Synmagmatic deformation patterns in the Old Woman Mountains, SE California

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. 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.