A large terrane extending along the Pacific margin of North America, from Vancouver Island, British Columbia, to south-central Alaska, is characterized throughout by similar sequences of Triassic rocks. These rocks, including a thick pile of tholeiitic flows and pillow lava (Nikolai Greenstone and Karmutsen Formation) capped with inner-platform carbonates (Chitistone Limestone, Whitestripe Marble, Kunga Formation, and Quatsino Limestone), overlie an upper Paleozoic andesitic arc sequence and Permian argillite and limestone. This coherent terrane, herein named Wrangellia, is juxtaposed against unlike sequences of Triassic and older rocks throughout its extent and is interpreted to be allochthonous. Paleomagnetic data obtained from the Nikolai Greenstone and published in a companion article by Hillhouse indicate that Middle and (or) Upper Triassic rocks in southern Alaska formed in low paleolatitudes, probably within 15° of the paleo-equator.A possible southeastern extension of Wrangellia occurs in the Hells Canyon region of eastern Oregon and western Idaho. This area contains the typical Triassic sequence of Wrangellia and has been interpreted by other geologists as allochthonous. Paleomagnetic data are lacking, however, to document its original latitude.

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Paleomagnetism and anisotropy of magnetic susceptibility (AMS) reveal pyroclastic fl ow patterns, stratigraphic correlations, and tectonic rotations in the Miocene Stanislaus Group, an extensive volcanic sequence in the central Sierra Nevada, California, and in the Walker Lane of California and Nevada. The Stanislaus Group (Table Mountain Latite, Eureka Valley Tuff, and the Dardanelles Formation) is a useful stratigraphic marker for understanding the post-9-Ma major faulting of the easternmost Sierra Nevada, uplift of the mountain range, and transtensional tectonics within the central Walker Lane. The Table Mountain Latite has a distinctively shallow reversed-polarity direction (I = −26.1°, D = 163.1°, and α 95 = 2.7°) at sampling sites in the foothills and western slope of the Sierra Nevada. In ascending order, the Eureka Valley Tuff comprises the Tollhouse Flat Member (I = −62.8°, D = 159.9°, α 95 = 2.6°), By-Day Member (I = 52.4°, D = 8.6°, α 95 = 7.2°), and Upper Member (I = 27.9°, D = 358.0°, α 95 = 10.4°). The Dardanelles Formation has normal polarity. From the magnetization directions of the Eureka Valley Tuff in the central Walker Lane north of Mono Lake and in the Anchorite Hills, we infer clockwise, vertical-axis rotations of ~10° to 26° to be a consequence of dextral shear. The AMS results from 19 sites generally show that the Eureka Valley Tuff fl owed outward from its proposed source area, the Little Walker Caldera, although several indicators are transverse to radial fl ow. AMS-derived fl ow patterns are consistent with mapped channels in the Sierra Nevada and Walker Lane.

We report paleomagnetic results from layered igneous rocks that imply substantial post mid-Cretaceous poleward motion of the Insular superterrane (western Canadian Cordillera and southeast Alaska) relative to North America. The samples studied are from the stratiform zoned ultramafic body at Duke Island, which intruded rocks of the Alexander terrane at the south end of the southeastern Alaska archipelago at about 110 Ma. Thermal and alternating field demagnetization experiments show that the characteristic remanence of the ultramafic rocks has high coercivity and a narrow unblocking temperature range just below the Curie temperature of magnetite. This remanence is likely carried by low-Ti titanomagnetite exsolved within clinopyroxene and perhaps other silicate hosts. The Duke Island intrusion exhibits a well-developed gravitational layering that was deformed during initial cooling (but below 540øC) into folds that plunge moderately to the west-southwest. The characteristic remanence clearly predates this early folding and is therefore primary; the Fisher parameter describing the concentration of the overall mean remanence direction improves from 3 to 32 when the site-mean directions are corrected by restoring the layering to estimated paleohorizontal. All samples exhibit a magnetic anisotropy that is strong but nonuniform in orientation across the intrusion, and we show that it has no significant or systematic effect on the sitemean directions of remanence. At least some of the anisotropy derives from secondary magnetite formed during partial serpentinization. The mean paleomagnetic inclination (56 ø ñ 10 ø) corroborates paleomagnetic results from five coeval silicic plutons of the Canadian Coast Plutonic Complex to the south and southeast and implies 3000 km (ñ 1300 kin) of poleward transport relative to the North American craton. Between mid-Cretaceous and middle Eocene time, the Insular superterrane and Coast Plutonic Complex shared a common paleolatitude history, with more poleward transport than coeval inboard terranes. Introduction The Canadian Cordillera, including southeastern Alaska, is divided into a series of physiographic/geologic provinces that are roughly parallel to the continental margin. Paper number 95TC01579. 0278-7407/95/95TC-01579510.00 Mountain Belt. Of these, both the Insular Belt (Terrane II) and the Intermontane Belt (Terrane I) are superterranes composed of smaller tectonostratigraphic terranes that are termed "suspect" because they are evidently allochthonous with respect to the North American craton [e.g., Monger et al., 1982]. The Coast Plutonic Complex and the Omineca Crystalline Belt are the broad loci of sutures joining these two allochthonous superterranes to each other and to the continent. The Intermontane superterrane accreted to North America in Middle Jurassic time, and the outboard Insular superterrane accreted to the Intermontane superterrane sometime between the Middle Jurassic and Late Cretaceous [Monger et al., 1982; van der Heyden, 1992; Rubin et al., 1990]. A regionall...

