|23 April 2008
GSA Release No. 08-29
Director of Education, Communication, & Outreach
MAY-JUNE 2008 GSA BULLETIN Highights
The May/June issue of GSA Bulletin is now online. Topics include the diversity of magmas in juxtaposed North Cascades chambers; studies of fossilized ungulate teeth constrain the age of the Sierras; the relationship between deglaciation and explosivity at Puyehue Volcano, Chile; how climate change– and land-use–erosion on alluvial fans may increase flooding hazards; determining best-use locations for high-resolution aeromagnetic surveys; fossil-shell records of climate change in Antarctica; and the Kushiro Submarine canyon, Japan.
Highlights are provided below. Representatives of the media may obtain complimentary copies of articles by contacting Christa Stratton, . Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA BULLETIN in articles published. Contact Christa Stratton for additional information or other assistance.
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The role of mantle delamination in widespread Late Cretaceous extension and magmatism in the Cordilleran orogen, western United States
Michael L. Wells, Dept. of Geoscience, University of Nevada–Las Vegas, Nevada 89154, USA; and Thomas D. Hoisch, Dept. of Geology, Box 4099, Northern Arizona University, Flagstaff, Arizona 86011, USA. Pages 515-530.
Extension (horizontal lengthening of rock) during plate convergence and mountain building is widely recognized, yet the causes of such extension remain controversial. Wells and Hoisch propose that the sinking of mantle from the base of the North American plate into the underlying asthenosphere explains many enigmatic yet prevalent aspects of the metamorphic, magmatic, and kinematic history of the ancient Late Cretaceous mountain belt (Selvier-Laramide orogen) of the western United States during the Late Cretaceous. Extension, heating, melting of crust, and perhaps rock uplift were widespread during a restricted time interval in the Late Cretaceous (75–67 Ma) along the axis of maximum crustal thickening within the Mojave Desert, and to a lesser extent, within the interior of the Great Basin to the north; similar processes may have been active in the magmatic arcs to the west. These processes are viewed as predictable consequences of the thermal, rheological, and dynamic state of the overlying crust following the sinking of mantle lithosphere beneath isostatically compensated mountain belts.
Late orogenic mafic magnetism in the North Cascades, Washington: Petrology and tectonic setting of the Skymo layered intrusion
Donna L. Whitney et al., Dept. of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA. Pages 531-542.
For more than 40 million years, the ancestral Cascade volcanic arc produced the same kind of magma—low-potassium granitic rocks similar to those produced in other continental magmatic areas. At the end of the arc’s life as an active magmatic system, however, the composition of the magma changed dramatically. About 50 million years ago, melt generated in the mantle rose through the crust and was emplaced in a shallow magma chamber, essentially untouched by the continental rocks through which it passed. Whitney et al. study this magma body, called the Skymo Complex, a remote and little-known layered mafic intrusion. At the same time, an enormous chamber of magma from the opposite end of the composition spectrum—high-potassium, high-silica magma—was emplaced nearby. Together, these two late-forming magma bodies represent the full range of diversity of compositions in the magmatic arc. Both magma chambers formed in a major strike-slip fault zone (the Ross Lake fault zone) and their generation and crystallization represents the final phase of the ancestral Cascade mountain building event. In fact, these magmas signal the collapse of the mountain system, as the crust extended, thinned, and cooled after tens of millions of years of thickening and heating.
Kinematics of Franciscan Complex exhumation: New insight from the geology of Mount Diablo, California
Kerstin Schemmann et al., Institut für Geologische Wissenschaften, Freie Universität Berlin, Malteser Str. 74-100, 12249 Berlin, Germany. Pages 543-555.
