Jonathan C. Aitchison

When and where did India and Asia collide

Timing of the collision between India and Asia is the key boundary condition in all models for the evolution of the Himalaya-Tibetan orogenic system. Thus it profoundly affects the interpretation of the rates of a multitude of associated geological processes ranging from Tibetan Plateau uplift through continental extrusion across eastern Asia, as well as our understanding of global climate change during the Cenozoic. Although an abrupt slowdown in the rate of convergence between India and Asia around 55 Ma is widely regarded as indicating the beginning of the collision, most of the effects attributed to this major tectonic episode do not occur until more than 20 Ma later. Refined estimates of the relative positions of India and Asia indicate that they were not close enough to one another to have collided at 55 Ma. On the basis of new field evidence from Tibet and a reassessment of published data we suggest that continent-continent collision began around the Eocene/Oligocene boundary (∼34 Ma) and propose an alternative explanation for events at 55 Ma.

Tectonic accretion and underplating of mafic terranes in the Late Eocene intraoceanic fore-arc of New Caledonia (Southwest Pacific): geodynamic implications

This paper deals with the tectonic events that result in the accretion of mafic terranes in the fore-arc region and a close juxtaposition of ultramafic rocks, low grade and high-grade mafic terranes in many collisional orogens. The example is taken from New Caledonia where tectonic accretion, subduction, underplating and obduction of mafic terranes took place during the late Eocene in an intra-oceanic forearc setting. The late Eocene tectonic complex comprised three major terranes: an overlying ultramafic, mainly harzburgitic allochthon named the Ophiolitic Nappe, an intermediate mafic, mainly basaltic off-scraped melange, composed of kilometre-scale slices of oceanic upper crust, called the Poya Terrane, parts of which have been metamorphosed into an eclogite/blueschist facies complex, the Pouebo Terrane; and a lower, continental basement formed by the Norkolk Ridge terranes. Based upon exhaustive sampling of the mafic terranes and field surveys, our tectonic, micropaleontologic and geochemical data reveal that Poya and Pouebo terranes rocks originally formed within one single Campanian to late Paleocene oceanic basin, floored by tholeiitic basalt associated with some minor seamount-related intraplate alkali basalt. The tholeiitic basalt displays a continuous range of compositions spanning between “undepleted” and “depleted” end-members; the former being volumetrically predominant. The overall geochemical and isotopic features indicate an origin from a prominently heterogeneous mantle source during the opening of a marginal basin, the South Loyalty Basin, which almost completely disappeared during Eocene convergence. The opening of this basin originally located to the east of the Norfolk Ridge was synchronous with that of Tasman Sea basin as a consequence of oceanward migration of the west-dipping Pacific subduction zone. Establishing the origin of the ultramafic Ophiolitic Nappe is beyond the scope of this paper; however, it appears to be genetically unrelated to the mafic Poya and Pouebo terranes. Although it was located in the Late Eocene fore-arc, the Ophiolitic Nappe and the corresponding oceanic lithosphere originated before the Late Cretaceous, to the east of the South Loyalty Basin in a back-arc setting; or alternatively in a much older, trapped basin. For reasons that remain unclear, a new east-dipping subduction started in the Eocene and consumed most of the South Loyalty Basin, forming the intra-oceanic Loyalty Arc. Due to a changing subduction regime (underplating of the Diahot Terrane?), the mafic slices that now form the Poya Terrane were tectonically accreted in the Loyalty fore-arc region and remained under low pressure-low temperature conditions (possibly at the subsurface) until the Norfolk Ridge reached the subduction zone diachronously. This resulted in the final obduction of the fore-arc area. The two-step obduction involved first the mafic complex forming the Poya Terrane and thereafter the lithospheric mantle that now forms the Ophiolitic Nappe. In contrast, pieces of the accretionnary complex were dragged down into the subduction zone, underplated at depth ca. 70 km and metamorphosed into high-temperature eclogite to form the Pouebo Terrane metamorphics that display the same geochemical features as the Poya Terrane basalt. A mid-to-late Eocene syntectonic piggy-back sedimentary basin (the Nepoui flysch basin) mainly filled with mafic clastic material and shallow water carbonates that record the progressive uplift of the fore-arc region due to the accretion and underplating of mafic ocean-related and other material. In contrast, a slightly younger foreland basin located upon the Norfolk Ridge (the Priabonian Bourail Flysch basin) received a massive input of detrital material derived from the Norfolk Ridge itself and a time-increasing amount of mafic, Poya-derived material that recorded the first step of obduction. Thereafter, the Bourail Flysch was overthrust by the Poya Terrane and finally by the Ophiolitic Nappe. At the same time, buoyancy-driven uplift and exhumation of the high-pressure metamorphic complex occurred as a consequence of the diachronous blocking of the subduction zone. Finally, for a short time a new subduction started along the west coast of New Caledonia and generated small amounts of active margin-related magmas. These events have resulted in the close juxtaposition of unmetamorphosed and highly metamorphosed mafic and ultramafic terranes that may represent a good pre-collision analogue of mafic/ultramafic belts found in many collisional mountain ranges.

