THE 3-D KINEMATICS OF DEFORMATION AT AN OBLIQUE RAMP, LEAMINGTON CANYON FAULT, CENTRAL UTAH

Kinematic differences in adjoining salients of a fold-thrust belt (FTB) are typically accommodated along transverse zones. The Leamington Canyon fault (LCF) is an example of an oblique transverse zone (likely an oblique ramp) that separates the Provo salient from the central Utah segment of the Sevier FTB in west central Utah. We used hanging wall quartzites of the folded LCF to determine 3-D strains, using the Fry technique on three mutually perpendicular sections. The average e₁/e₃ strain ratio is 1.3 and K-values (Flinn, 1962) range from plane strain to the flattening field. The 3-D strain ellipsoid long-axes vary in trend (90°- 115°) and plunge (1°- 68° to the NW). The 3-D strains clearly indicate that non-plane strains best describe the deformation at the LCF.

These plastic strains represent an early increment of deformation and reflect variations in transport over an oblique ramp, but the orientations of markers have been modified by subsequent folding of the LCF. The expected deflection of an assumed W-E regional transport direction by the oblique ramp was calculated using the kinematic model of Apotria et al (1992); the model results were then passively folded along the LCF trend. Regardless of the oblique ramp dip, the measured 3-D strain ellipsoid long-axes show more deflection toward the oblique ramp strike than the maximum possible from the kinematic model. The measured deflections are also supported by late phase transport directions (145°), determined from outcrop scale fault arrays with slickenlines.

These observations can be explained by two alternative kinematic models of salient formation: (1) deflection is primary as the fault grows in both transport and lateral directions (i.e. divergent flow) and (2) deflection is secondary, resulting from a combination of movement of the thrust sheet over the oblique ramp and superimposed components of local vertical-axis rotations.