July/August GSA Bulletin media highlights

07/07/04

Boulder, Colo. – The July/August issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Topics include: evidence from western Oregon that supports paleoclimatic change as a driver of the Eocene-Oligocene extinction; and comprehensive structural analyses of Yucca Mountain, NV, and their implications for seismic hazards and potential groundwater pathways.

Representatives of the media may obtain complimentary copies of articles by contacting Ann Cairns at acairns@geosociety.org. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA BULLETIN in stories published. Contact Ann Cairns for additional information or assistance.

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Deformation during terrane accretion in the Saint Elias orogen, Alaska
Ronald L. Bruhn, Department of Geology and Geophysics, University of Utah, Salt Lake City, Utah 84112-0111, USA, et al. Pages 771–787.
Keywords: terrane accretion, Saint Elias orogen, Alaska, transpression, deformation.

The Saint Elias Mountains of Alaska and Canada form one of the most spectacular landscapes in North America as the result of young and active mountain building and extensive glaciation. The natural beauty of the area has resulted in its preservation within the Wrangell–Saint Elias Park and Preserve and the Kluane National Park in adjacent Canada. Recent work on the structural geology of the region has shown how northward motion of a fragment of the North American continental margin has resulted in extensive folding, faulting, and uplift of the continental margin in the last 10 million years. In particular, this new work shows that the courses of the Bering and Malaspina Glaciers, the largest glaciers in North American, coincide with structural boundaries that subdivide the orogen into regions of different structure, topography, and styles of earthquake generation. In the southern part of the mountain belt, deformation is focused along and near the Fairweather strike-slip fault, in the middle part of the mountain belt underthrusting of the Yakutat Terrane beneath North American, and has formed a wide belt of folding and faulting similar in form to rumpling of a rug that is slid over a hardwood floor. At the western end of the mountain belt, deformation is associated with huge earthquakes like the M 9.2 Great Alaskan Earthquake of 1964, where the Yakutat Terrane is subducted beneath south-central Alaska.

Mafic enclaves densely concentrated in the upper part of a vertically zoned felsic magma chamber: The Kurobegawa granitic pluton, Hida Mountain Range, central Japan
Hajime Wada, Environmental System Science, Graduate School of Science and Technology, Shinshu University, Nagano 390-8621, Japan, et al. Pages 788–801.
Keywords: MME flotation, mafic microgranular enclaves, magma mingling, fractional crystallization, granitic pluton, Kurobegawa.

In island arc subduction zones, granitic magma chambers commonly lie beneath less silicic stratovolcanoes that are mainly composed of andesitic volcanic ejecta. Basaltic magma injection into the granitic chamber, followed by mixing between basalt and granite to form andesitic magma, has been interpreted to trigger the stratovolcano-forming eruptions.

The two million year old Kurobegawa pluton is one of the youngest igneous intrusions exposed at Earth's surface. Formed at depths ranging from 4 to 10 km, it is now exposed at 700–2900 m elevations in the Northern Japan Alps, having been exhumed by rapid tectonic processes and consequent uplift and erosion. The pluton represents a well-exposed cross section of a magma chamber beneath island-arc volcanoes. The cross section records voluminous injection of basaltic magma into the semi-solidified crystal mush at the bottom of a granitic chamber. The basalt fragmented and segregated into magma foams, eventually forming numerous isolated basaltic globules (mafic microgranular enclaves [MMEs]). The basaltic magma was originally hydrous and the MMEs underwent vapor exsolution during ascent, reducing the bulk density relative to that of the host granitic magma. The buoyant, vesiculated MMEs rose upward, resulting in their dense distribution throughout the pluton. The Kurobegawa Pluton is andesitic in bulk composition (granitic host plus MME), similar to typical island arc volcanoes, but it failed to produce true andesite because it solidified without effective mixing of the two magmas.

Forms and processes of two highly contrasting rivers in arid central Australia and the implications for channel-pattern discrimination and prediction
Stephen Tooth, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, SY23 3DB, UK, and Gerald C. Nanson, School of Geosciences, University of Wollongong, New South Wales, 2522, Australia. Pages 802–816.
Keywords: anabranching, channel pattern, dryland river, ephemeral flow, riparian vegetation, sediment transport.

