||5 January 2006
GSA Release No. 06-02
January / February Media Highlights:
The Geological Society of America Bulletin
Boulder, Colo. - The January-February issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes several newsworthy items. Topics include: cold vents and formation of near-seafloor massive gas hydrates; speleothems and high-resolution records of climate change; new dating of early Archean rocks in the Minnesota River Valley; Earth's rotation and movement of tectonic plates; and timing of volcanic activity and mountain building in northern Nevada.
Representatives of the media may obtain complimentary copies of articles by contacting Ann Cairns. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to GSA BULLETIN in stories published. Contact Ann Cairns for additional information or assistance.
Non-media requests for articles may be directed to GSA Sales and Service.
- Geophysical and geochemical signatures associated with gas hydrate-related venting in the northern Cascadia margin
Michael Riedel, Natural Resources Canada, Geological Survey of Canada-Pacific, Sidney, British Columbia V8L 4B2, Canada, et al. Pages 23-38.
- Keywords: gas hydrate, cold vent, seismic 2D/3D, piston coring, geochemistry, physical properties.
- The manuscript by Riedel et al. describes geophysical and geochemical observations near cold vents of the Northern Cascadia Margin that are associated with near-seafloor massive gas hydrate. Research on these cold vents was triggered by seismic observations of so-called wash-out zones in 1997 using a deep-towed seismic system. Subsequent multi-frequency and 3D seismic imaging showed complex structural settings and ring-like amplitude anomalies surrounding the cold vents. A combined coring and heat-flow study gave insight into the sedimentary and geochemical processes associated with methane venting. These studies were also complemented by seafloor video observations and sampling with the remotely operated vehicle ROPOS. This broad-based geophysical and geochemical data set has allowed us to comprehensively describe the processes associated with cold vents and the formation of near-seafloor massive gas hydrates.
- Deciphering igneous and metamorphic events in high-grade rocks of the Wilmington Complex, Delaware: Morphology, cathodoluminescence and backscattered electron zoning, and SHRIMP U-Pb geochronology of zircon and monazite
John N. Aleinikoff, U.S. Geological Survey, Denver, Colorado 80225, USA, et al. Pages 39-64.
- Keywords: Wilmington Complex, U-Pb geochronology, SHRIMP, zircon, monazite.
- Intrusive and extrusive rocks of the Wilmington Complex, Delaware, formed as an island arc sequence during the period 485-475 Ma. Detrital zircons from metasedimentary units of the Wilmington Complex were derived from 1.4-0.9 Ga crystalline basement rocks of North American (i.e. Laurentian) origin. The entire complex was metamorphosed to high grade ca. 430 Ma. These age data were determined by ion microprobe analyses of zircon and monazite of various morphologies and zoning patterns. The chemical composition of each rock was a determining factor in the shape and zoning characteristics of the dated minerals.
- Large kinetic isotope effects in modern speleothems
Patrick J. Mickler, Department of Geological Sciences, University of Texas, Austin, Texas 78712, USA, et al. Pages 65-81.
- Keywords: kinetic isotope effects, speleothems, oxygen isotopes, carbon isotopes, non-equilibrium, Barbados.
- The geochemistry of speleothems, which are secondary mineral deposits formed in caves, may be used to produce long, continuous, high-resolution records of climate change. The application of stable isotopes in speleothems requires an understanding of the extent to which speleothem calcite isotopic compositions reflect the compositions of the cave waters from which they precipitate. In order to test for equilibrium precipitation, modern speleothem calcite was grown on glass plates, so that the C and O isotopic composition of the calcite and the water from which it precipitated could be directly compared. The plates were placed on the top of three actively growing stalagmites in Harrison's Cave, Barbados. Only some of the plate C isotope values and none of the plate O isotope values correspond to equilibrium values, indicating kinetic isotope effects during speleothem calcite growth. On each plate, speleothem calcite was deposited with distinct O and C isotopic compositions that increase progressively away from the growth axis. The positive O versus C isotopic trends are likely a result of 18O and 13C enrichment in the HCO3- reservoir due to progressive CO2 degassing and CaCO3 precipitation. We compiled published speleothem stable isotope records with a global distribution and found that the majority of these records show a positive O and C isotopic covariation, similar to the results of our study. Speleothem stable isotope records may be influenced by kinetic isotope effects such that temperature-controlled equilibrium fractionation models alone can not directly explain the significance of the variations in these records.
- High-precision U-Pb geochronology in the Minnesota River Valley subprovince and its bearing on the Neoarchean to Paleoproterozoic evolution of the southern Superior Province
M.D. Schmitz, Department of Geosciences, Boise State University, Boise, Idaho 83725, USA, et al. Pages 82-93.
- Keywords: Minnesota River Valley, Superior Province, U-Pb, zircon, monazite, granites, collisional orogenesis, Archean, cratons.
