON HORNBLENDE GABBRO °5L.,2--;;5':-4--;5';:6---;5:';8.----,6:';0;--6;!:2;---.,;64;;---.,;6C6-*-*-~=~"""'?i'"'---­ • IVANPAH GRANITE ~ ROCK SPRING MONZODIORITE % Si02. /':,. ROCK SPRING MAFIC INCLUSIONS 70

TUOLUMNE 65 SERIES TUOLUMNE SERIES TREND OJ 0 60 u; cA

55

KESSLER SPRINGS ADAMELLITE 0 50 52 54 56 58 60 62 64 66 6B TH % Si 02 TUOLUMNE ROCK SPRING SERIES MAFIC INCLUSIONS TREND KESSLER SPRINGS 45 o ADAMELLITE ~ o 4 iF-" 40 I 0.2 0.4 0.6 O.B 1.0 FeO/FeO + MgO 0 52 54 56 5B 60 62 64 66 76 Q/" Si02-- Figure 6. Composition of batholith plutons in terms of Si0 2 and FeO/(FeO+MgO). Calc-alkaline (CA) and tholeiitic (TH) division from Miyashiro Figure 7. Harker variation diagrams for batholith (1974). Basis is weight %. All Fe as FeO. plutons.

212 Teutonia adamellite is rather iron-enriched and 300 depleted in CaO for its level of Si02. Even more unusual is the Jurassic Ivanpah granite. This plu­ ton has a compositional affinity to anorogenic granites rather than to margin-related batholiths. Its high alkali feldspar content (Fig. 3) is an ini­ tial indication of this fact borne out by its dis­ tinct low contents of CaO, A1 20 3 and Na20 and high K20. It also crystallized under somewhat different conditions of crystallization (see below). Presentl~ CALIfORNIA the Jurassic magmatic arc (Fig. 1) has not been well MONZONITES characterized and we are curious as to how repre­ sentative the Ivanpah granite is to this major pulse of magmatism. The Rock Spring monzodiorite is comparable in

many respects to similar alkali-rich plutons of early 500 1000 1500 2000 to middle Mesozoic age. As noted by Miller (1978) Sr (ppm) most of these of western North America have ages in the range of 170-230 m.y. and have "occasional" normative nepheline or minor quartz, hornblende 400 and/or clinopyroxene, a low to moderate silica con­ tent (53-63% Si02), high K20 and NaLO, and anoma­ " IVANPAH GRANITE lously high Sr (1000-2000 ppm) and Ba (1000-3000 ppm~ Although Miller (1978) hesitates to consider these 300 shoshonitic, they fit many shoshonite characteris­ tics (Morrison, 1980) including (1) low iron enrich­ KESSLER SPRINGS ment, (2) high alkalies, (3) K 0>Na 2 0, (4) high LIL ADAMELLITE CALIFORNIA 2 ~ MONZONITES (large ion lithophile) elements, (5) low Ti0 2 «1.3%) ~ 200 and variable to high A1 2 0 3 (14-19%). As depicted in Ii Figures 5-8, the composition of the Rock Spring monzodiorite falls well outside the compositional field for normal California monzonites. However it 100 does share in having low silica, early crystalliza­

tion of clinopyroxene, resorption of pyroxene to form '-"=''----lIVE OAK CANYON hornblende, red titaniferous biotite, and a minimum BLACK CANYON ROCK SPRING GRANODIORITE ® HORNBLENDE GABBRO MONZODIORITE Jurassic age. Moreover, it is compositionally simi­ O-J.-:=--~--~---~------r----,------, lar to some anomalous plutons of the monzonite belt, o 500 1000 1500 2000 2500 3000 particularly the San Bernardino Mountains monzonite Ba (ppm) (Miller, 1976). The latter is not as alkalic as normal California monzonites and has elevated initial Figure 8. Rb, Sr, and Ba data for batholith plutons. Sr isotopes indicative of crystal enrichment. Data points for two Rb-rich specimens of the Ivanpah granite are off of the scale of the Rb/Sr Magma Lineages and plot. Fractional Crystallization Inspection of the composition trends exhibited digestion. Secondly, in thin section they show all by the batholith plutons reveal a minimum of five the petrographic attributes of the pluton itself potential magma lineages. Hence, the composite including the presence of reddish, titaniferous nature of the batholith is due to the cogenetic biotite, hornblende with cores of clinopyroxene, and emplacement of several non-comagmatic suites. The magnetite inclusions in all major phases except separate lineages include (in approximate sequence plagioclase. Hence the mineralogy down to the in­ of age) (1) the Rock Spring monzodiorite, (2) the ferred crystallization sequence establishes a marked Ivanpah granite, (3) the Mid Hills and Teutonia affinity to the enclosing plutonic rock. In fact, adamellites, (4) the Kessler Springs adamellite and their composition (Table 3) brackets that of the most the Live Oak Canyon granodiorite, and (5) the silica­ primitive sample of the pluton (MH-4) which occurs undersaturated Black Canyon hornblende gabbro. along the northern edge. Combined with their fine grained texture, we suggest that the inclusions are Rock Spring Monzodiorite autoliths (cognate inclusions) of an early solidi­ fied chilled margin swept up in a resurgence of magma The Rock Spring monzodiorite exhibits the movement. Hence these inclusions and sample MH-4 largest range of composition of any of the batho­ likely approximate the parent melt which fractionated lith plutons (Fig. 5-8). To our surprise, the to form the rest of the series. rather abundant mafic inclusions (Fig. 4) fallon line with this lineage thus precluding their being Given the parental composition, the extent of random xenoliths. Several comagmatic models for the fractional crystallization can be estimated by lineage and inclusions are possible including (1) modeling the major element changes through removal of contamination, (2) xenolith metasomatism (i .e., their constituent mineral phases. The mineral xenolith being metasomatized to the extent that it is chemistry of these rocks is known (see next section) forced to be "on line"), (3) restite mixing, (4) and the results of the model are given in Table 4. cognate inclusions of a fractionation sequence. The The latter shows that the range of silica (55.5-64.1 former three are precluded on the basis of structure wt. %Si02) and other elemental changes including and texture. These inclusions occur throughout all an increase toward alumina saturation and modest iron members of the series and show no evidence of enrichment is due to 51% fractionation of plagioclase,

213 TABLE 4. RESULTS OF FRACTIONATION MOOELS FOR THE TEUTONIA BATHOLITH likely induced by plagioclase-dominated fractiona­ tion but occurred in two separate, although similar, A. Rock Spring ~lonzodiorite B. Mid Hills and Teutonia Adamellite magma systems. Parent I Differentiate Parent] Differentiate Actua1 2 flodel Actua ,4 Model Kessler Springs Adamellite and Live Oak Canyon 5i0 2 55.48 64.05 64.46 71.42 76.01 76.15 0.40 0.33 0.13 0.09 0.09 Granodiorite Ti0 2 I. II ------AlzO J 16.77 16.10 16.23 14.48 12.86 12.67 FeO 6.26 4.31 4.33 I. 82 0.53 0.53 M90 3.35 1.59 1.57 0.52 0.06 0.05 The Kessler Springs and Live Oak Canyon plutons MnO 0.125 0.110 0.050 0.09 0.05 0.08 CaO 5.80 4.03 4.22 2.27 0.27 0.84 are the most calcic and Sr-rich of the batholith. NazO 3.62 3.47 3.28 3.85 3.70 3.23 Both are small and quite separated plutons; hence K,O 2.68 2.83 2.68 3.58 4.86 4.54 they are not likely to be comagmatic. However, their TOTAL 96.38 98.73 98.80 98.16 98.43 98.18 close compositional affinity (Fig. 5-8) require ~~~WeO+l190) 0.651 0.731 0.734 0.778 0.898 0.914 that they are at least "distant cousins" formed 0.865 1.000 1.017 1.010 1.086 1.078 by an evolution that was similar with regard to Weight fraction of solid and residual liquid source material and post-fusion magmatic processes. X,olid 0.510 0.226 0.774 Xliquid 0.490 Depth of Magma Generation Weight percent of fractionated minerals 6 and Source Material Plagioclase 55.17 80.09 Biotite 23.82 Hornb 1ende 11.91 15.93 We presently lack the necessary trace element Cl i nopyroxene 7.36 Ilmenite 1.40 0.44 (e.g. REE) and isotopic data to thoroughly estimate Magnet; te 0.35 3.54 the nature of source material from which the batho­

'Sample MH-4 (Jrs) lith magmas may have been generated. Yet, some work­ 'Sample MH-J31 (Jrsq) ing models can be proposed at this stage of our 'Sample MH-122 (Kmh) 4Average of samples 1-34,1-122 (K+) ongoing investigation. Because we have been able sMolecular ratio of AlzO]/(CaO+NazO+KzO) to delineate the extent of fractionation or accumula­ 6Mi nera 1 compos iti ons froo mi croprobe da ta tion, and thus have some limits on parental magmas involved for the different series, then limits at least can be placed on the probable gross character of the source. biotite, hornblende, clinopyroxene, and Fe-Ti oxides. Although our trace element data base is not large Some insight to this problem is possible through (Table 3, Fig. 8), the modest changes in Rb, Sr and comparison of potential parental magma composition Ba are consistent with removal of these same to experimental work in the system Qz-Or-Ab-An-HzO. minerals. Compiled in the form of a grid (see Anderson and Cullers, 1978; Anderson, 1980), Figure 9 summarizes Ivanpah Gran ite present experimental data as well as that from the batholith. The usefulness of this approach is two­ The Ivanpah Granite exhibits a range in composi­ fold as melts that have changed little since fusion tion that must reflect, in part, alkali feldspar can be used to approximate depth of melt generation accumulation. The most primitive sample (1-112) has the lowest silica and highest K2 0 (71.1 and 8.99 Qz wt. %, respectively), The rock is not altered and such high potassium values are difficult to explain by any other igneous mechanism. Appropriately, this rock contains the lowest amount of modal quartz (21.2%) and highest amounts of alkali feldspar (59.4%). It also has the highest Sr and Ba, which is consistent as both of these trace elements are pre­ ferentially concentrated in alkali feldspar relative to liquid (Arth, 1976). Hence the parent melt for the Ivanpah granite must be somewhere in the middle Ab/An of this series. ~10deling of this fractionation indi­ cates 39% solid fractionation of 54.5% alkali feld­ spar, 25.5% plagioclase, and 20.0% hornblende. The latter phase accounts for the alumina saturation in the more differentiated members. Similar alkali feldspar accumulation in potassic granites has been documented by Anderson and Cullers (1978). IVANPAH GRANITE Teutonia and Mid Hills Adamellite The Teutonia and Mid Hills adamellites are so megascopically similar that we initially considered them to be the same pluton. The Harker variation Ab L------__-----"Or diagrams (Fig. 7) show that the two could constitute a single magma series involving an increase in SiO z and K2 0 and a decrease in Al z0 3 , FeO, MgO, and CaO. Figure 9. Normative quartz, albite, and orthoclase Modeling of these major element changes (Table 4) composition of bathol ith plutons compared to shows that the series could be produced by 22.6% experimental minimum melt compositions over a fractionation of plagioclase, hornblende, and Fe-Ti range of PHzO (=PTOTAL) and Ab/An ratio. Source oxides. Yet, perusal of the trace element data of grid experimental aata given in Anderson and (Fig. 8) reveals that such a model is not possible as Cullers (1978). Pluton symbols same as used the two plutons exhibit different tren~separated by in Figures 5-8. a compositional gap. The internal changes were

214 generation and a differentiation series should plutons, the range is An20 to An3'" For the Rock trend toward a minimum appropriate for the depth of Spring monzodiorite, the range is An29-37 except emplacement. This application involves several in the silica-richer portions which have plagioclase assumptions including: (1) absence of con­ rims as sodic as An22. Orthoclase contents are all tamination, (2) no accumulation, (3) rock composi­ low (Orl_3)' tion approximates a minimum melt, and (4) solid-melt equilibria occurred under vapor saturated conditions Perthitic alkali feldspar presents a special with a vapor phase approximating pure H20. We have analytical problem due to the effects of exsolution. attempted to preclude the effects of the first two Host compositions are potassic, ranging Or86-0r94. concerns in the above section. The fourth assumption We have attempted to retrieve the bulk composition may be a problem. Although most of the plutons of the alkali feldspar for two plutons (Mid Hills probably became saturated with an aquaeous vapor adamell ite and Ivanpah Grani tel by separate methods: phase during final stages of crystallization (as microprobe analysis utilizing a migrating and exemplified by the common occurrence of pegmatites defocused beam and atomic absorption analysis of in all plutons except the Rock Spring monzodiorite extracted feldspar. The results are similar ranging and Ivanpah granite), it is unlikely that they were Or78.S-79. Ab2o.8_19.6Ano.7_o.8 for the Ivanpah saturated during anatexis. Fortunately, it appears and Or73. 3-79. 3Ab19 .14-24. sAno. 7-2.6 for the Mid Hill. that vapor-undersaturated melting (Luth, 1969) displaces melt composition away from albite, sub­ Fe-Ti Oxides parallel to the isobaric minimum melt curves. The third assumption remains a larger problem, for if Magnetite is the predominant Fe-Ti oxide mineral anatexis involved several tens of percent of fusion, in the batholith. Ilmenite is rare to non-existent then the melts will have migrated away from a in all plutons except the Teutonia adamellite. For minimum melt composition. However the migration of the latter, the ilmenite was found to be too mangani­ a melt on the quartz, two feldspar isobaric univari­ ferous (up to 8.78% wt. %MnO) for use in thermometry ancy during progressive fusion is again parallel to calculations, having the composition Ilm79.3-87.7 the projected isobars. Hemo.I-I.2PyrI2. 1-19. 3' The magnetite compositions were all nearly pure Fe 304 and contained only minimal Inspection of Figure 9 reveals the uniform amounts of Ti0 2 (less than 0.16 wt. %), MnO (0.11­ nature of the Cretaceous granitic plutons in terms 0.19%), and Si0 2 (0.10-0.12%) indicating that sub­ of normative quartz, orthoclase, and albite. solidus reequilibration, a common feature of granitic Assuming that all represent an anatectic composition, rocks, had occurred. Despite the reequilibration, then it follows that all may have been formed by it is clear that the granitic plutons belong to the partial fusion of a quartz feldspathic source at an "magneti te-seri es" of Ishi ha ra (1977). Thi s has average of 7-10 Kb or 27-38 km. This depth is considerable bearing on the levels of oxygen fugacity indicative of a lower crustal origin. Although we and is addressed further below. prefer there was more confirming evidence, the dif­ ferentiation trend for the Teutonia adamellite (open Pyroxene circles) is anchored at a 2 Kb minimum indicating a level of emplacement of about 7-8 km. This is Clinopyroxene represents the initial mafic sili­ consistent with the epizonal nature of these plutons cate to crystallize in the Black Canyon hornblende and that they have intruded high into an overlying gabbro and the silica-poor members of the Rock Spring Paleozoic and Mesozoic section. The data for the monzodiorite. Pyroxene data is given in Table 5. Ivanpah granite is distinctly shifted to an Ab­ For the gabbro, the pyroxenes range in composition poorer composition. This may be due to fusion under from Ca44Fe13Mg43 to Ca41Fel6Mg43 and classify as drier conditions and/or a lesser percentage of fusion augite. For the monzodiorite, the pyroxene are both of a more calcic quartzofeldspathic source. Noting more calcic and iron richer and classify as salite. that the Qz-poorer data point(s) is due to alkali The compositional range is CasoFe12Mg38 to Ca48Fel2 feldspar accumulation, the cluster for most of the Mg 40 . pluton averages again at 7-10 Kb. Hornblende The Rock Spring monzodiorite does not have a composition that can be applied in the same manner Hornblende occurs as a primary phase in the as the above granitic plutons. However, it is clear Black Canyon hornblende gabbro, the Rock Spring from the position of the most primitive composition monzodiorite, the Mid Hills adamellite, and the that the parental melt was generated from a silica Kessler Springs adamellite. Never the sole mafic deficient source. Whether this source was eclogitic, mineral, this phase always coexists with clino­ as proposed by Miller (1978), or from LILE-enriched pyroxene or biotite, sphene, and magnetite. Accord­ mantle wedge (Gill, 1981) obviously needs testing ing to the classification of Leake (1978), the but remain as distinct possibilities. hornblende is an edenite to edenitic hornblende (Rock Spring) or magnesio-hornblende (all other DETAILED MINERALOGY AND ESTIMATED plutons). All are rather Mg-rich (Fe/Fe+Mg ranging CONDITIONS OF CRYSTALLIZATION 0.362-0.451). Hornblende in Black Canyon and Rock Spring is the most aluminous and least siliceous Mineral Chemistry (Table 5) reflecting the low activity of silica in these plutons. They also are the most titaniferous Feldspars (Ti0 2 to 1.73 wt. %) which likely is due to a higher temperature of crystallization relative to the Two feldspars, oligoclase to sodic andesine and granitic plutons. Studies by Raase (1974), Stephen­ perthitic alkali feldspar, coexist in all plutons son (1977), and Anderson (1980) have demonstrated except the Black Canyon hornblende gabbro. Plagio­ the temperature dependence of the Ti saturation clase compositions are noted for each pluton in limit in amphiboles. Table I and are rather predictable for each rock type. For the granite, adamellite, and granodiorite A remarkable aspect is the low contents of Al z0 3

215 TABLE 5. AVERAGE ANALYSIS OF CLINOPYROXENE AND HORNBLENDE FROM THE TEUTONIA BATHOLITH

P utan 1 Kbc Jrs Kbc Kmhe KmhD Jrs Jrso Krs Sarno le MH-100 MH5a MH-100 MH-9a MH-20a MH-5a I~H-l72 1-12 No. ' 2 3 1 2 2 3 3 3 I~i n. CPX CPX HBlD HBlD HBlD HBlO H8lD H8lD SiO, 50.51±.33 53,82±.27 45.44 47.41±.02 47.97+.05 44.71+.73 45.75+.77 49.83+.68 7.92±.28 8.25±.47 4. 85±. 40 A1 2 0 3 4.17±.02 .30±.14 9.10 5.57±.13 6.87±.13 1. 57±. 03 1.20±.19 .54±.04 Ti0 2 .8D±.15 .04±.02 1. 73 .95±.04 .8H.Ol Fe0 3 8.63±.51 7. 56±. 08 14.92 14. 53±. 23 15. 05±. 32 16. 36±. 40 16.92±1.20 14.47±1.17 M90 14. 38±.01 13. 55± .13 12.99 13.04±.25 13.11±.19 11. 65±. 34 11. 56±. 87 14. 32±1.04 MnO .29±.04 .78±.04 .31 1. 71±.03 .63±.03 .54±.01 .66±.06 .83±.01 CaO 19. 94±. 86 24. 15±1. 07 11. 73 11.39±.10 11.26±.31 11. 96±. 23 11.64±.23 11. 73±. 33 NazO .57±.04 .35±.07 1.18 1.55±.02 1. 13±.04 1.26±.08 1.05±.13 1. 13±.17 K,O ------.75 . 63±. 05 .80±.02 1.09±.05 1. 04±. 02 .54±.09 F1 ------.03 .63±.02 . 02±. 02 . 02±. 03 .13±.12 .51.±.06 C1 ------.09 .04±.01 .04±.01 .16±. 03 .15±.06 .Dl±.OI TOTAL 99. 33±. 25 100.54±.55 98.23 97 .14±. 27 97.64±.11 97.21±.49 98.26±.88 98.67±.34 Formula Units Per 4 Cations (CPX) or Charge = 46.000 (HBlO) 6.756±.056 6.815±.032 7. 252±. 026 Si IV 1. 883±. 018 1.993±.018 6.672 7. 117±.007 7. 093±. 013 ~~Vl . 117±.D18 · 007±. 007 1. 328 · 883±. 008 .907±.013 1. 244±. 056 1. 185± . 032 . 748±.026 · 066±. 019 · 006±. 011 .248 .103±.015 · 290±. 036 . 166±.059 · 264±. 069 · 086±. 066 Ti · 023±. 005 .001±.ODI .191 · 107±.005 .089±.001 . 179±.004 .134±.019 · 060±. 005 Fe .269±.017 · 234±. 003 1. 833 1. 825±. 027 1. 863±. 042 2. 068±. 063 2.109±.162 1. 762±.161 119 · 798±. 002 · 748±. 011 2.844 2.918±.058 2.891±.039 2.624±.082 2.566±.168 3. 106±. 194 Mn · 009±. 001 · D25±. 002 .039 .217±.OD3 · 078±. 003 · 069±. 002 · 083±. 007 .102±.002 Ca · 796±. 032 · 958±.037 1. 845 1.835±.015 1. 784±. 047 1. 9.37±. 037 1. 858±. 032 1. 829±. 038 Na .041±.003 · 025±. 005 .335 .451±.007 · 324±. 012 · 369±. 025 .317±.025 .317±.045 K ------.141 . 120±.009 .151±.005 .21O±.009 · 188±. 014 .100±.018 F1 ------.012 · 297±. 006 · 008±. 008 · 009±. 016 · 062±. 052 · 236±. 029 C1 ------.022 .01O±.003 · 009±. 001 · 042±. 009 .037±.015 · 001±. 002

