|2 February 2011
GSA Release No. 11-09
Director - GSA Communications & Marketing
GSA BULLETIN HIGHLIGHTS: NEW RESEARCH POSTED 21–28 JANUARY 2011
Boulder, CO, USA – GSA BULLETIN is now regularly posting pre-issue publication content — finalized papers that have not been assigned to an issue but are not under embargo. GSA invites you to sign up for e-alerts and/or RSS feeds to have access to new journal content the minute it is posted online. Go to http://www.gsapubs.org/cgi/alerts and enter your e-mail address to manage your subscriptions.
The following highlights are for articles posted 21-28 January 2011. View abstracts for these early GSA BULLETIN articles at http://gsabulletin.gsapubs.org/content/early/recent before they are assigned to an issue. You may also go to http://gsabulletin.gsapubs.org/, click on advanced search, and enter the digital object identifier (DOI) number listed after each paper.
Representatives of the media may obtain complementary 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. BULLETIN abstracts are available at http://geology.gsapubs.org/.
Non-media requests for articles may be directed to GSA Sales and Service, .
Topics for GSA Bulletin papers posted on 28 Jan. 2011 include findings suggesting that the Altyn Tagh fault of northwest Tibet slips at a uniform rate of 8-12 mm/yr; an examination of subfossil insects preserved in the lacustrine sediments of the Canadian Arctic; scale modeling of the Hebes Chasma, Mars; and a history of the advances in geoscience revealed in the Portuguese and Spanish explorations of the 1500s.
Topics for GSA Bulletin papers published online 26 Jan. include an assessment of the specific eruptive behavior of Cotopaxi, Ecuador; a report of new geochronologic and geochemical data for the Mesozoic McCoy Mountains Formation, western Arizona; evidence that puts to rest arguments against a glacial origin for the unusual rocks of the Ghuab Formation, Namibia; a new study on the California River and its role in carving the Grand Canyon; information on the Vars-Winchester esker, a mud-buried aquifer and aggregate source in the Champlain Sea basin near Ottawa, Canada; and a new geologic map of eastern and central Bhutan.
Topics for GSA Bulletin papers published online 21 Jan. include a proposal of the Alano section, Italy, as a potential candidate for defining the GSSP of the late Eocene Priabonian Stage; mineralization in the Thelon Basin, Canada; the Pleistocene sedimentary record of the Hawke Bay forearc; host-rock roofs and magma chambers in the Mooselookmeguntic igneous complex; Eocene basin fill in the Wind River Basin; the potential for landsliding in the Columbia River Basin; the Nile and the Negev; and the creeping segment of the San Andreas fault, California, USA.
Keywords: Tibet, Altyn Tagh fault, Canadian Arctic, chironomids, Valles Marineris, Mars, Hebes Chasma, Copernican Revolution, Portuguese and Spanish explorations, Plate Tectonic Revolution, Taupo Rift, New Zealand, Lesser Himalaya, Bhutan, zircon ages, Paleozoic, climate, Cotopaxi volcano, Ecuador, McCoy Mountains Formation, Colorado Plateau, Kailas Formation, Ghuab Formation, Namibia, California River, Grand Canyon, eskers, aquifer, Champlain Sea basin, Marguerite Bay, Antarctic Peninsula ice, Last Glaciation, Global Boundary Stratotype Section and Point, GSSP, Priabonian, Thelon Basin, Hawke Bay, New Zealand, Mooselookmeguntic igneous complex, Appalachians, Eocene, Wind River Basin, Columbia River Basin, Oregon, Nile, Negev, Onverwacht Group, Barberton greenstone belt, South Africa, Sierra Nevada, San Andreas fault.
Faulted terrace risers place new constraints on the late Quaternary slip rate for the central Altyn Tagh fault, northwest Tibet
Ryan D. Gold et al., Dept. of Geology, University of California, Davis, California 95616, USA. Published online 28 Jan. 2011; doi: 10.1130/B30207.1.
