No abstract

No abstract

The Pamirs represent the indented westward continuation of the northern margin of the Tibetan Plateau, dividing the Tarim and Tajik basins. Their evolution may be a key factor influencing aridification of the Asian interior, yet the tectonics of the Pamir Salient are poorly understood. We present a provenance study of the Aertashi section, a Paleogene to late Neogene clastic succession deposited in the Tarim basin to the north of the NW margin of Tibet (the West Kunlun) and to the east of the Pamirs. Our detrital zircon U‐Pb ages coupled with zircon fission track, bulk rock Sm‐Nd, and petrography data document changes in contributing source terranes during the Oligocene to Miocene, which can be correlated to regional tectonics. We propose a model for the evolution of the Pamir and West Kunlun (WKL), in which the WKL formed topography since at least ~200 Ma. By ~25 Ma, movement along the Pamir‐bounding faults such as the Kashgar‐Yecheng Transfer System had commenced, marking the onset of Pamir indentation into the Tarim‐Tajik basin. This is coincident with basinward expansion of the northern WKL margin, which changed the palaeodrainage pattern within the Kunlun, progressively cutting off the more southerly WKL sources from the Tarim basin. An abrupt change in the provenance and facies of sediments at Aertashi has a maximum age of 14 Ma; this change records when the Pamir indenter had propagated sufficiently far north that the North Pamir was now located proximal to the Aertashi region.

The plutonic rocks of the Antarctic Peninsula magmatic arc form one of the major batholiths of the circum-Pacific rim. The Antarctic Peninsula batholith is a 1350 km long by < 210 km wide structure which was emplaced over the period -240 to 10 Ma, with a Cretaceous peak of activity that started at 142 Ma and waned during the Late Cretaceous. Early Jurassic and Late Jurassic-Early Cretaceous gaps in intrusive activity probably correspond to episodes of arc compression. In a northern zone of the Antarctic Peninsula, the batholith intrudes Palaeozoic-Mesozoic low-grade meta-sedimentary rocks, and in a central zone it intrudes schists and ortho-and paragneisses which have Late Proterozoic Nd model ages and were deformed during Triassic to Early Jurassic compression. In a southern zone the oldest exposed rocks are Permian sedimentary rocks and deformed Jurassic volcanic and sedimentary rocks. All these pre-batholith rocks formed a belt of relatively immature crust along the Gondwana margin. With few exceptions, Jurassic plutons crop out only within the central zone: many are peraluminous, having 'S-like' mineralogies and relatively high 87 Sr/ 86 Sr i . They are considered to consist largely of partial melts of upper crust schists and gneisses and components of mafic magmas that caused the partial fusion. By contrast, Early Cretaceous plutons crop out along the length of the batholith. Few magma compositions appear to have been affected by upper crust, the bulk being compositionally independent of the type of country rock they intrude. They are dominated by metaluminous, calcic, Si-oversaturated, 1-type granitoid rocks with relatively low "Sr/^Sr,, intermediate-silicic compositions (< 5 % MgO). We interpret these to represent partial melts of basic to intermediate, igneous, locally garnet-bearing, lower crust. Contemporaneous mafic magmas (e.g. syn-plutonic dykes) form a more alkaline, Si-saturated series having higher 143 Nd/ 144 Nd at the same 87 Sr/ 86 Sr than the intermediate-silicic series, to which they are not petrogenetically related. The change from limited partial fusion of upper crust in Jurassic times to widespread partial fusion of lower crust in Early Cretaceous times is considered to be a result of an increasing volume of basaltic intrusion into the crust with time.

