GEOLOGY AND GEOCHEMISTRY OF THE VOLCANIC ROCKS IN THE RIVER MOUNTAINS, CLARK COUNTY, NEVADA AND COMPARISONS \-11TH VOLCANIC ROCKS IN NEARBY AREAS

Eugene I. Smith Department of Geoscience University of Nevada, Las Vegas Las Vegas, Nevada 89154

ABSTRACT to the west, low-angle faults are common in the volcanic section. These faults are probably Two chemically distinct suites of igneous rock readjustment structures in the upper plate. were formed in late-Miocene time in the River Mountains. Initial volcanic activity was alkalic in High-angle normal faults truncate the low-angle nature and built a stratovolcano of andesite and faults and are the dominant structural feature of the dacite in the southeastern part of the range. This range interior. These faults record a northeast­ episode of activity was followed by the eruption of southwest directed extension that was probably flows of subalkalic dacite. These lavas are commonly imposed on the range by the strike-slip faults of the well flow-banded and are interbedded with mud-flow Lake Mead Fault system. An earlier extensional event breccias, thin pyroclastic units, and flows of alkali of unknown origin is indicated by numerous east-west olivine-pyroxene basalt and andesite. Rare earth trending dikes. element data suggest that fractional crystallization was not important in producing either the alkalic or The strike-slip faults of the Lake Mead Fault subalkalic suites, rather they may have been formed system bound the River Mountains to the north and by the partial melting of a heterogeneous crust south and may be responsible for transporting the (probably Precambrian in age). The basalts were range as far as 40 km to the southwest from an probably derived from the mantle by partial melting. original site in the northern part of the Black Mountains of Nevada and Arizona. Stresses imposed on The River Mountains are chemically distinct when the range during strike-slip faulting may have compared to nearby volcanic ranges. The volcanic produced the discordant orientation of structural section in the River Mountains is more alkalic, and blocks within the range. displays a narrower range of Si02 values than volcanic rocks in either the McCullough or Eldorado INTRODUCTION Mountains. In addition, alkali basalts in the River Mountains show stronger iron enrichment than those in Areas adjacent to the Colorado River in Nevada, the other ranges. Also, the basalts of the River Arizona and California display numerous structures Mountains are nepheline normative, whereas those in that were formed during a major Tertiary-aged the McCullough and Eldorado Mountains are hypersthene extensional event. These structures include: normative. Both the McCullough and Eldorado ranges 1) low angle detachment faults well documented in the contain two major groups of lava flows that are \-Ihipple and Rawhide Mountains of California and separated by an ash-flow tuff. However, there are Arizona (Davis and others, 1979; Shackelford, 1976); small but important differences in chemistry that 2) listric normal and associated low angle faults in throw doubt on the direct correlation of units the Eldorado Mountains of Nevada (Anderson, 1971); between ranges. The distinctive chemistry of the 3) strike-slip faulting in the Lake Mead Region volcanic sections in the River, Eldorado and (Longwell, 1960; Anderson, 1973; and Bohannon, 1979); McCullough ranges suggests that each area records the and 4) numerous normal faults bounding and within development of a separate cogenetic assemblage of most of the ranges adjacent to the Colorado River volcanic rocks. (Longwell and others, 1965; Anderson, 1978; Steward, 1980). The structural geometry of the River Mountains is characterized by fault-bounded blocks each having The River Mountains lie within this extensional different structural orientation. This terrain, and are composed of a thick pile of pattern was probably imposed on the range by three Tertiary-aged dacites, andesites and basalts. They major structural events. The first and most represent one of four major volcanic ranges in the speculative of these events was the detachment of the western part of the Lake Mead area (Fig. 1); besides volcanic section and possibly part of the Precambrian the River Mountains, these include the McCullough complex from the Precambrian basement along a low­ Mountains (Longwell and others, 1965; Hewitt, 1956; angle fault. Evidence for this structural episode Bingler and Bonham, 1973; Anderson, 1977), the includes: 1) a low-angle fault on Saddle Island Eldorado Mountains (Anderson, 1971) and the Cleopatra (just to the east of the River Mountains) that is volcano in the northern part of the Black Range defined by well developed foliation (mylonitic (Anderson, 1973). Chemical data are presently fabric(?)) in Precambrian chlorite schist and available for the McCullough Range (Kohl, 1978; amphibolite. The fault separates an upper plate Johnson and Glynn, 1981), the Eldorado Range schist and gneiss terrain intruded by pegmatite, (Anderson, 1978; Johnson and Glynn, 1981), and the granite, rhyolite and basalt from a lower plate River Mountains (this report). intruded only by pegmatite. Since two lithologically and structurally different terrains are juxtaposed Previous investigations in the River Mountains along this fault, displacement was probably were mainly reconnaissance in nature. Longwell considerable. 2) Intensely brecciated volcanic and (1936) mapped a small part of the eastern River plutonic rocks to the west of Saddle Island may Mountains including the Precambrian rocks of Saddle reflect shattering during transport. These rocks Island as part of his study of the Boulder reservoir may lie just above the detachment fault. 3) Farther floor. Subsequently, he mapped the entire range, and

41 described the River Mountains stock as well as the limestone blocks that are wedged into the volcanic section just to the north of Boulder City (Longwell, 1963). McKelvey and others (1949) studied a small part of the northern River Mountains near the Three Kids manganese mine and recognized the anticlinal nature of the northern half of the range. As part of the site survey for the River Mountains tunnel, geolog~sts from the U.S. Bureau of Reclamation completed a detailed geological strip map along the route of the tunnel (Westcamp and Pavlovich, 1968). Anderson and others (1972) dated the rocks of the River Mountains as late-Miocene by the K-Ar technique, and later Anderson (1977) mapped a small part of the southern River Mountains near Boulder City as part of his work in the Boulder City IS' quadrangle. Recently Brenner and Glanzman (1979) suggested that the River Mountains are the site of a large caldera. This hypothesis was refuted by Bell and Smith (1980) in their geological study of the Henderson Quadrangle. The general geology of the Figure 1. Index map of the Lake Mead region. BC­ River Mountains and the relationship of strike-slip Boulder City. BCP-Boulder City pluton, CV­ faulting to volcanism were discussed by Smith (1979, Cleopatra Volcano, DP-Devil Peak, ED­ 1981). Eldorado Mountains, FM-Frenchman Mountain, LM-Lake Mead, M-McCullough range, RPP­ This paper is based on the current geological Railroad Pass Pluton, TM-Table Mountain, and geochemical research now being conducted at the VM-Virgin Mountains, \{RP-Wilson Ridge University of Nevada, Las Vegas. The paper presents Pluton. Lined areas represent the the first chemical data for the volcanic rocks of the approximate distribution of volcanic rocks River Mountains and speculates on the petrogenetic of Tertiary age. The shaded pattern and structural evolution of the mountain range. represents the approximate distribution of Chemical comparisons of the volcanic section in the intrusive rocks of Tertiary age. River Mountains with volcanic rocks in nearby ranges River Mountain (Bell and Smith, 1980) form the River show that each mountain range is composed of a Mountains stratovolcano in the southeastern part of chemically distinct volcanic pile and that each the mountain range. These andesite flows are cut by volcanic section probably originated from a different dikes and sills of dacite. Coring the stratovolcano source area. No key unit extending from range to is a quartz monzonite stock, that was dated by range was identified. This report is based on work Anderson and others (1972) at about 13 m.y. old. in progress, therefore interpretations may change as The stock is surrounded by an aureole of intensely more data becomes available. intruded and altered volcanic rock of the stratovolcano. This transition zone from stock to BRIEF DESCRIPTIONS OF MAJOR ROCK TYPES volcanic rock (the aureole) was first described by Longwell and others (1965). Wedged into the southern Volcanics of Powerline Road and Bootleg Wash part of the stratovolcano are blocks of well-bedded limestone, identified in part as the Banded Mountain The Powerline Road and Bootleg Wash volcanic Member of the Bonanza King Formation (Wernicke, sequences (Bell and Smith, 1980) represent the personal communication, 1981). The limestone blocks thickest accumulation of volcanic and volcaniclastic were partially consumed by intrusive rock related to rock in the River Mountains (Fig. 2). The Powerline the stock (Smith, 1981). These exposures probably Road section is composed of numerous flows of banded represent exotic blocks that were emplaced as a dacite containing plagioclase, biotite and hornblende result of strike-slip motion along the Lake Mead phenocrysts. Flows are interbedded with pyroxene and Fault System. The nearest source area for the olivine basalts, thick sections of mud-flow breccia, limestone lies 60 km to the northeast in the South thin pyroclastic units, and autobrecciated dacite Virgin Mountains (Fig. 1). The coeval relationship flows. Sources for the dacite flows are small between the emplacement of the limestone blocks, structurally elongated dacite domes. The source for volcanic activity, and strike-slip faulting suggests the basalt section is probably in the northern part that igneous activity and strike-slip faulting were of the range where a thick section of basalt, contemporaneous in this area (Smith, 1981). basaltic agglomerate and breccia crops out. To the east of the River Mountains stratovolcano The volcanics of Bootleg Wash underlie the is a thick section of steeply dipping dacite (well Powerline Road units, and are composed of an upper flow banded with phenocrysts of plagioclase, dacite flow containing plagioclase, biotite, hornblende and biotite) and basalt (augite and hornblende and quartz phenocrysts; a central plagioclase phenocrysts) that may be equivalent to epiclastic unit locally interbedded with a dacite the Powerline Road volcanic section. These flows flow; and a lower unit of andesite and andesite are separated from a thick section of breccia (mud­ breccia. The volcanics of Bootleg Wash are flow and autobreccia) by a low-angle fault. The restricted in occurence to the sourthern margin of upper plate breccias, named the volcanics of Teddy the interior valley (Fig. 2). Bear Wash by Bell and Smith (1980), are composed of numerous angular fragments (up to 1 m in size) of Volcanics of River Mountain and adjacent plagioclase-biotite dacite; little or no structure volcanic sequences (bedding or banding) is observed within this massive unit. The impressive cliffs along the eastern flank Porphyritic andesite flows of the volcanics of

42 :: Saddle • Isla

\ \ \ \ \ \ \

.-.-'"L.-· .--' -'- ,; -' q"'/' .;:,'iJ,/ ;.' / / / / / /'{ " 'j miles , iI

,i .--'/ Boulder ,/'-'-'-'-' City ,// .-,..--- ...---

Figure 2. General Geologic map of the River Mountains, Clark County, Nevada.

43 EXPLANATION numerous plagioclase-biotite bearing dacite dikes. Several low angle faults, originally described by Volcanics of 0]\':,\' Intrus~ve. rock Anderson (1973), separate highly altered and intruded Powerline Road -." lavas below from less highly altered and rarely Volcanics of B Exotic Blocks of intruded lavas and volcaniclastic rocks above. Because of similarities in lithology, I infer that t;;;;j Limestone Bootleg Wash the volcanics of Red Mountain are the highly altered equivalents of the volcanics of the River Mountain Alkali Basalts Limestone and stratovolcano. and Andesites Sandstone Precambrian Rocks on Saddle Island and adjacent brecciated exposures

Precambrian rocks crop out on Saddle Island (Fig. 2) and form two lithologically and structurally ~ Volcanics of Allochthon on distinct terrains. The northern part of the island ~ D Red Mountain Saddle Island is characterized by a basement of amphibolite and chlorite schist intruded by dikes and small stocks R..·.·.. ·.·.··. Volcanics of ~Autochthon on of: 1) red coarse-grained porphyritic granite U River Mountain ~ Saddle Island (quartz, orthoclase, biotite), 2) pegmatite (quartz, orthoclase, plagioclase, muscovite), 3) porphyritic River Mountain rhyolite (quartz, plagioclase, and biotite), and 4) DDikes basalt (plagioclase, pyroxene). Although the age of Stock these intrusions is unknown, a Precambrian age for the basalt and rhyolite dikes can be ruled out since Volcanic Source Illl11 Poss ible Volcanic several of them intrude a carbonate breccia of ~ Areas WU]Source Area probable Paleozoic age (described below). At Moon Cove near the northern tip of Saddle Island are General Y Dip-slip Fault several exposures of red quartzite, and carbonate ;/ Structural bearing breccia (Fig. 2). The quartzite is similar in lithology to the Cambrian Tapeats Sandstone; Orientation ./Low-angle Fault alternatively it may correlate with quartzite in the late-Precambrian section. The breccia contains Strike-slip clasts of limestone, granite and quartzite; the /I Fault dolomite clasts are almost certainly Paleozoic in age. These units lie unconformably on a Precambrian erosion surface of moderate relief, and 12 represent the ~nly "in situ" Paleozoic deposits in A this area. The southern half of the island is formed by chlorite schist, well foliat~d quartzo-feldspathic 0 gneiss and amphibolite and is intruded by pegmatite 10 " " .. dikes. Separating these terrains is a major low­ .. .. angle fault (strike N 80 E, dip 30 to the north) that 0/ D .. passes through the prominent saddle that gives the o N 8 island its name. This low-angle fault will be ::.:: .. discussed in detail in the structure section of this 6 .. paper . N .. 6 :\\V Z'" I>-\~~ :\\V Severe brecciation is common in volcanic and ~~~ '<>.;;'O~ plutonic rocks (probably Tertiary in age) just to the .. west and south of Saddle Island. Near the Boulder 4 .... Beach Marina (Fig. 2), low hills composed of intensely autobrecciated granite are intruded by dikes of brecciated biotite-bearing lamprophyre. .. Volcanics of Powerline Road Contacts between granite and dike rock can be traced o Volcanics of Bootleg Wash from clast to clast in the breccia in a continuous " Volcanics of the River Mountains manner. To the west of these outcrops are exposures D River Mountains Stock of brecciated dacite flows of Tertiary age, that are v Volcanics of Teddy Bear Wash O+--...:-::..;..:::.:-~~...:....::..:~..::..;..:=---,..:..::..--.-,-----.---r-.---i also intruded by lamprophyre dikes. 46 50 60 70 Weight Percent Si0 GEOCHEMISTRY 2 Figure 3a. Harker variation plot that differentiates Techniques between the alkalic and subalkalic suites. The field boundary is from Irvine and Major elements and Ba were analyzed by the Baragar (1971). inductively coupled argon plasma (ICAP) technique, of the mountain range just to the west of Lake Mead and x-ray fluorescence spectroscopy (XRF) was used are formed by these breccias. for Rb and Sr (for samples done at Technical Service Laboratories in Toronto). In five additional samples, Lying to the west of the River Mountain the major elements and Sr and Zr were analyzed by stratovolcano are the volcanics of Red Mountain (Bell Atomic Absorption Spectrophotometry and XRF in the and Smith, 1980). This unit is formed by highly Rock Chemistry Laboratory at the University of Nevada, altered and oxidized gray to light red porphyritic Las Vegas. The rare earth elements (REE) were lavas of intermediate composition, and is cut by obtained by instrumental neutron activation analysis

44 at the Phoenix Memorial Laboratory of the University River Mountain stock, but not the stock itself, show of Michigan. Accuracy for REE with concentrations alkaline affinities. On the other hand, dacites of less than 1000 ppm is 10 percent except for Nd, Tb the Powerline Road, Bootleg Wash and the intrusive and Yb which is 20 percent. The multielement rocks of the River Mountains Stock are distinctly standards G2, BCV-l, and HBVO-l were used as internal subalkaline (Fig. 3a). The two suites show several standards for all of the above determinations. A important differences in chemistry. For example, the total of 20 new chemical analyses were obtained for rocks of the alkalic suite are lower in CaO and Fe203 the River Mountain rocks (Table 1). Rock names are than comparable rocks of the subalkalic suite from the chemical classification of Irvine and (Figs. 3b and 3c). Within the subalkalic suite, Baragar (1971), and were determined by a Fortran IV rocks of the Bootleg Wash sequence can easily be program by Smith and Stupak (1978). distinguished from the dacites of the Powerline Road section by lower values of A1203, Sr and Ba (Figs. 4a Results for the dacites and 4b). The rocks of the alkalic suite represent the initial volcanic pulse in the River Mountains. Two chemically distinct suites of dacites were This episode was followed closely by the eruption of identified in the River Mountains. Rocks of the the rocks of the subalkalic suite. River Mountain stratovolcano, the Teddy Bear Wash breccias and several dikes that extend from the The dacites show light rare earth element l Table 1. New Chemical Analyses for Volcanic Rocks in the River Mountains.

Unit Tpb Lam Tpd Tpa Tbd Tba Trs Tra Trd Ttbd 2,3 No. 3 5 2

SiO~ 47.11 47.11 67.07 56.52 67.66 59.51 66.04 53.49 65.58 56.61 A1203 14.52 14.32 14.36 15.81 12.18 13.14 14.83 15.01 14.69 16.96 TiO? 1. 94 1. 73 0.48 0.90 0.57 1.14 0.53 1.08 0.48 0.79 Fe203 12.07 10.78 4.08 9.21 5.63 6.91 5.92 7.80 4.85 5.79 CaO 10.85 5.70 2.56 6.05 2.06 4.59 2.61 5.15 1. 39 4.39 MgO 7.08 6.88 0.91 3.89 0.42 1.41 1.43 3.75 1.12 3.03 MnO 0.15 0.13 0.06 0.11 0.01 0.09 0.06 0.09 0.04 0.07 Na20 2.95 3.53 3.90 3.74 2.40 3.03 3.95 4.12 4.73 3.98 K20 1.32 5.43 4.35 2.41 7.65 5.32 3.68 3.20 5.48 5.20 P205 0.88 1.68 0.16 0.59 0.22 0.50 0.18 0.65 0.12 0.50 LOI 1. 99 1.69 1.41 1.03 0.75 3.26 0.10 4.71 1.12 1.44 TOTAL 100.86 98.98 99.34 100.24 99.55 9,'0.90 99.33 99.05 99.60 98.77 Ba 2191. 63 3049.30 1472.62 2355.95 774.35 1120.70 1512.00 1375.50 1576.10 16211.50 Cr 186.52 62.78 49.20 88.06 175.83 98.33 177.36 161.85 92.35 35.62 Zr 312.00 172.00 374.00 Rb 38.83 126.05 175.39 99.63 199.26 160.66 135.80 99.15 127.84 139.04 Sr 1020.33 482.00 272.00 874.00 30.00 33.00 121.00 351. 00 66.00 405.00 La 125.24 153.64 58.80 120.24 47.66 53.55 52.27 74.51 57.69 75.94 Ce 274.19 360.14 109.24 258.52 102.74 120.25 101. 00 163.57 105.49 157.15 Nd 113.90 168.15 39.11 105.55 38.47 56.53 51.46 75.49 "35.14 77 .89 Sm 15.10 20.73 5.24 13.58 5.81 7.49 5.20 10.60 5.23 9.12 Eu 3.93 5.11 1.18 3.33 1.11 2.09 1.13 2.51 1.09 2.22 Tb 2.72 2.79 0.73 1.63 0.31 0.54 0.94 0.73 0.22 1.11 Yb 1.89 2.27 1. 52 2.37 1.42 2.00 1.77 2.44 1.63 1.69 Lu 0.33 0.35 0.26 0.37 0.22 0.29 0.27 0.36 0.29 0.30 Th 18.66 17.95 19.04 16.02 14.82 10.75 14.01 20.73 17.90 15.74 Hf 7.74 8.91 4.70 7.95 4.05 5.05 5.02 6.63 5.58 5.81 Sc 29.56 23.79 4.85 14.64 7.48 14.93 5.93 14.57 4.51 11.04 Na20/K20 2.23 0.65 0.95 1.55 0.31 0.57 1.07 1. 29 0.86 0.77 FeO/ (FeO+MgO) 0.63 0.61 0.82 0.70 0.93 0.83 0.81 0.68 0.81 0.66 Ba/Sr 2.15 6.33 5.41 2.70 25.81 33.96 12.50 3.92 23.88 4.01 Rb/Sr 0.04 0.26 0.82 0.11 6.64 4.87 1.12 0.28 1. 94 0.34 Sum REE 537.30 713.18 216.08 505.58 197.64 242.74 214.04 330.21 206.78 325.42 La/Lu 372.22 438.97 222.97 324.97 216.64 184.66 193.59 206.97 198.93 253.13 EU/Sm 0.27 0.25 0.23 0.25 0.19 0.28 0.22 0.24 0.21 0.24

lAnalyses performed by Technical Services Laboratories, Toronto, Canada; UNLV Rock Chemistry Laboratory; and the Phoenix Memorial Laboratory of the University of Michigan. 2 Number of analyses averaged

3Major elements as weight percent; trace elements as ppm. Unit symbols: Tpb = Powerline Road basalts, Lam = lamprophyre dike, Tpd = Powerline Road dacites, Tpa Powerline Road andesites, Tbd = Bootleg Wash dacites, Tba = Bootleg Wash andesites, Trs = River Mountains stock (quartz monzonite), Tra = andesites of the River Mountain stratovolcano, Trd = dacites of the River Mountain stratovolcano, Ttbd = dacite (clast from breccia in the Teddy Bear Wash sequence).

45 equivalence of REE patterns for the alkalic and subalkalic suites, and the lack of a prominent Eu anomaly suggests that feldspar and/or hornblende 14 dominated differentiation was not important for the generation of either suite or in the formation of one suite from the other. Rather, it is probable that both suites were derived from source rocks already 12 enriched in the light REE, such as the granites and gneisses of the Precambrian basement. This situation fi 6 is currently being modeled and the results will be 10 reported in another paper. (') 0 Results for the basalts and andesites N Ql U- 8 Basalts in the River Mountains are olivine ...t:: Ql normative and strongly alkalic in character. Iron U 0 0 Q; enrichment is greater than for the typical basin-and­ range alkalic basalts described by Leeman and Rogers 0... 6 ... (1970) (Fig. 6). REE patterns for the basalts show .t:: 6 0 Cl 6 0 extremely high light REE enrichment (Fig. 7). For 'w 60 0 $: example, La is enriched from 340 to 480 times that of 4 chondrites, and the lamprophyre dike shows 600 times enrichment. All rocks display steep slopes for the o River Mountains (Subalkalic) heavy REE, and either have a shallow Eu anomaly or 6 River Mountains (Alkalic) lack it entirely. These trends are similar to those 2 +----,....:---,-----,,---,--;--,----'---,------,----.----.---.--.----1 for alkalic basalts in other areas (for example, 46 50 60 70 Campbell and Gordon, 1980). The andesites are also Weight Percent Si0 2 strongly alkalic, but do not show the same extremes of light REE enrichment displayed by the basalts (Fig. 7). The fact that the basalts and andesites 12~------...., show greater light REE enrichment than the associated dacites suggests that the mafic rocks do not represent the parent material for the dacites (by either a process of partial melting or fractional 10 crystallization). The basalts and andesites probably formed independently of the dacites by partial melting of mantle material. 8 Conclusions 0 0 C1l 0 U Based on the chemical data presented above the 6 ...t:: following tentative scenario for the formation of the Ql U River Mountain volcanic suites can be presented. Q; 0 0... 1) Alkalic andesites and dacites formed the ... 4 .t:: o 0 River Mountain stratovolcano in the southeastern part Cl o Ow o of the range. Activity centered on the River $: Mountain stock resulted in several east-west trending o 2 o dikes that cut the western flank of the volcano.

o River Mountains (Subalkalic) 2) Subalkalic dacites with associated breccias " River Mountains (Alkalic) and thin pyroclastic units produced the voluminous O+----,----.-----,,---,--i--,----'---,------.,.---.----.--,-----,------i volcanics of Powerline Road. These units erupted 46 50 60 70 from numerous domes adjacent to the stratovolcano. Weight Percent Si0 2 During this event the River Mountain stock was intruded into the core of the stratovolcano. Dacite Figure 3b. The volcanic rocks of the alkalic suite eruptions were accompanied by extrusion of alkalic are generally lower in total iron (as basalts and andesites with sources mainly in the Fe203) than the rocks of the subalkalic northern part of the range. suite. 3c. The volcanic rocks of the alkalic suite CHEMICAL CO~ITARISONS WITH NEARBY RANGES in the River Mountains have lower values of CaO than similar rocks of the sub­ Modern chemical data is available for volcanic alkalic suite. rocks in several nearby ranges. Anderson (1971), and Johnson and Glynn (1981) reported analyses for the Eldorado Mountains, and Kohn (1978), and Johnson and Glynn (1981) provided chemical data for the enrichment, lack a prominent Eu anomaly, and have a McCullough range. Unfortunately there is an inherent relatively flat heavy rare earth pattern (Fig. 5). danger when comparing chemical data from different Modelling studies for the formation and subsequent rock laboratories, because of differences (sometimes differentiation of these suites are currently under­ major) in analytical precision and accuracy. As a way, so little can be said about their petrogenesis consequence, the reader should take care when at the present time. However, visual inspection of evaluating conclusions presented in this paper that the REE and major element plots allows several were made by comparing different data sets. We are speculative conclusions to be made. For example, the

46 attempting to rectify this problem by analyzing a large number of rocks from each volcanic range in the 12 Rock Chemistry Laboratory at UNLV. Hopefully, in the near future a common set of chemical data for volcanic rocks in the Lake Mead area will be 10 available for comparative studies.

Comparison of the River Mountains with the Eldorado and MCCullough Mountains 8 ('I) The following chemical characteristics o o· distinguish the volcanic rocks of the River Mountains N « 6 from those in the Eldorado and McCullough ranges: .... 6 • c:: Q) (J o " 0 1) The volcanic rocks of the River Mountains are 0 M Q; 0 limited in compositional range when compared to the 0.. " .... 4 Eldorado and McCullough samples (Fig. 8). Rocks with .c:: O'l .. a SiOZ content of greater than 69% are not present 'cu in the River Mountains, but are common in the Mt. 3: .. Davis, Patsy Mine and Tuff of Bridge Spring section 2 in the Eldorado range and in the Erie Tuff and o River Mountains Mt. Davis volcanics in the McCullough range. There .. Bootleg Wash Volcanics • Trachytes and latites of the McCullough Range is a compositional gap between 49 and 53% SiOZ for O+--.-,--'-r-,------,-,------.r--.--,....:-.--r--.-----l the River Mountains section. In comparison, it is 46 50 60 70 common for rocks of the Mt. Davis and Patsy Mine Weight Percent Si0 sequences in the other ranges to have SiOZ values 2 that fall within this gap. Most of the intermediate and felsic rocks of the River Mountains have SiOZ values between 57 and 69%, but only a few samples of the Patsy Mine and Mt. Davis volcanic sequences have B values of SiOZ that fall within this range. An important exception is the latite and trachyte dome­ 1000 flow complex in the McCullough range. These rocks, however, differ from the River Mountains samples by B being higher in AlZ03 and lower in normative albite A (Figs. 4a and 9). G A Z) Based on the alkali-lime index (Peacock index), the River Mountains samples are more alkalic than rocks of the other ranges (for the River G Mountains the alkali-lime index is 55; the Eldorado G Mountains 57 and the McCullough range 57). EQ, Q, G 3) Basalts in the River Mountains show stronger I:" iron enrichment than either the basalts of the C/) 100 McCullough or Eldorado Mountains (Fig. 6). Also the basalts of the River Mountains are nepheline GG normative, whereas basalts in the Eldorado and McCullough Mountains are hypersthene normative. In terms of total volume, basalts and andesites are comparatively rare when compared to the other ranges.

Overall chemical variation in the River Mountains is similar to that of the Eldorado range in '" Volcanics of Bootleg Wash that early flows (Patsy Mine volcanics) are alkalic G Subalkalic Dacites and later activity (Mt. Davis volcanics) is B Alkalic Basalts subalkalic (see Anderson, 1978). A similar history A Alkalic Andesites is weakly displayed in the McCullough rocks (Kohl, 10 -1-----,---,---,---r-----j 1978). It should be noted that the River Mountains 500 1000 1500 2000 2500 3000 lack a major ash-flow sheet that separates the alkalic and subalkalic suites of rocks such as that Ba(ppm) found in the other volcanic sections. Figure 4a. The volcanic rocks of Bootleg Wash can be Comparisons between the volcanic rocks in the distinguished from the other volcanic MCCullough Range and Eldorado Mountains rocks of the River Mountains by being lower in AlZ03. Also the trachytes and In the Eldorado Mountains, Anderson (1971) latites in the McCullough range (Kohl, divided the volcanic section into three mappable 1978) are higher in A1 Z0 3 than comparable units (from youngest to oldest): the Mt. Davis rocks in the River Mountains. volcanics, the Tuff of Bridge Spring and the Patsy 4b. The volcanic rocks of Bootleg Wash can be Mine volcanics. Kohl (1978) in his study of the distinguished from the other volcanic western McCullough range recognized the same three rocks of the River Mountains by having units, however he named the ash-flow unit the Erie lower values of Sr and Ba. Tuff, a name used earlier for the same unit by Barton

47 and Behre (1954) and by Hewitt (1956). A comparison 400 -,------, of chemical data of Anderson (1978) and Kohn (1978), and the recent data of Johnson and Glynn (1981) throws doubt on the direct correlation of units between ranges. Chemical evidence includes:

1) Mt. Davis volcanics in both ranges are distinctly bimodal in character, however more 100 silicic lavas are found in the McCullough range (up to 77% Si02) (Fig. 8).

