At continental strike-slip plate boundaries, such as the San Andreas fault system in California, one portion of continental lithosphere translates past another with primarily horizontal motion. This plate motion can be measured at the surface with GPS data and by offsets on shallow faults, including the slip produced by crustal earthquakes. However, the form of this motion in the mantle portion of the lithosphere is much less certain. Models of mantle deformation beneath strike-slip plate boundaries range from narrow shear zones that lie directly beneath the position of the plate boundary at the surface to broad zones of diffuse deformation that are more than ~100 kilometers wide. In this research project, seismic waves from distant earthquakes will be used to measure the properties of the continental lithosphere beneath two major strike-slip fault systems, the Northern Anatolian fault in Turkey and the Southern Alpine fault in New Zealand, and these results will be compared to prior work beneath the San Andreas. In particular, waves that convert from shear motion to compressional motion (or viceversa) at the base of the lithosphere will be used to constrain the thickness of the lithosphere and the gradient in seismic wave velocity between the colder, relatively rigid lithosphere and the warmer asthenosphere. If lithospheric thickness and/or the lithosphere-asthenosphere velocity gradient change over small horizontal length-scales beneath strike-slip plate boundaries, these findings would support the narrow shear zone model. For example, prior work indicates that shear at the base of the lithosphere beneath the central San Andreas is distributed across a zone that is less than 50 kilometers wide. More gradual horizontal changes in the properties of the mantle lithosphere would support the presence of broader plate boundary mantle shear zones. Numerical modeling of mantle deformation beneath strike-slip fault systems will aid in interpreting the seismologically-measured lithospheric properties. The results of this research will provide new insight on the physical properties of plate boundaries and how plate motion is accommodated within the continental lithosphere.
We propose to measure lithospheric thickness and the lithosphere-asthenosphere velocity gradient beneath the Northern Anatolian and Southern Alpine faults with seismic imaging primarily based on Sp phases that convert at the lithosphere-asthenosphere boundary (LAB), and we will employ numerical modeling of strike-slip plate boundary deformation to provide a physical framework for our results. This imaging approach directly constrains structure at the base of the mantle lithosphere, including possible variations in LAB structure due to through-going shear zones associated with shallow fault systems. In prior work on the LAB beneath the San Andreas fault system, we found that Sp phase amplitudes and the corresponding drop in shear-wave velocity at the LAB are systematically smaller on the western side of the plate boundary than to its east. In central California, the change in LAB velocity gradient occurs over a horizontal length scale of less than 50 km directly beneath the San Andreas fault. These results are consistent with the juxtaposition of mantle lithospheres with different properties across the central San Andreas fault across a narrow shear zone (< 50 km in width) that extends to the base of the lithosphere. Beneath the Northern Anatolian and Southern Alpine fault systems, Sp receiver functions will be stacked into 3D images to determine LAB discontinuity depths and amplitudes and whether LAB properties vary systematically across each of the faults. Constraints from Ps phases will also be assessed. Modeling based on synthetic seismograms will provide quantitative estimates of the vertical shear velocity gradients associated with the LAB and a robust assessment of how well Sp and Ps phases resolve lateral changes in LAB properties. Numerical modeling of lithospheric and asthenospheric deformation beneath the San Andreas, Northern Anatolian, and Alpine fault systems will allow us to integrate the results of the Sp and Ps imaging with constraints on mantle anisotropy from shear-wave splitting. It will also help us to interpret the seismological results in terms of lithosphere-asthenosphere viscosity contrasts and the role of deformation-induced anisotropy in creating apparent LAB properties. Comparison of results from the three fault systems will address several key questions. How wide are strike-slip shear zones in the deep continental lithosphere? What are the implications of LAB structure across strike-slip fault systems for the rheologies of the lithosphere and asthenosphere? What processes are responsible for the lateral contrast in LAB properties across the San Andreas fault system and potentially the Alpine and Northern Anatolian fault systems?