Schemmann et al. present new evidence on the exhumation of high P-T rocks of the Franciscan accretionary complex from the geology of Mount Diablo, California. Uplift and folding of the late Cenozoic Mount Diablo anticline have exposed the blueschist-facies Franciscan complex, which is juxtaposed against the relatively unmetamorphosed Coast Range Ophiolite across the Coast Range fault. Locally associated with this fault is a sheared zone of serpentinite (the lower part of the ophiolite). Shear-sense indicators such as S-C fabrics, rotated clasts, and slickenfibers record the deformation kinematics, which consistently indicate that the ophiolite moved down in a normal sense relative to the Franciscan rocks in the modern reference frame. Together with the metamorphic facies contrast, this indicates 6–18 km structural attenuation. After restoration of vertical axis rotation and late Cenozoic fold deformation, the kinematic relations restore to top-to-the-northeast motion on a northeast-dipping low angle Coast Range fault. Thus, normal displacement and ductile thinning were primarily responsible for the structural exhumation of high-pressure assemblages in the Franciscan Complex.
Unroofing, incision, and uplift history of the southwestern Colorado Plateau from apatite (U-Th)/He thermochronology
Brook E. Crowley et al., Dept. of Anthropology, University of California, Santa Cruz, California 95064, USA. Pages 588-598.
The age of uplift of the Sierra Nevada mountain range has been hotly debated, with most researchers split between two conclusions: (1) the majority of uplift occurred in the past 3–5 million years, or (2) the range has been a substantial topographic barrier for tens of millions of years, possibly since the dawn of the Cenozoic. Crowley et al. track how long the Sierra Nevada mountains have been a high range using oxygen isotopes from fossil ungulate teeth. Previous studies have examined oxygen isotope values on the lee side of the Sierras to infer past elevation from rain shadow effects. Unfortunately, results from these studies may have been confounded by global and regional climate shifts. Crowley et al. address this problem by comparing tooth enamel oxygen isotope values from both sides of the range. Their results suggest that the Sierra have had substantial, nearly modern elevation for at least the past 16 million years. Results from this study have important implications for the tectonic history of the western United States.
Eruptive history, geochronology, and magmatic evolution of the Puyehue-Cordon caulle volcanic complex, Chile
Brad S. Singer et al., Dept. of Geology and Geophysics, University of Wisconsin–Madison, 1215 West Dayton Street, Madison, Wisconsin 53706, USA. Pages 599-618.
Puyehue Volcano, in the southern Chilean Andes, has erupted a wide variety of basaltic to rhyolitic magma, most famously during the explosive eruption of rhyolite on 24 May 1960, just 38 hours after the main shock of the nearby magnitude 9.5 Great Chilean earthquake. A new set of field observations and mapping, supported by a large set of 40Ar/39Ar age determinations, geochemical analyses, and thorium isotope data reveal that Puyehue Volcano grew at twice the rate of other Andean volcanoes during the past 300,000 years. The last three cone-building events on Puyehue began during periods of deglaciation, suggesting a relationship between unloading of ice and ease of magma ascent. The volcano has become exceptionally bimodal in composition and more explosive over time. Despite the high flux of basalt during the past 300,000 years and a larger volume of rhyolite than at neighboring Andean volcanoes, no large silicic magma reservoir formed in the upper crust. Instead, Singer et al.’s data favor rapid ascent of several small bodies of basaltic and silicic magma from the lower crust, perhaps promoted by conduits that reflect regional strike-slip faulting beneath the volcano.
Controls on alluvial fan long-profiles
Jonathan D. Stock et al., U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA. Pages 619-640.
Floods exiting steep valleys onto lowlands tend to deposit sediment radially on alluvial fans. These fans are commonly occupied by people and infrastructure, but there is great uncertainty about flooding hazard because of the episodic nature of events. Stock et al. explore why fans are shaped the way they are by testing two longstanding hypotheses for fan shape. Using detailed new field data with sediment transport models, they show that the standard explanation that fan channel slopes are set by the largest grain size that a flow can transport is not consistent with field data. Instead, Stock et al. find support for an older hypothesis: fan channel slopes are set by the supply of sediment, with increased sediment supply leading to aggradation and steeper channel slopes. If this explanation is correct, increases in erosion from climate change or land use could create additional flooding hazards on alluvial fans throughout the western United States and other steeplands.
Rock magnetic characterization of faulted sediments with associated magnetic anomalies in the Albuquerque basin, Rio Grande rift, New Mexico
Mark R. Hudson et al., U.S. Geological Survey, Denver, Colorado 80225, USA. Pages 641-658.
