Boulder, Colo., USA - New Geosphere postings online on 7 and 16 May include additions to two special issues: CRevolution 2: Origin and Evolution of the Colorado River System II and The ANDRILL McMurdo Ice Shelf (MIS) and Southern McMurdo Sound (SMS) Drilling Projects. Other articles cover India-Asia collision; a Late Triassic snapshot in the U.S. Southwest; the Alabama and western Georgia Blue Ridge; and the Jemez Mountains volcanic field.
Abstracts for these and other Geosphere papers are available at http://geosphere.
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Preliminary balanced palinspastic reconstruction of Cenozoic deformation across the Himachal Himalaya (northwestern India)
A. Alexander G. Webb, Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana 70803, USA. First published on 16 May 2013, http://dx.
This study offers the first geometrically rigorous reconstruction of deformation in response to the India-Asia collision across a key portion of the western Himalaya. The reconstruction demonstrates the viability of important concepts pertaining to the dynamic evolution of crustal-scale contractional systems. First, despite variations in erosion and exhumation, the crystalline cores of mountain belts may be emplaced at depth. Second, zones of rapid uplift along the length of mountain belts in their interior may result from deep accretion and stacking of slices from the down-going collisional plate. Third, such stacking may occur in multiple systems at different crustal levels, all developing simultaneously. Finally, the reconstruction resolves much of the apparent mismatch between shortening estimates of two different types across the western Himalaya; i.e., between estimates based on reconstructing mountain belt deformation and estimates based on restoring plate motion using sea-floor magnetic anomalies.
Review and analysis of the age and origin of the Pliocene Bouse Formation, lower Colorado River Valley, southwestern USA
Jon E. Spencer et al., Arizona Geological Survey, 416 W. Congress Street, #100, Tucson, Arizona 85704, USA. First published on 16 May 2013, http://dx.
The lower Pliocene Bouse Formation in the lower Colorado River Valley (southwestern USA) consists of basal marl and dense tufa overlain by siltstone and fine sandstone. It is locally overlain by and interbedded with sands derived from the Colorado River. Jon E. Spencer and colleagues briefly review 87Sr/86Sr analyses of Bouse carbonates and shells and carbonate and gypsum of similar age east of Las Vegas that indicate that all of these strata are isotopically similar to modern Colorado River water. They also review and add new data that are consistent with a step in Bouse Formation maximum elevations from 330 m south of Topock Gorge to 555 m to the north. New geochemical data from glass shards in a volcanic ash bed within the Bouse Formation, and from an ash bed within similar deposits in Bristol Basin west of the Colorado River Valley, indicate correlation of the two ash beds and coeval submergence of both areas. The tuff bed is identified as the 4.83 million year old Lawlor Tuff derived from the San Francisco Bay region. Spencer and colleagues conclude, as have some others, that the Bouse Formation was deposited in lakes produced by first-arriving Colorado River water that entered closed basins inherited from Basin and Range extension, and estimate that first arrival of river water occurred about 4.9 million years ago. If this interpretation is correct, addition of Bristol Basin to the Blythe Basin inundation area means that river discharge was sufficient to fill and spill a lake with an area of ~10,000 square kilometers. For spillover to occur, evaporation rates must have been significantly less in early Pliocene time than modern rates of about two to four meters per year, and/or Colorado River discharge was significantly greater than the current ~15 cubic kilometers per year. In this lacustrine interpretation, evaporation rates were sufficient to concentrate salts to levels that were hospitable to some marine organisms presumably introduced by birds.
Porosity and density of the AND-1B sediment core, McMurdo Sound region, Antarctica: Field consolidation enhanced by grounded ice
F. Niessen et al., Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Am Alten Hafen 26, 27568 Bremerhaven, Germany. First published on 7 May 2013, http://dx.