Remnants of a Cretaceous intra-oceanic subduction system within the Yarlung–Zangbo suture (southern Tibet)

Extensive field investigations along the Yarlung–Zangbo suture zone in southern Tibet reveal the presence of now fragmented remnants of a south-facing intra-oceanic subduction system. This system developed within Tethys during the Cretaceous. The associated arc, forearc ophiolite, and subduction complex were emplaced onto the leading edge of India at the end of the Cretaceous. Rapid sedimentation in oblique-slip basins and disruption of water-saturated sediments into melange was widespread and concomitant with ophiolite emplacement. We describe the tectonic entities that developed during this previously unrecognized phase of Tethys–Tibet evolution and present a new model for the evolution of this portion of Tibet.

Eocene arc-continent collision in New Caledonia and implications for regional southwest Pacific tectonic evolution

New Caledonia preserves evidence that constrains models for the tectonic evolution of the southwest Pacific region. Onland geology reflects four main tectonic phases: (1) early Mesozoic development of subduction-related terranes and their accretion to the Gondwana margin; (2) Cretaceous passive margin development and sea-floor spreading during the Gondwana breakup; (3) foundering of an oceanic basin and the Eocene arrival of thinned Gondwana margin crust at a southwest-facing subduction zone, resulting in collisional orogenesis and obduction of an ophiolitic nappe from the northeast; and (4) detachment faulting during extensional collapse, resulting in unroofing of metamorphic core complexes. The last phase explains supposedly anomalous metamorphic gradients in the northeast of the island.

Aspects of the Tectonic Evolution of China

The subject of this Special Publication is one of the most interesting in global geoscience, the tectonic evolution of China. The assemblage of terranes that underlie this part of the world provides outstanding opportunities to elucidate global processes, and many of the factors that shape the Earths lithosphere are best exemplified by the geology of China and its immediately adjacent areas In addition, there are geological features that are particular and unique to the region. Some have been the focus of recent attention and have attracted international interest because of their global importance. This volume provides accounts of up-to-date research by Chinese and international geological teams on key aspects of the tectonic evolution of China and its surrounding areas. The papers describe the formation of the geological terranes that make up this part of east Asia, place constraints on plate tectonic models for their assembly and provide accounts of unique geological feature of the subcontinent.

The Zedong terrane: a Late Jurassic intra-oceanic magmatic arc within the Yarlung–Tsangpo suture zone, southeastern Tibet

An overturned sequence of igneous and volcaniclastic rocks crops out along the Yarlung–Tsangpo suture zone (YTSZ), in southeastern Tibet. These rocks are remnants of an intra-oceanic island arc, the Zedong terrane, that once lay between India and Asia. 40Ar/39Ar dating and U–Pb ion microprobe analyses reported here reveal that this arc was active during the Jurassic. U–Pb dating of zircon from a dacite breccia from the middle portion of the Zedong terrane, yields an age of 161±2.3 Ma (1σ). 40Ar/39Ar dating of hornblende from a cross-cutting andesite dyke yields the youngest age of 152.2±3.3 Ma (1σ). 40Ar/39Ar results from hornblende from andesite dykes and an andesite breccia from the upper portion yield a mean age of 156±0.3 Ma (1σ). Additional U–Pb and 40Ar/39Ar dating of zircon and hornblende from quartz diorite yields a mean age of 156.8±0.8 Ma (1σ). Geochronological data reported here and other published work indicate that the intra-oceanic subduction system to which the Zedong terrane belonged was active from at least Late Jurassic to Early to mid Cretaceous.

New constraints on the India-Asia collision: the Lower Miocene Gangrinboche conglomerates, Yarlung Tsangpo suture zone, SE Tibet

Abstract Lower Miocene conglomerates crop out along the length of the Yarlung Tsangpo suture zone on the southern margin of the Lhasa terrane. These conglomerates, known by various local names, are correlated herein as the Gangrinboche conglomerates. All units exhibit broadly similar stratigraphic histories and a basal depositional contact upon an eroded surface of rocks of the Lhasa terrane is ubiquitous. At most localities the tops of sections are either removed by erosion or truncated by north-directed thrusts. These conglomeratic molasse units developed in response to the India/Asia collision and record aspects of its development. In all units initial clast derivation was from the Lhasa terrane on the northern margin of the Yarlung Tsangpo suture zone. Up-section the first appearance of clasts derived from terranes within the suture zone and the northern margin of India, all of which lie to the south of any outcrops of Gangrinboche conglomerates, is observed. Although these units were previously thought to be Eocene, analysis of fossil and structural constraints indicates Early Miocene deposition. As development of the Gangrinboche conglomerates records a significant phase in the evolution of the India–Asia collision understanding of their age and stratigraphic evolution has wider implications for regional tectonic models.

