Opening Of Marginal Basins Research Articles

In search of Gondwana heritage in the Outer Melanesian Arc: no pre-upper Eocene detrital zircons in Viti Levu river sands (Fiji Islands)

Volcanic arcs of the Southwest Pacific, collectively referred to as the Outer Melanesian Arc, are generally thought to result from subduction of the Pacific Plate since the Late Cretaceous. Meanwhile, it is largely accepted that eastward roll-back of the old and dense oceanic plate allowed opening of marginal basins, which isolated large blocks of the former Gondwana margin. Incidentally, some ‘intra-oceanic’ volcanic arcs may have been nucleated on small continental fragments. Detrital zircons collected from sand banks in the mid-reaches of rivers from Viti Levu Island have been analysed for U–Pb geochronology and geochemistry, in order to search for a possible ancient continental arc basement, remnants of a Late Cretaceous arc, and determine the timing and evolution of Fiji arc magmatism. In contrast with some other places of the Outer Melanesian Arc (Solomon, Vanuatu), no pre-upper Eocene zircons have been found. Thus, Gondwana-derived fragments or Late Cretaceous–Paleocene arc remnants are unlikely to form the basement of Viti Levu. Zircon geochemistry confirms the purely intra-oceanic character of volcanic-arc magmatism as well. Variations in some trace-element ratios closely reflect the evolution of Viti Levu Arc from upper Eocene inception to upper Miocene climax and finally Pliocene intra-arc rifting and abandonment.

Development of framboidal pyrite in the Upper Permian marly limestone of the NE-Hungarian Darnó Hill

The NE Hungarian Darno Hill has a complex geology; it is built up by an accretionary melange complex, which contains Permian, Triassic and Jurassic sedimentary and magmatic blocks too. The succession can be well correlated with the NW Dinarides, and evidences of the different evolutionary stages of the Neotethyan Ocean (rifting, marginal basin opening, closure) were described too. Several ore indications are known from this area, but their genesis was by far not well reconstructed, mostly because of the complexity of the geological background. The present study deals with one of these indications found in deep drillings. A framboidal pyrite bearing Permian-Triassic marly limestone series was investigated, which was described earlier as a possible analogy of the Polish-German copper shales ( Kupferschiefer ). Based on detailed microscopic, EPMA and whole rock geochemical analyses, it was found, that besides the framboidal pyrite, euhedral pyrite overgrowths (both contain high amount of Au), disseminated euhedral and anhedral pyrite (which do not form framboids or overgrowths), anhedral chalcopyrite, galena and sphalerite were also found, causing a slightly enriched total metal content of 100-200 ppm. The minerals were formed under reducing, anoxic marine conditions, in more steps; after the synsedimentary processes, sulphides formed during early diagenetic/hydrothermal processes too. The found characteristics are typical to the weakly mineralized type of the Kupferschiefer .

Introduction to TETHYS—an interdisciplinary GIS database for studying continental collisions

The TETHYS GIS database is being developed as a way to integrate relevant geologic, geophysical, geochemical, geochronologic, and remote sensing data bearing on Tethyan continental plate collisions. The project is predicated on a need for actualistic model ‘templates’ for interpreting the Earth's geologic record. Because of their time-transgressive character, Tethyan collisions offer ‘actualistic’ models for features such as continental ‘escape’, collision-induced upper mantle flow magmatism, and marginal basin opening, associated with modern convergent plate margins. Large integrated geochemical and geophysical databases allow for such models to be tested against the geologic record, leading to a better understanding of continental accretion throughout Earth history. The TETHYS database combines digital topographic and geologic information, remote sensing images, sample-based geochemical, geochronologic, and isotopic data (for pre- and post-collision igneous activity), and data for seismic tomography, shear-wave splitting, space geodesy, and information for plate tectonic reconstructions. Here, we report progress on developing such a database and the tools for manipulating and visualizing integrated 2-, 3-, and 4-d data sets with examples of research applications in progress. Based on an Oracle database system, linked with ArcIMS via ArcSDE, the TETHYS project is an evolving resource for researchers, educators, and others interested in studying the role of plate collisions in the process of continental accretion, and will be accessible as a node of the national Geosciences Cyberinfrastructure Network—GEON via the World-Wide Web and ultra-high speed internet2. Interim partial access to the data and metadata is available at: http://geoinfo.geosc.uh.edu/Tethys/ and http://www.esrs.wmich.edu/tethys.htm. We demonstrate the utility of the TETHYS database in building a framework for lithospheric interactions in continental collision and accretion.

