THE CONSTANT-STRENGTH MODEL OF OVERPRESSURED ACCRETIONARY WEDGES: PRESENT-DAY STRENGTH-DEPTH OBSERVATIONS TO 14KM IN THE WESTERN TAIWAN ACCRETIONARY ORIGEN

Classic formulations of critical-taper wedge mechanics have commonly assumed the Brace-Kohlstedt (1980) strength-depth profile. However, this view assumes hydrostatic pore-fluid pressure, which does not hold in weak overpressured tectonic environments, as is extensively documented from petroleum borehole and seismic-velocity measurements in sedimentary basins. Evidence of overpressured fluids is also widely reported from accretionary wedges in both active and passive margins, such as Barbados and the Niger Delta. Nevertheless, the extent and nature of fluid-pressure weakening beyond drilling depths in accretionary complexes remains poorly documented and controversial given the lack of deeper direct measurement of stress and fluid pressure within regions that are actively deforming. Here we present new observations that limiting strength is regionally low (~40MPa) and nearly constant down through the brittle-ductile transition in the active Taiwan accretionary origin, in agreement with simple overpressure predictions. We determine a high-resolution 1D strength-depth profile in western Taiwan using seismologic stress-inversion methodologies based on perturbations of the stress field by the M7.6 Chi-Chi thrust-belt earthquake. This regional 1D profile from 3.5-14 km depth agrees with petroleum borehole fluid pressure measurements to 5.5 km depth and shows a reduction of integrated crustal strength by a factor of 4–5 relative to hydrostatic predictions. The one notable exception to the observation of constant strength with depth is an excursion to much higher (>150MPa) and heterogeneous stresses within ±1.5km of the Taiwan main subduction interface at ~10km depth, which lies close to the 1D brittle-ductile transition, beyond which limiting strength decays to ~10MPa. We suggest that a constant strength-depth profile may be more appropriate in deep overpressured settings than the common Brace-Kohlstedt hydrostatic prediction and discuss possible origins of the observed strength excursion.