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Block Rotation (block + rotation)
Selected AbstractsInterseismic Plate coupling and strain partitioning in the Northeastern CaribbeanGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2008D. M. Manaker SUMMARY The northeastern Caribbean provides a natural laboratory to investigate strain partitioning, its causes and its consequences on the stress regime and tectonic evolution of a subduction plate boundary. Here, we use GPS and earthquake slip vector data to produce a present-day kinematic model that accounts for secular block rotation and elastic strain accumulation, with variable interplate coupling, on active faults. We confirm that the oblique convergence between Caribbean and North America in Hispaniola is partitioned between plate boundary parallel motion on the Septentrional and Enriquillo faults in the overriding plate and plate-boundary normal motion at the plate interface on the Northern Hispaniola Fault. To the east, the Caribbean/North America plate motion is accommodated by oblique slip on the faults bounding the Puerto Rico block to the north (Puerto Rico subduction) and to the south (Muertos thrust), with no evidence for partitioning. The spatial correlation between interplate coupling, strain partitioning and the subduction of buoyant oceanic asperities suggests that the latter enhance the transfer of interplate shear stresses to the overriding plate, facilitating strike-slip faulting in the overriding plate. The model slip rate deficit, together with the dates of large historical earthquakes, indicates the potential for a large (Mw7.5 or greater) earthquake on the Septentrional fault in the Dominican Republic. Similarly, the Enriquillo fault in Haiti is currently capable of a Mw7.2 earthquake if the entire elastic strain accumulated since the last major earthquake was released in a single event today. The model results show that the Puerto Rico/Lesser Antilles subduction thrust is only partially coupled, meaning that the plate interface is accumulating elastic strain at rates slower than the total plate motion. This does not preclude the existence of isolated locked patches accumulating elastic strain to be released in future earthquakes, but whose location and geometry are not resolvable with the present data distribution. Slip deficit on faults from this study are used in a companion paper to calculate interseismic stress loading and, together with stress changes due to historical earthquakes, derive the recent stress evolution in the NE Caribbean. [source] Zircon sensitive high mass-resolution ion microprobe U,Pb and fission-track ages for gabbros and sheeted dykes of the Taitao ophiolite, Southern Chile, and their tectonic implicationsISLAND ARC, Issue 1 2006Ryo Anma Abstract The Late Miocene,Pliocene Taitao ophiolite is composed of a complete sequence of classic oceanic lithosphere and is exposed approximately 50 km southeast of the Chile triple junction, where the Chile Ridge subducts beneath the South American Plate. Gabbros and ultramafic rocks are folded into a complex pattern, but only evidence for block rotation has been reported in the overriding sheeted dyke complex. In the present study, sensitive high mass-resolution ion microprobe U,Pb and fission-track dating methods were applied to zircon crystals separated from gabbros and sheeted dykes. Two sets of radiometric ages of gabbros range between 5.9 ± 0.4 and 5.6 ± 0.1 Ma. These ages coincide within their error ranges and imply rapid intrusion and cooling of gabbros. The U,Pb age of a dacite dyke intruded into the sheeted dyke complex was determined to be 5.2 ± 0.2 Ma. These data indicate that the magmas of the Taitao ophiolite were formed during the 6 Ma Chile Ridge collision event and emplaced in a shorter period than previously thought. A short segment of the Chile Mid-oceanic Ridge must have been emplaced during the 6 Ma event. [source] Cenozoic Exhumation of Larsemann Hills, East Antarctica: Evidence from Apatite Fission-track ThermochronologyACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 2 2010Xuanhua CHEN Abstract: Does Cenozoic exhumation occur in the Larsemann Hills, East Antarctica? In the present paper, we conducted an apatite fission-track thermochronologic study across the Larsemann Hills of East Antarctica. Our work reveals a Cenozoic exhumation event at 49.8 ± 12 Ma, which we interpret to be a result of exhumation caused by crustal extension. Within the uncertainty of our age determination, the timing of extension in East Antarctica determined by our study is coeval with the onset time of rifting in West Antarctica at c.55 Ma. The apatite fission-track cooling ages vary systematically in space, indicating a coherent block rotation of the Larsemann Hills region from c.50 Ma to c.10 Ma. This pattern of block tilting was locally disrupted by normal faulting along the Larsemann Hills detachment fault at c.5.4 Ma. The regional extension in the Larsemann Hills, East Antarctica was the result of tectonic evolution in this area, and may be related to the global extension. Through the discussion of Pan-Gondwanaland movement, and Mesozoic and Cenozoic extensions in West and East Antarctica and adjacent areas, we suggest that the protracted Cenozoic cooling over the Larsemann Hills area was caused by extensional tectonics related to separation and formation of the India Ocean at the time of Gondwanaland breakup. [source] Emergency Medicine Subinternship: Does a Standard Clinical Experience Improve Performance Outcomes?ACADEMIC EMERGENCY MEDICINE, Issue 1 2008Christopher J. Lampe MD Abstract Background:, The emergency medicine (EM) subinternship provides a varied experience for senior medical students depending on gender, specialty choice, and interest. A didactic curriculum can be standardized, but the clinical component is difficult to control. Students can be directed to see patients with specific chief complaints. Objectives:, To assess whether a clinical requirement of 10 predetermined cases improves general knowledge as measured on an objective exam. Methods:, This was a prospective, nonrandomized, case-controlled study at a public teaching hospital. Students were assigned to the control group (CG) or test group (TG) by alternating block rotations over 6 months. The CG saw emergency department (ED) patients according to interest and faculty direction. The TG was also required to identify ten specific chief complaints. Patient encounters were recorded in computerized logs. A 10-question pretest assessed preexisting knowledge of each chief complaint, and a 40-question final exam tested general EM knowledge. Descriptive statistics measured demographic data. Groups were compared by Fisher's exact test. Difference in means testing was performed to see if pre- to posttest differences varied by group. Multivariate analysis controlled for gender and specialty choice. Results:, Eighteen CG students saw a mean of 57 patients, and 24 TG students saw a mean of 54 patients; 1 CG student (6%) and 7 TG students (31.8%) saw all 10 required cases (Fisher's exact test p = 0.044). Difference in means testing demonstrated a greater relative change in performance (13.4% points) by the TG relative to the CG on a general knowledge exam, compared with their performance on a brief pretest (p = 0.014). The authors performed multivariate regression controlling for pretest score, gender, and EM specialty choice, and neither gender nor intended EM specialty choice was a contributing factor to the improved performance. A greater relative change in performance (7% points) in the TG exam score was found when compared to the CG (p = 0.020). Conclusions:, Students who participated in the usual didactic curriculum and were required to see ED patients with representative chief complaints performed better on a general EM exam than those who employed common methods of choosing patients. [source] | ||||||||||