'American Association of Petroleum Geologists AAPG/Datapages'
Abstract
More than 3,000 gravity observations in the Northern Gulf province, including an underwater
gravity survey of the Salton Sea, show the over-all trend of isogal contours to be northwest, parallel to
the tectonic pattern dominated by the San Andreas fault system. Contours northeast of the trough
trend east, probably reflecting Transverse Range structures in this area. A prominent and linear gradient
of 5 mgal/km marks the Banning-Mission Creek fault in the Coachella Valley but dies out southeastward
at about the same point the surface trace disappears. The San Jacinto fault zone is characterized by a
series of maxima and minima that tend to confirm continuity of this fault zone to the Gulf of California.
A 15-20 mgal maximum over the Obsidian Buttes suggests a large anomalous mass at depth, or may be
related to contemporaneous metamorphism of the Tertiary sedimentary section that has recently been
observed in nearby steam wells. The regional gravity gradient indicates a crustal thickening northwest
from the Gulf of California; inferred crustal thicknesses are 32 km at the International Border and 40 km
at San Gorgonio Pass. Ten seismic refraction profiles in the Imperial and Coachella Valleys indicate
several throughgoing velocity zones, but we are unable to correlate these with known stratigraphic units.
The maximum thickness of sediments in the trough appears to be about 6.4 km (21,000 ft) just south
of the International Border, with basement becoming shallower both to the north and south. The Salton
trough has many geophysical and structural similarities to the Dead Sea rift, but the markedly en echelon
pattern of major faults in the Salton trough and Gulf of California appears unique. A particular problem
is presented by their orientation, which would suggest left-lateral displacement across the zone rather
than the right-lateral displacement that is known to characterize at least the northern end of the province