A magnetotelluric profile across the western boundary of the Salton Trough in northern Baja California, Mexico

Details A magnetotelluric profile across the western boundary of the Salton Trough in northern Baja California, Mexico A magnetotelluric profile across the western boundary of the Salton Trough in northern Baja California, Mexico

Mar 1989

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989pepi...53..376m&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 53, Issue 3-4, p. 376-383.

Physics

4

Scientific paper

Eighteen magnetotelluric (MT) stations were occupied in an east-west profile across the northern part of the Baja California Peninsula immediately south of the international border between Mexico and the U.S.A. The profile stretches across the Peninsular Ranges Batholith into the Salton Trough, a region of active crustal spreading. Data were collected in the frequency range of 0.001-25 Hz using the remote reference technique. We have conducted a preliminary modeling of the data by fitting both the transverse electric and the transverse magnetic apparent resistivity curves using a two-dimensional (2-D) forward modeling algorithm.
Across the batholith, the geoelectric model exhibits surface resistivity > 1000 Ω m, decreasing by an order of magnitude with depth. However, a very high conductivity has been modeled between depths of 12 and 32 km in an isolated block. This anomaly may be due to a zone of entrapped water.
In the Laguna Salada Basin on the western margin of the Salton Trough, the near-surface resistivities are of the order of tens of Ω m. In the center of the basin, at depths < 2.5 km, the resistivities fall to very low levels, probably owing to sediments saturated with saline water and with a high content of clays. These sediments are underlain by a resistive basement (500 Ω m) followed at depth by an 80 Ω m conductor at near 25-km depth. The deep conductor shallows towards the east, reflecting a thinning of the crust under the Mexicali Valley. The sounding measured closest to an intertrough granitic mountain range, the Sierra de los Cucapa, shows a complicated structure. The reduced thickness of the resistive crust beneath the Cucapa range may be geologically significant; for example, the MT data may be indicating that the Cucapa range is near the leading edge of a thrust sheet or probably shows the presence of three-dimensional (3-D) effects due to its finite length in the north-south direction.
The model for the Mexicali Valley near the center of the Salton Trough presents very low near-surface resistivities, some near 1 Ω m. In general, one sees approximately a horizontal interface between more and less conductive sediments at ~ 2-km depth. This contact deepens toward the center of the valley. The two layers correspond to clay-rich unconsolidated sediments saturated with saline water in the upper zone and consolidated sediments with limited pore space below. A systematic anisotropy present in all the soundings could be accounted for by the 3-D geometry of the Salton Trough which may enhance the electric field parallel to the structure.

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