Boulder, Colo., USA – GSA BULLETIN articles posted online between 10 Dec. and 21 Dec. 2012 include a new version of The Geological Society of America's Geologic Time Scale. This paper marks the beginning of a special series of invited papers in celebration of GSA's 125th Anniversary in 2013. Highlights are provided below.
1. A new/revised GSA geologic time scale.
2. Complex mammal fossil record of the Gran Barranca, Patagonia, Argentina.
3. A new and simple method for estimating the rate of sediment delivery to ancient basins.
4. A study of the provenance of volcanic material in ancient Roman masonry.
5. Comparison of different scenarios for the development of the Himalayan Brahmaputra River drainage.
6. A new discovery on Devonian plume-related magmatism in the Central Asian Orogenic Belt.
7. Substantial changes in ecosystem properties during the Holocene in northern Minnesota, USA.
8. A study of fault rocks in the West Antarctic Rift System, northern Victoria Land.
9. A survey of nearly 40 years of core-complex literature, along with a discussion of processes and questions and possible directions for future research.
10. Detailed pollen, dinoflagellate cyst, and magnetic susceptibility analysis of a marine sedimentary sequence of late Neogene from the Montemayor-1 core in the lower Guadalquivir Basin, southwestern Spain.
11. Comparison of pre- and post- Sept. 2010 magnitude 7.1 Darfield earthquake (South Island, New Zealand) LiDAR topographic surveys and property boundary surveys.
12. The first published study to interpret the corrugated mountain ranges of western Arizona as folds.
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The Geological Society of America has maintained and published a geologic time scale for its membership, the scientific community, and the public at large for 30 years. In this paper, J.D. Walker and colleagues present the newest version of the GSA time scale, which is fully updated, incorporating recent advances in understanding numerical ages and the overall succession of rocks and evolution of life on Earth's surface. This paper, one of a series of invited papers for GSA Bulletin in celebration of GSA's 125th anniversary, describes how geologic time scales have been established, including discussion of the underlying methods of determining geologic time. The paper also has an historical focus -- the first geologic time scale, for example, was published 100 years ago by Arthur Holmes -- but this contribution by Walker and colleagues focuses mostly on advances over the last 25 to 30 years. The authors also discuss some of the challenges and opportunities likely to arise over the next 25 years.
Patagonia, Argentina, is well-known for fossils of unique and extinct South American mammals. Gran Barranca, the "Great Cliff" has the most complete fossil record of the Cenozoic (Eocene-Miocene, 42-18.5 million years ago) in South America, and it contains abundant faunal remains entombed in datable volcanic rocks. Fossils at Gran Barranca demonstrate faunal evolution, and a record of ancient vegetation in the form of plant-silica is preserved. Regan E. Dunn and colleagues present a new chronology for the Sarmiento Formation based on high-resolution U-Pb isotopic dating techniques. The new dates provide key tie-points for magnetostratigraphy and allow for a new age model to date fossil samples. Because Gran Barranca is the most complete section of its age in South America, it serves as a rosetta stone for estimating ages of similar faunal assemblages found elsewhere. Our data along with other published ages for vertebrate sites in South America provide ages and durations of the so-called South American Land Mammal Ages. The new dates confirm that South America contains the oldest record of "grazer-like" tooth morphologies among Cenozoic plant-eaters globally, and that Gran Barranca preserves the only fossiliferous terrestrial section spanning the Eocene-Oligocene (about 34 million years) in the Southern Hemisphere.
Siliciclastic shelf margins are depositional features constructed from sediments that were eroded from continents and transported to the edges of deep basins such as oceans. Andrew Petter and colleagues demonstrate a new and simple method for estimating the rate of sediment delivery to ancient basins using the shelf-margin strata preserved within the basins. The method is based on equations for geomorphic modeling and utilizes the rates at which the ancient shelf margins grew to fill the basins as well as the 2-D shapes and dimensions of the shelf-margin bathymetric profiles. The rate of sediment delivery to ancient basins provides a way to reconstruct paleo-environmental conditions, such as climate change and tectonic movement, in the area where the sediments were eroded and produced. This method may also be useful for predicting the presence of hydrocarbon-bearing deepwater sandstones.
The paper presents an innovative analytical approach, based on the use of diagrams of selected trace elements, to the study of provenance of the volcanic material employed as the aggregate in ancient Roman masonry. Samples of pumice and lava from the mortars of Forum of Caesar and Forum of Trajan, employed in the concretes of these monuments, are analyzed and their compositions are compared to those of the volcanic deposits of central Italy, including the districts of Vulsini, Vico, Monti Sabatini, Vesuvius and Phlegrean Fields. Results of this study show that the builders mixed different kind of pumices in the mortars, coming either from deposits of the Monti Sabatini volcanic district near Rome, as well as from those of the Vesuvius and the Phlegrean Fields. Moreover, the peculiar geochemical signature of some pumice and lava samples suggests that these deposits were erupted by peripheral vents located on the southeastern slopes of Vesuvius near Pompeii. The study of the historical sources suggests that a systematic cultivation of lightweight volcanic material for the exportation to Rome occurred in this area during the late Republican era and the early Imperial age, and that its exploitation was re-established soon after the 79 AD eruption.
