VOL. 84,NO. B9 JOURNAL OF GEOPHYSICAL RESEARCH AUGUST 10, 1979 Paleomagnetism of Lower and Middle Cambrian Sedimentary Rocks From the Desert Range, Nevada STEPHEN L. GILLETT Department of Earth and Space Sciences, State University of New York at Stony Brook, Stony Brook, New York 11794 DAVID R. VAN ALSTINE Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125 Over 500 oriented samples werecollected from a homoclinal sequence of Lower and Middle Cambrian terrigenous and carbonate miogeoclinal stratain the Desert Range,Nevada. Extensive thermal demagne- tization wassuccessful in isolating similarcharacteristic magnetization directions in red-purple mudstones of the Wood Canyon Formation (Lower(?) and Lower Cambrian) and in gray limestones of the Carrara and Bonanza King Formations (Lower and Middle Cambrian).Lithologic and magnetic evidence suggest that these magnetizations were acquired penecontemporaneously with deposition. The similarityof the characteristic magnetization directions in these strata implies that little apparentpolar wanderoccurred with respect to North America from early Early throughmiddle Middle Cambrian time. The divergence of these directions from those from the partly coeval TapeatsSandstone of the Colorado Plateauprobably resulted from a net 36 ø clockwise rotation of the Desert Rangesection about a vertical axis.This rotation is probably due to mid-Tertiary oroflexuralbending but may in part have been caused by Mesozoic thrusting. The polarity of the geomagnetic field in early Middle Cambrian time appears to have been continuously reversed. Stratigraphic intervals of mixed polarity, possibly correlative with those found in the Tapeats, may be present in the Lower Cambrian Zabriskie Quartzite. INTRODUCTION during four sampling trips over 2 years.Of this total, about Afairly extensive body of paleomagnetic data from rocks of one tfiird come from upper Precambrian strata, and two thirds latest Precambrian and earliest Paleozoic age from the Gon- from Lower and Middle Cambrian strata. This paper focuses dwana continents [McElhinny and Embleton, 1976] suggests that ratesof apparentpolar wander (APW) were much higher during the late Precambrian and Early Paleozoicthan in the Mesozoicand Cenozoic. On this basis, McElhinnyet al. [1974] predicted that 'extensive studies of late Precambrian andCam- brian rocks of North America... will reveal a complicated apparentpolar wander path with a polar shift of at least 180ø. ' Whereas a relativelycontinuous APW path can be drawn for North America sincethe Early Ordovician [Van A lstineand de Boer, 1978], the APW path of the latest Precambrian and Cambrian remainspoorly known. To help extend the continuous North American APW path into these times, wehave undertaken a paleomagnetic investi- gation of upper Precambrian and Lower Paleozoic strata in the Cordilleran Geosyncline. A miogeoclinal sequence ex- posed in the Desert Range, Nevada, was chosenfor study because it is conformable from the upper Precambrian through Lower Ordovician,occurs in a generally homoclinal fault block, shows little if any evidence of regional meta- morphism, and is well exposed. Besides the advantage of strat- igraphic continuity, this miogeoclinal sequence is known to include the Precambrian-Cambrian boundary, unlike any se- quence on the craton. A disadvantage of this section is that it lieswithin a structurally complex area, sothat the probability of tectonic rotations about a vertical axis must be reckoned with. Such rotations may be corrected for, however,by calcu- lating the declinationcorrectionrequiredto make paleomag- netic polesfrom these rocksmatch poles of the sameagefrom the craton. The work reported in this and the companion paper [Van Alstineand Gillett, 1979]represents results from over 800 independently oriented samples collected in the Desert Range Copyright ¸ 1979by the American Geophysical Union. on the paleomagnetism of the Cambrian part of the section, with particular emphasison determining whether there has been postmagnetization rotation of the fault block about a verticalaxis. The companion paper includes results from the upper Precambrian part of the section. Both papers seekto elucidatethe pattern of geomagnetic reversals for their pos- sible utility in global stratigraphic correlation. REGIONAL GEOLOGIC SETTING OF THE DESERT RANGE The Desert Range is locatedin Clark County, southeastern Nevada (Figure 1). It is an eastwarddipping block which is much cut by faulting and which has beeninterpretedby Long- well [1945, 1960] to be the easternlimb of a broad north plunging anticline. Immediately east of the DesertRange is the SheepRange, part of a huge allochthonous plate which has overridden rocksof the Las VegasRange, farther to the east, on the Gass Peak Thrust. The large-scale thrusting occurred during the Sevier Orogeny in Late Cretaceous time [Fleck, 1970]. Burchfiel et al. [1970] have shownthat a belt of older (probably late Triassicto early Jurassic) thrust faults is ex- posed about 25 km westof the DesertRange. Normal faults, related to late Tertiary Basin and Range extension, separate the downfaulted eastsideof the Desert Range from the Sheep Range,and similarfaults probablydefine the western sideof the range.Because of its position with respect to the Mesozoic thrust belts,the Desert Rangeis probablyallochthonous, but the inferred basal thrust is hidden. To the souththe Desert Rangeis truncatedby the Las Vegas Shear Zone, a fault zone of right lateral displacement which is a major tectonic elementin southern Nevada [e.g.,Longwell, 1960, 1974;Stewart, 1967].The time of the major movement on this shear zone probablyliesin the range 17-11 m.y.B.P. [Ekrenet al., 1968; Longwell, 1974]. The pronounced westward bend of the strike of the bedsat the southern tip of the range has beeninterpreted as drag on this shear zone [cf. Burchild, Paper number 9B0430. 0148-0227 / 79/009 B-0430501.00 4475