Resolving the extent to which fault slip rates vary in space and time along major fault systems is a long-standing problem. Ryan Gold of the University of California at Davis and colleagues evaluate the steadiness of fault slip rate over the past 25 thousand years along the active, left-lateral Altyn Tagh fault, which defines the northwestern margin of the Tibetan Plateau in western China. Of the major Tibetan strike-slip faults, the left-lateral Altyn Tagh fault is among the most important, because of its length (less than 1200 km), longevity (49 million years ago), and total displacement (about 475 km). Gold and colleagues determined the age and magnitude of fault offset recorded by seven left-laterally faulted terrace risers at three sites spanning a 140-km-long fault reach by integrating surficial geologic mapping, topographic surveys, and geochronology. These results more than double constraints on the late Quaternary slip history along this first-order strike slip fault. The integration of their results with previous measurements suggests that the Altyn Tagh fault slips at a spatially and temporally uniform slip rate of 8-12 mm/yr. However, additional analysis is required to test how much temporal variability in slip rate is permitted by this dataset.
Chironomids record terrestrial temperature changes throughout Arctic interglacials of the past 200,000 yr
Yarrow Axford et al., Dept. of Earth and Planetary Sciences, Northwestern University, 1850 Campus Drive, Evanston, Illinois 60208, USA. Published online 28 Jan. 2011; doi: 10.1130/B30329.1.
Quaternary interglacial periods provide glimpses of a warmer Arctic and useful perspectives on possible future conditions, but records of Arctic terrestrial conditions over multiple interglacial periods are rare. Here, Yarrow Axford of Northwestern University and colleagues take advantage of a site in the Canadian Arctic where lacustrine sediments representing the past three interglacial periods are preserved in an extant lake. Axford et al. use subfossil insects (chironomids) preserved in this exceptional sedimentary archive to derive temperature reconstructions through the Holocene up to A.D. 2005, through the Last Interglacial and a portion of the penultimate interglacial. Data presented here represent the longest paleotemperature record thus far generated using chironomids. The existence of close modern analogs for ancient chironomid assemblages at Lake CF8 suggests that this method can provide useful paleotemperature estimates extending back hundreds of millennia.
Modeling the collapse of Hebes Chasma, Valles Marineris, Mars
M.P.A. Jackson et al., Bureau of Economic Geology, Jackson School of Geosciences, University of Texas, Austin, Texas 78713, USA. Published online 28 Jan. 2011; doi: 10.1130/B30307.1.
The Valles Marineris region of equatorial Mars is a zone of linear, parallel canyons more than two thousand kilometers long. The origin of these canyons (chasmata) has long been one of the most intriguing topics of martian geology. One of the smaller canyons, Hebes Chasma, is a canoe-shaped, closed depression 315-km long and 8-km deep, roughly five times the size of Arizona's Grand Canyon. Because it has no surface outlets, a central puzzle is how 100,000 cubic kilometers of missing material disappeared and where it went. M.P.A. Jackson of the University of Texas at Austin and colleagues present, for the first time, scale modeling and detailed mapping of Hebes Chasma to test the idea that the canyon collapsed like a giant sinkhole as material drained away below. By reproducing the canyon's main landforms, they found that the geologic evidence explained the origin of Hebes Mensa, a loaf-shaped mountain rising nearly 8 km from the canyon floor: As the surrounding material drained away, part of the canyon floor arched upward to form the mountain and raised deep layers almost level with the canyon rim. The materials removed by draining downward probably consisted of a mixture of salts, water ice, liquid water, and insoluble particles. The water that drained from the subsiding canyon burst out as floods, which scoured out the neighboring valleys -- Echus Chasma and Kasei Valles. If Hebes Chasma did form by collapse and subsurface drainage, this supports the idea that the upper few kilometers of at least part of the Highland crust of Mars grew by accretion of debris from volcanic eruptions and meteoritic impact and trapped significant amounts of water and ice.
The Portuguese and Spanish voyages of discovery and the early history of geology
Henrique Leitão and Walter Alvarez, Centro de História das Ciências, Faculdade de Ciências da Universidade de Lisboa, Edifício C4-Piso 3, 1749-016 Lisbon, Portugal. Published online 28 Jan. 2011; doi: 10.1130/B30368.1.
Geology is usually thought of as a young science, dating to around 1800, much younger than astronomy and physics, which trace their origins to Copernicus, about 1550. Perhaps this lack of an early pedigree explains why, when the sciences are enumerated, geology is often remembered only as an afterthought. Leitão and Alvarez argue that geology is as old a science as astronomy and physics, but that geologists have forgotten their own history. First, Leitão and Alvarez maintain that the Copernican Revolution was centered on geology as well as astronomy and physics, because Copernicus showed that Earth is a planet. Then they argue that the history of geology goes back even farther, to the Portuguese and Spanish explorations of the 15th century, which should be considered part of the history of geology because the Portuguese and Spanish mapped the continental coastlines, the patterns of the winds, and the declination of the Earth's magnetic field. The Portuguese and Spanish explorations are surprisingly analogous to the Plate Tectonic Revolution of the 1960s and 1970s -- the most recent great advance in understanding Earth. Geology is thus a venerable science, which lends weight to its understanding of Earth -- an understanding critical for the future of humanity.
