GSA release 02-42: Oct. Geology & GSA Today media highlights

September 30, 2002

Boulder, Colo.--The Geological Society of America's October issue of GEOLOGY contains a number of potentially newsworthy items. Topics include: new insights into the Permian-Triassic extinction from study of unusual Antarctic and Australian soils; another challenge to the Snowball Earth hypothesis based on new research in northern Oman; and new direct evidence for oceanic detachment faulting along the Mid-Atlantic ridge.

Highlights are provided below. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in stories published. Contact Ann Cairns at acairns@geosociety.org for copies of articles and for additional information or other assistance.

GEOLOGY

Low oxygen levels in earliest Triassic soils.
Nathan D. Sheldon and Gregory J. Retallack, Department of Geological Sciences, 1272, University of Oregon, Eugene, Oregon 97403, USA. Pages 919-922.
The Permian-Triassic extinction that preceded the ascent of the dinosaurs is the greatest extinction in the history of life. Observation and modeling of unusual Antarctic and Australian soils from that time suggest that methane released into the atmosphere from seafloor ices (clathrates) or permafrost drove high-latitude soil CO2 to near-tropical levels and soil oxygen levels to severe dysoxia. These twin conditions would have killed most or all non-microbial plant life, perhaps leading to greatly increased runoff onto the shallow continental shelves, exacerbating low oxygen levels there and contributing to the catastrophic loss of life both on land and in the ocean as terrestrial ecosystems collapsed.

Neoproterozoic snowball Earth under scrutiny: Evidence from the Fiq glaciation of Oman.
Philip A. Allen, Geological Institute, ETH-Zentrum, Sonneggstrasse 5, CH-8092 Zürich, Switzerland; et al. Pages 891-894.
Recent research in Oman suggests that some of the massive glaciations that affected Earth in the terminal phase of Precambrian time were characterized by high frequency cycles of glacial advance and retreat. Seven such cycles are preserved in the 1.5 km-thick Fiq unit of northern Oman. This throws into question the idea that all Neoproterozoic glaciations were global events involving several millions of years of shut-down of the water cycle.

Gas hydrate and mud volcanoes on the southwest African continental margin off South Africa.
Zvi Ben-Avraham, George Smith, Moshe Reshef, and Eric Jungslager, pages 927-930.
This paper describes the occurrence of gas hydrate and mud volcanoes on the continental margin off southwest Africa. It is the first time such features are reported south of the Walvis Ridge. The distribution of gas hydrate and mud volcanoes in this area is controlled by active faulting.

Boninitic volcanism in the Oman ophiolite: Implications for thermal condition during transition from spreading ridge to arc.
Tsuyoshi Ishikawa, et al., Pages 899-902.
Volcanic rocks called "boninites" were discovered in the Oman ophiolite, the world's largest fragment of ancient oceanic lithosphere exposed on Earth's surface. These boninites represent melts formed by partial fusion of hot, hydrous shallow mantle (>1250 °C at <30 km depth), and are likely to have been produced through the initiation of subduction young, hot oceanic lithosphere (and obduction future oman ophiolite) near spreading ridge. this impacts long-standing issue concerning mechanism ophiolite formation.

Seafloor imagery from the BIG'95 debris flow, western Mediterranean.
M. Canals, GRC Geociències Marines, Universitat de Barcelona, Barcelona E-08028, Spain; et al. Pages 871-874.
Based on state-of-the-art seafloor imagery, this paper provides clues to understand the behavior of submarine debris flows. This topic is of great importance for geohazard assessment, both for the oil and offshore industries, and for coastal communities that could be affected by potentially destructive waves generated by rapid sediment destabilization. The data image is the product of one of the youngest (ca. 11,500 calendar years B.P.) major mass-wasting events in the northwestern Mediterranean Sea: the 26 km3 BIG'95 debris flow deposit that covers 2,000 km2 of the geodynamically quiet Ebro continental slope and base-of-slope off Spain. Along the path of the debris flow, the following sections are distinguished: source area, proximal depositional area, blocky intermediate depositional area, and distal depositional area, each with different backscattering patterns. Low-backscatter patches represent large sediment blocks that moved while keeping their internal coherence, while high-backscatter alignments restricted to topographic lows, represent coarse sediment pathways separating the blocks and reaching the distalmost section. These results illustrate how coarse-grained deep sea depositional bodies can form and what the resulting geometry may be, thus also constituting an analogue to hydrocarbon reservoirs in similar settings.

