Boulder, CO, USA - Geology topics of interest include: landscape evolution of California's Sierra Nevada; Nevada's northwestern Basin and Range and its remarkable record of Cenozoic magmatism; turbidity currents and topography of Earth's submarine channels; sediment dynamics of the lower Mississippi River; microorganisms' catalytic effect on limestone formation; high-resolution topographic survey of offshore California's Eel Canyon; and impact of scoria-cone eruptions on nearby communities.
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- P-T-t data from central Nepal support critical taper and repudiate large-scale channel flow of the Greater Himalayn Sequence
Matthew J. Kohn et al., Department of Geosciences, Boise State University, Boise, Idaho 83725, USA. Pages 259-273.
Kohn et al. present a new synthesis of pressure-temperature conditions and pressure-temperature-time (P-T-t) paths for high-grade metamorphic thrust sheets associated with the Main Central Thrust, in the Langtang and Darondi regions, central Nepal. From structurally low to structurally high, major structures include the Lesser Himalayan Duplex, Munsiari Thrust, Main Central Thrust (thrust contact between the Greater and Lesser Himalayan Sequences), and Langtang Thrust. Overall, metamorphic and chronologic patterns are matched well by expectations of critical taper models, including (1) uniformly high pressures of metamorphism (8–12 kbar) for all structural levels and thrust movement along the paleo–Main Himalayan Thrust, (2) isobaric cooling from the peak of metamorphism for Greater Himalayan rocks (deep juxtaposition of thrust sheets), (3) “hairpin” P-T paths for Lesser Himalayan rocks, and (4) relatively slow cooling rates for Greater Himalayan rocks. However, observations contrast significantly with published channel flow models. Most generally, although channel flow may have initiated since ca. 10 Ma due to focused erosion above the Lesser Himalayan Duplex, it does not appear responsible for past transport and exhumation of the migmatitic core of the Himalaya.
Carson Pass–Kirkwood paleocanyon system: Paleogeography of the ancestral Cascades arc and implications for landscape evolution of the Sierra Nevada (California)
C.J. Busby et al., Department of Earth Science, University of California, Santa Barbara, California 93101, USA. Pages 274-299.
Tertiary strata of the central Sierra Nevada are dominated by widespread, voluminous volcanic breccias that are largely undivided and undated, the origin of which is poorly understood. These dominantly andesitic strata are interpreted to be eruptive products of the ancestral Cascades arc, deposited and preserved within paleocanyons that crossed the present-day Sierra Nevada before Basin and Range faulting began there. These strata are thus important not only for understanding the paleogeography of the Ancestral Cascades arc, but also for reconstructing the evolution of the Sierra Nevada landscape.
Detailed mapping and dating of previously undifferentiated Tertiary strata in the Carson Pass–Kirkwood area of the central Sierra Nevada allowed the authors to identify 6 unconformity-bounded sequences preserved within a paleocanyon as deep as 650 m cut into Mesozoic granitic basement. Busby et al. suggest that the early Miocene reincision records tectonism related to the onset of arc magmatism in the Sierra Nevada.
Tectonic and magmatic evolution of the northwestern Basin and Range and its transition to unextended volcanic plateaus: Black Rock Range, Nevada
Derek William Lerch et al., Feather River College, 570 Golden Eagle Avenue, Quincy, California 95971, USA. Pages 300-311.