2) The McCullough lavas have higher CaO contents 't:l'E (9-12%) than their Eldorado Mountain counterparts c: o (usually less than 8%). ..c: U '­ 3) The Tuff of Bridge Spring in the Eldorado .gc: Range is distinctly higher in Si0 (Fig. 8), and 2 ~ lower in Zr, Ba and Sr when compared to the Erie ..c: Tuff in the McCullough range (Table 2 and Fig. 10). Q) u 10 c: These differences suggest that either the Erie and o Bridge Spring tuffs are different units or that the U Erie Tuff represents a phase of the Tuff of Bridge Spring not recognized in the Eldorado Mountains. Alkalic Dacites

4) The Patsy Mine volcanics in the Eldorado Subalkalic Dacites Mountains have a greater compositional range (51-76% Si02 ) than the Patsy Mine lavas in the McCullough range (centered at 52%). Silicic lavas common in the Patsy Mine sequence in the Eldorado Mountains are apparently lacking in the McCullough range (Fig. 8). 1+--.-----,---,----,---,------1 La Ce Nd Sm Eu Tb Lu 5) Andesites and basalts in the McCullough range display greater enrichment in iron than similar rock types in the Eldorado Mountains (Fig. 6). Figure 5. Rare earth element plot comparing the alkalic and subalkalic dacites of the River In conclusion, important chemical differences Mountains. between the volcanic rocks in the River, McCullough and Eldorado Mountains make direct correlations of rock units between ranges doubtful. It follows that the volcanic sequences in each range erupted from F separate source areas. • River Mountains (-1 Eldorado Mountains Discussion '-' o McCullough Mountains Differences in chemical characteristics of a Basin and Range Basalts volcanic rocks between ranges suggests that each area from Leeman and records the development of separate volcanic piles Rogers(1970) each containing a cogenetic sequence of lavas. Development of these volcanoes was probably coeval and spans an age from 12-15 m. y. ago (Anderson and others, 1972). .. The Tuff of Bridge Spring in the Eldorado Mountains probably did not originate in the Lake Mead area. Flow direction studies using the .,'. ·CfZj,,--- techniques of Elston and Smith (1970), and Rhodes lit • ", ", and Smith (1972) suggest a source to the southwest " ... '------,' for this unit, possibly in the Mojave Desert of .;;A California (Brandon, 1979). The Erie Tuff in the McCullough Mountains may be a local unit originating in a latite and trachyte dome complex in the western part of the McCullough range as originally suggested by Hewitt (1956), or it may have as its source the volcanic center at Devil Peak (Fig. 1) described by Walker and others (1981).

The presence of an ash-flow unit between two predominately mafic sequences does not necessarily Figure 6. A(Na 0+K 0), M(MgO), F(FeO+O'9Fe 0 ) plot infer that the unit above the tuff is the Mt. Davis 2 2 2 3 comparing the volcanic rocks of the River, sequence and the unit below is the Patsy Mine McCullough and Eldorado Mountains. sequence. Supporting geochemical and geochronological data must be presented to substantiate these correlations.

48 Chemical differences in felsic rocks between degrees south to 40 degrees north. Preliminary ranges may be due to the partial melting of a analysis of foliation measurements suggests that the heterogeneous crust of Precambrian igneous and lower plate is folded with east-,.,rest axes metasedimentary rocks. The exposed Precambrian rocks predominating. Just to the south of the fault plane of southern Nevada vary considerably in composition (in the lower plate) a poorly developed foliation in from rapakivi granite (Volborth, 1962) to chlorite chlorite schist trends northwest (N 20 W) and dips to schist. Partial melting of such a crust may result the west (25 to 55 degrees). Adjacent to the in the observed compositional differences in felsic detachment fault chlorite schist and amphibolite rocks between ranges. Basalts and andesites in display a well developed foliation (mylonitic general show smaller compositional differences than fabric?) that is coplanar with the fault plane do the more silicic lavas, and may originate by the (N 80 E strike, 30 degrees north dip). Within this partial melting of a relatively homogeneous mantle. 10...,------, STRUCTURAL GEOLOGY McCullough Mountains 8 Low-angle faults 6 The River Mountains have been affected by at least three major structural events. The earliest 4 and most speculative of them is the westward motion of the entire range on a west-dipping detachment fault. The most impressive evidence for the 2 detachment surface is a major low-angle fault within the Precambrian complex exposed on Saddle Island on o+=-J.::..~=+-..I.-"""-+;....I.-+--bf the west shore of Lake Mead. The fault separates 44 48 52 56 60 64 68 72 76 80 lithologically different terrains (see section on rock types above) and may either represent the detachment plane itself or it may reflect a 10 detachment fault higher or lower in the stratigraphic section. Foliation developed in chlorite schist in - Eldorado Mountains the allochthonous terrain constantly strikes N50 E±lO 8 - degrees, and dips steeply (60 to 70 degrees to the - east). On the other hand, foliation in the lower 6 - plate is mainly east-west with dips varying from 50 - - - Q r.;-::;: ... 4 D 0 700~------' (!) I-- - r- c-- .c 0 0 0 ~~ ~ E 2 Ip 0 pO 0 10 0~~ 'f~ P :J 01 Pip P PP P Iplp 0 p p P rPl Z o I I I 44 48 52 56 60 64 68 72 76 80

100 10 -.------, River Mountains Q) 8 ·E "C <: o 6 .l: U "­<: 4 o '.. ~ 2 ...<: Q) p g 10 O+..L.:....j-:-I-+..:...L:.~yp_+..:..L:.._r_'_r-_r----l o U 44 48 52 56 60 64 68 72 76 80

Basalts Weight Percent Si02 Andesites

Figure 8. Histograms showing the distribution of SiO J values for volcanic rocks of the McCullougn Eldorado and River Mountains. For the McCullough and Eldorado Mountains D=Mt. 1+---,-----.-----,--,---,.------..., Davis volcanics, P=Patsy Mine volcanics, La Ce Nd Sm Eu Tb Lu stippled pattern=Tuff of Bridge Spring­ Erie Tuff. For the McCullough range L= Figure 7. Rare earth element plot comparing the latite, T=trachyte. For the River Mountains alkalic basalts and andesites of the River P=volcanics of Powerline Road, R=volcanics Mountains. of River Mountain, TB=volcanics of Teddy Bear Wash, B=volcanics of Bootleg Wash.

49 Table 2. Chemical Comparisons of the Tuff of Bridge Spring and the Erie Tuff Orthoclase e River Mountains

Erie Tuff o Trachytes and latites of the McCullough Range Ba 128 139 153 106 Sr 74 97 106 74 Zr 231 239 210 276 Ti 1790 2080 2080 2010

Tuff of Bridge Spring Ba 677 369 263 460 Sr 339 247 201 349

Zr 462 389 224 307 o Ti 3440 2800 2180 3520 o o c o 0 0 o Data from Johnson and Glynn (1981) COD mylontic(?) zone, pegmatitic dikes are rotated into 00 o the plane of foliation and in places well-developed o boudinage is formed (Fig. 11). The low-angle fault is only exposed at one locality, on the east side of Saddle Island. Here, well-foliated chlorite schist grades upward into a two meter thick zone of brecciated and altered schist. Above this zone is a band of finely brecciated rock about 20 em thick and Albite Anorthite above this band is the fault surface (commonly mineralized by chalcedony). At this locality the Figure 9. Normative orthoclase-albite-anorthite plot fault strikes N 80 E and dips 30 to the north. comparing the trachytes and latites of the HcCullough range to the volcanic rocks of The Tertiary section to the west of Saddle the River Hountains. Island was affected by the detachment faulting in two significant ways. First, volcanic and intrusive rocks just to the west of Saddle Island are highly shattered. These units are intruded by lamprophyre dikes that are also intensely brecciated. The entire Ba(75%) mass may be autobrecciated and may reflect an under­ lying detachment fault. Second, within the volcanic section in the River Hountains there are numerous low-angle faults. All of these faults strike to the northeast and dip to the west. They roughly parallel the detachment surface on Saddle Island, and may provide evidence for readjustment of the upper plate during detachment sliding (Fig. 12). /_,..-Tuff of /~. ":, Bridge Spring I" , In summary, the Tertiary volcanic section in the '-'~,,;) River Hountains and part of the Precambrian basement may have slid to the northwest (probably by gravity) off a Precambrian metamorphic complex. The age of detachment faulting is probably late-Tertiary since volcanic rocks (Hiocene in age) were faulted and brecciated during the sliding event. The fault on Saddle Island probably represents substantial displacement, since it juxtaposes lithologically and structurally distinct terrains, however, it mayor may not be the detachment surface itself. Hotion along the Saddle Island fault was distributed within Sr(75%) Zr the rocks of the two plates adjacent to the fault. Rocks of the upper plate behaved in a brittle manner and formed local zones of breccia. In the lower plate, chlorite schist and amphibolite developed a cigure 10. Ba, Sr, Zr plot (using the dat a of pervasive foliation. Farther from the fault in the Johnson and Glynn, 1981) comparing the upper plate, volcanic rocks riding above the detach­ Tuff of Bridge Spring in the Eldorado ment surface were autobrecciated, and slip adjustment Hountains to the Erie Tuff in the along low-angle faults occurred. HcCullough Range.

High-angle faults other detachment terrains, detachment and high-angle faulting are more closely related. For example, in In the River Hountains the low-angle faults are the Eldorado Hountains, high-angle faults apparently not continuous, but seem to be cut by a set of high­ translate into low-angle structures (Anderson, 1971), angle dip-slip faults (Fig. 2). This relationship and in the Whipple Hountains, high-angle faults in suggests that detachment and extension represent the allochthon terminate against the detachment separate, but possibly closely related events. In fault. The high-angle faults in the Whipple

50 Mountains may reflect the extension of the upper structural complexity of the River Mountains, plate during gravity sliding (Davis and others, therefore represents the complex interaction of 1979). detachment, high-angle and strike-slip faults.

In the River Mountains, most of the high-angle As suggested by Smith (1981) strike-slip faults strike to the northwest; faults in the western faulting and igneous activity may be coeval along part of the range dip steeply to the west (60 to 80 the southern margin of the River Mountains. Here, degrees), and faults in the eastern part of the range blocks of limestone, probably Cambrian in age, have dip steeply to the east. This fault geometry been wedged into the volcanic section along splays suggests that the range was extended in a northeast­ of the left-lateral strike-slip fault that bounds southwest direction, since normal faults usually form the range to the south. These carbonate blocks were perpendicular to the direction of maximum extensional subsequently cut by intrusions related to the River stress. This stress direction may be related to Mountains stock. Rocks of the stock were dated by forces imposed by the strike-slip faults of the Lake Anderson and others (1972) at 12.6 m.y. old. These Mead Fault System (discussed below). relationships strongly suggest that faulting and intrusive activity occurred contemporaneously about An earlier extensional event is revealed by a this time. set of east-striking dikes in the central and southern parts of the range (see Fig. 2). The origin SOURCE AREAS FOR VOLCANIC ROCKS of the north-south stress orientation required to produce these structures is at present unknown. Vent areas for the volcanic rocks are scattered These dikes are displaced by the northwest trending throughout the range. The River Mountains normal faults. stratovolcano is the source for most of the alkalic andesite and dacite; the stratovolcano may also be Strike-slip faults the vent area for some of the more recent subalkalic dacite flows. Numerous domes and dome complexes High-angle faults merge with arms of the Lake represent the source areas for the dacite flows of Mead fault system (Bohannon, 1979). This the Powerline Road sequence. The domes are normally relationship is clearly seen along the southern obsidian rich and are characterized by steeply margin of the range where dip-slip faults drag to the dipping flow foliation. Several of the domes are east as they approach a left-lateral strike-slip associated with thin pyroclastic units. Commonly the fault (Fig. 2). The relationship between the two flow banding within a dome is aligned subparallel fault sets is unclear along the northern margin of to the structural grain. This suggests that the range because of widespread cover by Tertiary and extrusion and extension may be contemporaneous in Quaternary-aged sediments. However, it is extremely some areas of the mountain range. probable that the range is separated from the Frenchman Mountain-Rainbow Gardens area to the north In the central part of the range there is a by a major fault (probably another arm of the Lake kinney-shaped area (2 km by 1 km in size) where rocks Mead Fault System) that trends parallel to Las Vegas are highly altered and locally mineralized (magnetite Wash (Bell and Smith, 1980). and quartz), and flow banding in dacite is usually steeply dipping (Fig. 2). This area may represent The River Mountains may have moved as far as the source for many of the dacite flows in the 40 km to the southwest as a result of motion along central and northern part of the River Mountains. the Lake Mead Fault System. Fault displacement was The source for the alkali basalts in the Powerline calculated by locating similar rock types on both Road section probably lies in a thick section of sides of the fault and measuring the amount of pyroxene basalt, breccia, and agglomerate preserved apparent offset. Specifically, intrusive rocks in a north-trending graben in the north-central part similar to those in the River Mountains stock crop of the range (Fig. 2). out in the Black Range of Nevada and Arizona (the Wilson Ridge Pluton of Anderson, 1978) (Fig. 1). Bell and Smith (1980) demonstrated that there is If the River Mountain stock is indeed the northward no structural or petrological evidence for a caldera extension of the Wilson Ridge Pluton then at least in the River Mountains as originally suggested by 40 km of strike-slip displacement is required to Brenner and Glanzman (1979). No major ash-flow produce the present outcrop configuration. sheets are evident in the mountain range and no nearby ash-flow units can be conclusively related to Fault-bounded blocks with different internal a source area in the River Mountains. structural orientations are an important characteristic of the River Mountains. Apparently SU11MARY the structural geometry of the upper plate in the River Mountains differs from other well kno\vn Initial alkalic volcanism formed a large detachment terrains. For example, the Eldorado Range stratovolcano in the southeast part of the River is characterized by volcanic ridges that consistantly Mountains. The volcano was mainly composed of strike north-south (Anderson, 1971). In the Whipple hornblende andesite and numerous sills and dikes of Mountains volcanic units are faulted but constantly biotite-hornblende dacite. Volcanism became more dip to the southwest (Davis and others, 1979), and in subalkalic with the eruption of the lava flows of the the Eagle Pass detachment in southeastern Arizona Bootleg Wash and Powerline Road sequences. The (Davis and Hardy, 1981), Tertiary units have a subalkalic flows originated from local domal sources, relatively constant orientation. The discordant and possibly from a major source area in the north­ orientations of structural blocks in the River central part of the range. Also, the stratovolcano Mountains may be the result of the late-stage may have been the source for some of the subalkalic internal disruption (as revealed by the rotation of lavas as evidenced by a plug of subalkalic quartz structural elements) of the mountain range by strike­ monzonite and monzonite in its core (the River slip faulting (a structural ingredient that is not Mountains stock). The subalkalic dacite flows are important in the other detachment terrains). The interbedded with alkali pyroxene-olivine basalts and

51 the Lake Mead area besides the River Mountains (the McCullough, Eldorado, Cleopatra, and Devil Peak volcanoes).

Three structural episodes affected the River Mountains. The first and most speculative event is the detachment of the range from the Precambrian basement along a major low-angle fault. The volcanics of the River Mountains according to such an interpretation would represent the upper plate of this detachment complex. Extension occuring mainly after detachment faulting produced north,,,est-trending, high-angle, dip-slip faults. The event was preceeded by an enigmatic north-south extensional episode revealed by numerous east-west oriented dikes. Finally the range was cut by arms of the Lake Mead fault system. During this deformational episode the River Mountains may have been transported 40 km from the northern part of the Black Range to their present location. The shear stresses imposed on the range during this structural event rotated blocks within the range thus forming the discordant structural Figure 11. Photograph of well-foliated chlorite pattern that is characteristic of the River Mountains. schist just below the low-angle fault on Along the strike-slip fault on the south margin of Saddle Island. Pegmatite dikes and quartz the range many of the normal faults show evidence of veins (just above box) form well-developed left-lateral drag, and blocks of Cambrian limestone boudin. For scale, the box is "ere wedged into the flanks of the River Mountains approximately 8 em wide. stratovolcano. The River Mountains therefore represent an alkalic ann subalkalic volcanic complex thick sections of mud-flow breccia. that was disrupted by low-angle, high-angle and strike-slip faults. Chemical data suggest that fractional crystallization was probably not important in ACKNOWLEDGEMENTS producing either the alkalic or subalkalic suites, rather they were generated by partial melting of a I especially thank John D. Jones and Ward Rigot heterogeneous crust of Precambrian rocks. of the Phoenix Memorial Laboratory, University of Michigan for completing the REE analyses. Financial Chemical data indicate that the River Mountains support by the Department of Energy (D.O.E.) to the are chemically distinct when compared to nearby Phoenix Memorial Laboratory through the University volcanic ranges. Volcanism in these nearby areas, Research Reactor Assistant Program and Reactor however, was coeval with the activity in the River Facility Cost Sharing Program made much of this work Mountains. During the period 12 to 17 m.y. ago there possible. Funding for many of the major element were at least four additional centers of activity in analyses was provided by a grant from the University Research Council of the University of Nevada, Las Vegas. This support is gratefully acknowledged. I would also like to thank several fellow geologists for helping me (either knowingly or unknowingly) formulate my ideas on the development of the River Mountains. Among them are R.E. Anderson, H.F. Bonham, J.W. Bell, Brian Wernicke, J.C. Kepper, and S.M. Rowland. I, however, take full responsibility for the conclusions presented in this paper. John Young, Charles Meyers, Joe Parolini and Mark Reese helped me occasionally in the field. I also thank Walter Raywood for keeping our analytical equipment in working order, and for helping me complete the chemical analyses done in our Rock Chemistry Laboratory.

REFERENCES CITED

Anderson, R.E., 1971, Thin skin distension in Tertiary rocks of southeastern Nevada: Geological Society of America Bulletin, v. 82, p. 42-58.

Figure 12. Photograph of a low-angle fault within the ------~7' 1973, Large magnitude late Tertiary strike­ Tertiary volcanic section in the River slip faulting north of Lake Mead, Nevada: Mountains. The fault strikes to the north­ U.S. Geological Survey Professional Paper 794, west and dips to the west and may reflect 18 p. the readjustment of the allochthon during detachment faulting. For scale, the light ______~-' 1977, Geologic map of the Boulder City patch in the upper right of the photograph IS-quadrangle, Clark County, Nevada: U.S. is approximately 10 em high. Geological Survey Quadrangle Map GQ-139s.

52 , 1978, Chemistry of Tertiary volcanic rocks Hewitt, D.F., 1956, Geology and mineral resources ------~i-n-the Eldorado Mountains, Clark County, Nevada, of the Ivanpah quadrangle, California and and comparisons with rocks from nearby areas: Nevada: U.S. Geological Survey Professional U.S. Geological Survey Journal of Research, Paper 275, 172 p. p. 409-424. Irvine, T.N., and Baragar, W.R.A., 1971, A guide to the chemical classification of volcanic rocks: Anderson, R.E., Longwell, C.R., Armstrong, R.L., and Canadian Journal of Earth Science, v. 8, p. 523­ Marvin, R.F., 1972, Significance of K-Ar ages 548. of Tertiary rocks from the Lake Mead region, Nevada-Arizona: Geological Society of America Johnson, Cady, and Glynn, Jack, 1981, Uranium Bulletin, v. 83, p. 273-288. resource evaluation, Las Vegas quadrangle, Nevada, Arizona and California: Department of Barton, P.B. Jr., and Behre, C.H., 1954, Energy, Final Report for Contract DE-AC-13­ Interpretation and evaluation of the uranium 76GJ01664, 64p. occurrences near Goodsprings, Nevada: U.S. Atomic Energy Commission., RME-3l19. Kohl, M.S., 1978, Tertiary volcanic rocks of the Jean-Sloan area, Clark County, Nevada and their Bell, J.W., and Smith, E.I., 1980, Geological map possible relationship to Carnotite occurrences of the Henderson Quadrangle, Clark County, in caliches: M.S. Thesis, University of Nevada: Nevada Bureau of Mines and Geology Map California, Los Angeles, unpublished, 116p. 67. Leeman, W.P., and Rogers, J.J.W., 1970, Late Cenozoic Bohannon, R.G., 1979, Strike-slip faults of the alkali-olivine basalts of the Basin-Range Lake Mead region of southern Nevada: in province, USA: Contributions to Mineralogy and Armentrout, J.J., Cole, M.R., and Terbest, Petrology, v. 25, p. 1-24. Harry: Cenozoic Paleogeography of the Western United States, Pacific Coast Paleogeography Longwell, C.R., 1936, Geology of the Boulder Symposium 3, The Pacific Section, Society of reservoir floor, Arizona and Nevada: Geological Economic Paleontologists and Mineralogists, Society of America Bulletin, v. 47, p. 1393­ p. 129-139. 1476.

Bingler, E.C., and Bonham, H.F., Jr., 1973, , 1960, Possible explanation of diverse Reconnaissance geologic map of the McCullough ------s~t-ructural patterns in southern Nevada: American Range and adjacent areas, Clark County, Nevada: Journal of Science, v. 258-A (Bradley Volume), Nevada Bureau of Mines and Geology Map 45. p. 192-203.

Brandon, C.F., 1979, Flow direction studies of the , 1963, Reconnaissance geology between Lake Tuff of Bridge Spring, Nevada: Senior Project, ----~M~e-a-d and Davis Dam, Arizona and Nevada: U.S. University of Wisconsin-Parkside (unpublished). Geological Survey Professional Paper 374-E, SIp.

Brenner, E.F., and Glanzman, R.K., 1979, Tertiary Longwell, C.R., Pampeyan, E.H., Bower, Ben, and sediments in the Lake Mead area, Nevada: in Roberts, R.J., 1965, Geology and mineral Ne~vman, G.W., and Goode, H.D., eds., 1979 deposits of Clark County, Nevada: Nevada Bureau Basin and Range Symposium: Rocky Mountain of Mines and Geology Bulletin 62, 2l8p. Association of Geologists and Utah Geological Association, p. 313-323. McKelvey, V.E., Wiese, J.H., and Johnson, V.H., 1949, Preliminary report on the bedded manganese Campbell, I.H., and Gorton, M.P., 1980, Accessory of the Lake Mead region, Nevada and Arizona: phases and the generation of light rare earth U.S. Geological Survey Bulletin 948-D, 101p. element enriched basalts - a test for disequilibrium melting: Contributions to Rhodes, R.C., and Smith, E.I., 1972, Directional Mineralogy and Petrology, v. 72, p. 157-163. fabric of ash-flow sheets in the northwestern part of the Mogollon Plateau, New Mexico: Davis, G.A., Anderson, J.L., Frost, E.G., and Geological Society of America Bulletin, v. 83, Shakelford, T.J., 1979, Regional Miocene p. 1863-1868. detachment faulting and early Tertiary mylonitization, Whipple-Buckskin-Rawhide Shackelford, T.J., 1976, Structural geology of the Mountains, Southeastern California and Western Rawhide Mountains, Mojave County, Arizona: Arizona: in Abbot, P.L., ed., Geological Unpublished Ph.D. dissertation, University of excursions in the southern California area, Southern California, Los Angeles, l75p. San Diego, San Diego State University, p. 75­ 108. Smith, E.I., 1979, Late Miocene volcanism in the River Mountains, Clark County, Nevada (abs.): Davis, G.H., and Hardy, J.J., 1981, The Eagle Pass Transactions of the American Geophysical Union, detachment, southeastern Arizona: product of v. 61, no. 6, p. 69. mid-Miocene listric (?) normal faulting in the southern Basin and Range: Geological Society , 1981, Contemporaneous volcanism, strike-slip of America Bulletin, v. 92, p. 749-762. -----faulting and exotic block emplacement in the River Mountains, Clark County, southern Nevada: Elston, W.E., and Smith, E.I., 1970, Determination Geological Society of America Abstracts with of flow direction of rhyolite ash-flow tuffs Programs, v. 13, no. 2, p. 107. from fluidal textures: Geological Society of America Bulletin, v. 81, p. 3393-3406.

53 Smith, E.I., and Stupak, W.A., 1978, A Fortran IV program for the classification of volcanic rocks using the Irvine and Baragar classification scheme: Computers and Geoscience, v. 4, p. 89-99.

Steward, J.H., 1980, The Geology of Nevada: Nevada Bureau of Mines and Geology Special Paper 4, 136 p.

Wall~r,J.D., Beaufait, M.S., and ZeIt, F.B., 1981, Geology of the Devil Peak area, Spring Mountains, Nevada: Geological Society of America, Abstracts with Programs, v. 13, no. 2, p. 112.

Volborth, A., 1962, Rapakivi-type granites in the Precambrian complex of Gold Butte, Clark County, Nevada: Geological Society of America Bulletin, v. 73, p. 813-831.

\vestcamp, D.P., and Pavlovich, R.L., 1968, Final construction geology report, River Mountains Tunnel and portal structure, volume 1: Bureau of Reclamation, Region 3, Southern Nevada Water Project, unpublished report, 88p.

