Research Interests

How plate motions are accommodated by deformation of the crust remains a fundamental problem with implications to interpreting seismic hazards, geodetic observations, landscape evolution and the rock record. My research aims to address this problem by focusing on relatively shallow crustal deformation in plate boundaries settings.

I focus on active, neotectonic (i.e., < 4 Ma) and ancient deformation because I believe that the temporal and spatial differences inherent in these deformation timescales provide exciting opportunities for discovery. In essence, barriers in one setting can be overcome by looking at other settings. For example, active plate boundaries enable us to examine deformation under relatively well-constrained boundary conditions. Unfortunately, most of the deforming volume in these settings cannot be sampled directly and our observations cover a timeframe that is infinitesimal. By contrast, ancient plate boundaries enable us to sample rocks from various structural levels, but we are forced to infer the boundary conditions that operated during deformation. Fortunately, in these settings we are also able to examine deformation process that have evolved over timescales that are relevant to an array of geologic questions. Moreover, ancient plate boundaries can be exploited to ascertain basic properties that prevailed during deformation (e.g., pressure, temperature, kinematics and fluid chemistry) by direct observation. In turn, our ability to constrain the boundary conditions that might have operated during deformation at ancient plate margins stands to improve by exploring ongoing deformation across the spectrum of active settings where the boundary conditions are known.

I am currently examining active and neotectonic deformation by inverting earthquake focal mechanism solutions and fault-slip data, respectively, for 3-dimensional strain geometry. I am using Rob Twiss’s (University of California at Davis) numerical adaptation of micropolar continuum theory that enables determination of the orientation and relative magnitudes of the strain-rates as well as the relative vorticity (i.e., the partial strain tensor). This approach is powerful because it allows block rotations to be quantified. There is extensive geologic evidence for such rotations, however documenting actively rotating blocks has been problematic, in part because of the inability of classical continuum theory to address rotations explicitly. In addition, micropolar theory does not require assumptions to be made regarding the rheology of the crust, a parameter that remains poorly constrained owing to the compositional and thermal heterogeneity of the crust, among other factors.

I am working on modern plate margins that display a range of tectonic boundary conditions. A primary goal of this work is establishing the relation between plate or block kinematics and shallow crustal deformation patterns. To this end I am working on the highly oblique Hikurangi convergent margin of New Zealand, the moderately oblique, Cascadia convergent margin of western North America, and the “transtensional” boundary between the Sierran microplate and the North America plate in the northern Mojave Desert of eastern California. In the latter area I am integrating shallow earthquake and fault-slip data with radiometic age and paleomagnetic data for young (< 4 Ma) lava flows to develop a kinematic model for the evolution of this young basin. This work will serve to better our understanding of the relation between active rotation surmised from the seismic record and rotations recorded geologically. Lastly, I am in the process of establishing a project to examine the seismogenic deformation associated with nearly normal convergence at the Costa Rica convergent plate boundary. The goal of this work will be to examine non-recoverable strain within the decollement zone (i.e., the seismogenic zone in parlance of the NSF Margins Program) and within the overriding forearc, which the geologic record suggests has deformed in response to the subduction of rough seafloor topography (e.g., seamounts).

My efforts to address the relation between plate kinematics and crustal strain patterns include an emphasis on both the scale of and the controls on strain partitioning. My work in the northern Mojave Desert, for example, shows partitioning of an oblique seismogenic deformation into two nearly orthogonal strain geometries. I see evidence for depth-dependent strain geometry over kilometer distances and depth-independent strain geometry over hundreds of meters. The latter result implies partitioning of large-scale deformation at fine spatial scales and suggests that single faults can accommodate two strain geometries during a continuous deformation (i.e., essentially instantaneously). This finding has implications to understanding active tectonic processes as well as the rock record (e.g., exhumed fault zones that record multiple slip directions).

My long-term plans focus primarily on plate margins and include: (1) examining a wider range of modern plate-boundary tectonic settings, (2) incorporating surface data into my analyses of active deformation, and (3) conducting field studies at ancient plate margins. Additional modern plate boundaries of interest include the “transtensional” Gulf of California, Taiwan, southwest Japan and Costa Rica. The sorts of surface data that I would like to begin incorporating in my analyses include geomorphic (e.g., digital elevation models, drainage geometries) and geodetic (e.g., INSAR, GPS) observations. These efforts will address the dynamic links between subsurface permanent deformation and surface geometries/displacements. My work on ancient plate margins will build on my past research in the Tertiary subduction complex of southwest Japan (the Shimanto belt) and will focus on paleokinematic indicators, penetrative strain markers and recorders of the physicochemical conditions of deformation. This component of my research is critical because by making direct observations in rocks whose modern analogs cannot be sampled, I expect to broaden our understanding of the deformation at active convergent margins. In turn, by documenting the 3-dimensional strain geometries associated with distributed seismicity at active convergent plate boundaries (as described above) I expect to shed light on the relation between active deformation and plate kinematics. This understanding will help us better interpret the outcrop-scale faults that are commonly observed exhumed convergent margin strata.