July-August GSA BULLETIN media highlights

Boulder, Colo. – The July-August issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes several newsworthy geology articles. Topics include: the impact of Mount St. Helens' 1980 eruption on river flows in neighboring basins; and new evidence for continental-scale ice sheets in the Cretaceous greenhouse world; and new insights into the relationship between environmental change and arroyos, the dry, steep-sided gullies common in the western U.S. and similar landscapes.

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 and the Geological Society of America in stories published. Contact Ann Cairns for additional information or assistance.

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The submarine volcanic succession of the basal complex of Fuerteventura, Canary Islands: A model of submarine growth and emergence of tectonic volcanic islands
Margarita Gutiérrez, Estudios del Terreno S.L., C/ España no. 21, locales 13 y 14, 38390 Santa Úrsula, Santa Cruz de Tenerife, Canary Islands, Spain, et al. Pages 785–804.

Keywords: basal complex, submarine volcanism, emergence of volcanic islands, Fuerteventura, Canary Islands.

This paper analyses the mechanisms of growth of volcanic islands in the context of tectonically active oceanic intraplate settings. The case study is the island of Fuerteventura (Canary Archipelago, eastern-central Atlantic Ocean). The results suggest that the emergence of these islands is more a consequence of uplift and deformation of their basement, than mere piling-up of volcanic sequences on a stable oceanic floor.

A 2500-yr-long paleoseismologic record of large, infrequent earthquakes on the North Anatolian fault at Çukurçimen, Turkey
Ross D. Hartleb, Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740, USA, et al. Pages 823–840.

Keywords: North Anatolian fault, paleoseismology, active tectonics, earthquake occurrence, San Andreas fault.

Trenches excavated across the North Anatolian fault at the village of Çukurçimen in north-central Turkey have revealed a complete record of large earthquakes on the fault for the past 2500 years. During this time, there is evidence for only five surface-rupturing earthquakes, and that these earthquakes occurred at relatively regular intervals. These results suggest that the relatively regular recurrence of rare, large-magnitude earthquakes may be the expected mode of earthquake occurrences on mature strike-slip faults in settings where such faults are structurally isolated from other major seismic sources.

Tectonic evolution of the Himalayan thrust belt in western Nepal: Implications for channel flow models
Delores M. Robinson, Department of Geological Sciences, University of Alabama, Tuscaloosa, Alabama 35487 USA, et al. Pages 865–885.

Keywords: Himalaya, thrust belt, Nepal, regional mapping, geochronology.

The Himalayan Mountains contain pockets of unexplored territory in extremely remote regions. Far western Nepal is one of these remote regions that researchers find difficult to access because of the lack of roads, problematic food, travel logistics, and potential for conflict with the Maoist revolutionaries. However, this paper sheds some light on the geologic history of this understudied region of Nepal. By hiking endless foot trails and traversing across the mountains, this team of geologists from the University of Alabama, University of Arizona, and University of Houston mapped approximately 25,000 km2 from the western border with India eastward to the Karnali River. In this paper, the geologists publish a map of the region, three cross-sections that illustrate the deformation that this region experienced, data from the field and from the laboratory, and a model for how the region evolved from essentially a flat coastal area to the most mountainous region in the world. The end result of this field expedition is an understanding of how much deformation occurred during the collision of the India with Asia, which began approximately 55 million years ago and continues through to the present. Approximately 75% shortening in some of the rock units in far western Nepal occurred during the collision. For example, if the rock units were 100 km long before deformation, now the units would only be 25 km long. The rocks that compose the Himalayan Mountains are extremely deformed and have a long and protracted geologic history.

Sevier orogenesis and nonmarine basin filling: Implications of new stratigraphic correlations of Lower Cretaceous strata throughout Wyoming, USA
Michael J. Zaleha, Department of Geology, Wittenberg University, Springfield, Ohio 45501-0720, USA. Pages 886–896.

Keywords: Gannett, Cloverly, Lakota, Sevier orogeny, Lower Cretaceous.

Lower Cretaceous rocks throughout Wyoming (approximately 100-140 million years old) record deposition from rivers and lakes in a basin east of the ancestral Rocky Mountains. Detailed correlation and dating of these strata have been problematic for a century. This is unfortunate because these strata have been used to date movements on major faults in the ancient mountains, evaluate the sedimentary response in the adjacent depositional basin, reconstruct the basin sinking history, evaluate the driving mechanisms of that sinking, and have been applied to basin modeling studies. Additionally, some researchers have contended that a large basin did not exist during initial deposition of these strata, and that deposition was not coincident with major faulting during early development of the ancestral Rockies. This study clearly shows that these strata were deposited in a major basin associated with the early development of the ancestral Rockies. The strata reflects two periods of high sediment supply, progressive erosion of the mountains, and movement on major faults. These events suggest renewed or accelerated mountain-belt uplift. Each of these periods was followed by a period of reduced sediment supply that may have been caused by (1) a change to a drier climate, possibly related to progressive mountain-belt uplift and development or intensification of an orographic rain shadow, or (2) a decrease in mountain-belt uplift rate and a reduction in mountain-belt relief.