54 THE MULE M0 UNTAIN S THRUS T IN THE t~ ULE M0 UNTAINS CALIF 0 R NIA AND ITS PROBABLE EXTENSION IN THE SOUTHERN DOME ROCK MOUNTAINS, ARIZONA; A PRELU~INARY REPORT

Richard M. Tosdal U.S. Geological Survey 345 Middlefield Road Menlo Park, California 94025 and Departm ent of Geological Sciences University of California Santa Barbara California 93106

ABSTRACT of thrust faults (Fig. 1). The brief description ofthese rocks and structures presented are preli min ary observations, and The ~1ule Mountains thrust in the Mule Mountains, 1'1111 be greatly expanded by further sturlies of the kine­ Calif., and its probable extension in the southern Dam e matics and regional relationships of the Mule r~ountain Rock Mountains, Ariz., is a southward dipping thrust fault thrust. that superposes Preca mbria n(?) or Mesozoic(?) granitic rocks and a Precam brian gneiss com plex over Early and (or) MULE MOUNTAINS Middle Jurassic(?) metavolcanic and volcaniclastic rocks. Mylonitic rocks and fabrics are present along the thrust Along the ,"1 ule Mountains thrust (Figs. 1, 2A), a zone fault, and synkinematic crystalloblastic fabrics in the of mylonitic rocks separates Precambrian(?) or M~sozoic(?) metamorphic rocks of the lower plate. In the southern porphyritic granodiorite and Preca mbrian gneiss of the Dome Rock Mountains, a gradational stratigraphic contact upper plate, from the weakly to moderately metamorphoserl between Early and (or) Middle Jurassic(?) metavolcanic and Early and (or) Middle Jurassic(?) rhyolitic to dacitic volcanic volcaniclastic rocks and structurally lower but younger and volcaniclastic rocks of the lower plate. The thrust fault upper Mesozoic metasedim entary rocks indicate these rocks, strikes north westerly and dips about 30 0 to 50 0 S[1. part of the Mc Coy Mountains Form ation and equivalent Livingston Hills Form ation, may be as old as Middle Penetrative foliation that rJeveloped in both the Jurassic(?) and not of Late Cretaceous(?) or younger age. lower-plate metavolcanic rocks and upper-plate blasto­ The age of movement along the thrust fault is constrainerl mylonitic granodioritic gneiss parallels that the in the to be post-M iddle Jurassic(?) and pre- middle Tertiary, and mylonitic rocks along the fault. The width of the zone of probably of late Cretaceous to early Tertiary age. blasto mylonitic granodioritic gneiss at the base of the upper-plate varies dram atic ally in thickness along the thrust INTRODUCTION fault, with foliation rapirlly diminishing in intensity structurally upward. All macroscopic trace of the foliation Thrust faulting, regional metamorphism, nappe has disappeared 300 m above the thrust, although localized form ation, and intrusion of synkinem atic granite are zones of blastomylonitic gneiss occur sporadically through­ important elements of the late Mesozoic to early Tertiary out the porphyritic granodiorite. Quartz rods and tral1s of tectonics of the eastern Mojave f)esert, Calif., and northern broken micas and feldspars present throughout the meta­ Sonoran Desert, Ariz. (Haml1ton, 1971; Pelka, 1973; Haxel morphic rocks define a penetrative lineation within the and Dillon, 1978; Howard and others; 1980; Reynolds and plane of the foliation that plunges to the south-southwest. others, 1980; Anderson and Ro wley, 1981; Cro well, 1981; A late stage north west-trending crenul atio n lineation Miller and others, 1981; r~ay and others, 1981). Throughout slightly folds the prim ary lineation. this region, crustal shortening of uncertain magnitude is recorded by the thrust fa ults; Reynolds and others (1980, The porphyritic granodiorite, a tabular body extending p. 50) suggest sam e tens of kilom eters of tectonic transport across the entire width of the mountain range (Fig. 2A), is on thrust faults in the area of Salome, Ariz. Varying bounded on the northeast by the Mule Mountains thrust, and structural thicknesses of mylonitic rocks are present along on the south by the gneiss com plex. With the exception of the thrusts. Prograde metamorphism of lower plate rocks the narrow zone adjacent to the Mule Mountain thrust, accom panied translation along the thrust faults; whereas scattered zones of blastom ylonitic rock that parallel the retrograde metamorphism occurred synkinematically at the thrust, and a structurally disturbed portion of the othervlise base of the upper plate. These thrust faults comprise two intrusive contact with the gneiss com plex, the porphyritic systems (Fig. 1): the outboard Vincent thrust, and a granodiorite has igneous textures and mineralogy. Cream recently recognized but incom pletely studied inboard syste In colored, com manly I'lith a pinkish tint, potassium feldspar (e.g. Crowell, 1981; Tosdal and Haxel1982). phenocrysts are the distinctive feature of the porphyritic granodiorite. These phenocrysts range fro m 1 c m to 6 em in The inboard system of thrust faults occurs within a size, and are variable both in their habit and distribution, north west-trending belt that extends at least fro m occurring as subparallel tabular crystals, blocky crystals northernmost Sonora and south-central Arizona (May and that com pose much of the granodiorite or as scattered, ran­ others, 1981) to the Salome-quartzsite-Blythe area of domly oriented crystals, usually of sm aller size. The age of Arizona and California (Reynolds and others, 1980). The the porphyritic granorliorite is unknown, but petrologic Mule Mountains thrust that superposes Precam brian(?) or resem blances to other dated plutonic rocks of southeastern r~ esozoic(?) pl uta nic roc ks ato p varyingly meta morphosed California suggest one of several Precam brian ages (Silver, Jurassic(?) volcanic and volcaniclastic rocks in the Mule 1968, 1971; L. To Silver, in Powell, 1981). However, Mountains, southeastern California (Pelka, 1973; Crowell, else where in the south I'lest-;-porphyritic granodiorite and 1981) and in the southern Dome Rock Mountains, south­ monzogranite plutons are also of Mesozoic age (Silver, 1971; western Arizona (this paper), is part of the inboard syste m L. T. Silver in Powell, 1981); hence, a Mesozoic age for the

55 ( ) ( \ \ \ ~\,;",~~I ") /. ( 7 ~~ Blythe (~ 1------\lO)------.::=.------~----,LQ:

Area of LC Figure 2A

40 I KILOMETERS

Figure 1. General geologic map of the lower part of Colorado River, California and Arizona, showing distribution of inhoard thrust system and associated rocks, and outhoard Vincent thrust system. Modified from Strand, (1962); ,Jenning (1967); Wilson and others, (1969); Miller (1970); Pelka, (1973); Dillon (1975); Haxel, (1977); Crowl, (1979); W. Hamilton (oral commun., 1981); CD, Castle Dome Mountains; CH, Chocolate Mountains; CM, Cargo Muchacho Mountains; CW, Chuckwalla Mountains; DR, Dome Rock Mountains; LC, Little Chuckwalla Mountains; LM, Laguna Mountains; KM, Kofa Mountains; MC, McCoy Mountains; MD, Middle Mountains MG, Muggins Mountains; ML, Mule Mountains; MMT, Mule Mountains thrust; MR, Maria Mounta ns (includes both Big and Little Maria Mountains); PL, Palomas Mountains; PM, Palen Mounta ns; PV, Palo Verde Mountains; TM, Trigo Mountains; TP, Trigo Peaks.

56 porphyritic granodiorite cannot be discounted. Polymetaillorphic melanocratic to leucocratic gneiss derived from granitic to granodioritic, gabbroic, and ultra mafic pl utonic roc ks, ann suborrlinate a fll ounts of EXPLANATION sedimentary(?) and (or) volcanic(?) protoliths form the majority of the rocks corn prising the upper plate. Crystal­ loblastic fabrics fornl ed under am phibolite facies conditions are characteristic of the gneiss com plex; although, exten­ Volcanic and sedimentary rocks (Tertiary) D sive areas of mylonitic fabrics, unrelaten to the ~~ule Mountains thrust, are present. A Precambrian(?) age is inferred for the gneiss co III plex. Granitic and sedimentary rocks (late Mesozoic and (or) early Tertiary) Certain rock types, ann their relative age relations within the gneiss cOin plex of the Mule Mountains are si milar to those characteristic of the Preca mbrian San Gabriel INBOARD THRUST SYSTEM terrane (Silver, 1971; L. T. Silver in Po well, 1981). The San Gabriel terrane is widely distributed in the eastern Trans­ verse Ranges, southeast Califol'l1ia, as far east as the Little UPPER PLATE Chuckwalla Mountains (Powell, 1981), located some 10 km west of the ~~ule Mountains (Fig. 1). Correlation of the ~~~If{i1 Sedimentary rocks (Paleozoic) gneiss complex of the Mule Mountains to the gneiss com­ posing the San Gabriel terrane has previously been suggested by Cro~lell (1981), but confirmation of this correlation must a \'iait further work. It is interesting to note that in the E"1~~ Gneiss and granite (Precambrian; Orocopia ~1ountains, 70 km west of the Mule r~ountains, the may include rocks of Mesozoic age) San Gabriel terrane forms the upper plate of the Vincent thrust (Cro~lell, 1981; PO~fell, 1981).

.,...,...... " ...""""" THRUST FAULTS A cofliugate syste m of gabbroic to granodioritic dikes are the youngest rocks of the upper plate. These rocks are converted to greenschists and blastolnylonite near and LOWER PLATE within the deform ation zone at the base of the upper plate. The age of these dikes are unkno ~1Il, but are provisionally I; :;;,""1 Sedimentary and metasedimentary assigned a Mesozoic age. :"'.' rocks (late Mesozoic) The rhyolitic to dacitic metavolcanic and volcaniclastic rocks of the 10~ler plate (Fig. 2A) consist of varying amounts of relict em bayed quartz, broken plagio­ Granitic rocks (Mesozoic) clase phenocrysts, and recrystallized pumice lapilli surrounded by a ground mass com posed of quartz, albite, white mica, epidote, chlorite, and tourm aline. Several white quartzite and quartzite-cl ast metaconglo merate beds are interstratified with the volcaniclastic strata. An Early and (or) Middle Jurassic(?) age is assigned to the metavolcanic rock sequence on the basis of their simllarity in appearance and petrology to volcanic rocks of known VINCENT THRUST SYSTEM Early to Middle Jurassic age elsewhere in southeastern California (Daniel Krum menacher, in Pelka, 1973), in south­ UPPER PLATE central Arizona (Wright and others, 1981), and northern Sonora, Mexico (Anderson and Silver, 1978). Crowell (1981) • Gneiss and gran,ite (Precambrian tentatively has correlated these metavolcanic rocks to ,,, ", "", and MeSOZOIc) weakly meta morphosed dacitic and andesitic volcanic rocks in the southernmost McCoy Mountains, located some 12 km north of the Mule Mountains; there, the meta morphosed

"V v v v Q CHOCOLATE MOUNTAINS THRUST dacitic and andesitic volcanic rocks structurally overlie the McCoy Mountains Formation of Mlller (1944; Pelka, 1973). Alter-natively, in the northern McCoy Mountains wea kly LOWER PLATE cleaved, rhyodacitic hypabyssal rocks of Early Jurassic age (Daniel Kru mmenacher, in Pelka, 1973), deposition ally Orocopia Schist (Late Cretaceous overlain by the McCoy Mountains Formation of Mlller(1944) and (or) early Tertiary) of late Cretaceous(?) or younger age, may prove to be a better stratigraphic correlation for the metavolcanic rocks in the Mule Mountains.

CONTACT SOUTHERN DOME ROCK MOUNTAINS FAULT The Dome Rock Mountains are com posed of upper Mesozoic metasedimentary rocks, considered to be equiv­ alent to the Mc Coy Mountains-Livingston Hills Form ations as defined by Harding (1980), and Early to Middle Jurassic metavolcanic and granitic rocks (Wllson and others, 1969, L. T. Silver, in Cro~ll, 1979). Scattered investigations of these rocks rcro~ll, 1979; Harding, 1980; Marshak, 1980;

57 114'49 Harding and others, 1982) have revealed highly corn plex and, as yet, incom pletely understood geologic relations. Athrust A fault in the southern Do me Rock Mountains (Figs., 1, 28) is generally on strike Itlith the Mule r'10untains thrust (Fig. 1), superposes si mII ar rocks (Fig. 28), and is inferred to be an extension of the Mule r~ ountains thrust. The thrust strikes east-northeast ilnd dips moderately south-southeast. The mylonitic foliation along the thrust parallels the crystal­ loblastic foliation in the lovler-plate rocks, and contains a penetrative quartz-rodding and mic a-strea king lineation 33' 32 that plunges soutlHlest. The bearing of this lineation is + + + + + + + simllar, but not identical, to the quartz-rodding and broken + + + + + rnica and feldspar lineation noted in the r1ule Mountains + + + + (Fig. 2A). +~(+r + ++++ + + +- + + The majority of the rocks corn prising the 00 In e Rock + + 110untains are wea kly to I~ oderately rn eta morphosed sand­ + + + + stone, siltstone, and conglorn erate (Crov/l, 1979; Marshak, + + 1980) of the 7,000-m-thick McCoy Mountains-Livingston :'::;:'-7\\7//==:~ Hills Form ations (Harding, 1980; Harding and others, 1982). ~fl \\=I/~ ~;/~ In the McCoy i~ountain, Calif., the central and southern -~ --//) \\ ~ fI-1' Dome Rock Mountains, Plornosa r~ountains, and the o 3 KILOMETERS Livingston Hllls, Ariz. (Fig. 1), the sedim entary or metasedi­ I I mentary rocks of the McCoy Mountains-Livingston Hills Form ations rest de position ally on or are interstratified with siliceous volcanic and metavolcanic rocks of known or inferred Early and (or) Middle Jurassic age (figs. 1, 2R, i1iller, 1970; Pelka, 19 73; Crov/l, 1979; Harding, 1980). In the southern Dome Rock Mountains (Fig. 28), the sequence is overturned, and the Early and (or) Middle Jurassic(?) metavolcanic rocks grade structurally dOI·lnward into the 33'29 metasedimentary rocks. This gradational contact between these metavolcanic rocks and upper Mesozoic metilsedi­ mentary rocks strongly implies the depositional age of the McCoy Mountains-Livingston Hllls Formations may be as old as Middle and (or) Late Jurassic (Harding, 1980) and not late Cretaceous(?) or· younger (Miller, 1970; Pelka, 1973). Recognition of a probable Jurassic depositional age for all, or at least the basal part, of the McCoy r~ ountains­ Livingston Hill Form ations has several im portant im plications for the stratigraphic relation of these sedim entary rocks to the Jurassic volcanic are, and to the Jurassic tectonics of the region.

TIMING OF MOVEMENT

EXPLANATION The youngest rocks involved in thrusting and meta morphis m are the weakly to rn oderately meta mor-' Volcanic and sedimentary rocks phosed Me Coy i10untains-Livingston Hllls Form ations of (middle Tertiary) late Cretaceous(?) or younger age (Miller, 1970; Pelka, UPPER PLATE 1973), although, a Middle and (or) Late Jurassic(?) age now seems more likely for these rocks. Middle Tertiary volcanic Gneiss (Precambrian) and sedim entary rocks, prior to limited and local movem ent along detachment faults (Frost and others, 1981; D. Sherrod and R. i1 Tosdal, unpub. datil, 1982) deposition ally overlay ~ Porphyritic granodiorite the Precambrian and Mesozoic crystalline rocks. Tilerefore, ~ (Precambrian or Mesozoic) existing data broadly constrains the movem ent of the Mule r~ountains thrust to late Mesozoic (late Jurassic(?)) and (or) MULE MOUNTAINS THRUST early Tertiary times.

LOWER PLATE Throughout western Arizona, crystalline thrusts of the inboard thrust system have been demonstrated to be late L»"~I Metasedimentary rocks (Upper Mesozoic) Cretaceous and (or) early Tertiary in age (R eynolds and others, 1980; Haxel and others, 1981). The simllarity between the r~ule Mountilins thrust and thrusts of the Metavolcanic rocks (Jurassic?) inboard thrust system suggests movem ent along the Mule i~ountains thrust may also be late Cretaceous and (or) early Contact Tertiary in age. ?",,,""" Gradational contact--Queried where uncertain Figure 2a. General ized geologic maps of the i~ule .~ Attitude of foliation and lineation Mountains thrust; A, northern Mule r10untains, Cal if., B, southern Dome Rock Mountains, ---? Fault--Dashed where approximately located; Ariz. See figure 1 for locations of these queried where uncertain rna ps •

58 SOME REGIONAL QUEST!()NS Crol'lell, ,J. C., 1981, An outline of the tectonic history of southeastern California, in Ernst, vi. G., ed., The SOlne 40 km south of the last outcrops of the geotectonic developm ent of California (\1. l~. Rubey southward-dipping Mule Mountains thrust is a simllar, but Volume No.1): Engle\'lOod Cliffs, N. J., Prentice Hall, disti net, myl onitic thrust fa ult, the Choc 01 ate Mountains p. 583-600. thrust of the Vincent thrust system. The Chocolate Crowl, lL ,J., 1979, Geology of the central Dome Rock I~ountains thrust places Precam brian granite and gneiss and riountains: Tucson, University of Arizona, M.S. thesis, Mesozoic granitic rocks over the ensimatic Orocopia Schist, 76 p. which consists of dOlninantly quartzofeldspathic schist and Dillon, ,J. T., 1975, Geology of the Chocolate and Cargo minor ferrom anganiferous III etachert, III etabasite, siliceous ~luchacho Hountains, southeasternmost California: marble, and meta-ultralnafic rock (Dillon, 1975; Haxel, Santa 8arbara, University of California, Ph. D. thesis, 1977). In its northernmost outcrops in the Chocolate and 405 p. Trigo Mountains (Fig. 1), the Chocolate Mountain thrust dips Frost, 1::. G., Caneron, T. E., Krum menacher, naniel, and varyingly north. Mov e ment alo ng the Choc 01 ate Mountains Martin, I). L., 1981, Possible reaccretionary wedge of an Early sOlIthern California, in Howell, D. G., iind McDougall, Jurassic(?) subduction zone, and subsequently, during the K. A., 1978, eds., Mesozoic paleogeography of the late Cretaceous and (or) early Tertiary, overridden by an 1'1 estern United States: Los Angeles, Society of allochthon that may have been exotic to North America Economic Paleontologists and Hineralogists, Pacific (Vedder and others, 1982), or part of the continental margin Section, Pacific Coast Paleogeography Symposiu:n 2, (Crowell, 1981)? The answers to these questions I'lhich p. 453-469. remain unresolved, may yet be found in the discontinuous Haxel, Gordon, Tosdal, R. M., Hay, n. J., and \'lright,.J. 1::., erosional windol'/s of Precam brian and Mesozoic 1981, Latest Cretaceous and early Tertiary thrust metamorphic and granitic rocks exposed between the Mule­ faulting, regional metamorphism, and granitic Dome Rock Mountains and the Chocolate-Trigo Mountains plutonism, south-central Arizona [abs.], ~ Howard, (Fig. 1). K. A., Carr, H. n., and Hiller, D. r1., eds., Tectonic framework of the Mojave and Sonoran neserts, Calif­ ACKN0 l~ LEO GEM EN TS ornia and Arizona: U.S. Geological Survey Open-File Report 81-503, p. 42-44. In particular I would like to thank Nobie Amamoto, Howard, K. A., Miller, C. F., and Stone, Paul, 1980, Karen Johnson, and Diane Stevens of the U.S. Geological Mesozoic thrusting in the eastern Mojave Desert, Survey for their generous help in preparation of the California [abs.]: Geological Society of America manuscript. Version of the manuscript were greatly Abstracts with Progra ms, v. 12, p. 112. improved through the freely given advice of J. C. Crowell, Jennings, C. I~., Compiler, 1967, Salton Sea sheet of Gordon Haxel, D. J. May, and R. E. Powell. Geologic map of California: Division of Mines and Geology, State of California, scale 1:250,000. REFERENCES Harshak, Stephen, 1980, A preliminary study of Hesozoic geology in the southern Dome Rock Mountains, Anderson, J. L., and Rowley, M. C., 1981, Synkinematic southwestern Arizona, in Jenney, J. P., and Stone, intrusion of peraluminous and associated metaluminous Claudia, eds., Studies m western Arizona: Arizona granitic mag mas, vlhipple Mountains, California: Geological Society Digest, v. 12, p. 123-133. Canadian Mineralogist, v. 19, pt. 1, p. 83-101. May, D. J., Tosdal, R. M., and Haxel, Gordon, 1981, Anderson, T. H., and Silver, L. T., 1978, Jurassic magmatism Imbricate thrust faulting of Late Cretaceous or early in Sonora, Mexico [abs.} Geological Society of Tertiary age, Organ Pipe Cactus National Monument, Am eric a Abstracts with Programs, v. 10, no. 7, p. 359. southwestern Arizona [abs.} Geological Society of Cameron, T. E., and Frost, E. G., 1981, Regional America Abstracts with Progra ms, v. 13, no. 2, p. 95. developm ent of mcdor antiforms and synform s Miller, D. M., Howard, K. A., and Anderson, J. L., 1981, coincident I'lith detach ment fa ulting in California, Hylonitic gneiss related to emplacement of a Arizona, Nevada, and Sonora, [M exico] [abs.]: Cretaceous batholith, Iron Mountains, southern Geologic al Society of America Abstracts with California, in Howard, K. A., Carr, M. D., and Miller, Programs, v. 13, no. 7, p. 421-422. eds., Tectonic framework of the Mojave and Sonoran

59 Deserts, California and Arizona: U.S. Geological Survey Open-Flle Report 81-503, p. 73-75. 1·1111er, F. K., 1970, Geologic map of the Quartzsite quadrangle, Yuma County, Arizona: U.S. Geological Survey Geologic Quadrangle Map GQ-841, 3 p., scale 1:62,500. Ml1l er, W. J., 1944, Geology of the Pal en Springs-Blythe strip, Riverside County, California: California Journal of Mines and Geology, v. 40, p. 11-72. Pelka, G. ,J., 1973, Geology of the Mc Coy and Palen Mountains, southeastern California: Santa Barbara, University of California, Ph. D. thesis, 162 p. Powell, R. E., 1981, Geology of the crystalline basement com plex, eastern Transverse Ranges, southern California: constraints on regional tectonics interpretations: Pasadena, California Institute of Technology, Ph. D. thesis, 441 p. Reynolds, S. ,J., Keith, S. B., and Coney, P. ,J., 1980, Stacked overthrusts of Precambrian crystalline baselTJent and inverted Paleozoic sections em placed over Mesozoic strata, west-central Arizona: Arizona Geological Society Digest, v. 12, p. 45-50. Silver, L. T., 1968, Precambrian batholiths of Arizona Cabs.} Geological Society of ,1\ merica, Cordllleran Section, Programs, 64th Annual Meeting, Tucson, Ariz., 1968, Progra ms, p. 109-11 O. 1971, Problems of crystalline rocks of the Transverse - Ranges Cabs.} Geological Society of Am eric a i\ bstracts with Progra ms, v. 3, no. 2, fl. 193-194. Strand, R. G., Com piler, 1962, San Diego-El Centro sheet of Geologic In ap of California: California Division of Mines and Geology, scale 1:250,000. Tosdal, R. M., and Haxel, Gordon, 1982, Two belts of late Cretaceous to early Tertiary crystalline thrust fa ults in southwest Arizona and southeast California Cabs.]: Geologic al Society of America Abstracts with Progra rn s, v. 14 Cin press]. Vedder, J. G. Ho well, D. G., and Mclean, Hugh, 1982, Stratigraphy, sedim entation, and tectonic accretion of exotic terranes, southern Coast ranges, California: (Paper presented at Hedberg Conference on Continental r1 argins, 1980). Wilson, E. 0., r100re, R. T., and Cooper, J. R., 1969, Geologic map of Arizona: Arizona Bureau of Mines and U.S. Geological Survey, scale 1:500,000. Wright, J. E., Haxel, Gordon, and May, D. J., 1981, Early Jurassic uranium-lead isotopic ages for Mesozoic supracrustal sequences, Papago Indian Reservation, southern Arizona Cabs.]: Geological Society of America Abstracts with Progra ms, v. 13, no. 2, p. 115.

60 REGIONAL GRAVITY AND ~~GNETIC EVIDENCE FOR A TECTONIC WEAKNESS ACROSS SOUTHWESTERN ARIZONA

Douglas p. Klein U.S. Geological Survey Denver, Colorado

ABSTRACT

The Basin and Range ~rovince of southwest Arizona Colorado Plateau (~) is distinguished from the Basin is traversed by a northwest-trending belt of magnetic and Range Province by generally larger amplitude and anomalies that separates a region of relatively high shorter dimension magnetic anomalies, and by a merging residual gravity to the northeast from a region of of anomalies into northeast trends that presumably gravity minimums to the southwest along the Mexican reflect Precambrian basement contrasts (Sauck, border. The magnetic anomalies form a fairly uniform 1972). To the southwest, these trends terminate at or zone of 20-30 km width whose magnetic intensity are attenuated on crossing T, and sou.thern Arizona is exceeds the otherwise generally quiet magnetic characterized by a northwest grain in the magnetic background pattern by 200-400 nT. The exposed rock pattern which follows the tectonic trends of Mesozoic associated with the magnetic anomalies consists and Tertiary age features (King, 1969). The residual predominantly of Mesozoic to Tertiary metamorphic, gravity (Fig. 3) is less definitive across T, but intrusive, and volcanic material that appears to cap there are patterns of northwest-trending anomalies in the more fundamental magnetic sources estimated by the Basin and Range Province that do not have an profile analysis to lie 6 km or more below the extension north of the transition T. These trends are surface. The anomalies may reflect a locus of mos t conspicuous south and southeas t of Phoenix and intrusion, metamorphism, or basement structural Morenci. The grouping of magnetic anomalies along MB defects that accompany a primary weakness between two and the broad, elongated gravity minimums southwest of different crustal regimes. ME are noteworthy parts of the general northwest geophysical grain of the Sonoran Basin and Range INTRODUCTION Province that will be discussed subsequently.

Regional aeromagnetic and gravity patterns in southern Arizona are examined for their relationship to regional geology and tectonics with particular focus on a belt of prominent magnetic anomalies (MB in Fig. 1) that trends southeast across the lower part of 0 Arizona from about lat. 35 N. on the Arizona-Nevada - 41'" State line. This area is part of the llasin and Range ~rovince generally east of California between the Colorado Plateau (CP) and Sonora, Mexico (see Fig. 1). For the purposes of identification, in contrast "". with the Great llasin region of Nevada (Gll) , this area will be referred to as the Sonoran Basin and Range Province (Sll).

The data for this study are the Residual Aeromagnetic Map of Arizona (Sauck and Sumner, 1970; Sauck, 1972) and the Residual Gravity Map of Arizona (Aiken, 1975, 1976). The aeromagnetic data was Now Moxlco acquired on north-south lines at 5-km spacing; the "". flight altitude was 9000 feet (2740 m) above sealevel except for a narrow strip bordering New Mexico that was flown at 11 ,000 feet (3350 m). These data are 3" appropriate for identifying geophysical features having dimensions greater than about 3 km, and are more than adequate to identify the geophysical signature of tectonic trends that measure in the range of tens of kilometers or more. Figure 1. Regional map showing the magnetic belt (MB), the Sonoran Basin and Range Province (SB), REGIONAL GEOPHYSICAL SETTING the Great Basin (GB) llasin and Range Province, the Colorado Plateau (~), and major volcanic A simplified version of Sauck and Sumner's (1970) fields: High Plateau (HP), San Juan (SJ), San residual magnetic intensity map is shown in Figure 2, Francisco (SF), Datil-White Mountain (D), and and Aiken's (1975) regional llouguer gravity map of Pinacate (P). Hachured line indicates eastern Arizona in Figure 1. The magnetic data show a clear edge of the Mesozoic-Tertiary Cordilleran geophysical separation across the transition T between orogenic belt (Drewes, 1978). To the west are two magnetic provinces that coincide roughly with the the Sierra Nevada (S) and Baja (B) intrusive Basin and Range, and Colorado Plateau Provinces. The complexes.

61 36°

35·

34°

33°

32· N RESIDUAL AEROMAGNETIC MAP of ARIZONA r'om Sauck &rid Sumner. 1970

Scale In mi"s 110·"'" RESIDUAL MAGNETIC INTENSITY 100 10203040 Contour intorval 20001 - Contours label@d in oT-:-1oo

[I]] Loss than 400 oT

Figure 2. Simplified residual aeromagnetic map of Arizona (modified from Sauck and Sumner, 1970), showing the inferred magnetic boundaries of the regions discussed in the text: the Colorado Plateau (CP); the transition zone (T); the Sonoran Basin and Range Province subdivided into regions, SBT (Sonoran Basin transition), SB (Sonoran Basin) and SBP (Sonoran Basin Papago area); the magnetic helt (MB), the extrapolated line of the Jemez Lineament (JL); the Ajo trend (AJ); and the Cananea Line (CL).

62 37'

35'

RESIDUAL GRAVITY MAP of ARIZONA

from Aiken, 1975

111 0 Scale in miles 110· RESIDUAL GR W 10 0 10 20 30 40 Conto Jf . AVITY " Interval 10 mglill ca Less than -20mgal

Figure 3 Bouguer map of Arizona (modified f rom Aiken, 197'5). SimplifiedZon residual from fi gravity es _T and _areME gure 2.

