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March-April GSA Bulletin media highlights

Geological Society of America

Boulder, Colo. - The March-April issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes several potentially newsworthy items. Topics include: first mapping of the North Sea's Silverpit impact crater, discovered in 2002; analysis of the Heise Volcanic Field in Idaho's Snake River Plain, which tracks along the Yellowstone hotspot; a new sediment transport study of the Yellowstone River, with implications for improved restoration and management efforts; and confirmation of a surface fault in the New Madrid seismic zone, now called the Bootheel Fault.

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U-Pb zircon geochronology of Paleoproterozoic plutons from the northern midcontinent, USA: Evidence for subduction flip and continued convergence after geon 18 Penokean orogenesis
Daniel K. Holm, Department of Geology, Kent State University, Kent, Ohio 44242, USA, et al. Pages 259-275.
Keywords: Penokean orogen, U-Pb zircon ages, geon 17 magmatism, Yavapai slab rollback, crustal stabilization.

New age dates on igneous rocks in the southern Lake Superior region indicate that a large amount of ancient magma was intruded into the area 50-100 million years after a period of crustal growth around 1850 Ma. The progression of magmatism across the region suggests that plate tectonics processes continued to play an important part in the development of the north-central United States for at least another 200 million years (from 1850 to 1650 Ma). The process of ancient crustal growth and continued convergence in the center of the North American craton is proposed to be similar to processes described for younger and more recently active crust.

Using U-Pb ages of Miocene tufa for correlation in a terrestrial succession, Barstow Formation, California
Jennifer M. Cole, Interdepartmental Doctoral Program in Anthropological Sciences, State University of New York at Stony Brook, Stony Brook, New York 11794-4364, USA, et al. Pages 276-287.
Keywords: geochronology, continental stratigraphy, uranium-lead method, calcite, lakes.

Sedimentary rocks provide important records of climate change, past life, and tectonic events. Few techniques permit direct dating of sedimentary minerals, which limits our ability to make terrestrial correlations. The goal of this study was to further test the feasibility of uranium-lead (U-Pb) dating of relatively young sedimentary carbonates. We collected five samples of uranium-rich tufa calcite associated with an ancient lake in the middle Miocene Barstow Formation, an important terrestrial succession due to abundant mammalian fossils and detailed chronology based on previous argon dating of igneous minerals from multiple volcanic ash horizons. For all samples, the concordance of the two U-Pb decay schemes and agreement with the preexisting chronology strongly suggests that the U-Pb calcite ages are accurate. Our most precise age has an uncertainty of less than one percent. These precise and accurate ages show that U-Pb dating of lacustrine carbonates is an exciting avenue for providing time constraints of sedimentary rocks and can be a powerful tool for correlation even in complex successions.

Timing and development of the Heise Volcanic Field, Snake River Plain, Idaho, western USA
Lisa A. Morgan, U.S. Geological Survey, Denver, Colorado 80225, USA, and William C. McIntosh, New Mexico Bureau of Mines and Mineral Resources, Socorro, New Mexico 87801, USA. Pages 288-306.
Keywords: eastern Snake River Plain, Heise volcanic field, uplift of northern Teton Range, Snake Range detachment, Arbon Valley Tuff, Heise Group.

The article defines the volcanic and tectonic history of the Heise volcanic field and surrounding areas based on field studies, new 40Ar/39Ar age determinations, geochemical, lithologic, and geophysical data. Large-volume silicic ignimbrites exposed within and on the margins of the eastern Snake River Plain erupted between 7.5-3.5 Ma from the Heise volcanic field from 4 nested calderas. This volcanic field is in the eastern Snake River Plain along the track of the Yellowstone hot spot and represents the eruptive center immediately preceding the present-day Yellowstone Plateau volcanic field. The volcanic stratigraphy also provides temporal control for tectonic events associated with late Cenozoic extension in the Snake Range and with uplift of the Teton Range, Wyoming. In the Snake Range, movement of large (¡Ý0.10 km3) slide blocks of Mississippian limestone exposed 50 km to the east of the Heise field occurred between 6.3 and 5.5 Ma and may have been catastrophically triggered by the caldera eruption of the 5.51 ¡À 0.13 Ma Conant Creek Tuff. This slide block movement of ~300 vertical meters indicates that the Snake Range had significant relief by at least 5.5 Ma. In Jackson Hole, the distribution of outflow facies of the 4.45 ¡À 0.05-Ma Kilgore Tuff related to eruption from the Kilgore caldera in the Heise volcanic field on the eastern Snake River Plain indicates that the northern Teton Range was not a significant topographic feature at this time.

