July-August GSA Bulletin media highlights

06/14/05

Boulder, Colo. – The July-August issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes several newsworthy items. Topics include: a possible 3-10-year warning mechanism for North American west-coast earthquakes; discovery of a 4600-year history of tsunamis on the Oregon coast; and present-day tilting of the Great Lakes region.

Representatives of the media may obtain complimentary copies of articles by contacting Ann Cairns at acairns@geosociety.org. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA BULLETIN in stories published. Contact Ann Cairns for additional information or assistance.

Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

Secular change in the Precambrian silica cycle: Insights from chert petrology
Robert G. Maliva, Camp Dresser and McKee, Inc., Fort Myers, Florida 33919, USA, et al. Pages 835–845.
Keywords: Precambrian, chert, diagenesis, silica cycle.

Chert, a form of microcrystalline silica, preserves a record of changes in the oceanic silica cycle of over time. The silica concentration in the oceans today is kept at relatively low levels by the actions of organisms that secrete siliceous skeletons. Prior to the evolution of silica-secreting organisms, oceanic silica concentrations were considerably higher than today. Comparative petrography of Phanerozoic and Precambrian cherts resulted in the identification of an earlier major change in oceanic silica concentrations, and thus chert deposition, occurring toward the end of Paleoproterozoic Era (~1.8 billion years ago). Oceanic silica concentrations during the Paleoproterozic were high enough so that some cherts were deposited largely by direct silica precipitation in open marine environments at or just below the seabed. Early direct silica precipitation in open marine environments did not occur at later times when silica concentrations were lower.

Evolution of a volcanic rifted margin: Southern Red Sea, Ethiopia
Ellen Wolfenden, Geology Department, Royal Holloway, University of London, Egham, Surrey, TW20 0EX, UK, et al. Pages 846–864.
Keywords: passive margin, magmatism, eastern Africa, rift.

Throughout earth history, continental plates subjected to forces applied within, beneath, and on their boundaries have ruptured to produce new ocean basins. A wealth of academic and industry data from ancient rupture sites (passive continental margins) provides physical and kinematic constraints on breakup processes, yet data from actively deforming rifts are also needed to understand the stretching and magmatic processes immediately prior to the onset of seafloor spreading. The seismically and volcanically active southern Red Sea margins in Ethiopia still remain above sea level, providing a natural laboratory for continental breakup studies. This area lies on or near a mantle plume and is marked by voluminous magmatism during the past 30 m.y. We made detailed measurements of volcanic and sedimentary rocks, fractures cutting these rocks, and drainage patterns across six transects of the southern Red Sea, and then extended our observations across a broader region using satellite imagery. Representative volcanic rock samples were dated using radiometric techniques to allow us to trace the evolution of the margin since initial massive outpourings of lavas at 30 Ma. The first faults formed ca. 29 Ma as the widespread flood basalts started to wane and were replaced by explosive eruptions. A few long faults accommodating large (>1 km) displacements formed to produce wedge-shaped basins that started to fill with volcanic material. Some volcanoes were located along these border fault systems. The spatial arrangement of border faults produced characteristic along-axis segmentation. As lithospheric thinning via stretching and heating progressed, faulting and volcanism moved to a narrow zone within the basin, and the border faults became inactive. Magmas intruded the crust and effectively stretched the plate, heating and weakening it in the process. Lavas piled up in this narrow weak zone, producing wedge-shaped sequences of lavas that bent the plate beneath their load. This new, "magmatic" along-axis segmentation is similar to that of slow-spreading mid-oceanic ridges, yet the plate has not yet ruptured. Our study of the southern Red Sea indicates that magmatic processes, rather than faulting, create much of the structure observed along successfully rifted margins and facilitate the breakup process.

Cretaceous-Tertiary shortening, basin development, and volcanism in central Tibet
Paul Kapp, Department of Geosciences, University of Arizona, Tucson, Arizona 85721-0077, USA, et al. Pages 865–878.
Keywords: Tibet, plateau formation, continental collision, underthrusting, suture, shortening.

This paper presents the first comprehensive overview of the history of Cretaceous-Tertiary shortening, basin development, and magmatism in a poorly studied region of central Tibet. We suggest that this history can be explained by large-scale northward underthrusting of southern Tibet beneath central Tibet along a reactivated Mesozoic suture zone during Early Cretaceous–early Tertiary time. Our results raise the possibility that central Tibet has been an elevated plateau since at least mid-Cretaceous time. Growing evidence for major pre-Cenozoic crustal thickening in Tibet requires revision to models of plateau formation that assume the thick crust and high elevation of Tibet are largely the consequence of India colliding with Asia during the past ~55 m.y.

