Boulder, CO, USA -- New research from three Geological Society of America journals is now online. Highlights for GSA BULLETIN (published online ahead of print on 19 Sept.), GEOSPHERE (with additions to two themed issues), and the October GSA TODAY science article are provided below.
Representatives of the media may obtain complimentary copies of GSA BULLETIN and GEOSPHERE articles by contacting Christa Stratton at the address above. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to the appropriate journals in articles published. Abstracts for all issues of GSA BULLETIN are available at http://gsabulletin.gsapubs.org/; GEOSPHERE abstracts are at http://geosphere.gsapubs.org/.
Non-media requests for articles may be directed to GSA Sales and Service, gsaservice@geosociety.org.
Abstracts for the following new GSA BULLETIN articles, which were published online ahead of print on 19 Sept., can be accessed at http://gsabulletin.gsapubs.org/content/early/recent.
GSA BULLETIN keywords: Ilchulbong, Jeju Island, Korea, UNESCO, Ries impact crater, Enkingen, Reunion plume, Marian plume, Kerala, India, Madagascan igneous rocks, White Mountains, California, Eureka Valley-Joshua Flat-Beer Creek (EJB) pluton
Ilchulbong tuff cone, Jeju Island, Korea, revisited: A compound monogenetic volcano involving multiple magma pulses, shifting vents, and discrete eruptive phases
Y.K. Sohn et al. (S.J. Cronin, corresponding), Dept. of Earth and Environmental Sciences and Research Institute of Natural Sciences, Gyeongsang National University, Jinju 660-701, Republic of Korea; doi: 10.1130/B30447.1.
Ilchulbong, "Sunrise Peak," rises dramatically from the coastal plain of Jeju Island, South Korea. The tuff-cone is a UNESCO World Heritage site with two-million visitors per year and hosts outstanding coastal exposures that form a type locality for the sedimentary depositional processes of phreatomagmatic basaltic eruptions. Its simple cone morphology was assumed to represent a single short-lived eruption. Reanalysis of the deposits by Y.K. Sohn of Gyeongsang National University, Korea, and colleagues has revealed that the eruption occurred in three separate phases, separated by breaks of days to weeks, and thus it is a compound structure. The first eruption pause was accompanied by vent migration of ~600 m, so that the basal part of Ilchulbong is the outer flanks of a formerly neighboring cone (since been removed by marine erosion). The second break was shorter, leading to a subtle discontinuity in the deposits, but no further vent migration. Detailed geochemistry showed that each eruption phase represented a distinct alkali basaltic magma pulse from a mantle source. These results demonstrate how deep magma system processes, up to 80 km below a volcano, may cause multiple-phase eruptions in small-volume basaltic volcanoes and how careful analysis of phreatomagmatic sequences are needed to detect potential shifts in eruption chemistry and vent location.
Petrography of the impact breccias of the Enkingen (SUBO 18) drill core, southern Ries crater, Germany: New estimate of impact melt volume
W.U. Reimold et al., Museum fur Naturkunde-Leibniz Institute for Evolution and Biodiversity Research at Humboldt University Berlin, Invalidenstrasse 43, 10115 Berlin, Germany; doi: 10.1130/B30470.1.
The Ries impact crater in southern Germany is arguably one of the best-preserved and best-studied impact structures in the world. There, not only the complete crater fill as well as an extensive sequence of post-impact sediments are preserved, but the impact breccias of the ejecta blanket are also widely preserved. A series of cored bore holes has made the Ries crater fill accessible for detailed geoscientific investigation. Recently, a new drill core has become available from the village of Enkingen close to the inner structural ring. W.U. Reimold of Humboldt University Berlin and colleagues present a detailed petrographic analysis of the impact breccia sequences intersected in this bore hole. Results are discussed in terms of breccia genesis, especially with regard to the controversial nature and formation of the type of breccia known as suevite. Nearly 50 vol percent of the within-crater impact breccias constitute impact melt -- significantly more than previously estimated -- resolving a major controversy. Suevite has been controversially debated as representing either a mainly clastic breccia with cogenetic impact melt particles or as a formation made up of a melt groundmass with clastic particles. These results favor the former, classical definition.
