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Proterozoic polydeformation in basement rocks of the Needle Mountains, Colorado

¹ Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061
² Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, Maryland 21218

Proterozoic rocks in the Needle Mountains include 1,755-m.y. old amphibolite-grade gneisses and 1,690-m.y.-old granites that comprise basement to the siliciclastic Uncompahgre Group. The mafic and felsic gneisses, which contain primary structures indicative of volcanic and plutonic protoliths, under-went two phases of isoclinal folding and foliation development prior to emplacement of the 1,690-m.y.-old plutons. Synkinematic garnet porphyroblast textures suggest that these fabrics developed during a progressive deformation event, D_B. Subsequent D_BC deformation caused folding of D_Bfabrics in the gneisses; development of a subvertical, east-striking foliation in the granites; and generation of a macroscopic sigmoidal foliation pattern throughout the area. D_BC structures in the basement are correlated with structures in the Uncompahgre Group and formed prior to pluton emplacement at ca. 1440 Ma. Gently east-plunging mineral lineations in the granites and quartz c-axis fabrics of both the gneisses and granites indicate subhorizontal extension on steeply dipping foliation surfaces during D_BC. Asymmetric c-axis fabrics and rare mesoscopic shear sense indicators in the gneisses record a component of dextral shear in domains of east-striking foliation and sinistral shear in areas of northeast-striking foliation. In contrast, symmetric c-axis fabrics in the granites suggest nearly coaxial extension. A model for D_BC involving the development of conjugate strike-slip shear zones in response to north-northwest shortening is most consistent with the kinematic and fabric-orientation data. Partitioning of deformation into subparallel zones of noncoaxial and coaxial deformation was controlled by the presence of a strong pre-existing anisotropy in the gneiss complex. The shortening direction during D_BC is consistent with that related to north-northwest-directed thrusting in rocks of similar age in central Arizona and northern New Mexico. Therefore, D_BC structures are tentatively interpreted to have accommodated transverse shortening and orogen-parallel extension within a foreland during regional crustal shortening between ca. 1690 and 1400 Ma.

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