Reconstructing the kinematic evolution of curved mountain belts: A paleomagnetic study of Triassic red beds from the Wyoming salient, Sevier thrust belt, U.S.A.

Abstract

Determining the kinematic history and mechanics of curved fold-and-thrust belts is fundamental to understanding the tectonic evolution of mountain systems. To better understand the development of a classic curved fold-and-thrust belt, we completed an integrated paleomagnetic and strain study of the Wyoming salient. Paleomagnetic data are reported here from 154 sites collected from red beds of the Triassic Ankareh Formation in the salient and nine sites collected from the relatively stable foreland. Red beds display three components with distinctly different magnetic behaviors: (1) a near-primary Triassic magnetization carried by hematite that is stable up to 680 °C (Tr component, 91 sites); (2) a Cretaceous chemical remagnetization carried partly by magnetite (K component, 32 sites); and (3) a recent viscous magnetization that is mostly removed by 350 °C. Site mean vectors for the Tr and K components show a high degree of scatter from expected Triassic and Cretaceous reference directions, suggesting significant tilt and rotation subsequent to magnetization acquisition. Restoration of tilt and folding for individual site means results in well-clustered shallow and moderate inclinations for the Tr and K components, respectively, and in variable declinations related to systematic vertical-axis rotations. Statistical analysis of declinations for both components indicates that ∼75% of present-day salient curvature resulted from secondary rotation, and ∼25% of primary curvature was likely related to sedimentary basin architecture. Analysis of individual thrust systems indicates a slightly greater component of rotation in more internal sheets (∼80%) compared to the frontal thrust sheets (∼65%), suggesting that rotations were concentrated near the leading edge of the propagating fold-and-thrust wedge, with only minor additional rotation of internal sheets. Transfer zones, oblique ramps, and more deformed overturned fold limbs display locally more complex patterns, which can be understood through careful structural analysis. When combined with internal strain data and regional structural relations, paleomagnetic data support a kinematic model of a progressive arc with curved thrust-slip paths and differential shortening that rotated early layer-parallel shortening fabrics and produced minor strike-parallel extension. This kinematic history likely reflects a combination of processes, including greater initial stratigraphic thickness and subsequent shortening and wedge propagation in the central part of the salient, presence of a weak basal detachment and fault-zone weakening that favored lower taper, and buttressing by Laramide foreland uplifts that formed along basement promontories at the north and south ends of the salient.