Philippe Patriat

Updated Interpretation of Magnetic Anomalies and Seafloor Spreading Stages in the South China Sea' Implications for the Tertiary Tectonics of Southeast Asia

We present the interpretation of a new set of closely spaced marine magnetic profiles that complements previous data in the northeastern and southwestern parts of the South China Sea (Nan Hai). This interpretation shows that seafloor spreading was asymmetric and confirms that it included at least one ridge jump. Discontinuities in the seafloor fabric, characterized by large differences in basement depth and roughness, appear to be related to variations in spreading rate. Between anomalies 11 and 7 (32 to 27 Ma), spreading at an intermediate, average full rate of ≈50 mm/yr created relatively smooth basement, now thickly blanketed by sediments. The ridge then jumped to the south and created rough basement, now much shallower and covered with thinner sediments than in the north. This episode lasted from anomaly 6b to anomaly 5c (27 to ≈16 Ma) and the average spreading rate was slower, ≈35 mm/yr. After 27 Ma, spreading appears to have developed first in the eastern part of the basin and to have propagated towards the southwest in two major steps, at the time of anomalies 6b-7, and at the time of anomaly 6. Each step correlates with a variation of the ridge orientation, from nearly E-W to NE-SW, and with a variation in the spreading rate. Spreading appears to have stopped synchronously along the ridge, at about 15.5 Ma. From computed fits of magnetic isochrons, we calculate 10 poles of finite rotation between the times of magnetic anomalies 11 and 5c. The poles permit reconstruction of the Oligo-Miocene movements of Southeast Asian blocks north and south of the South China Sea. Using such reconstructions, we test quantitatively a simple scenario for the opening of the sea in which seafloor spreading results from the extrusion of Indochina relative to South China, in response to the penetration of India into Asia. This alone yields between 500 and 600 km of left-lateral motion on the Red River-Ailao Shan shear zone, with crustal shortening in the San Jiang region and crustal extension in Tonkin. The offset derived from the fit of magnetic isochrons on the South China Sea floor is compatible with the offset of geological markers north and south of the Red River Zone. The first phases of extension of the continental margins of the basin are probably related to motion on the Wang Chao and Three Pagodas Faults, in addition to the Red River Fault. That Indochina rotated at least 12° relative to South China implies that large-scale “domino” models are inadequate to describe the Cenozoic tectonics of Southeast Asia. The cessation of spreading after 16 Ma appears to be roughly synchronous with the final increments of left-lateral shear and normal uplift in the Ailao Shan (18 Ma), as well as with incipient collisions between the Australian and the Eurasian plates. Hence no other causes than the activation of new fault zones within the India-Asia collision zone, north and east of the Red River Fault, and perhaps increased resistance to extrusion along the SE edge of Sundaland, appear to be required to terminate seafloor spreading in the largest marginal basin of the western Pacific and to change the sense of motion on the largest strike-slip fault of SE Asia.

Reconstruction of the Central Indian Ocean

Abstract Sixteen plate tectonic reconstructions are presented illustrating the development of the Central Indian Ocean from the latest Cretaceous (anomaly 29) to the present. The history of seafloor spreading in the Indian Ocean can be divided into five distinct tectonic epochs: 1. (1) latest Cretaceous-Early Paleocene (A29-A27), 2. (2) Middle Paleocene-Early Eocene (A27-A22), 3. (3) Middle Eocene-Late Eocene (A22-A18), 4. (4) Late Eocene-Early Miocene (A18-A8), and 5. (5) Early Miocene-Recent (A8-A5). During each of these tectonic epochs, subtle changes took place in the directions and rates of spreading along the Central, Southeast and Southwest Indian ridges. The most dramatic changes in plate motion occurred between A21 and A18 time due to the collision of India and Eurasia.

