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September 2010 Geology and GSA Today Highlights

21 August 2010 Geological Society of America, The (GSA)

From biofilms to carbon burial, the September Geology covers science all over the map. Highlights include studies of earthquake frequency along the San Andreas fault; using Iranian and Pakistani river sediment to understand Himalayan orogenesis; discovery of a thin-walled sponge in a Burgess Shale-type deposit near Stanley Glacier, British Columbia; and the science behind elevation gain of the southern African Plateau. GSA Today examines the possibility of a slab window within the subducting Chilean Ridge.

Keywords: Snowball Earth, fluid flow, Intertropical Convergence Zone, San Andreas fault, Laurentide Ice Sheet, Sino-Korean Craton, Singapore, Himalaya, Stephen Formation, Tyrrhenian Sea, Mediterranean, Kaapvaal craton, Portugal, Deccan basalt, biofilms, trace element partitioning, Nankai Trough, Rodrigues carbonate platform, Guatemala, Chile Ridge


  • Neoproterozoic ice ages, boron isotopes, and ocean acidification: Implications for a Snowball Earth

Simone A. Kasemann et al., Dept. of Geosciences, University of Bremen, 28334 Bremen, Germany. Pages 775-778.

The Neoproterozoic Earth underwent at least two severe glaciations, each extending to low paleomagnetic latitudes and punctuating warmer climates. An explanation for theses severe environmental changes is given by the snowball Earth hypothesis and predicts extreme glaciation, a progressive buildup of pCO2, and the rapid melt back and influx of atmospheric CO2 into the ocean under ultra-greenhouse conditions. Such CO2 transfer to the oceans should have caused a rapid decrease in seawater pH. Using boron isotope profiles and pH calculation for two low-latitude, glacial-interglacial sequences in Namibia, Kasemann et al. suggest two different environmental conditions. A negative B-isotope pattern for the younger post-glacial sequence indicates a transient decrease in ocean pH, suggesting a rapid drawdown of CO2 initiated at the start of the deglaciation, and supports inferences of a thick, global sea-ice shield with minimal air-sea gas exchange during glaciation. In contrast, B-isotope values suggest a largely constant ocean pH and no critically elevated pCO2 throughout the older post- and inter-glacial period, and lead to speculation that the ocean was not totally frozen and that the hydrological cycle was functioning.


  • Stress- and fluid-driven failure during fracture array growth: Implications for coupled deformation and fluid flow in the crust

Auke Barnhoorn et al., Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia. Pages 779-782.

Faults and small-scale fractures in Earth are not only the sites at which earthquakes are generated, but they also form easy pathways for fluid to migrate within Earth's crust. The interaction between fractures and fluids has a primary control on the formation of ore deposits, petroleum reservoir performance, and earthquake cycles. In an experimental study, Auke Barnhoorn of Australian National University and colleagues have demonstrated that high-pressure fluids can cause propagation of fractures away from the fluid reservoir, generating a fracture network that has different characteristics than a fracture network generated in fluid-absent rock. Furthermore, it is shown that the three-dimensional connectivity of fractures already occurs very early during tectonic loading. The pathways of this early fracture network are tortuous, potentially promoting ore mineralization, but also early leakage of oil and gas from the reservoir. Direct short-circuit pathways along the fracture network only occur later at failure of the rock (earthquake generation). The difference in the type of fracture network (fluid-driven or stress-driven) and the type of fracture connectivity (early tortuous pathways or later short-circuit pathways) will have an important control on ore-mineralization processes, the integrity of oil and gas reservoirs, and earthquake generation cycles.


  • Does Antarctic glaciation force migration of the tropical rain belt?

Ann Holbourn et al., Institute of Geosciences, Christian-Albrechts University, D-24118 Kiel, Germany. Pages 783-786.

