TRANSPRESSIONAL STRUCTURES IN NORTHERN SPECTER RANGE AND STRIPED HILLS, NEVADA: STRONG CONTRACTION WITHIN AN EXTENSIONAL OROGEN

Analyses of geologic maps and field observations in the Specter Range and the region to the north within the Nevada Test Site west of Mercury, Nevada, reveal four episodes of deformation. 1) The earliest event occurred during pre-Tertiary time, when north-trending folds were formed during east-directed thrusting that may have carried the rocks exposed in the Striped Hills onto a footwall presently exposed in the Specter Range and northern Spring Mountains. Hanging wall rocks in the Striped Hills are distinguished by the absence of Eureka Quartzite. 2) Early Miocene extension resulted in the formation of a basin, probably fault controlled, the full extent of which is unknown. 3) Middle Miocene deformation, occurring between approximately 16 and 10 Ma, is attributed to transpression between Mercury and Forty Mile Wash contemporaneous with displacement along the right-lateral Las Vegas Valley shear zone (LVVSZ). Contraction along the west-trending step may have accommodated tens of kilometers of displacement recorded along the LVVSZ. During shortening, Paleozoic and overlying Tertiary strata were uplifted. The contact between Early Miocene strata and Cambrian beds is strongly tilted and west-trending folds are present in the Horse Spring and Pavits Spring formations. Underlying Paleozoic units in the northern Specter Range record brecciation, thrusts that may verge either north or south and detachments, especially among carbonate strata of Bonanza King Formation. Left-lateral strike-slip faults, that cut the low-angle fault surfaces in the eastern Specter Range, are the youngest transpressional structures. During contraction, Paleozoic strata in the Striped Hills and overlying Miocene strata were upturned and fractured along north-striking normal faults, perhaps above a pre-existing thrust, which served as a decollement. 4) Post-10 Ma structures include the left-lateral Rock Valley fault and north-striking normal faults that cut older Tertiary features.