Understanding the processes of earthquake rupture and mountain building requires a quantitative understanding of how the earth responds to applied forces and boundary conditions. The tectonic plates consist of three primary mechanical layers: upper crust, lower crust, and upper mantle. The upper crust can be directly studied using a combination of geological and geophysical tools. The upper mantle has a relatively simple composition, and thus it is relatively easy to predict its mechanical behavior based on laboratory deformation experiments. The most poorly understood part of a tectonic plate, with respect to deformation, is therefore the lower crust.
Constraining lower crustal rheology is a first-order problem in tectonic analysis, highlighted by two separate, recent NSF-sponsored workshops. This problem is the focus of an ongoing study of lower crustal deformation that exploits the exceptional exposures in the Arunta block of central Australia: 100s of square kilometers of nearly continuous outcrop of rocks that record the high temperatures typical of the lower crust in active tectonic areas. The latter include a variety of typical lower crustal rock types, deformed at a range of depths at pressure and temperature conditions that resulted in granulite facies metamorphism prior to exhumation.
A major limitation to quantifying deformation in the lower crust is the typical lack of strain markers. Work to date demonstrates that strain gradients in the lower crust are recognized by: 1) comparison of length scales of heterogeneities in areas in which primary structures are preserved with those in which they are absent; and 2) determination of meso-scale fabric intensity, or degree of development of foliation and lineation. This approach is allowng high strain zones to be delineated. The latter record strain localization in the lower crust, similar to more widely recognized shear zones in the middle crust. This mapping, combined with strain and microstructural analyses, are being used to determine whether or not a weak mineral or rock type consistently localizes deformation ('weak element' concept). Deformation mechanisms of major minerals are being constrained by light and back-scattered electron petrography, electron back-scattered diffraction analysis of crystal lattice preferred orientations, and transmission electron microscopy. The range in lower crustal depths exposed in the Arunta block is allowing an evaluation of this 'weak element' concept to determine whether it generally characterizes lower crustal deformation or is applicable only to specific P-T conditions.
This grant is supporting a graduate student and an undergraduate field assistant for each year of the study. The integration of different approaches allows thee students to understand how different tools can be used to address a complex problem, and to participate in the integration of multi-disciplinary data. Recognizing the potential for this particular research project to unite elements of the structural geology and tectonics community that do not generally communicate regularly, this work will be used as a foundation for a workshop or conference focused on constraints on the rheology of continental lithosphere to be held in 2007 or 2008.
