Early Inception of the Laramide Orogeny in Southwestern Montana and Northern Wyoming: Implications for Models of Flat‐Slab Subduction

Timing and distribution of magmatism, deformation, exhumation, and basin development have been used to reconstruct the history of Laramide flat-slab subduction under North America during Late Cretaceous-early Cenozoic time. Existing geodynamic models, however, ignore a large (40,000-km(2)) sector of the Laramide foreland in southwestern Montana. The Montana Laramide ranges consist of Archean basement arches (fault-propagation folds) that were elevated by thrust and reverse faults. We present new thermochronological and geochronological data from six Laramide ranges in southwestern Montana (the Beartooth, Gravelly, Ruby and Madison Ranges, and the Tobacco Root and Highland Mountains) that show significant cooling and exhumation during the Early to mid-Cretaceous, much earlier than the record of Laramide exhumation in Wyoming. These data suggest that Laramide-style deformation-driven exhumation slightly predates the eastward sweep of magmatism in western Montana, consistent with geodynamic models involving initial strain propagation into North American cratonic rocks due to stresses associated with a northeastward expanding region of flat-slab subduction. Our results also indicate various degrees of Cenozoic heating and cooling possibly associated with westward rollback of the subducting Farallon slab, followed by Basin-and-Range extension. Plain Language Summary The Laramide region in the western U.S. is characterized by some of the highest topography in North America including the Wind River Range in WY and the Beartooth Range of WY and Montana. These ranges have fed detritus to surrounding basins for millions of years and contributed to modern ecosystems. These high topographic features and basins have significantly impacted paleoenvironmental conditions over geological time. The formation of these high-relief ranges has been linked to deep Earth, geodynamic, processes involving subduction of a flat slab under the North American Plate. Models of flat-slab subduction rely on the timing and pattern of deformation and exhumation of Laramide ranges, which remains poorly understood. Our study provides new data on the timing of deformation and exhumation of Laramide ranges in SW Montana and northern WY capable of testing current models of flat-slab subduction.NSF-Tectonics [EAR-1524151]6 month embargo; published online: 9 January 2019This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Evidence of a lacustrine origin for laminated marl of the Pliocene Bouse Formation, Milpitas Wash, Blythe Basin, lower Colorado River valley

The Pliocene Bouse Formation in the lower Colorado River trough locally contains laminated marl and claystone interpreted by O’Connell et al. (2017) to represent the spring-neap tide cycle during sediment deposition in an estuary. Tidal cycles, if present, should be detectable by Fourier spectral analysis of lamination thicknesses in continuous sequences of laminated sediments. To evaluate the tidal interpretation, we attempted to photograph several laminated sequences in the southern Bouse Formation (south of Blythe, California) so that thicknesses could be measured from the photographs. Only one sequence, in lower Milpitas Wash (California), was identified where thicknesses could be determined with adequate precision from field photography. Fourier analysis of that sequence failed to identify evidence of tides. Furthermore, electron-microprobe analysis determined that laminations consist of alternating claystone and marl, which is consistent with annual changes in lake chemistry and sediment sources rather than physical changes in sediment sorting and transport during tidal cycles. Fourier analysis of data presented by O’Connell et al. (2017) of two nearby laminated Bouse sequences interpreted as tidal rhythmites also failed to identify statistically significant evidence of tides.Documents in the AZGS Documents Repository collection are made available by the Arizona Geological Survey (AZGS) and the University Libraries at the University of Arizona. For more information about items in this collection, please contact [email protected]

Thermochronological and Geochronological Constraints on Late Cretaceous Unroofing and Proximal Sedimentation in the Sevier Orogenic Belt, Utah

