Showing papers in "Geosphere in 2011"

History of Cenozoic North American drainage basin evolution, sediment yield, and accumulation in the Gulf of Mexico basin

Abstract: The Cenozoic fill of the Gulf of Mexico basin contains a continuous record of sediment supply from the North American continental interior for the past 65 million years. Regional mapping of unit thickness and paleogeography for 18 depositional episodes defines patterns of shifting entry points of continental fluvial systems and quantifies the total volume of sediment supplied during each episode. Eight fluvio-deltaic axes are present: the Rio Bravo, Rio Grande, Guadalupe, Colorado, Houston-Brazos, Red, Mississippi, and Tennessee axes. Sediment volume was calculated from digitized hand-contoured unit thickness maps using a geographic information system (GIS) algorithm to sum volumes within polygons bounding interpreted North American river contribution. General age-dependent compaction factors were used to convert calculated volume to total grain volume. Values for rate of supply range from >150 km to <10 km3/Ma. Paleogeographic maps for eleven Cenozoic time intervals display the evolving matrix of elevated source areas, intracontinental sediment repositories, known and inferred drainage elements, and depositional fluvial/deltaic depocenters along the northern Gulf of Mexico basin margin. Patterns of sediment supply in time and space record the complex interplay of intracontinental tectonism, climate change, and drainage basin evolution. Five tectono-climatic eras are differentiated: Paleocene late Laramide era; early to middle Eocene terminal Laramide era; middle Cenozoic (Late Eocene–Early Miocene) dry, volcanogenic era; middle Neogene (Middle–Late Miocene) arid, extensional era; and late Neogene (Plio–Pleistocene) monsoonal, epeirogenic uplift era. Through most of the Cenozoic, three to four independent continental-scale drainage basins have supplied sediment to the Gulf of Mexico.

Cenozoic tectonic history of the Himachal Himalaya (northwestern India) and its constraints on the formation mechanism of the Himalayan orogen

Abstract: A central debate for the evolution of the Himalayan orogen is how the Greater Himalayan Crystalline complex in its core was emplaced during the Cenozoic Indo-Asian collision. Addressing this problem requires knowledge of the structural relationship between the South Tibet detachment fault (STD) and the Main Central thrust (MCT) that bound these rocks from above and below. The fault relationship is exposed in the Himachal Himalaya of northwestern India, where they merge in their updip direction and form a frontal branch line that has been warped by subsequent top-to-the-southwest shear deformation. To elucidate how the two major crustal-scale faults evolved in the western Himalaya, we conducted integrated geologic research employing field mapping, pressure-temperature ( P-T ) analyses, U-Pb zircon geochronology, trace and rare earth element (REE) geochemistry, and thermochronology. Our field study reveals complex geometric relationships among major thrusts with large-magnitude shortening within each thrust sheet. Three successive stages of top-to-the-southwest thrust development are recognized: (1) imbricate stack development, (2) translation of large thrust sheets along low-angle detachments and backthrusting along the STD, and (3) development of duplex systems via underplating. This kinematic process can be quantified by our new analytical data: (1) P-T determinations show 7–9 kbar and 450–630 °C conditions across the STD. The lack of a metamorphic discontinuity across the fault is consistent with a backthrust interpretation. (2) U-Pb zircon geochronology yields ca. 830 Ma and ca. 500 Ma ages of granitoids in the MCT hanging wall, ca. 1.85 Ga ages of granitic gneisses in both the MCT hanging wall and footwall, and 8–6 Ma ages of granitic pegmatites in the MCT footwall. These ages help define regional chronostratigraphy, and the youngest ages reveal a previously unknown intrusion phase. (3) Trace element and REE geochemistry of 1.85 Ga, 830 Ma, and 500 Ma granitoids are characteristic of remelted continental crust, constraining the protolith tectonic setting. (4) U-Pb geochronology of detrital zircon reveals that siliciclastic sedimentary sequences above the STD, below the MCT, and between these two faults have similar age spectra with Neoproterozoic youngest age peaks. This result implies that the STD and MCT each duplicated the same stratigraphic section. (5) Th-Pb geochronology of monazite included in MCT hanging-wall garnet yields Paleozoic and early Tertiary ages, indicating Paleozoic and early Tertiary metamorphism in these rocks. (6) The 40 Ar/ 39 Ar thermochronology of the K-feldspar from southern MCT hanging-wall rocks evinces cooling below 220–230 °C ca. 13–19 Ma or later, constraining the thrust development history. We use these results to derive a tectonic model of crustal shortening across the Himachal Himalaya involving early thickening, tectonic wedging emplacement of the Greater Himalayan Crystalline complex between the MCT and STD, and continued growth of the Himalayan thrust wedge by accretion of thrust horses from the Indian footwall.

Magma addition and flux calculations of incrementally constructed magma chambers in continental margin arcs: Combined field, geochronologic, and thermal modeling studies

Abstract: Incrementally constructed magma systems have been recognized from studies of the resulting plutons for more than three decades. However, magma addition rates, fluxes, growth durations, sizes of increments, and sizes and durations of the resulting magma chambers have been difficult to ascertain, emphasizing the need for a better understanding of how magmatic systems evolve. Our results from studies of plutons and arc sections in the North American Cordillera indicate that a large range exists in all of these values. Although arc sections and individual plutons clearly have dramatic temporal changes in volumetric magma additions, true volumetric flux calculations are particularly difficult to determine. Thus, although subduction beneath arcs may have active durations of hundreds of millions of years, volumetrically most magmatism is emplaced during magmatic flare-ups of ∼10–30 m.y. duration. Individual plutons and batholiths in these arcs can grow in

Modeling of submarine cyclic steps: Controls on their formation, migration, and architecture

Abstract: Submarine cyclic steps are a newly recognized manifestation of fundamental morphodynamic instability of Froude-supercritical flow over an erodible bed. There is a growing recognition of the global presence and importance of cyclic steps. An attempt was made here to: (1) outline submarine cyclic steps in the context of the sediment waves of various origins; (2) elucidate the physics and key parameters governing their formation, migration, and architecture; and (3) summarize selected numerical experiments on net-depositional and net-erosional cyclic steps in a useful form. The paper also addresses frequent terminology confusion between net-depositional cyclic steps and sediment waves in general.