We have used anisotropy of magnetic susceptibility (AMS) to define the flow fabric and possible source area of the Peach Springs Tuff, a widespread rhyolitic ash flow tuff in the Mojave Desert and Great Basin of California, Arizona, and Nevada. The tuff is an important stratigraphic marker from the Colorado Plateau to Barstow, California, a distance of 350 km; however, the location of its source caldera is unknown. Dated at 18.5 Ma by 4øAr/39Ar, the tuff erupted during the early stages of Miocene extension along the lower Colorado River. The thicker accumulations (> 100 m) occur at Kingman, Arizona, and in the Piute Mountains, California, on opposite sides of the Colorado River extensional corridor. Our AMS studies produced well-defined magnetic lineations in 30 of 42 sites distributed throughout the tuff. Typical ratios of the principal AMS axes are 1.01 for the magnetic lineation (kmax/kint) and 1.02 for the foliation (kint/kmin); the bulk magnetic susceptibility of the Peach Springs Tuff averages 2.0 x 10 -3 in the SI unit system. The subhorizontal lineations, which presumably parallel the flow directions, form a pattern radiating outward from the approximate center of the outcrop area. Magnetic foliations define an imbrication that generally dips away from the distal margins and toward the center of the outcrop of the tuff. The lineation and imbrication indicate a source region near the southern tip of Nevada. Defining the best intersection of the AMS lineations required restoration of major extension, strike-slip faulting, and associated tectonic rotation in the disrupted tuff. The optimum intersection of magnetic lineations lies in the southern Black Mountains of Arizona on the eastern side of the Colorado River extensional corridor. No caldera structures are known from that area, but the area contains thick sections of the Peach Springs Tuff above a silicic volcanic center. The caldera may be buried under younger deposits in the Mohave Valley of Arizona. Tertiary granite in the Newberry Mountains may represent a deeper level of the Peach Springs Tuff vent that has been exhumed by detachment faulting. 12,443 12,444 HILLHOUSE AND WELLS' MAGNETIC FABRIC OF PEACH SPRINGS TUFF 117 ø 31 10 Kingman Peach Springs 113ø36o • -' • Ariz. Fig. 1. Simplified geologic map of the Cenozoic volcanic rocks of the Mojave Desert and surrounding regions, California, Arizona, and Nevada. Volcanic rocks (mostly Miocene) are stippled; outcrops of the lower Miocene Peach Springs Tuff, as defined by Glazner et al. [1986], are in black. Known or suspected Tertiary plutons in Colorado River extensional corridor are shown be diagonal line pattern. Sampling sites for AMS study are numbered as in Tables 1 and 2. Localities mentioned in text: BM, Black Mountains; BR, Buckskin and Rawhide Mountains; CM, Castle Mountains; EM, Eldorado Mountains; MM, McCullough Mountains; LV, Lanfair Valley; MV, Mohave Valley; NM, Newberry Mountains; WM, Woods Mountains volcanic center. Solid lines are faults (ticks on downthrown side); arrows show sense of ...