63 Other geophysical data compliment the gravity and remarkably aligned with a southwest continuation of magnetic contrasts between the Colorado Plateau and the Jemez volcanic lineament (JL). The area of the Sonoran Basin and Range provinces. Crustal hypothetical intersection of the magnetic belt with thickness south of the transition T is 20-30 km, the trends AJ and JL shows a pronounced minimum in compared with 30 to more than 40 km (increasing residual gravity (Fig. 3) such as might be expected northeast) for the Colorado Plateau (Smith, 1978). over a major volcanic collapse structure. The The greatest change in crustal thickness is along the southwest and southeast segments of the magnetic belt general region of the magnetic transition T. This could be interpreted to show a northeast transverse transition also correlates with the average elevations offset of about 50 km in this region. Horizontal computed over 10 x 10 squares (Strange and Woollard, displacements along this direction and of this order 1964; and see Blackwell, 1978) which varies from less of magnitude have been suggested in southeast Arizona than 1 km to greater than 1.5 km northeast across a (Drewes, 1978), but supportive geological evidence is zone roughly marked by T. Heat flow reflects the lacking in southwest Arizona. major provinces by shOlving generally higher values in the Basin and Range Province (Blackwell, 1978). The continuous nature of the magnetic belt across Electrical conductivity averaged vertically to southwest Arizona, and possibly into southeast subcrustal depths similarly increases in the Basin and Arizona, suggests that the locus of anomaly sources Range Province compared with the Colorado Plateau as may reflect a "root zone" of a suite of rocks that may interpreted from geomagnetic variometer array have identifiable consistency in composition or age at measurements (Gough, 1974). the surface. A preliminary test of this possibility was performed by attempting to relate the magnetic GEOPHYSICAL SUBPROVINCES OF SOUTHHEST ARI7.0NA anomalies to mapped geologic units. Surface projections of probable source bodies were estimated Parts of the magnetic belt in southwest Arizona for the larger magnetic anomalies in the Sonoran Basin have been attributed to batholithic intrusion (Sauck, and Range Province whose residual contour closures 1972) but there is generally no obvious correlation of exceeded 500 nT. Table I shows the number of gravity and magnetic anomalies in the belt. The most anomalies associated with each of the magnetic important correlation in gravity data appears to be a subprovinces and with each of several generalized rock pronounced gravity low southwest of the magnetic belt types based on the geologic map of Arizona (Hilson and (Fig. 3) • The belt may be considered a separate others, 1969). The "ambiguous" category refers to subprovince, but based on its relationship to residual anomalies over alluvium or over complex suites of gravity it is probably more correct to consider it as different rock types. Most of these ambiguous a boundary between subprovinces. The transitional correlations were in southeast Arizona, east of long. subprovince (SBT) to the north of the magnetic belt 112° W. Table I does not differentiate between the generally shows more intense magnetic anomalies and "Laramide" (Cretaceous-Tertiary) rocks and the earlier higher residual gravity than does the region to the Mesozoic rocks because of possible inconsistencies south (SB and SBP). The gravity low to the southwest between the State geological map and more recent is over a region that is characterized by mixed geological mapping (Gordon Haxel, U.S. Geological Jurassic to mid-Cretaceous granitic rocks and Survey, pers. commun.). The major inconsistencies metamorphosed Jurassic-Triassic sedimentary rocks occur wes t of Tucson where new geological mapping (King, 1969). These rocks may be expected to have shows discrepancies with the older work, mainly lower intrinsic density, as well as to have been more between "Laramide" and earlier Mesozoic intrusions, susceptible to tectonic strains that could decrease and between Precambrian and Mesozoic metamorphic formation densities than the more prevalent rocks. Precambrian basement (King, 1969) north of the magnetic belt. Haxel and others (1980) document Tertiary-Mesozoic rocks make up the largest geological differences between the southern and number of source body-surface rock correlations for northern parts of the Papago area (SBP) that are in the Sonoran Basin and Range Province as a whole and accord with the hypothesis of a major variation of also for each of the geophysical subprovinces and the terrane type across the magnetic belt. magnetic belt. More than 35 percent of the larger anomalies in each subprovince are related to Tertiary­ An apparent discontinuity in the magnetic belt is Mesozoic rocks, whereas less than 20 percent are roughly centered on long. 1120 W. (SBP) and is bounded related to Quaternary and Tertiary basalts. Most of by two north-northwest magnetic trends. The the basalt associations occur along the magnetic belt westernmost trend (AJ) extends toward Ajo and in or just northwest of the Ajo area of southwest essentially terminates the magnetic highs of the Arizona. Precambrian rock contributes a relatively southwestern segment of the magnetic belt. The other small percentage to the association of surface rocks trend (CL) correlates with the Cananea Line to larger magnetic anomalies in general, and none for (Hollister, 1978, p. 124). The magnetic belt may the region southwest of the magnetic belt. The continue southeast of CL but it is noticeably predominance of Mesozoic-Tertiary age rocks in fragmented in the southeast corner of Arizona (Fig. association with the larger magnetic anomalies within 3) • The possibility that the apparent break in the and south of the magnetic belt is noteworthy; but belt may be related to thermal demagnetization rather except for the basaltic rock percentages, the results than being a structural defect in the belt can be in Table I do not identify any geological uniqueness argued from higher than normal drill-hole geothermal to the magnetic anomalies within the magnetic belt as gradients (Hahman and others, 1978) in the region of compared with the bordering subprovinces. long. 112° W. However, this geothermal evidence is weak because temperature sampling northwest of 1120 30' Although many surface outcrops seem to reasonably is sparse (Kenneth Hollett, U.S. Geological Survey, correlate with magnetic anomalies within the magnetic pers. commun.) and deep crustal electromagnetic belt, the half-width (10-15 km) of the belt indicates investigations that could provide complimentary that the main locus of sources may be at intermediate evidence are lacking. The AJ trend, that marks the crustal depths and that the surface rocks may be western boundary of the discontinuity, coincides with capping the more fundamental sources. Nine profiles a zone of recent volcanism (17 m. y. or less, Stewart across the magnetic belt were analyzed for depth-to­ and Carlson, 1978) and is in a region that is source based on dike and fault models using Herner's

64 method (1953; see also Hartman and others, 1971; Jain, of the anomalies that may be crustal intrusions, 1976). Figure 4 shows the profile lines, along with metamorphism or structural relief of buried crustal abbreviated aeromagnetic contours, and the location of units. The present aim is basically to point out the major known mineral deposits. The magnetic data were prominence of this belt and to document some possible sampled at 2-km intervals and are plotted on Figure correlative features. 5. Several depth solutions result from passing different filters over the profiles; generally, The magnetic belt is generally parallel to the confidence that reliable source parameters are found northwest-trending tectonic grain of southern is based on a clustering of solutions. '!'he method Arizona. This regional grain, or specific parts of provides a 2-dimensional zone in which the top of the it, has been identified as the "Texas Lineament" source is inferred. The horizontal dimension of this (Hertz, 1970). The significance of lineament "source zone" is indicated by a heavy line below the tectonics is a source of debate (Gilluly, 1976; anomaly in Figure 5; the vertical depth range of the Marshall, 1979), but one implication is a deep-seated zone is shown next to the heavy line. The "source crustal weakness that could give rise to magnetic zone" is taken as an approximation of the maximum contrasts either by displacement or by providing a depth to the top of the source because a wider conduit for intrusion. The magnetic belt is not magnetic body at shallower depth might also satisfy strictly a linear feature, but tectonic events that the anomaly. have occurred at numerous times at least since early Hesozoic time in southern Arizona (Coney, 1978) may The magnetic source depths (Fig. 5) show a large have distorted its original form. overall range along the belt, 6-31 km, but the medians (Fig. 4) average between 10 and 20 km depth. The An inferred Jurassic magma tic arc (Coney, 1978) results are consistent with indications from the crosses southern Arizona with a trend similar to that dimensions of the magnetic belt that near-surface of the magnetic belt. A magmatic arc would be rocks are not the fundamental sources of anomalies. expected to create a zone of intermediate-composition The surface rocks, however, may provide a clue to the igneous rocks against or within a sedimentary wedge age of the crustal feature that produces the anomalies that would produce a magnetic signature. A magmatic of the magnetic belt. The surface rocks may also in arc would also produce a deep-seated crustal flaw some cases be related to the deeper crustal feature as whose geophysical signature may remain relatively igneous derivatives or direct continuations with the stable over subsequent deformation events, and it deeper material. l<1Ould concentrate a source of intrusive material for subsequent differentiation into various crustal rocks DISCUSSION and, possibly, economic mineral concentrations.

The northwest-trending locus of large, possibly The fact that the Ajo, Lakeshore, and Silver Bell mid-crustal depth, magnetic contrasts that separates porphyry copper deposits, as well as several lesser two crustal regimes of apparently different average deposits, occur on the periphery of the magnetic belt density will eventually have to be put in the context indicates the possibility that the magnetic belt is of regional tectonics. Such a synthesis will require related to .a crustal flaw that guided magma, and further data and reasoning to infer the direct cause locally, economic concentrations of minerals to the

Table I. Correlations between magnetic anomalies and generalized surface geology in southern Arizona

Surface-rock Number of anomalies in each subprovince1 correlation HB SBT' SB + SBP Total

Quaternary and Tertiary basalts ----- 8 3 12

Lower Tertiary-Hesozoic metamorphic, intrusive & volcanic rocks ------16 10 13 39

Precambrian granites and granitic metamorphic rocks ------6 5 o 11

Subtotal ------30 18 14 62

Ambiguous ------12 q 29

Total ------42 27 22 91 I See Figure 2. -- For identification of subprovinces.

65 ®A 337 A

352 ~:o 33'

A A

112 ® 250 - greater tb.mn 500 nT - mlg'Detic sllOlDlllle.o - leflll thAn 300 nT 0 110 ..... no circle - ll!.lJlJ than $1 .Aercra.gnetlcs frOQ Sauck. and Sumner (1910) - $1-100 Klnernls frOllll Mardirod.llD (l973) A - gold-9ilver o - CUJ!IIIllati01l production to 1973 C - ~per - $100-2000 (rrllUoon - lead'-':lnc o - $1000-4000 Pigure 4. fup showing the location of magnetic profiles (a-h, see figure 5), mineral deposits Hardirosian (1973), and abbreviated magnetic contours (Sauck and Sumner, 1970). The profiles shoH interpreted source-locations (heavy-line segment) and source-depth in kilometers. upper crust. Major base-metal mineral deposits (Hardirosian, 1973) are shoHn on Figure 4. About 30 percent of these deposits are on or close to the edge . , - of the magnetic belt, Hhereas the area of this belt e. ~ :_ _ 290,T comprises about 15 percent of the region from T southHard. From the standpoint of random chance, this VN=-I• I~ is not a strong argument that the ore occurrences are _ 6 -24 km depth related to the belt but the possibility cannot be -- 6-20kmdepth dismissed. The mobilization of mineralized fluids lYOuld not have to have occurred simultaneously with the intrusions. The near-surface migration and I I deposition of minerals may be controlled by secondary I "I crustal features that could distribute deposits some a. I AI"",, 1(17ki distance laterally from the root zone. I I - 10-18 kmdepth I DreHes (1978) postulated the southeast I~ - 14 - 22 km depth continuation of the Cordilleran orogenic thrust belt I from the region of Nevada through the 10Her part of I I Arizona based on horizontal displacements of at leas t I 20 km, possibly more than 100 lan, 'in southeast -1- _ ~ Arizona. The timing of this thrusting Hould be partly - 10 -20km depth simultaneous Hith or l'lOuld postdate the Cretaceous ­ Tertiary (Laramide) orogenic event and its i/\J""~ " mineralization. If the source roots for the magnetic -- 9·31kmdepth belt are at depths approaching 10 km or more, then i. they are probably beneath the postulated thrust regime, and any surface correlations of rock suites to c. this root Hould be fortuitous unless the horizontal displacements involved Here close to the resolvable dimensions considered in the present data. The magnetic belt itself is not a critical argument for or

Figure 5. l1agnetic profile data (see figure 4) across

the magnetic belt shoHing the source-depth in 20 40 km kilometers, range of solutions and the anomaly Horizontal Scale peak-trough \qidth and amplitude. The vertical bars range from 300 to 600 nT. The left of the - 8 -18 km depth profile is alHays oriented to the south or to the southHest.

66 against major thrusting hecause a reLHively thin Dre"es, H. , 1978, The cordilleran orogenic helt upper crust regime may operate tYithout significantly bet'ween Nevada and Chihuahua: Geological Society affecting the deeper magnetic anomaly sources, and a of America llulletin, v. S9, p. 641-657. thicker thrust regime could create a quasi-linear Gtlluly, J., 1976, Lineaments--Ineffective guides to magnetic source as the result of thermal metamorphism ore deposit s: Economic Geology, v. 71, p. 1';07­ arising from crustal stress or as the result of 1514. dLsplacement of magnetic crustal units. Gough, D. 1., 1974, Electrical conductivity under \qestern North America in relation to heat flow, seismology and structure: Journal of Geomagetism CONCLUS IONS and Geoelectricity, v. 26, p. 105-1~1. Hahman, H. R., Sr., Stone, C., and 1'1itcher, J. C" The major magnetic feature of the U!lsin and Range 1978, Geothermal energy resources of 4.rizon!l, Province in south\qest Arizona is a northwest-trending preliminary map, geothermal map no. 1: Bureau of helt of magnetic anomalies that contr!lsts with a Geology and Hineral Technology, Geological Survey generally quiet magnetic pattern in this region. This Branch, University of Arizona, Tucson. helt has a general consistency in its average magnetic Hartman, R. R., Teskey, D. J., and Friedberg, J. L., morphology along most of its length with anomaly 1971, A system for rapid digi tal aeromagnetic amplitudes exceeding 300 nT across a zone 20-30 km interpretation: Geophysics, v. 36, p. 891-918. wide. Surface correlations with the anomalies of the Haxel, G., Hright, J. E., Hay, D. J., and Tosdal, R. magnetic belt are mostly Hesozoic and Tertiary age H., 1980, Reconnaissance geology of the Hesozoic rocks that may only be cappings to source roots at a and Lower Cenozoic rocks of the southern Papago maximum depth of about lO-;W km. There appears to he Innian Reservation, 4.rizona; A preliminary a transition in residual gravity across the helt that report: Arizona Geological Society Digest, v. suggests a regime of less dense crustal material to 12, p. 17-29. the soutlwest. The evidence is consistent \qith the Hollister, V. F., 1978, Geology of the porphyry copper speculation that this belt represents a tectonic deposits of the western hemisphere: Society of Heakness that separates two different &lsin and Range }lining Engineers, American Institute of llining, terranes. The magnetic belt may continue in southeast Hetallurgical, and Petroleum Engineers, Inc., New Arizona, east of a discontinuity in the region of 0 York, 219 p. long. 112 Iv. tYhere the helt intersects a hypothetical Jain, S., 1976, An automatic method of direct southHest continuation of the Jemez lineament. The interpretation of magnetic profiles: Geophysics, discontinuity is bounded to the west by a magnetic v. 41, p. 531-541. zone extending northwest toward Ajo, and on the east King, P. B., 1969, Tectonic map of North America, by the Cananea Line. sheet 2: U.S. Geological Survey. Harnirosian, C. A., 1973, Hining districts and mineral The trend of the magnetic belt in southVlest deposits of Arizon!l (exclusive of oil and gas), Arizona corresponds with the regional tectonic grain map: l1ining club of the southVlest, llox 17226, and correlates in a general sense with a magmatic arc Tucson, Ariz. proposed by Coney (1978). If the belt reflects !l zone Harshall, B., 1979, The lineament-ore association: of unique crustal intrusion, it could possibly be a Economic Geologv, v. 74, p. 942-940. source for the creation of differentiated fluids that Sauck, H. A. , 1972, Compilation and preliminary mineralized the region. However, in considering the interpretation of the Arizona aeromagnetic map: belt as a continuous unit across all southern Arizona, Ph.D. dissertation, University of Arizona, only about 30 percent of the known mineral deposits Tucson. are within or along its borders. Sauck, H. A., and Sumner, J. S., 1970, Residual aeromagnetic map of Arizona: Dept. of ACKNOtVLEOGEHENTS Geosciences, University of Arizona, Tucson. Smith, R. B., 1978, Seismicity, crustal structure, ann Professor John Sumner of the University of intraplate tectonics of the interior of the Arizona provided the regional magnetic and gravity western Cordillera, in Smith, R. B., and Eaton, maps used for this report. Data digitization and G. P., eds., Cenozoic tectonics and regional compilation were done \qith the assistance of Hike Baer geophysics of the western Cordillera: Geological and Hike Broker of the n.s. Geological Survey in Society of America Hemoir 152, p. 111-141. Denver. This study \qas performed in part as a 0 StetYart, J. H., and Carlson, J. E., 1978, Generalized preliminary geophysical analysis for the Ajo 1 x ,:>0 maps showing distribution, lithology and age of sheet; U.S. Geological Survey's "Conterminous U.S. Cenozoic igneous rocks in the western United Hineral Resource Assessment Program", (CUSHAP). States, in Smith, R. B., and Eaton, G. P., eds., Cenozoic~ectonics ann regional geophysics of the western Cornillera: Geological Society of REFf<:R"NCES CITfW America Hemoir 152, p. 263-264. Strange, H. E., and Hoollard, G. P., 1964, The use of Aiken, C. L. V., 1975, Residual Bouguer gravity geologic and geophysical parameters in the anomaly map of Arizona: Laboratory of evaluatton, interpolation, and prediction of Geophysics, University of Arizona, Tucson. gravity: Aeronautical Chart ann Information 1976, The analysis of the gravity anomalies of Center, USAF, St. wuis, llissouri Contract AF23 ------Arizona, Ph.D. dissertation, University of (601)-3879 (Phase 1), 225 p. Arizona, Tucson. Herner, S., 1953, Interpretation of magnetic anomalies Blackwell, D. D., 1978, Heat flow and energy loss in at sheet-like bodies: Sveriges Geologiska the western United States, in Smith, R. ll., and Undersok., Ser. C. C. Arsbok 43, N:06. Eaton, G. P. , eds. : Geological Society of Hertz, J. B., 1970, The Texas lineament and its America llemoir 152, p. 175-208. economic significance in southwest Arizona: Coney, P. J., 1978, The plate tectonic setting of Economic Geology, v. 65, p. 166-181. southeastern Arizona: New Hexico Geological Hilson, E. D., Hoore, R. T., and Cooper, J. R., 1969, Society Guidebook, 29th Fieln Conference, p. 285­ Geologic map of Arizona, scale 1 :500,000: U.S. 290. Geological Survey State Geologic }mp Series.

67

HINERAL OCCURRENCES IN THE I!HIPPLE HOUNTAINS WILDERNESS STUDY AREA, SAN BERNARDINO COUNTY, CALIFORNIA

James Ridenour, Phillip R. Hoyle, and Spencee L. Hillett u. S. Department of the Interior, Bureau of Mines Western Field Operations Center E. 360 Third Avenue Spokane, Washington 99202

ABSTRACT 1976), U. S. Bureau of Hines personnel conducted an examination of mines, prospects, and mineralized The Hhipple Mountains Wilderness Study Area (HSA) areas within and near the \fuipple Hountains Hilder­ encompasses approximately 85,100 acres in San ness Study Area (WSA). Field investigations were Bernardino County, California. An examination of conducted from mid-Harch to mid-Hay 1980, t-lith sub­ mines, prospects, and mineralized areas within and sequent short visits to the area in September 1980 near the study area was conducted during the spring and October 1981. During our investigations, 643 of 1980, by U. S. Bureau of Hines personnel under the hard rock samples and 39 petrographic samples were provisions of the Federal Land Policy and Hanagement collected from the 49 mines and prospects shown on Act. figure 1. Analytical results of these samples, detailed descriptions of sample localties, maps of U. S. Bureau of Hines statistical files indicate workings (where applicable), and background data that about $84,000 worth of copper-gold-silver ore regarding mineral potential of the study area, are was produced from mines in the Whipple mining contained in a report at the Bureau of Mines Western district, exclusive of Copper Basin, from 1906 to Field Operations Center, Spokane, \Iashington. 1969. Prospecting targets were the ubiquitous occur­ rences of chrysocolla, malachite, and hematite in A synopsis of our work plus geological, geo­ fracture zones both above and below the Whipple chemical, and geophysical investigations of the study detachment fault, the dominant structural feature in area conducted by the U. S. Geological Survey will be the HSA. published as a Summary Bulletin and ~W-series maps in the near future. The predominant element assemblage consists of barium-manganese-iron-copper-gold-silver and is found The purpose of this paper is to report our in a wide variety of host rocks ranging in age from observations of the study area's mineral occurrences Precambrian (?) to Hiocene. Petrographic analyses of with emphasis on their geologic setting, and to selected copper-gold-silver occurrences indicate comment on their time-space relationships to detach­ these elements are constituents of mineral assem­ ment faulting, probable origin, and mode of distribu­ blages largely composed of quartz, sericite, tion. chlorite, and sulfides which have been altered to secondary copper and iron oxide minerals. These Previous Investigations assemblages are indicative of an epithermal to perhaps lower mesothermal temperature range. Library and file research revealed many Structural analysis of fractures associated with references concerning geology and mineral resources these occurrences indicates distinct populations within and near the WSA. Davis and others (1980) above and below the detachment fault that are the present their latest findings regarding structure, result of crustal lengthening in the lower plate and stratigraphy, petrology, geochronology, kinematics of dislocation of the upper plate. The younger age detachment and listric faulting, and geologic history limit for these occurrences is believed to be the in the Ilhipple Hountains. The reader is referred to Osborne Hash Formation (mid-Hiocene); the older age this publication for a more detailed list of previous limit for these occurrences is thought to be early in investigations dealing with these subjects. Recent the dislocation process, or late Oligocene, as is published mapping includes Dickey and others (1980) suggested by a 24.5~0.7-m.y. K-Ar date of sericite in the southern part of the study area and a com­ from one of the localities. Collectively, these pilation of the Needles 1° X 2° sheet by Stone and indicate a temporal relationship between minerali­ Howard (1979). Site specific geological studies in zation and dislocation in the Whipple Mountains. the vicinity of the study area have been conducted by Frost (1980) and Anderson and Frost (1981). Early The conceptual mineralization model suggested by mining activity in the area has been recorded by Root these relationships is a low to moderate temperature (1909), Bancroft (1911), and Jones (1920). Specific hydrothermal system spatially related to the detach­ descriptions or notations of various mines, ment fault and chlorite breccia zone immediately prospects, and mineralized areas in the region are below it. Communication of fluids into the upper given by Bailey (1902), Turner (1907), Aubury (1908), plate was provided by listric fault and antithetic Graeff (1910), Stevens (1911), Hilson (1918), fault conduits formed during dislocation of the Cloudman and others (1919), Tucker (1921), Tucker and allochthon. The sources of metallic elements for Sampson (1930, 1931), Noble (1931), Trask and others this model are thought to include pre-dislocation (1943), Tucker and Sampson (1943), Eric (1948), Trask mineral deposits and syntectonic volcanic­ (1950), Wright and others (1953), Trengove (1960), metallogenic pulses. and Fleury (1961). Several previous property examina­ tions (unpublished) have been made by U. S. Bureau of INTRODUCTION Hines and California Division of Hines and Geology personnel. The tectonic history of the Vidal-Parker General Statement region has been presented by Carr (1981). A histori­ cal overview of mining in the California desert has Under the provisions of the Federal Land Policy been written by Vredenburgh and others (1981). and Management Act (Public Law 94-579, October 21, Studies on Cordilleran metamorphic core complexes, of

69 o (Y) ~ -l- I +

0." (/", 0-;> d",-,...y--r2lJ..-.7 1--~ ~!,n ","oX 'y~. ~ '<"" ___ ~-4.-____ ~ 'r

X War Eagle Mine ~ () o ~ <) r-- ~P~ r---- "-

f' X Blue Bird "\..~ Prospect F . kef ,o/ate Mine v.. rX.... XNlc \- ~ X. Lucky Green Group X "'ftPrQspect ~ ---.. --"" Prospect ~ Double MNo.---1X~ X Lizard Group Co ~ X p';;;;e>-' '\r Red Eagle Group X X Prospect P --r-34°15' ~ + + X Golden Arrow + X Dickie Blue Cloud \ Mine ... x ~/aCkjaCk Group

o 1 2 3 4 5 MILES X Riverview Mine I i \ I' I 'I I; i ! o 1 ~ 3 4 5 6 7 8 KI LOMETERS

Figure 1. Mines and prospects in the Whipple Mountains WSA. Ivhich the lihipple Hountains is but one, have been capped by a 2-to 25-cm-thick, resistant, flinty recently compiled by Crittenden and others (1980) and microbreccia (Davis and others, 1980, p. 112); Coney and Reynolds (1980). Schuiling's work (1978) consistent striae directions of N 50±10° E have on the mineral assemblages of the Sacramento been measured on its surface, indicating transport Hountains was used as a comparison to the assemblages direction of the allochthon. we observed in the lIhipple t1ountains. Salient geologic history of the Whipple Nining in the Study Area ~lountains, as documented by Davis and others (1980), includes Late Cretaceous and/or early Tertiary Bureau of Hines statistical files show that 5,123 mylonitization and metamorphism accompanied by tons of ore have been reported from 56 mines in the synkinematic intrusion of adamellite to tonalite Hhipple mining district beginning in 1906 and ending sheets or sills, follolVed by Oligocene (?) to middle in 1969. The historic value of the ore is estimated Niocene dislocation of the allochthonous terrane. at about $84,000; approximately 48 percent of the The lihipple llountains were asymmetrically domed after value came from gold and about 45 percent from dislocation, and erosion has removed most of the copper, with silver, lead, and zinc constituting the upper plate terrane from the core area, leaving only remaining seven percent of the ore's value. In ad­ a few isolated klippen of allochthonous rocks except dition to gold-copper ores, about 2,500 tons of in the east-central part of the study area. manganese IVere produced from five mines between liorld liar I and the 1950's. Huch of the mining activity Description of Assemblages has been limited to intermittent extraction of rela­ tively small, high-grade pockets of mineralized The commodity assemblages in the Whipple rock. Prospecting targets were the ubiquitous oc­ Hountains consist of barite-manganese-iron and currences of chrysocolla, malachite, and hematite in copper-gold-silver. The barite-manganese-iron assem­ fracture zones. Underground mining IVas accomplished blage is found, in significant amounts, almost primarily by sinking shafts and subsequent drifting exclusively in upper plate rocks. Copper-gold­ along the mineralized zones. Production records silver-bearing occurrences, on the other hand, are indicate mining activity was most prevalent prior to found in a IVide variety of host rocks in both 10lver liorld liar I and from the depression years through and upper plate positions and are megascopically liorld liar II. indicated by the presence of chrysocolla, malachite, and iron oxides. Associated with these commodities HINERAL OCCURRENCES is a pervasive chlorite-dominant propylitic altera­ tion. The barite-manganese-iron-copper-gold-silver General Statement assemblage and its alteration products suggest epi­ thermal temperatures associated IVith Tertiary volcan­ The investigation of mineral potential of the ism (Park and l~cDiarmid, 1970, p. 344-345). Eight Hhipple Hountains liSA has given us a unique oppor­ samples IVere selected for detailed petrographic de­ tunity to merge our observations regarding its scription from seven localities within and near the mineral deposits with recent information on meta­ study area. Descriptions of these samples are morphic core complexes in general and the lihipple summarized in Table 1. We believe these samples to Hountains in particular. Because detailed studies of be representative of the ubiquitous copper-gold­ the origin, temperatures of formation, geochemistry, silver-bearing occurrences we observed during our and age of the deposits are beyond the scope of our field investigations. investigation, we hope that our observations will stimulate more definitive studies of the economic Petrographically the mineral assemblages are mineral assemblages of this and other metamorphic composed largely of quartz, sericite, chlorite, and core complexes. sulfides (now mostly altered to secondary copper and iron oxide minerals). Although not all constituents Geologic Setting are always present in a given sample, where they do occur the sequence is fracturing and concomitant The dominant structural feature of the study area quartz-sericite-chlorite alteration followed by is a low-angle detachment fault IVhich juxtaposes deposition of sulfides. contrasting lithologic terranes across its surface. As described by Davis and others (1980, p. 79), the Although there are some important differences fault betIVeen the assemblages described here and those investigated by Schuiling (1978), IVe believe his •.• separates a lower-plate assemblage of assemblage B (quartz+minor pyrite+hematite+ Precambrian to t1esozoic or lower Cenozoic chrysocolla+barite) most closely approximates the igneous and metamorphic rocks and their Whipple Nountain assemblages. Temperatures of deeper, mylonitic equivalents from an formation of the Sacramento Hountains assemblages allochthonous, lithologically varied upper ranged from 120 0 C to more than 310 0 C (epithermal to plate. lower mesothermal); paragenetic sequences indicate that certain constituents IVere deposited in stages The allochthonous units (Davis and others, 1980, p. IVithin distinct ranges of temperature and salinities 80) (Schuiling, 1978, p. 53), perhaps indicating multiple stages of deposition. Overall alteration and ••• include Precambrian to Nesozoic crystal­ geologic setting of these assemblages appears line rocks, Paleozoic and tlesozoic meta­ correlative with the lfuipple Nountain assemblages. sedimentary rocks, Hesozoic metavolcanic rocks, and Tertiary volcanic and sedimentary Structural Patterns rocks. Structures that control the occurrences of the Immediately belolV the lihipple detachment fault lies assemblages described here include tension gashes, an altered and highly disturbed zone termed the shears, and faults. These occur throughout the study chlorite breccia zone (Frost, 1980). This zone is area in nearly all types of rock in both upper and

71 Table 1. Petrographic Summary

Name Setting Rock type Assemblages (minerals in decreasing order of abundance) Paragenetic Summary

Whipple Wash prospect Lower plate Chlori te breccia I quartz-chlori te-sericite-sulf ides-limoni te-clay Fracturing followed by simultaneous introduction of silica, sericite. chlori te-c lay, and sulfides.

