NON-UNIFORM SIMPLE SHEAR: A CASE STUDY, SOUTH VIRGIN MOUNTAINS, NEVADA

Examples of structures exhibiting non-uniform normal simple shear are common in regions that are characterized as highly extended, and integrating them into regional strain analysis and tectonic modeling is essential. In the South Virgin Mountains, detailed geologic mapping by Brady (1998) reveals two domains of west-tilted Paleozoic strata cut by down-to-east faults within a broad area of east-tilted Paleozoic through Tertiary strata cut by down-to-west faults. Each domain, about 2 km wide, of west-tilted rocks has one fault boundary against along-strike east-tilted rocks and one torsional transition into those rocks. The Paleozoic cover rocks in the eastern domain (the Perkins Spring block) are strongly and pervasively faulted and tilted. Strain analysis in those rocks in the lower 20 m of south- and west-facing escarpments reveals: 1) dominance of east-dipping faults (42 of 57); 2) faults dip from 30°to 90° (ave. 49°) and show no tendency to merge into a previously mapped detachment fault at the base of the block; 3) westerly stratal dips ranging from 20° to steeply overturned (ave. 56°); 4) no physical evidence of a west-vergent detachment fault; 5) anomalously low (ave. 20°) stratal dips in basal Cambrian Tapeats Sandstone, and concordant depositional contacts with underlying basement rock, precluding detachment between basement and cover; 6) slip lineations with rakes > 60° that are collinear with, but azimuthally opposed to, the regional average; and 7) slip lineations with rakes < 60° are southerly, showing that uniform-sense srike slip accompanied extension. Fault/fracture fabrics in underlying basement rocks show top-to-the-east strain. The along-strike torsional strain accommodation and the absence of any indication that intense down-to-the east simple shear is temporally separate from, or a subordinate attribute of, the regional down-to-the-west strain favor a multiple fixed tilt-axis tectonic model over a uniform-sense large-magnitude simple shear model.