In a new study on sedimentary rocks of the foothills of the Himalayas in northeastern India, François Chirouze and colleagues propose different scenarios for the development of the Brahmaputra River drainage over the past 13 million years. The interpretations are based on the chemical composition of the sedimentary rocks studied and the rates at which rocks in the Himalaya were eroded in the past. The chemical information was used to determine from which rocks in Himalayas the sedimentary rocks were originally derived. Erosion rates were estimated from the analysis of naturally uranium bearing apatite and zircon crystals in the sedimentary rocks. No evidence was found for a major change in erosion rates influenced by local or regional climate change, as proposed elsewhere. However, this study supports the hypothesis that the Brahmaputra changed its position over time, while erosion rates remained fairly constant. This work is compatible with similar studies conducted in the central Himalayas of Nepal.
Geochronology and geochemistry of basalts from the Karamay ophiolitic mélange in West Junggar (NW China): Implications for Devonian-Carboniferous intra-oceanic accretionary tectonics of the southern Altaids
Gaoxue Yang et al., Ministry of Education, Key Laboratory of Western China's Mineral Resources and Geological Engineering, Xi'an 710054, China. Posted online 10 Dec. 2012; http://dx.doi.org/10.1130/B30650.1.
According to a current tectonic model, the West Junggar (NW China) has been attributed to an intra-oceanic subduction system, which is illustrated by the fact that there are different models for a single subduction zone, arc-arc collision and ridge-subduction. However, in this paper, Gaoxue Yang and colleagues report new U-Pb age data from zircons and whole-rock major and trace element data from the basaltic units in the Karamay ophiolitic mélange. The data reveal the presence of Devonian OIB-type alkaline basalt in the Karamay ophiolitic mélange, which suggest an intra-oceanic setting. Furthermore, their data provides a new discovery on Devonian plume-related magmatism in the Central Asian Orogenic Belt.
Variable ecosystem response to climate change during the Holocene in northern Minnesota, USA
Kendra K. McLauchlan et al., Dept. of Geography, 118 Seaton Hall, Kansas State University, Manhattan, Kansas 66506, USA. Posted online 21 Dec. 2012; http://dx.doi.org/10.1130/B30737.1.
Abrupt climate changes affected the mid-continent of North America during the past 10,000 years, and similar climate changes will likely happen in the future. A small watershed in northern Minnesota, USA, showed only subtle dynamics during rapid climate changes that caused vegetation in the catchment to switch between pine forest, open grassland, and deciduous forest. The most substantial changes in ecosystem properties immediately followed deglaciation of the landscape, formation of the lake, and initial development of pine forests. The lack of response to subsequent dry and wet conditions afterward indicates relative resistance to abrupt climate change at later stages of ecosystem development.
Constraining the timing of fault reactivation: Eocene coseismic slip along a Late Ordovician ductile shear zone (northern Victoria Land, Antarctica)
G. Di Vincenzo et al., Istituto di Geoscienze e Georisorse, CNR, via Moruzzi 1, I Pisa, Italy. Posted online 21 Dec. 2012; http://dx.doi.org/10.1130/B30670.1.
The reactivation of faults or shear zones is a fundamental characteristic of deformation in the continental crust. Faults and shear zones give rise to long-lived zones of weakness which may repeatedly accommodate successive episodes of deformation. A better understanding of reactivation is crucial in order to determine whether a fault is extinct or just temporarily inactive, and this may have implications for seismic hazard assessment. This study by G. Di Vincenzo and colleagues exploits the potential of the 40Ar-39Ar dating method in conjunction with field analysis and electron microscopy down to the nanoscale to assess fault reactivation in fault rocks from the western shoulder of one of the world's largest rifts, the West Antarctic Rift System. The studied fault rocks experienced brittle deformation, with the development of friction-induced melts (pseudotachylyte), overprinting a ductile shear zone. Results reveal that coseismic faulting occurred about 50 million years ago, through coseismic reactivation of a Late Ordovician (460-440 million years) ductile shear zone, implying a period of apparent tectonic quiescence of as much as about 390 million years. The study highlights the inherent difficulty in investigating structures characterized by long periods of tectonic quiescence, for which an accurate evaluation of recurrent periods of fault movement is precluded.