Alluvial river channel patterns are usually categorized as straight, meandering, braided, or anabranched (anastomosed). Analysis of the diverse factors that give rise to these different river patterns is a key research topic for many earth scientists and engineers and commonly is used to reconstruct, characterize, or predict river behavior that occurs in response to environmental changes. Data from ephemeral rivers in central Australia, however, demonstrate the limitations of many traditional concepts and theories by showing how major transformations from straight, single channels to anabranching systems of multiple channels separated by numerous tree-lined islands can be induced by very subtle differences in water and sediment supply. These complex river transformations cannot be analyzed by traditional channel pattern discrimination plots, thus indicating that caution must be exercised before using these approaches to predict possible future river changes that may occur under scenarios of climate or land use change.

Eocene-Oligocene extinction and paleoclimatic change near Eugene, Oregon
Gregory J. Retallack, Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403, USA, et al. Pages 817–839.
Keywords: Eocene-Oligocene, Eugene, Oregan, fossil floras, ash-flow tuffs.

As recently as 34 million years ago, western Oregon was vegetated by tropical laurel forests and its seashores littered with cone shells, fig shells, and cockles now found no farther north than Baja California. After that, subtropical sea shells disappeared and the forests were dominantly such familiar temperate trees as oak, alder, and sycamore. With new radiometric dates and mapping of key volcanic ash beds, this record of extinction and climate change is carefully recalibrated. It was not an abrupt impact-induced extinction like that which terminated the dinosaurs, but rather a long-term vegetation and climate change. The timing of this paleoclimatic transition is not quite right for known changes in ocean currents and mountain uplift, and is more likely due to atmospheric carbon dioxide sequestration by newly evolving grasslands.

Cenozoic landscape evolution of the Convoy Range to Mackay Glacier area, Transantarctic Mountains: Onshore to offshore synthesis
David Sugden, Institute of Geography, School of GeoSciences, University of Edinburgh, Edinburgh, EH8 9XP, UK, and George Denton, Department of Earth Sciences and Climate Change Institute, Edward T. Bryand Global Sciences Center, University of Maine, Orono, Maine 04469, USA. Pages 840–857.
Keywords: Transantarctic Mountains, landscape evolution, East Antarctic Ice Sheet, paleoclimate.

There have been different views about the evolution of the Transantarctic Mountains and their role in the evolution of the Antarctic Ice Sheet. Some have pointed to a dynamic scenario with major ice sheet fluctuations, tectonic activity, and landscape dissection since the late Miocene and Pliocene. Others have favored long term stability since the mid Miocene. This paper presents geomorphological evidence of landscape evolution from the area of the Transantarctic Mountains immediately inboard of the site of the Cape Roberts drilling program. It confirms the view that the landscape is essentially inherited from the Miocene and that processes over the last 13.6 million years have achieved little change. A wider synthesis of the 260-km-long McMurdo sector of the Transantarctic Mountains and offshore coring and seismic results identifies an Eocene phase of denudation following rifting, a pulse of denudation, most rapid at 34–31 million years ago, an overriding ice sheet at 14.8–13.6 million years ago, and a hyper-arid climate from then to the present. Contrasts in morphology between different blocks in the Transantarctic Mountains reflect the varying location of the initial drainage divide following rifting, rather than subsequent tectonic activity.

Structural geology of the proposed site area for a high-level radioactive waste repository, Yucca Mountain, Nevada
Christopher J. Potter, U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225-0046, USA, et al. Pages 858–879.
Keywords: structural geology, Yucca Mountain, Nevada, faults, joints.

This paper summarizes the deformational history of volcanic rocks at Yucca Mountain, Nevada, where the U.S. Department of Energy has proposed a site for permanent underground storage of the United States' high-level radioactive waste. The proposed repository footprint, in north-central Yucca Mountain, includes the least-deformed part of the mountain. Geologic mapping of the site has demonstrated a hierarchy of fractures (faults and joints) that developed mainly between 14 and 11 million years ago. Block-bounding faults, the largest structures, remained sporadically active to within one million years before the present; these faults divide the mountain into numerous 1–4-km-wide panels of gently east-tilted volcanic strata. Intrablock faults, such as those present in the proposed repository volume, are commonly short and discontinuous. The frequency and characteristics of joints (planar breaks along which no displacement has occurred) vary greatly within the volcanic pile. These structural analyses contribute to the understanding of several important issues at Yucca Mountain, including seismic hazards and potential pathways for groundwater.

