Miocene Low-Angle Normal Faulting and Dike Emplacement, Homer Mountain and Surrounding Areas, Southeastern California and Southernmost Nevada

Home , Dead Mountains

Miocene low-angle normal faulting and dike emplacement, Homer Mountain and surrounding areas, southeastern California and southernmost Nevada

JON E. SPENCER* U.S. Geological Survey, 345 Middlefleld Road, Menlo Park, California 94025

ABSTRACT tions, differed radically from the state of that collectively accommodated as much as 50% stress in the upper plate, as inferred from to 100% extension of upper-plate rocks (Ander- Homer Mountain and surrounding regions fault geometry. Low-angle faulting and east- son, 1971). In many areas, normal faults within are within, or adjacent to, the western part of northeast-west-southwest distension of up- upper-plate rocks merge with, or are truncated a broad region of low-angle normal faults ex- per-plate rocks reflect regional reduction of by, a basal, subhorizontal fault often referred to posed within the lower Colorado River compression in the east-northeast-west- as a "detachment fault" (for example, see Davis trough. During middle Miocene time, upper- southwest direction and associated large- and others, 1980). The term "detachment fault" plate rocks in the Homer, Sacramento, Dead, scale east-northeast-west-southwest crustal is used here to indicate a low-angle normal fault and Newberry Mountains moved eastward or extension. In contrast, concave-upward flex- that formed at a low angle (for example, Wer- northeastward, relative to the lower plate, ure of the lower plate, in response to tectonic nicke and others, 1984; Reynolds and Spsncer, above single or multiple low-angle normal denudation and resultant isostatic uplift, is in- 1985). The interpretation that detachment faults faults. Deposition of coarse clastic sedimen- ferred to have produced local subhorizontal are rooted faults that accommodate crustal ex- tary rocks occurred during extensional fault- compression at shallow crustal levels in the tension (Wernicke, 1981; Howard and John, ing and was accompanied by, and closely lower plate that overwhelmed the regional 1983; Davis and others, 1983; Allmendinger followed by, eruption of basaltic volcanics. extensional stress and prevented emplace- and others, 1983) is accepted here. Upper-plate fault blocks of Miocene volcanic ment of dikes oriented perpendicular to the Within areas of low-angle normal faulting, and sedimentary irocks and older, underlying direction of regional extension. North-south upper-plate rocks are generally tilted in one crystalline rocks sire tilted gently to steeply to crustal extension during dike emplacement dominant direction over areas that range from the west or southwest. Low-angle normal appears to have resulted from minor diver- hundreds to thousands of square kilometres. The faults have complex, sinuous traces due to the gence of lower-plate crustal blocks as they transport direction of upper-plate rocks, relative irregular form of fault surfaces. Antiformal were displaced in a west-southwest direction to lower-plate rocks, was usually in the direction warping and uplifnt of the lower plate about a away from the Colorado Plateau. opposite to the tilt direction of upper-plate fault north-south trending axis, due to tectonic blocks. On the basis of tilt directions and other denudation and isostatic rebound, divided the INTRODUCTION criteria, upper-plate rocks are inferred to have regionally east-thickening extensional alloch- moved in an east-northeast direction, relative to thon into separate synformal and wedge- The northern part of the Colorado River the lower plate, in all areas of the northern Colo- shaped components. The Homer Mountain trough, extending for -200 km from the Eldo- rado River trough except the northern Eldorado area lies within, c>r adjacent to, the synformal rado and northern Black Mountains southward and northern Black Mountains where tilt direc- component. through the Whipple, Buckskin, and Rawhide tions suggest westward displacement of lpper- Lower-plate rocks are intruded by numer- Mountains, is a zone of major middle Tertiary plate rocks. ous, middle Miocene, east-west-trending low-angle normal faulting. Low-angle faults, ex- The origin of large-scale warps of detachment dikes, which are in turn intruded by subhori- posed in every range along the west side of the fault surfaces is not completely understood. In zontal dikes. Both sets of dikes are cut and Colorado River in the northern Colorado River the northern Colorado River trough, the : rregu- displaced by low-angle normal faults except trough, place moderately to steeply tilted, mid- lar, undulatory shape of detachment fault sur- on the west flank of the Newberry Mountains dle Tertiary volcanic and sedimentary rocks and faces reflects interference between two sets of where an east-west-trending dike cuts the Mesozoic and Precambrian crystalline rocks approximately perpendicular folds or warps basal low-angle normal fault. K-Ar geochro- over Phanerozoic and Proterozoic plutonic and (Davis and others, 1980; Frost, 1981; Cameron nologic and field data establish the approxi- metamorphic rocks. These faults are inferred to and Frost, 1981) and/or original irregularities of mate synchroneiity of dike emplacement and be normal faults because they juxtapose upper- fault surfaces (John, 1984). Large, broad warps low-angle faulting. The state of stress in the crustal rocks (including, in many cases, syntec- with axes perpendicular to fault-movement di- lower plate, as inferred from dike orienta- tonic clastic sedimentary rocks) with underlying rection appear to be primarily the proc.uct of igneous and metamorphic rocks that evidently isostatic rebound accompanying and imme- acquired some of their characteristics at middle- diately following tectonic denudation (Rehrig *Present address: Arizona Bureau of Geology and crustal depths. Upper-plate rocks may be cut by Mineral Technolog]', 845 North Park Avenue, Tuc- and Reynolds, 1980; Howard and others, son, Arizona 85719. numerous listric and/or planar normal faults 1982a, 1982b; Spencer, 1982, 1984). Shorter

Geological Society of America Bulletin, v. 96, p. 1140-1155, 12 figs., 1 table, September 1985.

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Figure 1. Simplified geologic map of the Homer Mountain area. Dashed line indicates approximate location of breakaway fault beneath Quaternary alluvium.

wavelength (3-12 km), foldlike irregularities stress remains perpendicular to the walls of the are potential indicators of the orientation of of uncertain age and origin have axes parallel dikes. Rehrig and Heidrick (1976) used the principal stresses responsible for warping of de- to the east-northeast direction of upper-plate geometry of intrusive bodies in Arizona to infer tachment faults. displacement. the direction of least compressive stress during This paper results from a field and K-Ar Study of dikes and dike swarms is a relatively both the late Mesozoic-early Tertiary Laramide geochronologic study of the Homer Mountain simple method of constraining the orientations orogeny and middle Tertiary crustal extension. area in the western part of the northern Colo- of the principal stresses at the time of dike em- Their results are consistent with stress orienta- rado River trough that was directed at under- placement. Field, experimental, and theoretical tions inferred from the geometry of faults that standing the temporal and mechanical relation- studies all indicate that dikes adjust their course formed during Laramide and middle Tertiary ships between detachment faulting, fault-surface of propagation so that the minimum principal tectonism. Dikes in areas of detachment faulting irregularities, and dike swarms. The study area

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Figure 2. Simplified geologic map of the southern end of the Piute Range. See Figure 1 for location. Geology from Spencer and Turner (1985).

includes Homer Mountain, the southern end of pioneering study by Longwell (1945). Recon- distribution of rock types and faults in the south- the Piute Range, the northern Sacramento naissance mapping by geologists of the Southern ern Eldorado, Newberry, and northern Dead Mountains, and parts of the Dead and Newberry Pacific Land Company in the late 1950s re- Mountains. Mountains (Fig. 1). All of these ranges have vealed the great extent and the complex, irregu- Davis and others (1980) documented, many been strongly affected by middle Miocene de- lar form of detachment faults along the west side of the distinctive characteristics of detachment tachment faulting and/or dike emplacement. of the northern Colorado River trough. The faults in their study of the Whipple-Buekskin- This study outlines the lithologies and structures Southern Pacific geologists recognized most of Rawhide Mountains area, and they compiled of the Homer Mountain area and presents K-Ar the geologic relationships and the distribution of and simplified the mapping of the Southern Pa- geochronologic data that constrain the timing of rock types in the Homer Mountain area. Unfor- cific geologists and their own mapping of areas faulting, sedimer.tation, volcanism, and dike tunately, they did not publish the results of their of detachment faulting along the northern Colo- emplacement. Mechanical analysis of processes investigation, and although their maps were rado River trough. This compilation (Davis and operating during detachment faulting suggests used in the compilation of the Needles 1° x 2° others, 1980, Fig. 2) revealed both the regional that stresses produced by warping and isostatic geologic map (Bishop, 1963), the low-angle extent and the complex, sinuous trace of de- uplift were a determinant of dike orientations. faults were not included. Studies by Anderson tachment faults. More recent studies of detach- (1971, 1977, 1978) and Anderson and others ment faults and related structures in the lower PREVIOUS STUDIES (1972) revealed both the large magnitude and Colorado River region are presented in Frost the Miocene age of extension at the north end of and Martin (1982). A recent study (Custis, Low-angle normal faults were recognized at the Colorado River trough. A reconnaissance 1984) of Homer Mountain and the southern the north end of the Colorado River trough in a study by Volborth (1973) outlined the general Piute Range covers field, geochemical, and struc-

