ABSTRACT The Willard-Paris-Meade thrust sheet is the oldest and westernmost sheet to develop in the Wyoming salient of the Sevier fold-and-thrust belt. This dominant thrust sheet was transported ~60–70 km eastward, but despite its importance to the overall shortening budget of the Cordilleran orogenic system, the timing for onset and cessation of major fault slip is poorly constrained. Synorogenic strata associated with emplacement of the sheet thicken westward, reaching a total thickness >2 km near the thrust front, and are divided into five intervals: (1) lower and (2) upper parts of the Aptian(?) to Albian Gannett Group (Ephraim and Bechler Formations); (3) Upper Albian Smiths, Thomas Fork, and Cokeville Formations and correlative Bear River Formation; (4) uppermost Albian to Cenomanian Sage Junction Formation and partly correlative Aspen Shale; and (5) Upper Cenomanian to Turonian Frontier Formation. Integrated sedimentologic, detrital zircon provenance, and subsidence analyses of these foreland basin strata record a protracted history of thrust sheet emplacement. Detrital zircon U-Pb age spectra, conglomerate clast types, and sandstone petrography record an unroofing sequence, with progressive erosion of Lower Mesozoic to Upper Paleozoic to Lower Paleozoic bedrock levels, starting in the more western Paris sheet and continuing across the Meade and Willard sheets. Synorogenic strata also contain young grains derived from the Cordilleran magmatic arc that bracket maximum depositional ages of: 115.2 ± 1.8 Ma and 116.2 ± 2.3 Ma in the upper Ephraim Formation to 107.9 ± 2.4 Ma in the overlying Bechler Formation, which were deposited during early thrust slip; 101.6 ± 0.7 Ma, 101.4 ± 0.6 Ma, and 98.8 ± 0.4 Ma in the Cokeville, Sage Junction, and Aspen Formations, respectively, which were deposited during increased subsidence and thrust slip rates; and 95.6 ± 1.4 Ma in the Frontier Formation, which was deposited during late slip, eastward thrust propagation, and early uplift of the Wasatch anticlinorium. The lower Ephraim Formation was deposited during the Early Cretaceous Cordilleran magmatic lull and lacks young zircon grains, such that timing for thrust initiation is not well constrained, ranging from ca. 130 to 120 Ma. Regional variations in stratigraphic thickness and sedimentation rates, combined with flexural modeling, indicate that a foredeep began developing in front of the Paris thrust and then migrated eastward as the Meade and Willard thrusts propagated and overrode their own synorogenic strata, with increasing rates of thrust loading and subsidence from ca. 105 Ma to 95 Ma.

Abstract Tectonic style of the late Mesozoic to early Cenozoic(?) Sevier orogenic belt in Utah was greatly affected by preexisting structural trends that date from late Precambrian rifting and fragmentation of the North American continent. The frontal zone of the thrust belt was superimposed on the edge of the late Precambrian craton (Cordilleran hingeline) that was marked by a system of prominent faults and structural highs, such as the Fillmore arch, the north-south-trending ancestral Wasatch and Ancient Ephraim faults, and the northeast-trending Leamington, Scipio, Cove Fort, and Paragonah lineaments. The renewed activity of these structures affected the geometries of the late Paleozoic Ancestral Rocky Mountain uplifts and basins, the extent of the Jurassic evaporitic Arapien basin, and the sedimentary pattern of the Cretaceous foreland basin. During the compressional Sevier tectonism, some of these fault zones were reactivated as tectonic ramps (e.g., the Ancient Ephraim fault) and tear faults (e.g., the Leamington fault). The Fillmore arch and other structural highs along the edge of the late Precambrian craton caused ramping of the inner Keystone, Pavant, Canyon, Paris, and Willard thrust sheets and telescoping of the frontal thrust sheets. During the Basin and Range extension, a major low-angle, normal fault (the Sevier Desert detachment) developed at the western side of the uplifted Fillmore arch. The northeast-trending Leamington, Cove Fort, and Paragonah lineaments were reactivated as left-lateral strike-slip faults. Mobilization of major fault zones that were inherited from late Precambrian rifting may account for some crustal shortening during the compressional Sevier orogeny and, conversely, for some crustal extension during the Basin and Range extensional event.