The building of pericratonic mountain ranges : structural and kinematic constraints applied to GIS-based reconstructions of SE Asia

Abstract Southeast Asia represents a valuable playground for studying the mechanisms of supra subduction mountain building. Fast relative plate velocities induce rapid geodynamic changes, so that the parameters for the convergence can be constrained, whereas the recently inactivated systems are still accessible. We make use of the GPS-controlled motion of major plates together with that of smaller units within the deformed belts, to explore the Neogene evolution of SE Asia. We present simplified synthetic sections for the structural control, and used GIS-based reconstructions to support the relative location of the plates and distances between them, so that at a given time, location of units can be confronted to the tectonic observations. Pericratonic mountain ranges are the result of long lasting convergence which was responsible for both the fragmentation of continental landmasses by the opening of marginal basins, and the shortening of these supra subduction units, which ultimately jammed them back against the motherland, or transferred them to the neighbouring plate. Because the obliquity of the convergence generated strain partitioning, new paired trench and strike-slip systems are created, moving blocks further away from the locus of accretion, so that continental or oceanic fragments may be chopped, transported and eventually severely sheared during final docking. We present simplified cross sections of the margins of the SE Asian regions prior and after the accretions of blocks and reconstructions at 2, 4, 6, 10, 15 and 20 Ma.

New constraints on the structure, thermochronology, and timing of the Ailao Shan‐Red River shear zone, SE Asia

New structural, petrographic, and 40Ar/39Ar data constrain the kinematics of the ASRR (Ailao Shan‐Red River shear zone). In the XueLong Shan (XLS), geochronological data reveal Triassic, Early Tertiary, and Oligo‐Miocene thermal events. The latter event (33–26 Ma) corresponds to cooling during left‐lateral shear. In the FanSiPan (FSP) range, thrusting of the SaPa nappe, linked to left‐lateral deformation, and cooling of the FSP granite occurred at ≈35 Ma. Rapid cooling resumed at 25–29 Ma as a result of uplift within the transtensive ASRR. In the DayNuiConVoi (DNCV), foliation trends NW‐SE, but is deflected near large‐scale shear planes. Stretching lineation is nearly horizontal. On steep foliations, shear criteria indicate left‐lateral shear sense. Zones with flatter foliations show compatible shear senses. Petrographic data indicate decompression from ≈6.5 kbar during left‐lateral shear (temperatures >700°C). 40Ar/39Ar data imply rapid cooling from above 350°C to below 150°C between 25 and 22 Ma without diachronism along strike. Along the whole ASRR cooling histories show two main episodes: (1) rapid cooling from peak metamorphism during left‐lateral shear; (2) rapid cooling from greenschist conditions during right‐lateral reactivation of the ASRR. In the NW part of the ASRR (XLS, Diancang Shan), we link rapid cooling 1 to local denudations in a transpressive environment. In the SW part (Ailao Shan and DNCV), cooling 1 resulted from regional denudation by zipper‐like tectonics in a transtensive regime. The induced cooling diachronism observed in the Ailao Shan suggests left‐lateral rates of 4 to 5 cm/yr from 27 Ma until ≈17 Ma. DNCV rocks always stayed in a transtensive regime and do not show cooling diachronism. The similarities of deformation kinematics along the ASRR and in the South China Sea confirms the causal link between continental strike‐slip faulting and marginal basin opening.

Overview of the Late Tertiary–Recent tectonic and palaeo‐environmental development of the Mediterranean region

