Small Ocean Basin Research Articles

Acoustic observations of heat content across the Mediterranean Sea

The ability to monitor the heat content of oceans over long distances is becoming increasingly important for understanding the role of oceans in climate change, for determining the variability of the state of the oceans, for operational ocean observing systems, and for studying large-scale ocean processes such as water-mass formation. Although the properties of the upper layers of the ocean can be routinely measured on large scales by satellite remote sensing (providing altimetric and infrared data) and with expendable probes dropped from commercial vessels, the deep interior of the ocean is more difficult to monitor. Ocean acoustic tomography1 is a promising technique for such applications, as it has the potential to provide systematic, instantaneous and repeated measurements of the ocean interior over large parts of an ocean basin. Here we demonstrate the capability of this technique for measuring the heat content across an entire (albeit small) ocean basin—the western Mediterranean Sea.

A New Frontier Province Offshore Northwest Greenland: Structure, Basin Development, and Petroleum Potential of the Melville Bay Area

In the Melville Bay area, offshore northwest Greenland, very large structures and sedimentary basins, which were predicted many years ago on the basis of magnetic and gravity data, have been confirmed by a recent reconnaissance seismic survey, with implications that are encouraging for petroleum exploration in the area. The Melville Bay area flanks a small ocean basin in Baffin Bay that is thought to have formed by oblique sea-floor spreading in the Eocene. There are two major, coast-parallel basins in the area. The inner basin, the Melville Bay Graben, is essentially a half graben with a maximum thickness of sediments exceeding 13 km. A complex fault- controlled ridge system separates this basin from the outer Kivioq Basin in which up to 7 km of sediments have accumulated. By analogy with onshore geology in the surrounding areas and well data from the continental shelves off southern west Greenland and Labrador to the south, it is expected that the first phase of rifting and sedimentation took place in the Early-middle Cretaceous, while a second phase of rifting took place in the latest Cretaceous and early Paleocene. Later, compression and inversion affected the northern part of the area, leading to the formation of large anticlinal structures. ©Copyright 1997. The American Association of Petroleum Geologists. All rights reserved.1Manuscript received January 30, 1996; revised manuscript received September 9, 1996; final acceptance January 21, 1997. 2GeoArctic Consulting Ltd., 1620 9th Street NW, Calgary, Alberta T2M 3L4, Canada. 3Nunaoil A/S, Pilestraede 52, DK-1112 Copenhagen, Denmark. 4Geological Survey of Denmark and Greenland, Thoravej 8, DK-2400 Copenhagen, Denmark. We wish to acknowledge the companies partaking in the KANUMAS project: BP, Exxon, Japan National Oil Company, Shell, Statoil, and Texaco, for allowing us the opportunity to publish this paper. Our thanks also to Rene Forsberg and Simon Ekholm, National Survey and Cadastre, for processing the gravity data for the paper, and to Alan Menelly (consultant) for discussions on the structural development of the area. Airborne gravity data were made available courtesy of John Brozena of the U.S. Naval Research Laboratory.

Ophiolites of the Kunlun Mountains, China and their tectonic implications

Three ophiolite belts, ranging in age from Cambrian to Triassic, provide valuable data on the tectonic evolution of the Kunlun Mountains which lie along the northern margin of the Tibetan Plateau. All of the ophiolites contain dismembered but nearly complete assemblages of peridotite, mafic and ultramafic cumulates, pillow and massive lavas and radiolarian cherts, (1) The oldest belt contains ophiolites of Cambrian age, but the initial spreading of the ocean basin most likely began in the Late Proterozoic. These ophiolites outcrop in the central part of the eastern Kunlun, extend for at least a few hundred kilometers, and are believed to reflect a small ocean basin. Closure of this ocean led the accretion of south Kunlun block to the north Kunlun block and Tarim craton. Ophiolitic lavas in this belt have island are affinities. (2) A second belt of early Carboniferous ophiolites extends nearly 600 km along the northern margin of the western Kunlun. Lavas in this belt are basalt, basaltic andesite and andesite, all of which have compositions characteristic of volcanic arcs. This ocean basin possibly developed on the basis of the early Paleozoic oceanic basin. Closure of this ocean basin by southward subduction in the early Permian produced a suite of calc-alkaline volcanic rocks in what is now the central part of the western Kunlun. (3) The third belt extends nearly 1200 km along the southern margin of the Eastern Kunlun and contains numerous ophiolites of Early Permian to Middle Triassic age. These ophiolites are highly tectonized, containing volcanic rocks with the geochemical characteristics of mid-ocean ridges, oceanic islands and volcanic arcs. This belt is tentatively interpreted as the suture zone between Gondwana and Eurasia.

