Paleomagnetism and Tectonic Rotation of the Lower Miocene Peach Springs Tuff: Colorado Plateau, Arizona, to Barstow, California

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Paleomagnetism and tectonic rotation of the lower Miocene Peach Springs Tuff: Colorado Plateau, Arizona, to Barstow, California

JQHNEWWH[LLHOUSE i V-S-Geological Survey, Menlo Park, California 94025

ABSTRACT detachment unexpectedly show no significant rotation. From this relation, we infer that rotations are accommodated along numerous low- We have determined remanent magnetization directions of the angle faults at higher structural levels above the detachment surface. lower Miocene Peach Springs Tuff at 41 localities in western Arizona and southeastern California. An unusual northeast and shallow mag- INTRODUCTION netization direction confirms the proposed geologic correlation of isolated outcrops of the tuff from the Colorado Plateau to Barstow, Remnants of block-faulted and tilted middle Tertiary volcanic and California, a distance of 350 km. The Peach Springs Tuff was appar- sedimentary sequences are exposed in numerous mountain ranges extend- ently emplaced as a single cooling unit about 18 or 19 Ma and is now ing westward from the Colorado Plateau of Arizona into the central exposed in 4 tectonic provinces west of the Plateau, including the Mojave Desert of southeastern California (Fig. la). Exposed in many of Transition Zone, Basin and Range, Colorado River extensional corri- these isolated Tertiary sections, there is a distinctive, lower Miocene ash- dor, and central Mojave Desert strike-slip zone. As such, the tuff is an flow tuff which Glazner and others (1986) correlated with the Peach ideal stratigraphic and structural marker for paleomagnetic assess- Springs Tuff of western Arizona (Fig. lb). The Peach Springs Tuff was ment of regional variations in tectonic rotations about vertical axes. originally described by Young and Brennan (1974) for exposures near From 4 sites on the stable Colorado Plateau, we have determined a Peach Springs on the western edge of the Colorado Plateau. Young and reference direction of remanent magnetization (I = 36.4°, D = 33.0°, Brennan (1974) used paleomagnetism to substantiate their lithologic corre- «95 = 3.4°) that we interpret as a representation of the ambient mag- lation of isolated exposures of the ignimbrite between Kingman and Peach netic field at the time of eruption. A steeper direction of magnetization Springs, Arizona, and they concluded that all were part of a single eruptive

(I = 54.8°, D = 22.5°, a95 = 2.3°) was observed at Kingman where the unit. Subsequent correlation of the tuff westward into the Mojave Desert tuff is more than 100 m thick, and similar directions were determined was based on stratigraphic position, the presence of megascopic sphene at 7 other thick exposures of the Peach Springs Tuff. The steeper and a characteristic blue schiller in sanidine, generally similar K-Ar ages component is presumably a later-stage magnetization acquired after averaging 18.3 m.y., and regional similarities in crystal chemistry. Gusa prolonged cooling of the ignimbrite. When compared to the Plateau and others (1987) have shown that the heavy-mineral suite of the Peach reference direction, tilt-corrected directions from 3 of 6 sites in the Springs Tuff is distinctive compared to other tuffs in the region. central Mojave strike-slip zone show localized rotations up to 13° in If the correlation of Glazner and others (1986) is correct, the Peach the vicinity of strike-slip faults. The other three sites show no signifi- Springs Tuff extends at least 350 km west of the Colorado Plateau and cant rotations with respect to the Colorado Plateau. Both clockwise crops out in five tectonic provinces, including the Colorado Plateau, Tran- and counterclockwise rotations were measured, and no systematic sition Zone, Basin and Range, lower Colorado River extensional corridor, regional pattern is evident. Our results do not support kinematic mod- and central Mojave Desert strike-slip zone (Figs, lb, lc). This region has els which require consistent rotation of large regions to accommodate been the locus of major Cenozoic tectonism, including several Miocene the cumulative displacement of major post-middle Miocene strike-slip episodes of crustal extension, unroofing of metamorphic core complexes, faults in the central Mojave Desert. Most of our sites in the Transition and late Cenozoic strike-slip faulting. In the eastern Mojave Desert, the Zone and Basin and Range province have had no significant rotation, Peach Springs Tuff predates a major extensional episode along the lower although small counterclockwise rotation in the McCullough and New Colorado River (Howard and John, 1987) and is found in numerous tilt York Mountains may be related to sinistral shear along en echelon blocks above detachment faults. In the central Mojave Desert, the tuff faults southwest of the Lake Mead shear zone. The larger rotations postdates an extensional event and laps up against tilt blocks of older occur in the Colorado River extensional corridor, where 8 of 14 sites volcanic rocks (Dibblee, 1964a; Dokka, 1983, 1986). After deposition of show rotations ranging from 37° clockwise to 51° counterclockwise. the tuff, the central Mojave Desert was cut by a set of northwest-trending These rotations occur in allochthonous tilt blocks which have been right-lateral faults that were approximately parallel to the San Andreas transported northeastward above the Chemehuevi-Whipple Moun- system (Dibblee, 1967a; Dokka, 1983). Garfunkel (1974) proposed a tains detachment fault. Upper-plate blocks within 1 km of the exposed tectonic model for the Mojave block in which the right-lateral faults

Geological Society of America Bulletin, v. 101, p. 846-863, 11 figs., 1 table, June 1989.

846

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TABLE 1. PALEOMAGNETIC RESULTS FROM THE PEACH SPRINGS TUFF IN ARIZONA, CALIFORNIA, AND NEVADA

Structural province Site Site Bedding N/N c "95 k I D >c Dc dl dD No. Site name Id. Lat. Long. Strike Dip

Colorado Plateau 1 Lookout Wash 6J254 35.20 246.57 0.0 0.0 9/10 4.6* 40.0 33.7 40.0 33.7 2 Truxton 6J264 35.46 246.38 32.8 1.7 E 10/10 2.9 282 36.3 31.7 36.3 31.7 3 Crazier Canyon 6J274 35.43 246.33 0.0 0.0 6/10 5.8 134 36.3 34.1 36.3 34.1 4 Fort Rock 6J284 35.23 246.74 117.9 4.7 SW 6/10 1.8* 28.3 32.3 33.0 32.5 Mean Reference 4/4 3.4 36.4 33.0

Transition Zone Kingman MO cut 5 Welded top 6J204 35.18 245.93 325.3 6.4 E 9/10 4.9 112 61.4 16.7 56.2 22.9 6 Welded middle 5H124 - 10/10 2.3 454 60.0 16.6 54.8 22.5 7 Welded base 6J184 - 5/5 1.4 2892 57.4 16.0 52.2 21.4 8 Unwelded base 6J189 * - - 4/4 5.0 344 43.8 27.2 38.1 29.7 -1.7 ± 4.8 -3.2 ± 6.1 9 Kingman Mesa 1 6J214 35.23 245.94 0.0 0.0 9/9 2.1 602 40.0 30.1 40.0 30.1 -3.6 ± 3.2 -2.9 ± 4.0 10 Kingman Mesa 2 6J224 " - - 10/10 2.6 346 47.0 26.6 47.0 26.6 -10.6 ± 3.4 -6.4 ± 4.6 11 Kingman Bridge 6J234 35.19 245.97 0.0 0.0 10/10 2.6 342 40.9 30.9 40.9 30.9 -4.5 ± 3.4 -2.1 ± 4.4 12 Flat Top 5HI14 34.79 245.97 0.0 0.0 7/10 6.7* 40.0 30.9 40.0 30.9 -3.6 ± 6.0 -2.1 ± 7.8

