Contrasts in integrated crustal strength drive the asymmetric distribution of topography and deformation within restraining bends

Curtis W. Baden, & George E. Hilley

Published August 15, 2018, SCEC Contribution #8773, 2018 SCEC Annual Meeting Poster #271

Restraining bends in the San Andreas Fault (SAF) generate zones of regional plate convergence along the Pacific-North American plate boundary. This causes uplift near them, and generates seismic activity as crust deforms. Interestingly, topography and deformation occur asymmetrically about these features along the SAF, which is not predicted by idealized elastic and plastic numerical models of deformation around these features. We propose that contrasts in crustal strength drive the localization of plastic deformation and uplift surrounding these bends, which ultimately generates the asymmetric topography observed. We test this idea using the finite element software Abaqus to simulate the impact of contrasts in material properties and boundary conditions on deformation and uplift around restraining bends. Specifically, we vary parameters of an elastoplastic rheology that yields according to a Drucker Prager or Mohr Coulomb constitutive model, and basal and horizontal boundary conditions, to determine each’s effect on modeled long-term deformations. These models are tailored after the Santa Cruz Mountains restraining bend, where previous work quantifies deformation using geodetic, geomorphic, thermochronologic, and geologic methods. These data provide a framework through which we assess the agreement of our models with existing measurements and observations.

Model results show that, under certain constitutive rules and boundary conditions, crust may be asymmetrically advected and uplifted along the modeled restraining bend, consistent with thermochronologic, topographic, and geomorphic observations within the Santa Cruz Mountains. We contrast these patterns of long-term deformation with those expected over geodetic time-scales by calculating velocities for a purely elastic medium at the end of each model simulation. We find that, in addition to producing asymmetric distributions of topography and deformation, irrecoverable off-fault failure may be essential in reconciling fault zone behavior observed over geodetic and geologic timescales.

Citation
Baden, C. W., & Hilley, G. E. (2018, 08). Contrasts in integrated crustal strength drive the asymmetric distribution of topography and deformation within restraining bends. Poster Presentation at 2018 SCEC Annual Meeting.

Related Projects & Working Groups
San Andreas Fault System (SAFS)