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7 July 2010
GSA Release No. 10-33
Contact:
Christa Stratton
Director - GSA Communications & Marketing
+1-303-357-1093
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GSA Bulletin Highlights

SEPTEMBER/OCTOBER 2010

Boulder, CO, USA - The September-October 2010 issue of GSA BULLETIN is now available online. Research collected from around the globe addresses fast erosion during floods; improving ancient Earth’s geological time scale fidelity; geologically recent tectonic plate movements; potential effects of volcanic activity on a California community; and an effort to identify the source of earthquakes in southern Alaska, among other topics.

Highlights are provided below. View abstracts at http://gsabulletin.gsapubs.org/content/current. Representatives of the media may obtain complimentary copies of GSA BULLETIN articles by contacting Christa Stratton at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA BULLETIN in articles published. Contact Christa Stratton for additional information or assistance.

Non-media requests for articles may be directed to GSA Sales and Service, .

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230Th/U dating of a late Pleistocene alluvial fan along the southern San Andreas fault
Kathryn E.K. Fletcher et al., Dept. of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA. Pages 1347-1359.

In this study, U-series dating of soil carbonate accumulated on gravel clasts provides a reliable, precise minimum age of 45.1 +/- 0.6 thousand years ago (2 sigma) for the Biskra Palms alluvial fan, Coachella Valley, California. Concordant ages for multiple sub-samples from individual carbonate coatings provide evidence that the U-238-U-234-Th-230 system has remained closed since carbonate formation. The U-series minimum age is used to assess published Be-10 exposure ages of cobbles and boulders (see Behr et al., companion paper in this issue, pages 1360-1377). All but one cobble age and some boulder Be-10 ages are younger than the U-series minimum age, indicating that surface cobbles and some boulders were partially shielded after deposition of the fan and have been subsequently exhumed by erosion of fine-grained matrix to expose them on the present fan-surface. Comparison of U-series and Be-10 ages indicates that the interval between final alluvial deposition on the T2 fan-surface and accumulation of dateable carbonate is not well resolved at Biskra Palms, however, the "time lag" inherent to dating via U-series on pedogenic carbonate can be no larger than about 10 thousand years old, the uncertainty of the Be-10-derived age of the T2 fan-surface. Dating of the T2 fan-surface via U-series on pedogenic carbonate (minimum age, 45.1 +/- 0.6 thousand years ago) and Be-10 on boulder-top samples using forward modeling (preferred age, 50 +/- 5 ka) provides broadly consistent constraints on the age of the fan-surface and helps to elucidate its post-depositional development.

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Uncertainties in slip-rate estimates for the Mission Creek strand of the southern San Andreas fault at Biskra Palms Oasis, southern California
W.M. Behr et al., Dept. of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA. Pages 1360-1377.

The rate of motion (slip rate) of the San Andreas fault over long time scales can be estimated using landforms that have been progressively displaced or offset along the fault. This requires measurement of both the amount of offset of the feature, and the time at which the feature formed. Behr and colleagues revisit a well-known slip rate site on the southern San Andreas and focus on the uncertainties in estimates of the geologic slip rate where it offsets an alluvial fan in southern California. They provide new estimates of the amount of offset of this fan aided by trench excavations, and new cosmogenic beryllium-10 age determinations from the tops of 12 boulders on the fan surface. Their results point to significantly larger geologic uncertainties than previously reported. The approach of rigorously addressing uncertainties is relevant to the current climate in the active tectonics community, which is focused on discrepancies between geologic and geodetic slip rates, i.e., whether these are real, and if so, what implications they have for temporal changes in plate boundary configurations, for loading during the earthquake cycle, and for seismic hazard. Behr et al. also address issues of cosmogenic sampling strategies, which have remained controversial within the literature. They compare boulder-top ages to those obtained from cobble-sized clasts and find that the boulder-tops yield much older ages that are consistent with uranium-series data from pedogenic carbonate (described in a companion paper in this issue by Fletcher et al., pages 1347-1359). This work emphasizes the value and necessity of applying independent geochronologic systems to landforms that clearly have complex geomorphic histories. The revised age and offset models from this study yield a range of slip-rates for the past 50,000 years of between 12 and 22 mm/yr, with the preferred range being 14-17 mm/yr; these rates are entirely compatible with short-term slip-rate estimates from geodetic models for this region, themselves subject to large uncertainties.

