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GEOLOGY
A band of precariously balanced rocks between the Elsinore and San Jacinto, California, fault zones: Constraints on ground motion for large earthquakes
James Brune, Rasool Aneeshehpoor (corresponding author), et al., University of Nevada, Reno, Seismological Laboratory, Reno, NV 89557-0141, USA. Pages 137-140.
A spectacular band of precariously balanced rocks extends from Riverside, California, to near Borrego Valley, California - halfway between the Elsinore and San Jacinto faults. These rocks place upper limits on the intensity of shaking, resulting from numerous large earthquakes (M ~ 7) along these two faults for thousands of years in the past (and also from two earthquakes in historic times, namely the 1899 and 1918 earthquakes). Based on field tests, photographic analyses, and computer modeling efforts, the estimated upper limit on toppling accelerations for these rocks is approximately 0.3 ± 0.1 g, roughly consistent with the median predicted ground motion for M = 7 earthquakes. This constraint is much lower than the ground motion predicted to occur once every 2475 years by the U.S. Geological Survey. These rocks provide vital information to constrain seismic hazard estimates (especially for hard rock sites, where few instrumental data are available). The data suggest that some of the assumptions that go into the hazard maps are incorrect (e.g., the assumed existence of random earthquakes on unknown faults, and the assumed variation in ground motion from one earthquake to the next).
Warmings in the far northwestern Pacific promoted pre-Clovis immigration to America during Heinrich Event 1
Michael Sarnthein, Christian-Albrechts-Universität zu Kiel, Institut für Geologisch-Palaontologisches, Kiel, Germany; et al. Pages 141-144.
America's first inhabitants were people from Asia who moved along the coasts of a now-submerged landbridge between the two continents, called Beringia. Archeological evidence suggests that the first people arrived in the Americas (down to Chile) as early as during early deglacial times, marked in Eurasia by the cold-arid climates of Heinrich stadial 1. But how did they move, and especially, why did they first move north from eastern Asia during this particularly cold stadial? First, well-dated centennial-scale sediment records from the subarctic northwest Pacific now show that the early deglacial 18.5–15.0 ka was marked by three short-term 5 °C warmings. They lasted 500-1500 years each and present the first evidence for a climatic seasaw mechanism between the subarctic North Pacific and North Atlantic during Heinrich stadial 1. The resulting favorable climate windows, when the shores of Beringia were free of sea ice during summer, may have promoted the long-shore emigration of pre-Clovis people from the then deteriorated, cold-arid monsoon climate of eastern Asia to the more climatically favorable Beringian and Aleutian shelf regions, and farther to the Americas.
Andrea Gebhardt, Alfred-Wegener-Institute of Polar and Marine Research, Geophysics, Bremerhaven ,Germany; et al. Pages 145-148.
Seismic data from the remote, 170-m-deep, 3.6-m.y.-old Lake El'gygytgyn (Chukotka, NE Siberia) revealed a crater structure underneath the lake that is comparable to other terrestrial impact craters. An uplift structure, formed by brecciated bedrock and interpreted as a central ring structure, could be identified in a depth of about 500 m below the seafloor. It is overlain by fallback breccia (material that was erupted during the impact event) and by lake sediments.
Interglacial clathrate destabilization on Mars: Possible contributing source of its atmospheric methane
Olga Prieto-Ballesteros, Centro de Astrobiología, Laboratory of Planetary Geology, Ctra. Ajalvir km. 4, Madrid 28850, Spain; et al. Pages 149-152.
Recently many discussions have been published about the detection of methane in the atmosphere of Mars. All of them consider the short life of methane in the Martian atmosphere and conclude that there is present volcanic activity or life producing this gas. Preito-Ballesteros et al. claim that there is not necessarily a current origin of methane; the source could have originated in past ages, been confined inside clathrate cages during glacial periods, and only recently been liberated when the water ice-rich deposits started to retreat poleward (0.4 m.y. ago).
Hot water: A solution to the Heart Mountain detachment problem?
Einat Aharonov, Weizmann Institute of Science, Rehovot 76100, Israel; and
Mark H. Anders (corresponding author), Columbia University, Department of Earth and Environmental Sciences, LDEO, Palisades, NY 10964, USA. Pages 165-168.
