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1. Laramide evolution of the San Juan Basin, New Mexico and Colorado: Paleocurrent and detrital-sanidine age constraints from the Paleocene Nacimiento and Animas formations

   https://doi.org/10.1130/GES02072.1  

   Cather, S. ; Heizler, H. ; Williamson, T.E. ( October 2019 , Geosphere)

   Understanding the tectonic and landscape evolution of the Colorado Plateau−southern Rocky Mountains area requires knowledge of the Laramide stratigraphic development of the San Juan Basin. Laramide sediment-transport vectors within the San Juan Basin are relatively well understood, except for those of the Nacimiento and Animas formations. Throughout most of the San Juan Basin of northwestern New Mexico and adjacent Colorado, these Paleocene units are mudstone-dominated fluvial successions intercalated between the lowermost Paleocene Kimbeto Member of the Ojo Alamo Sandstone and the basal strata of the lower Eocene San Jose Formation, both sandstone-dominated fluvial deposits. For the Nacimiento and Animas formations, we present a new lithostratigraphy that provides a basis for basin-scale interpretation of the Paleocene fluvial architecture using facies analysis, paleocurrent measurements, and 40Ar/ 39Ar sanidine age data. In contrast to the dominantly southerly or southeasterly paleoflow exhibited by the underlying Kimbeto Member and the overlying San Jose Formation, the Nacimiento and Animas formations exhibit evidence of diverse paleoflow. In the southern and western part of the basin during the Puercan, the lower part of the Nacimiento Formation was deposited by south- or southeast-flowing streams, similar to those of the underlying Kimbeto Member. This pattern of southeasterly paleoflow continued during the Torrejonian in the western part of the basin, within a southeast-prograding distributive fluvial system. By Torrejonian time, a major east-northeast–flowing fluvial system, herein termed the Tsosie paleoriver, had entered the southwestern part of the basin, and a switch to northerly paleoflow had occurred in the southern San Juan Basin. The reversal of paleoslope in the southern part of the San Juan Basin probably resulted from rapid subsidence in the northeast part of the basin during the early Paleocene. Continued Tiffanian-age southeastward progradation of the distributive fluvial system that headed in the western part of the basin pushed the Tsosie paleoriver beyond the present outcrop extent of the basin. In the eastern and northern parts of the San Juan Basin, paleoflow was generally toward the south throughout deposition of the Nacimiento and the Animas formations. An important exception is a newly discovered paleodrainage that exited the northeastern part of the basin, ∼15 km south of Dulce, New Mexico. There, an ∼130-m-thick Paleocene sandstone (herein informally termed the Wirt member of the Animas Formation) records a major east-flowing paleoriver system that aggraded within a broad paleovalley carved deeply into the Upper Cretaceous Lewis Shale. 40Ar/ 39Ar dating of detrital sanidine documents a maximum depositional age of 65.58 ± 0.10 Ma for the Wirt member. The detrital sanidine grains are indistinguishable in age and K/Ca values from sanidines of the Horseshoe ash (65.49 ± 0.06 Ma), which is exposed 10.5 m above the base of the Nacimiento Formation in the southwestern part of the basin. The Wirt member may represent the deposits of the Tsosie paleoriver where it exited eastward from the basin. Our study shows that the evolution of Paleocene fluvial systems in the San Juan Basin was complex and primarily responded to variations in subsidence-related sedimentary accommodation within the basin. 

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2. Evaluating the Shinumo-Sespe drainage connection: Arguments against the “old” (70–17 Ma) Grand Canyon models for Colorado Plateau drainage evolution

   https://doi.org/10.1130/GES02265.1  

   Karlstrom, Karl E. ; Jacobson, Carl E. ; Sundell, Kurt E. ; Eyster, Athena ; Blakey, Ron ; Ingersoll, Raymond V. ; Mulder, Jacob A. ; Young, Richard A. ; Beard, L. Sue ; Holland, Mark E. ; et al ( October 2020 , Geosphere)

   null (Ed.)

   Abstract The provocative hypothesis that the Shinumo Sandstone in the depths of Grand Canyon was the source for clasts of orthoquartzite in conglomerate of the Sespe Formation of coastal California, if verified, would indicate that a major river system flowed southwest from the Colorado Plateau to the Pacific Ocean prior to opening of the Gulf of California, and would imply that Grand Canyon had been carved to within a few hundred meters of its modern depth at the time of this drainage connection. The proposed Eocene Shinumo-Sespe connection, however, is not supported by detrital zircon nor paleomagnetic-inclination data and is refuted by thermochronology that shows that the Shinumo Sandstone of eastern Grand Canyon was >60 °C (∼1.8 km deep) and hence not incised at this time. A proposed 20 Ma (Miocene) Shinumo-Sespe drainage connection based on clasts in the Sespe Formation is also refuted. We point out numerous caveats and non-unique interpretations of paleomagnetic data from clasts. Further, our detrital zircon analysis requires diverse sources for Sespe clasts, with better statistical matches for the four “most-Shinumo-like” Sespe clasts with quartzites of the Big Bear Group and Ontario Ridge metasedimentary succession of the Transverse Ranges, Horse Thief Springs Formation from Death Valley, and Troy Quartzite of central Arizona. Diverse thermochronologic and geologic data also refute a Miocene river pathway through western Grand Canyon and Grand Wash trough. Thus, Sespe clasts do not require a drainage connection from Grand Canyon or the Colorado Plateau and provide no constraints for the history of carving of Grand Canyon. Instead, abundant evidence refutes the “old” (70–17 Ma) Grand Canyon models and supports a <6 Ma Grand Canyon. 