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e'-'W'DÌK^ feet meters I 1 QUATERNARY TERTIARY [^TERTIARY CONGLOMERATE I—1 TERTIARY 1 1 ALLUVIUM BASALT ^ (WITH SPARSE TUFF BEDS) ^ RHYOLITE

LOW-ANGLE NORMAL FAULT "^TET^ARY^KES110 R°CKS X K_Ar °ATE -1 HIGH-ANGLE NORMAL FAULT E§~PRECAM BRIAN CRYSTALLINE ROCKS Figure 3. Simplified geologic map and cross section of Homer Mountain. See Figure 1 for location. Geology from Spencer and Turner (1985).

turai aspects of dikes, their country rocks, and southern Piute Range are composed almost ex- Piute Range and equigranular to slightly porphy- deformations. clusively of Precambrian porphyritic biotite ritic granite and granodiorite at Homer Moun- granite and Mesozoic leucocratic granitic rocks. tain (Spencer and Turner, 1985). GEOLOGY OF THE HOMER The variably foliated Precambrian granite is lith- These Precambrian and Mesozoic granitic MOUNTAIN AREA ologically similar to variably foliated porphyritic rocks were intruded by numerous east-west- biotite granite of Precambrian age in the trending dikes during middle Miocene time. Homer Mountain and Southern Piute Range nearby Newbery Mountains (Volborth, 1973) The dikes are subvertical and most trend and Piute Mountains (Miller and others, 1982). N80°W ± 10° (Figs. 2 and 3). Three distinctive Lower-Plate Rocks. Lower-plate pre-Ter- Unfoliated Mesozoic granitic rocks include por- dike lithologies are recognized on the basis of tiary rocks at Homer Mountain and in the phyritic biotite granodiorite in the Southern phenocryst mineralogy. These are, in order of

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decreasing age: dirk gray dacite dikes with large tinctive mineralogies and the absence of transi- allow field classification and could be oaly ap- (up to 1 cm) bocks of fresh biotite; andesitic to tional lithologies suggest that each dike type was proximately classified as "silicic" or "interme- rhyolitic dikes containing phenocrysts of biotite, derived from a different source rather than from diate" on the basis of color or mineralogy hornblende, and K-feldspar rimmed with plagi- a single fractionating magma. Many dikes, how- (Spencer and Turner, 1985; see also Custis, oclase; and rhyolite quartz-porphyry dikes. Dis- ever, do not contain sufficient phenocrysts to 1984). Geochemical data are permissive of a

TABLE 1. POTASSIUM-ARGON GEOCHRONOLOCIC DATA FROM THE HOMER MOUNTAIN AREA

Sample Latitude Longitude Material 40 A- j Host rock k2o rad Age no. dated (%) (10 mol/g) (%) (m.y.)

Homer Mountain

HMI31BJ 3:>°00'50" 114°56'00" biotite silicic dike 7.77 1.89464 64.2 17.0 ± 0.2 7.67 HM152J 3:;°00'55- 114°56'08" biotite Precambrian 8.82 2.28145 52.8 17.9 ± 0.2 granite 8.81 HMI69T 3ii°01'54" 114°56'18" biotite dacite dike 8.20 2.14740 71.2 18.0 ± 0.2 8.25 HM239J 3!°01 '48" 114°55'25" biotite dacite dike 8.88 2.26299 62.2 17.6 ± 0.2 8.88 HM85IJ 34°58'53- 114°55'57" sanidine tuff 6.87 1.70529 85.1 17.2 ± 0.2 6.84 HM992J 35°02'19" 114°57'31" biotite Precambrian 8.95 10.4715 82.4 79.4 ± 0.6 granite 8.96 HM995J 35°02'27" 114°52'48' whole rock basalt 1.281 0.272961 57.6 14.6 ± 0.2 1.308

Southern Piute Range

HM346J 35'01'26' 115°00'45" whole rock basalt 1.868 0.383199 56.2 14.2± 0.2 1.855 HM493J 34"59'47" 115°02'25" hornblende silicic dike 0.616 0.170719 23.7 19.2 ± 1.0 0.614 HM575J 35"02'49" 115°03'00' whole rock basalt 1.661 0.349901 34.7 14.5 ± 0.2 1.687 HM990J 34c59'42" 115°04'22- biotite granodiorite 8.53 8.98285 83.8 71.7 ± 0.5 8.54 HM991J 35c00'43" 115°04'25- biotite granodiorite 8.94 9.00113 81.7 68.8 ± 0.5 8.90

Piute Wash Hills

PW759J 35°)8'42* 114°55'02- biotite granodiorite 9.10 9.24533 79.1 69.2 ± 0.5 9.11 PW993J 35°08'15" 1I4°54'51* whole rock silicic dike 5.46 1.11352 24.2 14.2 ± 0.3 5.41

Sacramento Mountains

SC338J 34°:i0'36" 114°55'02" biotite silicic dike 8.83 2.44318 74.4 19.1 ± 0.1

SC491J 34°<.9'52- 114°49'28" whole rock basalt 2.105 0.443015 47.6 14.6 ± 0.2 2.104 £ SC762T 34 M9'09" 114°46'25* biotite tuff 8.66 2.31567 65.9 18.5 ± 0.2 8.66 SC838J 34°49'46" 114°48'16" whole rock basalt 2.31 0.487031 83.0 14.6 ± 0.9 2.16 2.447* SC996J 34°51'33" 114°52'37" biotite Precambrian 7.04 1.61629 58.4 15.8 ± 0.2 gneiss 7.08

Newberry Mountains

NB6J 35011'29* 114°46'18" biotite silicic dike 7.90 1.74722 35.0 15.3 ± 0.2 7.90 NB78T 35°ll'41- 114°41'39" biotite silicic dike 6.72 1.80242 46.0 18.5 ± 0.3 6.73 NB81T 35°11'40" 114°41'37" biotite granite 7.81 1.64216 55.4 14.5 ± 0.2 7.82 NB994J 35°1 .'34" 114°46'30" biotite Precambrian 8.20 54.3268 95.5 409.9 ± 2.8 granite 8.20

Castle Mountains

CS137T 35=21-27- H5°05-06_ biotite tuff 8.65 1.78927 63.5 14.4 ± 0.2 8.59 CS138T 35°21'51- 115°04'00" sanidine rhyolite 10.12 1.85837 40.1 12.8 ± 0.2 plug 10.01

•Analysis courtesy of Paul Damon, Laboratory of Isotope Geochemistry, University of Arizona, Tucson, Arizona. Note: age calculation based on :iew decay and abundance constants (Steiger and Jager, 1977). Error in calculated age determined by method of Cox and Dalrymple (1967), assuming a standard deviation of 0.3% for tracer calibration. E:Tor in

potassium analysis is either sample standard deviation of K2O analyses or 0.5%, whichever is greater. With the exception of sample SC838J, two fractions of each sample were analyzed for K2O, and the average K20 content was used f Jr age

calculation. Because of the large spread in K20 analyses of sample SC838J. a third fraction was analyzed, resulting in an even larger spread and indicating that sample heterogeneity in K20 content is almost certainly the primary source of error in the age determination of this sample. The precision in the age determination of sample PW993J is probably at least an order of magnitude less than that given, due to very high Âr/Âr (-12) and low38Ar/Âr (-34) in the argon analysis of this sample. An error was made in entjring data on one of the argon tracer series (BD series) used for argon analysis. As a result, some dates given by Spencer (1983), Spencer and Turner (1983, 1985), Turner and others (1983) are slightly in error.

The corrected ages are given here. Mineral separations prepared by Dennis Sorg; K;0 analyses, by P. Klock. Potassium was measured by flame photometry, using a lithium internal standard. Argon extractions and analyses by J. E. Spencer and M. A. Pemokas. Argon measurements were made using standard techniques of isotope dilution. All analyses and separations were done at the U.S. Geological Survey, Menlo Park, California, in 1982.