Alternative tectonic models for the Late Palaeozoic-Early Tertiary development of Tethys in the Eastern Mediterranean region

Abstract A summary and discussion is given of alternative models of the tectonic evolution of the Tethyan orogenic belt in the Eastern Mediterranean region, based on recent information. Model 1 (Robertson & Dixon 1984). A single Tethyan ocean continuously existed in the Eastern Mediterranean region, at least from Late Palaeozoic onwards. The dominant influences were episodic northward subduction of Tethyan oceanic crust beneath Eurasia, and the northward drift of continental fragments, from Gondwana towards Eurasia. During the Mesozoic, the south Tethyan area was interspersed with Gondwana-derived microcontinents and small ocean basins. Ophiolites formed mainly by spreading above subduction zones in both northerly (internal) and southerly (external) oceanic basins during times of regional plate convergence, and were mainly emplaced as a result of trench-passive margin collisions. In a related model, Stampfli et al. (1991) argued for spreading along the North African margin in the Late Permian. Model 2A (Dercourt et al. 1986). Only one evolving Tethys existed. Triassic-Jurassic oceanic crust (Neotethys) formed in a single Tethyan ocean basin located north of Gondwana-related units. Spreading later formed a small ocean basin in the present Eastern Mediterranean Sea area during the Cretaceous. Jurassic and Cretaceous ophiolites formed at spreading ridges and record times of regional plate divergence. In an update version, Model 2B (Dercourt et al. 1993), spreading extended along the northern margin of Gondwana, with an arm extending through the south Aegean, splitting off a large microcontinent. Further spreading in the Cretaceous then opened the Eastern Mediterranean basin and fragmented pre-existing carbonate platforms. The Mesozoic ophiolites were seen as being mainly far-travelled from northerly (i.e. internal) orogenic areas. Model 3 (Şengör et al. 1984). Subduction in the Late Palaeozoic was dominantly southwards, beneath the northern margin of Gondwana in the Eastern Mediterranean. This subduction led to opening of Triassic backarc basins; and a rifted Gondwana fragment (Cimmeria) drifted across a pre-existing Tethys (Palaeo-Tethys) to collide with a passive Eurasian margin. In their model, a backarc basin (Karakaya Basin) rifted and then closed prior to collision of a Cimmerian microcontinent in the Mid Jurassic, and this was followed by renewed rifting of a small ocean basin in the Early Jurassic. Mesozoic ophiolites mainly formed above subduction zones; they were variously seen as far-travelled (in the ‘Greek area’), or more locally rooted (in the ‘Turkish area’). Recent evidence shows that difficulties exist in detail with all three models. However, four key elements are met in Model 1: dominantly northward subduction in the north; multiple ocean basins from Triassic onwards in the south; supra-subduction spreading of the major ophiolites; and emplacement from both northerly and southerly Mesozoic oceanic basins. Palaeomagnetism has played an important role, in setting the large-scale Africa-Eurasia relative motion framework and in providing tests for the tectonic affinities of smaller units, but such smaller-scale studies have often been compromised by the geological complexity and by the remagnetisation of tectonically thickened units.