Continental and oceanic core complexes
Donna L. Whitney et al., University of Minnesota, Minneapolis, Minnesota, 55455, USA. Posted online 21 Dec. 2012; http://dx.doi.org/10.1130/B30754.1.
Core complexes are domal geologic structures that form when continental and oceanic lithosphere extends and, as a result, formerly deep rocks rise toward the surface below normal faults. Because deep rocks are brought close to the surface, the formation of core complexes results in the transfer or large amounts of heat and material from deep to shallow. Core complexes offer a glimpse of the deep crust and upper mantle because these rocks are carried upward as the crust flows to fill the gap created by extension of the upper crust. Formation of these structures is related to crustal evolution, global element cycles, heat budgets of continents and oceans, and ore formation. In this review, Donna L. Whitney and colleagues provide a survey of about 40 years of core-complex literature, discuss processes and questions relevant to the formation and evolution of core complexes in continental and oceanic settings, highlight the significance of core complexes for lithosphere dynamics, and propose a few possible directions for future research.
Vegetation, sea-level, and climate changes during the Messinian salinity crisis
G. Jiménez-Moreno et al., Departamento de Estratigrafía y Paleontología, Universidad de Granada, Fuente Nueva s/n, 18002, Granada, Spain. Posted online 21 Dec. 2012; http://dx.doi.org/10.1130/B30663.1.
The Mediterranean Sea dried out partly or near completely during the Messinian around six million years ago (the "Messinian salinity crisis" or MSC). However, the relative role that tectonic processes and sea-level changes had, as triggers for restriction and isolation of the Mediterranean Sea from the open ocean, is still under debate. G. Jiménez-Moreno and colleagues present a detailed pollen, dinoflagellate cyst (dinocyst), and magnetic susceptibility analysis of a marine sedimentary sequence of late Neogene (between about 7.3 and 5.2 million years ago) from the Montemayor-1 core (lower Guadalquivir Basin, southwestern Spain). Their results show that significant paired vegetation and sea-level changes occurred during the Messinian, likely triggered by orbital-scale climate change. Important cooling events and corresponding glacio-eustatic sea-level drops are observed in this study at ca. 5.95 and 5.75 million years ago coinciding with the timing and duration of oxygen isotopic events TG32 and TG22-20 recorded in marine sediments worldwide. It is generally accepted that the onset of the MSC began about 5.96 million years ago; this study suggests that the restriction of the Mediterranean could have been triggered, at least in part, by a strong glacio-eustatic sea-level drop linked to a climate cooling occurring at the time of the MSC initiation.
Fault kinematics and surface deformation across a releasing bend during the 2010 MW 7.1 Darfield, New Zealand, earthquake revealed by differential LiDAR and cadastral surveying
Brendan Duffy et al., Dept. of Geological Sciences, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand. Posted online 21 Dec. 2012; http://dx.doi.org/10.1130/B30753.1.
The magnitude 7.1 Darfield earthquake on 4 September 2010 caused a 30-km-long surface rupture across the Canterbury plains in the South Island of New Zealand. The earthquake started on a steep, relatively small "blind" fault that ruptured into a fault intersection zone and triggered the rupture of the Greendale fault, which generated most of the energy of the earthquake. High-resolution (cm-scale) ground surface displacements in the area of the fault junction were determined by comparing pre- and post-earthquake LiDAR topographic surveys and property boundary surveys, supplemented by mapping of the fault scarp and earthquake-induced flooding. The study revealed exceptionally fine details of ground displacements, including subtle warping over hundreds of meters that was undetectable using traditional earthquake mapping techniques. The magnitude and direction of displacements were used to reconstruct how the faults surrounding the junction interacted with each other during the earthquake to create localized zones of extension and contraction. This study illustrates the value of multi-method investigations and provides a valuable real world example for comparison with dynamic simulations of rupture behavior on fault networks.
Development of extension-parallel corrugations in the Buckskin-Rawhide metamorphic core complex, west-central Arizona
John S. Singleton, Dept. of Atmospheric, Oceanic, and Earth Sciences, George Mason University, Fairfax, VA 22030. Posted online 21 Dec. 2012; http://dx.doi.org/10.1130/B30672.1.
Mountain ranges that have undergone large amounts of extension commonly form distinct ridges ("corrugations") that are parallel to the direction of maximum stretching. In western Arizona, these corrugated mountain ranges are as well defined as anywhere on Earth. Despite the significant amount of research that has been devoted to this area and extended regions in general, the origin of corrugations remains controversial. This study by John S. Singleton presents field data and observations from the Buckskin-Rawhide Mountains (east of Parker, Arizona) that indicate that the corrugations formed by folding during extension about 10 to 20 million years ago. This folding developed in response to NW-SE compression across the region. This is the first study to interpret the corrugated mountain ranges of western Arizona as folds.
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