Inferring stress directions from naturally deformed granular materials: Microstructural analysis of deformation bands near the San Andreas fault in Parkfield, California

Amberly Kroha, Carleton College
Sarah Titus, Carleton College
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Abstract

Deformation bands are well-developed in portions of the Etchegoin formation near Parkfield, California. The rocks are 1-2 km northeast of the San Andreas fault and are deformed within the Parkfield syncline. The deformation bands form in three sets: an early set that strikes WNW and dips moderately to the north and two later sets that are mutually cross cutting, strike NNW and NE, and dip steeply in two directions for each set. If they formed in their in-situ orientations, these later two sets of bands have strike-slip geometries. If they predate folding of the Parkfield syncline, they have normal geometries. Both kinematic conditions could be consistent with the broader tectonic context. There are few slip surfaces or other kinematic indicators that can be used to confirm the true band kinematics, nor are there useful cross-cutting relationships that suggest when the bands formed in relation to the syncline.

We use microstructural data from horizontally oriented thin sections to constrain the band kinematics and their timing of development from sites along the Parkfield syncline. Specifically, we digitize and measure the directions of concave-convex contacts and microfractures and we infer approximately contact-perpendicular, fracture-parallel force chains in sandstones and conglomerates. All types of microstructural data yield unimodal distributions that consistently trend N or NNE, which we infer as the maximum horizontal stress direction.

There are methodological and geological implications of these results. From a methodological perspective, we conclude that concave-convex contacts and microfractures are reliable paleostress indicators by themselves, without using them to infer force chains, because of the consistency of directions across data types. This is advantageous—digitizing directions from these datasets is less subjective and more efficient than using them to infer force chains. Further, these types of data may be amenable to digital image analysis techniques that would greatly decrease the time required for data collection. From a geological perspective, the N to NNE directions for the maximum horizontal stress are consistent with strike-slip rather than normal deformation band kinematics. The consistency from different sites along the syncline also indicates that bands form after folding, because sites from the flat-lying fold hinge and steeply dipping limbs give the same directions. Taken together, the microstructural analysis suggests a maximum horizontal stress direction at a moderate angle relative to the San Andreas fault near Parkfield.

Session

Deformation in the upper crust