The seismically active eastern and western margins of the northern Basin and Range have been studied extensively, yet the northwestern margin of the province remains incompletely understood. The Black Rock Range of northwestern Nevada straddles the transition from the Basin and Range province to the south and east, and flat-lying volcanic plateaus to the west. This poorly understood range preserves a remarkably complete record of Cenozoic magmatism and provides an important window into the pre-Miocene history of the unextended volcanic plateaus of northeastern California and southern Oregon. Geologic mapping and 40Ar/39Ar geochronology from the northern Black Rock Range document three significant episodes of Eocene to middle Miocene volcanism. Basaltic and rhyolitic volcanic products in the northern Black Rock Range span 35 to 16 Ma, with many of the Oligocene volcanic units derived from local vents and dikes. Despite the map-scale complexities of locally derived lava flows, the Cenozoic section is broadly conformable and dips gently (∼5?–10?) to the northwest. The region experienced no significant tilting between 35 and 16 Ma, with moderate tilting (∼5?–10?) and concomitant uplift occurring after 16 Ma. This tectonic history is consistent with that of the nearby Pine Forest and Santa Rosa Ranges, where low-temperature thermochronology documents footwall exhumation along the range-bounding normal faults after 12 Ma. The velocity structure of the crust beneath the northern Black Rock Range is constrained by a recent geophysical survey (seismic reflection-refraction-gravity) and contains gradients that correspond to basin depths predicted by our geologic mapping. Together with recently completed geological and geophysical studies from the surrounding region, Lerch et al.’s results suggest that the evolution of the northwestern margin of the Basin and Range was characterized by long-lived and voluminous volcanism without significant tectonism, followed by low magnitude (≤20%) extension along high-angle normal faults.
Late Cenozoic deformation along the northwestern continuation of the Xianshuihe Fault System, Eastern Tibetan Plateau
Shifeng Wang et al., Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Shuang Qing Road 18, P.O. Box 2871, Beijing 1000085, China. Pages 312-327.
The Xianshuihe fault system is a highly active left-lateral slip fault that has played an important role in accommodating late Cenozoic crustal deformation of the eastern Tibetan Plateau. The left-lateral displacement on the western portion of the fault system is about 80 km, based on geologic and geomorphic markers. Wang et al.’s observations on field relations and Landsat imagery suggest that the northwestern continuation of the Xianshuihe fault has had a complex evolutionary history. Fault evolution was dominated by transpressional and transtensional deformation, initiated in the middle Miocene (12 Ma) and the early Pleistocene, respectively. Fault trend change is possibly the main reason for the transfer system changing from compression to extension. The existence of the transfer system implies that the southeastern margin of the plateau is underwent clockwise rotation.
Evolution of the Mazatzal province and the timing of the Mazatzal orogeny: Insights from U-Pb geochronology and geochemistry of igneous and metasedimentary rocks in southern New Mexico
Jeffrey M. Amato et al., Department of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA. Pages 328-346.
Most of the rocks that are exposed on the surface of Earth are from the most recent 10% of geologic time, or younger than about 500 million years. In some places, faults have uplifted parts of the deeper and older crust. These uplifts allow geologists to investigate the older chapters in Earth history. In New Mexico, these older Precambrian rocks preserve a history of the assembly of the North American continent from smaller pieces of crust through the process of accretion. In southern New Mexico, radiometric dating of the mineral zircon in granites indicates that all of the rocks older than 1650 million years were deformed in a collision between these smaller pieces of crust, and those younger than this were emplaced following the collision. These rocks were likely part of a volcanic arc system similar to the Aleutian Islands. According to Amato et al., rocks dated at about 1630 million years ago were likely emplaced while the accreted crust began to stretch, similar to the Basin and Range province of the western U.S.
Regional 18O-depletion in Neoproterozoic igneous rocks of Avalonia, Cape Breton Island and southern New Brunswick, Canada
J. Potter et al., Department of Earth Sciences, the University of Western Ontario, London, Ontario, N6A 5B7, Canada. Pages 347-367.
The peri-Gondwanan terranes that comprise the northern and southern Appalachians have been the subject of intense study over the years. The recognition of individual tectono-stratigraphic packages and the linkage and relative juxtaposition of these areas throughout geological history have remained highly controversial. Potter et al. present oxygen-isotope results for granitic plutons and associated volcanic rocks from the peri-Gondwanan terranes of Cape Breton Island and New Brunswick. The Neoproterozoic rocks of the Avalonian terranes are characterized by distinct 18O-depletion, which is not observed in the inboard terranes. This geochemical behavior not only provides insight into the geological history of Avalonia, but may also be an effective means for identification of tectono-stratigraphic packages of Avalonian affinity in peri-Gondwanan terranes elsewhere in the Appalachian mountain belt and its equivalent in Europe.