This paper presents a new model that explains the emplacement mechanism responsible for the Heart Mountain detachment, one of the most spectacular and puzzling gravity slide features on Earth's surface. Part of its mystery is its massive size (over 3,400 km2) and the low-angle surface (~2°) it moved on. Aharonov and Anders present a quantitative framework that draws on recent discoveries relating to the role of water in fault zones and landslides. They suggest that the extensive dike network associated with the Eocene Absaroka Volcanics heated trapped waters near its base, resulting in over pressuring sufficient to initiate the catastrophic sliding that traversed across more than 45 km of Earth's surface.
Subsurface ice as a microbial habitat
Heidy M. Mader, University of Bristol, Department of Earth Sciences, Bristol, England BS8 1RJ, UK; et al. Pages 169-172.
Icy wildernesses on Earth and other planets, chemical signals of ancient climates, and life in extreme environments are all fascinating topics and they are encapsulated in the search for life in ice. This paper draws on geology, physics, chemistry, and microbiology to show conclusively for the first time that a viable habitat for bacterial life exists within glacial ice in the form of tiny veins of liquid water that surround individual ice crystals. Photographs show the bacteria lined-up along these veins, and calculations show that the water in the veins is rich in nutrients. These findings are totally different from the normal perception of bacteria entombed in chemically dilute solid ice. The results suggest that ice is a viable habitat for life on Earth and other icy planets.
Geological effects of tsunami on mid-ocean atoll islands: The Maldives before and after the Sumatran tsunami
Paul S. Kench, The University of Auckland, School of Geography and Environmental Science, Auckland, New Zealand; et al. Pages 177-180.
The geological effects of tsunami waves on mid-ocean atoll islands have previously been unreported. This article presents new and unexpected findings of the stability of reef islands based on a resurvey of 13 islands in the Maldives taken 6 weeks after the Sumatran tsunami. Results show the tsunami left distinct depositional and erosional signatures on all islands, but promoted no substantial island erosion and no significant change in island area (<5%). These findings challenge the popular perception of the physical vulnerability of reef islands and have global implications for mid-ocean reef island stability because they indicate that low-lying reef islands are physically robust landforms and that the geological impact of tsunamis on atoll islands is minor.
New constraints on the age and evolution of the Wishbone Ridge, southwest Pacific Cretaceous microplates, and Zealandia-West Antarctica breakup
Nicholas Mortimer, Institute of Geological and Nuclear Sciences, Crown Research, Dunedin, Otago 9000, New Zealand; et al. Pages 185-188.
Bathymetric surveying and rock dredging of a remote part of the southwest Pacific Ocean floor was done by the German research vessel Sonne in late 2002-early 2003. As expected, basalt lavas were obtained from the deep ocean floor, but rocks more typical of continents-dacite lava, granite and schist-were also found in three places. Analysis and dating of these rocks by German, New Zealand, and Australian workers has helped define the extent of the now-submerged continent of Zealandia and of extinct island arc volcano chains that once spanned this part of the Pacific. Geologists also now have a clearer picture of the major plate tectonic events that took place between the Pacific Ocean crust and the Gondwanaland supercontinent between 100 and 120 Ma. These events led to the formation of the Zealandia continent and its later submergence during breakaway from Antarctica.
Recognizing the Albian-Cenomanian (OAE1d) sequence boundary using plant carbon isotopes: Dakota Formation, Western Interior Basin, USA
Darren R. Gröcke, McMaster University, School of Geography & Earth Sciences, Hamilton, Ontario L8S 4K1, Canada; et al. Pages 193-196.
Scientific understanding of ocean-atmosphere interactions, and thus climate change in the geologic record, is hampered by natural processes that remove much of the terrestrial record. High-resolution stable-isotope analysis of fossil wood from a mid-Cretaceous (~100 m.y. ago) terrestrial section in Nebraska provides us with the ability to precisely correlate terrestrial with detailed oceanic records in a way that was not previously possible. Based on matching the shape of the oceanic and terrestrial curves it is evident that a portion was missing in the terrestrial curve, representing less than 500,000 yr. A global regression (sea-level fall) is known to have occurred during this time interval and would explain the missing part of the terrestrial curve. This short-lived regressive phase coincides with a breakdown in oceanic stratification and a marine extinction event. Gröcke et al. interpret this rapid change in sea level as a result of glacier formation; this is contrary to the current understanding of the mid-Cretaceous period, which is considered as a super-greenhouse period. Further investigation of terrestrial sequences will provide a greater understanding of the extent and duration of this sea-level event and whether in fact it was caused by glacial cycles in a greenhouse Earth.
