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September-October GSA Bulletin media highlights

Geological Society of America

Boulder, Colo. - The September-October issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes several newsworthy items. Topics include innovation in predicting extreme flooding and new chronology of major Neoproterozoic events. Two articles focus on offshore California's Monterrey Canyon, describing a trail of sand and gravel from shoreline to the canyon and offering new high-resolution images that illustrate the rapidity with which change takes place in the canyon.

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Semiannual patterns of erosion and deposition in upper Monterey Canyon from serial multibeam bathymetry
Douglas P. Smith, Division of Science and Environmental Policy, California State University Monterey Bay, Seaside, California 93933, USA, et al. Pages 1123-1133.
Keywords: Monterey, submarine, canyon, sediment, morphology, bathymetry.

Monterey Canyon, offshore from central California, is among the largest submarine canyons of the world. New advances in submarine imaging technology have made it possible to view the canyon as if all the water were removed. We have processed a variety of very high resolution images of the shallow portion of the canyon between the depths of 10 m (30 ft) and 275 m (900 ft). These new detailed views have lead to a clearer understanding of how this submarine canyon has evolved. Because we re-imaged the canyon over a 24 hour period and a 6 month period, we were able to see how fast the canyon is changing shape through erosion and deposition of sediment. Our results show that the canyon is far more active than previously believed. Newly discovered large sand dunes on the floor of the canyon migrated and completely changed their shapes in just six months. Our work suggests that the near-shore part of the canyon is in a long-term phase of enlargement, counter to previous studies in the region.

Trail of sand in upper Monterey Canyon: Offshore California
Charles K. Paull, Monterey Bay Aquarium Research Institute, Moss Landing, California 95039, USA, et al. Pages 1134-1145.
Keywords: submarine canyon, gravity flow, deep-sea fan, sediment transport, submarine channel.

The existence of submarine canyons has fascinated man since their discovery over 100 years ago. The existence of the canyons indicates that processes that occur in the channels of submarine canyons are capable of excavating features on the ocean floor that are equivalent in size to the Grand Canyon through which considerable volumes of sediment move from the continents into the deep sea. Because submarine canyons are covered in deep water, the traditional basic techniques of observation and sampling used in terrestrial geology cannot be easily applied. Thus, for most of the last century the lack of data on the canyon processes persisted because adequate technology was not available. In this paper, the results of using a Remotely Operated Vehicle (ROV), outfitted to carry a vibracoring system, are described. The combination of the ROV, which allows access to the seafloor, and the vibracoring system, which enables cores to be taken from the bottom, has revealed the nature of the sediments that occur within the axis of Monterey Canyon. Our ROV-deployed vibracoring system has enabled us to collect a grid of samples from the axis and flanks of upper Monterey Canyon (<1500 m water depth). These samples are providing the first detailed view of the facies associated with an axial channel within an active submarine canyon. These samples reveal a well-defined trail of sand and gravel extending along the axial channel from the shoreline to the abyss.

Differential compaction and subsidence in sedimentary basins due to silica diagenesis: A case study
Richard J. Davies, 3DLab, School of Earth, Ocean and Planetary Sciences, Park Place, Cardiff University, Cardiff, CF10 3YE, UK. Pages 1146-1155.
Keywords: differential compaction, opal A, opal CT, diagenesis, front, differential subsidence.

When sediments are laid down in basins, they compact when loaded by younger sedimentary layers deposited above. This is a fundamental geological process that occurs in all basins on Earth essentially due to gravity. If the amount of compaction within sedimentary strata varies laterally then the strata become deformed. Therefore without any compression or stretching of the basins, folds normally form. Geologists have been aware of this for nearly 100 years when it was found that sediment layers that draped hard, incompressible rocks developed a gentle, warped relief because elsewhere softer sediment was undergoing more compaction and subsidence. This ubiquitous process was called "differential compaction." It occurs in all sedimentary basins and generates much of the deformation that we can see within them.

Sediments deposited in all the oceans contain microscopic organisms that are made from silica. They die, are deposited on the seabed, and then undergo burial. At ~400-700 m of burial depth they undergo a transformation into a more compact form of silica, called opal CT. This paper uses three-dimensional visualization of the earth to show that this chemical transformation process causes differential compaction in sedimentary basins and therefore deformation. Since this type of silica is found globally, it is likely that it is responsible for deformation in many other sedimentary basins. So although geologists have been aware of deformation in sedimentary basins due to differential compaction, this is the first time that this process has been attributed to a thermally-induced chemical reaction. The deformational patterns that result are extraordinary.

