||July 1, 2002
GSA Release No. 02-36
July Media Highlights:
The Geological Society of America Bulletin
Boulder, Colo. - The July issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Of particular interest are late Holocene glacier fluctuations in the Wrangell Mountains of Alaska, and the story of climate change being revealed in the wake of the retreating ice; and numerical modeling of fluid flow and oxygen isotope exchange provides new insights into the hydrological evolution of contact-metamorphic aureoles. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to the GSA BULLETIN in stories published. Contact Ann Cairns for copies of articles and for additional information or assistance.
- Paleocene-Eocene syncontractional sedimentation in narrow, lacustrine-dominated basins of east-central Tibet
Brian K. Horton et al., Department of Earth and Space Sciences, University of California, Los Angeles, California 90095, USA. Pages 771-786.
- The Tibetan plateau is a remarkably flat topographic feature situated at an altitude greater than 5 km (16,000 feet). Although the timing and mechanisms of plateau uplift remain poorly understood, new geologic information on the depositional environments and geologic structures in eastern Tibet suggest that this part of the plateau was shortened and partially uplifted due to compression roughly 65 to 35 million years ago. Moreover, the ancient sedimentary environments in eastern Tibet at that time were characterized by low rates of deposition and relatively small lake systems, similar to the modern sedimentary environments in central Tibet. Therefore, the very large river systems that now exist in eastern Tibet, such as the Mekong and Yangtze Rivers, must have developed only during the past 35 million years.
- Numerical modeling of fluid flow and oxygen isotope exchange in the Notch Peak contact-metamorphic aureole, Utah
Xiaojun Cui et al., Department of Geological Sciences, University of Missouri, Columbia, Missouri 65211, USA. Pages 869-882.
- Fluid flow plays important roles in various geological processes. Oxygen isotope ratios are good indexes of fluid flow through rocks because oxygen is a major element in both rocks and fluids. This study is a two-dimensional numerical simulation of coupled fluid flow and oxygen isotope exchange during contact metamorphism, initiated by the emplacement of magma into the shallow crust of the Earth. Using geological constraints, the model predicts transient and heterogeneous fluid flow and oxygen isotope distribution, consistent with the observations from the Notch Peak contact metamorphic aureole, Utah. The results of this study provide new insights into the hydrological evolution of contact-metamorphic aureoles.
- Syndepositional thrust-related deformation and sedimentation in an Ancestral Rocky Mountains basin, Central Colorado trough, Colorado, USA
Richard G. Hoy and Kenneth D. Ridgway, Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, Indiana 47907-1397, USA. Pages 804-828.
- The Pennsylvanian-Permian Ancestral Rocky Mountains of western North America consist of large, basement-involved uplifts separated by structurally deep sedimentary basins. The tectonic configuration of the Ancestral Rocky Mountains has been difficult to place within a plate tectonic framework because the Pennsylvanian-Permian paleogeography of western North America is still poorly understood. An excellent location to study an Ancestral Rocky Mountains basin is in south-central Colorado where Pennsylvanian-Permian strata have been uplifted in the high peaks of the Sangre de Cristo Mountains. We interpret these strata as having been deposited in a flexural basin based on structural data that indicate syndepositional thrust-related deformation, and stratigraphic data that show that the basin was asymmetric, with coarsest deposits adjacent to the Sand Creek-Crestone thrust fault system.
- Metamorphic history of the southern Menderes massif, western Turkey
Donna L. Whitney, Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA, and Erdin Bozkurt, Department of Geological Engineering, Middle East Technical University, TR-06531 Ankara, Turkey. Pages 829-838.
- Western Turkey was assembled from rifted fragments of Africa/Arabia during the Alpine collision, which began tens of millions of years ago and is still tectonically active today. The folding and faulting related to the collisions and the modern extension that affects western Turkey and the Aegean region have created complex structures that are difficult to interpret, which has resulted in numerous models for their origin. This paper uses information from rocks heated and buried during collision and unroofed during extension to discuss and evaluate models for the tectonic evolution of western Turkey during Alpine mountain-building.
- Mafic injections, in situ hybridization, and crystal accumulation in the Pyramid Peak granite, California
R.A. Wiebe et al., Department of Geosciences, Franklin and Marshall College, Lancaster, Pennsylvania 17604, USA. Pages 909-920.
- The Pyramid Peak granite is a Jurassic pluton located in the northern Sierra Nevada batholith. Multiple injections of basaltic magma produced mafic sheets interlayered with the granite. Extensive mixing between basaltic and granitic magmas at the top of the mafic sheets indicates that the granitic pluton was constructed gradually by crystallization of an actively convecting, periodically replenished magma chamber. This mode of solidification is consistent with inferences from many volcanic eruptions. The granitic pluton, therefore, appears to represent a solidified magma chamber that, when active, was capable of producing volcanic eruptions.
- Late Holocene glacier fluctuations in the Wrangell Mountains, Alaska
Gregory C. Wiles et al., Department of Geology, College of Wooster, Wooster, Ohio 44691, USA. Pages 896-908.
- General ice retreat has dominated in Alaska during the 20th century, with a significant increase in ice wastage during the past two decades. In the wake of the retreating ice, a story of climate change is being revealed. The past fluctuations of nine glaciers in the Wrangell Mountains are dated primarily from forests previously overrun by ice and show intervals when the glaciers were more and less extensive than at present. These well-dated glacial histories are primarily a record of past multi-decadal to century-scale temperature changes. This interior Alaskan Wrangell glacial record compares well with similar glacial histories from coastal Alaska, revealing the structure of century-scale temperature variations over the past few thousand years.
- Welding and rheomorphism of phonolitic fallout deposits from the Las Cañadas caldera, Tenerife, Canary Islands
Carles Soriano et al., VIEPS, Department of Earth Sciences, Monash University, Wellington Road, Clayton, Victoria 3168, Australia. Pages 883-895.
- This paper describes the processes by which volcanic particles are deposited by air-fall mechanisms, sinter together, and behave as lava flows. This is possible due to the high temperatures of volcanic particles on landing from the ejected volcanic plumes. After landing, temperatures above 500 °C can remain for more than 50 days within the deposit. The study is undertaken on air fall deposits up to 2 million years old in a volcanic caldera on the Tenerife island.
To view abstracts for the GSA BULLETIN, go to www.gsajournals.org.
To obtain a complimentary copy of any GSA BULLETIN article, contact Ann Cairns.
The Geological Society of America