Fe/Fe+MD .252+.011 .238+.003 .392 .385+.008 .392+.009 .441+.021 .451+.035 .362+.036

lPluton abbreviation same as in Table 1 2Number of ana lyses averaged 'A11 Fe as FeO

for hornblende from the granitic plutons (wt. % A1 2 0 3 Intensive Variables ranging 4.85-6.87). Low Al hornblende seems to be characteristic of calc-alkaline metaluminous grani­ Some qualitative aspects pertaining to the toids of orogenic belts (see data reported by Dodge intensive variables (T, P, fH.O' fo 2 ) that character­ et al., 1968 for the Sierra Nevada batholith) ize the conditions of crystaliization can be in­ and contrast to the higher Al hornblende of ferred from the sequence of crystallization of the anorogenic granites (Anderson" 1980) which may constituent minerals. On an adamellite (monzo­ contain up to 9-11 wt. % A1 2 0 3 and classify as granite) similar to that of the Teutonia batholith, hastingsite to hastingsitic and edenitic hornblende. Maaloe and Wyllie (1975) determined the influence Although it is well recognised that the Al content of H2 0 content on crystallization sequence at may be temperature and pressure dependent (Raase, P=2Kb and T-fo 2 constrained to the Ni-NiO buffer. 1974), it must also be influenced by rock composi­ Although plagioclase was found to be the liquidus tion, mineral assemblage, and the composition of phase in all cases, the appearance of biotite (and coexisting phases. The controlling factor here may presumably other hydrous phases, when present), be the composition and abundance of plagioclase systemmati ca lly change from bei ng very 1ate under (W. M. Thomas, personal comm., 1982). Calc-alkaline dry conditions (H 2 0<1.4 wt. %) to early under wetter granitoids characteristically have plagioclase of conditions (H 2 0>3.0 wt. %). Of the major minerals greater abundance and An-richer composition relative in the granitic plutons of the batholith, plagio­ to their anorogenic counterparts. As shown by clase is always early followed by hornblende and Anderson (1980) for biotite, the anorthite component biotite thus indicating rather high levels of H2 0. in plagioclase effectively concentrates the available The co~non presence of pegmatites and aplites further Al at the expense of the aluminous ferromagesian suggests that H2 0 was high and that saturation silicates. probably occurred at least in the final stages of crystallization for most of the granitic plutons. Biotites As demonstrated so clearly by Ishihara (1977), The composition of biotite from the Teutonia Fe-Ti oxides are likewise an indication of the batholith is given in Table 6 and Figure 10. Their relative levels of oxygen fugacity. His "magnetite composition is uniform for most plutons with A1 2 0 3 series" refer to granitoids with high total Fe-Ti ranging 13.3 to 14.9 wt. % and Fe/(Fe+Mg) ranging oxides (0.1-2.0%) and a high proportion magnetite 0.394-0.491. They are similar to biotites reported to ilmenite. Coexisting biotite have both high for the Sierra Nevada batholith (Dodge et al., 1969), Mg/Fe and Fe 3+/Fe 2 + and are inferred to have crystal­ but tend to be Al poorer and Mg richer. In general, lized at high oxygen fugacity (f0 2 ?Ni-NiO). In there is very little excess Al after filling the contrast, "ilmenite-series" have low total Fe-Ti tetrahedral site (Table 6); hence the solution toward oxides «0.1%) and a high proportion of ilmenite. eastonite and siderophyllite is somewhat restricted. Their biotites are iron-rich, have low Fe 3+/Fe 2 +, Similar to the case for hornblende, the most titani­ and conversely, are inferred to have crystallized at ferous biotite occurs in the Rock Spring monzo­ a low oxygen fugacity (ca. QFM). As stated above, diorite. As noted earlier, the biotite of the the Teutonia batholith belongs to the magnetite­ Ivanpah granite stands apart from the rest of the series. A high level of oxygen fugacity is also batholith, being the most aluminous and iron-rich indicated by the early appearance of magnetite. (Fe/Fe+Mg averages 0.731). It also has the highest Anderson et al. (1980) have shown how the appearance content of flourine (2.23 wt. % F). of magnetite in a crystallization sequence correlates to calculated oxygen fugacity. In the Teutonia batholith, magnetite ·is one of the earliest phases to have crystallized.

216 TABLE 6. AVERAGE ANALYSES OF BIOTITE FROM THE TEUTONIA BATHOLITH

Pl utan 1 KIa Kks Kmhn Kmhe Kt Ji9 Jrs Jrsa Sampl e I-6d - MH- a MH-Ya - -1". MH-5a Mh- No.2 2 3 3 5 3 3 3 2 SiO, 37.54+.19 37.37+.78 37.81+.52 37. 49±. 33 38. 81±1. 02 35.34±.16 36.64±.41 36.94±.12 AhO, 14.94±.08 13. 65±. 40 14. 34±. 33 13.28±.64 13.83±.06 15. 55±. 44 14.01±.06 14. 96±. 02 riOz 2.77±.18 2.45±.09 2.90±.41 2.75±.63 3.55±.17 2. 67±. 22 4.11±.33 2. 83±. 02 FeO' 18. 66±. 09 16.91±. 23 17. 23±. 64 16. B9±. 49 16.90±.75 25.78±.47 19.85±.53 18. 45±. 41 M90 11.91±.35 14.61±.42 13. 32±. 44 13.16±.31 13.33±.17 5. 35±. 59 11.56±.16 12.04±.09 MnO .66±.04 .54±.12 · 43±. 06 1.14±.13 .62±.00 .80±.07 . 29±. 05 .43±.02 ZnO .13±.08 .13±.01 · 06±. 07 · 04±. 05 .06±.04 .11±' 06 .12±.14 .09± .01 CaD ---- .06±.03 .01±.02 .02±.01 .24±.09 .09±.15 . 01±. 01 ---- NazO .07±.01 .05±.03 · 08±. 03 · 05±. 03 .02±.02 .06±.04 .07±.04 .13±.01 K,O 10.17±.08 8.86±.34 9. 83±. 29 9.70±.32 6.55±.78 1O.01±.18 10. 39±.13 10. 04±. 08 FI .85±.03 1.14±.23 .65±.14 1.20±.24 1. 58±. 06 2.23±.41 .10±.05 .10± .01 Cl .01±.00 .02±.01 · 02±. 02 · 04±. 01 . 02±. 02 .15±.01 .21±.02 .07±.01 TOTAL 97.32±.30 96. 47±. 06 96. 38±. 27 95. 66±. 99 94. 84±. 36 97.15±.20 97.12±.45 96.00±.41 Formul a Un; ts Pe Charge = 22.0 0 Si IV 2. 799±.012 2. 840±. 061 2. 822±. 028 2. 834±. 022 2.867±.048 2. 725±. 023 2. 772±. 02B 2.800±.005 ~1VI 1.201±.012 1. 160±. 061 1. 17B±.028 1. 166±.023 1.133±. 048 1. 275±. 023 1. 22B±. 02B 1. 200±. 005 .113±.008 · 031±. 029 .083±.033 · 025±. 023 · Oll±. 043 · 139±. 069 .021±.021 .136±. 001 Ti .155±.010 . 137±. 004 .163±.023 .156±.037 · 197±. 010 .154±.012 .234±.017 .162±.002 Fe 1. 163± . 004 1.047±.015 1. 076±. 044 1. 068±. 027 1.044±.053 1. 662±. 035 1. 256±. 039 1. 170±. 025 M9 1. 324±. 041 1.612±.042 1. 480±. 043 1. 483±. 035 1. 467±.011 · 614±. 066 1. 303±.011 1. 360±. 012 Mn · 042±.003 · 034±. 007 · 027±. 004 · Oll±. 009 · 039±. 001 · 052±. 005 .019±.003 · 027±. 001 Zn .007±.005 .007±.001 · 003±. 004 · 002±. 003 · 003±. 002 · 006±. 003 · 007±. 008 · 005±. 000 Ca ---- · 005±. 002 · 001±. 002 .001±.001 · 019±. 007 .007±.012 · ODD±. 001 ---- Na .01O±.001 · 008±. 005 · 012±. 004 · 007±. 004 · 003±. 003 · 009±. 006 · 009±. 006 .019±.001 K · 968±. 007 · 837±. 035 · 936±. 031 · 936±. 032 .618±.079 · 985±. 021 1. 002±. 008 · 971±. 009 Fl · 200±. 005 · 267±. 052 · 153±. 033 · 287±. 056 . 363±.015 · 543±. 098 .023±.012 · 023±. 023 Cl · 002±.001 .002±.001 · 003±. 002 · 005±. 002 · 003±. 001 · 020±. 002 · 026±. 003 .009±.001

LVI 2.801±.017 2. 867±. 031 2. 832±. 036 2. 800±. 035 2. 840±. 026 2. 628±. 025 2. 839±, 025 2.860±.013 LA · 977±. 006 · 850±. 034 .949±.030 · 944±. 032 · 639±. 075 1.001±.016 1.009±.017 · 990±. 008 Fe/Fe+M9 · 468±. 009 · 394±. 006 .421+.012 I .418+.010 .416+.014 .731+.025 .491+.010 .463+.008 1Pluton abbrevlatlon same as In Table 1 2Number of analyses averaged 'All Fe as FeD

The now classic magnetite-ilmenite geothermo­ a problem; through an iterative process we have meter of Buddington and Lindsley (1964) is generally determined equilibrium magnetite compositions from regarded as one of the better means to calculate T the data of Buddington and Lindsley (1964). and foz. Unfortunately, as is the case for most granitic plutons, magnetite has undergone subsolidus Final estimation of f0 2 requires an independent reequilibration, a probable result of slow cooling. estimate of temperature. For the Rock Spring Biotite, coexisting with alkali feldspar and magne­ monzodiorite, a separate estimate is possible from tite, provides an alternative in estimation of T and the partitioning of Mg/Fe between clinopyroxene and fo z. The stability of annite component in biotite hornblende using the geothermometer developed by (Wones and Eugster, 1965; Wones, 1972) involves Kretz and Jen (1978). The result, 975±17°C, suggests 12 5 seven variables: PTOTAL ' temperature, fHzO' fo z, a rather low fo", of 10- • bars. This implies, aANNITE-BIO, aOR-KSP, and aFe304-MT which describe of course, that pyroxene, hornblende, and biotite the continuous reaction: were all equilibrium phases, a fact that we have not proven (recall the reaction relationship between Biotite + O2 Mg-richer biotite + alkali feldspar + hornblende and pyroxene).

magnetite + H2 0. For the granitic plutons, a lower limit of temperature is their wet solidus. As each stability For relatively hydrous epizonal granites, biotite field has pressure contours, then the solidus for acts more as an oxy-barometer as variations in pres­ granite to adamellite melts in equilibrium with sure, fH 0' and T are limited. Figure 11 shows the these biotites can likewise be plotted (Fig. 11). stability2 of biotite from the batholith calculated Additional temperature estimates can be made with the using the equation two-feldspar geothermometer of Stormer (1975) and Whitney and Stormer (1977). Although alkali feldspar -\log fo z = (7409/TOK) + 4.25 - log fH 0 + log aANN ­ has generally exsolved an ablite phase, we have 2 attempted for two plutons, the Mid Hills adamellite log aOR - log aMT and the Ivanpah granite, to retrieve the bulk composition of this feldspar (see above section on feldspars). Following the procedure suggested by where aANN = (Fe/WCT) 3(OH/(F + Cl + OH) )"'(K/(K+Na)). Whitney and Stormer for orthoclase, our data yields minimum temperatures of 642±13.1"C and 631±12°C, Similar application has been made by Czamanske and respectively. Thus, the stability fields depicted Wones (1973), Anderson (1980), Czamanske et al. in Figure 11 define a window in P-T-f0 2 space for (1981), and Anderson and Rowley (1981). Noted conditions of crystallization. Confirming our earlier, the batholith may have been emplaced at qualitative estimates, these "magnetite-series" about 2 Kb. Conservatively, we have calculated the granitic plutons have all rather uniformly crystal­ biotite stability fields over a 2-4 Kb pressure lized at hi9h levels of fo z, well above the Ni-NiO range. Assuming that PHzO ~ PTOTAL' we have deter­ buffer. Only the Ivanpah granite, with its systema­ mined the fHzQ for each pressure as a function of tically iron- and flourine-richer biotite, stands temperature wlth the fugacity coefficients of Burnham apart from the rest, having crystallized at about et al., 1969). The activity of alkali feldspar was an order of magnitude lower fo z. calculated for our data with the Margules parameters of Thompson and Waldbaum (1969). The fact that mag­ The black Canyon hornblende gabbro, like the netite has not retained its magmatic composition is monzodiorite, contains coexisting hornblende and

217 ANNITE SIDEROPHYLLITE clinopyroxene in a reaction relationship. For this 1.0 pair, the partitioning of Mg/Fe or Kd (Kd I I (Mg/Fe)CPX/(Mg/Fe)HBLD) averaged lower than that of the monzodiorite (Kd = 1.92 versus 2.51, respective­ ly) indicative of a lower temperature of equilibra­ tion according to the calibration of Kretz and Jen 0.8- - (1978). The resultant temperature, 942±9°C, is .00 above the solidus for a wet gabbroic melt at 2-4 Kb ~o (Lambert and Wyllie, 1968) and is within the IVANPAH GRANITE stability limit of amphibole in equilibrium with '" 0.61­ - such a magma. ::E + ROCK SPRING MONZODIORITE If SUMMARY "- ~ LIVE OAK CANYON GRANODIORITE I.L.'" 0.4 r- - The composite Teutonia batholith formed through the cogenetic emplacement of several non-cogmatic ~NIA AND MID HILLS ADAMELLITE suites involving seven Jurassic and Cretaceous plutons and six different magma lineages. Marginally metaluminous in composition, the granitic plutons 0.2- - may have formed by fusion of a lower crustal (27-38 km) quartzofeldspathic source. The potassic - Ivanpah gr~nite, its unusual composition further enhanced by alkali feldspar accumulation, may have I 0.01.0 been generated at approximately the same depth 1.5 2.0 involving either drier conditions and/or lesser AI ATOMS/II OXYGENS degrees of fusion of a more calcic quartzofeldspathic EASTONITE source. The Rock Spring monzodiorite, having a PHLOGOPITE shoshonitic affinity, appears to have been derived from a silica deficient source, either eclogitic Figure 10. Composition of biotite from batholith or enriched mantle. These conclusions, obviously plutons in terms of Fe/Fe+Mg and total Al (atomic preliminary, need testing by isotopic and further basis). Closed and half closed circles represent trace element studies. porphyritic and equigranular phases of the Mid Hills adamellite, respectively. Closed and open The plutons were emplaced into the upper crust triangles represent silica-depleted and silica­ (~7-8 km) and evolved from separate parental magmas richer phases of the Rock Spring monzodiorite, through fractional crystallization processes. The respectively. wide range of composition exhibited by the Rock Spring monzodiorite is consistent with 51% fractiona­ tion of plagioclase and constituent mafic phases. The granitic plutons underwent only moderate amounts of fractionation, alkali feldspar dominated for the Ivanpah granite, plagioclase dominated for all others. Reflecting their biotite composition and Fe-Ti oxide mineralogy, the granitic magmas crystal­ lized at high levels of oxygen fugacity near wet solidus temperatures. ACKNOWLEDGEMENTS

0> o This study has been supported by the Rudolph C. ...J I 15 Foss Endowment for Mineralogic Research (University of Southern California), a Geological Society of America Penrose Grant (to J.P.R.), and the Department of Geologlcal Sciences Graduate Research Fund (to G.M.B.). Discussions with Gregory A. Davis and Warren M. Thomas proved invaluable. Whitney A. Moore did a number of rock analyses and John Brand aided our effort through an atomic absorption analy­ 500 sis of alkali feldspar separates. Donna Martin TEMPERATURE °c and Daniel Krummenacher expertly coached one of us (G.M.B.) through the intricacies of K-Ar dating. Figure 11. Stability fields of biotite from the Many of the figures were drafted by Janet Dodds. Teutonia batholith. Lower and upper field Anne Snell bravely typed this manuscript into its boundaries represent a PHzO(=PTOTAL) of 2 and 4 Kb, final form. respectively. Range of temperature for the Rock Spring monzodiorite based on hornblende-clino­ REFERENCES CITED pyroxene thermometry. Lower range of temperature for the two gran itic fi e1ds is the wet so Ii dus Albee, A. L., and Ray, L., 1970, Correction factors for a wet granite or adamellite liquid in equi­ for electron microprobe analysis of silicates, librium with these biotites (see Anderson and oxides, carbonates, phosphates, and sulfates: Rowley, 1981). Anal. Chemistry, v. 42, p. 1408-1414.

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220

Plate 1 - A view looking to the west of the northernmost syncline (top photo), and (lower photo) a refolded fold from domain #4 (photos courtesy of Eric Frost).

222 STRUCTURAL ANALYSIS OF THE BIG MARIA MOUNTAINS, RIVERSIDE COUNTY, CALIFORNIA

M. Jane Ellis Union Oil Company 2323 Knoll Drive Ventura, California 93006

ABSTRACT Metamorphosed upper-greenschist grade Paleo­ zoic and Mesozoic metasediments are folded together with Precambrian basement and Mesozoic intrusive rocks into large (amplitude of at least 1 kilometer) east-west to north-northwest, south-southeast trending, south vergent recumbent folds. Structural analysis of small scale folds, refolded folds, lineation, foliation and boudinage from four domains within the range indicate that the large folds are a product of north-northeast directed underthrusting. K-Ar age dating (Martin and others, this volume) and the presence of Cretaceous (7) metavolcaniclas­ tic rocks within the folds indicate a mid-Cretaceous age for this event. A flat-lying thrust-fault exposed within Quien Sabe Point in the northern portion of the range and K-Ar age dates of mid-Tertiary age (Martin and others, this volume) found in the southern portion of the range, indicate the existence of a now eroded detachment surface overlying the present Big Maria Mountains. The detachment faulting caused a

I slight doming of earlier Mesozoic features. ; i ~1id ( to late Tertiary right-lateral strike­ '. slip faults cut the range with offsets of about \ , '. 2 km. These faults must be reconstructed in order \\ to give a clear picture of the Mesozoic folds. INTRODUCTION The Big Maria Mountains, located just north of Blythe, Southeastern California (figure 1), are composed of highly folded and thrust-faulted Pre­ cambrian, gneisses, Paleozoic metasediments of cratonal affinity (Hamilton, 1964b, 1971; Stewart Figure 1 - Location map. and Poole, 1975), and Mesozoic quartzites and meta­ volcaniclastic rocks. The metamorphism is upper the Whipple Mountains to the north. There are greenschist grade. strong suggestions that detachment faulting took place in the Big Marias. Detachment faulting has A.generalized map of the range (figure 2) shows been documented both to the north and south of the two maJor east-west trending, recumbent synclines Big Marias; in the Riverside Mountains (Hamilton, (amplitude 1 kilometer) and portions of the inter­ 1964a, Carr and Dickey; 1980, J. Lyle, this volume) vening anticline. Folding involves the basement and in the Trigo Mountains (Garner et al, this vol­ Paleozoic and Mesozoic sections. Thrust-faultin~ ume) respectively. A flat lying thrust fault has and extreme thinning of the rock units occurs been mapped by Hamilton (1964a) on Quien Sabe Point within the limbs of the synclines. Ages for unde­ in the northernmost portion of the range. The formed intrusive rocks that cut the folds are morphology of this fault strongly resembles a de­ 80:5:5.2 (biotite) m.y.B.P., (Martin and others, tachment fault (Frost, personal communication, 1981), thlS volume) placing the age of deformation at pre­ but K-Ar age dates (Martin and others, this volume) 80 m.y.B.P. The presence of the early Jurassic have not yet revealed a Mid-Tertiary thermal event. Aztec (7) Sandstone within the folds places a Mid-Tertiary K-Ar age dates have been found in the lowe~ limit on the folding, indicating that the southeastern portion of the range. The fault found f?ldlng took place during early to mid-Cretaceous on Quien Sabe Point in the northernmost portion of tlme. the range, and the resetting of age dates to Mid­ Tertiary time in the southernmost portion of the Another suite of K-Ar ages by D. Martin (Martin range leads to the conclusion that, prior to erosion and ?thers~ this volume) show a cluster during Mid­ a domal shaped detachment fault extended over the Tertlary tlme, the time of detachment faulting in range. This conclusion is supported by the domal

