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1. Provenance of upper Permian-lowermost Triassic sandstones, Wutonggou low-order cycle, Bogda Mountains, NW China: implications on the unroofing history of the Eastern North Tianshan Suture

   https://doi.org/10.1186/s42501-020-00067-9  

   Zheng, Dong-Yu ; Yang, Wan ( December 2020 , Journal of Palaeogeography)

   null (Ed.)

   Abstract This study investigates the provenance of sedimentary rocks in Bogda Mountains, NW China, and reconstructs the lithology and unroofing history of the Eastern North Tianshan Suture. Petrographic point counting data of sandstones and compositions of conglomerates of upper Permian-lowermost Triassic Wutonggou low-order cycle from Zhaobishan, North Tarlong, Taodonggou, and Dalongkou sections in the southern and northern foothills of Bogda Mountains were used to interpret the temporal and spatial variations of lithology of the Eastern North Tianshan Suture, which is the sediment source area. Three compositional trends were identified. A trend of upward-increasing quartz content and granitic pebbles in Zhaobishan section suggests a change from the undissected volcanic arc, accretionary wedge and trench setting to predominantly transitional volcanic arc and subordinate accretionary wedge and trench, in the eastern part of the Eastern North Tianshan Suture. In North Tarlong and Taodonggou sections, however, the lithic content decreases and the contents of quartz and granitic pebbles increase up sections. These trends indicate that the western part of the Eastern North Tianshan Suture changed from an undissected volcanic arc to the transitional volcanic arc, accretionary wedge and trench. No clear trend in the lithic-rich sandstones of the Dalongkou section indicates that sediments were derived from the undissected volcanic arc in the Eastern North Tianshan Suture and local rift shoulders. Compositional variations of studied rocks suggest that the Eastern North Tianshan Suture was an amalgamated complex with great spatial and temporal heterogeneities in lithology and experienced persistent unroofing during late Permian-earliest Triassic. This study reconstructs a key element of the Chinese Tianshan Suture and serves as an example to understand the unroofing processes of ancient sutures. 

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2. Stitch in the ditch: Nutzotin Mountains (Alaska) fluvial strata and a dike record ca. 117–114 Ma accretion of Wrangellia with western North America and initiation of the Totschunda fault

   https://doi.org/10.1130/GES02127.1  

   Trop, Jeffrey M. ; Benowitz, Jeffrey A. ; Koepp, Donald Q. ; Sunderlin, David ; Brueseke, Matthew E. ; Layer, Paul W. ; Fitzgerald, Paul G. ( November 2019 , Geosphere)

   The Nutzotin basin of eastern Alaska consists of Upper Jurassic through Lower Cretaceous siliciclastic sedimentary and volcanic rocks that depositionally overlie the inboard margin of Wrangellia, an accreted oceanic plateau. We present igneous geochronologic data from volcanic rocks and detrital geochronologic and paleontological data from nonmarine sedimentary strata that provide constraints on the timing of deposition and sediment provenance. We also report geochronologic data from a dike injected into the Totschunda fault zone, which provides constraints on the timing of intra–suture zone basinal deformation. The Beaver Lake formation is an important sedimentary succession in the northwestern Cordillera because it provides an exceptionally rare stratigraphic record of the transition from marine to nonmarine depositional conditions along the inboard margin of the Insular terranes during mid-Cretaceous time. Conglomerate, volcanic-lithic sandstone, and carbonaceous mudstone/shale accumulated in fluvial channel-bar complexes and vegetated overbank areas, as evidenced by lithofacies data, the terrestrial nature of recovered kerogen and palynomorph assemblages, and terrestrial macrofossil remains of ferns and conifers. Sediment was eroded mainly from proximal sources of upper Jurassic to lower Cretaceous igneous rocks, given the dominance of detrital zircon and amphibole grains of that age, plus conglomerate with chiefly volcanic and plutonic clasts. Deposition was occurring by ca. 117 Ma and ceased by ca. 98 Ma, judging from palynomorphs, the youngest detrital ages, and ages of crosscutting intrusions and underlying lavas of the Chisana Formation. Following deposition, the basin fill was deformed, partly eroded, and displaced laterally by dextral displacement along the Totschunda fault, which bisects the Nutzotin basin. The Totschunda fault initiated by ca. 114 Ma, as constrained by the injection of an alkali feldspar syenite dike into the Totschunda fault zone. These results support previous interpretations that upper Jurassic to lower Cretaceous strata in the Nutzotin basin accumulated along the inboard margin of Wrangellia in a marine basin that was deformed during mid-Cretaceous time. The shift to terrestrial sedimentation overlapped with crustal-scale intrabasinal deformation of Wrangellia, based on previous studies along the Lost Creek fault and our new data from the Totschunda fault. Together, the geologic evidence for shortening and terrestrial deposition is interpreted to reflect accretion/suturing of the Insular terranes against inboard terranes. Our results also constrain the age of previously reported dinosaur footprints to ca. 117 Ma to ca. 98 Ma, which represent the only dinosaur fossils reported from eastern Alaska. 