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The West Mariana Ridge, western Pacific Ocean: Geomorphology and processes from new multibeam data
James V. Gardner, University of New Hampshire, Center for Coastal & Ocean Mapping, 24 Colovos Road, Durham, New Hampshire 03824, USA. Pages 1378-1388.

In this study, the West Mariana Ridge and eastern Parece Vela Basin, a large region of a relict volcanic arc and adjacent deep-sea basin in the western Pacific Ocean, were mapped with a multibeam echosounder system. Gardner’s new data reveal a landscape dotted with extinct volcanoes, some aligned along the ridge crest but others trending at angles away from the ridge. These volcanoes should have subsided more than a kilometer because the ridge has been dormant for about 7 million years and therefore should have subsided as it cooled. However, the summits of several of the ridge volcanoes are at water depths of 500 m or shallower. This observation suggests the ridge may have been dormant for a long time, or may actually still be active. A surprising finding of the mapping is a series of very young channels that trend west, but not east, away for the ridge out onto the eastern-most Parece Vela Basin. The channels cut huge aprons of landslide debris, and the floors of the channels have been swept clean of the normal pelagic drape that is seen on the seafloor immediately adjacent to the channels. These observations suggest the channels are very young or even active sediment-transport paths, and that the landscape of the West Mariana Ridge has continued to be modified even though the ridge is thought to been inactive for a very long time.

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Linking river-flood dynamics to hyperpycnal-plume deposits: Experiments, theory, and geological implications
Michael P. Lamb et al., Jackson School of Geosciences, University of Texas, 1 University Station C1100, Austin, Texas 78712-0254, USA. Pages 1389-1400.

River plumes charged with sediment can plunge underneath seawater and run out along the ocean floor. The deposits of these plumes provide an important record of river dynamics through different climatic and tectonic settings. Lamb et al. present one of the first experimental flume studies aimed at testing whether river flood discharge can be unraveled from marine deposits. Counter to dominant theories, their results indicate that plume velocities can be uncorrelated or even anti-correlated with river discharge at certain seabed locations because of translation of the zone of plunging. An advection length scale of settling sediment is found to be an important control on hyperpycnal-plume deposits, where fine sediment (mud) is most likely to preserve rising and falling river discharge.

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Eruption chronology and petrologic reconstruction of the ca. 8500 B.P. eruption of Red Cones, southern Inyo chain, California
Brandon Browne et al., Dept. of Geological Sciences, California State University, Fullerton, California 92834, USA. Pages 1401-1422.

The 60-km-long Mono-Inyo volcanic chain of eastern California, stretching from Mono Lake in the north to a pair of basaltic scoria cones, known as Red Cones, located 5 km south of Mammoth Mountain, has produced ~20 explosive volcanic eruptions over the past 5000 years, the most recent of which occurred <500 years ago from vents in the central and northern portions of the chain. Although no eruptions have occurred in the southern portion of the chain for several thousand years, recent findings from geophysical studies suggest the presence of volatile-rich basaltic magma at depths of 10 to 20 km beneath Mammoth Mountain's southern flank near Horseshoe Lake and the southwest flank near Devil's Postpile National Monument. If this magma were to erupt at the surface, what would it be like in terms of eruption style? How much magma might be erupted? How long would the eruption last? And how might an eruption of this magma threaten the residents, infrastructure, and economy of the nearby community of Mammoth Lakes? One way to address these important questions is to investigate the products of the 8500-year-old eruption Red Cones, located only 5 km southwest of Mammoth Mountain, and use the results as a proxy for what may happen in the future if an eruption near Horseshoe Lake or Devils Postpile National Monument indeed occurs. Results from this study indicate that the Red Cones eruption lasted a minimum of 28 days, varied in eruption style from Hawaiian to Strombolian, and produced more than 10 million cubic meters of basaltic magma at temperatures of ~1200 degrees Celsius.

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Linking sedimentation in the northern Andes to basement configuration, Mesozoic extension, and Cenozoic shortening: Evidence from detrital zircon U-Pb ages, Eastern Cordillera, Colombia
Brian K. Horton et al., Dept. of Geological Sciences and Institute for Geophysics, Jackson School of Geosciences, University of Texas at Austin, Austin, Texas 78712, USA. Pages 1423-1442.

