||October 31, 2001
GSA Release No. 01-44
BOULDER, Colo. - The Geological Society of America's November issue of GEOLOGY contains a number of newsworthy items. Highlights from GEOLOGY and a summary of the science article for the November GSA TODAY are provided below. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY or GSA TODAY in stories published. Contact
Ann Cairns for copies of articles
and for additional information or assistance.
- Marine Cretaceous-Tertiary boundary section in southwestern South Dakota
Dennis O. Terry, Department of Geology, Temple University, Philadelphia, Pennsylvania 19122, USA; et al. Pages 1055-1058.
- The focus of this research is a zone of highly contorted and disrupted sediments in southwestern South Dakota that the authors interpret as a result of the Chicxulub asteroid impact event 65 Ma, which occurred at the end of the age of the dinosaurs. The research is not concerned with the potential effects of the impact on dinosaur extinction, but is instead focused on the recognition of the products of the impact event in areas far away from the impact site (in this case, approximately 2000 miles away). This zone of intense sediment deformation, up to 5 m thick, is the product of the seismic energy generated by the impact event. The authors have also recovered impact ejecta from the top of this zone of disruption. Of additional interest are the implications of this discovery. In particular, the zone of disruption, which marks the K-T boundary, is within marine delta deposits and is covered by an additional 25 meters of marine sediments originally interpreted as Cretaceous in age. The recognition of the zone of disruption as the K-T boundary would then make these overlying marine sediments Tertiary in age, which means that the sea that used to cover the western interior of North America persisted for a longer period of time than presently accepted. This in turn has implications for reconstructing the paleogeography of North America during this period of time.
- Organic haze in Earth's early atmosphere: Source of low-13C Late Archean kerogens?
Alexander A. Pavlov, Department of Geosciences, Pennsylvania State University, University Park, Pennsylvania 16802, USA; et al. Pages 1003-1006.
- Earth's ancient atmosphere, which existed 2.3-3.5 Ga, was very different in chemical composition from the present one. High concentrations of greenhouse gases would have been required to offset the low luminosity of the young sun. Enhanced CO2 levels are probably at least part of the solution, but CH4 may have played a significant role as well, particularly during the Late Archean era, 2.5-3.0 Ga, when methanogenic bacteria were almost certainly present. The authors suggest that in a methane-rich Archean atmosphere CH4 molecules could have been subject to polymerization, producing hydrocarbon aerosols (organic haze) similar to those in Titan's present-day atmosphere. The predicted flux of organic aerosols is of the same order of magnitude as the modern organic carbon burial rate. This haze would also have a peculiar signature in carbon isotopes that could account for the unique carbon isotope signature in organic-rich rocks of that age. Hence, this model could account for both the relatively warm Archean climate and the unusual geochemical record in ancient sediments.
- A decadal record of underflows from a coastal river into the deep sea
Kenneth S. Johnson, Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039-9644, USA; et al. Pages 1019-1022.
- The authors report observation of underflows of the Salinas River down to nearly 1200 m depth in Monterey Bay, California. Buoyant, muddy river plumes on the sea surface are the last visible link as rivers enter the ocean. These surface plumes usually travel only a small distance offshore before they lose their sediment. As a result, they do not remove much sediment from the coastal zone. However, a subsurface plume may also form if a river contains enough sediment to become denser than seawater. These river underflows contain both river water and large amounts of sediment. They have seldomly been observed directly in the ocean and are known primarily from estuaries and lakes, where the amount of sediment needed to make the river water plunge under the surface is much smaller than when rivers enter the ocean. The authors have observed five underflow events from the Salinas River during the past 12 years. Flow of the river can be tracked down to 1200 m depth below the surface with the remotely operated vehicle Ventana. These events may transport one-half, or more, of the sediment carried by the Salinas River off the continental shelf and into the deep sea. The underflows may carry much of the carbon required by deep-sea organisms and they may also transport large amounts of pesticides, such as DDT, which contaminate sediments of the Salinas Valley.
- Historical science, experimental science, and the scientific method
Carol E. Cleland, Department of Philosophy and Center for Astrobiology, University of Colorado, Boulder, Colorado 80309, USA. Pages 987-990.
- Beginning in grade school and extending through college, students are taught the scientific method. The scientific method is defined as providing a uniform characterization of the practices of all "good" science. It is based upon classical experimental science. Hypotheses about the remote past (the extinction of the dinosaurs or the origin of the universe, for example) cannot be tested in the manner of classical experimental science. Historical research is sometimes criticized as inferior on these grounds. This article, written by a philosopher, explains why historical research is not inferior to classical experimental research. In the first place, the scientific method of yore does not provide an adequate account of either historical or classical experimental research. Furthermore, the differences in methodology between experimental science and historical science are based upon a pervasive, time-asymmetric feature of nature. Insofar as each practice is keyed to exploit the information that nature puts at its disposal-and that information is different-neither can be said to be more objective or rational than the other.
- Strength of slightly serpentinized peridotites: Implications for the tectonics of oceanic lithosphere
Javier Escartin, Laboratoire de Géosciences Marines (CNRS FRE2316), case 89, Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris, France; et al. Pages 1023-1026.
