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U-Pb geochronologic and Hf isotopic analy ses have been conducted on detrital zircons extracted from 36 samples of Neoproterozoic through Triassic passive margin strata from western North America. The data serve as an improved reference for comparison with inboard strata that accumulated on the North American craton and outboard strata belonging to potentially displaced Cordilleran terranes. As expected, this reference documents signifi cant variations in ages and Hf isotope compositions both north-south and also through time. The data also provide insights into the provenance of Cordilleran passive margin strata. During Neoproterozoic, Cambrian, and Early-Middle Devonian time, most grains were shed from relatively local basement rocks and from Mesoproterozoic clastic strata containing 1.2-1.0 Ga grains that originated in the Grenville orogen. This pattern was interrupted during Ordovician time, when much of the Cordilleran margin was blanketed by detritus shed from the northern Canadian Shield. Beginning in Late Devonian time, and continuing through late Paleozoic and Triassic time, most regions were dominated by locally derived detritus (largely recycled from underlying strata), but also received 0.7-0.4 Ga grains that were shed from the Franklinian, Caledonian, Appalachian, and Ouachita-Marathon orogens. This pattern is complicated in southern transects as a result of mid-Paleozoic emplacement of off-shelf assemblages onto the continental margin (e.g., Antler orogeny) and construction of Permo-Triassic magmatic arcs along the margin. Our data also provide a robust record of the crustal evolution of western North America, with signifi cant production of juvenile crust during late Archean (3.0- 2.5 Ga) and Paleoproterozoic (1.78-1.6 Ga) time and phases of mainly crustal reworking at 2.0-1.78, 1.5-1.3, 1.2-1.0, and 0.6-0.2 Ga. This history is somewhat different from that of other continents, with western Laurentia comprising a greater overall proportion of juvenile crust, punctuated by greater degrees of crustal reworking between 2.2 and 1.78 Ga and 0.3-0.2 Ga. [ABSTRACT FROM AUTHOR]

The amalgamation of crustal blocks that composes the western cordillera of North America has a long history of deformation across a broad zone. Investigations spanning a range of scales in the region have sought to unravel details of its structure and evolution. By sampling the entire western portion of the United States, the deployment of the Transportable Array component of the Earth- Scope USArray provides broadband seismic data to study crustal structure across much of this deforming region. Receiver functions recorded across the western United States provide insight into the thickness of the crust and how it varies between tectonic provinces. The thickness of the crust varies from <30 km in the southern Basin and Range and along the west coast to >50 km beneath the Rocky Mountains in Colorado and Wyoming. Distinct crustal structures characterize the Basin and Range, Snake River Plain, the Sierra Nevada, and the active Cascade volcanic arc, suggesting that the recent tectonic processes that affected the region have shaped the crustal structure. In addition, characteristics of the crust appear to relate to the boundaries of, and structures within, the terranes that formed North America. Patterns of crustal thickness across an expansive region presented here allow for the crustal response to specific tectonic processes to be determined while still imaging small-scale structures that can be more difficult to identify in a continental- scale study. This provides a context for previous and future detailed studies to understand how observed crustal structures relate to the evolution of western North America. The amalgamation of crustal blocks that composes the western cordillera of North America has a long history of deformation across a broad zone. Investigations spanning a range of scales in the region have sought to unravel details of its structure and evolution. By sampling the entire western portion of the United States, the deployment of the Transportable Array component of the Earth- Scope USArray provides broadband seismic data to study crustal structure across much of this deforming region. Receiver functions recorded across the western United States provide insight into the thickness of the crust and how it varies between tectonic provinces. The thickness of the crust varies from <30 km in the southern Basin and Range and along the west coast to >50 km beneath the Rocky Mountains in Colorado and Wyoming. Distinct crustal structures characterize the Basin and Range, Snake River Plain, the Sierra Nevada, and the active Cascade volcanic arc, suggesting that the recent tectonic processes that affected the region have shaped the crustal structure. In addition, characteristics of the crust appear to relate to the boundaries of, and structures within, the terranes that formed North America. Patterns of crustal thickness across an expansive region presented here allow for the crustal response to specific tectonic processes to be determined while still imaging small-scale structures that can be more difficult to identify in a continental- scale study. This provides a context for previous and future detailed studies to understand how observed crustal structures relate to the evolution of western North America. [ABSTRACT FROM AUTHOR]

