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Plutons, swarms, geothermal energy, and active margins in transition

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Boulder, Colo., USA – New articles from the Geological Society of America's online-only journal, Geosphere, are now available. These new releases include three open-access articles, "Geochronological imaging of an episodically constructed subvolcanic batholith: U-Pb in zircon chronochemistry of the Altiplano-Puna Volcanic Complex of the Central Andes"; "Slab-rollback ignimbrite flareups in the southern Great Basin and other Cenozoic American arcs: A distinct style of arc volcanism"; and "Extraction of three-dimensional fracture trace maps from calibrated image sequences."

GEOSPHERE articles are available at http://geosphere.gsapubs.org/. Representatives of the media may obtain complimentary copies of GEOSPHERE articles by contacting Kea Giles at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GEOSPHERE in articles published. Non-media requests for articles may be directed to GSA Sales and Service, [email protected]

All recent articles are highlighted below.

Geochronological imaging of an episodically constructed subvolcanic batholith: U-Pb in zircon chronochemistry of the Altiplano-Puna Volcanic Complex of the Central Andes
Jamie M. Kern et al., Shanaka L. de Silva (corresponding), Kern and de Silva: College of Earth, Ocean, and Atmospheric Science, Oregon State University, Corvallis, Oregon 97331, USA. This article is OPEN ACCESS online at http://geosphere.gsapubs.org/content/early/2016/05/27/GES01258.1.abstract. Themed issue: PLUTONS: Investigating the Relationship between Pluton Growth and Volcanism in the Central Andes.

Throughout earth history, supereruptions have occurred in clusters known as ignimbrite flare-ups. These periods of prodigious explosive volcanism and their caldera volcanoes are the surface record of a growing plutonic system (batholith) at depth. Revealing the time signals of these flare-ups is critical for our understanding of how plate tectonics forms and consolidates continental crust, the development of geothermal energy and mineral resources, the hazards from supereruptions, and how batholiths are constructed. Applying U-Pb geochronology to microscopic zircon crystals, we reveal the 10 million year plutonic history of one the earth's major ignimbrite flare-ups in the Central Andes of Bolivia, Chile, and Argentina. By combining the age and chemical (chronochemistry) information from zircons with ages of eruptions we find that magma that fed supereruptions was stored for at least 400,000 years before eruption, quietly crystallizing zircon until eruption. The spatial and temporal pattern of the magma accumulations in the crust also reveals the episodic development of a batholith with an areal extent of about 70,000 square kilometers (similar to the exposed Sierra Nevada batholith) at depths of 5 to 10 km through synchronous accumulations of discrete magma bodies several thousands of cubic kilometers in volume. Ultimately, this forensic investigation using "chronochemical imaging" reveals how basaltic magma from the earth's mantle is processed within the continental crust to produce pulses of plutonism and supereruptions that eventually build batholiths.

Slab-rollback ignimbrite flareups in the southern Great Basin and other Cenozoic American arcs: A distinct style of arc volcanism
Myron G. Best et al. (Eric H. Christiansen, corresponding), both authors at Department of Geological Sciences, Brigham Young University, Provo, Utah 84602-4606, USA. This article is OPEN ACCESS online at http://geosphere.gsapubs.org/content/early/2016/05/27/GES01285.1.abstract. Themed issue: The 36-18 Ma southern Great Basin, USA, ignimbrite province and flareup: Swarms of subduction-related supervolcanoes.

The nature of volcanism along continental margins above subducting oceanic plates ranges widely. At one end of this spectrum are conical volcanoes built mostly of andesitic lavas that erupt at intervals of tens to hundreds of years in small volumes of generally less than a cubic kilometer. A recently recognized, contrasting style is less common and develops on thicker, farther inland crust. Less frequent eruptions occurring at intervals of hundreds of thousands to a few millions of years are violently explosive. Pyroclastic flows of silica-rich ash that are dispersed as much as 150 kilometers from the volcanic vent can cover areas of tens of thousands of square kilometers. The total volume of a single eruption can be thousands of cubic kilometers. Swarms of such supervolcanoes develop over as much as 20 million years in some fields. Explosive eruption of these huge volumes of ash create collapse calderas tens of kilometers in diameter over the evacuated subterranean magma chambers. This distinct style of continental margin volcanism, which has been recognized in the geologic record of southwestern North America and the central Andes of South America, accompanies rollback of subducting oceanic plates to a steeper dip beneath the continent. Unusually large amounts of hot basaltic magma generated in the mantle during this rollback invade the crust where the heating partially melts the thick crust to produce huge volumes of silicic magma that erupts explosively in an ignimbrite flareup.