Crescent Group Upper plate Quartz vein I quartz-chrysocolla-limoni te-malachite-serici te Fracturing followed by introduction of vein quartz and chalcopyri te accompanied by silification and sericitic alteration; further fracturing followed by oxidation of chalcopyri·te to limonite, chryso­ colla, and malachite, accompanied by deposition of chalcedony.

Lortie Group Upper plate Brecciated porphyritic grani te I quartz-c lay-limoni te-chrysocolla Brecciation followed by introduction of chalcopyrite and its alteration to limonite and chrysocolla; alteration of plagioclase to clay. I\)"'" Nickel Plate Mine Upper plate I Adamellite microbreccia quartz-clay-limonite-chrysocolla-sulfides I Microbrecciation followed by introductfon of quartz, pyrite, chalcopyrite, and sericite. Later argillic alteration of plagioclase.

Lucky Green Group Lower plate I Granitic cataclasite qua rtz-seri ci te-chalcopyri te-limonite-malachi te I Fracturing followed by quartz-sericite-sulffde minerali- zation; supergene alteration of chalcopyrite to lhnonite and malachi te.

Blue Bird Mine Lower plate I Graphic granite breccia quartz-seric i te-nontronite (? )-bari te-chrysocolla-chlori te-limonite I Brecciation followed by introduction of quartz. non­ tronite (?). sericite, barite, and sulfides. Super­ gene alteration of sulfides to limonite and chrysocolla.

New American Eagle Mine I Lower plate I Sadie dacite porphyry (dike?) quartz-chlori te-sulf ides-sericite-limonite Def ormation followed by introduction of quartz. sulfides. chlori te, and sericite. Supergene limonitic alteration.

New American Eagle Mine I Lower plate 1 Siliceous volcanic breccia quartz-limonite-malachite-clay Brecciation followed by introduction of quartz and sulfides. supergene alter<:1-tion of sulfides to limonite, malachite. and clay. lower plate position; they have been identified within a zone approximately 1200 feet both above and N below the detachment surface. Faults, particularly listric normal ones, can be traced for several miles; shear zones and tension gashes can usually be traced along strike for only a few hundred feet.

Two hundred and eighty-six attitude observations of mineralized structures were separated into lower and upper plate populations and analyzed by com­ puter. The resulting Pi diagrams (figure 2) show a concentration of poles in the lower plate oriented 'I N 26° H, 48° NE; upper plate structures show a dominant cluster at N 45° II, 28° NE, and a secondary cluster centered about N 46° II, 66° SH. Upper plate data confirm the suspected structural control of these occurrences by NE-dipping listric and sympathetic normal faults and Slrdipping antithetic and bedding plane faults. The dominant lower plate Pole density structural grain is within limits predictable for 00.1% tension fractures resulting from the N 50~0° E 01.2% direction of crustal extension and direction of = movement of the upper plate. o ~2.3% _3.4% DISCUSSION a. n 176 _4-5% N Our conclusions regarding the mineral potential ~5.6% of the Hhi pple l'lountains liSA were based on the _6·7% observation that the majority of the mineral occur­ rences observed at the mines and prospects we mapped and sampled were spatially related to the detachment fault and the premise that they are temporally re­ lated to the dislocation process. Because ground preparation is the dominant physical control on the mineral occurrences, a temporal link between the processes of ~islocatlon and mineralization seems intuitive. As younger rocks, e.g., Osborne lvash Formation, have not been deformed by the dislocation process and are not known to contain occurrences of the type described here, a younger age limit of l3.5~.0-m.y. (Kuniyoshi and Freeman, 1974) is placed on the mineralization and suggests that, if the processes were coeval, mineralization may have ceased with cessation of dislocation or shortly thereafter. He must also assume that if the mineralization process continued into Osborne Hash time, these rocks were unaffected. b. n 110 The older age limit of mineralization is harder to define. These occurrences are found in rocks that may be as old as Precambrian (Davis and others, 1980, p. 86) or as young as Niocene (Davis and others, 1980, p. 95). Paragenetic relationships at the Lucky Figure 2. Pi diagrams of mineralized fractures in Green Group indicate that mineralization resulteu a. upper plate and b. lower plate rocks in the during extensional fracturing discordant to earlier lIhipple Nountains HSA. mylonitic fabric; those at the pr03pect in lower Hhipple Hash indicate that sulfides formed during or these fluids was the detachment fault and chlorite after formation of the chlorite breccia. Finally, a breccia zone, and that the feeder conduits into the K-Ar date on sericite from the Nickel Plate Nine is upper plate were listric and antithetic faults. The placed at 24.5~0.7-m.y., indicating probable structurally lmver limit of mineralization, as indi­ formation of mineralization at this locality during cated by the study area's barren core, may simply early stages of dislocation. reveal the boundary beyond which confining pressures exceeded hydrostatic pressures of the hydrothermal In summary, we interpret the mineral assemblages system. as having been transported in a low to moderate temperature hydrothermal system, coeval with dis­ Because this model requires either predislocation location of the allochthon, and deposited in or syntectonic sources (or both) for the metallic favorable physical and geochemical environments. As elements such as copper, gold, and silver, we offer the dislocation process spanned some ~10 million the following possibilities: years and apparently was not a continuous process, as indicated by local erosion into the chlorite breccia 1. The work of Anderson and Frost (1981) zone followed by renewed dislocation (Davis and demonstrates that a Cretaceous adamellite, a others, 1980, p. 95), so must mineralization have significant piece of which is in Copper Basin, was also been somewhat discontinuous, if the two proces­ cut by the detachment fault and transported by the ses were coeval. He believe the main conduit for dislocation process into the l!hipple Nountains area.

73 Hhile this suggests its mineralization may signifi­ personal communication, regarding petrologic and cantly predate dislocation and, therefore, may have structural aspects of the study area. Conversations been transported, at least in part, to its present with Sherman Harsh, U.S. Geological Survey, have site, the hypogene minerals associated with this helped us understand the probable geochemical condi­ deposit, as described by Heidrick (1980, p. 20), tions under which the minerals were formed. He also indicate an epithermal, volcanic-derived assemblage, wish to thank Bill Rehrig, Phillips Petroleum the constituents of which are also present throughout Company, for sharing his observations of similar the Ivhipple llountains and apparently temporally mineral occurrences and for critical review of the associated with dislocation. Therefore, the Copper manuscript. Thanks are extended to our colleague Basin mineralization may be a consequence of Steve Hunts for his technical review and many helpful dislocation rather than a contributor to the process. suggestions for improvement of the manuscript. He thank Keith Howard and the U. S. Geological Survey 2. The extreme western part of the study area is laboratory for geochronological determination of a dominated by nonmylonitic rocks intruded by a specimen collected during our investigation. Petro­ northHest-trending dike s'varm whose lithologies graphic thin sections were prepared by Pacific appear to range from rhyolite to andesite. A dacite Petrographics, Spokane, Hashington; descriptions of (?) from this swarm has been dated at 4l.9~6.7-m.y. these were done by Koehler Geo-Research Laboratory, (Eric Frost, personal communication). In apparent Grangeville, Idaho. He are indebted to Clayton association Hith this dike swarm are podiform to Horlock and Julia Bosma for their assistance during lensoid and disseminated occurrences of sulfides at field investigations. Finally, appreciation is ex­ the D&\J Hine (Aubury, 1908, p. 337) and NeH American tended to the U. S. Bureau of Hines for allowing us Eagle Hine (Hilson, 1918, p. 4-6; Hright and others, to prepare and publish this paper. 1953, p. 65). Because the sulfides formed later than the host rock, as indicated by paragenetic relation­ REFERENCES CITED ships at the New American Eagle l.fine, and the dike SHarm is in the lmver plate and thus truncated by the Anderson, J. L., and Frost, E. G., 1981, Petrologic detachment fault, We infer that these occurrences may geochronologic, and structural evaluation of the have been one possible source for remobilized allochthonous crystalline terrane in the Copper metallic elements. Basin area, Eastern Hhipple Hountains, south­ eastern California: unpublished report to the 3. Volcanic activity in the area both prior to Hinerals Division, The Louisiana Land and and during dislocation has been documented by average Exploration Company, LakeHood, Colorado, 63 p. dates of 27.8~.8 and l7.7~0.6-m.y. for Gene Aubury, L. E., 1908, The copper resources of Cali­ Canyon and Copper Basin volcanic rocks, respectively fornia: California State Hining Bureau Bulletin (Anderson and Frost, 1981, p. 36). In addition, Carr 50, 366 p. (1981, p. 18) cites evidence of intermediate to Bailey, G. E., 1902, The saline deposits of Califor­ caIn-alkaline volcanism ranging in age from about 22 nia: California State Hining Bureau Bulletin 24,. to 18 m.y., and basaltic volcanism beginning about 15 216 p. m.y. ago in the Hhipple !!ountains and vicinity. Of Bancroft, Howland, 1911, Ore deposits in northern particular importance to this system is any syntec­ Yuma County, Arizona: U. S. Geological Survey tonic volcanism that could have supplied metals to Bulletin 451, 130 p. the transporting conduit. Carr, II. J., 1981, Tectonic history of the Vidal­ Parker region, California and Arizona, in Tec­ CONCLUDING COHHENTS tonic framework of the Hojave and Sonoran Deserts, California and Arizona: U. S. Geological Survey Hhile we are relatively comfortable with the Open-file Report 81-503, p. 18-20. mineralization model presented here, we realize that Cloudman, H. C" Huguenin, E., and Herrill, F. J. H., much further Hork is necessary to define its para­ 1919, San Bernardino County, in Hines and mineral meters and that its application to detachment resources of California: Report XV of the State terranes elseh'here may not be valid. Nevertheless, Hineralogist, Part VI, California State Hining using this model, we have defined areas in the Bureau, p. 775-889. Hhipple Hountains Hhere we believe there is a greater Coney, P. J., and Reynolds, S. J., 1980, Cordilleran likelihood of discovery of potentially significant metamorphic core complexes and their uranium mineral resources. Because of dominant structural favorability: U. S. Department of Energy Report control of the occurrences, we believe larger scale GJBX-258(80), 627 p. structures should be examined in detail and with Crittenden, H. D., Jr., Coney, P. J., and Davis, modern exploration methods and tools. In addition, G.H., eds., 1980, Cordilleran metamorphic core thicker sections of upper plate crystalline rocks may complexes: Geological Society of America Memoir be likely hosts for entrapment of ascending hydro­ 153, 490 p. thermal fluids generated during dislocation as well Davis, G. A., Anderson, J. L., Frost, E. G., and as favorable terrane in which to explore for signifi­ Shackelford, T. J., 1980, Hylonitization and de­ cant remnants of predislocation mineral deposits. tachment faulting in the Hhipple-Buckskin-Rawhide Mountains terrane, southeastern California and ACKNOHLEDGEHENTS western Arizona, in Cordilleran metamorphic core complexes: Geological Society of America Memoir He wish to acknowledge the assistance of the many 153, p. 79-129. individuals who contributed to our knowledge of the YDickey, D. D., Carr, H. J., and Bull, H. B., 1980, study area. Special thanks are due Eric Frost, San Geologic map of the Parker ffiv, Parker, and parts Diego State University, who not only provided most of of the Hhipple Hountains SH and Hhipple Hash the geologic and structural information necessary to quadrangles, California and Arizona: U. S. our understanding the occurrence of the study area's Geological Survey Hiscellaneous Investigations mineral deposits, but also tirelessly answered our Series Map 1-1124. questions and offered much encouragement. Lawford Eric, J. H., 1948, Tabulation of copper deposits in Anderson and Greg Davis, University of Southern California, in Copper in California: California California, contributed clarifying points, through Division of Mines Bulletin 144, Part 3,

74 p. 199-429. p. 421-609. Fleury, Bruce, 1961, The geology, origin, and eco­ Turner, H. Y., 1907, The sodium nitrate deposits of nomics of manganese at Roads End, California: the Colorado: Mining and Science Press, v. 94, unpublished M. A. Thesis, University of Southern p. 634-635. California, 96 p. Vredenburgh, L. M., Shumlmy, G. L., and Hartill, Frost, E. G., 1980. Appraisal design data, Structural R. D., 1981, Desert fever: An overviel, of geology and water-holding capability of rocks in mining in the California Desert Conservation the l/hipple Hash area, San Bernardino County, Area: Living Press Hest, Canoga Park, California: Hhipple Hash pump storage project: California, 323 p. United States Hater and POlver Resources Service, Hilson, P. D., 1918, American Eagle Mine: un­ P. O. 0-0l-30-D71l0, 88 p. published private report for the Eagle Sulphide Graeff, F. H., 1910, Nitrate deposits of southern Copper Company, 11 p. California: Engineering and llining Journal, Hright, L. A., Stewart, R. M., Gay, T. E., Jr., and v. 90, p. 173. Hazenbush, G. D., 1953, Mines and mineral Heidrick, Tom L., 1980, Examination of Copper Basin deposits of San Bernardino County, California, mineralization and alteration, in Mylonitization, in California Division of Mines, v. 49, nos. 1 detachment faulting, and associated mineraliza­ and 2, p. 49-192. tion, Hhipple Mountains, California, and Buckskin Nountains, Arizona: Arizona Geological Society 1980 Spring Field Trip Guide, p. 19-21. Jones, E. L., Jr., 1920, Deposits of manganese ore in southeastern California: U. S. Geological Survey Bulletin 710-E, p. 185-208. Kuniyoshi, S" and Freeman, T., 1974, Potassium­ argon ages of Tertiary rocks from the eastern Ho jave Desert: Geological Society of America Abstracts with Programs, v. 6, no. 3, p. 204. Noble, L. F., 1931, Nitrate deposits in southeastern California: U. S. Geological Survey Bulletin 820, 108 p. Park, C. F., Jr., and llacDiarmid, R. A., 1970, Ore deposits: 2d ed., II. H. Freeman and Company, San Francisco, 522 p. Root, H. A., 1909, llining in western Arizona and eastern California: The llining Ilorld, v. 30, no. 20, p. 929-932. Schuiling, II. T., 1978, The environment of formation of Cu+Ag-bearing calcite veins, Sacramento Mountains, California: unpublished 11. S. Thesis, University of Arizona, 60 p. Stevens, H. J., 1911, The Copper Handbook, v. 10: H. J. Stevens, Houghton, Michigan, 1902 p. Stone, Paul, and Howard, K. A., 1979, Compilation of geologic mapping in the Needles 1° X 2° sheet, California and Arizona: U. S. Geological Survey Open-file Report 79-388. Trask, P. D., 1950, Geologic description of the man­ ganese deposits of California: California Divi­ sion of Nines Bulletin 152, 378 p. '----~I~la--n-g~n:~~s~:~o~~t:·~fa~~l~~:~~~~~·aS~~m~:;~' report, in Nanganese in California: California Divisionof llines Bulletin 125, p. 51-215. Trengove, R. R., 1960, Reconnaissance of California manganese deposits: U. S. Bureau of Mines Report of Investigations 5579, 46 p. Tucker, H. B., 1921, Hines and mineral resources, Los Angeles Field Division, Part I, in Report XVII of the State Mineralogist - llining in California during 1920: California State Mining Bureau, p. 263-390. ~__~--_' and Sampson, R. J., 1930, San Bernardino County, Los Angeles Field Division, in Mining in California: Report XXVI of the Stat;/lineralo­ gist, v. 26, no. 3, California Division of Nines, p. 243-325. ____~--_' and Sampson, R. J., 1931, San Bernardino County, Los Angeles Field Division, in llining in California: Report XXVII of the State llineralo­ gist v. 27, no. 3, California Division of Mines, p. 243-406. ------;c-::-' and Sampson, R. J., 1943, Mineral resources of San Bernardino County, in Quarterly chapter of State Hineralogis t' s ReportXXXIX: California Journal of Nines and Geology, v. 39, no. 4,

75

TECTONIC IMPLICATIONS OF CENOZOIC VOLCANISM IN SOUTHEASTERN CALIFORNIA

Kent Murray California Energy Co~~ission 1111 Howe Avenue Sacramento, California

ABSTRACT Volcanic rocks of early-to-middle Cenozoic age in the record of igneous activity (for example, Arm­ crop out within a number of block-faulted mountain strong and others, 1969; Armstrong and Higgins, 1973; ranges in southeasternmost California. Geologic Cross, 1973; Armstrong, 1974; Cross and Pilger, 1974; mapping, and K-Ar age determinations of the volcanic Silberman and others, 1975; Snyder and others, 1976). rock within the region, have disclosed a major tran­ sition from calc-alkaline intermediate and silicic If generation of calc-alkaline volcanism is volcanism to fundamentally basaltic volcanism. The genetically related to subduction and basaltic magma­ space-time distribution of the various volcanic as­ tism to crustal extension, then the changes in igne­ sociations provide constraints on the plate inter­ ous activity with time must reflect changes in plate action history along the west coast of North America interactions. Based on recent detailed mapping and in early to middle Cenozoic time. newly obtained K-Ar age-determinations in the Choco­ late, Palo Verde and Picacho Mountains of southeast­ Volcanism of calc-alkaline intermediate and ern California (Crowe, 1973; Crowe and others, 1979; silicic composition was initiated approximately Murray and Crowe, 1976; Murray, 1979, 1980, 1981, 33 Ma and probably reflects an early Cenozoic sub­ 1982, in press), this paper evaluates the timing and duction system off the coast of western North America. nature of the regional calc-alkaline to basaltic The termination of subduction, inferred from the volcanic transition in southeastern California youngest calc-alkaline silicic volcanic activity, oc­ (Fig. 1) in terms of recent plate tectonic theory. curred from 25 to 22 Ma. A southeastward time­ This information is then used to test and refine de­ transgressive transition to fundamentally basaltic tailed volcanic-tectonic models (Lipman and others, volcanism is then recorded from 19 to 13 Ma across 1971; Christiansen and Lipman, 1972) which have been southeastern California. The age of the volcanic developed to interpret the complex space-time transition, in accord with previously published vol­ patterns of Cenozoic igneous and tectonic activity canic-tectonic models, overlaps with the probable in terms of plate configuration changes. age of inception of Basin and Range faulting, the probable age of inception of the San Andreas fault A fundamental part of the Lipman model system in southern California, and is consistent relates extension within the continental plate and with a southward migration of the Pacific-Cocos subsequent basaltic volcanism to collision of the Ridge. East Pacific Rise with a continental margin trench. The resulting direct contact of the American and I NTRODUCTI ON Pacific plates progressively terminated subduction and replaced it with a north-south propagating right­ The rapid development of plate tectonics theory lateral fault system. within the last decade has prompted renewed research of the volcanic history and geophysical characteris­ A corollary of the model is that a time lag tics of the Basin and Range province (Atwater, 1970; should exist between the cessation of subduction and Scholtz and others, 1971; Thompson and Burke, 1974). the cessation of arc magmatism. Active arc magmatism As a result of this research effort, the voluminous presumably reflects the presence of a descending Cenozoic volcanic rocks of the southern Basin and slab of subducted lithosphere directly beneath the Range have been interpreted in terms of plate inter­ site of the arc (Lipman and others, 1971, 1972; action between the North America, Pacific, and Faral­ McKee, 1971; Christiansen and Lipman, 1972). Where lon plates (for example, McKee, 1971; Lipman and subduction is arrested at the surface, the cessation others, 1971, 1972; Christiansen and Lipman, 1972; of arc magmatism will be deferred until the trail­ Noble, 1972; Cross, 1973; Cross and Pilger, 1974, ing edge of the detached descending slab has passed 1979). In these studies, calc-alkaline volcanic beneath the arc. Similarly, if subduction is renew­ rocks of broadly andesite composition are presumed ed, the inception of arc magmatism will reflect a to form from magmas generated along subduction zones similar lag ti~e required for the leading edge of by partial melting of oceanic crust, and/or by melt­ the descending slab to reach an appropriate depth. ing of the adjacent upper mantle (Green and Ringwood, This lag time is a function of spreading rates, in­ 1968; Hatherton and Dickinson, 1969; Ringwood, 1977). clination of the subduction zone, and promimity of By contrast, abundant basaltic and associated minor the descending slab to the spreading center (see for rhyolitic volcanism presumably reflects crustal ex­ example, Cross and Pilger, 1979; Pilger and Henyey, tension which permits the rise of denser magmas gen­ 1977). Consequently, the lag time could range from erated by partial melting in the asthenosphere one to several million years. (Scholtz and others, 1971; Christiansen and Lipman, 1972; Pilger and Henyey, 1979). IMPLICATIONS OF VOLCANISM From studies of space-time distributions of In the context of plate tectonics, there is a Cenozoic magmatic rocks in the western United States, fundamental difference between basaltic magmas, and several investigators have noted patterns of change andesitic to dacitic magmas because of their presumed

77 « z o N

"" O',y IMPERIAL co. "" l'". ~) 10 t

11;;,=oml===1='

I I it /' N .•• "!1' : USA· '. _.r--,f " ---~'., Y ~------MfXICO ~ ------..--

Figure 1. Location map of the area of study showing the general distribution of volcanic rocks in southeastern California (Strippled area), modified from Jennings (1977). Af: Algodones fault; B: Blythe; C: Chuckwalla Mountains; Cf: Chiriaco fault; If: Imperial fault; LC: Little Chuckwalla Mountains; MW: Milpital Wash.; M: Little Mule Mountains; NC: northern Chocolate Mountains; 0: Orocopia Mountains; PV: Palo Verde Mountains; SCf: Salton Creek fault; SAf: San Andreas fault; SHf: Sand Hills fault; SC: southern Chocolate Mountains; Y: Yuma, Arizona. origins; however, the orIgIn of high-silica rhyolite, United States however, reveal that the distribution primarily ignimbrites, is less certain (Cross and of rhyolite more closely resembles that of the in­ Pilger, 1979). As noted previously, calc-alkaline termediate-composition volcanic rocks than that magmas of intermediate composition generally are con­ of basalt (Cross and Pilger, 1979). Further, field sidered to be generated by partial melting of oceanic mapping and K-Ar ages of high-silica rhyolites and lithosphere or asthenosphere along subduction zones. intermediate-composition volcanic rocks in south­ Scholtz and others (1977) suggested that, owing to eastern California confirm the overlap in age and low fluid pressures and high density, basaltic mag­ stratigraphy of the intermediate and silicic com­ mas are less capable of penetrating the lithosphere position rock units and clearly distinguish them under compressional conditions. However, under ex­ from the patterns defined by the hasaltic rocks. tensional regimes, basaltic magmas rise more easily through the normally faulted and extended lithosphere. Volcanic fields in southeastern California Reduction of pressure by extension also may cause in­ (Fig. 1) show fundamental changes in configuration creased partial melting and increased volume of mag­ and position through time (Fig. 2 A-C). Cenozoic ma (Cross and Pilger, 1979). Christiansen and Lip­ calc-alkaline volcanism in southeastern California man (1972) argued that abundant late Cenozoic basal­ began in mid-Oligocene time (Fig. 2A) following a tic volcanism in the Basin and Range province was pronounced period of early Cenozoic magmatic closely associated with rhyolitic but not andesitic quiescence (Cross, 1973; Cross and Pilger, 1979). or dacitic volcanism producing a "bimodal suite". Space-time distribution of the youngest calc-alkaline Recent map plots of isotopic ages in the western activity, obtained from the stratigraphically

78 Fig. 28 would be slightly younger than the cessation of subduction. Subduction off the west coast of North America at the latitude of southeastern Cali­ fornia, therefore was probably terminated 22 to 25 Ma. Figure 2C indicates the late Cenozoic history of basaltic volcanism in southeastern California. These dates relfect the oldest basaltic volcanism in southeastern California following the transition in volcanic associations. , ,The distribution of isotopic ages as depicted 1n F1gure 2B and 2C therefore clearly indicates a middle to late Cenozoic age for the transition of arc magmatism typified by calc-alkaline volcanism to A an extensional environment typified by fundamental basaltic volcanism. Moreover, although Fig. 2C is based on limited K-Ar age determinations, it suggests that the transition was time-transgressive and mi­ grated progressively southeastward, into southeastern California. Plate Interactions Along the Western United States Inferred from the History of Igneous Activity It is apparent that the patterns of volcanic and tectonic activity in southeastern California in the early to middle Cenozoic should not merely reflect the interaction between the Pacific North America and parallel plates but can be used to constrain their movements, and refine tectonic models based on sea­ floor spreading data alone. B Early to middle Cenozoic intermediate and sili­ cic volcanism commenced in southeastern California about 33 Ma following pronounced magmatic hiatus that may mark the presence of a proto from Andreas fault system in coastal California (Crowe and others 1979; Cross and Pi 1ger, 1979). The occurrence of ' intermediate and silicic volcanism in the southern Basin and Range of California suggests that a shal­ lowing dipping subduction zone was in place off the west coast of California at this time. This data for N f~r example, supports Coney's (1972, 1916) asser­ tlon that the Farallon, North America relative 1 plate convergence was perpendicular to the trend of the continental margin opposite southern Cali­ fornia,and Arizona, between 40 and approximately 30 c Ma. P11ger and Henyey (1977, 1979) incorporating Atwater and Molnar (1973) work, presented modified plate reconstruction from 38 Ma to the present and compared these with an early Cenozoic palinspastic Figure 2. Index map of southeastern California restoration of the western United States, including east-west Basin and Range extension (their Fig. 4, adapte~ from Crowe and others, 1979; Outline of the Mounta1n ranges follows Figure 1. A. Distribution pg. 198). They concluded that the initial contact of K-Ar ages documenting the age of inception of of the Pacific plate with North America occurred calc-alkaline volcanism in southectstern California. opposite the southern Coast Ranges, the Transverse Ranges and the northern Continental Borderland be­ Ages shown are the oldest K-Ar ages for individual tween 29 and 21 Ma., or slightly later and farther volcanic centers; the true age of inception of vol­ north than the first contact predicted by Atwater and canism is older by a small but unknown amount. 8. r~o Distribution of the youngest K-Ar ages for the sili­ 1nar (1973). cic volcanic sequence of Crowe and others (1979) and Murray (1980; 1981). These ages represent the ces­ Plate interaction history subsequent to Atwater sation of intermediate and silicic volcanic activity (1970) and Atwater and Molnar (1973) have adequately descri~e~ plate interaction history subsequent to 1n southeastern California. They correspond to but the ln1t1al contact of the two plates. Two triple are,slightly younger than the probable age of termi­ junctions, separated by the San Andreas fault trans­ nat10n of subduction in coastal southern California. form was created, and the transform lengthened as C. Di~tribution of K-Ar ages for the oldest capping t~e triple junction continued to separate. Subduc­ basalt1c sequences following the transition. t10n of remnants of the Farallon plate terminated highest ignimbrite activity in southeastern pro~r~ssively northward and southward of the point C~lifor~ia, reflects the termination of arc magma­ of 1n1tlal contact, corresponding to the northward t1sm (F1g. 28). Due to the inferred lag time, how­ migration of the Mendocino fracture zone and the ever, between cessation of subduction and cessation southward migration of the Pacific-Cocos Ridge. Fig­ of arc magmatism, the isotopic ages indicated on ure 3 is a schematic presentation of the history of

79 uncertain. What is certain, is that the primarily post-20 Ma basaltic volcanism in the southern Basin and Range province of California is chemically tran­ ~'#I-I.>S~S_F-:Lo=A=-_G_~ sitional between alkalic and tholeitic associations, A PRESENT typical of the capping basalts of the Great Basin in Nevada (Lesman and Rodgers, 1970; Best and Brim­ hall, 1974; Vitaliano, 197D). Moreover, experimental work (Yoder, 1976), petrographic and chemical stu­ B 10 MY dies (Murray, 1981), and geophysical studies (Roy and others, 1968; Scholtz and others, 1971) all in­ dicate that the basaltic magma must have risen ra­ pidly to the surface and was extruded with a minimum C 25-22 MY of differentiation. Due to its low fluid pressure, and high density, these magmas are more capable of penetrating the lithosphere only under extensional regions. The timing, therefore of the transition from intermediate to silicic volcanic activity D 29-21MY to fundamentally basaltic associations, as suggested by the early Lipman model, also constrains the tim­ ing of Basin and Range faulting.

