RESEARCH COMMUNICATIONS CURRENT SCIENCE, VOL. 84, NO. 11, 10 JUNE 2003 1454 Age of the Karakoram fault activation: 40 Ar– 39 Ar geochronological study of Shyok suture zone in northern Ladakh, India Rajneesh Bhutani 1, *, Kanchan Pande and Nikhil Desai 2 Planetary and Geo-Sciences Division, Physical Research Laboratory, Navrangpura, Ahmedabad 380 009, India 1 Department of Earth Sciences, Pondicherry University, Kalapet, Pondicherry, India 2 Department of Geology, MS University of Baroda, Vadodara, India Shyok volcanics, from the Shyok suture zone in northern Ladakh, ranging from basalts to andesites are analysed for 40 Ar– 39 Ar isotopic systematics by step heating experiment. All samples, collected along the Nubra river, in the vicinity of Karakoram fault zone, yielded disturbed age spectra, reflecting subsequent tectono-thermal events. However, consistency in the pattern of the age spectra, particularly at the low tem- perature steps, indicate a strong tectono-thermal event between ~ 10 to ~ 20 Ma ago. Mica-segregate from a sheared granite of Karakoram fault zone near village Murgi has yielded an excellent plateau age of 13.9 ± 0.1 Ma. This age of Karakoram fault activation explains the consistent but disturbed age spectra of Shyok volcanics within the vicinity of the fault zone. The Karakoram fault activation in Shyok suture zone is therefore synchronous with the extensional tectonic regime within the Tibetan plateau. NORTH of the Ladakh batholith is characterized by the linear volcanic belts, and ophiolitic mélange, which sepa- rate it from the Karakoram batholith in the north (Figure 1). The ophiolitic mélange and associated flyschoidal and molassic sediments are believed to be representing the line of a subduction named as the Shyok Suture 1–4 (Figure 1). The Shyok suture zone in northern Ladakh is a highly tectonized zone as evidenced by highly deformed vol- cano-sedimentary rocks and ophiolitic mélange in the Nubra-Shyok valley 4 . Lithologically there are six major units of the Shyok Suture, which are more or less found to be in the form of tectonic slices between Ladakh batho- lith and Karakoram batholith. Ophiolitic Mélange, which marks the suture, mainly consists of serpentinized ultra- mafics, and meta-sediments. Shyok volcanics, which are supposed to be similar to Chalt volcanics of Kohistan sector 4,5 are a very heterogeneous sequence comprising of basalts to andesites. At most of the places the volcanics are greenish gray, fine-grained and massive in nature. They are tectonically sandwiched between the underlying Saltoro Molasse and are characterized by red and green shales, siltstones, conglomerate and breccias – and the Ophiolitic mélange in the west of Karakoram fault. In the East of Karkaoram fault Shyok volcanics are exposed from Panamik to Tirit villages intercalated with the metasediments. Explosive volcanism also has been repo- rted from the Shyok–Nubra valley, represented by Khardung Volcanics that overlie Ladakh batholith 6 . A thick succession of rhyolite, ignimbrite, dacite, agglo merate and volcanic breccia is exposed near the village Khardung. Several models have been proposed to explain the evo- lution of the island-arc terrain and formation of the Shyok Suture within the India–Asia collision zone 4,7–12 . However, the debate on the age of the suture and whether it is older or younger than the Indus suture, as well as the mode of subduction/suturing has not yet been settled. Brookfield and Reynolds 7 , and Reynolds et al. 8 suggested that the Shyok suture did not close until Miocene and the Indus suture closed earlier in the Late Cretaceous. Coward and Butler 9 , Petterson and Windley 10 , Treloar et al. 11 , Searle et al. 12 and Upadhyay et al. 4 favoured an early closure of the Shyok suture in Cretaceous. Rai 13 argued against any subduction along the Shyok suture. The present 40 Ar– 39 Ar study of the Shyok volcanics from the vicinity of Karakoram fault zone indicates that Karakoram fault has played a crucial role in the evolution of Shyok suture zone to the present-day tectonic setting. Regional strike slip faults in Himalaya–Tibet orogenic system are vital in understanding the uplift, deformation and overall accommodation of ~ 2500 km of crust into it 14,15 . Karakoram fault at the western margin of the Tibe- tan plateau (Figure 1) has been traced for more than thousand kilometers starting from north-west Tibet and cutting across the suture in north-west Himalaya to the south Kailas thrust system 15,16 . The Karakoram fault sys- tem has been visualized in different ways by different workers, e.g. as the major boundary in the west along which the Tibetan block has been extruding towards the east 14 , as a major strike slip fault which facilitated the indentation of Pamirs in the north 17 . It is also thought to have played a crucial role in the internal deformation of the Himalayan arc 18,19 . Timing of its activation is crucial in understanding its implication and its role in the overall evolution of India–Asia collision zone. Various indirect geological methods have been used to know the timing of this fault. Since it cuts across the 18 Ma leucogranites in the Karakoram batholith, it is supposed to be younger than 18 Ma 7 . Yin et al. 20 and Murphy et al. 16 proposed that it did not get activated until 13 Ma in the southern end where it offsets the 13 Ma-old Kailas Thrust system. Yin et al. further noticed that it has varying timings and amount of net slip along the strike, which they attributed to the southward propagation of faulting. Here we present a direct estimate of age of Karakoram fault activation in northern Ladakh 15 . *For correspondence. (e-mail: bhutani@vsnl.net)