genetic relationship between dike types (Custis, been displaced from an original high-level posi- sitional contact at the original base of the 1984), but isotope analyses are needed to de- tion where it formed part of a roof of Precam- conglomeratic rocks has been largely excised by termine conclusively if the different dike types brian granite above the Mesozoic plutonic the low-angle fault at the base of plate 3. The are related. intrusions. intrusive contact between Mesozoic granitic Biotite and hornblende from east-west-trend- Plate 3 is composed primarily of massive Ter- rocks and the originally overlying roof of Pre- ing dikes at 4 localities yielded K-Ar dates of 17 tiary conglomerate with minor conglomeratic cambrian crystalline rocks is inferred to be ex- to 19 Ma. In contrast, biotite from Precambrian sandstone, tuff, and fault blocks of Precambrian cised by the fault at the base of plate 2. Only and Mesozoic country rocks yielded Late Cre- and Mesozoic granitic rocks. These rocks are the fault at the base of plate 1 does not appear to taceous K-Ar dates in three of the four samples tilted predominantly to the west at dips gener- attenuate any part of the pre-existing structural dated. The fourth sample was collected closer to ally ranging from 20° to 60°, although eastward succession, which is consistent with its inferred the dikes than were the other three and yielded a tilts are present in the southeastern area of expo- small offset. date that falls within the 17-19-m.y. period of sures of plate 3 (Fig. 3). Conglomerate clasts, The direction of displacement of upper-plate dike emplacement (Figs. 2, 3; Table 1). These typically 2 to 50 cm in diameter but locally as rocks, relative to the lower plate, is inferred to be K-Ar analyses indicate that the dikes were in- large as 3 m, include abundant clasts of granite eastward. Eastward movement of plate 1 is indi- truded between — 17 and 19 m.y. ago into coun- similar to locally occurring Precambrian granite. cated by offset of a dike of the rhyolite of Homer try rock that, at sufficient distances from the Less-abundant clasts include Mesozoic granitic Mountain. Predominantly westward tilts of con- dikes, was not heated during dike emplacement rocks, similar to lower-plate granitic rocks at glomerates in plate 3 are also suggestive of east- above the blocking temperature for argon in Homer Mountain and in the southern Piute ward displacement. A block of Mesozoic biotite. Range, and sparse clasts of almost all local dike granodiorite(?) ~Vi km long located within the East-west-trending dikes and older granitic rocks. Monolithologic sedimentary breccias of southwestern part of exposures of plate 3 rocks are cut by thick, north-south-striking, Precambrian and Mesozoic granitic rocks are (Fig. 3) does not resemble Mesozoic granitic gently east-dipping, slightly irregular rhyolite locally present. These conglomeratic sedimen- rocks at Homer Mountain but is similar to the dikes at Homer Mountain (rhyolite of Homer tary rocks and breccias are interpreted as being biotite granodiorite forming much of the south- Mountain of Spencer and Turner, 1985). These syntectonic sediments deposited in extensional ern Piute Range. Correlation of these granitic dikes are younger than the 17 to 19 m.y.-old basins bounded on one or both sides by normal rocks suggests at least several kilometres of east-west-trending dikes and are cut by a de- faults. Sanidine from an interbedded tuff bed eastward displacement of this part of plate 3. A tachment fault that is overlain by basalt yielding yielded a K-Ar age of 17.2 ± 0.2 m.y. (Table 1), block of Precambrian granitic rock forming the a K-Ar date of 14.6 ± 0.2 Ma (Fig. 3), thereby suggesting that detachment faulting was under- northeastern corner of exposures of plate 3 is constraining the age of the rhyolite within a 2- to way at this time. intruded by rhyolite of Homer Mountain. 3-m.y. interval. The low-angle faults at the base of plates 2 Matching this rhyolite with rhyolite dikes to the Upper-Plate Rocks. At Homer Mountain, and 3 are interpreted as being normal faults be- west in the lower plate indicates at least 2 to 5 dikes and their country rocks are cut by gently cause each evidently excises part of a pre- km of eastward displacement of plate 3 in this south-dipping to east-dipping detachment faults existing vertical structural succession. The depo- area. that place three lithologically distinctive upper plates (in ascending order, plates 1, 2, and 3) above the lower plate (Figs. 3 and 4). Rocks along, and directly below, the detachment faults POST-TECTONIC are typically shattered, silicified, and altered to BASALT mineral assemblages characterized by chlorite, epidote, and hematite. Indurated, ledge-forming zones of cataclasite or microbreccia, characteris- PLATE 3 to ui tic of many detachment faults (for example, Davis and others, 1980; Phillips, 1982), are poorly developed to absent. The irregular shape PLATE 2 CE of the basal Homer Mountain detachment fault UI Q. defines an east-southeast-plunging synform CL PLATE I 3 flanked on each side by east-southeast-plunging antiforms (Figs. 3 and 4). Plate 1 consists of three fault-bounded blocks composed primarily of Mesozoic granitic rocks LOWER PLATE identical to those in adjacent parts of the lower KM O I plate. The largest and easternmost block is intruded by a gently east-dipping dike of the rhyo- Ml. O ' i lite of Homer Mountain that is interpreted as being the offset equivalent of a dike in the lower —ANTIFORM plate located ~ 1 km farther west (Fig. 3). Plate 2 is composed of Precambrian biotite —SYNFORM granite and irregular intrusions of rhyolite of Homer Mountain; these rocks are pervasively shattered in virtually all exposures. Unlike plate 1 and the lower plate, plate 2 contains no Meso- Figure 4. Structure map and cross section of Homer Mountain. This map covers approxi- zoic granitic rocks. Plate 2 is inferred to have mately the same area as does Figure 3. Fault symbols are the same as in Figure 3.

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Inferred eastward displacement of plate 2 is based on its structural position as a subhorizontal sliver between two plates that are each de- duced as having teen displaced eastward. Plate 2, however, is intruded by rhyolite of Homer Mountain, yet rhyolite does not intrude the lower plate to the west in the area presumably underlain by plate 2 before it was displaced. The highly irregular shape of rhyolite intrusions in plate 2, in marked contrast to the planar, gently dipping dikes in both plate 1 and the lower plate, indicates that stresses in plate 2 were quite different from stresses in underlying rocks at the time of rhyolite intrusion. This difference in intrusive geometry and inferred state of stress is interpreted as being the result of intrusion of rhyolite into plate 2 after it had been structurally detached from underlying rocks, and after stresses had been reoriented as a result of detachment. The rhyolite intruding plate 2 thus may have been emplaced late in the movement history of plate 2, and the rhyolite feeder dikes may now be concealed beneath it. Post-Tectonic Rocks. Flat-lying to gently tilted basalt rests on intrusive rocks of plate 3 at the northeast fool: of Homer Mountain. One continuous exposure of basalt rests on both plate 3 and the lower plate (Fig. 3). The contact with the lower plate is not well exposed but appears to be depositional. If the basalt is slightly faulted, displacement is probably minor in comparison to displacements elsewhere on the basal detachment fault because rocks along this contact are not affected by the brecciation and alteration that mark detachment faults elsewhere in the range. The basalt yielded a whole-rock K-Ar age of 14.6 ± 0.2 m.y. (Table 1). This indicates that low-angle faulting had ended, or nearly ended, by this time and that the basal detachment fault was exposed at the surface on the northeast flank of Homer Mountain at this time.

Piute Wash Hills Figure 5. Geologic map of Piute Wash hills (informal name). See Figure 1 for location. Fault symbols are the same as in Figure 3. An isolated grou p of hills surrounded by Qua- ternary alluvium, located about 10 to 15 km tics. A sample of a silicic dike yielded a whole- are similar to both types of granitic rock found north of Homer Mountain (Figs. 1 and 5), is rock K-Ar age of 14.2 ± 0.3 m.y. below the fault. here referred to as the "Piute Wash hills." Most Two fault blocks in the eastern Piute Wash of the exposed bedrock in these hills is com- hills contain no dikes and were juxtaposed with Northern Sacramento Mountains posed of the same biotite granodiorite that the granodiorite after dike emplacement. The makes up much of the southern Piute Range. lower fault block is bounded by northwest- Basal Fault Geometry. The structural geol- Biotite from the granodiorite yielded a K-Ar age trending, high-angle, normal(?) faults and is ogy of the northern Sacramento Mountains is of 69.2 ± 0.5 m.y., similar to the ages obtained composed of granite or granodiorite (not shown dominated by the Sacramento Mountains basal from the southern Piute Range. The granodiorite separately on Fig. 5) that is distinctly darker, due detachment fault (Figs. 6, 7, 8). This fault sepa- is intruded by numerous, approximately east- to abundant biotite, than the adjacent granodior- rates a lower plate of primarily Precambria:i gra- west-trending dikes that generally lack abun- ite. A structurally higher plate of massive nitic and gneissic rocks from overlying detached dant phenocrysts and could be only approxi- boulder conglomerate overlies both types of sheets and blocks of Miocene volcanic and sed- mately classified as "silicic" and "intermediate" granitic rock along a low-angle fault. The con- imentary rocks and local fault blocks of Pre- on the basis of color and weathering characteris- glomerate contains clasts of granitic rocks that cambrian crystalline rocks. Pervasive breccia-

Figure 6. Minimum-relief contour map of the Sacra- Ml. 0 I mento Mountains basal detachment fault in the northwest- KM 0 I 2 3 era Sacramento Mountains. This map is based on the fact that the elevation of now-eroded parts of the basal de- " DETACHMENT FAULT, DOTTED tachment fault was at least as high as exposed lower-plate WHERE rocks, and the elevation of buried parts of the detachment Zoo o ~°o) CONCEALED fault is at least as low as exposed upper-plate rocks. Con- 2200 s, \ ' N tours within lower-plate rocks indicate the minimum possible elevation of the basal detachment fault and were drawn to intersect the highest topographic points within the lower plate, such as the crests of ridges. Contours within upper-plate rocks indicate the maximum possible elevation of the basal detachment fault and were drawn to intersect the lowest topographic points within the upper plate, such as the bottoms of gullies and washes. At the point where contours cross the basal detachment fault, the contours are coincident with the elevation of the basal detachment fault. As upper-plate rocks occupy topographic lows, relief on the fault surface is considerable. See Figure 1 for location. Geology from Spencer and Turner (1983), Bonham and Tischler (1960), Tischler (1960), and Spurck (1960).