This study analyzes U-Pb isotopes in zircon mineral grains from 26 sandstone samples. These samples, from the northern Andes of Colombia, reveal sedimentation patterns during Paleozoic subsidence, Jurassic-Early Cretaceous extension, Late Cretaceous post-rift subsidence, and Cenozoic shortening and foreland-basin evolution. Additional U-Pb geochronological results for three basement samples indicate that the presumed Precambrian basement in the Eastern Cordillera is actually a product of early Paleozoic magmatism. Collectively, the new isotopic ages help address critical unknowns for tectonic reconstructions of the northern Andes, including (1) the age and nature of crystalline basement, (2) the role of possible Paleozoic orogenesis, (3) the timing and extent of Mesozoic rifting, and (4) the onset and tempo of Cenozoic shortening and surface uplift during the Andean orogeny.

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Oligocene-Miocene basin evolution in the northern Altiplano, Bolivia: Implications for evolution of the central Andean backthrust belt and high plateau
Bryan P. Murray et al., Dept. of Earth Science, University of California, Webb Hall, Santa Barbara, California 93106-9630, USA. Pages 1443-1462.

Murray and colleagues link deposition of upper Oligocene-lower Miocene basin fill in the central Andean backthrust belt of Bolivia to surface uplift of the Eastern Cordillera relative to the Altiplano. Nonmarine sedimentary lithofacies, provenance data, and growth stratal relationships indicate alluvial fan to braided fluvial deposition during coeval shortening and propagation of the backthrust belt toward the Penas-Aranjuez hinterland basin in the Altiplano. Such a progression of late Oligocene-early Miocene shortening along the Altiplano-Eastern Cordillera boundary likely reflects significant crustal thickening, potential isostatic uplift, and initial topographic expression of the eastern margin of the central Andean plateau.

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Rapid incremental assembly of the Monte Capanne pluton (Elba Island, Tuscany) by downward stacking of magma sheets
Federico Farina et al., Dipartimento di Scienze della Terra, Universita di Pisa, Via Santa Maria 53, 56126 Pisa, Italy. Pages 1463-1479.

Granite magmatism is one of the most important mechanisms contributing to the growth and geochemical differentiation of Earth’s crust. Many granitic intrusions are texturally and compositionally zoned and provide natural laboratories for the understanding the construction processes of continental crust. The Monte Capanne pluton (Elba Island, Italy) is characterized by the widespread occurrence of large crystals of K-feldspar (megacrysts), whose variation in size and abundance have been determined at 392 stations all over the pluton. Megacryst abundance variation allows the recognition of three zones (intrusive facies) characterized by low, intermediate, and high megacryst content. In this study a new geological map, based on K-feldspar megacryst distribution, reveals the composite structure of the pluton. Textural and compositional differences between facies suggest that they formed at depths as distinct magma pulses and rose through the crust to the emplacement level as three discrete injections (magma batches). The composite structure of the pluton reveals that it built up incrementally by downward stacking of three slightly different batches. Field evidence together with 3-D thermal modeling indicate that the three magma batches were emplaced in a short time sequence.

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High-precision U-Pb calibration of Carboniferous glaciation and climate history, Paganzo Group, NW Argentina
E.L. Gulbranson et al., Dept. of Geology, University of California, One Shields Avenue, Davis, California 95616, USA. Pages 1480-1498.

The ancient glacial history of a large continental landmass known as Gondwana has been considered sparse and fragmented. Nevertheless, icehouse climate stasis has long been inferred for the late Paleozoic (about 359 to 251 million-years-ago) from sedimentary records in the ancient tropics. Gulbranson et al. explore this history of glaciation and climatic change by high-precision U-Pb zircon calibration of existing and new field evidence for the presence or absence of ice and major climate transitions. Their results indicate at least three, rapid (1-8 million year), glacial events occurred on the southwestern margin of Gondwana that were separated by non-glacial intervals, thus indicating that the late Paleozoic ice age was more dynamic than previously considered. This study also documents substantial aridification of the study area as much as ten million years earlier than previously reported, coincident with the end of Carboniferous glaciation in this region.

