Heterogeneous Coulomb wedges: Influence of fluid pressure, porosity, and application to the Hikurangi subduction margin, New Zealand

摘要

Despite the widespread use of critical Coulomb wedge models, the effects of internal heterogeneities have been largely overlooked. Here we consider how spatial variations in pore pressure and porosity within an accretionary prism affect predictions of shear traction along the base of the prism and the internal slip surface geometry. Compared to Coulomb wedge models that use depth-averaged parameter values, the basal shear traction will be lower if (i) the pore pressure ratio increases monotonically with depth, (ii) the porosity decreases monotonically with depth, or (iii) the internal friction coefficient decreases monotonically with depth. The first two conditions can be considered first-order descriptions of many natural settings. To illustrate their effects, we consider two specific functional forms: a linear increase in the pore pressure ratio and an exponential decrease in porosity with depth within the prism. Both cases separately can be completely accounted for by simple modifications to the solution for a homogeneous Coulomb wedge. We show that including heterogeneity in the pore pressure ratio and the porosity, together or separately, can lead to large reductions in the basal shear traction, compared to wedges with relevant depth-averaged parameters. Finally, we apply our results to the Hikurangi margin at the North Island of New Zealand and show that lateral increases in the pore pressure ratio lead to further reductions in basal shear traction.