Subsurface temperatures and crustal strength changes within the seismogenic layer at Arroyo del Coyote in the Socorro seismic area, central Rio Grande rift, New Mexico
Marshall Reiter, New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA. Pages 307-318.
Keywords: Subsurface temperatures, seismogenic layer, elastic moduli, semibrittle zone, Arroyo Del Coyote.

At Arroyo del Coyote, near Socorro, New Mexico, the depths of events associated with an earthquake swarm are well determined. A close-by heat flow measurement allows a site specific approximation of the subsurface temperatures in the seismogenic layer. This information, and the likelihood of a largely granitic upper crust, is used to make qualitative predictions of relative crustal strength across the seismogenic layer from elastic and plastic models. It is suggested that at Arroyo del Coyote the seismogenic layer is a semi-brittle zone where crustal strength decreases with depth.

Evidence for Holocene Displacements on the Bootheel Fault (Lineament) in Southeastern Missouri: Seismotectonic Implications for the New Madrid Region
Margaret J. Guccione, Department of Geosciences, University of Arkansas, Fayetteville, Arkansas 72701, USA, et al. Pages 319-333.
Keywords: faults, neotectonics, paleoseismology, earthquake hazard studies, New Madrid Seismic Zone, liquefaction, Mississippi River valley, stratigraphy.

Recent work in the New Madrid seismic zone has confirmed the presence of a surface fault, termed the Bootheel Fault. Previously, a feature named the Bootheel Lineament was identified and was suspected to be a surface fault, but evidence to prove offset at the ground surface was elusive. The present study shows more than 13 meters of horizontal offset across a stream channel that crossed the lineament in the past 2-3000 years and approximately 3 meters of vertical offset along the lineament in the past 10,000 years and thus demonstrates that the lineament is a fault. Curiously, very few of the numerous microearthquakes in the seismic zone are along this fault, suggesting that it may be "locked" or is currently inactive. When movement does occur along the Bootheel Fault, it may trigger movement along other faults in the seismic zone and cause sequences of earthquakes, similar to those that occurred during 1811 and 1812.

Excision and the original low dip of the Miocene-Pliocene Bannock detachment system, SE Idaho: Northern cousin of the Sevier Desert detachment?
Stephanie M. Carney and Susanne U. Janecke, Department of Geology, Utah State University, Logan, Utah 84322-4505, USA. Pages 334-353.
Keywords: detachment, excision, Basin and Range province, breakup, isostatic folding, low-angle normal fault.

Low-angle normal faults (detachment faults) are important features because they accommodate large amounts of extension in mountain belts. Despite many years of study, questions persist about the original dip and evolution of low-angle normal faults. Geophysical and laboratory studies seem to suggest that they should not exist, yet there are many well-documented examples, especially the Basin and Range province of the western United States. Our field studies and structural analysis along the Bannock detachment system in SE Idaho provide compelling evidence for slip at low angles on a large, previously little known, low-angle normal fault. This research supports a growing body of work for active slip on low-angle normal faults.

The Bannock detachment system lies in the same structural position as the world famous but controversial Sevier Desert detachment system in central Utah. The Sevier Desert detachment cannot be examined directly because it is still deeply buried. The Bannock system, on the other hand, has been uplifted and exposed by subsequent faulting and erosion, and was examined directly in our study. The Bannock detachment system is somewhat smaller, shorter lived, and more disrupted by younger Basin and Range normal faults, but otherwise shares many characteristics with the Sevier Desert detachment system. Both top-to-the west systems formed at the western edge of an older mountain belt and "undid" some of the older mountain building. The basins above the faults evolved in a similar way and started out as large intact basins, but these later fragmented into smaller intermontane basins. Both detachment systems are geologically young and slip across them produced major lake basins in the down-dropped fault block. These lakes were salty and rich in salt minerals. Similar processes likely produced both detachment systems, and this work shows how former mountain belts often collapse to produce low basins above low-angle normal faults.

3D seismic reflection mapping of the Silverpit multi-ringed crater, North Sea
S.A. Stewart, BP Azerbaijan, c/o Chertsey Road, Sunbury on Thames, Middlesex TW16 7LN, UK, and P.J. Allen, Production Geoscience Ltd, Banchory AB31 5YR, UK. Pages 354-368.
Keywords: impact crater, North Sea, reflection seismic.