Tropical-subtropical precipitation patterns track seasonal migration of the Intertropical Convergence Zone, a narrow latitudinal zone of wind convergence that oscillates across the equator. Understanding the past evolution of this variable climate feature in the absence of large Northern Hemisphere ice sheets provides a perspective relevant to future variability on a warmer Earth. High-resolution upper-ocean temperature and salinity estimates derived from planktic foraminiferal delta-18O and Mg/Ca reveal sequential warming and freshening of the subtropical northwestern Pacific following Antarctic ice growth episodes during the middle Miocene epoch (15.7 to 12.7 million years ago). Ann Holbourn of Christian-Albrechts University and colleagues attribute the punctuated pattern of surface warming and freshening to successive northward movements of the Intertropical Convergence Zone, implying high sensitivity of tropical rain belts to the interhemispheric temperature gradient driven by high-latitude climate. This dynamic interaction has implications for future warmer climate regimes with differential warming of the Northern Hemisphere, as it may lead to changes in the latitudinal penetration of tropical Pacific moisture over southeast Asia.


  • Century-long average time intervals between earthquake ruptures of the San Andreas fault in the Carrizo Plain, California

Sinan O. Akciz et al., Program in Public Health and California Institute for Hazards Research, University of California, Irvine, California 92697, USA. Pages 787-790.

It was 153 years ago that the southern San Andreas fault last broke in a major earthquake. The lack of earthquakes along the southern San Andreas fault since at least the early 1900s, combined with studies on channels that were offset by the fault, suggest that past earthquakes along the Carrizo Plain section of the southern San Andreas fault were great (greater than M 7.8) events - similar to the 1857 earthquake - separated by long intervals (240-450 years) of dormancy. S.O. Akciz of the University of California-Irvine and colleagues present new data based on work supported by the National Science Foundation, U.S. Geological Survey, and the Southern California Earthquake Center. New earthquake evidence and radiocarbon analyses indicate that the average time interval between the last six earthquakes that ruptured the San Andreas fault in the Carrizo Plain is 88 plus or minus 41 years. This is less than the time since the most recent large earthquake (in 1857), less than all reported average intervals of prehistoric earthquakes along the entire San Andreas fault, and significantly shorter than the 235-year average used in recent seismic hazard evaluations. The new chronological data, combined with recent offset studies, imply that the magnitudes of the earthquakes since about A.D. 1360 that ruptured the southern San Andreas fault in the Carrizo Plain included both moderate and great events, with more frequency than previously thought.


  • Absolute chronology for major Pleistocene advances of the Laurentide Ice Sheet

Greg Balco and Charles W. Rovey II, Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA. Pages 795-798.

The defining feature of the last several million years of Earth's history has been the periodic advance and retreat of large ice sheets on the northern continents. These ice sheets left thick and extensive sedimentary deposits scattered all over North America and Eurasia; for example, throughout the central and eastern United States, north of a line extending approximately from Montana through Missouri, and east to Maine. Paradoxically, however, most of what we know about the history of these ice sheets is known from indirect evidence preserved in marine sediments. Although the terrestrial deposits in places like Missouri and Minnesota would provide much more direct evidence of what happened, we haven't been able to get this evidence because it has not been possible to accurately determine the age of the deposits. Balco and Rovey's goal is to fix this problem. To do this, they have developed a new technique that permits them to date terrestrial glacial deposits, and they have applied it to a sedimentary sequence in Missouri that records the largest advances of continental ice sheets into North America during the past 2.5 million years. This in turn helps to resolve a handful of outstanding questions about how Earth's great ice ages began and evolved.


  • Diachronous decratonization of the Sino-Korean craton: Geochemistry of mantle xenoliths from North Korea

Jin-Hui Yang et al., State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China. Pages 799-802.

Ancient cratons, characterized by thick lithospheric-mantle roots, are stable and normally unaffected by later magmatism or tectonic activity. Considerable geophysical and geochemical evidence, however, suggests that the Sino-Korean craton had been decratonized and that the ancient lithospheric mantle beneath the Chinese portion of the eastern Sino-Korean Craton was replaced by thinner, hotter, juvenile mantle during the Jurassic to early Cretaceous period. To decipher the history of the changes, including knowledge of both the lateral extent of the lithospheric modifications and the relative timing of change across the craton, Yang et al. studied the mineral compositions and Re-Os isotopic data for peridotite xenoliths from North Korean Triassic kimberlite and Tertiary basalts. Their data suggest that, from the Triassic period to the present, North Korea has been underlain by young, hot, and fertile lithospheric mantle, unlike typical cratonic lithospheric roots, but similar to the juvenile lithospheric mantle underlying Chinese portions of the craton. The Triassic eruption age of the kimberlite suggests the lithospheric changes may have occurred earlier than in China, indicating that lithospheric removal evolved from east to west. These results are most consistent with the conclusion that lithospheric loss was initially triggered by extension that followed the collision between the Sino-Korean and Yangtze cratons.


