Jean-Yves Royer

Digital isochrons of the world's ocean floor

We have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self-consistent set of global isochrons and associated plate reconstruction poles. The age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models. We have constructed an age grid with error estimates for each grid cell as a function of (1) the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid, (2) the distance of a given grid cell to the nearest magnetic anomaly identification, and (3) the gradient of the age grid: i.e., larger errors are associated with high age gradients at fracture zones or other age discontinuities. Future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere, the heat loss of the Earth, ridge-push forces through time, asymmetry of spreading, and providing constraints for seismic tomography and mantle convection models.

Revised plate motions relative to the hotspots from combined Atlantic and Indian Ocean hotspot tracks

We use an updated model for global relative plate motions during the past 130 m.y. together with a compilation of bathymetry and recently published radiometric dates of major hotspot tracks to derive a plate-motion model relative to major hotspots in the Atlantic and Indian oceans. Interactive computer graphics were used to find the best fit of dated hotspot tracks on the Australian, Indian, African, and North and South American plates relative to present-day hotspots assumed fixed in the mantle. One set of rotation parameters can be found that satisfies all data constraints back to chron 34 (84 Ma) and supports little motion between the major hotspots in this hemisphere. For times between 130 and 84 Ma, the plate model is based solely on the trails of the Tristan da Cunha and Great Meteor hotspots. This approach results in a location of the Kerguelen hotspot distinct from and south of the Rajmahal Traps for this time interval. Between 115 and 105 Ma, our model locates the hotspot underneath the southern Kerguelen Plateau, which is compatible with an age estimate of this part of the plateau of 115-95 Ma. Our model suggests that the 85°E ridge between lat 10°N and the Afanasiy Nikitin seamounts may have been formed by a hotspot now located underneath the eastern Conrad rise.

The tectonic history of the Tasman Sea: A puzzle with 13 pieces

We present a new model for the tectonic evolution of the Tasman Sea based on dense satellite altimetry data and a new shipboard data set. We utilized a combined set of revised magnetic anomaly and fracture zone interpretations to calculate relative motions and their uncertainties between the Australian and the Lord Howe Rise plates from 73.6 Ma to 52 Ma when spreading ceased. From chron 31 (67.7 Ma) to chron 29 (64.0 Ma) the model implies, transpression between the Chesterfield and the Marion plateaus, followed by strike-slip motion. This transpression may have been responsible for the formation of the Capricorn Basin south of the Marion Plateau. Another major tectonic event took place at chron 27 (61.2 Ma), when a counterclockwise change in spreading direction occurred, contemporaneous with a similar event in the southwest Pacific Ocean. The early opening of the Tasman Sea cannot be modeled by a simple two-plate system because (1) rifting in this basin propagated from south to north in several stages and (2) several rifts failed. We identified 13 continental blocks which acted as microplates between 90 Ma and 64 Ma. Our model is constrained by tectonic lineaments visible in the gravity anomaly grid and interpreted as strike-slip faults, by magnetic anomaly, bathymetry and seismic data, and in case of the South Tasman Rise, by the age and affinity of dredged rocks. By combining all this information we derived finite rotations that describe the dispersal of these tectonic elements during the early opening of the Tasman Sea.

Evidence for relative motions between the Indian and Australian Plates during the last 20 m.y. from plate tectonic reconstructions: Implications for the deformation of the Indo-Australian Plate

We use plate tectonic reconstructions to establish whether motions between India and Australia occurred since chron 18 (43 Ma). We test the Africa/Antarctica/Australia/India plate circuit closure at chrons 5 (10 Ma), 6 (21 Ma) and 13 (36 Ma) using a compilation of magnetic anomalies and fracture zone traces from the Southeast, Southwest, Central Indian and the Carlsberg ridges. Additional reconstructions at chrons 23 (55 Ma) and 26 (61 Ma) are used to estimate the overall motion between India and Australia. Relative motions between the Indian and Australian plates are estimated using the plate circuit India → Africa → Australia. A new statistical approach, based on spherical regression analyses, is used to assess the uncertainty of the “best-fitting” finite rotations from the uncertainties in the data. The uncertainty in a rotation is described by a covariance matrix directly related to the geometry of the reconstructed plate boundary, to the distribution and estimated errors of the data points along it. Our parameterization of the rotations allows for simple combination of the rotation uncertainties along a plate circuit path. Results for chron 5 are remarkably consistent with present-day kinematics in the Indian Ocean, except that the Arabian and Indian plates are found to be separate plates. Comparisons of the motions between the Indian and African plates across the Carlsberg Ridge with that between the Australian and African plates across the Central Indian Ridge evidence a significant counterclockwise rotation of the Australian plate relative to the Indian plate about a pole located in the Central Indian Basin. The determinations are consistent for chrons 26, 13, 6 and 5. Determination at chron 23 is different but questionable due to the small number of available data. We propose two alternative solutions that both predict convergence within the Wharton and Central Indian basins and extension in the vicinity of the Chagos-Laccadive Ridge. The first solution assumes that all the deformation in the equatorial Indian Ocean started 7 Ma ago as found by Ocean Drilling Program Leg 116. Hence all the determinations at different times represent the total motion between India and Australia. The averaged India/Australia Euler vector (chrons 5, 6, 13, and 26: 11.1°S, 78.0°, ω=3.54°) lies within the Central Indian Basin and yields a N-S contraction of 46±52 km at 85°, and 80±63 km at 90°. However, the difference of the India/Australia Euler vectors at chrons 5 and 6 suggests that the India/Australia convergence started between 10 and 21 Ma, following the continent-continent collision of India with Asia in the Early Miocene. The second averaged solution (chrons 6, 13, and 26: 5.2°S, 74.3°E, ω=5.93°) predicts a total N-S contraction of 123±73 km at 85°E, and 178±91 km at 90°E. Both models are compatible with the deformation pattern observed in the equatorial Indian Ocean.

