Claude Rangin

The active Main Marmara Fault

Abstract This paper presents selections from and a synthesis of a high resolution bathymetric, sparker and deep-towed seismic reflection data set recently acquired by the French Ifremer R.V. Le Suroit in an E–W deep trough that forms the northern half of the Sea of Marmara in NW Turkey. It includes the first high resolution complete bathymetric map of this area. A single, throughgoing dextral strike–slip fault system, which is the western continuation of the northern branch of the North Anatolian Fault, cuts this trough lengthwise and joins the 1999.8.17 Kocaeli earthquake fault with the 1912.8.09 Şarkoy–Murefte earthquake fault, both of which display strike–slip offset. In its eastern one fourth, the structure follows closely the northern margin of the deep trough, whereas in the west it hugs its southern margin. The eastern one fourth of the structure has a minor component of its displacement distributed across the deep trough owing to a possible original bend in the course of the dextral structure. The present course of the North Anatolian Fault in the Sea of Marmara originated some 2×105 a ago, by cutting across the older basin fabric generated by a dominant NNE–SSW extension before it began taking up major motion in the Pliocene.

Tectonic setting of Western Pacific marginal basins

Abstract Global kinematics as well as magnetic anomalies of marginal basins, with continental geology and paleomagnetic data as additional constraints, are used to present a set of reconstructions of the Western Pacific marginal basins between 56 Ma and the Present at key periods (56, 43, 32, 20, 12 and 3 Ma). Our model accounts for the rapid motion of “exotic terranes” along the whole of the Western Pacific convergent zone. Marginal basins appear to open in a great variety of tectonic settings, the two extreme examples being the Mariana trough where trench suction may be the predominant driving force and the South China Sea where intracontinental deformation appears to be the major mechanism. The study of marginal basins is a 3-D problem which must take into account the whole tectonic context (subduction related tectonics in cross-section and upper plate deformation in map view) and not only a 2-D problem (the classical trench-arc-back-arc problem).

Indosinian NW-trending shear zones within the Truong Son belt (Vietnam) 40Ar39Ar Triassic ages and Cretaceous to Cenozoic overprints

Abstract Several NW-trending ductile and partly mylonitic shear zones cross the Truong Son belt of Central Vietnam, along the Song Ma and Song Ca valleys and, north of the Kontum block, from Da Nang to Aluoi and Khe Sanh. High-grade metamorphic rocks of amphibolite facies, showing a retrograde evolution and consisting of ortho- and paragneisses, metavolcanics, amphibolites, marbles and quartzites are exposed along these structures. They display a homogeneous deformation pattern characterized by a generally steeply dipping foliation and a near-horizontal to gently plunging mineral stretching lineation, indicating a strike-slip tectonic regime of deformation. Along the southernmost fault zone at least, various and consistent kinematic indicators, including SC structures, asymmetric tails of porphyroclasts, prove that the strain and metamorphism have been generated by a phase of dominantly non-coaxial deformation with a dextral sense of shear. 40Ar39Ar dating, applied on the high-grade metamorphic rocks minerals occurring along these zones, provide plateau cooling ages, closely around 245 Ma, establishing that this event took place in the lowermost Triassic as an early phase of the Indosinian orogeny. The existence of Indosinian movements in Vietnam, as they have been defined by previous authors during the early century, is now accurately confirmed and this is the first insight in the occurrence of ductile strike-slip tectonics of Indosinian age along NW-SE fault zones. Well-expressed in Central Vietnam is a Cretaceous thermal and deformational overprint, marked by epimetamorphism, which took place between 90 and 120 Ma, as attested by low-temperature degassing ages. This Cretaceous event is not found further north in the Song Ma zone where younger ages appear as a result of the influence of Cenozoic shear movements along the Red River fault which displays 20–30 Ma ages. On the northern flank of Song Ca, in the Bu Khang-Phu Hoat core complex, comparable Oligocene-Lower Miocene ages (20–35 Ma), yielded by biotite and phlogopite, reflect a strong overprinting and attest to a rapid uplift of the basement.

Present-day crustal deformation around Sagaing fault, Myanmar

[1] Global Positioning System (GPS) measurement campaigns in Myanmar, conducted in 1998 and 2000, allow quantifying the present-day crustal deformation around the Sagaing fault system in central Myanmar. Both a regional network installed at four points within the country and a local 18-station network centered on the city of Mandalay across the Sagaing fault demonstrate that active deformation related to the northward motion of India is distributed across Myanmar in a platelet that extends from the western edge of the Shan Plateau in the east to the Andaman Trench in the west. In this platelet, deformation is rather diffuse and distributed over distinct fault systems. In the east, the Sagaing/Shan Scarp fault system absorbs 10 mm/yr). This GPS study combined with an on land geotectonic survey demonstrate that oblique slip of India along the rigid Sundaland block is accommodated by a partitioned system characterized by distribution of deformation over a wide zone. INDEX TERMS: 1206 Geodesy and Gravity: Crustal movements—interplate (8155); 1243 Geodesy and Gravity: Space geodetic surveys; 8110 Tectonophysics: Continental tectonics—general (0905); 8158 Tectonophysics: Plate motions—present and recent (3040); KEYWORDS: tectonics, GPS, fault, Southeast Asia