Associations between volcanic eruptions from Okataina volcanic center and surface rupture of nearby active faults, Taupo rift, New Zealand: Insights into the nature of volcano-tectonic interactions
P. Villamor et al., GNS Science, Lower Hutt 5040, New Zealand. Published online 28 Jan. 2011; doi: 10.1130/B30184.
P. Villamor of GNS Science, New Zealand, and colleagues assess the possibility of prehistoric large volcanic eruptions causing faults to rupture (that is, triggering large earthquakes) or large earthquakes triggering volcanic eruptions. In the northern sector of the Taupo Rift, New Zealand, large explosive volcanoes are located in close proximity to active faults. Villamor et al. have analyzed numerous exposures of fault planes displacing layers of volcanic ash to look for evidence of temporal coincidence between deposition of volcanic ash on a fault scarp and rupture of the fault. Villamor and colleagues found that for most cases (70%) there is no temporal coincidence. For 30% of the cases, however, they found that fault rupture occurred at the same time (or immediately prior or immediately after) deposition of volcanic ash. Their results indicate that earthquakes and volcanic eruptions in the Taupo Rift are independent phenomena but on occasion can trigger each other. This can occur because the forces produced by the movement of magma under the volcano, or by the release of energy during an earthquake, are transferred through Earth's crust within a relatively short distance. These forces can promote a volcanic eruption or a large earthquake that is already close to occurring.
Tectonostratigraphy of the Lesser Himalaya of Bhutan: Implications for the along-strike stratigraphic continuity of the northern Indian margin
Sean Long et al., Dept. of Geosciences, Princeton University, Princeton, New Jersey 08544, USA. Published online 28 Jan. 2011; doi: 10.1130/B30202.1.
Convergence between India and Asia has produced the Himalaya, which is one of Earth's most impressive orogenic features. Since the rocks that make up the Himalaya are deformed and translated along their entire east-west length, many questions remain regarding their original stratigraphic order and deposition timing. This problem is further compounded because geologic relationships observed in more thoroughly studied areas are extrapolated to less-studied areas, resulting in confusion in Himalayan literature regarding both structural and stratigraphic divisions. The country of Bhutan, which lies in the eastern quarter of the orogen, is one of these less-studied areas. Sean Long of Princeton University and colleagues build a stratigraphy for the frontal, Lesser Himalayan portion of the fold-thrust-belt in eastern Bhutan through a combination of geologic mapping, descriptions of field relationships, an extensive U-Pb detrital zircon data set, and delta-13C data. These data provide depositional age constraints, and allow for provenance interpretation and correlation along strike. These age constraints facilitate evaluating hypotheses proposed for the stratigraphic continuity of the northern Indian margin, and allow interpretation of eastern Himalayan tectonostratigraphy in the context of margin-wide depositional events.
Episodic zircon ages, Hf isotopic composition, and the preservation rate of continental crust
Kent C. Condie et al., Dept. of Earth and Environmental Science, New Mexico Tech, Socorro, New Mexico 87801, USA. Published online 26 Jan. 2011; doi: 10.1130/B30344.1.
U/Pb detrital zircon ages from global modern river sediments define eight peak clusters centered at 2700, 2500, 2010, 1840, 1600, 1150, 600, and 300 million years in age. Kent C. Condie of New Mexico Tech and colleagues suggest that these clusters, which correspond with times of supercontinent formation, are peaks in crustal preservation related to supercontinent formation. Although some new continental growth may occur during continental collisions, supercontinent assembly does not require an increase in the production rate of continental crust. Rather, Condie and colleagues suggest that the preservation rate increases by an increased probability of capture of both new and reworked continental crust in collisional orogens.
Modeling the early Paleozoic long-term climatic trend
Elise Nardin et al., Laboratoire des Mécanismes et Transfert en Géologie, CNRS-UPS-IRD, Observatoire Midi-Pyrénées, Toulouse 31400, France. Published online 26 Jan. 2011; doi: 10.1130/B30364.1.