223 ~1ill er and McKee, 1971). In the Pl umosa Mountai ns the metamorphosed Paleozoic rocks can be correlated with an equivalent unmetamorphosed Paleozoic sequence containing fossils (Miller and McKee, 1971). The Paleozoic and Mesozoic sequences have been des­ cribed by Emerson in this volume, and also Ellis

N (1981), Emerson (1981) and Tucker (1980). The Big Maria Mountains contain an additional Mesozoic unit not seen in the Little Maria Moun­ tains. An off-white, brown or greenish quartzite is found at the base of the Mesozoic section. The r upper portion is often stained red-orange. Fine g~ained quartz makes up about 90% of the quartzite. Mlnor amounts of sericite, microcline tremolite actinolite and epidote are visible in'this secti~n (Tucker, 1980). Field examination reveals the rare presence of cross-bedding. Cross-bedding and red­ T Tertiary dacite intrusion. staining in the upper portion of the unit suggestive @ Amphibolite, cuts folds of a paleosol, indicate that this quartzite may M Mesozoic metasediments correlate with the early Jurassic Aztec Sandstone. and intrusions The gypsum deposits of the Little Maria Moun­ ~~ Paleozoic metasediments tains are not seen in the Big Maria Mountains. Pc Precambrian basement Structural Analysis * Tertiary strike-slip faults \..1J Location of domains The Big Maria Mountains are characterized by large-scale recumbent folds and thrust faults. The folding style is Penninic. Fold axes trend west­ Figure 2 - Generalized geologic' map. northwest and are overturned to the south-southwest. The amp 1itude of the folds is at 1eas t 1 ki lometer shape of the range and gentle large-scale warping the synclines are exposed up to 12 kilometers in ' of the Mesozoic folds. length along strike. The large folds are defined by ~lack, tan ar.d white weathering Paleozoic meta­ The third event in the Big Marias was mid to sedlmentary rocks that contrast with the darker late-Tertiary right-lateral strike-slip faulting weathering basement which is also folded. The that is related to the San Andreas fault system. continuous cross sections of both synclines can be These faults have an offset of approximately 2 km seen in outcrops (plate 1). and cause the earlier Mesozoic folds to be exposed at differing structural levels. The synclines are both characterized by one Previous workers in the range have described thickened and one attenuated limb. The attenuated multiple complex Mesozoic deformations with no limbs are associated with northward-directed under­ systematic strike or orientation (Hamilton, 1971). thrusting. The attenuation is so extreme that the The suggestion has also been made that the degree entire Paleozoic section has been reduced from of metamorphism seen in the Big Maria Mountains approximately 1 kilometer to 22 meters thick. This is not due to a regional metamorphic event, but dramatic attentuation was first described by rather to the "heat of deformation" (Hamilton, Hamilton (1964b). 1964b) . Smal~-scale folds are extremely abundant, and This study was undertaken to determine if the present ~hroughout the range. They are best expose~ ln the Supai and Kaibab Formations, and in complex deformation seen in the Big Maria Mountains the Bnght Angel Shale. All of these units have a has a systematic geometry, if the deformation is the result of one or multiple deformational events high ductility contrast expressed by quartzite and and if it can be related to a regional tectonic ' marble (S~p~i .and K~ibab Formations) or quartzite event. a~d a ~enc:tlc SChlSt (Bright Angel Shale). These dlffer:ng :lthologies also weather differently, LITHOLOGY r~sultlng ln three-dimensional exposures of the mlnor-folds. Refolded folds are also exposed in The Grand Canyon sequence formation names this manner. have been applied to the metamorphosed Paleozoic section in the Big Maria Mountains (Hamilton, Boudins, lineations, and foliations are personal communication, 1980; Howard, 1980). This extremely well developed in the Supai and Kaibab sequence is also found in the Little Maria Moun­ Formations and in the Bright Angel Shale. This is tains (Emerson, 1981, this volume), Palen Pass d~e to the extreme ductility contrast, and the (~emar~e, 1981); Harquahala Mountains (Varga, 1977), hlgh degree of metamorphism. Rlverslde Mountains (Hamilton 1964a' Carr and Dickey, 1980; and Lyle, this ~olume)~ Buckskin An analysis of minor structural features Mountains (Frost, personal communication 1981) including small-scale folds, refolded folds, ' Kilbeck Hills (Jones, 1973; Evensen, 1973), Old' lineation, foliation and boudinage, was conducted Woman Mountains, Piute Hills (Stone and Howard in four domains. These four domains were chosen 1979), and in the Plomosa Mountains (Miller, 1979; from widely separated areas around the range to

224 Domain #1 Kaibab Fm 54 Poln1s give a representative view of the deformation within the entire range. Domains were chosen in areas 3 @ 1 where exposure is excellent, folds and other struc­ 6 3 2 tural elements are abundant, and the overall geolo­ 3 gic picture was understood (please note Figure 1 5 for location of domains). Geologic mapping was 2 2 3 1 1 SIMPLE HARMONIC ANALYSIS done within each domain, either by the author or 2 other San Diego State students (Bean, 1980, Good­ man, 1980; Tanges, 1980; Corones, Dowell, Garner, Domain#2 Supai Fm 66 Points r1i 11 er, f'~eitzner, Mul ke, Smith, and I.Jedenburn pers. I~ 8 1 commun., 1981). The minor structuralfeatures \"iere 5 A B C D FE then compared with the large-scale structural 9 4 features of the range discerned by using Hamilton's (1967) mapping as shown on the state map (Salton Sea 6 8 1 sheet), and by compiling all previous mapping done 1 2 by San Siego State students on the range. 1

Small-Scale Folds Domain #3 Bright Angel 49 P1s. Small-scale folds were classified by their 2 5 @ 3 1 geometry using both a visual harmonic analysis i 2 3 1 (Hudleston, 1973), and a Ramsay classification 4 1 system (Ramsay, 1967). Fold profiles are examined 1 in accordance with Hudleston's visual harmonic 1 2 analysis (Hudleston, 1973). These fold profiles are derived from a Fourier analysis of quarter­ wavelengths. Fold-profiles in the field were Domain #4 Kaibab Fm 50 Points 0 -Maximum compared to the standard folds by viewing them through a printed transparency. The results are 2 ~ 2 1 presented in Table 1. The dominant fold type in 5 5 5 1 all domains is lC, or a fold with a rounded hinge, a wide hinge angle, and a small wavelength. A 3 2 1 Ramsay classification (Ramsay, 1967) compares the 1 2 thickness of fold hinges to limbs. In a class lB, an ideal parallel fold, the hinge and limb thick­ nesses, are equal. A class 2, or ideal similar fold, has a thickened hinge relative to limbs. The Table 1 - Hudleston fold profiles. domains show a range from ideal parallel to ideal similar folds (Table 2). asymmetry is directed away from the slip line, the orientation of the slip line is projected upwards. The mechanism of folding can be deduced by examination of the style of folding and the develop­ Refolded Folds ment of cleavage and lineations. Although folds range from parallel to similar in form, similar Refolded folds were noted occasionally in the folds are more common. All domains examined contain Kaibab and Supai Formations. Patterns noted in the a rock type with a high ductility contrast (quart­ field are domal or elliptical in shape (plate 1). zite and limestone, or quartzite and schist). This The diameter of the refolded folds is usually about has led to well-developed folds without a pro­ 5 to 10 centimeters. Interference patterns of nounced axial surface cleavage. Lineations present refolded folds were examined to determine the re­ on the foliation surfaces are both parallel and lative directions of the two folding events forming perpendicular to the axes of major and minor folds. the refolded folds. The direction of movement of The folds and foliation both developed at the same the second fold set is within the axial surface of time. Folds are produced by folded foliation and the first fold set. Due to the geometry of the folds are enclosed in enveloping foliation surfaces. superposed folds, they are> compared to "sheath­ folds" described by f'1innigh (1979). Small-scale folds were examined according to both their asymmetries and orientations. An S-fold Sheath folds are formed in an environments of is defined as an S-shaped fold as seen in profile large shear strain. The large shear strains rotate view, when looking down the fold axis. A Z-shaped planar and linear structures parallel to the direc­ fold is similarly defined. An S-shape fold requires tion of extension. Anisotropies formed on the counterclockwise rotation for formation, a Z-shape surface or within early formed folds, such as a fold requires clockwise rotation. curved hinge, become nuclei for the development of sheath-folds. In the initial stages of development, The orientation of fold axes is plotted on the long axis of the strain ellipse is at 45° to lower hemisphere, equal area, stereonets, according the shear stress, and parallel to the plane contain­ to a method described by Hansen (1971). The fold ing the curved hinge. With progressive deformation, axes display a planar preferred orientation known the hinge is rotated until it is nearly parallel to as the slip plane and fall into two groups of the shear plane. Lineations become aligned with the opposite asymmetry, separated by a separation angle. direction of shear, and the fold becomes extended. The slip-line direction, or direction of tectonic The development of a sheath fold is shown in Figure transport, lies within the separation angle within 3. the plane of fold axes. In the case where the

225 RAMSEY FOLD CLASSIFICATION

DOMAIN FORMATION lB lC 2 3

Kaibab +1 54 Pts. 1 16 37 Supai +2 66 Pts. 12 17 30 5 Bright Angel +3 8 12 27 49 Pts. 2 Kaibab +4 49 Pts. 4 19 21 Development of a sheath-fold. From Minnigh, 1979.

1A 1B 1C Fi gure 3 All structural elements were measured within

the central, cherty portion of the Kaibab Formation. if Folds are defined by interbedded limestones and quartzites. These differing lithologies provide both the ductility contrast necessary for fold development and distinct weathering patterns enabl­ ing accurate measurements. Quartzitic layers weather to a dark brown color due to a desert var­ nish coating and are very resistant to erosion. Limestone layers are a tan color and have a lower relief than quartzitic layers. A visual harmonic analysis (Hudleston, 1973), and an examination of orthogonal thickness for 54 Table 2 - Ramsay classification. folds are shown in Tables 1 and 2. The folds are open with rounded hinges; they range from ideal Sheath folds have been described in association simi 1ar to idea 1 pa ra 11 e1 folds. The fo 1ds have an with mylonites, thrust faults and nappes (Minnigh, average amplitude of 10 centimeters. 1979). Because the style of deformation in the Big Maria Mountains includes both nappe structures and Axes of S-folds plunge generally to the east, thrust faults, sheath folds are not out of place. axes of Z-folds plunge to the west. Between these two groups a smaller number of both S- and Z-folds plunge to the north. A Hansen analysis of fold Foliation, Lineation, and Boudinage axes gives a slip-line direction of N16E, S16W Foliation, lineation, and boudinage (where (figure 4). Axial surfaces are subparallel to the available) measurements are plotted on stereonets foliation. for each domain. The foliation is defined by contrasting layers of quartzite and marble in the Lineation measurements have a maximum of 59, Supai and Kaibab Formations and quartzite and schist N4E. This is subparallel to the slip-line direc­ in the Bright Angel Shale. Lineation is defined by tion, and to the trend of the zone of overlapping striations visible on quartzite surfaces (plate 2) S- and Z-folds. Lineations (figure 4) also show a and by alignment of elongate minerals such as micas distribution on a great circle, reflecting the and amphiboles. development of lineation parallel to fold axes. The foliation envelopes minor folds and is A complex deformation history is shown by the oenerally parallel to both the major and minor presence of a small number of domes in this domain. axial surfaces (plate 3). Lineation is developed Minor undulations within the foliation surfaces and parallel to the fold axes of the minor folds. the overlap of S- and Z-folds may also suggest a complex history for the deformation. Domain 1 Formation of a dome-shaped fold requires that Domain #1 is located in the northwest portion the second deformation occurs within the axial sur­ of the range (figure 2). This area contains a face of the first-formed fold. The second direc­ homoclinally dipping, right side up, or no~mal, tion of movement shown by the refolded folds is sequence with an attitude of N86W, 60NE. (flgure ~, along the slip-line direction and is subparallel to poles to foliation). The southern portlon 0: thlS the lineation (figure 4). The refolded folds normal sequence is separated from the overlYlng (figure 4) may have been produced by a second defor­ Mesozoic metaarenite by a thrust fault with unknown mational event that was fortuitously oriented offset. The metaarenite dips gently to the north. within the axial surface of the first fold set. No major folds are visible within the domain. The Alternatively, such refolded folds could be produced relationship of this area to larger structures during progressive deformation as the folds flatten­ within the range is uncertain due to its proximity ed (limbs became parallel and hinges were domed). to a major strike-slip fault; it may be related to the northern syncline (see plate 1; figure 2). The development of such refolded folds or

226 Plate 2. Extremely well developed lineation. Lineation is shown by striations along a quartzite foliation surface.

Plate 3. A fold from domain #1. The black layer is quartzite, the lighter layer is limestone. Note that the axial surface is subparallel to the foliation.

227 dark layers within a bright-colored matrix. A Z-fold with an amplitude of 14 meters is pre­ sent in the eastern portion of domain #2. The asvm­ metry of this fold is congruent with the major fold, as well as minor folds measured (figure 5, Hansen B analysis) .

The attenuation is well reflected by the struc­ SLD = N16E, S16W SLD = N16E, S16W tural data. A stereonet plot of measured foliation G = N74\~, 42°W G = N74W, 42°W surfaces (figure 5) is tightly grouped with an atti­ 55 POINTS 55 POINTS tude of N70E, 33NW. The foliation is extremely planar and shows no evidence of refolding. Linea­ tions, visible as striations on quartzite surfaces, are very well developed (plate 2). They are also tightly grouped (figure 5, 25, N74W), reflecting both the attentuation and an absence of refolding. Axial surfaces (figure 5) are generally sub­ parallel to the foliation. An examination of sixty­ six fold profiles reveals that the folds are gen­ MAX = N16E, 7°NE MAX = N, 58°N erally open with rounded hinges. These folds show 2 POINTS 63 POINTS a wide distribution from ideal parallel to similar (tables 1 and 2). Axes of seventy-one folds were measured; of these, four were S-folds and sixty­ seven were Z-folds. The fold axes plot in one tight group subparallel to the lineations (figure 5). As only one group of fold axes is present within the domain, the position of S-folds is assumed to be 90 0 \ away from the Z-folds. The resulting slip-line direction is N18E, S18W. The presence of two boudin (figure 5) one Z-fold, and one lineation within the ) predicted position of S-folds supports the above assumption. One lineation and one boudin fall along the slip-line direction, indicating that the direc­ MAX = E-\~, 30 0 N MAX = N74W, 34°N tion of transport was N18E. 55 POINTS 54 POINTS Domain 3 Figure 4 - Domain #1. Lower hemisphere projections; Max = maximum, CI = contour interval (1% for all Domain #3, located southeast of domain #2, is figures), SLD = slip-line direction, G = best fit within the lower, unattenuated limb of a major syn­ girdle. A. Hansen Analysis, 0 = S folds, .. = Z cline. Domain #2 (attenuated section) is within the folds; B. fold axes; C. refolded axes; D. linea­ upper limb of the same structure (figure 2). All tions; E. poles to foliations; F. poles to axial structural elements are measured within the meta­ surfaces. morphosed Bright Angel Shale. Folds are defined by interbedded quartzites and schists, generally about "sheath-folds" by a progressive flilttening of pre­ 5 centimeters thick. A simple harmonic analysis of existing folds is illustrated in Figure 3. Small­ forty-nine fold profiles (Hudleston, 1973) shows a scale undulations within foliation surfaces may concentration (twenty folds) arcund type lC (table also be a result of this progressive deformation. 1). This is a wide hinge angle, short wavelength Axial surfaces, lineations, and foliation (figure fold, with a rounded hinge. A Ramsay classification 4) measurements are tightly grouped, indicating a (table 2) shows a broad range in fold type, from single deformational event. ideal parallel to similar, with a large grouping at ideal similar. Doma in 2 Poles to foliation and axial surfaces (figure Domain #2, located east of Black Hill (figure 2), is 6) are both tightly grouped and plot in the north­ wi~hin the overturned limb of a major recumbent syn­ east quardrant, indicating an absence of refolding. cllne, as shown in Plate 4. The limb is extremely Axial surfaces lie within the plane of foliation. attenuated, yet the entire Paleozoic sequence is still recognizable. The Paleozoic section ranges Lineations are defined by aligned micas along from 22 meters thick in the western exposure of this foliation planes, and by occasional striations on limb to 77 meters thick 5 kilometers to the east quartzite surfaces. Lineations are often noted not (Smith, personal communication, 1981). To the east to be parallel to a fold axis. These lineations of domain #2 along the strike of the axial surface, probably reflect friction between foliation surfaces the entire syncline is visible in cross-sectional during fold development. Kink bands are occasion­ view. The upper limb becomes thicker towards the ally visible along foliation planes within the hinge area where it joins the lower attenuated' limb schistose layers. The lineations are tightly The lower limb is approximately 800 meters thick .. grouped with a maximum of S53W, 36SW, indicating again, an absence of refolding (figure 6). All structural elements were measured within the Supai Formation. Foliation and The Hansen analysis of domain #3 (figure 6, folds are well exposed by interbedded quartzite and Hansen analysis and fold-axes) gives a slip-line dolomitic marble within this unit. Interbeds mea­ direction of S52W, N52E. The asymmetry of the folds sured were 2-5 centimeters thick and stand out as relative to the slip line is directed away from the

228 Plate 4 - The thinned Paleozoic section in an overturned limb of a syncline can be seen in the back~round ground as a striped band crossing the photograph, the dark colored rock in the foreground is a.port~on of the Paleozoic section within the upright limb of the same fold. Note the tremendous contrast 1n th1ckness.

slip line. A Hansen analysis requires the asymmetry should be noted that the formation boundories are a of minor folds to be directed towards the slip line. good indication of the intensity of deformation in This can be accomplished by projecting the slip line this area. Only traces of the unit above (strati­ to the upper hemisphere, through the center of the graphically below) the Kaibab can be seen--the rest stereonet. This gives a slip-line direction of has been squeezed out. N52E. Axial surfaces are also tightly grouped (fig­ Domain 4 ure 7) with a maximum orientation of 82, N87E. They are tightly grouped and show no evidence of refold­ Domain #4 is located in the east-central portion ing. Axial surfaces are parallel to subparallel to of the range, within a large parasitic fold on the the foliation. Lineations show a maximum at 82N upper overturned limb of a recumbent syncline (fig­ (figure 7). Lineations are not well developed, ure 2). This syncline is the same structure as probably due to a lack of attenuation in this area. noted in domains #2 and #3, but seen at a shallower Boudin show a maximum of 75, N05E. Axes of refolded depth due to faulting. All structural elements were folds also fall in this area, 69, N09E. measured within the central cherty portion of the overturned meta-Kaibab Formation. Folds and linea­ Refolded fold interference patterns are domal tions are both defined in the field as ~oted in to elliptical in shape. The direction of movement domain #1. The minor folds, especially refolded of the second fold set is within the axial surface folds, appear to be much more common than in the of the first fold set. The interference patterns Kaibab of domain #1. The degree of metamorphism and direction of movement suggest that the refolded was probably somewhat higher. folds are sheath folds (Minnigh, 1979). The eye­ shaped outcrop pattern is shown in Plate la: The poles to foliation have a maximum orienta­ A simple harmonic analysis of forty-nine fold tion of 68, N87E. They are tightly grouped and profiles (Hudleston, 1973) shows a wide hinge angle, show no evidence of refolding (figure 7). The rounded hinge and short wavelength fold. The folds foliation is parallel to formation boundaries. It range from ideal parallel to similar (table 1).