Extraction of three-dimensional fracture trace maps from calibrated image sequences
Thomas D. Seers, Department of Petroleum Engineering, Texas A&M University at Qatar, Qatar; and David Hodgetts, School of Earth, Atmospheric and Environmental Sciences, University of Manchester, Manchester M13 9PL, UK. This article is OPEN ACCESS online at http://geosphere.gsapubs.org/content/early/2016/06/16/GES01276.1.abstract.

Recent advances in 3D structural feature extraction now enable three-dimensional representations of structural lineaments and traces to be digitized from range and altimetry datasets. Current methods, however, are either restricted to outcrops with discontinuities that have well defined topographies or from 2.5D digital elevation models captured using airborne remote sensing. Here we present an approach for the extraction of 3D fracture maps, based upon optical ray tracing, which is capable of generating trace maps from arbitrary surfaces, whilst retaining the high feature resolution of optical image based techniques. We demonstrate that the 3D trace map datasets generated by our approach enable higher order fracture attributes, such as trace connectivity and (volumetric) fracture intensity to be derived directly from digital outcrop datasets. Such parameters are critical in the assessment of the mechanical strength of the upper crust and its fluid flow properties, having numerous applications within the geosciences.

Linking deep and shallow crustal processes during regional transtension in an exhumed continental arc, North Cascades, northwestern Cordillera (USA)
Robert B. Miller et al., Department of Geology, San Jose State University, One Washington Square, San Jose, California 95192-0102, USA. This article is online at http://geosphere.gsapubs.org/content/12/3/900.abstract.

The North Cascades is an exceptional region to evaluate potential linkages between processes operating at a wide range of crustal levels (0-35 km) during collapse of a thick continental magmatic arc. Eocene (55-45 million year ago) collapse during strike-slip faulting and extension of the crust was marked by magmatism, partial melting of the mid-crust, ductile flow, and exhumation of the metamorphic and plutonic core of the North Cascades synchronous with subsidence and rapid deposition in adjacent non-marine basins, and intrusion of dikes. Extension from Eocene dikes is oblique to the overall trend of the mountain belt and to the ductile flow direction in the metamorphic rocks. Thus, coeval strain was decoupled between the brittle and ductile crust. Arc-oblique to arc-parallel flow in the North Cascades contrasts sharply with E-W flow in metamorphic core complexes in northeastern Washington, Idaho, Montana, and southeastern British Columbia. This contrast probably resulted in part from right-lateral strike-slip along the plate margin and differences along the length of the belt in crustal thickness and temperature-controlled strength.

Cenozoic tilting history of the south slope of the Altyn Tagh as revealed by seismic profiling: Implications for the kinematics of the Altyn Tagh Fault bounding the northern margin of the Tibetan plateau Haifeng Zhao, School of Earth Sciences, Zhejiang University, Hangzhou 310027, China. This article is online at http://geosphere.gsapubs.org/content/12/3/884.abstract.

We identified a transition zone between the Altyn Tagh fault and the Qaidam Basin interior and termed it the Altyn Slope, based on a dense network of two- and three-dimensional seismic reflection profiles and isopach maps. We restored the tilting history of the Altyn Slope step by step, and accordingly proposed a two-stage evolution model for the Cenozoic kinematics of the Altyn Tagh fault, which provides new insight into the deformation mechanism of the northern Tibetan Plateau during the Cenozoic.

