Relation of deformation and multiple intrusion in the Death Valley extended region, California, with implications for magma entrapment mechanism

Abstract

The crystalline core of the Black Mountains crustal section, Death Valley, California, exposes a tremendous volume of Miocene plutonic rock intruded at a depth of 10–13 km. Few plutons were intruded above ∼10 km prior to unroofing by tectonic denudation. In the eastern part of the core, a brittle detachment fault separates predominantly Miocene hanging wall strata from the midcrustal, Miocene (11.6 and ∼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 ∼8.7 Ma silicic plutonic complex. The younger silicic complex throughout the crystalline core exhibits few ductile deformational structures. Published thermal and barometric studies from both plutonic bodies indicate similar midcrustal (10–13 km) emplacement depths at ambient temperatures just above 300°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 similar emplacement depths but contrasting styles of deformation of the two plutonic bodies further suggests that entrapment may have been controlled by a high‐strength barrier represented by the brittle‐ductile transition. Late hypabyssal intrusions and associated volcanism are linked to diachronous rapid unroofing of the range block; all show a northwest 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 magma ascent by fracture exploitation. Rheological stratification of the crust may have played an important, if not major, role in trapping magmas in the middle crust in this area.