MYLONITE MELANGES IN AN EXTENSIONAL SHEAR-ZONE IN THE BADWATER TURTLEBACK OF DEATH VALLEY, CALIFORNIA

The turtlebacks of the northern Black Mountains of Death Valley, California are generally characterized as extensional shear-zones. Our investigation focuses on two east-west trending canyons in the Black Mountains that expose the footwall of the Badwater Turtleback. In addition to involving a variety of lithologies, the shear-zone contains smaller zones of mylonite melange that may harbor an area of unusually high strain and late-stage deformation.

We describe these smaller zones as melanges because they exhibit a wide variety of material and structures at the mesoscopic scale. The smaller zones also contain broken fragments from all other lithologies with varying orientations. They incorporate basement mylonitic gniess and pegmatitic boudins in a matrix of calcite and dolomite marble. We also see fragments of mafic dikes present in the melange that otherwise cut the mylonite melange. A high strain is inferred from the very fine grained matrix of calcite surrounding the fragments. Late-stage deformation is inferred from the melange, involving fragments from all other mylonitic rocks in the shear-zone. Strong foliation is observed in some areas, while other parts of the melange show little-to-no structure. In thin section, similar variation can be seen. The strong foliation of some areas may be an indicator of strain localization. At the mineral scale there are very fine-grained areas of quartz and calcite that appear dynamically recrystallized, while in other areas very coarse-grained calcite with twinning can be seen in abundance.

With our ongoing work we hope to better constrain the timing, rate and nature of the deformation within the shear-zone. Further work at the meso scale, including mapping supplemented by stereophotographic models, and micro-scale work, including electron back-scatter diffraction and SEM imaging, will help determine these constraints.