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45444-B8
Dynamic Development and Reactivation of a Newly Discovered Frictional Plastic Deformation System
Understanding the origin and early geologic evolution of faults is important to petroleum exploration and reservoir development because faults serve as zones of weakness in the crust that are preferentially reactivated and rejuvenated through time. The reactivation of old zones of weakness can subsequently create conduits for the migration of fluids, and renewed fault displacement can influence the distribution and thickness of overlying sedimentary strata that create petroleum traps. The goal of this research is to examine the deep structure of an exhumed fault zone and document the geometry and dynamics of early fault development as compared to: (1) the pre-existing structure of mid- to lower-crustal metamorphic fabrics, and (2) the geometry of overprinting brittle faults formed during younger reactivation.
Fundamentally, most long-lived fault zones develop from seismically active faults in the incompletely understood middle crust where rock strength is qualitatively thought to be highest. However, the macroscopic evolution of faults known to have been seismically active in the middle crust is poorly known. In order to address this problem, this research focuses on the development of coeval pseudotachylyte and mylonite in the newly discovered Grizzly Creek shear zone exposed in Glenwood Canyon, Colorado. Pseudotachylyte is generated by friction during seismic slip at very high strain rates (~10-2/s) and is therefore a unique and rare fossil record of individual earthquakes. In contrast, mylonite forms at strain rates at least nine orders of magnitude lower (~10-11/s) and thus represents slow aseismic creep. The shear zone is one of only a dozen in the world that exhibit coeval and mutually overprinting pseudotachylyte and mylonite. The field site therefore definitively indicates that seismic and aseismic processes overlapped in the middle crust where earthquakes commonly nucleate. The shear zone is thus an extraordinary field laboratory that allows us to glimpse into the middle part of the crust and examine the nascent development of a seismically active fault zone, and relate the early structure to overprinting brittle faults that formed during younger reactivation that affected the overlying sedimentary rocks in the region.
Our early field work has successfully outlined the structural framework of this previously undiscovered shear zone, and will serve as a template for process-oriented studies in the next two years. The shear zone includes a 10-15-m-thick basal mylonite oriented 255/44°NW that truncates foliation and magmatic fabrics in the footwall. The mylonite is overlain by >400 m of moderately north-dipping tectonites dominated by protomylonitic to myonitic gneiss intercalated with pseudotachylyte fault veins and cm-scale ultramylonite bands. At the upper boundary of the shear zone, the high-strain tectonic fabrics grade into relatively undeformed rocks of the hanging wall. The shear zone separates supracrustal gneisses and coarse-to-megacrystic granitoids in the footwall (south block) from fine-grained, foliated granite and supracrustal gneisses in the hanging wall (north block). Mutually overprinting brittle and plastic fault rocks, including cataclasites, pseudotachylyte fault veins, mylonitized pseudotachylyte, and ultramylonite bands, record cyclic deformation by both seismogenic rupture and plastic creep within the shear zone. Mineral lineations on mylonitic foliation surfaces were found to plunge N-NE, and a variety of microscopic and mesoscopic shear-sense indicators conclusively document top-to-south reverse displacement.
The Grizzly Creek shear zone is truncated by the Cambrian-Precambrian nonconformity, which is underlain by a 1–2-m-thick paleoregolith containing altered pseudotachylyte. We thus definitively conclude that the shear zone is of Proterozoic origin. Furthermore, our data continue to support our initial hypothesis that it records cyclic seismogenic faulting and aseismic plastic flow at the mid-crustal, brittle-plastic transition during Proterozoic ~N-S shortening. Field mapping shows that the Grizzly Creek shear zone was brittley reactivated as a Laramide, south-vergent reverse fault with a displacement of >200 m. At present, early results suggest that the Proterozoic Grizzly Creek shear zone represents a persistent zone of weakness in the lithosphere that significantly impacted the Cenozoic structural evolution and present geomorphology of the region. Future work will further define the distribution and extent of reactivation and how it relates to the early structure of the shear zone.
To date, the project has involved three undergraduate researchers, including one SUMR scholar. One is now a graduate student and teaching assistant in geology at a regional research university. The others began research as freshmen and sophomores, and are preparing presentations for a regional conference, which should have a positive impact on their education and career goals. The research experience has benefited the students; they have become leaders in their classes and are a visible presence in the department on a daily basis. First-year research results will be presented in two forums at an upcoming national conference, and as a professional field trip for the Geological Society of America.