Colorado River Extensional corridor 13 Radio tower 6J174 34.60 245.63 146.7 48.5 W 10/10 10.2 24 0.1 33.9 43.8 24.2 -7.4 ± 8.6 -8.8 ±11.9 14 Bill Williams Mts. 6J164 34.36 245.89 127.0 66.5 SW 6/6 2.6 691 -17.5 32.9 51.1 31.1 3.6 ± 2.8 8.6 ± 4.5t 15 Parker 5H134 34.18 245.73 94.0 36.0 S 10/10 8.2* 19.4 56.8 35.0 70.4 1.4 ± 7.1 37.4 ± 8.7 16 Pyramid Butte 5H144 34.32 245.40 30.0 15.0 SE 10/10 2.9 284 49.3 21.3 49.2 38.9 5.6 ± 3.0 16.4 ± 4.8t 17 Snaggletooth I 5H165 34.59 245.35 145.0 52.0 SW 9/10 5.8 79 4.4 20.8 44.2 3.6 -7.8 ± 5.4 -29.4 ± 7.3 18 Snaggletooth II 7J041 34.56 245.37 149.0 87.0 SW 10/10 4.0 146 -46.1 27.8 33.7 33.5 2.7 ± 4.2 0.5 ± 5.1 19 Turtle Mountains 5H154 34.46 245.17 205.0 26.0 NW 11/11 4.5 104 22.8 72.8 40.3 60.7 -3.9 i 4.5 27.7 ± 5.8 20 Little Piute Mts. 5H176 34.65 244.94 232.5 12.0 NW 8/10 3.4 273 36.8 50.6 35.5 41.9 0.9 ± 3.8 8.9 ± 4.8 21 Chemehuevi Wash 7J051 34.43 245.56 105.8 31.4 S 8/9 8.4 44 15.2 350.5 42.7 341.6 -6.3 t 7.2 -51.4 ± 9.8 22 Havasu Lake NW 7J061 34.48 245.52 353.8 26.4 E 8/10 4.4 158 56.3 344.8 51.6 21.8 3.2 ± 4.0 -0.7 ± 6.5t 23 Aubrey Hills 7J071 34.42 245.73 145.2 25.6 SW 10/10 2.6 359 30.1 29.8 52.2 17.9 2.6 ± 2.8 -4.6 ± 4.7+ 24 Topock Gorge 7J176 34.65 245.55 193.5 30.0 E 6/10 15.7 19 9.2 45.3 23.5 37.4 12.9 ± 12.9 4.4 ± 14.1 25 The Needles SW 7J186 34.67 245.55 254.5 56.2 N 6/10 11.8 33 58.2 87.2 34.7 23.2 1.7 ± 9.8 -9.8 ± 12.0 26 The Needles NW 7J196 34.68 245.54 91.8 43.1 S 10/10 4.8 101 11.8 12.4 53.8 19.5 1.0 ± 4.3 -3.0 ± 7.3t

Basii n and Range zone 27 Piute Mts. I 6J132 34.76 244.86 18.1 30.9 S 10/10 7.7 41 35.4 337.3 50.4 3.6 4.4 ± 6.4 -18.9 ± 10.2t 28 Piute Mts. II 6J142 34.77 244.87 225.0 35.0 NW 21/21 2.2 214 54.6 65.7 51.8 16.0 3.0 ± 2.5 -6.5 ± 4.3+ 29 Danby 5H206 34.55 244.60 330.0 8.0 NE 10/10 4.5 118 48.5 35.1 41.2 38.2 -4.8 ± 4.5 5.2 ± 5.9 30 Essex 5H186 34.66 244.81 213.4 29.4 NW 8/10 78 29.5 47.3 33.2 29.3 3.2 ± 4.8 -3.7 ± 5.8 31 Clipper Mts. 5H226 34.77 244.53 260.0 8.0 N 10/10 135 42.6 36.5 36.9 31.8 -0.5 ± 4.3 -1.2 ± 5.4 32 Middle Hills 5H236 34.71 244.42 35.0 8.0 SE 8/10 767 40.4 25.4 41.2 32.3 -4.8 ± 3.2 -0.7 ± 4.0 33 McCullough Mts. 6J294 35.69 244.83 340.2 52.4 E 10/10 5.5 78 52.8 318.4 41.6 21.5 -5.2 ± 5.2 -11.5 ± 6.8 34 Castle Peaks 6J304 35.37 244.80 55.7 16.1 SE 10/10 888 36.6 16.5 45.5 28.3 -9.1 ± 3.0 -4.7 ± 3.8 35 Wildhorse Canyon 6J314 35.05 244.57 69.6 16.6 SE 8/10 4.5 156 28.7 26.7 39.1 35.5 -2.7 ± 4.5 2.5 ± 5.7

Central Mojave Desert Strike-slip zone 36 Bristol Mts. 6J122 34.73 244.08 125.7 23.9 SW 10/10 3.4 201 17.0 36.5 40.9 36.7 -4.5 ± 3.8 3.7 ± 4.9 37 Kane Wash 5H246 34.72 243.26 0.0 0.0 9/10 2.6 384 37.1 31.4 37.1 31.4 -0.7 ± 3.4 -1.6 ± 4.3 38 Pacific Mesa 7J001 34.60 243.83 89.5 9.0 S 10/10 1.5 1107 29.1 20.6 37.4 22.9 -1.0 ± 3.0 -10.1 ± 3.7 39 Broadwell Lake 7J011 34.80 243.99 213.2 15.5 NW 7/10 6.2 96 36.6 44.3 38.0 32.4 -1.6 ± 5.7 -0.6 ± 7.2 40 Sleeping Beauty 7J021 34.78 243.69 294.7 15.1 NE 5/10 2.6 870 49.3 49.9 35.3 44.6 1.1 ± 3.4 11.6 ± 4.2 41 Stoddard Wash 7J031 34.76 243.08 74.1 10.5 SE 9/10 3.3 246 38.7 38.2 44.2 46.1 -7.8 ± 3.8 13.1 ± 5.0

Note: N/Nc> number of specimens averaged/number of specimens collected; radius of 95% confidence circle in degrees; k. Fisher (1953) precision parameter for site-mean magnetization direction; I and D, inclination and declination of

site-mean, in degrees, before tilt correction; Ic and Dc, inclination and declination of site-mean, in degrees, after tilt correction; dl and dD, inclination and declination anomalies with respect to reference direction, in degrees, and 95% confidence limits, in degrees (Beck, 1980; Demarest, 1983), •Average of minor and major axes of confidence ellipse from analysis of remagnetization circles, t Later-stage magnetization; direction anomalies referred to site no. 6.

accommodated up to 30° of counterclockwise rotation in response to determined by the number of cooling units collected, may have been too north-south compression between the San Andreas and Garlock faults few in some studies to ensure complete time-averaging of the Miocene since middle Miocene time. In contrast, late Cenozoic faulting in the geomagnetic field. This could introduce substantial errors in the interpreta- Alvord and Cady Mountains in the northeast corner of the Mojave block tions of tectonic rotations. Moreover, the complex Tertiary history of is characterized by a set of left-lateral faults that trend east-west (Fig. lb). deformation has complicated the interpretation of these paleomagnetic In their palinspastic reconstruction of southern California, Luyendyk and data sets, which must average together results from rocks of diverse ages others (1985) proposed clockwise rotation for this region, analogous to and from different structural settings. rotations observed in the Transverse Ranges. Our approach has been to compare site magnetization directions Previous paleomagnetic studies in our sampling region focused pri- (after correcting for simple tilt of the bedding) to a reference direction for marily on sequences of Miocene volcanic rocks to test for tectonic the Peach Springs Tuff, calculated from sites on the relatively undeformed rotations (Burke and others, 1982; Calderone and Butler, 1984; Luyendyk Colorado Plateau. From the differences in magnetic declination, we infer and others, 1985; Calderone and others, 1986). These studies necessarily vertical-axis rotations resulting from late Cenozoic faulting in the diverse assumed that the sampling represented the full range of Miocene geomag- structural domains of the region. This single-flow method has been used netic secular variation and thereby gave a mean paleomagnetic pole cor- successfully for widespread basalt flows of the Columbia Plateau of responding to the ancient geographic pole. Although varying amounts of Oregon and Washington (Magill and others, 1982; Sheriff, 1984; Reidel tectonic rotation have been reported, the number of time samples, as and others, 1984). The Peach Springs Tuff and its correlative outcrops in

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5- \ NEVADA\ EXPLANATION Quaternary and Pliocene sedimentary rocks, Tectonic province boundary locail/ includes young mafic volcanic rocks Contact Mostly Miocene silicic, intermediate, and mafic volcanic rocks and associated High-angle fault—Doited where sedimentary deposits concealed. Arrows indicale direction ol relative movement Pre Tertiary rocks—Mostly Proterozoic Thrust fault—Sawteeth on upper AREA OF WAP crystalline rocks, late Precamb/ian- Paleozoic marine strata, and Mesozoic plate plutonic and metamorphic rocks Low-angle normal fault exposing metamorphic core complex a) (lined pattern)—Hachures on upper plate

Figure 1. Simplified geologic maps and schematic cross section of the Mojave Desert and surrounding regions in California, Arizona, and Nevada, (a) Geologic map modified from Jennings (1977), Wilson and others (1969), and Stewart and Carlson (1978). Tectonic province boundaries modified from Howard and John (1987) and Dokka (1983).

Figure 1. (Continued). (b) Distribution of the lower Miocene Peach Springs Tuff, modified from Glazner and others (1986). Dark patches show presently known outcrops; light stipple indicates presumed minimum extent of tuff. Tectonic provinces and structure same as part a; dotted lines indicate outlines of mountain ranges. Numbers are sites sampled for paleomagnetism (Table 1). AV, Alvord Mountains; BM, Black Mountains; BR, Bristol Mountains; BU, Bullion Mountains; CA, Cady Mountains; CB, Cerbat Mountains; CH, Chemehuevi Mountains; CL, Clipper Mountains; DR, Daggett Ridge; HM, Hualapai Mountains; LP, Little Piute Mountains; MC, McCullough Mountains; MM, Marble Mountains; NM, Newberry Mountains; NY, New York Mountains; OW, Old Woman Mountains; PI, Piute Mountains; PM, Providence Mountains; PR, Piute Range; SM, Ship Mountains; TM, Turtle Mountains; WH, Whipple Mountains. Areas of Figures 5, 7a, 7b, and 8 are outlined.

