December 21, 2000 GSA Release No. 00-31

January Geology and GSA Today Media Highlights

Following are media highlights from the January issue of GEOLOGY and a summary of the science article from the January issue of GSA TODAY, published by the Geological Society of America. Stories written regarding these articles are embargoed until January 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

Palynologically calibrated vertebrate record from North Dakota consistent with abrupt dinosaur extinction at the Cretaceous-Tertiary boundary
Dean A. Pearson et al.

Dinosaur extinction has been a very controversial subject since the publication of the asteroid impact hypothesis in 1980. This paper presents detailed evidence of the occurrence of dinosaur, bird, mammal, fish, amphibian, lizard, pterosaur, and crocodilian fossils relative to the K-T boundary in North Dakota. Dinosaurs are common in the youngest localities that preserve vertebrate fossils. This evidence is compatible with the idea that the extinction of the dinosaurs was caused by an extraterrestrial impact.

Ocean stagnation and end-Permian anoxia
Roberta M. Hotinski et al.

This paper presents results of a computer modeling study to assess the possible role of ocean chemistry and circulation in the end-Permian mass extinction. The cause of this extinction remains a mystery, but ocean chemistry is a leading candidate. Several authors have proposed that warm climatic conditions inhibited ocean mixing prior to the extinction, creating deep ocean waters devoid of oxygen that had lethal effects when brought into contact with the shallow marine environment. We find that warmth causes simulated ocean circulation that is reduced relative to a Permian scenario with a near-modern temperature distribution, but that circulation is not sluggish or stagnant. Oxygen levels, on the other hand, are indeed driven to extremely low levels, but by a combination of lowered oxygen content of ventilating waters and proportionally higher export of organic matter from high-latitudes, rather than by a decrease in physical mixing. Our results suggest that high-latitude warmth in the Late Permian could have contributed to the creation of a lethal deep water reservoir, but that this unusual deep water chemistry did not require stagnation of ocean circulation.

Cratered cobbles in Triassic Bundsandstein conglomerates in northeastern Spain: An indicator of shock deformation in the vicinity of large impacts
Kord Ernston et al.

This study reports the discovery of a new indicator of shock-wave effects from large impacts on Earth. Quartzite cobbles in outcrops of Lower Triassic conglomerates from the northeastern Spain display unusual features, including circular craters with central mounds surrounded by radial fractures. The conglomerates are also marked by intense fracturing down to microscopic scale. These features have traditionally been attributed to tectonic compression and pressure dissolution at cobble contacts. Cross sections through the cratered cobbles, however, reveal pervasive internal fracturing, detached segments along concave spall fractures, and zones marked y quartz grains with planar deformation features indicative of shock metamorphism. The authors produced similar features in impact experiments performed on artificial conglomerates. This supports the idea that these features were produced by deformation and spallation related to shock-wave propagation through the inhomogeneous deposits. The conglomerates are found in the vicinity of two mid-Tertiary impact structures in Spain the Azuara structure and the proposed Rubielos de la Cérida structure and the authors propose that such shock features in conglomerates provide a new, easily recognizable regional impact signature.

Modulation and amplication of climatic changes in the Northern Hemisphere by the Indian summer monsoon during the past 80 k.y.
H.R. Kudrass et al.

The monsoon of southeast Asia strongly affects the climate of the Northern Hemisphere. Change in salinity in a 80,000 year-long record from sediments in the northern Bay of Bengal document rapid and great changes of the intensity of the summer monsoon that are coeval with climatic changes recorded in the Greenland ice cores. A conceptual model depicts the great influence the monsoonal cycles have by changing the atmospheric concentrations of water vapor.

GPS estimate of relative motion between the Caribbean and South American plates, and geologic implications for Trinidad and Venezuela
John C. Weber et al.

This new study uses satellite surveying technology, the GPS (Global Positioning System), to precisely measure the relative motion between the Caribbean and South American plates. Unlike the motion between Earth's other plates, which is generally well-resolved, Caribbean-South American plate motion is very poorly resolved in previous studies that use traditional geologic methods. The new GPS result shows that the Caribbean plate slides eastward at 20 mm/yr along northern South America. This motion is taken up in a jerky fashion along the east-trending El Pilar fault in Venezuela, which has a long record of generating moderate to large earthquakes. In neighboring Trinidad, about 14 mm/yr of Caribbean-South American plate motion is concentrated along the Central Range fault, which is highly oblique to the plate motion vector, and was historically, and is currently, aseismic. The Central Range fault is thus either creeping aseismically, or locked and accumulating elastic strain.

