3-D GEOLOGIC MAPPING NEAR BLYTHE, CALIFORNIA UNVEILS THE ARCHITECTURE OF CONCEALED NEOGENE FAULTS

Bedrock mountain ranges surrounding Blythe, California expose a system of NW-striking faults that were active in the Late Miocene as part of the paleo-Eastern California shear zone. However, our understanding of how these faults interacted in a single coherent system is hampered by uncertain subsurface geometries across large basins covered by alluvial deposits that mostly postdate faulting. Here we present ongoing efforts to create a 3-D geologic map of concealed faults and basin deposits to better understand the tectonostratigraphic evolution of Neogene structural and depositional systems. We integrate data from geologic maps, gravity and magnetic surveys, and a compiled database of borehole records for a multi-method approach to constraining the subsurface geology.

A depth-to-bedrock inversion of gravity data reveals a NW-SE elongate basin beneath Blythe that is ~20-25 km long, ~4-7 km wide, and up to ~3 km deep with two localized depressions separated by a saddle-shaped intrabasin high. Gravity and aeromagnetic gradients parallel and sub-parallel to the regional trend of Neogene faults mark the basin margins. A linear southeast margin of the basin coincides with a ridge in the depth-to-bedrock gravity inversion and stepped northwest-striking gravity gradients mark the northeast margin of the basin. Fault kinematic data indicate that Neogene extensional strain changed orientation from NE-SW to E-W; this change caused a shift from normal to oblique and dextral faulting in the NW-striking faults, which could have facilitated basin development with a complex releasing fault geometry. Serial cross sections constructed parallel and orthogonal to the NW-SE-oriented basin axis constrain the major structures and lithologic surfaces at depth and provide a basis for 2-D geophysical modeling. Borehole data record detailed lithologies of the uppermost several hundred meters of the basin that host important groundwater resources. The basin contains Miocene to Quaternary alluvial deposits, including Pliocene sediments related to the integration of the Colorado River to the Gulf of California. This combination of geologic and geophysical data will be used to create a 3-D map showing the subsurface stratigraphy and fault geometries and will allow a better understanding of the evolution of the Neogene tectonic and depositional systems.