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Jan. GEOLOGY and GSA TODAY media highlights

Geological Society of America

Topics in the January issue of GEOLOGY include: evidence for Earth's earliest photosynthetic organisms functioning in a "hydrogen economy;" new geoarchaeological insights into ancient Phoenicia's harbors of Tyre and Sidon; results of a 40-year field study quantifying the connection between mechanical weathering and Earth's climate; and glacier advance in Pacific North America during the first millennium A.D. The GSA TODAY science article poses the question, "The long-term strength of continental lithosphere: "jelly sandwich" or "crème-brûlée"?"


Geoscience rediscovers Phoenicia's buried harbors
Nick Marriner, CEREGE CNRS, Geomorphology and Tectonics, Europole Méditerranéen de l'Arbois, Provence 13545, France; et al. Pages 1-4.

The exact locations of Tyre and Sidon's ancient harbors, Phoenicia's two most important city-states, have attracted scholarly interest and debate for many centuries. New research reveals that the ancient basins lie buried beneath the medieval and modern city centers. A network of sediment cores have been sunk into the cities' coastal deposits and studied using high-resolution geoscience techniques to elucidate how, where, and when Tyre and Sidon's harbors evolved since their Bronze Age foundations. In effect, ancient port basins are rich geological archives replete with information on human impacts, occupation histories, Holocene coastal evolution, and natural catastrophes. Dateable archeological and organic remains provide a chronology for this 8000-year-old story. Analyses identify various stages of harbor evolution from natural sheltered coves during the Bronze Age to human modified environments from the Phoenician period through Hellenistic, Roman, and Byzantine times. After the sixth and tenth centuries A.D., tectonic collapse, tsunamogenic impacts, and relative commercial decline meant that the harbors were no longer properly maintained, gradually buried beneath thick tracts of coastal sediment and lost until now. These findings have far-reaching implications for our understanding of Phoenician maritime archaeology and call for the protection of these unique cultural heritages.

Quiescent deformation of the Aniakchak Caldera, Alaska, mapped by InSAR
Oh-ig Kwoun, SAIC, USGS/EROS, Science Applications Branch, Sioux Falls, SD 57198, USA; et al. Pages 5-8.

Using satellite synthetic aperture radar interferometry (InSAR), scientists with Science Applications International Corporation (SAIC) and the U.S. Geological Survey found out that the 10-km-wide Aniakchak Caldera, Alaska, has been subsiding during 1992-2002. The maximum subsidence rate was estimated to be ~13 mm/yr near the center of the caldera. Combined with other in-situ observations, the deformation can be explained by the cooling or degassing of a shallow magma body (~4 km deep) and/or the reduction of the pore-fluid pressure of a cooling hydrothermal system. This volcanic deformation detected by InSAR indicates that the volcanic system is still active and, as such, requires close attention for the timely detection of any unforeseen hazard.

Collapse of large complex impact craters: Implications from the Araguainha impact structure, central Brazil
Cristiano Lana, Imperial College London, Earth Sciences, Royal School of Mines, London SW7 2BP, UK; et al. Pages 9-12.

The 40-km-wide Araguainha impact structure in central Brazil provides an extensive outcrop to study the structural evolution of all parts of a complex crater. While most craters of comparable size are buried by impact-related or post-impact sedimentary deposits, Araguainha is deeply eroded, and the exposed, detailed outcrop-scale structural features can be used to understand the structural evolution of large impact craters. This study explores structural evidence that provides constraints on the target rock movement during the crater collapse.

Oceanic-ridge subduction vs. slab break-off: Plate tectonic evolution along the Baja California-Sur continental margin since 15 Ma
F. Michaud, Géosciences Azur, UPMC-IRD, Villefranche sur mer 06235, France; et al. Pages 13-16.

The interaction between continent and active spreading centers is an important process that has shaped the pacific margins of the Americas. Off Baja California, Michaud et al. propose a new geodynamic evolution based on full bathymetry coverage and magnetic profiles from 23°N to 27°N. The data reveal a major clockwise rotation of the spreading direction. This reorganization of the oceanic plate is attributed to the break-off of the slab beneath Baja California.

