CENOZOIC TECTONIC EVOLUTION OF THE SOUTHERN BRISTOL MOUNTAINS, CA

The Southern Bristol Mountains (SBM), California, preserve a unique record of the Cenozoic tectonic evolution of a portion of the Mojave Province. Mapping within the SBM over 20 km along strike of the Southern Bristol Mountains fault zone (SBMFZ) at 1:10,000 scale allows subdivision of previously undivided Paleogene and Neogene units. Miocene tilt blocks preserve a cross section of sedimentary and volcanic rock sequences comprising: 1) an unequaled exposure of over 5 kilometers (sum along strike) of a deeply weathered Tertiary unconformity surface built upon exhumed Mesozoic and Proterozoic plutonic rocks; 2) a sedimentary succession of non-volcanic proximally sourced sandstone, fluvial and lacustrine volcanic sandstone, and an overlying 23.75 +/- 0.72 (2 sigma) Ma airfall tuff, unequivocally depositional upon the basement erosional surface; 3) a sequence of epiclastic and volcanic strata with little evidence of growth strata (within the resolution of the coarse bedding of the volcanic rocks) but that contains several internal angular unconformities indicating that tilting and moderate extension occurred in distinct phases relative to the ongoing volcanic activity; and 4) unconformable Peach Springs Tuff, in places overlying and preserving poorly indurated, gypsiferous, fine sands and muds and thin conglomerate beds likely contemporaneous with the fluvial system recorded by Lease et al (2009)’s Lost Marble gravel of the eastern adjacent Old Dad Mountains and Marble Mountains. The Miocene tilt blocks are cut, translated, and vertically offset by the SBMFZ. Onset of dextral shear in the SBMFZ is poorly constrained due to discontinuous later Neogene sedimentation, but the latest stages of slip are preserved by: A) unique marekenite clasts in a thick sequence of beheaded conglomerate, and B) unique silicified limestone clasts in oldest Quaternary desert varnished alluvial terraces offset up to 3.5 km from the limestone source.