IMBRICATE THRUST FAULTS AND TIGHT FOLDS IN NEOPROTEROZOIC FORMATIONS OF THE VIRGIN SPRING-AMARGOSA CHAOS OF SOUTHERN DEATH VALLEY, CA

The Virgin Spring phase of the Amargosa Chaos consists primarily of highly extended blocks of Neoproterozoic sedimentary formations. Though much of the lower section (Pahrump Group) has been dramatically thinned or is completely missing, the higher stratigraphic sections remain as large intact fault blocks. These blocks are less structurally chaotic and retain bedding and stratigraphic order. We conducted detailed mapping of these intact blocks; the structures discussed here are all exposed in one such continuous block.

We find evidence for compression (Mesozoic) in the Chaos prior to the well known extension by Wright and Troxel 1984. Our two main lines of evidence to support this consist of tight folding and imbricate thrust faulting. The tight folds throughout the Upper Johnnie and Sterling Quartzite formations, which have a 20-60° S45E trend and plunge, are diverse in fold morphology but are typically symmetrical, including: tight (~50% shortening) 40 m synforms, <10 m box folds, 50 meter sections of broad wavelength (1m) low amplitude (0.5 m) fold pairs, and 3-5 meter wide chevron folds. Imbricate thrust faults appear as a series of duplicated 5-17 m thick sections of upper Johnnie Formation. These faults appear to splay off a more continuous thrust fault with a hangingwall ramp anticline. The orientation of this fault, N20W and SW dipping, is sub-parallel to the folds to suggest contemporaneous formation during ~N45E directed compression. We also mapped a feature that appears to be a ~600m long klippe of older rocks over younger rocks possibly representing <2km of offset, but we have yet to finalize our interpretations of this feature. We suggest here the compressional deformation in the Amargosa Chaos is contemporaneous with the regional orogenic events during the late Mesozoic, adding to the previously mapped Mesozoic structures of the Death Valley region.