Between 540 and 400 million years ago (early Paleozoic), the climate was supposed to be relatively warm and equable. However, recent isotopic studies have revealed climatic instabilities. These major climatic changes may have provoked the initial biodiversification of modern marine life. Elise Nardin of Observatoire Midi-Pyrenees, France, and colleagues focus on the reconstruction of the early Paleozoic climate to quantify the climatic trends and explore its possible mechanisms. Their simulations with a coupled climate-biogeochemistry model have calculated low continental air temperature (~13.5 degrees Celsius) and low atmospheric CO2 pressure (14 to 10 Present Atmospheric Level). These results are in agreement with the sedimentologic record and isotopic reconstructions showing a long-term cooling trend between 480 and 450 million years ago, followed by a short but abrupt glacial period (445 to 430 million years ago). This long-term cooling trend may be mainly due to the drift of the continents across the tropical zone, which enhances runoff and promotes CO2 consumption via silicate rock weathering. The glacial interval may have been initiated or at least accelerated by a strong increase of the weathering of fresh volcanic rocks, which consumes about eight times more CO2 than the weathering of shield rocks.
Physical volcanology of the post–twelfth-century activity at Cotopaxi volcano, Ecuador: Behavior of an andesitic central volcano
Marco Pistolesi et al., Earth Science Dept., University of Pisa, Santa Maria, 53, Pisa, Pisa 56126, Italy. Published online 26 Jan. 2011; doi: 10.1130/B30301.1.
Volcanic hazard assessment based on tephro-chronological studies has been addressed at Cotopaxi volcano (Ecuador) by several papers. However, a detailed documentation of the physical parameters of eruptions of the very recent period is still lacking. This reconstruction by Marco Pistolesi of the University of Pisa and colleagues has allowed the authors to identify 21 main tephra beds that were related to documented volcanic events that occurred since the Spanish conquest, through cross-checking with the available chronicles and historical documents. The chronological framework was strengthened by new 14C dates that integrate previous data and substantially upgrade the information made available by previous studies. Data are used to review the model for the eruptive behavior of the past 2000 years suggesting that variations in eruption frequency, intensity, magnitude, and composition recorded in the Cotopaxi stratigraphy may reflect modulation by magma supply rate and the complex interplay of deep versus shallow magmatic processes. Pistolesi et al. thus provide new constraints on the specific eruptive history of Cotopaxi and they discuss these results with respect to the general knowledge of the behavior of andesitic volcanoes.
Age and tectonic setting of the Mesozoic McCoy Mountains Formation in western Arizona, USA
Jon E. Spencer et al., Arizona Geological Survey, 416 W. Congress Street, #100, Tucson, Arizona 85701, USA. Published online 26 Jan. 2011; doi: 10.1130/B30206.1.
The Upper Jurassic to Upper Cretaceous McCoy Mountains Formation is a several-kilometer-thick succession of siltstone, sandstone, conglomerate, and minor basalt exposed in an east-west-trending belt in southwestern Arizona and southeastern California. New geochronologic and geochemical data from Jon E. Spencer of the Arizona Geological Survey and colleagues indicate that the basaltic rocks in the lower McCoy Mountains Formation are about 154 million years old and were produced in a tectonic environment of rifting. Associated sands include a large component derived from unidentified Triassic igneous rocks, and a variably significant component of quartz-rich sands recycled from the Lower to Middle Jurassic, dominantly eolian sandstone units that are well known on the Colorado Plateau. Rifting is inferred to represent a northwestward extension of the Chihuahua-Bisbee rift belt in northeastern Mexico, southwestern New Mexico, and southeastern Arizona. The overlying middle McCoy Mountains Formation was possibly deposited during post-rifting, thermotectonic subsidence of the rift belt. The Upper Cretaceous, upper McCoy Mountains Formation contain arkosic sands that were partly derived from the developing Maria fold-and-thrust belt exposed along the northern flank of the McCoy basin.
Oligocene–Miocene Kailas basin, southwestern Tibet: Record of postcollisional upper-plate extension in the Indus-Yarlung suture zone
P.G. DeCelles et al., Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Published online 26 Jan. 2011; doi: 10.1130/B30258.1.
