Boulder, Colo., USA - Two new Geology articles this month are open access: "Steady rotation of the Cascade arc" and "Silica gel formation during fault slip: Evidence from the rock record." Other new articles cover everything from the discovery of fossilized whale "intestinal products" in central Italy to flooding as a result of the 2010 Eyjafjallajökull volcano eruption to new findings via the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) to using microfossils to reconstruct massive earthquakes in Cascadia.
Highlights are provided below. Geology articles published ahead of print can be accessed online at http://geology.gsapubs.org/content/early/recent. All abstracts are open-access at http://geology.gsapubs.org/; representatives of the media may obtain complimentary Geology articles by contacting Kea Giles at the address above.
Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to Geology in articles published. Contact Kea Giles for additional information or assistance.
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Steady rotation of the Cascade arc
Ray E. Wells and Robert McCaffrey, U.S. Geological Survey, 345 Middlefield Road, MS 973, Menlo Park, California 94025, USA. Published online 22 July 2013, http://dx.doi.org/10.1130/G34514.1.
This article by Ray E. Wells and Robert McCaffrey is open-access. Wells and McCaffrey write that the clockwise geologic displacement of the 16 million-year-old (m.y.) ancestral Cascade volcanic arc from the presently active volcanic chain in the northwestern U.S. and Canada is in the same sense and at nearly the same rate as the motions calculated from Global Positioning System (GPS) over the past 15 years. Motion of the ancestral arc can be explained by clockwise rotation of the leading edge of North America at 1.0 degree/m.y. over the magma source generated by the subducting plate, which is itself moving westward 1 to 4.5 km/m.y. as it slowly sinks into the mantle. The remarkable similarities between post-16 m.y. arc migration, paleomagnetic rotation, and modern GPS block motions indicate that the block motions from decadal GPS can be used to calculate meaningful long-term crustal strain rates and earthquake hazards.
Silica gel formation during fault slip: Evidence from the rock record
J.D. Kirkpatrick et al., Dept. of Geosciences, Colorado State University, 1482 Campus Delivery, Fort Collins, Colorado 80523, USA. Published online 22 July 2013, http://dx.doi.org/10.1130/G34483.1.
This open-access article by J.D. Kirkpatrick and colleagues discusses the dynamic reduction of fault strength as a key process during earthquake rupture. Many mechanisms for causing coseismic weakening have been proposed based on theory and laboratory experiments, including silica gel lubrication. However, few have been observed in nature. Here, Kirkpatrick and colleagues report on the first documented occurrence of a natural silica gel coating a fault surface at the Corona Heights fault slickenside in San Francisco, California, USA.
Comparison of microstructures in superplastically deformed synthetic materials and natural mylonites: Mineral aggregation via grain boundary sliding
Takehiko Hiraga et al., Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan. Published online 22 July 2013, http://dx.doi.org/10.1130/G34407.1
Very similar microstructure, that is, the same phase aggregation is found in both experimentally deformed synthetic materials and natural ultramylonite. The stress-strain-rate relationship, grain-size dependent flow strength, and the achievement of large tensile stain on the synthetic samples indicate that the samples creeped due to grain boundary sliding (GBS). As a result of GBS, grain-switching events allow dispersed phases to contact grains of the same phase oriented in the direction of compression. Mineral phase mixing through GBS, which helps to retain fine grain size in rocks due to grain boundary pinning, has been speculated to occur during formation of mylonites. However, the results presented here by Takehiko Hiraga and colleagues contradict this hypothesis because mineral aggregation through GBS promotes demixing rather than mixing of the mineral phases. GBS processes alone will not promote a transformation of well-developed monomineralic bands to polymineralic bands during mylonitization.
Sea-level-induced seismicity and submarine landslide occurrence
Daniel S. Brothers et al., U.S. Geological Survey, Coastal and Marine Science Center, 384 Woods Hole Road, Woods Hole, Massachusetts 02543, USA. Published online 22 July 2013, http://dx.doi.org/10.1130/G34410.1.