Geomagnetism and age study of Tyrrhenian seamounts

SUMMARY We discuss the results of measurements of the magnetic anomaly field of several seamounts located on the ocean floor and on the bordering continental slopes of the Tyrrhenian Sea. This geomagnetic study of seamounts is based on joint 3-D analyses of the magnetic anomalies and the bathymetry, on measurements of the magnetic properties of rock specimens and on geochronological data. The 3-D analysis of the magnetic anomalies of large volcanoes emplaced in the small oceanic basins of Vavilov and Marsili serves to shed light on the spatial distribution of the magnetized bodies by eliminating the effect of topography. A new magnetic anomaly map of the southern Tyrrhenian Sea is obtained by the merging of shipborne local data, measured on topographic highs, with data of regional type. The study of the magnetic patterns and age of the oceanic seamounts indicates that the volcanic activity mainly developed prior to chron C2 (> 2.02 Ma BP) on the Vavilov and Magnaghi edifices (Vavilov Basin), whereas the volcanism of a big oceanic seamount emplaced in the small basin of Marsili took place mainly during the period of chron C1 (0-0.78 Ma BP).

Mean Sea Level determination in small ocean basins from altimetry and tide gauge data

Mean Sea Level determination in small ocean basins from altimetry and tide gauge data

Late Variscan exhumation histories of the southern Rhenohercynian Zone and western Mid-German Crystalline Rise: results from thermal modeling

Thermal modeling techniques constrained by published petrological and thermo-chronometric data were applied to examine late orogenic burial and exhumation at a Variscan suture zone in Central Europe. The suture separates the southern Rhenohercynian zone from the Mid-German Crystalline Rise and traces the former site of a small oceanic basin. Closure of this basin during Variscan subduction and subsequent collision of continental units were responsible for different tectono-metamorphic evolutions in the suture's footwall and hanging wall. Relative convergence rates between the southern Rhenohercynian zone and western Mid-German Crystalline Rise can be inferred from the pressure-temperature-time evolution of the Northern Phyllite Zone. During Late Visean-Early Namurian times, horizontal thrusting velocities were at least 20 mm/a. Thermal modeling suggests that exhumation of the Mid-German Crystalline Rise occurred temporarily at rates of more than 3 mm/a. Such rapid exhumation cannot be produced by erosion only, but requires a substantial contribution of extensional strain. Exhumation by upper crustal extension occurred contemporaneously with convergence and is explained by continuous underplating of crustal slices and thrusting along faults with ramp-flat geometry. Finally, implications for the tectono-metamorphic history of the study area and the thermal state of the crust during late Variscan exhumation are discussed.

Metamorphism of the Bikou Group in the Shanxi‐Gausu‐Sichuan Border Region1

Abstract The Bikou Group on the Shaanxi‐Gansu‐Sichuan border is composed of Mid‐Late Proterozoic metamorphosed bimodal volcanic rocks and flysch sediments. Its metamorphism may be divided into the blueschist and greenschist facies. Three metamorphic zones, i.e. zones A, B, and C, may be distinguished on the basis of the field distribution of metamorphic rocks and the variation of b0 values of muscovite. Blueschists are characterized by coexistence of sodic amphiboles and epidote and occur as stripes or relict patches in extensive greenschists of zone A. Studies of metamorphic minerals such as amphiboles, chlorite, epidote and muscovite and their textural relationships indicate that blueschists and greenschists were not formed under the same metamorphic physico‐chemical conditions. The blueschist facies was formed at temperatures of 300‐400°C and pressures of 0.5–0.6 GPa. The greenschist facies in zones A and B has similar temperatures but its pressure is only 0.4 GPa or so. The transition from the blueschist to greenschist facies is a nearly isothermal uplift process. The rock and mineral assemblages of the Bikou Group indicate that the blueschist facies metamorphism of the group might be related to crustal thickening or A‐subduction accompanying the closure of an intracontinental small ocean basin.

Asian analogues for Precambrian tectonics?

