|1 December 2011
GSA Release No. 11-82
Director - GSA Communications & Marketing
GEOLOGY Highlights: New Research Posted in November
Boulder, Colo., USA – New GEOLOGY articles posted online ahead of print from 14-23 November report the discovery of tranquillityite, formally believed to originate on Earth’s Moon, in Australia’s Eel Creek Formation; new HiRISE images from Mars showing evidence of dune migration; the first-ever observed earthquake-eruption-landslide sequence at an active submarine volcano in the western Pacific; and a presentation of the first 3-D seismic velocity model of the Hong Kong region.
Keywords: Chile, subduction zones, carbon capture and storage, Hong Kong, Dangan Island Fault Zone, microbial mats, seismic reflection data interpretation, industry, global carbon cycle, El Niño-Southern Oscillation, Marca Shale, African Superplume, HiRISE, Mars, NW Rota-1m, Pacific, 2010 Darfield earthquake, New Zealand, Permian, mercury, small carbonaceous fossils, SCFs, Axial Volcanic Ridge, Bond Cycles, Brazil, Snowball Earth, Southwest Indian Ridge, Apollo 11, tranquillityite, Australia.
Highlights are provided below. Representatives of the media may obtain complementary copies of GEOLOGY articles by contacting Christa Stratton at the address above. Abstracts for the complete issue of GEOLOGY are available at http://geology.gsapubs.org/.
Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOLOGY in articles published. Contact Christa Stratton for additional information or assistance.
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Chilean flat slab subduction controlled by overriding plate thickness and trench rollback
Vlad C. Manea et al., Computational Geodynamics Laboratory, Centro de Geociencias, Campus Juriquilla, Universidad Nacional Autónoma de México, Querétaro, México. Posted online 14 Nov. 2011; doi: 10.1130/G32543.1.
How and why normally dipping subduction zones evolve into flat subduction zones has been long debated. It has been suggested that slab flattening in South America results from subduction of buoyant aseismic ridges or trenchward motion of the overriding plate. Nevertheless, numerical models suggest that, given the plate velocities for Nazca and South America, the actual ridge dimensions are too small to induce flattening of such large slab segments. Vlad C. Manea of the Universidad Nacional Autonoma de Mexico and colleagues present time-dependent numerical experiments that show how flat subduction can result from a combination of trenchward motion of thick cratonic lithosphere accompanied by trench rollback. They demonstrate that trenchward motion of cratonic lithosphere, 200-300 km thick, presently at ~700-800 km from the trench, reproduces a flat-slab geometry that fits the stress pattern, seismicity distribution, and the temporal and spatial evolution of deformation and volcanism in the Chilean flat slab region.
Two-phase fluid flow properties of cataclastic fault rocks: Implications for CO2 storage in saline aquifers
Christian Tueckmantel et al., Shell Global Solutions International B.V., Kessler Park 1, 2288 GS Rijswijk, Netherlands. Posted online 14 Nov. 2011; doi: 10.1130/G32508.1.
Carbon capture and storage in geological formations may provide an important contribution to the reduction of greenhouse gas emissions and the mitigation of global warming. Saline aquifers in Permo-Triassic rocks could provide large storage capacity for CO2 in Europe. There are, however, still large uncertainties regarding the exact volume of CO2 that can be stored in these formations and the rates at which CO2 can be injected. Faults could potentially provide major baffles to CO2 injection that would severely reduce feasible injection rates. Conventionally, the impact of fault rocks on fluid flow has been modeled by only taking into account their single-phase permeability, i.e., assuming only one fluid phase is present. Christian Tueckmantel of Shell Global Solutions International and colleagues collected data on the two-phase fluid flow properties of fault rocks similar to those likely to be found in potential storage sites offshore UK. They used this data to populate a CO2 injection model. Results indicate that faults could provide far more of a baffle to cross-fault flow of CO2 than would have been predicted based solely on the single-phase fault properties. Consequently, either more complex injection strategies may need to be adopted or more data acquired to ensure the absence of faults in aquifers selected for CO2 storage.
Three-dimensional tomographic model of the crust beneath the Hong Kong region
Shaohong Xia et al., CAS (Chinese Academy of Sciences) Key Laboratory of Marginal Sea Geology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou, 510301, China. Posted online 14 Nov. 2011; doi: 10.1130/G32537.1.
