Keywords: Gulf of Fonseca, Central America; Owens Valley, California; North Anatolian fault, Yaylabeli, Turkey, Canadian Cordillera, Canadian Shield, Tunisia.
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Forearc motion and deformation between El Salvador and Nicaragua: GPS, seismic, structural, and paleomagnetic observations
D. Alvarado et al., Dept. of Geoscience, University of Wisconsin, Madison, Wisconsin 53706, USA. Pages 3-21. Published online 17 Dec. 2010; doi: 10.1130/L108.1.
Measurements of present-day movements and past deformation at stations in El Salvador, Honduras, and Nicaragua are used to describe the present rates and directions of areas of El Salvador and Nicaragua between the Central America volcanic arc and coasts. The measured movements in both areas imply that all areas outboard from the volcanic arc move 15 plus or minus 2 mm per year toward the northwest. The movement is consistent with a recently posed hypothesis that many coastal areas of Central America are escaping to the northwest away from central Costa Rica, where a large oceanic ridge collides with the continent. The Gulf of Fonseca, which separates Nicaragua and El Salvador, is a seismically and volcanically active region with numerous mapped faults and frequent moderate-magnitude earthquakes that extend westward into adjacent areas of El Salvador. Detailed measurements, by Alvarado et al., of the regional faulting and a well-exposed, typical fault in this area indicate that the onshore and offshore deforming zones occur in response to movement across a roughly 40 km inland step in the volcanic arc, thereby offering a consistent picture of active deformation and crustal movements in the coastal areas of these two countries.
The role of low-angle normal faulting in active tectonics of the northern Owens Valley, California
Fred M. Phillips and Lisa Majkowski, Dept. of Earth and Environmental Science, New Mexico Tech, Socorro, New Mexico 87801, USA. Pages 22-36. Published online 17 Dec. 2010; doi: 10.1130/L73.1.
The Owens Valley of eastern California is an extensional graben. The mechanics of extension have traditionally been explained by means of high-angle normal faulting. However, this mechanism appears to be inconsistent with both the accepted tectonic structures of associated basins and with the expected kinematics of regional extension. Fred M. Phillips and Lisa Majkowski of New Mexico Tech have therefore reexamined several lines of evidence that bear on the fault structures bounding the northern Owens Valley.
Paleoseismologic evidence for the relatively regular recurrence of infrequent, large-magnitude earthquakes on the eastern North Anatolian fault at Yaylabeli, Turkey
Ozgur Kozaci et al., William Lettis and Associates Inc., Earth Science Consultants, 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596, USA. Pages 37-54. Published online 22 Dec. 2010; doi: 10.1130/L118.1.
Paleoseismologic trenches excavated across the eastern part of the North Anatolian fault at Yaylabeli, Turkey, provide evidence for five surface ruptures during the last 2000 years. Here Ozgur Kozaci Of William Lettis and Associates Inc. and colleagues interpret these events as (1) the historical 1939 Mw 7.9 earthquake; (2) the historical 1254 A.D. earthquake; (3) the historical 1045 A.D. earthquake; (4) an earthquake that occurred between 660 A.D. and 1020 A.D., most probably between 717 and 844 A.D.; and (5) an earthquake that occurred between 302 A.D. and 724 A.D., possibly the historical 499 A.D. event. Although one of the intervals they document is 685 years long (between the 1254 A.D. and 1939 A.D. earthquakes), the other three intervals are between 200 and 350 years long. Their results, which facilitate a rare opportunity to test the completeness of the paleoseismologic record at multiple sites, demonstrating reproducibility of the paleoearthquake record. These paleoearthquake data reinforce the idea of relatively regular recurrence of infrequent, large-magnitude earthquakes on the eastern section of the North Anatolian fault. They attribute this relatively simple behavior to the structural maturity of the North Anatolian fault and its relative isolation from other major seismic sources within the Anatolia-Eurasia plate boundary.
Testing modes of exhumation in collisional orogens: Synconvergent channel flow in the southeastern Canadian Cordillera
Felix Gervais and Richard L. Brown, Dept. of Earth & Planetary Sciences, McGill University, Montreal, Quebec H3A 2A7, Canada. Pages 55-75. Published online 22 Dec. 2010; doi: 10.1130/L98.1.
In the hinterland of many mountain belts lay rocks that were originally at a depth in excess of 30 km at temperatures above 700 degrees Celsius. Understanding the processes that lead to their exposure at the Earth's surface, i.e. how they are exhumed in scientific terms, represents a major challenge for geoscientists. In this contribution, Gervais and Brown present a test to distinguish the three main exhumation modes in mountain belts from the predictions of sophisticated numerical models. Applied to the hinterland of the southeastern Canadian Cordillera, our newly designed test rules out the commonly held view that these mid-crustal rocks were exhumed by large magnitude extension and gravitational collapse of the thickened belt on its own weight after a reduction of compressive stress. It also argues against the orogenic wedge model in which there is a continuous cycle of rocks that are accreted to the side of belt and removed from the top by erosion. The test rather points to a model of channel flow in which rock softening by partial melting induces the flow of a 10-15-km thick layer above a rigid ramp consisting of the old rocks of the Canadian Shield.
Structure of the Alima and associated anticlines in the foreland basin of the southern Atlas Mountains, Tunisia
P. Riley et al., Dept. of Geoscience, University of Wisconsin, 1215 W. Dayton St., Madison, Wisconsin 53706, USA. Pages 76-91. Published online 22 Dec. 2010; doi: 10.1130/L119.1.
P. Riley of the University of Wisconsin-Madison and colleagues study a series of folded rocks in southern Tunisia and suggest that the folds developed over reactivated faults. Tunisia, situated in North Africa along the Mediterranean coast, has undergone a diverse tectonic history. Between ~100 and 250 million years ago, the area was a large basin that was inundated by sediments. Much of the room created in order for the sediments to be deposited was formed due to normal faults. However, as North Africa and Europe began to converge after ~65 million years ago, the basins were inverted, causing the normal faults to become reverse faults and the basins to become the Atlas Mountains. Many of these faults do not breach Earth's surface, and what we see at the surface are folded rocks. Riley and colleagues suggest that the faults below the surface cause the rocks at the surface to fold, in the same manner as some structures in the western United States. The paper uses gravity data, seismic data, and geologic mapping to arrive at these conclusions.
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