Source of Oligocene to Pliocene sedimentary rocks in the Linxia basin in northeastern Tibet from Nd isotopes: Implications for tectonic forcing of climate
Carmala N. Garzione, Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA, and Department of Geological Sciences and Cooperative Institute for Research in Environmental Science, University of Colorado, Boulder, Colorado 80309, USA, et al. Pages 1156-1166.
Keywords: Nd isotopes, sedimentary provenance, loess, Tibetan Plateau, paleoclimate, unroofing.

The Nd isotopic and trace element composition of fine-grained sedimentary rocks in the Linxia basin in NE Tibet are used to determine the source of sediment to this region over the last 30 million years. This study suggests that wind-blown dust derived from central Asian deserts, such as the Taklamakan and Gobi, have contributed a significant volume of fine-grained sediment to the Linxia basin throughout its history. An increase in the range of Nd isotopic compositions at ca. 14 million years ago reflects the isotopic composition of source rocks in the NE margin of Tibet and suggests an increase in erosion of the margin of Tibet at that time. Enhanced erosion of the margin of Tibet slightly precedes a change in climate between ca. 13 and 12 Ma in the Linxia basin observed in the oxygen isotope record of lake carbonates. The change in the oxygen isotopic composition of lake carbonates has been interpreted as the result of reorganization of atmospheric circulation and an increase in aridity on the NE margin of the Tibetan plateau, perhaps associated with the plateau achieving an elevation sufficient to block moisture from the Indian and/or Pacific Oceans. Similar timing of erosion in NE Tibet and climate change suggests northeastward and eastward propagation of the plateau margin was responsible for the climate change between 13 and 12 Ma observed in the Linxia basin.

An integrated approach to flood hazard assessment on alluvial fans using numerical modeling, field mapping, and remote sensing
Jon D. Pelletier, Department of Geosciences, The University of Arizona, Tucson, Arizona, 85721, USA, et al. Pages 1167-1180.
Keywords: flood hazard, alluvial fan, surficial geology, remote sensing, numerical modeling.

Extreme flooding is a significant natural hazard in the western United States. Flood-hazard assessment is a challenge in the West because of the topographic complexity of the gently-sloping alluvial fans where many people live. In these areas, channels can shift and inundate new areas that have not been subject to inundation. Traditionally, hydrologists use numerical models to predict flood inundation. This technique works well but neglects channel changes over time scales of hundreds to thousands of years. In this paper, the authors combine four hydrologic and geomorphic techniques that can be used to determine flood hazards over a range of time scales from decades to thousands of years. These techniques are applied to two complex flood-prone areas in Arizona as a proof of concept.

Toward a Neoproterozoic composite carbon-isotope record
Galen P. Halverson, Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138-2902, USA and Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA, et al. Pages 1181-1207.
Keywords: Neoproterozoic, chemostratigraphy, carbon isotope, glaciation, geochronology.

During the Neoproterozoic Era (1000-542 Ma) Earth witnessed the breakup and reassembly of supercontinents, a series of extreme glaciations, an increase in atmospheric and marine oxygen concentrations, and the first appearance and diversification of macroscopic fauna. A great amount of integrated research, including biostratigraphy, physical stratigraphy, chemostratigraphy, and radiometric dating, has produced a geochronology for the final ~25 million years of the Neoproterozoic, helping to constrain the timing and pattern of early animal evolution. However, the chronology for the remainder of the Neoproterozoic, although improving, has remained obscure, and basic questions, such as the number of glaciations that occurred during the era, are highly controversial. In this paper we present a new composite carbon isotope record for the Neoproterozoic that is based mainly on detailed data sets from well-studied, carbonate-rich successions in northern Namibia and northeastern Svalbard (Norway). The basis for correlation between these successions is a new correlation scheme that rests upon the hypothesis that there were three Neoproterozoic glaciations. The result is a new framework that places the major events of the Neoproterozoic within a chronological, if not yet well time-calibrated, perspective. At the same time, the ?13C record is a basic measure of the biogeochemical changes that attended these events and a template for integrating other proxy records.

Late Cenozoic deformation and uplift of the NE Tibetan Plateau: Evidence from high-resolution magnetostratigraphy of the Guide Basin, Qinghai Province, China
Xiaomin Fang, Institute of Tibetan Plateau Research, Chinese Academy of Science, Beijing 100085, China, and National Laboratory of Western China's Environmental Systems, Ministry of Education of China and College of Resources and Environment, Lanzhou University, Gansu 730000, China, et al. Pages 1208-1225.
Keywords: Tibet, Neogene, magnetostratigraphy, Qinghai Province, plateau uplift.