Shaohong Xia of the Chinese Academy of Sciences and colleagues present the first three-dimensional seismic velocity model of the Hong Kong region, including the Dangan Island Fault Zone (DIFZ). This tectonic model provides the necessary empirical constraints for understanding the relationship between seismicity and crustal structures in the region, which has been affected by significant and destructive intraplate earthquakes. The results provide key evidence for hypotheses leading to further studies in the tectonics of the region and other passive margins, and is particularly timely because of the need for better subsurface seismotectonic knowledge of the densely populated, rapidly developing coastal regions around Hong Kong. The results and geologic interpretation are important to studying the regional tectonics and to understanding the relationship between fault zones and intraplate earthquakes. In addition, the new tectonic model of the Hong Kong region should appeal to policy makers who are involved in the development of the densely populated region of South China as well as to a broader readership interested in the geology of the international metropolis.
Possible early foraminiferans in post-Sturtian (716−635 Ma) cap carbonates
Tanja Bosak et al., Dept. of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Posted online 14 Nov. 2011; doi: 10.1130/G32535.1.
The naked and shelly ancestors of foraminifera, an ecologically important group of modern ameboid eukaryotes, were thought to be absent from the fossil record before 542 million years ago. However, fossils of tubular organisms similar to some modern foraminiferans are present in the recently discovered 716-635 million year-old microfossil assemblages. These microfossils lived in microbial mats that colonized the seafloor immediately after a low-latitude glaciation (the first Snowball Earth episode). The fossil tubes are slender (up to 1.3 mm long), their organic matrix is coated by a diverse assemblage of minerals, and they resemble modern small foraminiferans that bind sediment grains to form mineral-rich shells. The earliest putative representatives of modern foraminifera built shells by agglutinating minerals and colonized the seafloor much earlier than previously thought, and were present in ecosystems immediately after one the most dramatic climate changes in Earth history.
What makes an expert effective at interpreting seismic images?
C.E. Bond et al., Midland Valley Exploration, 144 West George Street, Glasgow G2 2HG, UK. Posted online 14 Nov. 2011; doi: 10.1130/G32375.1.
Interpretation of uncertain geological data underpins much of the world’s hydrocarbon exploration and future carbon minimization strategies (CO2 storage, radioactive waste disposal). It is therefore crucial to develop techniques and protocols that will improve interpretation of subsurface data. C.E. Bond of Midland Valley Exploration and colleagues asked 184 academic and industry experts to interpret a typical oil industry seismic reflection dataset and found that just over a third got the “right” answer. Bond and colleagues show that interpretational accuracy is significantly improved for experts educated to postgraduate level (regardless of years of experience). Furthermore, although only 18 of 184 experts validated their interpretation, these experts were almost three times as likely to produce the correct result. Our findings strongly suggest that significant improvements in the reliability of interpretations of geological structures could be made by increased postgraduate recruitment and by changes to industry workflows and quality assurance procedures to explicitly include validation techniques.
Does the global stratigraphic reproducibility of 13C in Neoproterozoic carbonates require a marine origin? A Pliocene-Pleistocene comparison
P.K. Swart, University of Miami, 4600 Rickenbacker Causeway, Miami, Florida 33134, USA and M.J. Kennedy, University of Adelaide, South Australia 5005, Australia. Posted online 14 Nov. 2011; doi: 10.1130/G32538.1.
Throughout geological time, variations in the ratio of 13C to 12C in carbonate rocks has been used to infer changes in the amount of buried organic carbon and other features of the global carbon cycle. One of the criteria used to establish that these variations were real, rather than a result of alteration, and hence related to the variation in the global carbon cycle was the presence of a carbon isotopic signal of similar magnitude on a global scale. However, this study by P.K. Swart of the University of Miami and colleagues shows that alteration of the carbon isotopic signal during Plio-Pleistocene sea-level changes can also produce global patterns in the carbon isotopic signal which are unrelated to the carbon cycle. Their research therefore suggests an alternative mechanism to explain the large carbon isotope anomalies which preceded the glaciations during Neoproterozoic time, popularly referred to as the Snowball Earth.
Elevated mantle temperature beneath East Africa
Tyrone O. Rooney et al., Dept. of Geological Sciences, Michigan State University, East Lansing, Michigan 48824, USA. Posted online 14 Nov. 2011; doi: 10.1130/G32382.1.
This work presents new mantle potential temperature estimates from volcanic rocks to probe the thermal state of the East African upper mantle. Our results are synthesized with seismological data to examine one of the mantle’s most striking features -- the African Superplume. We demonstrate that the mantle beneath East Africa has been characterized by elevated temperatures throughout the past forty million years, peaking during eruption of the Ethiopian-Arabian flood basalt province. When compared to other large igneous provinces, the thermal anomaly evident in our dataset falls at the low end of the global temperature range, despite the exceptionally slow seismic velocity mantle that characterizes the region. Our new thermal constraints have the implication that mantle seismic tomographic images from East Africa cannot, as has often previously been assumed, be explained simply by variations in temperature. We conclude instead that CO2- assisted melt production in the African Superplume must contribute significantly to the markedly slow seismic velocities observed.