(c) Schematic cross section of the Peach Springs Tuff between Peach Springs, Arizona, and Barstow, California (modified from Nielson and Glazner, 1986). Peach Springs Tuff (shaded unit) overlies tilted lower Miocene andesite, basalt, and fanglomerate with pronounced angular unconformity in the western part of the area and is cut by right-lateral faults.

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W 117° 116° 115° 114° 113°

CENTRAL MOJAVE DESERT BASIN AND LOWER COLORADO RIVER TRANSITION COLORADO STRIKE-SLIP ZONE RANGE EXTENSIONAL CORRIDOR ZONE PLATEAU

Daggett Newberry Cady Bristol- Ship Little Piute Chemehuevi Hualapal Ridge Mts Mts Marble Mts Mts Mts Mts Mts

+ + z /^oy + 7 O _ < + / + / > J + + + + + - + -P + + + + - O < +- — ? Barstow O Peach Springs

50 100 KILOMETERS I

EXPLANATION

Lower Miocene volcanic and sedimentary rocks Pre-Tertiary rocks

^ A 4 Peach Springs Tuff <1 Volcanic breccia Pz Paleozoic sedimentary rocks

O o O o Proterozoic and Mesozoic Fanglomerate Andesite flows + e> eoo + + igneous and metamorphic rocks I, I .I.J.I Basalt flows Sand and sandstone Other symbols

Volcanic tuff Contact

• ? Fault—Queried where uncertain; ® Arrow indicates direction of C) relative movement. Dot, south- moving block; cross, north-moving block

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the central Mojave Desert provide a unique opportunity to assess tectonic establish a reference magnetic direction derived from sites on the relatively rotations without requiring extensive sampling to obtain a time-averaged stable Colorado Plateau. We then extended the paleomagnetic sampling axial dipole field. The regional extent of the tuff makes it ideal to test 350 km to the west to cover the broader region of the Peach Springs Tuff hypotheses concerning large-scale rotation of the Mojave Desert (Garfun- that is now recognized by Glazner and others (1986). In the following kel, 1974; Luyendyk and others, 1985), block rotations in the southeastern discussion of paleomagnetic sampling, the site numbers (for example, no. Basin and Range province (Calderone and Butler, 1984; Calderone and 12) are keyed to the left-hand column of Table 1 and the locations are others 1986), and the kinematics of detachment faulting in the Colorado shown on Figure lb. River corridor. This report supersedes preliminary interpretations pre- sented by Wells and Hillhouse (1986, 1987) and Hillhouse and Wells Colorado Plateau (1986). The eastern distal edge of the Peach Springs Tuff laps onto the PALEOMAGNETIC SAMPLING OF THE Colorado Plateau between the Aquarius Mountains and the Hualapai PEACH SPRINGS TUFF Plateau (Young and Brennan, 1974; Goff and others, 1983; Lucchitta and Young, 1986). The tuff is interbedded with Miocene conglomerates, basalt In the original paleomagnetic study of the Peach Springs Tuff, Young flows, and andesites which once filled paleocanyons cut into the pre- and Brennan (1974) established the direction of remanent magnetization Tertiary section. The Tertiary and Paleozoic sections in this region are and used it to confirm correlations among 14 sample localities between nearly horizontal, although normal faults with throws of several hundred Kingman and Peach Springs, Arizona. They defined a distinctive magnetic meters have caused minor tilting in some areas. field direction for the tuff (mean inclination = 42.8°; declination = 32.3°; We sampled the Peach Springs Tuff at four localities along the west- a95 = 4.4°), which is substantially different from the average axial dipole ern edge of the Colorado Plateau (Fig. lb). Two sites (nos. 2, 3) are field direction for Miocene time. Young and Brennan's paleomagnetic data located along State Route 66 near Truxton, Arizona, and the others (nos. strengthened the regional correlation of the Peach Springs Tuff, because 1, 4) are just north of U.S. Interstate 40 between the Cottonwood Moun- the probability of two different-aged tuffs acquiring the same unusual tains and Cross Mountain. magnetic direction would be very low. We resampled the general area covered by Young and Brennan to Transition Zone identify the sources of moderate dispersion observed in their results and to Southwest of the Grand Wash Cliffs, which mark the western limit of the Colorado Plateau, there is a region of block-faulted ranges and a) NORTH grabens bounded by northwest-trending sets of normal faults. In this transi- -- 1

Figure 2. Results of alternating field treatment of Peach Springs Tuff, (a) Equal-area projection of remanent magnetization directions, showing removal of secondary components during alternating field (AF) treatment of specimens from the Peach Springs Tuff, site no. 9 (Table 1). (b) Orthogonal vector diagrams depicting the AF demagnetization of specimen 6J221 shown in part a. Left-hand diagram Horizontal plane shows projection of the magnetic vector 6J221 into the horizontal plane; right-hand diagram shows vertical plane. Note stabiliza- tion of the magnetization direction at AF fields above 30 mT (milliTeslas). NRM, natural remanent magnetization before AF treatment. Axis units: 1 division = 6.05 x 10"1 Ampere/meter.

NRM

tion between the Plateau and the Colorado River extensional corridor, strike-slip faulting, possibly contemporaneous with normal faulting, occurs Miocene volcanic and sedimentary rocks lie directly on an eroded base- along the north side of the New York Mountains and may be part of a ment of Precambrian gneisses and granitoids. system of en echelon sinistral-slip faults extending southwestward from the At Kingman, a thick accumulation of Peach Springs Tuff fills a basin Lake Mead shear zone (Anderson, 1973) into the Mojave Desert. floored by early Miocene volcanic and sedimentary rocks on the flanks of In the New York and McCullough Mountains to the north, the Peach the Hualapai and Cerbat Mountains. Minor normal faulting has gently Springs Tuff occurs near the bottom of the Miocene section and is overlain tilted the section, and major escarpments are eroded in the tuff along the by intermediate to mafic flows and laharic breccias (Miller and others, fault zones (Buesch and Valentine, 1986). A nearly complete section of the 1986; E. I. Smith, 1986, personal commun.). Site 33 in the McCullough tuff is exposed in the 100-m-deep roadcut south of Kingman along U.S. Mountains is in a 2-m-thick, distal ash-flow tuff that conformably overlies Interstate 40. There we collected 4 sites for paleomagnetism from a 70-m conglomerate and sandstone that dip steeply to the east. In the New York vertical section to test the consistency of the magnetic direction (nos. 5-8; Mountains, the gently dipping tuff overlies Proterozoic gneiss (no. 34); in see Fig. 5 below for site details). We also collected sites no. 9 and no. 11 in the Providence Mountains (no. 35), it dips gently beneath a 15.5-m.y.-old basal vitrophyre from localities where the tuff thinned over topographic tuff that was erupted from the adjacent Woods Mountains volcanic center highs and cooled more quickly (Buesch and Valentine, 1986). Additional (McCurry, 1986). In the Marble, Clipper, and Ship Mountains to the sites were collected in flat-lying tuff on the mesa (no. 10) north of King- south (nos. 29, 31, 32), the gently dipping Peach Springs Tuff is near the man and at Flat Top (no. 12), about 40 km south of Kingman. top of the Miocene section. The tuff overlies silicic and intermediate volcanic rocks and more steeply tilted fanglomerates and sandstones (Miller Colorado River Extensional Corridor and others, 1982; Knoll and others, 1986). The thickest exposure of Peach Springs Tuff (220 m) crops out in the Piute Mountains 10 km east of As defined by Howard and John (1987), the Colorado River exten- Essex, California, where it overlies older tuffs and sediments filling in a sional corridor is a domain of west-dipping tilt blocks within the Basin and surface of high relief. We sampled three sites near Essex, two from the tilt Range province, in which major Miocene extension has occurred on a blocks in the thick valley-fill section in the Piute Mountains (nos. 27, 28) system of east-dipping, low-angle detachment faults. Allochthonous blocks and one from tilted, vitrophyric, valley-margin facies in the Old Woman of Tertiary strata dip steeply into the detachment faults, below which are Mountains (no. 30). exposed metamorphic core complexes of mylonitic gneisses (Fig. la). A central belt of core complexes parallels the Colorado River from the Central Mojave Desert Strike-Slip Zone Newberry Mountains of Nevada to the Whipple and Riverside Mountains of California (Coney, 1980; Davis and others, 1980). The western part of our paleomagnetic transect, from the Bristol The Peach Springs Tuff was erupted during a major episode of exten- Mountains to Barstow, crosses a structural zone characterized by steeply sion, and the tuff was deposited, along with lower Miocene volcanic and tilted blocks which overlie low-angle detachment faults of early Miocene sedimentary rocks, on an erosional surface cut into pre-Tertiary plutonic age (Dokka, 1986). A pattern of younger strike-slip faults, mostly north- and metamorphic rocks (Davis, 1986; Nielson, 1986). The Tertiary west trending and having right-lateral offsets, is superimposed on the older section along with underlying pre-Tertiary crystalline rocks were tilted detachment terrane (Dibblee, 1967a; Bassett and Kupfer, 1964). The stra- and transported to the east on low-angle detachment faults sometime tigraphy in this part of the Mojave Desert consists primarily of lower between 19 Ma and 15 Ma (Howard and John, 1987). The resulting tilt Miocene andesitic flows, volcanic breccia, and pyroclastic rocks which blocks generally strike northwest and are dipping to the southwest with filled basins on an eroded terrane of Mesozoic granitoids and Proterozoic dips up to 90°. metamorphic rocks (Hewitt, 1954; Dibblee, 1967a). In ranges such as the We collected 14 paleomagnetic sites in the tilted section of the Peach Newberry, Cady, and Bullion Mountains (Fig. la, see also Fig. 7a below Springs Tuff with dips of 12° to 87° (Fig. lb; see also Fig. 8a below for for detail concerning faults), the lower Miocene strata were tilted to steep detailed site localities). Seven sites (nos. 13,17,18,22,24-26) are within a angles as blocks broke away from a headwall and were translated along a few kilometers of the exposed Chemhuevi detachment fault (John, low-angle detachment fault (Dokka, 1983, 1986). The detachment fault- 1987a), and five additional sites are in allochthonous blocks surrounding ing predates deposition of the Peach Springs Tuff, which overlies early the Whipple Mountains core complex (nos. 14-16,21,23). The remaining Miocene tilt blocks with a pronounced angular unconformity in several sites are from more gently dipping blocks in the western part of the mountain ranges. The distal, poorly welded tuff is interbedded with fan- extensional corridor, closer to the breakaway headwall in the Old Woman glomerates and basalt flows; it is nearly horizontal, except in localities Mountains (nos. 19, 20). adjacent to the major northwest-trending strike-slip faults. These major faults, such as the Lenwood, Camp Rock, and Calico faults, have had Basin and Range recent activity and show right-lateral offsets of several kilometers (Dibblee, 1967a; Dokka, 1983). Our sampling sites between the Colorado River extensional corridor We collected six paleomagnetic sites from mountain ranges in the and the central Mojave strike-slip zone are referred for convenience to the central Mojave Desert strike-slip zone. In the Bristol Mountains, site 36 Basin and Range province, although the province generally encompasses a was collected in the U.S. Interstate 40 roadcut from moderately dipping larger region (Fig. la; see Fig. 7b below for site details). The sampling area Peach Springs Tuff, where it overlies bedded sandstone. Our second site in is characterized by internally faulted and tilted mountain ranges composed the Bristol Mountains (no. 39) is in gently dipping, mesa-capping tuff east mostly of Proterozoic metamorphic rocks and late Precambrian, Paleo- of Broadwell Lake; the flow was originally mapped as "Quaternary or zoic, and early Mesozoic strata which have been deformed by Mesozoic Tertiary rhyolitic tuff (QTr)" by Dibblee (1967b). In the eastern Bullion thrusting, metamorphism, and plutonism (Stewart and Carlson, 1978; Mountains, we sampled the tuff on top of Pacific Mesa within the Jennings, 1977; Miller and others, 1982). Miocene volcanic and sedimen- Twentynine Palms Marine Corps base (no. 38). Dibblee (1967c) mapped tary rocks were deposited on an irregular erosional surface and were the unit as "Tertiary or Quaternary rhyolitic tuff (QTr)" underlain by subsequently moderately tilted by normal faults. Northeast-trending, gently dipping sandstone and conglomerate. In the Sleeping Beauty area of

