FURTHER PALEOMAGNETIC DATA FROM CHITRAL (EASTERN HINDUKUSH) - EVIDENCE FOR AN EARLY INDIA ASIA CONTACT

The Eastern Hindukush forms part of an elongate belt (''Central Domain '', collage of Cimmerian microcontinents) that encircles the northern part of the Indian subcontinent. A Gondwanan origin is commonly assume d for this belt, but a ''Laurasian'' origin for the Chitral region has been argued on palaeontological (Talent and Mawson, 1979) and palaeom agnetic (Klootwijk and Conaghan, 1979) grounds. The ''Laurasian'' view was based on a pilot study we undertook of Upper Devonian pisolitic i ronstones from a thrust sheet at Kuragh Spur in Chitral. Preliminary r esults showed a characteristic magnetization component [D = 318-degree s, I = 6.5-degrees, N = 7 (block samples), k = 14, alpha95 = 16.5-degr ees] indicating an equatorial palaeoposition. This component was thoug ht to be of primary origin and was interpreted in terms of a Late Devo nian ''Laurasian'' affinity of the Kuragh Spur rocks. This controversi al conclusion has been tested in the present more comprehensive study of the thrust pile of sedimentary rocks in the Reshun-Kuragh-Buni regi on of Chitral and the primary origin of the characteristic magnetizati on component refuted. Thermal demagnetization of 333 block samples fro m Middle to Upper Devonian variegated sediments, Permian quartz flysch , Permo-Triassic carbonates, and mid-Cretaceous redbeds showed two int erpretable components. A softer component of recent origin (A); and a harder characteristic component (B) of both normal and reverse polarit y whose mean direction [D = 314.1-degrees, I = 6.0-degrees, N = 4 (thr ust sheets), k = 198.2, alpha95 = 6.5-degrees] is comparable to the ch aracteristic component observed in our preliminary study. However, the universal presence of this component throughout the thrust pile prove s its overprint origin, which we attribute to initial India-Asia conta ct. Palaeomagnetic information pertinent to the controversy of a ''Lau rasian'' versus a Gondwanan origin of the Chitral region has not been obtained in this further study because primary magnetizations could no t be identified beyond doubt. Hence, we retract herewith our original conclusion of a Late Devonian ''Laurasian'' affinity of the Chitral re gion on the basis of the palaeomagnetic evidence. The secondary compon ent (B) comprises a suite of secondary magnetizations, acquired at equ atorial-to-low-northern palaeolatitudes, and is attributed to initial contact between Greater India and southern Asia. Component B has been observed previously in the Himalayan-Tibetan region, both north and so uth of the Indus-Tsangpo Suture zone. Identification is herein extende d to the Hindukush region north of the Northern Korhistan (or Shyok) S uture zone, which is a western continuation of the Indus-Tsangpo Sutur e. Comparison of this suite of collision-attributed equatorial palaeol atitude data from the India-Asia convergence zone with new palaeolatit ude constraints from the Ninetyeast Ridge on the northward movement of the Indian plate, contrained additionally by a recent minimal estimat e of the palaeogeographic northern extent of Greater India, indicates that initial contact between northwestern Greater India and southern A sia was established at, or before, the Cretaceous-Tertiary boundary. T he overprint origin of component B at about this time is further suppo rted by observations by Zeitler (1985) on rocks from the sampled area in Chital of partially reset zircon fission-track ages around 68-55 Ma . The NW-SE declination axis of component B indicates a 60-70-degrees counterclockwise rotation of the sampled thrust pile with respect to E urasia and a counterclockwise rotation between 10 and 30-degrees with respect to India. Some of the recent field components (A) show a compa rable rotation and indicate that the tectonic activity that led to the formation of the Hindukush-Pamir-Karakorum syntaxial zone has continu ed into recent times.