Direct evidence of active deformation in the eastern Indian oceanic plate

Conventional plate tectonics theory postulates that plates only deform on their boundaries. To the contrary, there is ample evidence of intraplate deformation in the equatorial Indian Ocean, west of the Ninetyeast aseismic ridge. Prior to this study, no direct evidence of deforma- tion east of the Ninetyeast Ridge was available. We present the results of a multipurpose geo- physical cruise showing that intraplate deformation also occurs in this area. Long, at least 1000 km, left-lateral north-south strike-slip faults are active and reactivate fossil fracture zones. This style of deformation is strikingly different from the east-west folds and reverse faults that affect the region west of the Ninetyeast Ridge. Contrasting processes of convergence at the northern plate boundaries can account for the two styles of deformation. West of the Ninetyeast Ridge there is a continent-continent collision, and east of the ridge oceanic lithosphere subducts along the Sumatra trench. The Ninetyeast aseismic ridge therefore appears to be a mechanical border separating two distinct deformed areas.

The Azores triple junction evolution since 10 Ma from an aeromagnetic survey of the Mid-Atlantic Ridge

Abstract In the past two decades several models have been presented to describe the evolution and the present structure of the Azores Triple Junction. These models were mainly based on morphological analysis of sea bottom topography, sparse magnetic profiling, sidescan sonar surveying over the plateaus and global plate kinematic considerations for the North Atlantic. In this paper we follow a different approach: from a detailed aeromagnetic survey covering both sides of the Mid-Atlantic Ridge between 37°N and 40°30′N the magnetic anomalies up to anomaly 5 are accurately identified, allowing careful modelling of the kinematics of this region for the past 10 Ma and thus establishing a coherent framework for the design of geophysical models for the Azores Triple Junction. The analysis of magnetic anomalies and the use of Fourier domain inversion techniques show that the ridge is made up of six segments, each one varying in length from 50 to 60 km. The more continuous section of the ridge can be defined from the first four northern segments, although the North Azores Fracture Zone right-offsets the ridge at 39°30′N, 29°40′W. The fifth and sixth segments are, respectively, right-offset by the Acor Fracture Zone (at 38°23′N, 30°15′W) and by the Princess Alice Fracture Zone (at 38°00′N, 30°50′W). Anomaly identifications using a two-dimensional model and plate tectonics reconstruction techniques allowed the calculation of rotation pole parameters. The results thus obtained reveal that, at least between anomalies 5 and 3 ( ∼ 10–3.85 Ma), the Azores displayed an independent motion relative to the neighbouring plates and after anomaly 2A (2.45 Ma) the Azores moved attached to the Eurasian plate. The triple junction (Azores-North America-Africa, or Eurasia-North America-Africa) moved northward from ∼ 38°00′N, 30°50′W to ∼ 38°20′N, 30°15′W (between 4 and 3A) and probably to 38°50′N, 30°00′W at anomaly 2A time. A detailed reconstruction model is presented.

The Southwest Indian Ridge between 49°15′E and 57°E: focused accretion and magma redistribution

Abstract Bathymetric, gravity, magnetic and backscattering strength data have been used to characterise the segmentation of an 800 km long portion of the ultraslow-spreading Southwest Indian Ridge (SWIR, full rate 14 mm/yr) between 49°15′E and 57°E. This analysis reveals that the segmentation defined by along-axis variations of depth and by occurrence of axial offsets does not systematically correspond to the segmentation determined by the along-axis variations of backscattering strength, mantle Bouguer anomaly (MBA) and amplitude of the central magnetic anomalies (CMA). At axial discontinuities with offsets larger than 15 km, thin crust and reduced volcanic production are suggested by the occurrence of MBA highs, almost non-existent CMA and 50% lower backscattering strength relative to the segment centres. By contrast, smaller non-transform discontinuities, with offsets smaller than 15 km, correspond to very weak variations or to no variation of the MBA, the CMA or the reflectivity of the seafloor, suggesting that there is little or no variation of volcanic production and crustal thickness associated with those small discontinuities. These small axial discontinuities bound low-relief bathymetric segments (500–700 m), corresponding to weak or no MBA lows (amplitude 1000 m), corresponding to large MBA lows (amplitude >30 mGal). We suggest that the magma supply to these low-relief segments is controlled by near-surface processes such as melt migration and/or crustal plumbing from adjacent high-relief segments. Pronounced MBA lows at high-relief segments are thought to correspond to spreading cells where magma supply is focused in the mantle. These spreading cells are spaced by about 100 km along the SWIR axis. We suggest that the spacing of spreading cells on slow-spreading ridges is primarily controlled by the spreading rate with larger spacing between spreading cells on ultraslow-spreading ridges than on slow-spreading ridges.

