BRITTLE TERTIARY DETACHMENTS ADJACENT TO THE SPECTER RANGE THRUST, SOUTHERN NEVADA

The Specter Range thrust (SRT) extends from Mercury to Amargosa, Nevada. As previously mapped the thrust separates a hanging wall, composed of slices of Cambrian Wood Canyon, Zabriskie, Carrara and Bonanza King that are tectonically thinned along sub-horizontal, stratigraphic detachments, from Ordovician carbonate rocks dipping south. Middle Paleozoic rocks in the footwall are cut by three principal detachments that may be parallel to bedding. The main detachments are, from lowest to highest; 1) at the base of the Eureka, 2) at the contact between the Eureka and Ely Springs, and 3) at the base of the upper Silurian dolomite. Stratigraphic relationships between the Antelope Valley and the Nine Mile formation also may record a detachment at the base of the Antelope Valley. Hundreds of meters of beds between major detachments commonly consist of granules to small boulders, although large boulders and mega blocks are locally common, especially in the dolomite unit. The breccia is pervasively developed along bedding surfaces and steep cross-cutting faults and fractures. The extensive cataclasis indicates deformation at low pressure and temperature. Vein networks are not prominent. The source of the detached units may be local or distant, as shown by large differences in thickness and ratio of quartzite to carbonate in the Eureka Quartzite. Furthermore, some detached bodies are slab like in contrast to the common north-trending folds characteristic of strata in the nearby Spring Mountains. The extensive and intensive transpressive Tertiary brecciation of carbonate rocks along a left-step of the dextral Las Vegas Valley shear zone in this region may provide fast pathways for the movement of groundwater from the Nevada Test Site southward.