Abstract A non-specialist view is that the Archaean crust comprises either tonalite/greenstone belt associations or gneissic terrains in granulite facies. Archaean crustal remnants lie as undeformed pods between ramifying Proterozoic mobile belts. The cratons themselves may have unconformable remnants of undeformed Proterozoic ‘platform’ sediments. Coward, Windley and others have already drawn attention to similarities between the accretionary collage which makes Central Asia and the tectonic patterns of the Precambrian. These similarities are developed and amplified in this paper. Central Asian basins such as Tarim, Junggar and their satellites have developed on stretched continental fragments now accreted within the Asian collage. This stretched lithosphere, having been equilibrated and strengthened, now forms resistant kernels around which the surrounding ramifying accretionary deformation belts have been moulded. Enclosed foreland basins have developed on the continental fragments in response to loading of the adjacent mountain belts, and great thicknesses of mostly Tertiary sediments have accumulated in them. The ‘mobile belts’ which surround the basins have been reactivated by later, distant, collision events with corresponding loading and sedimentation events in the basin. The Black Sea and South Caspian basins are back-arc small ocean basins incorporated within the collage. They also form resistant kernels around which the mountain chains are moulded. Deep erosion levels through these systems might provide broad analogues for Precambrian tectonic patterns, i.e. the Black Sea for the tonalite/greenstone belt terrains, the deep levels of Tarim for ‘reworked’ granulite terrains like John Sutton’s homeground of the NW Highland foreland. Sutton and Watson’s two orogenies are manifested as a ductile shear fabric associated with back-arc stretching (Scourian) reworked in zones after a substantial time interval (Laxfordian). Dykes like the Scourie dykes might be associated with stretching and incipient continental separation, and therefore be close in time with the ductile stretching as, arguably, the Scourie dykes are with the late Scourian (Inverian) deformation in Scotland.

Gondwanaland origin, dispersion, and accretion of East and Southeast Asian continental terranes

East and Southeast Asia is a complex assembly of allochthonous continental terranes, island arcs, accretionary complexes and small ocean basins. The boundaries between continental terranes are marked by major fault zones or by sutures recognized by the presence of ophiolites, mélanges and accretionary complexes. Stratigraphical, sedimentological, paleobiogeographical and paleomagnetic data suggest that all of the East and Southeast Asian continental terranes were derived directly or indirectly from the Iran-Himalaya-Australia margin of Gondwanaland. The evolution of the terranes is one of rifting from Gondwanaland, northwards drift and amalgamation/accretion to form present day East Asia. Three continental silvers were rifted from the northeast margin of Gondwanaland in the Silurian-Early Devonian (North China, South China, Indochina/East Malaya, Qamdo-Simao and Tarim terranes), Early-Middle Permian (Sibumasu, Lhasa and Qiangtang terranes) and Late Jurassic (West Burma terrane, Woyla terranes). The northwards drift of these terranes was effected by the opening and closing of three successive Tethys oceans, the Paleo-Tethys, Meso-Tethys and Ceno-Tethys. Terrane assembly took place between the Late Paleozoic and Cenozoic, but the precise timings of amalgamation and accretion are still contentious. Amalgamation of South China and Indochina/East Malaya occurred during the Early Carboniferous along the Song Ma Suture to form “Cathaysialand”. Cathaysialand, together with North China, formed a large continental region within the Paleotethys during the Late Carboniferous and Permian. Paleomagnetic data indicate that this continental region was in equatorial to low northern paleolatitudes which is consistent with the tropical Cathaysian flora developed on these terranes. The Tarim terrane (together with the Kunlun, Qaidam and Ala Shan terranes) accreted to Kazakhstan/Siberia in the Permian. This was followed by the suturing of Sibumasu and Qiangtang to Cathaysialand in the Late Permian-Early Triassic, largely closing the Paleo-Tethys. North and South China were amalgamated in the Late Triassic-Early Jurassic and finally welded to Laurasia around the same time. The Lhasa terrane accreted to the Sibumasu-Qiangtang terrane in the Late Jurassic and the Kurosegawa terrane of Japan, interpreted to be derived from Australian Gondwanaland, accreted to Japanese Eurasia, also in the Late Jurassic. The West Burma and Woyla terranes drifted northwards during the Late Jurassic and Early Cretaceous as the Ceno-Tethys opened and the Meso-Tethys was destroyed by subduction beneath Eurasia and were accreted to proto-Southeast Asia in the Early to Late Cretaceous. The Southwest Borneo and Semitau terranes amalgamated to each other and accreted to Indochina/East Malaya in the Late Cretaceous and the Hainanese terranes probably accreted to South China sometime in the Cretaceous.