An enigmatic crater several kilometers in diameter, buried hundreds of meters below the seabed of the North Sea, has been mapped in detail for the first time. The crater was discovered in 2002, but for the first time it has been thoroughly mapped in three dimensions, revealing detail of a spectacular set of ring structures around the 3-km-diameter central hole. This new work has reduced the chances that this structure can be explained by a terrestrial cause, leaving meteor impact as the leading contender. Perfect preservation of this structure, together with the detailed imaging, means that this complex type of impact structure, previously best known from the moons of Jupiter, can now be studied on our doorstep.

Photogrammetric techniques for analyzing displacement, strain, and structural geometry in physical models: Application to the growth of monoclinal basement uplifts
Mark P. Fischer and David P. Keating, Department of Geology and Environmental Geosciences, Northern Illinois University, DeKalb, Illinois 60115-2854, USA. Pages 369-382.
Keywords: physical model, monocline, basement uplift, photogrammetry, strain.

Geologists use scaled physical models to study and simulate a wide variety of processes ranging from lava flows to erosion and sedimentation to faulting and folding of upper crustal rocks. In the latter case, a 3-5-km-thick sequence of sedimentary rocks might be modeled as a 3-5-cm-thick cake of clay or sand. Folding and faulting are caused by the horizontal motion of a slow-moving ram. During such an experiment, geologists can actually observe the initiation, growth and interaction of faults and folds, something that cannot be done with real geological structures that often take millions of years to form. In this paper, Fischer and Keating present a novel experimental approach that makes physical modeling easier and more robust. The approach uses photogrammetry, the science of deriving 3D measurements from photographs, to precisely determine and track the evolution of folding and faulting in a homogeneous cake of clay. Although these specific experiments are aimed at understanding deformation in the upper crust, the technique could be used when modeling the flow of glaciers, the movement of tectonic plates, or the pre-eruptive inflation of volcanoes.

Midcrustal emplacement of the Sausfjellet pluton, central Norway: Ductile flow, stoping, and in situ assimilation
Gregory Dumond, Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA, et al. Pages 383-395.
Keywords: pluton emplacement, stoping, layered intrusions, Caledonides, Bindal Batholith.

This paper documents how magma chambers may form in the deep crust during collisional mountain building. The region studied represents a 450 million year old example of magmatism and mountain building similar to what may be presently occurring in the western Pacific (e.g., Indonesia) at about 25 km depth.

Mesozoic sedimentary evolution of the northwest Sichuan basin: Implication for continued clockwise rotation of the South China block
Qing-Ren Meng, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China, et al. Pages 396-410.
Keywords: Mesozoic, sedimentation, rotation, Sichuan basin, Qinling, Longmen Shan.

A study was carried out to investigate Mesozoic depositional environments and basin tectono-sedimentary evolution of the northwest Sichuan basin, which is geologically located in the northwest of the South China block. Based on our new sedimentary data and well-documented regional Mesozoic stratigraphic framework, this paper conducts a detailed analysis of the spatial and temporal variations of sedimentation and depocenter migration and uses them as a proxy for the basin subsidence history. The paper further explores the possible linkage between Mesozoic sedimentary evolution of the northwest Sichuan basin and coeval basin-margin tectonics. It is considered that the northwest Sichuan basin behaved as a passive margin from late Paleozoic to early Middle Triassic times and then evolved into a peripheral foreland basin in response to collision of the North and South China blocks since the late Middle Triassic. Sinistral transpressional deformation of the Longmen Shan belt led to flexural subsidence of the adjacent western Sichuan basin in Late Triassic time. Renewed basin-margin fold-thrust activity triggered recurrence of flexural subsidence of the northwest Sichuan basin since the Middle Jurassic, with the depocenter eventually shifting to the northwestern corner of the basin in the Early Cretaceous. A model is advanced that invokes clockwise rotation of the South China block as a driver for tectonic evolution of both the basin and adjoining structural belts.

Strike-slip structure and sedimentary basins of the southern Alpine Fault, Fiordland, New Zealand
Philip M. Barnes, National Institute of Water and Atmospheric Research (NIWA), Kilbirnie, Wellington, New Zealand, et al. Pages 411-435.
Keywords: Alpine Fault, New Zealand, strike slip, strike-slip basin, pull-apart, structure, bathymetry, subduction, Puysegur Trench, tectonic deformation, plate boundary, seismic reflection, transpression, earthquake potential.