Modeling deformation and salt tectonics in the eastern Mediterranean Ridge accretionary wedge
E. Costa, Dipartimento di Scienze della Terra, Parma, Italy, et al. Pages 880–894.
Keywords: Eastern Mediterranean, Mediterranean Ridge, Messinian evaporites, accretionary wedge, physical modeling, salt tectonics.

Africa and Europe have been approaching each other at an estimated speed of a few centimeters per year since about 40 Ma. In doing this, they have compressed the intervening rocks resting on the Africa Ocean floor, which, in turn, has been subducting below the European plate.

The compressed rocks form the Mediterranean Ridge. These rocks are also composed of a thick (about 3000 m) horizon of Messinian evaporites that were deposited between 5.96 and 5.33 million years ago, when the Mediterranean Sea almost desiccated due to its isolation from the Atlantic Ocean and peculiar climatic conditions.

We reproduced the formation of the eastern Mediterranean Ridge by means of physical models built up with silicone gel and overlying sand that reproduced rock salt and rocks lying above, respectively. These materials have the same rheology as the natural rocks they reproduce, but reduced strength, scaled down by the same scaling factor used for the other parameters.

Our models demonstrated that the whole Messinian horizon flowed as a viscous fluid over long deformation times, thus demonstrating it to be composed mainly of salt and minor anhydrite. The rock salt deformed very easily, whereas the other overlying rocks were harder to deform, mainly by faulting; therefore, the eastern Mediterranean Ridge shows a very peculiar deformation path evolving at a deforming rate faster than any other non-salty geodynamic setting. Our models also highlighted the fact that deformation affecting salt basins shows very peculiar geometry and kinematics.

Geomorphic constraints on surface uplift, exhumation, and plateau growth in the Red River region, Yunnan Province, China
L.M. Schoenbohm, Massachusetts Institute of Technology, Dept. of Earth, Atmospheric and Planetary Sciences, Cambridge, Massachusetts 02139, USA, et al. Pages 895–909.
Keywords: Asia, landscape evolution, Red River fault, Ailao Shan shear zone, erosion surface, fluvial geomorphology, incision.

In this paper, we examine the landscape morphology of the Red River region in southwestern China, identifying an upland, low-relief landscape continuous with a regional landscape, which was elevated through growth of the southeastern margin of the Tibetan plateau. The Red River, along with the other major rivers of the southeastern plateau margin, has deeply incised the upland landscape. We examine the morphology of 97 tributaries to the Red River and use these to reconstruct the paleo-drainage system developed on the upland landscape. Our analysis reveals that river incision, and consequently surface uplift, is ~1500 m near the Chinese-Vietnamese border, and river incision proceeded in two phases. Two-phase incision could reflect pulsed plateau uplift, but more likely reflects the interaction of regional climate change and plateau development. We also show that the right-lateral strike-slip Red River fault, which runs along the Red River, has a minor oblique extensional component along the northern part of the fault, with a maximum vertical displacement of ~750 m. This study provides important new constraints on plateau development and on the active faults of southwest China.

Coherent French Range blueschist: Subduction to exhumation in <2.5 m.y.?
M.G. Mihalynuk, British Columbia Geological Survey Branch, P.O. Box 9320, Stn Prov Govt, Victoria, British Columbia, V8W 9N3, Canada, et al. Pages 910–922.
Keywords: blueschist, Cache Creek, geochronology, structure, radiolarians, microfossils, obduction, exhumation, tectonics.

Squeeze a slippery pumpkin seed between your finger and thumb and what happens? It escapes--hopefully flying toward your intended target. By a similar process, coherent slabs of blueschist can escape a subduction zone. They are tectonically extruded from between buoyant crust entering the subduction zone (finger) and a continental backstop (thumb). Blueschists in the French Range of northwestern British Columbia (about 240 km east of Juneau, Alaska) were extruded so quickly that their velocity is difficult to accurately measure by contemporary geochronological techniques. White mica intergrown with the blueschist minerals is dated as 173.7 ± 0.8 million years old, and this is interpreted as the age of subduction zone metamorphism. The blueschist minerals were extruded southwestward onto a long-lived arc (Stikine) as it collided with another arc (Quesnel) that had been welded to the continental margin about 7 million years prior. Folds and faults thus formed were cut by intrusions at least as old as 172 million years. If the age data are correct, the journey from subduction zone to mountain belt took less than 2.5 million years, perhaps much less. This rapid rate might be attributable to subduction of an ocean plate beneath both segments of the conjoined Stikine-Quesnel arcs. As they rotated into parallelism and the intervening ocean crust was consumed beneath them, the ocean crust was bent until it finally ruptured, causing blueschist to be extruded and thrust southwestward, together with the oceanic slab, away from the continental backstop.