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the southern Cady Mountains (no. 40) and in Kane Wash of the Newberry Figure 3. Tilt-corrected site-mean directions of magnetization Mountains (no. 37), we sampled gently dipping or flat-lying Peach Springs (dots) and 95% confidence limits (circles) for the Peach Springs Tuff; Tuff where it overlies steeply tilted early Miocene volcanic strata with a equal-area projection of part of the lower hemisphere, northeast quad- major angular unconformity. Dibblee and Bassett (1966a) mapped the rant. Confidence circle about Colorado Plateau reference direction unit as "Miocene (?) rhyolitic tuff (Trt)" at Sleeping Beauty. At Kane shown by shaded area in parts a through e; shaded area in f is confi- Wash, Dibblee (1964a, 1964b) referred to the ash flow as "Miocene(?) dence circle around the steep reference direction from Kingman. tuff (Tst)." Dokka (1983,1986), Glazner (1986), and Bartley and Glazner Numbers beside site means are keyed to Table 1. Results grouped (1987) proposed that the steeply dipping, lower Miocene tilt blocks typical according to tectonic province (see Fig. la): (a) Colorado Plateau, of the central Mojave ranges (for example, the Newberry, Cady, and diamond denotes mean magnetic direction; GAD, geocentric axial di- Bullion Mountains) resulted from low-angle detachment faulting just prior pole direction; (b) transition zone; (c) Basin and Range; (d) central to eruption of the Peach Springs Tuff about 19 Ma. Finally, our site near Mojave Desert strike-slip zone; (e) Colorado River extensional corri- Daggett Ridge, south of Barstow, comes from one of the westernmost dor (sites 24 and 25 with large confidence limits are omitted); (f) outcrops of the Peach Springs Tuff where the tuff is folded along the steeply inclined component of remanent magnetization from thicker Lenwood fault (unit Tst of Dibblee, 1970). The least deformed exposures accumulations of the Peach Springs Tuff compared to steep reference are near Stoddard Wash (no. 41), about 1 km east of the main trace of the direction from Kingman (shaded); sites 5 and 7 from same area as site Lenwood fault, where we sampled the tuff in a series of low, en echelon 6 are omitted for clarity; (g) comparison of mean Plateau reference ridges separated from one another by minor splays of the main fault. direction (shaded) with mean geomagnetic field direction (star, GAD) and approximate envelopes containing 50% and 95% of the natural PALEOMAGNETIC METHODS secular variation, estimated for Miocene time (Mankinen and others, 1987). Using portable drilling equipment, we collected oriented specimens for paleomagnetic study from solid, fresh exposures of the Peach Springs Tuff. To minimize the effect of localized rotations due to jointing, we obtained about ten independently oriented cores at each locality from an area covering several hundred square meters. We used a solar compass to measure the azimuths of the core axes to an accuracy of ±1°. nT (nanoTeslas). In general, the intensity of magnetization decreased very To remove secondary components of magnetization, two pilot spec- little until the heating exceeded 450 °C; then the magnetization decreased imens from each locality were partially demagnetized in alternating fields sharply as the Curie temperature (580 °C) of magnetite was approached. (AF) in a device equipped with a self-reversing tumbler. In performing the In most of the specimens, about 30% of the total magnetization was AF treatments, we applied progressively higher peak fields in 12 steps up unblocked by heating in the range from 580 to 630 °C; beyond 630°, the to 100 mT (milliTeslas) and then analyzed orthogonal projections of the magnetization was reduced to the level of experimental noise. This distri- resultant vectors to determine components of magnetization (Fig. 2). Some bution of unblocking temperatures, which is typical of Great Basin ash- of the sample localities, particularly those in roadcuts, showed little evi- flow tuffs (Gose, 1970; Rosenbaum, 1986), results from high-temperature dence of spurious secondary magnetizations. In these cases, we chose four oxidation of the iron-titanium oxide minerals during emplacement of the steps from within the range of AF settings that best stabilized the magneti- ash flows. The oxidation process exsolves titanium from the spinel lattice zation direction of the pilot specimens, and we applied the same four field of titanomagnetite to form lamellae of ilmenite, leaving magnetite with a settings to the remaining untreated specimens. The optimum peak AF Curie temperature near 580 °C. The process may also produce titanohem- intensities generally ranged from 30 to 70 mT. Using principal component atite with Curie temperatures as high as 630 °C. analysis of the demagnetization data (Kirschvink, 1980), we determined The presence of magnetite-hematite mixtures in the Peach Springs the stable component of magnetization of the majority of specimens. Re- Tuff is also indicated by the remanence acquired when the specimens are sults from the individual specimens were then averaged together to obtain subjected to strong direct magnetic fields. We exposed several representa- the mean direction of magnetization for each locality (Table 1), and 95% tive specimens to direct fields up to 0.7 T (Tesla) and observed the acquisi- confidence limits about the mean directions were calculated (Fisher, tion of isothermal remanent magnetization (IRM). The IRM-curves show 1953). a steep rise of intensity up to 0.15 T, which is a characteristic of magnetite; At some of the natural exposures, the remanent magnetization is they show a more gentle rise beyond 0.15 T, which is consistent with the strongly overprinted by lightning as indicated by unusually strong mag- presence of hematite or titanohematite. netic intensities and widely scattered magnetic directions. Examination of Considerable time was spent at each sampling locality to determine the demagnetization paths from the pilot specimens showed that the sec- the attitude of the bedding of the Peach Springs Tuff. The strike and dip of ondary magnetization was not being removed completely, and so we used the ash flow were averaged from several horizontal indicators, such as the the method of converging remagnetization circles to obtain the underlying basal layering of the flow, underlying sediments, and fiamme from col- primary magnetization of the locality (Halls, 1978). Each specimen was lapsed pumice fragments within the ash flow. We corrected the paleomag- treated in 7-10 AF settings up to 100 mT, and a great circle was fitted to netic results for tilt of the bedding by rotating the mean magnetization the resultant demagnetization vectors using the least-squares method of vector about the strike by the amount of the dip (Table 1). Kirschvink (1980). The mean magnetic direction for the locality was After the remanent magnetic vectors have been corrected for tilt, obtained by solving for the best intersection of the remagnetization great rotations about the vertical axis can be inferred from the magnetic declina- circles (Onstott, 1980), and the 95% confidence ellipse was calculated tion. The simple tilt correction, however, can introduce errors in declina- according to the statistical model of Bingham (1974). tion, resulting in the calculation of "apparent" rotations (MacDonald, To examine the thermal stability of the magnetization, we selected 24 1980). These errors arise when the strata have been tilted about axes other representative specimens from the collection for thermal demagnetization than the horizontal line of strike. For example, a scissor fault may rotate in a low-field furnace. The heating steps were taken at 50 °C intervals from beds about an inclined axis. A plunging fold or beds crosscut by normal 100° up to 500° and were then decreased to 20° intervals from 500° to faults are common geologic structures that result from tilting about two or 680°. The specimens were heated in air in an ambient field of less than 5 more horizontal axes, possibly leading to the calculation of erroneous