Dome growth, collapse, and valley fill at Soufrière Hills Volcano, Montserrat, from 1995 to 2013: Contributions from satellite radar measurements of topographic change
D.W.D. Arnold et al., COMET (Centre for Observation and Modeling of Earthquakes, Volcanoes and Tectonics), School of Earth Sciences, University of Bristol, Queen's Road, Bristol, BS8 1RJ, UK. This article is online at http://geosphere.gsapubs.org/content/early/2016/05/27/GES01291.1.abstract.

In this article we use radar satellite imagery to record the changing shape of the British Overseas Territory of Montserrat, in the Caribbean due to the long-lived eruption of Soufrière Hills Volcano, which started in 1995. We measure volcanic deposits, such as ash and pyroclastic flows, which are up to 300 m thick. The radar imagery provides the first measurement of thickness all the on-land deposits, including the frequently cloud covered summit of the volcano. Continuously updated elevation maps of rapidly changing land surface at erupting volcanoes have numerous applications, including real-time hazard maps and estimating eruption rates.

Geotherms from the temperature-depth-constrained solutions of 1-D steady-state heat-flow equation
D. Ravat et al., Department of Earth and Environmental Sciences, University of Kentucky, Lexington, Kentucky 40506-0053, USA. This article is online at http://geosphere.gsapubs.org/content/early/2016/06/16/GES01235.1.abstract. Themed issue: Geothermal Energy from Sedimentary Basins: Challenges, Potential, and Ways Forward.

We formulate a method of constraining geotherms using Curie depths determined from the de-fractal method of spectral magnetic depth determinations. This method also yields the ratio of radiogenic heat production (A) to thermal conductivity (K). We show that the ratios A/K obtained in this manner are reasonable in comparison to their values either observed or previously modeled over regions dominated by granites.

Andean topographic growth and basement uplift in southern Colombia: Implications for the evolution of the Magdalena, Orinoco, and Amazon river systems
Veronica J. Anderson, Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, 2275 Speedway Stop C9000, Austin, Texas 78712, USA. This article is online at http://geosphere.gsapubs.org/content/early/2016/06/16/GES01294.1.abstract.

Uplift of a large basement block (Garzón Massif) in the northern Andes Mountains formed an orographic barrier (2500-3000 m elevation) that generated a major rain shadow and guided the evolution of the largest river systems of northern South America, including the Amazon, Orinoco, and Magdalena Rivers. Despite its pivotal role, the exhumation history of the Garzón Massif and its relationships to the structural evolution of the broader Eastern Cordillera remain unclear. The northern Andes underwent major Cenozoic shortening, with considerable deformation and topographic development in the Eastern Cordillera focused during late Miocene time. On the basis of widespread coarse-grained nonmarine sedimentation, previous studies have inferred that uplift of the Garzón Massif began during the late Miocene, coincident with rapid elevation gain elsewhere in the Eastern Cordillera. We take an integrated approach to better reconstruct Andean topographic growth and distinguish between exhumation and surface uplift of the Garzón Massif. We present new detrital zircon U-Pb (uranium-lead) provenance data, sandstone petrographic data, and paleoprecipitation data from upper Miocene clastic fill of the Neiva Basin within the adjacent Upper Magdalena Valley of the modern hinterland. In addition, new apatite fission-track (AFT) ages from the central segment of the Garzón Massif directly constrain its exhumation history over the past 15 million years (Myr). The results indicate that early exhumation may have initiated by ca. 12.5 Myr ago, but a substantial orographic barrier was not fully established until roughly 6-3 Myr ago, when over 1 km/Myr of material was exhumed. Thermal history modeling of the AFT data suggest diminished exhumation thereafter (3-0 Myr ago), during oblique Nazca-South America plate convergence. This exhumation history is consistent with paleontological data suggesting late Miocene divergence of the Amazon, Orinoco, and Magdalena river systems, with associated transcontinental drainage of the Amazon River.

Evidence for two Cretaceous superposed orogenic belts in central Mexico based on paleontologic and K-Ar geochronologic data from the Sierra de los Cuarzos
Michelangelo Martini et al., Instituto de Geología, Universidad Nacional Autónoma de México, Av. Universidad 3000, México D.F. 04510, México. This article is online at http://geosphere.gsapubs.org/content/early/2016/05/27/GES01275.1.abstract.

