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Geological Society of America

March/April 2010 GSA Bulletin Highlights

Boulder, CO, USA – The March-April 2010 GSA BULLETIN is online now "ahead of print." Topics span the globe, from the Greater Caucasus Mountains separating Azerbaijan and Georgia from Russia; to the Altyn Tagh fault zone, Bohai Bay Basin, Yangtze craton, and Tian Shan of China; the collision zone between India and the Himalaya; the Southern Uplands of Scotland; and on to the western U.S., covering central Idaho, Mammoth Mountain and Long Valley caldera, California, the King Lear Formation, Nevada, the Grand Canyon, and the Fountain Formation of Colorado.


Late Cenozoic deformation of the Kura fold-thrust belt, southern Greater Caucasus
A.M. Forte, Dept. of Geology, University of California, Davis, California 95616, USA. Pages 465-486.

It has long been speculated that much of the current convergence between the Arabian and Eurasian plates in the area between Turkey and Iran is accommodated by the shortening of the crust within the Greater Caucasus Mountains, which separate Azerbaijan and Georgia from Russia to the north. Forte et al. used imagery and digital elevation models from the ASTER satellite along with newly developed software for pseudo-3-D visualization of this data to investigate the potential that much of the convergence between Arabia and Eurasia in Azerbaijan and Georgia has instead been recently concentrated in a belt of topography at the southern margin of the Greater Caucasus called the Kura fold-thrust belt. Paring the remote sensing observations of bedding orientations with previously published geologic maps for the area allowed them to estimate that the Kura fold-thrust belt has absorbed 30%-40% of the convergence between Arabia and Eurasia since five million years ago. Their results suggest that the Kura fold-thrust belt is an important and active structure within the Arabia-Eurasia collision zone and has potential implications for seismic hazard investigations in Azerbaijan and Georgia.


Age, geochemistry, and tectonic implications of a late Paleozoic stitching pluton in the North Tian Shan suture zone, western China
Bao-Fu Han et al., Ministry of Education, Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and Space Sciences, Peking University, Beijing 100871, People's Republic of China. Pages 627-640.

In Earth's history, disappearance of an ocean between two continental blocks is thought to result in collision of the continental blocks and some remnants of the oceanic crust may be preserved in the collisional zone. Subsequently, the zone may be crosscut by granitic intrusions. So the timing of disappearance of the ocean and the collision of the continental blocks can be constrained by the youngest remnant of oceanic crust and the oldest granitic intrusion. Previous work revealed the youngest (325 million-year-old) remnant of the North Tian Shan Ocean between the Yili and Junggar blocks, North Xinjiang, western China, and in this study Han et al. report a zircon U-Pb age of 316 million years old for the Sikeshu granitic intrusion which crosscuts the collisional zone between the Yili and Junggar blocks and thus places a crucial upper age bound for the time of disappearance of the North Tian Shan Ocean and collision between the Yili and Junggar blocks. After that time, North Xinjiang was in another stage of evolution.


The lower Lesser Himalayan sequence: A Paleoproterozoic arc on the northern margin of the Indian plate
Matthew J. Kohn et al., Dept. of Geosciences, Boise State University, Boise, Idaho 83725, USA. Pages 323-335.

The lower Lesser Himalayan Sequence figures heavily in understanding the Indo-Asian collision and in plate reconstructions of the about 1800-million-year-old supercontinent Columbia. Previous studies indicated that lower Lesser Himalayan rocks reflected deposition on a passive margin. Five lines of evidence from Kohn et al., however, including textures, ages, chemistry, and geographic distributions of outcrops, instead indicate these rocks represent part of a volcanic arc. Lesser Himalayan ages are distinct from those in continental India, so intrusions do not represent Indian plate basement. These data help in reconstructing India's position in the hypothesized supercontinent Columbia, and in deciphering distributions of rocks prior to the Indo-Asian collision.


SHRIMP U-Pb dating of recurrent Cryogenian and Late Cambrian-Early Ordovician alkalic magmatism in central Idaho: Implications for Rodinian rift tectonics
K. Lund et al., U.S. Geological Survey, MS 973, Denver Federal Center, Box 25046, Denver, Colorado 80225, USA. Pages 430-453.

