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Description
Eight magnetotelluric (MT) stations in northern Baja California were analyzed. These stations form a profile across the Laguna Salada basin on the western side of the Salton Trough. The primary objectives of the study were to: (1) interpret the MT soundings in order to construct a geoelectric model using previous geophysical findings to aid in interpretation, (2) construct a gravity model along the same profile from previously published gravity data, (3) implement a preliminary tensor stripping analysis in order to remove the effects of topography and conductive basin sediments on the observed data and (4) interpret deep seated geologic structure. The MT data were modeled using a two-dimensional computer algorithm. Analysis of various magnetotelluric parameters indicate that some stations were affected by three-dimensional geometries. However, an overall reasonable agreement between calculated and observed data was achieved. In the geoelectric cross-section, a zone of relatively high conductivity is modeled at a depth of 16 km below the Laguna Salada basin. The zone of relatively high conductivity underlies a zone of relatively intermediate conductivity. A residual Bouguer anomaly was modeled along the MT profile using parameters from the geoelectric model to construct the gravity model. A good match between observed and calculated gravity data was achieved. For the Laguna Salada basin, the zones of high conductivity and intermediate conductivity (geoelectric model) correspond to zones of high density and intermediate density in the gravity model. Results from the tensor stripping analysis, on the MT data, indicate the the deep conductive parts of the geoelectric model are not the result of conductive sediments in the adjacent Mexicali Valley or the result of distorted fields from the near surface conductive sediments in the Laguna Salada basin. Results from the geoelectric model indicate that the sediments in the Laguna Salada basin extend down to a depth of 3 km with very conductive sediments (2 Ω-m) the lower part of the basin. Hot, saline groundwater may exist in the sediment pore spaces for this lower part of the basin. Relatively non-saline groundwater probably exist in the sediments for the shallower part of the basin which have an apparent resistivity of 10 Ω-m. Results from the integrated models suggest that sediments in the Laguna Salada basin overlie a transition zone. The upper part of the transition zone may consist of hydrothermally altered or metamorphosed sediments which are relatively more resistive because of the closing off of pore spaces in the sediments. These rocks could overlie highly fractured crystalline rocks intruded by magma which was differentiated from the mantle. The dense, conductive body at a depth of 16 km may indicate the presence of melt. The Sierra de los Cucapas are modeled as a resistive block which extends down to a depth of 14 km and overlies a zone of intermediate resistivity. The root of the Sierra de los Cucapas could be responsible for a downwarp of mantle convective paths causing a local mantle upwarp or ripple below the Laguna Salada basin to the west.