A source-to-sink analysis incorporating geochronometric and thermochronometric data from the Sevier fold-thrust belt (SFTB) and proximal synorogenic strata of the Canyon Range Conglomerate (CRC) and Indianola Group (IG) provides new insights into orogenic exhumation, erosional unroofing, and the interplay between thrusting and coarse clastic deposition in the Cretaceous Cordilleran foreland basin of western North America. Zircon (U-Th)/He ages from the Pavant and Nebo thrust sheets record significant Cenomanian cooling indicative of synchronous exhumation and thrusting along a large segment of the SFTB in central and northern Utah. Detrital zircon (U-Th)/He (DZHe) ages indistinguishable from depositional ages from the Cenomanian Dakota Formation and lower CRC also record rapid unroofing of the SFTB and synchronous deposition. DZHe data from wedge-top deposits of the CRC record two significant unroofing episodes: Albo-Cenomanian exhumation of the Pavant thrust and progressive unroofing of the Canyon Range culmination. For the IG, the presence of Paleozoic DZHe ages along with Paleozoic-Mesozoic DZ U-Pb ages in the Cenomanian Sanpete Formation suggests derivation from Paleozoic to Jurassic strata exhumed in the frontal Pavant and Nebo thrust sheets. After the Cenomanian episode of rapid exhumation, proximal foredeep strata recorded a widespread DZ provenance shift in the Turonian. Short DZHe lag time values from Campanian CRC and IG deposits reveal rapid exhumation of the SFTB during the Campanian. The synchroneity of major shortening and Campanian and Cenomanian changes in foreland basin architecture and provenance supports models proposing that active shortening in the fold-thrust belt coincides with coarse clastic influx in foreland basins.6 month embargo; first published: 23 May 2020This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Thermochronological and Geochronological Constraints on Late Cretaceous Unroofing and Proximal Sedimentation in the Sevier Orogenic Belt, Utah

A source-to-sink analysis incorporating geochronometric and thermochronometric data from the Sevier fold-thrust belt (SFTB) and proximal synorogenic strata of the Canyon Range Conglomerate (CRC) and Indianola Group (IG) provides new insights into orogenic exhumation, erosional unroofing, and the interplay between thrusting and coarse clastic deposition in the Cretaceous Cordilleran foreland basin of western North America. Zircon (U-Th)/He ages from the Pavant and Nebo thrust sheets record significant Cenomanian cooling indicative of synchronous exhumation and thrusting along a large segment of the SFTB in central and northern Utah. Detrital zircon (U-Th)/He (DZHe) ages indistinguishable from depositional ages from the Cenomanian Dakota Formation and lower CRC also record rapid unroofing of the SFTB and synchronous deposition. DZHe data from wedge-top deposits of the CRC record two significant unroofing episodes: Albo-Cenomanian exhumation of the Pavant thrust and progressive unroofing of the Canyon Range culmination. For the IG, the presence of Paleozoic DZHe ages along with Paleozoic-Mesozoic DZ U-Pb ages in the Cenomanian Sanpete Formation suggests derivation from Paleozoic to Jurassic strata exhumed in the frontal Pavant and Nebo thrust sheets. After the Cenomanian episode of rapid exhumation, proximal foredeep strata recorded a widespread DZ provenance shift in the Turonian. Short DZHe lag time values from Campanian CRC and IG deposits reveal rapid exhumation of the SFTB during the Campanian. The synchroneity of major shortening and Campanian and Cenomanian changes in foreland basin architecture and provenance supports models proposing that active shortening in the fold-thrust belt coincides with coarse clastic influx in foreland basins.6 month embargo; first published: 23 May 2020This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Andean Mountain Building and Foreland Basin Evolution During Thin‐ and Thick‐Skinned Neogene Deformation (32–33°S)

