RELATION OF DEFORMATION AND MULTIPLE INTRUSION IN THE DEATH-VALLEY EXTENDED REGION, CALIFORNIA, WITH IMPLICATIONS FOR MAGMA ENTRAPMENT MECHANISM

The crystalline core of the Black Mountains crustal section, Death Val ley, California, exposes a tremendous volume of Miocene plutonic rock intruded at a depth of 10-13 lan. Few plutons were intruded above simi lar to 10 lan prior to unroofing by tectonic denudation. In the easter n part of the core, a brittle detachment fault separates predominantly Miocene hanging wall strata from the midcrustal, Miocene (11.6 and si milar to 8.7 Ma), plutonic complex and Early Proterozoic basement. In the west, mylonitic lineations and foliations, locally well-developed within the 11.6 Ma intermediate-mafic Willow Spring pluton, are cut by dikes which emanate from an similar to 8.7 Ma silicic plutonic comple x. The younger silicic complex throughout the crystalline core exhibit s few ductile deformational structures. Published thermal and barometr ic studies from both plutonic bodies indicate similar midcrustal (10-1 3 lan) emplacement depths at ambient temperatures just above 300 degre es C. The significant difference in densities; of these magmas argues against a density control for magma entrapment. Also, the country rock above and below the plutonic complex contains no apparent differences (from field observations) that would suggest a change in density. The depth of entrapment corresponds well with the expected depth for the crustal strength maximum determined from laboratory experiments. The s imilar emplacement depths but contrasting styles of deformation of the two plutonic bodies further suggests that entrapment may have been co ntrolled by a high-strength barrier represented by the brittle-ductile transition. Late hypabyssal intrusions and associated volcanism are l inked to diachronous rapid unroofing of the range block; all show a no rthwest progression paralleling the regional extension direction. Thus when migration of magma through the high-strength barrier did occur, it was apparently related to increased strain rates which allowed magm a ascent by fracture exploitation. Rheological stratification of the c rust may have played an important, if not major, role in trapping magm as in the middle crust in this area.