Interactions between turbidity currents and topography in aggrading sinuous submarine channels: A laboratory study
Kyle M. Straub et al., Department of Earth, Atmosphere, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Building 54-824, Cambridge, Massachusetts 02139, USA. Pages 368-385.
Mapping of the ocean seafloor has revealed ubiquitous channels constructed by turbidity currents, mixtures of water, and sediment that flow down continental margins as sediment-gravity flows. These channels can exceed 3000 km in length and play an important role in setting the surface slope of continental margins. Affiliated sediment deposits make up the largest accumulations that host significant hydrocarbon resources on Earth. The morphologies of submarine channels are similar to rivers, yet little is know about processes that set their lengths. Using laboratory experiments Straub et al. documented current interactions with channel bends for depositional turbidity currents. Reported observations aided their interpretation of environmental conditions associated with channels constructed by sediment-gravity flows in the deep ocean to the surface of Titan.
Estuarine controls on fine-grained sediment storage in the Lower Mississippi and Atchafalaya Rivers
John J. Galler, Department of Earth and Environmental Sciences, Tulane University, New Orleans, Louisiana 70118, USA; Mead A. Allison, Present address: Institute for Geophysics, University of Texas, 10100 Burnet Road (R2200), Austin, Texas 78758-4445, USA. Pages 386-398.
Galler and Allison’s study of sediment dynamics in the lower (tide influenced) Mississippi River in Louisiana has recognized several phenomena previously unobserved in this system. A significant part of the river's yearly suspended sediment load is deposited ephemerally on the channel floor for several months each year during low water discharge periods. Two mechanisms were identified. The first is a freshwater storage of thin (<30 cm thick) mud layers in shallow (<20 m) water areas that appears to result from organic particle aggregation and settling during the reduced downstream water velocities associated with low discharge. The second is associated with penetration of a saline salt wedge upstream as a dense bottom layer. Up to several meters of mud are deposited on the channel floor underneath this wedge, which serves to mantle the bottom from the high energy of the overlying freshwater layer. Each of these storage mechanisms is responsible for 10–15% of the annual sediment budget, which has major implications for the timing and character of sediment discharged to the adjacent ocean—river sediment discharges are based upon gauges located in many cases far upstream of the land-ocean interface to remove the influence of tides.
Initiation of gravitational collapse of an evaporite basin margin: The Messinian saline giant, Levant Basin, Eastern Mediterranean
J.A. Cartwright, 3DLab, School of Earth, Ocean and Planetary Sciences, Cardiff University, Main Building, Park Place, Cardiff CF10 3YE, UK; and M.P.A. Jackson, Bureau of Economic Geology, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78713, USA. Pages 399-413.
Regional 2D and 3D seismic data from the Levant Basin reveal the updip extensional component of thin-skinned gravity tectonics on this continental margin. Because of its youth (5–7 Ma), the earliest stages of deformation of this salt basin are preserved without the severe structural overprinting common on more mature, giant salt-tectonic systems. Extension detaches onto and within Messinian evaporites up to 1800 m thick. By structural restoration, Cartwright and Jackson reconstructed the tectonic evolution of the Messinian evaporites using paleobathymetric constraints from wells and seismic profiles. This analysis suggests that the Mediterranean drawdown during the Messinian salinity crisis was ~800 m. The 10- to 15-km–wide extensional domain tracks the landward pinch-out of the mobile Messinian evaporites against an older Late Miocene scarp. Diachronous extension began in the center of the margin in the mid-Pliocene, then spread northward in the late Pliocene, then finally southward in the early Pleistocene. Extension continues today on many of the most landward faults. Extensional strain varies greatly along strike from <1 km to as much as 12–15 km. Comparing observations with four end-member conceptual models, Cartwright and Jackson infer that both extension and seaward salt flow thinned the evaporite margin and its overburden. Both processes were triggered by a combination of uplift of the continental shoulder of the Dead Sea Rift and subsidence in the Mediterranean basin. At least three factors controlled variations in extension: (1) degree of tilting of the salt wedge, related to the interplay of coastal uplift and basin subsidence; (2) presence of pre-Messinian canyons overlain by landward salients of salt; and (3) variations in evaporite facies and the proportion of siliciclastic admixture.
