Polyphase suprastructure deformation in metasedimentary rocks of the Uncompahgre Group: Remnant of an early Proterozoic fold belt in southwest Colorado
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, F 1 mesoscopic folds, and cleavage S 1 . Continued crustal shortening during phase 2 resulted in upright macroscopic folding (F 2 ) and cleavage (S 2 ) 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 F 3 fold and an S 3 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.