February 23, 2001 GSA Release No. 01-03

March Media Highlights:
Geology and GSA Today

EFFECTIVE IMMEDIATELY, MONTHLY MEDIA HIGHLIGHTS AND THE ARTICLES TO WHICH THEY REFER ARE NOT UNDER GSA EMBARGO.

Following are highlights from the March issue of GEOLOGY and a summary of the science article from the March issue of GSA TODAY, published by the Geological Society of America. Please discuss articles of interest with the authors before publishing stories on their work, and make reference to GEOLOGY or GSA TODAY in stories published. Contact Ann Cairns at GSA to request advance copies of articles and for additional information or assistance.

GEOLOGY

Slumping and a sandbar deposit at the Cretaceous-Tertiary boundary in the El Tecolote section (northeastern Mexico): An impact-induced sediment gravity flow
Ana R. Soria et al.

This article analyses landslides (slumps) affecting, and produced during, the sedimentation of the Cretaceous-Tertiary (K/T) boundary deposit and the sedimentary processes of this deposit in a new section of northeastern Mexico (the El Tecolote section). Both the synsedimentary slumps (first slumps discovered on land) and the sedimentary processes reflect destabilization and collapse of the continental margin. The authors document these processes as important and extensive evidence for the bolide impact effects of the Chicxulub event, which has been shown to be responsible for the K/T mass extinction.

Role of Panama uplift on oceanic freshwater balance
Gerald H. Haug et al.

One major element in today's climate system is a conveyor-like ocean circulation that delivers heat and moisture to the northern Atlantic. The motor of the conveyor is the salt difference between the major ocean basin, with higher salt contents in the Atlantic. Our data suggest that the modern Atlantic-Pacific salinity contrast of about 1 per mil became fully established at 4.2 Ma. This is interpreted as the result of restricted surface-water exchange between the tropical Atlantic and Pacific in response to the shoaling of the Central American Seaway. As a consequence, the Atlantic and Pacific surface ocean circulation regime changed, as did the freshwater balance between the major ocean basins. Simultaneous shifts in benthic carbon isotope records in the Caribbean Sea suggest an intensification in North Atlantic thermohaline circulation. These results suggest that the formation of the Isthmus of Panama caused several new ocean-atmosphere feedback mechanisms that have affected climate since the early Pliocene.

Probabilistic assessment of volcanic hazard to radioactive waste repositories in Japan: Intersection by a dike from a nearby composite volcano
Frank V. Perry et al.

The authors have attempted to assess the stability of potential radioactive waste sites. They have focused on sites near active volcanism, because these are under consideration in Japan for the burial of such waste. Obviously barring burying radioactive waste on the flanks of a volcano, the authors address the stability of sites in volcanic or near-volcanic settings over long periods (e.g., 100,000 years). To do this they simulated such a setting by using radial dike arrays located in Colorado. Dikes would obviously compromise the stability of a burial site, and so the goal would be to bury radioactive waste where dikes would not form. They analyzed dikes in terms of their proximity to volcanos, and determined that the mineralogy of the dike, as well as position relative to the volcano, exerts a considerable and predictable (quantifiable) influence on the probability of a dike intersecting a potential burial site.

First evidence for Archean continental crust in northern Vietnam and its implications for crustal and tectonic evolution in Southeast Asia
Chin-Ying Lan et al.

The South China continent is always believed to be a "young" continent. Young means the crustal age of South China is younger than 2.5 billion years. This paper presents the first convincing evidence for the existence of rocks older than 2.5 billion years on South China. It's not that young after all. This finding will force scientists to re-evaluate the geologic history and tectonic evolution of this region.

Metamorphic diamonds: Mechanism of growth and inclusion of oxides
L.F. Dobrzhinetskaya et al.

Using a variety of analytical tools, researchers from the University of California at Riverside and Los Alamos National Laboratory report that unusual metamorphic microdiamonds from Kazakhstan, ranging in size from 1 to 100 microns, have imperfect, skeletal crystalline forms. Rather than the perfect forms common in larger diamonds, these diamonds are often like a "house of cards" or like a form of gypsum called "desert rose." Within the diamonds is a wide variety of even more tiny oxide inclusions that suggest the water-rich fluid from which they grew carried impurities from Earth's mantle in addition to the metamorphic sedimentary host. The imperfect morphologies are all consistent with a simple model implying growth from a supersaturated fluid. The microdiamonds were born during continental collision, 530 million years ago, when sediments from the Earth's surface were forced down a subduction zone to depths of more than 120 km (where diamond is stable) and then rose back to the surface over tens of millions of years.

Cretaceous demise of the Moa plate and strike-slip motion at the Gondwana margin
Rupert Sutherland and Chris Hollis

The authors analyzed fossils from the Deep Sea Drilling Site north of New Zealand and found that the seafloor there formed about 130 to 145 million years ago. This makes it the oldest known seafloor in the South Pacific, and requires that a previously unrecognized oceanic spreading ridge was active in Cretaceous time. Although the site is currently at a latitude of about 30 degrees S, the fossils and magnetic data show that it formed at high latitudes, adjacent to the Gondwana supercontinent. Comparison with data from Australia shows that considerable strike-slip motion, parallel to the margin of the supercontinent, occurred during Cretaceous time. This is the first time that such motion has been reliably identified and quantified.

A recipe for microcontinent formation
R. Dietmar Müller et al.

Accreted continental terranes are ubiquitous in the geological record. Yet the processes that lead to microcontinent formation are poorly understood. Based on a global analysis of geophysical data from the ocean floor, the authors suggest that microcontinents may form by "jumps" of mid-ocean ridges towards volcanic hotspots. Hotspot-triggered microcontinent formation may have been an important mechanism since the Proterozoic for creating continental terranes, which were later accreted. Microcontinents grow in volume due to volcanism and sedimentation, contributing to crustal growth once they become accreted after onset of subduction and ocean basin closure.

GSA TODAY

Central Andean ore deposits linked to evolving shallow subduction systems and thickening crust
Suzanne Mahlburg Kay and Constantino Mpodozis

This paper summarizes the complex and intimate interactions among the diverse processes of arc magmatism, flat-slab subduction, and mineralization. It presents an important new model for formation of some of the largest copper and gold deposits on Earth, those that are associated with Miocene magmatic arc volcanism in the central Andes of South America. The model suggests that these major ore deposits formed above shallowly dipping segments of the active subduction zone that carries the Nazca plate under the South American continent. Importantly, the deposits developed during Miocene times, when the dip of the subducting slab was either shallowing or steepening. The model suggests that a key ingredient for magmatism and ore formation is release of fluids linked to hydration of the mantle and lower crust above a progressively shallower and cooler subducting oceanic slab. Another key is stress from South American-Nazca plate convergence that results in crustal thickening and shortening. Fluids for mineralization are released as the crust thickens and hydrous fluids are released from amphibole-bearing mineral assemblages. The model is also applicable to major ore deposits that formed in the western United States, when similar, flat-slab subduction systems existed in the early Tertiary.

*To view abstracts and the complete table of contents of GEOLOGY, as well as that of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN, see www.gsajournals.org. To obtain full text of these articles and articles from back issues, contact Ann Cairns.

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