tion and chloritic alteration characterize crystalline rocks for up to several tens of metres below the fault. The northern Sacramento Mountains are divided into two north-south-trending, broadly antiformal uplifts of lower-plate crystalline rock by a synformal keel of upper-plate rocks (Fig. 1). The synformal keel represents the southward continuation of the synform defined by upper-plate rocks exposed on the flanks of Piute Valley, whereas the antiform on the east flank of the synformal keel represents the southward continuation of the antiform defined by the axis of the Dead and Newberry Moun- tains (for example, Davis and others, 1980, Fig. 2). Lower-plate crystalline rocks exposed in the northwestern Sacramento Mountains represent an isolated, nearly circular uplift that is not part of a regionally defined antiformal uplift. In this area, two sets of wave-like irregularities are defined by the complex form of the basal detachment fault (Fig. 6). A north-south-trending, doubly plunging, broadly antiformal warp is only crudely defined; shorter-wavelength, east-

northeast- to northeast-trending, doubly plung- * K-Ar DATE ing antiformal and synformal irregularities are well developed. The average wavelength of the LOW-ANGLE FAULT northeast-trending synforms and antiforms is

iygp] TERTIARY VOLCANIC Figure 7. Simplified geologic map of part of ^^ ANDSED. ROCKS the northwestern Sacramento Mountains. Ml. 0 aPRECAMB., UNDIVIDED See Figure 1 for location. Geology from KM 0 TERTIARY DIKE Spencer and Turner (1983).

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~3-4 km, and the amplitude of some of these west-trending dike swarm in the northwestern southern exposures. The dikes are truncated by features is at least 3'X) m. Sacramento Mountains is composed of nu- the basal detachment fault, and no dikes have The origin of the east-northeast- to northeast- merous silicic to intermediate dikes that in- been recognized above the fault. trending antiforms and synforms is uncertain. trude lower-plate Precambrian crystalline rocks Biotite from an intermediate-composition The basal fault may have formed with the anti- (Fig. 7). As at Homer Mountain, the dikes have dike yielded a K-Ar age of 19.1 ± 0.1 m.y. forms and synforms as original features, or these sharp, planar contacts, locally chilled margins, (Fig. 7; Table 1). Biotite from gneissic country features are folds. In either case, if these folds or and contain few inclusions of country rock, sug- rock, collected at a freeway roadcut outsid; the grooves formed before, or during, faulting, they gesting that dike emplacement was accommo- area of the main dike swarm, yielded a younger would not have been an impediment to upper- dated entirely by basement dilation and not by K-Ar age of -15.8 ± 0.2 m.y. These ages are plate movement if the direction of displacement assimilation of country rock. Dikes of the swarm difficult to interpret. The similarity of dike orien- was parallel to the iixes of the irregularities. are oriented approximately N70°W in most tations in the Sacramento Mountains to dike Dike Swarms. A major, approximately east- areas but change orientation to east-west in orientations at Homer Mountain suggests that

uj IOOO t

Figure 8. Simplified geologic map and cross section of part of the northern Sacramento Mountains. See Figure 1 for location. Sources of data: (1) Spencer and Turner (1983), (2) McClelland (1982), (3) Collier (1960a), (4) Bonham and Tischler (1960), (5) Spurck (1960), and (6) Collier (1960b).

lliiifiiiiiMtiiiinn dikes of both areas were emplaced during the Miiiiiiiiiiimi'Hiiii m.y. Assuming that conglomerate deposition um •••iitiiiit'i'iiii BASALT OF FLATTOP MOUNTAIN iHiiurmiiiiiifiiiiii same tectonic regime and therefore are of similar iiiiuiiififiumrififH I4.6t0.l M.Y., K-Ar was synchronous with faulting, this K-Ar age age. indicates that low-angle faulting, tilting, and sed- CONGLOMERATE WITH Upper-Plate Rocks. Upper-plate rocks, ex- INTERBEDDED TUFF AND imentation were underway at this time. posed over a large area within the central part of BASALT(14.610.9 M.Y., K-Ar), AND The contact between plates 1 and 2 is inter- SPARSE SILTSTONE AND the northern Sacramento Mountains, form two SEDIMENTARY BRECCIA preted as being a fault on the basis of its detached sheets (plates 1 and 2, in ascending moderate- to high-angle discordance to locally order) of Miocene volcanic and sedimentary discernable, moderately to steeply west-dipping rock, and local fault blocks and slivers of Pre- bedding in plate 2. This interpretation is also cambrian crystalline rock (Fig. 8). Plate 1 is SANDSTONE AND CONGLOMERATE based on the sharp lithologic contrast between composed primarily of a sequence of ash-flow ASH-FLOW TUFF volcanic and fine-grained volcaniclastic sedi-

tuffs and is tilted gently to the west. The basal U SANDSTONE AND CONGLOMERATE ments below the fault and boulder conglomerate fault cuts gradually upsection to the west so that with predominantly granitic and gneissic clasts plate 1 is entirely eliminated by faulting in the above the fault (see also McClelland, 1982,

western part of the map area where plate 2 rests ASH-FLOW TUFF 1984). The contact separating plates 1 and 2 is directly on the lower plate (cross section, Fig. 8). not obviously a fault in virtually all exposures, 100 A large sliver of Precambrian(?) crystalline rock however, because it is approximately parallel to 3- 18.510.2 M.Y., K-Ar structurally beneath plate 2 in westernmost ex- bedding in plate 1, and rocks along it are not posures of upper-plate rocks could be a piece of crushed, altered, or appreciably brecciated. SANDSTONE AND CONGLOMERATE a detached sheet of crystalline rock that in the WITH SPARSE SILTSTONE AND Normal faults accommodating rotation and ex- central Sacramento Mountains underlies plates LIMESTONE tension of plate 2, but not cutting the fault at its S 0 1 and 2 (McClelland, 1982, 1984). PRECAMBRIAN GRANITE AND base, are probably numerous but are difficult to GNEISS Plate 1 in the central part of the northern recognize and map due to the massiveness and Sacramento Mountains is composed of a basal Figure 9. Structure-stratigraphy column lithologic homogeneity of most of plate 2. unit of clastic sedimentary rock and interbedded for the central part of the northern Sacra- Flat-lying basalt at Flattop Mountain, yield- limestone overlain by a thick sequence of ash- mento Mountains (area of Fig. 8). ing a K-Ar age of 14.6 ± 0.2 m.y., rests uncon- flow tuff and sparse interbedded sandstone and formably on tilted conglomerate of plate 2 and is conglomerate (Fig. 9). Biotite from a thick tuff interpreted as postdating detachment faulting bed in the lower part of the tuff sequence yielded 1982). More detailed geochronologic study is and associated tilting. The contact at the base of a K-Ar age of 18.5 ± 0.2 m.y. (Figs. 8, 9; Table needed to resolve the contradiction between the basalt of Flattop Mountain is -250 to 300 m 1). Laterally equivalent tuffs and sedimentary these two dates. above the basal detachment fault. Because the rocks, as well as older parts of the section that Plate 2 is composed of massive boulder con- basalt immediately postdates detachment fault- have been removed by faulting in the northern glomerate and sedimentary breccia, with less ing, the subhorizontal basal detachment fault Sacramento Mountains, are exposed in the cen- abundant tuff, basalt, conglomeratic sandstone, appears to have been not much more than 300 tral Sacramento Mountains where they are in- sandstone, and siltstone. A whole-rock sample m below the ground surface at the time of latest truded by a latite intrusion that yielded a K-Ar of one interbedded basalt flow, now tilted 35° to fault movement. Modern examples of such shal- biotite date of 21.6 ± 1.7 Ma (McClelland, 40° to the west, yielded a K-Ar age of 14.6 ± 0.9 low, subhorizontal normal faults are unknown

QUATERNARY ALLUVIUM

™ | TERTIARY BASALT

| tcu | TERTIARY CONG., UPPER

TERTIARY CONG., LOWER, WITH INTERBEDDED BASALT

PRECAMBRIAN GNEISS 0.5 I 0.5

Figure 10. Geologic map of the Dead Mountains reverse fault and surrounding area. See Figure 1 for location. Teeth on reverse fault are on upthrown block.

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in the modern Basin and Range province, suggesting that structural styles of extensional deformation changed during late Cenozoic time (Zoback and others, 1981).

