January-February 2010 GSA Bulletin highlights

Boulder, CO, USA - The January-February 2010 GSA BULLETIN is online now "ahead of print." Topics cover the globe, with studies in Peru, Ecuador, China, Chile, South Africa, Spain, Tenerife, Dominican Republic, the Appalachian basin, the New England Avalon zone, the Green River Formation, and the Llano Uplift, Texas. The issue's first two papers cover the Sevier thrust belt; other papers examine the Sudbury impact crater; the Lonar Crater, India; GIS assessments of post-fire debris-flow hazards in central Idaho; and Franciscan metagraywackes of San Francisco, California.

Reconstructing the kinematic evolution of curved mountain belts: A paleomagnetic study of Triassic red beds from the Wyoming salient, Sevier thrust belt, U.S.A.
Arlo Brandon Weil et al., Dept. of Geology, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010, USA. Pages 3-23.

Most active and ancient mountain belts display map-view curvature over a range of scales, yet mechanisms responsible for developing such curvature remain incompletely understood. Determining the origins of curved mountain belts is critical for understanding the tectonic and paleogeographic evolution of continents. At the root of this problem is when and how mountains acquire curvature during complex and protracted deformation histories. By integrating structural and paleomagnetic studies in the Wyoming salient of the Sevier mountain belt Weil et al. have been able to constrain its 3-D kinematic evolution and interpret processes responsible for producing the belt's present-day architecture. The Wyoming salient began with minor primary curvature, which then underwent progressive secondary rotation penecontemporaneous with mountain building. Rotation was related to curvature of fault slip directions, differential shortening, and wrenching. Processes that gave rise to this kinematic evolution include (1) variations in initial thickness and strength of foreland basin-fill stratigraphy, (2) feedback with basins that were formed in front of, and eventually incorporated into, the growing mountain belt, and (3) interaction with foreland uplifts along the salient ends. Their approach of integrating regional structural and paleomagnetic studies can be applied to other mountain belts to better understand the processes that produce curved orogens.

Reconstructing the kinematic evolution of curved mountain belts: Internal strain patterns in the Wyoming salient, Sevier thrust belt, USA
Adolph Yonkee and Arlo Brandon Weil et al., Dept. of Geosciences, Weber State University, Ogden, Utah 84408, USA; ayonkee@weber.edu. Pages 24-49.

Most active and ancient mountain belts display map-view curvature over a range of scales, yet mechanisms responsible for developing such curvature remain incompletely understood. Determining the origins of curved mountain belts is critical for understanding the tectonic and paleogeographic evolution of continents. At the root of this problem is when and how mountains acquire curvature during complex and protracted deformation histories. By integrating structural and paleomagnetic studies in the Wyoming salient of the Sevier mountain belt Yonkee and Weil have been able to constrain its 3-D kinematic evolution and interpret processes responsible for producing the belt's present-day architecture. The Wyoming salient began with minor primary curvature, which then underwent progressive secondary rotation penecontemporaneous with mountain building. Rotation was related to curvature of fault slip directions, differential shortening, and wrenching. Processes that gave rise to this kinematic evolution include (1) variations in initial thickness and strength of foreland basin-fill stratigraphy, (2) feedback with basins that were formed in front of, and eventually incorporated into, the growing mountain belt, and (3) interaction with foreland uplifts along the salient ends. Their approach of integrating regional structural and paleomagnetic studies can be applied to other mountain belts to better understand the processes that produce curved orogens.

The Sudbury impact layer in the Paleoproterozoic iron ranges of northern Michigan, USA
W.F. Cannon et al., U.S. Geological Survey, Reston, Virginia 20192, USA. Pages 50-75.