Ophiolites of the Hengduan Mountains, China: characteristics and tectonic settings

The Hengduan Mountains contain Neotethyan and Palaeotethyan ophiolites. Dengqen ophiolite is representative of Neotethyan ophiolite. Its extrusive series are typical fore-arc boninite and are considered to have formed in a fore-arc basin setting. It was emplaced at the end of the Jurassic. The Palaeotethyan ophiolite is widely distributed over the Hengduan Mountains. It is composed mainly of metamorphic periootite (mainly lherzolite), gabbro and basalt. Cumulative ultramafic rocks and sheeted dike complex are rarely exposed. Basalt in most of Palaeotethyan ophiolites have geochemical features similar to MORBs. Basalts in most associated sedimentological evidence, most of the Paleotethyan ophiolites (except Changning-Menglian ophiolite in big ocean basins) were formed in small ocean basins.

The petrogenesis and tectonic setting of lavas from the Baft Ophiolitic Mélange, southwest of Kerman, Iran

A Late Cretaceous ophiolite complex in the Baft area, southwest of Kerman, Iran, is characteristic of the Central Iranian Ophiolitic Mélange Belt, which wraps around the Lut Block. Despite the extensive tectonic disruption of the Baft complex, most ophiolitic lithologies are present and many original igneous contacts are preserved. A lack of cumulate gabbros within the sequence suggests that a large and continuous magma chamber did not exist beneath the Baft spreading axis. Geochemical data confirm the presence of two distinct compositional groups in the mafic lavas: (1) tholeiitic basalt and (2) transitional tholeiitic basalt. The tholeiitic lavas are similar to typical mid-ocean-ridge basalt compositions, whereas the transitional tholeiites are similar to intraplate basalts. The available data suggest that the Baft ophiolite complex formed in a small ocean basin, possibly at or near a ridge–transform intersection. Emplacement may have occurred as a result of conversion of the transform fault to a subduction zone during a change in relative plate motion. A ridge–transform setting is compatible with the intraplate character of some of the transitional basalts, which probably represent off-axis (seamount) magmatism, marked by the absence of cumulate gabbros and the presence of a serpentinite mélange cut by basaltic dykes. The ridge–transform model suggests formation of the ophiolite in a narrow ocean basin separating the Sanandaj-Sirjan microcontinent from the Central Iran Block in Late Cretaceous time.

A comparison between classical cross-over techniques and a stochastic-deterministic approach

An alternative approach to classical cross-over adjustment was developed for small ocean basins such as the Baltic, the Mediterranean and the North Sea. This alternative approach is compared to a classical cross-over method in order to show advantages and disadvantages for each method in the three test areas. The comparison is done using simulated altimeter data which include white noise as a first step towards actual observations. The alternative approach depends on the availability of several selected tide gauges for which space coordinates (such as GPS data) are available. Several test configurations are considered for which associated criteria are established.

