Three-dimensional kinematic history at an oblique ramp, Leamington zone, Sevier belt, Utah

Abstract

Differences in kinematics in adjoining salients in fold–thrust belts are typically accommodated along bounding transverse zones. The Leamington zone is an oblique ramp that accommodates slip between structures in the Provo salient and the central Utah segment of the Sevier fold–thrust belt. The Leamington zone consists of the Leamington Canyon thrust, associated second-order asymmetric folds and an out-of-syncline reverse fault. The Leamington Canyon thrust is a rotated, SE-dipping thrust fault with top-to-southeast motion; it represents an oblique ramp of the folded Canyon Range thrust. Fold tightening of the syncline SE of the folded Leamington Canyon thrust trace during later emplacement of underlying structures, caused out-of-syncline reverse faulting in the fold-core. Analysis of the finite strain, minor fault populations, and conjugate fracture sets were used in interpreting the 3-D kinematic history along the Leamington zone. Restored maximum stretching directions of finite strain ellipsoids trend eastward overall. Inferred motion planes from populations of slickenlines on minor faults are consistent with overall southeastward motion. Acute bisectors of conjugate fracture sets may reflect more southward directions later in the deformation history. These superimposed deformations, with local relative chronologies of plastic and brittle structures using cross-cutting relationships, indicate that the Leamington zone area shows overall clockwise changes in kinematic directions from E to ESE to SSE during successive pulses of deformation. The temporal changes in kinematic directions along the Leamington zone most likely reflect variations in kinematics over the oblique ramp as the overall easterly displaced 3-D fold–thrust belt wedge interacted with the pre-existing oblique ramp structure, with small magnitudes of superimposed local vertical-axis rotations during later folding and fold-tightening of the Leamington Canyon thrust.