II North American Plate Relation of Basin and Range Faulting to North S SF LA GM ::: > constant American-Farallon Plate Interaction E > 30MY 1111111111111111111111111111111 mollon

,,~~arallon.Plate Formation of the southern Basin and Range pro­ "'7L motion vince during the late Cenozoic may also be related Pacilic Plate to North American-Farallon plate interactions as implied above. Atwater (1970) suggested that extension within the Basin and Range province as a whole, was a result of broadly distributed, right­ Figure 3. Schematic model of plate interactions as­ lateral shear stress originating by the Pacific- suming the constant motion model of Atwater (1970). North American plate interaction along the San An­ The coast is approximated as parallel to the San dreas fault. Scholtz and others (1971), presented Andreas fault as in A~wa:er (1970) figure 5. Ini­ an alternative explanation for Basin and Range tec­ tials represent cities: S, Seattle; SF, San Fran­ tonism, hypothesizing that the termination of sub­ cisco; LA, Los Angeles; G, Guaymas; M, Mazatlan. duction removed the compressional forces acting Symbols follow McKenzie and Parker (1967): single across the Cordillera, allowing back-arc extension. lines are transform faults, double lines are spread­ In the detailed tectonic model of Christiansen and ing centers, and hatched lines are zones of subduc­ Lipman (1972) the authors adopted Atwater's hypo­ tion (usually trenches). Large arrows show motion thesis as a genetic relationship between the Basin of plates with respect to the pacific plate being and Range tectonism, and the northward lengthening held fixed. Captions show the time represented by of the San Andreas transform. Similarly, Snyder and each sketch in million of years before present and others, 1976, interpreted the Basin and Range to the distance that the North American plate must sub­ represent an ensialic interarc basin, accepting the sequently be displaced to reach its present position relationship of crustal extension to the lengthening with respect to the Pacific plate (adopted from At­ of the San Andreas transform. In the majority of water, 1970). Termination of subduction, begin- those models, the authors also relate the onset of ning 29 to 21 Ma (Figure D) eventually lead to basaltic volcanism with crustal extension. As dis­ cessation of arc volcanism back-arc extension and ul­ cussed above, a direct relationship is implied be­ timately basaltic volcanism. tween the termination of subduction, the initiation of strike-slip faulting, Basin and Range extension this plate interaction history (following Atwater, and basaltic volcanism. If this implication is va­ 1970) based on the work of Pilger and Henyey (1977, lid, then a close temporal and spatial correspondence 1979). The north to south trend of cessation of should exist between the various volcanic and struc­ intermediate and silicic volcanism of southeastern tural features manifested by the changing of tec­ California (Fig. 2B) correspond to the history tonic regimes. There appears to be well-documented predicted by these modern plate reconstructions evidence relating the timing and spatial association (assuming a small time lag) which describe a pro­ of the onset of basaltic volcanism to Basin and Range gressive southward termination of subduction, cor­ faulting and northward migration of the Mendocino responding to the southward migration of the Pacific­ triple junction. In the northern Basin and Range, Cocos Ridge (Figure 3 C-A). McKee (1971) and Noble (1972) both recognized that the onset of Basin and Range faulting was closely Whereas middle and late Cenozoic intermediate followed by the widespread inception of basaltic vol- . and silicic volcanism in the southern Basin and Range canism. Similarly, McKee and Silberman (1975) em­ province is apparently related directly to post-40 phasize a correspondence of major extension and ba­ Ma interaction of the North America-Farallon plates saltic volcanism at 16 to 17 Ma., throughout much and Pacific plate, post-20 Ma basaltic volcanism is of the Great Basin at that time. Snyder and others apparently related to crustal extension. Whether (1976) adopted the Christiansen and Lipman model and crustal extension however, is caused by back arc armed with a wealth of new K-Ar age determinations, spreading due to the release of compressive stress closely documented the correspondence in the timing following the termination of subduction (Scholtz of the onset of basaltic volcanism within the Great and others, 1971), or by a slowdown in the absolute Basin with the generally northward migration of the plate motion of North America due to interaction with Mendocino triple-junction, and concluded that the re­ the Farallon plate (Cross and Pilger, 1979), is un- lationship was too close to be ignored.

80 Direct application of Christiansen and Lipman's a close spatial and temporal relationship does exist 1972 model to the southern Basin and Range province between the termination of subduction, initiation of however, has led to many disparities in both the tim­ strike-slip faulting, Basin and Range extension and ing and spatial association of Basin and Range tec­ basaltic volcanism. tonism and the timing of the onset of basaltic vol­ canism (Cross and Pilger, 1979; Crowe and others, SUMMARY 1979) . An early hypothesis proposed by Christiansen and As summarized by Cross and Pilger (1979) right Lipman (1972), suggesting a genetic relationship lateral shear deformation occurs only along the between termination of middle Cenozoic subduction, western part of the Basin and Range province. The formation of the San Andreas fault, inception of eastern Basin and Range apparently is deforming by Basin and Range faulting and subsequent onset of east-west extension and associated left lateral in­ fundamentally basaltic volcanism still appears to tercontinental transform faulting. In addition, be valid in southeasternmost California if con­ Basin and Range faults extend farther north than the straints can be imposed on the timing and location landward projection of the Mendocino fracture zone of the initial interaction between the Farallon and (the northern terminus of the San Andreas fault). North American plates. Recent detailed mapping of Thus, Cross and Pilger suggest that there is no ob­ Cenozoic volcanic complexes in southeastern vious correspondence between contemporary Basin and California in concert with plate reconstructions, Range deformation and deformation along the San recent sea-floor spreading data and a palinspastic Andreas fault. In addition, the Lipman and Christ­ restoration of the western United States provides the iansen (1972) model based on plate reconstructions necessary constraints to refine the Lipman model. The of Atwater (1970), predicts extensional faulting, refined model results in an initial collision of the marked by the transition to fundamentally basaltic Farallon and North American plates occurring opposite volcanism to migrate northwestward into southeastern the Transverse Ranges between 29 and 21 Ma. This col­ Ca 1iforni a at 26 to 24 Ma. (correspondi ng to the lision yielded the formation of two triple-point northward movement of the Mendocino triple junction junctions, one moving northwestward, the Mendocino at this time and initiation of Basin and Range fracture zone; the other moving generally southward faulting). Extensional faulting, however, appears corresponding to the Pacific-Cocos Ridge. It is the to take place either much earlier (pre-32 Ma.; southward migration of the Pacific-Cocos Ridge and Crowe and others, 1979) or much later (21-17 Ma.; corresponding progressive southward termination of Murray, 1980, 1981, 1982). Further, the age of arc magmatism that is apparently documented by the transition to fundamentally basaltic volcanism as changing volcanic terranes of southeastern California. determined throughout southeastern California (Crowe and others, 1979) is clearly much younger varying REFERENCES CITED from 22 to 13 Ma (Fig. 2C). Moreover, as indicated in Fig. 2C, the transition to fundamentally basaltic Armstrong, R. L., 1974, Magmatism, orogenic timing, volcanism in southeastern California is actually in and orogenic diachronism in the Cordillera from a southeasterly direction, indicated by the south­ Mexico to Canada: Nature, v. 247, p. 348-351. eastward younging in the ages of the capping volcanic sequence (Fig. 2C). Armstrong, R. L., Eckel, E. B., McKee, E. H., and Noble, D. C., 1969, Space-time relations of DISCUSSION Cenozoic silicic volcanism in the Great Basin of the western United States: Am. Jour. Sci., The age of transition to fundamentall basaltic v. 267, p. 478-490. volcanism in southeastern California overlaps with the probably mid-Miocene age of inception of movement Armstrong, R. L., and Higgins, R. E., 1973, K-Ar on the San Andreas fault south of the Transverse datin9 of the beginning of tertiary volcanism Ranges (Crowell, 1973; Ehlig and others, 1975) in the Mojave Desert, California: Geol. Soc. and, this is in accord with the general concepts of America Bull., v. 84, p. 1095-1100. the Lipman model. The Lipman model however was based on limited sea floor'spreading data and little detail­ Atwater, T., 1970, Implication of plate tectonics ed field mapping, particularly in the southern Basin for the Cenozoic tectonic evolution of western and Range province of California. Recent mapping of North America: Geol. Soc. America Bull., Cenozoic volcanic complexes in southeastern Cali­ v. 81, p. 3513-3533. fornia (Crowe, 1973; Murray and Crowe, 1976; Crowe and others, 1979; Murray, 1979, 1980, 1981, 1982 in Atwater, T., and Molnar, P., 1973, Relative motion press) and more work involving modern plate recon­ of the Pacific and North America plates deduced struction theory (Cross and Pilger, 1979; Pilger and from sea-floor spreading in the Atlantic, Indian, Henyey, 1977, 1979) can be used to refine the Lipman and South Pacific Oceans, in Kovach, R. L., and model while still supporting its basic tenets. As an Nur, A., eds., Proceedings of the Conference example, field mapping in the Palo Verde Mountains, on Tectonic Problems of the San Andreas fault Black Hills, and Little Mule Mountains of southeast­ system: Stanford University Pubs. Geol. Sci., ern California (Murray, 1980, 1981, 1982 in press) v. 13, p. 136-148. have established a close spatial and temporal rela­ tionship between termination of intermediate and Best, M. G., and Brimhall, W. H., 1974, Late Cenozoic silicic composition volcansim, the initiation of alkali basalt magmas in the western Colorado Basin and Range faulting, and the subsequent onset Plateaus and the Basin Range, Transition Zone, of basaltic volcanism. These relationships suggest U.S.A., and their bearing on mantle dynamics: that Basin and Range faulting was initiated at Geol. Soc. America Bull., v. 85, p. 1677-1690. approximately 17 Ma. Reconnaissance mapping through­ out southeastern California tend to support these Christiansen, R. L., and Lipman, P. E., 1972, Ceno­ relationships on a regional level. There thus zoic volcanism and plate tectonic evolution of appears to be adequate field evidence to suggest that western United States, Part 2, Late Cenozoic:

81 Royal Soc. London Philos. Trans., Ser. A, p. 5031-5310. v. 271, p. 249-284. Jennings, C. W., 1977, Geologic map of California: Coney, P. J., 1972, Cordilleran tectonics North Am­ Calif. Div of Mines and Geology, California erican plate motion: Am. Jour. Sci., v. 272, geologic map series. p. 603-628. Leeman, W. P., and Rogers, J. J. W., 1970, Late Ceno­ ----, 1976, Plate tectonics and the Laramide Orogeny: zoic alkali-olivine basalts of the Basin-Range New Mexico Geol. Soc. Spec. Pub. 6, p. 5-10. province: Contr. Mineralogy and Petrology, v. 35, p. 1-24. Cross, T. A., 1973, Implications of igneous activity for the early Cenozoic tectonic evolution of Lipman, P. W., Prostka, H. J., and Christiansen, western United States: Geol. Soc. America Abs. R. L. 1971, Evolving subduction zones in the with Programs, v. 6, p. 587. western United States, as interpreted from igneous rocks: Science, v. 174, p. 821. Cross, T. A., and Pilger, R. H. Jr., 1974, Space/time distribution of late Cenozoic igneous activity ----, 1972, Cenozoic volcanism and plate tectonic in the western United States, Geol. Soc. America evolution of western United States, Part 1; Abs. with Programs, v. 6, p. 702. early and middle Cenozoic: Royal Soc. London Philos. Trans., Ser. A, v. 271, p. 217-248. -----, 1979, Origin of Basin-Range extension: Back arc spreading controlled by North American plate Mcl(ee, E. H., 1971, Tertiary igneous chronology of motion: Geol. Soc. America Abs. with Programs, the Great Basin of western United States­ v. 11, p. 74. implication of tectonic models: Geol. Soc. America Bull., v. 82, p. 3497. Crowe, B. M., 1973a, Tertairy volcanism east of the San Andreas fault, southeastern California: McKee, E. H., and Silberman, M. L., 1971, Geochron­ Stanford Univ. Publ. v. XIII, Proceedings of ology of the Great Basin of the western United the conference on tectonic problems of the San States-implications of tectonic models: Geol. Andreas fault system, Kovach, R. L., and Nur, A., Soc. America Bull., v. 82, p. 2317-2328. eds., p. 334-338. Murray, K. S., 1979, Cenozoic geology of the Palo Crowe, B. M., 1973b, Cenozoic volcanic geology of Verde Mountain Volcanic Field: Geol. Soc. the southeasternmost Chocolate Mountains, America Abs. with Programs, v. 12, p. 489. California (Ph.D. thesis): University of California, Santa Barbara, 117 p. ----. 1980, Implications of Cenozoic volcanism in the Palo Verde Mountain volcanic field, southeastern ----, 1975 a, Probable age of inception of basin­ California: South Coast Geological society range faulting in the southeasternmost Chocolate Guidebook, p. 256-259. Mountains, California: Geol. Soc. America Bull., v. 89, p. 259-264. ----, 1981, Tectonic implication of space-time patterns of Cenozoic volcanism in the Palo Verde Crowell, J. C., 1973, Problems concering the San Mountain Volcanic Field, southeastern California Andreas fault system in southern California, (Ph.D. thesis): University of California, in Kovach, R. L., and Nur, A., eds., Pro­ Davis, 132 p. ceedings of the Conference on Tectonic Problems of the San Andreas Fault System: Stanford ----,1982, in press, K-Ar Age Bracket on the Initia­ University, Pub. 13, p. 125-235. tion of Basin and Range faulting in southeastern California: Ischron West. Damon, P. E., and Mauger, R. L., 1966, Eperiogeny­ orogeny viewed from the Basin and Range Pro­ Murray, K. S., and Crowe, B. M., 1976, Geology of vince: Soc. Mining Eng. Trans., v. 235, the northern and central Chocolate Mountains, p. 99-112. southeastern California: Geol. Soc. America Abs. with Programs, v. 8, p. 1023-1024. Ehlig, E. B., Ehlert, K. W., and Crowe, B. M., 1975, Offset of the Upper Miocene Caliente and Mint Noble, D. C., 1972, Some observations on the Ceno­ Canyon formations along the San Andreas zoic volcano-tectonic evolution of the Great fault in southern California: California Div. Basin, western United States: Earth and Mines Spec. Rept. 188, p. 83-92. Planetary Sci. Letters, v. 17. p. 142-150. Elston, W. E., and Damon, P. E., Coney, P. J., Pilger, R. J. Jr., and Henyey, T. L., 1977, Plate tec­ Rhodes, R. C., Smith, E. I., and Bikerman, M., tonic reconstructions and the Neogene tectonic 1973, Tertiary volcanic rocks, Mogollon-Datril and volcanic evolution of the California Border­ province, New Mexico and surrounding region: land the Coast Ranges: Geol. Soc. America Abs. Geol. Soc. America Bull., v. 84, p. 973-982. with Programs, v. 9, p. 482. Green T. H., and Ringwood, A. E., 1958, Genesis of ----, 1979, Pacific-North-American plate interaction the calc-alkaline igneous rock suite: Contr. and Neogene volcanism in coastal California: Mineraology Petrology, v. 18, p. 105-162. Tectonphysics, v. 57, p. 189-209. Hatherton, T., Dickinson, W. R., 1969, The relation­ Ringwood, A. E" 1977, Petrogenesis in island arc ship between andesite volcanism and seismicity systems in Talwani, Manik, W. C. III, eds., in Indonesia, the lesser Antilles, and other is­ Island Deep Sea Trenches and Back Arc Basins: land areas: Jour. Geophys. Research, v. 74, Am. Geophys. Union, Maurice Ewing Ser. 1, p.

82 311-331. Rotstein, Y., 1974, Geophysical investigations in southeastern Mojave Desert, California (Ph.D. thesis), University of California, Riverside, California, 175 p. Roy, R. F., Blackwell, D. D., and Birch, F., 1968, Heat generation of plutonic rocks and continen­ tal heat flow provinces; Earth and Planetary Sci. Letters, v. 5, p. 1-12. Scholtz, C. H., Barazangi, M., and Sbar, M. L., 1971, Late Cenozoic evolution of the Great Basin, western United States, as an ensialic interarc basin: Geol. Soc. America Bull., v. 82, p. 2979-2990. Sheridan, J. F., Stuckless, J. S., and Fodor, R. B., 1970, A Tertairy silicic cauldron complex at the northern margin of the Basin and Range province, central Arizona. U.S.A.: Bull. Volcan., v. 34, p. 649-662. "'- Silberman, M. L., Noble, D. C., and Bonham, H. F., 1975, Ages and tectonic implications of the transitions of calc-alkaline andesite to basal­ tic volcanism in the Great Basin and Sierra Nevada: Geol. Soc. America Abs. with Pro­ grams, v. 7, p. 375.

Snyder, W. S., Dickinson, S. R., and Silberman, M. L., 1976, Tectonic implications of space-time patterns of Cenozoic magmatism in th~ western United States: Earth and Planetary Sci. Letters, v. 32, p. 91. Thompson, G. A., and Burke, D. B., 1974, Regional geophysics of the Basin and Range province: Ann. Rev. Earth and Planetary Sci. Letters, v. 2, p. 213-238. Vitaliano. C. J., 1970, Capping volcanic rocks: west­ central Nevada: Bull. Volcani. v. 34, No.2, p. 617-635. Yoder, H. S. Jr., 1976, Generation of Basaltic magma: Washington. D. C., National Academy of Sciences, 265 p.

83

PRELIMINARY REPORT ON DIKING EVENTS IN THE MOHAVE MOUNTAINS, ARIZONA

JOHN K. NAKATA

U.S. Geological Survey 345 Middlefield Road Menlo Park, California 94025

INTRODUCTION Young gabbro and andesitic dikes, class 1;

Swarms of per-Tertiary and Tertiary dikes intrude The Class dikes appear to be basaltic Precambrian crystalline rocks in the Mohave Mountains, (gabbroic) and andesitic in composition. Prominent western Arizona. This range lies in a terrane of exposures of the gabbro occur in the northeastern Oligocene(?) and Miocene detachment faulting described portion of the area mapped, where the dikes trend N. by Davis et al. (1980), Carr (1981), and Rehrig 200 -30 0 W., dip vertically, and truncate both the (1981). The dike swarm records repeated magmatism and leucocratic suite 4 and the young mafic dikes 3. In apparent northeast-southwest crustal distension. This this area they are medium-grained, hypidiomorphic paper briefly summarizes the characteristics and granular mela-gabbros. Fresh surfaces are dark orientation of the dikes. grayish green but weather to dark brown. The andesite and andesite porphyries are grayish green on fresh Six different diking events are currently surfaces and weather to light brown to pale orange and recognized in the Mohave Mountains Fig.1 ). occur on the western side of the range. Both of these Radiometric ages for the dikes are not currently rock types truncate the young mafic dikes,3, but their available, but ages can be inferred from similar dikes ages relative to each other and to the trachytic­ in adjacent areas. Crosscutting relations establish textured dikes, 2, have not been established. relative ages, and dikes are assigned numbers that increase with age (Fig. 2). The most abundant are Trachytic-textured dikes, class 2; dikes of Tertiary age that represent four intrusive events. The pre-Tertiary dikes include Cretaceous(?) The traychytic-textured dikes contain prominent granite intrusions and pre-Cretaceous (Precambrian?) plagioclase phenocrysts 2 to 12 rom in length in a dikes. The following descriptions of dike grayish andesitic (?) matrix (these may be related to characteristics are based primarily on field the "jackstraw porphyry" described by Pike and Hansen observations and are summarized in Table 1. this volume). The most prominent exposures are

1 14° 22.5 southeast of the intersection of the V-shaped fault, where the dikes trend southeastward and truncate large old ophitic diabase dikes. Elsewhere, they also cut the young mafic dikes, 3.

Young mafic dikes, class 3;

The young mafic dikes are most abundant east of the V-shaped bifurcating fault; they occupy a zone approximately 7 km wide trending N. 65 0 _88 0 W. and dip moderately to steeply to the east. They are gray to black on fresh surfaces but weather to a dark brown. These weathered surfaces are often pockmarked owing to differentially weathered phenocrysts. The dikes range in width from 0.3 to 10m; the average width is 2.3 m. Dikes less than 0.5 m wide are mostly fine grained. Those of greater width are commonly symmetrically zoned and have chilled margins, an intermediate porphyritic zone, and a medium-grained core •

Leucocratic suite, class 4; 00' This suite includes a variety of dike types that cut the old ophitic diabase and are in turn cut by the young mafic dikes. Dikes of this suite that cut Miocene volcanic rocks on the west and south sides of the Mohave Mountains establish a maximum age. The FIGURE 1. INDEX TO QUADRANGLE MAPS OF THE rocks are aplite, rhyolite, dacite and andesite(?) with textures ranging from hypidiomorphic granular to MOHAVE MOUNTAINS AREA. THE SHADED AREA aphanitic. The granular outcrops weather to knoblike forms typical of desert areas, while the fine-grained REPRESENTS LIMITS OF MAPPING. equivalents form spires. They are cream to light gray on fresh surfaces but weather to pale orange and all shades of brown. Dark-gray chill zones are common and grade inward toward a light-colored central zone.

85 ALLUVIAL COVER

o -<\

N +34° 32'

0 114 17' 30"

I---~I 1 Kilometer o I-----~I 1 Mile

FIGURE 2. DISTRIBUTION OF DIKES IN THE MOHAVE MOUNTAINS, ARIZONA

86 EXPLANATION

FAULTS DIKES

oo Young gabbro and andesitic dikes BLOCKS

~A Trachytic - t extured dikes

Lib V oung mafic . Blocks of apparently rotated dikes !3F4'] dIkes , ../)..l2.; Leucocratic suite dikes

~ Granitic dikes

® Old ophitic dIabase. dikes

35' 00" +

0 114 05' 00' I

87 TABLE 1. SUMMARY OF DIKE CHARACTERISTICS

(J) (f) W --l - (..)1- (f) (f) -w FIELD NAME RELATIVE AGE CRITERIA W a: z ATTITUDES z

Gabbro N. 30 0 W Bot h cut I he young mafic & dark gray: weathers YOUNG DIKES Gabbro: verticle dip bul their rela t ive position 10 dark brown Gabbroic rocks (3- 5) with respect 10 class 0 Andesite: gray-green; A n Ii e sit eN. 4 os.. 5 0° W. + moderate to steep weathers Andesitic roc l( s d ik es rema in unc Ie a r 10 light 8 dips to the NE brown and pale-orange

Gr a y; X ::; 3.5 N. 60~850W TRAC HY TIC -T EX TURED Cut y Dung mafic ,& weathers 10 brown moderate to steep co ® + (1- 9) CO dip s tot he NE MIOCENE t 6.5 % I GTa y 10 b lac k; N. 70~8cPW. X = 2.3 Truncate all o Ide r dik es moderate to steep YOUNG MAFIC & weathers 10 dark b Town + (0.3- 1 0) dips 10 the NE

Cream 10 light-gray; N. 60~80oW. X.;:. 6 weathers pale-orange 10 moderate dips 8] LEUCOCRATIC SUITE cuI old ophit ie diabase ® brown to the NE + ( 1-20)

Contains xenoliths of Light gra y; N. 200 W. steep dips to the NE GJ GRANITE old ophitic diabase CRETACEOUS weathers 10 light-brown ( I-C) + I i PRE-CRETACEOUS Olive-gray; X'" 3.7 OLD Col by leucocratic ~uite N. 20°_40° W OPHITIC DIABASE ® PRECAMBRIP,N ? weathers fo olive-brown high dips to the NE end young diabase + (1-20 ) \ These dikes are found throughout the Mohave Mountains All the faults portrayed in figure 2 cut the dike but are most abundant on the west side of the range. swarm. If these faults are Miocene in age like other They strike N. 500 _80 0 W. and dip predominantly to the major faul ts in the region (Davis et al. , 1980; northeast, but their strike changes significantly to Carr, 1981), this establishes a Miocene age for the east-west at the northern end of the range. The dikes four youngest dike swarms. The dikes are not only range in thickness from 1 to 20+ m and average 6 m in faulted, but in places show anomalous trends width. A dense swarm of these dikes in the southwest suggesting that they are part of rotated landslide part of the map area occupies as much as 90 percent of blocks. the rock; the densest patch is stippled on the map (Fig.2). This area was not included in the dike-width ACKNOWLEDGMENTS calculations. I would like to express my appreciation to Keith Granitic dikes, class 5; Howard for his helpful comments. Granitic intrusions ar~ limited to the northern REFERENCES part of the map area. The medium-grained hypidiomorphic-granular rock consists of quartz, Carr, W. J., 1981, Tectonic history of the Vidal­ feldspar, biotite, and hornblende. These granite Parker region, California and Arizona, in Howard, K. dikes connec t with a small stock (not shown in fig. A., Carr, M. D., and Miller, D. M., eds., Tectonic 2). The granite contains xenoliths of the old ophitic framework of the Mojave and Sonoran Deserts, diabase and the Precambrian gneisses that range in California and Arizona: U.S. Geological Survey Open­ diameter from 10 to 30+ em. The granite is cut by File Report 81-503, p.18-20. both class 4 and 3 dikes. Lithologic similarity to granites to the west in the Mojave Desert suggest a Davis, G. A., Anderson, J. L., Frost, E. G., and Cretaceous age (John, 1981). Shackelford, T. J. , 1980, Mylonitization and detachment faulting in the Whipple-Buckskin-Rawhide Old ophitic diabase, class 6; Mountains terrane, southeastern California and western Arizona: Cordilleran metamorphic core complexes: Dikes that resemble pre-Cretaceous diabases in Geological Society of America Memoir 153, p.79-129. the nearby Whipple Mountains (Davis et al.,1980) and the Precambrian diabases of the Hualapai Mountains to John, B. E., 1981, Reconnaissance study of Mesozoic the northeast are the oldest in the Mohave plutonic rocks in the Mojave Desert region, in Howard, Mountains. These dikes are characterized by a medium­ K. A., Carr, M. D., and Miller, D. M., eds., Tectonic grained ophitic and subophitic texture. Local framework of the Mojave and Sonoran Deserts, resistant outcrops are patinated a dark purplish California and Arizona: U. S .Geological Survey Open­ brown, but typical exposures have negative relief and File Report 81-503, p.48-50. consist of olive-brown surface float. These diabases occur as steeply and shallow dipping dikes and sills, Rehrig, W.A., 1981, Principal tectonic effects of the however only the dikes are portrayed in Fig. 2. They mid-Tertiary orogeny in the Sonaran Desert province, trend approximately east-west in the eastern part of in Howard, K A., Carr, M. D., and Miller, D. M., eds., the range, curve northwestward in the west-central Tectonic framework of the Mojave and Sonoran Deserts, portion, then bend around again to an east-west strike California and Arizona: U. S.Geological Survey Open­ in the northwestern part of the area mapped. They File Report 81-503, p.90-92. have an average thickness of 3.7 m (with a range of 1 to 20 m) and dip at moderate to steep angles to the east. Small remnants throughout the range suggest that these dikes may have been more extensive than recognized but are obscured by the overprint of Tertiary dikes.

STRUCTURAL SUMMARY

The distribution of the dikes can be used to infer certain significant structural events in the Mohave Mountains. After the intrusion of the pre­ Cretaceous (Precambrian?) diabase, the dikes were deformed into an S-shape. This warping does not seem to affect the Cretaceous(?) granitic intrusions, so the deformation may be older. The granitic dikes were recognized only in the northwestern Mohave Mountains where they bring up xenoliths of the older diabase and Precambrian gneiss.

The oldest Tertiary diking event intruded leucocratic rocks and distended the Mohave Mountain block in a direction N. 10 0 -300 E. These were followed by the intrusions of the class 3,2, and 1 dikes that have basically the same trend as the leucocratic suite. In a southwest to northeast transect across the range, measurements showed that the Tertiary dikes occupy 16.5 percent of the area. This amount records NNE-SSW crustal distension of a least 1.75 km.