Composite alkalic plutonic suites and tuffaceous diamictite, although discontinuously exposed across central Idaho in roof pendants and inliers within the Idaho batholith and Challis volcanic-plutonic complex, define the more than 200-km-long northwest-aligned Big Creek-Beaverhead belt. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dates on these igneous rocks provide direct evidence for the orientation and location of the Neoproterozoic-Paleozoic western Laurentian rift margin in the northern U.S. Cordillera. Dating delimits two discrete magmatic pulses about 665-650 million years ago and 500-485 million years ago at the western and eastern ends, respectively, of this belt. Together with the nearby 685 million year old volcanic rocks of the Edwardsburg Formation, there is a 200 million year history of recurrent extensional magmatic pulses along the belt. A similar history of recurrent uplift is reflected in the stratigraphic record of the associated miogeoclinal and cratonal platform basins, suggesting that the Big Creek-Beaverhead belt originated as a border fault during continental rift events. The magmatic belt is paired with the recurrently emergent Lemhi Arch and narrow miogeoclinal facies belts and it lies inboard of a northwest-striking narrow zone of thinned continental crust. These features define a northeast-extending upper-plate extensional system between southeast Washington and southeast Idaho that formed a segment of the Neoproterozoic-Paleozoic miogeocline. This segment was flanked on the north by the St. Mary-Moyie transform zone (south of a narrow southern Canadian upper-plate margin) and on the south by the Snake River transfer zone (north of a broad Great Basin lower-plate margin). These are the central segments of a zigzag-shaped Cordilleran rift system of alternating northwest-striking extensional zones offset by northeast-striking transfers and transforms. The data substantiate polyphase rift and continental separation events that included (1) pre- and syn-Windermere rifting, (2) Windermere margin subsidence, (3) late Ediacaran-Cambrian rifting, and (4) well-developed late Ediacaran-Devonian passive margin subsidence and deposition. Timing and geometries support synchronous but opposing divergence along Cordilleran and Atlantic rifts with a junction in Southern California-Sonora.


New 40Ar/39Ar ages reveal contemporaneous mafic and silicic eruptions during the past 160,000 years at Mammoth Mountain and Long Valley caldera, California
Gail A. Mahood et al., Dept. of Geological and Environmental Sciences, Building 320, 450 Serra Mall, Stanford University, Stanford, California 94305-2115, USA; . Pages 396-407.

Predictions of eruptions at volcanoes in the circum-Pacific "Ring of Fire" have improved with the experience of the last several decades at volcanoes such as Mount St. Helens, Pinatubo, and, most recently, Redoubt, in Alaska. But what about the eruptions a thousand times more voluminous from rhyolitic "supervolcanoes," which spread thick ash continent-wide? Fortunately for humans, "supervolcanoes" erupt infrequently, but as a result we know less about the triggers and precursors to these catastrophic events. Long Valley caldera (and adjacent Mammoth Mountain, a popular ski area in east-central California) and Yellowstone are the two "restless" calderas in the U.S., characterized by earthquake swarms, ground uplift, and gas emissions indicating shallow intrusion of basaltic magma. A new 40Ar/39Ar study by Mahood et al. of young basaltic and rhyolitic lavas shows that Mammoth Mountain and lavas in the northwest quadrant of the Long Valley caldera are considerably younger than previously thought (younger than or equal to 68,000 years old). As a result of this high-precision dating they can identify four eruptive sequences over the past 160,000 years; in each, basaltic and rhyolitic lavas erupted contemporaneously from spatially associated vents. This suggests that intrusion of basalt into the shallow crust triggered eruptions of rhyolitic magma. If the seismic unrest and deformation of the last three decades is a result of basalt injected beneath Mammoth Mountain and the western third of the Long Valley caldera, then there is the possibility of spatially associated small-volume silicic eruptions, which would typically be considerably more explosive. The new dating also demonstrates that in the last 40,000 years, eruptions have occurred along a N-S linear trend less than 10 km wide, limiting the zone most subject to volcanic hazards.


The Lower Cretaceous King Lear Formation, northwest Nevada: Implications for Mesozoic orogenesis in the western U.S. Cordillera
Aaron J. Martin et al., Dept. of Geology, University of Maryland, College Park, Maryland 20742, USA; martinaj@geol.umd.edu. Pages 537-562.

The paper by Martin et al. presents new structural, sedimentologic, and shallow seismic data that confirm that the mostly clastic King Lear Formation was deposited in a half-graben, not a thrust-bounded basin as previously interpreted. Their interpretation has profound implications for understanding the tectonic evolution of the western U.S. Cordillera during the Jurassic and Cretaceous. First, it constrains shortening deformation to have been completed prior to deposition of the King Lear Formation at about 125 million years ago. This constraint is important in the ongoing debate about whether the western U.S. experienced continuous or punctuated shortening throughout the Jurassic and Early Cretaceous. Second, they show that north-central Nevada was probably a regional topographic high in the Early Cretaceous, an earlier constraint than previously published. Both new constraints are important for furthering our understanding of the tectonic evolution of the western U.S. Cordillera during the Mesozoic.


