THE GEOMETRY, KINEMATICS AND DEFORMATION CHARACTERISTICS OF THE LEAMINGTON CANYON TRANSVERSE ZONE, CENTRAL UTAH

Differences in thrusting in adjoining salients of a fold-thrust belt (FTB) are typically accommodated along transverse zones. The Leamington Canyon fault (LCF) in west central Utah is an example of a transverse zone that extends 50km obliquely across the trend of the Sevier FTB and separates the Provo salient from the central Utah segment. It trends ENE-WSW, dips steeply toward SSE, and places Proterozoic and upper Paleozoic quartzites over Paleozoic sandstones and limestones. It has been variously interpreted either as a thrust fault with up-dip motion or as a tear fault with right-lateral movement.

Mesoscopic and microscopic structures in the LCF zone were analyzed to evaluate its kinematics and deformation characteristics. Small-scale fault arrays define the motion plane (M-plane) to be steeply dipping and trending NW-SE. This is supported by the orientation of small-scale folds, with fold-axes parallel to the LCF trend and gentle SW plunge. The sense of shear in the hanging wall of the LCF, from the acute angles of cleavage-bedding intersections and, at the microstructural scale, from asymmetric cross girdles of quartz C-axis fabrics, shows top-to-southeast down-dip motion. Microstructures in the hanging wall quartzites include intragranular and transgranular cracks, zones of cataclasis, recystallization, undulose extinction, grain boundary migration, serrated grain boundaries, deformation lamellae and pressure solution features. These features indicate crystal plastic deformation accompanied by diffusive mass transfer early in the deformation history, followed later by dominant cataclasis as the rocks were brought closer to the surface. The strain in the transport plane in both the hanging wall and the footwall of the LCF shows low shear strains (g=0.1 - 0.25) and low stretch (e=0.8 - 1.1). These early plastic strains are due to initial LPS modified by fault parallel shear accompanied by flattening. The orientation of strain ellipse long-axes with respect to the fault further supports our kinematic interpretation. From all the evidence, we conclude that the LCF is a rotated (folded?) thrust fault dipping southeast with top-to-southeast down-dip motion, and interpret it to be a lateral ramp of the folded Canyon Range thrust to the south.