Geochemistry and paleomagnetism of Late Cretaceous mafic dikes in Kerala, southwest coast of India in relation to large igneous provinces and mantle plumes in the Indian Ocean region
T. Radhakrishna, Center for Earth Science Studies, Trivandrum 695 031, India; and Mathew Joseph; doi: 10.1130/B30288.1.
Occurrences of magmatic rocks of about 65-70 Ma and 85-90 Ma age have come to light recently in south India. The ages are comparable with the age of Reunion plume derived large igneous province of Deccan, India and the Marion plume derived Madagascan igneous rocks, less known in India. This study by T. Radhakrishna of the Center for Earth Science Studies, India, and colleague Mathew Joseph of the Indian Geological Survey presents new geochemical and palaeomagnetic results on the dykes in Kerala state, far south of Deccan, in India. The geochemistry of the dykes, so near in age, is quite distinct. Comparisons between the 65-70 Ma dykes and the Deccan and between the 85-90 Ma dykes and the Madagascan rocks are quite remarkable. Deeper garnet lherzolite mantle source produced the 85-90 Ma dykes, while a shallow spinel lherzolite gave rise to the 65-70 Ma dykes. The paper reports a 85-90 Ma pole for India. The pole data from the 65-70 Ma dykes compare well with the highly refined Deccan super pole. The paleolatitudes of 65-70 Ma dykes are not easily distinguishable with the effects of true polar wander. The 85-90 Ma dykes paleolatitude suggest a southward migration of Marion plume in conformity with the Global Mantle Circulation Models, but differ with the stationary plume position proposed by Torsvik et al (1990) using Madagascan data. Furthermore, the study places southern India adjacent to the northern Madagascar in the pre-drift continental assembly and do not support many reconstructions that correlate the "shear" zones across India and Madagascar.
Metamorphism and fluid flow in the contact aureole of the Eureka Valley–Joshua Flat–Beer Creek pluton, California
Peter I. Nabelek, Dept. of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA; and Sven S. Morgan; doi: 10.1130/B30425.1.
Peter I. Nabelek of the University of Missouri and colleague Sven S. Morgan investigate metamorphism and fluid flow around granite plutons in the White Mountains of eastern California during the Jurassic and Cretaceous eras. At that time, this region was of part of an Andean-type arc system due to subduction of an oceanic plate. Nabelek and Morgan use mineral assemblages in sedimentary rocks to determine the pressures and temperatures at which they were metamorphosed during emplacement of the Eureka Valley-Joshua Flat-Beer Creek composite pluton. The heat generated by the magma within the growing pluton may have enhanced deformation of the metamorphosed sedimentary wall rocks to their current subvertical orientation. The authors also used stable isotope ratios of oxygen and carbon in calcite and dolomite to determine the amount of fluid that has flowed through the vertically oriented rock formations. They found that fluid flow through the rocks was highly heterogeneous and limited by the rocks' low permeabilities. This was particularly true of marbles. In one part of the pluton's aureole, focused fluid flow has promoted melting of heated schists. Subsequent to growth of the pluton by multiple magmatic pulses, most of the schists around it were altered by continued hydrothermal activity as additional plutons were emplaced in the region.
The October issue of GEOSPHERE is online now at http://geosphere.gsapubs.org/. Themes include "Exploring the Deep Sea and Beyond" (E. Eke et al; C.K. Paull et al.) and "Origin and Evolution of the Sierra Nevada and Walker Lane" (du Bray and John; A.G. Sylvester). Other articles include studies of the Picuris-Pecos fault, New Mexico; and giant garnet crystals from Gore Mountain in the Adirondacks.
GEOSPHERE keywords: breaching, Monterrey Submarine Canyon, ancestral Cascades, Sierra Nevada batholith, Picuris-Pecos fault, garnet, Gore Mountain, Ruby Mountains, field mapping, Tibet
Field-scale numerical modeling of breaching as a mechanism for generating continuous turbidity currents
Esther Eke et al., Dept. of Civil and Environmental Engineering, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801, USA; doi: 10.1130/GS607.1.
"Breaching" is a generic term that is often used to describe the process of retrogressive erosion and eventual failure subsequent to the overtopping of embankments, dams, dikes, and sand barriers in closed estuaries. In this work, however, "breaching" is used in a much more restrictive sense. Here it refers to a kind of relatively slow, subaqueous retrogressive failure that was apparently first identified by the Dutch dredging industry. This mechanism has remained largely unexplored, and yet evidence exists to link breaching to the formation of continuous turbidity currents (subaqueous bottom flows driven by suspended sediment) in the deep sea or in any other submarine setting where relatively clean fine sand can accumulate (e.g., some river deltas). Once this mode of failure is instigated, it can continue for many hours. Breaching is common in densified sands. Sand grains typically mix with the ambient fluid, forming a turbidity current that runs along the steep breach face, and then down the gentler slope that forms at the base of the steep face. In this study, we report on numerical and physical modeling of breach-generated turbidity currents. The numerical model has been first developed and tested at laboratory scale. It has then been applied to establish the feasibility of a natural breach-generated turbidity current in a field setting, using a generic example based on the Monterey Submarine Canyon.
High-resolution bathymetry of the axial channels within Monterey and Soquel submarine canyons, offshore central California
Charles K. Paull et al., Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, California 95039-9644, USA; doi: 10.1130/GS636.1.