A giant meteorite struck the earth near what is now Sudbury, Ontario, 1,850 million years ago. It created a crater estimated to have been as large as 250 km in diameter. Many geologic features near Sudbury record this cataclysmic event, but more widespread effects outside of the crater have not been recognized until recently. The rock layer described by Cannon et al., named the Sudbury impact layer, has been discovered at eleven sites across the northern peninsula of Michigan ranging from about 500 to 700 km from ground zero at Sudbury, and has been shown to contain the geologic record of the meteor impact's effects on this area. Features formed by extreme shock pressures that can be created only by large meteor impacts have been found in mineral grains within the layer and provide conclusive proof of the link to a giant impact. The impact layer records several different processes, all of which occurred within minutes to hours after the impact. Material blasted from the crater and carried through the atmosphere makes up part of the layer. This rapidly traveling material, upon falling to Earth's surface continued to race outward from the crater as a "ground surge," which scoured the surface over which it traveled. The impact almost surely generated giant tsunami waves which swept across much of the area reworking surficial materials. An intense earthquake, probably larger than any recorded in human history, was also generated by the impact. Its effects in Michigan are recorded as giant submarine slump deposits in which recently deposited and only partly consolidated sediments were mobilized and slid down submarine slopes to form distinctive deposits of dismembered and broken rock layers. The area of this study was mostly a shallow sea at the time of the impact. The nature of the sedimentary rocks that were being deposit changed profoundly at the moment of the impact indicating that the effects of the impact had substantial and long-lasting consequences to the earth's surface over a wide region around the crater.

Forearc basin formation in the tectonic wake of a collision-driven, coastwise migrating crustal block: The example of the North Andean block and the extensional Gulf of Guayaquil-Tumbes Basin (Ecuador-Peru border area)
César Witt and Jacques Bourgois, Observatoire Oceanologique de Villefranchesur-Mer, UMR Geosciences Azur, (UPMC-IRD-CNRS), BP48, 06235 Villefranche/Mer, France. Pages 76-95.

In a study by Witt and Bourgois, the northward tectonic escape or drifting of the North Andean block controlled the tectonic evolution of the Gulf of Guayaquil-Tumbes basin, at least for the past about 1.8-1.6 million years. Industrial multichannel seismic and well data document that east-west to east-northeast low-angle detachment normal faults accommodated the main subsidence step along the shelf area during the late Pliocene-Quaternary times (1.8-1.6 million years ago to the Present). Two tectonic regimes showing different styles and ages controlled the evolution of the southern Ecuador and northern Peru continental margin and shelf. The about north-south extensional regime along the shelf area is related to NAB drift, whereas the east-west extensional regime along the continental margin results from tectonic erosion at depth. The Gulf of Guayaquil-Tumbes basin is not a classical pull-apart basin, and exemplifies a particular type of pull-apart basin basically controlled by (1) detachments extending downward across the brittle crust, and (2) the plate coupling along the subduction décollement, which controls the inward segmentation of deformation.

Geology of Lonar Crater, India
Adam C. Maloof et al., Dept. of Geosciences, Princeton University, Guyot Hall, Washington Road, Princeton, New Jersey 08544, USA. Pages 109-126.

Lonar crater, India, is one of the youngest and best preserved impact structures on Earth. It is a 1.88-km diameter simple crater formed entirely within the Deccan traps (a large volcanic province consisting of many layers of basalt flows), making it a useful analogue for small craters on other terrestrial planets and the Moon. In this study, Maloof et al. present a topographic model and a geological map of Lonar crater and the surrounding area. Using radio-carbon dating techniques they attempt to constrain the maximum formation age of the crater. At the crater rim, the upper lava flows have been turned up and folded over onto the surrounding terrain, however the hinge of this fold is preserved around only 10 to 15% of the crater. They find that large displacements along tear faults in the crater walls are not characteristic of small craters in basalt. The continuous ejecta blanket deposited on the surrounding terrain is reasonably well preserved and in some regions is overlain by small, glassy impact spherules, with aerodynamic shapes resulting from traveling through the air while cooling. By studying the preserved thickness distribution of the ejecta, the location and size of clasts of ejecta and underlying materials, and the boundary between the two, we can infer that the ejecta blanket was emplaced in a large debris-flow that traveled along the ground. The ejecta profile resembles the ejecta structures observed on Mars, indicating similar processes may be active on both planets.

Predicting the probability and volume of postwildfire debris flows in the intermountain western United States
Susan H. Cannon et al., U.S. Geological Survey, Box 25046, Denver Federal Center, MS 966, Denver, Colorado 80225, USA. Pages 127-144.

Methods for assessing the potential for debris flows from basins burned by wildfires over extensive areas are needed to rapidly assess hazards and to prioritize locations for pre-fire restoration efforts. Cannon et al. describe a set of models that rely on data that are readily available immediately after a fire, and can be implemented in a Geographical Information System (GIS) to assess post-fire debris-flow hazards. The assessments identify the probability that given basins will produce debris flows and estimate the potential volume of the debris flows at the basin outlet. For a given storm event, or set of storms, this approach addresses two of the fundamental questions in debris-flow hazard assessment: Where might debris flows occur and how big might they be? The mapping approach identifies those basins that are most prone to the largest debris-flow events and thus provides information necessary to prioritize areas for post-fire erosion mitigation, warnings, and pre-fire management efforts throughout the intermountain western United States.

