FISSION-TRACK AND AR-40 AR-39 EVIDENCE FOR EPISODIC DENUDATION OF THEGANGOTRI GRANITES IN THE GARHWAL HIGHER HIMALAYA, INDIA/

In order to document quantitatively the cooling and denudation history of the Higher Himalayan granites bordering the Tethyan sedimentary zo ne, fission-track (FT) apatite and Ar-40/Ar-39 mica ages have been det ermined on the Gangotri leucogranites and biotite granites in the Garh wal region of India. Gangotri is the source area of the Ganges River a nd 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 Gangot ri 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 similar to 1800 m cooled through 130 +/- 10 degrees C within only similar to 1 million years during the Late Pliocene. An average denudation rate of similar to 2 mm/yr is estimated for the past 2.4 million years. From the Gango tri granites, we also report a muscovite Ar-40/Ar-39 age of 17.9 +/- 0 .1 Ma and a biotite age of 18.0 +/- 0.1 Ma. These reflect cooling of t he rocks through 300-350 degrees C, probably related to an Early Mioce ne pulse of denudation caused by a basement-cover detachment (the Mart oli Normal Fault) above the leucogranites. Time-temperature pathways i ndicate 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, in dicating a distinct pulse of rapid denudation in the Late Pliocene-Qua ternary. 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 consider ed secondary to the role of tectonic forcing, and indeed produced a po sitive feedback to the primary cause. Although our study is confined t o 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 d enudation - one major pulse in the Early Miocene, which was mainly tec tonic denudation, and another in the Late Pliocene-Quaternary, which w as mainly erosional. The latter is well recorded by apatite FT data, a nd is consistent with the hypothesis that rapid uplift and denudation of the Himalayan rocks may have influenced the initiation of the ice a ges in the northern hemisphere.