Peter D. Clift

Palaeogeographic and palaeotectonic evolution of the Eastern Mediterranean Neotethys

Abstract Integrated basin-scale field and laboratory studies form the basis of detailed discussions of three important areas of the Eastern Mediterranean Neotethys, here termed the Pindos ocean (Greece), the Antalya ocean (SW Turkey) and the Cyprus ocean. These three areas preserve remnants of interconnected Mesozoic small ocean basins, sited along the north margin of Gondwanaland. The most powerful tools available for basin analysis are biostratigraphy, facies trends, tectonic discrimination of basalts and structural evidence of emplacement directions. It is shown that, despite some important differences in the timing of events, individual oceanic basins went through essentially predictable stages, including rifting, spreading, subduction/accretion, displacement/emplacement and collision. An idealised model of small ocean basin evolution can, thus be envisaged. However, in detail, individual areas show considerable palaeogeographic variety, particularly in the size, distribution and shape of carbonate platforms, margins and seamounts and no one area exemplifies Neoethyan evolution as a whole. Rifting in each of the three areas took place in Late Permian? to Mid Triassic time, followed by continental break-up in the Late Triassic. The Pindos ocean developed into a significantly wide small ocean basin (ca. 1000 km) by the Early Jurassic, while the Antalya ocean remained narrower (less than 500 km). The rifted continental fragments subsided and were capped by carbonate platforms that were fringed by slope and deep-water basinal sediments. Intra-oceanic carbonate platforms were present in both the Pindos and Antalya oceans. Ophiolites formed above an inferred west-dipping subduction zone in Greece in the Mid Jurassic, followed by displacement and metamorphic sole formation, then regional emplacement onto a microcontinent, the Pelagonian Zone, in the Late Jurassic. Above-subduction zone spreading took place in the Late Cretaceous in the Cyprus ocean and, probably also in the Antalya ocean basin. Diachronous collisions closed both the Pindos and Antalya oceans by the Early Tertiary, while the Cyprus ocean basin survived as an oceanic remnant until Quaternary-Recent time. Finally, an attempt is made to position the Pindos, Antalya and Cyprus oceanic units in the context of our understanding of the Eastern Mediterranean Neotethys.

Seven Million Years of Glaciation in Greenland

Glacial till, glaciomarine diamictites, and ice-rafted detritus found in marine cores collected off the shore of southeast Greenland record multiple Late Cenozoic glaciations beginning in the Late Miocene. Distinct rock assemblages and seismic stratigraphic control correlate the diamictites with glaciation of the southeast Greenland margin. Glaciers advanced to the sea during several intervals in the Pliocene and Pleistocene. North Atlantic glaciation may have nucleated in southern Greenland rather than further north because of the high mountains and the high levels of precipitation in this region.

Erosional response of South China to arc rifting and monsoonal strengthening; a record from the South China Sea

Abstract Ocean Drilling Program sampling of the distal passive margin of South China at Sites 1147 and 1148 has yielded clay-rich hemipelagic sediments dating to 32 Ma (Oligocene), just prior to the onset of seafloor spreading in the South China Sea. The location of the drill sites offshore the Pearl River suggests that this river, or its predecessor, may have been the source of the sediment in the basin, which accounts for only ∼1.8% of the total Neogene sediment in the Asian marginal seas. A mean erosion depth of ∼1 km over the current Pearl River drainage basin is sufficient to account for the sediment volume on the margin. Two-dimensional backstripping of across-margin seismic profiles shows that sedimentation rates peaked during the middle Miocene (11–16 Ma) and the Pleistocene (since 1.8 Ma). Nd isotopic analysis of clays yielded ϵNd values of −7.7 to −11.0, consistent with the South China Block being the major source of sediment. More positive ϵNd values during and shortly after rifting compared to later sedimentation reflect preferential erosion at that time of more juvenile continental arc rocks exposed along the margin. As the drainage basin developed and erosion shifted from within the rift to the continental interior ϵNd values became more negative. A rapid change in the clay mineralogy from smectite-dominated to illite-dominated at ∼15.5 Ma, synchronous with middle Miocene rapid sedimentation, mostly reflects a change to a wetter, more erosive climate. Evidence that the elevation of the Tibetan Plateau and erosion in the western Himalaya both peaked close to this time supports the suggestion that the Asian monsoon became much more intense at that time, much earlier than the 8.5 Ma age commonly accepted.