The relationships between global climate change and marine geohazards remains poorly understood. This study by Daniel S. Brothers and colleagues investigates compelling linkages between rapid sea-level rise, bending of the lithosphere, and stress loading of crustal faults. Rupture of such faults may induce slope failure and generate submarine landslides, thus offering a new explanation for the temporal coincidence between many submarine landslides and rapid sea level rise between 16,000 and 8,000 years ago.
Reconciling disparate estimates of total offset on the southern San Andreas fault
Michael H. Darin and Rebecca J. Dorsey, Dept. of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403-1272, USA. Published online 22 July 2013, http://dx.doi.org/10.1130/G34276.1.
The total amount of motion along a fault can be estimated by offset geologic markers that were once continuous across the fault. There are various pairs of cross-fault markers along the San Andreas fault in southern California that suggest different amounts of offset ranging from 160-240 km. These estimates assume that the markers were offset solely by sliding along the fault. In this paper, Michael H. Darin and Rebecca J. Dorsey reinterpret one pair of offset markers and use a simple geometric model to show that fault block rotation adjacent to the fault can account for a small but significant amount of the apparent offset, thus reducing the amount of sliding required to displace the formerly continuous markers. Their model provides a new estimate of ~200 km of total offset on the San Andreas fault within the Salton Trough region, which is consistent with all other geologic data from various offset markers. This new lower estimate of total slip on the southern San Andreas fault implies that more slip is required on other faults in California and Arizona, in order to satisfy estimates of the total amount of motion between the Pacific and North American plates.
Erosion of the Tsangpo Gorge by megafloods, Eastern Himalaya
Karl A. Lang et al., Dept. of Earth and Space Sciences, and Quaternary Research Center, University of Washington, Box 351310, Seattle, Washington 98195, USA. Published online 22 July 2013, http://dx.doi.org/10.1130/G34693.1.
Karl Lang and colleagues present new detrital zircon U-Pb provenance data from large magnitude "megaflood" deposits immediately downstream of the Yarlung-Tsangpo River Gorge in the easternmost Himalaya. These data support the previous hypothesis that Tibetan lakes restrained by glacial ice and debris within the Yarlung-Tsangpo drainage episodically evacuated through this steep, narrow gorge. These extreme flood events were capable of transporting a considerable amount of material from steep channel-adjacent hillslopes, focusing erosion within the gorge. Although megaflood frequency remains unconstrained, these data support a mechanism to substantially contribute to the Quaternary erosion of the Yarlung-Tsangpo Gorge.
The role of multiple glacier outburst floods in proglacial landscape evolution: The 2010 Eyjafjallajökull eruption, Iceland
Stuart A. Dunning et al., Geography, Engineering and Environment, Northumbria University, Newcastle upon Tyne NE1 8ST, UK. Published online ahead of print 30 July 2013, http://dx.doi.org/10.1130/G34665.1.
In 2010, Eyjafjallajökull volcano in Iceland erupted, causing global disruption due to the dispersing ash cloud. Less well known are the series of floods (known by the Icelandic term "jökulhlaup") generated by the melting of parts of the volcano ice-cap. Stuart A. Dunning and colleagues had a unique opportunity to survey the flood routing prior to the flooding as monitoring indicated an imminent eruption, and to then return after the eruption ended. They used laser scanning and time-lapse camera imagery to create detailed 3D models of the landscape to quantify change. The jökulhlaups completely in-filled a lake at the foot of Gigjökull glacier with sediment totaling more than 17 million cubic meters -- enough to cover nearly 315 NFL pitches 10 m deep in a mix of ash, rock, and ice. Contrary to prior models assuming that the largest events dominate the proglacial landscape, much change occurred during a series of comparatively small jökulhlaup events. It is erosion and deposition from these events that dominate the current surface landscape.