Hudson et al.’s case study of well-exposed sedimentary strata demonstrates that their variations in magnetic properties are sufficient to cause aeromagnetic anomalies spatially associated with the San Ysidro fault in the Rio Grande rift of New Mexico. Hudson et al. highlight geologic factors such as sediment source areas, depositional environments, and post-depositional preservation and alteration of magnetic minerals as important aspects in determining where high-resolution aeromagnetic surveys will be successful in mapping faults within sedimentary basins.
Eocene climate record of a high southern latitude continental shelf: Seymour Island, Antarctica
Linda C. Ivany et al., Dept. of Earth Sciences, Syracuse University, Syracuse, New York 13244, USA. Pages 659-678.
The Eocene Epoch, from roughly 55 to 34 million years ago, records a time of significant global change for our planet. Early in the Eocene, Earth was much warmer than today, particularly at high latitudes. Palm trees grew in the northern Rocky Mountains, crocodiles swam above the Arctic Circle, and forests, rather than glacial ice, characterized the polar regions. The transition from this warm, equable world to one in which continent-scale ice sheets blanketed the poles took place during the course of the Eocene. The record of this climate transition is reasonably well-known from sediments preserved at low and mid-latitudes, but very little is known about the timing and magnitude of cooling on Antarctica—mostly because any potential sedimentary archives are now covered by ice and inaccessible. Only one place on the continent is known to contain a reasonably complete record of the Eocene that is available for study—the strata exposed on Seymour Island, a tiny island off the coast of the Antarctic Peninsula. Ivany et al. collected fossil shells from shallow-marine sands and muds through the whole section, and looked at the chemistry of those shells through time to construct a record of climate change through the Eocene. The ratio of the oxygen-18 to oxygen-16 isotopes in the shell material yields an estimate of the temperature of the water in which the shells grew. These data show that Antarctica experienced a shift toward cooler conditions ~41 million years ago, in concert with a proposed change in ocean circulation resulting from the opening of a marine gateway between Antarctica and South America. This cooling is associated with a dramatic change in the Antarctic biota both on land and in the sea. A number of species became extinct, and ecological differences between earlier and later faunas suggest limitation by cold temperatures and/or low productivity. The record of climate change recovered from the Seymour Island section provides important new information in the quest to understand the operation of Earth’s climate system and the development of our present-day pole-to-equator thermal gradient.
Insights into the tectonomagmatic evolution of NW Mexico: Geochronology and geochemistry of the Miocene volcanic rocks from the Pinacate area, Sonora
Jesús Roberto Vidal-Solano et al., Departamento de Geología, Universidad de Sonora, Apdo. Postal 847, 83000 Hermosillo, Sonora, México, and Pétrologie Magmatique, Université Paul Cézanne (Aix-Marseille 3), Case Courier 441, 13397 Marseille Cedex 20, France. Pages 691-708.
The oldest volcanic rocks in El Pinacate volcanic field of Sonora, called pre-Pinacate event, record the history of the volcanic activity in the region. The age, mineral, and chemical data of old volcanic rocks, located next to edge of the recent volcanic field, allow to the identification of two episodes (ca. 20 Ma and 12–15.5 Ma). The volcanic evolution in space and time indicates that (1) the volcanic activity is located over the western limit of the North American Craton; and (2) the magmas related to the pre-Pinacate event were progressively changing to more deeper sources until generating huge volumes of lavas that could easily reach the surface during the past million years, building the recent volcanic field.
Late Cenozoic glacier-valvano interaction on James Ross Island and adjacent areas, Antarctic Peninsula region
Michael J. Hambrey et al., Centre for Glaciology, Institute of Geography & Earth Sciences, Aberystwyth University, Ceredigion, Wales SY23 3DB, UK. Pages 709-731.
