Ea. Campbell et Be. John, CONSTRAINTS ON EXTENSION-RELATED PLUTONISM FROM MODELING OF THE COLORADO RIVER GRAVITY HIGH, Geological Society of America bulletin, 108(10), 1996, pp. 1242-1255
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.