Dating Transpression and Extrusion at Mid-Crustal Depths

Theoretical models for transpression and extrusion in obliquely convergent orogens make numerous predictions regarding crustal behavior that have yet to be systematically field tested. A likely zone of midcrustal 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: simultaneous vs. sequential deformation across the system. Mapping and structural analysis at a range of scales demonstrate that the transpression/ extrusion system in the northern Appalachian orogen contains classical elements of transpression models and elements proposed for more complex systems of triclinic symmetry: a central N-S striking, steeply inclined zone of extrusion bounded in this case by simple shear zones of opposite displacement sense (sinistral/normal on the west; dextral/reverse on the east). Similar kinematics along boundaries and lack of overprinting relationships is consistent with the three elements being components of a simultaneous partitioned dextral transpression zone. Precise dating of deformation within the three components of the system is necessary before other aspects of transpression/extrusion models are examined for viability. Three geochronologic methods on strategically selected samples will be applied to test for contemporaneity within the transpression/extrusion system: (1) U-Th-Pb ion microprobe and in situ electron probe microanalysis (EPMA) of monazite from metapelites in the bounding and central zones, in which generations of monazite growth can be tied to specific foliations, lineations, and vorticity gauges in tectonites that define the kinematics of each zone; (2) U-Pb ion microprobe and high precision ID-TIMS analysis of zircon in variably deformed crosscutting leucopegmatites in the central zone. All microanalyses will be guided by and interpreted in the context of CL/BSE imaging and/or EPMA elemental mapping. 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 pure vs. simple shear. Intellectual Merit: The PI is unaware of any study that has attempted to directly test the implicit assumption related to absolute timing of deformation in the zone of transpression and bounding high strain zones via high-precision geochronology. The study area is very well characterized from prior mapping and structural analysis and is an ideal natural laboratory for this test. The mid-crustal setting provides a link between deeper lithospheric processes and shallower crustal phenomena observable in active transpressional orogens. Preliminary geochronologic analysis using the approaches proposed here demonstrates that the methodology is practicable and that there is a strong likelihood of unequivocal outcomes for the specific system. These outcomes are transferrable to other systems and broadly relevant for mechanisms of mid-crustal continental deformation involving oblique convergence and transpression in ancient exhumed or active systems. The research directly addresses several of the major research frontiers outlined at the summer 2012 SERC/NSF Cutting Edge workshop at Williams College. Broader Impacts The project will support the professional development of a young career scientist as a postdoctoral research associate, a graduate student working on an M.S. thesis project in close collaboration with the PI and postdoctoral associate, an undergraduate student working as a mentee with the postdoc and graduate student, and a local high school earth science teacher working with all participants. This research team approach ensures effective engagement of all participants, who will each be involved to varying extent on aspects of the project: field work, sample collection and preparation; microprobe imaging and characterization; and geochronology (although post-doc only for zircon IDTIMS analysis and EPMA analysis). All data generated by this research will be archived at open source sites for public access. The results will be disseminated by publication in peer-reviewed journals and by presentation at professional meetings.