New Geosphere themed issue: The anatomy of rifting
This is a NASA image created by Jesse Allen, Earth Observatory, using data provided courtesy of the MODIS Rapid Response team. Credit NASA
Studies of a wide variety of extensional processes, ranging from plate thinning to magma intrusion, have helped scientists understand how continents are broken apart to form ocean basins. However, deformation processes vary significantly during the development of continental rifts and mid-ocean ridges. In addition, ocean ridges are offset along their length by major transform faults, the initiation of which is poorly understood.
Data documenting active processes have proven difficult to obtain because most ridges are submerged with only rare portions of the divergent plate boundary being exposed on land. Therefore current knowledge about the length and time scales of magmatism and faulting during rift evolution as well as the mechanisms of initial development of mid-ocean ridges and transforms is limited. In this themed issue, Carolina Pagli and colleagues present contributions that document the wide variety of processes acting at divergent plate boundaries and transforms in order to synthesize some of the most relevant research topics about plate extension and to identify the important questions that remain unanswered.
FEATURED ARTICLE
Introduction: Anatomy of rifting: Tectonics and magmatism in continental rifts, oceanic spreading centers, and transforms
Carolina Pagli et al., Dipartimento di Scienze della Terra, Università di Pisa, Via S. Maria 53, 56126 Pisa, Italy. New themed issue: Anatomy of Rifting: Tectonics and Magmatism in Continental Rifts, Oceanic Spreading Centers, and Transforms. This article is OPEN ACCESS online at http://dx.
Related Article
Upper mantle structure of the southern Arabian margin: Insights from teleseismic tomography
Félicie Korostelev et al., Sorbonne Universités, UPMC and Université Paris, CNRS, Paris, France. New themed issue: Anatomy of Rifting: Tectonics and Magmatism in Continental Rifts, Oceanic Spreading Centers, and Transforms. This OPEN ACCESS article is online at http://dx.
In this open-access study recently published in Geosphere, Félicie Korostelev and colleagues image the lithospheric and upper asthenospheric structure beneath the central and eastern parts of the northern Gulf of Aden rifted passive continental margin with seismic stations to evaluate the role of transform fault zones on the evolution of magma-poor continental margins. Their results suggest that locally buoyant mantle creates uplift and dynamic topography on the rift margin that affects the course of seasonal rivers and the sedimentation at the mouth of those rivers. Korostelev and colleagues also suggest that the dynamic topography and recent volcanism in the central and eastern Gulf of Aden could be due to small-scale convection at the edge of the Arabian plate and/or in the vicinity of fracture zones.
Other papers recently published in Geosphere are highlighted below:
Crustal-scale tilting of the central Salton block, southern California
Rebecca J. Dorsey and Victoria E. Langenheim, Dept. of Geological Sciences, 1272 University of Oregon, Eugene, Oregon 97403-1272, USA and U.S. Geological Survey, MS 989, 345 Middlefield Road, Menlo Park, California 94025, USA. This article is online at http://dx.
The San Andreas fault system in southern California is a wide zone of strike-slip faulting where the Pacific and North America plates have been grinding past each other for the past ~8 million years. The Salton block is a large, 35-km wide block of continental crust bounded by the southern San Andreas fault on the northeast and the San Jacinto fault on the southwest. This integrated geologic, geophysical, and geomorphic study by Rebecca Dorsey and Victoria Langenheim of the central Salton block (southern Santa Rosa Mountains and Coachella Valley) provides evidence that this piece of crust has tilted to the northeast about a horizontal axis since the geologically young initiation of the San Jacinto fault about 1.2 million years ago. This strain pattern is unexpected and includes, paradoxically, large vertical displacements across two major strike-slip faults. Northeast tilting is a predicted result of oblique convergence and vertical loading of the Salton block across a northeast-dipping San Andreas fault in Coachella Valley. This interpretation is consistent with the results of recent geodetic, seismic, and modeling studies that indicate a dipping strike-slip fault (often assumed to be vertical), and highlights the large effect that a small angle of obliquity (less than 10 degrees) can have on vertical crustal displacements over geologic time in major transform fault zones.
A new model for Quaternary lava dams in Grand Canyon based on 40Ar/39Ar dating, basalt geochemistry, and field mapping
Ryan S. Crow et al., Dept. of Earth and Planetary Sciences, University of New Mexico, Albuquerque, New Mexico 87131, USA. Themed issue: CRevolution 2: Origin and Evolution of the Colorado River System II. This article is online at http://dx.
