Dating Transpression and Extrusion at Mid-Crustal Depths

University Of Kentucky Research Foundation, Lexington KY

Investigators

Abstract

Theoretical models for transpression and extrusion in continental collisional zones have evolved markedly since the original model of Sanderson and Marchini (1984), driven largely by modeling and analysis of real zones of transpression. A zone of mid-crustal transpression and extrusion exhibiting a range of structures that record markedly different states of finite strain suggestive of contraction, wrenching, and extension serves as a natural laboratory to test a fundamental aspect of transpression and extrusion models: testing via precise geochronology whether simultaneous vs. sequential deformation occurred across the system. Mapping and structural analysis at a range of scales demonstrate that the transpression/extrusion system in the northern Appalachian orogen contains elements of classical transpression models, elements proposed for more complex systems of triclinic symmetry, and elements not predicted by models. Linked kinematics along boundaries and lack of overprinting relationships are consistent with some elements being components of a simultaneous partitioned dextral transpression zone, whereas other elements appear to have developed later in the deformation history or outlasted deformation in the other zones. Geochronologic methods applied to strategically selected samples will be used to determine absolute timing of deformation within the transpression/extrusion system. Pseudosection analysis and conventional mineralogic geothermobarometry will be completed in order to obtain temperature constraints for interpreting geochronology and depth constraints for the present level of exposure of the former mid-crustal setting. Strain and vorticity analysis in each element of the system will be performed in order to assess contributions of coaxial vs. simple shear. Mountain belts such as the Appalachians are the expression of colliding continental plates. As a result of the compressional stresses in collision zones, the crust may be uplifted vertically and/or pushed aside laterally (extruded). Full understanding of the collision history and mechanism of mountain belt formation requires (1) examination of structural and mineralogical features in rocks that are expressed at the map (kilometers to hundreds of kilometers), outcrop (meters to hundreds of meters), hand sample, and the microscopic scale; and (2) determination of the absolute age of geologic events via radiometric dating methods. Our previous research in the northern Appalachians of central Massachusetts identified features suggestive of oblique (glancing blow), rather than orthogonal (head-on) collision, which is the prevailing paradigm. The proposed research will involve field and laboratory studies by a team of geoscientists, including a faculty member, post-doctoral research associate, graduate student, and undergraduate student from the University of Kentucky, working in collaboration with faculty at three other institutions and a secondary earth science educator from the local school system, to collect, measure, and interpret the data necessary to distinguish between the two collision alternatives, and to carry out the geochronology needed to determine precisely when the deformation of the crust occurred. The project activities will further the professional development of the junior geoscientists and the research results will be disseminated at professional meetings and in peer-reviewed journals.