Globally synchronous Marinoan deglaciation indicated by U-Pb geochronology of the Cottons Breccia, Tasmania, Australia
C.R. Calver et al., Mineral Resources Tasmania, PO Box 56, Rosny Park, Tasmania 7018, Australia. Published online ahead of print 30 July 2013, http://dx.doi.org/10.1130/G34568.1.
The apparently global distribution of Marinoan glacial deposits inspired the "snowball Earth" hypothesis, and prompted designation of the top of the type Marinoan glacials in South Australia as the Global Stratotype Section and Point ("golden spike") for the base of the terminal Proterozoic, Ediacaran System. However, horizons suitable for radioisotopic dating are lacking in the stratotype section and correlated sequences on mainland Australia. Ash beds suitably placed to directly and precisely date the Cryogenian-Ediacaran transition have so far been found only in Namibia (635.5 plus or minus 0.8 million years ago) and south China (635.2 plus or minus 0.8 million years ago). In this paper, C.R. Calver and colleagues show that a probable reworked volcaniclastic horizon at the very top of the Cottons Breccia, a Marinoan glacial correlative on King Island, Tasmania, has yielded an abundant population of juvenile zircons dated (by U-Pb on zircon, using chemical abrasion-thermal ionization mass spectroscopy) at 636.4 plus or minus 0.5 million years ago. Equivalence to the ash bed dates from Namibia and China supports correlation of those strata to the Australian type sections, and globally synchronous deglaciation at the beginning of the Ediacaran Period, and is consistent with the "snowball Earth" hypothesis.
A hematite-bearing layer in Gale Crater, Mars: Mapping and implications for past aqueous conditions
A.A. Fraeman et al., Dept. of Earth and Planetary Sciences, Washington University in St. Louis, St. Louis, Missouri 63130, USA. Published online ahead of print 30 July 2013, http://dx.doi.org/10.1130/G34613.1.
The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) instrument on the Mars Reconnaissance Orbiter collected data in a spatially overlapping mode that show hematite, an iron oxide mineral, is present on the top of a layered ridge located about 3 km away from the Curiosity rover's proposed arrival point at Mount Sharp's base. The hematite formed either through leaching of local material in neutral to acidic waters or through mixing of anoxic groundwaters with a more oxidizing water body or atmosphere. These formation hypotheses can be tested using Curiosity's payload, and both scenarios indicate that the ridge was a site of past active iron oxidation. In similar environments on Earth, iron oxidation is almost exclusively mediated by microorganisms. This hematite ridge therefore represents a specific site where concentrated and localized iron oxidation occurred, and is a prime location to search for signs of past habitability.
Dynamic pore-pressure variations induce substrate erosion by pyroclastic flows
O. Roche et al., Laboratoire Magmas et Volcans, Université Blaise Pascal, CNRS UMR6524, IRD R163, 5 rue Kessler, F-63038 Clermont-Ferrand, France. Published online ahead of print 30 July 2013, http://dx.doi.org/10.1130/G34668.1.
Pyroclastic flows are ground-hugging dense mixtures of gas and particles generated during volcanic eruptions. Field evidence shows that they can entrain blocks from underlying substrates formed by earlier geological events, yet, counter-intuitively, they are less likely to erode unconsolidated layers of fine particles. O. Roche and colleagues report laboratory experiments that reproduce these seemingly contradictory observations and also offer a means to infer pyroclastic flow velocity. Experiments demonstrate that the sliding head of a granular flow generates a dynamic upward pore pressure gradient at the flow-substrate interface. Associated upward air flux is enough to fluidize a smooth substrate of fines, so that particles are not entrained individually but the substrate instead results in small shear instabilities. In contrast, coarse particles forming a non-fluidized rough substrate are lifted at a critical upward force due to the pore pressure gradient, according to their individual masses, which provides a basis for a model to calculate the flow velocity. Application to the 18 May 1980 pyroclastic flow deposits at Mount St. Helens gives velocities of about 9 to 13 meters per second at about 6 to 7 km from the vent on gentle slopes (less than 4 to 6 degrees), in agreement with field observations at this volcano and others.