Mapping granite and gneiss in domes along the North Himalayan antiform with ASTER SWIR band ratios
Doyle R. Watts, Department of Geological Sciences, Wright State University, Dayton, Ohio 45435, USA, et al. Pages 879–886.
Keywords: Himalaya, dome, granite, ASTER, SWIR band ratio.

Watts et al. use images from the ASTER sensor on the EO1 satellite to map gneiss domes in Tibet that are found just south of the main suture between Asia and India along the North Himalayan antiform. The images show the disposition of the domes with considerable precision allowing the investigators to map them with unprecedented accuracy. They developed a method to distinguish between the older gneiss cores and the recent granites that are associated with the generation of these features. In the course of this investigation, they found unmapped granite along the same trend. This work shows that the amount of granite exposed in the gneiss domes increases westward from the Kangmar dome.

Stratigraphic and geochemical evolution of an oceanic arc upper crustal section: The Jurassic Talkeetna Volcanic Formation, south-central Alaska
Peter D. Clift, Department of Geology and Petroleum Geology, University of Aberdeen, Aberdeen AB24 3UE, UK, et al. Pages 902–925. Keywords: sedimentology, arc volcanism, subduction, geochemistry, isotope geology.

Although the chemistry of rocks indicates that most of the crust that forms the modern continents is formed in subduction zone settings, there is a significant mismatch between the composition of arc crust and typical continental crust. However, understanding of arc crustal composition has been limited by poor knowledge of the deep structure of these features and because most modern examples lie in deep water in remote, inaccessible parts of the oceans. The Talkeetna Arc in South Central Alaska exposes one of the most complete arc sections known. In particular, the upper crustal sections are well preserved and complete. In this study, the first detailed documentation of the rocks and chemistry of this unique series of rocks is published, showing that this arc behaved very much like a modern western Pacific volcanic arc, with deep water conditions and explosive eruptions prevailing until the arc collided with the margin of North America around 160 Ma. The work confirms that the apparent mismatch between continental and arc crust is real and that arc crust is likely transformed into continental crust during the collision process itself. The study shows that the history of explosive volcanism can be accurately reconstructed from the sediment record.

Isotopic and structural constraints on the location of the Main Central thrust in the Annapurna Range, central Nepal Himalaya
Aaron J. Martin, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, et al. Pages 926–944.
Keywords: Himalaya, Nepal, tectonics, Nd-143/Nd-144, U/Pb, microstructure.

With a minimum of 150 km of slip, the Main Central thrust (MCT) in the Himalaya is one of the largest thrust faults in the world, and it is also one of the best studied. One fundamental challenge in Himalayan geology, however, is the accurate location of the MCT. Because rocks on both sides of the MCT were the same rock type prior to metamorphism, and because these rocks were deformed and metamorphosed in a similar manner, it is difficult to visually distinguish hanging wall and footwall rocks. Fortunately for geologists, the MCT juxtaposes rocks with different ages and chemical characteristics, so differences in these attributes can be exploited to identify hanging wall and footwall rocks, and thus pinpoint the location of the fault. The authors of this paper use Neodymium isotopes from crushed and powdered whole rocks and uranium-lead ages from detrital zircons, combined with field mapping and microstructural analyses, to accurately locate the MCT across most of the Annapurna Range in central Nepal. This work shows that any zone of mixing between hanging wall and footwall rocks along this large-magnitude thrust is thin. The work also enhances models of the tectonic evolution of the Himalaya by improving the placement of microstructural, geochronologic, and thermobarometric data into tectonic context.

Early Mesozoic thrust tectonics of the northwest Zhejiang region (Southeast China) Wenjiao Xiao, State Key Laboratory of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China, and Haiqing He. Pages 945–961. Keywords: NW Zhejiang area, turbidites, deep-water basin, multifold duplexes, structural styles, Early Mesozoic orogeny. Sedimentological and tectonic analyses of the NW Zhejiang region of South China, located near the suture zone of the Yangtze and Cathaysian blocks, provides clues for how East Asia formed. Some 240 million years ago, the region had a paleo-slope that then shifted from deep sea to a continental environment, recording an important convergent event associated with ancient amalgamation of various blocks. Associated field thrust structures are described in detail, and all the sedimentological and tectonic analyses, together with other geological data, are used to constrain a new tectonic model in relation to the Early Mesozoic archipelago paleogeography of South China and formation of this portion of East Asia

Evidence for 65 km of dextral slip across Owens Valley, California, since 83 Ma
Andrew R.C. Kylander-Clark, Department of Geological Sciences, University of North Carolina, Chapel Hill, North Carolina 27599, USA, et al. Pages 962–968.
Keywords: Owens Valley, Coso Range, Sierra Nevada, slip rates, geochronology, geochemistry.