Preferential mantle lithospheric extension under the South China margin

Abstract Continental rifting in the South China Sea culminated in seafloor spreading at ∼30 Ma (Late Oligocene). The basin and associated margins form a classic example of break-up in a relatively juvenile arc crust environment. In this study, we documented the timing, distribution and amount of extension in the crust and mantle lithosphere on the South China Margin during this process. Applying a one-dimensional backstripping modeling technique to drilling data from the Pearl River Mouth Basin (PRMB) and Beibu Gulf Basin, we calculated subsidence rates of the wells and examined the timing and amount of extension. Our results show that extension of the crust exceeded that in the mantle lithosphere under the South China Shelf, but that the two varied in phase, suggesting depth-dependent extension rather than a lithospheric-scale detachment. Estimates of total crustal extension derived in this way are similar to those measured by seismic refraction, indicating that isostatic compensation is close to being local. Extension in the Beibu Gulf appears to be more uniform with depth, a difference that we attribute to the different style of strain accommodation during continental break-up compared to intra-continental rifting. Extension in PRMB and South China slope continues for ∼5 m . y . after the onset of seafloor spreading due to the weakness of the continental lithosphere. The timing of major extension is broadly mid-late Eocene to late Oligocene (∼45–25 Ma ), but is impossible to correlate in detail with poorly dated strike–slip deformation in the Red River Fault Zone.

Development of the Indus Fan and its significance for the erosional history of the Western Himalaya and Karakoram

Correlation of new multichannel seismic profiles across the upper Indus Fan and Murray Ridge with a dated industrial well on the Pakistan shelf demonstrates that ;40% of the Indus Fan predates the middle Miocene, and ;35% predates uplift of the Murray Ridge (early Miocene, ;22 Ma). The Arabian Sea, in addition to the Makran accretionary complex, was therefore an important repository of sediment from the Indus River system during the Paleogene. Channel and levee complexes are most pronounced after the early Miocene, coincident with an increase in sedimentation rates. Middle Eocene sandstones from Deep Sea Drilling Project Site 224 on the Owen Ridge yield K-feldspars whose Pb isotopic composition, measured by in situ ion microprobe methods, indicates an origin in, or north of, the Indus suture zone. This observation requires that India-Asia collision had occurred by this time and that an Indus River system, feeding material from the suture zone into the basin, was active soon after collision. Pleistocene provenance was similar to that during the Eocene, albeit with greater contribution from the Karakoram. A mass balance of the erosional record on land with deposition in the fan and associated basins suggests that only ;40% of the Neogene sediment in the fan is derived from the Indian plate.

Large‐scale drainage capture and surface uplift in eastern Tibet–SW China before 24 Ma inferred from sediments of the Hanoi Basin, Vietnam

Current models of drainage evolution suggest that the non-dendritic patterns seen in rivers in SE Asia reflect progressive capture of headwaters away from the Red River during and as a result of surface uplift of Eastern Asia. Mass balancing of eroded and deposited rock volumes demonstrates that the Red River catchment must have been much larger in the past. In addition, the Nd isotope composition of sediments from the Hanoi Basin, Vietnam, interpreted as paleo-Red River sediments, shows rapid change during the Oligocene, before similar to 24 Ma. We interpret this change to reflect large-scale drainage capture away from the Red River, possibly involving loss of the middle Yangtze River. Reorganization was triggered by regional tilting of the region towards the east. This study constrains initial surface uplift in eastern Tibet and southwestern China to be no later than 24 Ma, well before major surface uplift and gorge incision after 13 Ma.

Ages and magnetic structures of the South China Sea constrained by deep tow magnetic surveys and IODP Expedition 349

