Boulder, Colo., USA – Topics in this current batch of Geology articles posted ahead of print include the puzzle of parallel mountain chains; 25 years on the East Pacific Rise; unique episodes in Earth's history; turbidity currents; computer models; Wilson cycles; salt structure beneath the sea bed; the North Scotia Ridge; El Hierro, Canary Islands; sand-sized sub-spherical silica grains; bank pull or bar push; kaolinitic paleosols; Earth's youngest, hottest rocks; 3-D thermo-mechanical numerical models; and the Bohemian Massif.
Open Access Papers:
1. Sublithospheric small-scale convection -- A mechanism for collision zone magmatism by L. Kaislaniemi et al.
2. Cenozoic tectonic history of the South Georgia microcontinent and potential as a barrier to Pacific-Atlantic through flow by Andrew Carter et al.
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Rift flank uplift at the Gulf of California: No requirement for asthenospheric upwelling
Chris Mark et al., Department of Earth Science and Engineering, South Kensington Campus, Imperial College London, London SW7 2AZ, UK. Published online ahead of print 24 Jan. 2014; http://dx.doi.org/10.1130/G35073.1.
The Gulf of California is a continental rift system that forms part of the boundary between the Pacific and North American tectonic plates. Like many rifts, the gulf is bordered by parallel chains of mountains. The existence of such mountainous landscapes at rifts is puzzling; as the continental crust floats on the denser mantle beneath, mountains should form at zones of crustal collision and thickening, rather than rift zones where the crust thins. Several competing models exist to explain this phenomenon; crucially, these make different predictions of the relative timing of uplift and crustal extension during the rifting process. In this study, Chris Mark and his colleagues were able to use the well-preserved rift landscape along the western margin of the Gulf of California to date the timing of uplift and extension. Their results indicate that a mechanism called flexural isostasy was responsible for the uplift. This long-standing hypothesis envisages Earth's crust as an elastic sheet, which flexes upward in response to the removal of a load -- here, the thinning of the crust during rifting. Mantle upwelling beneath the mountains, posited by previous studies to explain mountain formation, is shown to be unnecessary.
Magmatic eruptions and iron volatility in deep-sea hydrothermal fluids
Nicholas J. Pester et al., Department of Earth Sciences, University of Minnesota, Minneapolis, Minnesota 55455, USA. Published online ahead of print 24 Jan. 2014; http://dx.doi.org/10.1130/G35079.1.
Few mid-ocean ridge segments are better characterized than 9 to 10 degrees north on the East Pacific Rise (EPR). Twenty-five years of intensive, integrated research has provided a wealth of geophysical and geochemical data, elucidating processes associated with vigorous hydrothermal activity. These data include a comprehensive 17-year time series of hydrothermal vent fluid chemistry, reflecting two recorded episodes of volcanism separated by a decade of steady-state chemical and physical conditions. Both magmatic events manifest unusually dilute fluids, or vapors, due to extreme degrees of phase separation. Nicholas Pester and colleagues emphasize that despite significant decreases in all other metals, these dilute vapors maintain surprisingly high dissolved Fe concentrations. They present experimental data that helps explain this phenomenon, demonstrating how Fe solubility is unexpectedly enhanced in low-density vapors. The observation that Fe fluxes generally remain consistent throughout eruption, recovery, and steady-state time periods may have implications for the succession and temporal evolution of vent related fauna.
Ti-in-zircon thermometry and crystallization modeling support hot Grenville granite hypothesis
D.P. Moecher et al., Department of Earth and Environmental Science, University of Kentucky, Lexington, Kentucky 40506, USA. Published online ahead of print 24 Jan. 2014; http://dx.doi.org/10.1130/G35156.1.
Claims of unique episodes in Earth's history (meteorite impacts; the oldest fossils) require marshalling evidence in support of those claims. This paper supports the claim that the Grenville orogeny, the mountain building event associated with assembly of the supercontinent Rodinia that occurred from 1.3 to 1.1 billion years ago, produced the hottest granite magmas in Earth history. The temperature of the granite is measured from the zirconium content of the magma, and the amount of titanium in the mineral zircon, a common accessory mineral in the granite. High temperature favors more zirconium being dissolved in the magma (which produces more zircon crystals, i.e., what is referred to as high zircon fertility) and more titanium in the zircon crystals. The titanium content was measured via Sensitive High Resolution Ion MicroProbe ("SHRIMP") method. When hot Grenville granites erode and produce sediment, they also produce an abundance of detrital zircon crystals, which show up in sandstones and sediment from all across North America and can be used to infer the ultimate source of the sand grains comprising the sandstones.
Distal turbidites reveal a common distribution for large (>0.1 km3) submarine landslide recurrence
Michael A. Clare et al., National Oceanography Centre, Southampton, European Way, Southampton SO14 3ZH, UK. Published online ahead of print 24 Jan. 2014; http://dx.doi.org/10.1130/G35160.1.