Structural and anisotropy of magnetic susceptibility (AMS) evidence for oblique impact on terrestrial basalt flows: Lonar crater, India
Saumitra Misra et al., Indian Institute of Geomagnetism, Navi Mumbai 410 218, India. Pages 563-574.

Asteroid impact is a common geological process that shapes the surfaces of rocky (or icy) planetary bodies in our Solar System. The hypervelocity (faster than 11 km/s) impacts of asteroids create some circular to elliptical depressions on the target planetary bodies called impact craters. Most of the rocky planetary bodies in our Solar System have basaltic crusts. Out of the two known terrestrial impact craters that are excavated in basaltic target rocks, the Lonar crater in India is fully accessible and possibly the best studied crater. It can be taken as an example to evaluate planetary impact cratering processes. The present radiometric ages suggest that the Lonar crater was formed ~50 thousand years ago in the undeformed and sub-horizontal Deccan Traps that erupted at ~65 million years ago. Two of the important aspects of crater research are the nature and projectile path of the impactor. Misra et al. have successfully applied a new technique - the Anisotropy of Magnetic Susceptibility (AMS) along with satellite imagery and structural geological studies on the small Lonar crater (~1.8 km diameter) to evaluate the projectile path of its impactor. Their studies of satellite images show that the rim of Lonar crater is almost circular while the continuous ejecta around the crater rim, estimated to be in its pristine shape, can be best enveloped by an ellipse having an east-west major axis. The ejecta materials show greater spread towards the west compared to other directions. When compared with experimental results, the distribution of ejecta and the shape of the crater rim suggest that the impactor asteroid hit the pre-impact target from the east at an angle between 30 and 45 with the horizon. Their AMS data also suggest that the target basalt about 2 km west of the crater is highly shocked due to oblique impact from the east compared to the unshocked target basalt from an equal distance in the east. Oblique impact from the east also resulted in the vertical or sub-vertical attitudes of target basalts along the western sector of the Lonar crater, whereas in other sectors of crater rim they dip gently outwards.


Numerical modeling of the late Cenozoic geomorphic evolution of Grand Canyon, Arizona
Jon D. Pelletier, Dept. of Geosciences, University of Arizona, Gould-Simpson Building, 1040 East Fourth Street, Tucson, Arizona 85721-0077, USA. Pages 595-608.

Grand Canyon was carved by the Colorado River starting between 6 and 5 million years ago and continuing to today. In this paper, Pelletier describes a numerical model that quantifies how quickly erosion of the canyon took place and how downward erosion, lateral erosion (cliff retreat) and the response of the underlying crust has occurred over time.


Imbricated ocean-plate stratigraphy and U-Pb zircon ages from tuff beds in cherts in the Ballantrae complex, SW Scotland
Yusuke Sawaki et al., Dept. of Earth and Planetary Science, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro, Tokyo 152-8551, Japan. Pages 454-464.

Ocean plate stratigraphy (OPS) is the fundamental, first-order structure of accretionary orogens that are forming today on the Pacific margins. It consists of cherts that were deposited first under deep-sea pelagic conditions at or near a mid-oceanic ridge, later during transport of the ocean floor towards a subduction zone, and finally the cherts were overlain by clastic infill in the trench. Movement of oceanic lithosphere, which provides key information for reconstruction of paleogeography, is recorded in active margins, and particularly at a subduction zone. One of the active margins of the Iapetus Ocean was the Southern Uplands of Scotland that contains a Lower Palaeozoic accretionary prism. Structures in mélanges and décollement zones in the Ordovician (northern) belt of the Southern Uplands accretionary prism resulted from southeast-directed thrusting. The Ballantrae Complex is located on the north of the Southern Uplands Fault. The Ballantrae Complex was considered as one of the Ordovician ophiolites and contains imbricated cherts at Bennane Head in the west of the Ballantrae Complex. However, in spite of numerous studies, the Ballantrae Complex has never been interpreted in the light of a modern understanding of comparative accretionary prism. Ancient plate motions are usually estimated from geological, geochronological and paleomagnetic data. Sawaki et al. present detailed structural relations, new U-Pb ages of zircons from tuffs interbedded in cherts to the south of Bennane Head, and a discussion on the movement of the Iapetus oceanic lithosphere in the Palaeozoic. Ocean plate stratigraphy (basalt, chert, clastics) of the Balcreuchan Group in the Ballantrae Complex is repeated by layer-parallel thrusts to form duplex structures south of Bennane Head. Although mutually distant, five tuff beds, all in chert, have similar U-Pb zircon ages of about 470 million years ago. The geometrical polarity of the duplexes and the zircon ages provide new constraints on the tectonic evolution of the accretionary wedge of the Ballantrae Complex. Northward thrusting of the duplexes suggests that subduction was from the northwest to the southeast, a polarity that is consistent with the northward younging of the volcanic arcs of the Ballantrae Complex.


