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The July 2016 issue of Geology is now online


Boulder, Colorado – The July 2016 issue of the Geological Society of America's flagship journal, Geology, includes two open-access features: "Pre-Mississippian tectonic affinity across the Canada Basin-Arctic margins of Alaska and Canada," by David W. Houseknecht and Christopher D. Connors; and "Hydrothermal alteration of seafloor peridotites does not influence oxygen fugacity recorded by spinel oxybarometry," by Suzanne K. Birner and colleagues.

Other topics include "Recent volcanic resurfacing of Venusian craters"; "Insights into cyanobacterial fossilization in Ediacaran siliciclastic environments"; "Extending Alaska's plate boundary: Tectonic tremor generated by Yakutat subduction"; and "Vegetation control allows autocyclic formation of multiple dunes on prograding coasts."

All articles are highlighted below.

GEOLOGY articles are online 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, [email protected]

Pre-Mississippian tectonic affinity across the Canada Basin-Arctic margins of Alaska and Canada

David W. Houseknecht and Christopher D. Connors, U.S. Geological Survey, 956 National Center, Reston, Virginia 20192, USA; Geology Dept., Washington and Lee University, 204 West Washington Street, Lexington, Virginia 24450-2116, USA. This article is OPEN ACCESS online at

Reflection seismic data from the Arctic margins of Alaska and Canada reveal thrust faults and folds in pre-Mississippian (older than 359 million years [Ma]) rocks that indicate northward tectonic movement in Alaska and eastward tectonic movement in Canada during a Middle Devonian to Early Mississippian (about 390 to 350 Ma) mountain-building event. The similarity in age and structure of these rocks indicates that the Alaskan and Canadian margins likely were adjacent parts of a continental plate (Laurentia) prior to being separated by tectonic opening of the Canada Basin. The conclusion that these margins are conjugates clarifies the possible geometry of Canada Basin opening, which remains one of the least understood and most controversial aspects of Arctic Ocean tectonics.

Hydrothermal alteration of seafloor peridotites does not influence oxygen fugacity recorded by spinel oxybarometry

Suzanne K. Birner et al., Dept. of Geological Sciences, 450 Serra Mall, Building 320, Stanford University, Stanford, California 94305, USA. This article is OPEN ACCESS online at

In the Pacific Ocean, north of New Zealand, the collision of two tectonic plates has created the Tonga Trench, a massive gash in the sea floor, miles deep. The walls of this trench expose Earth's interior, below the crust. In a study published this week in Geology, a multi-institutional team of scientists document how, despite large degrees of chemical degradation, these rocks still have a lot to tell us about Earth. Interactions with seawater degrade rocks sitting on the ocean floor, calling into question the information they can give us. But scientists are very interested in the story these rocks have to tell. For example, the oxygen content of Earth's deep interior has important implications for understanding how the planet and atmosphere have changed through time. The rocks recovered from Tonga are unique in that some of them are perfectly fresh, with essentially no degradation from seawater. Using this unique suite of rocks, ranging from fresh to corroded, this team was able to show that some chemical properties of the original minerals are unaffected by alteration. This result allows scientists to analyze a wider range of seafloor rocks — and learn more about Earth's interior.

Recent volcanic resurfacing of Venusian craters

Jennifer L. Whitten and Bruce A. Campbell, Center for Earth and Planetary Studies, Smithsonian Institution, MRC 315, PO Box 37012, Washington, D.C. 20013-7012, USA. This article is online at

Ejecta from impact craters on Venus are a major source of fine-grained materials across the planet. The distribution of these materials in tesserae, highly deformed landforms that may be the oldest materials on Venus, can aid in compositional studies of tesserae. Currently, the composition of tesserae is unknown, but is key to understanding whether water played an important role in the geologic evolution of Venus early in its history. Thus, tesserae are ideal sites for future landed missions, but efforts must be made to avoid contaminating surface materials from craters. We developed a method to detect contaminating crater ejecta and find that these materials are unevenly distributed in the tessera, which indicates that selection of future landing sites will have to take into account ejecta distribution in tesserae. In one particular location these ejecta are preserved in tesserae, but the crater that sourced the ejecta cannot be observed. Through analysis of the site geology we infer that the source crater was resurfaced by volcanic activity within the past 80 m.y., and suggest that similar fine-grained ejecta deposits may have built up over time in other tesserae across Venus.