Submarine landslides can be far larger than those on land, and are important for moving sediment across our planet. They can generate tsunamis and may destroy expensive oil and gas structures. Fast moving, large volume (greater than 0.1 cubic kilometers) landslides may disintegrate to create sediment avalanches, known as "turbidity currents." These may travel hundreds of kilometers and can break important communication cables. It is important to understand their timing and triggers. This study identifies that landslide timings for three different deep sea sites are near-random. The likelihood of a large disintegrating slide occurring in a region does not depend on the time since the last. In contrast with the findings of many studies, non-random processes such as sea level change are shown to not exert a dominant control on landslide timing. This is the first study to analyze hundreds of submarine landslides using statistical analysis. It is thought that large submarine landslides are either caused by a random trigger such as large magnitude earthquakes, or more likely that a combination of factors is responsible. Intriguingly, this suggests that the frequency of large landslide-triggered flows is unlikely to change significantly due to future sea level rise.
Sublithospheric small-scale convection -- A mechanism for collision zone magmatism
L. Kaislaniemi et al., Department of Earth Sciences, Durham University, Science Labs, DH1 3LE Durham, UK. Published online ahead of print on 10 Feb. 2014; http://dx.doi.org/10.1130/G35193.1. OPEN ACCESS.
The collisions of continents typically produce orogenic uplift, that is, mountain building, and volcanism, such as in the today's Alpine-Himalayan orogeny. The volcanism associated with these collisions is distinctively different from the volcanism observed during the subduction prior to the collision. Unlike the subduction zone volcanism, the causes for the volcanism during and after the collision are poorly understood, but most hypothesized explanations relate this change in the chemical composition and geographical distribution of the volcanism to cessation of the active subduction, thickening of the lithosphere during collision, and subsequent destabilization of the lithosphere. With the help of numerical computer models, we have demonstrated in our study that the high volumes of water input into the upper mantle during the prior subduction might trigger the volcanism later, during the collision. Even though major part of this water is removed early in the subduction zone volcanism, only small amounts of leftover water are needed to later destabilize the bottom of the lithosphere with the strength decreasing effect of the water. This kind of destabilization leads to small pieces of lithosphere to drip off to the mantle below, thus causing the irregular patterns of basaltic volcanism observed at collision zones.
Seismological evidence for a fossil subduction zone in the East Greenland Caledonides
Christian Schiffer et al., Department of Geoscience, Aarhus University, Høegh Guldbergs Gade 2, 8000 Aarhus, Denmark. Published online ahead of print on 10 Feb. 2014; http://dx.doi.org/10.1130/G35244.1.
The North Atlantic is an area of great interest both in terms of resource exploration and as a natural geoscience laboratory. The region is considered a text book example of the so-called "Wilson cycle" where an ocean closed about 400 million years ago and continents involving Greenland and Norway collided leading to the formation of the large Caledonian mountain belt of Himalayan dimensions. About 350 million years later; i.e. 55 million years ago, the mountain belt rifted apart, and a new ocean formed, the present Atlantic. This is one of the best understood Wilson cycles, yet important aspects of this sequence of events are still poorly understood. Schiffer et al. present a seismological study that images a structure dipping eastward from the base of the Earth's crust close to the present ice rim in East Greenland down to a depth of 100 km or more beneath the present coast. This structure is interpreted as an inherited remnant of the collision of continental plates 400 million years ago. It is the first such image of a full-lithosphere structure in Greenland and Norway and it is key evidence for the interpretation of the tectonic history of the North Atlantic region.
Internal structure, kinematics, and growth of a salt wall: Insights from 3-D seismic data
Christopher A-L. Jackson et al., Basins Research Group (BRG), Department of Earth Science and Engineering, Imperial College, London SW7 2BP, UK. Published online ahead of print on 10 Feb. 2014; http://dx.doi.org/10.1130/G34865.1.
Structures formed by the flow of salt hold many secrets regarding their internal geometry. Exposed rock outcrops are typically too small to allow direct study of their three-dimensional complexity and data that image the subsurface of the Earth typically yield low-resolution images. In this study we use spectacular, three-dimensional seismic reflection data from offshore Brazil to image the internal structure of a salt structure (diapir) buried 2-4 km below the seabed. We are able to demonstrate that the internal structure of salt diapirs is considerably more complex than previously thought, thus challenging the paradigm that these giant structures form due to simple inward flow and thickening of ductile salt.
Cenozoic tectonic history of the South Georgia microcontinent and potential as a barrier to Pacific-Atlantic through flow
Andrew Carter et al., Department of Earth and Planetary Sciences, Birkbeck, University of London, Malet Street, London WC1E 7HX, UK. Published online ahead of print on 10 Feb. 2014; http://dx.doi.org/10.1130/G35091.1. OPEN ACCESS.