In this article, we present evidence for the correlation of two distinct geologic markers that are right-laterally offset by 65 km across Owens Valley, California: the Golden Bear and Coso dikes. The age, chemistry, geologic relations, and isotopic trends are similar in these rocks as well as rocks that surround them. Because these dikes are perpendicular to the valley that offsets them, we conclude that they have been offset, in a dextral sense, by 65 km, since they were emplaced, roughly 83 Ma. If all of this motion has occurred since the inception of current motion in the valley, then rates must have been higher in the past. Alternatively, motion could have been episodic, with a significant proportion of offset occurring much earlier.

Reinitiation of subduction and magmatic responses in SW Japan during Neogene time
Jun-Ichi Kimura, Department of Geoscience, Shimane University, Matsue 690-8504, Japan, et al. Pages 969–986.
Keywords: Neogene, back-arc basin opening, volcanism, SW Japan, subduction reinitiation.

Development of a volcanic arc in SW Japan over the past 25 million years is discussed based on the synthesis of published works on volcanology, igneous petrology, geochemistry, and tectonics. Subducted plates beneath the arc changed ca. 17 Ma from old Pacific Plate to young Philippine Sea Plate (PSP), coinciding with the opening of the Japan Sea backarc basin (BAB). Reinitiation of subduction of the young and hot PSP generated slab melts, and high-magnesium andesite activity took place in the forearc between 15 and 12 Ma. Further subduction of PSP lead to reheating and melting of the slab by penetration into hot mantle asthenosphere having been upwelled during BAB opening. This resulted in cessation of rear-arc alkali basalt activity and onset of adakitic dacite activity in the Quaternary.

Age of the Los Ranchos Formation, Dominican Republic: Timing and tectonic setting of primitive island arc volcanism in the Caribbean region
Stephen E. Kesler, Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA, et al. Pages 987–995.
Keywords: Los Ranchos Formation, U-Pb, Island-arc tholeiite, Hispaniola, Caribbean, plate tectonics.

This paper deals with an unusual rock sequence known as the Los Ranchos Formation, which formed at the start of volcanism in the Greater Antilles. Similar rock sequences are found in most other islands in the Greater Antilles, and ages for them are poorly known. New U-Pb isotope analyses show that these rocks are about 110 million years old and that they formed during a period of widespread anoxic conditions in the world ocean. Viewed in the context of plate tectonic models for the Caribbean, this age suggests that the Los Ranchos Formation and related volcanic islands might have contributed to ocean anoxia by blocking the seaway between North and South America in early Cretaceous time.

Evidence for possible precursor events of megathrust earthquakes on the west coast of North America
Andrea D. Hawkes, Centre for Environmental and Marine Geology, Dalhousie University, Halifax, Nova Scotia B3H3J5, Canada, et al. Pages 996–1008.
Keywords: precursor, megathrust earthquake, foraminifera, thecamoebian, marsh/forest transition.

For many civic planners, the time frame of 300–500 years as a periodicity for the return time of megathrust earthquakes on the west coast of North America is too long to plan by. However, in this paper, we report a possible precursor mechanism that appears to have a 3–10 year time frame that provides 3–10 years' warning time, which is a realistic time frame for civic planners. The precursor was detected using changes in microfossil assemblages (protozoans that leave a fossil record--in this case, foraminifera and thecamoebians) in cores from Alaska and Oregon. The assemblages change in response to small elevational deviations a few years before mega-thrust earthquakes that occurred in coastal forests and salt marshes along the west coast of North America. This technique provides a reliable and economical tool that could save thousands of lives and millions of dollars of property damage in large cities like Vancouver, British Columbia; Portland, Oregon; or Seattle, Washington.

Tsunami history of an Oregon coastal lake reveals a 4600 yr record of great earthquakes on the Cascadia subduction zone
Harvey M. Kelsey, Department of Geology, Humboldt State University, Arcata, California 95521, USA, et al. Pages 1009–1032.
Keywords: tsunami, Cascadia subduction zone, paleoseismology, plate boundaries, subduction zone.

