Coseismic growth of basement-involved anticlines: The Northridge-Laramide connection

Seismogenic thrust faults in the Transverse Ranges of southern California may provide analogues for interpreting kinematic data from some Laramide anticlines in the southern Rocky Mountains. The 1994 Northridge earthquake (M_w 6.7) occurred on a southwest-dipping blind thrust fault beneath the Transverse Ranges and resulted in incremental growth of a basement-involved fault-propagation fold. Analyses of geodetic data and aftershock focal mechanisms show that deformation of the hanging-wall block consisted of two major components: (1) a large-scale elastic strain, characterized by horizontal shortening normal to the fault and horizontal lengthening parallel to the fault, that was imposed by reverse slip on the underlying blind thrust during the main shock, and (2) postseismic cataclastic flow with a geometry comparable to the coseismic strain, including horizontal, fault-parallel lengthening of the hanging-wall block, which was accommodated at least in part by distributed aftershock activity. Fault- and fold-parallel lengthening strains similar to those observed during the Northridge earthquake have been obtained by inversion of fault-slip data from some Laramide basement-involved anticlines. The Northridge analogue suggests that distributed brittle faulting of the crystalline basement may reflect aftershock deformation during incremental growth of the Laramide folds. If so, then the Laramide structures provide examples, now exposed by erosion, of the mature internal structure of the active, basement-involved folds in the Transverse Ranges.