apparent rotations. During the paleomagnetic sampling at each site, we dips are greater, as in the Colorado River extensional corridor, there is examined the local structure for faults and folds to assess the validity of the greater potential for errors in declination to arise. Nevertheless, the local simple tilt correction. At two-thirds of the sites, the dip angles are too small structures are generally more easily explained by simple tilt rather than by (<30°) to produce substantial apparent rotations (Scott, 1984). Where the polyphase deformation.

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PALEOMAGNETIC RESULTS FROM THE 3a), transition zone (Fig. 3b), Basin and Range (Fig. 3c), and the central PEACH SPRINGS TUFF Mojave Desert strike-slip zone (Fig. 3d). Group 2 directions are from the Colorado River extensional corridor and show inclinations of -40°; decli- In Table 1, we summarize the paleomagnetic directions and nations span a broad range in the northwest and northeast quadrants associated 95% confidence limits obtained from 41 sites within the Peach (Fig. 3e). Group 3 directions are clustered in the northeast quadrant with Springs Tuff. As would be expected from a welded tuff (Gromme and steeper inclinations near 55°, mainly from sites in the Colorado River others, 1972), the remanent magnetization is very stable and yields well- extensional corridor (Fig. 3f). defined mean paleomagnetic vectors. After AF cleaning, results from spec- We interpret the mean direction (inclination = 36.4°, declination = imens were rejected if the magnetic directions diverged noticeably from 33.0°, a95 - 3.4°) from four sites on the tectonically stable Colorado the main cluster; the rate of rejection of results was -10% of the total Plateau as a recording of the geomagnetic field direction at the time of number of specimens. More than three-quarters of the sites gave confi- eruption of the Peach Springs Tuff. These four sites are in the distal margin dence radii about the mean of less than 6°. Although minor slippage of of the tuff, which presumably cooled rapidly below the range of magnetic blocks along fractures may account for some of the scatter of directions blocking temperatures. Within Group 1 mentioned above, there are 14 within a given site, lightning is most likely the major source of remanent site-mean directions having confidence circles that overlap the confidence magnetic noise. The larger confidence limits (10°-15°) and most of the circle of the Plateau direction (Figs. 3a-3d). These sites are distributed rejected results are from natural outcrops that have a longer exposure to throughout the sampled region, some occurring in each of the structural lightning strikes. When the severity of lightning contamination required provinces. This observation strongly supports the hypothesis that the the analysis of demagnetization circles, a circular 95% confidence limit was welded tuff is the same volcanic unit from Barstow to the Colorado calculated by averaging the minor and major radii of the Bingham confi- Plateau and that the tuff acquired a uniform direction of magnetization dence ellipse; these sites are flagged by an asterisk in Table 1. shortly after emplacement. The somewhat unusual magnetic direction Figure 3 is a series of equal-area diagrams showing the tilt-corrected strengthens this argument, because the likelihood of several tuffs acquiring magnetic directions obtained from the Peach Springs Tuff in each of the the same direction at different times is very small. The reference direction structural provinces. Three groups of magnetic directions are apparent. from the Peach Springs Tuff is in the outer fringe of the natural variation of Group 1, having directions clustered in the northeast quadrant with an the geomagnetic field in contrast to the field's more common direction inclination of -40°, is typical of results from the Colorado Plateau (Fig. near the axial dipole direction (Fig. 3g), which is appropriate for Miocene and younger time (Mankinen and others, 1987). Compilations of global paleomagnetic data from Miocene and younger lavas show that such large departures from the axial dipole direction occur in less than 5% of the lavas (McElhinny and Merrill, 1975). Therefore, the geomagnetic field would

NORTH maintain this unusual direction for only a short time before moving back toward the axial dipole direction.

Within Group 1, there are relatively small departures in declination from the reference direction. The departures in the Basin and Range and in

NOT CORRECTED the strike-slip zone range up to 13° and are equally distributed on both FOR TILT sides of the reference direction. In the Colorado River extensional corridor, where we identified the directions of Group 2, the inclinations are similar to the inclination measured on the Colorado Plateau. The deviations in G declination, however, are very large, ranging up to 51° (Fig. 3e). Again, the declinations are equally divided between clockwise and counterclockwise deviations from the reference declination. We intepret the declination

L anomalies as resulting from tectonic activity in which fault-bounded -WEST - EAST- blocks have been rotated about vertical axes. The sense of rotation and the pivot angles are highly variable within each structural province. Using the fold test of Graham (1949), we can demonstrate that the Peach Springs Tuff was magnetized before it was tilted, providing a con- straint on the age of magnetization. The classic fold test requires sites on CORRECTED opposite limbs of a fold; the test involves a comparison of the angular FOR TILT dispersion of magnetic directions before and after correcting the results for tilt of the strata. As in our study, the test can be applied to sites in a number IT of tilted, fault-bounded blocks. A substantial decrease in angular dispersion following tilt corrections, as depicted in Figure 4, indicates that the magnetization of the Peach Springs Tuff was acquired before the tuff-bearing

70° 50° blocks were tilted. For clarity we have limited the data in Figure 4 to results from the Basin and Range zone (nos. 29-35) and the central INCLINATION Mojave Desert strike-slip zone (nos. 36-41). Blocks within the Colorado Figure 4. Lower hemisphere, equal-area projection of site-mean River extensional corridor have undergone large tectonic rotations in addi- directions of magnetization (dots) and associated 95% confidence lim- tion to being tilted, and so the bedding corrections reduce the dispersion in its (circles) from the Peach Springs Tuff before (top diagram) and inclination while large variations in declination remain. after (bottom diagram) correcting for tilt of the bedding. Sites are in To compensate for dispersion in declination added by vertical-axis variably tilted strata from the Basin and Range and the central Mojave rotations, we performed the fold test using only magnetic inclinations from Desert strike-slip zone. tilted areas (24 sites) in Groups 1 and 2. We calculated the angular

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70° 50° 30°

35°12'30"

Tps \Tst Qs a Tsb Tps EXPLANATION Tps Qs Surticial deposits (Quaternary) Tps N Qs Tuffaceous sandstone and tut) Tst (Miocene) »Tst/ Tps Tst> Tps Peach Springs Tuff (lower Miocene) t-Tst "jgk Sandstone and tuff, basalt flows, and Tps cinder cones (lower Miocene) Quartz monzonite and gneiss qm Tps (Proterozoic) 35°10' Contact

Tps High-angle fault—Dashed where approximate

a^ Paleomagnetic sample site—Numbers refer to paleomagnetic data and Table 1

1 14°05' 114°02'30"

Figure 5. Geologic map of the vicinity of Kingman, Arizona, from Buesch and Valentine Paleomagnetic data: specimen (dot), site-mean (square), 95% confidence limit of mean (1986). Paleomagnetic sample sites and tilt-corrected magnetic directions from the Peach (circle); equal-area projection of part of the lower, northeast quadrant. Springs Tuff are shown along with the stratigraphie section along U.S. Interstate 40.

