Description
From 1990 to 1992, as part of the Pacific-Arizona Crustal Experiment (PACE), researchers from San Diego State University and the University of California, Riverside, occupied 16 magnetotelluric (MT) sites along a 70+ km portion of the transect connecting sites in the eastern Salton trough and the Colorado River, and at 11 sites in the nearby Milpitas Wash. Analyses and modeling of the MT data resolved several important geologic features of the area: (1) the lithosphere of the Chocolate Mountains is resistive to at least 100 km depth, (2) the crust of the Milpitas Wash is conductive across its center, to the depth of resolvability, and flanked by resistive rocks in the Midway and Little Mule Mountains, (3) the middle crust of the Chocolate Mountains contains conductive bodies of tabular and filamental prisms, and (4) the crust and mantle beneath the eastern Salton Trough sites is conductive to the depth of resolvability. Crustal resistivities in the Chocolate Mountains crust typically are > 320 ohm-m and are highly resistive in the deep crust. The uppermost 1 km in the Chocolate Mountains crust spans from < 10 ohm-m to nearly 100 ohm-m, representing fine elastics and weathered crystalline rocks, and the lowest resistivity values representing fault-related mineralizations, which are known to have economic importance. The geoelectric models for the Milpitas Wash show a basin geometry, with elastic and volcaniclastic deposits marked by resistivities of < 100 ohm-m to ~4 km depth. Seismic reflection images this geometry as sediment-filled troughs over steeply tilted basement slabs, separated by normal faults which merge at depth with a roughly horizontal zone of detachment related ductile deformation. The mid-crustal zone of high conductivity coincides with the top detachment deformation. Lower resistivity in the Milpitas Wash in comparison to the Chocolate Mountains for similar crustal depths and lithologies is due to the more extreme tectonism in the Milpitas Wash during the Miocene. The Chocolate Mountains/Milpitas Wash area was subjected to great compression, uplift, and extension in the Cretaceous and early Paleogene. A similar tectonic history has been inferred for the central and southern Coast Ranges, where subducted oceanic crust has underplated attenuated continental crust of various basement terranes. Geoelectrical and seismic studies of the Coast Ranges show significant parallels to the Chocolate Mountains, of which the resistive lower crust and mantle do not match up with any presently adjacent regions, or that of the southern Basin and Range province in general. The resistive portion of the lithosphere (i.e., the eastern three-fourths) of the study area is interpreted to represent a relic, land-locked terrane sliver of highly retrograde metamorphosed and deformed Franciscan-like accretionary wedge, underlain by oceanic lithosphere. This was likely tectonically underplated to the overriding plate, uplifted, and partially unroofed during the late Cretaceous-early Paleogene. Oligocene-Miocene extensional and wrench tectonism has caused relative uplift of these deep crustal units, and subsequent subsidence of the Milpitas Wash.