Effects of seafloor diagenesis on planktic foraminiferal radiocarbon ages

Jody Wycech et al., Dept. of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA. This article is online at

Radiocarbon (14C) ages obtained from the inorganic carbon of calcareous microfossils (foraminifera) preserved in deep-sea sediments are commonly used to reconstruct ocean-climate change and carbon cycle dynamics of the past 30 thousand years. However, the post-depositional effects of chemical reactions occurring at and/or below the seafloor (i.e. early diagenesis) on the marine 14C chronometer are poorly constrained. Wycech et al. address this issue by comparing 14C ages and trace element ratios of planktic foraminifera with white, opaque shells deemed well preserved by traditional standards to those with exquisitely preserved translucent shells. This novel approach to a longstanding conundrum reveals that opaque shells yield 14C ages invariably older and trace element ratios consistently higher than those of translucent shells. These unprecedented results indicate that the use of translucent foraminiferal shells enhances reproducibility and accuracy of 14C ages by minimizing the deleterious effects of diagenesis. Thus, the findings reported by Wycech et al. serve as a cautionary tale since white, opaque foraminifera are common in deep-sea sediments, and 14C ages derived from their ostensibly well-preserved shells can lead to substantial discrepancies in the timing of recent climate events recorded in the deep-sea sedimentary archive and reconstructions of past ocean circulation.

Insights into cyanobacterial fossilization in Ediacaran siliciclastic environments

Sharon A. Newman et al., Dept. of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. This paper is online at

During the Ediacaran Period (which occurred approximately 635 to 541 million years ago), fossils of photosynthetic microorganisms and fossilized microbial mats were frequently preserved in sandy environments. This type of fossilization occurs infrequently today and is less common than the preservation of microbes in finer-grained sediments. To understand how microbes can be preserved in sand, our study experimentally fossilizes photosynthetic microbes (cyanobacteria) in the presence of beach sand. We show that clay minerals can coat cyanobacterial cells within days, in fully oxygenated conditions. The most exceptional fossilization is expected when oceanic silica is higher than modern-day values, there is a steady delivery of suspended clay minerals, and there is an absence of sediment bioturbation. Similar conditions may have preserved photosynthetic microbial communities during the Ediacaran Period, and at other times throughout Earth's history.

Uranium irradiation history of carbonado diamond; implications for Paleoarchean oxidation in the São Francisco craton (South America)

Charles W. Magee, Jr., Research School of Earth Sciences, Australian National University, Canberra, 0200 ACT, Australia. This article is online at

Carbonado is an obscure type of ancient diamond found in South America and Africa. Over the past 25 years, a number of studies have shown that this diamond has been exposed to radiation from uranium decay. New results tie these radiation studies together, and connect them with studies of rivers that flowed through Brazil billions of years ago to determine when the uranium and diamond interacted. The data show that the uranium was mobile in the sedimentary environment 3.32 billion years ago. Because uranium requires a certain amount of oxygen to be mobile, these new results may indicate that microbes in surface waters of the early Earth were producing oxygen in rivers and streams at that time. Carbonado diamond therefore reveals the earliest traces of oxygen in surface waters at a time when the ancient atmosphere did not contain oxygen.

Control of silicate weathering by interface-coupled dissolution-precipitation processes at the mineral-solution interface

Encarnación Ruiz-Agudo et al., Departamento de Mineralogía y Petrología, Universidad de Granada, 18071 Granada, Spain. This paper is online at

The mechanism of surface altered layers [SALs] or leached layers formation during weathering of silicate minerals is controversial and hinges on understanding the evolution of the composition of the fluid at the dissolving mineral surface. Here we present in-situ data on the evolution of the interfacial fluid composition during dissolution of wollastonite (CaSiO3), obtained using interferometry and micro pH and ion-selective electrodes. Steep concentration gradients develop at the mineral interface as soon as it makes contact with the solution. This interfacial fluid becomes supersaturated with respect to amorphous silica that then precipitates and forms a surface coating, limiting fluid access to the mineral surface and hence affecting the dissolution rate. The thickness of the concentration gradient and the formation or not of SALs depends on the relative rates of mass transport and surface reaction in the system; this effect could contribute to the discrepancy between dissolution rates measured in the field and in the laboratory. Our results have implications for predictions of silicate weathering rates and hence climate evolution, as different assumptions on dissolution mechanisms affect calculations on CO2 drawdown during weathering and consequent effects on estimates of past and future evolution of global mean temperatures.

