Modeling of active anticlines and associated blind thrusts in central California.

摘要

Recent literature investigating active folding indicates that crustal-scale anticlines grow primarily through slip on underlying faults. Such studies use the geometry and uplift rates of active fault-related folds to infer fault slip rate based upon an assumed kinematic relationship between fault slip and particle motion in the surrounding crust. Our method focuses on the mechanics of deformation in order to examine the development of fold growth through time from a physical perspective using a boundary element model of flexural slip folding called BEAFS (Boundary Element Analysis of Flexural Slip). In many cases, the shallow geometry (<5km) of natural folds are well constrained by subsurface data, however, the geometry of the causative blind thrust faults is often not well imaged by existing geophysical methods. By comparing our numerical simulations with published subsurface and geomorphic data on naturally occurring folds we can determine the extent to which physical parameters and mechanisms control fold evolution.;We present model results for the underlying faults at Kettleman Hills South Dome and Kettleman Hills North Dome in the San Joaquin Valley of Central California. Blind thrust faults associated with actively growing anticlines pose a significant global seismic hazard. Thus, these anticlines are of particular interest as they are the near the sites of two recent earthquake events---a Mw=6.5 earthquake in 1983 at Coalinga and a Mw=6.1 in 1985 at Kettleman Hills North Dome. We compare our fault geometries, fault slip rates, and uplift rates determined from our numerical models to published seismic data and reconstructions of these folds and find that the underlying fault can be modeled by a simple ramp-flat-ramp geometry. Our models suggest that the mechanism of folding for Kettleman Hills is primarily due to buckling, with our models requiring very low friction between layer contacts in order to encourage slip.;From our models, the structure at Kettleman Hills North Dome is younger and shows fold growth during earthquakes that is aligned along the forelimb and away from the fold axis. In both cases, we find evidence for fold growth due to fault slip, as well as through buckling. Slip along the fault ramps at the South Dome is determined using BEAFS to be ~1 mm/yr for the South Dome and ~0.6 mm/yr at the North Dome. For the South Dome, the far-field slip rate along the detachment is about 2.5 mm/yr and 0.8 mm/yr at the North Dome. In both cases, this far-field slip rate is less than the slip along the ramp. Finally, we present an examination of parameters that have the greatest influence on our models---namely layer friction, fault burial depth, fault geometry, and mechanical layering---and examine the effect of deposition rate on the growth strata of each anticline.