229 B A B SLD = N52E, S52W SLD = N18E, S18W SLD = N76W, S67E SLD = N52E, S52W 23°Nl~ G = N38W, 32°SW G = N30E, G = ~BOE, 23°Nv) G = N38W, 32°SvJ 71 POINTS 71 POINTS 50 POINTS 50 POINTS o = S FOLDS 0= S FOLDS CI = 1% 6 = Z FOLDS A = ZFOLDS a ti~ o

C o o MAX = S53W, 36"SW MAX = N59W, 28°SW MAX = N74W, 25°NW MAX = N68W, 33°NW CI = 1% = 1 CI = 1% CI = 1% CI % 68 POINTS 52 poans 67 POINTS 62 POINTS

\1 @~ F

MAX = N60E, 11 °Nt,! MAX = a) N65\» , 28°NW MAX = N54W, 58°SW 0 CI = 1% b) N85E, 50 NE CI = 1% 70 POINTS 8 POINTS 50 POINTS Figure 5 - Domain #2, Lower hemisphere projections: Figure 6 - Domain #3. Lower hemisphere projections; Max = maximum, CI = contour interval (1'% for all Max = maximum, CI = contour interval (1% for all figures), SLD = slip-line direction, G = best fit figures), SLD = slip-line direction, G = best fit girdle. A. Hansen analysis; B. fold axes; C. girdle. A. Hansen analysis; B. fold axes; C. lineations; D. poles to foliation; E. poles to 1ineations; D. poles to fol iation; E. poles to axial surfaces; F, boudin. axial surfaces. The Hansen analysis for this domain (figure 7) in table 3. The Hansen analysis, or slip-line shows a distinct division between S- and Z-folds, directions have been placed in one column in with only a few folds overlapping. The slip-line order to show their congruence. The direction is N22E-S22W. Although the best fit gir­ main points from thE diagrams are: (1) axial dle dips to the south, the lineations, boudins, and surfaces and foliation trend east-west and dip to refolded axes all trend to the north. The position the north, with the exception of domain #3, which of the sheath-fold axes is especially important, appears to be reoriented due to Tertiary faulting, .They plot between the two groups and to the north and (2) lineations, refold axes and the direction of (figure 7), 69, N09E. As in domain #1, the refolded tectonic transport is to the north-northeast in all folds are domal to elliptical in shape and probably domains. This indicates that the major folds ar--e-­ formed as part of one continuous deformation, If the result of north-northeast directed tectonic the refolded folds are included on the Hansen transport. analysis (as part of the same deformation), the bes t-fit gi rd 1e swi ngs to the north. The res uIt is Reconstruction of Tertiary faul tinCj resul ts in a slip-line direction of N22E (figure 7). the schematic cross-section (figure 8). The westward plunging fold axes of the major folds The above assumption may be unnecessary if you create an outcrop pattern that has previously been consider the location of the domain within a large mistaken for a complex twisting or refolding of parasitic fold. The parasitic fold probably axial surfaces. developed in the later stages of fold formation, and may be congruent with the development of the refold­ Attenuated limbs are associated with the north­ ed folds in a progressive deformation. If you un­ ward thrusting of a nappe. In order to produce fold the parasitic fold, you must rotate the best­ southwest-verging folds with northward directed fit girdle about a horizontal axis to reorient the tectonic transport, the mode of deformation has to early formed folds. The girdle moves to the north be north-northeast underthrusting. and the slip-line direction becomes N22E. SUMMARY AND CONCLUSIONS Summary The Big Maria Mountains contain Precambrian. A compilation of data from domains 1-4 is given Paleozoic, and Mesozoic rocks that are together

230 ___~N

A B

SLD = N22E, S22W SLD = N22E, S22W P"M G = N76W, 62°SW G = N76W, 62°SW 60 POINTS 60 POINTS 0= S FOLDS A = Z FOLDS Schematic croBs-sBctlon. M Mesozoic metosedlments. ~ 1 ml. P Paleozoic metasediments. Pc-M Precambrian basement and mesozoic Intrusions.

Figure 8 - Schematic cross-section through both synclines after restoration of late Teritary C D faulting. MAX = N09E, 69°NE MAX = N, 82°N This indicates that the mid-Cretaceous event seen CI = 1% 53 POINTS in the Big Maria Mountains is a part of a larger 4 POINTS CI = 1% regi ana 1 even t. Mid Tertiary detachment faulting is probably present beneath Quien Sable Point. South of Quien Sabe Point the detachment fault extends to the south, over the present surface of the range. Its effect is noted in the southern portion of the range by the presence of 39 m.y.B.P. K-Ar ages (Martin and others, 1981; Martin, this volume). E F The westward plunge of the Mesozoic synclines may be due to the doming of the range during detach­ MAX = N78W, 78°N MAX = N60W, 28°NE ment faulting. CI = 1% CI = 1% 67 POINTS 62 POINTS Late Tertiary strike-slip faulting has offset earlier-formed structures throughout the range. Figure 7 - Domain #4. Lower hemisphere projections; Differing structural levels are thus exposed within Max = maximum, CI = contour interval (n; for all the range, making it necessary to undo the faulting figures), SLD= slip-line direction, G = best fit before earlier events can be properly studied. girdle. A. Hansen analysis; B. fold axes; C.refold axes; D. lineations; E. poles to foliation; F. poles ACKNOWLEDGEMENTS to axial surfaces. I wish to thank Eric Frost and Daniel folded into at least two large south-southeast Krummenacher for their help and guidance during verging synclines and anticlines. The style of this project; Donna Martin for all of her K-Ar folding is Penninic with major attentuation on some dating and for answering my many questions con­ limbs. Fold axes of major and minor folds trend cerning this article. I also with to thank Cary east-west. Hansen fold analyses, combined with Bean, Joe Corones, Devon Dowell, Ford Garner, other structural elements examined in four domains Gary Goodman, Lynda Meitzner, Vic Miller, Gary around the range, indicate a north-northeast tecto­ Mulke, Gary Wedenburn, Mark Smith, and Sue Tanges nic transport direction. Only one Mesozoic defor­ for their geologic mapping within the Big Maria mational event appears to be recorded despite the Mountains. A special thanks to Donna Meznarich apparent complexity of folding. Small-scale refold­ for her drafting and Pattie Brannen for typing. ed folds are the product of progressive flattening during one continuous deformation. Numerous K-Ar REFERENCES CITED cooling ages of 80.5~5 m.y.B.P. on igneous rocks that cut the folds and the presence of Cretaceous Bean, C. S., 1980, Metasediment and basement com­ (7) metavolcaniclastic rocks within the folds place plex mapping in the southeastern Big Maria the age of deformation as probably mid-Cretaceous. Mountains, Riverside County, California: Asymmetry of the major folds and the degree of meta­ Unpublished Senior Report, San Diego State morphism indicate that the folding resulted from University, 18 p. north to northeast-directed underthrusting, probably Carr, W. J., and Dickey D. D., 1980, Geologic map related to the shallowly dipping Farallon plate. of Vidal, California and Parker, Ariz., SW quadrangle: U. S. Geol. Survey Map 1-1125, Mesozoic deformation of this style, orienta­ Scale 1:24,000 (1 sheet). tion and timing has also been documented to the west Demaree, R., 1981, Geology of Palen Pass area, of the Big Maria Mountains in the Little Maria Riverside County, California: Unpublished Mountains (Emerson 1981 a, b, and this volume), Master's Thesis, San Diego State University, Palen Pass (Demaree, 1981), Palen Mountains (LeVeque, 136 p. 1981 and this volume). To the north similar fea­ Ellis, M. J., Frost, E. G., and Krummenacher, D., tures are seen in the Arica Mountains (Baltz, this 1981, Structural analysis of multiple defor­ volume) and Riverside Mountains (Lyle, this volume). mational events in the Big Maria Mountains,

231 ._---- o~ Multiple Refolded- <::)o{:' Slip-line Foliation Axia I-surface Lineation Deformation Boudin Refold Axes Fold Shape

1 N16E, S16W E-W, 30N N74j,J,34N N, 58N YES N16E, 7NE Dome 45 Pts. 5 Pts. 54 Pts. 63 Pts. 2 Pts.

a)N65vl,28 1v 2 N18E,S18W N70E,33NW N60E, llNW N74H,25NW ? b)N85E,58tE

71 Pts. 52 Pts. 70 Fts. 60 Pts. 8Pts. -_._-----

3 N52E,S52H N59W,28S1v N54H,58SH S53W,36SW NO

50 Pts. 62 Pts. 50 Pts. 67 Pts.

4 N22E, S22W N87E,68N N60W,28NE N,82N YES N05E,75NE N09E,69NE Ellipse 60 Pts. 52 Pts. 62 Pts. 53 Pts. 3 Pts . 4 Pts. .- -_._-_._------

Table 3 - Compilation of data.

Ri versi de County Ca 1iforn i a (abs.): Geo 1. Soc. , 1964b, Nappes in southeastern California America Abstracts, V. 13, no. 2, p. 54. --~(a~b-s-.): Geol. Soc. America Abstracts, p. 274. Ell is, M. J., 1981 , Structural ana lys i sand regi ona 1 __~-",' 1967, Geologic map of California, Salton significance of complex deformational events Sea Sheet (Olaf P. Jenkins, ed.), Jennings, in the Big Maria Mountains, Riverside County, C. W., comp.: Calif. Div. Mines and Geology California: Unpublished Master's Thesis, Scale 1:25,000 (1 sheet). San Di ego State Uni vers ity, 109 p. , 1971, Tectonic framework of southeastern Emerson, W. S., 1981a, Geological and deformational ---~ornia (abs.): Geol. Soc. America characteristics of the Little Maria Mountains, Abstracts, Cordilleran Section, pp. 130-131. Ri vers i de County, Cal ifornia: Unpub 1i shed Hansen, E. F., 1971, Strain facies; Berlin, Springer Master's Thesis, San Diego State University, Verlag, 207 p. 98 p. Hudleston, P. J., 1973, Fold morphology and some , 1981b, Mesozoic and Cenozoic deformation geometric theories of fold development: Tec­ --a~n~dr-?igneous activity in the Little Maria tonophysics, v. 16, pp 1-46. Mountains, Riverside County, California: in Jones, J. A., 1973, Geology of the northern Kilbeck Tectoni c Framework of the Mojave and Sonora Hills and an adjacent portion of the Old Woman Deserts, California and Arizona, U.S.G.S. Mountains, eastern Mojave desert, San Bernar­ Open File report 81-503 p. 35. dino County, California: Unpublished Masters Emerson, W. S., and Krummenacher, D., 1981, Geolo­ Thesis, University of Southern California, gical and deformational characteristics of 11 0 p. the Little Maria Mountains, Riverside County, Martin, D. L., Krummenacher, D., and Frost, E. G., California: Geol. Soc. of America, Abst. with 1981, K-Ar dating in the Big Maria Mountains, programs, v. 13, no. 2. p. 54. Granite Mountains, and Palen Pass; thermal Evenson, W. A., 1973, Geology of the southern Kil­ history rates of cooling and uplift, and tec­ beck Hills and an adjacent portion of the Old tonic implications (abs.): Geol. Soc. America Woman Mountains, eastern Mojave Desert, San Abstracts, Cordilleran Section, p. 70. Bernardino County, California: Unpublished Miller, C., 1979, Map and petrographic analysis of Master's Thesis, University of Southern Cali­ the lateral changes within the metaarenite in fornia, Los Angeles 51 p. the NW part of the Big Maria Mountains, River­ Frost, E. G., 1981 , Mid-Tertiary Detachment Faulting side County, California: Unpublished Senior in the Whipple Mountains, California and Buck­ Report, San Deigo State University, 27 p. skin Mountains, Arizona and its relationship Miller, f'. K., and McKee, E. H., 1971, Thrust and to the Development of ~1ajor Antiforms and Syn­ strike-slip faulting in the Plomosa Mountains, forms: G.S.A. abstracts, v. 13, no. 2, p. 57. southwestern Arizona: Geol. Soc. America Bull., Goodman, G., 1980, Geology of a northwestern portion v. 82, pp. 717-722. of the Big Maria Mountains, Riverside County, Minningh, L. D., 1979, Structural analysis of sheath­ California: Unpublished Senior Report, San folds in a meta-chert from the Western Italian Di ego State Uni versity, 15 p. Alps: Jour. of Structural Geology, v. 1, no. Hamilton, W., 1960, Structure in the Big Maria 4, pp. 275-282. Mountains of southeastern California: Geol. Ramsay, J. G., 1967, Folding and fracturing of rock~ Survey Research Short Papers in the Geological New York, McGraw-Hill Book Company, 568 p. Sciences- #126, pp. B 277-78. Stewart, J. H., and Poole, F. G., 1975, Extension of the Cordilleran Miogeosynclinal Belt to the San

232 Andreas Fault, southern California: Geol. Soc. America Bull., v. 86, pp. 205-212. Stone, P., and Howard, K. A., 1979, Compliation of geologic mapping the Needles 1° by 2° sheet, California and Arizona: U. S. Geol. Survey Open File Report 79-388, Scale 1:250,000 (2 sheets). Tanges, S. E., 1980, Geology of an eastern portion of the Big Maria Mountains, Riverside County, California: Unpublished Senior Report, San Diego State University, 25 p. Tucker, R. S., 1980, A reconnaissance of metamor­ phism in the Big Maria Mountains, Riverside County, California: Unpublished Master's Thesis, San Deigo State University, 138 p. Varga, R. J., 1977, Geology of the Socorro Peak area, western Harquahala Mountains: Circular 20, Arizona Bureau of Geology and Mineral Technology, Geological Survey Branch, 30 p.

233

PRELllliNARY REPORT ON THE STRUCTURE AND STRATIGRAPHY OF THE SOUTHERN LITTLE HARQUAHALA MOUNTAINS, YU}~ COUNTY, ARIZONA

Stephen M. Richard Department of Geosciences University of Arizona Tucson, Arizona 85721

ABSTRACT

Precambrian through Tertiary rocks in the south­ ern Little Harquahala Mountains record a complex his­ tory of Mesozoic and Tertiary deformation. Precam­ brian quartz monzonite is overlain by: 1) about 1000 m of Paleozoic strata correlated with the Balsa, Abrigo, Martin, Redwall, Supai, Coconino and Kaibab Formations; 2) up to 1000 m of Mesozoic dacitic to rhyolitic volcanic and volcaniclastic rock; 3) at least 750 m of Mesozoic lithofeldspathic sandstone, conglomerate and siltstone. Probable high-angle faulting prior to deposition of the Mesozoic sand­ stone is indicated by rapid facies changes, massive conglomerate and basal onlap onto older units to the southeast. Subsequently, the strata were folded into a large southeast-vergent fold limb. This fold was Figure 1. Location map showing study area and points refolded about steep axes trending N-NE. In Late referred to in text, numbered as follows: (1) Buck­ Cretaceous time the deformed rocks were thrust over skin Mtns.; (2) Harcuvar Mtns.; (3) Vulture Mtns.; Mesozoic clastic, volcaniclastic and volcanic rocks (4) Harquahala Mtns.; (5) Plomosa Mtns.; (6) Dome along the Hercules thrust. Mesozoic strata below the Rock Mtns.; (7) Hovatter Road; (8) Buckeye-Salome Rd.; Hercules thrust are lithologically and stratigraphic­ (9) Big Horn Mtns.; (10) Eagle Tail Mtns.; (11) Gran­ ally different from Mesozoic strata above the fault. ite Wash Mtns.; (12) Ranegras Plain. Mesozoic structures are strongly overprinted by Ter­ tiary(?) NW-dipping, moderate to low-angle, normal­ section. Depositionally above the Paleozoic rocks separation faults and associated northerly trending are Mesozoic volcanic and volcaniclastic rocks and faults. The youngest structures are north- to north­ Mesozoic lithofeldspathic sandstones. On the south, west-trending, near-vertical oblique- or strike-slip the sedimentary rocks overlie an assemblage of al­ faults with an associated northeast-dipping normal tered crystalline rocks of uncertain age along the fault. One of the near-vertical faults cuts poorly complex steep to low-angle Sore Fingers fault zone indurated east-dipping Tertiary(?) gravel. (Fig. 2). On the north, the Precambrian quartz mon­ zonite structurally overlies Mesozoic clastic, vol­ INTRODUCTl ON caniclastic and volcanic rocks informally known as the Harquar section. The lower plate Mesozoic rocks The Little Harquahala Mountains are located are lithologically and stratigraphically different within the Basin and Range province in west central from Mesozoic rocks in the upper plate. The Harquar Arizona. Access to the area is excellent, either by section is intruded in the northern part of the range the Hovatter Road, which connects Salome with 1-10 by the Granite Wash Granodiorite, dated at 65 m.y. through the western edge of the study area, or by the (Damon, 1968) and 69 m.y. (Eberly and Stanley, 1978). Buckeye-Salome Road through the northeast edge of the Along the western edge of the range, southwest-dipping area (Fig. 1). volcanic rocks of probably Miocene age overlie the Harquar section. The range is structurally bounded The Little Harquahala Mountains occupy an area on the northeast by an inferred northwest-trending of overlapping Mesozoic and mid-Tertiary tectonism. oblique-slip fault in the vicinity of Centennial The purpose of this project is to determine the Wash. A complete summary of the regional geology of structural geometry of Paleozoic rocks in the Little west central Arizona is presented by Reynolds (1980, Harquahala Mountains in an attempt to define the ki­ and this volume). nematics of Mesozoic and Tertiary deformation in the area. To this end, a geologic map of the southern STRATIGRAPHY part of the range was made. The base map used was a 1:12,000 enlargement of part of the Hope, Arizona 15' The Little Harquahala Mountains contain rock series U.S.G.S. quadrangle (1961). This paper con­ units ranging in age from Precambrian to Tertiary. tains descriptions and preliminary interpretations of The pre-Cenozoic stratigraphic column includes ap­ the rocks and structures of the Little Harquahala proximately 1000 m of Paleozoic rocks, a highly vari­ Mountains. A more complete discussion will be pre­ able thickness of Mesozoic volcanic and volcaniclas­ sented in a forthcoming circular to be published by tic rocks up to 900 m thick and Mesozoic lithofeld­ the Arizona Bureau of Mines and Mineral Technology. spathic sandstones with a maximum exposed thickness of 750 m. For lithologic descriptions of the Paleo­ GENERAL GEOLOGY zoic and Mesozoic section in the central part of the area, see the accompanying stratigraphic column (Fig. Precambrian quartz monzonite in the central part 4). In addition, a variety of igneous and metamorph­ of the range is overlain by a highly faulted north­ ic rocks of uncertain age are exposed in the Sore east-trending, steeply dipping cratonic Palezoic Fingers area, and Precambrian quartz monzonite and