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East-west extension in the NW Indian Himalaya
Esther Hintersberger et al., Institut fur Geowissenschaften, Universitat Potsdam, Karl-Liebknecht-Strasse 24, 14476 Potsdam, Germany. Pages 1499-1515.

The northward movement of the Indian tectonic plate toward Eurasian landmasses leads to shortening which creates the high mountains of the Himalaya and Tibet. However, there is growing evidence for east-west extension in the Himalaya and the Tibetan Plateau, perpendicular to the orientation of the main mountain chain in the central part of the Himalaya. Hintersberger and colleagues provide multiple field evidence and interpretation of high-resolution satellite imagery for ongoing east-west extension in the northwest Indian Himalaya. This is new and important information, because it is generally assumed that extension in the Himalaya and in the Tibetan Plateau is either limited to local graben or domal structures in the Central Himalaya or to the plateau region. However, investigations by Hintersberger et al. demonstrate that extensional tectonics is a pervasive, long-lasting phenomenon that has been active in the northwest Indian Himalaya for at least ~16 million years. Their observations are used as an independent data set to evaluate existing theories about extension in the Himalaya, since most of those models are based on data from the Central Himalaya. The conclusions are also relevant for the ongoing discussion about the role of the Karakorum fault as a decoupling structure between the Himalaya and the Tibetan Plateau.

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Suprasubduction-zone ophiolite generation, emplacement, and initiation of subduction: A perspective from geochemistry, metamorphism, geochronology, and regional geology
John Wakabayashi et al., Dept. of Earth and Environmental Sciences, California State University, Fresno, California 93740, USA. Pages 1548-1568.

This study presents geochemical data giving insight into the process of subduction initiation and formation of rock suites known as ophiolites. Ophiolites are on-land remnants of oceanic lithosphere, and most well-known ophiolites apparently formed above a subduction zone; this is called a suprasubduction-zone (SSZ) setting. Thin sheets of high-temperature metamorphic rocks, known as metamorphic soles, crop out structurally beneath many SSZ ophiolites. Such rocks may have formed during subduction initiation beneath young, hot, oceanic lithosphere. High-temperature metamorphic rocks from the Franciscan Complex of California may represent a metamorphic sole beneath the SSZ Coast Range ophiolite (CRO). Geochemical data indicate the parent rocks (protoliths) of the high-temperature rocks formed in an SSZ environment, requiring the existence of a pre-Franciscan subduction zone, whereas later-subducted, lower-temperature oceanic rocks apparently formed at mid-ocean ridges or seamounts. The CRO and Franciscan high-temperature rock protoliths apparently formed over a pre-Franciscan subduction zone that dipped westward, away from North America. After west-dipping subduction ceased, high-temperature Franciscan rocks were metamorphosed as east-dipping subduction began beneath the CRO, emplacing the ophiolite and eventually leading to its exposure. Other metamorphic soles apparently have SSZ protoliths, suggesting that this model may be globally applicable.

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Processes, rates, and time scales of fluvial response in an ancient postglacial landscape of the northwest Indian Himalaya
Daniel E.J. Hobley et al., School of GeoSciences, University of Edinburgh, Edinburgh, Scotland EH9 3JW, UK. Pages 1569-1584.

Rates of landscape change and sediment production are known to have increased during the Late Cenozoic (the last few million years). This change is also known to have happened at the same time as major climatic change on Earth, as glaciers grew for the first time over large areas of the northern hemisphere. However, how the presence of glaciers might affect the growth and evolution of mountain ranges is presently poorly understood, particularly in terms of how fast change will happen as the landscapes respond to the retreat of the ice, and how rivers deal with the large quantities of glacial debris left behind. This study discusses what happens when rivers reoccupy a mountain landscape that has relatively recently been abandoned by glaciers, looking in detail at the Ladakh area of the western Himalayas. They show that the rivers are beginning to reshape the landscape toward an unglaciated form and to move the loose material left by the glaciers downstream, as would be expected from what we already know, but that the style in which the river moves this material is not what would be expected based on preexisting work. In particular, they show that the rivers will take well over one million years to reshape the valleys and free all the sediment left by the glaciers, much slower than previous estimates might suggest, and many times longer than the expected time taken for glaciers to go through cycles of advance and retreat. This new information means that we can understand better the geological records of past ice ages, as well as the way modern high-mountain landscapes are likely to respond to current glacial melting.