Tibet is often called the "roof of the world." It has an unusually high average elevation of more than 14,000 ft (4667 m) in the central and southern parts and more than 8000 ft (2000 m). in the northeast. The processes by which these high elevations are achieved, however, are unclear. The Guide Basin in northeastern Tibet today is a high-elevation area that appears to have been an isolated (land-locked?) lake environment for much of its history during the last 20 million years. This paper documents the ages of the sedimentary strata deposited during this history, as well as the time constraints on the deformation and uplift of this part of Tibet. From these temporal constraints, an evolutionary scenario can be constructed of the development of this intramontane basin as a far-field response to the continuing plate-convergence between the Indian subcontinent and central Asia. The appearance of the Yellow River in this part of the world appears to be a relatively recent event, and is marked by subsequent incision of the river into the basin sediments of some 2700 ft (900 m) in the last 1.8 million years.

Basin physiography and tectonic influence on sequence architecture and stacking pattern: Pleistocene succession of the Canoa Basin (central Ecuador)
Claudio Di Celma, Dipartimento di Scienze della Terra, Università di Camerino, I-62032 Camerino (MC), Italy, et al. Pages 1226-1241.
Keywords: Pleistocene, Canoa Formation, Tablazo Formation, hiatal shell beds, taphonomy, paleoecology, falling-stage sequence set.

This paper shows that the analysis of taphonomic and paleoecologic features of condensed shellbeds may provide a means of identifying discontinuities and stratal surfaces where these are obscure. In particular, in conjunction with traditional lithofacies analysis, these allow for better resolution of sequence subdivisions of siliciclastic successions, and identification of the internal architecture of systems tracts at a more detailed scale than possible with sedimentary facies alone. Comparative analysis of the studied equatorial cyclothems with other Pleistocene sequences developed within temperate settings reveals similar internal arrangement, interpreted in terms of similar accommodation history, despite markedly different climatic and tectonic settings and syndepositional regimes. This suggests that the development of a condensed shell bed is possible also at low latitude and is not just as a largely temperate-latitude phenomenon and, moreover, that a global process of glacio-eustatic sea-level change was the fundamental control upon the general deepening-shallowing-up facies succession of each cyclothem. In the studied succession, taphonomic and paleoecologic features of condensed shellbeds and characteristics of primary sedimentary facies are influenced by basin physiography during sedimentation, whereas the long-term stacking pattern of depositional sequences is controlled by syn-sedimentary basin uplift.

Characteristics of internal contacts in the Tuolumne Batholith, central Sierra Nevada, California (USA): Implications for episodic emplacement and physical processes in a continental arc magma chamber
Jirí ák, Institute of Geology and Paleontology, Charles University in Prague, Prague 128 43, Czech Republic, and Czech Geological Survey, Prague 11821, Czech Republic, and Scott R. Paterson, Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740, USA. Pages 1242-1255.
Keywords: batholith, pluton, intrusive contacts, emplacement, Sierra Nevada, Tuolumne Batholith.

Our study focuses on internal contacts in the Tuolumne Batholith, central Sierra Nevada, California, which represents a large magma chamber. We show that the contacts are highly variable, often very complex boundaries formed by multiple processes. Fractional crystallization, km-scale mixing within broad transition zones, voluminous magmatic stoping along sharp contacts, and downward return flow of older magma units were important large-scale processes along these contacts during chamber construction. Our results indicate that multiple processes are likely during emplacement of large magma bodies within one another and that it is unlikely that evidence for all internal processes during batholith construction are preserved. We also argue that that fairly large magma chambers existed in this batholith and thus, that large accumulations of magma may exist in upper crustal chambers for significant periods of time.

Geologic and geophysical investigation of two fine-grained granites, Sierra Nevada Batholith, California: Evidence for structural controls on emplacement and volcanism
Sarah J. Titus, Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA, et al. Pages 1256-1271.
Keywords: Sierra Nevada Batholith, shear zones, emplacement, structural controls, volcanism.

The Johnson Granite Porphyry and the Silver Pass Porphyry are two small granitic bodies found in Yosemite National Park and the John Muir Wilderness, respectively. Both granites intruded at the end of the assembly of the Sierra Nevada magmatic arc during the Cretaceous period, about 85 million years ago. Based on structures that we observe in these rocks in the field as well as precise measurements of variations in gravity associated with the bodies, we believe that the geographic position of these two granites was controlled by regional shear zones, which are the deeper crustal extension of major faults seen at the surface of the earth. These Cretaceous shear zones had predominantly strike-slip movement, like the present-day San Andreas fault, but the current level of exposure of the igneous bodies and the faults corresponds to a deeper, ductile deformation regime instead of a brittle regime. Furthermore, we believe that these intrusions may represent a link between magma chambers and surface volcanoes during the Cretaceous. Our findings suggest that the location of present-day volcanism, like that occurring in the Cascades or in Sumatra, may be controlled by regional structures like shear zones.

