Taking a Fault's Temperature

Plus more Geology articles published online ahead of print in August–September 2016

Boulder, Colo., USA: Ever think about taking a fault's temperature? What would you learn? A unique experiment where temperature was continuously measured for nearly a year inside the fault that made the catastrophic 2011 magnitude 9.0 Japan Earthquake reveals the thermal signature of pulses of water squirting out of fractures in response to other earthquakes on neighboring faults.

The experiment required measurements more than 7 km (4.5 miles) beneath the sea surface in a borehole observatory stretching nearly a kilometer (more than a half mile) underground as part of the Integrated Ocean Drilling Program's Japan Trench Fast Drilling Project. The results illustrate how water pressure within fault zones, which influences the susceptibility of faults to slip, can be disturbed by earthquakes on other faults. The observation of interactions between faults during the aftermath of a major earthquake helps scientists gain a better understanding of the processes that control earthquake occurrence.

This research, supported by a grant from the Gordon and Betty Moore Foundation, was conducted by researchers from Texas A&M University and the University of California Santa Cruz.

FEATURED ARTICLE
In situ observations of earthquake-driven fluid pulses with-in the Japan Trench plate boundary fault zone
Patrick M. Fulton and Emily E. Brodsky; Center for Tectonophysics, Dept. of Geology and Geophysics, Texas A&M University, College Station, Texas 77843, USA; Dept. of Earth and Planetary Sciences, University of California, Santa Cruz, California 95064, USA. This article is OPEN ACCESS online at http://geology.gsapubs.org/content/early/2016/08/31/G38034.1.abstract.

GEOLOGY articles are online http://geology.gsapubs.org/. Representatives of the media may obtain complimentary articles by contacting Kea Giles at the e-mail 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. Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.

Other recently posted GEOLOGY articles are highlighted below:

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The origin of Earth's first continents and the onset of plate tectonics
Alan R. Hastie et al., School of Geography, Earth and Environmental Sciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. This article is OPEN ACCESS online at http://geology.gsapubs.org/content/early/2016/08/31/G38226.1.abstract.

Unlike all other rocky planets in our solar system the Earth has liquid water oceans, is composed of oceanic and continental crust and has plate tectonics. These distinguishing features are considered to be interlinked and only by understanding how they are connected can scientists appreciate how the Earth has tectonically developed and acquired marine and terrestrial ecosystems to support abundant life over 4 billion years. A crucial question, yet to be answered, is how were the first continents formed ~4 Ga? Resolving this question is essential because the growth and recycling of continental crust over the past 3.5-4.0 billion years has resulted in the chemical and thermal modification of Earth’s mantle, hydrosphere, atmosphere and biosphere. Here we investigate the hypothesis that all of the oldest continental crust formed through the partial melting of thick and hot oceanic crust in wet, flat subduction zones, implying that in the presence of early liquid oceans worldwide plate tectonics was active from ~4 Ga.

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The mechanism of partial rupture of a locked megathrust: The role of fault morphology
Qiang Qiu et al., Earth Observatory of Singapore, Nanyang Technological University, Singapore. This article is OPEN ACCESS online at http://geology.gsapubs.org/content/early/2016/08/31/G38178.1.abstract.

Assessment of seismic hazard relies on estimates of how large an area of a tectonic fault could potentially rupture in a single earthquake. Vital information for these forecasts includes which areas of a fault are locked and how the fault is segmented. Much research has focused on exploring downdip limits to fault rupture from chemical and thermal boundaries, and along-strike barriers from subducted structural features, yet we regularly see only partial rupture of fully locked fault patches that could have ruptured as a whole in a larger earthquake. Here we draw insight into this conundrum from the 25 April 2015 Mw 7.8 Gorkha (Nepal) earthquake. We invert geodetic data with a structural model of the Main Himalayan thrust in the region of Kathmandu, Nepal, showing that this event was generated by rupture of a décollement bounded on all sides by more steeply dipping ramps. The morphological bounds explain why the event ruptured only a small piece of a large fully locked seismic gap. We then use dynamic earthquake cycle modeling on the same fault geometry to reveal that such events are predicted by the physics. Depending on the earthquake history and the details of rupture dynamics, however, great earthquakes that rupture the entire seismogenic zone are also possible. These insights from Nepal should be applicable to understanding bounds on earthquake size on megathrusts worldwide.