In 1869, John Wesley Powell was the first geologist to recognize that a series of lava flows dammed the Colorado River in western Grand Canyon. Despite almost 150 years of study since then, there is still a lot to learn about age, structure, and failure mechanism of the dams and the impact they had on the Colorado River and Grand Canyon. In this paper for Geosphere, Ryan S. Crow and colleagues use a range of techniques to date 17 individual dams and reconstruct their original extent. The largest dams were at least 135 km long and more than 395 m high. Based on those reconstructions and field observations, Crow and colleagues suggest that the dams failed in tens to hundreds years in most cases. The longest dams may have existed for about a thousand years. The upstream portions of dams failed quickly, in some cases catastrophically, while stable downstream dam segments were dismantled by the Colorado River more slowly.
Tectonic evolution of the Mesozoic South Anyui suture zone, eastern Russia: A critical component of paleogeographic reconstructions of the Arctic region
Jeffrey M. Amato et al., Dept. of Geological Sciences, New Mexico State University, MSC 3AB, P.O. Box 30001, Las Cruces, New Mexico 88003, USA. This article is online at http://dx.
Jeffrey Amato and colleagues conducted a study of a Mesozoic suture zone in Northeastern Russia where two continental blocks collided sometime in the Cretaceous period. The South Anyui suture zone is a critical component of the tectonic history of the Arctic region because the Arctic Ocean basin (Amerasian basin) in the region of the North Pole has only existed for the last 130 million years. Prior to this, the continents surrounding the North Pole were linked in some uncertain configuration. Studying the suture zones in the region helps understand the geology of the region. Previously, this suture zone was linked to a similar zone in Alaska. Amato and colleagues use geologic evidence and a seismic line in Russia to suggest that the two zones may not be directly correlative and have constructed an animated movie of their tectonic model to illustrate a possible history for the region from 200 million years ago until 120 million years ago.
Deformation and magma transport in a crystallizing plutonic complex, Coastal Batholith, central Chile
Jeffrey R. Webber et al. (Keith A. Klepeis, corresponding author), Dept. of Geology, University of Vermont, Delehanty Hall, 180 Colchester Avenue, Burlington, Vermont 05405-0122, USA. This article is online at http://dx.
This study examines how batholiths, which form part of the deep plumbing systems of volcanic arcs on the continents, form. Studies such as this one are important because the transport of magma to form batholiths is one of the primary mechanisms that moves heat and mass through the Earth's lithosphere. A thorough understanding of these mechanisms is essential to our ability to unravel the timing and styles of volcanism in continental arcs. In this paper we show how magma chambers that reside below the Earth's surface can be rejuvenated by multiple episodes of magma injection. These events mobilize the magma in melt-rich currents. This data reveals the three-dimensional patterns of this flow and document how geological structures that commonly are preserved within these magma chambers form.
Unlocking the correlation in fluvial outcrops by using a DOM-derived virtual datum: Method description and field tests in the Huesca fluvial fan, Ebro Basin (Spain)
Rubén Calvo and Emilio Ramos, GEOMODELS Institute, Group of Geodynamics and Basin Analysis (GGAC), Dept. of Stratigraphy, Paleontology, and Marine Geosciences, University of Barcelona, Barcelona 08028, Spain. This article is online at http://dx.
This paper presents a new method based on the terrestrial laser scanning technology that provides a high degree of stratigraphic control when characterizing outcrops of fluvial sediments as analogs of hydrocarbon reservoirs.
Geothermal energy characterization in the Appalachian Basin of New York and Pennsylvania
George R. Stutz et al. (Teresa E. Jordan, corresponding author), Cornell Energy Institute, Snee Hall, Cornell University, Ithaca, New York 14853, USA. This article is online at http://dx.
In eastern and central North America, the geothermal heat flow is insufficient to be economically viable for electricity generation, yet in theory there are many opportunities for direct use of lower temperature geothermal resources. This paper explores the potential that suitable grades of geothermal energy exist at comparatively shallow depth in some parts of Pennsylvania and New York within the Appalachian sedimentary basin. This study analyzed nearly 8000 archival petroleum industry temperature-depth well data to calculate the surface heat flow at the location of each well. Using interpolation methods, a spatially refined map of surface heat flow shows that over much of the Appalachian Basin of New York and Pennsylvania the heat flow exceeds 50 mW/m2. Regions of enhanced heat flow appear to exist in south central NY and in two northeast-elongate regions in central and western PA. The study highlights sources of major uncertainty in the data and in the analysis, and recommends follow-up studies to improve their accuracy.