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3. Grand Canyon provenance for orthoquartzite clasts in the lower Miocene of coastal southern California

   https://doi.org/10.1130/GES02111.1  

   Sabbeth, Leah ; Wernicke, Brian P. ; Raub, Timothy D. ; Grover, Jeffrey A. ; Lander, E. Bruce ; Kirschvink, Joseph L. ( October 2019 , Geosphere)

   null (Ed.)

   Abstract Orthoquartzite detrital source regions in the Cordilleran interior yield clast populations with distinct spectra of paleomagnetic inclinations and detrital zircon ages that can be used to trace the provenance of gravels deposited along the western margin of the Cordilleran orogen. An inventory of characteristic remnant magnetizations (CRMs) from >700 sample cores from orthoquartzite source regions defines a low-inclination population of Neoproterozoic–Paleozoic age in the Mojave Desert–Death Valley region (and in correlative strata in Sonora, Mexico) and a moderate- to high-inclination population in the 1.1 Ga Shinumo Formation in eastern Grand Canyon. Detrital zircon ages can be used to distinguish Paleoproterozoic to mid-Mesoproterozoic (1.84–1.20 Ga) clasts derived from the central Arizona highlands region from clasts derived from younger sources that contain late Mesoproterozoic zircons (1.20–1.00 Ga). Characteristic paleomagnetic magnetizations were measured in 44 densely cemented orthoquartzite clasts, sampled from lower Miocene portions of the Sespe Formation in the Santa Monica and Santa Ana mountains and from a middle Eocene section in Simi Valley. Miocene Sespe clast inclinations define a bimodal population with modes near 15° and 45°. Eight samples from the steeper Miocene mode for which detrital zircon spectra were obtained all have spectra with peaks at 1.2, 1.4, and 1.7 Ga. One contains Paleozoic and Mesozoic peaks and is probably Jurassic. The remaining seven define a population of clasts with the distinctive combination of moderate to high inclination and a cosmopolitan age spectrum with abundant grains younger than 1.2 Ga. The moderate to high inclinations rule out a Mojave Desert–Death Valley or Sonoran region source population, and the cosmopolitan detrital zircon spectra rule out a central Arizona highlands source population. The Shinumo Formation, presently exposed only within a few hundred meters elevation of the bottom of eastern Grand Canyon, thus remains the only plausible, known source for the moderate- to high-inclination clast population. If so, then the Upper Granite Gorge of the eastern Grand Canyon had been eroded to within a few hundred meters of its current depth by early Miocene time (ca. 20 Ma). Such an unroofing event in the eastern Grand Canyon region is independently confirmed by (U-Th)/He thermochronology. Inclusion of the eastern Grand Canyon region in the Sespe drainage system is also independently supported by detrital zircon age spectra of Sespe sandstones. Collectively, these data define a mid-Tertiary, SW-flowing “Arizona River” drainage system between the rapidly eroding eastern Grand Canyon region and coastal California. 

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4. South China Sea Rifted Margin

   https://doi.org/10.14379/iodp.proc.367368.2018  

   Sun, Z. ; Jian, Z. ; Stock, J.M. ; Larsen, H.C. ; Klaus, A. ; Alvarez Zarikian, C.A. ( September 2018 , Proceedings of the International Ocean Discovery Program)

   null (Ed.)