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Criticality in the planform behavior of the Ganges River meanders
P.A. Carling et al., Geography and Environment Dept., University of Southampton, Southampton SO17 1BJ, UK. This article is online at http://geology.gsapubs.org/content/early/2016/08/31/G38382.1.abstract.

Natural rivers are not straight but meander across their floodplains. Over time, the channel bends get tortuous and it becomes increasingly difficult for the flow to pass rapidly down the valley. River bank erosion is worse on the outside of the bends until during floods a short channel is cut across the neck of a bend. The river then adopts this new shorter channel as a new main channel, which is much straighter than the old meandering pattern. The old meanders are abandoned as backwaters. However, the straight course then starts to meander again only to repeat the cut off process after a period of time. This cyclic behavior is not an example of a river "out-of-control" but a natural means by which the river "self-organizes" its channels to dissipate its flow-energy efficiently. In our paper we provide a simple explanation for how this process develops, using meanders on the Ganges River, in a heavily populated area of south Asia, as an example. Importantly for river management and protection of human life we argue that such channel behavior is predictable. The method usefully might be applied to world’s large rivers where people and infrastructure are at risk from floods annually.

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Wyoming on the run—Toward final Paleoproterozoic assembly of Laurentia
Taylor M. Kilian et al., Dept. of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, Connecticut 06511, USA. This article is online at http://geology.gsapubs.org/content/early/2016/08/31/G38042.1.abstract.

New magnetic, geochronology, and geochemical data from Wyoming redefine the structure of North America's basement rocks and when they assembled into their present-day arrangement. Conspicuously young ages (ca. 1.715 Ga) associated with deformation in southeast Wyoming craton argue for a more protracted consolidation of North America (Laurentia), long after peak metamorphism in the Trans-Hudson orogen to the north (~1.81 Ga). It was previously thought that the Wyoming craton was part of North America before the Superior craton; we argue that this is not the case, challenging the current understanding of North America's beginning ca. 1.7 Ga.

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Detecting magma-poor orogens in the detrital record
G.J. O’Sullivan et al., Dept. of Geology, Trinity College Dublin, Dublin 2, Ireland. This article is online at http://geology.gsapubs.org/content/early/2016/08/31/G38245.1.abstract.

This study has determined uranium-lead (U-Pb) ages of apatite and rutile grains from modern-day sands in the French Broad River of North Carolina and Tennessee, a river that samples the southern Appalachian mountain chain. These ages are compared to previously published age data from zircon and monazite grains from the same river. Zircon is by far the most commonly used mineral for dating sedimentary grains to determine their source region. However, the zircon U-Pb record is biased towards specific rock-types such as granites, and only forms at very high temperatures, which excludes magma-poor, low-medium temperature mountain-building events (orogens). Zircon in the French Broad River does not detect the last major phase of Appalachian mountain building, (the 320 million year old “Alleghanian” orogeny), which represents the collision of Laurentia (which includes North America) with Africa, and formed the supercontinent Pangea. This study shows that the apatite and rutile U-Pb systems are far more sensitive at detecting magma-poor, low-temperature orogens than zircon or monazite, as they fully record all phases of Appalachian mountain building, including the Alleghanian event. When determining the source of sediment shed from magma-poor orogens, far more useful and unique information can be obtained using apatite and rutile than is possible with zircon.

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Experimental evidence that clay inhibits bacterial decom-posers: Implications for preservation of organic fossils
Sean McMahon et al., Dept. of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, Connecticut 06511, USA. This article is online at http://geology.gsapubs.org/content/early/2016/08/31/G38454.1.abstract.

