Tectonics of the Uinta recess in the Sevier fold-thrust belt, Utah

The Sevier fold-thrust belt makes a pronounced concave-to-the-foreland (syntaxial) map-view curve, known as the Uinta recess, at the west end of the E-W-trending Uinta-Cottonwood (U/C) arch in north-central Utah. As a consequence, traces of Sevier-age faults trend subparallel to the north and south flanks of the arch. Because these zones trend at a high-angle to the regional structural grain of the Sevier belt, we refer to them as transverse zones. I conducted a structural analysis of major E-W-trending Sevier fault segments adjacent to the U/C arch (namely, segments of the Charleston transverse zone (CTZ) on the south and of the Mount Raymond transverse zone (MRTZ) on the north) in order to address the following outstanding problems: (1) How do thrust belt transverse zones develop and behave? (2) What is the origin of the Uinta recess? Did the recess originate during Sevier thrusting or because of Laramide uplift of the U/C arch? and (3) Why did the recess develop where it did?; The CTZ consists of a complexly branching array of faults and folds that resembles a flower structure formed by left-lateral strike-slip movement. Laramide uplift of the U/C arch shallowly tilted structures to the south and subsequent Tertiary oblique extension with a right-lateral component of shear, reactivated and formed new faults within the zone. I suggest that the E-W trend of the CTZ developed when it acted as a tear zone accommodating differential foreland propagation of the Sevier belt to the north with respect to the belt to the south. In contrast, the MRTZ originated as an ESE-verging, low-angle fold-thrust system. Laramide uplift of the U/C arch tilted the MRTZ to the north, so that faults within the zone presently trend E-W. Later, Tertiary extension also reactivated and formed new faults within the zone. Thus, in sum, the recess originated during Sevier thrusting, but was amplified by later Laramide and Tertiary tectonism.; Regional synthesis of stratigraphic data and study of thrust-belt analog models suggests that the map-view geometry of the Sevier fold-thrust belt was influenced during formation, at the site of the future U/C arch because the site marked an abrupt change in the thickness of the sedimentary basin in which the fold-thrust belt formed. Basin geometry, in turn, was affected by reactivation of a long-lived basement fault system. Abrupt thickening of sedimentary strata to the south of this fault zone led to the development of the CTZ, for thrust systems propagate further to the foreland where strata are thicker. Thinning of strata adjacent to the north flank of the basement fault zone similarly retarded foreland propagation of the thrust belt, but because the thinning was not so abrupt as on the south side, an ESE-verging low-angle thrust and fold system developed, rather than a tear. In sum, the Uinta recess reflects both variation in sedimentary basin thickness prior to the origin of the Sevier fold-thrust belt, and the affects of later (Laramide) uplift and Tertiary extension.