GROWTH OF FAULT-CORED ANTICLINES BY FLEXURAL SLIP FOLDING (Invited Presentation)

Blind faults underlying actively growing anticlines are a significant source of global seismic hazard. In many cases we have observational constraints on the shallow fold geometry, but the main difficulty is identifying the geometry of the deep seismogenic fault within and below the fold. In practice, blind fault geometry and estimates of fault slip rates need to be inferred indirectly through models. It is often assumed that crustal-scale anticlines grow primarily by slip on underlying faults and the geometry of the fold reflects directly the geometry of the causative fault. The implicit or explicit assumption is that distributed off-fault deformation is negligible. On the other hand, we know from the relatively mature study of multilayer folding that initial perturbations in strata without a fault subjected to horizontal shortening can be significantly amplified through buckling of the layers accommodated by flexural slip. We have developed boundary element model of fault-related folding in viscoelastic layers within frictional contact to investigate the physical conditions under which fault-cored anticlines form. Under this assumption, the rock is assumed to behave instantaneously as an elastic solid containing a random distribution of cracks, and stress is relaxed over time due to penetrative pressure solution creep. We show for a fault-tip fold geometry that the buckling of sedimentary layering can significantly amplify and localize folding. We also show that under zero or low layer friction, fault-tip folds are rounded and nearly concentric in shape whereas under higher friction, folds grow inter nearly flat-topped anticlines by kink bands forming both limbs. Of particular importance to seismic hazard, we find that distributed deformation in the form of flexural slip is far from negligible. As the fold grows, off-fault deformation increases steadily until more than 50% of the deformation is distributed through the fold. We compare model results with data from Kettleman Hills Anticline in central Calfiornia. We show that the shallow fold geometry is matched well with a fault-tip fold model with low friction at layer contacts. The model reproduces well both the geometry of the fold and growth strata and surface deformation recorded with leveling data from the 1985 Mw=6.1 Kettleman Hills earthquake.