The Kailas basin developed during late Oligocene-early Miocene time along the Indus-Yarlung suture zone in southwestern Tibet. Correlative coarse-grained syntectonic strata similar to the Kailas Formation crop out along a >1300 km length of the Indus-Yarlung suture zone, suggesting that the basin-forming mechanism recorded by the Kailas Formation was of regional significance and not exclusively related to local kinematics near the southeastern end of the Karakoram fault. P.G. DeCelles of the University of Arizona and colleagues propose that extension of the southern edge of the Eurasian plate was caused by southward rollback of underthrusting Indian continental lithosphere, followed by slab break-off. Alternating episodes of hard and soft collision, associated with regional contraction and extension, respectively, in the Tibetan-Himalayan orogenic system may have been related to changing dynamics of the subducting/underthrusting Indian plate.
An ice grounding-line wedge from the Ghaub glaciation (635 Ma) on the distal foreslope of the Otavi carbonate platform, Namibia, and its bearing on the Snowball Earth hypothesis
Eugene W. Domack, Dept. of Geosciences, Hamilton College, 198 College Hill Road, Clinton, New York 13323, USA; and Paul F. Hoffman. First published online 26 Jan. 2011; doi: 10.1130/B30217.1.
The Ghuab Formation in north-central Namibia was deposited some 635 million years ago and represents ancient sedimentation under the direct action of a grounded ice sheet and ice shelf. Eugene W. Domack of Hamilton College and Paul F. Hoffman of Harvard and the University of Victoria document the unusual setting of a glacial deposit (the Ghuab Formation) that interrupts warm-water carbonate deposition. Given the low relief and latitudinal position of the platform, across which the ice mass accreted and flowed, the shift to and out of glacial conditions represents a dramatic reorganization and oscillation of Earth's climate system before the widespread emergence of multicellular life. The evidence presented by Domack and Hoffman puts to rest arguments against a glacial origin for the unusual rocks of the Ghuab Formation and instead places them in a deep slope position where they could only have accreted when global sea levels were dramatically lower. Their presence therefore also points to a global buildup of ice masses on most, if not all, of the continents at the time allowing for incursion of the ice mass deep onto the slope surrounding the ancient continental platform.
The California River and its role in carving Grand Canyon
Brian Wernicke, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125, USA. First published online 26 Jan. 2011; doi: 10.1130/B30274.1.
Anyone who stands at the rim of Grand Canyon is confronted with one of the most humbling spectacles in the solar system - a high, featureless plateau interrupted by a mile-deep chasm. Its origin has been controversial ever since John Wesley Powell's historic navigation of the Colorado River in 1869. A long-held consensus is that the canyon is 6 million years old and was carved by the Colorado River. Brian Wernicke of the California Institute of Technology critically examines new data collected over the last three years suggesting instead that the canyon was incised between 70 and 80 million years ago by a river called the California River. The California River flowed from southwest to northeast, opposite the direction of the modern Colorado, from mountainous highlands in California down to a coastal plain in what is now the interior of the high-standing Colorado Plateau.
Sequence stratigraphy of a glaciated basin fill, with a focus on esker sedimentation
Don I. Cummings et al., Geological Survey of Canada, Ottawa, Ontario K1A 2A9, Canada. First published online 26 Jan. 2011; doi: 10.1130/B30272.1.
Eskers are sand and gravel ridges common to glaciated terrain. In addition to being aggregate sources and, commonly, groundwater aquifers, they contain clues as to how the last great ice sheets decayed, and how meltwater from the ice sheets sorted, partitioned, and redistributed sediment. According to Don I. Cummings of the Geological Survey of Canada and colleagues, signatures in the Vars-Winchester esker, a mud-buried aquifer and aggregate source in the Champlain Sea basin near Ottawa, Canada, suggest that the esker formed during multiple higher-magnitude floods as the ice front retreated across the Champlain Sea basin. The conduits in which the esker formed were large - similar in diameter to the esker itself - and ran along the base of the glacier. The esker consists of sand and gravel, but the dominant component transported through the conduits was mud, which bypassed the conduits and deposited in the Champlain Sea as muddy rhythmites. Ice-front retreat superimposed the impermeable muddy rhythmites over the sand and gravel of the esker aquifer. Esker aquifers formed as such may have a tendency to self-seal.
Geometry and crustal shortening of the Himalayan fold-thrust belt, eastern and central Bhutan
Sean Long et al., Dept. of Geosciences, Princeton University, Princeton, New Jersey 08544, USA. Published online 26 Jan. 2011; doi: 10.1130/B30203.1.