Pleistocene reduction of polar ice caps: Evidence from Cariaco Basin marine sediments
R.Z. Poore and H.J. Dowsett

Microfossil assemblages and stable isotope measurements in high-accumulation rate marine sediments from the Cariaco Basin, Venezuela, provide evidence for an extended interval of warm climate and associated high sea level about 400,000 years ago during Marine Isotope Stage (MIS) 11. The study supports controversial geologic evidence that polar ice caps have been smaller than today under the natural range of Pleistocene interglacial climates leading to sea level rise of as much as 20 m above modern sea level. Current sea level rise estimates of 13 to 94 cm over the next 100 years assume polar ice caps will remain essentially stable over century time scales. This study suggests that the current interglacial climate will continue into the future and that any anthropogenic climate warming will accelerate the natural process of ice cap decay resulting in increased rates of sea level rise.

Thermal ionization mass spectrometry U-series dating of a hominid site near Nanjing, China
Jian-xin Zhao et al.

High-precision and reliable dating of key hominid sites is crucial for a better understanding of human evolution, particularly for the resolution of a long-term debate between out-of-Africa and multiregional evolution schools regarding the origin of modern human species. To obtain a better constraint on the origin, evolution, and destination of Asian Homo erectus, we carried out mass spectrometric U-series dating of speleothems from Tangshan Cave, where Nanjing Man, a typical H. erectus morphologically correlated with Peking Man at Zhoukoudian, was discovered. Our results, combined with ecological and paleoclimatic evidence, indicates that Nanjing Man should be at least 580 thousand years old, or more likely lived during the glacial oxygen isotope stage 16 (~620 thousand years before present). Such an age estimate, which is ~270 thousand years older than previous electron-spin-resonance (ESR) and alpha-counting U-series dates, has significant implications for the evolution of Asian H. erectus. Tooth dentine and enamel samples from the coexisting fossil layer yield significantly younger apparent ages, with that of the enamel sample being only less than a quarter of the minimum age of Nanjing Man. This suggests that U uptake history is far more complex than existing models can handle. As a result, great care must be taken in the interpretation of ESR and U-series dates of fossil teeth.

Crustal structure and tectonics from the Los Angeles basin to the Mojave Desert, southern California
G.S. Fuis et al.

The Los Angeles region is underlain by a network of active faults, including many that are deep and do not break Earth's surface. These hidden faults include the previously unknown one responsible for the devastating January 1994 Northridge earthquake, the costliest quake in U.S. history. So that structures can be built or strengthened to withstand the quakes that are certain in the future, the Los Angeles Region Seismic Experiment (LARSE) is locating hidden earthquake hazards beneath the region to help scientists determine where the strongest shaking will occur. Images of the sedimentary basin beneath the San Gabriel Valley show that its depth reaches 3 miles, 50% more than earlier estimates. Because deeper sedimentary basins have greater shaking potential, earthquake hazards in the San Gabriel Valley need to be reevaluated. Another major finding is a strongly reflective zone located deep beneath the San Gabriel Mountains. This zone begins at about 12 miles depth near the vertical San Andreas Fault and rises in a ramp-like fashion southward toward the Los Angeles Basin. It appears to connect to the fault system responsible for the 1987 magnitude 5.9 Whittier Narrows earthquake, which occurred on a blind thrust fault. This reflective zone is interpreted as a "master" blind thrust fault that transfers stress and strain upward and southward to a network of faults in the San Gabriel Valley and Los Angeles Basin. The LARSE data obtained so far have provided answers that could not have been obtained in any other way. The knowledge gained will be used to make new and existing structures in the region better able to withstand earthquakes.

GSA TODAY

Erosion, Himalayan geodynamics, and the geomorphology of metamorphism
Peter K. Zeitler et al.

Using the Himalayan Mountains, this paper discusses the interaction of erosion with structural and petrological processes in the evolution of mountain belts. The Himalayas contain two unique corner zones, or syntaxes, where active metamorphic massifs are co-located with deep river gorges and some of the most extreme relief on Earth. Geologic, petrologic, and geophysical zonations around the massifs, together with the magnitude of erosional fluxes that occur in these regions, leads them to conclude that erosion plays an integral role in collisional dynamics. At multiple scales, erosion exerts an influence on par with such fundamental phenomena as crustal thickening and extensional collapse. Erosion can mediate the development and distribution of both deformation and metamorphic facies, accommodate crustal convergence, and locally instigate high-grade metamorphism and melting.

*To view the complete table of contents of Geology, as well as that of the Geological Society of America Bulletin, see the Coming Attractions page.

*Full text Geology articles and the science article from GSA Today are available on the first of each month at our Online Journals page.

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