Synmagmatic deformation patterns in the Old Woman Mountains, SE California

Abstract

Synmagmatic structures within granitic plutons may provide insight into the interplay between plutonism and regional deformation at mid-crustal levels. Within the Late Cretaceous Old Woman pluton (Mojave Desert, SE California), synmagmatic structures include; magmatic fabrics parallel to wall rock foliations, melt-filled shear zones, folded gneissic xenoliths and schlieren with axial planar magmatic fabric. Deformation continued below granite solidus temperatures in shear zones on the pluton margins. These structures permit reconstruction of a sheet-like pluton that was emplaced under regional extension. Taken together with published thermochronometric data the synmagmatic structures constrain the role played by magmatic events in the collapse of a thickened crustal section. The post-plutonic cooling history suggests rapid uplift and denudation ending by 65 Ma. We argue that extensional deformation overlapped and post-dated pluton emplacement and may have contributed to tectonic unroofing. We suggest that magma emplacement can trigger subsequent extensional collapse in overthickened crustal blocks.