||October 1, 2002
GSA Release No. 02-43
October Media Highlights:
The Geological Society of America Bulletin
Boulder, Colo.-The October issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Of particular interest: origins of the Yellowstone hotspot; paleoseismology of Alaska and implications for understanding and managing earthquake risk; and climate change in the northeastern United States.
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.
- Interplay between tectonics and glacio-eustasy: Pleistocene succession of the Crotone basin, Calabria (southern Italy)
F. Massari, Dipartimento di Geologia, Paleontologia e Geofisica, Università di Padova, Via Giotto 1, 35137 Padova, Italy; et al. Pages 1183-1209.
- Keywords: Pleistocene, cyclothems, basin analysis, neotectonics, eustasy, chronology.
- The Pleistocene is characterized by high-amplitude, high-frequency, and marked asymmetry of sea-level changes, in turn controlled by astronomically forced climatic changes. It is known that these changes imply long-lived falling stages punctuated by minor oscillations, related to the growth of the ice caps, and very fast rising stages marking the transition from glacial to interglacial conditions.
- The analysis of climate changes and sea-level fluctuations in recent sediments is a topic of great interest in the scenario of international research in the field of earth and allied sciences. The main interest for this topic stems from social-economic demands (need of forecasting short-term and long-term climatic changes, analysis of the impact of climatic and sea-level changes on the natural environment and human activities).
- The availability of extensive outcrops and thick subsurface successions of Pleistocene sediments in various Italian areas provides the rare opportunity to study in high resolution an important record of global Pleistocene changes. The areas offering such a chance occur very rarely in the world (New Zealand, Japan, California, in addition to Italy), and the Italian record is particularly interesting and offers spectacular exposures. For these reasons the Italian Quaternary record drew the attention of stratigraphers since the beginning of the last century. Until recent times, the study of marine Quaternary successions in Italy was hampered by the lack of a reliable chronologic frame. In the last 5-10 years important advancement has been made by the development of a detailed biostratigraphic scheme based on calcareous nannofossils. It has been demonstrated through an integrated approach that in shelf deposits chronologic models can be built up which are comparable in resolution and reliability to those established in Mediterranean deep-sea and oceanic sediments. In some cases the response to short-term climatic changes can be documented in such shallow-water settings.
- Several examples of thick Pleistocene successions significantly affected by synsedimentary tectonics are present in several basins of the Adriatic-Ionian margin of Italy, linked to the recent evolution of the Apenninic chain. They were locally brought to the surface by recent important uplift movements. The growth of tectonic structures can substantially influence the internal architecture of sedimentary bodies by originating unconformities and particular stratal geometries. However, the high degree of chronologic resolution that can be attained in these successions allows a discrimination of tectonic and eustatic signals. Furthermore, by combining this information with that provided by cyclostratigraphy, it is possible to quantify the deformation rate of tectonic structures. These data are of critical importance in the analysis of neotectonics and in the modeling of geologic risks linked to the subsidence and seismicity.
- Correlation of Permian and Triassic deformations in the western Great Basin and eastern Sierra Nevada: Evidence from the northern Inyo Mountains near Tinemaha Reservoir, east-central California
Calvin H. Stevens, Department of Geology, San Jose State University, San Jose, California 95192, USA; and Paul Stone, U.S. Geological Survey, Menlo Park, California 94025, USA. Pages 1210-1221.
- Keywords: deformation, Great Basin, Permian-Triassic, Sierra Nevada, tectonics.
- Study of sedimentary rocks at Tinemaha Reservoir near Big Pin, California, has provided key chronological and structural constraints for linking Permian and Triassic deformational events in the White-Inyo Range in the western Great Basin with those in the eastern Sierra Nevada. Five major deformational events are represented in this region during this relatively narrow time span. We speculate that these intense and tectonically diverse events were the result of changing stress regimes associated with changing plate interactions, which culminated in establishment of an east-dipping subduction zone and an associated magmatic arc in the latter part of the Late Triassic.
