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Research Article| May 01, 1990 Polyphase suprastructure deformation in metasedimentary rocks of the Uncompahgre Group: Remnant of an early Proterozoic fold belt in southwest Colorado CHARLES W. HARRIS CHARLES W. HARRIS 1Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 Search for other works by this author on: GSW Google Scholar Author and Article Information CHARLES W. HARRIS 1Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1990) 102 (5): 664–678. https://doi.org/10.1130/0016-7606(1990)102<0664:PSDIMR>2.3.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation CHARLES W. HARRIS; Polyphase suprastructure deformation in metasedimentary rocks of the Uncompahgre Group: Remnant of an early Proterozoic fold belt in southwest Colorado. GSA Bulletin 1990;; 102 (5): 664–678. doi: https://doi.org/10.1130/0016-7606(1990)102<0664:PSDIMR>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search 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. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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