December Geology and GSA Today highlights

November 27, 2000

Following are highlights from the December issue of GEOLOGY and a summary of the science article from the December issue of GSA TODAY, published by the Geological Society of America. Stories written regarding these articles are embargoed until December 1. We ask that you discuss articles of interest with the authors before publishing stories on their work, and that reference be made to GEOLOGY or GSA TODAY in stories published. Please contact Ann Cairns at GSA to request advance copies of articles and for additional information or assistance.

GEOLOGY

Geochemical evidence for an impact origin for a Late Archean spherule layer, Transvaal Supergroup, South Africa.
Bruce M. Simonson et al.

Thin layers rich in sand-size spherules of former silicate melt have been identified in five different sedimentary formations in Western Australia and South Africa. The layers range in age from roughly 2.63 to 2.49 billion years old and have all been interpreted as reworked ejecta from impacts by large extraterrestrial bodies such as asteroids or comets. This was originally based on geologic comparisons with younger layers of impact ejecta, especially the Cretaceous-Tertiary boundary layer, but an impact origin for two of the four spherule layers found in Australia is supported by anomalously high concentrations of iridium and other platinum group elements (PGEs) (reported in Simonson et al., Geology, 1998, v. 26, p. 195-198). New analyses of samples from the one late Archean layer found to date in South Africa indicate that it, too, is enriched in iridium and other siderophile elements relative to associated tuffs, carbonates, and shales. Moreover, the relative abundance of the PGEs in the spherule layer is very similar to those in chondritic meteorites. This suggests up to about 1% extraterrestrial material is present in the South African spherule layer and provides support for an impact origin.

Brittle strain regime transition in the Afar depression: Implications for fault growth and seafloor spreading.
Anupma Gupta and Christopher H. Scholz

There aren't many places in the world where you can watch an ocean basin form, which is why East Africa has captured the attention of many geologists. Using new high-resolution satellite maps of topography, the authors studied a population of faults in the Afar depression of Ethiopia and Djibouti. These faults are especially interesting to study because different regions in this setting are at different stages in the transition from continent to ocean basin. The authors found that certain fault dimensions could be linked to the total amount of stretching, which is controlled by tectonic forces. The way these related variables change is an important constraint in modeling the physics of faulting and earthquakes.

Mass failure of the North Atlantic margin triggered by the Cretaceous-Paleogene bolide impact.
Richard D. Norris et al.

The impact of a 10-km-diameter bolide at the end of the Cretaceous (65 Ma) not only precipitated one of the five largest mass extinctions of all time, but also generated a Richter Scale 10-13 earthquake. We find evidence from deep sea drilling that much of the eastern seaboard of North America (from the Bahamas to Nova Scotia) catastrophically failed and generated vast flows of debris into the deep sea as a result of the quake. The resulting mass flows reached as much as 1200 km offshore and accumulated in thickness as much as 25 m. The mass flows cover an area up to 3.9 million square kilometers, making them easily the most widespread mass failure deposits on Earth.

Regional warming: Pliocene (3 Ma) paleoclimate of Europe and the Mediterranean.
A.M. Haywood et al.

This paper examines the past climatic character of the mid-Pliocene warm period over the European and Mediterranean regions. The robustness of climate simulations, derived from an advanced numerical climate model, is evaluated by reference to available geological evidence. Europe and the Mediterranean ca. 3 Ma can be described as both seasonally and annually warmer and wetter than present. The cause for these changes relate to reduced land and sea ice cover in the Northern Hemisphere, which in turn affected regional atmospheric pressure systems and wind strengths. In particular, increasing wind strengths and surface wind stress over the North Atlantic Ocean facilitated greater oceanic heat transport by producing a more vigorous flow of heat transporting surface ocean currents such as the Gulf Stream. If future climate warming reduces Northern Hemisphere land and sea ice cover, Europe and the Mediterranean may experience a climatic scenario akin to what occurred 3 million years ago.

Estimation of fault propagation distance from fold shape: Implications for earthquake hazard assessment.
Richard W. Allmendinger and John H. Shaw

As an unexposed ('blind") thrust fault propagates, folding of the layers of rock is accompanied by earthquakes. The shape of these folds may record how far the fault tip has propagated relative to the amount of displacement of the blocks on either side of the fault. This information can tell us where in the Earth a fault nucleated. By applying this approach to the Puente Hills thrust-Santa Fe springs anticline in the Los Angeles basin, the authors show that the fault nucleated in the same part of the Earth as the subsequent Whittier Narrows earthquake.

Dead delta's former productivity: Two trillion shells at the mouth of the Colorado River.
Michal Kowalewski et al.

Biological productivity of the Colorado River delta is about 5% of what it was before the mighty Colorado's water was diverted for human uses. Since the 1930's there's been a drastic reduction in the number of shellfish that inhabit the river's estuary because dams, diversions, and irrigation projects have reduced the flow of nutrient-laden fresh water to the Colorado Delta in Mexico. The shellfish formed a vital part of the food chain in the area and were likely an important food supply for migratory waterfowl. Islands composed entirely of gleaming white clam shells line the lower reaches of the Colorado Delta. Satellite images and field data indicate that at least two trillion clam shells make up the area's beaches and islands. Virtually all these shells are less than 1000 years old, according to radiocarbon and amino acid dating. An average of about 6 billion clams inhabited the delta before the river's natural flow was interrupted. Clam densities formerly reached 50 specimens per square meter (5 per square foot). Now, recent surveys reveal far fewer, only 3 per square meter (0.3 per square foot). Although a few highprecipitation years since 1981 have resulted in occasional flow to the delta, the findings suggest that these rare events have not restored the ecosystem to its original productivity. The methods pioneered in this study can be used to estimate the prehistoric productivity of coastal ecosystems in other parts of the world. Such estimates will be especially valuable in areas where no biological surveys were made before humans modified the habitat.

GSA TODAY

Beneath Yellowstone: Evaluating plume and non-plume models using teleseismic images of the upper mantle.
Gene Humphreys et al.

This paper provides a synthesis of new geophysical data on the state of Earth's mantle at depths down to 700 km beneath Yellowstone; one of the most complete images, anywhere on Earth, of the nature of the mantle beneath a continental hotspot. This data set provides an opportunity to understand the mantle processes that give rise to the spectacular tectonic uplift, earthquakes, volcanism, and hydrothermal springs and geysers that characterize Yellowstone and other hotspots. The findings are surprising and controversial. Yellowstone has traditionally been interpreted to result from motion of the North American plate over a stationary mantle plume, where the plume represents a "jet" of hot mantle rising thousands of kilometers from near the core-mantle boundary. In contrast, the new teleseismic data from Yellowstone, combined with geologic data, can only be explained if there is appreciable upper mantle convection beneath Yellowstone. A plume may still be involved, but the data do not require it, and instead it may be possible to understand this hotspot entirely in terms of upper mantle flow, driven in part by subduction of the Juan de Fuca plate beneath North America.
-end-
*To view the complete table of contents of GEOLOGY, as well as that of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN, see http://www.geosociety.org/pubs/cattract.htm.

*Full text GEOLOGY articles and the science article from GSA TODAY are available on the first of each month on the GSA Web site, http://www.geosociety.org/pubs/journals.htm.

Contact: Ann Cairns
303-447-2020 ext. 156
acairns@geosociety.org

Geological Society of America

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