  • Punctuated eustatic sea-level rise in the early mid-Holocene

Michael I. Bird et al., Earth and Environmental Science, James Cook University, PO Box 6811, Cairns, Queensland 4870, Australia. Pages 803-806.

This study presents a new high-resolution sea-level curve for Singapore covering the past 8900 years, corroborated by the independent proxy records. The results suggest that sea level rose at a rapid rate of 1.8 meters per century until 8100 years ago. After this time, there was a near cessation in the rate of sea-level rise between 7800 and 7400 years ago, followed by further rise of 4-5 meters that was complete by 6500 years before the present. Bird et al. suggest that this period of relatively stable sea level during the early to mid-Holocene enabled modern deltas to advance, providing a highly productive environment for the establishment of coastal sedentary agriculture. The periods of rapid sea-level rise before and after the period of stability may have served to catalyze focused episodes of human coastal migration in the early mid-Holocene into fairly narrow time intervals.


  • Locating earliest records of orogenesis in western Himalaya: Evidence from Paleogene sediments in the Iranian Makran region and Pakistan Katawaz basin

Andrew Carter et al., Joint Research School of Earth Sciences, University College London and Birkbeck, University of London, London WC1E 7HX, UK. Pages 807-810.

The Himalayan mountain belt has long been the subject of intense study, yet despite this effort the early growth history of the Himalaya remains poorly understood because direct evidence is missing due to removal of bedrock records by extensive erosion. To overcome this difficulty, researchers have turned to exploring sedimentary deposits of the large river systems that drained the growing mountain belt. In this study, Andrew Carter of University College London and colleagues examined the provenance of 20-40-million-year-old sedimentary rocks exposed in western Pakistan and southern Iran, considered related to an ancient form of the River Indus drainage. Results confirm the sediments were originally derived from the developing western Himalaya, which opens up a new archive for researchers who want to understand how the early Himalayas developed. First results point to a different style of growth compared to the eastern Himalaya.

  • A new Burgess Shale-type assemblage from the "thin" Stephen Formation of the southern Canadian Rockies

Jean-Bernard Caron et al., Dept. of Natural History-Palaeobiology, Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada. Pages 811-814.

The Burgess Shale in Yoho National Park (Canadian Rockies, British Columbia) is world-famous for its soft-bodied fossils. These fossils are critical for our understanding of the origin and early evolution of animal life during the Cambrian explosion. In the type area (Yoho National Park), Burgess Shale fossils occur along the basinal edge of a paleoescarpment within the "thick" Stephen Formation. It was traditionally assumed that the paleoescarpment played a critical role in preservation or in the recruitment of the biota. This study by Caron et al. shows that similar fossils occur in a ramp setting in a lateral equivalent of the thick Stephen Formation but with no evidence of an escarpment, ~40 km southeast of the type area near Stanley Glacier (Kootenay National Park). This study not only extends the geographic and stratigraphic range of the Burgess Shale biota, but also extends the type of environmental setting in which Burgess Shale-type fossils occur. At least eight new soft-bodied taxa have been discovered, including a new large predatory arthropod. The lateral equivalent of the thick Stephen Formation has an extensive distribution in the Canadian Rockies, with great potential for the discovery of new fossil species.


  • Spreading pulses of the Tyrrhenian Sea during the narrowing of the Calabrian slab

Benjamin Guillaume et al., Dipartimento di Scienze Geologiche, Universita Roma Tre, Rome, Italy. Pages 819-822.