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Origin of sackung uphill-facing scarps in the Saint Elias orogen, Alaska: LIDAR data visualization and stress modeling
Zhiyong Li et al., Faculty of Earth Sciences, China University of Geosciences, Wuhan, 430074, People’s Republic of China. Pages 1585-1599.

The coastal mountains of southern Alaska and the Yukon rise dramatically from the sea because of on-going collision between the North American continental plate and a small microplate. This region creates some of the world's greatest earthquakes and has the potential to create destructive tsunamis that may travel throughout the Pacific Ocean basin. Identifying the sources of earthquakes within the mountains is of interest both for basic research into the origin of mountainous landscapes as well as better documentation of natural hazards. This study investigates the origin of numerous young fault scarps that have previously been described as the results of active tectonics on the one hand, and gravitational collapse of mountain blocks on the other. In the first case, the scarps would reflect folding above large, earthquake-generating faults at depth, but in the latter case the scarps would be superficial features in the landscape. Scarps in one mountain block were investigated using high-resolution topographic data collected by airborne laser mapping, field observations, and computer modeling of stresses acting as the result of gravity and tectonic force fields. The results demonstrate that the scarps formed by outward rotation and toppling of sedimentary layering in the upper 100 meters of the mountain, and are therefore not directly related to active folding and buried earthquake faults. However, strong ground shaking caused by large earthquakes may enhance the toppling process which is primarily caused by gravitational stress in the steep-sided, previously glaciated mountains.

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Contrasting bedrock incision rates from snowmelt and flash floods in the Henry Mountains, Utah
Joel P.L. Johnson et al., The University of Texas at Austin, Dept. of Geological Sciences, 1 University Station C9000, Austin, Texas 78712, USA. Pages 1600-1615.

How fast can rivers erode into bedrock, how erosive are flash floods, and how do slot canyons form? A new study addresses these questions through environmental monitoring of floods and bedrock erosion along a canyon near the Henry Mountains in southeast Utah. They found that local erosion rates during an individual flood can be fast: Up to about 0.5 m of vertical incision along a short and steep channel reach was caused by 23 days of flow and sediment transport from upstream mountain snowmelt. This rate is much faster than the well-constrained landscape erosion rate in this region of ~0.4 mm/year, when averaged over millennial timescales. The rapid short-term erosion rate was caused by a prolonged but moderate discharge flow event which was interpreted to transport relatively low concentrations of coarse sediment. In contrast, a flash flood with a peak discharge nearly 10 times larger than the peak snowmelt flow caused negligible bedrock erosion but extensive alluvial deposition. The shape of the bedrock channel that eroded is very similar to larger slot canyons nearby along the Escalante River, suggesting similar mechanisms for their development.

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What happens when the ocean is overheated? The foraminiferal response across the Paleocene-Eocene Thermal Maximum at the Alamedilla section (Spain)
Laia Alegret et al., Departamento de Ciencias de la Tierra e Instituto Universitario de Investigacion de Ciencias Ambientales de Aragón, Universidad de Zaragoza, 50009 Zaragoza, Spain. Pages 1616-1624.

Global warming and a major perturbation of the carbon cycle occurred during the Paleocene-Eocene transition, about 55 million years ago, triggering acidification of the oceans and rapid changes in terrestrial and marine organisms. Alegret et al. investigate this period of Earth's history in the Alamedilla section (Spain), where changes in microfossil assemblages and in their test size have been related to the effects of carbon input and warming, as well as to interspecific competition, suggesting a high degree of complexity in the ocean structure.

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A late Miocene–early Pliocene chain of lakes fed by the Colorado River: Evidence from Sr, C, and O isotopes of the Bouse Formation and related units between Grand Canyon and the Gulf of California
Jennifer A. Roskowski et al., Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Pages 1625-1636.

The Colorado River has great importance for water and other resources in the U.S. Southwest. In recent years, it has become clear that the Colorado River has a near-unique origin among studied world rivers: It came into existence as a single through-flowing stream about five and a half million years ago, but the river propagated downward from the Colorado Plateau to the Gulf of California, into a preexisting landscape of desert basins. It formed large lakes in these basins, which became saline from evaporation in the warm climate, and in some cases supported organisms that live in salty water. The authors show that strontium, oxygen, and carbon isotopes in the sediments deposited in the lake basins can be used to document the geographical extent of a chain of large Colorado River-fed saline, freshwater lakes from Las Vegas south to the U.S.-Mexico border. They also use the isotopic studies to characterize the environments in the lakes, and demonstrate that no seawater was involved in the system. The lakes disappeared over the succeeding period of 0.1 to 1 million years as the river filled the basins with sediment and cut through rock barriers that originally separated the basins.