Modeling the magma plumbing system of Vulcano (Aeolian Islands, Italy) by integrated fluid inclusion geobarometry, petrology and geophysics
A. Peccerillo, Università degli Studi di Perugia, Dipartimento di Scienze della Terra, Piazza Università, Perugia I-06123, Italy; et al. Pages 17-20.

Failure to correctly forecast volcanic eruptions depends heavily on the lack of knowledge on modalities of magma ascent in active volcanoes. Therefore, modeling the internal structure and mantle dynamics in the active volcanoes is a primary objective of igneous petrology and volcanology. Multidisciplinary studies are necessary to achieve this objective. In this paper, a model is presented for the internal structure of Vulcano, an explosive active system in the southern Tyrrhenian Sea. For the first time, an integrated petrological, geochemical, geophysical, and fluid-inclusion approach was undertaken, which suggests magma ponding and crystallization at various pressures, and an arrival of magmas at shallow depths shortly before eruptions. Such a model is of paramount importance for establishing better strategies of volcano monitoring and for forecasting future eruptions.

Does the Great Valley Group contain Jurassic strata? Reevaluation of the age and early evolution of a classic forearc basin
Kathleen D. Surpless, Trinity University, Geosciences, San Antonio, Texas 78212, USA; et al. Pages 21-24.

The Mesozoic Great Valley sedimentary basin of California is the world's archetypal ancient forearc basin and reconstructing its evolution and development has proven essential for understanding the geologic history of California as well as the development of other sedimentary systems. Determining sedimentary age is critical to reconstructing the basin's history, and has long been established by fossil data in the Great Valley. However, the new detrital zircon results presented here suggest that the fossil ages may be too old, calling into question the well-established sedimentation history of this basin, as well as the utility of the index fossils, used in the Great Valley and globally, for age information.

Dissected hydrologic system at the Grand Canyon: Interaction between deeply derived fluids and plateau aquifer waters in modern springs and travertine
Laura J. Crossey, University of New Mexico, Department of Earth and Planetary Sciences, Albuquerque, NM 87131-1116, USA; et al. Pages 25-28.

This paper presents a new model for the origin of Grand Canyon's spectacular Quaternary travertine deposits. It integrates a broad range of geochemical tracers on modern spring water and gas chemistry, as well as the more ancient travertines. Crossey et al. have identified a previously unrecognized deeply derived saline and CO2-rich fluid ("lower world" water) that is present in the regional carbonate aquifer system. These fluids contain mantle-derived helium and Sr isotopic signatures indicative of deep derivation and deep circulation, demonstrating an unsuspected link between the mantle and the surface water system. They propose a model whereby the lower world components are conveyed upward via both magmatism and seismicity along normal faults. Though small in volume, the CO2-rich fluids contribute salinity and important trace elements such as arsenic and uranium in addition to delivering large amounts of dissolved carbonate to spring outlets where massive travertines accumulate. These waters cause degradation of water quality that is societally important in the arid west. Because the travertines record both paleohydrologic and neotectonic influences, they are an important resource for understanding interplay of tectonic and climatic changes.

Magnetic record of Milankovitch rhythms in lithologically noncyclic marine carbonates
Diana K. Latta, Lehigh University, Department of Earth and Environmental Sciences, Bethlehem, Pennsylvania 18015, USA; et al. Pages 29-32.

Rock magnetic variations record cyclicity within lithologically homogeneous, basinal, lime mudstones of the Lower Cretaceous San Angel Limestone, northeastern Mexico. Variations in the concentration of fine-grained ferromagnetic minerals occur at frequencies consistent with Milankovitch orbital rhythms. Magnetic mineral composition, grain size distributions, and grain shapes from digested samples are consistent with fine detrital magnetite in atmospheric dust. Prevailing winds and the proximity of the Cretaceous basin to an African aeolian source support the encoding of orbitally modulated changes in wind intensity or source area aridity fluctuating at the 20 k.y. precessional time scale. Rock magnetic measurements offer great potential to calibrate the pace of depositional processes in carbonates and to investigate high-frequency orbitally driven climate change in carbonates throughout geologic time.