Pore networks in continental and marine mudstones: Characteristics and controls on sealing behavior

Abstract: Mudstone pore networks are strong modifiers of sedimentary basin fluid dynamics and have a critical role in the distribution of hydrocarbons and containment of injected fluids. Using core samples from continental and marine mudstones, we investigate properties of pore types and networks from a variety of geologic environments, together with estimates of capillary breakthrough pressures by mercury intrusion porosimetry. Analysis and interpretation of quantitative and qualitative three-dimensional (3D) observations, obtained by dual focused ion beam–scanning electron microscopy, suggest seven dominant mudstone pore types distinguished by geometry and connectivity. A dominant planar pore type occurs in all investigated mudstones and generally has high coordination numbers (i.e., number of neighboring connected pores). Connected networks of pores of this type contribute to high mercury capillary pressures due to small pore throats at the junctions of connected pores and likely control most matrix transport in these mudstones. Other pore types are related to authigenic (e.g., replacement or pore-lining precipitation) clay minerals and pyrite nodules; pores in clay packets adjacent to larger, more competent clastic grains; pores in organic phases; and stylolitic and microfracture-related pores. Pores within regions of authigenic clay minerals often form small isolated networks (

Oblique convergence, arc-parallel extension, and the role of strike-slip faulting in the High Himalaya

Abstract: Arc-parallel extension is an important component of the active deformation of the Himalaya. This extension is accommodated via arc-perpendicular normal faults linked to arc-parallel strike-slip faults. Analysis of ~130 global positioning system geodetic velocities indicates >3 cm yr –1 of arc-parallel extension of the Himalaya. Several models have sought to explain Himalayan arc-parallel extension and strike-slip faulting, including lateral extrusion of Tibet, oroclinal bending of the Himalaya, radial spreading of Tibet and the Himalaya, and variably oblique convergence between India and the Himalaya. Predictions of each model are tested against structural and geodetic observations. These tests indicate that the oblique convergence model best describes Himalayan extensional and strike-slip deformation.

Spatial variations in focused exhumation along a continental-scale strike-slip fault: The Denali fault of the eastern Alaska Range

Abstract: 40 Ar/ 39 Ar, apatite fission-track, and apatite (U-Th)/He thermochronological techniques were used to determine the Neogene exhumation history of the topographically asymmetric eastern Alaska Range. Exhumation cooling ages range from ∼33 Ma to ∼18 Ma for 40 Ar/ 39 Ar biotite, ∼18 Ma to ∼6 Ma for K-feldspar minimum closure ages, and ∼15 Ma to ∼1 Ma for apatite fission-track ages, and apatite (U-Th)/He cooling ages range from ∼4 Ma to ∼1 Ma. There has been at least ∼11 km of exhumation adjacent to the north side of Denali fault during the Neogene inferred from biotite 40 Ar/ 39 Ar thermochronology. Variations in exhumation history along and across the strike of the fault are influenced by both far-field effects and local structural irregularities. We infer deformation and rapid exhumation have been occurring in the eastern Alaska Range since at least ∼22 Ma most likely related to the continued collision of the Yakutat microplate with the North American plate. The Nenana Mountain region is the late Pleistocene to Holocene (∼past 1 Ma) primary locus of tectonically driven exhumation in the eastern Alaska Range, possibly related to variations in fault geometry. During the Pliocene, a marked increase in climatic instability and related global cooling is temporally correlated with an increase in exhumation rates in the eastern Alaska Range north of the Denali fault system.

High-resolution bathymetry of the axial channels within Monterey and Soquel submarine canyons, offshore central California

Abstract: High-resolution multibeam bathymetry and chirp (compressed high-density radar pulse) seismic data acquired from an autonomous underwater vehicle outline in unprecedented detail the shape and near subbottom character of the axial channels within upper Monterey and Soquel Canyons (offshore California, USA). In Monterey Canyon, the bathymetric data span water depths from 100 m to >2100 m, and include the confluence with Carmel Canyon at ∼1900 m water depth. The bathymetric data for Soquel Canyon begin close to the canyon head at 100 m water depth and extend down to the intersection with Monterey Canyon. The seafloor within the axis of Monterey Canyon is covered with sediment fill out to 910 m water depth. Below this water depth exposures of underlying strata are common, presumably because of decreasing sediment drape and generally increased erosional resistance of the pre-canyon host strata. The seafloor within the axial channel of upper Soquel Canyon is smooth and contains horizontally layered sediment fill. In contrast, the sediment fill within the incised portions of the axial channel of Monterey Canyon is characterized by distinctive crescent-shaped bedforms down to the limit of the surveys. These differences in morphology and texture correspond with the contrasting cohesive strength of the sediments filling these canyons and the increased propensity for weakly cohesive sands and gravels in Monterey Canyon to fail. Episodic movement of coarse-grained sediments down Monterey Canyon maintains a longitudinal gradient of ∼1.6°. The more cohesive fine-grained sediments in Soquel Canyon stabilize the seafloor and maintain a substantially higher longitudinal gradient (3°–6°) than that measured in Monterey Canyon. The textural and lithologic data, plus previously published observations, indicate that upper Monterey Canyon is currently active, whereas upper Soquel Canyon appears to be inactive as a coarse sediment transport conduit. Episodic seabed sediment failures in active submarine canyons are hypothesized to control the gradient of the axial channel. The propensity for sediment failure in weakly cohesive coarse-grained sediments results in shallower horizontal gradients compared to submarine canyons stabilized by more cohesive fine-grained sediments.

