Jean-Claude Sibuet

Geological evolution of the tethys belt from the atlantic to the pamirs since the LIAS

Abstract We discuss nine palinspastic geological maps (Plates 1–9), at 1 20,000,000 scale, which depict the evolution of the Tethys belt from the Pliensbachian (190 Ma) to the Tortonian (10 Ma). A Present structural map (Plate 10) is shown for comparison at the same scale with the same conventions. Our reconstructions are based on a kinematic synthesis (Savostin et al., 1986), a paleomagnetic synthesis (Westphal et al., 1986) and geological compilations and analyses concerning in particular the western domain (Ricou et al., 1986), the eastern passive margins (Kazmin et al., 1986a), the eastern active margins (Kazmin et al., 1986b), the Black Sea-Caspian Sea basins (Zonenshain and Le Pichon, 1986) and the ophiolites (Knipper et al., 1986).

Kinematic evolution of the Tethys belt from the Atlantic ocean to the pamirs since the Triassic

Abstract We present an updated series of kinematic reconstructions of the major plates around the Tethys from the Atlantic Ocean to the Pamirs between the Early Jurassic and the Present. This set is used elsewhere as a basis for paleogeographic maps of the entire region. The problems related to the positions of the continents in the Lower Triassic are also discussed. No direct analyses of magnetic anomalies and fracture zones in the Atlantic have been made. Rather, all available poles and rotations have been tested in order to eliminate or minimize possible kinematic errors. The reconstructions are shown for nine key geological periods which correspond to well recognized magnetic anomalies, except for the Jurassic-Cretaceous boundary and the Cretaceous-Tertiary boundary which correspond to interpolated positions. Paleolatitudes have been drawn using the study of Westphal et al. (1986). An attempt has been made to take into account the displacements caused by formation of the continental margins and basins by stretching. The resulting relative vector of motions along the northern boundary of the Tethys shows a significant change 80 m.y. ago. Left-lateral motion with compression dominates before whereas right-lateral motion with compression dominates after. To the east, rates of motion vary by a factor of three with time and four maxima can be clearly related to tectonic events in the Late Jurassic, Late Cretaceous, Eocene and post Middle Miocene. To the west, north of Apulia, on the contrary, the motion rate has not changed significantly since the Early Cretaceous and is close to 1 cm/yr as an average. These rather complex adjustments in rates and directions of relative motion are produced in great part through a complex migration of the Africa-Eurasia pole of rotation and seem to be mostly governed by the tectonics of the Tethys plate boundary.

Okinawa trough backarc basin' Early tectonic and magmatic evolution

The Okinawa Trough, lying between Japan and Taiwan, is a backarc basin formed by extension within the continental lithosphere behind the Ryukyu trench-arc system. Stress directions associated with the two last extensional phases in the southwestern Okinawa Trough have been deduced from a comparison with analog modeling: the direction of extension is N 150 o for the Pleistocene phase of extension (2-0.1 Ma) and N 170 o for the late Pleistocene-Holocene phase of extension (0.1-0 Ma). The present-day Ryukyu volcanic arc, a narrow continuous feature extending from Japan to Taiwan, is located on the eastern side of the Okinawa Trough, 80-100 km above the Wadati-Benioff zone, the minimum depth for emplacement of arc magmatism. Scarce present-day backarc volcanism appears in the middle and southern Okinawa Trough within linear en echelon bathymetric depressions. A N045 o oriented seamount volcanic chain cuts across obliquely the southwestern Okinawa Trough and lies in the direct line of extension of the Gagua ridge, a N-S linear volcanic feature of the Philippine Sea plate. Associated with this extension of the Gagua ridge, a large reentrant located at the base of the Ryukyu prism, the uplift of part of the Nanao forearc basin and the deformation of the sedimentary arc suggest that the voluminous cross-backarc volcanism could be tied to the subduction of the Gagua ridge located there at a depth of 80-100 km beneath the backarc basin. A second area of anomalous volcanism has been identified in the middle Okinawa Trough in the ENE extension of the Daito ridge, a WNW-ESE 400-km-long volcanic feature of the Philippine Sea plate. We suggest that the Gagua and Daito ridges initially induced stress at the base of the arc which is still brittle and cracks propagated through the overlying brittle lithosphere, allowing magmas with arc affinities to erupt at the seafloor. This excessive magmatism reaches the seafloor through conduits which preferentially follow in their shallowest portion the crustal normal faults of the backarc rifts. The Okinawa Trough is consequently still in an early stage of evolving from arc type to backarc activity.

