Geomorphic Criteria for Establishing the Kinematics of Active Fault-Bend Folds

Karl J. Mueller

Under Review 2001, SCEC Contribution #428

Geomorphic features produced by erosional and deposional processes are used as criteria for identifying recent growth on active fault bend folds. Better understanding of these features is driven by the societal need for characterizing blind thrust earthquake hazards beneath active folds in southern California. Erosional features such as stream drainage networks and emergent marine terraces allow areas of recent folding to be identified where active axial surfaces lie at the top of fold limbs. Folded drainage networks on the San Joaquin Hills, in the southern Los Angeles Basin, are used to differentiate active vs. inactive axial surfaces, constraining subsurface blind thrust geometry in a region with little subsurface control. Emergent marine terraces, besides providing an absolute reference frame for uplift are used to confirm the location of active axial surfaces identified by the folded drainage networks. Depositional features, such as alluvial fans and shingled fan ridges are used to identify active axial surfaces located at the base of fold limbs. Kinematic information provided by shingled fan ridges, on Wheeler Ridge anticline, a wedge thrust structure in the southern San Joaquin Valley, allow three dimensional folding vectors to be resolved. For this fold the ratio of dip to strike slip is ~ 5.6.

Mueller, K. J. (2001). Geomorphic Criteria for Establishing the Kinematics of Active Fault-Bend Folds. Journal of Structural Geology, (under review).