3-D STRAIN SYMMETRY AND VORTICITY OF FLOW ALONG THE MOINE THRUST, NORTHWEST SCOTLAND: IMPLICATIONS FOR THRUST SHEET EVOLUTION AT MID-CRUSTAL LEVELS

Traditional models of ductile thrust sheet emplacement that assume either simple shear or some other form of plane strain as the dominant deformation symmetry are remarkably simple, especially given the observed variation of mylonitic fabrics observed within ductile thrust sheets. L, S, and L-S tectonites, which are believed to approximate varying amounts of orogen parallel stretching and constriction, are observed both together and within distinct separate zones. This may indicate that simple shear or plane strain models cannot meaningfully describe ductile thrust kinematics. Although some studies have suggested that these variations may represent apparent rather than real strains, either as a result of volume loss or through superposition of multiple deformation episodes, current analytical techniques for quantifying strain magnitude, strain symmetry, and vorticity of flow in three dimensions may allow separation of the former from the latter.

The Moine thrust zone from the north Sutherland coast to the Knockan region represents the perfect setting to test these ideas because: 1) the lower to mid-crustal ductile Moine thrust is well-exposed, and the position of this structure has been well-documented by numerous BGS and academic geologic maps: 2) mylonites derived from rocks of the foreland and overlying Moine nappe are lithologically well-suited for numerous strain symmetry and vorticity techniques: 3) the temperatures of deformation (~350-550° C) in the mylonite zone and overlying Moine nappe produced brittle deformation and rotation of feldspar and other “rigid” grains often in a plastically deforming quartz or phyllosilicate matrix: 4) the mylonitic foliation is sub-parallel to the pervasive fabrics (D2) in the overlying Moine nappe in this region, which has been attributed to a single progressive deformation event.

By quantifying 3-D strain symmetry and vorticity of flow along an orogenic scale ductile thrust, insight may be gained on the kinematics of ductile thrust deformation and movement of material during emplacement. This quantification will also allow proposed models of ductile thrusting and deformation to be tested, including: 1) gravity spreading and flow, 2) orogenic wedge extrusion, 3) dislocation models, 4) surge and slack zones, and 5) flow pertubation folding.