Structure of the Sierra Nevada from receiver functions and implications for lithospheric foundering

Abstract: Receiver functions sampling the Sierra Nevada batholith and adjacent regions exhibit significant variations in the structure of the crust and upper mantle. Crustal Vp/Vs values are lower in the core of the batholith and higher in the northern Sierra Nevada, portions of the Basin and Range, and near young volcanic fields in the eastern Sierra Nevada and Owens Valley. P- to S-wave conversions from the Moho vary from high amplitude and shallow (>25% of the direct P-arrival amplitude, 25–35 km depth) along the eastern Sierra Nevada to low amplitude and deep (

Hydrodynamic mechanism for the Laramide orogeny

Abstract: The widespread presumption that the Farallon plate subducted along the base of North American lithosphere under most of the western United States and ∼1000 km inboard from the trench has dominated tectonic studies of this region, but a number of variations of this concept exist due to differences in interpretation of some aspects of this orogeny. We contend that five main characteristics are central to the Laramide orogeny and must be explained by any successful hypothesis: thick-skinned tectonism, shutdown and/or landward migration of arc magmatism, localized deep foreland subsidence, deformation landward of the relatively undeformed Colorado Plateau, and spatially limited syntectonic magmatism. We detail how the first two elements can be well explained by a broad flat slab, the others less so. We introduce an alternative hypothesis composed of five particular processes: (1) a more limited segment of shallowly subducting slab is created by viscous coupling between the slab and the Archean continental keel of the Wyoming craton, leaving some asthenosphere above most of the slab; (2) dynamic pressures from this coupling localize subsidence at the edge of the Archean Wyoming craton; (3) foreland shortening occurs after the subsidence of the region decreases gravitational potential energy, increasing deviatoric stresses in lithosphere beneath the basin with no change to boundary stresses near the subduction zone or changes to basal shear stress; (4) shear between the slab and overriding continent induces a secondary convective system aligned parallel to relative plate motion, producing the Colorado Mineral Belt above upwelling aligned along the convection cell; (5) the development of this convective system interrupts the flow of fresh asthenosphere into the arc region farther west, cutting off magmatism even in segments of the arc not over the shallowly dipping slab.

ASTER spectral analysis and lithologic mapping of the Khanneshin carbonatite volcano, Afghanistan

Abstract: Advanced Spaceborne Thermal and Reflection Radiometer (ASTER) data of the early Quaternary Khanneshin carbonatite volcano located in southern Afghanistan were used to identify carbonate rocks within the volcano and to distinguish them from Neogene ferruginous polymict sandstone and argillite. The carbonatitic rocks are characterized by diagnostic CO3 absorption near 11.2 μm and 2.31–2.33 μm, whereas the sandstone, argillite, and adjacent alluvial deposits exhibit intense Si-O absorption near 8.7 μm caused mainly by quartz and Al-OH absorption near 2.20 μm due to muscovite and illite. Calcitic carbonatite was distinguished from ankeritic carbonatite in the short wave infrared (SWIR) region of the ASTER data due to a slight shift of the CO3 absorption feature toward 2.26 μm (ASTER band 7) in the ankeritic carbonatite spectra. Spectral assessment using ASTER SWIR data suggests that the area is covered by extensive carbonatite flows that contain calcite, ankerite, and muscovite, though some areas mapped as ankeritic carbonatite on a preexisting geologic map were not identified in the ASTER data. A contact aureole shown on the geologic map was defined using an ASTER false color composite image (R = 6, G = 3, B = 1) and a logical operator byte image. The contact aureole rocks exhibit Fe2+, Al-OH, and Fe, Mg-OH spectral absorption features at 1.65, 2.2, and 2.33 μm, respectively, which suggest that the contact aureole rocks contain muscovite, epidote, and chlorite. The contact aureole rocks were mapped using an Interactive Data Language (IDL) logical operator. A visible through short wave infrared (VNIR-SWIR) mineral and rock-type map based on matched filter, band ratio, and logical operator analysis illustrates: (1) laterally extensive calcitic carbonatite that covers most of the crater and areas northeast of the crater; (2) ankeritic carbonatite located southeast and north of the crater and some small deposits located within the crater; (3) agglomerate that primarily covers the inside rim of the crater and a small area west of the crater; (4) a crater rim that consists mostly of epidote-chlorite-muscovite–rich metamorphosed argillite and sandstone; and (5) iron (Fe3+) and muscovite-illite–rich rocks and iron-rich eolian sands surrounding the western part of the volcano. The thermal infrared (TIR) rock-type map illustrates laterally extensive carbonatitic and mafic rocks surrounded by quartz-rich eolian and fluvial reworked sediments. In addition, the combination of VNIR, SWIR, and TIR data complement one another in that the TIR data illustrate more laterally extensive rock types and the VNIR-SWIR data distinguish more specific varieties of rocks and mineral mixtures.

New insights into the morphology, fill, and remarkable longevity (>0.2 m.y.) of modern deep-water erosional scours along the northeast Atlantic margin

Abstract: A series of large-scale erosional scours are described from four modern deep-water canyon and/or channel systems along the northeast Atlantic continental margin. Regional-scale geophysical data indicate that most scours occur in zones of rapid flow expansion, such as canyon and/or channel termini and margins. High-resolution images of the scours cover ∼25 km² at 2 × 2 m pixel size, and were obtained at depths of 4200–4900 m using Autosub6000, an autonomous underwater vehicle equipped with an EM2000 multibeam bathymetry system. Sedimentological and microfossil-based chronological data of scour fills and interscour areas were obtained via accurately located piston cores that targeted specific sites within imaged areas. These core data reveal a number of key findings. (1) Deep-water scours can be very long lived (>0.2 m.y. ) and may undergo discrete phases of isolation, amalgamation, and infilling. (2) Deep-water scours can develop via a composite of cutting and filling events with periodicities of between tens of thousands and hundreds of thousands of years. (3) Immediately adjacent scours may have strikingly different sedimentological histories and do not necessarily evolve contemporaneously. (4) Scour infills are typically out of phase with sedimentation in intrascour areas, having thin sands internally and thick sands externally, or thick muds internally and thin muds externally. (5) Erosional hiatuses within scour fills may represent hundreds of thousands of years of time, and yet leave little visible record. Four distinct morphologies of scour are identified that range from 40 to 3170 m wide and 8 to 48 m deep: spoon shaped, heel shaped, crescent shaped, and oval shaped. Isolated scours are shown to coalesce laterally into broad regions of amalgamated scour that may be several kilometers across. The combined morphosedimentological data set is used to examine some of the putative formative mechanisms for scour genesis.