The Himalayan orogen provides a type example on which a number of models of the causes and consequences of crustal deformation are based and it has been suggested that it is the site of a variety of feedbacks between tectonics and erosion. Within the broader orogen, fluvial drainages partly reflect surface uplift, different climatic zones and a response to crustal deformation. In the eastern Himalaya, the unusual drainage configuration of the Yarlung Tsangpo-Brahmaputra River has been interpreted either as antecedent drainage distorted by the India-Asia collision (and as such applied as a passive strain marker of lateral extrusion), latest Neogene tectonically-induced river capture, or glacial damming-induced river diversion events.Here we apply a multi-technique approach to the Neogene paleo-Brahmaputra deposits of the Surma Basin (Bengal Basin, Bangladesh) to test the long-debated occurrence and timing of river capture of the Yarlung Tsangpo by the Brahmaputra River. We provide U-Pb detrital zircon and rutile, isotopic (Sr-Nd and Hf) and petrographic evidence consistent with river capture of the Yarlung Tsangpo by the Brahmaputra River in the Early Miocene. We document influx of Cretaceous-Paleogene zircons in Early Miocene sediments of the paleo-Brahmaputra River that we interpret as first influx of material from the Asian plate (Transhimalayan arc) indicative of Yarlung Tsangpo contribution. Prior to capture, the predominantly Precambrian-Paleozoic zircons indicate that only the Indian plate was drained. Contemporaneous with Transhimalayan influx reflecting the river capture, we record arrival of detrital material affected by Cenozoic metamorphism, as indicated by rutiles and zircons with Cenozoic UPb ages and an increase in metamorphic grade of detritus as recorded by petrography. We interpret this as due to a progressively increasing contribution from the erosion of the metamorphosed core of the orogen. Whole rock Sr-Nd isotopic data from the same samples provide further support to this interpretation. River capture may have been caused by a change in relative base level due to uplift of the Tibetan plateau. Assuming such river capture occurred via the Siang River in the Early Miocene, we refute the "tectonic aneurysm" model of tectonic-erosion coupling between river capture and rapid exhumation of the eastern syntaxis, since a time interval of at least 10 Ma between these two events is now demonstrated. This work is also the first to highlight U-Pb dating on detrital rutile as a powerful approach in provenance studies in the Himalaya in combination with zircon U-Pb chronology.

New Rb‐Sr and K‐Ar geochronological data are presented for the majority of known pre‐Cenozoic outcrops in Thurston Island, the Jones Mountains, and the western Eights Coast, which collectively represent the basement geology of the Thurston Island crustal block of West Antarctica. Almost all are of calc‐alkaline igneous or metaigneous rocks, and indicate long‐standing proximity to a magmatic arc. The observable history began with Late Carboniferous (309±5 Ma) emplacement of mantle‐derived orthogneiss precursors in eastern Thurston Island. Nd model ages from these and later igneous rocks suggest that the underlying crust is no older than about 1200–1400 Ma throughout the area. A variety of cumulate gabbros was emplaced soon after gneiss formation, followed by crust‐contaminated diorites that have Triassic mineral cooling dates of 240–220 Ma. In the nearby Jones Mountains, the oldest exposed rock is a muscovite‐bearing granite with an Early Jurassic age of 198±2 Ma; its initial 87Sr/86Sr ratio of 0.710 and ϵNdt values of −5 to −7 indicate either anatexis or, at least, a high degree of crustal input during magma genesis. This belongs to a suite of such granites known throughout the Antarctic Peninsula and related to earliest rifting of the Gondwana supercontinent. The subsequent evolution of the Thurston Island area was dominated by I‐type magmatism, apparently in two major episodes at 152–142 Ma (Late Jurassic granites) and 125–110 Ma (Early Cretaceous bimodal suite). Most of these magmas had initial 87Sr/86Sr ratios of 0.705–0.706 and ϵNdt values of +2 to −4 and were derived from slightly enriched mantle or from juvenile lower crust. They are thought to signify subduction of Pacific Ocean floor as in the adjacent parts of West Antarctica, although the Late Jurassic episode was of greater intensity in Thurston Island than elsewhere. The Cretaceous magmatism was intense and of Andean‐type. Between 100 and 90 Ma, volcanism in the Jones Mountains became predominantly silicic, with increasing incorporation of crustal components (initial 87Sr/86Sr ratios of 0.706–0.709 and ϵNdt values of −3 to −6), as subduction‐related magmatism ceased in this part of the margin.

12The Moine Thrust Zone in the Scottish Highlands developed during the Scandian 13Event of the Caledonian Orogeny, and now forms the boundary between the 14 Caledonian orogenic belt and the undeformed foreland. The Scandian Event, and the 15 formation of the Moine Thrust Zone, have previously been dated by a range of 16 isotopic methods, and relatively imprecise ages on a suite of alkaline intrusions 17 localised along the thrust zone have provided the best age constraints for deformation. 18Recent BGS mapping has improved our understanding of the structural relationships 19 of some of these intrusions, and this work is combined with new U-Pb dates in this 20 paper to provide significantly improved ages for the Moine Thrust Zone. Our work 21shows that a single early intrusion (the Glen Dessarry Pluton) was emplaced within 22 the orogenic belt to the east of the Moine Thrust Zone at 447.9 ± 2.9 Ma. A more 23 significant pulse of magmatism centred in the Assynt area, which temporally 24 overlapped movement in the thrust zone, occurred at 430.7 ± 0.

No abstract