Sean Long of Princeton University and colleagues present a new geologic map of eastern and central Bhutan and four balanced cross sections through the Himalayan fold-thrust belt. Major structural features, from south to north, include a single thrust sheet of Subhimalayan rocks above the Main Frontal thrust; the upper Lesser Himalayan duplex system; the lower Lesser Himalayan duplex system; the structurally lower Greater Himalayan section above the Main Central thrust (MCT) with overlying Tethyan Himalayan rock in stratigraphic contact in central Bhutan and structural contact above the South Tibetan detachment in eastern Bhutan; and the structurally higher Greater Himalayan section above the Kakhtang thrust.
Ice-stream retreat and ice-shelf history in Marguerite Trough, Antarctic Peninsula: Sedimentological and foraminiferal signatures
Aoibheann A. Kilfeather et al., Dept. of Geography, Durham University, Science Site, South Road, Durham, DH1 3LE, UK. Published online 21 Jan. 2011; doi: 10.1130/B30282.1.
Sea-level rise 14,000 years ago caused the rapid floatation of the Antarctic Peninsula ice-sheet margin in Marguerite Bay on the west side of the peninsula. The sea-level rise is associated with the melting of ice sheets in North America and Europe at the end of the Last Glaciation, indicating that northern hemisphere deglaciation had a direct effect on the Antarctic Peninsula ice sheet. The decoupled ice sheet formed a floating "ice shelf." This ice shelf persisted for about 1,000 years, and would have acted as a buffer to fast-flowing ice on land. However, changing ocean currents around the peninsula brought warm waters onto the continental shelf, causing the ice shelf to melt and break up and the ice sheet to retreat further toward land. These findings by Aoibheann A. Kilfeather of Durham University and colleagues are important in understanding oceanographic controls on ice sheet retreat, especially in the context of recent observations of ice-shelf collapse and accelerated glacier flow on the east side of the Antarctic Peninsula.
Integrated biomagnetostratigraphy of the Alano section (NE Italy): A proposal for defining the middle-late Eocene boundary
Claudia Agnini et al., Dipartimento di Geoscienze, Universita di Padova, Via G. Gradenigo, I-35131 Padova, Italy. Published online 21 Jan. 2011; doi: 10.1130/B30158.1.
Claudia Agnini of Universita di Padova and colleagues focus on a middle-upper Eocene marine sedimentary succession outcropping near the village of Alano di Piave in the Venetian Southern Alps (northeast Italy), which is the type area of the Priabonian Stage. The section is easily accessible, crops out continuously, is unaffected by any structural deformation, is rich in calcareous plankton, and contains an expanded record of the critical interval for defining the Global Boundary Stratotype Section and Point (GSSP) of the Priabonian. Recently, Agnini et al. have proposed the Alano section as a potential candidate for defining the GSSP of the late Eocene Priabonian Stage. The development of a standard Global Chronostratigraphic Scale, based on rigorous agreement upon stratigraphic principles, terminology, and classificatory procedure is a long standing objective of the International Commission on Stratigraphy. The proposal of the Alano section as the GSSP for the Priabonian Stage does represent an improvement in the establishment of "common language," adding another piece of the puzzle to the Chronostratigraphic Time Scale.
Fluorapatite in the Paleoproterozoic Thelon Basin: Structural-stratigraphic context, in situ ion microprobe U-Pb ages, and fluid-flow history
W.J. Davis et al., Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada. Published online 21 Jan. 2011; doi: 10.1130/B30163.1.
Large sedimentary basins within the interior of continents host significant mineral deposits including uranium. Economic mineralization results from precipitation of ore minerals from fluids that flowed within the basins. W.J. Davis of the Geological Survey of Canada and colleagues describe a method to determine when these fluids were active in the past and how this relates to mineralization events, with a specific application to the 1.8-billion-year-old Thelon Basin in northern Canada.
Controls on active forearc basin stratigraphy and sediment fluxes: The Pleistocene of Hawke Bay, New Zealand
Fabien Paquet et al., UMR Geosciences, Centre National de la Recherche Scientifique, Universite de Rennes 1, Campus de Beaulieu, 35042 Rennes cedex, France. Published online 21 Jan. 2011; doi: 10.1130/B30243.1.