The southern Central Andes recorded retroarc shortening, basin evolution, and magmatic arcmigration during Neogene changes in subduction. At 31?33°S, above the modern flat‐slab segment, spatialand temporal linkages between thin‐ and thick‐skinned foreland shortening, basement‐involvedexhumation of the main Cordillera, and lower‐crustal hinterland thickening remain poorly resolved. Weintegrate new geochronological and thermochronological data for thrust sheets and Neogene foreland basinfill with structural, sedimentological, and passive seismic results to reconstruct the exhumation historyand evaluate potential geometric linkages across structural domains. 40Ar/39Ar ages for volcanic horizonsand zircon U‐Pb ages for synorogenic clastic deposits in the Manantiales Basin constrain the minimumduration of synorogenic sedimentation to ~22?14 Ma. Detrital zircon age distributions record sequentialunroofing of hinterland thrust sheets until ~15 Ma, followed by eastward (cratonward) advance of thedeformation front, shutoff of western sediment sources, and a shift from fluvial to alluvial fan deposition at~14 Ma. Apatite (U‐Th)/He cooling ages confirm rapid exhumation of basement‐involved structural blocksand basin partitioning by ~14?5 Ma, consistent with the timing of the Manantiales facies and provenanceshifts and a coeval (~12?9 Ma) pulse of thin‐skinned shortening and exhumation previously identified in theeastern foreland. Late Miocene?Pliocene (~8?2 Ma) cooling ages along the Chile‐Argentina border point tohinterland uplift during the latest stage of Andean orogenesis. Finally, geophysical constraints on crustalarchitecture and low‐temperature thermochronometry results are compatible with a hybrid thin‐ andthick‐skinned décollement spanning retroarc domains.Fil: Mackaman Lofland, Chelsea. Jackson School Of Geosciences; Estados Unidos. Department Of Geological Sciences; Estados UnidosFil: Horton, Brian K.. Jackson School Of Geosciences; Estados Unidos. Institute For Geophysics; Estados UnidosFil: Fuentes, Facundo. YPF; ArgentinaFil: Constenius, Kurt N.. University of Arizona; Estados UnidosFil: Ketcham, Richard A.. Jackson School Of Geosciences; Estados UnidosFil: Capaldi, Tomas N.. Jackson School Of Geosciences; Estados Unidos. Department Of Geological Sciences; Estados UnidosFil: Stockli, Daniel F.. Jackon School Of Geosciences; Estados Unidos. Department Of Geological Sciences; Estados UnidosFil: Ammirati, Jean Baptiste. Universidad de Chile. Facultad de Ciencias Físicas y Matemáticas. Departamento de Geología; Chile. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; ArgentinaFil: Alvarado, Patricia Monica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; Argentina. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Departamento de Geofísica y Astronomía; ArgentinaFil: Orozco Chirino, Paola Gabriela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Juan. Centro de Investigaciones de la Geosfera y Biosfera. Universidad Nacional de San Juan. Facultad de Ciencias Exactas Físicas y Naturales. Centro de Investigaciones de la Geosfera y Biosfera; Argentin

Clumped‐Isotope Geothermometry and Carbonate U–Pb Geochronology of the Alta Stock Metamorphic Aureole, Utah, USA: Insights on the Kinetics of Metamorphism in Carbonates

To assess thermal and kinetic influences on atomic mobility and mineral (neo)crystallization, clumped‐isotope abundances of calcite and dolomite were measured alongside dolomite cation ordering and U–Pb dates, across metamorphic grade within the c. 35–30 Ma Alta stock contact metamorphic aureole, Utah, USA. Average Δ47 values of dolomite inside the metamorphic aureole reflect the blocking temperature of dolomite (300°C–350°C) during cooling from peak temperatures. Dolomite Δ47 values outside the metamorphic aureole record a temperature of ∼160°C. At the talc isograd, dolomite Δ47 values abruptly change, corresponding to a decrease of ∼180°C over <50 m in the down‐temperature direction. This observed step in dolomite Δ47 values does not correlate with cation ordering in dolomite or U–Pb dates, neither of which correlate well with metamorphic grade. The short distance over which dolomite Δ47 values change indicates strong temperature sensitivity in the kinetics of dolomite clumped‐isotope reordering, and is consistent with a wide range of clumped‐isotope reequilibration modeling results. We hypothesize that clumped‐isotope reordering in dolomite precedes more extensive recrystallization or metamorphic reaction, such as the formation of talc. Dolomite U–Pb analyses from inside and outside the metamorphic aureole populate a single discordia ∼60 Myr younger than depositional age (Mississippian), recording resetting in response to some older postdepositional, but premetamorphic process.Key Points:Dolomite clumped isotopes record an abrupt ∼180°C decrease, over <50 m, at the talc isograd of the Alta contact metamorphic aureoleThis step feature suggests strong thermal sensitivity in clumped‐isotope reordering that precedes talc neocrystallizationClumped‐isotope geothermometry may provide new insights into processes controlling metamorphism and reaction kinetics in carbonatesPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/167127/1/ggge22474.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/167127/2/2020GC009238-sup-0001-Supporting_Information_SI-S01.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/167127/3/ggge22474_am.pd