Southwestern Dead Mountains

A steeply east-dipping reverse fault on the west flank of the southern Dead Mountains places Precambrian gneiss over steeply tilted to overturned granite -clast conglomerate (Fig. 10). Sedimentary structures indicate that the stratigraphic top direction in the conglomerate is to the west. Vertically dipping Tertiary sedimentary and volcanic rocks are unknown in nearby ranges, as are reverse faults, suggesting that the anomalously steep attitude of the sedimentary rocks is the result of movement on the adjacent reverse fault. The steeply tilted conglomerate is uncon- formably overlain by gently west-dipping conglomerate that contains clasts of gneiss, volcanic rock, and reworked red sandstone and conglomerate. This upper conglomerate unit is interpreted as postdating reverse faulting an associated tilting of the lower conglomerate unit. A basalt flow is either faulted against (Keith How- ard, 1984, written commun.), or overlies (the contact is poorly exposed), the gently west- dipping upper conglomerate unit. Figure 11. Geologic map of the central Newberry Mountains (slightly modified from Vol- A basalt flow interbedded with the lower borth, 1973). K-Ar data from Table 1 and from Anderson and others (1972) and Volborth conglomerate yielded a whole-rock K-Ar age of (1973) (ages recalculated for new decay and abundance constants by method of Dairymple, 12.2 ± 0.3 m.y.; the basalt flow in contact with 1979). See Figure 1 for location. Fault symbols are the same as in Figure 3. the gently dipping conglomerate yielded an age of 12.1 ± 0.3 m.y. (John Nakata, 1984, written commun.). These ages suggest that a period of ple, 1979). At a fifth locality, biotite from a complete argon loss from Precambrian biotite compressional faulting and related sedimenta- north-south-trending dike yielded a K-Ar age of during Mesozoic and/or Cenozoic thsrmal tion affected the Dead Mountains 2 to 3 m.y. 18.5 ± 0.3 m.y. Biotite from a sample of granitic events. K-Ar data indicate that the upper and after faulting and sedimentation had ended in country rock, collected several tens of metres lower plates of the central Newberry Mountains nearby ranges. away from the dike, yielded a younger age of had contrasting thermal histories prior to the 14.5 ± 0.2 m.y. (Table 1; Fig. 11). Although the time (~ 16 to 18 m.y. ago) when the lower plate Central Newberry Mountains age of the dike is similar to ages obtained from cooled through the blocking temperature for other localities in the lower plate, the age of the argon in biotite and muscovite. This is inter- The central Newberry Mountains consist of country rock is 2 to 4 m.y. younger. It therefore preted as indicating that the upper and lower a lower plate of Mesozoic or Cenozoic leu- seems that the country-rock age is anomalous plates were not juxtaposed until after ~ 16 to 18 cocratic granitic rocks and Precambrian gneiss for unknown reasons. All other geochronologic m.y. ago. and an upper plate of Precambrian biotite granite data indicate that dikes and granitic rocks in the An east-west-trending "andesitic" dike cuts and locally exposed Tertiary volcanic rocks lower plate of the central Newberry Mountains the low-angle fault that juxtaposes the two plates (Fig. 11). The two plates are separated by a are at least as old as ~ 17-18 m.y. on the west flank of the Newberry Mountains broadly arch-shaped detachment fault that dips Upper-plate rocks on the west flank of the (Volborth, 1973). Biotite from this dike yielded outward on the flanks of the range (Volborth, central Newberry Mountains, composed primar- a K-Ar age of 15.3 ± 0.2 m.y. (Table 1; Fig. 11). 1973; Mathis, 1982). Numerous north-south- ily of the same Precambrian biotite granite that This date is interpreted as being the crystalliza- trending dikes intrude the lower plate (Vol- is exposed over a wide area at Homer Mountain tion age of the dike because a sample or Pre- borth, 1973). and in the southern Piute Range, yielded K-Ar cambrian country rock in the upper plate, Four samples of lower-plate granitic rocks biotite ages of 146.8 ± 4.3 m.y. in the north- collected -150 m from the dike, yielded the yielded biotite or muscovite K-Ar ages between western Newberry Mountains (Volborth, 1973) 410 ± 3.0 m.y. K-Ar biotite age (Fig. 11). 16 and 18 m.y. (Anderson and others, 1972; and 410 ± 3.0 m.y. in the southwestern New- Detachment fault movement and final j uxta- Volborth, 1973; ages recalculated using new berry Mountains (Table 1; Fig. 11). Both of position of the upper and lower plates is there- decay and abundance constants after Dalrym- these dates are interpreted as reflecting partial or fore constrained to have occurred prior to

15.3 ± 0.2 m.y. ago on the west flank of the UNEXTENDED\ SYNFORMAL WEDGE-SHAPED Newberry Mountains. AREA ^^UPPER PLATE UPPER PLATE

Castle Mountains

The Castle Mountains, located at the north end of the Piute Range along the California- OJ CJl Nevada border (Fig. 1), are composed primarily o of middle Tertiary volcanic rocks that are folded öl into a broad, open anticline trending N35°E. The volcanic rocks rest depositionally on Precambrian crystalline rocks exposed in the core of the anticline at the northeast end of the range (Hewett, 1956; Bingler and Bonham, 1973). Rhyolite plugs with near-vertical flow foliation intrude moderately tilted volcanic rocks on the flank of the anticline and are interpreted as having been emplaced during the later part of folding (Turner and others, 1983; Ryan Turner, 1984, written commun.). Sanidine from a rhyolite plug yielded a K-Ar age of 12.8 ± U1 0.2 m.y.; biotite from a tilted dacite flow yielded o o a K-Ar age of 14.4 ± 0.2 m.y. (Turner and o others, 1983; Table 1). II5°00' II4°30' Summary of Structural and Geochronologic A B Relationships

Low-angle faults in the Homer Mountain area are interpreted as normal faults because they attenuate the pre-existing vertical structural and stratigraphic sequence. Both depositional and in- Figure 12. Simplified geologic map and cross section of the Newberry Mountains and trusive contacts are excised by faulting at Homer surrounding area. The distribution of upper-plate rocks and the form of the basal detachment Mountain, and depositional contacts are excised fault define four structural domains. Each domain is labeled at the top of the figure and is by faulting in the Sacramento Mountains. In the separated from adjacent domains by the trace of the basal detachment fault. Hypothetical Newberry Mountains, the intrusive contact with internal structure of the synformal upper plate, as shown in cross section, is drawn to resemble rocks interpreted as forming the roof of the imbricate tilt blocks of other areas of detachment faulting. Fault symbols are the same as in Mesozoic and/or Cenozoic plutonic complex Figure 3 and are dashed where inferred but concealed. appears to have been excised by low-angle faulting. In this area, upper-plate rocks in which biotite yields Jurassic or older K-Ar dates now rest on granitic rocks containing biotite and musco- upper-plate fault blocks probably resulted from in the Homer and northern Sacramento Moun- vite that yield only Miocene K-Ar dates. This is eastward to northeastward displacement and in- tains to have occurred prior to ~ 14.6 m.y. ago, interpreted as the product of Miocene detach- ternal extension of upper-plate rocks. The folds and in the western Newberry Mountains, prior ment faulting that juxtaposed upper-crustal or grooves of the basal detachment fault in the to ~ 15.3 m.y. ago. rocks with crystalline rocks originally at signifi- northwestern Sacramento Mountains suggest East-west-trending dikes were emplaced be- cantly greater depths. These data, plus the sim- displacement toward N60°E or S60°W (±15°). tween ~17 and 19 m.y. ago in the southern ilarity of rocks and structures in the Homer Detached and tilted conglomerates in the Piute Range, Homer Mountain, and northwest- Mountain area to areas of low-angle normal Homer Mountain area are interpreted as being ern Sacramento Mountains. A single K-Ar age faulting elsewhere in the Basin and Range prov- syntectonic sediments deposited in basins (17.2 ± 0.2 m.y.) on syntectonic volcanics at ince, support the interpretation that low-angle formed by extension above detachment faults. Homer Mountain is within the range of dates faults in the study area are normal faults. Two K-Ar ages (14.6 ± 0.9 and 17.2 ± 0.2 m.y.) obtained from the east-west-trending dikes, sug- Regional relative eastward displacement of of volcanic rocks interbedded with these con- gesting that faulting was synchronous with em- upper-plate rocks is indicated by several types of glomerates are thus interpreted as indicating that placement of at least the younger east-west- evidence. A gently east-dipping dike is offset faulting was underway at this time. K-Ar data trending dikes. The age of initial fault move- -1 km to the east by the basal low-angle fault at indicate that faulting was underway at about the ment is poorly constrained, however. It is Homer Mountain. Although tilt directions are same time in the Newberry Mountains. Termi- possible that low-angle faulting began during, or locally variable, the overwhelming preponder- nation of fault movement west of the crest of the even before, dike emplacement and that sedi- ance of westward to southwestward tilts of Dead-Newberry Mountains arch is constrained mentary deposits associated with initial low-