Planet-wide sand motion on Mars
Nathan T. Bridges et al., Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Road, Laurel, Maryland 20723, USA . Posted online 14 Nov. 2011; doi: 10.1130/G32373.1.
Nathan T. Bridges of Johns Hopkins University and 12 co-authors describe new observations showing that many sand ripples and dunes (collectively termed "bedforms") move up to a few meters per year. These findings are surprising given the previous paradigm of infrequent sand motion due the very low pressure of Mars' atmosphere (less than 1% that on Earth). Only with the advent of high resolution (down to 25 cm/pixel) images from the HiRISE camera has bedform migration become apparent. All bedforms in the north polar sand sea appear mobile, with many other dunes and ripples at other latitudes moving as well. The average dune migration rate is similar to that of some of the slower dunes on Earth. The most likely explanation is that wind gusts, although rare, are of sufficient strength and frequency to mobilize large volumes of sand. Bedforms that appear immobile may be composed of coarser grains, requiring a past climate with a thicker atmosphere to move. In any case, the current climate of Mars is sufficient to mobilize many bedforms seen on the planet.
El Niño-Southern Oscillation variability from the Late Cretaceous Marca Shale of California
Andrew Davies et al., National Oceanography Centre Southampton, School of Ocean and Earth Science, University of Southampton, Southampton SO14 3ZH, UK. Posted online 23 Nov. 2011; doi: 10.1130/G32329.1.
The El Nino phenomenon (also known as the El Nino Southern Oscillation or ENSO) is the world's pre-eminent mode of inter-annual climate variability. It has strong impacts globally, not just in those continents bordering the equatorial Pacific Ocean where it originates. Changes in the recent behavior of ENSO, together with evidence from the geological past, have been used to propose that present climate warming trends may lead to a permanent El Niño that would have potentially devastating consequences. Some evidence from past warm climates has been used to support this "permanent El Nino" scenario for the future, however others do not. One of the warmest episodes in earth history was the "greenhouse" period of the Cretaceous. We studied laminated sediments from the Panoche Hills in California that, like tree rings, preserve seasonal-scale records of climate variability and from this derived the first seasonally resolved climate record from the Cretaceous. Analysis of these records shows that there was strong climate variability at El Nino frequencies in the Cretaceous. On the basis of this evidence of a robust El Nino during one of the warmest periods in Earth history, a change to a future "permanent El Nino' does not appear likely.
Millennial-scale climate cycles in Permian-Carboniferous rhythmites: Permanent feature throughout geologic time?
Daniel R. Franco et al., Dept. of Geophysics, National Observatory, Rua General José Cristino 77, 20921-400 Rio de Janeiro, RJ, Brazil. Posted online 23 Nov. 2011; doi: 10.1130/G32338.1.
Daniel R. Franco of Brazil's National Observatory and colleagues analyze Permo-Carboniferous laminated rocks of glacial origin from southern Brazil, and suggest that the so-called "Bond Cycles" - abrupt, millennial scale cycles of warming and cooling reported as largely responsible for major climate shifts over the last 100,000 years - were also present during the Late Paleozoic Ice Age. This interpretation provides a new perspective in understanding the key mechanisms that drive climate change through Earth's history, and for predicting future global climate change.
Submarine landslide triggered by volcanic eruption recorded by in situ hydrophone
W.W. Chadwick, Jr., et al. Hatfield Marine Science Center, Oregon State University, Newport, Oregon 97365, USA. Posted online 23 Nov. 2011; doi: 10.1130/G32495.1.
This paper by W.W. Chadwick of Oregon State University and colleagues presents the first-ever observed earthquake-eruption-landslide sequence at an active submarine volcano in the western Pacific, named NW Rota-1. The sequence was documented by a combination of repeated seafloor mapping, dives with remotely operated vehicles, and acoustic monitoring by an in-situ hydrophone (which fortunately managed to survive these rather catastrophic events!). NW Rota-1 is one of only two places in the world where deep-sea eruptions have been discovered and witnessed, and where low-level eruptive activity appears to be ongoing over many years. We know very little about active submarine eruptions and their impacts because they are so difficult to observe while they are happening, which makes these results from NW Rota-1 truly extraordinary. An earthquake swarm recorded near NW Rota-1 in early 2009 represented an intrusion of new magma below the volcano and lead to a large eruption that lasted almost 4 days and produced 10 times the acoustic energy of the background level of activity. This unusual eruption in mid-August 2009, in turn, triggered a landslide that moved material from near the summit to the lower slopes of the volcano, showing that eruptions and landslides are in a close feedback relationship during the growth of submarine arc volcanoes.