- High precision U-Pb ages constrain the voluminous intrusion of Neoarchean (2591-2603 Ma) late- to post-kinematic granitic plutons into the Mesoarchean (ca. 3.4 Ga) high-grade gneisses of the Minnesota River Valley (MRV) subprovince of the southern Superior craton. The synchrony and rapidity of deep crustal melting and plutonism, and coeval high-grade metamorphism throughout the MRV terranes, effectively constrain the suturing of the Mesoarchean continental MRV terrane fragments to the southern margin of the MRV terranes and represent a discrete and terminal phase of Archean continental growth along the southern Superior craton margin. Subsequent rifting of this margin in the Paleoproterozoic is dated at ca. 2.07 Ga by major WNW-trending tholeiitic diabase dikes in the MRV and the correlative Fort Frances (Kenora-Kabetogama) dike swarm of northern Minnesota and adjoining Canada.
- SHRIMP study of zircons from Early Archean rocks in the Minnesota River Valley: Implications for the tectonic history of the Superior Province
M.E. Bickford, Department of Earth Sciences, Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244-1070, USA, et al. Pages 94-108.
- Keywords: Minnesota River Valley, Archean, zircons, SHRIMP, geochronology, U-Pb.
- Although it has been known since the 1970s that rocks exposed in the Minnesota River Valley of southwestern Minnesota are among the oldest in North America, only recently have modern techniques of uranium-lead dating revealed how truly ancient and complex they are. Using the sensitive high-resolution ion microprobe (SHRIMP) at Stanford University, M.E. Bickford, of Syracuse University, and his colleagues Joe Wooden (U.S. Geological Survey) and Bob Bauer (University of Missouri) have shown that the oldest components of these complex rocks are 3500 million years old and that episodes of emplacement of new molten material occurred at 3420, 3380, 3140, and 2600 million years ago. The SHRIMP instrument allows measurement of uranium-lead ratios, from which ages can be calculated, from portions, less than 0.020 mm in diameter, of zircon crystals, showing complex growth zones corresponding to these magmatic events.
- Interest in such ancient parts of Earth's crust has intensified since the discovery of rocks greater than 4000 million years old in northern Canada and Australia. Newly discovered ancient rocks, 3500 to 3800 million years old, occur in northern Manitoba, in the Assean Lake area, and in the Porpoise Cove area of northern Quebec. If these and the Minnesota River Valley rocks are the disrupted parts of an ancient continent, intervening younger rocks of the Superior Province of Canada, Minnesota, and Michigan — only 2700 to 3200 million years old! — may have formed as the ancient continent rifted apart. It is in this way that the history of the formation of Earth's continental crust is unraveled.
- Syncollisional basin development in the Appalachian orogen — The Saint-Daniel Mélange, southern Québec, Canada
Jean-Michel Schroetter, Institut National de la Recherche Scientifique–Eau, Terre et Environnement, Québec, PQ G1K 9A9, Canada, et al. Pages 109-125.
- Keywords: Appalachians, Québec, ophiolite, syncollisional basin, mélange.
- This paper describes and reinterprets the sedimentary and stratigraphical history of the Saint-Daniel Mélange, an orogen-scale basin of the Québec Appalachians. The regional stratigraphic framework and relationships with underlying and overlying rock units have been established on the basis of six detailed stratigraphic sections from the Thetford-Mines, Asbestos, and Mont-Orford Ophiolitic complexes. The results suggest that: (1) the Saint-Daniel Mélange unconformably overlies various levels of the Southern Québec ophiolites and represents the base of a syncollisional basin developed in a forearc setting; (2) it is made up of laterally discontinuous units recording the emplacement of ophiolites onto the Laurentia continental margin; (3) it records a transition from ophiolite-dominated to continental sources, indicating the progressive exhumation of both the ophiolite and the continental margin; and (4) it is stratigraphically overlain by a 7-km-thick sequence of synorogenic flysch (the Magog Group).
- Timing of Cenozoic volcanism and Basin and Range extension in northwestern Nevada: New constraints from the northern Pine Forest Range
Joseph P. Colgan, Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA, et al. Pages 126-139.
- Keywords: Basin and Range, Pine Forest Range, volcanism, geochronology, fission-track dating.