A stable and hot Turonian without glacial {delta}18O excursions is indicated by exquisitely preserved Tanzanian foraminifera
Kenneth G. MacLeod et al., Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA. Published online ahead of print 30 July 2013, http://dx.doi.org/10.1130/G34510.1.
A shift over a few centuries from the icehouse climate state in which humans evolved to a greenhouse climate similar to that of Late Cretaceous (~100 to 65 million years ago) is an often repeated, cautionary prediction of the likely consequence of continued burning of fossil fuels and other anthropogenic additions to atmospheric CO2. The corollary, that understanding previous greenhouse times might help predict future conditions, has justified many Late Cretaceous studies. The resolution of these studies has increased to the point where temporal variability in greenhouse climates can be examined. Widespread warmth is generally accepted to have existed during the Late Cretaceous, but times of growth of continental ice sheets have also been proposed. To test this "greenhouse glacial" hypothesis, Kenneth G. MacLeod and colleagues measured the oxygen isotopic composition of more than 1,000 samples of exceptionally well-preserved shells of single celled organisms (foraminifera) from Tanzania to estimate temperatures spanning a proposed Late Cretaceous glacial interval. Results indicate that hot and remarkably stable conditions prevailed along coastal east Africa during the entire interval examined. There are no indications for growth of glacial ice at this time, and these results support an interpretation that the Earth is effectively ice-free during greenhouse times.
Implications for late Grenvillian (Rigolet phase) construction of Rodinia using new U-Pb data from the Mars Hill terrane, Tennessee and North Carolina, United States
John N. Aleinikoff et al., U.S. Geological Survey, MS 963, Denver, Colorado 80225, USA. Published online ahead of print 30 July 2013, http://dx.doi.org/10.1130/G34779.1.
New ion microprobe U-Pb geochronology of zircon and monazite from high grade gneisses of the Mars Hill terrane in western North Carolina and eastern Tennessee, USA, indicate that these rocks, previously considered to be 1.8 billion-year-old meta-igneous rocks, are really 1.0 billion-year-old metasedimentary rocks. Thus, they do not represent a fragment of ancient Amazonian crust and were not accreted to Laurentia during the Ottawan phase of the Grenvillian orogeny 1.05 billion years ago. Instead, according to John N. Aleinikoff and colleagues, these rocks probably are composed of detritus eroded from Amazonian crust; the sediments were deposited late in the development of the Rodinia supercontinent, no earlier than about 1.02 Ga, and were metamorphosed at about 0.98 Ga. As such, they are part of a long belt of recently discovered and described late Grenvillian metasedimentary rocks that extends from at least the Smoky Mountains to northern Virginia.
Magma chamber-scale liquid immiscibility in the Siberian Traps represented by melt pools in native iron
Vadim S. Kamenetsky et al., ARC Centre of Excellence in Ore Deposits and School of Earth Sciences, University of Tasmania, Hobart, TAS 7001, Australia. Published online ahead of print 30 July 2013, http://dx.doi.org/10.1130/G34638.1.
Exceptional preservation of glass inclusions in intrusive rocks of the Siberian Large Igneous Province evidences evolutionary processes of tholeiitic basalts, the most common terrestrial magmas. Silicate liquid immiscibility between aluminosilicate and iron-rich paired melts is recorded by melt pools in native iron. These unique snapshots of magma evolution prove that cooling and crystallizing basaltic magma reaches a two-liquid stability field, and two contrasting silicate melts split at large-scale. The recognition of this mechanism in the Earth's largest magmatic province is critical for understanding common compositional bimodality in other continental magmas and origin of orthomagmatic iron-titanium-phosphorus ore deposits.