Evidence for a ridge subduction event in the Ordovician rocks of north-central Maine
Adam Schoonmaker, Department of Earth and Atmospheric Sciences, University at Albany, State University of New York, Albany, New York 12222, USA, and William S.F. Kidd. Pages 897–912.

Keywords: Bean Brook gabbro, Chain Lakes Massif, ridge subduction, basalt geochemistry, Taconic ocean, Dunnage zone.

Geochemical and field evidence of mafic basalts and gabbros intrude deformed mélange and meta-sedimentary rocks in north-central Maine, and indicate that a ridge subduction event occurred during the early Ordovician closure of the Appalachian ocean. This is one of a very few pre-Meszoic examples of ridge subduction thus far identified, and the only one from the northern Appalachians.

Deglacial and postglacial sedimentary architecture in a deeply incised paleovalley-paleofjord - The Pennsylvanian (late Carboniferous) Jejenes Formation, San Juan, Argentina
Mason Dykstra, Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA, et al. Pages 913–937.

Keywords: mass transport, deglaciation, fjord, climate change, Carboniferous, Argentina.

Deglacial successions preserve particularly high-resolution records of the process of climate warming through Earth's history. This paper details a deglacial succession of sedimentary rocks within an ancient fjord setting from the Late Carboniferous (ca. 310 million years ago) of western Argentina. Prior to the deglaciation, the fjord consisted of a glaciated mountain valley that fed down into a lake near the head of the valley. During climate warming, sea levels rose and flooded the valley, causing the glacial ice to rapidly float on the seawater, breaking up and melting the valley glacier system. This released large amounts of sediment previously trapped in the glacial ice into the valley, where it was deposited as a thick sandstone bed. The deposition of this bed combined with the rapidly rising sea levels then caused the failure of a number of the fjord-side slopes in large-scale underwater landslides. The rough fjord-bottom topography created by these landslides played a major part in controlling how subsequently deposited sediment was distributed throughout the fjord. This type of climatically-controlled glacial melting and sedimentation may be commonly seen along fjord-dominated coastlines if current global warming trends continue.

Peak flow responses to landscape disturbances caused by the cataclysmic 1980 eruption of Mount St. Helens, Washington
Jon J. Major, U.S. Geological Survey, Cascades Volcano Observatory, Vancouver, Washington 98683, USA, and Linda E. Mark. Pages 938–958.

Keywords: Mount St. Helens, volcano, eruption, streamflow, hydrology, landscape disturbance, hydrologic response, tephra, landslide, debris flow, watershed.

This paper describes a detailed analysis of how and for how long the great 1980 eruption of Mount St. Helens affected river flows in neighboring basins. That eruption caused an abrupt, widespread, and devastating landscape disturbance. Several basins around the mountain were variously disturbed by the largest landslide in recorded history, a sweeping volcanic blast that destroyed over 200 square miles of mature forest, large debris flows that reamed riparian corridors and deposited several feet of gravelly sand on valley floors, and ash fall that blanketed the landscape near the volcano with up to 20 or more inches of pumiceous silt, sand, and gravel. Despite such cataclysmic reconfiguration of the landscape, Major and Mark found the landscape to extraordinarily hydrologically resilient. They found that post-eruption storm-flow discharges typically had larger magnitudes and generally rose to peak more rapidly than did pre-eruption storm flows. However, they found that the hydrological responses of the disturbed basins were distinctly seasonal and short lived: the landscape disturbances affected chiefly autumn and winter storm flows for a period of about 5 years. Their findings also suggest that eruption-induced modifications to river channels, and not just to changes in hillslope hydrology, likely played a prominent and additional role that affected the hydrological response, a finding not previously reported in studies of other forms of landscape disturbance such as forest practices, wildfire, urbanization, and mining.

Headwater channel dynamics in semiarid rangelands, Colorado high plains, USA
Gregory E. Tucker, Cooperative Institute for Research in Environmental Sciences (CIRES) and Department of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA, et al. Pages 959–974.

Keywords: arroyos, climate, erosion, gullies, networks, streams.

