USE OF GEOPHYSICAL AND REMOTE SENSING DATA FOR A BETTER UNDERSTANDING OF THE HYDROLOGIC ROLE OF FAULTS IN THE MOJAVE DESERT

It has been suggested that in the Basin and Range area, Mojave Desert, the dextral strike slip fault systems (e.g., Helendale, Lenwood, Camp Rock) are acting as barriers for groundwater movement, as evidenced by local differences in water levels across a number of these faults. This suggestion is inconsistent with findings in tectonically similar settings elsewhere; fault-related brittle deformation enhances rock porosity and could potentially facilitate groundwater movement along fault planes and shear zones, making these structures act as conduits rather than barriers for groundwater movement. To investigate whether these faults are barriers or conduits, an integrated (geophysics, remote sensing, field) study was conducted in the Mojave Basin: (1) the Helendale fault traces were first mapped with precision using geologic maps, false color images from ASTER VNIR bands, areal photographs, and SRTM digital elevation data; (2) Geophysical transects (Very Low Frequency [VLF]; and magnetometer) were carried out along the Helendale fault, starting from the NW corner of the study area where the fault cuts through the granitic mountains (Granite & Fry Mountains – Mesozoic (240-65mya) (NW), across the Este Basin, and ending in the SE corner of the study area, where the fault intersects the San Bernido mountains. Eleven sites were surveyed with 14 transects in 3 days. Very Low Frequency (VLF) measurements across the fault traces in the mountainous areas showed significant radio field dip angles (up to 60% tilt) consistent with the presence of sub-vertical, sheet-like conductors in the shallow subsurface, whereas in the valleys no appreciable VLF anomalies were seen probably because water table was far below the reach of the VLF (several 10’s of meters). For many of the acquired VLF tilt profiles, the peaks coincided with changes in the magnetic profiles that are to be expected across faults which juxtapose rocks with varying magnetic properties. One interpretation for our preliminary data sets is that the Helendale fault and most likely other dextral faults in the area (e.g., Lenwood, Camp Rock) are channeling groundwater from the surrounding mountains to the lowlands (Lucerne Valley) and hence could represent additional groundwater influx to the Alluvial and Old Woman Springs Aquifers that are unaccounted for in earlier models.