- Structural segmentation, inversion, and salt tectonics on a passive margin: Evolution of the Inner Kwanza Basin, Angola
Michael R. Hudec and Martin P.A. Jackson, Bureau of Economic Geology, University of Texas, Box X, University Station, Austin, Texas 78713-8924, USA. Page 1222-1244.
- Keywords: Angola, basement tectonics, Kwanza Basin, passive margins, salt tectonics, segmentation.
- The Kwanza Basin, Angola, is divided into the Inner and Outer Kwanza salt basins, separated by a chain of basement highs on which salt is thin or absent. North- to northwest-trending basement faults segment the Inner Kwanza Basin. These structures probably acted as transfer faults during rifting, and have been repeatedly reactivated since that time. Reactivation has produced three fold-and-thrust belts near basement uplifts, and has also controlled the evolution of salt structures. Salt structural styles change dramatically across the transfer faults. This paper describes the overall basin architecture, presents evidence for the transfer faults and their reactivation, and discusses the impact of basement reactivation on salt tectonics.
- Upper-mantle origin of the Yellowstone hotspot
Robert L. Christiansen, U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA; et al. Pages 1245-1256.
- Keywords: helium, hotspots, mantle plumes, tomography, upper mantle, Yellowstone National Park.
- Hotspots represented by age-progressive volcanic chains, commonly within a tectonic plate, typically are ascribed to convective plumes from the base of Earth's mantle. Such a deep-mantle plume hypothesis for Yellowstone does not explain or is contraindicated by a variety of observations, including geologic and tectonic setting and seismic tomography. The Yellowstone magmatic system appears to extend no deeper than ~200 km below the surface. Furthermore, Yellowstone's helium-isotopic ratios, commonly regarded as indicative of a deep-mantle origin, are explicable by upper-mantle processes. The Yellowstone magmatic system can be considered to represent a deep-mantle plume only by invoking numerous coincidences and special hypotheses. It is more likely that it originates from prolific partial melting of the upper mantle controlled by regional tectonics and local structures and sustained by thermal feedback processes.
- Tectonic history of the Altyn Tagh fault system in northern Tibet inferred from Cenozoic sedimentation
A. Yin, Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California 90095, USA; et al. Pages 1257-1295.
- The Tibetan Plateau is the largest high-altitude land mass on Earth. Its average elevation is ~5000 m and has a dimension >1500 km both in the north-south and east-west directions. When the Tibetan Plateau began to be uplifted remains uncertain, but our work along the northern rim of the plateau suggests that the onset of uplift started ~42 millions years ago, which is 30 millions earlier than previously thought.
- Paleoseismology at high latitudes: Seismic disturbance of upper Quaternary deposits along the Castle Mountain fault near Houston, Alaska
Peter J. Haeussler, U.S. Geological Survey, 4200 University Drive, Anchorage, Alaska 99508, USA; et al. Pages 1296-1310.
- Keywords: Alaska, Castle Mountain fault, liquefaction, paleoseismology, Quaternary, trenching.
- Fundamental to understanding the earthquake hazard of a region is the desire to evaluate which faults in Earth's crust can produce earthquakes, how large those earthquakes will be, and how often earthquakes will occur. In southcentral Alaska, where half the population of the state resides, studies show there are essentially two regions in the crust that produce earthquakes. The first is the so-called Benioff zone, along which earthquakes delineate the boundary between the North American continental crust and the underthrusting Pacific plate. The Pacific plate is sliding to the northwest beneath southcentral Alaska at a rate of ~5.5 cm/yr. Earthquakes along this zone produced the Great 1964 magnitude 9.2 earthquake. The other source of earthquakes for southcentral Alaska is in the upper crust around and beneath Cook Inlet.
