The relationship between continental collision and deformation along lithosphere-scale (hundreds of km-long) intracontinental strike-slip faults remains intensely debated. Debate persists because despite what we know about this important class of faults at the surface, we know comparatively little about how these structures work in the lower crust. Questions regarding the degree of strain localization, the lateral extent of rheological heterogeneities, and the physical nature of major intracontinental faults at depth will be answered "in situ" in this project by studying a well-exposed deep crustal shear zone and its wall rocks, i.e., the >400 km-long intracontinental Grease River shear zone in the western Canadian Shield (exhumed from ~30 to 40 km-paleodepths). Preliminary data support the hypothesis that at ca. 1.92-1.90 Ga, lower crust NW of the Grease River shear zone was melt-weakened at the same time that lower crust SE of the Grease River shear zone was dehydrated and strong. This ?rheological dichotomy? may be analogous to lower crust beneath the steep topographic gradients and major intracontinental fault systems that mark the margins of the Tibetan Plateau, e.g., the Altyn Tagh and Longmen Shan ranges.
This project will provide field-based constraints on the role of rheological heterogeneity in facilitating strain localization across a major intracontinental strike-slip fault from the lower crust's perspective. Anticipated results include: (1) resolving the Pressure-Temperature conditions and paths for metamorphic tectonites in the Grease River shear zone and its wall rocks, (2) testing the contemporaneity of deformation, shearing, and partial melting using in situ Th-U-total Pb dates on monazite via EPMA, and (3) constraining a >500 m.y. history of shear zone reactivation via high-precision ID-TIMS U-Pb dates on zircon from a suite of cross-cutting and variably-deformed dikes.
Broader impacts of this project include: (1) implementing a program of 6th-8th grade teacher preparation and enhancement, and (2) public outreach via classroom visits and panel discussions that are all focused on faulting and earthquakes in intracontinental systems. These efforts will directly impact teachers and students who live in the immediate vicinity of unexposed intracontinental faults of the New Madrid seismic zone ? the most seismically active region east of the Rocky Mountains. Two professional development summer institutes (workshops + field trips) will be provided to teachers utilizing inquiry-based modules to be developed by the PI. Implementation and assessment of these modules will impact middle school Earth Science frameworks in Arkansas (and nationally via on-line dissemination of the modules). These efforts will be in collaboration with the University of Arkansas' Center for Math and Science Education. Training and education will be provided to at least one Ph.D. student, one M.S. student, and several undergraduate students who will be integrally involved in the research and outreach efforts. This project directly impacts the new geoscience Ph.D. program at the University of Arkansas. Benefits to the EarthScope community are also anticipated, as this project uses an exposed deep crustal shear zone in the Canadian Shield to inform our understanding of the structure, rheology, and timing of a major intracontinental strike-slip fault as it may have existed in the lower crust near the Moho.