Earthquakes on the Cascadia subduction zone, which is the offshore fault that runs from northern California to southern Canada, generate destructive tsunamis. When these tsunamis spill over into low-lying coastal lakes, they can leave a record of their disturbance as layers of imported beach sand and wood debris preserved in the lake mud. Using this technique, four paleoseismologists (Harvey Kelsey, Humboldt State University; Alan Nelson, U.S. Geological Survey; Eileen Hemphill-Haley, Humboldt State University and Rob Witter, William Lettis and Associates) have discovered a 4,600-year record of tsunamis that have invaded the southern Oregon coast. During this time, fourteen tsunamis have entered Bradley Lake, an 18–20-foot-high lake dammed behind coastal sand dunes. Each tsunami carries beach sand and marine organisms into the lake. Detailed study of these sand layers in the lake shows that the tsunamis consisted of multiple waves that overtopped the sand dunes. Based on the height of the sand dune dam, the paleoseismologists estimate that the tsunamis were at least 18 to 20 feet high, with the larger tsunamis much higher than this minimum height. Time intervals between tsunamis have been as long as about 1,200 years and as short as a few decades. The tsunamis (and hence accompanying earthquakes) appear to come in clusters; for instance, between ca. 850 BC and 250 AD, there were no tsunamis, yet from AD 250 to AD 950, there were at least four. The most recent tsunami was 300 years ago and was preceded by a 700–800 year time gap with no tsunamis. The tsunami of 300-years-ago, which other geologists have determined was caused by a subduction zone earthquake that broke the seafloor between northern California and southern British Columbia, may be the start of a new cluster of closely spaced-in-time subduction zone earthquakes and tsunamis. Although the tsunamis are documented at this one site, it is likely that each tsunami affected a much larger coastal area, encompassing all or most of the northern California to southern Canada coastline. Also, Cascadia tsunamis can travel across the Pacific Ocean and lead to hazards at other Pacific coast localities.

Submarine landforms and the reconstruction of fast-flowing ice streams within a large Quaternary ice sheet: The 2500-km-long Norwegian-Svalbard margin (57°–80°N)
D. Ottesen, Geological Survey of Norway, N-7491 Trondheim, Norway, et al. Pages 1033–1050.
Keywords: Barents Sea Ice Sheet, Scandinavian Ice Sheet, glacial lineations, ice flow, ice-sheet dynamics, paleo-ice stream, seafloor morphology.

The paper describes the morphology of the seafloor on the 2500 km long Norwegian shelf, and is based mainly on regional and detailed bathymetric data sets. A suite of glacial "landforms" uncovers the dynamic behavior of the last great ice sheets over the Norwegian shelf, the Barents Sea, and the areas around Svalbard during the last glacial maximum (about 20,000 years ago). The last ice sheet appears to have reached the shelf edge along the whole margin. Glacial lineations in the cross-shelf troughs show that fast-flowing ice streams moved here and represented the most important ice drainage pathways towards the shelf edge. Different types of recessional moraines and glaciotectonic features are described from the shelf areas.

Late Quaternary eolian and alluvial response to paleoclimate, Canyonlands, southeastern Utah
Marith C. Reheis, U.S. Geological Survey, MS-980, Federal Center, Box 25046, Denver, Colorado 80225, USA, et al. Pages 1051–1069.
Keywords: Colorado Plateau, dunes, eolian dust, paleoclimate, paleosols, soil nutrients.

Studies on dune and alluvial deposits in the area of Canyonlands National Park, Utah, show that the thin dune deposits preserve a 40,000-year record of dune deposition and soil formation. Fossil pollen and soil properties show that dune deposition occurred during both relatively wet and dry intervals when landscape and vegetation changed. Soils formed on the dune sands and alluvium contain infiltrated eolian dust partly from sources west of the Colorado Plateau and are important to soil fertility and plant growth.

Present-day tilting of the Great Lakes region based on water level gauges
Andrι Mainville and Michael R. Craymer, Natural Resources Canada, Geodetic Survey Division, 615 Booth Street, Ottawa, Ontario, K1A 0E9, Canada. Pages 1070–1080.
Keywords: Great Lakes, postglacial rebound, water level gauges, least-squares analysis, trend surface analysis, water management.
Using monthly mean water levels at 55 sites around the Great Lakes, a regional model of vertical crustal motion is computed for the region. Compared to previous studies over the Great Lakes, fifteen additional gauge sites, monthly water level data from all seasons instead of the four summer months, and eight additional years of data between 1860 and 2000 are used. The regional model is found to agree best with the ICE-3G global isostatic model while the ICE-4G model displays smaller gradients around the Great Lakes.