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The sedimentary architecture of outburst flood eskers: A comparison of ground-penetrating radar data from Bering Glacier, Alaska, and Skeioararjokull, Iceland
Matthew J. Burke et al., School of Applied Sciences, Northumbria University, Newcastle upon Tyne, NE1 8ST, UK. Pages 1637-1645.

Eskers are ridges of sand and gravel that record the infilling of ice-walled river channels and have been used to infer the dynamics and hydrology of former ice sheets. Hypotheses regarding their origin have focused on analysis of ancient eskers that understandably lack rigorous constraints on depositional timescale. As such, the conditions under which ancient eskers formed remains a matter of debate. Burke and colleagues used ground-penetrating radar to investigate the controls on the formation of eskers that were generated during relatively well constrained glacial outburst floods at Bering Glacier, Alaska and Skeioararjokull, Iceland. These eskers are currently the only known examples that can be unequivocally linked to rapid deposition during high-magnitude glacial outburst floods. The similar form and sedimentary architecture of the eskers suggests glacial outburst floods generate distinct depositional signatures in eskers. Identifying these signatures in ancient eskers will help assess the nature of meltwater drainage systems during retreat of the North American and European ice sheets following the Last Glacial Maximum.

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Whole-rock Pb and Sm-Nd isotopic constraints on the growth of southeastern Laurentia during Grenvillian orogenesis
Christopher M. Fisher et al., Dept. of Earth and Environmental Sciences, Vanderbilt University, Nashville, Tennessee 37235, USA. Pages 1646-1659.

The configuration of continents on Earth’s surface has changed continuously throughout its history, a result of processes geologists refer to as plate tectonics. Reconstructing the more recent history of these plate movements, including the formation of "supercontinents," relies on a combination of paleomagnetic and fossil correlation studies. However, these techniques are difficult, if not impossible, to apply to reconstructions of earlier parts of Earth’s history. In these situations geologists must rely on geochemical data to establish a set of "fingerprints" for different pieces of crust around the globe. These fingerprints allow geologists to identify pieces of crust that did not form in their current position and determine the likely "parent" continent of an out-of-place piece of crust. These continent-fragment connections can help in the reconstruction of plate movements over time. In this study, researchers applied such a fingerprinting approach (using Pb and Nd isotopes and U/Pb zircon ages) to ancient rocks within the Appalachian Mountains of southeastern North America to address a long-standing debate about the origin of these rocks. Their results suggest that a large fragment of another continent was added to North America about one billion years ago, stretching from Georgia to New York, and as far west as the Tennessee-North Carolina border. This piece of crust was likely a piece of South America, transferred to North America when it collided with the western edge of the ancient core of South America, during the formation of a supercontinent that geologists refer to as Rodinia.

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Extraordinary transport and mixing of sediment across Himalayan central Gondwana during the Cambrian-Ordovician
Paul M. Myrow et al., Dept. of Geology, Colorado College, Colorado Springs, Colorado 80903, USA. Pages 1660-1670.

Grains of the mineral zircon were separated from sandstone samples taken from Cambrian and Ordovician rocks from along the entire length of the Himalaya and dated for this study. Their ages represent a census of the sorts of the rocks exposed on Earth’s surface at the time. The specific ages indicate river transport of sediment for thousands of kilometers within the ancient supercontinent at this time, Gondwanaland. Possible sources for the sediment include mountain belts along the eastern part of Africa, parts of the present-day Arabian Peninsula, and both Antarctica and Australia. The spectra of ages for these samples are remarkably similar, which suggests that very large river systems, operating in the absence of land plants, mixed grains from many sources and deposited them over great areas of the continental margin.

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Quaternary reactivation of the Kern Canyon fault system, southern Sierra Nevada, California
Elisabeth S. Nadin and Jason B. Saleeby, Division of Geological and Planetary Sciences, California Institute of Technology, MS 100-23, Pasadena, California 91125, USA. Pages 1671-1685.

