STAMP ACROSS THE EASTERN HIMALAYA: AGES AND STAGES OF BURIAL, TRANSFORMATION AND EARLY EXHUMATION

Structural and metamorphic pressure-temperature-time history data together provide insight into the tectonic mechanisms by which crustal rocks are buried, transformed and transported in plate collision zones. Rates of prograde and retrograde metamorphism, especially in high-grade metamorphic rocks, are commonly determined from differences in temperature and time recorded by minerals that grow at different stages in the metamorphic evolution and/or by minerals that have different closure temperatures to the diffusion of their daughter products.

The Greater Himalayan Sequence (GHS) forms the high-grade metamorphic core of the Himalayan orogen. The mechanisms by, rates at, and timescales over which these rocks were buried, transformed and exhumed within an overall compressional setting are still much debated, yet are important for providing insight into middle and lower crustal processes in continent-continent collision zones. New monazite U-Th-Pb and muscovite Ar/Ar data from the highest structural levels of the GHS Arunachal Pradesh, in conjunction with previously reported data from Sikkim and Bhutan, suggest that the timing of peak metamorphism and rates of exhumation-related cooling appear to get younger further eastwards in the orogen. Monazites associated with peak metamorphism and/or melting reactions yield ages of ca. 16-11 Ma in Arunachal, compared with 15-13 Ma in NW Bhutan¹ and 26-23 Ma in N. Sikkim². Muscovite Ar/Ar ages mirror this younging trend, yielding ca. 7 Ma in Arunachal Pradesh, compared with 13-11 Ma in Bhutan³ and 13-12 Ma in Sikkim⁴.

We have combined recent advances in petrogenetic modeling, trace element fingerprinting techniques, and diffusion modelling to yield robust insights into the evolution and cooling history of the GHS in the Eastern Himalaya. These data suggest a higher degree of complexity in the architecture and mechanisms of formation and exhumation of the eastern Himalayan GHS than is reported for the central portions of the orogen, with corresponding implications for formation and exhumation mechanisms.

References: ^1. Warren et al., 2012, Chem. Geol. ^2. Rubatto et al., 2012, CMP. ^3. Kellett et al., 2009, Lithosphere. ^4. Kellett et al., 2013, Tectonics.