Seismicity and sedimentation rate effects on submarine slope stability

U.S. ten Brink et al., U.S. Geological Survey, Woods Hole, Massachusetts 02543, USA. This article is online at

Earthquakes are often thought to trigger submarine landslides. However, this paper shows that the more earthquakes that occur in an area, the fewer the number of submarine landslides. The authors suggest that this relationship exists because shaking from earthquakes causes submarine sediments to become more compact and stronger, making the sediments less likely to fail and cause submarine landslides.

Extending Alaska's plate boundary: Tectonic tremor generated by Yakutat subduction

Aaron G. Wech, Alaska Volcano Observatory, U.S. Geological Survey, 4230 University Drive, Anchorage, Alaska 99508, USA. This article is online at

Subduction zones are tectonic plate boundaries where one plate slides beneath another. These boundaries create giant megathrust faults that are responsible for Earth's largest earthquakes, help produce magma that forms volcanoes at the surface, and generate many smaller quakes within the downgoing plate as it bends and sinks into the mantle. At the eastern edge of the Alaska-Aleutian subduction zone, neither the tectonics nor the resulting seismicity pattern or source of volcanism is well understood. The earthquakes tracking the subducting Pacific plate terminate abruptly, and there is little evidence for a subducting plate beneath the Wrangell volcanic field further east. This paper looks at subtle, continuous tremors resulting from slow earthquakes on the megathrust boundary. The tremors map out a plate boundary that continues past the known edge of the fault and display a trend from periodic to more continuous slip near the Wrangell volcanic field. The result extends the subduction zone fault and may merge with an aseismic slab beneath the Wrangell mountains, which would explain the origins of volcanism there. The result also means the plate boundary fault continues past the 1964 rupture zone, which has implications for the possibility of a future megathrustearthquake further east.

Dynamic links among rock damage, erosion, and strain during orogenesis

.G. Roy et al., Earth and Climate Sciences, University of Maine, 5790 Bryand Global Sciences Center, Orono, Maine 04469, USA. This article is online at

We have shown that river erosion can play a critical and previously underestimated role in the shaping of large mountain ranges when considering a link between rock strength and its susceptibility to erosion. Using an innovative coupled modeling technique, we track the pattern and magnitude of deformation as tectonic stresses drive collision between plates, topography controls patterns of rainfall, and rivers erode into the surface, using different degrees of scaling between rock strength and erodibility. Our results suggest that not only does the strength-erodibility link explain many common landscape patterns, but it can increase the concentration of deformation by almost double. As a consequence, faults that form in actively deforming mountain ranges tend to remain active for a longer period of time.

Vegetation control allows autocyclic formation of multiple dunes on prograding coasts

Laura J. Moore et al., Dept. of Geological Sciences, University of North Carolina, 104 South Road, Mitchell Hall, Campus Box 3315, Chapel Hill, North Carolina 27515, USA. This article is online at

Multiple dunes, also known as dune ridges, occur throughout the world on prograding (seaward extending) coastlines and have long been a subject of interest, especially as potential recorders of past conditions and events. Despite this, the processes that form multiple dunes have remained enigmatic. For decades, multiple dunes have been assumed to arise from changes in conditions external to the beach-dune system, such as shifts in in climate, sea level, land use and tectonic activity that can alter the rate of sediment supply from nearby rivers to the coast. In contrast, Moore et al. combine a modern long-term record of multiple dune growth from the Pacific Northwest and a modified version of a recently developed coastal dune model, and find that multiple dunes can emerge from dynamics within the beach-dune system itself — interactions between ecological and physical processes in the presence of a seaward moving shoreline are sufficient to create multiple dunes through time. The findings of Moore et al. imply that caution is required when using dune ridges as proxies for past changes in climate, sea level, land use, and tectonic activity as the relationship between external events and multiple dune formation may not be one-to-one as previously thought.

Variable El Niño-Southern Oscillation influence on biofacies dynamics of eastern Pacific shallow-water carbonate systems

Alexander F. Humphreys et al., Dept. of Chemical and Physical Sciences, University of Toronto, Toronto, Ontario M5S, Canada. This article is online at

Humphreys et al. used statistical models to investigate the dominant environmental controls on the distribution of modern shallow water carbonate sediments of the Galápagos Archipelago, Ecuador, and the Gulf of California, Mexico. Results show ENSO to play a dominant role in the composition and evolution of Galápagos sediments, while nutrients, as a result of upwelling, primarily determine the sediment distribution within the Gulf of California.