Deformation associated with a continental normal fault system, western Grand Canyon, Arizona
Philip G. Resor, Department of Earth and Environmental Sciences, Wesleyan University, Middletown, Connecticut 06459, USA. Pages 414-430.
The Grand Canyon presents a spectacular natural laboratory for studying processes associated with extension of continental crust. Resor uses differential GPS surveying to quantify the geometry of folded Esplanade sandstone layers surrounding normal faults that offset these same layers up to ~225 m vertically. GPS measurements reveal a systematic relationship between faulting and folding that is consistent with patterns of deformation predicted by mechanical models of slip on planar faults that terminate at ~1km depth. These results suggest that the folds formed in association with slip on the nearby normal faults rather than Laramide reverse faults as previously suggested. The relatively shallow depth of the modeled faults is a result of the relatively narrow fold widths (~1.5 km) and may be explained by confinement of faults within the Paleozoic sedimentary section or localization of deformation by weakening effects associated with folding fractured and layered strata. A general analysis illustrates that similar “reverse-drag” folds of moderate dip are expected to form in association with slip on planar faults of finite extent and do not require curved fault profiles as is commonly assumed; a result that has the potential to impact our estimates of hydrocarbon volumes, crustal extension, and earthquake hazards associated with continental normal faults.
Geothermal anomalies in the crust and upper mantle along southern Rocky Mountain transitions
Marshall Reiter, New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA. Pages 431-441.
Heat flow data compliment other geophysical studies along geological transitions by contributing to the understanding of the cause of gravity anomalies and seismic velocity anomalies. Local variability in heat flow data can be smoothed along profiles traversing from peak to trough values by filtering high frequency components. Filtered heat flow and gravity data can be fitted by a low order since series to estimate approximate depths of corresponding anomaly sources. Because of the peak to trough distances present in Reiter’s study, anomaly source depths in the crust and upper mantle can be estimated. Along the profiles in this study, crossing from the Colorado Plateau and Great Plains to the highest regions of the Southern Rockies, some transitions indicate both upper mantle-lower crustal and crustal thermal sources; other transitions indicate either primarily mantle or primarily crustal thermal sources. The estimated depths of these thermal sources are often coincident with depths suggested for gravity anomaly sources and seismic velocity anomalies. This observation indicates the contribution of elevated temperatures in the development of gravity anomaly sources in seismic velocity anomalies.
Microbial Biomass: A Catalyst for CaCO3 Precipitation in advection-dominated transport regimes
Michael T. Kandianis et al., Department of Geology, University of Illinois at Urbana-Champaign, 1301 W. Green St., Urbana, Illinois 61801, USA. Pages 442-450.
Forming the limestone needed to construct a new statehouse, monument, or master bathroom would take a fraction of the time if microorganisms were available to lend a hand. This revelation is not a rising architectural trend, but is rather the latest discovery of Kandianis et al. They have determined that the cell surfaces and biofilms of microorganisms, which adhere to the surface of calcium carbonate in flowing marine and freshwater spring systems, can accelerate limestone formation by more than a factor of twenty. Most notably, this research shows that microorganisms elicit a catalytic effect whether they are alive or dead via a process called biologically induced mineralization. For years chemists and materials scientists have been investigating the impact that single proteins, amino acids, and other organic compounds have on calcium carbonate production, but never before has a study confirmed that the most abundant form of organic matter on the planet may be responsible for helping to build rock deposits that cover the globe. This research therefore indicates that changes in the rate of calcium carbonate formation are intimately linked with changes in microbial abundance, a fact which has implications for materials development and refines our ability to identify biological "fingerprints" in rocks from Earth and other planets.
The origin of the adakite–high-Nb basalt association and its implications for post-subduction magmatism in Baja California, Mexico
Paterno R. Castillo, Scripps Institution of Oceanography, University of California-San Diego, La Jolla, California 92093-0212, USA. Pages 451-462.
