THE ROLE OF BASAL TRACTION IN MODELING NEOGENE DEFORMATION IN THE EASTERN CALIFORNIA SHEAR ZONE, COAL VALLEY AREA, MINERAL AND LYON COUNTIES, NEVADA

Shallow deformation in the west-central Great Basin, Nevada and California, reflects a combined response to regional extension and superposed lateral ductile flow of substrate (e.g., Oldow, 1992). Flow can be at a high angle to the extension direction, analogous to vertically partitioned Tertiary strain in the Ruby Mountains, Nevada. Stratigraphic and structural mapping in the Coal Valley area, NE of Bridgeport, California, reveals a heterogeneous array of late Miocene (9-7.4 Ma.) structures including extensional tilt blocks, some with contemporary shallow-axis hinge folds; vertical pop-up structures; contractional (?) folds with steep axes; shallow-axis folds of diverse orientation; normal faults with widely varying strike and/or strongly curved surface traces; and reverse faults and associated overturned beds, some parallel to nearby normal faults. These diverse and seemingly incompatible structures are the strongest deformation in the Cenozoic rocks in the area, and they coincide with early development of the Eastern California Shear Zone. Because they are so restricted in time, the heterogeneity is unlikely to have resulted from shifts in the regional stress state. Therefore, we propose a tectonic model in which the regionally extending shallow crust is subjected to coeval basal tractional forces resulting from lateral ductile flow of substrate at a high angle to the extension direction. Spatially variable steep-axis folding and reverse faulting reflect local structural buttressing as well as velocity gradients in the flow system. The timing is consistent with our geologic observations in the northern Wassuk Range, and the style is consistent with vertically partitioned strain patterns we recognize in geologic and high-resolution aeromagnetic data to the south in and surrounding the Adobe Hills. The youthfulness of the deformation is consistent with a large body of geologic data from the region, but is at variance with recently reported interpretations of Ar/Ar geochronologic data, apatite fission-track data, and U-Th/He thermochronologic data, that hypothesize major extensional deformation 3-6 m.y. earlier. We see this inherent inconsistency not so much a question of whether the early deformation occurred, but whether it represented major extension