Rasoul B. Sorkhabi

Fission-track and 40Ar39Ar evidence for episodic denudation of the Gangotri granites in the Garhwal Higher Himalaya, India

Abstract In order to document quantitatively the cooling and denudation history of the Higher Himalayan granites bordering the Tethyan sedimentary zone, fission-track (FT) apatite and 40 Ar 39 Ar mica ages have been determined on the Gangotri leucogranites and biotite granites in the Garhwal region of India. Gangotri is the source area of the Ganges River and lies in the midst of the highest Himalayan peaks in India. A total of 15 apatite ages from a vertical profile (2580–4370 m) on the Gangotri granites yields FT ages in the range of 1.5 ± 0.6 to 2.4 ± 0.5 Ma, indicating that the rock column with a relief of ∼1800 m cooled through 130 ± 10°C within only ∼1 million years during the Late Pliocene. An average denudation rate of ∼2 mm/yr is estimated for the past 2.4 million years. From the Gangotri granites, we also report a muscovite 40 Ar 39 Ar age of 17.9 ± 0.1 Ma and a biotite age of 18.0 ± 0.1 Ma. These reflect cooling of the rocks through 300–350°C, probably related to an Early Miocene pulse of denudation caused by a basement-cover detachment (the Martoli Normal Fault) above the leucogranites. Time-temperature pathways indicate that the cooling of the rocks in the Late Pliocene-Quaternary was five to six times the magnitude of cooling between 18 and 2 Ma, indicating a distinct pulse of rapid denudation in the Late Pliocene-Quaternary. We interpret these young apatite ages and fast denudation as a geomorphic response (increased erosion and cooling) of the rocks to a major pulse of tectonic uplift in the Higher Himalaya shortly before 2.4 Ma. The effect of climatic cooling on this denudation is considered secondary to the role of tectonic forcing, and indeed produced a positive feedback to the primary cause. Although our study is confined to the Garhwal region, it is probable that other granitic bodies of the Higher Himalaya bordering the Tethyan sedimentary rocks, and forming the loftiest summits in the Himalaya, have also experienced episodic denudation — one major pulse in the Early Miocene, which was mainly tectonic denudation, and another in the Late Pliocene-Quaternary, which was mainly erosional. The latter is well recorded by apatite FT data, and is consistent with the hypothesis that rapid uplift and denudation of the Himalayan rocks may have influenced the initiation of the ice ages in the northern hemisphere.

Time-temperature pathways of himalayan and trans-himalayan crystalline rocks: A comparison of fission-track ages

Abstract Fission-track (FT) dates of apatate and zircon recording, respectively, paleo-temperatures of 100–120°C and ∼230°C provide a useful tool for understanding the thermotectonic history of the Himalaya, which is the worlds loftiest and most dynamic orogenic belt, brought about by the continent-to-continent collision of the Indian and Asian plates. Systematic FT ages are now available for the crystalline rocks of the Higher Himalaya and Trans-Himalaya in Pakistan and N.W. India, and some FT ages have been reported from southern Tibet, the Karakorum and the Kunlum. These FT data are synthesized and discussed here in order to assess the contribution of FT thermochronology to Himalayan geology. FT zircon and apatite ages from the Higher Himalayan Crystalline rocks are confined to the Neogene. For example, in the Zanskar region of N.W. India and the Babusar-Kaghan region of Pakistan, zircon ages are 13–17 Ma and apatate ages range from 4–10 Ma (mostly ∼6 Ma) indicating an exhumation of at least 8 km for these rocks since the Middle Miocene and average unroofing rates of ∼ 6 mm yr −1 . In the northwestern Himalaya, overall the FT ages become younger towards the Nanga Parbat Massif, which is the promontory of the Indian plate and has experienced a rapid Quaternary uplift and exhumation. Other areas of young uplift in the Himalayan include antiformal domes in Zanskar, such as the Kishtwar Window. The bulk of the Cretaceous-Paleocene Trans-Himalayan crystalline rocks, which are situated to the north of the Indus-Tsangpo Suture Zone and were generated from the consumption of the Tethyan ocean beneath Asia, seem to have cooled rapidly through paleo-temperatures of 200–500°C in the Eocene as a result of uplift and erosion that affected the Trans-Himalayan Batholith after the India-Asia collision. Apatite ages from these rocks in western Kohistan, southern Ladakh, and Lhasa are Early-Middle Miocene. In the Karakorum (western Tibet) and the Kunlun (northern Tibet), Miocene apatite ages demonstrate young tectonics of the ranges beyond the Himalaya related to the India-Asia convergence.

Cooling age record of domal uplift in the core of the Higher Himalayan Crystallines (HHC), southwest Zanskar, India

The cooling and tectonic history of the Higher Himalayan Crystallines (HHC) in southwest Zanskar (along the Kishtwar-Padam traverse) is constrained by K-Ar biotite and fission-track (FT) apatite and zircon ages. A total of nine biotite samples yields ages in the range of 14–24 Ma, indicating the post-metamorphic cooling of these rocks through ∼ 300°C in the Miocene. Overall, the ages become younger away from the Zanskar Shear Zone (ZSZ), which marks the basement-cover detachment fault between the HHC and the Tethyan sedimentary zone, towards the core of the HHC. The same pattern is also observed for the FT apatite ages, which record the cooling of the rocks through ∼ 120°C. The apatite ages range from 11 Ma in the vicinity of the ZSZ to 4 Ma at the granitic core of the HHC. This pattern of discordant cooling ages across the HHC in southwest Zanskar reveals an inversion of isotherms due to fast uplift-denudation (hence cooling) of the HHC core, which is, in turn, related to domal uplift within the HHC. The Chisoti granite gneiss is the exposed domal structure along the studied traverse. Cooling history of two granite gneisses at the core of the HHC is also quantified with the help of the biotite, zircon and apatite ages; the time-temperatures thus obtained indicate a rapid pulse of cooling at ∼ 6 Ma, related to accelerated uplift-denudation of the HHC core at this time. Long-term denudation rates of 0.5–0.7 mm/yr are estimated for the high-grade rocks of the Higher Himalaya in southwest Zanskar over the past 4.0–5.5 m.yr.

“Roof of the Earth” Offers clues about how our planet was shaped

The Tibetan Plateau and its surrounding mountains—the Himalaya on the south, the Kun Lun on the north, and the Pamir and Karakoram on the west—comprise the largest, loftiest, and youngest highland on the Earth (Figure 1). All of the worlds peaks higher than 7000 m (with the lone exception of Anchomuma in the Bolivian Andes) as well as the largest concentration of alpine glaciers are found in this vast region that geographers have dubbed “the Roof of the World.” The Himalaya-Tibet region is virtually the “water tower” of Asia: it supplies freshwater for more than one-fifth of the worlds population, and it accounts for a quarter of the global sedimentation budget. Since 1985, a series of international workshops have been held to discuss the latest geoscientific research in the Himalaya-Karakoram-Tibet region. The eleventh workshop was held in Flagstaff, Ariz., from April 29 to May 1, 1996. It was the first to be held in North America. Nearly 120 people from 12 different countries attended.