235 amphibolite gneiss underlie the Paleozoic section. conglomerate at the top of unit 5 are probably Meso­ The rock units are divided into five major groups re­ zoic but are too thin and poorly exposed to map flecting the geologic development of the area. These separately. are: 1) Precambrian basement consisting of intrusive and metamorphic rocks; 2) Paleozoic cratonic sedi­ Mesozoic Rocks ments; 3) Mesozoic continental deposits; 4) the Sore Fingers Complex; and 5) Cenozoic deposits which are Two distinct Mesozoic sequences are present in only briefly described. Thicknesses of rock units the Little Harquahala Mountains--the Harquar section were determined using measurements from the geologic and the southern Little Harquahala section. Formal map. nomenclature for these rocks is lacking. A Mesozoic age is inferred from stratigraphic position above Precambrian Rocks Paleozoic rocks and involvement in late Cretaceous deformation (see Tectonic Interpretations). Granitic rocks occupying the northeast boundary of the map area are depositionally overlain by the The Harquar section includes volcanic and sedi­ Balsa Quartzite and thus are known to be of Precam­ mentary rocks lying below the Hercules thrust. These brian age. North of Martin Peak, gneissic rocks in­ were not studied in detail. Within the area mapped, truded by this granite crop out over a small area and porphyritic andesite flows overlie lithic sandstone, are also considered Precambrian. siltstone and conglomerate. The section is distin­ guished from the southern Little Harquahala section Quartz monzonite underlying the Balsa Quartzite by the more intermediate composition of the volcanic is ubiquitously altered in the vicinity of the uncon­ rocks, the greater abundance of conglomerate and the formity to an assemblage of light green argillized or predominance of volcanic clasts in the conglomerate. epidotized feldspar set in a red stained argillic groundmass with abundant quartz eyes. Sericite and The southern Little Harquahala section is de­ epidote are common. In less intensely altered zones, scribed in the stratigraphic column (Fig. 4). Con­ further from the unconformity, the quartz monzonite glomerates and rapid facies changes at the base of consists of a medium-grained quartz, plagioclase and the lithofeldspathic sandstone unit indicate a period minor biotite groundmass with 1-3 em potassium feld­ of deformation and erosion prior to deposition of the spar phenocrysts. Some of the alteration at the con­ sandstone. The contact between the sandstone and un­ tact may be due to pre-Balsa weathering, but the derlying volcaniclasts is conformable along a presence of similar alteration within the Balsa re­ northeast-trending zone southeast of the Needle. In quires post-Paleozoic chemical changes as well. The this area the volcaniclastic rocks fine upward into a contact between the Balsa and quartz monzonite com­ shale horizon overlain by the sandstones. To the monly is faulted. south, a rapid facies change occurs, possibly involv­ ing telescoping of facies on hidden faults, and the Amphibolite gneisses consisting of medium­ base of the section becomes conglomeratic. The vol­ grained hornblende and plagioclase crystals occur at canic and volcaniclastic units apparently pinch out, the nortm,est edge of the map area above the Hercules and in the Limestone Hills (Fig. 2), a massive lime­ thrust (Fig. 2). Near-vertical, northeast-trending stone conglomerate overlies Paleozoic rocks at the foliation in these gneisses is characteristic of base of the sandstones. The contact there is sheared early proterozoic gneisses in west central Arizona and is inte~preted to be a minor fault. Thinning of (Reynolds, 1980). This foliation is disrupted and the volcanic unit, coarsening of the basal sandstone folded within 10-15 m of the Hercules thrust. section and overlap onto Paleozoic rocks are indica­ tive of uplift in the southern part of the area dur­ Paleozoic Rocks ing or after deposition of the volcanics and before deposition of the Mesozoic sandstones. A cratonic Paleozoic section overlies the Pre­ cambrian basement in the Little Harquahala }lountains. Sore Fingers Crystalline Complex The stratigraphy of these rocks resembles the south­ east Arizona Paleozoic section in its lower part and The Sore Fingers crystalline complex is an in­ the Grand Canyon section in its upper part. Miller formal name assigned to an assemblage of intrusive (1970) described a similar section in the southern and metamorphic rocks in the southern part of the map Plomosa Mountains and noted its resemblance to the area. The complex is named after two low hills in section in the Little Harquahala Mountains. He rec­ the southernmost Little Harquahala Mountains called ognized the Balsa, Abrigo, Martin, Escabrosa, Supai, the Sore Fingers on the Hope 15' quadrangle. The Coconino and Kaibab Formations. Varga (1977) report­ complex is bounded by faults on the northwest, south­ ed an essentially identical section in the western west and northeast and covered by alluvium on the Harquahala Mountains, except the Abrigo and Martin southeast. The age of the complex is uncertain but Formations are apparently absent due to a bedding is probably Precambrian and Mesozoic. plane fault. Varga (1977) favored correlation of the carbonate unit below the Supai with the Redwall Lime­ The most abundant lithology in the Sore Fingers stone instead of the Escabrosa Limestone. In the ab­ complex is a quartz monzonite porphyry. Equant to sence of definitive evidence for either correlation, slightly elongate light flesh-pink potassium feldspar I have chosen to continue Varga's usage. Except for phenocrysts up to 8 em long occur in a groundmass of this change, Miller's (1970) correlations are used in 1-5 mm quartz, plagioclase and altered biotite. This this report. rock has yielded a minimum age of 140 m.y. (K-Ar, biotite, Rehrig and Reynolds, 1980). Slight altera­ The Kaibab Formation in the Little Harquahala tion is concentrated along joints throughout the in­ Mountains is unique in western Arizona. Miller trusion but is locally intense and extensive, con­ (1970) and Varga (1977) described strata resembling verting large areas of quartz monzonite to a dense units 1 and 2 of this report, but units 3, 4 and 5 black siliceous alteration product in which 1-3 mm are absent in all other sections described in west quartz eyes and 3-5 mm white feldspar "spots" are all central A~izona. Quartz-chert sandstone and that is left of the original texture. Silicification

236 Figure 2 Generalized Geologic Map of the Southern Little Harquahala Mountains, Yuma County, Scale 1:62500 o mile

Geology by Stephen M. Richard, 1981

Base from U.S. Geological Survey Hope 15' Quadrangle. Contour Interval 200 feet

CENOZOIC

Aluvium IQTg I \vell indurated gravel

Poorly indurated tilted gravel Breccia

I1ESOZOIC IMzvl Volcanic and volcaniclastic rocks IMZS I Lithofeldspathic I MZhl Harquar section

PALEOZOIC

Bolsa through Supai formations, undivided ~ Contacts- definite, approximate, inferred or concealed I pzuj Coconino and Kaibab formations Faults- 'vith dip, ball 011 dmmthrc1\Vn side, arrOlvs shaH PRECAHBRIAN relative motion, barbs on upper plClte. 20 t '=:;' ! bLb ___ •••••••••••• " ." IP£ul Quartz monzonite Attitudes of bedding- inclined, vertical, overturned ---"--30 -----v ~60 PRECAI1BRIAN OR HESOZOIC SORE FINGERS C0l1PLEX Cleavage- ~ inclined Iqmpl Quartz monzonite porphyry Folds- Axial trace of anticline, syncline, overturned Imqml Hetamorphosed and altered quartz monzonite syncline -r- f't ~ Diorite G Hetamorphic rocks * ~ Slightly prophyritic qnartz monzonite DIKES md >r+-i' Hicrodiorite a ~ Aplite

237 and biotitization seem to be the major effects of the On lap of Mesozoic clastic rocks across a thin alteration. The porphyritic quartz monzonite is in­ Mesozoic volcanic sequence and across a major truded by a diorite or quartz diorite in the north­ northeast-trending fault in the Limestone Hills (Fig. east part of the complex ane by a number of small 2) suggests that the fault was related to uplift of bodies of equigranular quartz monzonite, only one of Paleozoic rocks, ",hich ",ere a source for clasts in "'hich is sho"'n on the geologic map (Fig. 2). In the the basal part of the clastic sequence. At the very northern part of the complex, the porphyritic quartz least, major movement on the largest northeast­ monzonite is in both gradational and fault contact trending fault in the Limestone Hills predates shear­ ",ith slightly metamorphosed and altered porphyritic ing along the base of the clastic unit. granite. This unit is characterized by red staining, zones "'ith "'eak crystalloblastic foliation, slightly The early large-scale fold is apparent in the rounded pink-red feldspar phenocrysts and abundant general decrease in dip of strata from vertical and quartz veins. Foliation attitudes are not consistent overturned beds in the northeast- to moderately over the area; the fabric seems to be a very local south-dipping beds in the south. The axis of this phenomenon. fold is subhorizontal and trends east-northeast. Southeast vergence is indicated by northwest-dipping The porphyritic quartz monzonite is intruded in­ overturned beds and by extrapolation from the western to an extremely heterogeneous assemblage of meta­ Harquahala Mountains. igneous and meta-sedimentary rocks. Contacts, ",here not faulted, are gradational, ",ith interleaving of The second folding event is evident in the various lithologies, and locally appear migmatitic. change from northeast to southeast strike on Martin Porphyritic quartz monzonite, variably altered or fo­ Peak and south of the Needle (Fig. 2). Antiformal liated "'ith gradational contacts, is a "'idespread synclines and synformal anticlines in the highly component of the metamorphic terrane. These rela­ faulted area east of Martin Peak are also believed to tions suggest that the quartz monzonite may be de­ be related to this event. Axes are moderately rived, at least in part, from partial melting of the northeast-plunging to vertical but are difficult to metamorphic rocks. The metamorphic rocks are gener­ determine because of the earlier folding event. ally quartzo-feldspathic ",ith minor biotite, altered to chlorite and muscovite. Strong alteration ob­ The Hercules thrust places Precambrian quartz scures contact relationships every",here. Pods and monzonite on Mesozoic volcanic and clastic rocks of lenses of black microdiorite are common. Foliation the Harquar section north of the Paleozoic outcrop is locally strong but generally is ",eakly developed belt. The fault dips gently to the south"'est. Foli­ and irregular. ation in gneissic rocks above the fault is folded, and a north",est-trending south",est-dipping cleavage TERTIARY ROCKS is strongly developed in clastic rocks belo", the fault. Tertiary rock units include a breccia and t",o overlying gravels. The breccia occurs in a Cleavage with a similar orientation and charac­ north"'est-trending zone north of the Needle ",hich is ter is present in Mesozoic sandstone south of the believed to be a northeast-dipping lo",-angle fault Needle, in rocks along the northwest-trending steep zone. It is underlain by Paleozoic rocks east of the faults bounding the Sore Fingers complex, along the Needle fault (Fig. 2) and by Precambrian quartz monzo­ fault bounding the klippe of volcanic rock lying on nite to the "'est. The breccia is composed of crushed the Sore Fingers Complex and along a fault within Paleozoic clasts ranging from brecciated blocks sev­ the Sore Fingers Complex. As cleavage is not folded eral meters long to angular pebbles. The rock is and is not axial planar to folds, its development strongly cemented by calcite or silica. East of the evidently postdated the folding. Needle fault, a poorly indurated gravel overlies the breccia, dipping 15° to the east. Near the fault, Northeast-striking, north",est-dipping, lo",-angle the gravel contains boulders of Supai Formation up to normal-separation faults cut Paleozoic and Mesozoic 1 m in diameter along ",ith clasts of other Paleozoic rocks (Figs. 2,3). Dips of the faults vary from 0 to lithologies and Precambrian quartz monzonite; it be­ 40°. Major faults of this type placed a large klippe comes finer grained up-section a",ay from the fault. of Paleozoic rocks on Precambrian quartz monzonite West of the Needle fault, the breccia and Precambrian north of the Needle, extended the Paleozoic section quartz monzonite are overlain by an untilted "'ell­ in the area bet",een the Needle and Martin Peak and in indurated gravel composed of Paleozoic clasts up to the Northeast Hills and juxtaposed Mesozoic and crys­ about 40 cm in diameter. talline rocks along the Sore Fingers fault. North"'est- to north-striking, east-dipping steep to STRUCTURE lo",-angle faults are associated with the normal­ separation faults. They are characterized by strong­ Six deformation events are recognized in the ly brecciated fault zones. North",est-dipping normal­ southern Little Harquahala Mountains. From oldest to separation faults in general cannot be correlated youngest they are: 1) probably high-angle faulting across these structures, indicating that they may act before deposition of the Mesozoic lithofeldspathic as tear faults. sandstone; 2) large-scale south- to southeast-vergent folding; 3) refolding of earlier folds about steep The youngest structures in the area are a set of north-northeast plunging axes; 4) thrust faulting; 5) northwest- to north-trending high-angle strike- or northeast-trending, moderate- to lo",-angle normal oblique-slip faults. The Hovatter Road fault and the faulting "'ith associated high- and lo",-angle faults; Needle fault show left separation downthro",n on the 6) north- to northwest-trending strike- or oblique­ northeast, and the Northeast Hills fault shows right slip faulting "'ith an associated north",est-trending separation do",n on the southwest (Fig. 2). The lo",-angle normal fault. These structures "'ill be de­ northeast-dipping breccia zone north of the Needle is scribed in chronologic order. believed to represent a detachment on ",hich the North­ east Hills moved northeast off the central axis of the range. Although the breccia zone apparently is

238 A

2000' Qal Pzl Mzv I i-- ~!..SK:?0jFi1JJ}!j;jji};ji<:0~@'~~= P£u I MZ5 -- 1000' \, ----?-? ---- - a ? ? ? AI I-- SORE FINGERS COMPLEX---

Qal 2000'

1000' qmp qmp qm

1 8 8 qrpp A-A' SCALE, SECTIONS A-A' and 8-8' 2000'

I\) w IO:"L 1000' 1000' CD o 1000'

Pzu .--: . I C .....~~ C ---- ..<7" SCALE, SECTION C-C' 2000' , .' ~l1JK ~2000'Qal :;;;478//T-- 500'L ,,0,,', ,I p. Mzv ," 'j zu 1500' 1 / I 1500' a , 0 500

Figure 3: Geologic Cross Sections For explanation see Fig. 2. Southern Little Harquahala Mountains A\T Strike slip fault. movement A- away from viewer T- toward viewer Y(/':·. Upper Paleozoic Coconino Sandstone '~LITHOFELDSPATHIC BrO'~l ~ .::-'7 ..~, SANDSTONE: weathering, grey fresh sandstone, dark grey siltstone and silty shale, and pebble to cobble conglomerate; clasts in conglomerate mostly Paleozoic ., '. :,,: '...: ~ . -==- ~ .: lithologies; sequence fines upward from coarse sand and conglomerate at base to mostly '~;",:",:,,:"'" siltstone at top; lenticular sand bodies; cross bedding present, not common; complex re­ . ',.~::. . lations at base--conformable and unconformable contacts present. 0000 -: ;.: -.--~ .; .. ' . ~ .. '.~-, .. . ~ .. '" () ~ ;)::>' ... ~ ...... ' - . ~ .. ..

" . .:: :.'::' 0000 0 ••.

SEDIMENTS DERIVED FROM QUARTZ PORPHYRY: light grey or grey green epiclastic conglomerate and sandstone; massive, similar in appearance to quartz porphyry, distinguished by clastic textures, rare quartzite clasts, less prominent phenocrysts and rare magnetite lamina; sequence fines upward.

QUARTZ PORPHYRY: massive light grey green or grey intrusive and extrusive quartz porphyry; altered plagioclase, biotite or hornblende and quartz phenocrysts in very fine grained w groundmass; locally intrnsive into Lower Volcanic Unit, but stratiform shape of body and cover of lithologically similar epiclastic sediments suggests extrusive origin.

-LOHER VOLCANIC UNIT: maroon and purple silicic flows; aphanitic groundmass with minor quartz and feldspar phenocrysts, flow banding present; agglomerates; laminated and massive tuff; red volcanic lithic sandstones interbedded in southwest; local red conglomerate at base-­ clasts include volcanic rocks, porphyritic granite and Paleozoic lithologies. KAIBAB LHIESTONE: Unit 5: basal tan silty sandstone, cherty light pink grey limestone, ,vith quartz-chert sandstone, conglomerate and mudstone of probable Mesozoic age at top. Unit 4: thick bedded light grey limestone; cherty and fossiliferous; gastropods, Chaetetes corals and brachiopods; abundant chert at top. Unit J: even, medium bedded dark grey limestone; poorly preserved fusilinids and large gastropods present. Unit 2: massive cherty medium grey limestone; crinoid grainstone; large Productid brach- I~~~~!'~ iopods abundant in upper part. ' E Unit 1: basal sandy dolomite grades up into cherty fossiliferous dolomitic limestone, with .... "~." ..... '., &\ large crinoid columnals, echnoid spines and brachiopods; capped by tan silty sandstone; ., .' :.', :',' probable Torm,eap Formation equivalent. :..2:.. ::':',.::: OCONINO SANDSTONE: "hite vitreous medium grained well sorted quartzite; mostly very thin ~.. bedded, plane laminated; med. to large scale troughy cross beds present. SUPAI FORI-fATION: interbedded "hite vitreous quartzite, calcareous sandstone, maroon muds tone, limestone or dolomite; thick bedded; lenticular beds; medium scale troughy cross bedded sands tone beds present; prominent dark brmvn varnished outcrops. REm/ALL LTMESTONE: lm,er thick bedded light grey limestone and dark grey dolomite form prom­ inent bands; upper massive cherty light grey limestone, variably dolomitized; variably pre served crinoid grainstone at top; local thin karst breccia at base; karsted zone at top. fARTIN FORI-fATION: brm~l, grey and tan dolomite and dolomitic limestone; chocolate brmvn sand-.' beds at base; one or two coarse, very poorly sorted sandstone beds in section; carbonate beds laminated, massive and mottled. « ABRIGO FOR}fATION: dark grey, maroon and grey green sandy shale, contact gradational at base; ~JIIII~~ij; 0'jBOLSAbioturbationQUARTZITE:in maroonsome thingreysiltsfeldspathictone beds.sandstone; grit and pebble conglomerate at base; , a.. '~'.~' 'T':.7· 8 ~:~;u~o~o~~ne grained sandstone with siltstone partings up section; planar tabular cross P-C 1\\ ..... ,/\1\/ PRECAMBRIAN ~UARTZ MONZONITF.

FIGURE 4. STRATIGRAPHIC COLUMN

240 cut by the Needle fault, it lies on quartz monzonite that lOI,-angle faults which place Precambrian on Pa­ west of the fault and on Paleozoic limestone east of leozoic rocks are truncated by the Granite Wash the fault, requiring movement on the Needle fault be­ Granodiorite (S. Reynolds, pers. comm., 1981), which fore the breccia developed. The two faults thus have has yielded K-Ar biotite ages of 65 and 69 m.y. overlapping periods of activity. (Damon, 1968; Eberly and Stanley, 1978). Biotite from sheared granite directly above the Hercules TECTONIC INTERPRETATIONS AND DISCUSSION thrust in the northern Little Harquahala }[ountains yielded a K-Ar age of 66 m.y. (Rehrig and Reynolds, The geologic history of the Little Harquahala 1980). The amount and direction of tectonic trans­ Mountains begins in the Proterozoic(?) with the depo­ port are not known. The presence of older structures sition of volcanic and sedimentary rocks which were mentioned above may have provided enough relief on metamorphosed and then intruded by one or more gener­ the basement surface that great vertical throw was ations of porphyritic granitic rock. During Paleozoic not required to place Precambrian on Mesozoic rocks time a cratonic section was deposited. The Coconino in the Late Cretaceous. However, the difference in sandstone is the only formation that shows evidence of }[esozoic clastic rocks above and below the fault re­ continental deposition. Karst horizons above the quires significant lateral transport. Martin Formation and the Redwall Limestone indicate periods of subaerial erosion, but there is no evi­ Northwest-dipping normal-separation faults cut­ dence of major Paleozoic orogenic activity. ting the Paleozoic section are subject to various in­ terpretations. Apparent north to northwest transport The presence of chert pebbles and igneous intru­ is more consistent with northerly transport direc­ sive rock clasts in the basal Mesozoic section im­ tions indicated for Late Cretaceous thrust faults in plies a period of uplift and locally extensive ero­ the western Harquahala Mountains (Reynolds, Keith and sion following deposition of the Kaibab Limestone. Coney, 1980) than with regional northeast extension Preservation of several hundred feet of Kaibab strata indicated by extensive southwest-dipping mid-Tertiary not reported in adjacent areas indicates that the strata in the area (Rehrig, Shafiqullah and Damon, Little Harquahala Mountains were not eroded as deeply 1980; Scarborough and Wilt, 1979). Interpretation as as those areas. }[esozoic volcanic rocks, thought to thrust faults requires post-thrust northwest tilting represent the Jurassic arc in this area, thin to the of the faults. The northeast-trending arch, which southeast through non-deposition, erosion, tectonic nOI, forms the Harquahala Mountains (Reynolds, this thinning or some combination of these. They are volume), provides a possible means to achieve this present in the southern Plomosa Mountains (Miller, tilting. However, independent evidence for extension 1970) but are not exposed and are probably absent in of this arch into the Little Harquahala Mountains the western Harquahala Mountains. Again, the Little presently is lacking. I have chosen to interpret Harquahala Mountains were the site of thicker accumu­ these faults as mid-Tertiary normal faults because of lation than nearby areas. These relations are inter­ their complex, discontinuous geometry, association preted to be the result of early Mesozoic high-angle with highly brecciated tear-like faults and absence faulting as a result of I,hich the Little Harquahala of undeformed crosscutting dikes. Mountains occupied a down-dropped block. Activity on these faults during or just after Jurassic(?) vol­ The relationship between the Hercules thrust and canic activity suggests that these structures may be the Sore Fingers fault is a key problem. The atti­ related to the Mojave-Sonora megashear, which was al­ tude of the Sore Fingers fault, its brecciated char­ so active during Or just after Jurassic arc magmatism acter and the probable younger on older juxtaposition (Kluth, pers. comm., 1981). Mesozoic(?) lithofeld­ suggest that it is correlative with the northwest­ spathic sandstone was deposited across these older dipping normal-separation faults. Rock assemblages structures, ending the major Phanerozoic period of which are lithologically similar to the Sore Fingers deposition in the region. Complex or which are similarly altered are unknown in the Little Harquahala, Harquahala and Granite Wash Subsequent history of the study area involves Mountains. A major hidden contact between the Sore several deformational events but no significant depo­ Fingers Complex and the Harquar section below the sition. Large-scale south-vergent folds are believed Hercules thrust is necessary. Considering the al­ to be correlative with similar structures observed in tered condition and probable depth of intrusion of the western Harquahala Mountains (Varga, 1977; the coarse crystalline rocks of the Sore Fingers Com­ Reynolds, Keith and Coney, 1980). Similar fold struc­ plex and the unmetamorphosed character of the Harquar tures are known in the Big Maria (Krummenacher et al., section, a buried unconformity or fault is the most 1981); Little Maria (Emerson, 1981); Old Woman probable candidate. Further mapping in the area is (Howard, 1981); and Clark Mountains (Burchfiel and required to resolve this problem. Davis, 1971) of California. Folds in the Harquahala Mountain area may represent the eastern termination North- to northwest-trending oblique- or strike­ of a belt of middle Mesozoic compressional structures slip faults are Tertiary(?) in age. The Needle fault characterized by large-scale basement involved cuts east-dipping Tertiary(?) gravel north of the folding. Needle. This assignment requires that the brecciated zone associated with the Needle fault is Tertiary(?) Folds with steep north-northeast plunging axes as well. have not been reported in adjacent areas. They are similar to drag structures expected along strike-slip In summary, structures in the Little Harquahala faults, but the absence of complementary folds on the Mountains are interpreted to indicate Mesozoic high­ opposite side of appropriately oriented structures is angle faulting, southeast-vergent folding, folding not consistent with this hypothesis. These folds re­ about steep north-northeast plunging axes and Late main an enigma. Cretaceous thrust faulting. These structures are overprinted and obscured by chaotic Tertiary struc­ The Hercules thrust is pre-Late Cretaceous in tures including early northeast-trending northwest­ age. Correlation of fabrics from the Harquahala dipping normal-separation faults and later north- to Mountains and the Granite Wash Mountains suggests northwest-trending strike- or oblique-slip faults.