Submarine canyons are conduits through which huge volumes of sediment are transported from the continent to the deep sea. Because of the difficulty of mapping complicated features on the ocean floor, the processes that occur within submarine canyons remain poorly understood. Charles K. Paull of the Monterey Bay Aquarium Research Institute and colleagues report on the use of an unmanned free-swimming robotic vehicle to map the floor within Monterey Canyon, off the coast of central California. When launched from a ship, this vehicle dives to the seafloor, and proceeds to navigate near the bottom through the sinuous canyon floor channel using sonars to make extremely high-resolution maps of the seafloor. This technology has enabled the channel on the floor of Monterey Canyon to be mapped at an unprecedented level of detail and to water depths never before seen (2.2 km). The maps reveal the detailed shape of the axial channel within this canyon for the first time and show the existence of a surprising number of large bedforms that were previously unknown. While the origins of many of the features illustrated in this paper are still poorly understood, knowledge of their existence provides new constraints on the process that occur within these channels.
Petrologic, tectonic, and metallogenic evolution of the Ancestral Cascades magmatic arc, Washington, Oregon, and northern California
Edward A. du Bray and David A. John, U.S. Geological Survey, MS 973, Box 25046, Denver Federal Center, Denver, Colorado 80225, USA; doi: 10.1130/GS669.1.
Present-day High Cascades arc magmatism was preceded by ~40 m.y. of nearly co-spatial magmatic activity represented by the ancestral Cascades arc in Washington, Oregon, and northernmost California. The composition and spatial distribution of ancestral-Cascades-related magmatism varied considerably through time and along the length of the arc. Compositions of the earliest erupted magmas reflect development beneath thin crust, whereas compositions of later erupted magmas suggest involvement of thicker, more evolved crust as subduction-related magmatic processes became well established in this region. Mineral deposits associated with ancestral Cascades arc rocks are uncommon and most are small and low grade relative to those found in other continental magmatic arcs. The dearth, small size, and low grade of deposits, especially in the southern two-thirds of the ancestral arc, probably reflect the prevalence of extensional tectonics within this arc domain during this magmatic episode. In contrast, the Washington arc domain developed in a mildly compressive regime that was more conducive to magmatic processes and hydrothermal fluid channeling critical to deposit formation. Small, low-grade porphyry copper deposits in the northern third of the ancestral Cascades arc segment also may be a consequence of more mature continental crust, including a Mesozoic component, beneath Washington north of Mt. St. Helens.
The nature and polygenetic origin of orbicular granodiorite in the Lower Castle Creek pluton, northern Sierra Nevada batholith, California
Arthur Gibbs Sylvester, Dept. of Earth Science, University of California, Santa Barbara, California 93106, USA; doi: 10.1130/GS664.1.
This paper describes and explains the origin of a remarkable outcrop, a "pipe," of mafic granodiorite and basalt within a granodiorite pluton of Cretaceous age in the northern part of the Sierra Nevada batholith near Soda Springs, California. In the basketball court-sized outcrop, some semi-rounded, football-sized blocks of basalt -- enclaves -- each have a 10-15 mm thick mantle of white rock -- aplite -- whose plagioclase and biotite crystals are concentrically oriented parallel to the margins of each enclave. The mantled enclaves are enclosed in the mafic granodiorite of the pipe. The relationships among the four rock types (granodiorite, basalt, aplite, and mafic granodiorite) require successive mingling among four distinctly different magmas during the upward rise of a molten mass within a pipe in the granodiorite pluton. Such pipes are not uncommon in granitic plutons, but the aplitic mantles are unusual and so also are instances of mingling of more than two magmas. The origin of the aplite mantles requires immersion of the basaltic enclaves in an aplitic magma after granodiorite and basaltic magmas mingled but before the mantled enclaves were subsequently entrained in mafic granodiorite magma of the pipe. The rocks and their relationships in the pipe may reflect a heat transfer process from the depths of a cooling pluton.
Provenance evidence for major post-early Pennsylvanian dextral slip on the Picuris-Pecos fault, northern New Mexico
Steven M. Cather et al., New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA; doi: 10.1130/GS649.1.
It has long been known that major lateral (strike-slip) faults are present in northern New Mexico. These faults bound the eastern edge of the Colorado Plateau and have facilitated the northward movement of the plateau relative to the Great Plains by about 100 km (60 miles). There has been much disagreement among geologists about when this movement took place. In this paper, Steven M. Cather of the New Mexico Institute of Mining and Technology and colleagues examine the composition of sediments that were shed off one of these faults (the Picuris-Pecos fault near Santa Fe and Taos), where it formed an ancient mountain front in the ancestral Rocky Mountains during the middle of the Pennsylvanian epoch, about 310 million years ago. Comparison of these sediments to their source areas shows that all of the lateral movement on this fault (about 37 km or 23 miles) occurred since 310 million years ago. Movement primarily occurred late in the ancestral Rocky Mountain deformation about 270-310 million years ago and/or during the Laramide deformation that formed the modern Rocky Mountains about 50-75 million years ago.