F. Niessen and colleagues present a study of density and porosity for the 1285-m-long AND-1B core recovered from a flexural moat in the McMurdo Sound (Antarctica) in order to interpret sediment consolidation in an ice-proximal location on the Antarctic shelf. Various lithologies imply environmental changes from open marine to subglacial, and are numerically expressed in high-resolution whole-core wet-bulk density (WBD). Grain density data interpolated from discrete samples range from 2.14 to 3.85 g/cm3 and are used to calculate porosity from WBD in order to avoid the 5% to 15% overestimation and underestimation of porosities obtained by standard methods. The trend of porosity extends from 0.5 near the top (Pleistocene) to 0.2 at the bottom (Miocene). Porosity fluctuations in different lithologies are superimposed with 0.2 to 0.3 in sequences younger than about one million years and 0.5 to 0.8 in Pliocene diatomites. The AND-1B porosities and void ratios of Pliocene diatomites and Pleistocene mudstones exhibit a large negative offset compared to modern lithological analogs and their consolidation trends. This offset cannot be explained in terms of the effective stress at the AND-1B site. The effective stress ranges from 0 to 4000 kPa in the upper 600 m, and reaches 13,000 kPa at the base of the AND-1B hole. Niessen and colleagues suggest that an excess of effective overburden stress of ~1700 and ~6000 kPa to explain porosities in Pliocene diatomites and Pleistocene mudstones, respectively. This is interpreted as glacial preconsolidation by subsequently grounded ice sheets under subpolar to polar, followed by colder polar types of glaciations. Information on Miocene consolidation is sparse due to alteration by diagenesis.
The Early Mesozoic Cordilleran arc and Late Triassic paleotopography: The detrital record in Upper Triassic sedimentary successions on and off the Colorado Plateau
N.R. Riggs et al., School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, Arizona 86011, USA. First published on 7 May 2013, http://dx.
A volcanic arc grew along the southwest coast of North America in Permian-Triassic time, between about 280 and 200 million years ago. One of the best ways to understand how this arc developed is to look at the sedimentary rocks that were deposited in river systems as it eroded, especially the durable grains that carry a record of the volcanic material that originally incorporated them. This paper by N.R. Riggs and colleagues provides results from a study of zircon grains from three Triassic sedimentary units, one each in northern and southern Arizona and one in eastern California, which together provide a Late Triassic snapshot of the rivers that drained off this volcanic arc. The chemical composition of the zircons can also be used to show how groups of the grains are likely related.
Late to post-Appalachian strain partitioning and extension in the Blue Ridge of Alabama and Georgia
Mark G. Steltenpohl et al., Dept. of Geology and Geography, Auburn University, Auburn, AL 36849, USA. First published on 7 May 2013, http://dx.
Kinematic analysis of the Goodwater-Enitachopco and Alexander City faults document that dextral strains in the Alabama and western Georgia Blue Ridge are partitioned much farther toward the foreland than is reported to the northeast, likely as a consequence of the southern Appalachian master décollement having passed obliquely across a several kilometer step up along the Cartersville transform. The top-to-the-south-southeast normal-slip component of movement along the Goodwater-Enitachopco fault is unusual, considering its position far toward the foreland. Loose timing constraints for this extensional event (late Carboniferous to Early Jurassic) leave room for several tectonic explanations, but Mark G. Steltenpohl and colleagues favor the following: (1) Late Pennsylvanian to Early Permian crustal thickening created a wedge of Blue Ridge rocks bound above by the Goodwater-Enitachopco, below by the décollement, and to the northwest (present-day direction) by a topographically steep mountain front; (2) further convergence and crustal thickening caused this wedge to gravitationally collapse with southward-driven motion; and (3) Mesozoic rifting reactivated some of the faults as the Gulf of Mexico began to open.
Spatial and temporal trends in pre-caldera Jemez Mountains volcanic and fault activity
Shari A. Kelley et al., New Mexico Bureau of Geology and Mineral Resources, New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, New Mexico 87801, USA. First published on 7 May 2013, http://dx.
Nearly continuous eruption of lavas and tuffs over the last 10 Ma in the Jemez Mountains volcanic field (JMVF) on the western margin of the Rio Grande rift provide a unique opportunity to examine the interplay between faulting and volcanism along a rift margin. New 40Ar/39Ar dates on JMVF lavas and tuffs are coupled with the first comprehensive evaluation of the history of fault activity between 10 and 2 million years ago in this region to document a complex east to west to east pattern of faulting and volcanism through time in the northern JMVF. This pattern requires a reorientation of volcanic center alignment from a NE to a more northerly-striking trend, an episode of rift widening, and reactivation of previously unmapped Laramide structures. In addition, the new ages, combined with detailed mapping of both volcanic rocks and the Santa Fe Group, document significant pulses of faulting, erosion, and deposition during middle Miocene time and during late Miocene time along the Cañones fault zone, a significant rift bounding structure that is exposed in the northern wall of the Valles caldera.