ABSTRACTThe Late Tertiary history of the Mediterranean region exemplifies processes of ocean basin closure and continental collision, as determined from integrated land and marine evidence. During the Mesozoic–Early Tertiary, tectonic settings were dominated by evolution of Neotethys. This ocean generally widened eastwards, with a number of oceanic strands in the Eastern Mediterranean area. Great diversity of tectonic settings and palaeo‐environments developed during the Tertiary closure history of these oceanic basins.In the Eastern Mediterranean region, more northerly Neotethyan strands were closed by the Mid Tertiary, while oceanic crust remained in the south in the present Eastern Mediterranean Sea area.Northwards subduction of the remaining southerly Neotethyan strand was probably active by the Early Miocene. Different areas exhibit different stages of convergence and ocean basin closure. In the east, the amalgamated Eurasian plate had collided with the Arabian margin (Africa) by the Late Miocene, while oceanic crust still persisted further west. Steady‐state subduction during the Late Tertiary gave rise to the Mediterranean ridge, as a substantial mud‐dominated accretionary wedge. In the Aegean area, sufficient northward subduction took place to activate arc volcanism and pervasive back arc extension, short of marginal basin opening. In the easternmost Mediterranean, only limited subduction took place, associated with supra‐subduction zone extension (e.g. in Cyprus). Today, steady state‐subduction continues only locally, where vestiges of Neotethys remain (e.g. Herodotus abyssal plain).In the Western Mediterranean area, suturing of the African and Eurasian plates initially took place in the Betic region (Early–Mid Tertiary), where the Neotethys had existed only as a narrow connection with the Central North Atlantic. In the Central Mediterranean region, where the Western Neotethys was wider, northward subduction was active, apparently as early as the Late Cretaceous. In a widely accepted interpretation, an Andean‐type magmatic arc developed along the southern margin of Europe and was then rifted off in the Late Oligocene‐Early Miocene, to form the Corsica‐Sardinia Block, opening the North Balearic marginal basin in its wake. The migrating subduction zone and microcontinent then collided diachronously with North Africa‐related continental units (North Africa and Apulia) from Late Oligocene‐Early Miocene, giving rise to collisional thrust belts in the Northern and Southern Apennines and along the North African continental margin (i.e. the Maghrebian chain) to the Betic‐Rif area. From the Early Miocene onwards, a separate subduction system became active, related to removal of Neotethyan oceanic crust to the southeast (Ionian Sea), fueling suprasubduction zone extension and opening of the Tyrrhenian Sea. ‘Orogenic collapse’ is an alternative mechanism of such extension, and is widely believed to have caused divergent thrusting in the Betic and Rif regions of the westernmost Mediterranean, at the same time as crustal extension and subsidence of the Alboran Sea.

A Late Palaeozoic‐Early Mesozoic marginal basin along the actives sourthern continental margin of Eurasia: Evidence from the central Pontides (Turkey) and adjacent regions

AbstractRemnants of two ‘Palaeotethyan’ oceanic basins are exposed in the Central Pontides of northern Turkey, separated by a continental sliver and an oceanic arc. The southern basin corresponds to the main Tethys (‘Palaeotethys’), which partially closed in Early Mesozoic time following northward subduction under the southern, active continental margin of Eurasia.The northern basin (Küre Complex) opened above the ‘Palaeotethyan’ subduction zone as a marginal basin, following rifting of a continental fragment (Istanbul fragment) from Eurasia. Marginal basin opening apparently dates from the Late Palaeozoic in the east (Küre basin) and from the Triassic in the west (Kocaeli basin). Basin closure was achieved by southward subduction‐accretion, in pre‐Late Jurassic times, leaving ‘Neotethys’ open to the south.Counterparts of the Küre Complex are found in the adjacent Crimea (Taurian Series), Istranca (Zabernevo Complex), Dobrogea (Nalbant flysch) and Caucasus (pre‐Late Jurassic Southern Slope Basin) regions. Basin opening was accompanied by oceanic crust genesis, at least in the Pontides and Caucasus. Closure before Mid‐Jurassic time was achieved by subduction‐accretion processes, whereby oceanic crust and deep‐sea sediments (including sulphides) were detached and structurally assembled, while oceanic basement was subducted. Marginal basin opening and closure is seen as one in a series of events along a long‐lived, active south Eurasian continental margin.

