CAREER: Identifying the Dominant Controls on Strain Localization in the Lower Crust

University Of Maine, Orono ME

Investigators

Abstract

This project combines a study of crustal rheology with efforts to improve student understanding of the properties controlling the mechanical behavior of Earth materials. We integrate the two components of the project by developing a framework for university faculty, K-12 teachers, and graduate students in both Earth sciences and education to collaborate during field-based research and classroom resource development. The distribution of rock strength in the crust is a fundamental control on strain and corollary processes such as fluid flow, topographic evolution, and seismicity. Without being able to accurately characterize the strength distribution, we cannot accurately predict how the crust will respond to internal and external driving forces. Despite recent progress constraining the general mechanisms involved in strain-related weakening, we do not know which weakening processes dominate at different levels of the crust. Using the southwestern Grenville Province of Ontario, Canada, as a natural laboratory, we will combine structural, petrological, and geochronological methods, followed by analytical and numerical modeling sensitivity analysis, to determine the relative importance of different weakening mechanisms during the development of well-exposed km-scale shear zones there. We will test the following hypotheses: 1) Reaction and textural weakening are of subequal importance in shear zone development; 2) Grain size reduction is commonly not a major weakening factor at deep crustal levels; 3) Geological evidence does not require shear heating as a component in shear zone formation; and 4) Although initial heterogeneities are important for controlling the stress distribution in the crust, km-scale shear zones form due to a change in constitutive relationships. Earth's topography and the pattern of rock units exposed at the surface result almost entirely from the combination of erosion and the heterogeneous movement of rocks within the crust and mantle. In this project, we focus on identifying what controls patterns of rock movement, or deformation, in the lower portions of continental crust. By identifying the processes that control how rocks change strength during periods of mountain-building, and therefore how the lower crust accommodates the large-scale deformation induced by the movement of tectonic plates, we will improve our ability to predict and explain the evolution of Earth's surface and related processes such as fluid flow and seismicity. The physical controls on the mechanical properties of geological materials are well known. Yet many students still appear unsuccessful in understanding those core ideas in Earth Science courses, despite their inclusion in science standards. Moreover, due to a relative lack of formal studies, the geoscience community has not developed a full picture of student misconceptions and effective pedagogical strategies related to those concepts. To address this, we will use a concept inventory approach to identify student needs at the 6th-9th grade and introductory college levels in physical science content areas that underlie concepts related to deformation, then develop, implement, and evaluate the impact of instructional resources that address those needs. The principles that govern rock deformation are the same as those that govern the deformation of ice, metals, ceramics, and composite materials. Thus, students will be able to apply gains in their understanding derived from this project more broadly to fields such as glaciology and mechanical engineering. We will use the vertical integration of students, pre- and in-service K-12 teachers, and faculty as a pathway to improve student attitudes to science and engineering and to assist pre- and in-service teachers in maintaining currency in research-driven advances in the geosciences and geoscience education.