Megacrystic Gore Mountain-type garnets in the Adirondack Highlands: Age, origin, and tectonic implications
James M. McLelland and Bruce W. Selleck, Dept. of Geology, Colgate University, Hamilton, New York 13346, USA; doi: 10.1130/GS683.1.
Giant garnet crystals from Gore Mountain in the Adirondacks are known to collectors around the world. These garnet deposits are also the source of high-quality abrasive used for polishing giant telescope mirrors. Similar, though less impressive garnet occurrences are known elsewhere in the Adirondacks. Why did the garnets grow so big? And when did they form during the complex geological evolution of the Adirondacks? This new research by James M. McLelland and Bruce W. Selleck of Colgate University shows that the garnets grew so large because the rocks were invaded by fluid that allowed chemical components to move through the garnet rock easily, thus permitting the growth of large crystals. The fluids came from nearby intrusions of molten granite, and the granite crystallized about 1.04 billion years ago, just when the garnets were growing in the nearby rock. The granite intrusion and garnet growth happened near the end of the mountain-building event that marked the assembly of the ancient supercontinent called Rodinia.
Episodic growth of a Late Cretaceous and Paleogene intrusive complex of pegmatitic leucogranite, Ruby Mountains core complex, Nevada, USA
K.A. Howard et al., U.S. Geological Survey, MS-973, 345 Middlefield Road, Menlo Park, California 94025, USA; doi: 10.1130/GS668.1.
How do large bodies of granite form from melts? Geochronologic advances have spurred renewed interest in this long-vexing question and the related questions of how long it takes to generate and to emplace the melts. In a report integrating provocative field observations and U-Pb dating evidence for an intrusive complex of pegmatitic granite in Nevada, K.A. Howard of the U.S. Geological Survey and colleagues propose an expanded time dimension for protracted evolution of large igneous systems and construction of batholith-sized granite bodies. The authors show that the intrusive complex was constructed piecemeal by myriad small bodies inflating the host rocks, and find that zircon and monazite crystals record episodic igneous crystallization from about 90 to 30 million years ago. The proposed construction of the intrusive complex from melting and partial remelting episodes over this very long interval would span changes in tectonic environment from the Sevier orogeny to Paleogene extension.
Late Eocene crustal thickening followed by Early-Late Oligocene extension along the India-Asia suture zone: Evidence for cyclicity in the Himalayan orogen
Ran Zhang et al., Dept. of Earth and Atmospheric Sciences, University of Houston, Houston, Texas 77204-5007, USA; doi: 10.1130/GS643.1.
The study by R. Zhang of the University of Houston and colleagues presents extensive field mapping and analytical results from a remote region of Tibet along the suture zone between India and Asia. Results indicate two cycles of crustal shortening and extension are archived in the rocks along the India-Asia suture zone in southwest Tibet. Each cycle lasted for approximately 20 Ma. The regional nature of this event shows that orogens cycle through periods of energy accumulation and energy dissipation over their lifespan.
The October GSA TODAY science article looks at what a Miocene river in the Kaibab Plateau reveals about the ancestral Colorado river and Grand Canyon. All GSA TODAY articles are open access at http://www.geosociety.org/pubs/.
A Miocene river in northern Arizona and its implications for the Colorado River and Grand Canyon
Ivo Lucchitta et al., U.S. Geological Survey, Flagstaff, Arizona 86001, USA, and Museum of Northern Arizona, Flagstaff, Arizona 86001, USA. Pages 4 doi: 10.1130/G119A.1
The Miocene Crooked Ridge River traverses part of the Kaibab Plateau in northern Arizona in eroded relief -- the riverbed now forming the crest of a ridge that can be traced for almost 100 km. The river's southwesterly course can be followed upstream at least as far as the San Juan Mountains of Colorado and is entirely unrelated to present-day drainage. Its downstream course was probably across the Kaibab Arch in a valley that roughly followed the present eastern Grand Canyon.
The history of the Crooked Ridge River places constraints on that of the Colorado and its integration through the Grand Canyon. According to Ivo Lucchitta of the U.S. Geological Survey and colleagues, it does not support the continuation of the ancestral Colorado into lakes in Arizona or Utah, or its integration through the Grand Canyon by lake spillover. Instead, it suggests that the Four Corners region has been lowered through erosion by 1-2 km and that the regional drainage pattern has been reconfigured twice since the ancient river flowed.
Journal
Geological Society of America Bulletin