Abstract

The Colorado River gravity high is a positive gravity anomaly coincident in location with the Colorado River extensional corridor, a zone of extreme middle Tertiary crustal extension in the southern Basin and Range. The high extends for ≈150 km along the west side of the Colorado River and has a magnitude of 10–20 mGal above the regional level. The source of this gravity high has previously been interpreted as either mafic intrusion or dense material added to the middle crust via lateral flow during extension. Detailed modeling of the gravity high is now possible due to acquisition of new gravity data that thoroughly define the shape of the anomaly at and around its highest point. The peak of the high is 20 mGal higher than the ambient level of the anomaly, and is in the southern Sacramento Mountains, California.

A small (≈50 km2) middle Tertiary pluton composed of mixed and mingled diorite and granite crops out in the southern Sacramento Mountains at the location of the peak of the gravity high. Surface density measurements indicate that the Miocene diorite has the highest density in this area: a maximum density of 2.91 g/cm3 and a mean density (including measurements of mingled diorite and granite) of 2.75 g/cm3. This pluton is interpreted to be the minor surface expression of a larger, deeper source for the Colorado River gravity high.

Forward modeling of the Bouguer gravity data is based on surface structure and density contrasts documented by recent geologic mapping and on interpretations of the deeper levels of the crust from the seismic experiments of PACE, CALCRUST, and COCORP. Interpretation of a regional profile crossing the peak of the gravity high yielded several possibilities, from which one preferred model was chosen on the basis of geologic data. The preferred model is that of intrusion of mantle-derived melt which acquired an upward-decreasing density distribution due to mixing and mingling with magma derived from the continental crust. The magma conduit is linked through the entire crust and shows no sign of offset or “decapitation” by a detachment fault system. In three dimensions, the Colorado River gravity high can be interpreted as the geophysical signature of an ≈150-km-long, 10- to 20-km-wide zone of intrusion into the area of maximum crustal extension. The magmatic contribution to extension in this region was primarily in the form of large, subvertical, dike-like intrusions, rather than sill-like or horizontally distributed underplated material. The inflated lower-middle crust, interpreted from seismic data, may be due to either (1) lateral material flow into the zone of maximum extension, which mixed and mingled with intrusions from the mantle, or (2) ponding of some intruded material between the lower and middle crust. In either case, the ultimate cause of the gravity high is intrusion of mantle-derived melt incorporated with melt derived from continental crust, which is now exposed as mixed and mingled granite and diorite in the southern Sacramento Mountains.

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