Cenozoic right-slip faulting along the eastern margin of the Pamir salient, northwestern China
Eric Cowgill et al., Dept. of Geology, University of California, Davis, California, 95616, USA. Pages 145-161.

The Tibetan Plateau is the largest region of convergent continental deformation on Earth, and provides a unique opportunity to understand how continents deform during continent-continent collision. Key to this problem is understanding the geometry and sense of motion along the major (250- to 1000-km-long) fault systems that define the boundaries of the plateau, such as those which flank the Pamir range along the northern edge of the Tibetan Plateau at the western end of the India-Eurasia collision zone. New geologic mapping reported by Cowgill et al. suggests that the eastern flank of the Pamir is defined by a ~350-km-long fault system that has accommodated both right-lateral strike-slip and east-west shortening. This system appears to have accommodated late Cenozoic separation of the Pamir from mountain ranges to the east during south-directed intracontinental subduction beneath the leading edge of Tibet. Correlation of major faults suggests total slip along the system is likely on the order of ~280 km. This offset estimate implies long term slip rates of 7-15 mm/yr along the fault system, when combined with previous data that indicate deformation along the east flank of the Pamir started between the Late Eocene and Early Miocene. These results imply that the first-order structures on the western and eastern flanks of the Pamir are asymmetric. In contrast to the strike-slip and shortening along the east side as reported here, previous work has shown that deformation in the west was accommodated by anti-clockwise vertical axis rotation of the Pamir over the eastern margin of the Tajik basin generally interpreted to reflect northwest-directed radial thrusting.

Geochemical variations in igneous rocks of the Central Andean orocline (13°S to 18°S): Tracing crustal thickening and magma generation through time and space
Mirian Mamani et al., Abteilung Geochemie, Geowissenschaftlichen Zentrum der Universitat Gottingen, Goldschmidtstrasse 1, D-37077 Gottingen, Germany. Pages 162-182.

The Central Andes of Northern Chile and Southern Peru are characterized over the past 20 million years by a series of zones where magmas have formed and ascended through the crust. Many so called "intrusions" and volcanic centers are associated with ore deposits (gold, silver, copper). Mamani et al. combine a large number of new major and trace element analyses and (Pb-Sr-Nd) isotopes and geochemical analyses (n = 420) with a compilation of published data (total 1635) to trace the systematic changes in magma composition through time and space. Based on this, the authors relate the compositional changes in the magmas to the tectonic evolution of the active Andean continental margin and the increasing thickness of the crust during the formation of the present Andes during the last 35 million years.

Exhumation of Mesoarchean TTG gneisses from the middle crust: Insights from the Steynsdorp core complex, Barberton granitoid-greenstone terrain, South Africa
Cristiano Lana et al., Dept. of Geology, Geography and Environmental Studies, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa. Pages 183-197.

The Barberton granitoid-greenstone terrain in South Africa is a classic dome-and-keel province characterized by km-scale gneiss domes and elongate keels of largely folded supracrustal rocks of the Barberton greenstone belt. Much of the structural grain of the main greenstone belt relates to a long episode of post-orogenic extension, following the assembly of the Barberton greenstone belt at 3230 million years ago. Lana et al. present key metamorphic and structural data from granitoid-greenstone contacts in the oldest and one of the best preserved parts of the terrain. Their results are consistent with the presence of a cool and rigid continental nucleus in the MesoArchean that was exhumed from crustal depths of 30-40 km, shortly after tectonic collision. Peak metamorphic textures, associated fabric development, and consistent granitoid-up, greenstone-down kinematic indicators point to the exhumation of metamorphic core complexes along an extensional detachment below the low-grade main greenstone belt.

Tracing exhumation of the Dabie Shan ultrahigh-pressure metamorphic complex using the sedimentary record in the Hefei Basin, China
Shaofeng Liu et al., State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, China; and College of Geosciences and Resources, China University of Geosciences, Beijing 100083, China. Pages 198-218.

