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Microcracks in New England granitoids: A record of thermoelastic relaxation during exhumation of intracontinental crust

Brett J. Nadan¹ and Terry Engelder^1,

¹ Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16827, USA

Correspondence: E-mail: engelder{at}geosc.psu.edu

The upper intracontinental crust carries an excess horizontal compression, a remnant stress that arises because exhumation-related thermoelastic relaxation of deeper horizontal stress lags behind the reduction in overburden stress. This remnant stress appears in Earth stress data as an interchange in orientation of vertical ₂ and horizontal ₃ so that the ratio of least compressive horizontal stress (S_hmin) to vertical compressive stress (S_v) is >1 in much of the top 2 km of intracontinental crust. In theory, rocks exhumed from beneath 2 km should carry some record of this stress interchange, and this record is found in the orientation and density of healed, filled, and open micro-cracks in exhumed New England granitoids. Fluid inclusion planes (FIP) of older, healed microcracks are the best developed in a vertical orientation, and younger filled and open microcracks are best developed in the horizontal plane. Lateral unloading during initial isobaric cooling from the solidus of laterally constrained granite allows early microcrack growth once horizontal tension on the microscopic scale develops in response to vertical compression from the overburden load. During exhumation, further relaxation of lateral compressive stress takes place by a combination of decompression and cooling so that S_hmin/S_v <1. Such behavior preserves a horizontal compression at depths <2 km where horizontal microcracks are found. Excess horizontal compressive stress, a remnant of incomplete relaxation, carries upward right to the bedrock surface where near-surface structures such as stress-relief buckles and topographically related sheet fractures are found. This excess compression is consistent with the abundance of thrust fault focal mechanisms found in the top 2 km of intracontinental crust east of the Rocky Mountain front and south of the U.S. border.

Key Words: granite • microcracks • exhumation • remnant stress • thermoelastic relaxation

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