At about 20 km depth crustal rocks show a transition from the brittle behavior characteristic of the upper crust to ductile behavior characteristic of most of the Earths interior. The stress required to deform the ductile rock immediately below the brittle-ductile transition (BDT) is likely to approximate the bulk strength of the brittle crust immediately above the BDT. Rocks that have deformed by ductile processes can preserve microscale features in their crystalline structure that record the stress level during deformation. If these rocks have subsequently been brought to the surface (exhumed), we can use microstructural measurements to determine the strength of the crust around the BDT. The goal of this work is to obtain better estimates of the strength of the upper crust. Our measurements will allow us to reconstruct strength profiles of the continental crust. These will provide reliable data for assessing the strength of the tectonic plates in areas of active deformation. In particular, our results will improve mechanical models of the way continental crust responds to plate motions, with implications for mountain-building processes, the formation of sedimentary basins that host economically valuable reserves of petroleum and mineral resources, and the generation of earthquakes.

The upper 20-25 km of the continental crust is the coldest and strongest part of the tectonic plates, and because cold rock is brittle, it generates most of the earthquakes. The strength and mechanical properties of the upper crust are fundamental to understanding the way the plates respond to forces acting on their boundaries and to forces generated internally by gravity. The bulk strength of the crust is difficult to measure directly, however; estimates vary by about a factor of ten, because of uncertainties about the strength of faults, the effect of fluids within the crust, and the temperature gradient through the crust. In areas of continental rifting and extension, such as the Basin & Range Province of the western USA, rocks have been exhumed from the ductile middle crust, cooling through the BDT as they rise to the surface. During exhumation and cooling, deformation becomes increasingly localized into narrow shear zones. As a result, different parts of the rock body record stress levels from different depths. We plan to make measurements of microstructural features, mineral chemistry, and isotopic composition, to determine the stress-temperature-time history of rocks from areas affected by recent extensional tectonics in SE California and in southern Spain. Electron backscatter diffraction will be used to determine the grain size, grain misorientation, and crystallographic preferred orientation of dynamically recrystallized quartz-bearing rocks for stress measurements. Element exchange between different minerals will be used to calculate chemical equilibria that are functions of pressure and temperature. Several different radiogenic isotopic systems will be used to constrain the temperature during exhumation and cooling. Earthscope has provided generous support for the geochronology component of the work.

Project Report

@font-face { font-family: "?? ??"; }@font-face { font-family: "Cambria Math"; }@font-face { font-family: "Cambria"; }p.MsoNormal, li.MsoNormal, div.MsoNormal { margin: 0in 0in 0.0001pt; font-size: 12pt; font-family: Cambria; }.MsoChpDefault { font-size: 10pt; font-family: Cambria; }div.WordSection1 { page: WordSection1; } The Earth’s lithosphere is the strong, ~100 km thick outer layer of the Earth, which controls the process of plate tectonics. Knowledge of the strength and mechanical properties of the lithosphere is essential to understanding how plate tectonics works, and why it is such a distinctive feature of our planet. The purpose of this project was to estimate the strength of the lithosphere in the region between 15 and 40 km depth. At these depths rocks are warm enough that they deform in a ductile fashion (much like slowly stretched silly putty) and the minerals became stretched and thinned, rather than breaking up along faults and fractures. We examined rocks from the Whipple Mountains in eastern California, and in the coastal region of southern Spain, which in both areas have been brought up from these depths to the surface. As these rocks approached the surface, they cooled down, and were affected by motion on the deep roots of brittle faults that extend downward from the surface. As a result, the rocks have preserved a record of the conditions they experienced as they moved from the warm, deep, and ductile lower crust through the bottom of the seismogenic (earthquake-producing) layer – the so called brittle-ductile transition. This project focused in particular on characterizing these rare exposures of the brittle-ductile transition. We used a suite of geological tools to quantify the temperature, pressure, depth, geometry, and mechanical strength in this region, in an effort to understand its mechanical behavior and its influence on the earthquake cycle. This work involved several visits to the two field areas, during which we made careful and detailed observations of the rock structure and composition, and collected samples for microstructural and laboratory analysis. Our results suggest that the rocks and structures both above and below the brittle-ductile transition are essentially strong, and that the middle crust is thus an important feature in controlling large-scale plate tectonics. During the course of the project, a female PhD student, Whitney Behr, received support and interdisciplinary training in Earth Science. Dr. Behr now holds an appointment at Assistant Professor level in the Jackson School of Geociences at the University of Texas at Austin. Several undergraduate students were also involved in the project as field assistants, and in one case as a research assistant working on the microstructure of some of our samples. Results of this project have been presented at numerous scientific conferences and several papers have been published in high impact peer-reviewed journals.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Stephen S. Harlan
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University of Southern California
Los Angeles
United States
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