HR: 16:45h
AN: GP54A-04 INVITED    [Abstracts]
TI: What do remagnetizations tell us about regional thermo-tectonic processes in the India-Asia collision zone?
AU: * Appel, E
EM: erwin.appel@uni-tuebingen.de
AF: Inst. f. Geowiss., Sigwartstr. 10, Tübingen, 72076, Germany
AU: El Bay, R
EM: rachida.el-bay@uni-tuebingen.de
AF: Inst. f. Geowiss., Sigwartstr. 10, Tübingen, 72076, Germany
AU: Dunkl, I
EM: istvan.dunkl@geo.uni-goettingen.de
AF: Geowiss. Zentrum, Goldschmidtstr. 3, Göttingen, 37077, Germany
AU: Ding, L
EM: dinglin@mail.igcas.ac.cn
AF: Inst. of Tibetan Plateau Research, P.O.Box 2871, Beijing, 100085, China
AU: Antolin, B
EM: borja.antolin@uni-tuebingen.de
AF: Inst. f. Geowiss., Sigwartstr. 10, Tübingen, 72076, Germany
AU: Schill, E
EM: schille@uni-mainz.de
AF: Inst. f. Geowiss., Becherweg 21, Mainz, 55099, Germany
AU: Waldhör, M
EM: terrana.geophysik@t-online.de
AF: Inst. f. Geowiss., Sigwartstr. 10, Tübingen, 72076, Germany
AB: Remagnetization is a common phenomenon throughout the Himalayan Range. Stable secondary remanences mostly reside in pyrrhotite formed and magnetized during last metamorphism. Many sites have samples with both normal and reverse polarities and single specimens even reveal antipodal directions during thermal demagnetization. Therefore the time span of remanence blocking was obviously sufficiently long to average on paleosecular variation. K-Ar data on illites provide a time constraint for remanence acquisition; ages ranging from Eocene/Oligocene to Oligocene/Miocene in the western and central Himalaya, respectively. The pyrrhotite remanences record net displacements since remanence acquisition. It has been demonstrated that they can contribute to decipher the regional thermo-tectonic orogenic evolution. Block rotations about vertical axes show that oroclinal bending and rotational shortening between India and Asia are the dominating processes in the western and central part of the Himalayan arc. Less data are available from the eastern part of the collision zone. First paleomagnetic results indicate that in this region large-scale material flow around the East Himalaya Syntaxis becomes significant. The transition between the western and eastern realm seems to be located in western to central Nepal. It is interesting that the general pattern of paleomagnetic block rotations match with present-day rotations derived from GPS velocities suggesting that recent crustal movements are representative also for longer-term geological processes. This is particularly important for models of the evolution of the Tibetan Plateau in which decoupling of horizontal movements of the upper crust and mantle, enabled by a soft partially molten middle crust, plays a major role. The age and nature of the secondary pyrrhotite remanences is crucial for interpretations. Their origin is either thermo-remanent related to cooling or thermo-chemical related to the pro-grade or peak metamorphic phase. Coinciding results for remanence directions of formations with peak temperatures higher and lower than the Curie temperature of pyrrhotite, respectively, suggest that remanence acquisition was close to cooling in either case. West of ca. 89°E pyrrhotite remanences clearly postdate main Himalayan folding (F1) evidencing that F1 phases stopped before metamorphic cooling. East of the Yadong Graben scatter of remanence directions may indicate that significant small-scale deformation continued after cooling in this region. In a number of areas along the Himalayan arc in situ directions show distinct small circle distributions which document large-scale tilting processes after cooling or synchronous to cooling. Application of small circle methods is essential for analysis and interpretation. Associated tilt axes predominantly show an E-W trend parallel to the main Himalayan structures; however, some are oriented closer to N-S. It is therefore likely that these late-orogenic tiltings are related to the widespread formation of the Miocene North-Himalayan gneiss domes. In the context of a partially molten middle crust beneath the Tibetan Plateau the domes could be an expression of channel flow towards frontal regions of high erosion. Paleomagnetism is a potential tool to "map" doming structures in the crust also in regions without visible indications at surface.
DE: 1525 Paleomagnetism applied to tectonics: regional, global
DE: 1533 Remagnetization
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting