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

地球物理学和地质学研究已经对走滑系统的近地表过程有了比较清晰的认识，但对走滑系统的深层结构仍然知之甚少。特别是上地壳在断层上的刚体运动向下转化为中、下地壳流动的方式，至今仍未得到解决。阿拉斯加南部楚加奇山脉始新世韧性走滑系统的一个壮观的下倾暴露允许调查发生在这些更深层次的走滑系统的变形。在这一地区，走滑系统可以追溯到从接近其熔融温度变形的片麻岩到在脆-韧性转变温度附近变形的千枚岩;所有这些岩石都具有相同的一般成分。本研究项目建立在以前的工作上，这个系统来检查底部驱动的附着（下地壳中的流动向上转移到上地壳）与顶部驱动的拆离（上地壳独立于下地壳移动）在中-下地壳变形的发展中的作用。为了解决这一问题，本研究项目将结合基于野外的研究来表征构造几何学和相对年代学，地质年代学来限制变形序列的年龄，微构造研究来分析复杂构造中不同构造层次的变形机制和变形历史，有限应变研究来表征累积变形，和数值模拟来检查产生所观察到的特征的机械过程。该项目的总体目标是明确解决伴随着走滑系统发展的岩石圈力学分层问题，并建立一个工作模型，说明在复合体演化的不同阶段，底部驱动附着与顶部驱动剥离的重要性。滑动断层对地球上包括北美西部、新西兰、中东和中国在内的许多地区的10亿多人造成了重大的地震危险。因此，更好地理解地球岩石圈运动如何驱动走滑区地震周期是地球科学研究的重要目标。虽然人们对走滑行为了解很多，但对这些断层更深层次的作用过程却知之甚少。该项目利用了始新世（约5000万至5600万年）走滑断层的深层暴露，以揭示这些过程。特别是，断层系统的力学行为的应变率，温度和流变学的变化方面的确定将放置在活动的走滑断层系统内的中地壳发生的过程的重要约束。

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.