Surface rupture during the 2010 Mw 7.1 Darfield (Canterbury) earthquake: Implications for fault rupture dynamics and seismic-hazard analysis
M. Quigley et al., Dept. of Geological Sciences, University of Canterbury, 8140 Christchurch, New Zealand. Posted online 23 Nov. 2011; doi: 10.1130/G32528.1.
Earthquake-induced surface ruptures provide important insights into the behavior of active faults and are used to assess seismic hazard. The 2010 Mw 7.1 Darfield earthquake in New Zealand occurred on a previously unidentified fault network and generated a ~29.5 km long surface rupture. High-accuracy measurements of surface displacements along the Greendale fault were obtained using field investigations and airborne LiDAR data. Maximum (5.3 plus or minus 0.5 m) and average net surface displacements (2.5 plus or minus 0.1 m) are anomalously large for a fault of this length and earthquake of this Mw. This relates to an increased ability to characterize surface displacements beyond the brittle cracking zone using modern technology, a concentration of co-seismic slip at relatively shallow (less than 3-5 km) depths, and a moderately high stress drop of ~14 MPa that we attribute to the rupture of a ‘strong fault’ with an earthquake recurrence interval of sufficient duration to allow significant interseismic 'healing'. Analogous regions on the fringes of active origins are likely to be similarly characterized by 'strong' faults that are prone to higher-than-average stress drops, earthquake 'clustering', and complex and/or 'hidden' ruptures. These characteristics pose major hazards to urban centers, as exemplified by the 2010-2011 Canterbury (Christchurch) earthquake sequence.
Latest Permian mercury anomalies
Hamed Sanei et al. Geological Survey of Canada, 3303-33rd Street NW, Calgary, Alberta T2L2A7, Canada. Posted online 23 Nov. 2011; doi: 10.1130/G32596.1.
New research shows that catastrophic mercury emissions are associated with the largest volcanic eruptions in Earth's history (the Siberian Trap eruptions), which may have caused toxic ocean conditions and ultimately contributed to Earth's greatest extinction event in the Latest Permian (~252 million years ago). This study, by Hamed Sanei of the Geological Survey of Canada and colleagues provides evidence that an important "self-cleaning" marine process, which deposits mercury into bottom sediments by organic matter scavenging, broke down, allowing catastrophic toxic mercury levels to develop. However, the oceans started an alternative recovery process when switched to extreme euxinic (hydrogen sulphide bearing) conditions allowing sulphide to capture and settle down mercury from ocean water. This sulphide drawdown process marked the beginning of a self mitigation process that led to gradual recovery from toxic conditions. This study provides a new concept of both the drivers for the largest extinction in Earth's history and recovery processes. It also begs the question of if there are significant mercury loading events associated with other major eruption phases in earth history. As well, the results of this study have direct bearing on a current debate as to the impact of modern mercury loading and organic matter scavenging theory on marine ecosystems.
Small carbonaceous fossils (SCFs): A new measure of early Paleozoic paleobiology
N.J. Butterfield and T.H.P. Harvey Dept. of Earth Sciences, University of Cambridge, Downing Street, Cambridge, Cambridgeshire CB2 3EQ, UK. Posted online 23 Nov.; doi: 10.1130/G32580.1.
Most organisms have little likelihood of ending up in the fossil record, partly because they lack mineralized hard-parts, partly because their constituent parts disarticulate before final burial. In rare circumstances, such as the celebrated Burgess Shale, these hurdles have been effectively by-passed, offering an extraordinary view onto early animal life. This same rarity, however, precludes any useful application of Burgess Shale-type data to larger scale paleobiological questions, such as evolution and extinction dynamics or environmental/biogeographic distribution. In this study, N.J. Butterfield and T.H.P. Harvey of the University of Cambridge identify a new and largely untapped source of paleontological data -- "small carbonaceous fossils" (SCFs) -- that adds significantly to our understanding of early Paleozoic evolution. Although mostly disarticulated, SCFs document the presence of diverse 'soft-bodied' organisms across a much wider range of environments than represented by Burgess Shale-type macrofossils, and are proving sufficiently common to record first-order macroecological and macroevolutionary patterns. The authors identify three entirely new middle Cambrian occurrences of 'soft-bodied' fossils, demonstrate a global distribution of the Burgess Shale problematicum Wiwaxia, and document the oldest known occurrence of crustacean arthropods. SCFs also significantly expand the record of various shelly fossils, and provide compelling new evidence for the reality of the Cambrian explosion.
