George Hilley, PhD, Stanford University
During this reporting period, we have been exploring the relationship between processes that construct topography above high-angle reverse faults whose spatial distribution and constitutive properties are controlled by pre-existing heterogeneities in crustal properties that reflect the tectonic history of an area. This was accomplished using models in which we embedded a series of finite-width zones of reduced friction and cohesion into the Gale geodynamic model, and then forced the plastically deforming crust to undergo contraction by prescribing velocities on the edges of the model. Our principle results are: 1) When multiple pre-existing structures of different frictional properties exist within the modeled crust, deformation is initially localized along the weakest structure. However, as topography is built atop this weak structure with progressive contraction, deformation additionally becomes localized on stronger structures that lack topography. 2) The contrast between the frictional properties of the modeled pre-existing shear zones determines the required topography that must be built atop the weak structure to initiate failure of the frictionally stronger structure. Thus, the timing at which deformation migrates between structures depends on this frictional contrast. 3) We continue to implement erosion rules in the Gale model, but have faced some challenges with successfully implementing the alluvial transport rule in this model. 4) Research conducted during this reporting period was presented at an invited lecture at the Geological Society of America Annual Meeting in Denver in October, 2013. 5) We are writing up the results of our comparison between deformation styles and controls on topography and kinematics in a submission to the Journal of Geophysical Research, Solid Earth.
Key Results of PRF-Funded Research During This Reporting Period
We have two principal results to report during this reporting period, which we report in detail below:
1) When multiple pre-existing structures of different frictional properties exist within the modeled crust, deformation is initially localized along the weakest structure. However, as topography is built atop this weak structure with progressive contraction, deformation additionally becomes localized on stronger structures that lack topography. This results from the fact that increased body forces in the crust that are manifestations of high topography change the fault loading conditions as topography is built. This portion of the study was carried out using the Gale geodynamic model, in which shear zones of prescribed thicknesses, cohesions, and frictions were embedded into an otherwise homogeneous crust. We then drove these sets of models using constant velocity inflow boundary conditions at the edge of the model, and scaled the material viscosities to approximate a rigid crust that deformed when yield stresses, defined by a Drucker-Prager constitutive rule, were exceeded. In these models, the overall state of stress in the vicinity of the failing structures increased in a direction opposite to the shear stresses driving failure of the structure, which progressively increased the background state of stress in the modeled crust as failure was discouraged. Eventually, the background stress state required to sustain motion along the weakest structure increased sufficiently to drive failure along other, frictionally stronger pre-existing structures. Thus, we were able to demonstrate a clear causal association between the progressive construction of topography, increases in the background state of stress in the crust, and failure of other, stronger structures within the modeled crust. To our knowledge, this is the first geodynamic study to be able to show the importance of pre-existing structures in controlling deformation, which has been a long-standing observation of field geologists.
2) The contrast between the frictional properties of the modeled pre-existing shear zones determines the required topography that must be built atop the weak structure to initiate failure of the frictionally stronger structure. Thus, the timing at which migration of deformation between structures occurs depends on this frictional contrast. On the other hand, the localization of deformation within the modeled crust is largely controlled by the prescribed location of the preexisting flaws. Our parametric study included an analysis of the impact of the contrast in frictional properties of the different pre-existing structures on the geometry of the topography of the simulated mountain belt, as well as the timing of initiation of slip along individual modeled structures. We found that large frictional contrasts between structures produce steep, high-elevation topography above the frictionally weak structures. These scenarios also require more accrued total model displacement to initiate the frictionally stronger structures relative to the low-contrast cases. Background crustal stress levels are lower in models with low frictional contrasts relative to models in which this contrast is high, which is consistent with our findings from (1).
Dissemination of Results
We have had / are pursuing two primary outlets to disseminating our project’s results. First, Hilley gave an invited lecture that presented this reporting period’s research results at the 2013 Geological Society of America Annual Meeting in Denver, CO (a recording of this invited lecture can be viewed at http://pangea.stanford.edu/~hilley/Talks/GSATalk2013.m4v). Secondly, Cruz and Hilley are currently in the process of preparing a manuscript for submission to the Journal of Geophysical Research-Solid Earth on this material.
We continue to couple a bedrock incision and sediment deposition model into the Gale geodynamic model. Thus far, the implementation of one of these erosion rules (the alluvial transport rule) into the Gale model has posed some challenges. This rule controls basin deposition adjacent to eroding ranges, and so is likely important for determining the role that changes in reverse-fault footwalls may play in controlling the state-of-stress around the failing shear zones. These additions have become more onerous due to the fact that the NSF-CIG program no longer provides the dedicated programming-staff support for this software package. We will continue to work on this implementation through the grant period.