   The primary objectives of International Ocean Discovery Program (IODP) Expedition 367/368 to the northern South China Sea (SCS) margin were to (1) examine its history of continental breakup and (2) compare it with other nonvolcanic or magma-poor rifted margins with the broader goal of testing models for continental breakup. A secondary objective was to further our understanding of the paleoceanographic and environmental development of the SCS and southeast Asia during the Cenozoic. Four primary sites were selected for the overall program: one in the outer margin high (OMH) and three seaward of the OMH on distinct, margin-parallel basement ridges. These three ridges are informally labeled A, B, and C and are located in the continent–ocean transition (COT) zone ranging from the OMH to the interpreted steady-state oceanic crust (Ridge C) of the SCS. The main scientific objectives include the following: Determining the nature of the basement in crustal units across the COT of the SCS that are critical to constrain style of rifting, Constraining the time interval from initial crustal extension and plate rupture to the initial generation of igneous ocean crust, Constraining vertical crustal movements during breakup, and Examining the nature of igneous activity from rifting to seafloor spreading. In addition, the sediment cores from the drill sites targeting primarily tectonic and basement objectives will provide information on the Cenozoic regional environmental development of the Southeast Asia margin. Site U1499 on Ridge A and Site U1500 on Ridge B were drilled during Expedition 367. Expedition 368 was planned to drill at two primary sites (U1501 and U1503) at the OMH and Ridge C, respectively, but based on drilling results from Expedition 367, Expedition 368 chose to insert an alternate site on Ridge A (Site U1502). In addition, Expedition 368 added two more sites on the OMH (Sites U1504 and U1505). Expedition 367/368 completed operations at six of the seven sites (U1499–U1502, U1504, and U1505). Site U1503, however, was not completed beyond casing without coring to 990 m because of mechanical problems with the drilling equipment that prevented the expedition, after 25 May 2017, from operating with a drill string longer than 3400 m. New alternate Site U1504, proposed during Expedition 367, met this condition. Original Site U1505 also met the operational constraints of the 3400 m drill string (total) and was an alternate site for the already-drilled Site U1501. At Site U1499, we cored to 1081.8 m in 22.1 days with 52% recovery and then logged downhole data from 655 to 1020 m. In 31 days at Site U1500, we penetrated to 1529 m, cored a total of 1012.8 m with 37% recovery, and collected log data from 842 to 1133 m. At Site U1501, we cored to 697.1 m in 9.4 days with 78.5% recovery. We also drilled ahead for 433.5 m in Hole U1501D and then logged downhole data from 78.3 to 399.3 m. In 19.3 days at Site U1502, we penetrated 1679.0 m in Holes U1502A (758 m) and U1502B (921 m), set 723.7 m of casing and cored a total of 576.3 m with 53.5% recovery, and collected downhole log data from 785.3 to 875.3 m and seismic data through the 10¾ inch casing. At Site U1503, we penetrated 995.1 m and set 991.5 m of 10¾ inch casing, but no cores were taken because of a mechanical problem with the drawworks. At Site U1504, we took 40 rotary core barrel (RCB) cores over two holes. The cored interval between both holes was 277.3 m with 26.8% recovery. An 88.2 m interval was drilled in Hole U1504B. At Site U1505, we cored 668.0 m with 101.1% recovery. Logging data was collected from 80.1 to 341.2 m. Operations at this site covered 6.1 days. Except for Sites U1503 and U1505, all sites were drilled to acoustic basement. A total of 6.65 days were lost due to mechanical breakdown or waiting on spare supplies for repair of drilling equipment, but drilling options were severely limited from 25 May to the end of the expedition by the defective drawworks limiting deployment of drill string longer than 3400 m. At Site U1499, coring ~200 m into the interpreted acoustic basement sampled sedimentary rocks, possibly including early Miocene chalks underlain by Oligocene polymict breccias and poorly cemented gravels of unknown age comprising sandstone pebbles and cobbles. Preliminary structural and lithologic analysis suggests that the gravels might be early to late synrift sediment. At Site U1500, the main seismic reflector corresponds to the top of a basalt sequence at ~1379.1 m. We cored 149.90 m into this volcanic package and recovered 114.92 m (77%) of sparsely to moderately plagioclase-phyric basalt comprising numerous lava flows, including pillow lavas with glass, chilled margins, altered veins, hyaloclastites, and minor sediment. Preliminary geochemical analyses indicate that the basalt is tholeiitic. Sampling of the Pleistocene to lower Miocene sedimentary section at Sites U1499 and U1500 was not continuous for two reasons. First, there was extremely poor recovery in substantial intervals interpreted to be poorly lithified sands, possibly turbidites. Second, we chose to drill down without coring in some sections at Site U1500 to ensure sufficient time to achieve this site’s high-priority deep drilling objectives. The upper Miocene basin sequence, which consists of interbedded claystone, siltstone, and sandstone can be correlated between the two sites by seismic stratigraphic mapping and biostratigraphy. At Site U1501 on the OMH, coring ~45 m into the acoustic basement sampled prerift(?) deposits comprising sandstone to conglomerate of presumed Mesozoic age. These deposits are overlain by siliciclastic synrift sediments of Eocene to Oligocene age followed by primarily carbonaceous postrift sediments of early Miocene to Pleistocene age. Site U1502 on Ridge A was cased to 723.7 m. No coring was attempted shallower than 380 m to save operational time and because of low expectations for core recovery in the upper Plio–Pleistocene sequence. At this site, we recovered 180 m of hydrothermally altered brecciated basalts comprising sheet and pillow lavas below deep-marine sediments of Oligocene to late Miocene age. At Site U1503 on Ridge C, 991.5 m of casing was installed in preparation for the planned deep drilling to ~1800 m. No coring was performed due to mechanical failures, and the site was abandoned without further activity except for installation of a reentry cone. Coring at Site U1504 on the OMH, located ~45 km east of Site U1501, recovered mostly foliated, greenschist facies metamorphic rocks below late Eocene(?) carbonate rocks (partly reef debris) and early Miocene to Pleistocene sediments. At Site U1505, we cored to 480.15 m through Pleistocene to late Oligocene mainly carbonaceous ooze followed at depth by early Oligocene siliciclastic sediments. Efforts were made at every drill site to correlate the core with the seismic data and seismic stratigraphic unconformities interpreted in the Eocene to Plio–Pleistocene sedimentary sequence prior to drilling. The predrilling interpretation of ages of these unconformities was in general confirmed by drilling results, although some nontrivial corrections can be expected from detailed postexpedition work on integrating seismic stratigraphic interpretations with detailed bio- and lithostratigraphy. As a result of the limited length of drill string that could be deployed during the later part of Expedition 368, the secondary expedition objectives addressing the environmental history of the SCS and Southeast Asia received more focus than originally planned, allowing Site U1505 (alternate to Site U1501) to be included. Despite this change in focus, Expedition 367/368 provided solid evidence for a process of breakup that included vigorous synrift magmatism as opposed to the often-favored interpretation of the SCS margin as a magma-starved margin or a margin possibly overprinted at a much later stage by plume-related magmatism. In this broader perspective, Expedition 367/368 accomplished a fundamental objective of the two-expedition science program. 

