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1. Oligocene-Neogene lithospheric-scale reactivation of Mesozoic terrane accretionary structures in the Alaska Range suture zone, southern Alaska, USA

   https://doi.org/10.1130/B35665.1  

   Waldien, Trevor S.; Roeske, Sarah M.; Benowitz, Jeffrey A.; Twelker, Evan; Miller, Meghan S. (August 2020, GSA Bulletin)

   null (Ed.)

   Abstract Terrane accretion forms lithospheric-scale fault systems that commonly experience long and complex slip histories. Unraveling the evolution of these suture zone fault systems yields valuable information regarding the relative importance of various upper crustal structures and their linkage through the lithosphere. We present new bedrock geologic mapping and geochronology data documenting the geologic evolution of reactivated shortening structures and adjacent metamorphic rocks in the Alaska Range suture zone at the inboard margin of the Wrangellia composite terrane in the eastern Alaska Range, Alaska, USA. Detrital zircon uranium-lead (U-Pb) age spectra from metamorphic rocks in our study area reveal two distinct metasedimentary belts. The Maclaren schist occupies the inboard (northern) belt, which was derived from terranes along the western margin of North America during the mid- to Late Cretaceous. In contrast, the Clearwater metasediments occupy the outboard (southern) belt, which was derived from arcs built on the Wrangellia composite terrane during the Late Jurassic to Early Cretaceous. A newly discovered locality of Alaska-type zoned ultramafic bodies within the Clearwater metasediments provides an additional link to the Wrangellia composite terrane. The Maclaren and Clearwater metasedimentary belts are presently juxtaposed by the newly identified Valdez Creek fault, which is an upper crustal reactivation of the Valdez Creek shear zone, the Late Cretaceous plate boundary that initially brought them together. 40Ar/39Ar mica ages reveal independent post-collisional thermal histories of hanging wall and footwall rocks until reactivation localized on the Valdez Creek fault after ca. 32 Ma. Slip on the Valdez Creek fault expanded into a thrust system that progressed southward to the Broxson Gulch fault at the southern margin of the suture zone and eventually into the Wrangellia terrane. Detrital zircon U-Pb age spectra and clast assemblages from fault-bounded Cenozoic gravel deposits indicate that the thrust system was active during the Oligocene and into the Pliocene, likely as a far-field result of ongoing flat-slab subduction and accretion of the Yakutat microplate. The Valdez Creek fault was the primary reactivated structure in the suture zone, likely due to its linkage with the reactivated boundary zone between the Wrangellia composite terrane and North America in the lithospheric mantle. 

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2. Determining the relationship between the Garlock fault and the Eastern California shear zone through detailed digital mapping and age characterization of faulted landforms, Southeastern California

   https://doi.org/10.32469/10355/97090  

   Murphy, Taylor R; Bidgoli, Tandis (August 2023, University of Missouri)

   Southeastern California is known for complex fault networks that accommodate strain from Pacific-North American plate convergence. The 250-km-long, left-lateral Garlock fault is integral to this system, yet its overall kinematic role within the plate boundary and relationship with faults of the Eastern California shear zone/Walker Lane belt remain poorly understood. A key area that has not been adequately studied is a 15-km stretch of the eastern Garlock fault, at its intersections with the right-lateral Brown Mountain fault and left-lateral Owl Lake fault. This segment of the fault lies within the China Lake Naval Air Weapons Station and U.S. Fort Irwin boundaries, which have restrictions on civilian access and portions of which contain unexploded ordnance, making them unsuitable and unsafe for field investigations. The purpose of this project is to use a combination of high-resolution LiDAR topographic data, remotely sensed imagery, and published geochronology data to map and establish the ages of faulted landforms along this portion of the eastern Garlock fault. The inaccessibility of this area makes it ideal for the application of remote-sensing techniques. A range of surface analysis techniques were used to differentiate and map Quaternary units in the study area. Geomorphic surface properties were determined from physiographic roughness and surface reflectance data, established from analysis of LiDAR, radar backscatter, and visual-near and short-wave infrared multispectral and hyperspectral reflectance datasets. The ages of faulted landforms were established using two approaches: (1) fault scarp and terrace riser degradation analysis and (2) a surface property-age model that links remotely sensed surface properties to new and published ages of alluvial surfaces in the region. A final goal of the study was to determine the slip rate along this segment of the Garlock fault and other faults in the map area. To accomplish this, offset landforms, such as terrace risers and channels, were analyzed in the context of the new age determinations. The results will be compared to published slip rate estimates for the region in order to better understand the Garlock fault's role within the plate boundary and how plate boundary strain is being accommodated in such an intraplate setting. 

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3. Geology of the Chugach-Prince William Terrane in northern Prince William Sound, Alaska.

   Garver, J.I. (April 2019, Proceedings of the Keck Geology Consortium.)

   This six-student project focused on the geology of the Chugach and Prince William terranes in northern Prince William Sound, Alaska. The Chugach-Prince William (CPW) composite terrane is a Mesozoic- Tertiary accretionary complex that is well exposed for ~2200 km in southern Alaska and is inferred to be one of the thickest accretionary complexes in the world (Plafker et al., 1994; Cowan, 2003). The CPW terrane is bounded to the north by the Border Ranges fault, which shows abundant evidence of Tertiary dextral strike slip faulting, and inboard terranes of the Wrangellia composite terrane (Peninsular, Wrangellia, Alexander) (Pavlis, 1982; Cowan, 2003; Roeske et al., 2003). Throughout much of the 2200 km long belt of the CPW terrane it is bounded by the offshore modern accretionary complex of the Alaskan margin, but east of Prince William Sound the Yakutat block is colliding into the CPW and this young collision has significantly affected uplift and exhumation of inboard rocks. 