Recent predictions by the United Nations Intergovernmental Governmental Panel on Climate Change indicate that temperatures are likely to rise over the next century by several degrees. The geological record indicates that the last time Earth experienced such temperatures was before the major ice sheets developed. Thus, understanding how the Antarctic ice sheet has changed through time is important for predicting its future response to global warming. The northern Antarctic Peninsula and adjacent James Ross Island are particularly sensitive to climate change. Hambrey et al. document the changes in a glaciological regime that ranged from much more extensive glaciation around 6 million years ago to more restricted ice-cap conditions over James Ross Island today. The interaction of glaciers with underlying volcanoes has produced a distinctive suite of sediments that, in combination with the precise dating afforded by the volcanic rocks, allows us to derive a detailed glacial record spanning the past several million years.
Polyphase tectonothermal history recorded in granulitized gneisses from the north Qaidam HP/UHP metamorphic terrane, western China: Evidence from zircon U-Pb geochronology
Jianxin Zhang et al., Institute of Geology, Chinese Academy of Geological Sciences (CAGS), Beijing 100037, People’s Republic of China, and Dept. of Earth Sciences, National Cheng Kung University, Tainan 701, Taiwan. Pages 732-749.
High-grade gneisses of the north Qaidam high-pressure/ultrahigh-pressure metamorphic terrane enclose minor eclogites and ultramafic rocks. In combination with petrological data, sensitive high-resolution ion microprobe U-Pb geochronology on the granulitized gneisses in the Luliangshan and Xitieshan, western north Qaidam Mountains, reveals a polyphase tectonothermal history including Early Neoproterozoic and Early Paleozoic events. The rocks investigated by Zhang et al. are two granulite-facies paragneisses and one orthogneiss that both surround garnet peridotite in the Luliangshan, and two paragneisses that enclose eclogite in the Xitieshan. The inherited zircon cores from paragneiss and orthogneiss yield ages between ca. 1000 Ma and ca. 2500 Ma, representing Mesoproterozoic to Archaean source material for these gneisses. A ca. 900 Ma age obtained from one orthogneiss and one paragneiss is interpreted as the age of simultaneous Early Neoproterozoic magmatism and metamorphism. This Early Neoproterozoic tectonothermal event is similar in age to the Jinning orogeny, which is commonly recognized in the metamorphic basement of the south China block and suggests that the north Qaidam Mountains has an affinity to the south China block. Early Paleozoic metamorphism is recorded in all gneiss samples. In conjunction with cathodoluminescence imagery and mineral inclusions, U-Pb dating of zircons reveals two Early Paleozoic age groups: ca. 450 Ma represents the age of high-pressure granulite metamorphism (Grt + Rt inclusions in zircon; kyanite porphyroblasts), whereas ca. 425 Ma reflects the time of medium-pressure, granulite-facies metamorphism (Pl + Sil inclusions in zircon) and associated anatexis during decompression. The ages obtained suggest that the granulite-facies metamorphism lasted for ~25 million years and was related to the Early Paleozoic continental collision between the Qilian and Qaidam blocks, and to subsequent thermal relaxation and exhumation.
Physiographical and sedimentological characteristics of submarine canyons developed upon an active forearc slope: The Kushiro Submarine Canyon, northern Japan
Atsushi Noda et al., Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, Central 7, 1-1-1Higashi, Tsukuba, Ibaraki 305-8567, Japan. Pages 750-767.
Comprehensive geological surveys have revealed the physiographical and sedimentological developments of submarine canyons upon a tectonically active margin. The Kushiro Submarine Canyon has a generally straight and deeply excavated course of more than 230 km in length upon the active forearc slope of the Kuril Trench in the northwest Pacific. The upper reach of the canyon (~3250 m of thalweg water-depth; shallower than outer-arc highs) shows a concave-upward longitudinal profile and evidence of frequent turbidity currents, suggesting that the erosional effect of currents are balanced by a reduction in gradient to achieve a spatially equilibrated erosion rate. The lower reach of the canyon (~3250–7000 m thalweg water-depth, deeper than outer-arc highs), in contrast, largely reflects the morphology of the forearc slope along the canyon, which has been deformed by subduction-related tectonics. Noda et al. show that interrelationships between canyon erosion and sedimentation and tectonic processes along the forearc slope are important in the physiographical development of submarine canyons along active forearc margins.
Review abstracts for these articles at http://www.gsajournals.org/perlserv/?request=index-html&issn=0016-7606.