Rates and mechanisms of Mesoarchean magmatic arc construction, eastern Kaapvaal craton, Swaziland
Blair Schoene and Samuel A. Bowring, Dept. of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, Massachusetts 02139, USA. Pages 408-429.

Within the oldest core of the Kaapvaal craton (about 3.7-3.1 billion years old) in eastern South Africa and Swaziland, evidence is preserved for the construction and amalgamation of one of Earth's oldest continents. Remarkably, some of the sedimentary, volcanic and plutonic rocks from this region remain undeformed or metamorphosed, and thus provide unique insight into processes occurring in the early Earth. Schoene and Bowring provide insight into the time scales of magmatic intrusion with unprecedented temporal precision, using new methods of U-Pb zircon dating, and link this with field mapping and geochemistry to characterize an episode of Mesoarchean magmatism. Integrating this period of magmatism and deformation into existing tectonic models for the region, these new data support the hypothesis that the eastern Kaapvaal craton formed through subduction and accretion of young continental fragments by plate tectonic processes similar to those found today.


Analysis of the Wallowa-Baker terrane boundary: Implications for tectonic accretion in the Blue Mountains province, northeastern Oregon
Joshua J. Schwartz et al., Dept. of Geology and Geophysics, University of Wyoming, Laramie, Wyoming 82071, USA. Pages 517-536.

Schwartz et al. explore the boundary between the Wallowa island arc terrane and the Baker melange terrane in northeastern Oregon. This boundary is significant because it records an episode of intense faulting and folding, which they suggest is related to the collision of two island arc terranes (Wallowa and Olds Ferry) in the Middle to Late Jurassic. They propose that this terrane boundary is an example of a broad zone of imbrication of arc crust tectonically mixed within an accretionary complex. It may provide an on-land, ancient analogue to the actualistic arc-arc collisional zone developed along the margins of the Molucca Sea of the central equatorial Indo-Pacific region.


Late Paleozoic tectonics and paleogeography of the ancestral Front Range: Structural, stratigraphic, and sedimentologic evidence from the Fountain Formation (Manitou Springs, Colorado)
Dustin E. Sweet and Gerilyn S. Soreghan, School of Geology and Geophysics, University of Oklahoma, 100 E. Boyd Street, Suite 810, Norman, Oklahoma 73019, USA. Pages 575-594.

Approximately 300 million years ago, during the Late Paleozoic, Colorado was mountainous. But these so-called "ancestral Rocky Mountains" were very different from the modern Rockies. For example, the tectonic forces that created the ancestral Rocky Mountains are puzzling because the mountains formed in the interior of the continent, rather than along a plate boundary, and trended at a high-angle to the nearest such boundary, the well-known Ouachita-Marathon belt that forms the southwestern continuation of the Appalachian chain. New data documented by Sweet and Soreghan show that the ancestral Rockies were marked by sporadic uplift and a final phase of subsidence. The ancestral Rocky Mountains are recognized not by topography, because it no longer exists, but rather by the presence of large piles of sediments that were shed off the ancestral Rockies and accumulated adjacent to the highlands. The Fountain Formation exposed along the modern Front Range of Colorado represents sediments shed off the ancestral Front Range during the late Paleozoic, and preserve a record of this ancient mountain-building event. Sweet and Soreghan document three discrete phases of sedimentation in the Fountain Formation that record (1) initiation of mountain uplift, (2) reinvigoration of uplift and (3) cessation of uplift accompanied by an enigmatic post-tectonic subsidence phase. Comparison of the timing for the formation of the ancestral Rocky Mountains as recorded in this study to other regions with similar constraints indicates that uplift of the ancestral Rockies ceased earlier in eastern Colorado than western Colorado and New Mexico. This pattern is consistent with stresses that were imparted onto the continent from the continent-to-continent collision along the south central margin of the United States. However, the tenuous age controls on the sediments also allow for a relatively synchronous end to uplift, which would require geologists to rethink tectonic models for the formation of the ancestral Rocky Mountains.


