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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 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 rela- tion, 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 iso- lated 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/6/846/3380644/i0016-7606-101-6-846.pdf by guest on 27 September 2021 PALEOMAGNETISM AND TECTONIC ROTATION, ARIZONA TO CALIFORNIA 847 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.
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