The Iapetan rifted margin of southern Laurentia

Abstract: 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 identification of synrift rocks and structures, as well as continental-shelf and off-shelf 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 fill 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 upper-plate 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-fill 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 reflects 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.

The natural range of submarine canyon-and-channel longitudinal profiles

Abstract: We differentiated 20 submarine canyon-and-channel longitudinal profiles across various types of continental margins on the basis of relative convexity or concavity, and according to their similarities to best-fitting mathematical functions. Profiles are visually differentiated into convex, slightly concave, and very concave groups, each of which generally corresponds with a continental-margin type and distinct depositional architecture. Profile groups generally reflect the competing influences of uplift and construction of depositional relief of the seafloor and its degra da tion by erosion related to mass wasting. Longitudinal-profile shape provides a basis for classifying deep-sea sedimentary systems, linking them to the geomorphic processes that shape continental margins.

Late Cretaceous subduction initiation on the eastern margin of the Caribbean-Colombian Oceanic Plateau: One Great Arc of the Caribbean (?)

Abstract: Detailed mapping on the Leeward Antilles islands of Aruba, Curacao, Bonaire, and La Blanquilla has led to a reassessment of their stratigraphic, magmatic, and structural evolution. In general, each island preserves its own distinct sequence of geologic events. The Cretaceous geology of Aruba and Curacao consists of a mafic igneous complex, long interpreted to represent exposures of the Caribbean-Colombian Oceanic Plateau (CCOP), intruded by 89–86 Ma arc-related plutons and dikes. The rocks on both islands that are interpreted as remnants of the CCOP underwent a period of subaerial erosion in the Late Cretaceous, but subsequently their geologic histories diverge significantly in terms of their stratigraphic and structural evolution. Mapping on Bonaire has resulted in a major revision to the Cretaceous bedrock geology. Instead of a single stratigraphic unit (Washikemba Formation) the island contains two stratigraphic units separated by a northwest-trending fault. The southwest side of the fault consists of an arc-related Early to Late Cretaceous volcaniclastic section cut by shallow level intrusions, whereas the northeast side is composed of Early to Late Cretaceous epiclastic/hemipelagic strata that are locally cut by small arc-related mafic intrusions. La Blanquilla represents the southernmost exposure of the Aves Ridge which is a remnant arc separated from the modern arc of the Lesser Antilles by the Grenada back arc basin. The bedrock geology consists of two Late Cretaceous arc-related plutons. The geologic evolution of the Leeward Antilles when combined within a broader context of Caribbean tectonics leads us to a tectonic model involving three distinct arcs rather than a single “Great Arc” of the Caribbean as an explanation for the geodynamic evolution of the CCOP and its fringing arc system.

3D characterization of sandstone by means of X-ray computed tomography

Abstract: Due to technological development, stateof-the-art high-resolution X-ray computed tomography (CT) systems can be developed, enabling the internal visualization of geomaterials in three dimensions. However, in order to obtain structural information one also needs proper three-dimensional (3D) analysis software. In this paper, the potential for petrographic purposes of high-resolution X-ray CT in combination with the 3D analysis software Morpho+ is explored for a Belgian sandstone sample. The advantage of the CT technique is the fact that it is an ideal tool to characterize the internal structure of a rock in three dimensions in a nondestructive way while a limitation of this CT technique is that only small samples can be combined with a high spatial resolution and therefore often many samples will need to be scanned in order to obtain representative volumes. The relationship between sample size and obtained spatial resolution are discussed as well as the infl uence on the spatial resolution exerted by some important technical aspects like the used X-ray source and detector. This paper focuses in detail on the structures that can be determined by means of micro- and high-resolution X-ray CT in combination with 3D algorithms.

Jurassic volcanic and sedimentary rocks of the La Silla and Todos Santos Formations, Chiapas: Record of Nazas arc magmatism and rift-basin formation prior to opening of the Gulf of Mexico

Abstract: Stratigraphic relationships, detrital zircon provenance, U-Pb and 40 Ar/ 39 Ar geochronology, and trace element geochemistry in volcanic and sedimentary rocks of the Sierra homocline of central Chiapas near La Angostura reservoir in Mexico document an extensive pulse of Early–Middle Jurassic arc magmatism in rocks that overlie and intrude the Permian–Triassic Chiapas massif. Upper Jurassic rift-basin strata unconformably overlie the volcanic rocks and the massif. A Pliensbachian U-Pb (zircon) SHRIMP (sensitive high-resolution ion microprobe) age from porphyritic andesite (191.0 ± 3.0 Ma), Early to Middle Jurassic 40 Ar/ 39 Ar dates from andesitic dikes, U-Pb grain ages of detrital zircons in overlying strata (196–161 Ma), and previously reported K-Ar dates indicate that subduction-related magmatism occurred in the western portion of the Maya block from Early to latest Middle Jurassic time. We assign the volcanic rocks to the La Silla Formation, which correlates with the informal Pueblo Viejo andesite of the Cintalapa and Uzpanapa regions to the northwest. La Silla magmatism predates opening of the Gulf of Mexico Basin. The Todos Santos Formation, which overlies La Silla Formation, was deposited in extensional basins during the early stages of gulf opening. We recognize a lower El Diamante Member of the Todos Santos, consisting of red fluvial sandstone, mudstone, and minor conglomerate containing primarily volcanic-lithic detritus; this member is characterized by a nearly unimodal Jurassic detrital zircon age population that indicates a Callovian or younger depositional age. Volcanic activity continued into the upper part of the El Diamante Member, but with a more mafic character. We also recognize an upper member, which we term the Jerico Member. This member is characterized by thickly bedded, coarse-grained pebbly arkose intercalated with several thick intervals (tens of meters) of conglomerate and pebbly sandstone. Sandstone petrology indicates a source in the granitic rocks of the Chiapas massif, with a tendency to show deep-seated sources and a diverse zircon population in the upper part of the section. The upper Todos Santos Formation in the study area is gradational into the overlying San Ricardo Formation (Kimmeridgian–Tithonian). The La Silla Formation was deposited in volcanic-complex environments, with a clear lack of differentiated volcanic rocks. Fluvial strata of the El Diamante Member were deposited in a mud-rich sinuous river system. The Jerico Member was deposited in large, sand-rich fluvial systems, which probably represent deposits of rift-axis trunk streams; conglomerate facies were deposited in adjacent and interfingering alluvial fan systems. We suggest that the stratigraphic record of the western Maya block records a transition from volcanic arc to intra-arc basin and subsequently to rift basin during Pliensbachian to Oxfordian time.

