SYSTEMATICALLY FRACTURED CLASTS IN THE TITUS CANYON FORMATION: EVIDENCE FOR LATE-STAGE BRITTLE DEFORMATION IN THE BOUNDARY CANYON DETACHMENT SYSTEM

Pervasive sub-parallel, clast-scale fractures occur within conglomerates of Eocene-Miocene synextensional sedimentary strata of the Titus Canyon Formation in the Grapevine and Funeral Mountains of southwestern Nevada and southeastern California. Clasts show pressure solution depressions, cross-cut by subsequent point-contact deformation and sub-parallel fractures, which document transition from ductile to brittle deformation and which constrain conditions that produced the unusually systematic brittle deformation. We propose that clast deformation occurs in 2 phases. Phase 1 involves pressure solution, occurring at relatively low temperatures of 90±10°C and at depths of approximately 2-5 km. With rapid uplift and erosion, Phase 2 conditions emerge, leading to sub-parallel fractures. Modeling clasts as Eshelby inclusions with an internally uniform stress field show that sub-parallel fracturing occurs with minimum confining pressure between 2 to 30 MPa. This corresponds to relatively shallow depths of less than 1250 meters. With progressive deformation from the same remote stress during Phase 2 conditions, shear fractures coalesce from sub-parallel fractures. Through kinematic analysis, we infer minimum compressive stress in a NW/SE orientation and the maximum compressive stress near 225°, 50° SW. Investigation of units below the Titus Canyon Formation show slickenlines with similar orientations that cross-cut ductile fabrics below regional detachments. Units above the Titus Canyon Formation suggest that conditions leading to sub-parallel fractures were either no longer present by Late Miocene time or that younger units were deposited during deformation, but were not exposed to the appropriate conditions required to induce fracturing. Altogether, these data suggest a common source for the stress field responsible for the deformation. Our inferences of the stress field orient the maximum compressive stress near normal to the orientation of sub-parallel regional detachments. This possibly indicates the influence of near-field stresses associated with brittle rupture events on the fault as it was exhumed, or some other driving stress. We propose the Boundary Canyon detachment fault and Monarch Spring fault as likely candidate sources of this stress field.