The formation and subsequent development of the Antarctic Circumpolar Current, one of the key processes that help to keep Antarctica cold by isolating it from warmer ocean waters, is poorly understood. The modern current is steered by regional bathymetry, passing through gaps in the North Scotia Ridge and diverting around the eastern end of South Georgia. How this ridge developed and whether it was a topographic barrier in the past is unknown. To answer this we investigated the uplift history of the South Georgia micro-continental block, the exposed part of the ridge. Using geochronology methods we discovered that South Georgia remained connected to South America until 45-40 million years ago when it separated and travelled over 1000 kilometers to the east driven by faulting and opening of the Scotia Sea. Our data show that during its passage South Georgia was submerged, buried under a thick pile of sediments until 10 million years ago when it started to uplift. We concluded that South Georgia could have only been a major topographic barrier to ocean currents after this recent uplift.
Mixing in mantle magma reservoirs prior to and during the 2011� eruption at El Hierro, Canary Islands
Marc-Antoine Longpré et al.. School of Earth and Environmental Sciences, Queens College, City University of New York, Flushing, New York 11367, USA; and Earth and Planetary Sciences, McGill University, Montréal, Québec H3A 2A7, Canada. Published online ahead of print on 10 Feb. 2014; http://dx.doi.org/10.1130/G35165.1.
Our ability to forecast volcanic eruptions depends on how well we understand the links between volcano monitoring signals measured at the surface and the subterranean magmatic processes that cause them. The first historical eruption at El Hierro, Canary Islands, took place in 2011-2012 and was associated with extensive seismicity and ground deformation. This study uses detailed chemical analyses of lava samples produced by the eruption to infer the architecture of the magmatic system beneath El Hierro. We find that two distinct magma batches hybridized at upper mantle depths before intruding the lower crust and causing pre-eruptive seismicity. Chemical zoning in olivine crystals indicates that the timing of magma mixing at depth correlates with period of pre-eruptive and syn-eruptive seismicity. Our data also imply that magma stalled in the lower crust on its way from the mantle to the surface, in agreement with the subhorizontal propagation of earthquakes in September 2011. The calculated depths of mantle magma reservoirs correspond to depths of syn-eruptive earthquakes, suggesting that these reservoirs became unstable after a few weeks of eruptive activity. These results demonstrate the relationship between deep-seated magmatic processes and seismicity at El Hierro, and will help the interpretation of similar data at other volcanoes.
Primary silica granules—A new mode of Paleoarchean sedimentation
Elizabeth J.T. Stefurak et al., Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA. Published online ahead of print on 10 Feb. 2014; http://dx.doi.org/10.1130/G35187.1.
The silica cycle in Precambrian oceans was markedly different than the modern system due to the absence of silica-biomineralizing organisms and differences in silica inputs. Geologists have long thought that amorphous silica may have precipitated as a primary mineral in Archean oceans, but unambiguous evidence of this process has been elusive. Elizabeth Stefurak and colleagues report the common occurrence of sand-sized sub-spherical silica grains within many 3.5-3.2 Ga cherts, which provide direct evidence of primary marine silica precipitation. These "silica granules" appear to be a type of chemical sand grain unique to Precambrian time and promise new insights into the genesis of cherts, microfossil preservation potential, and the Archean silica cycle.
Bank pull or bar push: What drives scroll-bar formation in meandering rivers?
Wietse I. van de Lageweg et al., Faculty of Geosciences, Department of Physical Geography, Utrecht University, P.O. Box 80115, 3508 TC Utrecht, Netherlands. Published online ahead of print on 10 Feb. 2014; http://dx.doi.org/10.1130/G35192.1.
One of the most striking features of meandering rivers is quasi-regular ridges of the point bar. The scroll bar ridges can be seen from the air and are an excellent record of the history of the river. A series of these scroll bars forms a point bar, which is the basic building block of meandering rivers and a potential reservoir for water, oil and gas. A key unresolved problem is whether scroll bars are formed as a consequence of erosion of the outer bend, referred to as bank pull, or due to inner bend deposition, referred to as bar push. Most of the action in natural rivers occurs during floods making it hard to safely distinguish what is cause and what is effect. We use experimentally formed meandering rivers to isolate the effects of sediment supply and bank erosion. We find that channel widening caused by bank erosion causes deposition of new scroll ridges along the inner bend point bar, whereas scroll bars cannot be forced by sediment pulses. Thus channel width variations along meander bends cause bank pull, which explains scroll bar formation and is important to better understand the internal structure of meander deposits.
Late Neoproterozoic Baltic paleosol: Intense weathering at high latitude?
