[1] A range of ages have been proposed for the timing of India-Asia collision; the range to some extent reflects different definitions of collision and methods used to date it. In this paper we discuss three approaches that have been used to constrain the time of collision: the time of cessation of marine facies, the time of the first arrival of Asian detritus on the Indian plate, and the determination of the relative positions of India and Asia through time. In the Qumiba sedimentary section located south of the Yarlung Tsangpo suture in Tibet, a previous work has dated marine facies at middle to late Eocene, by far the youngest marine sediments recorded in the region. By contrast, our biostratigraphic data indicate the youngest marine facies preserved at this locality are 50.6-52.8 Ma, in broad agreement with the timing of cessation of marine facies elsewhere throughout the region. Double dating of detrital zircons from this formation, by U-Pb and fission track methods, indicates an Asian contribution to the rocks thus documenting the time of arrival of Asian material onto the Indian plate at this time and hence constraining the time of India-Asia collision. Our reconstruction of the positions of India and Asia by using a compilation of published palaeomagnetic data indicates initial contact between the continents in the early Eocene. We conclude the paper with a discussion on the viability of a recent assertion that collision between India and Asia could not have occurred prior to ∼35 Ma.

S U M M A R YSeventeen sites were drilled from ca. 53 Ma old mafic dykes intruded in the Linzizong Formation of the Linzhou Basin for palaeomagnetic studies. From 10 sites a higher coercivity component demagnetized between 20 and 100 mT could be isolated. Detailed rockmagnetic analyses reveal Ti-rich titanomagnetite as the remanence carrier, which indicates that the rock is not much altered and the remanent magnetization is likely of primary origin. This is supported by a positive fold test. Tilt correction could be performed by the bedding of overlying fluvial lacustrine sediments and tilt angles of ignimbrite columns post-dating the dyke emplacement. Bedding corrected directions give an overall mean direction of D/I = 12.3 • /27.2 • (α 95 = 10.6 • , k = 21.7, N = 10) corresponding to a palaeolatitude of 14.4 • ± 5.8 • N. Comparison with previous Cretaceous data mainly from the Takena Formation yields a stable position of the Lhasa terrane during Cretaceous and Early Eocene. The difference between expected palaeolatitudes determined from the APWP of Eurasia and observed ones reveals relative northward movement of the Lhasa terrane of ca. 1847 ± 763 km since early Eocene. This is attributed to indentation of India into Asia and implies a considerable amount of north-south crustal shortening. Together with the extent of 'Greater India', we can derive an age for the India-Asia collision between ca. 53-49 Ma with a 95 per cent confidence limit of ± 6 Ma.

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Palaeomagnetic, rock magnetic and geochronological investigations were carried out on the Abor volcanics of Arunachal Pradesh, NE India. A Late Palaeozoic formation age for part of the Abor volcanics cannot be excluded based on K–Ar whole rock dating. Low-temperature thermochronometers – zircon (U–Th)/He and fission track analyses – yield a maximum burial temperature of c. 150–170 °C during Late Miocene. ZFT thermochronology of the Yinkiong and Miri Fms. indicates a post-Paleocene and post-Jurassic deposition age, respectively. This infers that the volcanic rocks intercalating or intruding them are not part of the Late Palaeozoic sequence but represent one or more, latest Cretaceous to Tertiary event(s). Therefore the Abor volcanics are connected to at least two separate events of volcanism. From palaeomagnetic sites, two characteristic magnetic remanence components were separated: a low-coercivity-component demagnetized below 20 mT and a high-coercivity-component demagnetized between 15 and 100 mT. Fold tests support a secondary origin of both components. Thermochronological and rock magnetic analyses indicate a low-grade overprint event between India–Asia collision and Miocene, which probably represents the time of remanence acquisition. The high-coercivity-component shows a trend of clockwise declinations, which is likely related to vertical-axis rotations of the eastern Himalayas due to eastward extrusion of the Tibetan Plateau.