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angle faulting, if they existed, have been MECHANICS OF DIKE east-northeast-west-southwest extension, how- displaced relatively eastward and are now bur- EMPLACEMENT ever, the least compressive stress must have been ied or largely eroded away. abruptly reoriented from a north-south orienta- The structure of upper-plate rocks in the tion to an east-northeast-west-south west orien- REGIONAL CONTEXT OF Homer Mountain area indicates that the direc- tation — 17 m.y. ago. This is inconsistent with MIDDLE MIOCENE DEFORMATION tion of upper-plate movement and extension and K-Ar data from Homer Mountain that suggest at of the inferred least compressive stress was least some temporal overlap between dike em- The distribution of detached rocks in much of east-west to east-northeast-west-southwest dur- placement and detachment faulting. In addition, the northern Colorado River trough defines four ing middle Miocene faulting. This is entirely con- the 15.3-m.y.-old, east-west-trending dike in north-south-trending structural belts or domains sistent with evidence from other ranges in the the western Newberry Mountains is almost (Fig. 12). Each belt is separated from adjacent lower Colorado River trough in which upper- 2 m.y. younger than the youngest K-Ar date on belts by the trace of a basal detachment fault. plate rocks have been highly distended in the east-west-trending dikes at Homer Mountain, From west to east, these belts are: (1) an area same direction (for example, Anderson, 1971; indicating that stresses responsible for emplace- not significantly affected by extensional faulting, Davis and others, 1980; papers in Frost and ment of east-west-trending dikes were present, (2) a detached and distended synform of upper- Martin, 1982). In contrast, the orientation of at least intermittently, over a period of time en- plate rocks, (3) an. antiformal uplift of lower- dikes and other structures in the Homer Moun- compassing much or all of the known duration plate rocks flankel by outward-dipping low- tain area indicates that the least compressive of detachment faulting in the Homer Mountain angle faults, and (4) a wedge-shaped allochthon stress was oriented approximately north-south area. above an east-dipping, rooted detachment fault or vertical in the lower plate. Major temporal If detachment faulting began before dike em- (Spencer, 1984). and/or spacial changes in the orientations of placement, as permitted, but not required, by the On the basis of correlation of the basal de- principal stresses must account for these conflict- geochronologic data, then the state of stress in tachment faults or fault zones throughout the ing paleostress indicators. upper-plate rocks differed radically from the Homer-Sacramento-Dead-Newberry Moun- The east-west-trending dikes in the Homer coexisting state of stress in the lower plate. I tains, the entire arei is inferred to be a distended Mountain area represent significant extension in propose that subregional east-west compression aggregate of upper plates above a warped but the north-south direction that must have been within lower-plate rocks, in the area of east- structurally coherent lower plate. The arched accommodated by regional tectonic movements. west-striking dikes, overwhelmed the regional form of the Newberry-Dead-northeastern Sac- Dike swarms at Homer Mountain and in the east-northeast-west-southwest extensional ramento Mountains is interpreted as being the southern Piute Range represent 10% dilation of stresses that were responsible for detachment product of isostatic uplift and warping following country rock over north-south transects that, at faulting and major crustal extension. The east- relative eastward displacement of upper-plate Homer Mountain, are equal to the entire 7-km west-trending dikes were emplaced when aNS rocks now forming the Black Mountains from width of exposed lower-plate rock. Comparable (the north-south-directed stress) was less than an original position closer to, or even on top of, dilation by dike intrusion occurred over a north- both ay (the vertically directed stress) anc. CTEw these ranges. The synformal shape of the basal south distance of 6 km in the northwestern Sac- (compression is here considered to be positive). detachment fault beneath, and on, the flanks of ramento Mountains. Several percent dilation The presence of subhorizontal dikes at Homer Piute Valley is also inferred to be the product of has also occurred in the Piute Wash hills. A total Mountain indicates that, at other times, was warping due to tectonic denudation (Spencer, of ~ 1.4 km of basement dilation is demonstrable less than both aN$ and aEW. In either case, aEW 1984). The isolated, approximately domal uplift over a 42-km-long north-south transect (average was not the minimum compressive stress, sup- of lower-plate rocks in the northwestern Sacra- 3.5% extension) that crosses all three areas, but porting the inference that the lower plate in the mento Mountains is structurally anomalous be- this is a minimum because much of the lower Homer Mountain area was in an anomalous cause it lies within the domain of the synformal plate is not exposed but is probably also in- state of east-west compression. As outlined upper plate. truded by dikes. This is one to two orders of below, both north-south dilation and east-west The basal detachment fault at Homer Moun- magnitude greater strain than can be accommo- compression in the lower plate are possible sec- tain is the eroded trace of a segment of the dated by elastic compression of surrounding ondary products of major denudational faulting breakaway fault th at separates the broad area of country rocks without producing a state of stress and crustal extension in an east-northeast-west- detachment faulting in the lower Colorado that would favor subhorizontal dike orienta- southwest direction. River trough from the unextended area to the tions. west. The breakaway projects southward under Models of processes that controlled dike Dike Accommodation by Secondary alluvium along the east flank of the Piute Moun- orientation are dependent on inferences concern- Divergence tains, may crop out between the Little Piute and ing the relative timing of initiation of dike em- Piute-Old Woman Mountains, and may crop placement and detachment faulting. If dike Major crustal extension in a belt along the out again farther south between the Turtle and emplacement began before detachment faulting, northern part of the lower Colorado River Mopah Mountains (Davis and others, 1980; then stresses produced by crustal flexure accom- trough from Lake Mead to the Whipple Moun- Howard and others, 1982b). North of Homer panying tectonic denudation and isostatic uplift tain area accommodated westward to south- Mountain, the breakaway is represented by the did not influence stresses and resultant dike westward displacement, relative to the Colorado detachment fault in the Piute Wash hills. North orientations, at least not during early dike em- Plateau, of the lower plate in the Homer f foun- of the Piute Wash hills, the location of the placement. There seems to be no particular rea- tain area and of the structurally continuous un- breakaway is unknown, and it may change son why north-south extension should, or should extended area including the Old Woman-Piute geometry and character where it projects into an not, occur prior to detachment faulting. If the Mountains, Piute Range, and McCu Hough area of opposite tilt directions in the vicinity of east-west-trending dikes were emplaced in a re- Mountains. If extension occurred simultane- the Highland Spring Range. gional stress regime that existed prior to regional ously in this belt, then slight deviations; from

strictly parallel directions of extension along the and concave-upward flexure as a result of prox- sion slows or ceases, but flexure due to differen- belt would have resulted in secondary compo- imity to denuded areas. The boundary between tial denudation and isostatic uplift continues, nents of convergence or divergence at high an- synform and antiform in the Newberry Moun- emplacement of east-west-trending dikes will