Side-valley tributary fans in high-energy river floodplain environments: Sediment sources and depositional processes, Navarro River basin, California
J.L. Florsheim, Geology Department and Center for Integrated Watershed Science and Management, University of California, Davis, Davis, California 95616, USA, et al. Pages 923–937.
Keywords: floodplains, rivers, tributary, fan, debris flow, Pacific Northwest.

This paper addresses the sediment sources and depositional processes that form floodplains in high-energy systems where both hillslope-floodplain and channel-floodplain interactions are fundamentally important. Side-valley tributary fans are an important component of the sediment assemblage that characterizes the floodplains of main axial valley rivers in high-energy mountain systems. There are two sources of sediment to the fan systems within the floodplain ecotone including the side-valley tributary and overbank flow from the main axial channel. The processes that supply this sediment includes (1) flows from the tributary when there is no overbank flow from the main channel as well as down-fan transfer of this sediment; (2) overbank flow from the axial valley river; and (3) mixing of sediment from both sources. Of particular interest is both the mixing and interlayering of sediment from these disparate sources within the floodplain environment.

True three-dimensional trishear: A kinematic model for strike-slip and oblique-slip deformation
Ernesto O. Cristallini, Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET–Argentina) and Laboratorio de Tectónica Andina, Departamento de Ciencias Geológicas, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina, et al. Pages 938–952.
Keywords: trishear, strike-slip, numerical model, fault-propagation deformation, oblique-slip, fractures.

This paper presents a new model for analyzing rock deformation in three dimensions, which can be used to predict the amount of deformation and orientation of possible fracture planes within fault zones.

Tectonic evolution of the northeastern Pamir: Constraints from the northern portion of the Cenozoic Kongur Shan extensional system, western China
Alexander C. Robinson, Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, 90095-1567, USA, et al. Pages 953–973.
Keywords: Pamir, tectonics, metamorphism, age determination, extension, crustal shortening.

Investigations along in the northeastern Pamir along the northern portion of the Kongur Shan extensional system have identified several tectonic events from the Early Mesozoic to the present. High-grade metasediments and plutonic bodies in the hanging wall of the active Kongur Shan extensional system record Triassic to Early Jurassic metamorphism synchronous with pluton emplacement, interpreted to be related to northward subduction of the Paleotethys beneath Asia. Early Cretaceous crustal thickening and shortening is recorded in the northern portion of the footwall of the extensional system where high-grade Late Cretaceous schists were emplaced over low-grade rocks along a northeast-directed thrust about 100–110 million years ago. A final period of crustal thickening occurred in the Late Miocene, with footwall gneisses of the Kongur Shan normal fault recording high-grade metamorphism about 9 million years ago. Crustal thickening was immediately followed by initiation of east-west extension about 7–8 million years ago. The Kongur Shan normal fault has accommodated about 34 km of east-west extension, with magnitudes decreasing toward the northern and southern ends of the extensional system.

Coral reef complexes at an atypical windward platform margin: Late Quaternary, southeast Florida
Barbara H. Lidz, U.S. Geological Survey, St. Petersburg, Florida 33701, USA. Pages 974–988.
Keywords: windward carbonate margin, coral reef complexes, evolution, Florida Keys, outlier reefs, paleotopography, progradation, Quaternary, sea levels, shelf-edge models.