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dispersion from inclinations using the method of McFadden and Reid (1982). The reduction of dispersion after making the tilt corrections is significant at the 99% confidence level (McElhinny, 1964). We conclude that the magnetization predates tilting, which occurred before 12 Ma in the McCullough Mountains (Weber and Smith, 1987), but the tilting may be younger near strike-slip faults in the central Mojave Desert. The magnetic directions of Group 3 (Fig. 3f ), characterized by inclinations of approximately 55°, occur locally within the extensional corridor, the Basin and Range, and the Transition Zone. The steeper direction is typically found in the thicker accumulations of the Peach Springs Tuff, where the original topography allowed ponding of the ash flow. Two exceptions are sites nos. 14 and 16, where the tuff is 30 m thick and yet records the steeper direction. At Kingman (nos. 5-7), the Piute Mountains (nos. 27,28), and along the Colorado River (nos. 22,23,26), the thickness of the tuff exceeds 60 m. The tuff rarely exceeds 50 m in thickness elsewhere in the study area. Thicker accumulations of the tuff would cool more slowly, perhaps slowly enough to allow appreciable secular variation of the field to occur during emplacement and cooling of the tuff. This process could explain the contrast of magnetic directions between the widespread, generally thin accumulations and the locally thicker sections. Evidence from Kingman, where U.S. Interstate 40 follows a deep cut in the Peach Springs Tuff, indicates that later lock-in of the magnetization is a reasonable explanation for the steeper magnetic component (Fig. 5). Complete exposure in the roadcut allowed us to collect paleomagnetic samples from a vertical section spanning about 70 m of the tuff. The lower 18 m of the ignimbrite is moderately indurated, but the matrix and pumice lumps are unwelded. The next 20 m are composed of densely welded, devitrified matrix containing flattened pumice clasts and volcanic rock UNBLOCKING TEMPERATURE DISTRIBUTION fragments. This part of the tuff forms the lower prominent cliffs in the 2 mesas around Kingman. The lower cliff-forming unit is capped by a 10-m- b) DC -0.8 High- J thick zone of less resistant tuff, which is in turn overlain by another Z Low-temperature temperature r II o cliff-forming unit. The layering reflects differing degrees of welding and o N vapor-phase alteration within a single eruptive unit (Buesch and Valentine, w CK •0.4 1986). In our paleomagnetic sampling of these horizons within the Peach CO o • _l 1 Springs Tuff, we discovered a significant variation of the magnetization . i . h direction near the base (Fig. 5). The unwelded base (no. 8) yielded a magnetic direction which is indistinguishable from the Plateau reference TEMPERATURE C°C) direction, given the size of the confidence circles. At progressively higher levels (nos. 7, 6, 5), however, the magnetic inclination steepens to about Figure 6. Dual directions of remanent magnetization observed in 55°, and the declination swings to a more northerly azimuth, giving direc- the unwelded, ash-flow tuff at Kingman, Arizona, site no. 8 (Table tions significantly different (at the 95% confidence level) from the reference 1). (a) High-temperature component from specimen 6J190 is com- direction. The direction from the higher levels agrees well, particularly in pared to Colorado Plateau reference direction, and low-temperature inclination, with the sites from thick sections along the Colorado River and component is compared to magnetic direction from middle welded in the Piute Mountains. zone, site no. 6. Circles show 95% confidence limits; equal-area projec- Results from thermal demagnetization reveal two components of tion of part of lower, northeast quadrant, (b) Histogram showing magnetization in the unwelded base of the ignimbrite, supporting the unblocking temperature distributions of low- and high-temperature hypothesis of late-stage cooling (Fig. 6a). Heating specimens from the components from thermal demagnetization of specimen 6J190. NRM, basal layer to 400 °C removed a secondary component similar to the natural remanent magnetization. high-temperature component observed in specimens from the welded flow interior. In the temperature range from 400° to 630°, the remaining magnetization of the tuff closely matched the Plateau reference direction. In- ences in remanent directions. Finally, as the flow interior acquired its terpretation of the unblocking temperature distribution (see the example in remanent magnetization, downward diffusion of heat would raise the Fig. 6b) suggests the following magnetization sequence. In the unwelded temperature of the basal layer to about 400 °C, superimposing the steeper basal layer, the high-temperature component was locked in immediately secondary component as a partial thermoremanent magnetization on the by cooling near the substrate. Unusual thickening of the tuff due to pond- original high-temperature magnetization. ing created a large reservoir of heat, requiring a long period of dissipation Our other sample localities in the transition zone yield magnetic before the flow interior could cool below the magnetic blocking tempera- directions in accord with the Plateau reference direction (Fig. 3b) with the ture. Based on Holocene rates of secular variation and assuming that a exception of site no. 10, which is intermediate between the Plateau refer- volcanic flow of such thickness would cool in about 100 yr (Riehle, 1973), ence and the steeper direction. Because the tuff does not exceed a thickness the ambient field could change sufficiently to explain the observed differ- of 20 m at these localities, generally rapid cooling of the tuff prevented the

acquisition of the steeper magnetic component that was found in the 1-40 exerted by the Lenwood fault. In the Bullion Mountains, Pacific Mesa (no. roadcut. For example, the relatively thin accumulations of Peach Springs 38) is bounded by a steep fault that dies out 5 km north of the mesa, and Tuff at Flat Top (no. 12) and the mesa (no. 9) northeast of Kingman sedimentary rocks at the base of the mesa are cut by a low-angle fault yielded mean directions and confidence limits that overlap the reference (Bassett and Kupfer, 1964), providing a localized rotational mechanism. direction from the Plateau. Local faulting near the sampling site at Sleeping Beauty (no. 40) may be In the following sections, we examine in more detail the tectonic responsible for the rotation observed in the Cady Mountains (Glazner, rotations inferred from paleomagnetism of the Peach Springs Tuff. The 1986). declination discordances and their 95% confidence limits are listed in Table Our results do not support tectonic models, such as the one proposed 1. At the majority of sites, the declination discordance is referred to the by Garfunkel (1974), calling for post-middle Miocene strike-slip faulting Colorado Plateau reference direction; rotations inferred from our paleo- to produce rotations of large-scale crustal blocks in the Mojave Desert. In magnetic data are therefore relative to the Colorado Plateau. The rotations his model, the region is divided into a number of narrow blocks, about 20 can be referred to the North American craton as well, because Oligocene km wide and bounded by active strike-slip faults, such as the Camp Rock, and younger rotation of the Colorado Plateau is less than 2°, as interpreted Lenwood, and Ludlow faults. Garfunkel (1974) estimated that the faults from gravity data in the Rio Grande rift (Hamilton, 1978; Cordell, 1982). and intervening blocks have rotated counterclockwise up to 30° in re- Furthermore, limited paleomagnetic data from Miocene volcanic rocks sponse to north-south shortening of the wedge-shaped region between the above and below the Peach Springs Tuff on the Colorado Plateau show Garlock and San Andreas faults. According to the model, the Garlock less than 5° clockwise rotation, although the confidence limit is large fault acts as a fixed buttress at the northern boundary of the fault blocks; (Young and Brennan, 1974). Where the later-stage steep component of the counterclockwise rotation is necessary to accommodate the slip across magnetization was observed, we have calculated the declination discor- faults within the strike-slip zone, where Garfunkel estimated the cumula- dance using as a reference the magnetic direction from the middle welded tive offset to be as great as 125 km. In contrast, Dokka (1983) argued that zone at Kingman (no. 6). Rotations inferred from the later-stage magneti- no more than 38 km of cumulative offset has occurred in the central zation are also relative to the Colorado Plateau, because site no. 8 in the Mojave Desert, about 50% to 30% of Garfunkel's estimate for total slip base of the tuff at Kingman shows no significant rotation relative to the across the same region. This lower estimate is still sufficient to allow a Plateau. substantial regional rotation, measurable by paleomagnetic methods. The confidence limits reflect the internal consistency of magnetic The Peach Springs Tuff, which is cut by the northwest-trending fault directions at each site and at the reference locality; the limits do not factor systems, shows no consistent rotation of the region as a whole, according in uncertainties in the tilt corrections or the lock-in time of the magnetiza- to our paleomagnetic results. Hence, we conclude that Garfunkel's kine- tion. Although most of the inclination discordances are not significant at matic model is incorrect for post-19 Ma faulting in the Mojave Desert the 95% confidence level, we believe that some are due to errors in the tilt block. This is consistent with Dokka's (1987) observation that the major corrections and the magnetization lock-in time. Regarding tilt corrections, strike-slip faults are discontinuous, which would also invalidate the re- the lack of well-bedded underlying sediments at some localities introduces gional fault-rotation model. In the absence of regional rotation, the cumu- uncertainties of approximately ±5° in the dip angle. Three-quarters of the lative offset of the Mojave strike-slip faults must be accommodated in inclination discordances are negative, and the average of the site inclina- another way, perhaps by bending of the Garlock fault or by north-south tions (38°) is 2° steeper than the Plateau reference inclination. This might shortening within the fault blocks. reflect at some sites a slight smearing of magnetic directions between the Luyendyk and others (1985) proposed a fault-rotation model that Plateau reference and the later-stage magnetization documented at King- predicted large clockwise rotations in Neogene volcanic rocks of the Al- man. If so, an additional uncertainty of 5° (half the difference in declina- vord Mountains region of the northeastern Mojave Desert (Fig. lb). They tion between the two reference directions) should be factored into the modeled the region as a series of elongate fault blocks bounded to the west declination discordances. and south by the northwest-trending blocks of the central Mojave Desert. According to the model, the Alvord Mountain block and one of its bound- DISCUSSION: TECTONIC ROTATIONS aries, the Cady fault, were rotated clockwise into their present east-west alignments in response to strike-slip movement in the surrounding region. In the strike-slip zone of the central Mojave Desert, the rotations Ross and others (1987), reporting results of a paleomagnetic study of inferred from the declination discordance (Fig. 7a) indicate that the region Neogene volcanic rocks in the southern Cady Mountains and Alvord has not been rotated uniformly since emplacement of the Peach Springs Mountains, found evidence of clockwise rotation (52° to 60°) in the Tuff. Rotations are negligible in the Newberry Mountains in the heart of east-west structural domain. Because our site (no. 40) in the Peach Springs the strike-slip zone and are not significant in the Bristol Mountains at the Tuff is in the same area sampled by Ross and others (1987), we conclude eastern edge of the zone. Azimuthal rotations of 10°-13° were detected in that clockwise rotation south of the Cady fault after 19 Ma is limited to the Cady Mountains, Bullion Mountains, and near Barstow, although the 12° ± 4°. Young strike-slip faulting possibly accounts for a fifth of the sense of rotation is not consistent among the sites. amount of rotation observed by Ross and his coworkers in the southern In contrast to the 10.1° counterclockwise rotation measured in the Cady Mountains, but another mechanism is needed to account for the Bullion Mountains, clockwise rotations of 11.6° and 13.1° were measured remainder. Ross and others (1987) have suggested that detachment fault- in the Cady Mountains and near Barstow, respectively. Published geologic ing prior to deposition of the Peach Springs Tuff is the appropriate mecha- mapping and our own field observations show that all 3 sites are in nism, because many of the sampled volcanic flows are in steeply tilted northwest-trending fault zones, with fractures cutting the tuff less than 100 blocks above proposed detachment faults of widespread extent (Dokka, m from the sampled localities. In each case, the rotations can be explained 1986; Glazner, 1986; Glazner and others, 1988). The Peach Springs Tuff, by drag associated with the nearby fault zones. In Stoddard Wash (no. 41), which is tilted only 15°, was deposited after regional extension and major the Peach Springs Tuff is exposed repeatedly in a stepped series of ridges block-tilting had occurred. separated by minor faults related to the right-lateral Lenwood fault. In five of nine sites in the Basin and Range, no detectable rotation Clockwise rotation of these small ridges is consistent with the shear couple with respect to the Colorado Plateau has occurred (Fig. 7b). In the remain-