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4. Age and tectonic setting of the Paleocene Glacier Island volcanic sequence of the Orca Group in Prince William Sound, Alaska.

   https://doi.org/10.1130/abs/2019CD-329372  

   Noseworthy, C. (April 2019, Proceedings of the Keck Geology Consortium.)

   Southern Alaska has a long history of subduction, accretion, and coastwise transport of terranes (Coney et al., 1980; Monger et al., 1982; Plafker et al., 1994). The Chugach-Prince William (CPW) terrane is about 2200 km long and extends through much of southern Alaska (Plafker et al., 1994) (Fig. 1A). The inboard Chugach terrane can be divided into two parts, a mélange and sedimentary units that are Permian to Early Cretaceous in age and a turbidite sequence that is from the Upper Cretaceous (Plafker et al., 1994). In the Prince William Sound area, the outboard Prince William terrane is comprised of Paleocene to Eocene turbidites and associated basaltic rocks of the Orca Group (Davidson and Garver, 2017), and the turbidites of the inboard Chugach terrane are known as the Valdez Group. The turbidites are intruded by the Sanak-Baranof Belt (SBB), a group of 63-47 Ma plutons that are progressively younger to the east. The Border Ranges fault system marks the northern boundary of the CPW terrane, separating the Chugach terrane from the Wrangellia composite terrane and the Contact fault separates the Chugach and Prince William terrane (Fig. 1; Plafker et al., 1994). There are three ophiolite sequences in the Orca Group: Knight Island (KI), Resurrection Peninsula (RP), and Glacier Island (GI) (Fig. 1B). The KI ophiolite contains a sequence of massive pillow basalts, sheeted dikes, and a minor amount of ultramafic rocks (Tysdal et al, 1977; Nelson and Nelson, 1992; Crowe et al., 1992). The RP ophiolite is a typical ophiolite sequence and has interbedded Paleocene turbidites (Davidson and Garver, 2017). Paleomagnetic data gathered from the RP ophiolite indicated a mean depositional paleolatitude of 54° ± 7° which implies 13° ± 9° of poleward displacement (Bol et al., 1992). These data suggest that the RP ophiolite was translated northward to its current position after being formed in the Pacific Northwest, and thus the CPW terrane may have been originally located at 48-49° north and at 50 Ma was transferred 1100 km to the north by strike-slip faulting (Cowan, 2003). However, an opposing hypothesis suggests that the terrane has not experienced significant displacement and formed in Alaska due to a now-subducted Resurrection plate (Haeussler et al., 2003). KI and RP ophiolites have traditionally been assumed to be oceanic crust that was tectonically emplaced into the CPW terrane (Bol et al., 1992; Lytwyn et al., 1997). However, a more recent study suggests a hypothesis that the ophiolites originated in an upper plate setting and formed due to transtension (Davidson and Garver, 2017). Previous workers have used discriminant diagrams to identify the volcanic rocks of KI ophiolite and RP ophiolite as mid-ocean ridge basalts (Lytwyn et al., 1997; Miner, 2012). This project presents new geochemical and geochronological data from the GI ophiolite to determine its age and tectonic setting. The purpose of this study is to compare the data from GI with the data from KI and RP, and the comparison of the geochemical data will allow for a greater understanding of the tectonic setting of southern Alaska. 

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5. The Ordovician Thores volcanicn island arc of the Pearya Terrane from northern Ellesmere Island formed on Precambrian continental crust

   https://doi.org/https://doi.org/10.1016/j.lithos.2021.105999  

   Majka, J.; Kosminska, K.; Bazarnik, J.; McClelland, W.C. (April 2021, Lithos)

   null (Ed.)

   Ion microprobe U–Pb zircon dating of intermediate to felsic rocks coupled with bulk-rock geochemistry analyses and compared to previously published data shows that the Thores Suite of the Pearya Terrane of northern Ellesmere Island (Arctic Canada) represents an Early Ordovician (c. 490–470 Ma) suite formed in an island arc setting. Interestingly, three out of five dated samples contain abundant xenocrystic zircon that have ages spanning from c. 2690 Ma to c. 520 Ma. The vast majority of xenocrystic zircon are Precambrian in age and typical of Laurentia. The youngest well-pronounced age cluster around 580–570 Ma is inferred to be an expression of the Timanide Orogen, traditionally ascribed to Baltica. This geochronological dataset provides new insight on the origin of the Thores Suite of the Pearya Terrane, which was traditionally thought to be formed due to the M'Clintock orogenic event and commonly treated as independent from Caledonian tectonism. We suggest that the Thores island arc formed on a sliver of continental crust within the Iapetus Ocean. The timing of igneous activity recorded by the Thores Suite is consistent with other island arcs and subduction-related metamorphic units that occur within the Caledonides of northern Scandinavia and Svalbard. Hence, we suggest that the Thores volcanic island arc was closely associated with age equivalent arcs developed within the northern Iapetus Ocean. Its juxtaposition with the other successions of the Pearya Terrane is explained by a large-scale, left lateral, strike-slip system operating along the northeastern margins of Baltica and Laurentia, coeval with the main collision between the two continents. This strike-slip system was responsible for the juxtaposition of multiple terranes with contrasting Precambrian histories that can be traced in the present day High Arctic, e.g. in southwest Svalbard and the Pearya Terrane. 

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