Fabien Paquet of France's Centre National de la Recherche Scientifique and colleagues investigate the Pleistocene sedimentary record of the tectonically active Hawke Bay forearc domain of the Hikurangi subduction margin of New Zealand. Interpretation of an extensive seismic reflection dataset that is correlated with marine cores and onshore geological maps allow Paquet et al. to identify the detailed stratigraphic architecture of the past 1.1 million years.
Coupled deformation and metamorphism in the roof of a tabular midcrustal igneous complex
S.E. Johnson et al., Dept. of Earth Sciences, University of Maine, Orono, Maine 04469-5790, USA. Published online 21 Jan. 2011; doi: 10.1130/B30269.1.
Host-rock roofs above mid-crustal intrusions preserve evidence for the mechanisms that facilitate vertical ascent/growth of magma chambers. Such roofs are poorly represented in the geological record, and they commonly show little evidence for vertical chamber growth being accommodated by grain-scale creep mechanisms and associated foliation development. The Mooselookmeguntic igneous complex in the Maine Appalachians is a gently dipping, tabular-shaped body with an extensively exposed host-rock roof. This roof preserves a thick, 600-meter emplacement-related strain gradient with a deformational fabric that evolves through all stages of crenulation cleavage to become an intense, non-differentiated foliation approximately parallel to the intrusive contact. Thus, vertical growth of the tabular intrusion was largely accommodated by dissolution-precipitation creep, and S.E. Johnson of the University of Maine and colleagues provide useful insight into mechanisms of chamber growth and the strain-dependent rheological evolution of crustal rocks.
Sedimentology, detrital zircon geochronology, and stable isotope geochemistry of the lower Eocene strata in the Wind River Basin, central Wyoming
Majie Fan et al., Dept. of Geosciences, University of Arizona, Tucson, Arizona 85721, USA. Published online 21 Jan. 2011; doi: 10.1130/B30235.1.
The shallow subduction of the Farallon plate beneath the western United States during Late Cretaceous through Eocene time has been commonly accepted as the tectonic cause for the Precambrian basement-cored uplifts in Wyoming, but mechanisms responsible for developing the individual structures in the upper crust remain unclear. Critical information about the timing of uplifts, their paleoelevations at the time of uplift, and the temporal relationships have yet to be documented at regional scale. Majie Fan of the University of Arizona and colleagues present a multidisciplinary study of the early Eocene basin fill in the Wind River Basin, with the purpose of improving understanding to the timing of uplifts, process of basin evolution, and changes in paleoelevation and paleoclimate. The depositional environments changed from alluvial fans to braided rivers in early Eocene time, associated with the formation of a confined valley in the northwestern corner of the basin by exhuming the surrounding ranges. The paleoclimate was humid, and the atmosphere pCO2 decreased in the early Eocene. The paleoelevation of the basin floor was about 500 m above sea level, and the local relief between the surrounding ranges and basin floor was 2.3 plus or minus 0.8 km. Up to 1 km of post-early Eocene regional net uplift is required to form the present landscape in central Wyoming.
Controls on large landslide distribution and implications for the geomorphic evolution of the southern interior Columbia River basin
Elizabeth B. Safran et al., Environmental Studies Program, Lewis & Clark College, Portland, Oregon 97219, USA. Published online 21 Jan. 2011; doi: 10.1130/B30061.1.
Large landslides (larger than 0.1 square kilometers in surface area) are important agents of landscape change. While most common in rugged mountain ranges, large landslides can also be widespread in areas with only a few hundred of meters of topography. The distribution of large landslides in such places has been relatively little studied, and this forms a gap in the understanding of how landslides affect landscape evolution in different geologic settings. To address this gap, Elizabeth B. Safran of Lewis & Clark College and colleagues documented <400 landslides between 0.1 and ~40 square kilometers in size across ~140,000 square kilometers of eastern Oregon, in the semi-arid, southern interior Columbia River basin. The mapped landslides form a band 50-100 km wide, oriented northwest-southeast. Landslides mostly occur where even a small amount of topography (~100 m) coincides with contacts between strong rocks on top and weak rocks below. Landslide distribution is not controlled by the local degree of faulting, while about 10% of mapped landslides may be associated with rock folds. Thick piles of strong rock limit landsliding to the northeast, while flat topography limits landsliding to the southwest. Safran et al.'s results suggest that future landslides will occur where sediment accumulation followed by lava flows created the layering conductive to landsliding. In such areas, episodic landsliding may last for hundreds of thousands of years or more, producing slopes of about 7-13 degrees and delivering a range of landslide types to the region’s rivers.