89

COMPLEX TERTIARY STRATIGRAPHY AND STRUCTURE MOHAVE MOUNTAINS, ARIZONA: A PRELIMINARY REPORT

by

J.E.N. Pike and V.L. Hansen U. S. Geological Survey Department of Geology Menlo Park, CA 94025 University of Montana Missoula, MT 59801

ABSTRACT STRATIGRAPHY General Relationships Tertiary volcanic and sedimentary rocks of the Mohave Mountains, Arizona are divisible into three The Tertiary rocks of the Mohave Mountains, sequences: an oldest (early Miocene) unit of mafic and Arizona, include several tilted and faulted volcanic silicic flows, pyroclastic and sedimentary rocks, a and sedimentary sequences, one or more intrusive middle unit of similar rock types, but dominantly centers, and extensive dike swarms that intrude pre­ sedimentary rocks, and a youngest unit that consists Tertiary and Tertiary rocks (Nakata, 1982, this principally of fanglomerate and thin silicic and volume). The deformed volcanic/sedimentary units basaltic flows. The middle and youngest units trend southeast from The Needles at the Colorado River probably are middle Miocene in age. Except for the to the Bill Williams Mountains along the western flank uppermost layer of the oldest unit, individual strata of the range (Figure 1). Isolated fault-bounded have little lateral continuity, and lithologies vary slivers of Tertiary volcanic and sedimentary rocks with locality. The oldest unit is complicated further occur north of the gneiss and granite core of the by repeated, sometimes voluminous intrusion. The range, but no Tertiary sequences have been found in oldest unit lies unconformably on Precambrian gneiss the interior of the range, or along its northeast along the western flank of the range. Where contact flank. relations are exposed, the middle unit is separated from the overlying and underlying sequences by angular unconformities. In several localities the youngest Our preliminary work indicates that the Miocene unit overlies the oldest unit in marked angular volcanic and sedimentary rocks are divisible into unconformity. three units (Figure 2): the oldest is steeply tilted and dips are commonly to the south or southwest. It Too few radiometric ages have been determined lies unconformably on Precambrian gneisses along the from the Mohave Mountains to establish the timing of western and southwestern margin of the range, and eruptive and tectonic events. A small number of K-Ar consists dominantly of interbedded mafic and silicic ages from nearby ranges and of two Mohave Mountains lava flows and tuffs, with subordinate sedimentary rocks suggest that mafic volcanism and fault-induced units. Mafic flows are more common in the lower part deformation occurred between 21 and 18 m.y. ago. of this section and the top is marked in places by a Renewed faul ting, volcanism and rapid sedimentation blue-sanidine-bearing tuff. None of the rock types in occurred following silicic intrusion into the oldest this unit has great lateral extent in the Mohave unit, possibly before deposition of the Peach Springs Mountains. The middle unit (Figure 2) overlies the Tuff of Young and Brennan (1974) about 18 m.y. ago. oldest sequence in angular unconformity. This unit Determination of the timing of these events in the normally dips more steeply at the base than at the top of the sequence, although we have measured dips up to Mohave Mountains may aid understanding of low-angle 0 extensional (II detachment") faul ts and related 45 locally in the upper levels of the sequence. Like structures in the Mohave and Whipple Mountains, and the oldest unit, this unit consists of laterally other nearby mountain ranges. discontinuous flows, tuffs, and sedimentary rocks; unlike the oldest unit sedimentary rocks are dominant. The youngest unit (Figure 2) overlies both SETTING the oldest and middle units with marked angular unconformities. The contact of the youngest over the The Mohave Mountains of western Arizona, a oldest unit is exposed in the areas of the Lake Havasu northwest-trending range, is composed largely of pre­ City dump and Standard Wash; that of the youngest over Tertiary metamorphic and igneous rocks. Tertiary the middle unit is exposed south of Tumarion Peak, and volcanic and volcaniclastic sedimentary rocks comprise also in Standard Wash (Figure 1). The youngest unit a small part of the range. The Mohave Mountains are comprises alluvial fan materials derived dominantly separated from the Chemeheuvi and Whipple Mountains, from the older volcanic-sedimentary rock sequences, both composed largely of similar pre-Tertiary rocks, and less commonly from Precambrian rocks; dips in this by the Colorado River. To the north are extrusive unit range from 50 to 25 0 • Locally the fanglomerate Tertiary volcanic rocks that comprise the southern end is interleaved with and overlain by very gently­ of the Black Mountains, and to the northeast are the dipping silicic tuffs and flows, and capped by thin Hualapai Mountains, an outlier of the Colorado basalt flows. Plateau. A fault separates the Mohave Mountains from the Bill Williams Mountains to the southeast. This All three sequences can be identified along the range too is composed largely of pre-Tertiary gneiss, entire western flank of the Mohave range, but no amphibolite, and granitic rocks. single lithologic member is present everywhere, so there is no reliable marker for any of the units.

91 EXPLANATION GEOLOGIC SYMBOLS + Antiel ine "'- Contacts pTu + Syncline '" Stri ke and di p of strata TVS~ r Nanna 1 faul t - bar on downth rawn si at:: "" Overturned strata ~ ~ Low-angle fault Vertical strata /~ \ <1:>- I Q Faul t ,)f unknm,n character ;;< Altitude location faults and contacts dashed where a,'proximate

ROCK UNITS Quaternary deposits \ , Tvs Tertiary volcanic and pou Q sedimem:ary un; ts Tertiary 'i(l,~rusive N centers pTu Pre-Tertiary rocks, l1r,div;cJEJ

Cl'QBSman Peak pGu Precambrian r-oc~~s, 'it pou undi vi ded 157& n pGg Precambrian granitic F'ock5.

fla p A,'ea

G 1000 SCALE (meters)

Figure 1. Sketch map of locations and generalized geologic relationships, Mohave Mountains, Arizona

Lithologies of each rock type vary from northwest to Elsewhere, rhyolitic flows and breccia form the southeast, as does the type of alteration. Thus, lowermost part of this unit. Coarser-grained rocks although the sequences are generally similar, rocks at apparently were derived entirely from granitic and each 10ca11ty have distinct characteristics. metamorphic rocks, or from both volcanic and Intrusion and accompanying alteration of the sequences metamorphic/granitic materials. Clasts in the basal further complicates the stratigraphic record. conglomerates invariably resemble the Precambrian crystalline rocks they unconformably overlie. A common red-weathered zone in the underlying gneiss, Volcanic and Sedimentary Rocks and red matrix in basal Tertiary sedimentary layers, Oldest unit suggest deep pre-Miocene weathering.

The oldest volcanic-sedimentary unit ranges from Other lithologies in the lower part of the oldest 600 to 1200 m in thickness and comprises interbedded unit are arkosic and tuffaceous sedimentary rocks, mafic and silicic flows, silicic (and rarely mafic) boulder conglomerates and megabreccias, abundant mafic tuffs and volcanic breccias, and sedimentary rocks lava flows, and interbedded platy silicic flows. that range from arkosic sandstone and conglomerate to Chemical analyses of these rock types are not yet rocks dominantly composed of reworked volcanic available, but in the field the flows appear to be materials. A whole-rock K-Ar age of 21.1 m.y. andesite and platy dacite. All the rocks are (Conoco, unpublished data) has been derived from a characterized by abundant plagioclase and one or a mafic flow near the base of this sequence. Dips on combination of the mafic minerals clinopyroxen'e, strata in the oldest unit range from 450 to vertical, hornblende, and biotite. Many of the mafic flows have and locally are overturned. In general, dips are platy plagioclase phenocrysts ("turkey-track" or steepest near the basal contact, although overturned " jackstraw" porphyry) . Finer-grained equivalents strata are present locally near the top of the commonly have equant dark-green clinopyroxene, and sequence. equant spots of iron clay, probably altered from olivine. Whether or not these volcanic rocks In some localities the basal strata are originally had calc-alkaline affinity, or were a sandstones, shales, and conglomerates that range from basal t-rhyo11te bimodal association, is difficult to several to several hundred meters thickness. assess because the rocks are extremely altered.

92 QUATER:-iARY DEPOS I TS Fang]\lmeralt2

TERT JARY UNITS (~liucene) Unl"llnfnrmity

basalt flows --rhyolite flows; welded tuff

Youngest Unit

{ ~==="'-<----__I__:::::::e::::;w:::C::::l:::C:::riX

Unconformity I- ,--quartz- and reworked tuff-sandstone --purplish shale or claystone with thin 1------; airfall tuff or reworked tuff interbeds Niddle Unit --conglomeratic sandstone

[ _ ----boulder conglomerate L-==::::~:=2=====d-_-lahardeposits Unconformity Tuff of Peach Springs Unconformity? airf3ll tuff with basal t interbeds silicic intrusive rocks of the intrusive centers welded tuff tuffaceous volcanic breccia andesite (altered basalt?) and dacitic flows Interbedded airfall tuff and sandstone _---- dacitic intrusions Oldest Unit

----- andesite (altered basalt?) andesHe (basa1 t?) intrusions and dacitic fJows _C=:,-!__pyroxene-plagioclase porphyry with flow­ aligned phenocrysts

rhyolite fluw and volcanic breccia __basal sedimentary units: conglomerate, aphyric andesite(?)­ arkose, tuffaceous sandstone fl{lwS Unconformity

PRE-TERTIARY ROCKS

HESOZOJC eremite Plutons

PRECAflBR IAN

Undifferentiated gneiss

figure 2. Diagrammatic geologic column of pre-Tertiary and Tertiary (Miocene) rock types

Very coarse boulder conglomerates, and Young and Brennan (1974). Inclusion of Peach Springs megabreccias are ubiquitous throughout the exposures Tuff into the oldest unit is tentative; it may of the oldest unit. Sizes of clasts range from unconformably overlie the tuff and basalt unit. several centimeters to 6 m; clasts of the largest size commonly are a distinctive unfoliated Precambrian Unlike most of the other parts of the oldest pophyritic granite characterized by large, elongate, volcanic and sedimentary unit, the Peach Springs Tuff flow-aligned feldspars. Potential sources are rare in is a distinctive and persistent stratigraphic marker the present Mohave Mountains, where this lithology in the field. It occurs in the Mohave Mountains area commonly has foliated structure or is augen gneiss, only between The Needles and a major northeast­ but the porphyritic granite crops out widely in the trending faul t north of the Lake Havasu City dump Whipple, (Davis and others, 1980), Bill Williams, and (provisionally named the "Castle Rock" faul t for ease Hualapai Mountains. of description, Figure 1), and in the Aubrey Hills. Samples of the Peach Springs Tuff from two localities The uppermost part of the oldest unit clearly on the Colorado Plateau have been dated at 16.9 and comprises a basalt-rhyolite association: silicic (and 18.3 m.y. (Damon and others, 1964; 1966), and a silicified) airfall tuffs interbedded with basalt similar-appearing unit 32 krn west of Mohave Mountains flows and basaltic tuffs (and cinder cones), capped by (Sawtooth Range, adjacent to the Chemehuevi Mountains) a blue-sanidine and sphene-bearing welded ash-flow yields an age of 18.1 m.y. on sanidine (determined by tuff, that we believe to be the Peach Springs Tuff of R.F. Marvin; K.A. Howard, personal communication).

93 Carr and others (1980) reported whole-rock K-Ar ages may be similar; there the basalt floHs are well­ for basalt flows that overlie and underlie Peach preserved. We believe that this unit may be Springs Tuff near Pyramid Butte, west of the Whipple correlated with the interbedded fanglomerates and Mountains. The bracketing ages are 14.5 m. y. on the flows of Osborne Wash, 35 km south of Standard Hash overlying and 17 m.y. on the underlying flows. (Eberly and Stanley, 1978).

Throughout the oldest unit, features such as Intrusive Center complex brecciation and deformation of tuffaceous sedimentary rocks are associated with brecciated and Silicic intrusion of the oldest unit dominates in highly al tered mafic lavas. This association several parts of the area north of the Lake Havasu indicates that the sediments were wet when the lavas City dump (Figure 1). The mos t abundant intrusive overrode them. Also, syndepositional intrusion within bodies are dikes and irregular masses of latite(?) and the oldest unit is indicated by dismemberment and dacite(?) porphyry, which are cut by rhyolite or partial assimilation of sedimentary units by mafic rhyodacite dikes, and locally by thin mafic dikes. In (andesite or basal t) lavas. Where sedimentary units the areas of most concentrated intrusion (patterned in were assimilated by intrusions, coarser particles such Figure 1) the silicic intrusive bodies are pervasively as conglomeratic layers rarely can be traced, only altered, as are the host rocks. slightly deflected from the original strike direction, wi thin the engulfing red-al tered "andesite" (al tered Abundant silicic dikes intrude the gneisses of basalt?) or dacite. Most of these intrusions are the interior Mohave Mountains (leucocratic suite of conformable, bu t some crosscutting mafic and dacitic Nakata, 1982, this volume), and are in turn intruded dikes occur. by young mafic dikes. The area of densest intrusion Middle unit of these dikes lies southeast of the Tertiary intrusive center along strike. The dikes and intrusive Basal strata of the middle unit are dominantly bodies within the oldest unit may represent the same sedimentary, al though locally there are volcanic intrusive event as the leucocratic suite. breccias and lahars. The sequence of strata in the middle unit is varied, like the oldest unit. Measured Dating of the intrusions is in progress. An age thicknesses range from 300 m in an area near the of 16.5 m.y. was reported by Martin and Frost (1981, Colorado River where the Tertiary units are overlain unpublished 'data) for plagioclase from a latite that by Qua ternary deposi ts, to 600 m adjacent to the apparently intrudes the older sequence near the Crossman Peak faul t. Basal sedimentary rocks are "Castle Rock" fault. He have not seen intrusions in largely derived from mafic volcanic rocks, and include the middle unit, but thin rhyolitic dikes intrude boulder conglomerate with blue-sanidine tuff silicic flows of the youngest unit near the city dump, fragments. Clasts of granitic and metamorphic rocks where the middle unit is missing. He assume that also occur, and increase in abundance higher in the these dikes were feeders for the flows in which they section. The middle unit contains probable landslide are found. deposits that reflect local source regions. In some STRUCTURE areas these deposits are extensive ledges of brecciated Precambrian rocks, while in other areas Two major faults divide the Tertiary volcanic and large isolated lenses of Peach Springs Tuff are sedimentary sequences of the Mohave Mountains into interbedded with tuffaceous sandstone. Other three structural regions. The northern region is lithological members incl ude thin airfall tuffs characterized by faulted folds, the central region by interbedded with red shale or claystone (probably lake rock units that tilt dominantly to the south, and bed deposits), and large thicknesses of poorly­ contain abundant intrusions, and the southern region consolidated tuffaceous sandstone layers. Dips by mul tiple repetitions of southwest-dipping section 0 0 measured on this unit range from 12 to 45 locally. across normal faults (Figure 1). The "Castle Rock" No ages have yet been determined on volcanic rocks of faul t separates the northern and central structural the middle unit. regions. It is unexposed except for a breccia zone at its southwest end (K.A. HOHard, personal communication). This fault strikes N65E, and Youngest unit apparently dips about 60 0 to the north but its true nature is unknoHn. The fault displaces a distinctive This unit comprises fanglomerate with a Precambrian gneiss unit, as Hell as the overlying distinctive purplish (volcanic-derived) matrix. The oldest volcanic and sedimentary unit and intrusive contacts of the voungest unit with both the oldest and Tertiary rocks, by about 3000 m along strike. The middle units ar'e inconsistently exposed. Near the southern faul t (Crossman Peak faul t of Howard and city dump the fanglomerate overlies south-dipping others, 1982, this volume) dips gently southeast and strata of the oldest unit. North of Standard Wash the separates Precambrian gneisses of the central Mohave youngest unit overlies northeast-dipping rocks of t?e Mountains from contrasting Precambrian granitic rocks oldest unit, but adjacent to Standard Wash, and In that are overlain by all three Tertiary volcanic and rocks adjacent to the Crossman Peak fault, the sedimentary units. youngest unit bevels southwest-dipping rocks of the middle unit; fragments of reworked middle-unit conglomerate occur in the lowest fanglomerate The northernmost structural region occupies the layers. Dips in the fanglomerate vary from southHest area west and south of Tumarion Peak, and extends to northeast locally. In both the city dump and northwest through The Needles, and into the Chemehuevi Standard Wash areas fanglomerate apparently is Mountains west of the Colorado River. At the interleaved Hith or overlain by thin floHs. Near the northeast boundary of this region, the oldest unit city dump these include rhyolite to rhyodacite flows, uncornfomably overlies Precambrian basement with dips volcanic breccias, airfall tuff, welded ashflow tuff, in the basal strata that are vertical to overturned to and a thin cap of basalt (now represented only by lag the northeast. Both the oldest and middle units are boulders). The sequence is not as well-knoHn in the deformed into a series of folds, dominantly synclines area of Mohave Springs Mesa and Black Mountain, but wi th northwes t- trending axes. The apparent

94 thicknesses of both units are greater in southwest­ flank of the Mohave Mountains. This seems to require dipping than in northeast-dipping synclinal limbs. localized sediment sources as well as localized Wavelengths of the folds are in the range of 1000-2000 tectonic events during accumulation of the volcanic m, but the folds are broken by faults sub-parallel to and sedimentary sequences. the strikes of the fold axes. The faults cut out the anticlines with only one exception; the thicknesses of Rocks of the oldest Tertiary unit appear to have the oldest and middle units exposed in this anticline been deposited in an interval between about 21 and 18 are considerably less than those in the southwest­ m.y. ago. During this time volcanism alternated with dipping limbs of the synclines. rapid sedimentation, probably along active fault margins where debris flows and landslides were Exposure of one fault surface together with the common. The abundance of granite boulders in these stratigraphic pattern of displacements suggest that deposits requires that the source was an unroofed all the northwest-striking offsets may be northeast­ crystalline terrane. In some localities sedimentary dipping normal faults. Outliers of the oldest unit rocks in the basal part of the oldest unit were northeast of the folds probably are erosional remnants derived dominantly from volcanic sources and are of blocks tilted to the southwest and bounded by associated with some of these coarse granite-clast similar normal faults. Just south of Tumarion Peak conglomerates. For the deposition of this the faul ted folds are truncated by a low-angle faul t association, multiple sources, including a source of that strikes northeast and dips southeast under the earlier volcanic materials, must have been available. Tertiary rocks. This fault can be traced northeast along strike into pre-Tertiary gneisses and granites. Intrusion was an important part of volcanism throughout deposition of the oldest unit. Some time Southeast of the "Castle Rock" fault in the following formation of the airfall tuff-basalt central structural region are steeply-tilted strata of association that lies beneath the Peach Springs Tuff the oldest unit that unconformably overlie quartz­ of Young and Brennan (1974) there was abundant biotite and granitic gneisses along the entire west centralized intrusion of mostly silicic rocks within flank of the range to the Crossman Peak faul t. The the oldest unit. These events probably occurred at unconformable contacts locally are faulted and the same time as voluminous silicic diking within the intruded by dacite porphyry dikes. This sequence here underlying crystalline rocks. is the host for the silicic rocks of the intrusive center. The basal sediments and flows strike The existence of a distinct angular unconformity dominantly east - west and dips are very steep (to the between the oldest and middle units indicates that south) to overturned at the basal contact; the middle tilting began during or after deposition of the oldest unit is not exposed. Here the youngest unit dips very unit, and continued during accumulation of the middle shallowly southwest, overlapping both the steeply­ unit. As a result the early Miocene rocks are tilted dipping oldest unit and intrusive bodies, but the dominantly to the southwest or south (at some places fanglomerate apparently does not cont'3.in clasts overturned), with dips becoming gentler toward the top derived from rocks of the intrusive suite. of the sequence. The presence of overturned beds locally in the upper strata of the oldest unit may be In the southern structural region, south of the due to intrusion or to local faults, rather than Crossman Peak fault, relations among Precambrian regional deformation. Near the city dump, tilting of granitic and dioritic rocks and all three Tertiary the oldest sequence apparently occurred before units are repeatedly exposed in a series of southwest­ intrusion of dacite and latite porphyries. These tilted fault blocks. As in the Tumarion Peak area, intrusions did not extend north into the Turnarion Peak these are probably normal faults that dip to the area, and may have been emplaced before deposition of northeast, and merge into the low-angle Crossman Peak the Peach Springs Tuff. fault at depth. Here, steeply-dipping to overturned strata of the oldest unit are unconformably overlain Tertiary units in the central structural region, by a thick section of the steep- to moderately-dipping outside of the intrusive center, are the least middle unit At the mouth of Standard Wash the complexly deformed. The oldest unit is simply tilted youngest unit locally overlies both the oldest and to the southwest, with minor tear faulting, and there middle units. Here, the middle unit is bevelled by are few observed repetitions of the section along nearly-horizontal layers of fanglomerate that forms normal faul ts. In contrast, both the northern and the base of the youngest unit. Toward the Colorado southern structural regions are more complexly River, units that are equivalent to the oldest and deformed. Although the northern structural region is middle units in Standard Wash are repeated in the both folded and faul ted and the southern region is Aubrey Hills, probably by a normal fault buried under unfolded, the dominant structures in both are alluvium. southwest-dipping Tertiary units that occur in faulted blocks separated by northeast-dipping normal faults. INTERPRETATION The oldest Tertiary unit and its unconformable Tertiary volcanic and sedimentary sequences contacts in the central structural region are steeply unconformably overlie Precambrian metamorphic and tilted. There is no indication of any major faults granitic rocks all along the western flank of the that separate these volcanic and sedimentary units Mohave Mountains. Our work in Miocene rocks and that from the central core of the Mohave Mountains. This of others in the gneisses and granitic rocks of the implies that the underlying crystalline rocks also are range indicate that the Tertiary units overlie tilted to the southwest, and broken on high-angle Precambrian rocks on southeast-dipping low-angle normal faults. John and Howard (1982) report evidence faults at the north and south ends of the range (K.A. that rocks of the central and eastern Mohave Mountains Howard, personal communication; Howard and others, ("Crossman Peak plate") represent progressively deeper 1982, this volume). The variations in lithologies, crustal levels exposed by this tilting. If their structural style, and the occurrence of angular interpretation is correct, the Precambrian rocks of discordances within the Tertiary section, noted above the Mohave range that lie between the "Castle Rock" occur within a distance of only 6 kID along the western faul t and the Crossman Peak fault have been tilted steeply southwest as a block.

95 The northern and southern structural regions both are underlain by shallow southeast-dipping faults. ACKNOv~EDGEMENTS Precambrian rocks are exposed where the "Castle Rock" fault displaces them; we presume that the steep He thank Keith Howard, Jon Spencer, and Howard "Castle Rock" fault and the underlying low-angle fault Hilshire for stimulating discussions in the field, and intersect at depth. Similarly, we assume that the for helpful reviews of this paper. Thanks also to northeast-dipping normal faul ts of the southern Richard Knox, John Goodge, John Nakata, Steve Reneau, structural region merge at depth with the illlderlying and Tova Diamond for observations of and opinions Crossman Peak fault. Thus, both the "Castle Rock" about the complex field relations we have all fault and the Crossman Peak fault are zones that confronted in the Mohave Mountains. separate blocks or plates of Tertiary and underlying Precambrian rocks of the northern and southern structural regions from the rocks of the "Crossman Peak plate". The plates overlying the low-angle REFERENCES faul ts are internally broken on steep normal faults that allowed differential tilting of individual Carr, H.J., Dickey, D.D., and Quinlivan H.D., 1980, blocks. Thus, south of the Crossman Peak fault the Geologic map of the Vidal NH, Vidal Junction and Precambrian and Tertiary sequences are repeated parts of the Savahia Peak quadrangles, San numerous times by normal faul ts, but a traverse from Bernardino County, California: US Geological west to east on the north side of the Crossman Peak Survey Miscellaneous Investigations Series, 1­ fault crosses progressively deeper crustal levels, 1126, 1:24,000. possibly as much as 9 km of original crustal thickness Eberly, L.D.and Stanley, T.B. Jr., 1978, Cenozoic (Howard and others, 1982, this volume). stratigraphy and geologic history of southwestern Arizona: Geological Society of America Bulletin, In the northern structural region folding of the v. 89, p. 921-940. Tertiary rocks produced a more complicated structure Damon, P.E., and others, 1964, Correlation and than that in the southern or central structural chronology of ore deposits and volcanic rocks ­ regions. The deformational history is not yet clear, Annual Progress Report No. COO-689-42, Contract but we tentatively assume that folding followed the AT(11-1)-689, to US Atomic Energy Commission: initial tilting and intrusion of the oldest unit, and Tucson, Arizona, Geochronology Labs, University depositon of the middle unit. The folding may be of Arizona, p. 1-28. related to drag from recurrent movements along the Damon, P.E., and others, 1966, Correlation and "Castle Rock" fault. Apparent movement along the chronology of ore deposi ts and volcanic rocks ­ fault is not large, and the disappearance of the Peach Annual Progress Report No. COO-689-60, Contract Springs Tuff to the south of it, and of the middle AT( 11-1 )-689, to Research Div., US Atomic Energy unit in the central structural region, remain a Commission: Tucson, Arizona, Geochronology Labs, problem. University of Arizona, p. 26-29. Davis, G.A., Anderson, J .L., Frost, E.G., and The time of deposition of the youngest volcanic Shackelford, T.J., 1980, Mylonitiz~tion and and sedimentary unit is not yet well-known. Our detachment faul ting in the Whipple-Rawhide­ correlation of this unit with similar rocks at Osborne Buckskin Mountains terrane, southeastern Wash implies ages that range from about 8 - 21 m.y., California and western Arizona, in Crittenden, with most clustered in the range 12 - 16 m.y. (Eberly M.D. Jr., Coney, P.J., and Davis, G.A. ,eds., and Stanley, 1978; Martin and Frost, 1981, unpublished Cordilleran Metamorphic Core Complexes: data). In the Whipple Mountains, like the Mohave Geological Society of America Memoir 153, p. 79­ range, rocks associated with fanglomerate of Osborne 130. Wash are only slightly tilted, and thus postdate major Howard, K.A., Goodge, J., and John, B.E., 1982, deformational events. La ter rhyolite or rhyodacite Detached crystalline rocks in the Mohave, Buck, intrusions probably provided feeders for the silicic and Bill Williams Mountains, this volume. flows of the youngest unit. Following extrusion of John, B.E. and Howard, K.A., 1982, Multiple low-angle these rocks, and a few thin basalts, tilting ceased in Tertiary faults in the Chemehuevi and Mohave the middle Miocene. Mountains, California and Arizona (abs.), Geological Society of America Abstracts with In the Whipple-Rawhide-Buckskin Mountains to the Programs, Cordilleran Section, in press. southeast, the style of mid-Tertiary deformation is Martin, D.M. and Frost, E.G., 1981, Compilation of K­ similar to what we have observed in the Mohave AI' isotopic ages in the Hhipple Mountains and Mountains. In the Whipples Mountains the deformation adjoining ranges wi thin a portion of the is ascribed to transport of the overlying rocks along California Arizona detachment terrane: San low-angle "detatchment" faul ts that presumably result Diego State University Institute of Geochronology from regional extension (Davis and others, 1980). (unpublished data). These faults and the rocks overlying them have Nakata, J .K., 1982, Preliminary report on diking relations similar to those at the south end of the events in the Mohave Mountains, Arizona: this Mohave Mountains. It is not yet clear how detatchment volume. faulting in the Whipple Mountains is related to Young, R.A. and Brennan, H.J., 1974, Peach Springs deformation in the Mohave -Mountains as a whole, but Tuff: Its bearing on structural evolution of the dating of Mohave flows and intrusions may help to Colorado Plateau and development of Cenozoic define the timing of such events. drainage in Mohave County, Arizona: Geological Society of America Bulletin, v. 85, p. 83-90.

96 DIKE SWARMS AND LOW-ANGLE FAULTS, HOMER MOUNTAIN AND THE NORTHWESTERN SACRAMENTO MOUNTAINS, SOUTHEASTERN CALIFORNIA 1

Jon E. Spencer and Ryan D. Turner U.S. Geological Survey 345 Middlefield Road Menlo Park, CA 94025

rocks, over Phanerozoic and Precambrian plu­ ABSTRACT tonic and metamorphic rocks (e.g., Davis and others,1980). Allochthonous rocks commonly Homer Mountain and the Sacramento Moun­ are cut by numerous listric and/or planar tains are within an extensive terrane along normal faUlts which merge with or are cut by the lower Colorado River characterized by a basal detachment fault. These normal numerous low-angle (detachment) faults of faults may accommodate as much as 50% to 100% Tertiary age. Two major 10li-angle faults at extension of rocks above the detachment fault Homer Mountain displace crystalline rocks as (Anderson, 1971). well as Tertiary conglomerates, whereas the two low-angle faults mapped in the northwest­ Alloch thonous rocks commonly are ti 1 ted ern Sacramento Mountains displace primarily in one dominant direction over areas that Tertiary volcanic and sedimentary rocks. range from hundreds to thousands of square Tilt directions of allochthonous rocks are kilometers. The transport direction of al­ generally westward. At Homer Mountain, a lochthonous rocks usually is inferred to be dike is 'offset one to three kilometers east­ in the direction opposite to the tilt direc­ ward by movement on the lowest detachment tion of allochthonous fault blocks. Based on fault. The middle detachment fault at Homer this and other criteria, detached rocks ap­ Mountain juxtaposes rocks of almost complete­ pear to have moved westward in the Eldorado ly different lithologies, indicating major Mountains, and northeastward in the Whipple displacement. Two detachment faults in the Mountains. Tilting of detached fault blocks northwestern Sacramento Mountains are paral­ and other indicators of movement direction lel or subparallel to bedding in the lower are not as well-known in the intervening allochthon. ranges, but available evidence seems to indi­ cate that a reversal in tilt direction occurs East-west trending, subvertical dikes in the southern Eldorado Mountains near are numerous in autochthonous rocks at Homer Searchlight, Nevada (Longwell, 1963; Vol­ Mountain and the northwestern Sacramento borth, 1973), and that allochthon transport Mountains, and are cut by several gently east in most areas has been toward the east or dipping, north-south striking dikes at Homer northeast. Mountain. Both dike sets are cut by the low­ angle faul ts. The east-west orientation of Within the detachment fault terrane, the the oldest dike set indicates that the least dip direction and form ·of the detachment compressive stress was in the north-south faul ts is highly variable, as is the struc­ direction at the time of dike emplacement. tural complexity of allochthonous rocks. The This state of stress is consistent with in­ variable and locally highly irregular form of ferred maximum east to northeast directed detachment faul t surfaces may be the result compression during much of Mesozoic and early of folding or warping (Davis and others, Tertiary time. Detachment faults may be in­ 1980; Frost, 1981; Cameron and Frost, 1981) terpreted as products of middle or late Ter­ and/or of irregularities in detachment fault tiary extensional tectonics. The subhori­ surfaces at the time of formation. Our pre­ zontal orientation of the younger dike set, sent lack of understanding about the nature indicating minimal overburden and absence of and origin of detachment faults is compounded any tensional stress, may be interpreted in by the great structural compleXity of detach­ the context of either tectonic regime. ment fault terranes.