Paleomagnetic implications on the evolution of the tethys belt from the atlantic ocean to the pamirs since the triassic

Abstract We first re-examined the apparent polar wander curves for stable Eurasia and Africa since the Triassic. These curves were then combined together with curves of North and South America according to the kinematics of the Atlantic ocean and a synthetic polar wander curve was given. Then, most of the paleomagnetic results from the Tethys mobile belt, from the Atlantic to the Pamirs, were analysed. Several groups of plates, microplates and blocks can be seen. First, relatively stable regions like Maghreb and Sicily, which have not moved much. Then we have a group formed by Iberia, Sardinia, Italy and, to a lesser extent, Corsica and the Western and Central Alps. For these blocks, movements are anticlockwise rotations chiefly driven by the anticlockwise rotation of Africa, but they are sometimes stronger. To the east, a major change takes place. The north of the Aegean Sea and the Ionian zone are clockwise rotated and these rotations are recent: Oligocene-Miocene for the first part, Pliocene to the present for the second part. A major problem arises in Turkey, Caucasus and Iran. Paleomagnetic results indicate a position far to the south of Eurasia, and, at the same time, geological evidence is in favour of a position close to Eurasia. We discuss these discrepancies.

Magnetic evidence for slow seafloor spreading during the formation of the Newfoundland and Iberian margins

There is considerable debate concerning the nature and origin of the thin crust within the ocean^continent transition (OCT) zones of many passive non-volcanic continental margins, located between thinned continental and true oceanic crust. This crust is usually found to be underlain by upper mantle material of 7.2^7.4 km/s velocity at shallow depths (1^2 km). It has been proposed that such crustal material could have originated either by exhumation of upper mantle material during rifting of continents or by slow seafloor spreading. One of the examples of occurrence of such a crust are the conjugate margins of Newfoundland and Iberia. Here we present an interpretation of magnetic data from these regions to show that their OCT zones are underlain by crustal material formed by slow seafloor spreading (6.7 mm/yr) soon after Iberia separated from the Grand Banks of Newfoundland in the late Jurassic. Similarities in the magnetic anomalies and velocity distributions from these regions with those from the Sohm Abyssal Plain, a region lying immediately south of the Newfoundland Basin and formed by seafloor spreading at a similar rate of spreading, give further support to such an interpretation. The idea that these regions were formed by unroofing of upper mantle during rifting of Iberia from Newfoundland may be likely but the presence of weak magnetic anomalies in these regions, which bear all the characteristics of seafloor spreading anomalies, makes it difficult to ignore the possibility that these regions could be underlain by oceanic crust formed during slow seafloor spreading. The similarities in velocity structure and the presence of small amplitude magnetic anomalies both across this pair of conjugate margins of the North Atlantic and that of the Labrador Sea suggest that this OCT velocity structure may be the norm rather than the exception across those passive non-volcanic margins where the initial seafloor spreading was slow. Furthermore, the existence of similar velocity distributions along a few active spreading centers raises the possibility of formation of similar crust across slow spreading ridges. fl 2000 Elsevier Science B.V. All rights reserved.

The fit of the continents around the North Atlantic Ocean

Abstract A new reassembly of the continents around the North Atlantic Ocean is presented. The first criterion used for this reassembly is the identification of the structural framework related to the opening which consists of marginal fracture zones generated by offsets of the Rift. The Africa—North America, Eurasia—Greenland, Greenland—North America and Eurasia—North America adjustments are successively discussed. It is argued that the adjustments are best made at the 3000-meters isobath between Africa and North America and at the 2000-meters isobath for the younger rifts. The difference is attributed to subsidence and modification of continental margins with time. The importance of the Late Paleozoic tectonic phase in determining the subsequent pattern of Mesozoic rifting is emphasized.

Exhumed mantle-forming transitional crust in the Newfoundland-Iberia rift and associated magnetic anomalies

Transitional zones located between Iberia and North America formed during continental rifting and mostly consist of exhumed mantle. In this study we show that ages of exhumed mantle at Ocean Drilling Program (ODP) sites 1068 and 1070 in the Iberia Abyssal Plain and site 1277 in the Newfoundland Basin are similar to ages determined from magnetic lineations created by serpentinization during mantle exhumation. On the basis of paleomagnetic and geological data and a comparison with a fossil ocean-continent transition in the Alps, we envisage a first episode of mantle serpentinization during which a strong component of magnetization was acquired followed by a second episode occurring at the contact with cold seawater, and which only affects the upper tens of meters of the exhumed mantle. The inversion of magnetic data (Euler deconvolution) shows that magnetic sources are N-S trending horizontal cylindrical bodies located within the highly serpentinized upper crust. Therefore the serpentinization process is able to produce magnetic lineations formed in a similar way to those formed by seafloor spreading. Within transition zones, sequences of magnetic anomalies can provide information concerning the timing of the emplacement of crust, but not on its nature (oceanic versus exhumed mantle). This discovery enables us to date the exhumation of mantle rocks in transition zones and allows kinematic reconstructions of the final stages of continental rifting. During rifting the deep distal continental margins and the adjacent transitional zones in the Newfoundland-Iberia rift system were formed by ultraslow extension from early Berriasian to late Valanginian–early Hauterivian and by slow extension from early Hauterivian to the late Aptian–early Albian boundary. Therefore transitional zones share many similarities with slow and ultraslow spreading midoceanic ridges

