IMPLICATIONS OF THREE-DIMENSIONAL THRUST SURFACE GEOMETRY ON THRUST SHEET KINEMATICS: A STUDY ALONG THE MOINE THRUST, NW SCOTLAND

In map pattern, fault traces in fold-thrust belts (FTBs) are typically curvi-linear, often comprised of a series of salients and recesses suggesting that in three-dimensions, faults have non-planar geometries. Such three-dimensional "irregularities" in the fault surface may have important implications for the overall kinematics of FTBs. We report quartz c-axis patterns, grain-shape fabrics, and microstructures for footwall quartzites and hanging wall quartz-phyllosilicates from a salient-recess pair in the Moine thrust zone between Loch Srath nan Aisinnin and the Stack of Glencoul, NW Scotland. Within the recess, the footwall samples > 53 m normal distance (nd) from the fault trace have quartz c-axis fabrics which resemble a hybrid of a small circle girdle and a symmetric cross-girdle both centered about the normal to foliation thus indicating a general flattening strain. Flinn's K-values from deformed relict grains range between 0.0524 and 0.659, and the octahedral shear strains (e_s) range between 1.10 and 1.47. Samples closer to the thrust, (within 8 m nd), also within the recess, are completely recrystallized and exhibit asymmetric c-axis patterns (some with vorticity parallel maxima) and Regime 3 type microstructures. The long axes of these recrystallized quartz grains are gently plunging and oriented subparallel to the regional transport direction. Within the hanging wall, the recrystallized quartz grains have their long axes parallel to the regional transport direction. Moreover, quartz overgrowths form radially about opaque grains within the foliation plane, further supporting the inference of general flattening strains. Within the salient, footwall samples, ranging from 3.7 to 110 m nd from the thrust surface, have relict grains which demonstrate lower e_s (0.55 - 1.01) than seen in the adjoining recess, and have K-values (0.40 - 1.56) more closely approximating plane strain deformation. Furthermore, these samples exhibit weak to undeveloped quartz c-axes patterns. At 1 m nd, relict grains are still preserved, exhibiting Regime 2 type microstructures, an e_s of 1.21, and a symmetric cross-girdle c-axis fabric. The lateral variation seen between the salient and the recess are reflexive of the variable kinematics within non-planar thrust systems and therefore thrust fault kinematics in general.