Detrital zircon U-Pb ages of sandstones in continental red beds at Valle de Huizachal, Tamaulipas, NE Mexico: Record of Early-Middle Jurassic arc volcanism and transition to crustal extension

Abstract: Continental strata and volcanic rocks of the type Huizachal Group in Valle de Huizachal record arc magmatism and subsequent crustal extension prior to seafl oor spreading in the Gulf of Mexico. The older La Boca Formation consists of two informal members, a lower unit of siliciclastic and volcanic rocks discordantly overlain by a predominantly siliciclastic upper member. The younger La Joya Formation is an upwardfi ning, alluvial-braided fl uvial succession with a basal conglomerate. U-Pb detrital-zircon ages (n = 576) from six Huizachal Group sandstones (fi ve from La Boca and one from La Joya) consist of four groups indicating a mixed provenance: (1) Grenville grains (~1.3‐1.0 Ga) derived from Gondwana (Novillo Gneiss); (2) earlymiddle Paleozoic grains (430‐300 Ma) derived from peri-Gondwanan accreted rocks (Granjeno Schist); (3) Permo-Triassic grains (296‐222 Ma) derived from volcanic and plutonic rocks (West Pangaean arc) and/or turbidites (Guacamaya Formation); and (4) Early-Middle Jurassic grains (199‐ 164 Ma), locally derived from the Nazas arc. Groups 1‐3 increase in abundance upsection as a result of unroofi ng of Jurassic volcanic and sedimentary carapace from uplifted basement. The Huizachal Group records three stages in the pre-breakup history of Gondwana: (1) The lower member of La Boca Formation (maximum depositional age 184‐183 Ma; Pleinsbachian) indicates Nazas arc activity; (2) the upper member (maximum depositional age 167‐163 Ma; Bathonian‐Callovian) indicates continued arc magmatism as early crustal extension formed horsts that supplied basement grains to an incipient rift basin; and (3) the La Joya Formation represents late rift basin development and widespread exposure of fl anking basement rocks. Although our La Joya sample lacks a coherent age group of young grains, its single youngest grain age (164 ± 3 Ma; Callovian) is consistent with its stratigraphic position beneath inferred Oxfordian strata.

Pre-Andean deformation of the Precordillera southern sector, southern Central Andes

Abstract: This paper presents a detailed investigation of the structure and evolution of the Precordillera southern sector (Argentina). We document the development and successive reactivation of regional and discrete structural grain through time, and discuss the existence of a large-scale mechanical anisotropy present in the lithosphere. Our kinematic studies indicate that the Permian orogeny generated a doubly vergent fold-and-thrust belt of transpressive deformation, where strain was partitioned into two different types of deformation domains. The west-vergent western domain was characterized by partitioned transpression with shortening dominating, and a strike-slip–dominated subdomain. The east-vergent eastern domain was characterized by pure contractional deformation. Our model for the Late Permian to Early Triassic evolution of the Precordillera involves a north-northwest–trending weakness zone affected by north-northeast–directed extension, generating an area with transtensional deformation during the Choiyoi volcanism development. Later, during the Triassic generation of the Cuyana rift basin, the northeast stretching direction was orthogonal to the rift trend, indicating pure extensional deformation. We propose a model where the clear parallelism between the distribution of an inferred early Paleozoic suture zone, a north-northwest–trending late Paleozoic belt, and Permian–Triassic rift-related magmatism indicates the reactivation of a north-northwest–trending long-lived lithospheric weakness zone.

Petrologic, tectonic, and metallogenic evolution of the Ancestral Cascades magmatic arc, Washington, Oregon, and northern California

Abstract: Present-day High Cascades arc magmatism was preceded by ∼40 m.y. of nearly cospatial magmatism represented by the ancestral Cascades arc in Washington, Oregon, and northernmost California (United States). Time-space-composition relations for the ancestral Cascades arc have been synthesized from a recent compilation of more than 4000 geochemical analyses and associated age data. Neither the composition nor distribution of ancestral Cascades magmatism was uniform along the length of the ancestral arc through time. Initial (>40 to 36 Ma) ancestral Cascades magmatism (mostly basalt and basaltic andesite) was focused at the north end of the arc between the present-day locations of Mount Rainier and the Columbia River. From 35 to 18 Ma, initial basaltic andesite and andesite magmatism evolved to include dacite and rhyolite; magmatic activity became more voluminous and extended along most of the arc. Between 17 and 8 Ma, magmatism was focused along the part of the arc coincident with the northern two-thirds of Oregon and returned to more mafic compositions. Subsequent ancestral Cascades magmatism was dominated by basaltic andesite to basalt prior to the post–4 Ma onset of High Cascades magmatism. Transitional tholeiitic to calc-alkaline compositions dominated early (before 40 to ca. 25 Ma) ancestral Cascades eruptive products, whereas the majority of the younger arc rocks have a calc-alkaline affinity. Tholeiitic compositions characteristic of the oldest ancestral arc magmas suggest development associated with thin, immature crust and slab window processes, whereas the younger, calc-alkaline magmas suggest interaction with thicker, more evolved crust and more conventional subduction-related magmatic processes. Presumed changes in subducted slab dip through time also correlate with fundamental magma composition variation. The predominance of mafic compositions during latest ancestral arc magmatism and throughout the history of modern High Cascades magmatism probably reflects extensional tectonics that dominated during these periods of arc magmatism. Mineral deposits associated with ancestral Cascades arc rocks are uncommon; most are small and low grade relative to those found in other continental magmatic arcs. The small size, low grade, and dearth of deposits, especially in the southern two-thirds of the ancestral arc, probably reflect many factors, the most important of which may be the prevalence of extensional tectonics within this arc domain during this magmatic episode. Progressive clockwise rotation of the forearc block west of the evolving Oregon part of the ancestral Cascades magmatism produced an extensional regime that did not foster signifi cant mineral deposit formation. In contrast, the Washington arc domain developed in a transpressional to mildly compressive regime that was more conducive to magmatic processes and hydrothermal fluid channeling critical to deposit formation. Small, low-grade porphyry copper deposits in the northern third of the ancestral Cascades arc segment also may be a consequence of more mature continental crust, including a Mesozoic component, beneath Washington north of Mount St. Helens.