Exceptionally preserved fossils reveal the fine details and soft, delicate features of long-extinct organisms. Such fossils are commonly found in clay-rich rocks, and many examples are coated by clay minerals. Here, we argue that there may be a connection between the remarkable preservation of these fossils and the presence of clay minerals in the environment where they formed. Post-mortem decay of animal tissues is largely driven by bacteria. Many bacteria are sensitive to the presence of clay minerals. Our experiments show that some animal-decaying bacteria grow poorly in the presence of very fine mineral particles, especially those of clay composition. Clays of different compositions affect the bacteria differently. We conclude that the abundance and composition of clay minerals in the environment, and their interaction with the bacteria involved in decay, might have contributed to exceptional preservation and played a role in shaping the distribution of exceptionally well preserved fossils through time.

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The Neogene de-greening of Central Asia
Jeremy K. Caves et al., Earth System Science, Stanford University, Stanford, California 94305, USA. This article is online at http://geology.gsapubs.org/content/early/2016/09/12/G38267.1.abstract.

There remains substantial controversy regarding when and why Central Asia became one of the most arid regions on Earth. We take a novel approach and use continental-scale maps of carbon isotopes as recorded in carbonate that formed in ancient soils to unravel the hydrology and climate of Asia over the past 20 million years. We use this information to infer how primary productivity (i.e. the amount of carbon fixed by plants) has varied in Asia. We find that Central Asia has always been remarkably arid, but it has become even more arid over the past 20 million years. This increase in aridity implies that vegetation cover has decreased—hence, Central Asia has become less and less “green”. Some of this “de-greening” is due to the uplift of the Tian Shan and Altai mountain ranges, which have steadily decreased the amount of moisture that can reach Central Asia. An additional mechanism is the general cooling of the planet over the past 20 million years, which likely reduced the amount of moisture that reaches Central Asia and shortened the growing season. Our study thus suggests that primary productivity—as measured on a continental-scale—is sensitive to both global climate and to tectonics.

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Sub–ice shelf ironstone deposition during the Neoproterozoic Sturtian glaciation
Maxwell Lechte and Malcolm Wallace, School of Earth Sciences, University of Melbourne, Elgin Street, Parkville, VIC 3010, Australia. This article is online at http://geology.gsapubs.org/content/early/2016/09/12/G38495.1.abstract.

The Sturtian glaciation occurred around 700 million years ago and is considered to be one of the most extreme glacial events in Earth history, potentially encompassing the entire planet. Sediments left behind by this glaciation can be found globally, and often contain iron-rich layers known as ironstones. These enigmatic deposits are the first example of widespread iron deposition anywhere on Earth in over one billion years, and similar ironstones have not been deposited since. Since oceanic iron content is essentially incompatible with abundant oxygen, these ironstones have profound implications for our understanding of the evolution of oceanic oxygen and, by extension, the proliferation of life on Earth. New evidence from Namibia and Australia shows that these ironstones were deposited underneath a floating ice-shelf. Circulation beneath this ice-shelf enhanced the mixing of seawater rich in dissolved iron with glacial fluids containing oxygen, leading to the precipitation of iron on the seafloor. These deposits suggest that global glaciation likely played a key role in invigorating the oceans during times of relatively low oceanic oxygen, and led to the brief return of oceanic iron deposition.

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Evidence for Eocene-Oligocene glaciation in the landscape of the East Greenland margin
Thomas Bernard et al, Géosciences Rennes, Université de Rennes 1 and CNRS UMR 6118, 35042 Rennes, France. This article is online at http://geology.gsapubs.org/content/early/2016/09/12/G38248.1.abstract.

Greenland and Antarctica now have the largest ice sheets on Earth but were once completely ice-free. Climate change strongly impacts the spatial distribution of ice masses at the Earth’s surface and the last 50 million years were a period of global climate cooling. The presence of large ice masses has implications for paleoclimatology, paleo-oceanography and paleo-geodynamics, but dating the actual onset of continental glaciation has remained a real challenge. Based on new themochronological data and landscape analysis, the study by Bernard et al. investigates when the fjords of East Greenland, the largest on Earth, were first carved out by ice. The results show a clear acceleration of erosion rates around 30 million years ago, synchronous with a major temperature drop at the Eocene-Oligocene transition. The nature of the eroded rocks is consistent with the first ice rafted debris found in oceanic sediments dated as 30 to 38 million years old. In addition to dating the onset of continental ice in East Greenland, this study also reveals that ice extent rapidly expanded over hundreds of kilometers, implying either the connection of a network of glaciers or an existing smaller ice sheet.