Tectonics of the western margin of the Shan plateau (central Myanmar): implication for the India -Indochina oblique convergence since the Oligocene

Abstract We present new data and interpretations on the Neogene tectonics of the Shan scarp area (central Myanmar) and its relationship with the India–Indochina oblique convergence. We describe ductile and brittle fabrics associated with the major features in this area, the Mogok Metamorphic Belt (MMB), the Shan scarp and the Sagaing fault. From these data we identify a succession of two tectonic regimes. First, a dominant NNW–SSE-trending extension, marked by ductile stretching that characterizes the MMB, and associated N70E brittle normal faults. Later, from Middle or Upper Miocene to the Present, these structures were cross-cut by brittle right-lateral faults, among which the most important are the N20W transpressive Shan scarp fault zone and the N–S Sagaing fault. To explain this transition from a dominant transtensive to a transpressive stress regime, that occurred during Miocene, we place our data within a larger geodynamic context. We suggest that, like the intraplate deformation in the Indian Ocean, the end of spreading in the South China sea, the opening of the Andaman basin or the end of subduction within the Indo-Burma range, the change in the tectonic regime in central Myanmar could be in response to a major Miocene regional plate kinematic reorganization.

Magmatic response to abrupt changes in geodynamic settings: Pliocene—Quaternary calc-alkaline and Nb-enriched lavas from Mindanao (Philippines)

Abstract Mindanao, the largest island in the southern Philippine archipelago, is a composite of at least two terranes; one with Eurasian affinity (western Mindanao) and the other belonging to the Philippine Mobile Belt (eastern Mindanao), of Philippine Sea plate affinity. The island is surrounded by three subduction zones that have been installed only in the past 4 m.y. Prior to this, the two terranes were separated by an ocean that disappeared continuously by subduction of its two edges beneath western and eastern Mindanao, where mostly typical arc magmatic rocks, dated at 30 Ma, 19−15 Ma, 12−11 Ma and 7−4 Ma were emplaced. The suturing of the two terranes occurred at ca. 5 Ma. Following this major structural reorganization, abrupt changes are recorded in the The geochemical diversity of magmatic types in Mindanao is attributed to: 1. (1) the highly heterogenous character of their mantle source, which contains variable amounts of metasomatic pargasite and phlogopite, and, possibly, an additional OIB component that could contribute to Nb enrichment of NEB; 2. (2) the contribution of melts from the ubducted oceanic crust; these melts are either emplaced directly on the surface (adakites) or act as metasomatic agents leading to a Nb-enriched mantle, a probable source of NEB. Garnet and amphibole fractionation could also account for additional variations in the MREE and the HREE.

Indochina Peninsula and the collision of India and Eurasia

The collision of India with Asia results in a rapidly changing stress pattern around both Himalayan syntaxes. The maximum compressive stress, which is north- south north of the indenter, fans radially through nearly 180° around the syntaxes to accommodate the necessary strike-slip motion on the edges of the indenter. Thus, the whole Indochina Peninsula was swept by the migrating stress and associated strain pattern as the eastern syntaxis moved to the north. As a result, the maximum horizontal component of the stress field is predicted to have been first oriented east-west, then north-south. We propose that this changing stress field controlled the opening of the sedimentary basins and of the South China Sea as well as the direction of motion of the major strike-slip faults.

Geodynamic evolution of the Taiwan-Luzon-Mindoro belt since the late eocene

Abstract The structural framework of the Taiwan-Luzon-Mindoro belt (or festoon) is described, following three major transects: the Luzon transect with active subduction and active island arc; the Taiwan transect with active collision; the Mindoro transect with active subduction and inactive collision. Based on this geological study and on available geophysical data, a model for the geodynamic evolution of this portion of the Philippine Sea and Eurasia Plates boundary is proposed in a succession of reconstructions between the Late Eocene and the Present. The major geodynamic events are: 1. (1) beginning of the opening of the South China Sea (S.C.S.) in Lower Oligocene times, contemporaneous with obduction of the Zambales and Angat ophiolites on Luzon. 2. (2) subduction of a Mesozoic (?) oceanic basin along the proto-Manila trench from the Upper Oligocene to the Lower Miocene. 3. (3) obduction of the South China Sea oceanic crust onto the Chinese and Reed Bank—Calamian passive margins in Middle Miocene time (14–15 Ma) related to a major kinematic reorganization (end of opening of the S.C.S.). 4. (4) beginning of collision between the Luzon microblock and the two margins of the S.C.S. in the Upper Miocene (~ 7 Ma); collision is still active in Taiwan whereas it stopped in Mindoro during the Pliocene.