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0 10 20 KILOMETERS 1 1 I

EXPLANATION

QT Surficial sedimentary rocks (Pliocene and Quaternary) -i—— High-angle fault—Dashed where approximately located; dotted where concealed. Arrows indicate direction of relative Qv Volcanic flows (Quaternary) movement. Ball and bar on downthrown side

i M i i i Low-angle fault—Hachures on upper plate [Tps J Peach Springs Tuff (lower Miocene) t=i <=* <=] Tectonic province boundary

Continental volcanic and sedimentary rocks (Miocene) '/=: Quaternary volcanic vent

pT Plutonic, metamorphic, and sedimentary rocks (pre-Tertiary)

ing sites, we observed small but significant counterclockwise rotations created by failure to account for polyphase tilting about different axes. (nos. 27, 28, 33, 34). The largest rotation of 19° is in the Piute Mountains (1) In most tilt domains in the extensional corridor, the strike of bedding in a small tilt block with an anomalous trend and a complicated structural parallels the fault boundaries of the tilted blocks and implies, to first order, history (G. Heilman, 1987, personal commun.). Small counterclockwise that simple tilting along the boundary faults is the primary deformation rotations in the New York Mountains and the McCullough Mountains to mechanism (Howard and John, 1987; their Fig. 3). This relationship holds the north (nos. 33, 34) may be a response to slip along en echelon left- for most, but not all, of our sampling localities, as determined from de- lateral faults southwest of the Lake Mead shear zone (Anderson, 1973; tailed 1:24,000 scale geologic mapping (John, 1987a; Nielson, 1986; Niel- Weber and Smith, 1987). Rotation in the resulting sinistral shear couple son and Turner, 1986; Carr and others, 1980; Dickey and others, 1980) would be accommodated by the closely spaced north-northwest-trending and our own field studies. (2) The mapping also shows that plunging folds faults that cut across the ranges. Ron and others (1986) also found coun- are rare in the Tertiary sequences and are not likely to be an unrecognized terclockwise rotations of 29° in late Miocene volcanic rocks in the vicinity source of tilting. We did sample a moderately plunging syncline at the of Lake Mead and proposed a similar mechanism to accommodate the Needles on the Colorado River (nos. 25, 26), but no significant interlimb rotations. dispersion of magnetic directions was observed upon unfolding about the The largest declination discordances with respect to the Colorado local strike. Whenever possible, we selected site localities along continuous Plateau are found in the tilt blocks of the Colorado River extensional strike ridges. (3) No significant correlation between the angle of dip of the corridor. After correcting for tilt of the beds, the inferred rotations range tuff and the declination anomaly is observed (Fig. 9), which indicates that from 51° counterclockwise to 37° clockwise (Table 1 and Fig. 8). There large dip corrections are not introducing apparent rotations (MacDonald, are three reasons to believe that the declination discordances reflect true 1980; Scott, 1984). A plot of discordant declinations against the plunge of tectonic rotations about the vertical axis, rather than apparent rotations poles to bedding (Chan, 1988) for all sites in the extensional corridor

Figure 7. Geologic maps and paleomagnetic results from the Peach Springs Tuff. See Figure lb for locations. Shaded pie slices with tick marks show rotation and 95% confidence limits on rotation relative to Colorado Plateau; site numbers are in italics. Number in circle indicates amount of tectonic rotation in degrees; positive numbers indicate clockwise rotations, negative numbers denote counterclockwise rotations, (a) Central Mojave Desert strike-slip zone, California; geology modified from Bortugno and Spittler (1986), Bassett and Kupfer (1964), Dibblee (1964a, 1964b, 1966,1967b, 1967c, 1970), Dibblee and Bassett (1966a, 1966b).

Figure 7. (Continued). (b) Basin and Range region, California and Nevada, geology modified from Stewart and Carlson (1978), Jennings (1977), Miller and others (1986), D. M. Miller (1986, written commun.), and E. I. Smith (1986, oral commun).

10 20 KILOMETERS _j I

generally shows no linear trends that would indicate rotation about an There are, however, some possible correlations between the amounts inclined axis, with the possible exception of four sites (nos. 15,16,19,20). of rotation and local structure. The senses of rotation seem to follow the Although the declination discordances of the four sites could be explained strike of the tilt blocks, with clockwise rotations occurring in the northeast- by a common rotation axis plunging 45°, we see no evidence of a steeply striking blocks and counterclockwise rotations occurring in most of the plunging structure in the broad region defined by the four sites. northwest-striking blocks (Fig. 10). This implies that differential rotation

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114° 00'

34° 30'

Figure 8. Tectonic map, cross section, and paleomagnetic results from the Peach Springs Tuff, Colorado River extensional corridor, California and Arizona (modified from Howard and John, 1987). (a) Map units designated by Roman numerals are tilt-block domains; higher numerals indicate progressively greater tilt and northeast transport of the allochthons above the basal detachment. Blocks consist predominantly of pre-Tertiary crystalline rocks with lesser Tertiary volcanic and sedimentary rocks. BR, Breakaway fault; CH, Chemehuevi detachment fault; WM, Whipple Mountains detachment fault. Paleomagnetic site numbers are in italics (Table 1); rotations depicted as in Figure 7. (b) Maximum observed tectonic rotations (in degrees) for tilted domains are listed above schematic cross section of the area shown in part a.

followed tilting on block-bounding faults, which are the primary control occur in tilt domains III and V. These domains consist of broken-up, small on the strike of beds. The amount of rotation may also correlate to some tilt blocks which Howard and John (1987) interpret to have been trans- extent with tilt domains defined by Howard and John (1987) in the ported significant distances eastward above the Chemehuevi-Whipple extensional corridor (Fig. 8a). The larger rotations, up to 51° at site no. 21, Mountains detachment fault (Fig. 8b). Smaller rotations are measured in