East Antarctic ice stream tributary underlain by major sedimentary basin
Jonathan L. Bamber, University Bristol, School of Geographical Sciences, Bristol, Avon BS8 1SS, UK; et al. Pages 33-36.

During the Austral summer of 2001-2002, an airborne geophysical survey was undertaken in the interior of East Antarctica. Ice penetrating radar measurements of bedrock elevation were obtained along with magnetic anomaly data. These data were used to infer the presence of a 3-km-deep sedimentary basin beneath one of the fast flowing tributaries of Slessor Glacier. The basin is believed to be of marine origin. The presence of the sedimentary basin appears to be influencing the flow of the overlying ice and suggests that this part of East Antarctica may have undergone a substantial change in glaciological conditions in the past few million years.

Hydrogen-based carbon fixation in the earliest-known photosynthetic organisms
Michael M. Tice and Donald R. Lowe, Stanford University, Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA. Pages 37-40.

Determining the metabolic processes employed by ancient organisms is a critical goal for the study of early evolution, yet it has proven difficult to accomplish. Much previous work has focused on dating the rise of oxygen-producing photosynthetic organisms. In this paper, Tice and Lowe present geochemical evidence suggesting that the earliest-known (3,416-million-year-old) photosynthetic organisms actually consumed hydrogen, and instead of producing oxygen as a waste product, they produced water. These results provide a tantalizing hint of an ancient biological "hydrogen economy" that operated in the absence of oxygen.

Unusually Cu-rich magmas associated with giant porphyry copper deposits: Evidence from Bingham, Utah
Daniel P. Core, Geoinformatics Exploration, Geology, Perth, Western Australia 6005, Australia; et al. Pages 41-44.

Formation of giant porphyry copper deposits (PCDs) requires either highly efficient collection of Cu from large volumes of magma or unusually Cu-rich parent magmas. Support for the second of these possibilities has been discovered in the form of mafic enclaves with abundant bornite and chalcopyrite in the Last Chance stock, one of the parent intrusions of the giant Bingham PCD, Utah, United States. Mineral assemblages and compositions indicate the Last Chance enclaves are made up of phases that crystallized from the intrusion, and that the intrusion was unusually enriched in Cu. The magma could have become enriched in Cu during crystallization or could have been formed by melting areas of the lower crust enriched in Cu. Possible Cu-rich source regions are a subcrustal mafic intrusion with sulfide cumulates, or a deeply buried metamorphic terrane containing Cu deposits such as those in the Curaca Valley (Brazil) or Okiep (South Africa). Heterogeneous distribution of Cu-Fe sulfides in an Okiep-type source terrane would produce local PCDs such as Bingham or large accumulations of Cu-Fe sulfides, possibly in the form of cumulates in subcrustal intrusions at convergent margins, could produce giant PCD provinces such as those in Indonesia, Papua New Guinea, and central Chile.

Rock-slope failure and the river long profile
Oliver Korup, Swiss Federal Research Institutes WSL/SLF, Natural Hazards, Davos, Grisons CH-7260, Switzerland. Pages 45-48.

At time scales of millions of years, river incision into bedrock is commonly proposed as the dominant process for shaping mountain relief in regions of high uplift, such as the Himalayas, Taiwan, or the Southern Alps of New Zealand. In current concepts of mountain belt evolution, hillslopes respond to fluvial downcutting by rapid landsliding, thus adjusting to critical gradients. This study suggests that at time scales of several hundreds to thousands of years, however, the opposite may be also possible, and that large landslides may control patterns of river incision and deposition. It shows that many mountain river channels are often the steepest and endowed with the highest inferred erosion potential where they had to cut through large piles of debris from massive rock-slope failures. These former rockslide dams act as long-lived plugs, above which sediment accumulates, gradually flattening river slope, whereas channels incising through these dams have to regain the original profile elevation. Since many studies rely on the interpretation of river long profiles as a means to deduce long-term climatic or tectonic influences on mountain evolution, it may be necessary to detect and eliminate from these analyses the more transient effects of large landslides on mountain river channels.

The role of river-suspended material in the global carbon cycle
Sigurdur R. Gislason, University of Iceland, Institute of Earth Science, Reykjavik IS-101, Iceland; et al. Pages 49-52.