The Mindanao collision zone: a soft collision event within a continuous Neogene strike-slip setting

Abstract Two volcanic belts are presently juxtaposed on Mindanao Island in the southern Philippines. Southward, the collision is still active in the Molucca Sea which is commonly regarded as a region of doubly verging subduction, plunging eastward below the Halmahera arc and westward below the Sangihe arc. In the Molluca Sea, tectonic features related to the incipient collision appear only in the very thick sediments of the basin, and the morphology of the parallel Halmahera, Talaud and Sangihe ridges is closely controlled by recent N-S strike-slip faults. Among these faults, the Philippine Fault is a neotectonic feature crosscutting the Agusan-Davao Basin which seals tectonic events not younger than Eocene. In addition, the Central Cordillera shows strong similarities with the Pacific Cordillera for both stratigraphy and tectonic evolution, and several indications favour a Eurasian margin affinity for the Daguma Range (Southern and Eastern Kudarat Plateau that may be part of the Sangihe arc, as inferred for the Zamboanga Peninsula and the Northern Arm of Sulawesi. Thus the island of Mindanao can be divided into two composite terranes, the western one (northward extension of the Sangihe arc) being restricted to the Kudarat Plateau and the Zamboanga Peninsula. The apparent continuation of the Sangihe arc into the Central Cordillera of Mindanao is thus the result of post collision tectonics. The portion of the suture where the collision is completed curves westward north of the southern peninsula and extends beneath the sediments of the Cotabato Basin or the volcanic plateaus of the Lanao-Misamis-Bukidnon Highlands. In the northern part, the contact is linear and suggests, together with the absence of compressional deformation, a docking of the eastern oceanic terrane (Philippine Mobile Belt-Halmahera arc) against the western continental terrane (Zamboanga-Daguma) in a strike-slip environment. Prior to Early Pliocene, the eastern and the western terranes were subject to different tectonic regimes with direction of extension perpendicular to the present one. From Late Pliocene to present, both terranes are affected by NNE and E-W compression.

Investigation of the tectonics of the Main Marmara Fault by means of deep-towed seismic data

Abstract We report a study of the active principal deformation zone (PDZ) of the Main Marmara Fault (MMF) in the sub-basins of the Sea of Marmara by means of deep-towed seismic (Pasisar) as well as multi-beam bathymetry data collected by Ifremers r/v Le Suroit in September 2000. Our main objective is to investigate the active deformation within the uppermost sedimentary layers with a much higher resolution than hitherto has been available. To the west of the Sea of Marmara, the PDZ is located along the southern flank of the Tekirdag Basin where the sediments are affected by steep reverse faults dipping toward the north. E–W strike–slip faults are also observed in the central part of the Tekirdag Basin and thrusting occurs along its N50°-trending margin. We interpret these structures in terms of a slight clockwise rotation in the basin north of the PDZ. To the east, the PDZ enters the Central Basin and follows a steep scarp along the southern flank of a tectonic depression. The scarp consists of an en echelon fault system with a normal component. These faults are combined with small parallel anticlines and synclines that extend along the southern portion of the depression. The northern scarp of the depression is made of a mixed system of faults with both normal and reverse components associated with anticlines and synclines. These faults are best interpreted as right lateral strike–slip faults with a vertical component that is dominantly normal. These faults and the sigmoid shape of the depression are compatible with a clockwise rotation above the PDZ. This recent tectonic structure appears to be superimposed over a pre-existing graben that is now essentially deactivated. The PDZ continues eastwards, out of the Central Basin, as a N50°E-trending NW-verging thrust system toward the Kumburgaz Basin that is located on a restraining bend of the PDZ. This shortening zone consists of two main N60°E-trending branches. The northern one is more pronounced and composed of two successive restraining bends. The southern branch is smoother and forms a gently curved connection between the two segments of the PDZ. This suggests that the PDZ migrates southward to cut through this restraining bend. Further east, the PDZ enters the Cinarcik Basin along its northern scarp. Active deformation observed on the Pasisar profiles along the 290°-trending eastern portion of this scarp consists of N110±5°-trending dextral strike–slip faults connecting short segments of active N130–140° normal faults that control elongated depocenters. Along the southern flank of the Cinarcik Basin, the E–W Izmit fault enters the basin from the east. Pasisar data confirm the extension of the Izmit strike–slip fault into the Cinarcik Basin and the large development of normal faulting along the southern flank of the basin. Some of the normal faulting observed here may be related to horse tail termination of the Izmit fault, while most of it is the expression of strain partitioning.