GEOLOGY OF THE CHICAGO PASS AREA, CA-NV: IMPLICATIONS FOR REGIONAL TECTONICS

New field mapping and analysis of existing work in the Resting Spring and northern Nopah Ranges provides the basis for re-interpretation of an area that has been important in regional reconstructions of the Death Valley extensional terrane. Most recently, Snow and Wernicke (2000, Am. J. Sci.) suggested the Chicago Pass “Thrust” represented an extensional reactivation of a thrust ramp and was part of a regional detachment with up to 7 km of slip. This study attempts to re-examine the area through construction of several balanced cross-sections across Chicago Pass representing a number of alternative interpretations, with cross-section balancing and restoration aided by use of the GeoSec2D software package (2002, Paradigm Geophysical). Preliminary results suggest relatively modest extension (<2 km) along a normal fault beneath alluvial deposits in Chicago Pass, with slip probably increasing to the south along the front of the Nopah Range. This reconstruction correlates the Shaw and Baxter thrusts across Chicago Valley, and suggests that the Chicago Pass Thrust represents a thrust klippe. An important constraint on the area’s history is placed by cross-cutting relationships of Tertiary fanglomerates that depositionally overlap the Chicago Pass Thrust, but without age information the significance of this overlap is debatable. Uncertainties arise from alternative interpretations of isolated exposures of Stirling megabreccias lying atop younger rocks just south of Chicago Pass which may represent either normal fault klippe or landslide deposits.