Strain distribution, strain history, and kinematic evolution associated with the formation of arcuate salients in fold-thrust belts: The example of the Provo salient, Sevier orogen, Utah

The Provo salient of the Sevier fold-thrust belt in central Utah has a prominent arcuate shape in map view with the thrust traces strongly convex toward the foreland. We use the Provo salient as an example for understanding the kinematics of the formation of salients. The kinematic information from the preserved mesoscopic and microscopic structures related with the Cretaceous Sevier orogeny are used to infer two superimposed phases of deformation that define the early and the late directions of transport for formation of the Provo salient. These new three-dimensional kinematic results, together with available data from the Sheeprock thrust sheet and along the Leamington transverse zone, are used to distinguish between five end-member models of salient formation; namely, "bow and arrow rule," "orocline," "divergent transport," "tear fault boundaries, and "lateral or oblique ramp boundaries." The tectonic transport directions from the long-axis orientations of the finite strain ellipsoids yield trends generally parallel to the overall W-E transport direction (080°-117°) in the middle of the Provo salient, in the Midas, East Tintic, and Tintic Valley thrust sheets, with e ₁ /e ₃ axial ratios of 1.19-1.36. The Charleston-Nebo thrust sheet, in the foreland portion of the Provo salient, shows consistent NE (026°-062°) transport directions and axial ratios ranging from 1.19 to 1.46. Along the Leamington transverse zone, the long-axes orientations from strain ellipsoids trend E to ESE (091°-122°), and strain axial ratios range from 1.13 to 1.50. Octahedral strain values (ε _s ) are relatively greater in the hinterland thrust sheet (viz. Sheeprock thrust sheet; ε _s = 0.12-0.83) than other thrust sheets (ε _s = 0.13-0.43). Analysis of fracture populations of the late (open) fractures within the Provo salient demonstrates that the late tectonic transport directions trend almost parallel to the early W-E transport directions in the middle of the salient, indicating that they remained constant during successive phases of crustal shortening. However, analyses of fracture populations from two different periods of fractures (viz.fractures with slickenlines and late open fractures) based on cross-cutting relationships indicate that late stage transport directions changed temporally along the edges of the salient. Overall, the variations in directions of both the early and the late transport show a divergent pattern. These variations in transport direction in different parts of the Provo salient suggest the possibilities of either lateral or oblique ramp boundaries or divergent transport. Detailed three-dimensional kinematic study along the Leamington transverse zone shows that the original E-W transport direction is progressively changed by interaction with the oblique ramp, suggesting that a model of modification at lateral or oblique ramp boundaries best explains the structure and kinematics at the edges of the Provo salient. In contrast, three-dimensional strain in the Sheeprock thrust sheet shows stretching perpendicular to the transport direction in the back end of the sheet and stretching parallel to the transport direction at the front of the sheet, indicating the possibility of divergent transport. We suggest that the Sheeprock thrust sheet evolved by divergent transport in the hinterland portion of the salient without any interaction with lateral or oblique ramp structures during relatively strong convergence, while areas of the frontal thrust sheets (e.g., Charleston-Nebo thrust) experienced a different mode of salient formation with W-E transport being modified by lateral or oblique ramp boundaries during relatively weak convergence.