Stratigraphy, geochronology, and geochemistry of the Laramide magmatic arc in north-central Sonora, Mexico

Abstract: The Laramide magmatic arc in the Arizpe-Mazocahui quadrangle of north-central Sonora, Mexico, is composed of volcanic rocks assigned to the Tarahumara Formation and several granitic plutons that intrude it. The arc was built over juxtaposed crustal basements of the Caborca and Mazatzal provinces. A basal conglomerate of the >4-km-thick Tarahumara Formation overlies deformed Proterozoic igneous rocks and Neoproterozoic to Early Cretaceous strata, thus constraining the age of a contractional tectonic event that occurred between Cenomanian and early Campanian time. The lower part of the Tarahumara Formation is composed of rhyolitic ignimbrite and ash-fall tuffs, andesite flows, and interbedded volcaniclastic strata, and its upper part consists of rhyolitic to dacitic ignimbrites, ash-fall tuffs, and volcaniclastic rocks. The Tarahumara Formation shows marked lateral facies change within the study area, and further to the north it grades into the coeval fluvial and lacustrine Cabullona Group. The age of the Tarahumara Formation is between ca. 79 and 59 Ma; the monzonitic to granitic plutons have ages of ca. 71–50 Ma. The informally named El Babizo and Huepac granites, La Aurora and La Alamedita tonalities, and the Puerta del Sol granodiorite compose the El Jaralito batholith in the southern part of the area. Major and trace element composition of the Laramide igneous rocks shows calc-alkaline differentiation trends typical of continental magmatic arcs, and the isotope geochemistry indicates strong contribution from a mature continental crust. Initial 87Sr/86Sr values range from 0.70589 to 0.71369, and eNd values range from –6.2 to –13.6, except for the El Gueriguito quartz monzonite value, –0.5. The Nd, Sr, and Pb isotopic values of the studied Laramide rocks permit comparison with the previously defined Laramide isotopic provinces of Sonora and Arizona. The El Gueriguito pluton and Bella Esperanza granodiorite in the northeastern part of the study area along with plutons and mineralization of neighboring northern Sonora have isotopic values that correspond with those of the southeastern Arizona province formed over the Mazatzal basement ([Lang and Titley, 1998][1]; [Bouse et al., 1999][2]). Isotopic values of the other Laramide rocks throughout the study area are similar to values of provinces A and B of Sonora ([Housh and McDowell, 2005][3]) and to those of the Laramide Pb boundary zone of western Arizona, while the Rancho Vaqueria and La Cubana plutons in the northernmost part of the area have the isotopic composition of the Proterozoic Mojave province of the southwestern United States. These data permit us to infer that a covered crustal boundary, between the Caborca block with a basement of the Mojave or boundary zone and the Mazatzal province, crosses through the northeastern part of the area. The boundary may be placed between outcrops of the El Gueriguito and Rancho Vaqueria plutons, probably following a reactivated Cretaceous thrust fault located north of the hypothesized Mojave-Sonora megashear, proposed to cross through the central part of the area. [1]: #ref-53 [2]: #ref-13 [3]: #ref-50

Birth of the Sierra Nevada magmatic arc: Early Mesozoic plutonism and volcanism in the east-central Sierra Nevada of California

Abstract: Granitic and volcanic rocks in the east-central Sierra Nevada, western United States, record the earliest stages of magmatism in the eastern Sierra Nevada magmatic arc, allowing us to examine magma sources and connections between plutonic and volcanic processes in the initial stages of arc construction. The Scheelite Intrusive Suite is one of the largest in the Sierra Nevada region, and is composed of the Wheeler Crest Granodiorite, granite of Lee Vining Canyon, and Pine Creek Granite. The Pb/U zircon ages from each unit of the suite suggest assembly between 226 and 218 Ma. The Scheelite Intrusive Suite is a high-K calcic or calc-alkalic suite, compositionally broadly similar to the nearby Late Cretaceous Tuolumne and John Muir Intrusive Suites, though plutons of the Scheelite Intrusive Suite are consistently Ca and Fe rich and lower in Na. Although Triassic granodiorites are isotopically quite similar to nearby Late Cretaceous intrusive suites, the trend toward more isotopically primitive granites is in contrast to the constant or more whole-rock radiogenic Sr trends observed in younger intrusive suites. Along the western margin of the Scheelite Intrusive Suite, the basal Mesozoic volcanic section in the Saddlebag Lake pendant includes silicic volcanic rocks that are in part coeval and potentially comagmatic with Triassic plutonic rocks. Widespread quartz-phyric ash-flow tuffs of Black Mountain, Saddlebag Lake, and Greenstone Lake yield Pb/U zircon ages of 232, 224, and 219 Ma, indicating that felsic ignimbrite volcanism commenced earlier and continued during emplacement of the 226–218 Ma Scheelite Intrusive Suite. Ash-flow tuffs are hydrothermally altered but have high field strength element abundances and Nd isotopic compositions, suggesting affinity to the relatively felsic parts of the Wheeler Crest Granodiorite and the granite of Lee Vining Canyon.