The ocean‐continent boundary off the western continental margin of Iberia: Crustal structure west of Galicia Bank

A seismic refraction transect across the Galicia Bank continental margin shows that the original continental crust thins westward from 17 to 2 km immediately east of a margin-parallel peridotite ridge (PR). immediately west of the PR, oceanic crust is only 2.5-3.5 km thick, but farther west (oceanward) it thickens to 7 km. The PR caps a similar to 60-km-wide lens-shaped serpentinized peridotite body underlying both thinned continental and thin oceanic crust. When superimposed on a reflection time version of the velocity model, the S reflector is clearly intracrustal at its east end. Westward, S cuts down to lower crustal levels, eventually coinciding with the top of the serpentinized peridotite lens (original crust-mantle boundary). These observations render almost impossible the seafloor exposure of the PR by S acting as a top-to-the-west detachment fault. Numerical models of melting and borehole subsidence information constrain our rifting model. The easternmost continental crust experienced a total stretching factor of 4.3 (most likely in two stages); it probably occurred over similar to 25 m.y., with the highest Fate of stretching at the beginning of the main earlier rift phase (Valanginian; 141-135 Ma). The 3 (4.7) km thick continental crust (depending on whether serpentinized peridotite is assigned to crust or mantle), which may include melt products, requires stretching factors 6 more than 11 (7) and a rift duration of more than 25 (13) m.y. The thin oceanic crust immediately west of the PR is explained by conductive cooling of the mantle during the long prebreakup stretching phase, which temporarily caused reduced melting immediately after breakup.

East Asia plate tectonics since 15 Ma: constraints from the Taiwan region

Abstract 15 Ma ago, a major plate reorganization occurred in East Asia. Seafloor spreading ceased in the South China Sea, Japan Sea, Taiwan Sea, Sulu Sea, and Shikoku and Parece Vela basins. Simultaneously, shear motions also ceased along the Taiwan–Sinzi zone, the Gagua ridge and the Luzon–Ryukyu transform plate boundary. The complex system of thirteen plates suddenly evolved in a simple three-plate system (EU, PH and PA). Beneath the Manila accretionary prism and in the Huatung basin, we have determined magnetic lineation patterns as well as spreading rates deduced from the identification of magnetic lineations. These two patterns are rotated by 15°. They were formed by seafloor spreading before 15 Ma and belonged to the same ocean named the Taiwan Sea. Half-spreading rate in the Taiwan Sea was 2 cm/year from chron 23 to 20 (51 to 43 Ma) and 1 cm/year from chron 20 (43 Ma) to 5b (15 Ma). Five-plate kinematic reconstructions spanning from 15 Ma to Present show implications concerning the geodynamic evolution of East Asia. Amongst them, the 1000-km-long linear Gagua ridge was a major plate boundary which accommodated the northwestward shear motion of the PH Sea plate; the formation of Taiwan was driven by two simple lithospheric motions: (i) the subduction of the PH Sea plate beneath Eurasia with a relative westward motion of the western end (A) of the Ryukyu subduction zone; (ii) the subduction of Eurasia beneath the Philippine Sea plate with a relative southwestward motion of the northern end (B) of the Manila subduction zone. The Luzon arc only formed south of B. The collision of the Luzon arc with Eurasia occurred between A and B. East of A, the Luzon arc probably accreted against the Ryukyu forearc.

The Ocean--Continent Transition in the Uniform Lithospheric Stretching Model: Role of Partial Melting in the Mantle [and Discussion]

The role of partial melting in the uniform lithospheric stretching model of continental margin formation is explored. It is shown that the transition from continental lithosphere stretching to oceanic accretion is most probably controlled by the production of a significant amount of partial melting in the asthenosphere immediately below the lithosphere, which requires stretching factors larger than 3. It is also shown that, at stretching factors exceeding 2, the law of subsidence is significantly changed by the presence of partial melt in the underlying asthenosphere. The implications for the existence of deep continental margin basins on thinned continental crusts are examined. The Armorican deep continental margin basin is taken as an example.