The Falkland Islands: A key element in Gondwana paleogeography

Stratigraphic and sedimentological fieldwork on the Falkland Islands confirms that they were originally part of South Africa. East and West Falkland have contrasting geological and deformation histories and are separated by the Falkland Sound fault, which marks a significant deformation edge to the Cape Fold Belt. The degree of structural deformation and metamorphism in West Falkland is substantially less than that of East Falkland, adjustments to differential shortening between the two islands being reflected by structures developed along the Falkland Sound fault. Prior to fragmentation of Gondwana, the Falkland Platform and Maurice Ewing Bank were positioned in the Natal Valley, adjacent to the east coast of South Africa. This predrift reconstruction shows that the Falkland Islands have subsequently been rotated by 180° relative to South Africa and complements recent paleomagnetic data. There was a complete overlap of the Maurice Ewing Bank and Falkland Platform to the exclusion of the Falkland Plateau Basin which is a small oceanic basin formed during the Jurassic fragmentation of Gondwana. Extension in this basin was accommodated by movements along the Aghulas Falkland Fracture Zone and the Gastre Fault System. Predrift restoration of these faults shows that Patagonia was positioned closer to both Africa and Antarctica. The Falkland Plateau Basin was an integral component of the West Gondwana Jurassic microplate system and provides an invaluable insight into the evolution of its western margin. At some time prior to the main opening of the South Atlantic, the Falkland Platform was rotated clockwise by 120° against the Agulhas Falkland Fracture Zone, such that the Falkland Islands were translated some 300 km to the south. The driving force for this rotation is not known but could possibly have been initiated by periodic stress buildups in the Falkland Plateau Basin as the Gastre Fault System, within continental crust, became unable to accommodate the continuous dextral shear in the Agulhas Falkland Fracture Zone. The Falkland Islands represent an outboard extension of the Eastern Cape and are significant for understanding the paleogeography of both the Cape and Karoo supergroups. East Falkland is closely similar to South Africa in its geology, while West Falkland shows significant contrasts with both the Devonian and Permian successions being more proximal in character but still demonstrate the eastward continuation of the Cape and Karoo basins.

Ferrodiorites from the Mid-Cayman Rise

The Cayman Trough in the Caribbean is a very small ocean basin (1000 km x 110 km) that is surrounded by older terranes, some of which are continental (Case et al., 1990). The spreading rate at the Mid-Cayman Rise is extremely slow (possibly 1.2 cm a -I for the last 5.5 Ma Rosencrantz et al., 1988). Magmas contributing to the basalts that form the Cayman Trough potent ia l ly formed f rom several sources, including non-asthenospheric ones. This region, therefore, provides an ideal area to investigate the processes that occur in magma chambers beneath ocean ridges as the magmas here potentially have the greatest ranges in isotopic ratios and tracedement patterns.

Stratigraphic Constraints on the Development and Timing of Arc-Continent Collision in Northern Papua New Guinea

ABSTRACT The Finisterre Mountains of northern Papua New Guinea are the site of an active, southeast-propagating collision between the Finisterre terrane and the Australian continental margin. The clastic sedimentary sequence on the southern flank of the range records the history of the collision. The age of initial collision can be deduced from the second of two provenance shifts that occur in the sequence, as revealed by a study of 69 dated sandstones. The first shift, at about 16-18 Ma, was from volcanolithic sediments to mixed-provenance sediments rich in quartz and metasedimentary lithic fragments. These latter sediments were probably derived from the orogenic belt active along the Australian continental margin at that time. At 3.0-3.7 Ma the volcanic source was rejuvenated. Initial collis on at this time uplifted the Finisterre terrane, causing the provenance shift. Here we break the stratigraphic sequence into three units: the Sarawaget beds, the Erap Structural Complex, and the Leron Formation. The Sarawaget beds are Middle Eocene to Early Miocene volcanolithic sandstones that were deposited as clastic aprons un the flanks of the Finisterre volcanic are. Volcanic activity ceased in the Early Miocene. The Erap Complex is divided into two clastic subunits, the Sukurum and Nariawang units, and one basaltic subunit, the Gorambampan unit. The Sukurum unit consists of turbidites deposited south of the extinct Finisterre Arc from the latest Early Miocene through the Early Pliocene. Much of the sediment in the unit was derived from low- to medium-grade metasedimentary units on the active northern margin of the Australian continent. Renewed subduction i the Late Miocene or Pliocene occurred along the New Britain Trench, just south of the remnant Finisterre Arc, closing an eastward-widening small ocean basin. The Sukurum unit probably represents an accretionary prism that formed at this subduction zone. Scattered seamounts that were scraped off the downgoing plate are preserved as the Gorambampan unit. Initial collision of the Finisterre terrane with the continental margin at 3.0-3.7 Ma uplifted the Paleogene volcanic rocks, which were rapidly eroded and deposited as the volcanolithic turbidites of the Nariawang unit. Southeastward propagation of the collision tip caused incorporation of the Nariawang unit into the accretionary wedge. Rapid uplift raised the terrane foreland above sea level sometime after 1.3 Mo. The fluvial, lacustrine and marginal-marine deposits of the Leron Formation accumulated in a terrane foreland basin that developed south of the collision zone. Further propagation of the thrust front toward the foreland incorporated the Leron Formation into the fold-and-thrust belt. Rapid uplift of the Finisterre Range and voluminous volcanolithic sedimentation continue to the present. In the Finisterre Range, arc-continent collision is marked by a provenance shift from metasedimentary and quartzose detritus to volcanic detritus. In another modern arc-continent collision on Taiwan, collision has caused a shift from volcanic to metasedimentary detritus. We attribute this discrepancy to the presence of the Finisterre terrane, an extinct arc, in the forearc region of the Finisterre arc-continent collision and the absence of a similar terrane in the forearc area on Taiwan. The very different evolution of sandstone provenance in the active Taiwan and Finisterre collision zones shows that local factors can profoundly influence the provenance histories of collision zones. No one model of sediment provenance evolution can characterize all collision zones, whether modern or ncient.