This paper documents the structure, strike-slip basins, and earthquake potential of the southern 250 km of the Alpine Fault in New Zealand. The research contributes to improved understanding of Earth's continental strike-slip faults and is highly topical in terms of plate boundary tectonics and implications for seismic hazard research. Although Harold Wellman discovered this 850-km-long fault more than half a century ago, it was only 4 years ago that the offshore section of the fault was identified by a French team of workers using bathymetric data. Following this discovery, this paper is the first to integrate subsurface structure from new seismic reflection data together with new bathymetric information in order to present a detailed account of the structure and basins on the fault. There are few places on Earth where such faults lend themselves to these types of investigations using marine geophysical technology. The description of the major basins and fault segments will establish nomenclature for the Alpine Fault that will exist for many years to come, and results will have significant implications, not just for research on the Pac-Aus plate boundary and hazard assessment in New Zealand, but for workers in other regions of continental deformation. These include implications for understanding of the structure and evolution of strike slip faults and their sedimentary basins in general.

Mechanisms controlling rupture shape during subcritical growth of joints in layered rocks
Laura Savalli and Terry Engelder, Department of Geosciences, The Pennsylvania State University, University Park, Pennsylvania 16802, USA. Pages 436-449.
Keywords: joints, fracture, crack, rupture, crack velocity, plumose morphology.

The rupture velocity of joints in the crust has been debated. One camp holds that joints are fast phenomena, with their ruptures moving at nearly the speed of sound in rock. The other camp holds that joints are much slower with a rupture speed that creeps along over hours if not days. Laura Savalli and Terry Engelder's paper, "Mechanisms controlling rupture shape during subcritical growth of joints in layered rocks," presents evidence for the slow growth of joints in Earth's crust. Their arguments are based on surface morphology of joints that indicates a stable, chemically controlled rupture called subcritical crack propagation. In general, the earth creeps as it deforms over geological time, and evidence suggests that joint growth is consistent with a geological rate rather than a high speed growth.

Fast strain rates during pluton emplacement: Magmatically folded leucocratic dikes in aureoles of the Mount Stuart Batholith, Washington, and the Tuolumne Intrusive Suite, California
Markus Albertz, Department of Earth Sciences, University of Southern California, Los Angeles, California 90089-0740, USA, et al. Pages 450-465.
Keywords: dikes, aureole, strain rates, plutons, emplacement, cooling.

This paper provides estimates on velocities of ductile rock shortening associated with Cretaceous magmatism in the Mount Stuart Batholith, Washington, and the Tuolumne Intrusive Suite, California. Using finite strain analysis on felsic dikes that were folded under magmatic conditions and thermal modeling of maximum durations available for shortening, we calculate strain rates that span a wide range of numerical values (10-2 to 10-13 s-1), some of which are several orders of magnitudes higher than rates considered typical for regional deformation (10-13 to 10-15 s-1). The study raises the question: which grain-scale deformation mechanisms are mechanically feasible and can accommodate such fast strain rates? Field and microstructural observations suggest that micro-fracturing and melt-assisted granular flow may be the best candidates for faster-than-normal strain rates. Moreover, extrapolating our extremely fast strain rates to long-term bulk aureole shortening estimates indicates that some aureoles may be characterized by pulsating high strain rate surges. In any case, we propose that under certain circumstances (e.g. thin magmatic dikes, short cooling times and hence fast folding) magmatically folded dikes in pluton aureoles can be used as evidence for fast host rock strain rates during pluton emplacement.

U-Pb detrital-zircon geochronology of northern Salinian basement and cover rocks
David L. Barbeau, Jr., University of Arizona, Department of Geosciences, Tucson, Arizona 85721, USA, et al. Pages 466-481.
Keywords: tectonics, Cordillera, California, provenance, Sur Series, depositional age.

This article reports the ages of more than 600 individual zircon sand grains recovered from ancient sandstones that now belong to an enigmatic fragment of continental crust in central coastal California. Our understanding of the composition and tectonic history of this crustal block ¡ª named "Salinia" ¡ª has been largely inhibited by intense metamorphism and widespread igneous activity within the block. Using a new technique of isotopic analysis, this study generated a large dataset that suggests Salinia previously resided in the Mojave Desert region, that Salinian basement rocks may be younger than previously thought, and that sediment was contributed to Salinia from several different source terranes in the heterolithic southwestern part of North America.

Structural evolution of a major Appalachian salient-recess junction: Consequences of oblique collisional convergence across a continental margin transform fault
James F. Tull and Christopher S. Holm, Department of Geological Sciences, Florida State University, Tallahassee, Florida 32306, USA. Pages 482-499.
Keywords: Appalachians, transverse zone, oblique ramp, orogenic curvature, Blue Ridge, Talladega.

