Abstract

Isolated remnants of supracrustal rocks of the Uncompahgre Group in the Needle Mountains and Uncompahgre Gorge area have been subjected to polyphase deformation and greenschist-facies metamorphism. Phase 1 structures (recognized only in the West Needle Mountains) include bedding-parallel, ductile shear zones, F1 mesoscopic folds, and cleavage S1. Continued crustal shortening during phase 2 resulted in upright macroscopic folding (F2) and cleavage (S2) development. In addition, basement-cover contacts were folded. Further shortening during phase 3 deformation resulted in conjugate shearing of cover rocks with the development of both a macroscopic F3 fold and an S3 crenulation cleavage. In addition, basement-cover contact zones were activated as phyllonitic, oblique-slip, reverse faults. Conjugate shearing resulted in lateral offset of stratigraphic units, with loss of section. Conjugate reactivation of phase 1 ductile deformation zones was manifested by oblique dextral and sinistral slip.

The structural evolution of the Uncompahgre Group records the transition from a ductile, north-directed, thin-skinned, fold-thrust belt to the formation of a basement-involved "megamullion" structure which was subjected to conjugate strike-slip faulting to accommodate further shortening. The results of this study indicate that the basal unit of the Uncompahgre Group was parautochthonous on basement, a departure from some previous interpretations. This implies that the metasedimentary cover rocks are younger than surrounding ca. 1750 Ma basement gneisses and ca. 1690 Ma granitoids. The over-all style of deformation may be analogous to the deep foreland suprastructure of an orogenic belt that developed between ca. 1690 Ma and 1600 Ma in the southwestern United States.

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