High-resolution three-dimensional imaging and analysis of rock falls in Yosemite Valley, California

Abstract: We present quantitative analyses of recent large rock falls in Yosemite Valley, California, using integrated high-resolution imaging techniques. Rock falls commonly occur from the glacially sculpted granitic walls of Yosemite Valley, modifying this iconic landscape but also posing signifi cant potential hazards and risks. Two large rock falls occurred from the cliff beneath Glacier Point in eastern Yosemite Valley on 7 and 8 October 2008, causing minor injuries and damaging structures in a developed area. We used a combination of gigapixel photography, airborne laser scanning (ALS) data, and ground-based terrestrial laser scanning (TLS) data to characterize the rock-fall detachment surface and adjacent cliff area, quantify the rock-fall volume, evaluate the geologic structure that contributed to failure, and assess the likely failure mode. We merged the ALS and TLS data to resolve the complex, vertical to overhanging topography of the Glacier Point area in three dimensions, and integrated these data with gigapixel photographs to fully image the cliff face in high resolution. Three-dimensional analysis of repeat TLS data reveals that the cumulative failure consisted of a near-planar rock slab with a maximum length of 69.0 m, a mean thickness of 2.1 m, a detachment surface area of 2750 m 2 , and a volume of 5663 ± 36 m 3 . Failure occurred along a surfaceparallel , vertically oriented sheeting joint in a clear example of granitic exfoliation. Stress concentration at crack tips likely propagated fractures through the partially attached slab, leading to failure. Our results demonstrate the utility of high-resolution imaging techniques for quantifying far-range (>1 km) rock falls occurring from the largely inaccessible, vertical rock faces of Yosemite Valley, and for providing highly accurate and precise data needed for rock-fall hazard assessment.

Tectonic and climatic influence on the evolution of the Surveyor Fan and Channel system, Gulf of Alaska

Abstract: Present-day seafl oor morphology and sediment distribution in the deep-water Surveyor Fan, Gulf of Alaska, is dominated by the >700-km-long Surveyor Channel, an anomaly in a system with no major fl uvial input or shelf canyons. The sediment supply instead has been provided by glacial erosion in the stillactive Chugach‐St. Elias orogen, and glacial transport across the shelf. Glaciation has periodically increased in the St. Elias Range since the Miocene, but began dominating erosion and spurred enhanced exhumation since the mid-Pleistocene transition, at ~1 Ma. Ice associated with this glacial intensifi cation carved cross-shelf sea valleys that connect the St. Elias Range to the Surveyor Fan. The direct deposition of newly increased terrigenous sediment fl ux into the fan triggered the formation of the Surveyor Channel and its growth across the Surveyor Fan. Through the formation of the Surveyor Channel, climate events created three major differentiable sequences across the Surveyor Fan that we mapped using seismicrefl ection profi les. The change in morphology observed throughout the sequences allows us to characterize the infl uence that a glaciated orogen can have in shaping margin processes and the sediment pathway from source to sink. We show that the large variation in sediment fl ux between glacial- interglacial cycles together with sea valley formation leads to a glacial shelf transport process not typical of a fl uvial system. This glacial shelf transport along with the channel terminus in the Aleutian Trench makes the Surveyor Fan and Channel morphologically one of the most unique systems in the world.

Measuring currents in submarine canyons: Technological and scientific progress in the past 30 years

Abstract: The development and application of acoustic and optical technologies and of accurate positioning systems in the past 30 years have opened new frontiers in the submarine canyon research communities. This paper reviews several key advancements in both technology and science in the field of currents in submarine canyons since the1979 publication of Currents in Submarine Canyons and Other Sea Valleys by Francis Shepard and colleagues. Precise placements of high-resolution, high-frequency instruments have not only allowed researchers to collect new data that are essential for advancing and generalizing theories governing the canyon currents, but have also revealed new natural phenomena that challenge the understandings of the theorists and experimenters in their predictions of submarine canyon flow fields. Baroclinic motions at tidal frequencies, found to be intensified both up canyon and toward the canyon floor, dominate the flow field and control the sediment transport processes in submarine canyons. Turbidity currents are found to frequently occur in active submarine canyons such as Monterey Canyon. These turbidity currents have maximum speeds of nearly 200 cm/s, much smaller than the speeds of turbidity currents in geological time, but still very destructive. In addition to traditional Eulerian measurements, Lagrangian flow data are essential in quantifying water and sediment transport in submarine canyons. A concerted experiment with multiple monitoring stations along the canyon axis and on nearby shelves is required to characterize the storm-trigger mechanism for turbidity currents.

Introduction: New developments in Grenville geology: In honor of James McLelland

Abstract: The Grenville Province exposes the interior of an ancient mountain belt whose scale rivals the modern Himalayan-Alpine Orogen ([Rivers, 2008][1]; [McLelland et al., 2010][2]). The geology of this vast tectonic collage holds a record of mid-Proterozoic continental margin dynamics, including the

Revised regional correlations and tectonic implications of Paleoproterozoic and Mesoproterozoic metasedimentary rocks in northern New Mexico, USA: New findings from detrital zircon studies of the Hondo Group, Vadito Group, and Marqueñas Formation

