Mid-Crustal Structure of Strike-Slip Systems from a Down-Plunge Exposure in the Chugach Metamorphic Complex, Alaska

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Geophysical and geological studies have produced a relatively clear picture of the near surface processes in strike-slip systems, yet the deep structure of strike-slip systems remains poorly understood. In particular, the manner in which upper-crustal rigid-body motion on faults is transferred downward into middle and lower crustal flow remains unresolved. A spectacular down-plunge exposure of an Eocene ductile strike-slip system in the Chugach Mountains in southern Alaska allows investigation of the deformation that occurs at these deeper levels of strike slip system. In this area, the strike-slip system can be traced from gneiss deformed near its melting temperature to phyllites deformed at temperatures near the brittle-ductile transition; all within rocks of the same general composition. This research project builds on previous work on this system to examine the role of bottom driven attachment (flow in the lower crust is transferred upward to the upper crust) versus top-driven detachment (the upper crsut moves independently of the lower crust) in the development of mid- to lower-crustal deformation. To address this problem, this research project will use a combination of field-based studies to characterize structural geometry and relative chronology, geochronology to constrain the age of the deformational sequence, microstructural studies to analyze deformational mechanisms and deformational histories at different structural levels in the complex, finite strain studies to characterize accumulated deformation, and numerical modeling to examine the mechanical processes that have produced the observed features. The overall goals of this project are to develop a clear resolution of the mechanical stratification of the lithosphere that accompanied the development of the strike-slip system and develop a working model for the importance of bottom-driven attachment versus top-driven detachment during the different phases in the evolution of the complex.Strike-slip faults pose a major earthquake hazard for well over a billion people on Earth in many regions including western North America, New Zealand, the middle East, and China. Thus, a better understanding of how motion in the Earth''s lithosphere drives the earthquake cycle in strike-slip regions is an important objective of earth science research. Although much is known about strike-slip behavior, very little is known about the processes that operate at the deeper levels of these faults. This project takes advantage of a deep level exposure of an Eocene (approximately 50 to 56 million years old) strike-slip fault in order to shed light on these processes. In particular, determination of the mechanical behavior of the fault system in terms of variations in strain rate, temperature, and rheology will place important constraints on the processes that occur in the middle crust within active strike-slip fault systems.