60 We obtained unexpected paleomagnetic results from detailed sampling adjacent to the exposed core complex of the Chemehuevi Mountains. 50 - We collected 5 sites from variably oriented tilt blocks of tuff within 1 km of the Chemehuevi detachment fault (nos. 18, 22, 24-26), and none of Transition Extensional them shows any significant rotation (although the uncertainties are large corridor " zone for nos. 24 and 25 due to incomplete removal of lightning-induced rema- and Basin and nence). In contrast, sites in topographic lows between the elevated meta- • Range morphic core complexes show the largest rotations (nos. 15,17,19,21). A seismic reflection profile from the Old Woman Mountains to the Cheme- huevi Mountains (Frost and Okaya, 1986) shows gently dipping reflec- tions that project toward the surface exposures of the detachment in the Chemehuevi Mountains and the headwall exposed in the Old Woman Mountains. Tilt blocks with large rotations, although topographically low, presently lie at high structural levels above the detachment surface. We suggest that tectonic rotations of both senses could occur more easily at 30 40 50 60 high structural levels in tilt blocks separated from the main detachment by DIP OF BEDS, IN DEGREES numerous low-angle faults. The lack of rotation in blocks resting directly Figure 9. Comparison of amount of tectonic rotation versus dip on the detachment may result from the geometry of the detachment sur- of the Peach Springs Tuff for tilt blocks in the Colorado River exten- face. John (1987b) described the surface as having mullions of all scales, sional corridor (triangles) and surrounding regions. Greatest variation up to 100 m in amplitude, aligned in the direction of transport. The blocks occurs in the extensional corridor, but no correlation of amount of may have glided, as if on rails, along the deeply grooved surface (Fig. 11). rotation to dip angle is evident. Site numbers refer to Table 1 and In the Turtle Mountains (no. 19), strike-slip faulting after extension Figure lb. Sites 24 and 25 omitted due to large confidence limits. may have contributed to the 28° clockwise rotation of the site. The northeast-striking volcanic rocks form a ridge that has been cut by two sets of faults. Part of an older northwest-trending ridge-bounding fault is askew of its regional strike by 20° in the clockwise sense and lies between a tilt domain II, which consists of structurally thicker tilt blocks closer to the younger pair of east-west faults (Nielson and Turner, 1986, p. 32). Al- breakaway fault that forms the headwall to the west. Small rotation of 9° though mapped separation on the east-west faults is mostly dip slip, some also occurs in domain IV, the steeply tilted Mohave Mountains block, sinistral slip is possible and could have accommodated small-scale clock- which has been transported several tens of kilometers to the northeast. The wise Riedel shear rotation of the fault-bounded block (see, for example, block has remained an intact piece of mostly Proterozoic crust, about Ron and others, 1984). The rotation of site (no. 15), which is part of a 10 km thick, with an overlying veneer of Tertiary volcanic rocks (Howard landslide (G. A. Davis, October, 1988, oral commun.), may merely reflect and John, 1987). structure of limited areal extent. The rotations may result from drag along transfer zones between In summary, the rotations of upper-plate tilt-blocks in the extensional upper-plate domains that have undergone different rates of northeastward corridor are both clockwise and counterclockwise and may be correlated transport. For example, the arcuate rotation pattern around the eastern end with the strike of tilt blocks. Most of the blocks (1 to 10 km in length) of the Whipple Mountains may reflect more rapid movement of blocks off the rising Whipple dome as compared to regions on the northern and southern flanks. A similar pattern is not observed around the Chemhuevi structural dome, however. ROTATION Clockwise 40°-- 15* Figure 10. Tectonic rotation versus >19 strike of the Peach Springs Tuff for tilt blocks in the Colorado River extensional corridor. 20 -- I" Vertical bars represent 95% confidence limits \20 STRIKE on declination anomaly given in Table 1. If L14 ^ 24 we discount the results of sites 24 and 25, I,}"! if H 1- W 90°' 60 23* which were strongly overprinted by light- '26 f 30 221 30 60 90° E *13 025 ning, an approximate correlation may hold between strike of beds and the amount of 20 -- rotation. This suggests to us that vertical axis >17 rotation followed tilting of the beds on normal faults. Site numbers refer to Table 1 and 40 -- Figure lb. 21 Counterclockwise 60°-L

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Figure 11. Block diagram of envi- ronment of tectonic rotation in Colorado River extensional corridor. Large arrow shows direction of transport of upper- plate blocks. Blocks rotate clockwise or counterclockwise above low-angle normal faults, depending upon local structure. For example, drag along transfer fault zones that accommodate differential block transport may produce rotation.

Detach NE with mullions

appear to have been rotated about vertical axes during gliding on local, 2. Miocene and younger strike-slip faulting in the central Mojave low-angle normal faults at structurally high positions above the Desert between Barstow and the Bristol Mountains has produced localized Chemeheuvi-Whipple Mountains detachment. Rotation may have been rotations up to 13°. The rotations vary from clockwise to counterclock- generally driven by differential rates of tectonic transport in the extensional wise, and some areas are not rotated significantly. Localized drag along allochthons. Some rotations may have been accommodated by small-scale strike-slip fault zones is the most likely cause of post-19 Ma rotations in strike-slip faulting or by recent landsliding in a few localities. this region, whereas earlier clockwise rotations are primarily the result of early Miocene detachment faulting, as suggested by Ross and others CONCLUSIONS (1987). Paleomagnetic data from the Peach Springs Tuff contradict kinematic models (for example, Garfunkel, 1974) that call for consistent We have determined directions of remanent magnetization from a rotation of major lateral fault sets, such as the northwest-trending system in broad distribution of sites in the lower Miocene Peach Springs Tuff, an the central Mojave Desert. extensive welded ash flow, that is found throughout the central and eastern 3. In the Basin and Range, most sites have undergone no significant Mojave Desert (Young and Brennan, 1974; Glazner and others, 1986). azimuthal rotation, although small counterclockwise rotation in the New After treatment in alternating fields to remove the magnetic effects of York Mountains and McCullough Mountains may be related to move- lightning, the tuff yielded well-defined, stable directions of magnetization. ment along en echelon left-lateral faults southwest of the Lake Mead shear Correction for tilt of the bedding substantially reduces the angular disper- zone. sion of site-mean directions, indicating that the magnetization was ac- 4. In the Colorado River extensional corridor, tectonic rotations are quired before the tuff underwent tilting of Miocene and younger age. This large, ranging from 51° counterclockwise to 37° clockwise. The larger positive fold test and the high unblocking temperatures (560-630 °C), as rotations affect tilt-blocks that have been transported eastward above the determined by thermal demagnetization of the natural remanence, affirm Chemehuevi-Whipple Mountains detachment fault. Surprisingly, blocks of the high reliability of the tuff as a recorder of the geomagnetic field at the the upper plate within 1 km of the exposure of the detachment surface time of eruption. The thinner accumulations of the tuff, such as exposures show little rotation, perhaps due to anti-rotational resistance provided by on the Colorado Plateau, preserve an unusual direction (inclination = 36°, large mullions and grooves in the detachment surface. declination = 33°), useful for long-range correlation of isolated outcrops. A steeper component of magnetization was detected in the Peach ACKNOWLEDGMENTS Springs Tuff generally in areas where the ash flow exceeds 60 m in thickness. Closely spaced sampling of a thick section near Kingman, Arizo- We thank K. A. Howard (U.S. Geol. Survey) for suggesting this na, shows that the steeper component is a later-stage magnetization, prob- project and for his enthusiastic support. We also thank J. E. Nielson, ably reflecting changes in the geomagnetic field during prolonged cooling D. M. Miller, R. W. Simpson (all from the U.S. Geol. Survey) and A. of the ponded tuff. Glazner (Univ. North Carolina) and G. Calderone (Univ. Arizona/U.S. The Peach Springs Tuff has proved to be a useful stratigraphic Geol. Survey) for unpublished data and guidance during this study. We marker, and its magnetization provides a frame of reference for assessing benefitted from field guidance given by D. C. Buesch (Univ. California, tectonic rotations in the Mojave Desert. Comparison of tilt-corrected Santa Barbara), E. I. Smith (Univ. Nevada, Las Vegas), and R. A. Young magnetization vectors from sites in the Mojave Desert with results from (SUNY, Geneseo). Special thanks are owed to V. L. Pease (U.S. Geol. the stable Colorado Plateau leads to the following conclusions. Survey) for assistance in the field and laboratory. David Parks (U.S. Geol. 1. The Peach Springs Tuff is a single eruptive unit that extends Survey) also assisted with the laboratory work. We appreciate thoughtful westward from the Colorado Plateau, Arizona, to Barstow, California; the reviews given by Robert Butler (Univ. Arizona), Keith Howard and Jon paleomagnetism confirms the geologic correlations of the tuff by Glazner Hagstrum (both at the U.S. Geol. Survey), Bruce MacFadden (Univ. and others (1986). Florida), and D. C. Buesch.

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MANUSCRIPT RFCEIVEDBYTHF SOCIFTY MAY 6, 1988 Mesozoic paleogeography of the western United States: Los Angeles. Society of Economic Paleontologists and RFVISF.D MANUSCRIPT RECFIVFD DFCFMBFR 31. 1988 Mineralogists. Pacific Section, p. 33 70. MANUSCRIPT ACCEPTED DFCFMBFR 31, 1988 Printed in U.S.A.

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