- The vast Great Basin of Nevada and Utah formed during the last 40 million years as the state of California moved west away from Utah, leaving in its wake the evenly spaced high mountains and dry valleys that geologists call the Basin and Range Province. The details of how this happened remain unclear, however, particularly the timing and amount of faulting that formed the mountains. When did the different mountains start forming? How many episodes of faulting took place? Did volcanic eruptions happen at the same time? We can't answer all of these questions yet, but our new research provides another piece of the puzzle by establishing the relative timing of volcanic activity and mountain building in northern Nevada. We used a variety of methods to do this, including field studies, measuring the ages and composition of rocks in the lab, and dating the timing of faulting using the cooling ages of nearby rocks (thermochronology). Our new data show that, prior to the late Miocene (about 12 million years ago), there were no big mountains in northwestern Nevada — it probably resembled the high lava plains of southern Oregon, buried under 1-2 km of volcanic ash and lava flows. Beginning ca. 12 Ma, these rocks were broken and tilted as major mountain ranges began to form, a process that probably continued into the recent geologic past. Our results should be of broad interest to anyone studying how and why the Basin and Range Province developed over time, and locally of interest to anyone studying volcanism and/or faulting in northwestern Nevada.
- Eocene volcanism above a depleted mantle slab window in southern Alaska
Ronald B. Cole, Department of Geology, Allegheny College, Meadville, Pennsylvania 16335, USA, et al. Pages 140-158.
- Keywords: Eocene, volcanism, Alaska, Talkeetna Mountains, slab window, adakite.
- A unique episode of volcanism occurred in southern Alaska about 50 million years ago. This volcanic activity took place when a hole was formed beneath the crust of southern Alaska after a mid-ocean spreading ridge collided with the continental margin. Magmas from beneath the sea floor were erupted through the hole and formed extensive lava flows and small explosive eruptions. This episode of volcanism was associated in time with important tectonic events along southern Alaska that included gold mineralization, major fault activity, and a shift in tectonic plates.
- Development and dismemberment of a Middle Devonian continental-margin submarine fan system in east-central California
Calvin H. Stevens and Tina Pelley, Department of Geology, San Jose State University, San Jose, California 95192, USA. Pages 159-170.
- Keywords: Devonian, Sierra Nevada, Inyo Mountains, sequence stratigraphic analysis, submarine fan system, strike-slip faulting, continental margin.
- A submarine fan system of Middle Devonian age developed along the middle Paleozoic continental margin in east-central California. It apparently is unique for a Paleozoic fan in the exposure of all elements of the system. The fan itself, the main distributary channel, and the lower interchannel slope are represented in sedimentary roof pendants in the eastern Sierra Nevada. To the east, in the Inyo Mountains, a middle slope channel, lower and middle to upper interchannel slopes, two upper slope channels, and the shelf are exposed. Similar successions of Middle Devonian rocks in extreme west-central Nevada and southeastern California can be utilized to estimate displacements on several major strike-slip faults in this region.
- Plutonism in three orogenic pulses, Eastern Blue Ridge Province, southern Appalachians
Brent V. Miller, Department of Geological Sciences, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3315, USA, et al. Pages 171-184.
- Keywords: U-Pb, geochronology, zircon, monazite, Southern Appalachians, plutons.
- High-precision U-Pb radiometric dating of five plutons along the western flank of the Southern Appalachian mountain belt in western North Carolina and northern Georgia has revealed the timing of their crystallization from magmas. Previously, plutons in this region were thought to be related to only the first two of the three major tectonic events that shaped the geological architecture of the southern Appalachians. We show that three plutons, previously believed to correspond to the second phase of tectonism, are instead part of the third, latest phase. The recognition of voluminous plutonism of this age explains the long-recognized, widespread thermal metamorphism, the heat source for which was previously unknown. Additionally, one of the newly-dated plutons of this latest phase is cut by a thrust fault that was a major structural feature in the assembly of the southern Appalachians, and thus constrains at least the latest movement on this fault to post-date the crystallization of the pluton.
- Geologic and U-Th-Pb geochronologic evidence for early Paleozoic tectonism in the Kathmandu thrust sheet, central Nepal Himalaya
G.E. Gehrels, Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, et al. Pages 185-198.
- Keywords: Himalaya, tectonics, geochronology, orogeny, early Paleozoic.
- The Kathmandu area of central Nepal is underlain by a regionally extensive thrust sheet that carries mainly metasedimentary rocks southward over lower grade strata of the Indian craton and the Himalayan foreland basin. Previous work has shown that rocks of the thrust sheet were metamorphosed and deformed during Tertiary orogenesis related to India-Asia collision. Our work demonstrates, however, that these rocks were initially deformed, metamorphosed, intruded by syntectonic plutons, uplifted, and eroded during early Paleozoic time. The occurrence of similar rocks and relations in many other regions of the Himalaya suggests that at least some of the deformation, metamorphism, plutonism, uplift, and erosion in the orogen may be of Paleozoic age, and unrelated to India-Asia collision. The regional significance of this early Paleozoic tectonism needs to be incorporated into future syntheses of Himalayan tectonics.
- The westward drift of the lithosphere: A rotational drag?
B. Scoppola, Dipartimento di Matematica, Università Tor Vergata, Roma, Italy, et al. Pages 199-209.
- Keywords: Earth's rotation, westward drift, lithosphere, asthenosphere viscosity, decoupling.