- Serpentinites are rocks formed by the complete alteration of mantle rocks in the presence of water. They are found along continental faults, such as the San Andreas fault, and at the seafloor, being particularly abundant near slow spreading ridges such as the Mid-Atlantic ridge. These rocks are extremely weak, and their presence could explain the apparent weakness of serpentine-bearing faults, such as some sections of the San Andreas. They may also favor the formation and evolution of detachments recently identified at oceanic basins. The new deformation experiments on partially serpentinized peridotites demonstrate that slight amounts of serpentinization can have a profound effect on the strength of the rock. The presence of less than 15% of serpentine reduces the strength of mantle rocks by more than a factor of two. Consequently, very small amounts of alteration of the lithospheric mantle can substantially weaken the lithosphere, promote the localization of strain along faults, and eventually the formation of detachments in oceanic environments.
- Propagation of the thrust system and erosion in the Lesser Himalaya: Geochemical and sedimentological evidence
Pascale Huyghe, Maison des Géosciences, Université Joseph Fourier, BP 53, 38041 Grenoble, France; et al. Pages 1007-1010.
- Sedimentological and Nd isotope data of two sections of the sub-Himalaya of western Nepal are used as new constraints for understanding the erosion history of the Himalaya. Throughout the deposition of the middle and upper members of the Siwalik Group, the Lesser Himalaya contribution to the total detrital input progressively increased from less than 20% to 40%. The increasing proportion of Lesser Himalaya sediments started ca. 10-8 Ma and is associated with a coarsening of the maximum grain size at both microscopic and macroscopic scales. Tectonics of the Lesser Himalaya thrust system would have controlled the exhumation of the Lesser Himalaya rocks and would have begun 12-10 Ma, taking into account the delay for denudation. Together with other studies, these data restrict the onset of movement on the Lesser Himalaya thrust system to less than 3 m.y. along more than 1750 km. This short time frame implies a lateral propagation rate far too high for the propagation of a single crustal thrust; thus, the authors suggest instead the simultaneous initiation of several thrusts ahead of the Main Central thrust ca. 12 Ma. The authors propose that a rapid rise of the Tibetan plateau at that time could be responsible for such a tectonic evolution of the Himalaya belt. This regional rising could be the prime cause of the increase of sediment influx ca. 11 Ma around the Himalaya.
- Oroclinal bending and evidence against the Pangea megashear: The Cantabria-Asturias arc (northern Spain)
Arlo B. Weil, Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48103, USA; et al. Pages 991-994.
- The Cantabria-Asturias arc of southwestern Europe is a highly curved mountain belt that formed along the ancient plate boundary between Gondwana and Laurussia during the assembly of the Pangea supercontinent. New paleomagnetic data refute earlier models of arc formation that concluded secondary rotation of only 50% of present-day arc curvature. The new data show that the arc was originally a linear north-south-trending belt that was buckled and bent in a two-stage tectonic history. This history represents two regional collision phases that ended with a north-south compression phase caused by the northward movement of Africa and its collision with Europe during the final amalgamation of Pangea. This tectonic scenario argues against a previously proposed 3500 km fault system that existed between the southern Gondwana continent and the northern Laurussia continent in earlier reconstructions.
- The California Arc: Thick Granitic Batholiths, Eclogitic Residues, Lithospheric-Scale Thrusting, and Magmatic Flare-Ups
Mihai Ducea, University of Arizona.
- The Sierra Nevada Mountains and related areas of California provide one of the best-studied examples of a continental margin arc, where large volumes of granitic rock were intruded into the crust above a subduction zone. This paper provides a new view of the arc: its deep structure, its magmatic history, and the sources of the granites. The arc was active from 220 to 80 Ma, but it had two major magmatic flare-ups that lasted only 10-20 m.y. The second of these (100-85 Ma) was the big one-it resulted in emplacement of about 80% of the entire batholith. The original geometry of the batholith has been modified by the Basin and Range extension in the Cenozoic, but Ducea reconstructs its original form using surface exposures (mostly upper crustal), scattered outcrops of rocks from the deeper part of the arc, and xenoliths that were brought to the surface by younger volcanism, all in combination with previously published seismic data. The present crust is 33 km thick, but arc granites extend through this entire depth range. Since some granites have been eroded off the top of the Sierra, and since granites are derived by differentiation of more mafic materials, he argues the crust must have been about 70 km thick when it formed; much like the Andes today. The residue of granite production has since been recycled back into the mantle, but the xenolith data show that the residue was likely a mixture of granulite and eclogite, with the latter being more important than previously thought. The flare-ups are thought to have been instigated by major periods of deformation when the arc overrode North America; the eventual cessation of magmatism in the arc is inferred to be related to depletion of the deep mantle source of granitic components. Overall, this paper clarifies the nature of continental margin arcs at a fundamental level.
*To view abstracts for the current issues of GEOLOGY and GSA TODAY, go to www.gsajournals.org.
To obtain a complimentary copy of any GEOLOGY article, contact Ann