This manuscript reports the results of a 40-year-long field study in north-eastern Iceland quantifying the connection between mechanical weathering and Earth's climate. The main result of this study is that the feedback between mechanical weathering and global climate is a major climate moderating process over geologic time scales.

Deep electrical structure of the northern Cascadia (British Columbia, Canada) subduction zone: Implications for the distribution of fluids
Wolfgang Soyer, Geosystem srl., Milano 20133, Italy, and Martyn Unsworth, University of Alberta, Physics, Avadh Bhatia Physics Lab, Edmonton, AB T6G2J1, Canada. Pages 53-56.

Oceanic tectonic plates subducted beneath continents in active subduction zones contain large amounts of water, contained in the rocks' mineral structure as well as in pore spaces between minerals. Under increasing pressure and temperature, water gets squeezed out of pores, and minerals that contain a lot of water become unstable. By metamorphic processes reorganizing the rock matrix, close to all of the water originally stored in the plate is released and migrates upward into overlying material. How far down the water can be pulled within the subducting plate, which changes it does induce in the continental crust and mantle above once released, and where it moves at which velocity all may be factors that govern the subduction process to a large extent. This paper presents the analysis of electromagnetic variation data that were collected in the Vancouver area of Northern Cascadia, Canada. Containing information on subsurface electric current systems induced by atmospheric magnetic field variations, the data allow researchers to map the distribution of electrical conductivity beneath Earth's surface. Elevated conductivity was found throughout the area east of the volcanic arc, beneath the high plateau of British Columbia, extending from a depth of 30 km down to at least 60 km. Water, either as a free fluid or by promoting partial melting of mantle material, is known to dramatically increase the electrical conductivity. Therefore, the conductivity distribution is largely interpreted to reflect the fluid distribution. While the percentage of water required to account for the conductivity increase is under 1%, the study shows that the continental mantle in subduction zones contains enough water to drastically change its physical properties - and the high electrical conductivity is very likely linked with a low viscosity: a very crucial parameter in plate tectonics.

Expansion of alpine glaciers in Pacific North America in the first millennium A.D.
Alberto V. Reyes, University of Alberta, Earth and Atmospherics Sciences, Edmonton, Alberta T6G 2E3, Canada; et al. Pages 57-60.

Buried trees and soils unearthed from sites recently exposed by twentieth century glacier recession provide evidence for a previously undocumented period of first millennium A.D. glacier expansion in British Columbia (Canada) and Alaska. For more than 30 years, geological records of glacier fluctuations have been used as a framework against which other evidence for long-term climate change, such as data from lake, ocean, and ice cores, is evaluated. A regional glacier advance in Pacific North America during the first millennium A.D. has not been documented before and, together with other geological evidence for climate change, is consistent with recent arguments for millennial-scale climate variability in the region over the last several thousand years.

GSA TODAY Science Article

The long-term strength of continental lithosphere: "jelly sandwich" or "crème-brûlée"?
E.B. Burov, Laboratoire de Tectonique, University of Paris 6, 75252 Paris Cedex 05, France, and A.B. Watts, Department of Earth Sciences, University of Oxford, Oxford, OX1 3PR, UK. Pages 4-10.

Is Earth's continental crust stronger or weaker than underlying denser mantle? Where does the strength reside that holds up mountain ranges or keeps tectonic plates whole? E.B. Burov and A.B. Watts address this long-standing debate in the January 2006 issue of GSA Today. They address two contrasting models, nicknamed jelly sandwich (weak lower crust between strong mantle and strong upper crust) and crème brûlée (all the strength in the top layer), different models that have arisen because of conflicting results from elastic thickness and earthquake data. By modeling data from gravity measurements to estimate the effective elastic thickness of the lithosphere where the crust and mantle lithosphere is flexed down by, for instance, the weight of the Himalayas, Burov and Watts find that the elastic thickness is mainly within the mantle part. Thermomechanical modeling also supports their conclusion that the crème-brûlée model is unable to explain either the persistence of mountain ranges or the integrity of subducting slabs. They therefore conclude that a jelly sandwich model is more widely applicable than a crème-brûlée model.


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