Palaeomagnetic research in Southeast Asia: progress, problems and prospects

Southeast Asia is a complex assembly of allochthonous continental terranes now far removed from their sites of origin together with island arcs, accretionary complexes and small ocean basins (Fig. 1). The boundaries between these terranes are marked by major fault zones or by sutures recognised by the presence of ophiolites, melanges and accretionary complexes. Stratigraphical, sedimentological, palaeobiogeo-graphical and palaeomagnetic data suggest that most, and probably all, of the Southeast Asian continental terranes were derived directly or indirectly from Gondwana. These terranes were assembled between the Late Palaeozoic and Cenozoic, but the precise times of rifting from Gondwana and timings of amalgamation and accretion are still contentious. Cenozoic modification of the region involves major strike-slip faulting, rotations and spatial displacements of crustal blocks, and the development of marginal basins. Palaeomagnetic data are vital for constraining the movements of crustal blocks in this region and ongoing systematic regional palaeomagnetic research is critical for an improved understanding of the geological evolution of Southeast Asia.

Geochemistry of the Start Complex greenschists: Rhenohercynian MORB?

AbstractThe meta-igneous greenschists of the Start Complex, SouthDevon, are composed of a mineralogically uniform, but texturally variable, actinolite-epidote-albite assemblage with retrogressed variants containing chlorite, muscovite, sphene, carbonate and oxidized opaque minerals. Geochemically they represent a suite of relatively primitive tholeiites, exhibiting mild differentiation, depleted incompatible element abundances, and variable light rare-earth-element-depletion patterns comparable to modern basalts from normal spreading ridge segments (N-MORB). As the Start greenschistsexhibit a number of chemical similarities to the nearby Upper Palaeozoic Lizard ophiolite, and MORB-type clasts within the Rhenohercynian Zone generally, they may also represent local Variscan ocean crust, which floored smalloceanic basins that separated the Old Red Sandstone continent from the Armorican microplate to the south. The Start Complex could thus represent a previously unrecognized oceanic component to the Variscan orogenic belt (Rhenohercynian Zone) of Northwest Europe.

The origin of hydrothermal metalliferous sediments associated with the early Mesozoic Othris and Pindos ophiolites, mainland Greece

