Are segmented fracture zones weak? Analytical and numerical models constrain anomalous bathymetry at the Clarion and Murray fracture zones

Abstract

Between 150°W and 135°W, the Clarion Fracture Zone (FZ) coalesces from six discrete FZ traces to a single FZ over a period of ∼30 million years, coinciding with a change in plate motion that placed the associated transform fault (TF) in compression. Between 160° and 157°, the Murray FZ is also comprised of several closely-spaced (<50 km) discrete FZs and the associated TF experienced similar transpressional motion. Analysis of newly collected bathymetry data along the Clarion and Murray FZs reveals FZ structures that are inconsistent with predictions from a simple locked fault thermal subsidence model. These structures include FZ-bounded lithosphere that dips towards the old side of the FZ, whereas locked fault models predict it should dip towards the young side, and reversed fault scarp relationships where younger lithosphere lies deeper than older lithosphere. We investigate these anomalous observations using a combination of analytical and numerical models of two closely-spaced fracture zones to test how FZ strength and tectonic motions (compression or extension) affect the evolution of the intra-transform lithosphere. Our models predict that interior blocks within a segmented FZ, initially in isostatic equilibrium, can develop anomalous tilts and scarp depth relationships if the intervening FZs are relatively weak. Model results suggest that unusually low apparent coefficients of friction (<0.01) and tectonic compression are required to reproduce the magnitude of reverse tilt and scarp reversals observed in the Clarion and Murray FZs. We suggest that these low apparent friction coefficients might be the result of near lithostatic pore-fluid pressures associated with compression and alteration processes. Our results provide a tool to constrain histories of tectonic compression across FZs in places where rotation poles or other measures of plate motion may not be well constrained. Our results also imply that FZs are tectonically weak regions of hydrothermal alteration that may carry large volumes of volatiles into the mantle when subducted.