Fault Slip and Exhumation History of the Willard Thrust Sheet, Sevier Fold‐Thrust Belt, Utah: Relations to Wedge Propagation, Hinterland Uplift, and Foreland Basin Sedimentation

Zircon (U‐Th)/He (ZHe) and zircon fission track thermochronometric data for 47 samples spanning the areally extensive Willard thrust sheet within the western part of the Sevier fold‐thrust belt record enhanced cooling and exhumation during major thrust slip spanning approximately 125–90 Ma. ZHe and zircon fission track age‐paleodepth patterns along structural transects and age‐distance relations along stratigraphic‐parallel traverses, combined with thermo‐kinematic modeling, constrain the fault slip history, with estimated slip rates of ~1 km/Myr from 125 to 105 Ma, increasing to ~3 km/Myr from 105 to 92 Ma, and then decreasing as major slip was transferred onto eastern thrusts. Exhumation was concentrated during motion up thrust ramps with estimated erosion rates of ~0.1 to 0.3 km/Myr. Local cooling ages of approximately 160–150 Ma may record a period of regional erosion, or alternatively an early phase of limited... (see full abstract in article)

Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya

Geochemical and geochronological analyses provide quantitative evidence about the origin, development and motion along ductile faults, where kinematic structures have been overprinted. The Main Central Thrust is a key structure in the Himalaya that accommodated substantial amounts of the India–Asia convergence. This structure juxtaposes two isotopically distinct rock packages across a zone of ductile deformation. Structural analysis, whole-rock Nd isotopes, and U–Pb zircon geochronology reveal that the hanging wall is characterized by detrital zircon peaks at c. 800–1000 Ma, 1500–1700 Ma and 2300–2500 Ma and an εNd(0) signature of –18.3 to –12.1, and is intruded by c. 800 Ma and c. 500–600 Ma granites. In contrast, the footwall has a prominent detrital zircon peak at c. 1800–1900 Ma, with older populations spanning 1900–3600 Ma, and an εNd(0) signature of –27.7 to –23.4, intruded by c. 1830 Ma granites. The data reveal a c. 5 km thick zone of tectonic imbrication, where isotopically out-of-sequence packages are interleaved. The rocks became imbricated as the once proximal and distal rocks of the Indian margin were juxtaposed by Cenozoic movement along the Main Central Thrust. Geochronological and isotopic characterization allows for correlation along the Himalayan orogen and could be applied to other cryptic ductile shear zones

Stratigraphic, geochronologic, and geochemical record of the Cryogenian Perry Canyon Formation, northern Utah: Implications for Rodinia rifting and snowball Earth glaciation

The Cryogenian Perry Canyon Formation, formally named herein with a type section located in northern Utah, consists of a 0.3- to 1.5-km-thick succession of diamictite- and volcanic-bearing strata that record glacial events and early rifting along wester

Mesozoic Tectonics of the Northern Wasatch Range, Utah

The geometry and chronological development of regional structures in the Idaho-northern Utah-Wyoming fold and thrust belt are well constrained (Armstrong and Oriel, 1965; Royse and others, 1975; Dixon 1982; Lamerson, 1982; Wiltschko and Dorr, 1983; Oriel, 1986), making this an excellent area to study kinematics of thrust systems. Major thrusts cut up section to the east and transported Paleozoic and Mesozoic shelf and miogeoclinal strata relatively eastward above a regional decollement. Large-scale fault-bend and fault-propagation folds with concentric and kind geometries are associated with the thrusts. Synorogenic conglomerates record the successive emplacement of thrust sheets from west to east during the Mesozoic to early Tertiary Sevier orogeny. The fold and thrust belt forms a salient convex to the east, and is separated from thrust systems in central Utah by the Cottonwood and Uinta arches. The four major thrust systems exposed in northern Utah are from west to east the Willard, Ogden, Crawford, and Absaroka systems (Figs. 1 and 2). The frontal Hogsback thrust is exposed to the east in Wyoming. Basement rocks are absent in the frontal thrust sheets but are incorporated into thrust sheets in the Wasatch Range (Bell, 1951; Bruhn and Beck, 1981; Yonkee, 1990) and farther west (Snoke and Miller, 9188). The frontal thrust sheets are unmetamorphosed, but metamorphic grade locally reaches greenschist facies in the Wasatch Range (Yonkee, 1990), and increases in overall grade to the west (Snoke and Miller, 1988)

Timing of Exhumation, Wheeler Pass thrust sheet, southern Nevada and California: Late Jurassic to middle Cretaceous evolution of the southern Sevier fold-and-thrust belt