Sirle Liivamagi et al., Department of Geology, Tartu University, 50411 Tartu, Estonia. Published online ahead of print on 10 Feb. 2014; http://dx.doi.org/10.1130/G35209.1.
When compared to its younger analogues, Precambrian paleosols are typically rare, hard to recognize and tend to be metamorphosed. Baltic paleosol is an unmetamorphosed Neoproterozoic paleosol with deeply weathered structure and kaolinite-Fe-oxyhydroxide rich composition in the uppermost part. Its composition and structure is similar to modern tropical Oxisols and would therefore indicate weathering in a warm and humid climate. However, the Baltica (paleo)continent was located at high-to-medium latitudes with typically cool-to-temperate climate in Neoproterozoic, around 550-610 million years ago when this paleosol was formed. Although the kaolinitic paleosols could result from long lasting weathering under temperate climate, then it is most likely that the lateritic Baltic paleosol was developed during an abrupt and intensive weathering event due to temporarily elevated temperatures and increased CO2 levels that accelerated soil formation at the termination of Neoproterozoic snowball glaciations or possibly in relation to Shuram-Wonoka isotope event.
Earth's youngest known ultrahigh-temperature granulites discovered on Seram, eastern Indonesia
Jonathan M. Pownall et al., SE Asia Research Group, Department of Earth Sciences, Royal Holloway University of London, Egham TW20 0EX, UK. Published online ahead of print on 10 Feb. 2014; http://dx.doi.org/10.1130/G35230.1.
When rocks forming the Earth's crust get hot, they undergo metamorphism and may melt. Rocks recording temperatures greater than 900 degrees Celsius (ultrahigh-temperature [UHT] conditions) are very rare and are known from only about 50 locations worldwide. The vast majority of those rocks are many hundreds of millions of years old and the conditions that produced the extreme temperatures are not known. Hence there is no agreement on models to account for UHT metamorphism. The discovery of 16-million-year-old UHT rocks -- the youngest identified on Earth -- on the remote island of Seram in eastern Indonesia show how UHT conditions were generated in the recent past. On Seram, the crust has been stretched and broken so that the rocks of the underlying mantle are currently exposed at the surface. During stretching, juxtaposition of the hot mantle rocks against the crust caused UHT metamorphism. The stretching and thinning of the crust was driven by rapid eastward movement of a subduction zone in response to sinking of old and cold oceanic crust into the mantle. A similar mechanism could account for some of the UHT rocks found in ancient mountain belts.
Subduction initiates at straight passive margins
F.O. Marques et al., Geology Department, University of Lisbon, 1749-016 Lisbon, Portugal. Published online ahead of print on 10 Feb. 2014; http://dx.doi.org/10.1130/G35246.1.
Subduction initiation at a passive margin of Atlantic type is a critical step of the Wilson cycle -- the cyclical opening and closing of ocean basins caused by movement of lithospheric plates. Subduction initiation has been investigated by means of two-dimensional (2-D) numerical modelling, as if the passive margins were straight. However, straight margins rarely occur on Earth. The use of 3-D models is therefore critical in the modeling of spontaneous subduction initiation at realistic, curved passive margins. We used gravitationally driven 3-D thermo-mechanical numerical models with a visco-plastic rheology and a passive margin with a single curved section in the middle. The models show that the curvature angle can control subduction initiation: the greater the angle, the more difficult the subduction initiation. Most importantly, the 3-D thermo-mechanical models provide an in-depth physical understanding of the processes. Specifically, we find that pressure gradients, arising from density differences between oceanic and continental rocks, drive subduction initiation and strongly influence the timing. Temperature also plays a major role because it affects viscosity and density. The main difference between straight and curved margins is that the orientation of the pressure gradient in 3-D is no longer constant, thus producing a horizontal, along-margin component of flow.
Anatomy of a diffuse cryptic suture zone: An example from the Bohemian Massif, European Variscides
Karel Schulmann et al., Czech Geological Survey, Centre for Lithospheric Research, Klárov 3, 118 21 Prague 1, Czech Republic; and EOST, Institut de Physique de Globe, UMR 7516, Université de Strasbourg, 1 rue Blessig, 67084 Strasbourg, France. Published online ahead of print on 10 Feb. 2014; http://dx.doi.org/10.1130/G35290.1.
The fate of the lower plate during continental collision can be examined in deeply eroded orogens such as the late Paleozoic Variscan belt in continental Europe. In particular, the Bohemian Massif at its eastern extremity preserves well the evolution of an Andean-type orogen involved in continental collision. This process included relamination of subducted light felsic material rich in radioactive elements underneath a dense mafic lower crust of the upper plate. This led to gravity-driven overturns and overprinting of the original suture by a broad zone of mixed upper and lower plate materials. In the studied example, this zone of interaction repeatedly reappears within the orogen, forming a so-called "diffuse cryptic suture zone."
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