FORMATION OF THE STRONG WHIPPLE DETACHMENT FAULT, CALIFORNIA, IN THE BRITTLE-PLASTIC TRANSITION

The Whipple detachment fault (WDF) formed progressively at ~9-10 km depth and ~300-400°C in the zone of maximum crustal strength (brittle-plastic transition), by linking of small-scale analogs (minidetachments). Minidetachments formed by brittle slip and cataclasis along pre-existing mylonitic shear (C) planes when epidote crystallized and strengthened the mylonites (Selverstone et al, 2012). Most C planes were coated with chlorite so initial minidetachment friction was low (~0.3), but newly formed quartz+feldspar-dominated cataclasite and ultracataclasite probably had "Byerlee" friction of 0.6-0.85. Fluid pressure dropped from lithostatic to hydrostatic upon embrittlement, and reidel shears formed. Thus, embrittlement strengthened the minidetachments and aided in keeping the brittle-plastic transition strong. Accumulating slip, during upward propagation as the footwall was exhumed, smoothed the WDF and contributed to the fault core and surrounding damage zone.

Minidetachments, and thus the evolving WDF, were ~45° from the maximum principal stress (S1), so slipped under maximum ambient shear stress. Using late-mylonitic, steady-state differential stress (157 MPa) from quartz paleopiezometry (Behr and Platt, 2011, 2013) shows that the WDF formed under applied shear stress of ~80 MPa; thus, the WDF was a strong fault. A Mohr diagram, derived from these numbers and the depth is presented. Initial WDF dip, constrained by experimental ranges of static and internal friction, was ~16-26°, consistent with the angle between the mylonite front and the WDF. The maximum principal stress when the WDF formed was plunging ~60-70° NE. Rotation of S1 away from vertical was probably due to asymmetric tectonic boundary conditions applied to the base of the continental lithosphere (probably Orocopia Schist underplated after Laramide subduction erosion had removed all or most continental mantle lithosphere). Stress rotation thus was caused by (a) vertical buoyancy (Spencer and Chase, 1989) due to upward asthenospheric flow into an opening slab window and/or (b) basal shear traction (Yin, 1989) due to middle or lower crustal top-NE mylonitic flow. The model probably applies directly to other lower Colorado River core complexes; aspects are likely relevant to other strong detachments.