Toothpaste & Tibet: deformation, anatexis and doming in the eastern Tethyan Himalaya

A.B. Aikman

Deformation, exhumation and isostasy are arguably three of the most important processes governing the geological evolution of continents. They control not only the distribution and style of topography, outcrop and continental sedimentation, but are also thought to influence the behavior of the atmospheric and oceanic domains as well (Crowley and Burke, 1998). Studying these processes by definition considers time scales that are difficult to measure directly. It is therefore judicious to examine cases where they are actively occurring, and at rates which maximize the signal to noise ratio. To this end, active orogenic zones provide the acme opportunity.

The Himalayan orogen is part of the Alpine-Himalayan chain, the world's largest active orogenic belt. Nearly a century of research (e.g. Argand, 1924) has yielded a plethora of evolutionary models, however many important aspects this disquisition still defy consensus. The eastern THS have received relatively little attention with respect to frontal parts of the orogen. Our studies in this area are already providing valuable new insights into outstanding problems.

The Tethyan Himalayan Series (THS) is a thick sequence of generally low grade metasediments outcropping along the northern margin of the Himalayan arc. East of ca. 90_E, the THS is bounded to the south by the Greater Himalayan Crystallines (GHC) and to the north by the Indus-Tsangbo Suture (ITS). The sequence comprises the most northerly segment of the Himalayan fold and thrust belt, and is thought to have originated as continental margin deposits on the northern margin of the Indian Shield prior to closure of the Tethys Ocean (e.g. Yin and Harrison , 2000).

Figure 1: Digital Elevation Model of the Himalaya and southern Tibet , derived from the USGS SRTM data set.

 

Figure 2: Digital Elevation Model the Hiamalaya at ca. 92°E, derived from the USGS ETOPO2 data set.

Tectono-structural and metamorphic histories recorded in the Tethyan metasediments and North Himalayan Domes provide a window into the evolution of the North Himalayan crust and to the early history of Indo-Asian collision. Furthermore, the THS may also be regarded as part of the roof zone of a proposed Himalayan Orogenic Channel (Lee et al., 2000; Aikman et al., 2004; Lee et al., 2004).

We document the first evidence of Eocene leucogranite plutonism (ca. 45 Ma) along the Himalayan arc. We interpret this relationship as placing a lower bound of ca. 45 Ma on the timing of south vergent deformation in the THS. Geochemical data from the Yala-Xiangbo Igneous Complex suggest that conditions under which Miocene partial melting occurred in the central eastern-Tethys were distinct from those documented for other High Himalayan Leucogranites (HHL) or North Himalayan Granites (NHG). Isotopic and geochronological data suggest that the eastern Tethyan middle crust includes material distinct from that seen elsewhere in the Himalaya . We interpret our results as consistent with underthrusting of parts of the Eurasian forearc up to 120 km beneath the eastern Tethys, by movement along the Great Counter Thrust (GCT).

References

Aikman, A., Harrison, T., and Lin, D. (2004). Preliminary results from the yala-xiangbo dome, se tibet . Himalayan Journal of Sciences, 2(4):91.

Argand, E. (1924). La tectonique de l'asie. In International Geological Congress Report Session 13/1.

Crowley , T. and Burke, K., editors (1998). Tectonic Boundary Conditions for Climate Reconstructions, volume, Oxford monographs on geology and geophysics No. 39. Oxford University Press.

Lee, J., Hacker, B., Dinklage, W., Wang, Y., Gans, P., Calvert, A., Wan, J., Chen, W., Blythe, A., and McClelland, W. (2000). Evolution of the kangmar dome, southern tibet ; structural, petrologic, and thermochronologic constraints. Tectonics, 19(5):872–895.

Lee, J., Hacker, B., and Wang, Y. (2004). Evolution of north himalayan gneiss domes: structural and metamorphic studies in mabja dome, southern tibet . Journal of Structural Geology, 12(2297-2316).

Yin, A. and Harrison, T. (2000). Geologic evolution of the himalayan-tibetan orogen. Annual Review of Earth and Planetary Sciences, 28:211–280.