Evolution of the southwest Indian Ridge from the Late Cretaceous (anomaly 34) to the Middle Eocene (anomaly 20)

Abstract The determination of the motion of Antarctica relative to Africa is particularly important when considering the breakup of Gondwana. Two models have been proposed that describe the pattern of seafloor spreading between Africa and Antarctica during the Late Cretaceous (starting at chron 34, 84 Ma) through to the Middle Eocene (chron 20, 46 Ma). In the first model, the motion of Antarctica relative to Africa can be simply described by a rotation about a single pole of rotation. In the second model, which we favor, the relative motion of Antarctica and Africa is more complex, and a major change in spreading direction between chron 32 (74 Ma) and chron 24 (56 Ma) times is required. In this paper we present ten plate tectonic reconstructions of the Southwest Indian Ridge that were produced using a new compilation of magnetic, bathymetric and satellite altimetry data, in combination with interactive computer graphics. These reconstructions illustrate that spreading directions started to change at chron 32 (74 Ma). Between chrons 31 and 28 (69-64 Ma), spreading was very slow (

Three‐dimensional gravity study of the Mid‐Atlantic Ridge: Evolution of the segmentation between 28° and 29°N during the last 10 m.y.

During the high-resolution survey SARA (Segmentation Ancienne de la Ride Atlantique), Sea Beam bathymetry, magnetic, gravity, and seismic reflection data were collected on the flanks of the Mid-Atlantic Ridge, between 28° and 29°N. This survey was designed to provide off-axis information (up to approximately 10 m.y.) and to complement a detailed on-axis survey carried out in 1988 and 1989 (Sempere et al., 1990) between the Atlantis and Kane fracture zones. A previous gravity study had revealed the existence of “bulls-eye” shaped gravity lows centered on the axial segments and gravity highs centered on the non-transform discontinuities (Lin et al., 1990). We carried out a three-dimensional calculation of the mantle Bouguer anomaly in order to investigate if the axial pattern of circular anomaly lows can be followed on the flanks of the spreading center. The off-axis gravity anomalies are characterized by anomaly lows over the centers of the segments and anomaly highs over the discontinuities. After correcting for the gravity effect of lithospheric cooling away from the ridge, the segmentation configuration determined from gravity data appears to be very similar to that deduced from bathymetry. Off-axis bathymetry is characterized by southward pointing, V-shaped basins which indicate that the traces of the nontransform discontinuities do not follow plate motion flow lines and that each ridge segment advances and retreats continuously. However, the gravity trace of the discontinuities is always slightly offset with respect to the bathymetric lows: northward on the western flank and southward on the eastern flank. The sense of offset between the gravity and bathymetric traces appears to be related to the right-stepping nature of the axial discontinuities.

Magmato-tectonic cyclicity at the ultra-slow spreading Southwest Indian Ridge: Evidence from variations of axial volcanic ridge morphology and abyssal hills pattern