After the break-up of Pangea, the Pacific margin of North America was involved in numerous different orogenic events (e.g., Sonoma, Sevier, Laramide, etc.) that progressively shaped the landscape and exerted a primary control on the climate, floral and faunal distribution of this continental landmass. The superposition of these orogenic events in space during time rendered the tectonic history of the North America Cordillera difficult to decipher. Our new stratigraphic, paleontologic, and geochronologic data document that the North America Cordillera exposed in central Mexico was developed at least during two different tectonic pulses. One of these tectonic pulses took place at ca. 115 Ma and was produced by the collision of an intraoceanic volcanic massif to the Pacific margin of North America. This pulse was followed by a second one that started at ca. 93 Ma and lasted for about 50 million years, producing a high-relief mountain chain that is known as the Sierra Madre Oriental. The cause for this second tectonic pulse is still object of discussion. An increase in the velocity of convergence between North America and the adjacent Pacific plate seems to be the most plausible reason.

Detrital-zircon geochronology of the Sawtooth metamorphic complex, Idaho: Evidence for metamorphosed lower Paleozoic shelf strata within the Idaho batholith
Chong Ma, Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA. This article is online at http://geosphere.gsapubs.org/content/early/2016/06/23/GES01201.1.abstract. Themed issue: Active Margins in Transition — Magmatism and Tectonics through Time: An Issue in Honor of Arthur W. Snoke.

This research article utilizes isotopic age-dating of detrital zircons from the metamorphosed sedimentary rocks of the Sawtooth Range immediately southwestern of Stanley, Idaho, to reveal the age spectra of zircons in those rocks. The age spectra obtained for a group of rock samples are then statistically compared to and correlated with those of age-known rocks from elsewhere in the U.S. Cordilleran mountain belt. This allows us to establish the depositional ages of the highly altered sedimentary rocks in the study area within the Idaho batholith. The results show that those rocks were likely deposited in the Cambrian and Middle Ordovician. The identification of lower Paleozoic shelf sedimentary rocks in the Idaho batholith region suggests that the Cordilleran passive margin sediments was continuous along the western margin of the ancient North America.

Origin and heterogeneity of pore sizes in the Mount Simon Sandstone and Eau Claire Formation: Implications for multiphase fluid flow Peter S. Mozley et al., Department of Earth and Environmental Sciences, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801, USA. This article is online at http://geosphere.gsapubs.org/content/early/2016/06/23/GES01245.1.abstract.

Rocks below ground are not always entirely solid, but instead often have void spaces within them that can be of considerable societal importance. A variety of fluids can be stored in these voids (also known as pores), including groundwater in aquifers, and oil and gas in hydrocarbon reservoirs. A team of scientists at Sandia National Laboratories and New Mexico Tech has just published the results of a detailed analysis of the characteristics of void spaces in the Mount Simon Sandstone in the Geological Society of America's journal Geosphere (insert link to article here). The Mount Simon Sandstone occurs throughout the Midwestern United States, where it is a target horizon for a variety of fluids, including: (1) wastewater produced from industrial operations, including fracking; (2) CO2 captured before entering the atmosphere, so as not to contribute to greenhouse gases; and (3) compressed air that is injected using solar and wind energy for later use as an energy source (picture filling a balloon with air, and then releasing it to turn a propeller). The Mount Simon Sandstone is a favored horizon for wastewater and CO2 storage because it is the lowermost (deepest) sedimentary formation in the area, occurring just above older (Precambrian) rocks that have much lower porosity. Thus, if you want to keep fluids as far away from the atmosphere or shallow groundwater as possible, in many places the Mt. Simon is the ideal geologic formation in which to inject the fluids, as it is the deepest unit with sufficient void spaces to store the material. The scientists examined the microscopic pore structure of the Mt. Simon Sandstone using microscopes and a laboratory technique that involves injecting mercury into the rock under high pressure. They determined that the formation has undergone a complex geologic history that resulted in the formation of unusually variable pore sizes. They concluded that this high degree of variability could lead to enhanced trapping of non-aqueous fluids, such as oil, CO2, and air. This is good news for those wishing to use the formation for wastewater disposal and CO2 storage, since it means that the fluids will migrate upward more slowly than normal. It is, however, not a favorable characteristic for compressed air energy storage, since it could contribute to an irregularly shaped "air bubble" configuration, which is less efficient for this application. The study was funded by the U.S. Department of Energy.