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5. Cenozoic slip along the southern Sierra Nevada normal fault, California (USA): A long-lived stable western boundary of the Basin and Range

   https://doi.org/10.1130/GES02574.1  

   Lee, Jeffrey ; Stockli, Daniel F. ; Blythe, Ann E. ( April 2023 , Geosphere)

   The uplift history of the Sierra Nevada, California, is a topic of long-standing disagreement with much of it centered on the timing and nature of slip along the range-bounding normal fault along the east flank of the southern Sierra Nevada. The history of normal fault slip is important for characterizing the uplift history of the Sierra Nevada, as well as for characterizing the geologic and geodynamic factors that drove, and continue to drive, normal faulting. To address these issues, we completed new structural studies and extensive apatite (U-Th)/He (AHe) thermochronometry on samples collected from three vertical transects in the footwall to the east-dipping southern Sierra Nevada normal fault (SNNF). Our structural studies on bedrock fault planes show that the SNNF is a steeply (~70°) east-dipping normal fault. The new AHe data reveal two elevation-invariant AHe age arrays, indicative of two distinct periods of cooling and exhumation, which we interpret as initiation of normal faulting along the SNNF at ca. 28–27 Ma with a second phase of normal faulting at ca. 17–13 Ma. We argue that beginning in the late Oligocene, the SNNF marked the now long-standing stable western limit, or break-away zone, of the Basin and Range. Slip along SNNF, and the associated unloading of the footwall, likely resulted in two periods of uplift of Sierra Nevada during the late Cenozoic. Trench retreat, driven by westward motion of the North American plate, along the Farallon–North American subduction zone boundary, as well as the gravitationally unstable northern and southern Basin and Range pushing on the cold Sierra Nevada, likely drove the late Oligocene- aged normal slip along the SNNF and the similar-aged but generally local and minor extension within the Basin and Range. We posit that the thick proto–Basin and Range lithosphere was primed for late Oligocene extension by replacement of the steepening Farallon slab with hot and buoyant asthenosphere. While steepening of the Farallon slab had not yet reached the southern Sierra Nevada by late Oligocene time, we speculate that late Oligocene slip along the SNNF reactivated a late Cretaceous dextral shear zone as the Sierra Nevada block was pulled and pushed westward in response to trench retreat and gravitational potential energy. The dominant middle Miocene normal fault-slip history along the SNNF is contemporaneous with high-magnitude slip recorded along range-bounding normal faults across the Basin and Range, including the east-adjacent Inyo and White mountains, indicating that this period of extension was a major regional tectonic event. We infer that a combination of slab-driven trench retreat along the Juan de Fuca–North America subduction zone boundary and clockwise rotation of the southern ancestral Cascade Range superimposed on continental lithosphere pre-conditioned for extension drove this episode of middle Miocene normal slip along the SNNF and extension to the east across the Basin and Range. Transtensional plate motion along the Pacific–North America plate boundary, and likely a growing slab window, continued to drive extension along the SNNF and the western Basin and Range, but not until ca. 11 Ma when the Mendocino triple junction reached the latitude of our northernmost (U-Th)/He transect. 

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