Arc deformation and marginal basin opening: Japan Sea as a case study

We discuss the opening mechanism of the Japan Sea in Miocene time using (1) tectonic and published paleomagnetic data along the eastern margin from the north of Hokkaido Island to Sado Island, (2) a mechanical model which is tested by small‐scale physical modeling, and (3) crustal structure and bathymetric features in the Japan Sea which constrain our kinematic model and preopening reconstructions. Our main conclusions are the following. The eastern margin of the Japan Sea was, as a whole, a dextral shear zone about 100 km wide. This conclusion is supported by the existence of a ductile dextral shear zone in Central Hokkaido (Hidaka Mountains) and associated brittle deformation in western Hokkaido and northeastern Honshu. The stress field during the opening (which ended about 12 Ma ago at the end of the middle Miocene) changes from right‐lateral transpression in the north to right‐lateral transtension in the south. The western margin, along the Korean peninsula, during the same period, also was an active dextral shear zone. Paleomagnetic results indicate that clockwise rotations occurred in the south during the opening and counterclockwise rotations in the north. We propose a model of right‐lateral pull‐apart deformation with clockwise rotations of rigid blocks in the southern transtensional domain and counterclockwise rotations in the transpressional one. Small‐scale physical models show that the clockwise rotation in transtension is possible provided that the eastern boundary (Pacific side) is free of stress. The opening stopped and compression subsequently began about 12 Ma ago. Finally, we show that the dextral shear, which is distributed over the whole Japan Sea area, is accommodated by N‐S trending right‐lateral faults and rotation of blocks located between these right‐lateral faults.

Right‐lateral shear along the Northwest Pacific Margin and the India‐Eurasia Collision

Right‐lateral shear along the eastern margin of Asia, from the Eocene to the Present has led to the opening of pull‐apart basins, intracontinental such as the Bohai basin, or oceanic such as the Japan Sea. We suggest in this paper that this right‐lateral shear is a consequence of indentation of Asia by India. As in small‐scale analog experiments, we conclude that antithetic wrench faults accommodate the counterclockwise rotation of large domino blocks between two major left‐lateral shear zones (Tien Shan‐Baikal‐Stanovoy for the northern one, and Qin Ling for the southern one). We discuss the compatibility of this mechanism, which involves a rather small amount of extrusion, with the fast eastward expulsion described for southeast Asia. We re‐emphasize the role played in the opening of marginal basins by the Pacific subduction as a free boundary to the east.

America-Eurasia plate boundary in eastern Asia and the opening of marginal basins

The opening of the Arctic Ocean during the past 55 Ma resulted in relative rotation of America with respect to Eurasia about a pole located in eastern Siberia, near the plate boundary. The extensional plate boundary enters deep inside the Eurasian continent up to the rotation pole. On the opposite side of the pole, on the Pacific side of the plate boundary, compressive tectonics are recorded along Sakhalin and Hokkaido. From the Oligocene to Middle Miocene, the relative movement was accommodated by strike-slip motion along Sakhalin and Hokkaido although the rotation pole was not located at a significatively different position from now. In this paper we explain this by independent motion of the southernmost tip of the American plate towards the Pacific margin which behaves as a free boundary. This oceanward motion resulted in an extension of the American plate giving rise to the wedge structure of the Okhotsk Sea. The Japan Sea opened as a pull-apart basin along the strike-slip boundary; finally the increasing extension in the Okhotsk Sea led to the opening of the oceanic Kuril Basin.

Relative plate motions and the formation of marginal basins

The location and timing of activity in marginal basins are considered in the context of relative plate motions. The motion of the Pacific and Farallon plates are reconstructed relative to the overriding Eurasian and South American plates for two models: Antarctica as one and as two plates (converging over the interval from 59 to 36 m.y. B.P.). The two‐plate model (the more consistent) indicates that over the last 80 m.y. the motion of the Pacific plate relative to Eurasia was first westward (80–59 m.y. B.P.), then northward (59–36 m.y. B.P.), and then westward again (36 m.y. B.P. to present). For those Pacific marginal basins where magnetic anomalies have been identified, there is a pattern related to the direction of motion of the Pacific. Those basins active during the time period of northward motion of the Pacific opened in a north‐south direction, and most of those which were active during the recent time period of westward motion of the Pacific opened a east‐west direction. No basins were active during the time of change of direction. This strong correlation between the opening of marginal basins and the relative motions of the converging plates suggests that changes in these relative motions are a factor in the episodicity of marginal basins.

Extension behind island arcs and motions relative to hot spots

The spatial distribution and episodic nature of behind‐arc spreading is as yet unexplained. The present sites of rapid marginal basin opening well‐developed enough to be producing unmistakable oceanic crust are the Tonga‐Kermadec, Mariana, and Scotia arcs and the Andaman Sea. In relation to a hot spot reference frame the major plate lying behind the trench in all these spreading arc systems moves away from the subduction zone (retreats). Taking into account the behind‐arc spreading, almost all of the oceanic subduction zones advance in the hot spot frame, i.e., move toward the side from which the subducted slab approaches. This leads to the suggestion that marginal basin opening may be related to a component of ‘trench suction’ in the driving forces of plate tectonics.