241 ACKNO\.JLEDGEHENTS Reynolds, S. J., 1980, Geologic framework of west­ central Arizona: Ariz. Geol. Soc. Digest, I would like to thank Peter Coney, Bill v. 12, p. 1-16. Dickinson and Steve Reynolds, who introduced me to the geology of west central Arizona, for their en­ Reynolds, S. J., Keith, S. B. and Coney, P. J., 1980, couragement, assistance, and hours of discussion. Stacked overthrusts of Precambrian crystalline Conversations with Lucy Harding, Stan Keith and Rick basement and inverted Paleozoic sections em­ Leveque have been a continuing source of inspiration. placed over Mesozoic strata, west central Field assistance, moral support and a stream of ideas Arizona: Ariz. Geol. Soc. Digest, v. 12, p. were provided by Dawn Harvey, Kerry Inman, Bill 45-52. Jefferson and Nancy Riggs. Financial support was provided by GSA Grant #270880, NSF Grant #8018500, Scarborough, R. and Hilt, J. C., 1979, A study of awarded to Peter Coney and Lucy Harding, and a re­ uranium favorability of Cenozoic sedimentary search assistantship from the Arizona Bureau of Ge­ rocks, Basin and Range province, Arizona, Part I, ology and Hineral Technology. This paper 'vas re­ General geology and chronology of pre-late Hio­ vie'.Jed by S. Calvo, H. R. Dickinson, K. F. Inman, cene Cenozoic sedimentary rocks: Ariz. Bur. H. S. Jefferson, T. Lmvton and N. Riggs. Geol. Min. Tech., Open-file Rpt. 79-1 or U.S.G.S. Open-file Rpt. OFR-79-l429, 101 p. REFERENCES Varga, R. J., 1977, Geology of the Socorro Peak Area, Burchfiel, B. C. and Davis, G. A., 1971, Clark Houn­ ,"es tern Harquahala Mountains: Ariz. Bur. Geol. tain thrust complex in the Cordillera of South­ Hin. Tech., Circ. 20, 20 p. eastern California, in Elders, H. A. (ed.), Geo­ logical Excursions in Southern California: Univ. Calif., Riverside, Campus Huseum Contrib., no. 1, p. 1-28.

Damon, P. E., 1968, Correlation and chronology of ore deposits and volcanic rocks, in Annual Progress Report COO-689-l00, Contract AT(11-1)-689 to U.S. Atomic Energy Commission: University of Arizona, Geochronology Labs, Tucson, 75 p.

Eberly, L. D. and Stanley, T. D., Jr., 1978, Cenozoic stratigraphy and geologic history of southwest­ ern Arizona: Geol. Soc. America Bull., v. 89, p. 921-940.

Emerson, H. S., 1981, Hesozoic and Cenozoic deforma­ tion and igneous activity in the Little Haria Hountains, Riverside County, California (abs.), in Tectonic Framework of the Hojave and Sonoran Deserts, California and Arizona: U.S. Geol. Survey, Open-file Rpt. 81-503.

Howard, K. A., 1981, Hesozoic framework of the south­ eastern Hojave Desert region, California: Geol. Soc. America, Abstr. with Program, v. 13, p. 62.

Krurnrnenacher, D. et al., 1981, Hiddle Mesozoic com­ pressional tectonics and Tertiary extensional overprint in the Big Maria, Little Maria, River­ side, and Arica Mountains and Palen Pass areas of Riverside County, California: Geol. Soc. America, Abstr. with Programs, v. 13, p. 492.

Hiller, F. K., 1970, Geologic map of the quartzite quadrangle, Yuma County, Arizona: U.S. Geol. Survey Geologic Quad. Hap GQ-84l, 1:62,500.

Rehrig, H. A. and Reynolds, S. J., 1980, Geologic and geochronologic reconnaissance of a northwest­ trending zone of metamorphic core complexes in southern and western Arizona: Geol. Soc. America, Hem. 153, p. 131-157.

Rehrig, H. A., Shafiqullah, H. and Damon, P. E., 1980, Geochronology, geology and listric normal faulting of the Vulture Hountains, Haricopa County, Arizona: Ariz. Geol. Soc. Digest, v. 12, p. 89-110.

242

244 GEOLOGIC DEVELOPMENT AND LATE MESOZOIC DEFOR11ATION OF THE LITTLE MARIA 11OUNTAINS, RIVERSIDE COUNTY, CALIFORNIA

Hilliam S. Emerson Tenneco Oil P.O. Box 9909 Bakersfield, CA 93309

ABSTRACT

The Little Maria Mountains form a complex, Hollastonite. Until recently, little attempt has north,,,est-trending range Hithin Riverside County, been made to correlate the rock types or structural California. Highly deformed and moderatley meta­ features of the range to kno,·m stratigraphic morphosed (middle to upper greenschist grade) sequences and structural trends found elseHhere in Paleozoic cratonal strata, correlative ,,rith the the region. Grand Canyon sequence, and Mesozoic metasedimentary rocks are found in tectonic contact Hith Precambrian The purpose of this study was to produce a granitic and gneissic basement. These rocks Here reconnaissance geological map (Figure I and 2) of deformed and perhaps remobilized during Latest the entire Little Haria Mountains in order to Jurassic-Earliest Cretaceous time in response to determine the structural and stratigraphic relation­ subduction beneath southHestern North America. ships of the units within the mapped area and to Large-scale, east-Hest trending recumbent folds determine their regional significance. An under­ formed during this deformational event. A Hansen standing of both the structural and stratigraphic fold analysis Has conducted in tHO locations in the history of this area should aid in our understanding Little Haria llountains Hithin the Muav Formation, a of Cordilleran tectonics Hithin the soutffi,restern Cambrian metasedimentary unit consisting largely of United States. calcitic and dolomitic marbles, locally interlayered ,,rith chert. The resulting slip lines, or directions General Geology of transport, trend roughly north-south Hith a shalloH northerly plunge. Tight clustering of data Highly deformed and moderately metamorphosed points suggests that one continuous deformation is (middle to upper greenschist grade) Paleozoic responsible for all the east-Hest trending folds cratonal strata, correlative with the Grand Canyon Hhich verge to the south-southHest, as Hell as the sequence, and Mesozoic metasedimentary rocks are sub-parallel foliation. Attenuation and thrusting found in tectonic contact Hith Precambrian granitic may be a late-stage feature of this deformational and gneissic basement in the Little Maria Mountains. event. The close parallelism of the thrust-fault Both the Hesozoic and Paleozoic metasedimentary plane and the fold structures indicates that the rocks define the major structural feature of the failure of the folds to further flatten and range, Hhich is the east-west trending Little Maria attenuate resulted in fold separation along a syncline. A thrust fault at the base of the thrust plane. Igneous rocks related to three Paleozoic section juxtaposes an attenuated and separate intrusive events invade both the meta­ upside-down section containing both Paleozoic and sedimentary rocks and the basement complex. The Mesozoic metasedimentary rocks with the base of the Hidland plutons, composed largely of granodiorite Little Haria syncline (Figure 3a,b). Another fault at Hith large feldspar phenocrysts, intrude the l,rest­ the northern edge of the range thrusts pre-Paleozoic central Little llaria llountains. Further northHard, crystalline rocks from the north over the Little the range is intruded by a series of dioritic dikes Haria syncline. Bedding-plane faults and minor and sills and the Little Maria pluton Hhich is thrust faults are present in both the Little Maria composed predominantly of adamellite. Normal dip­ syncline and the attenuated, reversed (upside-down) slip faults Hith offsets of generally less than 10 section~ meters riddle the range and may have been formed during mid-Tertiary extension of this region. THO Late Cretaceous dioritic and granitic rocks right-separation faults transect the range and may intrude a large part of the Little Maria syncline be related to the groHth of late Cenozoic Pacific­ and the attenuated, reversed sequence. These rocks North America transform faults dominated by the San form the northern and southern llidland plutons, Andreas fault system. numerous dikes and sills, and the younger Little Maria pluton. INTRODUCTION LITHOLOGY The Little llaria llountains occupy 155 square kilometers of the Hojave Desert in southeastern Pre-Paleozoic Rocks California. Located in Riverside County, the range is 32 kilometers northl,rest of Blythe and adj acent Pre-Paleozoic crystalline rocks are exposed to the nOH-deserted mining tOHn of Midland, Hhich over a large area in the northeastern part of the in an earlier part of the century produced the Little Haria lIountains and are composed predominantly largest single source of gypsum in California. of metamorphosed granitic augen gneiss and lesser Understandably, the previous geologic ,wrk in the amounts of schist. Large porphyroplasts of micro­ range is concerned primarily Hith the metamorphic cline up to 2 em by 3 em stand out of a foliated history and occurrence of economic minerals found matrix defined largely by the alignment of biotite in the range, such as gypsum, manganese and flakes. This unit may be equivalent to the 1.4-1.5

245 2~\ 27

",

33 34 35 1 \'~~

o Z I KII ~ EARLY TERTIARY GRANITIC ROCKS

~ LATE CRETACEOUS GRANITIC ROCKS ____ CONTACT 9 IlIIIIl CRETACEOUS OIORITIC ROCKS ~FAULT m MESOZOIC METASEOIMENTARY ROCKS ~ THRUST FAULT IZ] PALEOZOIC METASEOIMENTARY ROCKS \.._ AXIAL TRACE OF ~ PRE-PALEOZOIC CRYSTALLINE ROCKS -, SYNCLINE

Figure 2. Generalized geologic map of the northern Little Maria Hountains. b.y.B.P. rapakivi granite that forms much of the Bright Angel Shale Precambrian crust in this area (Silver and others, 1977). Large bodies of this rock have been The Cambrian Bright Angel Shale, like the documented in the Hhipple Mountains by J.L. Anderson Tapeats Quartzite, is found only in the northern (Davis and others, 1980). The pre-Paleozoic base­ Little Maria Mountains. It crops out in the ment rocks are in thrust-fault contact with both northern extension of the Little Maria Syncline on Paleozoic and Mesozoic metasedimentary rocks to the the overturned northern limb, where it is found as south and are intruded by early Tertiary granitic a highly deformed and weathered slope-forming unit. rocks to the west. Metamorphism of the Bright Angel Shale has trans­ formed it into interbedded pelitic schist and Paleozoic Rocks quartzite. This pelitic schist possesses an excellent cleavage, which gives this unit a charac­ Tapeats Quartzite teristic green to silver sheen when viewed normal to the cleavage plane because of the abundance of Tapeats Quartzite represents the basal, lowest biotite and sericite. Thin, platy fragments Cambrian unit of the Paleozoic section. Found only invariably litter the slope when the Bright Angel in the northern Little llliria Mountains, the Tapeats Shale is present. Quartzite appears everywhere exposed in thrust fault contact with younger Mesozoic sediments. Muav Marble Metamorphosed to greenschist facies (Tucker, 1980), the Tapeats is a medium- to fine-grained quartzite, The Lower Cambrian Muav Marble is a mixture of light to reddish gray in color, and contains many lithologic types and occurs as an impressive slightly darker layers indicating bedding. cliff-forming unit exposed along much of the south-

246 21

N 30 i o 2 __ CONTACT ~FAULT I2;2;l LATE CRETACEOUS GRANITIC ROCKS ~ THRUST FAULT 1m MESOZOIC METASEDIMENTARY ROCKS ~_ AXIAL TRACE OF D PALEOZOIC METASEDIMENTARY ROCKS -1!l' SYNCLINE

Figure 3. Generalized geologic map of the southern Little Maria Mountains. Detailed explanations are found "ithin the text.

,,,estern face of the northern Little Haria Mountains. RedHall Marble THO general units Hithin the Muav Marble can be recognized. The lOHer unit is a gray, medium­ grained calcite marble. It appears to be a The Mississippian RedHal·l Marble is a massive, restricted facies 50 m thick in the Little Maria snoH-Hhite, coarse-grained calcite marble. Found Mountains but depositionally thinned or tectonically only on the overturned northern limb of the Little removed in the Big Maria Mountains (Krummenacher, Maria syncline, the RedHall Marble appears up to 1981, pers. comm.). This distinctive medium-gray 40 m thick but may be rendered locally absent due to marble appears everyHhere as a cliff-forming unit at attenuation produced during deformation. the base of the Muav Marble and serves as an excellent marker bed. T4e bulk of the Muav Marble Supai Formation lies stratigraphically above this gray marble and consists of a variety of lithologic rock types. The Pennsulvanian to Lm"er Permian Supai Though predominantly a calcite marble, the unit Formation is a sequence of interbedded quartzite, also contains interbedded dolomitic marble, calcitic and dolomitic marble and calc-silicate rocks quartzite, and epidote chlorite schist. The marbles exposed in both the northern and southern Little are buff, cream, pink and gray in color Hith a lillria Mountains. Interbedded green-gray, medium­ coarse to medium grain size. The bedding of the grained quartzite alternated Hith thicker horizons Muav Marble is often defined by thin (generally less of flesh pink to medium tan, medium-grained than 20 cm) interbedded gray quartzite layers, calcitic and dolomitic marble Hith abundant tremolit~ Hhich Heather to a dark brmm color. These layers This unit forms steep cliffs, Hhich possess a are more abundant tOHards the base of the unit distinctive dark black desert varnish characteristic Hhere small-scale isoclinal folding is common. of no other formation.

247 Figur~ 3a. Northeast vi~w of the overturned northern limb of the Little Maria Syncline (Paleozoic units-Pzs) 1n thrust contact w1th overturned !1esozoic gneissic schist (iMz) and an overturned sheared and thinned sequence of Paleozoic metasediments(iPz ).

Hermit Shale Kaibab Limestone

In the southern Little Maria Mountains, a The metamorphosed Late Permian Kaibab Limestone porcellaneous chlorite schist is locally present is found only in the southern Little Maria Mountains stratigraphically above the metamorphosed Supai where it is exposed as a 325 m thick sequence of Formation and may represent the metamorphosed calcitic and dolomitic marble, chert, and quartzite. equivalent of the Lower Permian Hermit Shale. Thin This uppermost Paleozoic Formation also plays host schists derived from the Hermit Shale were difficult to a considerable amount of economically mineable to distinguish from the underlying Supai Formation agricultural gypsum and anhydrite evaporites with and were mapped as such due to limited exposures. the bulk of the literature on this area reporting on its occurrence (Surr, 1911; Jones, 1919; Hess, Coconino Sandstone 1920; Tucker and Sampson, 1919; Ver Planck, 1950, 1952, 1957;- and Shklanka, 1963) (Fig. 4). The Lower Permian Coconino Sandstone is limited to a 10 m section exposed only in the southern LOHer Units Hithin the Kaibab Limestone range Little Maria Mountains. It is composed of massive from massive to poorly bedded calcite marbles Hith and bedded quartzite, which despite metamorphism occasional calc-silicate quartzite and gypsum beds. and deformation, locally displays wedge-planar Higher up section, the occurrence of dolomitic and cross bedding. and Hollastonite marble, gypsum and anhydrite evaporties and cherty layers increases. The metamorphosed Coconico Sandstone may be divided into two separate units based largely on Meso zoic Rocks color (Tucker, 1980). The lower unit is a pale green, planar-bedded quartzite, with sericite Undifferentiated Metasedimentary Rocks developed along cleavage planes giving it a characteristic sheen. The upper unit is a strongly Mesozoic metasedimentary rocks are exposed as a jointed, ledge-forming quartzite that can be locally thick 400 m section forming the core of the Little massive or well bedded. llaria syncline and as a tectonically emplaced 1000 ill

248 Figure 3b. West-northwest vie" of the overturned Mesozoic (iMz) and attenuated Paleozoic metasediments (iPz) in thrust contact with the overturned Paleozoic limb of the Little Maria Syncline (Pzs).

thick overturned section in the northern Little diorite has been metamorphosed and deformed, and Maria Mountains. Determination of a stratigraphic appears· as a fine-grained, dark green, mildly sequence applicable to both northern and southern foliated rock, which can sometimes resemble Mesozoic exposures of Mesozoic metasedimentary rocks in the metasedimentary rocks. Sharp intrusive contacts Little Maria Mountains is difficult primarily due to serve to distinguish the two rock types. variable metamorphism and deformation. Midland Plutons Mesozoic metasedimentary rocks in the core of the Little Maria syncline are composed of quartzite, The Midland plutons make up a substantial quartzite conglomerate, metaarenite, and meta-tuff. portion of the Little Maria Mountains. Located in The presence of green minerals such as actinolite, both the northern and southern parts of the range, green hornblende, epidote and chlorite give the the Mesozoic meta-igneous rocks intrude and mildly unit a characteristic green color. deform both Paleozoic and 11esozoic metasedimentary rocks. The pluton is composed of an assortment of A second terrane is found as part of the granitoids, including granodiorite, adamellite, reversed, attenuated section, directly north of the granite aplite and pegmatite (Lafferty, 1981). overturned limb of the Little Maria syncline. Here These rocks often posses a well-developed tectonic the Mesozoic metasedimentary rocks have been meta­ foliation and range in texture from augen gneiss to morphosed and deformed to green fine-grained schist. schist. Prophyroblasts of microcline up to two­ These rocks are intruded and irregularly feldspath­ square centimeters can be found in the augen ized by sill-like igenous bodies that have been gneiss, though generally they are smaller. transformed into fine-grained gneiss. Little Maria Pluton Dioritic Rocks The Little Maria Pluton is a separate, younger Rocks of dioritic composition are found as plutonic body present at the northernmost end of stocks, dikes and sills primarily in the northern the Little Maria Mountains. It is a highly Little Maria Mountains. These rocks intrude leucocratic pluton composed largely of adamellite Paleozoic and Mesozoic metasedimentary rocks both with lesser amounts of syenogranite and pegmatite. parallel and at high angles to the foliation. The Relatively sharp contacts are found where these

249 Figure 4. Southwest view of steeply dipping Paleozoic metasediments (pz) intruded on the right by the Midland Pluton (MF). Economic gypsum and anhydrite deposits occur within the Kaibab Limestone. Mining activity can be seen on the left-central portion of the photo.

rocks intrude Paleozoic and Mesozoic metasedimentary the Little Maria syncline lies a series of rocks as well as pre-Paleozoic crystalline rocks. imbricately stacked, reversed and attenuated wedges, The marked absence of a well-developed tectonic which were tectonically emplaced along northwest-to foliation indicates that this is a relatively northeast-dipping thrust faults. A partially young (post-deformational) igneous intrusion. exposed cross section of this attenuated, reversed sequence (Figure 3) yields an amplitude of approxi­ STRUCTURAL GEOLOGY mately 60 meters.

