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1. Late Jurassic paleogeography of the U.S. Cordillera from detrital zircon age and hafnium analysis of the Galice Formation, Klamath Mountains, Oregon and California, USA

   https://doi.org/10.1130/B36810.1  

   Surpless, Kathleen D.; Alford, Ryan W.; Barnes, Calvin; Yoshinobu, Aaron; Weis, Natalee E. (August 2023, Geological Society of America Bulletin)

   The Upper Jurassic Galice Formation, a metasedimentary unit in the Western Klamath Mountains, formed within an intra-arc basin prior to and during the Nevadan orogeny. New detrital zircon U-Pb age analyses (N = 11; n = 2792) yield maximum depositional ages (MDA) ranging from ca. 160 Ma to 151 Ma, which span Oxfordian to Kimmeridgian time and overlap Nevadan contractional deformation that began by ca. 157 Ma. Zircon ages indicate a significant North American continental provenance component that is consistent with tectonic models placing the Western Klamath terrane on the continental margin in Late Jurassic time. Hf isotopic analysis of Mesozoic detrital zircon (n = 603) from Galice samples reveals wide-ranging εHf values for Jurassic and Triassic grains, many of which cannot be explained by a proximal source in the Klamath Mountains, thus indicating a complex provenance. New U-Pb ages and Hf data from Jurassic plutons within the Klamath Mountains match some of the Galice Formation detrital zircon, but these data cannot account for the most non-radiogenic Jurassic detrital grains. In fact, the in situ Cordilleran arc record does not provide a clear match for the wide-ranging isotopic signature of Triassic and Jurassic grains. When compiled, Galice samples indicate sources in the Sierra Nevada pre-batholithic framework and retroarc region, older Klamath terranes, and possibly overlap strata from the Blue Mountains and the Insular superterrane. Detrital zircon age spectra from strata of the Upper Jurassic Great Valley Group and Mariposa Formation contain similar age modes, which suggests shared sediment sources. Inferred Galice provenance within the Klamath Mountains and more distal sources suggest that the Galice basin received siliciclastic turbidites fed by rivers that traversed the Klamath-Sierran arc from headwaters in the retroarc region. Thus, the Galice Formation contains a record of active Jurassic magmatism in the continental arc, with significant detrital input from continental sediment sources within and east of the active arc. These westward-flowing river systems remained active throughout the shift in Cordilleran arc tectonics from a transtensional system to the Nevadan contractional system, which is characterized by sediment sourced in uplifts within and east of the arc and the thrusting of older Galice sediments beneath older Klamath terranes to the east. 

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2. Orogens of Big Sky Country: Reconstructing the Deep‐Time Tectonothermal History of the Beartooth Mountains, Montana and Wyoming, USA

   https://doi.org/10.1029/2022TC007541  

   Ronemus, Chance B.; Orme, Devon A.; Guenthner, William R.; Cox, Stephen E.; Kussmaul, Christopher A. L. (January 2023, Tectonics)

   Abstract Archean rocks exposed in the Beartooth Mountains, Montana and Wyoming, have experienced a complex >2.5 Gyr thermal history related to the long‐term geodynamic evolution of Laurentia. We constrain this history using “deep‐time” thermochronology, reporting zircon U‐Pb, biotite⁴⁰Ar/³⁹Ar, and zircon and apatite [U‐Th(‐Sm)]/He results from three transects across the basement‐core of the range. Our central transect yielded a zircon U‐Pb concordia age of 2,805.6 ± 6.4 Ma. Biotite⁴⁰Ar/³⁹Ar plateau ages from western samples are ≤1,775 ± 27 Ma, while those from samples further east are ≥2,263 ± 76 Ma. Zircon (U‐Th)/He dates span 686.4 ± 11.9 to 13.5 ± 0.3 Ma and show a negative relationship with effective uranium—a proxy for radiation damage. Apatite (U‐Th)/He dates are 109.2 ± 23.9 to 43.6 ± 1.9 Ma and correlate with sample elevation. Multi‐chronometer Bayesian time‐temperature inversions suggest: (a) Cooling between ∼1.90 and ∼1.80 Ga, likely related to Big Sky orogeny thermal effects; (b) Reheating between ∼1.80 Ga and ∼1.35 Ga consistent with Mesoproterozoic burial; (c) Cooling to ≤100°C between Mesoproterozoic and early Paleozoic time, likely reflecting continental erosion; (d) Variable Paleozoic–Jurassic cooling, possibly related to Paleozoic tectonism and/or low eustatic sea level; (e) Rapid Cretaceous–Paleocene cooling, preceding accepted proxies for flat‐slab subduction; (f) Eocene–Miocene reheating consistent with reburial by Cenozoic volcanics and/or sediments; (g) Post‐20 Ma cooling consistent with Neogene development of topographic relief. Our results emphasize the utility of multi‐chronometer thermochronology in recovering complex, non‐monotonic multi‐billion‐year thermal histories. 