The Kern Canyon fault of the southern Sierra Nevada, California, has a storied history spanning 100 million years. Since the 1930s, this fault has been considered long dead. Recent investigations include groundwork and aerial surveys of the fault line, and reveal that the fault has ruptured within the past few thousand years. This discovery, paired with an abundance of (low-magnitude) earthquakes along the fault, means that the fault is active. There is a potential for future large earthquakes along the Kern Canyon fault, posing a flood threat to the city of Bakersfield, with a population of 800,000, which lies downstream of a dam that crosses the fault. The study also shows that faults can lie quiet and can reactivate under different stress conditions of the crust over a 100-million-year span of time.

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Maximum depositional age and provenance of the Uinta Mountain Group and Big Cottonwood Formation, northern Utah: Paleogeography of rifting western Laurentia
Carol M. Dehler et al., Dept. of Geology, Utah State University, Logan, Utah 84322, USA. Pages 1686-1699.

Precambrian sedimentary rocks record information about ancient landscapes and their tectonic settings (>542 million years ago). However, few of these rocks have been dated numerically, and therefore it is difficult to understand how these and other Precambrian sedimentary rocks related to one another in space and time. In this study two sandstone units located east of Salt Lake City, Utah, have recently been determined to be about 770-740 million years old and record the presence of a large transcontinental river system flowing westward to a west-facing ocean margin at this time. Knowing the age of these strata allows correlation with other strata of this age range in other parts of the world, and, together, they record the initial separation of a supercontinent know as Rodinia.

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Testing the limits of Paleozoic chronostratigraphic correlation via high-resolution (<500 k.y.) integrated conodont, graptolite, and carbon isotope (delta-13C) biochemostratigraphy across the Llandovery-Wenlock (Silurian) boundary: Is a unified Phanerozoic timescale achievable?
Bradley D. Cramer et al., Division of Earth History, School of Earth Sciences, The Ohio State University, 125 S. Oval Mall, Columbus, Ohio 43210, USA. Pages 1700-1716.

The ability to tell time in Earth history is a function of the time period under investigation. Whereas the relative order and duration of comparatively recent geologic events can be determined to a level of plus or minus a few days to months to years, investigating the deep past is more difficult. The order of events in older parts of Earth history typically can be deciphered only to within plus or minus a million years or worse. The work of Cramer et al. has begun to demonstrate that resolving the deep time record of Earth history to a level approaching more recent intervals is possible. This order-of-magnitude improvement in the ability to tell relative time (i.e., order of events) in the distant past was made possible through the integration of multiple lines of investigation and approaching the ancient geologic record in a manner similar to researchers of more recent intervals of time. As a result, their work has begun to demonstrate that it may be possible to produce a geologic time scale of equal resolution and fidelity for perhaps as much as the past 500 million years.

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Coupling volcanism and tectonics along divergent plate boundaries: Collapsed rifts from central Afar, Ethiopia
Valerio Acocella, Dipartimento Scienze Geologiche Universita Roma Tre Largo San Leonardo Murialdo, 1, Roma, 00146, Italy. Pages 1717-1728.

Divergent plate boundaries consist of rift zones characterized by active extension and volcanism. On Earth, these processes can be best appreciated along the Ethiopian Rift extending continental crust, the Central Afar extending transitional crust, and the Iceland extending oceanic crust. Here, volcanic activity usually has a limited impact on rift architecture and evolution, being associated with moderate eruptions from fissures or collapse calderas. However, Central Afar is characterized by the emplacement of a km-thick volcanic sequence (the "Stratoids"), the largest ever recognized along divergent plate boundaries so far, responsible for the emission of ~7000 km cubes of magma in ~1.5 million years. This study considers the effect of such an eruption on the rift in Central Afar. The rift sides show distinctive features, as steep topographic gradients coinciding with inward tilted crustal blocks, resulting from the collapse induced by magma withdrawal during the emplacement of the Stratoids. This portion of Afar shows how the entire rift architecture is shaped by voluminous fissure eruptions. The result is a collapsed rift that is a portion of the rift undergoing collapse, similarly to calderas. A collapsed rift represents an end-member type of volcano-tectonic activity, where the width of the erupting reservoir balances that of the active rift zone.

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