Mountain belts often display a curved (alternating concave and convex) architecture rather than that of a straight or linear chain. The Appalachian Mountain belt is a prime example of such a feature. Extending from near Cartersville, Georgia, to the Georgia, Alabama, and Tennessee state boundaries, there exists the most pronounced change/deflection in curvature of the Appalachian mountain belt. This feature exists across the entire width of tectonically deformed rocks native to the North American continent and is likely inherited from tectonic transport over an initially offset continental margin. This zone of deflection, named the Cartersville Transverse Zone, likely represents surfaces offset laterally and vertically that yielded space for depositional basins to accrue and provided the locus of an irregular continental margin surface allowing for rocks from different structural depths to be tectonically thrust up and over offset ramps to the same tectonic level. This is the cause for many abrupt geological changes across this zone and is responsible for the offset of valleys and ridges along this transverse zone. There is also a general change in the directional trend of these features and it is shown as a broad regional deflection of curvature in the Appalachian Mountain belt.

Subglacial recharge into the Western Canada Sedimentary Basin ¡ª Impact of Pleistocene glaciation on basin hydrodynamics
Stephen E. Grasby and Zhouheng Chen, Geological Survey of Canada, Natural Resources Canada, Calgary, Alberta T2L 2A7, Canada. Pages 500-514.
Keywords: Subglacial recharge, hydrogeology, Western Canada Sedimentary Basin, eskers.

New research shows that when North America was covered by ice during the last glaciation there was a significant movement of subglaciatial meltwater into the underlying sedimentary rocks. This water dissolved salt beds within the rock and is now flowing back to surface as salt springs in central Manitoba and northeastern Alberta (two central Canadian Provinces). The influx of glacial meltwater into the sedimentary rocks caused a significant change in the movement of groundwater through the sedimentary basin. A model is proposed where ancient sea water trapped in deep layers of sedimentary rocks has been pushed back and forth through time in response to changing conditions at either end of the sedimentary basin.

Suspended sediment sources and transport distances in the Yellowstone River basin
Peter J. Whiting, Department of Geological Sciences, Case Western Reserve University, Cleveland, Ohio 44106, USA, et al. Pages 515-529.
Keywords: sediment transport, erosion, Be-7, Pb-210, Yellowstone National Park, fluvial geomorphology.

The fine sediment that can cloud the Yellowstone River after a rain or during snowmelt may be carried in the water for hundreds of miles before settling onto the stream bottom or bank. The findings of a recent study tracking the sediment's movement could guide efforts to improve water quality. Researchers collected sediment at nine locations along the river, from near the northeast entrance of Yellowstone National Park to Billings, Montana. They relied upon a naturally occurring radionuclide to identify the sources of the sediment and to track its progress. The samples show that in the headwaters, where the stream is shallow, sediment travels just a few miles, but that in the deeper waters downstream, it travels several hundred miles. Researchers also found that high in the watershed, half the fine sediment comes from erosion of the soil surface and half from erosion of the banks. Below the mountains, however, erosion of the banks accounts for more than three-quarters of the sediment. These findings can be used to prioritize restoration efforts and to better manage activities that have the potential to produce sediment that enters rivers.

Provenance of Jurassic Tethyan sediments in the HP/UHP Zermatt-Saas ophiolite, western Alps
Nancy J. Mahlen, Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA, et al. Pages 530-544.
Keywords: Western Alps, Zermatt, metasediments, Sm/Nd, Rb-Sr, rare earth elements, provenance.

The Alps formed through closure of the Piemont-Ligurian ocean basin during latest Cretaceous to early Tertiary collision of the European and African/Apulian plates. Metasediments of the Zermatt-Saas ophiolite represent the fine-grained Piemont-Ligurian ocean-floor and basin sediments that were metamorphosed to high pressure conditions. Rare earth element concentrations and Rb-Sr and Sm-Nd isotope data are used here to investigate element mobility and to determine provenance and tectonic setting of the metasediments of the Zermatt-Saas ophiolite. Strontium isotopes were nearly completely homogenized for most of the Zermatt-Saas metasediments during early Tertiary metamorphism, whereas the rare earth elements appear to have remained undisturbed. Provenance of the metasediments appears to have been derived from local sources. Furthermore, the similarity in dispersion of Nd isotope compositions of the metasediments and their likely source terranes suggests that the metasediments reflect deposition in small, isolated basins early in the formation of the Piemont-Ligurian ocean.


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