Cataclastic Strain in the Footwall Rocks of the Mormon Point Turtleback, Death Valley

Abstract We explore an example of distributed brittle deformation in a core complex exposed in the Black Mountains of Death Valley in California, USA, and distinguish between cataclastic flow, common within fault zones, and a more general and inclusive phenomenon of cataclastic strain. In canyon traverses that cut hundreds of meters into footwall rocks within the Mormon Point turtleback slip surfaces are pervasive at the outcrop scale. Several relationships indicate most of the slip surfaces formed during and after turtleback formation. With slip surface densities typically greater than 1 per meter traverse and offsets from centimeters to meters, crude minimum estimates of bulk shear strain angle components range from ~10 to 60°, indicating a non-rigid footwall. While the kinematics are generally consistent with the established dextral transtensional environment, they vary considerably spatially. Local preferred orientations vary from shallowly to steeply dipping surfaces with the entire range of dip- to strike-slip movement. The variation suggests a domainal character to the cataclastic strain at outcrop and cliff-side scales. Internal strain within larger blocks bounded by larger-magnitude slip surfaces may be accommodated by distributed lower-magnitude slip. Earlier ductile fabrics in syn-kinematic intrusive rocks indicate a ductile to brittle transition in footwall deformation consistent with the known exhumation history. In Sheep Canyon, a marble tectonite zone separates footwall rocks of the Copper Canyon turtleback to the northeast with significantly less cataclastic strain from those of the Mormon Point turtleback to the southwest which have an abundance. Such spatial variation of footwall cataclastic strain as a function of core complex geometry and lithology may be an important phenomenon in some core complexes and in other settings and is worth further investigation.