The Southern Uplands Accretionary Prism: Implications for Controls on Structural Development of Subduction Complexes
Published:January 01, 1982
J. K. Leggett, D. M. Casey, 1982. "The Southern Uplands Accretionary Prism: Implications for Controls on Structural Development of Subduction Complexes", Studies in Continental Margin Geology, J. S. Watkins, C. L. Drake
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Subduction complexes preserving extensive coherent strata beneath lower slope sediments can be more useful than those dominated by melange for studying the sequence of deformation caused by accretion. In the southern Uplands, a 70 to 80-km-wide coherent accretionary complex comprising imbricated thin ocean-floor (basalt, metalliferous sediment, radiolarian chert, black graptolitic shale) and thick trench (volcaniclastic greywacke) deposits records 50 to 60 million years of accretion during northward subduction of Iapetus oceanic crust under Ordovician and Silurian southern Scotland, then part of the southern margin of ancient North America (Laurentia). Like many more recent coherent accretionary terranes, the Southern Uplands complex appears to have developed about a slow convergence-high sediment input subduction zone. Competency contrasts in the subducting sequence controlled the mesoscopic and macroscopic structural style and determined the type of strata accreted. Detailed mapping of three new map areas shows a generalized tectonic history in which compressive stress is taken up by: 1) initial decollement (offscraping) commonly at or above a black shale unit (the Moffat Shale Group) underlying the greywackes; 2) local fold development, during and after isolation of discrete linear packets of offscraped strata; 3) variable cleavage development, commonly transecting folds; 4) strain-hardening, tightening, and eventual locking of each packet so that deformation switches to a new packet at the base of the inner trench slope; 5) subsequent more-or-less bedding-parallel (commonly intense) imbrication within packets as they are uplifted and rotated.
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Studies in Continental Margin Geology
"Studies in Continental Margin Geology" contains papers from a research conference co-sponsored by AAPG and the University of Texas Institute for Geophysics held in Galveston, Texas in 1981. Rapid advances in the understanding of continental margin geology were taking place during the time period, based on major improvements in the quality and availability of regional seismic surveys plus other fields such as organic geochemistry. For the first time it was becoming common to have a visual characterization of tectonic processes at significant depths below the surface. Twenty-seven papers are presented that deal with field investigations of continental margin structure and stratigraphy. The geographic areas of study are global in nature and many of the descriptive results are derived from modern seismic investigations in areas where that type of data had not previously been available in commercial publications. Fifteen of the papers focus on rifted margins and the other twelve concern convergent margins. Twelve papers are model investigations of a variety of margin environmental processes, related to subjects such as depositional environments, biostratigraphy, organic matter deposition, and oil and gas occurrences as a function of the plate tectonic setting. An additional nine papers model the thermal and mechanical tectonic processes involved in the structural development along continental margins.