Combined analyses of deep tow magnetic anomalies and International Ocean Discovery Program Expedition 349 cores show that initial seafloor spreading started around 33 Ma in the northeastern South China Sea (SCS), but varied slightly by 1-2 Myr along the northern continent-ocean boundary (COB). A southward ridge jump of approximate to 20 km occurred around 23.6 Ma in the East Subbasin; this timing also slightly varied along the ridge and was coeval to the onset of seafloor spreading in the Southwest Subbasin, which propagated for about 400 km southwestward from approximate to 23.6 to approximate to 21.5 Ma. The terminal age of seafloor spreading is approximate to 15 Ma in the East Subbasin and approximate to 16 Ma in the Southwest Subbasin. The full spreading rate in the East Subbasin varied largely from approximate to 20 to approximate to 80 km/Myr, but mostly decreased with time except for the period between approximate to 26.0 Ma and the ridge jump (approximate to 23.6 Ma), within which the rate was the fastest at approximate to 70 km/Myr on average. The spreading rates are not correlated, in most cases, to magnetic anomaly amplitudes that reflect basement magnetization contrasts. Shipboard magnetic measurements reveal at least one magnetic reversal in the top 100 m of basaltic layers, in addition to large vertical intensity variations. These complexities are caused by late-stage lava flows that are magnetized in a different polarity from the primary basaltic layer emplaced during the main phase of crustal accretion. Deep tow magnetic modeling also reveals this smearing in basement magnetizations by incorporating a contamination coefficient of 0.5, which partly alleviates the problem of assuming a magnetic blocking model of constant thickness and uniform magnetization. The primary contribution to magnetic anomalies of the SCS is not in the top 100 m of the igneous basement.

Pre-Miocene birth of the Yangtze River

The development of fluvial systems in East Asia is closely linked to the evolving topography following India–Eurasia collision. Despite this, the age of the Yangtze River system has been strongly debated, with estimates ranging from 40 to 45 Ma, to a more recent initiation around 2 Ma. Here, we present 40Ar/39Ar ages from basalts interbedded with fluvial sediments from the lower reaches of the Yangtze together with detrital zircon U–Pb ages from sand grains within these sediments. We show that a river containing sediments indistinguishable from the modern river was established before ∼23 Ma. We argue that the connection through the Three Gorges must postdate 36.5 Ma because of evaporite and lacustrine sedimentation in the Jianghan Basin before that time. We propose that the present Yangtze River system formed in response to regional extension throughout eastern China, synchronous with the start of strike–slip tectonism and surface uplift in eastern Tibet and fed by strengthened rains caused by the newly intensified summer monsoon.

Continent‐ocean interactions within the East Asian Marginal seas

Interactions between continents and oceans are a frontier area for the Earth sciences in the 21st century. An AGU Chapman Conference, Continent-Ocean Interactions within the East Asian Marginal Seas, examined the nature of these interactions in the marginal seas of east Asia. The objective was to highlight both recent advances, and especially the contributions made by the Ocean Drilling Program (ODP)semi; as well as to identify key future science goals. The types of continent-ocean interactions discussed were wide-ranging, including climate-tectonic interactions, continental-oceanic climate linkages, and the material flux from the rivers of Asia to the ocean, as well as how continental tectonic evolution since the India-Asia collision has influenced the tectonics of the western Pacific and vice-versa. The marginal seas of east Asia form the transition between the worlds largest continent and its largest ocean, and are major repositories of information on the interaction between the two.

Fluvial landscapes of the Harappan civilization

The collapse of the Bronze Age Harappan, one of the earliest urban civilizations, remains an enigma. Urbanism flourished in the western region of the Indo-Gangetic Plain for approximately 600 y, but since approximately 3,900 y ago, the total settled area and settlement sizes declined, many sites were abandoned, and a significant shift in site numbers and density towards the east is recorded. We report morphologic and chronologic evidence indicating that fluvial landscapes in Harappan territory became remarkably stable during the late Holocene as aridification intensified in the region after approximately 5,000 BP. Upstream on the alluvial plain, the large Himalayan rivers in Punjab stopped incising, while downstream, sedimentation slowed on the distinctive mega-fluvial ridge, which the Indus built in Sindh. This fluvial quiescence suggests a gradual decrease in flood intensity that probably stimulated intensive agriculture initially and encouraged urbanization around 4,500 BP. However, further decline in monsoon precipitation led to conditions adverse to both inundation- and rain-based farming. Contrary to earlier assumptions that a large glacier-fed Himalayan river, identified by some with the mythical Sarasvati, watered the Harappan heartland on the interfluve between the Indus and Ganges basins, we show that only monsoonal-fed rivers were active there during the Holocene. As the monsoon weakened, monsoonal rivers gradually dried or became seasonal, affecting habitability along their courses. Hydroclimatic stress increased the vulnerability of agricultural production supporting Harappan urbanism, leading to settlement downsizing, diversification of crops, and a drastic increase in settlements in the moister monsoon regions of the upper Punjab, Haryana, and Uttar Pradesh.