The Iapetan rifted margin of southern Laurentia includes the northeast-striking Blue Ridge, Ouachita, and Marathon rifts, which are offset by the northwest-striking Alabama-Oklahoma and Texas transform faults, framing the Alabama and Texas promontories and the Ouachita and Marathon embayments of the continental margin. Interpretations of the original trace, structural style, and age of the rifted margin rest on identifi cation of synrift rocks and structures, as well as continental-shelf and offshelf sedimentary deposits on the passive margin. Both late Paleozoic Ouachita- Appalachian allochthons and post-orogenic Atlantic-Gulf passive-margin deposits cover the Iapetan rift margin, necessitating the use of data from deep wells and geophysical surveys along with geologic maps of the exposed Ouachita-Appalachian thrust belts to characterize the synrift and post-rift rocks and structures. The continental margin and passive-margin shelf strata are primarily in the footwall of the Ouachita allochthon; however, some Ouachita thrust faults displaced shelf-margin basement and cover. Appalachian thrust faults imbricate synrift fi ll of the intracratonic Birmingham graben and the passive-margin shelf. Palinspastic restoration of thrust-belt structures uses balanced cross sections to locate the original trace of the Iapetan margin. Thickness and subsidence history of the passive-margin successions, as well as a general lack of preserved synrift deposits, indicate an upperplate structure along the Blue Ridge rift on the Alabama promontory and along the Ouachita rift on the Texas promontory. The upper plate on the Texas promontory is conjugate to a lower-plate rift structure on the Argentine Precordillera. Although data are limited, the evolution of the passive margin along the Marathon rift in the Marathon embayment suggests a lower-plate structure. Geophysical modeling supports a steep continental margin along the Alabama-Oklahoma transform, and a similar structure can be inferred for the Texas transform. The Blue Ridge rift north of the Alabama promontory is dated by synrift volcanic rocks as young as 564 Ma, and passive-margin transgression beginning in earliest Cambrian is documented along the Alabama promontory and farther north. The age of the Ouachita rift is documented by the 530-539 Ma synrift volcanics of the transform-parallel intracratonic Southern Oklahoma fault system, by Early Cambrian synrift sediment along the conjugate rift margin in the Argentine Precordillera, and by late synrift graben-fi ll of Early to early Late Cambrian age in the rift-parallel intracratonic Mississippi Valley and Birmingham graben systems, as well as by subsidence history of the passive margin on the Texas promontory. The diachroniety of rifting refl ects an inboard shift from the Blue Ridge rift to the Ouachita rift along the Alabama-Oklahoma transform and rifting of the Argentine Precordillera from the Ouachita embayment. [ABSTRACT FROM AUTHOR]

Tectonic processes, flexure due to crustal loading, and dynamic mantle flow each impart a unique imprint on topography and geomorphic responses over time scales of 104 to 106 yr. This paper explores the mobility of regional drainage divides as a key geomorphic metric that can distinguish between the various processes driving crustal deformation in the greater Yellowstone region of the northwestern United States. We propose a new analysis that quantifies the differences between the location of the presentday drainage divide from divides synthetically generated from filtered topography to determine the relative impact of tectonic and dynamic mantle influences on landscape development. The greater Yellowstone region is an opportune location for this investigation because contrasting models have been proposed to explain the parabolic shape of elevated topography and active seismicity that outline the imprint of hypothesized hotspot activity. Drainage divides synthesized from topography filtered at 50, 100, and 150 km wavelengths within the greater Yellowstone region show that the locations of the actual and synthetic Snake River drainage divides are controlled by both dynamic and flexural mechanisms in the eastern greater Yellowstone region, but by flexural mechanisms only in the western greater Yellowstone region. The location of the actual divide deviates from its predicted position in the filtered topography where tectonic controls, such as active faults (e.g., Centennial and Teton faults), have uplifted large footwall blocks. Our results are consistent with the notion of a northeastward-propagating greater Yellow stone region topographic and seismic parabola, and suggest that Basin and Range extension follows from, rather than precedes, greater Yellowstone region dynamic topography. Furthermore, our analysis suggests that eastward migration of the Snake River drainage divide lags behind the continued northeastward propagation of high-standing topography associated with the Yellowstone geophysical anomaly by 1–2 m.y. [ABSTRACT FROM AUTHOR]

The first indication of a potential deep-subsurface kimberlite-like material in the Appalachian Basin, without surface expression, has been discovered in oil and gas well drilling samples from a deep well drilled near Zanesville, Ohio, USA. The well was drilled into the Cambrian Sauk Sequence within a localized fault-bounded graben, where a zone of ultramafic rocks was encountered. The zone of ultramafic rocks is within Sauk carbonates at 1720 m below sea level and is not highly anomalous on geophysical wire-line well logs. Petrographic analyses of grain-mounted samples show a 20 m zone of ultramafic rock material near the top of the Conasauga Group. Well cuttings from the ultramafic zone include relatively fresh phenocrysts of phlogopite with calcite, apatite, and titaniferous magnetite, in a secondary matrix of amphibole, chlorite, Fe-oxides, and possibly serpentine. The rocks have undergone deuteric alteration and probably later hydrothermal alteration. A 20-m-thick, localized, natural gas—bearing sandstone unconformably overlies (Knox unconformity) the Sauk Sequence above the ultramafic zone. Structural mapping indicates that the localized sandstone is confined to a small seismic-defined graben originating in the faulted Precambrian Grenville basement complex. Mapping of newly acquired gravity and magnetic data does not indicate the presence of an anomaly coincident with the graben, suggesting that the ultramafic material must be a very small, localized intrusive body (pipe or diatreme). Alternative interpretations include a thin intrusive sill, extrusive tuff deposits, Grenville-aged tectonic fault slice, or eroded igneous deposits. Subsurface mapping of units above the Sauk Sequence (Ordovician Black River Group-Trenton Limestone) shows no significant variation in structural contours above the feature. Although the igneous material has not been isotopically dated, the stratigraphic position suggests that it was intruded prior to Middle Ordovician time. [ABSTRACT FROM AUTHOR]