The subduction process implies the progressive sinking within the mantle of a lithospheric plate underneath another one. This process characterized the tectonic evolution of the central Mediterranean Sea during the past 10 million years. The sinking of the Ionian slab, accommodating the slow but continuous convergence between Africa and Eurasia, produces slab rollback and, in turn, backarc opening. The opening of the Tyrrhenian Sea, which has been punctuated by short-lived episodes of oceanic accretion on separate small backarc basins during the Pliocene-Pleistocene, is not totally understood. Previous theoretical models argued that such transient behavior is likely related to the progressive narrowing of the Ionian slab. Guillaume et al., simulating in a laboratory the subduction of a dense plate within the upper mantle (by using properly scaled analog materials), show that an abrupt decrease in the width of the subducting plate may be responsible for a short-term acceleration of the retreat velocity of the subducting plate hinge, and thus confirm that slab narrowing is a plausible mechanism to explain the tectonic history of the Tyrrhenian Sea.


  • Active surface deformation and sub-lithospheric processes in the western Mediterranean constrained by numerical models

Eugenie Perouse et al., Laboratoire Geosciences Montpellier CNRS (Centre National de la Recherche Scientifique)-Universite Montpellier 2, 35095 Montpellier, France. Pages 823-826.

The present-day geodynamics of the Mediterranean Sea region is the result of the convergence between the Eurasian and the African plates. Several driving forces have been proposed to explain the complex horizontal motion, including crustal block interaction and/or sub-crustal processes related to subduction dynamics. Eugenie Perouse of Universite Montpellier and colleagues present a new numerical model to account for the unexpected surface motion in the Rif region (northern Morocco, western Mediterranean), measured by global positioning system (GPS). To test the influence of sub-crustal driving forces, Perose et al. apply a horizontal traction on a small area (patch) at the base of an elastic plate model. They find that the surface motion in the Rif is the combined effect of the Eurasia-Africa convergence, low rigidity of the Alboran sea domain, and a south-southwest-directed horizontal traction applied beneath the Rif. They further suggest that the modeled horizontal traction beneath the Rif could represent the coupling zone between the overlying plate and the retreating western Mediterranean slab.


  • (U-Th)/He thermochronometry constraints on unroofing of the eastern Kaapvaal craton and significance for uplift of the southern African Plateau

Rebecca M. Flowers and Blair Schoene, Dept. of Geological Sciences, University of Colorado, Boulder, Colorado 80309, USA. Pages 827-830.

The timing and causes of the more than one-kilometer elevation gain of the southern African Plateau are widely debated. Rebecca Flowers and Blair Schoene, both at the University of Colorado, report the first apatite and titanite (U-Th)/He thermochronometry data for southern Africa to resolve the unroofing history across a classic portion of the major escarpment that encircles the plateau. The study area encompasses about 1500 meters of relief within the eastern Kaapvaal craton. Titanite dates are Neoproterozoic. Apatite dates are Cretaceous. Thermal history simulations confirm Mesozoic heating followed by accelerated cooling in mid- to Late Cretaceous time. The data limit Cenozoic temperatures east of the escarpment to about 35 degrees Celsius, and appear best explained by temperatures within a few degrees of the modern surface temperature. These results restrict Cenozoic unroofing to less than about 850 meters, and permit negligible erosion since the Cretaceous. If substantial uplift of the southern African Plateau occurred in the Cenozoic as advocated by some researchers, then it did not cause the majority of post-Paleozoic erosion across the eastern escarpment. Significant Mesozoic unroofing is coincident with large igneous province activity, kimberlite magmatism, and continental rifting within and along the margins of southern Africa, compatible with a phase of plateau elevation gain due to mantle buoyancy sources associated with these events.


  • Efficient burial of carbon in a submarine canyon

D.G. Masson et al., National Oceanography Centre, Southampton SO14 3ZH, UK. Pages 831-834.

Significant quantities of organic material (wood and other plant fragments) are transported to the ocean by rivers each year. Burial of this plant material in seabed sediments, preventing it being metabolized by marine animals, is the final step in one of the natural processes that lock away CO2 in the geological record. However, the efficiency of the burial process on the ocean floor, dependent on the type of organic material and on how long it is exposed to oxygen in the seawater, is extremely variable, making it difficult to estimate carbon burial in marine sediments on a global scale. D.G. Masson of the UK’s National Oceanography Centre and colleagues show that large quantities of fine-grained sediment and organic matter are trapped in a submarine canyon off Portugal. This creates a local "sink" of organic carbon on a continental margin where carbon burial is otherwise low. However, the burial process is spatially highly variable, and a complete understanding of sedimentary processes and organic material input at several spatial scales is necessary to obtain reliable estimates. This underlines the difficulty in extrapolating local studies to a global scale.