Dry, steep-sided gullies (also called arroyos) are common erosional features in rangelands of the western United States and in many similar landscapes around the world. It is important to understand the degree to which these features reflect environmental change. This paper explores the origin and development of these features using data collected from two study areas in the Colorado high plains, USA. Repeat aerial photography covering over sixty years suggests that typical propagation rates range widely but can be as much as 1–2 meters per year on average. Recent case studies suggest that much of the erosion in this region is driven by summer thunderstorms, which can deliver very rapid rates of runoff over a small area. The dominant pattern of erosion is one in which a near-vertical channel head migrates up-valley, and is replaced by a gradually widening channel downstream. This pattern is shown to be consistent with observed flood hydrology and material properties. Analysis of the hydraulics and the resisting effects of vegetation suggest that the following conditions are both necessary and sufficient to explain these erosional forms: (1) a resistant vegetation layer (for example, grasses) growing on a cohesive but highly erodible soil or rock; (2) high flow variability (e.g., long dry periods punctuated by intense thunderstorms); and (3) a high volume fraction of fine-grained material in the soil or rock. We argue that, over time, repeated episodes of gullying by flash floods contribute significantly to shaping the gentle ridge-valley topography in which these floods occur. In particular, we interpret the concave-upward shape of the valleys in the study region as a trade-off between increasing flood frequency and decreasing flood effectiveness downstream.

Detection of Late Cretaceous eustatic signatures using quantitative biostratigraphy
James S. Crampton, Institute of Geological & Nuclear Sciences, P.O. Box 30-368, Lower Hutt, New Zealand, et al. Pages 975–990.

Keywords: biostratigraphy, correlation, eustasy, Late Cretaceous, New Zealand, quantitative analysis, sequence stratigraphy, unconformities.

Paleontology is a data-rich science--so rich, in fact, that most geological dating using fossils has traditionally been based on just a minor subset of the total information available. This practice limits the resolution that can be achieved in dating, a particular problem when testing contentious theories about the existence of continental-scale ice sheets in the Cretaceous period, a time of global greenhouse climate. To demonstrate the presence of such ice sheets, it is necessary to prove that sea-level fluctuations resulting from apparent changes in ice volume were global and synchronous. In this paper we use a newly-developed quantitative approach to integrate very large amounts of paleontological data and to date the geological evidence for sea-level changes in New Zealand. We show that some Cretaceous sea-level changes in New Zealand were apparently synchronous (with 92% probability) with changes in eastern United States, thereby supporting hypotheses of Cretaceous polar continental glaciation in the Cretaceous greenhouse world.

Oligocene to Holocene erosion and glacial history in Marie Byrd Land, West Antarctica, inferred from exhumation of the Dorrel Rock intrusive complex and from volcano morphologies
Sergio Rocchi, Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, I56126 Pisa, Italy, et al. Pages 991–1005.

Keywords: erosion anomalies, erosion rate, glacial erosion, volcanic morphology, West Antarctic Ice Sheet.

Climatic evolution of Antarctica during the last 50 million years and the history of the West Antarctic ice sheet provide clues to an understanding of recent and future evolution of Earth's climate. In Marie Byrd Land (West Antarctica), landscape evolution related to the erosion of volcanic cones and deep-seated igneous rocks has shed light on the timing of climate deterioration in West Antarctica. Two main periods of worsening conditions are indicated, one at ca. 27–29 million years ago, when it appears that the West Antarctic ice sheet first formed, and another at ca. 15 million years ago, when the climate became truly polar.

Kinematic models of fluvial terraces over active detachment folds: Constraints on the growth mechanism of the Kashi-Atushi fold system, Chinese Tian Shan
K.M. Scharer, Department of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403, USA, et al. Pages 1006–1021.

Keywords: strath terrace, active folding, intracontinental mountains, fault-related fold, landform evolution.

The relationship between differential uplift and shortening across a detachment fold is dependent on the mechanism of fold growth, but the evolution of detachment folding is difficult to discern from the final shape of a fold. We advance existing kinematic models of detachment folding by showing that fluvial terraces, successively emplaced across growing detachment folds, reveal the fold mechanism in their final geometries. The kinematic models show that distinct geometries are created when folds form by limb rotation and/or hinge migration during steady, aggrading, or incising conditions. Furthermore, the terrace geometries can be used to understand the temporal evolution of axial surfaces. Applying these models to a series of active detachment folds on the southern margin of the Chinese Tian Shan, we infer that the mechanism varies spatially and temporally; the folds are currently growing by a combination of limb rotation in the cores of the folds and hinge-zone migration at the flanks of the folds.

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