On-axis deep tow side scan sonar data are used together with off-axis bathymetric data to investigate the temporal variations of the accretion processes at the ultra-slow spreading Southwest Indian Ridge. Differences in the length and height of the axial volcanic ridges and various degrees of deformation of these volcanic constructions are observed in side scan sonar images of the ridge segments. We interpret these differences as stages in an evolutionary life cycle of axial volcanic ridge development, including periods of volcanic construction and periods of tectonic dismemberment. Using off-axis bathymetric data, we identify numerous abyssal hills with a homogeneous size for each segment. These abyssal hills all display an asymmetric shape, with a steep faulted scarp facing toward the axis and a gentle dipping volcanic slope facing away. We suggest that these hills are remnants of old split axial volcanic ridges that have been transported onto the flanks and that they result from successive periods of magmatic construction and tectonic dismemberment, i.e., a magmato-tectonic cycle. We observe that large abyssal hills are in ridge sections of thicker crust, whereas smaller abyssal hills are in ridge sections of thinner crust. This suggests that the magma supply controls the size of abyssal hills. The abyssal hills in ridge sections of thinner crust are regularly spaced, indicating that the magmato-tectonic cycle is a pseudoperiodic process that lasts ~0.4 m.y., about 4 to 6 times shorter than in ridge sections of thicker crust. We suggest that the regularity of the abyssal hills pattern is related to the persistence of a nearly constant magma supply beneath long-lived segments. By contrast, when magma supply strongly decreases and becomes highly discontinuous, regular abyssal hills patterns are no longer observed.

A survey of the Southwest Indian Ridge axis between Atlantis II fracture zone and the Indian Ocean Triple Junction : Regional setting and large-scale segmentation

The study of very low-spreading ridges has become essential to ourunderstanding of the mid-oceanic ridge processes. The Southwest Indian Ridge(SWIR) , a major plate boundary of the world oceans, separating Africa fromAntarctica for more than 100 Ma, has such an ultra slow-spreadingrate. Its other characteristic is the fast lengthening of its axis at bothBouvet and Rodrigues triple junctions. A survey was carried out in thespring of 1993 to complete a multibeam bathymetric coverage of the axisbetween Atlantis II Fracture Zone (57° E) and the Rodrigues triplejunction (70° E). After a review of what is known about the geometry,structure and evolution of the SWIR, we present an analysis of the newalong-axis bathymetric data together with previously acquiredacross-axis profiles. Only three transform faults, represented byAtlantis II FZ, Novara FZ, and Melville FZ, offset this more than 1000 kmlong section of the SWIR, showing that the offsets are more generallyaccommodated by ridge obliquity and non-transform discontinuities. From comparison of the axial geometry, bathymetry, mantle Bouguer anomaly and central magnetic anomaly, three large sections (east of Melville FZ, between Melville FZ and about 65°30′ E, and from there to the Rodrigues triple junction) can be distinguished. The central member, east of Melville FZ, does not resemble any other known mid-oceanic ridge section: the classical signs of the accretion (mantle Bouguer anomaly, central magnetic anomaly) are only observed over three very narrow and shallow axis sections. We also apply image processing techniques to the satellite gravity anomaly map of Smith and Sandwell (1995) to determine the off-axis characteristics of the Southwest Indian Ridge domain, more especially the location of the triple junction and discontinuities traces. We conclude that the large-scale segmentation of the axis has been inherited from the evolution of the Rodrigues triple junction.

Reappraisal of the Arabia^India^Somalia triple junction kinematics

Abstract We propose alternative kinematics for the Arabia–India–Somalia triple junction based on a re-interpretation of seismological and magnetic data. The new triple junction of the ridge–ridge–ridge type is located at the bend of the Sheba Ridge in the eastern gulf of Aden at 14.5°N and 56.4°E. The Owen fracture zone (Arabia–India boundary) is connected to the Sheba Ridge by an ultra-slow divergent boundary trending N80°E±10° marked by diffuse seismicity. The location of the Arabia–India rotation pole is constrained at 14.1°N and 71.2°E by fitting the active part of the Owen fracture zone with a small circle. The finite kinematics of the triple junction is inferred from the present-day kinematics. Since the inception of the accretion 15–18 Ma ago, the Sheba Ridge has probably receded ∼300 km at the expense of the Carlsberg Ridge which propagated northwestward in the gulf of Aden, while an ultra-slow divergent plate boundary developed between the Arabian and Indian plates. The overall geometry of the new triple junction is very similar to that of the Azores triple junction.