Glacioeustatic changes in the early and middle Eocene (51–42 Ma): Shallow-water stratigraphy from ODP Leg 189 Site 1171 (South Tasman Rise) and deep-sea δ18O records
Stephen F. Pekar, Queens College, School of Earth and Environmental Sciences, 65-30 Kissena Boulevard, Flushing, New York 11367, USA and Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA, et al. Pages 1081–1093.
Keywords: Eocene, eustasy, Ocean Drilling Program, Leg 189, stratigraphy, foraminifer, sea level, Australia, Site 1171.

The timings of water-depth changes (i.e., sequence boundaries) observed in shallow-water sediments obtained from cores obtained from the Ocean Drilling Program Leg 189 Site 1171 (South Tasman Rise) compare well with other stratigraphic records (New Jersey, USA, and Northwest Europe) and δ18O increases (inferred ice volume increases/sea level decreases) from deep-sea records. This suggests that significant (>10 m) global sea level changes occurred between 51 and 42 million years ago, a time in which Earth has long been considered a Greenhouse World. The synchronous nature of sea-level decreases from globally distal sites and δ18O increases indicates a global control and that glacioeustasy (i.e., global sea-level change from ice volume) was operating in this supposedly ice-free world. This is supported by previous modeling studies and atmospheric CO2 estimates showing that the first time CO2 levels decreased below a threshold that would support the development of an Antarctica ice sheet occurred ca. 51 Ma. Estimates of sea-level amplitudes range from ~20 m for the early Eocene (51–49 Ma) and ~25 m to ~45 m for the middle Eocene (48–42 Ma).

Assembling an ignimbrite: Compositionally defined eruptive packages in the 1912 Valley of Ten Thousand Smokes ignimbrite, Alaska
Judy Fierstein, Volcano Hazards Team, U.S. Geological Survey, MS 910, 345 Middlefield Road, Menlo Park, California 94025, USA, and Colin J.N. Wilson. Pages 1094–1107.
Keywords: Novarupta 1912 eruption; ignimbrite; Valley of Ten Thousand Smokes; Katmai; rhyolite; dacite; andesite.

This study shows the Valley of 10,000 Smokes (VTTS) ignimbrite was emplaced by nine sequential compositionally distinct flow packages, mostly within the first 11 hours of the 3-day eruption. Mapped flow-package distributions show how their emplacement is an interplay between flow volumes, energy, and topography, with ongoing compaction greatly controlling channeling of subsequent, lower-energy and smaller-volume flows. Thus, this ignimbrite was clearly not emplaced by a single, slow-moving flow, which challenges a popular theoretical notion that this ash-flow sheet was deposited from a single, continuously aggrading outpouring of flow material. This has implications for interpretations of the emplacement dynamics of the ignimbrite, underscoring that any theoretical modeling of emplacement should be made on the products of individual packages, not on the ash-flow sheet as a whole. Long considered the "classic, slow-moving, simple end-member," this new view of the VTTS ignimbrite provides a different perspective that is likely applicable to other ignimbrites as well.

Cosmogenic exposure dating of late Pleistocene moraine stabilization in Alaska
Jason P. Briner, INSTAAR, University of Colorado, Boulder, Colorado 80309-0450, USA, et al. Pages 1108–1120.
Keywords: cosmogenic exposure dating, moraine, Alaska, last glacial maximum, penultimate glaciation.

This paper provides data that allow us to constrain when glaciers reached and retreated from their advances of the last Ice Age. The records come from across the state of Alaska and show that there are differences in the timing of the advance and retreat of Alaskan glaciers. This information lets us hypothesize about why glaciers behaved differently, which we feel is mostly likely due to gradients in precipitation across the state that were different during the Ice Age than today. The data also allow us to scrutinize an emerging popular dating tool, called cosmogenic exposure dating, that scientists are using across the globe to date glacier behavior. Cosmogenic exposure dating establishes how long boulders have been sitting on the surfaces of large piles of glacial debris left behind from the last Ice Age. Our relatively large dataset shows that the settling of these piles of debris strongly influences the boulder ages and tends to make an age population too young. The results in this paper on the differences in timing of Alaskan glaciers during the Ice Age, and how best to interpret cosmogenic exposure ages, will be of interest to a broad array of scientists, especially those interested in Ice Age glaciers and climate change and in the exposure dating technique.

Source: Eurekalert & others

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