Regional-Scale Low-Angle Normal Fault Friction and Cohesion Constrained From Mohr-Coulomb Models of Active and Abandoned Range-Front Faults in Papua New Guinea

Low-angle normal faults (LANFs) are poorly understood, as their slip at <30° dips appears inconsistent with Byerlee friction and Andersonian stresses. Rider blocks are fault slices formed when the uppermost part of a LANF is abandoned in favor of a new, steeper fault. Mohr-Coulomb modeling of rider blocks can constrain fault frictional and cohesive strength provided known fault geometries. Analytical modeling shows that LANF viability depends on fault geometry and on the frictional and cohesive strength of the fault relative to that of the surrounding crust. Modeling of the Gwoira rider block, located atop the Mai'iu LANF, southeastern Papua New Guinea, indicates that this fault segment is frictionally weak (0.04 ≤ µf ≤ 0.17) in the shallow crust, with neither non-Andersonian stresses nor supra-hydrostatic fluid pressures required. Such weakness favors slip localization onto a single fault, enabling rapid slip rates at shallow dips. We show that slip at a second site, which lacks a rider block, does not require a weak fault because of a large fault/crust cohesion contrast and a rapid steepening of dip on the convex-upward fault from only 21° at the surface to >30° down-dip. From our results, we characterize four possible styles of faulting on convex-up normal faults: (a) total fault slip, (b) footwall damage, (c) fault partial abandonment, and (d) total fault abandonment. Our results reveal that LANF faulting style depends on the strength of the fault compared to the surrounding crust, fault geometry, and the distribution of differential stress required for slip as a function of depth.