HR: 17:30h
AN: T24A-07    [Abstracts]
TI: Timing of Deformation and Crustal Structure of the North Himalaya Constrained by 45 Ma Granitoid Plutonism
AU: * Aikman, A B
EM: amos.aikman@anu.edu.au
AF: Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 Australia
AU: Harrison, T M
EM: director.rses@anu.edu.au
AF: Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200 Australia
AU: Lin, D
EM: dinglin@mail.igcas.ac.cn
AF: Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing, ACT 100029 China
AB: Models for the development of the Himalayan fold and thrust belt (HFTB) depend on knowledge of the timing and style of deformation within and between major litho-tectonic units. Furthermore, hypotheses for the evolution of the Indo-Asia collision make widely varying predictions about the structure of the Himalayan crust. A persistent obstacle in assessing such models has been the paucity of data in the ca. 55-25 Ma period following initiation of collision. Studies of North Himalayan granitoid plutons constrain the timing of deformation in the structurally highest and inferred earliest accreted surface- outcropping segment of the HFTB and provide insight into the crustal structure of the northern Himalaya. The North Himalaya (NH) comprise a sequence of predominantly low-grade metasediments that originated as passive margin deposits north of the Indian Shield prior to closure of the Tethys Ocean. The sequence is bounded to the south by the South Tibetan Detachment (STD), a segmented normal- sense movement zone cropping out along the peaks of the high Himalaya, and to the north by the Indus Tsangpo Suture (ITS), which marks the surface boundary between rocks of Indian and Eurasian continental affinity. The Dala granites (DG) outcrop in elliptical patterns within greenschist facies metasediments of the central NH at ca. E92°. Structural mapping indicates the surrounding country rocks experienced at least 50% strain in the form of east-west trending folds prior to intrusion of the DG. Crystallization of the DG has been dated by SHRIMP U-Pb geochronology of zircon at 45±2.9 Ma (MSWD = 2.7). Biotite K-Ar and K-feldspar ⁴⁰Ar/³⁹Ar thermochronology indicate that the DG cooled to ambient upper crustal temperatures (ca. 300°C) by ≤slant42 Ma, and remained isothermal until their rapid exhumation at ca. 15 Ma. Neither the DG, nor their andalusite-hornfels contact aureoles show evidence of significant deformation. To our knowledge, the DG are the largest known Eocene granitoids anywhere in the Himalayan chain and provide the first constraint on the timing of bulk shortening in the eastern NH (i.e.~>45 Ma). The Gangdese Batholith (GB) is an Andean complex which formed above a north-dipping subduction zone on the southern edge of Eurasia. The DG are situated ca. 100 km south of the ITS, however they are geochemically similar to the GB. Zircon and monazite saturation temperatures are calculated at 765 ±15°C and 768±15°C respectively, but single-grain crystallisation temperatures (estimated from zircon Ti thermometry) range from 650-850°C, with no obvious age correlation. This range of single-grain temperatures may be explained in conjunction with Sr-Nd isotopic measurements indicating that the DG formed by mixing of a mafic magma with a component derived from Paleozoic or older crustal material, such as Indian cratonic basement. Granitic magmatism has been documented in the GB as young as 16-10 Ma, combined, these data strongly suggest that the detachment point of Indian lithosphere was situated at least 100 km south of the present-day ITS shortly after the end of oceanic subduction, and cannot have advanced more than ca. 100 km northwards between the mid-Eocene and the late-Miocene. Furthermore, these data argue strongly against models requiring that the Indian lithosphere be substantially underthrust beneath the southern Tibetan Plateau, either as a means of thickening the crust, or providing insulation to promote development of a partially molten layer.
DE: 1037 Magma genesis and partial melting (3619)
DE: 1099 General or miscellaneous
DE: 1199 General or miscellaneous
DE: 8100 TECTONOPHYSICS
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: Fall Meeting 2005