Coupled textural and compositional characterization of basaltic scoria: Insights into the transition from Strombolian to fire fountain activity at Mount Etna, Italy
Margherita Polacci, INGV, INGV-sezione di Catania, , Catania 95123, Italy; et al. Pages 201-204.
Vigorous explosive activity has strongly marked the eruptive behavior at Mount Etna over the last five years and severely impacted the overall economy of the eastern part of Sicily. This paper deals with the explosive activity that characterized Mount Etna in the first half of 2000, during which Strombolian explosions and fire fountains combined and generated the most significant example of paroxysmal activity in the known history of the volcano. Polacci et al. present data on the texture and glass composition of scoria emitted during this period of activity. The results are used to provide new insights into the eruptive mechanisms of Etna in 2000, as well as to understand the general dynamics of explosive activity at active basaltic volcanoes.
Explorer deformation zone: Evidence of a large shear zone and reorganization of the Pacific-Juan de Fuca-North American triple junction
Robert P. Dziak, Oregon State University/NOAA, CIMRS, Hatfield Marine Science Center, Newport, Oregon 97365, USA. Pages 213-216.
Recently collected seafloor bathymetry and a 12-year record of earthquakes recorded using U.S. military hydrophones are interpreted by Dziak to show evidence that the Explorer plate, a section of the Juan de Fuca plate located off Vancouver Island (British Columbia., Canada), thought to be being pushed beneath North America along the Cascadia Subduction Zone, is changing. This change is a result of a reorganization of the Juan de Fuca-Pacific-North American triple junction that is causing the Explorer plate to become a transform zone, with motion along the fault that is similar to that of the San Andreas. This is important because it affects scientific understanding of the current state of seismic hazard in the region, implying the nature of the subduction boundary off Vancouver Island is changing and may be smaller than previously thought.
Resolving vertical tectonics in the San Francisco Bay area from permanent scatterer InSAR and GPS analysis
Roland Bürgmann, University of California, Earth and Planetary Science Dept., Berkeley, CA 94720-4767, USA; et al. Pages 221-224.
Although the horizontal deformation associated with the San Andreas fault system in the San Francisco Bay Area is now fairly well known from dense global positioning system (GPS) networks, the vertical motions are not well understood. This paper uses a new approach, combining precise GPS-measured horizontal station velocities with permanent scatterer Interferometric Synthetic Aperture Radar measurements to determine a detailed image of vertical motions in the region. While much of the inferred uplift and subsidence appear to be associated with nontectonic source processes, such as landslides, settling sediments, and subsurface fluid transport, tectonic uplift is found at restraining bends or step-overs along the major strike-slip faults. Active uplift rates of ~1mm/yr in these regions suggest rapid convergence rates and may indicate significant earthquake hazard associated with the underlying active thrust faults.
GSA TODAY Science Article
Evidence for El Niño-like conditions during the Pliocene
Ana Christina Ravelo, et al., Ocean Sciences Department, University of California, Santa Cruz, California 95064, USA.
A key question for understanding global climate change is how the El Niño–Southern Oscillation phenomenon, which has a massive impact on tropical and extra-tropical conditions, will respond to global warming. One of the best means to evaluate this question is to examine the behavior of the tropical Pacific during prior warm intervals, such as 2 to 5 million years ago in the Pliocene warm interval, which was demonstrably warmer than present conditions and was geologically recent enough to have very complete and well-dated oceanic records. A controversy about tropical Pacific conditions during the Pliocene has been brewing since Science published two articles in 2005 that posited opposite conclusions about the Pliocene Pacific. Each used temperature proxy data derived from fossil shells of planktonic single-celled organisms called foraminifera. The first Science article, by Oxford scientists Rickaby and Holloran, argued that the Pliocene Pacific Ocean temperature structure and climate was La Niña-like. The second article, five months later, by University of California at Santa Cruz scientists Wara, Ravelo, and Delany, used a much more extensive data set to argue for an El Niño-like Pliocene. The controversy generated notice in the press, including the New York Times. In their GSA Today article, Ravelo, Denkens, and McCarthy review key issues for the tropical Pacific and present new data sets that support an El Niño-like Pliocene Pacific. Their synthesis of the Pliocene inspires the question of whether El Niño years likely may dominate a warmer future.
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