WHY SO BLUE? THE DISTRIBUTION, ORIGIN, AND DEFORMATION OF ELECTRICALLY CONDUCTIVE STRATIGRAPHIC UNITS WITHIN THE NORTHEAST MOJAVE STRIKE-SLIP PROVINCE

Since 2010, airborne electromagnetic investigations within the northeast Mojave Strike-slip Province have collected 2750 line-kilometers of data, producing a detailed picture of subsurface resistivity down to 500 m depth in at least eight of the groundwater basins within the Fort Irwin National Training Center, CA. While the data density varies considerably from basin to basin, examining the resistivity models in two and three dimensions reveals extensive electrically-conductive (blue) regions characterized by their low resistivity (1-10 Ω·m). Exposures of Mesozoic plutonic rocks which underlie the region are consistently resistive, as are early Miocene volcanic flows and domes, suggesting that the high conductivity is associated with clastic basin fill or fine-grained volcanic deposits. Aside from isolated occurrences of Quaternary playa and groundwater-discharge deposits, the conductive ‘blueite’ appears to predate the Quaternary alluvium, and in the western basins is interfingered with early Miocene volcanic flows.

Given the complex structural and depositional framework of the region, it is unlikely that the conductive ‘blueite’ represents a single depositional source or provenance. Correlations between conductive units and volcaniclastic (tuffaceous) material exist in the western basins, whereas Tertiary lacustrine deposits show a greater degree of correlation in the eastern basins. Laboratory conductivity and X-ray diffraction measurements on a suite of older clastic rocks suggest the high conductivity is due to mineralogical clay within older Tertiary deposits. Given the near absence of conductive Quaternary deposits in the region, we speculate the clay formed in a less-arid pre-Pleistocene environment. Sparse paleoclimate data are consistent with this interpretation.

Conductive deposits appear to predate some or all of the post-11 Ma deformation along the west-trending sinistral and northwest-trending dextral faults that dissect the region. Deformation is associated with numerous faults, but is most prominent near the eastern terminus of the Garlock fault zone where it meets the Southern Death Valley Fault zone. Here, compressional features (pop-up structures, fault-propagation folds, etc.) are interpreted from the resistivity models and can be linked to surface features.