- Plate tectonics is well established, but what moves the plates? What drives the lithosphere is still under debate (e.g., the slab pull, the ridge push, or the mantle convection). This paper shows how Earth's rotation can be a fundamental ingredient in moving plates, which have a net rotation roughly westerly directed. The tidal drag associated with an ultra-low asthenospheric viscosity, and mantle downwelling, could explain why plates move and why they present a net "westward" rotation. An astronomical contribution to plate tectonics could also explain the persistent asymmetries between "west" versus "east or northeast" directed subduction zones, and the asymmetries on rifts flanks.
- Multimethod detrital thermochronology of the Great Valley Group near New Idria, California
Pieter Vermeesch, Department of Geological and Environmental Sciences, Stanford University, Braun Hall, room 320, 450 Serra Mall, Stanford, California 94305-2115, USA, et al. Pages 210-218.
- Keywords: detrital thermochronology, fission track, Great Valley Group, serpentinite, Sierra Nevada, exhumation.
- Technological advances have made it possible to date the thermal evolution of individual grains of sediment. The Great Valley Group near New Idria and Coalinga (California) provides a spectacular example of the power of this method. Combining U/Pb and fission track dating on detrital zircon and apatite tells a story spanning more than 100 million years. The sedimentary rocks presently exposed in the Diablo Range, west of California's Central Valley, are the erosional products of a steadily but rapidly exhuming Cretaceous Sierra Nevada.
- They were buried up to 10 km deep in the Great Valley forearc basin, which was associated with the subduction of the Pacific underneath the North American plate. 14 million years ago, the active Pacific margin became a transform one, as the San Andreas fault propagated to the latitude of the New Idria-Coalinga area. This caused minor intrusive activity and the formation of the New Idria serpentinite diapir. The serpentinite body rapidly exhumed from about a 20 km depth, coming to the surface as a rising hot protrusion, heating the Great Valley Group on the way up and spreading at once over the countryside. The protrusion formed spectacular deposits of sedimentary serpentinite (Big Blue Formation) and potentially provided the heat source for petroleum generation in the shallow Vallecitos basin, immediately adjacent to New Idria.
- Fracture-driven intrusion and upwelling of a mid-crustal pluton fed from a transpressive shear zone — The Rieserferner Pluton (Eastern Alps)
Ralph Wagner, Institut für Geowissenschaften Wissenschaften, Freie Universität Berlin, Malteserstrasse 74-100 Haus B, 12249 Berlin, Germany, et al. Pages 219-237.
- Keywords: Eastern Alps, syntectonic plutons, ascent mechanisms, shear zones, emplacement mechanisms, pluton roofs.
- The Rieserferner Pluton was emplaced at a depth of 12-15 km into steeply dipping greenschist facies mylonitic rocks of the Austroalpine basement, just south of the Tauern Window (Eastern Alps). Intrusion occurred during N-S-directed shortening and E-W horizontal extension in front of the rigid Southern Alpine Indenter. Tonalitic melt ascended through a feeder channel preserved in the steep southern part of the Rieserferner Pluton, within and adjacent to the steep mylonitic foliation of a major Tertiary shear zone, the Defereggen-Antholz-Vals (DAV). The transition from melt ascent along a subvertical conduit to final emplacement into a gently dipping body involved melt-induced hydrofracturing to form a subhorizontal, tabular pluton that protruded northward from the DAV into the previously folded country rocks. During the late stage of emplacement, buoyant upwelling of the partly recrystallized magma induced doming of the pluton roof as well as vertical ductile shortening of the immediately overlying country rocks. Magma pressure, therefore, locally exceeded the lithostatic pressure.
- Latest Cretaceous to Miocene deformation events in the eastern Sierra Madre del Sur, Mexico, inferred from the geometry and age of major structures
A.F. Nieto-Samaniego, Universidad Nacional Autónoma de México, 76001 Querétaro, Qro., Mexico, et al. Pages 238-252.
- Keywords: southern Mexico, Laramide orogeny, Oaxaca shear zone, strike-slip faulting.
- This article reviews the deformation events that occurred in southern Mexico during the past 65 m.y. Folds and faults in the study area record three deformation events, with the activity of the structures migrating from west to east. A thick crustal block made up of Precambrian and Paleozoic metamorphic rocks acted as a rigid element during deformation. Most of the structures are located around the rigid block, forming a belt of faults and folds.
To view abstracts for the GSA BULLETIN, go to www.gsajournals.org.
Representatives of the media may obtain a complimentary copy of any GSA BULLETIN article by contacting Ann Cairns.
Other non-media requests for articles should be directed to
GSA Sales and Service, 1-888-443-4472.
The Geological Society of America
3300 Penrose Place - PO Box 9140
Boulder, CO 80301-9140, USA
To unsubscribe from GSA's media distribution list, notify Ann Cairns.