Arc volcanism or magmatism along plate convergent boundaries is due to the subduction of an oceanic slab under another tectonic plate. Some of the volcanic activities during the past 13 Ma in Baja California, Mexico are unusual because their volcanic rocks are identical to those erupted along an active plate convergent boundary although subduction of the oceanic slab under Baja California had ceased ~13 Ma. Among the young arc volcanic rocks occurring in Baja California are adakite and high-Nb basalt (HNB). The currently accepted model for the close spatial and temporal association of these two arc rock suites in Baja California is due to melting of the subducted slab, similar to the model for the origin of adakite–HNB associations elsewhere. The model claims that melting occurs at the edges of the small gaps or windows that were formed in the subducted slab directly beneath Baja California. This paper proposes alternative models for the origin of adakite, HNB, and post-subduction magmatism in Baja California. The new models propose that the compositionally heterogeneous Pacific upper mantle, instead of the subducted slab, is the source of HNB.
Evidence for superelevation, channel incision, and formation of cyclic steps by turbidity currents in Eel Canyon, California
Michael P. Lamb et al., Department of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA. Pages 463-475.
A new high-resolution topographic survey of a prominent submarine canyon, Eel Canyon, offshore California, has revealed evidence for active erosion and deposition by turbidity currents. Morphologic features such as channels, in-channel cyclic steps, and submarine fans suggest that sediment-laden underflows travel at great speeds (>10 m/s) and are capable of overflowing a 280-m-high wall within the canyon, which acts as an obstruction to abyssal-bound currents. This may indicate a partial avulsion of the main canyon by turbidity currents and a shift in the locus of deposition of Eel River sediment.
Effects of Scoria-Cone eruptions upon nearby human communities
M.H. Ort et al., Environmental Sciences and Geology, P.O. Box 4099, Northern Arizona University, Flagstaff, Arizona 86011, USA. Pages 476-486.
Scoria cone eruptions occur in many places around the world, including scores of locations in North America. Each eruption makes a new volcano, so they are the most common volcanic landform on Earth. Their relatively small size (up to a thousand feet high and a mile wide) and their relatively low level of explosivity compared to stratovolcanoes, such as Mount St. Helens or Mount Rainier, have led to few studies of their potential hazards. Stratovolcano eruptions tend to affect large areas, so that, although few people may live on the volcano, distant population centers are at risk. Scoria cones may erupt where people are living and thus profoundly affect people within a limited area. In this paper, the effects of the eruptions of two scoria cones, Parícutin (1943-52 AD) in central Mexico and Sunset Crater (~1075 AD) in northern Arizona, upon nearby human populations are examined using geologic and archaeological techniques. Significant agricultural communities lived near each volcano and the eruptions’ effects upon agriculture were distinct. In the wetter environment of Parícutin, the scoria primarily limited agriculture and led to the abandonment of a large area. At Sunset Crater, the scoria acted as a mulch and, although land near the volcano was abandoned, previously non-arable land became fertile. Those communities in which evacuation decisions were made at family or small group levels generally did better in following decades than those that were forcibly evacuated and/or separated into new communities. Ort et al. shed light on possible priorities for responding to small volcanic eruptions that occur where people live.
Integrated stratigraphy of the Oligocene pelagic sequence in the Umbria-Marche basin (Northeastern Apennines, Italy): A potential Global Stratotype Section and Point (GSSP) for the Rupelian/Chattian boundary
Rodolfo Coccioni et al., Istituto di Geologia e Centro di Geobiologia dell’Università degli Studi “Carlo Bo,” Campus Scientifico, Località Crocicchia, 61029 Urbino, Italy. Pages 487-511.
Coccioni et al. present a synthesis of an integrated stratigraphic study carried out in the pelagic succession of the Umbria-Marche Apennines of Italy which provide the means for an accurate and precise radiometric calibration of the Oligocene time scale. The Monte Cagnero section stands out as a potentional candidate for defining the Cahttian Global Stratotype Section and Point and some reliable criteria are proposed for marking the Rupelian/Chattian boundary.