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3. Petrography and geochemistry of the impact to postimpact transition layer at the El'gygytgyn impact structure in Chukotka, Arctic Russia

   https://doi.org/10.1111/maps.13243  

   Maierhofer, Karin ; Koeberl, Christian ; Brigham‐Grette, Julie ( March 2019 , Meteoritics & Planetary Science)

   The 3.6 Ma El'gygytgyn impact structure, located in northeast Chukotka in Arctic Russia, was largely formed in acidic volcanic rocks. The 18 km diameter circular depression is today filled with Lake El'gygytgyn (diameter of 12 km) that contains a continuous record of lacustrine sediments of the Arctic from the past 3.6 Myr. In 2009, El'gygytgyn became the focus of the International Continental Scientific Drilling Program (ICDP) in which a total of 642.4 m of drill core was recovered. Lithostratigraphically, the drill cores comprise lacustrine sediment sequences, impact breccias, and deformed target rocks. The impactite core was recovered from 316.08 to 517.30 meters below lake floor (mblf). Because of the rare, outstanding recovery, the transition zone, ranging from 311.47 to 317.38 m, between the postimpact lacustrine sediments and the impactite sequences, was studied petrographically and geochemically. The transition layer comprises a mixture of about 6 m of loose sedimentary and volcanic material containing isolated clasts of minerals and melt. Shock metamorphic effects, such as planar fractures (PFs) and planar deformation features (PDFs), were observed in a few quartz grains. The discoveries of silica diaplectic glass hosting coesite, kinked micas and amphibole, lechatelierite, numerous impact melt shards and clasts, and spherules are associated with the impact event. The occurrence of spherules, impact melt clasts, silica diaplectic glass, and lechatelierite, about 1 m below the onset of the transition, marks the beginning of the more coherent impact ejecta layer. The results of siderophile interelement ratios of the transition layer spherules give indications of the relative contribution of the meteoritical component.

    

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4. AGE AND PROVENANCE OF CRETACEOUS TO PALEOCENE CLASTIC ROCKS OF THE YAKUTAT GROUP IN GLACIER BAY NATIONAL PARK, ALASKA

   Bryan, R.P. ; Garver, J.I. ; and Davidson, C. ( April 2023 , Geological Society of America Abstracts with Programs)

   The Upper Cretaceous to Paleocene Yakutat Group contains a flysch unit and a mélange unit with an unknown source terrane. The provenance of detrital zircons may be the key to understanding the age of clastic units, their source terrane, and correlative rocks along the margin. Two samples were collected from remote and difficult to access areas in Glacier Bay National Park, and these samples can be compared to samples from Harlequin Lake, Russell Fiord, and Yakutat Bay to the north. We dated detrital zircons using standard LA-ICPMS methodology. A sample of Yakutat Group flysch (YGf) from the Grand Plateau Glacier is from quartzofeldspathic turbidites adjacent to the Grand Plateau pluton. It has an MDA of ~66 Ma (Maastrichtian-Paleocene), and the grain-age distribution is dominated by a broad mid-Cretaceous population with ages from ~91 to ~114 Ma, it also has a Jurassic component at ~166. A unique attribute of this sample is that 23% of the zircons are Precambrian with a bimodal population at ~1397 Ma and ~1702 Ma. A sample of sandstone from the Yakutat Group mélange (YGm) from Lituya Bay, was collected from an assemblage of dark lithic sandstones interbedded with basalt, and dark-gray bedded chert. This sample has an MDA of ~108 (Albian), and its grain-age distribution is dominated (88%)by Jurassic dates ranging from ~156 to ~188 Ma. Both samples can be correlated to similar dated units in the area in and around Yakutat Bay. The YGf sample is correlative to the primary zircon facies common to arkosic rocks in both the Yakutat Group flysch and mélange, which we refer to as the Russell zircon facies, with an MDA range from 61-72 Ma, and distinctive Precambrian populations. The YGm sample is more complicated, but it appears to belong to the Shelter Cove zircon facies, dominated by mid-Cretaceous lithic sandstones that occur only in the mélange. The Yakutat terrane has been translated along the margin of the Cordillera, and candidate correlative rocks are to the south. We are intrigued that similar facies with similar grain-age distributions occur in the Western Mélange Belt in the North Cascades foothills in WA. We evaluate the correlation and connection between the Yakutat and the WMB and post Paleocene translation of part of this once contiguous unit. 

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5. Data report: major and trace element and Sr-Nd-Pb-Hf isotope composition of three granite clasts from Hole U1501D in the South China Sea (IODP Expedition 367/368/368X)

   https://doi.org/10.14379/iodp.proc.367368.202.2022  

   Straub, S.M. ; Gomez-Tuena, A. ; Dorais, M.J. ( April 2022 , Proceedings of the International Ocean Discovery Program)

   During International Ocean Discovery Program Expeditions 367/368/368X, Hole U1501D was cored on the continental shelf (2846 meters below sea level) in the northern South China Sea (SCS). In Hole U1501D, sediments were recovered from 433.5 to 644.3 meters below seafloor (mbsf) and the acoustic basement was penetrated at 598.91 mbsf. The acoustic basement is a stratigraphic boundary at which late Eocene Cenozoic sediments likely unconformably overlay heterolithic Mesozoic sandstones that are intercalated with rare siltstones and subordinate conglomerate with pebble- and cobble-sized igneous clasts of proximal provenance. Here, we present major and trace elements and Sr-Nd-Pb-Hf isotope data of a fine-grained granite pebble, a medium-grained granite cobble, and a porphyritic volcanic pebble. The data show that these clasts are relics of the Mesozoic subduction-related magmatism that was active along the southeast Asian margin prior to the Cenozoic rifting. The Pb isotope composition of the clasts partially overlaps with the enriched Cenozoic mid-ocean-ridge basalt type and intraplate basalts of the SCS. However, the clasts are distinct from the Cenozoic basalt volcanism in Sr-Nd-Hf isotope space. Thus, Sr-Nd-Hf isotope systematics of the Cenozoic basalts might be useful in detecting traces of crustal contamination in the earliest rift basalts of the SCS that may have erupted through the Mesozoic continental basement. 

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