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3. Cenozoic tectono-thermal history of the southern Talkeetna Mountains, Alaska: Insights into a potentially alternating convergent and transform plate margin

   https://doi.org/doi.org/10.1130/GES02008.1  

   Terhune, P.; Benowitz, J.; O'Sullivan, P.; Gillis, R.; Freymuller, J. (January 2019, Geosphere)

   The Mesozoic–Cenozoic convergent margin history of southern Alaska has been dominated by arc magmatism, terrane accretion, strike-slip fault systems, and possible spreading-ridge subduction. We apply 40Ar/39Ar, apatite fission-track (AFT), and apatite (U-Th)/He (AHe) geochronology and thermochronology to plutonic and volcanic rocks in the southern Talkeetna Mountains of Alaska to document regional magmatism, rock cooling, and inferred exhumation patterns as proxies for the region’s deformation history and to better delineate the overall tectonic history of southern Alaska. High-temperature 40Ar/39Ar thermochronology on muscovite, biotite, and K-feldspar from Jurassic granitoids indicates postemplacement (ca. 158–125 Ma) cooling and Paleocene (ca. 61 Ma) thermal resetting. 40Ar/39Ar whole-rock volcanic ages and 45 AFT cooling ages in the southern Talkeetna Mountains are predominantly Paleocene–Eocene, suggesting that the mountain range has a component of paleotopography that formed during an earlier tectonic setting. Miocene AHe cooling ages within ~10 km of the Castle Mountain fault suggest ~2–3 km of vertical displacement and that the Castle Mountain fault also contributed to topographic development in the Talkeetna Mountains, likely in response to the flat-slab subduction of the Yakutat microplate. Paleocene–Eocene volcanic and exhumation-related cooling ages across southern Alaska north of the Border Ranges fault system are similar and show no S-N or W-E progressions, suggesting a broadly synchronous and widespread volcanic and exhumation event that conflicts with the proposed diachronous subduction of an active west-east–sweeping spreading ridge beneath south-central Alaska. To reconcile this, we propose a new model for the Cenozoic tectonic evolution of southern Alaska. We infer that subparallel to the trench slab breakoff initiated at ca. 60 Ma and led to exhumation, and rock cooling synchronously across south-central Alaska, played a primary role in the development of the southern Talkeetna Mountains, and was potentially followed by a period of southern Alaska transform margin tectonics. 

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4. Kinematic Evolution of the Tangra Yumco Rift, South-Central Tibet

   Reynolds, Aislin R; Laskowski, Andrew K (December 2022, AGU Fall Meeting 2022, held in Chicago, IL, 12-16 December 2022, id. T25B-06.)