Structural evolution of the Yushu-Nangqian region and its relationship to syncollisional igneous activity, east-central Tibet
Matthew S. Spurlin, Department of Earth and Space Sciences, and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, 90095-1567, USA, et al. Pages 1293-1317.
Keywords: Asia, China, Tibetan Plateau, thrust belts, strike-slip faults, syncollisional magmatism.

Scientists have long been wondering about how the Tibetan plateau, the largest elevated land mass on Earth, has been constructed. Some thought that the plateau was raised by heating from below or by pumping of viscous materials below Tibet as the Indian subcontinent moved northward against Asia. This study shows that at least the northern Tibetan plateau, its rise, was mainly accomplished by piling of the uppermost part of the crust.

Geochemical evidence for a subducted infant arc in Franciscan high-grade-metamorphic tectonic blocks
Aniki Saha, Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA, et al. Pages 1318-1335.
Keywords: Franciscan, arc, subduction, geochemistry, high-grade metamorphism.

Metamorphic rocks of the Franciscan Formation such as blueschists and ecologites that are found along the San Andreas Fault System in western California are possibly the best samples of ancient subduction zones. These rocks are believed to come from depths of 20 to over 50 km, and are thought to represent an original package of rocks consisting of sediments and altered oceanic crust. Our geochemical analysis of these rocks, including several trace elements and the isotopic compositions of some of these elements, reveals that before subduction and metamorphism, the original rocks were formed over a pre-Franciscan subduction zone in an island arc setting. In other words, the sources of the Franciscan metamorphic rocks that we analyzed represent the crust of a young island arc that formed just west of the ancient California coast in the Pacific Ocean floor.

40Ar/39Ar thermochronological evidence for formation and Mesozoic evolution of the northern-central segment of the Altyn Tagh fault system in the northern Tibetan Plateau
Yu Wang, Geologic Laboratories Center and Department of Geology, China University of Geosciences, Beijing 100083, China, et al. Pages 1336-1346.
Keywords: 40Ar/39Ar dating, thermochronology, Altyn Tagh fault system, Paleozoic-Mesozoic, mechanism, sinistral strike-slip motion.

The Altyn Tagh fault system has been a focused study area on the tectonic uplift and continental dynamics of the Qinghai-Tibetan plateau in the last two decades. To better constrain the probable timing of formation and evolution of the Altyn Tagh sinistral strike-slip system in the Mesozoic, in this paper, a 40Ar/39Ar thermochronological study, has been carried out in the north-central segment of the Altyn Tagh fault system, the northern margin of the Qaidam Basin, and the eastern Kunlun orogenic belt. Muscovite, biotite, and K-feldspar separated from mylonite, granite, pegmatite, and metamorphic rocks have been analyzed. The range of 40Ar/39Ar data and structural evidence indicates that a peak metamorphic event in terranes bordering the Altyn Tagh fault system occurred between 450-420 Ma. At ca. 250-230 Ma there is evidence for initial sinistral strike-slip shearing. Sinistral strike-slip deformation occurred later along the Altyn Tagh fault system at 165-160 Ma and 100-89 Ma, respectively. The regional tectonic and cooling process indicates that the initial formation of the Altyn Tagh sinistral slip fault system occurred in a latest Permian-Early Triassic time frame and was coupled with, or related to, suturing in the northern margin of the Qaidam Basin and the Kunlun orogenic belt. Cooling events along the Altyn Tagh fault system between 165-160 Ma and 100-89 Ma were accompanied by differential closure along the Bangong Lake-Nujiang suture zone in its eastern and western sectors during the Middle-Late Jurassic and Early Cretaceous, respectively.

Contrasting tectonothermal domains and faulting in the Potomac terrane, Virginia-Maryland--discrimination by 40Ar/39Ar and fission-track thermochronology
Michael J. Kunk, U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225, USA, et al. Pages 1347-1366.
Keywords: Potomac terrane, argon geochronology, muscovite, tectonic assemblage, Plummers Island fault.

This study describes the timing of assembly of rock units in the Appalachians of Northern Virginia, just to the west of Washington, D.C., including the rocks at Great Falls National Park. The study uses argon and fission-track dating together with petrology in determining the thermal history(ies) of these complex rocks. Our results indicate that the rocks were assembled hundreds of millions of years later than had previously been thought and that the rocks in the western part of the study area have been uplifted by a kilometer or more in the last 144 m.y. This uplift may be continuing in the present day.


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