Multiple episodes of hematite mineralization indicated by U-Pb dating of iron-ore deposits, Marquette Range, Michigan, USA

Birger Rasmussen et al., Dept. of Applied Geology, Curtin University, Kent Street, Bentley, WA 6102, Australia. This article is online at

New research on iron-rich sedimentary rocks, known as iron formations, from the Lake Superior region of North America links the timing of iron ore formation to the growth of a mountain chain more than 1.85 billion years ago. The Marquette Range of Michigan, USA, is historic because it was in Negaunee in 1844 that surveyor William Burt discovered iron ore in what was to become one of the richest iron ore fields in the world. Despite the economic significance of the field, it has proven virtually impossible to date ore formation, hindering our understanding of how the iron was concentrated into high-grade ore. In this study, we identified minute uranium-bearing phosphate minerals intergrown with ore minerals (e.g. hematite). By dating these accessory minerals, the study established a link between iron mineralization and the growth of a mountain chain that developed during collision between an island arc and the North American continent more than 1.85 billion years ago. The iron ore was upgraded and concentrated in two subsequent upheavals at 1.80 and 1.77 billion years ago, producing the high-grade ore mined since 1848. The findings provide a new perspective on this important source of iron, which formed by repeated episodes of fluid channeled through the crust during regional-scale mountain building.

Rapid variation in upper-mantle rheology across the San Andreas fault system and Salton Trough, southernmost California, USA

Shahar Barak and Simon L. Klemperer, Dept. of Geophysics, Stanford University, 397 Panama Mall, Stanford, California 94305, USA. This article is online at

Though the San Andreas fault is Earth's iconic earthquake fault and forms a substantial part of the Pacific-North America plate boundary, its structure at depth has remained poorly known. Stanford University's detailed analysis of the way seismic waves from distant earthquakes vary across southernmost California, and their demonstration that there must be aligned magma-filled cracks at geologically shallow depths, changes our view of how fault slip is distributed at depth, and must be considered in future estimates of earthquake hazard.

Mediterranean outflow pump: An alternative mechanism for the Lagomare and the end of the Messinian Salinity Crisis

Alice Marzocchi et al., School of Geographical Sciences, University of Bristol, Bristol BS8 1SS, UK. This article is online at

The marine connection between the Mediterranean Sea and the North Atlantic Ocean has not always looked like the Straits of Gibraltar today. Around 5-6 million years ago, progressive restriction of the exchange between the two basins caused a substantial desiccation of the Mediterranean Sea, leading to one of the most dramatic events in its geologic history: the so-called Messinian Salinity Crisis. As a consequence, a layer of salt more than one kilometer thick still lies beneath the Mediterranean basin today. The debate on how this extreme event ended is still very much open. In particular, understanding the role played by the connection with the Paratethys — former Black and Caspian seas — during the final stage of the crisis, which was characterized by brackish water conditions, represents a major challenge for scientific interpretation. Here, we propose an alternative mechanism for the end of the Messinian Salinity Crisis, based on hydrologic calculations from an ocean-atmosphere-vegetation numerical model. We suggest that this resulted from the re-establishment of Mediterranean Outflow into the Atlantic Ocean, through a slow refilling of the Mediterranean Sea with Atlantic water, rather than the traditionally invoked waterfall-like reflooding of the basin.

Cryogenian intraplate magmatism along the buried southern Laurentian margin: Evidence from volcanic clasts in Ordovician strata, Marathon uplift, west Texas

Richard E. Hanson et al., School of Geology, Energy and the Environment, Texas Christian University, Fort Worth, Texas 76129, USA. This article is online at

In this paper we present isotopic dates and chemical data from volcanic clasts contained in sedimentary strata that occur in west Texas and consist of detrital material derived from the ancient southern margin of Laurentia, the ancestral core of North America. Our data show the volcanic clasts originated at least partly from an igneous province active 700 million years (m.y.) ago and have compositions typical of magmatic rocks emplaced during continental rifting. This is the first report of igneous activity of this type occurring this far back in geologic time along the southern Laurentian margin, which is mostly deeply buried beneath younger sedimentary strata. Previous work by many geologists has shown that igneous activity of the same type occurred from 850 to 540 m.y. ago along all the other margins of Laurentia. The magmatism occurred during breakup of the ancient supercontinent Rodinia, and our new data suggest that Laurentia may have been entirely encircled by rift-related magmatism as Rodinia began to fragment. This in turn supports models that heat rising up from Earth's mantle, from which the rift-related magmas were derived, played a major role in causing the supercontinent to break apart.