- Within the past 5 million years or so Earth's crust has folded and faulted, producing folds that trap oil and gas beneath the inlet, as well as earthquakes in the core of the folds. The only one of these faults that comes to Earth's surface is the Castle Mountain fault, which runs along the south side of the Talkeetna Mountains near Palmer and Houston. The authors performed trenching studies of the Castle Mountain fault to determine when and how often earthquakes have occurred on the fault. The scientists had trenches dug across the fault trace in nine locations, and they carefully mapped the walls and collected samples for radiocarbon dating. In a study published in the October issue of the Geological Society of America Bulletin, they found evidence that the fault produced about four earthquakes in the past 2700 years, which indicates an earthquake recurrence interval of ~675 years. Moreover, the last earthquake on the fault occurred around 600-700 years ago and thus it appears that the fault may be ready for another earthquake.
- Unfortunately, geologists and seismologists have yet to be able to predict the exact time that any earthquake will occur, but studies like this one more clearly identify the hazard and enable planners to mitigate the risk. "For example, in California, if you build within six miles of an active fault you have to build a building in a special way," says Haeussler. "A similar strategy could be taken here, where the Castle Mountain fault runs through populated areas."
- The study also noted that the timing of megathrust earthquakes, like the 1964 magnitude 9.2 earthquake, is similar to the inferred times of faulting on the Castle Mountain fault. This suggested to the authors that these very large earthquakes may cause earthquakes to occur on the Castle Mountain fault. Scientists in California have recently noted that movement on one fault during an earthquake can cause the stresses to increase on other faults, resulting in a higher probability of earthquakes.
- Lastly, the study found that the types of features seen on the walls of the trenches are quite different from the features commonly studied in the lower 48 states, where most trenching studies have been done. Haeussler said, "It's a zone of goo," when describing the appearance of the fault zone. Most other trenches have been mapped where it is not nearly as wet beneath the ground surface, and the evidence of flowage of sand in the Castle Mountain fault trenches indicates the ancient earthquakes took place when the ground was not frozen. Haeussler and colleagues also found that freeze-thaw action, the almost impenetrable root mat, and variations in local vegetation affected what they observed on the trench walls. They anticipate the study will be come a benchmark for further trenching studies in Alaska or at high latitudes.
- Late glacial-Holocene atmospheric circulation and precipitation in the northeast United States inferred from modern calibrated stable oxygen and carbon isotopes
Matthew E. Kirby, Department of Earth Sciences, Heroy Geology Laboratory, Syracuse University, Syracuse, New York 13244, USA; et al. Pages 1326-1340.
- Keywords: Quaternary, Holocene, isotopes, atmosphere, precipitation, lakes.
- Climate has changed in the past, and climate will change in the future. How we as a society adapt to further climate change greatly depends on what we know about climate dynamics. Key to our knowledge of climate dynamics is a well-developed working knowledge of past climate variability. Understanding past climate variability, and its dominant modes of stability, or lack thereof, provides an important baseline of knowledge through which we can interpret future climate direction.
- In the October issue of GSA Bulletin, we present a high-resolution (decadal-scale) record of past climate change from the heavily populated and economically vital northeastern United States, a place where future climate change is certain to have profound effects. Using a modern calibration methodology, we infer past positions of the polar front jet stream and early summer precipitation amounts. Our research indicates a high degree of polar vortex variability over the past 15,000 years. As an example, the average winter position of the polar front shifted more than 600 km (6° latitude) in a matter of decades; the first occurrence happened at 11,600 years ago and lasted for nearly 1300 years, the second happened at 5200 years ago and lasted another 2000 years.
- How would such a shift impact the northeastern United States today? Similarly, precipitation is characterized by dramatic variability over the past 15,000 years. Considering the importance of annual precipitation to groundwater recharge and freshwater reservoirs, it is important that we better understand the past dynamics of the precipitation cycle for future evaluation. Non-ice core, decadal-resolution continental archives of past climate change are rare, especially from populated regions. We hope our research demonstrates the importance of developing these types of records from similarly populated regions in the near future.
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