33-10 Deformation intensity adjacent to the San Andreas fault along the Parkfield syncline, California

Session: Going with the Shear - New Insights into Lithospheric Extensional and Strike-Slip Systems (Posters)

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Abstract:

The Parkfield syncline is a 25-km long fold northeast of the San Andreas fault near Parkfield, California. The fold has a steeply dipping southwest limb, a moderately dipping northeast limb, and an axial trace that is 10˚ counterclockwise from the San Andreas fault strike. The youngest folded unit is the Plio-Pleistocene Etchegoin Formation, which includes brown and blue sandstones, pebble conglomerates, and poorly exposed clay and siltstones. These rocks are exposed between 0.5 and 5 km from the San Andreas fault along the syncline.

Deformation bands are well developed in sandstones of the Etchegoin Formation, especially within blue sandstone lenses. These structures are observed as individual bands and as clusters of bands. In all locations along the fold, including nearly horizontal outcrops near the fold hinge, there are two sets of steeply dipping and mutually cross-cutting bands. The two populations define a conjugate set with a dihedral angle of 50˚ to 70˚. In lieu of observable kinematic indicators, we use this geometry to infer right-lateral strike-slip motion on the NNW-to-NS-striking set and left-lateral motion on the NE-striking set. Restoring bedding to horizontal reduces the consistency of deformation band geometries between outcrops, so we interpret that bands developed during and/or after folding.

We use scanlines to characterize the intensity of deformation at stations with varying distances from the San Andreas fault and in different lithologies. In blue sandstones between 3.5 to 5 km from the fault, the intensity is ~10 deformation bands per meter. At sites within 2 km from the San Andreas fault, this value increases to ~20 deformation bands per meter for blue sandstones, but is ~10 bands per meter for brown sandstones. In this near-fault area, two blue sandstone outcrops contain impressive meter-scale clusters where the intensities increase to 65 to 75 bands per meter. In all transects, the right- and left-lateral sets are equally well developed and band clusters account for at least 75% of the deformation bands in each transect. These structures are useful for characterizing the kinematics, intensity, and history of deformation along the Parkfield syncline. These data demonstrate that, despite forming within contractional map-scale structures in the transpressional San Andreas fault system, outcrop-scale structures still record a strong influence of the strike-slip component of deformation.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

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