Anoxic oxidation of chromium

Christopher Oze et al., Dept. of Earth System Science, Stanford University, Stanford, California 94305-2115, USA. This article is online at

In The Cedars of Northern California, ultramafic rocks produce hydrogen peroxide when interacting with water. The hydrogen peroxide subsequently reacts with chromium containing mineral in the rocks and generates hexavalent chromium, a compound toxic to humans and ecosystems. This formerly unknown process alters interpretations of oxygen concentrations in Earth's geologic past and offer new pathways to assess the origins of life on Earth. Rock-derived hydrogen peroxide may also lead to hexavalent chromium contamination of local groundwater.

A shale-hosted Cr isotope record of low atmospheric oxy-gen during the Proterozoic

Devon Cole et al., Dept. of Geology and Geophysics, Yale University, New Haven, Connecticut 06511, USA. This article is online at

The emergence and expansion of animal life on Earth represents a dramatic shift in the structure and complexity of the biosphere. A lack of firm constraints on surface oxygen levels during Earth's middle history (~1 to 2 billion years ago) has resulted in heated debate as to whether the rise and earliest diversification of animals was directly linked to a change in environmental oxygen levels or, simply reflects the timing genetic innovation independent of a direct environmental trigger. Here, we present chromium (Cr) isotope data from marine siliciclastic rocks (shales) that provide evidence for minimal Cr oxidation leading up to the diversification of eukaryotes and the rise of animals ~ 700 — 800 million years ago. This observation requires very low oxygen levels (

U-Pb geochronology of calcite-mineralized faults: Absolute timing of rift-related fault events on the northeast Atlantic margin

Nick M.W. Roberts and Richard J. Walker, NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth NG12 5GG, UK and Dept. of Geology, Leicester University, University Road, Leicester LE1 7RH, UK. This article is online at

Continental fault systems have the potential to act as barriers or conduits for fluid flow, hence the timing of their formation and subsequent reactivation is important for geological resources such as hydrocarbons, economic minerals, and aquifers. Many regional-scale fault systems lack readily available techniques to provide absolute chronological information. Calcite mineralization occurs in crustal faults in many geological settings, and can be suitable for U-Pb geochronology. The calcite system has remained under-utilized because traditional techniques that involve dissolving entire calcite crystals require uncommonly high uranium concentration; this technique has not been successful in the application of dating faults. Uranium and lead are distributed heterogeneously throughout calcite crystals; thus, higher uranium domains can be targeted using high spatial-resolution sampling techniques. Here we present a novel application of in-situ laser ablation inductively coupled mass spectrometry (LA-ICPMS) to basaltic fault rock geochronology in the Faroe Islands, NE Atlantic margin. The new absolute ages highlight a previously unrecognized protracted brittle deformation within the newly developed continental margin. LA-ICPMS U-Pb calcite geochronology represents an important and novel method that will have wide application to a range of geological settings, to constrain the absolute timing of fault and fluid-flow events within Earth's upper crust.

Temporal variations of helium isotopes in volcanic gases quantify pre-eruptive refill and pressurization in magma reservoirs: The Mount Etna case

Antonio Paonita et al., Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Via Ugo La Malfa 153, 90146 Palermo, Italy. This article is online at

Eruptive activity at active basaltic volcanoes is mainly the result of deep magma uprise that pressurizes magma chambers, in turn forcing the resident magma in the chamber to move up and erupt at the surface. One of the signals that a deep magma is uprising is the increase of helium isotope ratio measured in fluids emitted at the surface (fumaroles, soil gas vents, etc.), as it is well known that deep magmas are characterized by helium isotope ratios higher than magmas stored in the shallow crust. However, up to now this parameter has been used only as a qualitative indicator of primitive magma contribution. Starting from a matchless, 12-yr-long time series of helium isotopes in gases emitted from Mt Etna volcano (Italy), Paonita et al. present an unprecedented use of this parameter to get quantitative information on physics of magmatic systems. They quantify the flow of magma entering a magma chamber and the related pressure buildup at depth. The resulting values of magma chamber volume deformations are well correlated with those obtained by independent geodetic studies. This approach can be extended to other volcanic systems showing temporal variations of the helium isotope ratios.