Folding The fold axes of the large-scale folds trend from east-west to northwest-southeast with a Metamorphosed Mesozoic and Paleozoic metasedi­ variable subhorizontal plunge. The plunge direction mentary rocks form a sequence of upright to over­ is complicated by doming, or arching, of the major turned and tectonically stacked, attenuated, and structure. The north-northeast limb is either isoclinal folds. Large-scale recumbent folding is greatly overturned or steeply dipping while the most evident in the major structural feature of the southern limb possesses gentle to moderate dips. range, the Little Maria syncline. It is well defined by formational boundaries, which, despite extreme Small-scale folds are found within the meta­ thickening and thinning of strata, remain surprising­ sedimentary units on the exposed limbs of large­ ly intact. Digitations within the Little Maria scale folds. The folds are defined largely by a syncline occur in several locations, producing color contrast between light tan and dark brown upright sequences locally within the larger recumbent desert-varnished layers. These layers reflect a limbs. strong lithologic variance or ductility contrast, the light tan layers being composed of calcitic The asymmetric nature, limited exposure, igneous and dolomitic marbles, the dark brown layers being intrusion, and tectonic deformation of the large­ composed of recrystallized chert (Fig. 5). scale folds complicate amplitude and wavelength measurements. The Little Maria syncline received Small-scale folds exhibit broadly open to relatively little tectonic deformation and its tightly isoclinal behavior. Hinge thickening is wavelength is estimated to be between three and five common and is reflected by the abundant Class lC kilometers. In contrast, directly to the north of folds and the lesser Class 2 and 3 folds (Ramsay,

250 Figure 5. Small-scale folds within the Kaibab Limestone as exposed in the Little Maria Mountains.

1967). Wavelengths and amplitudes range from appears responsible for folding. In much of the several centimeters to several tens of meters but metasedimentary section, the ductility contrast generally are less than one meter. While the appears to be low and layer-parallel shortening interlimb angle may vary greatly, the fold amplitude was probably the dominant response to stress, is usually greater than the fold wavelength, resulting in relatively planar beds.

The mechanism of folding, both on a large and small scale, is largely by flexural flow. The Kinematic Significance of Folding ductility contrast between interbedded chert and marble layers appears to have been high enough in A fold analysis was conducted in two locations the Muav Formation to cause tight isoclinal folding in the northern Little Maria Mountains. Both fold and hinge thickening within layers. In areas where locations are within the Muav Formation, the chert layers are largely absent, passive flow Cambrian metasediments possessing well-defined

251 small-scale folds. Approximately 60 fold orienta­ axes. This may be related to the regional develop­ tions were measured in order to perform a Hansen ment if major antiforms and synforms in this region (1971) analysis. For a detailed discussion of the during mid~Tertiary time (Frost, 1981). The methods and results of this analysis the reader is proximity of the Little Maria Mountains to areas referred to Emerson (198lb). where the antiforms and synforms are well developed (Hhipple and Riverside Mountains, Quin Sabe Point of In both fold locations the resulting slip lines, Big Maria Mountains) suggest that this same deforma­ or directions of transport, trend roughly north-south tional style may extend into the Little Maria with a shallow northerly plunge (Emerson, 1981a). A !fuuntains as well. Broad warping of the Mesozoic south to soutmvest vergence is exhibited by the folds around northeast-southwest trending fold axes large-scale folds lvhich reflects a north-northeast­ is clearly displayed in both the Big Maria (Ellis ward underthrusting of the southern limb beneath and others, 1981; Martin and others, 1981) and the northern limb. This direction of overturning is Riverside Mountains (lyle, 1981, pers. comm.). This opposity that seen in most of the foreland fold and would indicate that the refolding of the Little thrust belt, where transport is toward the continent. Maria syncline may be a separate and distinct In the Little Haria Hountains, h01vever, transport deformational event due to mid-Tertiary extensional of the upper plate is t01vard the continental margin, tectonics. suggesting that the Little Harias reflect deforma­ tion related to underthrusting beneath the North Faults American continent. Hidespread deformation due to faulting is Such an underthrusting event appears to be extremely common in the Little Maria Mountains. recorded in several surrounding mountain ranges Thrust, strike-slip, normal and bedding-plane faults within southeastern Calfironia and western Arizona. disrupt both crystalline and metasedimentary rocks. This is reflected by subhorizontal fold axes and Foliation planes and contact surfaces seldom extend axial surfaces, which facilitate the exposure of for more than several tens of meters before they are these folds over long distances. South to southwest­ disrupted. Despite this deformation, the general verging, large-scale folds interpreted to have continuity of formational boundaries remains formed by underthrusting are also found in the Palen relatively intact. Pass area (Demaree, 1981) and the Big Maria Mountains (Ellis, 1981). Only the thrust and right-separation faults could be Sh01Vll on the geologic map, primarily due to Several authors have suggested multiple folding the large scale at which the geology was mapped, as events in the Maria Mountains (Hamilton, 1971; well as the much greater amount of offset these Shklanka, 1963) on the basis of fold style. The faults displayed. Bedding-plane and normal dip-slip results of the Hansen fold analysis conducted in the faults literally riddle the range but the amount of Little Maria Mountains cannot substantiate separate movement is relatively small compared ldth the move­ events. The tight clustering of data points and the ment along the thrust and right-separation faults. parallel relationship of axial surfaces and For a much more detailed treatment of the faulting foliation suggest that one continuous Mesozoic in the Little Maria Mountains the reader is deformation is responsible for these subhorizontal, referred to the map and text of Emerson (198lb). slightly northward-dipping features. That this Mesozoic deformational event may have completely Thrust and bedding-plane faults are assigned a refolded previous events is not discounted, however. late Mesozoic age, possibly Late Cretaceous, and Multiple fold events do seem to present in the occurred prior to the intrusion of granitic rocks adjacent Riverside (Lyle, pers. comm., 1981) and which crosscut these features. These faults formed Arica Hountains (Baltz, pers. comm., 1982). as a result of extreme shear strains developed in the fold limbs during intense folding. Northwest­ Small- scale folds occasionally appear refolded trending normal faults are assigned a mid-Tertiary but are also attributed to a continuous deformation age and fit into a regional picture of northeast­ and are similar to sheath folds as described by southwest directed extension. These faults occur at Minnigh (1980). As a deformation continues, an high angles to bedding and reflect the more brittle initially straight hinge line begins to arch or dome. character of the rocks during this deformational By this method, the axial surface and the slip period. Two large right-lateral, strike-slip faults planes all remain subparallel. are of mid- to late Tertiary age and are attributed to a pattern of regional fractures associated with Thrust faulting and bedding-plane faults the San Andreas system. represent deformational events that are probably coeval with this Mesozoic deformational event. As DYNM1IC SIGNIFICANCE OF GEOLOGICAL AND DEFORHATIONAL the folds developed in the Little Maria Mountains, CHARACTERISTICS OF THE LITTLE ~MRIA MOUNTAINS particularly intense shear strains developed in the fold limbs along the bounding zones of the competent The distribution of major rock types as well as layers. Thrust faults resulted when the fold limbs the major structural features of the Little Maria were unable to further flatten and attenuate. The Mountains represent a complex geologic and deforma­ extreme attenuation resulted in the juxtaposition of tional hisotry. Mesozoic metasedimentary rocks against Cambrian metasedimentary rocks in one thrust fault in the The Paleozoic Era was marked by fluctuating sea northern Little Maria Mountains. levels producing a series of epeiric seas, which covered much of southwestern continental North A second distinct event l"hich is not apparent America. This resulted in platform sedimentation of in the Hansen fold analysis due to the close spacing evenly bedded limestone, dolomite, sandstone and of fold locations and large amplitude of folding is shale, correlative with units best exposed today in the deformation of the axial surface of the Little the Grand Canyon. A significant change in the Maria syncline around northeast-southwest trending depositional environment occurred during the Mesozoic

252 Era. This is due primarily to the formation of a faults (Coney, 1978) dominated by the San Andreas northwest-trending magmatic arc which extended fault system. Alternatively, these right separation across the area from southern Arizona to northwestern faults may be largely normal in their sense of offset California (Hamilton, 1969; Burchfiel and Davis, 197~ and reflect the mid-Tertiary extensional regime. 1975). Island-arc sedimentation resulted in the deposition of conglomerate, tuff, sandstone and REFERENCES volcanic rocks. These Mesozoic rocks cover a broad region and make up significant portions of the Burchfiel, B.C., and Davis, G.A., 1972, Structural Little Maria, Big Haria, Riverside, Palen and McCoy framework and evolution of the southern part of Mountains. the Cordilleran orogen, western United States: Am. Jour. Sci., v. 272, p. 97-118. The initiation of subduction beneath southwest­ llurchfiel, B. C., and Davis, G. A., Nature and con­ ern North America in Latest Triassic time also trols of Cordilleran orogenesis, western United initiated widespread deformational and magmatic States: Extensions of an earlier synthesis: activity (Hamilton, 1969; Burchfiel and Davis, 1972, Amer. Jour. Sci., v. 275-A, p. 363-396. 1975). The Little Haria syncline is interpreted to Carr, H. J., and Dickey, D. D., 1980, Geologic map of have formed during this event. The plastic style of the Vidal, California, and Parker SI;, California­ deformation indicates that crustal softening by high Arizona quadrangles: U. S. Geol. Survey Hap temperature extended into the craton in southeastern 1-1125. California (Burchfiel and Davis, 1975). Attenuation Coney, P., 1978, Mesozoic-Cenozoic Cordilleran plate and thrusting may be a late-stage feature of this tectonics, in Smith, R.B., and Eaton, G.P., event. The close parallelism of the thrust-fault eds., Cenozoic tectonics and regional geophysics plane and the fold structure indicates that the of the western Cordillera: Geol. Soc. America failure of the folds to further flatten and Mem. 152, p. 33-50. attenuate resulted in fold separation along an Davis, G.A., Anderson, J.L., Frost, E.G., and incipient thrust plane. The south-southwest Shackelford, T.J., 1980, Mylonitization and vergence of large-scale folds is attributed to detachment faulting in the Hhipple-Buckskin­ north-northeastward underthrusting of a south"estern Rawhide Mountains terrane, southeastern downgoing slab, just as it is in the Palen Pass area California and western Arizona; in Crittenden, (Demaree, 1981) and the Big Maria Mountains (Ellis M.D., Jr., Coney, P.J., and Davi~ G.H., eds., and others, 1981). This deformation appears to be Cordilleran Metamorphic Core Complexes, Geol. related to stresses developed in the North American Soc. America Memoir 153, p. 79-129. plate during massive northeastward underthrusting of Demaree, R., 1981, Geology of Palen Pass, Riverside the Pacific plate (Burchfiel and Davis, 1972, 1975; County, California (M.S. thesis; San Diego Coney, 1978; Krummenacher and others, 1981). State Univ.) 136 p. Ellis, M.J., 1981, Structural analysis and regional Granitic rock of the northern and southern significance of complex deformational events in Midland plutons belong to a widespread igenous the Big Maria Mountains, Riverside County, event that is well documented throughout southeastern California (M.S. thesis; San Diego State Univ.), California and western Arizona. Much, but not all, 109 p. of the folding and thrusting occurred prior to the Ellis, M.J., Frost, E.G., and Krummenacher, D., 1981, emplacement of these Late Cretaceous granitic rocks Structural analysis of multiple deformational which cross-cut these deformational features. A events in the Big Maria Mountains, Riverside well-developed foliation present in both of the County, California; Geol. Soc. of America, Midland plutons, which parallels the fold structure Abstracts with Programs: v. 13, no. 2, p. 54. and thrust faults, indicates an ongoing deformation Emerson, H.S., 1981a, Mesozoic and Cenozoic Deforma­ during emplacement of these igneous rocks. tion and Igneous Activity in the Little Maria Mountains, Riverside County, California, in The emplacement of the Little Maria pluton in Tectonic Framework of the Mojave and Sonoran the northern part of the range post-dates the major Deserts, California and Arizona, U.S.G.S. Open deformational event and, as a result, the pluton File report 81-503, p. 35. lacks a well-developed foliation. Emerson, H.S., 1981b, Geological and deformational characteristics of the Little Maria Mountains, Normal sip-slip faults may be related to Riverside County, California (M.S. thesis, San northeast-southwest extension in mid-Tertiary time. Diego State Univ.) 98 p. A similar geometry of extensional faulting is seen Emerson, H.S., and Krummenacher, D., 1981, Geological in ranges to the north and east of the Little Haria and deformational characteristics of the Little Mountains. Pervasive northwest-striking normal Maria Mountains, Riverside County, California; faults are seen in the Turtle, Hopah, Riverside, Geol. Soc. of America, Abstracts with Programs; Whipple, Arica and Buckskin Hountains (Frost, 1981, v. 13, no. 2; p. 54. pers. comm.). Similar northwest-trending normal Frost, E.G., 1981, Mid-Tertiary detachment faulting faults may also be present in the Big Haria in the Ifhipple Mountains, California and Buck­ Mountains (Ellis and others, 1981). The presence skin Mountains, Arizona, and its relationship to of detachment faulting in the adjacent Riverside the development of major antiforms and synforms; (Carr and Dickey, 1980; Lyle, this volume) and Big Geol. Soc. of America Abstracts with Programs, Maria Mountains (Ellis and others, 1981; Martin and v. 13, no. 2, p. 57. others, 1981) may also suggest a northeast-southwest Hamilton, H., 1969, l1esozoic California and the extensional regime in the Little Maria Hountains underflow of Pacific mantle: Geol. Soc. during mid-Tertiary time. America Bull., v. 80, p. 2409-2430. Hamilton, H., 1971, Tectonic framework of southeast­ Right-separation faults in the Little Maria ern California: Geol. Soc. America Abstracts Mountains may reflect a widespread change in with Programs, v. 3, no. 2, p. 130-131. Cordilleran tectonics and may be related to the Hansen, E., 1971, Strain Facies: New York, Springer­ growth of the Pacific-North America transform Verlag, 207 p.

253 Hess, F.L., 1920, California, in Stone, R.W., and others, Gypsum deposits o~the United States: U.S. Geol. Survey Bull. 697, p. 78-79. Jones, E.L., Jr., 1919, Deposits of manganese ore in southeast California: U.S. Geol. Survey Bull. 7l0-E, p. 185-208. Lafferty, M., 1981, A reconnaisance geochemical, geochronological and petrological investigation of granitoids in the Big and Little Maria Mountains and Palen Pass, Riverside County, California (ll.S. thesis, San Diego State Univ.) 147 p. Krummenacher, D., Martin, D.L., Baltz, R.t!., Demaree, R.G., Ellis, M.J., Emerson, W.S., Kies, R.P., Lafferty, H.R., Lyle, J.H., and Frost, E.G., 1981, Middle Hesozoic compressional tectonics and Tertiary extensional overprint in the Big Haria, Little Haria, Riverside, and Arica Mountains and Palen Pass areas of Riverside County, California: Geol. Soc. of America Abstracts with Programs, v. 13, no. 7. Hartin, D., Krummenacher, D., and Frost, E.G., 1981, K-Ar dating in the Big Haria Hountains, Granite Hountains and Palen Pass; Thermal history, rates of cooling and uplift and tectonic implications: Geol. Society of America, Abstracts with Programs, v. 13, no. 2, 94 p. Minnigh, L.D., 1980, Structural analysis of sheath­ folds in a meta-chert from the Western Italian Alps: Journal of Structural Geology, v. 1, no. 4, p. 275-282. Ramsay, J.G., 1967, Folding and fracturing of rocks: New York, HcGraw-Hill Inc., 568 p. Silver, L.T., Bickford, H.E., Van Schmus, W.R., Anderson, J.R., Anderson, T.H., and Madaris, L.G., Jr., 1977, The 1.4-1.5 b.y. transcontin­ ental anorogenic plutonic perforation of North America: Geol. Soc. Anlerica Abstracts with Programs, v. 9, no. 7, p. 1176-1177. Shklanka, R., 1963, Repeated metamorphism and deformation of evaporite-hearing sediments, Little Maria Mountains, California: (Ph.D. thesis) Stanford, California, Stanford Univ., 127 p. Surr, G., 1911, Gypsum deposits in the Maria Mountains of California: Mining World, v. 34, p. 787-790. Tucker, R.S., 1980, A reconnaissance study of meta­ morphism in the Big Maria Mountains, Riverside County, California: (M.S. thesis; San Diego State Univ.) 124 p. Tucker, W.B., and Sampson, R.J., 1929, Riverside County: Calif. Div. Mines Rept. 25, p. 468-526. Ver Planck, W.E., 1950, Gypsum, in Mineral Commodi­ ties of California: Calif. Div. Mines Bull. 156, p. 227-230. Ver Planck, W.E., 1852, Gypsum in California: Calif. Div. Mines Bull. 163, 151 p. Ver Planck, W.E., 1957, Gypsum, in Hineral commodi­ ties of California: Calif.lDiv. Mines Bull. 176, p. 231-241.

254

I\> (J1 en

Figure 1. Eastward view of Savahia Peak in the western \n'ipple Mountains. Tilted Miocene volcanic units appear relatively coherent as they dip to the southwest and are truncated by the detachment fault along the base of the peak. A closer examination shows that Savahia is riddled with a complex system of faults which help to explain the enigma of extension and rotation in the detachment terrane. Klippe is two-and-one-half kilo­ meters across and 350 meters high. GEO~ffiTRIC ANALYSIS OF UPPER-PLATE FAULT PATTERNS IN THE HHIPPLE-BUCKSKIN DETACHHENT TERRANE, CALIFORNIA AND ARIZONA

\-larren H. Gross and F. L. "Bud" Hillemeyer Department of Geological Sciences San Diego State University San Diego, California 92182

ABSTRACT INTRODUCTION

According to several current models, upper-plate The presence of a regionally developed, 10H­ extension associated "lith the 10H-angle detachment angle normal fault, or detaclment fault, throughout fault of the Hojave-Sonoran desert is facilitated by the eastern Hojave desert and extending from south of a series of "idely spaced normal faults. It is Las Vegas to Sonora is Hell documented (Anderson, impossible to explain the observed rotation of upper­ 1971, 1977, 1978; Davis and others, 1979, 1980; Davis plate blocks along such Hidely spaced faults Hhether and Coney, 1979; Rehrig and Reynolds, 1980; Crittenden they are planar faults or faults that flatten at and others, 1980). The detacl1l'lent fault, Hhich devel­ depth called listric faults. Our field investigations oped during the Oligocene to middle Hiocene, separates indicate that extension and rotation are facilitated Precambrian through early Tertiary igneous and meta­ by a complex system of normal faults "hich are pene­ morphic rocks from an allochthonous, lithologically tratively developed Hithin the major rotated blocks. varied upper plate of Precambrian throu~h Tertiary The recognition of closely spaced faults "ith small units (Davis and others, 1980). The detachment fault offsets is essential in explaining the observed exten­ appears to be a result of regional crustal extension. sion and rotation. Host faults appear to be planar As such, models of hOH upper-plate fault geometries to curviplanar. Systems of planar faults that actually Hork may also be applicalbe to other areas approximate listric faults appear to accommodate of crustal extension such as continental rift margins most of the rotation, "ith true listric faults playing (Fig. 2a and b) and the Basin and Range province a lesser role. VieHing upper-plate fault blocks as (Fig. 3). Unlike the Basin and Range province and rigid crustal blocks is an over simplification of continental rift margins, the detachment terrane upper-plate structure. More realistically, these offers exceptional exposures of the normal faults rotated "fault blocks" should be vieHed as fault­ associated Hith crustal extension. This is especial­ block complexes. Hodels of hOH upper-plate fault ly true in the Hhipple and Buckskin llountains ,·,here geometries in the detachment terrane actually "ork the regional geology and structure has already been may also be applicable to other areas of extension extensively studied by Davis and others (1979, 1980) such as the Basin and Range province and continental and Anderson and others (1979). rift margins.

s N o o L;.Hole400A

Trevelyan Meriadzek 15

12.1 )

3.1 3.6

4.9

10km

82

Figure 2a. A geological cross section constructed from seismic reflectors from the Bay of Biscay (a continental rift margin) shoHing listric faults flattening into a subhorizontal Zone of decollement. This type of crustal extension may be analogous to the crustal extension observed in the Hojave-Sonoran detachment terrane. HOHever, the faults in this region cannot be directly observed at depth. Studies are limited to the interpretation of seismic profiles, Hhich may have insufficient resolution to yield clear-cut interpretations. From Hontadert and others (1979).

257 SOT W E SOT

6_ 5km _6

Figure 2b. Interpretation of a seismic profile from the Bay of Biscay showing listric to curviplanar normal faults bottoming into a subhorizontal zone of decollement. Again, observations are limited to sub­ surface geophysical data. The evidence for listric faults in this cross section is not so strong as to preclude other interpretations. From l10ntadert and others (1979).

