||April 23, 2001
GSA Release No. 01-10
Following are highlights from the May issue of GEOLOGY and a summary of the
science article from the May issue of GSA TODAY, published by the Geological Society
of America. Please discuss articles of interest with the authors before publishing
stories on their work, and please make reference to GEOLOGY or GSA TODAY in stories
published. Contact Ann Cairns at GSA
to request advance copies of articles and for additional information or assistance.
- Early Paleozoic tectonism within the East Antarctic craton: The final suture between east and west Gondwana?
S.D. Boger et al. Pages 463-466.
- East Antarctica, together with India and Australia, has traditionally been interpreted to have formed the stable eastern half of Gondwana. These continents were thought to have collided ca. 1000 Ma during the construction of an earlier supercontinent, Rodinia, and to have remained intact until Gondwana rifted apart and the modern southern continents formed ca. 25040 Ma. However, new age data from the southern Prince Charles Mountains east Antarctica suggest that this may not have been the case. Instead, it appears that east Gondwana consisted only of Australia and part of east Antarctica. The remainder of east Antarctica and India formed a separate and previously unrecognized continent that did not form part of east Gondwana or Rodinia as they are currently reconstructed. Instead, this Indo-Antarctic continent formed a separate cratonic block that was sutured between east and west Gondwana during the amalgamation of Gondwana ca. 500 Ma.
- Rapid localization of the PacificNorth America plate boundary in the Gulf of California
Michael Oskin et al. Pages 459-462.
- The San Andreas fault is one of the best understood strike-slip plate boundaries in the world, except where it enters the Gulf of California. Until now, no one knew exactly when Baja California was first ripped from mainland Mexico. By finding and correlating identical volcanic rocks of different ages on each side of the Gulf, we determined the evolution of the San Andreas. Our results indicate that the San Andreas abruptly jumped into the Gulf ca. 6 Ma, and that Baja California has moved at least 255 km (160 miles) away from Mexico since that time. Much of the work was done on the Isla Tibur Biosphere Reserve with the help of Seri Indians.
- Minimal climatic control on erosion rates in the Sierra Nevada, California
Clifford S. Riebe et al. Pages 447-450.
- Climate is widely thought to regulate erosion rates. However, the relationships
among precipitation, temperature, and erosion rate have remained speculative because
long-term erosion rates have been difficult to measure. We used accumulations
of cosmogenic nuclides in stream sediment to measure long-term erosion rates at
climatically diverse sites in the Sierra Nevada, California. Our sites span 20180
cm/yr in annual precipitation and 415 °C in mean annual temperature. Average
erosion rates vary by <2.5-fold across these sites and are not correlated with
climate, indicating that climate only weakly regulates non-glacial erosion rates
in mountainous granitic terrain.
- Late Quaternary arroyo formation and climate change in the American southwest
Michael R. Waters and C. Vance Haynes. Pages 399-402.
- Arroyos are deeply entrenched channels that occur on the floors of many valleys in the arid and semiarid southwestern United States. Arroyos were first noted in the mid-nineteenth century and their formation was largely attributed to overgrazing by cattle and other human impacts on the environment. Because these arroyos cut through and exposed older valley floor sediments, geologists were able to examine the exposed stratigraphic record in which they discovered paleoarroyo channels. This discovery indicated that arroyos had formed in the past and that their creation was likely linked to climate change. The debate about what caused the initiation of historic and prehistoric arroyo cutting has gone on for nearly a century with no resolution. Waters and Haynes show that arroyos were essentially absent in the desert environment from 15 ka to ca. 8 ka. As climate and vegetation changed ca. 8 ka, arroyos began to form. The most intense period of arroyo formation has been during the past 4 k.y., with six episodes of arroyo cutting and filling. Furthermore, these periods of arroyo cutting and filling occurred at the same time in different desert basins. Arroyo formation appears to be linked to repeated dry-wet cycles, which in turn are linked to El Niño and non-El Nño conditions. During dry periods, water tables dropped and vegetation cover was reduced in desert basins, making the ground more susceptible to erosion. When a wet interval followed and flooding occurred, arroyo formation ensued. Stratigraphic sequences of arroyo cutting and filling offer excellent paleoclimatic records for desert environments because desert basins are sensitive to major and short-term changes in climate.
- El Niño floods and culture change: A late Holocene flood history for the Rio Moquegua, southern Peru
Francis Magilligan and Paul S. Goldstein. Pages 431-434.