U-series and 40Ar/39Ar ages of Holocene volcanic rocks at Changbaishan volcano, China

Frank C. Ramos et al.; Dept. of Geological Sciences, New Mexico State University, Las Cruces, New Mexico 88003, USA. This article is online at

Accurately dating Holocene volcanic rocks poses many challenges but is critical to assessing magmatic evolution and hazard risks at highly active volcanoes. Here we use Ra/Th and 40Ar/39Ar geochronology to date very young eruptions at Changbaishan volcano, northeastern China, a recently active stratovolcano responsible for one of the most voluminous eruptions in the past ~2000 yr. For Holocene eruptions, 40Ar/39Ar ages are consistently older than those of both independently determined ages and maximum Ra/Th ages. Overall, Ra/Th ages are most consistent with historical accounts and indicate inaccurate 40Ar/39Ar ages that are due to extraneous argon in various forms. Ra/Th geochronology also confirms the highly active nature of Changbaishan and supports the continued presence of trachytic magma residing under the volcano that appeared more than ~1100 yr ago.

Nanomineralogy as a new dimension in understanding elusive geochemical processes in soils: The case of low-solubility-index elements

Michael Schindler, Dept. of Earth Sciences, Laurentian University, Sudbury, Ontario P3E 2C6, Canada; and Michael F. Hochella Jr., Dept. of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, USA. This article is online at

Nanomineralogical studies of mineral surface coatings in soils reveal insights into biogeochemical processes that heretofore were not known to exist. This is a new dimension in understanding past and present biogeochemical processes in soils, and in this study it is a way to better understand the behavior of low-solubility-index elements such as Al, Ti, and Zr. Soils were sampled from selected sites in Sudbury (Ontario, Canada) that have been affected by acidification and particulate matter emissions from base-metal smelters with subsequent remediation within the past century. These anthropogenic processes have affected an entire landscape, but are now recorded in assemblages of nano-size phases that can be only studied using a combination of focused ion beam technology (for sample preparation) and high-resolution analytical transmission electron microscopy (for phase identification). A first generation of clay minerals (pre-acidification phase), their partial replacement by nano-size hematite and amorphous silica (anthropogenic acidification), and a second generation of clay minerals (post-acidification, including soil remediation) are products of changes in soil biogeochemical processes during these natural and anthropogenic-induced weathering stages. Complex assemblages of nanophases formed prior to the second generation of clay minerals depict underlying mechanisms for the mobilization and sequestration of the low-solubility-index elements Zr and Ti under acidic conditions. The occurrence of baddeleyite (ZrO2), anatase (TiO2), and the Magneli phases Ti4O7 and Ti5O9 (all present at the nanoscale) suggest an influx of nanocolloidal Zr and Ti oxides during weathering of smelter-derived particulate matter. Kelyshite {NaZr[Si2O6(OH)]}, authigenic zircon (ZrSiO4), and kleberite [Fe3+Ti6O11(OH)5] are most likely products of the sequestration of the Zr- and Ti-bearing nanocolloids.

Stick-split mechanism for anthropogenic fluid-induced tensile rock failure

Mirko van der Baan et al.; Dept. of Physics, University of Alberta, Edmonton, Alberta T6G 2G7, Canada. This article is online at

Fluids play a critical role in natural and human-induced rock failure. It is unclear, however, if propagation of a tensile fracture is inherently an episodic or continuous process. For example, typical average propagation speeds of hydraulic fracture tips on the order of 1-10 m/min suggest continuous crack growth, possibly at subcritical stress intensities. In contrast, using field observations and numerical and mathematical analyses, we show that fracture growth due to anthropogenic hydraulic fracturing is most likely to occur in an episodic fashion, characterized by stick-split behavior that is analogous to stick-slip motion of earthquakes. The stick-split mechanism is regulated by cyclic variations in fluid pressure near the crack tip, in which each successive failure produces a local pressure drop that temporarily halts or slows fracture propagation. A pressure drop results in partial fracture closure, producing noncontinuous fracture propagation through a process that is reminiscent of hand clapping. Rupture speeds for individual failure events are on the order of the shear-wave velocity of the medium; thus, continuous crack growth is not a likely mechanism for anthropogenic hydraulic fracturing treatments despite slow average tip propagation speeds.



Kea Giles
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