Direct geological evidence for oceanic detachment faulting: The Mid-Atlantic Ridge, 15°45 N.
C. J. MacLeod, Department of Earth Sciences, Cardiff University, Cardiff CF10 3YE, UK; et al. Pages 879-882.
In the mid 1990s scientists carrying out sonar surveys of the Mid-Atlantic Ridge axis discovered spectacular corrugations covering hundreds of square kilometers on the upper surfaces of flat-topped highs near the ridge axis. The striations were exactly parallel to the spreading direction and occurred over a wide range of scales. An interpretation that quickly became popular was that these structures are striations and grooves on the surfaces of immense near-horizontal extensional faults (detachments) that cut through the entire lithosphere. Such detachments could potentially accommodate separation of the plates in the absence of any magma supply from the mantle over timescales in excess of a million years, and provide a mechanism by which deep levels of the crust and the shallow mantle may be exposed on the seafloor. Until now, attempts to verify the detachment fault hypothesis for the corrugated massifs have been dogged by the inability to access them directly: submersible dives have shown that their surfaces are largely covered by sediment or basaltic rubble. Using a recently developed seabed rock corer, however, MacLeod et al. successfully drilled 63 sites on a corrugated massif near the Fifteen-Twenty fracture zone. They discovered heavily deformed rocks with sub-horizontal shear fabrics on the flat striated surface but not beneath it, confirming the detachment model and demonstrating that deformation-and hence motion between Africa and America-was concentrated onto a single fault plane less than 100 m thick for a period of more than a million years. They show that, contrary to previous expectation, the fault rocks are all composed of hydrous secondary minerals such as talc, formed at very low temperatures (300°C or less), with no record of any earlier, high-temperature deformation. These minerals, which formed as seawater percolated along the fault beneath the ridge axis, are extremely weak and slippery, thus lubricating the fault zone and localizing deformation onto a single detachment surface.

Impulsive alluviation during early Holocene strengthened monsoons, central Nepal Himalaya.
Beth Pratt, Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA; et al. Pages 911-914. Rain can lead to increased erosion as well as to pulses of river deposition. Despite the fact that ~9000 to 6000 yr B.P. Asian monsoon precipitation was higher than it is today, we infer a period of alluvial deposition in the central Himalayas. Exposure ages of bedrock and the composition of river deposits indicate that this region underwent a cycle of river deposition and reincision in response to the elevated water discharge. It appears that the additional water destabilized hill slopes, flushing them of easily eroded material, and temporarily overwhelming the river's carrying capacity. This finding highlights the complex response of the Earth system to climate change.

Sr isotope tracing of aquifer interactions in an area of accelerating coal-bed methane production, Powder River Basin, Wyoming.
C.D. Frost, B.N. Pearson, K.M. Ogle, E.L. Heffern, and R.M. Lyman, pages 923-926.
The Powder River Basin of northeastern Wyoming provides a third of the nation's supply of coal, and it also hosts a significant coalbed-methane resource. Coalbed-methane production involves withdrawal of groundwater in order to depressurize coal seams and desorb methane. It is important to understand the degree of hydraulic connection between coal seams and adjacent aquifers, because leakage has environmental consequences, and also can make it difficult to adequately depressurize coal and recover gas. In this study we evaluate the Sr isotopic ratio of groundwater as a potential fingerprint of waters from coal and silicate aquifers and a monitor of aquifer interactions. Our results show that waters from coal and sandstone aquifers located more than ~5 km from the basin margins have distinctive Sr isotopic compositions. Moreover, we are able to demonstrate that the Sr isotopic ratio is useful in identifying wells that contain mixed waters, whether due to well construction or to incomplete aquifer isolation. Continued, periodic measurement of the Sr isotopic ratio in groundwaters of the Powder River Basin should be helpful in monitoring changes in groundwater hydrology related to coalbed-methane activity.

Active detachment of Taiwan illuminated by small earthquakes and its control of first-order topography. Sara Carena, John Suppe, and Honn Kao, pages 935-938. Using novel 3D computer techniques plus 110,000 very small earthquakes that "illuminate" faults, we mapped the previously predicted master sliding surface under the active Taiwan mountain belt and into the mantle to depths of 30-60 km. The shape of this master fault surface in 3D appears to control the first-order topography of the mountain belt, in particular the position of the regional drainage divide.

GSA TODAY

Proterozoic Prism Arrests Suspect Terranes: insights into the ancient Cordilleran margin from seismic reflection data.
Dave Snyder, Geological Survey of Canada, 615 Booth Street, Ottawa, Ontario K1A 0E9, Canada; et al.
The Canadian Cordillera is one of the principal regions from which the hypothesis of exotic, accreted, or suspect terranes was developed a few decades ago. This hypoythesis states that North America grew by addition of microplates along the Cordilleran (western) margin in the Mesozoic Era. In a dramatic modification to this hypothesis, new field mapping and seismic reflection profiling reveal a vast volume of Proterozoic strata of largely North American affinity along the margin, leaving little room for the suspect terranes. The Proterozoic strata, deposited in at least three distinct periods between 1.85 and 0.54 billion years ago, form a reflective tectono-depositional prism or wedge that has a volume greater than a million cubic kilometers, extends over 1000 kilometers in length, and makes up most of the crust of the Cordillera. The suspect terranes apparently grounded upon and were arrested by this metamorphosed sedimentary prism during a complex interplay of thrusting and strike-slip displacements 190 to 170 million years ago. Thus the Cordilleran suspect terranes, excepting Stikinia, have shallow roots only a few kilometers deep with no deep crust or mantle attached.
-end-
To review the abstracts for these articles, go to www.gsajournals.org. To obtain a complimentary copy of any GEOLOGY article, contact Ann Cairns at acairns@geosociety.org. To review the complete table of contents for the October issue of GEOLOGY, go to http://www.gsajournals.org/gsaonline/?request=get-toc&issn=0091-7613&volume=030&issue=10

Geological Society of America

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