- The 1997-98 El Niño generated large floods throughout southern Peru, especially
in inland locations along Rio Moquegua. By using a combination of remote sensing,
hydraulic modeling, field surveying, and stratigraphic analyses, we have estimated
the magnitude and frequency of this event and determined a late Holocene flood
history for main-stem and tributary sections. Modeling indicates a peak discharge
of 450 m3/s, with an estimated recurrence interval between 50 and 100
yr. Flood deposits for two large floods, dated to cal. A.D. 690 and cal. A.D.
1300, respectively, exist in a small tributary system and temporally bracket a
- Miocene high-pressure metamorphism in the Cyclades and Crete, Aegean Sea, Greece: Evidence for large-magnitude displacement on the Cretan detachment
Uwe Ring et al. Pages 399-402.
- The aim of this paper is to determine the magnitude of displacement on a major fault, the Cretan detachment, which has torn apart or extended a mountain belt that was situated in the region of the present Aegean Sea. Based on our reconstruction, an extensional displacement of >100 km occurred on the Cretan detachment. This is one of the greatest displacement magnitudes ever reported from a detachment fault. The huge amount of horizontal extension caused vertical thinning of the mountain belt, which led to topographic lowering; as horizontal extension proceeded, it resulted in the formation of the Aegean Sea.
- Huge CO2-charged debris-flow deposit and tectonic sagging in
the northern plains of Mars
Kenneth L. Tanaka et al. Pages 427-430.
- Could the northern plains of Mars have been infilled by highland debris in a single, extraordinary event? The authors analyze Mars Global Surveyor topographic data and suggest that a debris deposit as much as 23-km-thick infilled the northern lowlands within a millennium. Water-bearing debris flows cannot account for the amount of erosion and the distances of travel required toassemble this deposit. The authors suggest that carbon dioxide within source rocks may have provided the necessary lubrication. The weight of the deposit was sufficient to depress the northern lowlands and uplift the areas marginal to the deposit by hundreds of meters.
- Neotectonics of the southeastern Reelfoot rift zone margin, central United States, and implications for regional strain accommodation
Randel Tom Cox et al. Pages 419-422.
- The Reelfoot rift zone contains the New Madrid fault system, the major seismic hazard in the mid-continental United States. New Madrid faults are characterized by ongoing low-level seismic activity that indicates North American crust west of the fault system is sliding northeastward along the faults. Results from a recent Global Positioning System (GPS) survey by other authors show little or no movement along the longest segment of the New Madrid fault system (occupying the center of the Reelfoot rift). This previous study suggests that seismic hazard posed by New Madrid faults may be overestimated. However, we present evidence that the southeastern Reelfoot rift margin may be the fault currently accommodating much of the northeastward movement of the western crust (rather than the central rift fault). A sharp break in topography follows the southeastern rift margin for >150 km, and we have documented surface faulting of geologically young sediments along this topographic break. Our interpretation of active fault movement in the region is consistent with current GPS data.
- Fault-Related Folding in California's Northern Channel Islands Documented by Rapid-Static GPS Positioning
Nicholas Pinter et al.
- This paper uses global positioning system data to measure the heights of coastal terraces in the Northern Channel Islands of California, south of Santa Barbara. Using these terraces, the paper helps resolve the patterns and magnitudes of late Quaternary (<100,000 years) deformation on the islands. In particular, their data show that late Quaternary deformation on Santa Rosa Island involved several hundred meters of surface-rupturing left slip, but 15+ meters of vertical motion taken up by folding, resulting in anticlinal growth of at least 0.12 m/ka. The Santa Rosa Island fault represents an en echelon segment of the larger Transverse Ranges Boundary fault system and the Northern Channel Islands antiform. These deformation measurements suggest that the uplift of the islands is occurring over a smoothly curved thrust ramp rather than a fault-bend fold. This reinterpretation is important, as current earthquake-hazard assessments on this and other buried reverse faults depend entirely on assumptions unique to the fault-bend fold geometry. The paper is also a summary of "rapid-static" GPS positioning, which can give centimeter-level measurement precision with just 10 minutes of measurement time, and hence is an excellent tool for neotectonic studies of this type and other high-precision applications.
*To view abstracts and the complete table of contents of GEOLOGY, as well as
that of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN, see www.gsajournals.org.
To obtain full text of these articles and articles from back issues, contact Ann