RHEOLOGICAL CONTROLS ON DAMAGE INTENSITY, SAN JACINTO FAULT, SOUTHERN CALIFORNIA

We are studying two sites along the San Jacinto fault (SJF), southern California, offset by ~ 19 km of dextral displacement. Each site represents portions of a dismembered positive flower structure. At the first site, Wellman Ranch, damage within sandstones of the Pleistocene Bautista Fm are characterized by heterogeneously developed intragranular fragmented quartz grains. Such grains form a jig-saw puzzle texture and reveal little evidence for shearing; hence, they represent pulverization at the grain scale. We measured damage intensity as a function of the number of fractures/grain. The number of quartz grains containing >10 fractures/grain was greatest in sandstones with <18% matrix. Damage intensity measured in this way did not correlate with spatial position relative to the fault core, and did not vary with porosity. Notably, intragranular fractures are opened and likely formed from dilation. The above characteristics suggest that intragranular fracturing was controlled primarily by Hertzian mechanics, and that variations in the proportions of matrix is a primary rheological control of intragranular fracturing. This interpretation is consistent with the observation that outside the influence of the SJF, sandstones within the Bautista contain no textural evidence for intragranular fractures. Moreover, estimates of confining pressures at the Wellman site are ~6 MPa, and estimated exhumation depths is ~220 m. Hence, we favor the idea that at low confining pressures, shear wave stress loading generated by passing earthquake ruptures is the primary cause of the observed intragranular fracturing. In order to evaluate whether or not such features exist elsewhere along the SJF we have started a detailed field and laboratory study of the Bautista sandstones at the second site, north of Hog Lake. We will present the results of this work at the poster session.