Eruptive hummocks: Building blocks of the upper ocean crust
Isobel Yeo et al. Department of Earth Sciences, Durham University, Durham, DH1 3LE, UK. Posted online 23 Nov. 2011; doi: 10.1130/G31892.1.
As tectonic plates move apart new oceanic crust is produced at mid-ocean ridges. At slow spreading ridges, like the Mid-Atlantic Ridge, this spreading is typically marked by an Axial Volcanic Ridge (AVR), large, elongate, composite volcanoes formed by successive eruptions building on top of each other. Using a combination of methods to study a typical slow-spreading ridge segment at 45 degrees N on the Mid-Atlantic ridge (and following the work of other authors), Isobel Yeo of Durham University and colleagues find that these eruptions take the form of small, monogenetic hummocks. Using new methods of high-resolution (~1 m) bathymetry acquisition, they find that these hummocks are commonly prone to gravitational collapse (approximately 33% have collapsed), producing extremely steep scarps and converting a large percentage of their volume (typically ~40%) to rubble. Because AVRs are the main site of volcanism at slow-spreading mid-ocean ridges, it follows that they are the location for the production of the upper oceanic crust. As such the addition of this higher porosity rubble deposits may increase average upper crustal porosity by several percent, contributing greater than 0.5 km s-1 to seismic velocity decrease in the upper oceanic crust and possibly acting as one of the dominant mechanisms for increasing porosity in upper slow-spread oceanic crust.
First active hydrothermal vents on an ultraslow-spreading center: Southwest Indian Ridge
Chunhui Tao et al., Key Laboratory of Submarine Geosciences, Second Institute of Oceanography, SOA, Hangzhou 310012, China. Posted online 23 Nov. 2011; doi: 10.1130/G32389.1.
The ultraslow-spreading Southwest Indian Ridge is a major tectonic province, representing one of the important end-member mid-oceanic-ridge types for its very slow and oblique spreading. This ridge also provides the only known route for migration of chemosynthetic deep-sea vent fauna between the Atlantic and Indian Oceans. Chunhui Tao of China’s Second Institute of Oceanography and colleagues investigate the first active high-temperature hydrothermal field found on any ultraslow mid-ocean ridge worldwide. Located on Southwest Indian Ridge, it consists of three zones extending ~1000 m laterally, and it is one of four recently discovered active and inactive vent sites within a 250-km long magmatically robust section. Results presented here provide the first direct evidence for potentially widespread distribution of hydrothermal activity along ultraslow-spreading ridges -- at least along magmatically robust segments. This implies that the segment sections with excess heat from enhanced magmatism and suitable crustal permeability along slow and ultraslow ridges might be the most promising areas for searching for hydrothermal activities. Tao and colleagues also not that it is surprising that the special vent fauna appear to indicate some complex affinity to those on the Central Indian Ridge, southern Mid-Atlantic Ridge, and the southwest Pacific Ocean.
Tranquillityite: The last lunar mineral comes down to Earth
Birger Rasmussen et al., Dept. of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia. Posted online 23 Nov. 2011; doi: 10.1130/G32525.1.
Tranquillityite was first discovered in mare basalts collected during the Apollo 11 Lunar Mission to the Mare Tranquillitatis (Sea of Tranquility). Until now, the mineral has since been only in returned Moon samples and lunar meteorites, with no Earth counterpart. However, Birger Rasmussen of Curtin University and colleagues have now identified tranquillityite in six dolerite dikes and sills from Western Australia. This terrestrial tranquillityite occurs as clusters of fox-red laths closely associated with baddeleyite and zirconolite in quartz and potassium-feldspar intergrowths in late-stage interstices between plagioclase and pyroxene. Its composition is relatively uniform, and its habit and chemistry are consistent with tranquillityite found on the Moon. It has a face centered-cubic subcell, similar to that of annealed lunar tranquillityite. Rasmussen et al. used sensitive high-resolution ion microprobe (SHRIMP) U-Pb geochronology for tranquillityite from sills intruding the Eel Creek Formation, northeastern Pilbara Craton, to determine an approximate age of 1064 million years (plus or minus 14 m.y.). This age indicates that the previously undated sills belong to the ca. 1070 Ma Warakurna large igneous province, extending the geographic range of this mafic complex. The date also provides a new minimum age (older than 1.05 billion years) for the intruded sedimentary rocks, which were previously thought to be Neoproterozoic. Examination of dolerite from Western Australia suggests that tranquillityite is a relatively widespread, albeit volumetrically minor, accessory mineral and, where sufficiently coarse, it represents an exceptional new U-Pb geochronometer.