U-Pb (SHRIMP) and 40Ar/39Ar geochronological constraints on the evolution of the Xingxingxia shear zone, NW China: A Triassic segment of the Altyn Tagh fault system

The development of structures and their age along the segment of the Altyn Tagh fault system, and the eastward extension of the Tianshan orogenic belt, remain speculative. Recent investigations by Wang et al. on the structural framework, granitic intrusions, and metamorphic rocks in the eastern Tianshan and adjacent areas show that the northeast-striking Xingxingxia sinistral ductile shear zone, northwest China, is sub-parallel to the Altyn Tagh fault zone and is superposed on the eastern Tianshan orogenic belt. U-Pb zircon SHRIMP dating, and muscovite, biotite and K-feldspar 40Ar/39Ar thermochronology indicate that sinistral shear along the Xingxingxia shear zone initiated at about 240-235 million years ago, broadly at the same time as initial formation of the Altyn Tagh fault zone, but later than initiation of dextral strike-slip motion along the ~east-west-trending eastern Tianshan orogenic belt at about 270-245 million years ago. Formation of the Xingxingxia ductile shear zone was associated with Gondwanaland convergence along the southern margin of the Eurasian continent during the late Permian-early Triassic.


The Tertiary evolution of the prolific Nanpu Sag of Bohai Bay Basin, China: Constraints from volcanic records and tectono-stratigraphic sequences
Yuexia Dong et al., PetroChina Jidong Oilfield Company, Tangshan 063004, China). Pages 609-626.

The Bohai Bay Basin, located on the eastern Asian margin, is the second largest oil production basin in China. It contains numerous depressions and sags among which the Nanpu Sag has become particularly important because of significant oil discoveries in recent years. Geologically and tectonically, however, the rifting mechanism and geodynamic evolution of the Tertiary Basin remain uncertain. Based on detailed volcanic and stratigraphic records, Dong et al. have identified five tectono-stratigraphic sequences produced by episodic continental rifting, and four basin dynamic evolutionary phases. A diapiric upper mantle upwelling model is proposed to explain the dynamics that controlled the multiple rifting processes, the cyclic volcanism, and the periodic tectonic evolution of the Sag and the greater Bohai Bay Basin.


Geologic correlation of the Himalayan orogen and Indian craton: Part 1. Structural geology, U-Pb zircon geochronology, and tectonic evolution of the Shillong Plateau and its neighboring regions in NE India
An Yin et al., Dept. of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095, USA. Pages 336-359.

Despite being the highest mountain range in the world, the geologic origin of rocks that make up the Himalayan Range is not clear. Yin et al. present new results on the ages and types of rocks that constitute the northern Indian subcontinent. This work suggests that the Himalayan rocks are most likely composed of Indian rocks rather than Tibetan rocks from the north as some researchers have suggested.


Geologic correlation of the Himalayan orogen and Indian craton: Part 2. Structural geology, geochronology and tectonic evolution of the Eastern Himalaya
An Yin et al., Dept. of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095-1567, USA. Pages 360-395.

The classic view for the development of the Himalayan Range is that the India-Asia collision has caused the uppermost layer of sedimentary rocks overlying the Indian continent to be piled up by faults. However, Yin et al. suggest that the mountain range was constructed by faults that cut deep into the basement of the Indian continent and brought the deeply buried rocks to the surface. This new discovery fundamentally changes the way we think about the forces that created the most active mountain range in the world.


Evolution of the Hongzhen metamorphic core complex: Evidence for Early Cretaceous extension in the eastern Yangtze craton, eastern China
Guang Zhu et al., School of Resource and Environmental Engineering, Hefei University of Technology, Hefei 230009, China. Pages 506-516.

The Hongzhen metamorphic Core Complex in the eastern Yangtze craton, eastern China was first recognized through structural studies. Brittle normal faulting and basin rifting in the hanging wall were developed during the formation of the Core Complex. The exposed Paleoproterozoic Dongling Complex in the footwall is widely overprinted by a detachment ductile shear zone, which has consistent southwest-plunging mineral elongation lineations and top-to-southwest shear indicators. Structural analysis indicates that they formed within a low-angle, southwest-dipping, extensional shear zone at mid-crustal levels. Four muscovite grains separated from mylonites within the shear zone yielded 40Ar/39Ar plateau ages ranging from 128.5-126.1 million years ago. They are interpreted as cooling ages of the shear zone associated with the Core Complex. It is proposed that the Core Complex was initiated as a mid-crustal, low-angle extensional shear zone with top-to-southwest shear sense at about 145 million years ago, and the shear zone was then warped and uplifted by the emplacement of the Hongzhen Granite at 122 million years ago. Zhu et al. have demonstrated that the eastern Yangtze Craton was also involved in the Early Cretaceous extension widely occurring in the eastern China continent. A northeast-southwest extensional direction during the Early Cretaceous is indicated by the Core Complex.

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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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