Affirming life aquatic for the Ediacara biota in China and Australia
Shuhai Xiao et al., Dept. of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA. Published online ahead of print 30 July 2013, http://dx.doi.org/10.1130/G34691.1.
The Ediacara biota (579-541 million years old) has been long championed as a snapshot of the marine ecosystem on the eve of the Cambrian explosion and provides important insights into the early evolution of animals. A recent reinterpretation of the eponymous Ediacara Member of South Australia as paleosols and Ediacara fossils as lichens or microbial colonies that lived on terrestrial soils, if correct, questions the relevance of the Ediacara biota to our understanding of early marine ecosystems. This reinterpretation, however, is not supported by comparative paleobiological and functional morphological analysis. The Ediacara Member shares a number of fossil forms with Ediacaran-age assemblages preserved in unequivocally marine sediments elsewhere in the world, including marine black shales in South China. In addition, Ediacara fossils show no morphological adaptions to address the most fundamental challenges for terrestrial life, for example, mechanical support and desiccation. Thus, comparative paleobiological and functional morphological data support the conventional interpretation that the Ediacara biota records the marine ecosystems just prior to the Cambrian explosion of animals.
Water and the composition of Martian magmas
J. Brian Balta (corresponding) and Harry Y. McSween, Jr., Dept. of Earth and Planetary Sciences, University of Tennessee, 1412 Circle Drive, Knoxville, Tennessee 37996, USA. Published online ahead of print 30 July 2013, http://dx.doi.org/10.1130/G34714.1.
Shergottites are the most common type of martian meteorite. Their composition is basaltic, similar to igneous rocks from Iceland and Hawaii, but with some important differences. When the shergottites are dated using common isotope-decay techniques, they generally are found to be only a few hundred-million years old. On Mars, the only sources of igneous rocks of that age are the large volcanoes, such as Olympus Mons, which are also made of basalt. But, when J. Brian Balta and Harry Y McSween Jr. compare the shergottites to measurements of the composition of those volcanoes by orbiting spacecraft, they find that they do not match, particularly in their silica contents. Despite the shergottites matching the volcanoes in age, the volcanoes appear to be made of a different type of basalt from most of the meteorites in our collection. Magmas can dissolve small amounts of water in them, and that water can change the silica contents of magmas in a way that could explain both the volcanoes and the shergottites. Balta and McSween propose that the shergottites represent magmas generated with water and the volcanoes represent drier magmas. Magmas similar to the shergottites could therefore have been a major source of the water present on the martian surface early in its history.
Syn-tectonic, meteoric water–derived carbonation of the New Caledonia peridotite nappe
Benoît Quesnel et al., Géosciences Rennes, Université Rennes 1, UMR 6118 CNRS, 35042 Rennes Cedex, France. Published online ahead of print 30 July 2013, http://dx.doi.org/10.1130/G34531.1.
Ultramafic rocks, originated in the mantle, are sometimes carried upon continents through the process of obduction. The weathering of such rocks under warm and wet climates results in the formation of laterites associated with nickel ore deposits. A typical example is provided by the peridotite nappe of New Caledonia, SW Pacific. Carbonation of the ultramafic rocks is also indicated by abundant veins of magnesite (MgCO3) found along the serpentine sole of the nappe. In this study, exceptional outcrops recently exposed in the Koniambo Massif allow Benoit Quesnel and colleagues to document (1) the synkinematic character of many magnesite veins with respect to pervasive shear deformation of the sole, providing the first known example of syntectonic carbonation of an ultramafic nappe; and (2) the meteoric origin of the fluids from which the veins have been formed, by means of stable isotopic analyses, which indicates that carbonation and laterization represent complementary records of meteoric water infiltration. Taken together, these results suggest a scenario in which synlaterization tectonic activity has enhanced water infiltration through the nappe, leading to widespread carbonation of the serpentine sole. This calls for renewed examination of other magnesite-bearing ultramafic nappes worldwide in order to establish whether active tectonics is commonly a major agent for carbonation.
