Growth of Wheeler Ridge anticline, California: geomorphic evidence for fault-bend folding behaviour during earthquakes

Unique landforms, which we term fold-growth terraces and alluvial fan ridges, are interpreted to be produced by kink-band migration of the front limb of Wheeler Ridge anticline, an active fault-bend fold in the southern San Joaquin Valley. Based on a folding-related model, these features form by onlap of alluvium across an active axial surface along the tip of a thrust-wedge structure. Alluvial fan ridges record the sense of slip on the underlying blind thrust, which is composed of both thrust and left-lateral components. Our model predicts formation of fold-growth terraces by upward propagation of an active axial surface through onlapped alluvium during a folding ‘event’ to produce a prism of translated but otherwise undeformed sediment. Alternative hypotheses for the terraces include flexural slip faulting, solifluction and erosive processes. The width of fold limb between adjacent terraces is equivalent to fault displacement at depth, which we interpret to be related to slip caused by earthquakes on the underlying blind thrust. Earthquake magnitudes derived from the terraces are much larger than expected for the length of the active fold and thrust belt, and may indicate a kinematic link with the San Andreas fault that drives shortening in the region.