DISTRIBUTED BRITTLE SLIP DEFORMATION IN THE DEATH VALLEY TURTLEBACK NEAR MORMON POINT

We address strain characteristics within major antiformal corrugations, or turtlebacks, below crustal-scale detachments for the Death Valley Turtlebacks (Black Mountains). These turtlebacks are associated with a well-known and much debated corrugated extension-transtensional detachment zone that separates a footwall of lower-middle crustal rocks from hanging wall perched basins. The detachment zone is dominated by brittle cataclasis that locally overprints plasticly formed mylonites. Sheep Canyon near Mormon Point provides provides a more deeply incised and continuous outcrop transect into the footwall of one of the turtlebacks starting from the detachment exposed near its mouth. Footwall rocks consist of host Precambrian basement (granitic and mafic gneisses, marbles) with a significant volume of Miocene intrusives. The syntectonic intrusive suite includes hypabyssal dikes and more irregular intrusive bodies, both of which show variable but significant degrees of brittle deformation within and below the detachment indicating coeval faulting, magmatism, and mineralization. Significant distributed brittle slip deformation (DBSD) features were found to be pervasive up to a point that is a kilometer below the projected detachment level. DBSD surfaces display epidote and chlorite mineralization, and several centimeter or less thick fault rocks with observed offsets in the outcrops in the mm to tens of meters range. In The footwall DBSD kinematics are more complex than those in the detachment, and are locally variable but include expected dextral and extensional components. Three or more slip plane preferred orientations occur in an outcrop, forming complex networks. Preliminary work with scaled and oriented digital outcrop models indicates that the shear fracture and fault density significantly exceeds 5 m²/m³. This is a minimum estimate due to the challenge of documenting the numerous smaller slip surfaces. While the 3D rock strain associated with DBSD is unconstrained, component shear strain estimates typically exceed 1. Such footwall strain magnitudes are sufficient to significantly modify overlying detachment and basin geometries, perhaps contributing to corrugation development by bulk brittle flow. We will continue the work on footwall DBSD strain in the Death Valley and other areas.