Pure shear dominated high-strain zones in basement terranes

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Conference Proceeding

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High-strain zones are common in basement terranes, and understanding their tectonic significance requires quantitative knowledge of deformation kinematics. We report on strained rocks from different tectonic settings that record pure shear dominated (W m < 0.4) deformations. Mylonitic rocks derived from Mesoproterozoic basement granitoids are exposed in the Lawhorne Mill high-strain zone in the Virginia Blue Ridge. Chemical and mineralogical differences between the leucogranitoid protolith and mylonite are consistent with ∼50% volume loss during deformation. Minimum finite strains in XZ sections range from 4:1 to 7:1, and three-dimensional strains plot in the field of apparent flattening; however, with volume loss these rocks likely experienced bulk plane strain. The R s /Θ and quartz c-axis vorticity gauges yield Wm values of 0.0-0.6. Fabric asymmetries normal to both foliation and lineation are consistent with modest triclinic deformation symmetry. Mylonitic rocks from the Lawhorne Mill high-strain zone record a pure shear dominated deformation that produced ∼70% contraction across the zone with only minimal displacement parallel to the zone (<0.5 km). Pure shear dominated high-strain zones occur in a variety of mid-crustal settings. Ultramylonites from metamorphic core complexes in Arizona record very low vorticity values (W m < 0.4). Well-foliated, steeply dipping, upper amphibolite facies rocks from the Coast shear zone in British Columbia are characterized by orthorhombic fabrics formed during pure shear dominated deformation that accommodated crustal contraction. These zones differ from simple and general shear zones because displacement across these zones is minimal relative to the overall finite strain. However, zonenormal shortening and zone-parallel stretching are significant in pure shear dominated zones. Steeply dipping zones formed in contractional settings serve to effectively shorten and thicken the crust across basement massifs, whereas gently dipping zones formed in extensional settings thin the crust. © 2007 The Geological Society of America. All rights reserved.

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Memoir of the Geological Society of America

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