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S U M M A R YRock magnetic analyses were performed on basaltic-andesitic dyke samples from the Lhasa Block, which were previously used for palaeomagnetic investigations of the India-Asia collision. The aim of the study is to prove whether antipodal directions separated in these samples represent different polarities of the Earth's magnetic field or a self-reversal magnetization. Reflected light microscopy, domain pattern observations and energy dispersive X-ray analyses revealed two generations of titanomagnetite (TM) grains, one consisting of largesized (∼20 µm to more than 100 µm) and inhomogeneous Ti-poor TM, and another with small (smaller than ∼10 µm) and rather homogeneous Ti-rich TM grains. Partial thermoremant magnetization experiments and reflected light microscopy show that magnetostatic or superexchange interaction between these two phases is unlikely. Temperature dependence of saturation magnetization and low-temperature curves of isothermal remanent magnetization disprove a possible N-type behaviour of the samples. A self-reversal due to ionic reordering can be ruled out as it requires a high degree of oxidation of the Ti-rich TMs, which was not observed. In summary, a self-reversal magnetization is very unlikely in the studied dyke samples, and therefore the observed antipodal directions most probably represent a record of different polarity epochs of the Earth's magnetic field. This conclusion implies that the time of remanence acquisition in the studied dykes was sufficiently long to average out paleosecular variation, supporting the significance and reliability of the palaeomagnetic results.Key words: Paleomagnetism applied to tectonics; Reversals: process, timescale, magnetostratigraphy; Rock and mineral magnetism. I N T RO D U C T I O NThe evolution of the Himalaya-Tibetan orogen is of particular interest for understanding continental collision processes causing crustal uplift and associated climate change. Palaeomagnetism can provide direct constraints for the age of the India-Asia collision by comparing palaeolatitudes of the pre-collisional continental margins of India and Eurasia. The extent and latitudinal position of the passive northern Indian margin was determined by Patzelt et al. (1996) and recently confirmed by Yi et al. (2011). Some earlier results from the Lhasa Block (Westphal et al. 1983; Achache et al. 1984;Chen et al. 1993), which formed the southern rim of Eurasia before collision, were published in the 1980s and 1990s. A recent paper of Aitchison et al. (2007), postulating a 34 Ma age for the India-Asia continental collision, revived an intensive discussion on the evolution of the collision and stimulated several new palaeomagnetic studies on the Lhasa Block (Chen et al. 2010;Dupont-Nivet et al. 2010;Liebke et al. 2010;Sun et al. 2010;Tan et al. 2010). The new results largely improved the database and allowed a more significant determination of the position of Eurasia's southern margin. A compilation of palaeolatitudes supports a collision age between about 60 and 48 Ma. Alm...

Summary Knowing the pre-collisional extent of the northern Indian Plate margin (‘Greater India’) is vital to understanding the tectonic evolution of the India-Asia collision and the formation of the Himalayan-Tibetan orogen. However, suitable geological units for paleomagnetic investigations along the Himalayan belt are limited, which makes it difficult to reconstruct Greater India during the pre-collisional period in Late Cretaceous to Paleogene. Often the paleomagnetic results from the Zongpu Formation at Gamba in southern Tibet (∼88.5° E) were used for estimates of Greater India, but their validity was recently questioned. As a contribution to closing the data gap, we performed a paleomagnetic study of the Paleocene/Lower Eocene Dibling limestone (DL) in the western Tethyan Himalaya of Zanskar (34.0° N/76.6° E). The results from 27 sites revealed a well grouping (k = 71.7) syntectonic magnetization with best grouping at 52 per cent unfolding. The remagnetization of the DL was acquired shortly after ∼54 Ma, at the latest at ∼49 Ma, and is probably carried by fine-grained magnetite formed during the early orogenic phase. Assuming proportional tilting of the fold limbs, the corresponding paleolatitude of 11.8 ± 2.4 ° N suggests a maximum Greater India extent of 810 ± 420 km and a first continental contact with the southern Eurasian margin at ∼12° N in the western part of the suture zone. The tectonostratigraphic equivalence of the DL with the Zongpu Formation at Gamba and a great similarity in their magnetic properties supports a secondary origin of the Gamba results. Through understanding the mechanism of remagnetization in the DL, an early orogenic remanence acquisition is also indicated for the Zongpu Formation, and thus the Gamba results deserve further credit for Greater India reconstructions. However, we notice a large inconsistency of the available Late Cretaceous and Paleogene paleolatitude data from the Tethyan Himalaya by up to ∼20°, corresponding to differences of up to ∼2000 km in the size of Greater India. These discrepancies require further paleomagnetic work in the Tethyan Himalaya, and in particular we recommend comparative studies at same locations and of same units.