Nd isotopic compositions of the Tethyan Himalayan Sequence in southeastern Tibet

Abstract

The Himalayan orogen consists of three major lithologic units that are separated by two major north-dipping faults: the Lesser Himalayan Sequence (LHS) below the Main Central Thrust (MCT), the Greater Himalayan Crystalline Complex (GHC) above the MCT, and the Tethyan Himalayan Sequence (THS) juxtaposed by the South Tibet Detachment fault (STD) over the GHC. Due to widespread metamorphism and intense deformation, differentiating the above three lithologic units is often difficult. This problem has been overcome by the use of Sm-Nd isotopic analysis. The previous studies suggested that the LHS can be clearly distinguished from the GHC and THS by their Nd isotope compositions. However, the lack of detailed and systematic Sm-Nd isotopic studies of the THS across the Himalaya in general has made differentiation of this unit from the nearby GHC impossible, as the two appear to share overlapping Nd compositions and model ages. To address this problem, we systematically sampled and analyzed Nd isotopes of the THS in southeastern Tibet directly north of Bhutan. Our study identifies two distinctive fields in a ε Nd -T DM plot. The first is defined by the ε Nd (210 Ma) values of −3.45 to −7.34 and T DM values of 1.15 to 1.29 Ga from a Late Triassic turbidite sequence, which are broadly similar to those obtained from the Lhasa block. The second field is derived from the Early Cretaceous meta-sedimentary rocks with ε Nd (130 Ma) values from −15.24 to −16.61 and T DM values from 1.63 to 2.00 Ga; these values are similar to those obtained from the Greater Himalayan Crystalline Complex in Bhutan directly south of our sampling traverse, which has ε Nd (130 Ma) values of −10.89 to −16.32 and Nd model ages (T DM ) of 1.73 to 2.20 Ga. From the above observations, we suggest that the Late Triassic strata of the southeast Tibetan THS were derived from the Lhasa block in the north, while the Early Cretaceous strata of the THS were derived from a source similar to the High Himalayan Crystalline Complex or Indian craton in the south. Our interpretation is consistent with the existing palaeocurrent data and provenance analysis of the Late Triassic strata in southeastern Tibet, which indicate the sediments derived from a northern source. Thus, we suggest that the Lhasa terrane and the Indian craton were close to one another in the Late Triassic and were separated by a rift valley across which a large submarine fan was transported southward and deposited on the future northern margin of the Indian continent.

Relicts of the Early Cretaceous seamounts in the central-western Yarlung Zangbo Suture Zone, southern Tibet

“…The accretionary prism is dominantly composed of the Triassic to Eocene sediments. However, in the eastern part of this suture, the Late Triassic Langjiexue Group is different from the strata of the accretionary prism, as these might have been derived from the Lhasa terrane to the north rather than the India to the south (Dai et al, 2008;. The Yarlung Zangbo ophiolitic belt consists of disrupted ophiolitic massifs, which represent remnants of part of Neo-Tethyan ocean lithosphere.…”

Geologic correlation of the Himalayan orogen and Indian craton: Part 2. Structural geology, geochronology, and tectonic evolution of the Eastern Himalaya

“…As the normal stratigraphic sequence was severely modifi ed by the Paleogene shortening, the South Tibetan detachment and Main Central thrust may have variable older-over-younger and younger-overolder relationships across the faults. In southern Tibet directly north of Bhutan, the South Tibetan detachment places Cretaceous strata over Greater Hima layan Crystalline Complex units (Pan et al, 2004;Dai et al, 2008), whereas in Bhutan to the south, the South Tibetan detachment places Neoproterozoic and Cambrian strata over the Greater Himalayan Crystalline Complex. This relationship suggests that the South Tibetan detachment cuts up section of its hanging-wall strata in its northward transport direction, and this relationship is inconsistent with normal-fault but consistent with thrust-fault geometry.…”

Provenance of the Liuqu Conglomerate in southern Tibet: A Paleogene erosional record of the Himalayan–Tibetan orogen

“…However, recent studies have reported southward paleocurrent directions within the Langjiexue Group (Li et al, 2003). Whole-rock ε Nd (T) values (Dai et al, 2008) and detrital zircon ages obtained from these rocks (Aikman et al, 2008) differ from those of the Tethyan Himalaya (Gehrels et al, 2003;Hu et al, 2010) and the Xigaze forearc basin (Wu et al, 2010). This ambiguity has resulted in debate as to whether the Langjiexue Group should be assigned to the Tethyan Himalaya of the Indian plate Pan and Ding, 2004;Aikman et al, 2008), the sedimentary sequences of the Asian plate (Dai et al, 2008), or the Yarlung-Zangbo mélange (Yin, 2006;Aitchison et al, 2007a).…”