Metalliferous sediments and sulphides are associated with the Jurassic Othris and Pindos ophiolites, central and northern Greece. These ophiolites formed at an Early Mesozoic spreading ridge within a small. Pindos ocean basin created by rifting of the northern margin of Gondwanaland. Ubiquitous terrigenous silt was derived from adjacent continental margins mainly the Pelagonian microcontinent to the east. In the Othris area, disseminated, massive and vein sulphides occur within and above MORB-type pillow lavas and volcaniclastic sediments. Sulphide was precipitated in quartz veins within fault-controlled stockworks. Episode (with local epidosite) is locally very abundant in extrusives immediately underlying thick SiFe-rich hydrothermal sediments. Massive sulphides were precipitated as small fault-controlled bodies. that are directly overlain by Fe-rich mudstones and Fe-rich cherts, strongly depleted in Mn. Mudstones, relatively enriched in hydrothermal Fe, Mn, Cu, Zn, Pb, REE and Ba, were ponded into small fault-controlled hollows around the fringes of the sulphide-precipitating field. Elsewhere (e.g. northern Othris), manganese-rich layers in ribbon radiolarites are strongly enriched in Mn and trace-elements. Metal-enriched deposits are also found within slices of mid-ocean ridge-type lavas associated with melange (Avdella Melange) structurally underlying the Pindos ophiolite, NW Greece. Sulphides there are restricted to tiny massive deposits overlying pillow lava and sulphide veins within ophiolitic extrusives. Pillow lavas are overlain by volcanic talus breccias, shed from submarine fault scarps. Overlying metalliferous mudstones are enriched in Fe and other hydrothermal constituents, probably derived from nearby sulphide precipitating zones and mixed with lithogenous sediments. The metalliferous deposits preserve a variety of hydrothermal deposits within an Early Mesozoic small ocean basin. The Pindos sediments reflect mixing of ferruginous hydrothermal constituents with terrigenous sediment, possibly during the earlier stages of basin opening. The Othris sulphide and ferruginous oxide-sediments record evidence of a small, high-temperature hydrothermal field, formed near the spreading axis when the ocean neared its maximum width. Manganese-rich cherts in the northern Othris area reflect widespread accumulation of hydrothermal constituents on the abyssal plain, derived from axial high-temperature hydrothermal fields and/or related to low-temperature (off-axis?) discharge.

Development of concepts concerning the Troodos ophiolite and adjacent units in Cyprus

Abstract The foundations for study of the Troodos ophiolite and adjacent units in Cyprus were laid down by systematic mapping by the Cyprus Geological Survey Department (1952–1970). Prior to the late 1950s, Cyprus geology was interpreted in classic geosynclinal terms. The first, integrated description of the Troodos was given by Gass & Masson Smith in 1963. Publication of the Vine & Matthew’s (1963) sea-floor spreading hypothesis was followed by Ian Gass’s key interpretation in 1968 of the Troodos as oceanic lithosphere formed by sea-floor spreading. During the early 1970s, the dominant view of the Troodos was of a mid-ocean ridge, formed by spreading at a narrow, linear spreading zone, fed by a single, axial magma chamber. However, early geochemical studies suggested that genesis could have taken place above a subduction zone. During the 1980s, drilling related to the International Cyprus Crustal Study Project stimulated much new field work. By 1990 a popular view of the Troodos was that it had formed above a northward-dipping subduction zone during the earlier stages of convergence within an oceanic basin, similar in some respects to the origin of SW Pacific forearcs. Others favoured an origin as a marginal basin formed by spreading above a southward-dipping subduction zone, either in the vicinity of Cyprus, or located in central Turkey. In whatever scenario, it is generally accepted that the spreading fabric was unstable and was fed by small, multiple magma chambers. The extension outpaced magma supply and this favoured extensional detachment faulting, mainly near the base of the sheeted dykes. The Troodos lithosphere was also bounded by an oceanic fracture zone to the present south. Studies of the Late Cretaceous-Recent sediment cover have documented deep-sea sedimentation and later uplift, related to Africa-Eurasia convergence. In W Cyprus, the adjacent Mamonia Complex was pieced together as a Mesozoic passive margin bordering a small ocean basin, although whether this was a Red Sea-type basin, or a backarc marginal basin is still debated. The Mamonia lavas and sediments were juxtaposed with the Troodos ophiolite by processes, variously interpreted as subduction/accretion, oceanic transform faulting, strike-slip, collision and/or palaeorotation of a Troodos microplate. The Kyrenia Range in N Cyprus also includes remnants of Mesozoic passive margin; it was emplaced in stages apparently by a combination of strike-slip and thrusting. On a regional scale, the Troodos ophiolite preserves part of one of a number of inferred small oceanic basins within the Mesozoic Tethyan area of the Eastern Mediterranean.