A CASE AGAINST THE ‘BRITTLE-DUCTILE' DICHOTOMY – CONTEMPORANEOUS DISCRETE AND DISTRIBUTED DEFORMATION IN THE BORDERLANDS OF THE SAN ANDREAS FAULT, DURMID HILL, CA

East of the Salton Sea, Pleistocene strata of the Brawley Formation of Kirby and others (2007) are exposed in the Durmid Hill structural culmination adjacent to the San Andreas fault (SAF). Broadly, the culmination is a doubly-plunging, asymmetric anticlinorium with a highly-strained NE side near the SAF and a less-strained SW side farther from it. Mappable structures here are (1) step-shaped, thrust faults that cut strata at low angles to bedding, (2) fault-bend and fault-tip folds associated with thrusts, (3) detachment folds, (4) oblique-slip faults, some with stepped shapes, that cut strata at moderate to high angles to bedding, (5) folds associated with oblique-slip faults, and (6) boudined beds. Mappable deformation is accommodated by (1) joints and veins in sandstones and ash layers, (2) deformation bands and breccia zones in sandstones, (3) arrays of small faults in sandstones, siltstones, and ash layers, (4) diffusive mass transfer in gypsiferous shales, and (5) particulate flow in massive ash layers. Cross-cutting and overprinting relationships indicate that all structures contribute to dextral shearing parallel to the SAF and flattening normal to it. Of particular interest are late, map-scale zones of sinistral shearing that trend 15-35° to the SAF. Sinistral zones have orientations comparable to Riedel X shears, but they are not discrete faults and no other macroscopic Riedel shears occur here. The late origin, diffuse character, and orientation of the sinistral zones are consistent with formation parallel to directions of maximum sinistral shearing within a zone of distributed, incremental general shearing.

With widespread jointing, fracturing, veining, and faulting, one could say deformation here was ‘brittle.’ With shearing by diffusive mass transfer in shales and particulate flow in ash layers, one could say deformation here was ‘ductile.’ Do the arrays of fractures, veins, and faults in zones of distributed sinistral shearing support brittle or ductile interpretation? Rather than deciding whether to call deformation ‘brittle’ or ‘ductile,’ describing joints, fractures, and faults discrete structures and folds, shear zones, and fabrics distributed structures enables geologists to focus on the rheological implications of structures and microstructures not how to name them.