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Temperature and salinity of the Late Cretaceous Western Interior Seaway
Sierra V. Petersen et al., Earth and Environmental Sciences Dept., University of Michigan, 2534 C.C. Little Building, 1100 N. University Avenue, Ann Arbor, Michigan 48109, USA. This article is online at http://geology.gsapubs.org/content/early/2016/09/12/G38311.1.abstract.

The Western Interior Seaway was a shallow and expansive body of water that covered much of the central United States during the Late Cretaceous, at times connecting the Gulf of Mexico to the Arctic. It was one of a few shallow oceans in existence at the time for which there is no modern equivalent. Typical methods of estimating past ocean conditions have failed in this instance due to uncertainty in the salinity and oxygen isotopic composition of water in the seaway. In this study, we apply a new geochemical method (the clumped isotope paleothermometer) that can robustly determine past temperature and estimate salinity in the seaway, and we compare the findings to new global climate model simulations. We find that during the Late Campanian to Late Maastrichtian (~74-66 Ma), temperatures were between 5°C and 21°C in the Western Interior Seaway and surrounding environments. We estimate that the seaway waters were fresher than the Gulf of Mexico. Estimated salinities become progressively fresher if deep marine samples from the seaway are compared to nearby shallower marine and estuarine environments. Model simulations corroborate new data.

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Boron isotopic discrimination for subduction-related serpentinites
Céline Martin et al., Dept. of Earth and Planetary Sciences, American Museum of Natural History, New York, New York 10024, USA. This article is online at http://geology.gsapubs.org/content/early/2016/09/12/G38102.1.abstract.

The occurrence of serpentinite from both exhumed subduction channel mélange (i.e., mantle wedge) and ophiolite is not uncommon in paleo-suture zones, but distinguishing them and their tectonic origin can be difficult. A new method of discrimination, based on boron isotopes in serpentine from both mélanges and ophiolite, as well as on mica and pyroxene from the metamorphic and vein-rock blocks embedded within the mélanges, has been developed. The metamorphic and vein samples have mainly negative δ11B, in the same range as the serpentine from the mélanges. Besides being the most negative δ11B values ever measured in serpentinite, comparable values from vein minerals indicate that the same fluid serpentinized the overlying mantle. In contrast, serpentine samples from the ophiolite have positive δ11B, consistent with hydration by seawater-derived fluids. We hypothesize that the negative signature of exhumed mélange serpentine is indeed the norm and that the B isotopic signature can be a useful tool to discriminate the tectonic origin of serpentinization in paleo-suture zones.

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15 September 2016
GSA Release No. 16-50

Kea Giles Managing Editor,
GSA Communications
+1-303-357-1057
kgiles@geosociety.org

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IODP vessel

After the March 2011 Mw 9.0 Tohoku-oki Earthquake, the Deep-sea Drilling Vessel (D/V) Chikyu, seen here off-shore Japan, installed a sub-seafloor temperature observatory through the plate boundary fault as part of the Integrated Ocean Drilling Programs' Japan Trench Fast Drilling Project. Fulton and Brodsky present analysis of the observatory data to reveal signals interpreted to be the thermal signature of pulses of water squirting out of fractures in response to aftershocks on neighboring faults. The observation of interactions between faults during the aftermath of a major earthquake helps scientists gain a better understanding of the processes that control earthquake occurrence. See the open-access article, "In situ observations of earthquake-driven fluid pulses within the Japan Trench plate boundary fault zone," by Fulton and Brodsky. Click on the image for a larger version. Photo Credit: JAMSTEC/IODP.