  • Tectonic setting and timing of the final Deccan flood basalt eruptions

Peter Hooper et al., Volcano Dynamics Group, Dept. of Earth and Environmental Sciences, Open University, Walton Hall, Milton Keynes, Buckinghamshire MK7 6AA, UK. Pages 839-842.

The ultimate causes of the huge, rapidly erupted basaltic lavas of continental flood basalt provinces are controversial, as are the temporal relationships of the volcanism with the often-associated lithospheric extension. This paper by Peter Hooper of Open University and colleagues clarifies the relative timing of extension with the Deccan flood basalt eruptions. Initial east-west extension, which eventually led to separation of the Seychelles from the Indian continent, is recorded by the Panvel flexure and associated north-south listric faults and shear zones. The relative ages of three distinct groups of dikes cutting the youngest (Wai) subgroup of the Deccan basalt flow stratigraphy and the extension-related shear zones, using new 40Ar/39Ar ages, demonstrate the sequence of events. Extension began at 64-65 million years ago after the main Deccan eruptions, including the earliest Wai subgroup flows (~ 65.5 million years ago), but before intrusion of the north-south-oriented lamprophyric dikes (64.9 million years ago) and spilitized basalts of the Mumbai Volcanics (64.5 million years ago). Magma generation could not have been triggered by secondary convection at the edge of the rifted continent because the main eruptions predate the formation of those rifts. Nor could rifting have caused the earlier main flood basalt eruption. Hooper et al. conclude that a hot mantle plume was able to ascend and decompress sufficiently to cause extensive melting before extension began.


  • Nanoparticle factories: Biofilms hold the key to gold dispersion and nugget formation

Frank Reith et al., University of Adelaide, Centre for Tectonics, Resources and Exploration, School of Earth and Environmental Sciences, Adelaide, SA 5000, Australia. Pages 843-846.

Frank Reith of the University of Adelaide and colleagues demonstrate that bacterial biofilms play a key role in mediating the environmental dispersion of gold via the formation of nanoparticles. By using state-of-the-art microanalysis techniques, they are also able to elucidate the fundamental mechanism leading to the growth of gold nuggets under Earth surface conditions.


  • Reading the mineral record of fluid composition from element partitioning

Vincent J. van Hinsberg et al., Hydrothermal Geochemistry Group, Dept. of Earth and Planetary Sciences, McGill University, Montreal, Quebec H3A 2A7, Canada. Pages 847-850.

Earth is the "blue planet," with more than 70% of its surface covered by water and the equivalent of up to four oceans of water in its interior. This abundance of water has a profound impact on the processes that shape our planet, from ore formation to plate tectonics, as well as the origins and development of life. To understand this impact, it is necessary to know the compositions of this fluid. At present, such information is largely unavailable, because direct samples of fluid are rare, especially for the early Earth and the deep Earth interior. Here, Vincent van Hinsberg of McGill University and colleagues propose a different approach in which the composition of the fluid is reconstructed from that of minerals, based on the characteristic trace element partitioning between minerals and aqueous fluids. They show experimentally that this partitioning is systematic and is controlled by the mismatch in the element's radius and charge compared to the ideal element. Unlike fluids, minerals with preserved compositions are readily available in the geological record, and this approach therefore provides a powerful and widely applicable tool to reconstruct fluid compositions for the full range of Earth environments and for its earliest history.


  • Oxidative sulfur cycling in the deep biosphere of the Nankai Trough, Japan

N. Riedinger et al., Max Planck Institute for Marine Microbiology, Celsiusstrabe 1, 28359 Bremen, Germany. Pages 851-854.

It has long been known that sulfur cycling plays an important role within marine surface sediments. However, the influences of sulfur cycling in the deep subsurface biosphere remains poorly understood. Thus, it is important to investigate the sulfur cycle in these sediments, although this task can be challenging due to strong overprinting of the primary signals over time. One part in the sulfur cycle puzzle is the oxidation of sulfur in environments where no or limited oxidants are available. The record of this process in deep subsurface marine sediments is usually overprinted by the dominant process of sulfur reduction in these deposits. In their study, Riedinger et al. found evidence for ongoing oxidative sulfur cycling in deep subsurface marine sediments sampled during the Integrated Ocean Drilling Program Expedition 316 onboard the drilling vessel Chikyu. The finding is related to the highly dynamic sedimentary and tectonic regime at Nankai Trough, Japan, which provides an environment that sustains this biogeochemical process over long durations of time. The geochemical fingerprint they found for oxidative sulfur cycling in these sediments could also be used as a marker to identify this biogeochemical process in other marine deep biospheres and ancient deposits.