Paleoseismologic evidence for large-magnitude (Mw 7.5-8.0) earthquakes on the Ventura blind thrust fault: Implications for multifault ruptures in the Transverse Ranges of southern California
Lee J. McAuliffe et al., Dept. of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA. This article is online at http://dx.
Detailed analysis of geological and geophysical data, together with the latest dating techniques reveal the ages of two (or more) of the most recent earthquakes on the Ventura fault in southern California. These earthquakes, dated at between 235 years ago and 805 +/- 75 years ago (likely near the end of that interval), and 4065 and 4665 years ago, are determined by examining the geometry of folded sedimentary strata above the tip of the fault. The amount of uplift preserved in the rock record indicate that these two earthquakes were in the magnitude range of 7.5 to 8.0, and likely near the higher end of that range. For an earthquake of such magnitude to occur on the Ventura fault, the rupture likely involved multiple adjacent faults within southern California. The proximity of these faults to major population centers, including the greater Los Angeles region, and the potential for tsunami generation during ruptures extending offshore along the fault system, highlight the importance of understanding the complex behavior of these faults for future seismic hazard assessment.
Heat flow and thermal modeling of the Appalachian Basin, West Virginia
Z.S. Frone et al., Roy M. Huffington Dept. of Earth Sciences, Southern Methodist University, 3225 Daniel Avenue, Dallas, Texas 75223, USA. This article is online at http://dx.
Heat flow and temperatures-at-depth are an important parameters in delineating areas of high geothermal energy potential. In the past, continental heat flow measurements have been limited to a small number of boreholes due to the small number of wells with available equilibrium temperature logs. Bottom hole temperatures have been used in the past to calculate 1D heat flow and extrapolated to determine temperatures-at-depth, however these values are subject to large uncertainties due to poor data quality. The methods used in this paper use the theory of regional heat flow provinces where the heat flow at the base of the upper crust and the thickness of the upper crust are known from regional heat flow studies. These constrains are then used in a Monte Carlo simulation to determine the heat flow at the base of the basin that best fits all the available temperature data. From this analysis surface heat flow and temperatures at depth for a 2D section of the Appalachian Basin are determined. Producing heat flow and temperature-at-depth models is a crucial first step in defining the geothermal energy potential of a region. These data can then be used as a baseline from which further development questions related to feasibility, engineering, and economics can be solved.
Topographically driven fluid flow within orogenic wedges: Effects of taper angle and depth-dependent permeability
Ryan M. Pollyea et al., Dept. of Geology and Environmental Geosciences, Northern Illinois University, 1425 W. Lincoln Highway, DeKalb, Illinois 60115, USA. This article is online at http://dx.
Regional-scale groundwater flow within foreland basins is important for a number of societally significant geological processes, including oil/gas migration and accumulation, earthquake generation, and Mississippi Valley Type ore deposits. As a result, groundwater flow in these environments has been a subject of geological research for decades. Nevertheless, investigators have yet to quantify how the combination of variable topographic slope and depth-decaying permeability influence fluid system evolution over geologic time. In this work, Ryan Pollyea and colleagues use a numerical modeling experiment to refine the conceptual understanding of regional-scale groundwater flow when topographic slope increases from one degree to three degrees to nine degrees, while permeability decays systematically with depth. Their results suggest that (1) depth-dependent permeability severely limits the penetration depth of infiltrating water within a steeply dipping foreland basin, (2) fluid system evolution within a gently dipping foreland basin is governed by local scale topography superimposed on the regional gradient, (3) the influence of sub-basin topography on local-scale fluid circulation is suppressed as the regional topographic gradient increases, and (4) the spatial distribution of groundwater residence time is fundamentally different when topographic slope exceeds 3 degrees. Although the models presented here are generic in nature, these results provide a quantitative framework for understanding the relationship between topographic driving potential and reservoir permeability, and will likely be useful in deciphering the evolution of ancient geological fluid systems.
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All GEOSPHERE articles are available at http://geosphere.
Contact: Kea Giles
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kgiles@geosociety.org
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