CONSTRAINTS ON EXTENSION-RELATED PLUTONISM FROM MODELING OF THE COLORADO RIVER GRAVITY HIGH

The Colorado River gravity high is a positive gravity anomaly coincide nt in location with the Colorado River extensional corridor, a zone of extreme middle Tertiary crustal extension in the southern Basin and R ange. The high extends for approximate to 150 km along the west side o f the Colorado River and has a magnitude of 10-20 mGal above the regio nal level, The source of this gravity high has previously been interpr eted as either mafic intrusion or dense material added to the middle c rust via lateral flow during extension. Detailed modeling of the gravi ty high is now possible due to acquisition of new gravity data that th oroughly define the shape of the anomaly at and around its highest poi nt. The peak of the high is 20 mGal higher than the ambient level of t he anomaly, and is in the southern Sacramento Mountains, California. A small (approximate to 50 km(2)) middle Tertiary pluton composed of mi xed and mingled diorite and granite crops out in the southern Sacramen to 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/cm(3) and a mean de nsity (including measurements of mingled diorite and granite) of 2.75 g/cm(3). This pluton is interpreted to be the minor surface expression of a larger, deeper source for the Colorado River gravity high. Forwa rd modeling of the Bouguer gravity data is based on surface structure and density contrasts documented by recent geologic mapping and on int erpretations of the deeper levels of the crust from the seismic experi ments of PACE, CALCRUST, and COCORP, Interpretation of a regional prof ile crossing the peak of the gravity high yielded several possibilitie s, 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 offse t or ''decapitation'' by a detachment fault system, In three dimension s, the Colorado River gravity high can be interpreted as the geophysic al signature of an approximate to 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 hor izontally distributed underplated material. The inflated tower-middle crust, interpreted from seismic data, may be due to either (1) lateral material flow into the zone of maximum extension, which mixed and min gled with intrusions from the mantle, or (2) pending of some intruded material between the lower and middle crust. In either case, the ultim ate cause of the gravity high is intrusion of mantle-derived melt inco rporated with melt derived from continental crust, which is now expose d as mixed and mingled granite and diorite in the southern Sacramento Mountains.