Abstract: Detrital zircon ages from quartzite and metaconglomerate in the Tusas and Picuris Mountains in northern New Mexico reveal new information about age and provenance trends within a >1000 km 2 Proterozoic sedimentary basin and provide a critical test of regional correlations. Samples from the Paleoproterozoic Vadito and Hondo groups are dominated by a single detrital zircon population with age probability peaks that range from 1765 to 1704 Ma. Minor Archean and ca. 1850 Ma age probability peaks were also recognized in some samples. Close correspondence between detrital zircon ages and the age of surrounding basement rocks indicates predominately local sources, and we interpret systematic shifts in peak ages with stratigraphic position to represent changes in local sources through time. Similarities of age spectra support previous correlation of stratigraphic units between discontinuous exposures of the Hondo Group. We interpret that these supracrustal rocks were deposited in a single basin that we refer to as the Pilar basin. Two samples of the Marquenas Formation, a pebble to boulder conglomerate previously correlated with the ca. 1700 Ma Vadito Group, are dominated by Paleoproterozoic detrital zircon with age probability peaks at 1707 and 1715 Ma in the middle and upper units, respectively. Unlike the Vadito and Hondo group samples, the Marquenas Formation also contains abundant ca. 1700–1600 Ma zircon derived from Mazatzal-aged sources to the south and Mesoproterozoic zircon with age probability peaks at 1479 and 1457 Ma. Weighted averages of 1477 ± 13 Ma and 1453 ± 10 Ma for the youngest detrital zircon in the middle and upper Marquenas Formation provide new maximum depositional age constraints, indicating that it is not part of the Vadito Group. The minimum age is not well constrained but is interpreted to be ca. 1435 Ma on the basis of the timing of regional metamorphism and deformation previously documented in the Picuris Mountains. These data represent the first evidence of sedimentation directly associated with ca. 1.4 Ga regional metamorphism, plutonism, and deformation in the southwestern United States and provide an important new constraint on the tectonic evolution of southern Laurentia during this time.

New insights on the evolution of the Lyon Mountain Granite and associated Kiruna-type magnetite-apatite deposits, Adirondack Mountains, New York State

Abstract: The integrated approach of field work, microscopy, whole-rock and mineral-scale geochemistry, and in situ U-Th-Pb zircon geochronology has proven to be useful for recognizing the type, timing, and sequence of complex Na and K fluid alteration related to the development of Kiruna-type magnetite-apatite deposits and the tectonic evolution of the granites that host these deposits. The Lyon Mountain Granite in the northeastern Adirondack Mountains of New York State has undergone multiple episodes of hydrothermal fluid alteration and Fe mineralization. Perthite granite containing ubiquitous 1060–1050 Ma zircon grains was overprinted by potassic alteration, which in turn was overprinted by pervasive Na alteration. During the Na alteration, preexisting orebodies, consisting of magnetite, clinopyroxene, and apatite, were overprinted and remobilized to form new deposits that contain magnetite, apatite, quartz, and zircon. The U-Th-Pb zircon geochronology data suggest that the Lyon Mountain Granite intruded the Adirondack Highlands during the Ottawan orogeny between ca. 1060 and 1050 Ma. However, subsequent alteration obscured much of the prehistory of the LMG. Amphibolite layers within the Lyon Mountain Granite and granitic dikes and pegmatites that crosscut the foliation in the Lyon Mountain Granite have been dated between 1045 and 1016 Ma. These ages coincide with previous published zircon age data from second-generation orebodies associated with Na alteration.

Late Eocene crustal thickening followed by Early-Late Oligocene extension along the India-Asia suture zone: Evidence for cyclicity in the Himalayan orogen

Abstract: The timing of geologic events along the India-Asia suture in southern Tibet remains poorly understood because minimal denudation prevents widespread exposure of structurally deep rocks. In this study, we present geologic maps of two structurally deep domes, cored by mylonitic orthogneisses, across the India-Asia suture zone in southwestern Tibet. New U-Pb zircon ages and rock textures indicate that core orthogneisses are originally Gangdese arc rocks that experienced Late Eocene prograde metamorphism, probably during crustal thickening. Crosscutting leucogranite sills underwent northwest-southeast extension related to slip along a brittle ductile shear zone here designated the Ayi Shan detachment. The timing of shear along detachment is bracketed by zircon U-Pb ages of 26–32 Ma for these pre- to syn(?)-extensional leucogranites, and by a 40 Ar/ 39 Ar muscovite age of 18.10 ± 0.05 Ma for a rhyolitic dike. This rhyolite dike crosscuts a widespread siliciclastic unit that was deposited across the detachment, which we correlate to the Kailas Formation. The Great Counter thrust defines the surface trace of the India-Asia suture zone; it cuts the Kailas Formation, and is in turn cut by the Karakoram fault. A new 40 Ar/ 39 Ar muscovite age of 10.17 ± 0.04 Ma for the Karakoram fault footwall is consistent with published thermochronologic data that indicate Late Miocene transtension in southwestern Tibet.

Stratigraphy and age of Upper Jurassic strata in north-central Sonora, Mexico: Southwestern Laurentian record of crustal extension and tectonic transition

Abstract: Stratigraphy, sedimentology, and geochronology of the Upper Jurassic Cucurpe Formation in north-central Sonora, Mexico, provide new insights into Late Jurassic rifting along the southwestern margin of Laurentia. The Cucurpe Formation is the fill of the Altar-Cucurpe Basin. This basin developed upon attenuated crust of the Triassic–Middle Jurassic continental arc and was part of the Arivechi-Cucurpe seaway; a narrow marine embayment oriented parallel to and located west of the Chihuahua trough. The Cucurpe Formation unconformably overlies Middle Jurassic arc assemblages and represents upward-coarsening marine prodeltaic deposits. New U-Pb zircon geochronology and a Kimmeridgian ammonite ( Idoceras cf. I . densicostatum ) constrain its age to between ca. 158 and 149 Ma. Detrital zircon ages from the unconformably overlying Lower Cretaceous Bisbee Group indicate a maximum depositional age of 139 ± 2 Ma (2σ error), demonstrating a hiatus of at least 10 m.y. between Jurassic and Cretaceous strata. Detrital zircon ages and petrographic data indicate the provenance of Cucurpe Formation and lowermost Bisbee strata. The lower part of the Cucurpe was derived dominantly from Middle Jurassic volcano-sedimentary successions. The upper part of the Cucurpe Formation was largely derived from syneruptive volcanic material equivalent to the Ko Vaya volcanic suite of southern Arizona and northern Sonora. Lowermost Bisbee strata were derived from Middle Jurassic arc rocks and exhumed Caborcan basement, Paleozoic–Lower Jurassic sedimentary cover, and Lower Cretaceous intermediate volcanic rocks. Revised stratigraphy of the Cucurpe-Tuape region indicates that several conglomeratic units, formerly interpreted as Late Jurassic pull-apart basin deposits, are not of Late Jurassic age.