The upper plate, Hhich appears to have been ex­ Since rotation and extension of the upper-plate tended a minimum of several tens of kilometers along blocks appears to be facilitated by this system of the detachment surface, is characterized by a system normal faults, knowing the true geometries of the of normal faults Hith large offsets. Extension along faults plays an important role in our understanding such faults has been accompanied by back-rotation of of upper-plate tectonics. Almost all upper-plate large upper-plate blocks, resulting in tilts of up to normal faults merge with, but do not offset the 70°. Despite the pronounced antiformal shape of the detachment fault (Davis and others, 1980; Frost and mountain ranges, transport is not radial, aHay from Hartin, 1982). This is spectacularly illustrated in lower-plate structural highs, but is uniformly to the the Whipple !Vash gorge area about 15 km to the north­ northeast (Davis and others, 1980). This is spectac­ east of Savahia Peak (Fig. 4). Early investigations ularly illustrated at Savahia Peak where back rotation of major normal faults in the detachment terrane to the southHest of a thick sequence of Tertiary resulted in interpretations that the faults flatten volcanic rocks indicates northeast transport up the at depth and curve into a detachment surface or zone \vhipple Hountain antiform (Fig. 1). of decollement (Shackelford, 1976, 1980; Davis and others, 1979, 1980; Rehrig and Reynolds, 1980; Anderson, 1971). A listric fault geometry was sug­ gested for many of the faults since rotation of a fault block is most easily visualized along curved faults. Hore recent studies (Frost, 1980, 1982; Davis and Anderson, 1982; Carr and others, 1980) suggest that many of the faults are planar and do not A B C o E F flatten at depth. This is supported by field evidence since many of the normal faults are observed to be nearly planar and can be traced to within a meter or TILTED BLOCK two of the detachment surface where they abruptly terminate showing little, if any, sign of flattening. ~~ The geometry of such straight faults makes the ..------...... - rotation of large upper-plate blocks difficult to LISTRIC FAULT explain. Rotation of fault blocks along widely spaced normal faults, such as illustrated in figure o 50 km 5, requires simultaneous faulting along subparallel APPROXIMATE HORiZoNTAL AND VERTICAL SCALE faults in a domino-like fashion. The schematic cross Figure 3. Schematic diagrams shmving three section of figure 6 shows that faulting along such generalized models of basin-range structure: (1) the parallel, straight faults is capable of producing horst and graben model Hhich involves a brittle the observed rotation. However, the greater the crustal slab being pulled apart by a plastically amount of extension (and therefore the greater the extending substratum into a series of systematically amount of rotation) that the upper plate undergoes, spaced dOHndropped horizontal prisms termed grabens, the more serious the spatial consequences become. (2) the tilted block model relating the structure to Figure 7 shoHS that similar volume problems are fragmentation of a brittle upper crust into a system associated with rotation along listric faults. It is of buoyant blocks rotated along straight normal apparent that it is impossible to balance cross faults into a plastic substratum, and (3) the listric sections that show rotation along widely spaced lis­ fault model relating the structure to rotation of tric or straight faults. Volume problems cannot be crustal blocks along downHard flattening normal explained by upper-plate abrasion along the detachment faults to a common decollement zone (Stewart, 1980). surface sin~e geochemical studies by Heidrick and Again, as in the Bay of Biscay, studies of normal !Vilkins (1980) indicate that breccias associated with fault geometrics in the Basin and Range province are the detachment fault were derived from lower-plate limited by the level of surface exposure. rocks.

258 Figure 4. VieH of east Hall of Hhipple Hash gorge shm.,ing a Hell exposed example of the fault geometry seen repeatedly in the area. A steeply dipping, nearly planar normal fault (A) contacts the detachment fault (B) at a high angle but does not cut it. Upper-plate volcanic rocks have been back-rotated along the normal fault to a dip of about 30°. Bedding in upper-plate rocks can be traced to the detachment fault shoHing that brecciation of the upper plate has not occurred. Such a fault geometry seems unable to account for the observed rotation of upper-plate blocks. Distance across photo is about 100 meters.

Sea Level 1000' 800'

~'OOO' 1600' "- O-'Sc:C~AL"E~ ------~2400'

Figure 5. This cross section through the eastern Hhipple llountains is typical of cross sections Hhich have been draHn through areas of crustal extension in that it cannot be balanced. j,hile the geology and structural relationships shmvn are Hell documented, the upper-plate fault blocks cannot be rotated to their pre-faulting position Hithout creating large void spaces in the section. Such cross sections are useful for displaying lithologic correlations and large-scale structural features, but they should not be vieHed as an explanation for ho,., upper-plate rotation and extension has occurred. Cross section from Frost (1982) as modified from thesis mapping by Steve Lingrey and Karl Evans.

259 To date, models to explain thin-skinned crustal extension and associated fault-block rotation have relied upon ",idely spaced normal faults ",ith large offsets. The inability of "'idely spaced normal faults alone to explain upper-plate extension in the Hojave­ Sonoran detachment terrane led us to believe that the fault geometries are more complicated. For this reason ",e began an investigation to obtain a better understanding of upper-plate fault geometries and to UPPER-PLATE EXTENSION ~ develop a more realistic scheme for ho", these fault geometries facilitate extension and fault-block rotation in the detachment terrane. Field investiga­ tions for this study ",ere primarily conducted at Savahia Peak in the ",estern lfuipple Hountains (Fig. 8). Outcrops from other areas of the detachment terrane ",here fault geometries are easily observed ",ere also investigated.

DISCUSSION OF FIELD INVESTIGATIONS (90%) The detachment surface at Savahia is largely covered by recent talus accumulations, but the peak offers an exceptionally ",ell exposed cross section of upper-plate rocks (Fig. 1). These rocks, wostly Tertiary volcanic units, are stacked in a nearly homoclinally dipping fashion, ",ith tilts of 50-700 to the south",est, and are truncated in this direction by the detachment surface. From a distance, these planar beds appear relatively undisturbed. Upon closer exam­ ination it is apparent that extensive fracturing and brecciation is characteristic of the upper-plate rocks Figure 6. Schematic cross section illustrating at Savahia. This suggests that the rocks have been problems associated with extension by movement along subjected to a penetrative brittle deformation. "idely-spaced planar faults. Serious volume problems (void spaces) result from fairly small amounts of A number of predominately north",est-striking, extension. northeast-dipping normal faults ",ere recognized at Savahia. Orientation and sense of movement for most of these ",ere determined on fault surfaces exhibiting slickensides and moderately to exceptionally ",ell­ developed striations. These polished surfaces are not preserved for a great length of time since, upon exposure, the preferential ",eathering of feldspars LISTRIC FAULTS results in a rough pitted surface. The existence of a fe", faults in the more inaccessable areas of the rugged peak ",ere inferred from photographs. Due to the intense fracturing and homogeneous nature of the rocks, faults generally cannot be traced over dis­ tances of about 50 meters, ho",ever, all faults that

v ",ere observed are flat over the length of their ex­ v v v v v v v v v posure. Offsets are not measurable on any of the v V V v v v v V v v observed faults, but are probably on the order of a DETACHMENT FAULT fe", tens of meters at most, and much less in most cases. This supposition is based on the apparent integrity of bedding visible in photographs of Savahia (Fig. 1).

These normal faults, ",hich are synthetic with respect to the major normal faults that bound Savahia, 0 dip through angles of 20-65 • There is no apparent pattern to the degree of dip, rather the faults, if extended sufficiently, appear to intersect at various angles (Fig. 9). He believe that the degree of dip may be related to the age of the fault as suggested by Proffett (1977). In such a case, first-formed faults ",auld be rotated to a shallo",er dip by movement along subsequent faults. He cannot, ho",ever, confirm this hypothesis ",ith the nature of the exposures at Savahia.

Figure 7. Schematic cross section illustrating Evidence of antithetic faults is suprisingly problems associated with extension along ",idely scarce. Striations ",ere observed along one bedding­ spaced listric faults. In addition to the volume plane fault and others could perhaps be justified by problems, unreasonably shaped fault blocks are arguing for offset along south",est-dipping joints, produced. but our investigations suggest that antithetic faults

260 ..

WHIPPLE MTNS

SAVAHIA PEAK

~.. +

Figure 8. Index map showing locations of areas cited in text (Modified from Davis and others, 1980).

NE sw

Figure 9. Schematic cross section of Savahia peak showing geometry of faults observed. Solid bars indicate location of fault measurement. Dotted lines represent obscured fault traces assuming straight faults. Bedding dips 60-70 0 to the southwest.

261 Figure 10. An example of the type of faulting seen in the upper plate of the Whipple Mountains. One stratigraphic sequence has been repeated several times by a series of subparallel synthetic faults and smaller antithetic faults. Hote the penetrative development of both types of faults and the presence of both planar and curved faults. This is an example of the type of small-scale faulting which may be present throughout the detachment terrane. Such faults may be obscured in most areas due to the weathering characteristics of the rocks. This appears to be the case at Savahia Peak. Outcrops such as this one present a strong argument against viewing upper-plate rotation and extension in terms of rigid crustal blocks. This outcrop is located in the southern ~fhipple Mountains. Photo courtesy of Eric Frost. are either much less abundant than synthetic faults homogeneity of the units makes it impossible to or have such small offsets that they are not observ­ recognize fault displacements without the presence of able. This aggrees with the observations made by these features. Pervasive fault sets with each of T. Shackelford (pers. commun., 1981) in the upper­ these three geometries; (1) synthetic normal, (2) plate rocks of several areas to the northeast of antithetic normal, and (3) oblique-slip, can be coaxed Savahia Peak and suggests that this may be typical of from the fracture pattern observed in the rocks at much of the detachment terrane. It should be noted Savahia. here that some faults which appear to be synthetic may have formed as steeply-dipping antithetic faults Arguments for the penetrative development of and have since been rotated along subsequent faults faults at Savahia are supported by observations made so that they become apparent synthetic faults. Two in other areas. Figure 10 shm,s an outcrop of upper­ oblique-slip faults with a dominantly strike-slip plate rocks in the southern Whipple Mountains in offset were also identified in this study. which synthetic and antithetic normal faults are pene­ tratively developed. It seems likely that faults at One of the most striking characteristics of the Savahia are similarly penetratively developed but that rocks at Savahia is the pervasive jointing. Numerous the nature of the rocks is such that evidence of joint sets are penetratively developed. There are small-scale faulting is quickly obliterated by weath­ particularly well developed joint sets with the ering of the rocks. orientations of the major faults. It appears that there has been displacement along many of the abundant Another area that clearly shows relationships "joints", possibly to the extent of producing a fault that may be obscured at Savahia is located in the spacing of a few centimeters or less in many areas. Buckskin 110untains, 15 kilometers northeast of Parker, The offsets along such small faults may not have been Arizona (Fig. B). This outcrop yields an excellent great enough to produce the polished surfaces and cross-sectional exposure of upper-plate rocks about striations required for field verification. The a meter above an exposure of the detachment surface.

262 30~

10' =

'..., ... v ... '0' "'" 10'

Figure 11. Line drawing of faults observed in an outcrop (25 meters across) of upper-plate rocks in the southern Buckskin Mountains. Base of outcrop is about one meter above an excellent exposure of the detachment fault. The pattern of intersecting synthe­ tic normal faults seen in this outcrop is similar to the fault pattern ovserved at Savahia Peak as shown in figure 9.

Figure 11 is a detailed field sketch showing the geometry of faults observed in the outcrop. The draw- ing shows a network of curviplanar normal faults with 0 northeast dips of 30-70 intersecting at various angles. This example of intersecting synthetic normal faults is representative of the fault patterns we believe are penetratively developed throughout Savahia Peak.

CONCLUSIONS

It is obvious from our earlier discussion that attempts to explain upper-plate extension along widely spaced faults, whether flat or listric, have been inadequate. Our field investigations, combined "ith the recognition of volume problems associated ~ ~ with rotation of rigid fault blocks, suggest that ®~" 11,1,,' II h h " attempts to explain the rotation of upper-plate blocks ...... ". " .." "",,,~ by widely spaced normal faults is an unrealistic and A"" " " simplistic approach to the problem. Normal faulting within the upper plate is much more extensive than has previously been recognized - to the extent that microfaults are penetratively developed. Such pene­ trative deformation appears to be essential in pro­ ducing the rotation and extension in upper-plate rocks throughout the detachment terrane.

Ironically, it seems that the abundance of faults is responsible for relatively few of them having been observed. The presence of penetrative faulting (and thus great numbers of faults) means that offsets along individual faults need be very small, on the order of a few centimeters or less. In rocks such as those at Savahia Peak, most of these faults are indistinguish­ Figure 12. Schematic diagram showing the devel­ able from joints. Very large faults certainly do opment of upper-plate faulting from time 1 through exist, but the fault blocks which they bound (such as time 3. As shown in these cross sections, the initial Savahia Peak) are not rigid blocks, rather they must fault geometries are simple, but as extension proceeds be penetratively deformed via microfaults. new faults develop to produce progressive back rota­ tion into the detachment surface. The final result is It is interesting to note that no listric faults a series of intersecting planar faults that, acting were observed in the course of this investigation. simultaneously, accomodate upper-plate extension and The curviplanar faults of the outcrop from the Buck­ rotation. This system of faults can be viewed as an skins cannot be considered listric because their approximation of a listric fault.

263 Figure 13. Intersecting planar normal faults producing rotation of upper-plate rocks immediately above the detachment fault (subhorizontal white line). This type of normal fault pattern is similar to the fault patterns that produce the rotation in the schematic diagram of figure 12. Exposed face at the right side of the photo­ graph is about 325 meters high. The detachment fault is exposed for a short distance in the center of the photo­ graph and is exposed for a distance of several meters in the area to the left of the photo. Photo courtesy of Eric Frost. irregular traces steepen as frequently as they Figure 12 is a simplified diagram showing how such a flatten. Similarly, preliminary interpretations of process might occur. This pattern of intersecting upper-plate fault geometries by others (Frost, 1979; planar faults is seen along the walls of the Hhipple Davis and others, 1979, 1980) suggesting that faults Hash gorge (Fig. 13). were listric have proved to be largely unsupportable (Frost, 1982; Davis and Anderson, 1982). These later He would also like to point out that simiplified studies have found that where the faults are well cross sections such as figure 5 and terms such as exposed for distances of several hundred meters, the "fault blocks" or "rotated blocks" tend to entrench fault surfaces appear nearly planar. Some of the in ones mind the idea of large rigid crustal blocks faults in this terrane are listric but most flatten rotated along widely spaced normal faults. It would only within a few meters of the detachment fault. be more accurate to view rotation along a system of faults and the "fault block" as a penetratively Such a fault geometry makes rotation of upper­ deformed fault-block complex. plate blocks particularly difficult to explain.. It 0 seems that rotations of 30-70 would require faults In conclusion, we wish to stress the following that flatten with depth. A possible solution to the points concerning upper-plate structure of the problem of rotation along straight faults is suggested detachment terrane: by exposures in the \fuipple Hash gorge (Frost, 1982). 1. Fault blocks should not be viewed as rigid Here the rotation is facilitated by a multitude of crustal blocks. planar faults, which, acting together as. a system, 2. Faults have various shapes and orientations. approximate listric faults. Initially, rotation may 3. Faults are penetratively developed. be facilitated along a fairly simple fault system, 4. Offsets along most faults are very s!'lall. but as the amount of rotation increases a more cODlplex 5. Listric faults (~stricto) are of rela­ system of faults is generated. As rotation proceeds, tively !'linor importance. originally high-angle normal faults become rotated to 6. Rotation is facilitated along a system of lower-angle faults along subsequently developed faults. planar faults. This type of fault development allows progressive back He do not pretend to be able to visualize the details rotation of upver-plate rocks as extension occurs. of such a complex system of faults. He do feel,

264 ho",ever, that Ive have not overstated the complexity California area, Abbott, P. 1.., ed., San Diego of the true fault geometry. Field evidence suggests State University, San Diego, California. and supports such penetrative deformation. Davis, G. A., Anderson, J. 1.., Frost, E. G., and Shackelford, T. J., 1980, Mylonitization and ACKNOHLEDGMENTS detachment faulting in the Ifhipple-Buckskin­ Rmvhide Mountains terrane, southeastern The authors are particularly grateful for the California and ",estern Arizona, in Tectonic input ",e received from Eric Frost. It ",as Eric ",ho significance of metamorphic core~omplexes of brought to our attention the problems of upper-plate the North American Cordillera, Crittenden, fault geometries ",hich ",e have addressed in this M. D., Jr., Coney, p. J., and Davis, G. H., paper. His suggestions and editing of this manuscript eds., Geol. Soc. America Memoir 153. have proved invaluable. Thanks also goes to Ruth Davis, G. H., and Coney, P. J., 1979, Geologic Hillemeyer for typing the preliminary and final development of the Cordilleran metamorphic manuscripts. He are indebted to Marathon Oil for core complexes: Geology, v. 7, p. 120-124. financial support of this investigation. Frost, E. G., 1980, Appraisal design data structural geology and ",ater-holding capability of rocks REFERENCES CITED in the Ifhipple Hash area San Bernadino County, California: Consulting for the United States Anderson, R. E., 1971, Thin-skinned distension in ",ater and po",er resources service, Boulder Tertiary rocks of southeastern Nevada: Geol. City, Nevada. Soc. America Bull., v. 82, p. 43-58. Frost, E. G., 1982, Structural style of detachment Anderson, R. E., 1977, Geologic map of the Boulder faulting in the l~lipple Mountains, California, City IS-minute quadrangle, Clark County, and Buckskin Mountains, Arizona: Arizona Nevada: U. S. Geological Survey, Geol. Quad. Geological Society Digest 15. Map GQ-139s. Heidrick, T., and Hilkins, 1980, Mylonitization, Anderson, R. E., 1978, Geologic map of the Black detachment faulting, and associated mineral­ Canyon IS-minute quadrangle, Mohave County, ization: Hhipple Mountains, California, and Arizona, and Clark County, Nevada: U. S. Buckskin Mountains, Arizona: Arizona Geol. Geological Survey, Geol. Quad. Map GQ-1394. Soc. spring field trip guide. Anderson, J. 1.., Podruski, J. A., and Ro",ley, M. C., Montadert, 1.., Roberts, D. G., Charpal, O. de, and 1979, Petrological studies in the "suprastruc­ Guennoc, P., 1979, Rifting and subsidence of tural" and "infrastructural" crystalline rocks the northern continental margin of the Bay of of the Ifhipple Mountains of southeastern Biscay, in Initial Reports of the Deep Sea California: Geol. Soc. of America Abstracts Drilling Project, L. Montadert, D. G. Roberts ",ith Programs, v. 11, no. 3, p. 66. et al., v. 48, p. 1026-1060, U. S. Government Carr, H. J., Dickey, D. D., and Quinlivan, H. D., Printing Office, Hashington. 1980, Geologic map of the Vidal NH, Vidal Proffett, J. M., Jr., 1977, Cenozoic geology of the Junction and parts of the Savahia Peak SH and Yerington district, Nevada, and implications Savahia Peak quadrangles, San Bernardino for the nature and origin of Basin and Range County, California: U. S. Geol. Survey Map faulting: Geol. Soc. America Bull., v. 88, 1-1126. p. 247-266. Crittenden, M. D., Jr., Coney, P. J., and Davis, Rehrig, H. A., and Reynolds, S. J., 1980, Geologic G. H., eds., 1980, Tectonic significance of and geochronologic reconnaissance of a metamorphic core complexes of the North north",est-trending zone of metamorphic com­ American Cordillera: Geol. Soc. America plexes in southern Arizona, in Tectonic Memoir 153, 490 p. significance of metamorphic core complexes Davis, G. A., and Anderson, J. 1.., 1982, Diverse of the North American Cordillera, Crittenden, mechanisms, since mid-Cretaceous time for M. D., Jr., Coney, P. J., and Davis, G. H., northeast-south",est crustal extension in the eds., Geor. Soc. America Memoir 153. Colorado River area bet",een Arizona and Shackelford, T. J., 1976, Structural geology of the California: Geol. Soc. of America Abstracts Ra"'hide Mountains, Mojave County, Arizona: ",ith Programs, v. 14. Unpub. Ph.D. dissertation, University of Davis, G. A., Anderson, J. 1.., Frost, E. G., and Southern California, Los Angeles, California, Shackelford, T. J., 1979, Regional Miocene 175 p. detachment faulting and early Tertiary (?) Ste",art, J. H., 1980, Regional tilt patterns of late mylonitization Ifhipple-Buckskin-Ra",hide Cenozoic basin-range fault blocks, ",estern Mountains, southeastern California and ",estern United States: Geol. Soc. America Bull., Arizona, in Geologic excursions in the southern v. 91, p. 460-464.

265 Vie" looking west of repetitions of the Palen Pass thrust fault, Palen Pass, southeastern California. The light-colored rocks that form the knob in the foreground are overturned limestones of the Kaibab Limestone that are structurally above structurally interleaved arenites of the Palen Formation and porphyritic rhyo­ dacite. The Coxcomb Mountains are on the skyline.

266