Research

Hot Laramide! The onset of the Laramide Orogeny in Southern California and the development of a regionally extensive intra-arc shear zone within the Southern California Batholith

Collaborators: Josh Schwartz (CSUN) and Keith Klepeis (Univ. Vermont)

The purpose of the work is to reconcile the controversial beginnings of the Laramide orogeny by testing conflicting hypotheses about initial Laramide processes. We focus on the frontal arc region of the Southern California Batholith, which will allow us to establish a structural, geochronologic, and geochemical foundation for resolving conflicting models for upper-plate processes associated with the flat-slab paradigm and the initiation of the Laramide orogeny. My group is working on the ~200 km- long intra-arc shear zone system to understand the magmatic-tectonic interaction during the Late Cretaceous arc flareup. We are working on the Black Belt Shear Zone, the Cucamonga Shear Zone, the Alamo Mountain Shear Zone, and the Tumamait Shear Zone. The goal is to use microstructural, EBSD, and CVA analysis to interpret the kinematic deformation geometry of the individual shear zones that together comprise the Southern California Batholith Intra-Arc Shear Zone, and to date the timing of fabric development with titanite petrochronology.

Building the Statewide California Earthquake Center (SCEC) Community Rheology Model: what insights do mid-crustal shear zones give into San Andreas Fault system rheology?

Collaborators: Josh Schwartz (CSUN) and Sinan Akciz (CSUF)

A critical new research objective in the Southern California Earthquake Center (SCEC)'s SCEC5 research initiative program is the development of the Community Rheologic Model (CRM), whereby key research priorities include building a provisional rheology model around a geologic framework for Southern California. The rheology model requires consideration of whether strain is strongly localized on or near faults and their downward continuation as shear zones, or if strain is broadly distributed across the plate boundary in Southern California (SCEC Community Rheology Workshop, 2018). This question must be considered in three dimensions, as the degree of strain localization varies with depth in response to many factors, including temperature, rock composition, and fault/shear zone geometry. Thus, characterizing the rheology of faults and shear zones requires direct examination of rocks at a variety of structural depths. Surface geology provides crucial constraints on brittle fault rheology in the shallow crust, but characterizing ductile fault rheology in the deep crust is far more problematic. We cannot directly sample the deep crustal shear zones that are the downward continuation of the fault zones that define the San Andreas Fault (SAF) system. Critical insights into ductile shear zone rheology must be achieved by other means. As a solution, we propose a field- and lab-based study of an exposed shear zone that is an ideal analog for the deep crustal shear zone associated with the Coachella Valley segment of the SAF system. The purpose of our study is to directly observe and sample this analog shear zone to place geologic constraints on the rheology of deep-crustal shear zones for direct incorporation into the CRM. We focus our work on the Eastern Peninsular Ranges Mylonite Zone (EPRMZ), a 100 km-long shear zone between Palm Springs and the Gulf of California (Sharp, 1967, 1979; Todd et al., 1988), where the shear zone developed at middle crustal depths within quartzofeldspathic crystalline rocks (Goodwin and Renne, 1991). This regional-scale shear zone is the most appropriate rheologic analog for the deep crustal shear zone that is the downward continuation of the SAF as it approaches the Salton Sea. The importance of understanding localized ductile shear zone rheology is underscored by recent work suggesting that shear zone fabric development is a crucial parameter in controlling the effective viscosity and strength of the crust below the seismogenic zone (Shinevar et al., 2018).

The role of plagioclase as a rheology-controlling mineral in lower oceanic crust: reconciling naturally and experimentally deformed plagioclase datasets

Collaborators: Greg Hirth (Brown University). Plagioclase is regarded as a rheology-controlling mineral both in oceanic and continental lower crust, but the deformation mechanisms of plagioclase are not as well understood as other rheology-controlling minerals of the upper crust (quartz) and mantle (olivine). For example, experimental studies of deformed olivine identify multiple high-temperature deformation mechanisms (e.g., diffusion creep, dislocation-accommodated grain boundary sliding (DisGBS), and dislocation creep), but there is not an equivalent experimental dataset for plagioclase though it also deforms by these deformation mechanisms.  Specifically, there are no experimental DisGBS flow law data for plagioclase, though naturally deformed plagioclase has been shown to deform by DisGBS (Svahnberg and Piazolo, 2010; Miranda et al., 2016). Another problem is that plagioclase DisGBS may be under-recognized in nature due to the similarity of its microstructural traits with other grain-size-sensitive deformation mechanisms such as diffusion creep in the absence of comprehensive microstructural investigation, so there is need to carefully document all microstructural traits that distinguish the two mechanisms. I have worked with Greg Hirth (Brown University) to document evidence for DisGBS in naturally deformed plagioclase of lower oceanic crust.