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Description
Magnetotelluric (MT) soundings and dc resistivity data are used to infer the geoelectric structure of the Salton Trough, particularly, the region near the Salton Sea Geothermal Field (SSGF). The near-surface (<2 km) in the vicinity of the SSGF is divided into three general layers: a thin (<600 m) surface resistive layer (2-5 ohm-m) that is absent in the SSGF, a conductive layer (<1 ohm-m) that becomes anomalously conductive (<0.25 ohm-m) beneath the SSGF, and a resistive basement of altered sediments that is much shallower (<1 km) beneath the geothermal zone than elsewhere in the Trough. The conductive zone (<0.5 ohm-m) extends south from the SSGF. The areal extent of the conductive zone coincides with that of the Brawley Seismic Zone and with the axis of lowest elevation within the Trough. One possible explanation for the low resistivity is that the seismicity may permit the flow of hypersaline brines by maintaining the fracture permeability, thereby, decreasing the resistivity. An alternative explanation is that there are more clays and evaporite deposits deposited in the center of the valley compared with the edges. The MT data near the SSGF as well as other MT data and results are used to develop a regional geoelectric model of the Salton Trough and surrounding regions. The older crust adjacent to the Trough is shown to be resistive (greater than 1000 ohm-m in the Peninsular Ranges to the west and at least 200 ohm-m to the east beneath the Chocolate Mountains). The resistive material is underlain by a deep (20 km) conductive zone (40-200 ohm-m beneath the Peninsular Ranges and 10-50 ohm-m beneath the Chocolate and Cargo Muchacho Mountains). The deep (>4 km) resistive basement beneath the Salton Trough is estimated to be as low as 10 ohm-m in the north near the Salton Sea increasing to approximately 50 ohm-m in the south near Cerro Prieto. The new crust beneath this basement remains at this resistivity to the greatest depths observed (> 25 km). Three-dimensional MT effects of the basin explain much of the discrepancy between the observed data and that synthesized from two-dimensional models. Both the local and regional results of this study concur with the results of previous seismic and gravity studies.