   We interpret the kinematics of the Tangra Yumco (TYC) rift by evaluating spatiotemporal trends in fault displacement, extension onset, and exhumation rates. We present new geologic mapping, U-Pb geochronology, zircon (U-Th)/He (ZHe) thermochronology, and HeFTy thermal modeling results that are critical to testing dynamic models of extension in Tibet. The TYC rift is bounded by two NNE striking (~N10°E-N35°E) high angle (~45-70°) active normal faults that alternate dominance along strike. Footwall granodiorites show foliation, slip lineation, and fault plane striation measurements indicative of northeast directed oblique sinistral-normal slip. In North and South TYC, hanging wall deposits are cut by a series of active high-angle normal faults which likely sole into a master fault at depth, while in central TYC, hanging wall deposits display synthetic graben structures potentially indicative of low-angle faulting. Analysis of ~50 samples collected across key structural relationships in and around TYC yield 14 mean U-Pb dates between ~59-49 Ma and ~190 single-grain ZHe dates between ~60-4 Ma with spatial trends in ZHe data correlating strongly with latitude. Samples from Gangdese latitudes show a concentration of ~28-15 Ma ages, while those north of ~29.8° latitude yield both younger (~9-4 Ma) and older (~59-45 Ma) ages. We interpret (1) Gangdese Range samples reflect exhumation during contraction and uplift along the GCT peaking at ~21-20 Ma, (2) ~9-4 Ma ages reveal extension timing along fault segments experiencing significant rift-related exhumation, and (3) ~59-45 Ma ages represent un-reset or partially-reset samples from fault segments that have experienced lesser magnitudes of rift exhumation. HeFTy thermal models indicate a two-stage cooling history with initial slow cooling followed by accelerated cooling rates in Late Miocene-Pliocene time (~13-4 Ma) consistent with prior results from TYC and other Tibetan rifts. Our data are consistent with a segment linkage fault evolution model for the TYC rift, with underthrusting of Indian lithosphere likely related to the northward acceleration of rifting. Future work will utilize advanced HeFTy modeling including U-Pb and apatite fission track data to further constrain the exhumation history of TYC and test dynamic models of extension for southern Tibet. 

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5. Uplift and exhumation of the Russell Fiord and Boundary blocks along the northern Fairweather transform fault, Alaska

   Schartman, A.; Enkelmann, E.; Garver, J.I.; Davidson, C.M. (January 2019, Lithosphere)

   Cooling ages of tectonic blocks between the Yakutat microplate and the Fairweather transform boundary fault reveal exhumation due to strike-slip faulting and subsequent collision into this tectonic corner. The Yakutat and Boundary faults are splay faults that define tectonic panels with bounding faults that have evidence of both reverse and strike-slip motion, and they are parallel to the northern end of the Fairweather fault. Uplift and exhumation simultaneous with strike-slip motion have been significant since the late Miocene. The blocks are part of an actively deforming tectonic corner, as indicated by the ~14–1.5 m of coseismic uplift from the M 8.1 Yakutat Bay earthquake of 1899 and 4 m of strike-slip motion in the M 7.9 Lituya Bay earthquake in 1958 along the Fairweather fault. New apatite (U-Th-Sm)/He (AHe) and zircon (U-Th)/He (ZHe) data reveal that the Boundary block and the Russell Fiord block have different cooling histories since the Miocene, and thus the Boundary fault that separates them is an important tectonic boundary. Upper Cretaceous to Paleocene flysch of the Russell Fiord block experienced a thermal event at 50 Ma, then a relatively long period of burial until the late Miocene when initial exhumation resulted in ZHe ages between 7 and 3 Ma, and then very rapid exhumation in the last 1–1.5 m.y. Exhumation of the Russell Fiord block was accommodated by reverse faulting along the Yakutat fault and the newly proposed Calahonda fault, which is parallel to the Yakutat fault. The Eocene schist of Nunatak Fiord and 54–53 Ma Mount Stamy and Mount Draper granites in the Boundary block have AHe and ZHe cooling ages that indicate distinct and very rapid cooling between ca. 5 Ma and ca. 2 Ma. Rocks of the Chugach Metamorphic Complex to the northeast of the Fairweather fault and in the fault zone were brought up from 10–12 km at extremely high rates (>5 km/m.y.) since ca. 3 Ma, which implies a significant component of dip-slip motion along the Fairweather fault. The adjacent rocks of the Boundary block were exhumed with similar rates and from similar depths during the early Pliocene, when they may have been located 220–250 km farther south near Baranof Island. The profound and significant exhumation of the three tectonic blocks in the last 5 m.y. has probably been driven by uplift and erosional exhumation due to contraction as rocks collide into this tectonic corner. The documented spatial and temporal pattern of exhumation is in agreement with the southward shift of focused exhumation at the St. Elias syntaxial corner and the southeast propagation of the fold-and thrust belt. 

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