Zircon (U-Th)/He (ZHe) thermochronologic data for 46 samples from the Wheeler Pass thrust sheet, which carries thick passive-margin strata and is discontinuously exposed across the southern part of the Sevier fold-and-thrust belt, record Late Jurassic cooling related to exhumation during early thrust slip, and local middle Cretaceous cooling related to footwall basement imbrication. Within the frontal part of the Wheeler Pass sheet exposed in the northwest Spring Mountains, ZHe ages decrease from ca. 160 to 140 Ma over a paleodepth interval of ∼6 to 9 km in Devonian to Lower Cambrian strata, interpreted to record exhumation and enhanced cooling above a frontal ramp, followed by slow cooling. A similar pattern of ZHe ages occurs over a more limited paleodepth range exposed within the Resting Spring Range. To the south in the Nopah Range, ZHe ages of ca. 140 Ma in Lower Cambrian strata record early cooling, followed by slow cooling until ca. 100 Ma. ZHe ages in underlying Paleoproterozoic basement rocks decrease from ca. 100 to 85 Ma over a paleodepth interval of ∼9 to 12 km, interpreted to record uplift, exhumation, and cooling during footwall basement imbrication as slip was transferred eastward onto the Keystone thrust system. Late Jurassic slip on the Wheeler Pass thrust overlapped with deformation along the NW-trending East Sierran thrust system, early hinterland crustal thickening, and growth of the Sierra magmatic arc, and may have been correlative with early deformation in the Central Nevada thrust belt. Slip on the Wheeler Pass thrust preceded major slip on other thrust sheets that carried thick passive-margin strata in more northern parts of the Sevier belt, probably reflecting the influence of different initial widths and tapers of the passive-margin sedimentary wedge

Tectono-stratigraphic framework of Neoproterozoic to Cambrian strata, west-central U.S.: Protracted rifting, glaciation, and evolution of the North American Cordilleran margin

Stratigraphic, geochronologic, and geochemical patterns of Neoproterozoic to Cambrian sedimentary and volcanic rocks in Utah, Nevada, and SE Idaho record a dynamically evolving landscape along the North American Cordillera margin, which included: (1) initial development of intracratonic basins with deposition of siliciclastic strata of the Uinta Mountain Group from ~. 770 to 740. Ma; (2) early rifting and volcanism along a N-S (present day geographic coordinates) basin system with deposition of diamictite-bearing strata of the Perry Canyon and related formations from ~. 720 to 660. Ma; (3) early, broad subsidence with deposition of mature siliciclastic strata of the lower Brigham and McCoy Creek groups from ~. 660 to 580. Ma; (4) final rifting, volcanism, and transition to drift with deposition of variably immature siliciclastic strata of the Prospect Mountain and correlative formations from ~. 570 to 520. Ma; and (5) regional subsidence along a passive margin with deposition of Middle Cambrian to Devonian carbonate-rich strata. The Uinta Mountain Group comprises fluvial to marine, feldspathic to quartzose sandstone, conglomerate, and mudstone, with detrital zircon (DZ) patterns recording a mix of local basement sources to the N and distal Laurentian sources to the SE. The lower Perry Canyon and related formations contain variably feldspathic sandstone, quartz-pebble diamictite deposited during an older glacial episode, and mudstone, with DZ patterns recording a mix of distal sources, local basement sources, and sediment recycling during early rifting. The upper Perry Canyon and related formations contain mafic volcanic rocks, polymict diamictite deposited during a younger glacial episode, volcaniclastic wacke, and mudstone, with DZ patterns recording local basement sources along an evolving rift margin and felsic volcanism from ~. 700 to 670. Ma. Mafic volcanic rocks and trachyte to rhyolite clasts in diamictite have geochemical signatures typical of continental rifting. The lower Brigham and McCoy Creek groups contain mostly mature quartz arenite deposited in shallow marine environments, with DZ patterns recording distal Laurentian sources. The base of the Prospect Mountain and correlative formations is marked by an influx of feldspathic, coarse-grained sediment derived from local basement sources and ~. 570-540. Ma basalt volcanism, which was followed by deposition of subfeldspathic strata with dominant 1.7-1.8. Ga DZ grains, recording sources from the SE rift margin and a marked decrease in distal sources during uplift of the Transcontinental Arch. Overlying carbonate-rich strata were deposited in shallow marine settings, with episodic influx of siliciclastic sediment derived from basement exposed during regressions. Stratigraphic thickness-age relations of Neoproterozoic to early Paleozoic strata are consistent with two episodes of rifting concentrated at ca. 700-670. Ma and 570-540. Ma along western Laurentia, leading to final development of a passive margin. Early rifting was incomplete with an estimated 25-40% extension of initially thick lithosphere that was weakened by igneous activity. Final rifting of previously thinned lithosphere involved an estimated 20-35% additional extension, renewed igneous activity, and thermal thinning of mantle lithosphere, with localized extension culminating in final separation along the continental margin. Stratigraphic, geochronologic, and available paleomagnetic data are consistent with linkage of western Laurentia to Australia-East Antarctica within Rodinia, followed by protracted rifting and drift during Cambrian time