Testing the use of microfossils to reconstruct great earthquakes at Cascadia
S.E. Engelhart et al. -- B.P. Horton, corresponding: Sea Level Research, Institute of Marine and Coastal Sciences, School of Environmental and Biological Sciences, Rutgers University, New Brunswick, New Jersey 08901, USA. Published online ahead of print 30 July 2013, http://dx.doi.org/10.1130/G34544.1.
In 1700, a massive earthquake struck the west coast of North America. Though it was powerful enough to cause a tsunami as far as Japan, a lack of local documentation has made studying this historic event challenging. In a study lead by S.E. Engelhart, Benjamin Horton, Professor in the Institute of Marine and Coastal Science at Rutgers University, and colleagues have helped unlock this geological mystery using a fossil-based technique. Their work provides a finer-grained portrait of this earthquake and the changes in coastal land level it produced, enabling modelers to better prepare for future events. The Cascadia Subduction Zone runs along the Pacific Northwest coast of the United States and up to Vancouver Island in Canada. This major fault line is capable of producing megathrust earthquakes 9.0 or higher, though this trait was only discovered within the last several decades from geology records due to a dearth of observations or historical records. The Lewis and Clark expedition would not make the first extensive surveys of the region for another 100 years, and contemporaneous aboriginal accounts were scarce and incomplete. To provide a clearer picture of how the earthquake occurred, Horton and his colleagues applied a technique they have used in assessing historic sea level rise. They traveled to various sites along the Cascadia Subduction Zone, taking core samples from up and down the coast and working with local researchers who donated preexisting data sets. The researchers' targets were microscopic fossils known as foraminifera. Through radiocarbon dating and an analysis of different species' positions within the cores over time, Horton and colleagues were able to piece together a historical picture of the changes in land and sea level along the coastline. This research revealed how much the coast suddenly subsided during the earthquake. This subsidence was used to infer how much the tectonic plates moved during the earthquake.
Synchronous Oligocene-Miocene metamorphism of the Pamir and the north Himalaya driven by plate-scale dynamics
Michael A. Stearns et al., Earth Science Department, University of California, Santa Barbara, California 93106, USA. Published online ahead of print 30 July 2013, http://dx.doi.org/10.1130/G34451.1.
The Pamir and Himalaya are both parts of the India-Asia continent collision, and both have exposed deep crustal rocks ideal for investigating processes operating deep within continent collisions. Mineral dates and chemistry provide both a time record and a petrologic record of the collision at both localities. Both locations experienced metamorphism from 28 to 15 million years ago. Mineral compositions indicate a transition from crustal thickening to thinning 20 million years ago at both places. These events coincide with two tearing events of the subducting Indian plate inferred from tomography.
Enigmatic, biogenically induced structures in Pleistocene marine deposits: A first record of fossil ambergris
Angela Baldanza et al., Department of Earth Sciences, University of Perugia, Piazza Università, 06123 Perugia, Italy. Published 30 July 2013, http://dx.doi.org/10.1130/G34731.1.
This article deals to the discovery of unknown, enigmatic trace fossils in marine clay deposits about 1.75 million years ago in central Italy (Umbria Region). Some hypotheses about their origin are considered, but the most convincing explanation stands in the close similarity with large masses of present-day ambergris (known as "floating gold"), a solid, waxy, dull grey or blackish flammable substance usually associated with sperm whales. Preliminary chemical data reveal the presence of organic molecules compatible with mammalian gastric or intestinal activity. Squid beaks are also found. Most of the geological, paleontological, and chemical results allow the identification of these structures as intestinal products of sperm whales living about 1.75 million years ago. At this time, they represent the only known example worldwide of Pleistocene sperm whale "coprolites," and enhance the knowledge about the cetacean frequentation of the ancient Mediterranean Sea.
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Geology