  • Holocene reef accretion on the Rodrigues carbonate platform: An alternative to the classic "bucket-fill" model

M.J. O'Leary and C.T. Perry, Dept. of Environmental and Geographical Sciences, Manchester Metropolitan University, Chester Street, Manchester M1 5GD, UK. Pages 855-858.

Most models of reef and carbonate platform evolution envisage a scenario whereby reef accretion along the platform margin produces a rigid framework structure that encloses a deeper water lagoon (widely referred to as the "empty bucket" model). In this model, subsequent infilling of the lagoon occurs through successive phases of sand apron progradation--with sediment sourced from the high carbonate productivity (and topographically elevated) rim. In this study by M.J. O'Leary and C.T. Perry of Manchester Metropolitan University, core records obtained from across the Rodrigues carbonate platform (southwestern Indian Ocean) demonstrate a very different mode of platform evolution. On Rodrigues, growth of the reef rim appears to have lagged behind rising post-glacial sea levels, adopting a catch-up mode of growth. In contrast to the "bucket-fill" model, the primary mode of platform filling has occurred not through the development of the reef rim and subsequent export of sediment into the empty lagoon, but predominantly through reef progradation from the platform interior seaward. This catch-up growth mode allowed sufficient open water influence within the platform interior to promote active reef growth in the period prior to the reef rim being emplaced. Given that such catch-up growth modes are widely recognized on many reef/platform margins, O'Leary and Perry suggest this very different platform evolutionary model may have widespread applicability.


  • Crustal assimilation no match for slab fluids beneath Volcan de Santa Maria, Guatemala

Brian R. Jicha et al., Dept. of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA. Pages 859-862.

Geochemical and isotopic data from the Santa Maria-Santiaguito volcanic complex in northwestern Guatemala indicate that the magma beneath this explosive stratovolcano was generated by much different processes than previously envisioned. Trends in the isotopic data also suggest that the magma that erupted in 1902, the second-largest eruption of the 20th century, was produced because of interaction with the crust beneath the volcano, which had a composition similar to that of the ocean seafloor, not the granites which predominate in this region. The findings of Brian R. Jicha of the University of Wisconsin and colleagues highlight the fact that investigations into the magma-generation processes of active volcanoes will benefit by considering the magmatic history recorded by all products of long-lived volcanic centers, not just the most recent.

GSA Today Science Article

  • Subduction of the Chile Ridge: Upper Mantle Structure and Flow

R.M. Russo et al., Dept. of Geological Sciences, University of Florida, Gainesville, Florida, USA. Pages 4–10.

In its broadest sense, plate tectonics is a product of the long-term cooling of Earth. Hot mantle rises at mid-ocean spreading ridges, forming oceanic lithosphere that, once cooled, sinks back down into Earth's interior at subduction zones. But at several places on Earth, mid-ocean spreading ridges come into contact with subduction zones. As is pointed out in a recent paper in GSA TODAY by Raymond Russo of the University of Florida and his colleagues, these "ridge-trench" intersections present us with a major conundrum: What happens when a spreading ridge, with its young, hot, upwelling mantle, enters into a subduction zone where lithosphere is sinking back into the depths of Earth? To answer this question, Russo and his team of researchers turned their focus on the Chilean mid-ocean spreading ridge, which is currently being subducted beneath southwestern South America. Through an in-depth geophysical survey, they demonstrate that the oceanic plates on either side of the Chilean ridge continue to move apart at depth beneath South America, opening a broad "slab window." Remarkably, deep, hot mantle appears to be erupting up through this window, plastering the underside of southern South America. These first-ever images of a slab window provide direct evidence for a process (slab-window formation) that may explain much about the tectonics and magmatism of continental margins throughout Earth's history.

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