Topographic constraints on magma accumulation below the actively uplifting Uturuncu and Lazufre volcanic centers in the Central Andes
Jonathan P. Perkins et al., Department of Earth and Planetary Sciences, University of California Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA. This article is online at http://geosphere.gsapubs.org/content/early/2016/06/16/GES01278.1.abstract. Themed issue: PLUTONS: Investigating the Relationship between Pluton Growth and Volcanism in the Central Andes

In the Central Andes of South America, remote observations from InSAR satellites have led to the recognition that a number of its volcanoes are not only actively deforming, but are some of the fastest growing features in the southern hemisphere. This has been cause for concern, as two of the volcanoes, Uturuncu in southern Bolivia and Lazufre southward along the Chile-Argentina border, are potentially connected to large magma reservoirs beneath the surface that could eventually source catastrophic eruptions. Although satellite observations have provided detailed constraints on deformation for the past 15-20 years, this represents only a snapshot in time of how these systems are evolving. In order to extend this record of deformation into the geologic past and better understand the connection between the current unrest at Uturuncu and Lazufre and their magmatic histories, in this paper we look for signs of surface deformation that are preserved in the topography around the volcanoes. For example, Uturuncu is flanked by ancient lake shorelines that provide horizontal markers at the time they were formed (about 16 thousand years ago). At Lazufre, where lake shorelines are less common, we use a simple-but-novel technique and measure the flow direction of pre-historic lava flows to learn about where high topography existed in the geologic past. The two case studies at Uturuncu and Lazufre highlight very different behaviors of their underlying magmatic plumbing systems. At Uturuncu, which is thought to be connected to one of the largest known magma reservoirs on Earth (the Altiplano-Puna Magma Body), the topography appears largely unchanged since the late Pleistocene. This suggests that the surface is likely quick to relax from any perturbation from below (perhaps due to the heating of the Earth's crust from such a large magma reservoir). At Lazufre, however, our measurements suggest that the topographic dome that characterizes this volcanic complex has persisted for over 0.5 million years. Furthermore, the pattern of volcanic vents at Lazufre suggests it has been actively inflating throughout its history, and therefore Lazufre is likely a highly evolved pre-caldera magmatic system. Taken together, our work demonstrates that the forensic tools of geomorphology may be useful for understanding how large volcanic systems grow over their lifetime.

Provenance evolution during progressive rifting and hyperextension using bedrock and detrital zircon U-Pb geochronology, Mauléon Basin, western Pyrenees
Nicole R. Hart et al., Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, 2275 Speedway C9000, Austin, Texas 78712, USA. This article is online at http://geosphere.gsapubs.org/content/early/2016/06/23/GES01273.1.abstract.

Rifting is the process by which the continental crust stretches, extends, and can break apart forming an ocean basin. During such extension, erosion and transport of sediments leads to a sedimentary system that varies three-dimensionally due to the mixing of different sedimentary materials. The response of sedimentary systems to tectonics during progressive rifting along magma-poor, hyper-extended margins, where the crust is stretched and thinned to less than or equal to 10 km without extensive volcanism, have not been studied in detail because many are in subsurface and submarine environments. The Mauléon Basin of the western Pyrenees is a unique example of such a system as it offers exposures of the upper mantle, crust, and sedimentary units that in most other places are inaccessible. In this study ~5800 new zircon U-Pb ages were obtained from rock units deposited before (prerift), during (synrift) and after (postrift) rifting. Zircon U-Pb ages obtained from prerift units constrain the ages of crystallization of western Pyrenean plutons, metamorphism of the lower crust, and the source of sediments deposited throughout Paleozoic and early Mesozoic time. The variation of detrital zircon U-Pb ages obtained from synrift and postrift units constrain the evolution of the Mauléon Basin during and after rifting. These observations are used to define a general model for sediment routing during rifting along magma-poor continental margins.

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