Abstract

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, DB. Subsequent DBC deformation caused folding of DBfabrics in the gneisses; development of a subvertical, east-striking foliation in the granites; and generation of a macroscopic sigmoidal foliation pattern throughout the area. DBC 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 DBC. 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 DBC 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 DBC is consistent with that related to north-northwest-directed thrusting in rocks of similar age in central Arizona and northern New Mexico. Therefore, DBC 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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