Flow perturbation folding in shear zones
Published:January 01, 2007
G. I. Alsop, R. E. Holdsworth, 2007. "Flow perturbation folding in shear zones", Deformation of the Continental Crust: The Legacy of Mike Coward, A. C. Ries, R. W. H. Butler, R. H. Graham
Download citation file:
Variable patterns of displacement in shear zones may result in arcuate fold and fault traces as recognized by Mike Coward and co-workers in the early 1980s. Within such deformation zones, localized perturbations in flow may undergo acceleration (surging flow) or deceleration (slackening flow) with respect to the adjacent regions. Such flow cells may govern the orientation and geometries of folds and fabrics and thereby provide evidence of the scale and nature of deformation associated with heterogeneous flow in the high-strain zones. The length/width ratio of individual flow cells (measured in the direction of flow) may vary from <1 to >1 for situations when flow cells are, respectively, dominated by layer-parallel shear (LPS) or layer-normal shear (LNS). Folds initiating at high angles to transport are associated with LPS, whereas LNS may generate folds with slight clockwise (sinistral LNS) or anticlockwise (dextral LNS) trends relative to flow. Continued progressive deformation may subsequently modify and reduce angular relationships between folds and fabrics, but these geometric obliquities are generally preserved. Using examples from the Moine metasediments of northern Scotland, we show that folding displays predictable geometric patterns that can be related to the development of flow perturbation cells associated with Caledonian ductile thrusting under mid-crustal conditions. The differing relative timing of folds and individual ductile thrusts reflects the complexity of flow cells within ductile imbricates and additionally highlights the progressive foreland-directed propagation of ductile thrusting. These geometric relationships developed during contractional shear are compared with those generated in extensional systems, to provide an overall framework for the study of perturbation patterns. The geometric arrangement of mean fold axial planes about the flow direction results in their intersection forming parallel to the transport direction. This relationship permits transport directions to be calculated via the axial-planar intersection method (AIM), and also allows comparison with other techniques devised primarily for the study of soft-sediment deformation and palaeoslope analysis.
Figures & Tables
Deformation of the Continental Crust: The Legacy of Mike Coward
This Special Publication, in memory and celebration of the work of Professor Mike Coward, is about the deformation of the continental lithosphere. The collected papers discuss geometry, structural principles, processes and problems in a wide range of tectonic settings and thereby reflect the breadth of Coward's interests. They encompass the evolution of Precambrian basement gneiss terrains, the geometry and evolution of thrust systems, basement involvement and structural inheritance in basins, syn-orogenic extension, salt tectonics, the implication of structural evolution on hydrocarbon prospectivity and structural controls on mineralization. Examples are drawn from the Lewisian and Moine Thrust Belt of NW Scotland, the Italian Apennines, NW Himalayas, the Cyclades, Oman, Zagros Mountains, Colombian Cordillera, Carpathians, North Sea, offshore Brazil, regional studies of the Irumide Belt (central Africa), Taurus Mountains (Turkey), greater South America, and from the Witwatersrand Basin of South Africa and the Antler Orogeny of SW USA.