The paper by Liu et al. presents the exhumation of the Dabie Shan ultrahigh-pressure (UHP) metamorphic complex in central China using the sedimentary record in the Hefei basin. Previous studies mainly demonstrated the timing of the first surface exposure of ultrahigh-pressure metamorphic rocks in the Dabie Shan based on the discovery of eclogite pebbles, SHRIMP dating of zircons within gravels, etc. However, little is known about the spatial and temporal patterns of exhumation of the different rock units in Dabie Shan and their contribution to basin sediments as well as to further constraints on episodic thrust events within the Dabie Shan. Liu et al. document a study of Jurassic sedimentary rocks in the Hefei basin as they relate to erosion of the Dabie Shan. Specifically, space-time changes within sedimentary sequences are correlated with exhumation events of the Dabie Shan complex based on multi-proxy provenance analyses, including Nd isotopic and geochemical studies. This has allowed us to identify the distribution and contribution of source terrain lithologies to the Jurassic basin sediments in order to make inferences about exhumation of the Dabie Shan. Provenance analyses of the sediments in the Hefei basin clearly demonstrate that the depth and rate of exhumation in the Dabie Shan and the North Huaiyang fold and thrust belt increases from the west to the east. The study results are helpful for scientists to understand the exhumation of the Dabie Shan UHP metamorphic complex and a kind of method of proxy provenance analyses.

Magmatism and early-Variscan continental subduction in the northern Gondwana margin recorded in zircons from the basal units of Galicia, NW Spain
Jacobo Abati et al., Departamento de Petrologia y Geoquimica and Instituto de Geologia Economica, Universidad Complutense and Consejo Superior de Investigaciones Cientificas, 28040 Madrid, Spain. Pages 219-235.

The article by Abati et al. uses a radioactive decay method (uranium-lead) to measure the time elapsed since several important geologic events took place. The subduction of the external edge of the Gondwana continental margin took place 371 million years ago, and it marks the beginning of the collision of the two great continents of that period, Laurentia and Gondwana, giving rise to the formation of the Paleozoic supercontinent Pangea.

Geomorphic controls on lacustrine isotopic compositions: Evidence from the Laney Member, Green River Formation, Wyoming
Amalia C. Doebbert et al., Dept. of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA. Pages 236-252.

In a study by Doebbert et al., the oxygen isotope record from the Laney Member of the Green River Formation provides evidence that the evolution of upstream drainage networks influences the isotopic composition of water in downstream basins. A large, rapid, geographically widespread negative shift in the oxygen isotope composition of the Laney Member coincides with evidence for lake-type change, and is poorly explained by regional climate or local elevation changes. Mass-balance modeling and geologic evidence support the upstream introduction of water with low isotopic values as a plausible cause for this shift in the lake basin record. Stable isotope studies of non-marine sedimentary deposits are a repository of information commonly used to interpret paleoclimate and paleoelevation. However, drainage networks are often dynamic, and this study demonstrates the need to understand drainage history in order to accurately interpret non-marine sedimentary records.

Proterozoic granites of the Llano Uplift, Texas: A collision–related suite containing rapakivi and topaz granites
Daniel S. Barker and Robert M. Reed, Dept. of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, 1 University Station C1100, Austin, Texas 78712-0254, USA. Pages 253-264.

Granites of central Texas have served as building and decorative stone throughout the United States, from the L.A. County Courthouse to the American Museum of Natural History in New York City, and as far as Reykjavik, Iceland, and the financial district of London. For the first time, a comprehensive study of these granites by Barker and Reed shows their variety, ages, and chemical compositions. These granites resulted from collision 1.2 to 1.1 billion years ago between what is now North America and an unknown continent that left the scene before it could be identified.

Late Devonian glacigenic and associated facies from the central Appalachian Basin, eastern United States
David K. Brezinski et al., Maryland Geological Survey, 2300 St. Paul Street, Baltimore, Maryland 21218, USA. Pages 265-281.