The role of the Nile in initiating a massive dust influx to the Negev late in the middle Pleistocene
Rivka Amit et al., Geological Survey of Israel, 30 Malkhe Israel Street, Jerusalem 95501, Israel. Published online 21 Jan. 2011; doi: 10.1130/B30241.1.
The late middle Pleistocene initiation of loess accretion in the Negev Desert, Israel, resulted from a chain of events starting with the exposure of shallow offshore Nile delta sands beginning about 180,000 years ago, during a period of glacially lowered eustatic sea level. This exposure allowed the formation of the dunes of the Sinai-Negev erg and the eastward movement of the dunes under the gusty glacial-time winds in northern Sinai. This eastward dune advance occurred by eolian saltation that generated the coarse silts that accumulated downwind as loess. Rivka Amit of the Geological Survey of Israel and colleagues postulate that the absence of earlier Negev loess is not a result of interglacial erosion as previously proposed, but instead is due to the relatively recent convergence of the above conditions necessary for loess formation.
Fragmentation and dispersal of komatiitic pyroclasts in the 3.5-3.2 Ga Onverwacht Group, Barberton greenstone belt, South Africa
M. Thompson Stiegler et al., Dept. of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA. Published online 21 Jan. 2011; doi: 10.1130/B30191.1.
M. Thompson Stiegler of Stanford University and colleagues analyze the 3.5-3.2-billion-year-old Onverwacht Group of the Barberton greenstone belt, South Africa, which contains thick sequences of komatiitic tuff, lapilli tuff, and lapillistone representing proximal to distal volcanic settings. Volcaniclastic komatiites in the southern part of the belt include: (1) areally extensive sheets of silicified, massive to normally graded ash and accretionary lapilli interpreted as pyroclastic fall deposits, and (2) thick, widespread, carbonatized beds of lapilli and minor ash deposited by subaqueous sediment flow and pyroclastic fallout.
Chronology of late Cenozoic volcanic eruptions onto relict surfaces in the south-central Sierra Nevada, California
Fred M. Phillips et al., Dept. of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA. Published online 21 Jan. 2011; doi: 10.1130/B30000.1.
Early twentieth-century geologists observed extensive areas of subdued topography in the otherwise strongly dissected landscape of the southern Sierra Nevada that they ascribed to periods of stability following episodes of orogenic uplift. From this evidence, they developed an uplift history for the Sierra Nevada. This conceptual model for landscape evolution of the Sierra has largely been rejected or ignored since the middle of the twentieth century, based in considerable part on observations indicating that these surfaces can be explained as dynamic landforms actively developing in response to conditions of the relatively recent geological past and that long-term erosion rates are much too high to permit pre-Quaternary surfaces to be preserved. Fred M. Phillips of the New Mexico Institute of Mining and Technology investigated the chronology of basaltic flows and ejecta on several such apparent relict surfaces located west of Bishop, California, USA.
Geologic versus geodetic deformation adjacent to the San Andreas fault, central California
Sarah J. Titus et al., Dept. of Geology, Carleton College, 1 N. College St., Northfield, Minnesota 55057, USA. Published online 21 Jan. 2011; doi: 10.1130/B30150.1.
The San Andreas fault system marks the boundary between two tectonic plates - the large, primarily oceanic Pacific plate and the small, continental Sierra Nevada microplate. This study by Sarah J. Titus of Carleton College and colleagues is focused on one section of the San Andreas fault in central California, known as the creeping segment, where the two plates move past each other at a steady rate without generating large earthquakes. This slip rate, however, does not match the full plate motion rate between the Pacific plate and Sierra Nevada microplate. Therefore, Titus et al. document how and where additional deformation occurs in central California to meet the full rate. They address this problem from two points of view. The first - a geologic point of view - examines the long-term record of deformation recorded by geologic structures, such as faults and folds, that have developed in the rocks next to the San Andreas fault. The second - a geodetic point of view - examines the short-term record of deformation using GPS to measure how stations on either side of the fault have moved over the past 10-15 years. When the observations from each view point are integrated, we can better understand the style and magnitude of off-fault deformation in central California, which ultimately has implications for earthquake hazard prediction.