gles to the direction of regional extension. tains corresponds to the eastward transition cease as aNS becomes greater than CTv, resulting Alternatively, if extension was diachronous from east-west-trending dikes to north-south- in a state of stress above the neutral surface that along the belt, rotations of the unextended area trending dikes (Fig. 11), further supporting the favors subhorizontal dike orientations. The tra- would occur, and deviations from parallelism inference that flexure helped to control dike jectories of the principal stresses within a cross within a set of arcuate vectors describing the orientation. section through a tapered lower plate undergo- extension and rotational movement would also Within an area of synformal flexure, in- ing concave-upward flexure were modeled pre- result in secondary convergence or divergence. creased subhorizontal compression at shallow viously (Spencer, 1982, Fig. 5). They indicate In either case, a secondary component of diver- crustal levels, due to flexure, is balanced by sub- that, in the absence of horizontal extension in a gence could have accommodated emplacement horizontal extensional stress (reduced compres- direction perpendicular to detachment fault of east-west-trending dikes in the Homer Moun- sion) at deeper crustal levels. The upper area of movement, the least compressive stress changes tain area. The amount of dilation represented by compressional stress is separated from the lower orientation gradually from horizontal at the the dikes is minor in comparison to the tens of area of extensional stress by the subhorizontal rheological base of the elastic-plastic lower plate kilometres of extension that occurred in the neutral surface, within which flexure causes no to almost vertical at the top. Vertical, north- northern Colorado River trough (Davis and compression or extension. In the Homer Moun- south-striking dike orientations are predicted at others, 1980; Wernicke and others, 1982). For tain area, flexure-generated stresses would thus the base of the lower plate, but as dikes pass this reason, only slight deviations from parallel- have favored north-south dike orientations upward through the neutral surface, they should ism in the direction of major extension would be below the neutral surface and east-west dike gradually roll over into near-parallelism with the necessary for dike emplacement. Indeed, it orientations above the neutral surface. North- overlying detachment fault. Furthermore, all would probably be surprising if minor, second- south dilation due to divergent regional-exten- dikes should roll over so that they dip away ary crustal movements, such as those repre- sion vectors would have favored east-west dike from the breakaway fault. This is exactly what is sented by the dikes in the Homer Mountain orientations at all depths, however. The pre- observed at Homer Mountain, where north- area, did not occur in a sinuous, extensional dicted dike geometry is thus a grid of vertical south-striking dikes dip -20° to the east. The orogenic belt. dikes below the neutral surface and only east- remarkable similarity between predicted and west dikes above. observed orientations further supports the hypothesis that stresses produced by flexure were a Effects of Crustal Flexure The problem with this hypothesis for control major determinant of dike orientation. of dike geometry is that initial emplacement of Laterally variable tectonic denudation and north-south-trending dikes below the neutral LATE-DETACHMENT TO isostatic uplift is considered to be a major cause surface in the Homer Mountain area would have POSTDETACHMENT STRUCTURES of warping of the lower plate in the Homer accommodated east-west extension and thereby Mountain area, as well as in many other areas of reduced the flexure-generated extensional stress Reverse faulting in the Dead Mountains and detachment faulting in the southwestern United and caused the neutral surface to move upward folding in the Castle Mountains could also be States (Howard and others, 1982b; Spencer, in the area above the dikes. In addition, heating products of east-west compression due to 1984). Regional warps produced by denudation of mid-crustal rocks by magma intrusion would concave-upward warping, even though geo- and uplift consist of a broad synform-antiform have weakened them, also causing stress reduc- chronologic data indicate that these structures pair with axes oriented approximately perpen- tion and elevation of the neutral surface. If these formed after low-angle faulting had ended in dicular to the direction of fault movement. processes of stress reduction had been highly ef- nearby areas. Although available evidence indi- Bending of an elastic beam or plate produces fective and operative over a large area, the neu- cates that extensional faulting and denudation compression on the concave side of the flexure tral surface would have risen upward just ahead ended in the area of synformal warping and tension on the convex side. Subhorizontal of the advancing, north-south-striking dikes ~ 14.5 m.y. ago, denudation of the east flank of compression in the upper part of the lower plate until both reached the top of the flexed slab. the Newberry, Dead, and northeastern Sacra- is thus the predicted result of concave-upward The significance of this process of elevation of mento Mountains, and associated isostatic uplift flexure in the area of synformal warping. Furth- the neutral surface is dependent on the size and of these ranges, could have continued for per- ermore, small amounts of flexure produce large extent of the intruding magma body. A small haps another 1 to 2 m.y. This hypothetical tran- stresses, because rock is quite inelastic. feeder stock or plug, for example, would cause sition to late-stage, one-sided denudation would East-west-trending dikes in the Homer only minor, local changes in the stresses caused have occurred as a result of formation and uplift Mountain area are all within, or adjacent to, the by flexure, whereas a large pluton could com- of the Dead-Newberry-northeastern Sacramen- area of synformal warping (Fig. 12). The distri- pletely release flexure-generated stresses over a to Mountains arch (hereafter referred to as the bution of east-west-trending dikes, and the large area. In either case, the neutral surface "Dead-Newberry arch"), which acted as a bar- complete absence of north-south-trending, would be restored to near its initial depth after rier to lateral movement of upper-plate rocks in steeply dipping dikes in the same area, are evi- solidification and cooling and after a small the area of synformal warping (Spencer, 1984). dence that east-west-directed compression due amount of flexure. The numerous petrologic Late-stage detachment faulting on the east side to concave-upward flexure helped to control suites of dikes in the Homer Mountain area are of this arch would represent the southward con- dike orientation. The east-west orientation of inferred as representing multiple episodes of tinuation of a zone of major extensional faulting, dikes within the unextended area adjacent to the magma intrusion, possibly from small magma recognized by Anderson (1971) in the Eldorado breakaway is consistent with a flexure-generated bodies at depth, each intrusive event causing and northern Black Mountains, that was active stress model because unextended areas adjacent minor transient elevation of the neutral surface. between -13.5 and 15 m.y. ago (Anderson and to the breakaway would have undergone uplift If north-south dilation due to divergent exten- others, 1972).

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This hypothetical transition would have been surface irregularities at Homer Mountain appear major stress reorientations over a short period of marked by termination of extensional faulting to be original features rather than folds because time. This alternate model requires that the within the detached synform and by formation the shape of upper plates 1 and 2 suggests that least compressive stress, was initially oriented of a new breakaway fault above the Dead- they were scalloped out of the autochthon, leav- north-south during emplacement of east-west- Newberry arch and within the previously dis- ing behind the fault surface irregularities. If the trending dikes, changed to east-west for exten- tended upper plate. Continued denudation and irregularities are folds, they apparently did not sional faulting, changed to subvertical before uplift of one flank of the arch would have form during emplacement of east-west-trending emplacement of subhorizontal dikes, and caused synformal flexure of the other, inactive dikes because dikes emplaced during folding changed back to east-west for continued exten- flank. The flank of the antiform adjacent to the would be expected to have a regional strike per- sional faulting. Finally, a north-south orienation synformal upper plate would thus have been pendicular to fold axes or to change orientation of (73 is indicated by the locally postfaulting; east- transformed from a site of antiformal flexure to at boundaries between antiforms and synforms. west dike in the Newberry Mountains, and a a site of synformal flexure as a result of the In contrast, the east-west-trending dikes in the vertical orientation is suggested by apparent late- change to one-sided denudation. Concave- Homer Mountain area are at a slight to moder- extension to postextension folding and reverse upward flexure within, and on, the flanks of ate angle to "fold" axes and do not change orien- faulting. All of these regional stress reorienta- Piute Valley, occumng in response to one-sided tation at synform-antiform boundaries. On the tions must have occurred within a several- denudation and associated isostatic uplift of the whole, data from this study are mildly suppor- million-year period, and magmatism musl have Dead-Newberry arch, is a possible mechanism tive of the interpretation that east-southeast- to occurred only during anomalous regional states for the compression that caused reverse faulting east-northeast-trending fault-surface irregulari- of stress in which a3 was not oriented in the in the Dead Mouniains and folding in the Castle ties at Homer Mountain and in the northwestern east-west to east-northeast-west-southwest di- Range and perhaps determined the orientation Sacramento Mountains are original features rection of primary crustal extension. This seems of the east-west-striking dike on the west flank rather than folds. This interpretation is not unlikely, and a model in which stresses in lower- of the Newberry Mountains. meant to apply to other areas of east-northeast- and upper-plate rocks differed, but coexisted, within a regional tectonic regime of east- Late-stage, one-sided denudation also ac- trending folds or grooves such as those in the northeast-west-south west extension is favored counts for the distribution of rock types in the Whipple-Buckskin Mountains that have a here. upper plate of the Newberry Mountains. Upper- greater wavelength and more regular spacing and form. plate volcanic rocks are in contact with the Regional considerations are suggestive of, but lower plate only a t the east foot of the central The mechanical model proposed here for dike do not require, east-northeast-west-south west Newberry Mountains; only upper-plate crystal- emplacement and deformation requires some extension in the Homer Mountain area (luring line rocks are in contact with the lower plate on amount of concave-upward flexure prior to dike emplacement of the 17- to 19-m.y.-old. east- the crest and west side of the range (Mathis, emplacement and requires major, approximately west-trending dikes. To the south, major exten- 1982; Volborth, 1973). Late-stage, one-sided east-west extension during dike emplacement. sion was underway in the Whipple Mountains denudation could have brought shallow-level, Concave-upward flexure is necessary to produce area at this time (Davis and others, 1982; Teel Tertiary volcanic rocks down against the lower stresses that will prevent emplacement of dikes and Frost, 1982). To the north, synchronous plate at the east foot of the Newberry Moun- oriented perpendicular to the direction of large- formation of a broad, shallow sedimentary basin tains. magnitude regional extension. Major extension in the Lake Mead area (Bohannon, 198^ ) was East-west compression in the area of synfor- in a direction approximately parallel to the dikes probably due to minor, incipient crustal exten- mal warping, due to concave-upward warping is necessary for secondary divergence to ac- sion. An intermediate amount of extension in accompanying isostatic rebound, also could commodate dike emplacement. Both processes the northern Colorado River trough at the lati- have continued after termination of all detach- must have operated simultaneously to account tude of the Homer Mountain area maj have ment faulting and tectonic denudation. This for the anomalous orientation of the east- occurred during this time interval. The north- would occur as a result of viscoelastic relaxation west-trending dikes in the Homer Mountain ward-decreasing rate of east-northeast-west- of the ductile middle crust that resists flexure area. Subhorizontal dike orientations were fa- southwest extension envisioned here would have initially but gradually flows in response to devia- vored when secondary divergence did not occur, resulted in minor clockwise rotation of the lower toric stresses prod uced by tectonic denudation and approximately north-south-trending verti- plate and unextended area to the west, relative and flexure. Uplift and flexure thus will continue cal dikes would have been favored in the ab- to the Colorado Plateau. after termination of tectonic denudation, and sence of concave-upward flexure. Variable isostatic rebound and associated structures related, to east-west compression Geochronologic data do not, however, accu- warping of the lower plate in the northern Colo- might result from postdenudation isostatic rately constrain the age of initial extensional rado River trough divided the regional exten- rebound. faulting in the Homer Mountain area. As a re- sional allochthon into separate synform il and sult, speculation about an alternate model in wedge-shaped components separated by a belt DISCUSSION AND CONCLUSIONS which dike emplacement began first is war- of antiformal uplifts of the lower plate. Warping ranted. A model in which dike emplacement appears to have exerted strong control over the Map-scale irregularities of the basal detach- and other apparently compressional deforma- state of stress in the lower plate. Application of ment fault surface in the northwestern Sacra- tions occurred during distinct periods of anom- principles of mechanics to the warping process mento Mountains trend east-northeast to north- alous regional stress orientations is not favored provides an explanation for a number of anom- east, whereas those at Homer Mountain trend here because it is at odds with some of the alous structural features, including dike orienta- east-southeast or due east. Some of the fault- geochronologic data and would require several tions, reverse faults, and folds, all of which are