Within the central Appalachians, the origin of an enigmatic, 360-million-year-old layer of rock has long puzzled geologists. Brezinski et al. provide evidence that this layer is the product of extensive glaciers that once covered much of the eastern United States at the end of the Devonian. This hypothesis contradicts long-held beliefs that at that time in Earth's history eastern North America was subtropical in its paleo-latitude. The enigmatic deposits display characteristics that are also seen in recently glaciated regions, including deposits formed beneath actively advancing and melting glaciers, and in laterally equivalent environments, including glacial lakes and outwash rivers. The paper also documents, through the study of fossil soils, that areas of the Appalachians not covered by glacial ice were subjected to increased rainfall and shallow permafrost. Furthermore, the impact of this Appalachian event was manifested by a global drop in sea level that produced deeply eroded river channels, not only within the Appalachians, but also Europe. Finally, the precise timing of the Appalachian glacial episode and sea level drop is contemporaneous with, and may have contributed to, one of Earth's most significant biotic crises, known as the Hangenberg extinction event.

Detrital zircon evidence for progressive underthrusting in Franciscan metagraywackes, west-central California
Cameron A. Snow et al., Exploration and Production Technology, Apache Corporation, 2000 Post Oak Boulevard, Suite 100, Houston, Texas 77056, USA. Pages 282-291.

In a study by Snow et al., radiometric age data collected from rocks located in the San Francisco Bay area of California have demonstrated that in convergent margin settings, thrust belts become progressively younger at depth. This phenomenon is the result of progressively offscraping oceanic sediments at subduction zones -- these data support observations made by earlier researchers from subduction zones at various locales.

Contrasting origins of serpentinites in a subduction complex, northern Dominican Republic
Benoit-Michel Saumur et al., Dept. of Earth Sciences, University of Ottawa, 140 Louis Pasteur, Ottawa, Ontario, K1N 6N5, Canada. Pages 292-304.

Serpentinites are hydrated Mg-rich rocks, and are abundant in many oceanic subduction zones where oceanic crust has been subducted. Saumur et al. examined serpentinites in northern Dominican Republic and found two different origins: hydrated oceanic peridotites and subarc mantle peridotites. Hydrated oceanic peridotites form large bodies and also melanges along the northern coast of Dominican Republic. They were once exposed on the ocean floor of the old Caribbean oceanic plate and hydrated by sea water. The forearc mantle peridotites were once at deep levels in the mantle and were hydrated by water released from subducted oceanic crust. They occur along major faults that formed during the collision of the Caribbean plate with the Bahamas platform after subduction stopped. These buoyant serpentinites ascended from the base of the mantle wedge, above the subducted slab, to shallow crustal levels; the hydrated serpentinites in turn assisted creep along major faults, allowing movement without seismic activity.

Origin of accretionary lapilli within ground-hugging density currents: Evidence from pyroclastic couplets on Tenerife
R.J. Brown et al., Volcano Dynamics Group, Dept. of Earth and Environmental Sciences, Open University, Walton Hall, Milton Keynes, MK7 6AA, UK. Pages 305-320.

Volcanic ash clouds commonly contain ash aggregates -- small pea-sized balls of fine-grained ash (volcanic "hailstones"). Detailed fieldwork by Brown et al. on numerous pyroclastic layers on Tenerife, Canary Islands, has revealed that the occurrence and distribution of different types of these ash aggregates varies within different types of pyroclastic deposits. This has allowed the unraveling of how ash aggregates grow and evolve during transport within pyroclastic density currents ("pyroclastic flows"). The results may be useful in hazard assessments at volcanoes. Similar ash aggregates occur in the deposits of giant meteorite impacts and the results of this research may help decipher what happens during catastrophic impact events.

Cambro-Ordovician paleogeography of the Southeastern New England Avalon Zone: Implications for Gondwana breakup
M.D. Thompson et al., Dept. of Geosciences, Wellesley College, Wellesley, Massachusetts 02481-8203, USA. Pages 96-108.

By integrating magnetic directions with precise U-Pb dates, Thompson et al. demonstrated (in a 2007 GSA Bulletin paper) that West Avalonia, comprising southeastern New England and geologic equivalents bordering maritime Canada all the way to eastern Newfoundland, lay off the African portion of the super-continent Gondwana approximately 595 million years ago. The present paper uses the same approach to show that West Avalonia was beginning to separate from Gondwana at 490-488 million years ago, consistent with previous findings from East Avalonian localities in Great Britain. As West and East Avalonia jointly drifted farther from Gondwanan towards ultimate collision with ancestral North America (Laurentia), the Rheic Ocean opened in its wake.

Highlights are provided below. To review abstracts, go to http://gsabulletin.gsapubs.org/content/early/recent. 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, gsaservice@geosociety.org.

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