Bacterial community composition in thermophilic microbial mats from five hot springs in central Tibet

Despite detailed study of selected thermophilic taxa, overall community diversity of bacteria in thermophilic mats remains relatively poorly understood. A sequence-based survey of bacterial communities from several hot spring locations in central Tibet was undertaken. Diversity and frequency of occurrence for 140 unique 16S rRNA gene phylotypes were identified in clone libraries constructed from environmental samples. A lineage-per-time plot revealed that individual locations have evolved to support relatively large numbers of phylogenetically closely related phylotypes. Application of the FST statistic and P test to community data was used to demonstrate that phylogenetic divergence between locations was significant, thus emphasizing the status of hot springs as isolated habitats. Among phylotypes, only the Chlorobi were ubiquitous to all mats, other phototrophs (Cyanobacteria and Chloroflexi) occurred in most but not all samples and generally accounted for a large number of recovered phylotypes. Phylogenetic analyses of phototrophic phylotypes revealed support for location-specific lineages. The alpha, beta and gamma proteobacteria were also frequently recovered phyla, suggesting they may be abundant phylotypes in mats, a hitherto unappreciated aspect of thermophilic mat biodiversity. Samples from one location indicated that where phototrophic bacteria were rare or absent due to niche disturbance, the relative frequency of proteobacterial phylotypes increased.

Geochemical evolution and tectonic significance of boninites and tholeiites from the Koh ophiolite, New Caledonia

The Central Chain ophiolites in New Caledonia are fragments of a supra-subduction zone (SSZ) ophiolite, now preserved from the upper layered gabbros through to volcanics and overlain by pelagic cherts and a thick Middle Triassic to Upper Jurassic volcaniclastic sequence. Most of the fragments were formed by a single tholeiitic magmatic episode, but one of these, the Koh ophiolite, was formed by two tholeiitic magmatic episodes separated by boninites. The first event in the Koh ophiolite formed cumulate gabbros, dolerites, plagiogranites, and the first pillow lava sequence from a tholeiitic magma with strong depletion in the light rare earth elements (LREE) and abnormally low TiO2 (0.5% at Mg#=60). Shortly after their eruption, these tholeiitic lavas were overlain by a high-Ca boninitic unit with a basal section of boninite pillows, flows, and breccias and an upper section of boninitic dacites and tuffs. The last magmatic phase involved eruption of evolved tholeiitic basalts, as pillows above the boninites and as dykes and sills intruding the older plutonic and volcanic sections of the ophiolite. This second phase of tholeiitic magmatism is compositionally distinct from the first and is closest to back arc basin basalts (BABB) erupted during the early rifting history of modern back arc basins. The boninitic volcanics belong to a high-Ca series with slightly lower SiO2, Al2O3, and TiO2 compared to those from modern island arc systems, and they lack the positive Zr spike relative to adjacent rare earth elements (REE) in normalised element variation patterns. These boninites were formed shortly after the production of back arc basin crust represented by the depleted tholeiites and shortly before a second spreading event which caused 40–60% extension of the initial basin crust and eruption of the upper tholeiites. The dominance of BABB-like tholeiites throughout the Central Chain ophiolites in New Caledonia, the restricted occurrence of boninites, and the stratigraphy and chemistry of the Koh ophiolite suggest that the boninites were erupted in response to an exceptional tectonic situation. We suggest that this boninite generation event was driven by adiabatic decompression of hot depleted mantle residual from the production of the lower tholeiites, during initiation of rifting of young oceanic crust intimately associated with propagation of a back arc basin spreading centre. The occurrence of a thick blanket of calc-alkaline volcaniclastic sediments above the ophiolite indicates proximity to a mature arc and suggests that the Koh boninites were not associated with the initiation of subduction. A close modern analogy for the Koh ophiolite exists on the Hunter Ridge protoisland arc between southernmost Vanuatu (New Hebrides island arc) and the Fijian islands; there, high-Ca boninites lacking positive Zr spikes occur together with low-Ti tholeiites and more typical BABB tholeiites where the southern spreading centre of the North Fiji Basin is propagating into the protoarc crust of the Hunter Ridge.

Precise radiolarian age constraints on the timing of ophiolite generation and sedimentation in the Dazhuqu terrane, Yarlung-Tsangpo suture zone, Tibet

Well-preserved, abundant radiolarians provide high-precision biostratigraphic age constraints on the timing of the eruption of ophiolitic basalts exposed along the Yarlung–Tsangpo suture zone in southern Tibet. Dazhuqu terrane ophiolites were generated in an intra-oceanic supra-subduction zone setting within a relatively short (<10 Ma) interval from late Barremian to late Aptian. Accumulation of sediments upon the newly generated ophiolite initially occurred in a series of discrete rift-controlled sub-basins associated with various spreading centres. An increasing flux of arc-derived volcaniclastic sediment up-section indicates nearby volcanic arc activity. The Dazhuqu terrane developed in an intra-oceanic setting within Tethys where it was isolated from any continental influence.