Bernardino County, California: Southern Pacific Land Company, un- California, University of Southern California, 94 p. not easily explained in an extensional tectonic published map, scale 1:24,000. Miller, C. F., Howard, K. A., and Hoisch, T. D., 1982, Mesozoic thrusting, setting. Cameron, T. E., and Frost, E. G., 1981, Regional development of major anti- metamorphism, and plutonism, Old Woman-Piute Range, southeastern forms and synforms coincident with detachment faulting in California, California, in Frost, E. G., and Martin, D, L., eds., Mesozoic-Cenozoic Arizona, Nevada, and Sonora: Geological Society of America Abstracts tectonic evolution of the Colorado River region, California, Arizona, with Programs, v. 13, p. 421-422. and Nevada: San Diego, California, Cordilleran Publishers, p. 561-581. ACKNOWLEDGMENTS Collier, J. T., 1960a, Geologic map of township 8 north, ranges 21 and 22 east, Phillips, J. C., 1982, Character and origin of cataclasite developed along the San Bernardino base and meridian, San Bernardino County, California: Whipple detachment fault, Whipple Mountains, California, in Frost, Southern Pacific Land Company, unpublished map, scale 1:24,000. E. G., and Martin, D. L., eds., Mesozoic-Cenozoic tectonic evolution of My research in the Homer Mountain area 1960b, Geologic map of township 8 north, ranges 19 and 20 east, San the Colorado River region, California, Arizona, and Nevada: San Bernardino base and meridian, San Bernardino County, California: Diego, California, Cordilleran Publishers, p. 109-116. was conducted while I was a National Research Southern Pacific Land Company, unpublished map, scale 1:24,000. Rehrig, W. A., and Heidrick, T. L., 1976, Regional tectonic stress during the Council postdoctoral Research Associate at the Cox, Allan, and Dalrymple, G. B., 1967, Statistical analysis of geomagnetic Laramide and late Tertiary intrusive periods. Basin and Range province, reversal data and the precision of potassium-argon dating: Journal of Arizona: Arizona Geological Society Digest, v. 10, p. 205-228. U.S. Geological Survey in Menlo Park, Califor- Geophysical Research, v. 72, p. 2603-2614. Rehrig, W. A., and Reynolds, S. J., 1980, Geologic and geochronologic recon- Custis, K. H., 1984, Geology and dike swarms of the Homer Mountain area, naissance of a northwest-trending zone of metamorphic core complexes nia, in 1981-1982. I especially thank Keith San Bernardino County, California [M.S. thesis]: Northridge, California, in southern and western Arizona, in Crittenden, M. D., Jr., Coney, P. J., Howard for sponsoring this research and for his California State University, 168 p. and Davis, G. H., eds., Cordilleran metamorphic core complexes: Geo- Dalrymple, G, B., 1979, Critical tables for conversion of K-Ar ages from old to logical Society of America Memoir 153, p. 131-157. encouragement, support, and interest through- new constants: Geology, v. 7, p. 558-560. Reynolds, S. J., and Spencer, J. E., 1985, Evidence for large-scale transport on Davis, G. A., Anderson, J. L., Frost, E. G., and Shackelford, T. S., 1980, the Bullard detachment fault, west-central Arizona: Geology, v. 13, (in out all phases of this research project. I also Mylonitization and detachment faulting in the Whipple-Buckskin- press). thank Ryan Turner, Jackie Huntoon, and Henri Rawhide Mountains terrane, southeastern California and western Ariz- Spencer, J. E., 1982, Origin of folds of Tertiary low-angle fault surfaces, south- ona, in Crittenden, M. D., Jr., Coney, P. J., and Davis, G. H., eds., eastern California and western Arizona, in Frost, E. G., and Martin, Wathen for assistance in the field; Jerry Von Cordilleran metamorphic core complexes: Geological Society of Amer- D. L., eds., Mesozoic-Cenozoic tectonic evolution of the Colorado ica Memoir 153, p. 79-130. River region, California, Arizona, and Nevada: San Diego, California, Essen and Marty Pernokas for assistance with Davis, G. A., Anderson, J. L., Martin, D. L., Krummenacher, D., Frost, E. G., Cordilleran Publishers, p. 123-134. K-Ar dating; and Dennis Sorg for providing and Armstrong, R. L., 1982, Geologic and geochronologic relations in 1983, Miocene dike emplacement and low-angle faulting in the north- the lower plate of the Whipple detachment fault, Whipple Mountains, ern Sacramento Mountains, Homer Mountain, and adjacent areas, Cali- mineral separates. Dave Pollard is gratefully ac- southeastern California: A progress report, in Frost, E. G., and Martin, fornia and Nevada: Geological Society of America Abstracts with D. L., eds., Mesozoic-Cenozoic tectonic evolution of the Colorado Programs, v. 15, p. 384. knowledged for several helpful discussions con- River region, California, Arizona, and Nevada: San Diego, California, 1984, Role of tectonic denudation in uplift and warping of low-angle cerning the mechanics of dike emplacement and Cordilleran Publishers, p. 408-432. normal faults: Geology, v. 12, p. 95-98. Davis, G. A., Lister, G. S., and Reynolds, S. J., 1983, Interpretation of Cordil- Spencer, J. E., and Turner, R. D., 1983, Geologic map of part of the north- crustal flexure. Discussions with Keith Howard, leran core complexes as evolving crustal shear zones in an extending western Sacramento Mountains, southeastern California: U.S. Geo- orogen: Geological Society of America Abstracts with Programs, v. 15, logical Survey Open-File Report 83-614, scale 1:24,000. Dave Miller, Robert Powell, Jane Pike, G. A. p. 311. 1985, Geologic Map of Homer Mountain and the southern Piute Davis, Lawford Anderson, Barbara John, Bill Frost, E. G., 1981, Mid-Tertiary detachment faulting in the Whipple Mtns., Range, southeastern California: U.S. Geological Survey Miscellaneous Calif., and Buckskin Mtns., Ariz., and its relationship to the develop- Field Studies Map MF-1709, scale 1:24,000. McClelland, Steve Reynolds, and Eric Frost im- ment of major antiforms and synforms: Geological Society of America Spurck, W. H., 1960, Geologic map of township 9 north, ranges 19 and 20 east, Abstracts with Programs, v. 13, p. 57. San Bernardino base and meridian, San Bernardino County, California: proved my understanding of the eastern Mojave Frost, E. G., and Martin, D. L., eds., 1982, Mesozoic-Cenozoic tectonic evolu- Southern Pacific Land Company, unpublished map, scale 1:24,000. region. Finally, I thank Keith Howard and Gor- tion of the Colorado River region, California, Arizona, and Nevada: Steiger, R. H., and Jäger, E., 1977, Subcommission on geochronology: Conven- San Diego, California, Cordilleran Publishers, 608 p. tion on the use of decay constants in geo- and cosmochronology: Earth don Haxel for helpful reviews of an earlier ver- Hewett, D. F., 1956, Geology and mineral resources of the Ivanpah quadrangle, and Planetary Science Letters, v. 36, p. 359-362. California and Nevada: U.S. Geological Survey Professional Paper 275, Teel, D. B„ and Frost, E. G„ 1982, Synorogenic evolution of the Copper Basin sion of this paper. 172 p. Formation in the eastern Whipple Mountains, San Bernardino County, Howard, K. A., and John, B. E., 1983, Extensional faulting through the upper California, in Frost, E. G., and Martin, D. L., eds., Mesozoic-Cenozoic crust, California-Arizona border: Geological Society of America Ab- tectonic evolution of the Colorado River region, California, Arizona, stracts with Programs, v. 15, p. 309. and Nevada: San Diego, California, Cordilleran Publishers, p. 275-285. REFERENCES CITED Howard, K. A., Goodge, J. W., and John, B. E., 1982a, Detached crystalline Tischler, M. S., I960, Geologic map of township 10 north, ranges 19 and 20 rocks of the Mojave, Buck, and Bill Williams Mountains, western east, San Bernardino base and meridian, San Bernardino County, Cali- Allmendinger, R. W„ Sharp, J. W., VonTish, D., Serpa, L., Brown, L., Kauf- Arizona, in Frost, E. G., and Martin, D. L., eds., Mesozoic-Cenozoic fornia: Southern Pacific Land Company, unpublished map, scale man, S., Oliver, J., and Smith, R. 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