Abstract

Investigation of the tectonic geomorphology of active folding over buried reverse faults at the San Emigdio Mountain front, southern San Joaquin Valley, California, provides insight concerning the tectonic and geomorphic development of mountain fronts produced by active folding and faulting. Monoclinally flexed gravels with dips as great as 50° and a minimum age of about 65 ka provide evidence of late Pleistocene deformation at the active range front. Studies of the surface folding of alluvial fans and fluvial terraces indicate a Holocene vertical deformation rate of 1.9–3.0 m/k.y. at the active range front and 0.8–1.3 m/k.y. ∼2 km basinward. Geomorphic evidence also indicates that the locus of active folding and vertical deformation along the northern flank of the San Emigdio Mountains has migrated and continues to migrate basinward. This evidence includes a relict mountain front, now within the uplifted block, 5 km from the present active mountain front, and the existence of recently initiated folds in the active alluvial fan 2 km basinward from the mountain front. Northward migration of tectonic activity results in the progressive widening of the uplifted block as the location of active folding moves basinward. This migration of tectonic activity appears to occur through the onset and subsequent increase of vertical deformation along more northerly folds and faults accompanied by reduction and eventual cessation of activity along the older, more southerly structures.

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