INTRODUCTION Detachment faults are often thought to represent upper crustal accommodation of Low-angle (detachment) faults are ex­ crustal extension, although some workers have posed in every mountain range along the west suggested that these faults represent slip side of the Colorado River area between the surfaces at the base of gigantic gravity Eldorado Mountains, 100 km north of the study slides (e.g., Davis and others, 1980). If area (Anderson, 1971), and the Whipple Moun­ the crustal extension hypothesis is correct, tains, 80 km southeast of the study area (Da­ we would expect, at least initially, a re­ vis and others, 1980). Throughout this area, gional drop in the magnitude of subhorizontal these enigmatic subhorizontal faults typical­ compression at the time of extensional fault­ ly place tilted middle Tertiary volcanic and ing. The gravitational sliding hypothesis sedimentary rocks, and older crystalline requi res no parti~ular stress regime in au­ tochthonous rocks. Thus, determination of 1This report has not been reviewed for con­ the orientation and relative magnitude of formity with U.S. Geological Survey editorial stresses in· autochthonous rocks, at the time standards or stratigraphic nomenclature. of detachment faulting, should provide in-

97 sight into the cause of faulting.

Dikes can be useful indicators of the orientation of stresses in country rock at the time of dike emplacement. Experimental and theoretical studies indicate that, during ma gm a em p 1 acemen t, d ike 13 13 h 0 u 1 dad just the i 1" course of propagation so that the minimum principal stress (Cl ) remains perpendicular 3 to the walls of the dike. Application of this principle to ancient dike swarms may yield information about the nature and orien­ tation of past stress regimes. For example, Muller and Pollard (1977), following the ori­ ginal analysis of Ode (1957), were able to determine the orientation of a regional stress field, the origin and orientation of a local stress field, and the relative magni­ tude of the stresses in each, by fi tting the pattern of the Spanish Peaks dike swarm to calculated stress trajectories.

In detail, features such as dikes must be analyzed carefully to distinguish between the effects of regional stresses and of lo­ cally generated stresses or other factors influencing dike orientation. Stresses gen­ erated locally, for example by a stock or volcanic neck, will be less effective at greater distances. Consistency of dike ori­ entation over a large area is thus a good indication that regional stresses were the only significant factor controlling dike ori­ entation.

Previous geologic field mapping at Homer Mountain (Bonham and others, 1960) and the northwestern Sacramento Mountains (Bonham and Tischler, 1960; Collier, 1960; Spurck, 1960) consists of a reconnaissance survey by the Southern Pacific Land Company. As indicated by their maps, and supported by this study, Homer Mountain and the northwestern Sacramen­ to Mountains are similar in that both areas contain detachment faults of Tertiary age and numerous dikes that locally are cut by these faults. This is a report on an ongoing study directed at determining the form of the de­ tachment faults, the structure of the alloch­ thons, and the age, abundance, and composi­ tion of the dikes. Ul tima tely, we hope to use dike swarms in the study area as paleo­ stress indicators, and possibly to clarify understanding of the nature of detachment faulting. Figure 1. Sketch map showing location of study areas. Detachment faults placing DIKE SWARMS IN THE STUDY AREA Tertiary or primarily Tertiary rocks over older crystalline rocks, based primarily on East-West Trending Dikes mapping by the Southern Pacific Land Company, are represented by hatchured lines (hatchures East-west trending dikes are numerous in on upper plate). Detachment faults at Homer the autochthon and lowest allochthon of Homer Mountain and west of the Little Piute Mountain (Fig. 1, Fig. 2) and over several Mountains, and connecting dashed lines, square kilometers immediately south of Inter­ represent the westernmost extent of known state Highway 40 in the northwestern Sacra­ detachment faults in this part of the lower mento Mountains (Fig. 3). These subvertical Colorado River trough. There are no known dikes are remarkably consistent in orienta­ detachment faults within 30 to 50 kilometers tion. They trend about N80W ±5° in the au­ west of this boundary, which may be tochthon of Homer Mountain, and N75W ±10 o in interpreted as a breakaway for detached fault the northwestern Sacramento Mountains. Dikes blocks to the east. typically are 2 to 15 meters thick and 200 to 2,000 meters long. Field mapping and aerial photograph analysis of a 5 kilometer long, Figure 2. Simplified geologic map and cross­ north-south transect across Homer Mountain section of Homer Mountain.

98 -.:::- ~ UNMAPPED gr-m ".... /' .:=:::::-- /~~

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gr-m

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GRANITIC AND SEDIMENTARY ROCKS, GRANITIC AND METAMORPHIC ROCKS, TERTIARY RHYOLITIC C'l UNDIFFERENTIATED, OF THE MIDDLE UNDIFFERENTIATED, AND MAFIC AND VOLCANIC DIKE ROCK c:J 8 ltiJ AND UPPER ALLOCHTHONS ~ FELSIC DIKES, UNDIFFERENTIATED, ROCKS dike OF THE LOWER ALLOCHTHON AND AUTOCHTHON UNMAPPED

UNMAPPED

Qa

N Qa

~ ~gr-m

o KM. 2 3 I I I o MI. 2 B QUATERNARY ALLUVIUM

[,'T'VS' ",] TERTIARY VOLCANIC ... ~. LOW-ANGLE FAULT, AND SEDIMENTARY ROCKS, DOTTED WHERE INFERRED . ',. UNDIFFERENTIATED

gr-m GR. ANITIC AND METAMORPHIC ~. ROCKS, UNDIFFERENTIATED dike (mafic and felsic, undiff.)

Figure 3. Simplified geologic map of dike swarms and detachment faults on the west side of the northwestern Sacramento Mountains.

100 indicates that 11% of the autochthon is com­ DETACHMENT FAULTS IN THE STUDY AREA posed of fairly evenly distributed dikes. A similar 5 kilometer transect through the Sac­ Detachment Faults at Homer Mountain ramento Mountains indicates that 13% of the basement is composed of dike rock. Sharp, At least three detachment faults are planar intrusive contacts, chilled margins, exposed on the south and east flanks of Homer and virtual lack of inclusions within dikes Mountain. There appears to be only limited indicate that dilation of country rock rather offset across the upper and lovrer detachment than assimilation was the primary mechanism faults, but a major lithologic contrast of dike emplacement. across the middle detachment fault indicates that it has undergone significant displace­ At Homer Mountain, the dikes intrude an ment. The lowest allochthon pinches out to assemblage of unfoliated, pegmatitic granitic the west so that the middle allochthon rests rock of inferred Mesozoic or Precambrian age directly on the autochthon on the south flank and older pendants and inclusions of dark, of Homer Mountain (Fig. 2). The lovrest de­ variably foliated, porphyritic granite simi­ tachment fault offsets both the middle and lar to Precambrian porphyritic granite mapped upper detachment faults. by Volborth (1973) in the nearby Newberry Mountains. Country rocks in the northwestern Lovrest detachment fault Sacramento Mountains include a variety of gneissic and metamorphic rocks of unknown The lowest detachment fault has an arcu­ protolith. There is no evidence that struc­ ate trace that is concave to the southeast, tural anisotropies in these rocks had any and is inferred to be synformal and gently influence on dike orientations. southeast plunging (Fig. 2). The limbs of this synformal detachment fault dip inward at Although dike compositions are highly about 10 0 to 30°. Dikes in the autochthon variable, several basic lithologic types can beneath the limbs of the synformal fault do be distinguished. These include fine-grained not appear to be warped, suggesting that the to aphanitic, dark green mafic dikes and tan shape of the lovrest detachment fault origi­ to brown weathering, aphanitic, silicic dikes nated at the time of formation and vras not with less than 40% phenocrysts. Silicic produced by later folding or vrarping. dikes with greater than 40% phenocrysts are sparse in the northvrestern Sacramento Moun­ Correlation of a north-south trending tains but numerous at Homer Mountain vrhere rhyolitic dike in the lovrest allochthon vrith they include several suites. Based on modal either of two dikes of identical composition mineralogy, these suites include biotite rhy­ in the autochthon indicates about 1 to 3 kil­ olite, biotite quartz latite, biotite (±horn­ ometers of eastvrard displacement (Fig. 2). blende) andesite, and basalt.Biotite from an This amount of inferred displacement is con­ andesite dilfault trace indicates that east-vrest trending dikes also if not for the presence of chloritic breccias are exposed over a several square kilometer along the fault which, although locally ab­ exposure of granitic rock located about 10 sent, may be as much as 5 to 10 meters thick. kilometers north of Homer Mountain. Thus, east-vrest trending dikes are exposed discon­ The lower allochthon is primarily repre­ tinuously over a 40 kilometer long north­ sented by two elongate, north-south striking south transect. The consistency of dike ori­ faul t blocks. Field evidence suggests that entation over this larp;e area may be inter­ at least three of the four north-south trend­ preted as indicating that regional stresses ing faults that bound these two fault blocks vrere the only significant factor controlling dip gently to moderately outward from the dike orientation. faul t blocks, as shovrn in the cross-section of Figure 2. These three faul ts are trun­ North-South Trending Dikes cated by the lowest detachment fault, whereas the easternmost north-south striking fault Several rhyolitic dikes vrith north-south cuts the lovrest detachment fault. striking, gently east dipping (200 ±5°) con­ tacts are present in the autochthon and lovr­ Our interpretation of this fault pattern est allochthon at Homer Mountain. Unlike the is shown in the cross-section of Figure 2. east-vrest trending dikes, these dikes change We consider the two faults on the west flank greatly in thickness along strike and have of the each fault block to be tilted segments irregular contacts (Fig. 2). Hovrever, the of the middle detachment fault. The fault on north-south trending dikes also have sharp the east flank of the western fault block is contacts, chilled margins, and generally lack interpreted as a lovr-angle normal fault ac­ inclusions, suggesting that assimilation vras commodating extension above the lovrest de­ not an important process in dike emplacement. tachment faul t, vrhereas the normal faul t on These dikes clearly crosscut east-west trend­ the east flank of the east faul t blo.:lk is ing dikes, and therefore are younger. An interpreted as a high-angle normal fault that impure hornblende separate from one of these post-dates movement on the lower and middle dikes yielded a K-Ar age of 41.0 ±16.0 m.y. detachment faults. (Donna Martin, pers. commun., 1982). Middle detachment fault

101 Exposures of the middle detachment fault almost total lithologic dissimilarity between include an east-west trending segment on the the autochthon and lower allochthon, support southwest flank of Homer Mountain \oIhere the the interpretation that the Tertiary rocks middle allochthon rests directly on the au­ are separated from older crystalline rocks by tochthon, and two north-south trending seg­ a major detachmen t faul t, as indicated by ments along the west sides of two west-tilted Collier (1960). Both lower and upper detach­ fault blocks where the middle allochthon ment faults are parallel or subparallel to rests on the lower allochthon (Fig. 2). bedding in the lower allochthon (Fig. 4). Brecciation is a characteristic feature of the rocks along the middle detachment fault, The lower allochthon is a sequence of in places extending for tens or even hundreds interbedded volcanic flows, tuffs, and vol­ of meters into the middle allochthon above caniclastic sandstone that dips gently to the the fault. Chloritic alteration of fault west and is about 150 meters thick (Fig. 4B). breccias may be as well-developed above the The youngest unit in the sequence is a vol­ fault as below it. caniclastic sandstone (Ts S ) that is exposed only at the southern end of the map area The middle allochthon is composed pri­ (Fig. 4). At other locations the sandstone marily of dark, variably foliated, porphyri­ is missing and is inferred to have been trun­ tic ~ranite and poorly-sorted conglomerate cated by the upper detachment fault, \oIhich with abundant cobbles and boulders of the usually occurs at the top of a cliff-forming dark, porphyritic granite. Similar dark tuff unit (Tv4)' granite occurs locally as pendants and inclu­ sions in the leucocratic ~ranitic rocks of The upper allochthon is composed of the autochthon and lower allochthon. A major poorly bedded to massive cobble- and boulder­ lithologic contrast thus occurs across the conglomerate and sedimentary breccias. The middle detachment fault, suggesting that conglomerate contains clasts of gneiss and large net displacement has occurred on this dark granite, whereas the breccia is derived fault. This inference is supported by the from gneiss and dark granite, as well as dark fact that dikes, which are abundant in the (basaltic?) volcanic rocks. Sandstone is autochthon and lower allochthon, are absent completely absent at lower stratigraphic in the middle allochthon. levels, but is increasingly abundant at high­ er levels. Some moderate sized pendants of dark granite and its gneissic equivalents occur on The contact at the base of the conglom­ the west side of Homer Mountain and near Sig­ erate locally cuts across section in the low­ nal Hill in the southern Piute Range. These er allochthon, and is also a major lithologic may represent remnants of a once continuous boundary between underlying volcanic rocks or semi-continuous roof of dark granite above and their fine-grained sedimentary deriva­ younger, leucocratic plutonic rocks. This tives, and overlying conglomerate and sedi­ roof is the inferred source area of the mid­ mentary breccias that are generally poor in dle allochthon. The middle detachment might volcanic clasts. We interpret this contact have carried part of this roof down onto its as a low-angle fault, and not an unconform­ foundation of younger plutonic rocks by per­ ity, because volcanic clasts are rare in the haps 5 to 20 km of horizontal displacement basal part of the conglomerate. In addition, and perhaps .5 to 3 km of vertical displace­ preliminary data indicates that the conglom­ ment. eratic rocks are tilted much more steeply than the underlying volcanic and volcani­ Upper detachment fault clastic rocks of the lower allochthon. The upper cliff forming part of TV4' where locat­ A low-angle fault above the middle al­ ed immediately below the inferred low-angle lochthon is exposed over a small area on the fault, is not brecciated or chloritized, al­ southeast flank of Homer Mountain, where it though locally it is pervasively fractured is truncated by the lower detachment fault and has slickenside lineations on fracture (Fig. 2). The upper allochthon is composed surfaces. of conglomerate with cobbles and boulders of dark granite, and is lithologically indistin­ A system of northwest trending high­ guishable from conglomerates in the middle angle faults cuts the upper detachment fault allochthon. Based on this lithologic simi­ and forms a graben within the allochthonous larity, we infer that net movement on this rocks (Fig. 4). Total stratigraphic throw on faul t is small in comparison to net movemen t the graben-bounding faults is about 50 to 70 on the middle detachment fault. meters. Mapping by the Southern Pacific Land Company (Collier, 1960), and interpretation Detachment Faults in the Sacramento of aerial photographs suggest that these Mountains faults do not cut the basal detachment fault. The graben may have formed by extension above Detailed mapping in the area between the basal detachment fault. If this inter­ Eagle Peak and Flattop Mountain in the north­ pretation is correct, it suggests that move­ western Sacramento t40untains indicates that ment on the upper detachment fault ended be­ at least two major allochthons, composed al­ fore the last movement on the 10Her detach­ most entirely of Tertiary volcanic and sedi­ ment fault. mentary rock, are present above an autochthon of Precambrian crystalline rock. The pre­ DISCUSSION sence of, fault gouge and well-developed chloritic breccia in Precambrian rocks im­ Dike SHarms mediately below the lower allochthon, and the

102 Rehrig and Heidrick (1972,1976) used Low-Angle (Detachment) Faults the geometl'Y of magmatic bodies as paleo­ stress indicators to clarify undel'standing of The inference of regional northeast to Mesozoic and Cenozoic tectonics in southel'n east directed transport of allochthonous and we s tel'n Al' i z 0 n a . The y s tat i s tica 11 y an­ fault blocks along the west side of the lower "11yzed the orientations of a lal'ge number of Colorado River, as determined from tilt dir­ subvertical dikes, veins, and elongate stocks ections, is supported by offset indicators at emplaced during the late Cretaceous-early Homer Mountain. Offset of a north-south Tertiary Laramide orogeny and during middle trending dike indicates that the lowest al­ and late Tertial'y time. They interpreted lochthon was displaced 1 to 3 km eastward by this data as indicating that the least com­ movement on the lowest detachment faul t, al­ pressive stress «() 3) was perpendicular to though the north-south component of movement the planar or tabular form of these magmatic is not discernable from this offset marker. bodies at the time of emplacement. Laramide However, tilted fault blocks of the lower age magmatic features typically strike N70E allochthon strike north-south, suggesting ±20o , whereas late Tertiary magmatic features that the north-south component of displace­ typically strike N20W ±20°. This indicates ment was negligible, and that offset was due that principal stress orientations changed east. approximately 90° during early or middle Ter­ tiary time. This interpretation is consis­ Inferred east to northeast sense of tent with the widely held notion, based on transport of allochthonous fault blocks is styles of faulting, that the maximum compres­ further supported by the presence of a break­ sive stress «() 1) was oriented east-west to away along the west margin of this detachment northeast-southwest during the Laramide oro­ fault terrane, as first recognized by Davis geny, and that () ~ was oriented east-west to and others (1980) in the Mopah Range west of northeast-southwes~ during middle and late the Whipple Mountains. A breakaway is geo­ Tertiary basin and range extension. metrically similar to the scarp at the uphill end of a landslide, and is the fault trace In the study area, the east-west trend­ that separates detached and displaced alloch­ ing dikes are inferred to represent a state thonous fault blocks from their adjacent au­ of stress in the upper crust in which the tochthonous equivalents. The Mopah Range minimum compressive stress (Cl ) was oriented 3 breakaway, as depicted by Davis and others perpendicular to a vertical, roughly east­ (1980), is a concave upward fault that dips west striking plane. The subhorizontal dikes moderately at the surface and flattens with are inferred to represent a state of stress depth toward the northeast where it becomes a in which the minimum compressive stress (Cl ) basal detachment fault. If this geometry is 3 was subvertical. Both dikes sets are thus applicable to breakaway faul ts in general, consistent with an inferred east directed then more deeply eroded and exhumed breakaway maximum compressive stress (°1 ) for much of faults will have gentler dips. Mesozoic and early Tertiary time based on the widespread existence of east to northeast The trace of the Mopah Range breakaway directed thrust faults of this age (Miller is inferred to extend northward under Quater­ and McKee, 1971; Burchfiel and Davis, 1971, nary alluvium toward the east flank of the 1977; Burchfiel and others, 1974; Howard and Old Woman and Piute Mountains where the Lit­ others, 1980; Reynolds and others, 1980). tle Piute Mountains are detached and dis­ placed eastward (Keith Howard, pel's. commun., However, the age of dikes in the study 1981). From the east flank of the Piute area is uncertain. An 11.5 ±1.8 m.y. K-Ar Mountains, the breakaway may extend northward age on biotite from one of the east-west under Quaternary alluvium along the west trending dikes is suggestive of a Miocene age fl3.nk of the northern Sacr3.mento Mountains, for dike emplacement, yet hornblende from a and may be represen ted farther nOY'th by the north-south trending dike that clearly is main (middle) detachment fault at Homer Moun­ younger based on cross-cutting relationships tain (Fig. 1). Alth9ugh correlation of yielded a K-Ar age of 41.0 ±16.0 m.y.. Both breakaway faults and the inference of region­ ages are Tertiary, indicating that Tertiary al northeast to east directed movement on heating and/or c~oling affected the area. detachment faults are both speculative, it is However, it is not known if, for example, the certain that every range between this region­ hornblende contains excess argon and both al breakaway and the Colorado River contains dikes are Miocene in age, or perhaps both major detachment faults, whereas no detach­ dikes are Mesozoic but have undergone a Mio- ment faults are known to exist in any of the cene thermal disturbance causing partial ranges immediately to the west. (hor'nblende) or complete (biotite) argon loss. Four biotite separates from granitic The main, or middle detachment fault at rocks from the Newberry Mountains, located 25 Homer Mountain is inferred to have accommo­ km northeast of Homer Mountain, all yielded dated eastward displacement of allochthonous middle Miocene ages (Anderson and others, rocks based on (1) demonstrable eastward dis­ 1972, Volborth, 1973). Thus, Miocene thermal placement of a lower (and related?) detach­ effects have been significant in this area, ment fault, and (2) westward tilts of con­ and may have caused resetting of K-Ar ages at glomerates in the middle allochthon. The Homer Mountain, although, if both dike sets present southward dip of the detachment fault at Homer Mountain are Mesozoic, there are no may be the result of warping and folding known intrusions exposed at Homer Mountain about an east-west trending axis during fault that could be related to a Miocene thermal movement, similar to folding of detachment event. fault surfaces in areas to the southeast

103 A~

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:::::::..... :.:; II!! ilJ;> ... ". df!J c) ...... - ' :O:~·~·;'·4:"; ..... " . ::::: : :

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1p£u PRECAMBRIAN IGNEOUS AND META- I MORPHIC ROCKS, UNDIFFER NTIATED

Figure 4A. (other page) Simplified geologic map of Figure 4B. Cross-section and columnar section of area the central part of the northwestern Sacramento Moun­ shown in Figure 4A. tains between Eagle Peak and Flattop Mountain. Geo­ logic unit symbols are keyed in Figure 4B, except Tb = Tertiary basalt, and Qa = Quaternary alluvium.

105 (Frost, 1981; Cameron and Frost, 1981). and suggested as structural features of the The middle detachment fault is inter­ study area, indicate conditions in which C>N preted as a low-angle normal fault because was greater than 0-v' This condi tion would part of an approximate vertical section of not favor emplacement of dikes with an east­ upper crustal rocks is missing across this west, subvertical orientation. fault; i.e., that part represented by the int~usive contact between Mesozoic and/or CONCLUSION Tertiary leucocratic granitic rocks and over- lying, intruded Precambrian granite and Detachment faults in the study area rep­ gneiss. In some places, the Precambrian resent the westernmost extent of the detach­ rocks are missing as well, and Tertiary con­ men t faul t terrane along the lower Colorado glomerates are juxtaposed against underlying River trough. Movement directions of alloch­ leucocratic granite rocks. thons are uncertain in the northwestern Sac­ ramento Mountains, although west tilted con­ The lowest detachmen t faul t in the glomerates in the upper allochthon and a northwestern Sacramento Mountains is inter­ northwest trending graben in both allochthons preted as a low-angle normal fault because suggest east to northeast translation and the depositional contact at the base of the extension. Unlike allochthons in the north­ Tertiary section is omitted across the fault. western Sacramento Mountains, allochthons at The nature of the upper detachment fault is Homer Mountain contain crystalline rocks, and uncertain because we do not know the relative clear evidence of eastward translation. ages of the rocks in each allochthon. These inferences are all consistent with regional Structural features of Homer Mountain eastward to northeastward displacement on and the northwestern Sacramento Mountains are detachment faults in and around the study interpreted as representing two contrasting area, and with the existence of a headwall or tectonic regimes. The orientation of east­ breakaway along the west side of this area. west trending dikes is interpreted as the result of east-west directed compression in Relationship of Dikes to Detachment Faults an Andean type orogenic province of Mesozoic and early Tertiary age. The subhorizontal In an extensional tectonic environment, orientation of the younger di.kes is consis­ the least compressive stress (6""3) should be, tent with strong, east-west directed compres­ at least initially, in the direction of ex­ sion during Mesozoic or early Cenozoic time, tension. Rehrig and Heidrick (1976), for but a similar state of stress, resulting from example, found that dikes of mi ddle and late autochthon flexure following low-angle fault­ Tertiary age were generally subvertical and ing and tectonic denUdation, may have de­ north to northwest striking, consistent with veloped during middle and late Tertiary time no~mal fault orientations indicating east to (Spencer, this volume). The development of northeast directed crustal extension in detachment faults may be interpreted as the southern and western Arizona. In detachment result of crustal extension during middle and fault terranes, the direction of extension is late Tertiary time. However, the interpreta­ generally inferred to be the direction of tion that the study area is part of a de­ movement on detachment faults, which is east­ nUded, uphill end of a mega-gravity slide can west at Horner Mountain and approximately in not be refuted by our data. the same direction in surrounding areas. In Arizona, two regional dike sets, the However, secondary effects of detachment older oriented N70E ±200 and the younger ori­ faulting may include major changes in the ented N20W ±200, are both rotated about 20 to state of stress in au tochthonous rocks. The 30 degrees in a counterclockwise di rection stress due to overburden «()"7"" v) is clearly relative to the strike of the two dike sets reduced by tectonic denudation, while the in the study area. Tilt directions and other subhorizontal stress in the direction of al­ offset indicators suggest northeastward lochthon transport (C:;-E in the study area) translation of allochthons in areas to the may increase due to isostatic uplift and at­ southeast of the study area (Davis and tendant flexure of the aut'ochthon (Spencer, others, 1980; Rehrig and others, 1980; Rehrig this volume). Both of these effects may lead and Reynolds, 1980), a direction about 20° to to conditions favoring subhorizontal dike 40° more northerly than inferred eastward orientations instead of north-south, subver­ displacement of allochthonous rocks in the tical dike orientations. study area. These differences in orientation may reflect the arcuate trace of both the East-west trending dikes at Homer Moun­ Mesozoic magmatic arc and the Tertiary basin tain and the northwestern Sacramento Moun­ and range province around the western corner tains indicate 01 was oriented north-south, of the Colorado Plateau. at the time of diRe emplacement, which is 90 degrees from the inferred orientation of (J 3 ACKNOWLEDGEMENTS based on movement directions of nearby de­ tachment faults (assuming detachment faults We especially thank Keith Howard for his reflect crustal extension). While the in­ continued support and interest in this pro­ crease in 0E due to autochthon flexure may ject, and Donna Martin for K-Ar analyses. We favor emplacement of dikes with a subverti­ also thank Bill McClelland and Jane Pike for cal, east-west trending orientation, the re­ helpful discussions in the field, and Henri duction of 0 v would not. The east-west to Wathen for assistance in the field and for northeast-southwest trending folds documented typing this paper. This paper was reviewed by Frost (1981) and Cameron and Frost (1981), by Robert Powell and Jane Pike.

106 REFERENCES CITED 1980, Mesozoic thrusting in the eastern Mojave Desert, California (abs.): Geo­ Anderson, R. E., 1971, Thin-skinned disten­ logical Society of America Abstracts sion of Tertiary rocks of southeastern with Programs, v. 12, p. 112. Nevada: Geological Society of America LongHell, C. R., 1963, Reconnaissance geology Bulletin, v. 82, p. 43-58. between Lake Mead and Davis Dam, Ari­ Anderson, R. E., Longwell, C. R., Armstrong, zona-Nevada: U. S. Geological Survey R. L., and Marvin, R. F., 1972, Signifi­ Prof. Pap. 374-E, 51 p. cance of K-Ar ages of Tertiary rocks Miller, F. K., and McKee, E. H., 1971, Thrust from the Lake Mead region, Nevada-Ari­ and strike-slip faulting in the Plomosa zona: Geological Society of America Mountains, southHestern Arizona: Geo­ Bulletin, v. 83, p. 273-288. logical Society of America Bulletin, v. Bonham, H. F., Jr., and Tischler, M. S., 82, p. 717-722. 1960, Geology and mineral resources of Muller, D. H., and Pollard, D. 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