Classic windward carbonate platform margins are steeply inclined. The windward margin of Florida is distinct. The shelf slopes gently seaward over several kilometers from the Florida Keys shoreline to about 40 ft below sea level. The shelf edge is ringed with 100-ft-high coral reefs on a 130-ft-deep upper-slope terrace. New models that provide new perspectives on regional evolution over the past 325,000 years have been developed by combining a newly compiled chronology of dated geologic events with surface and subsurface contours traced from high-resolution seismic profiles. The contours are correlated with intervals of coral growth during interglacial (high) sea levels and coral absence during glacial (low) sea levels. Addressing origin, timing, growth, and changes in shelf-edge surface landforms at four offshore sites, the models show uneven subsurface topography, upward and landward reef-complex buildups, and a previously unreported, geologically rapid seaward progradation of the shelf edge over the most recent 7,000 years. The models have significant implications for interpretation of ancient analogues. The terms backstepped reef-complex margin, backfilled prograded margin, and coalesced reef-complexes margin are proposed for sections exhibiting suitable signatures in the stratigraphic record.

Geologic events dated locally and from around the world indicate that marine transgressions flooded the Florida shelf 13 times during the past 325,000 years. Seven, including the present transgression, produced coral reefs. Two primary architectures built the shelf and margin: coral ridges and sediment-filled swales developed discontinuously on the outer shelf, and coral reefs and backreef troughs developed discontinuously at the margin. The shelf was dry land during a prolonged period of lowered sea level from about 77,800 to 9,600 years ago. New perspectives put forth in this paper are applicable to studies of other reef-rimmed carbonate platforms, whether windward or leeward, modern or ancient.

U-Pb zircon and geochemical evidence for bimodal mid-Paleozoic magmatism and syngenetic base-metal mineralization in the Yukon-Tanana terrane, Alaska
Cynthia Dusel-Bacon, U.S. Geological Survey, Mineral Resources, Menlo Park, California 94025, USA, et al. Pages 989–1015.
Keywords: U-Pb zircon, geochemistry, Alaska, mid-Paleozoic magmatism, Cordilleran tectonics, Cordilleran VMS deposits.

New ion microprobe U-Pb zircon ages and whole-rock trace-element geochemical data for mafic and felsic metaigneous rocks of the Yukon-Tanana terrane in east-central Alaska help define the tectonic setting of mid-Paleozoic magmatism and volcanic-associated zinc-lead-silver mineral deposits in the Northern Cordillera. U-Pb ages for felsic rocks in the Bonnifield mining district in the Alaska Range, and both felsic and mafic rocks in the nearby Yukon-Tanana Upland, indicate that bimodal (felsic and mafic) magmatism occurred in late Devonian to early Mississippian time (378–353 million years ago). Trace-element contents of mafic metaigneous rocks in both areas, and of felsic metavolcanic rocks that host the largest volcanic-associated mineral deposits of the Bonnifield district, indicate that the magmas were generated in an extensional tectonic setting. Most other felsic metaigneous rocks from the Alaska Range and the Yukon-Tanana Upland have geochemical signatures that are similar to those of both average upper-continental crust and continental-margin arc rocks generated within continental crust. Our new age and geochemical data, together with the absence of intermediate-composition magmatic rocks, generally found in most arcs, and the widespread occurrence of interlayered carbon-rich sediments indicative of deposition within a restricted marine basin or submerged continental margin, strongly suggest that prolonged late Devonian to early Mississippian magmatism in east-central Alaska resulted from attenuation of the ancient continental margin of western North America, rather than development of an arc, as proposed by many others.

Geologic evolution of the Xolapa Complex, southern México: Evidence from U-Pb zircon geochronology
Mihai N. Ducea, University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA, et al. Pages 1016–1025.
Keywords: Xolapa Complex, arc magmatism, U-Pb zircon, geochronology, deformation.

Rocks exposed along the coastal mountain range of Pacific southern México make up the enigmatic Xolapa Complex, a sequence of intrusive and metamorphic rocks whose origin is highly debated. We studied the Xolapa Complex along three transects: inland from Acapulco, Puerto Escondido, and Puerto Angel. The main purpose of this study was to determine the age of the Xolapa Complex, using zircon U-Pb geochronology. Analyses were performed using a recently developed technique--laser-ablation multicollector inductively coupled mass spectrometry--which allows us to investigate the age of single zircon grains, or fractions of a grain, and thus helps in unraveling complex histories of crystal growth. We show that the Xolapa Complex developed as a series of magmatic arcs along the Pacific margin of Central America and that magmatism was highly episodic, with peaks during the Jurassic and Eocene-Oligocene. We also show that the Xolapa Complex appears not to be a far traveled sequence of rocks, as has been previously suggested.

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