TERTIARY TRANSPRESSIONAL STRUCTURES IN THE NEVADA TEST SITE REGION, SOUTHERN NEVADA

Thrusts and folds that occur at constraining bends along two sets of strike-slip faults in the Nevada test site region show diverse directions of vergence that suggest the presence of positive flower structures. Transpressional structures include: 1) thrusts that carry Cambrian and older strata onto middle and upper Paleozoic strata, 2) narrow panels of overturned beds, 3) folds, and 4) stratigraphic detachments characterized by very strong brecciation. The lateral faults, which cut pre-Tertiary contractional structures, are different with respect to: 1) strike, northerly vs. northwesterly; 2) sense, left- vs right-lateral; and 3) inferred ages, Late Oligocene-Early Miocene (?) vs mid-Miocene. In western Rock Valley, Middle Miocene (~16 and 10 Ma) deformation is attributed to transpression along a west-trending constraining step between Mercury and Forty Mile Wash contemporaneous with right-lateral displacement along the Las Vegas Valley shear zone (LVVSZ). Contraction is recorded by west-trending folds in beds of Miocene Pavits Spring and underlying units equivalent to Horse Spring. Paleozoic units in the northern Specter Range record contractional structures, of inferred Miocene age, including: 1) extensive brecciation along detached beds of Bonanza King Formation, 2) westerly-trending folds and the Specter Range thrust, a major south-vergent structure, and 3) northeast-striking, left-lateral strike-slip faults, that cut the sub-horizontal detachment surfaces in the eastern Specter Range. West- and northwest-directed “hinterland vergent” structures (e.g. Mine Mountain fault, CP thrust system and Panama thrust), may also have formed in response to mid-Miocene transpression. South of the Specter Range, Paddys fault, “Johnnie thrust” and Montgomery thrust have characteristics of style and/or vergence that set them apart from typical Mesozoic thrusts and suggest formation as transpressional structures. North-striking, left-lateral faults along the western side of Yucca Flat include: Tippinip, Area 13, and Carpetbag. Contractional structures at right-bends along these faults (e.g. Quartzite Ridge) record transpression. The left-lateral faults, which are kinematically unrelated to the right-lateral faults, are correlated with structures of Late-Oligocene-Early Miocene age.