Asymmetric sea-floor spreading caused by ridge-plume interactions

Crustal accretion at mid-ocean ridges is generally modelled as a symmetric process. Regional analyses, however, often show either small-scale asymmetries, which vary rapidly between individual spreading corridors, or large-scale asymmetries represented by consistent excess accretion on one of the two separating plates over geological time spans. In neither case is the origin of the asymmetry well understood. Here we present a comprehensive analysis of the asymmetry of crustal accretion over the past 83u2009Myr based on a set of self-consistent digital isochrons and models of absolute plate motion,. We find that deficits in crustal accretion occur mainly on ridge flanks overlying one or several hotspots. We therefore propose that asymmetric accretion is caused by ridge propagation towards mantle plumes or minor ridge jumps sustained by asthenospheric flow, between ridges and plumes. Quantifying the asymmetry of crustal accretion provides a complementary approach to that based on geochemical and other geophysical data, in helping to unravel how mantle plumes and mid-ocean ridges are linked through mantle convection processes.

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 (

Evidence for long-term diffuse deformation of the lithosphere of the equatorial Indian Ocean

The presence of large earthquakes, east–west-striking folds and thrust faults in sediments, and east–west-striking undulations of wavelength 200u2009km in topography and gravity shows that the equatorial Indian Ocean is the locus of unusual deformation. This deformation has been interpreted as a diffuse boundary between two tectonic plates. Seismic stratigraphy and deep-sea drilling at two locations in the Bengal fan indicate that the deformation began 7.5–8.0u2009Myr ago,,. Here, however, we show, using plate reconructions, that motion across this diffuse oceanic plate boundary began more than 10u2009Myr earlier than previously inferred and that the amount of north–south convergence across the boundary through the central Indian basin has been significantly greater than the convergence estimated from seismic profiles. The relative plate velocity accommodated across the central Indian basin has varied with time and has been as fast as ∼6u2009mmu2009yr −1 — about half the separation rate of Earths slowest-spreading mid-ocean ridge. The earliest interval of measurable motion, which began more than 18u2009Myr ago, may coincide with rapid denudation of the Tibetan plateau from ∼21u2009Myr to 15–17u2009Myr ( ref. 16). The present motion across the central Indian basin began no earlier than 11u2009Myr — following an earlier interval of slower motion from 18 to 11u2009Myr — and may have begun at ∼8u2009Myr, when the Tibetan plateau is thought to have attained its maximum elevation,.

Evolution of the Louisiade triple junction

We derived new finite rotations for the opening of the Coral Sea using revised magnetic anomaly interpretations and fracture zone data from a gravity anomaly grid based on from satellite altimetry. These rotations differ from the finite rotations that describe the opening of the Tasman Sea; this confirms the existence of a triple junction between the Australian Plate, the Mellish Rise, and the Louisiade Plateau active during the opening of the Coral Sea (62 to 52 Ma). Magnetic anomalies, fracture zones visible on the gravity grid, and strike-slip faults indicate that extension occurred between the Mellish Rise and the Louisiade Plateau, and extensional and transform motion occurred between Australia and the Mellish Rise (attached to the Chesterfield and Kenn Plateaus). The configuration of the triple junction from chron 27 to 26 was either ridge-ridge-ridge (RRR) or ridge-fault-fault (RFF). At chron 26 (58 Ma) the triple junction had a RFF configuration and migrated southward as the relative motion between the Louisiade Plateau and the Mellish Rise was transferred to the boundary between the Mellish Rise and the Kenn Plateau. The gravity low between the Kenn Plateau and the Mellish Rise is interpreted as a strike-slip fault active from about 57 to 52 Ma. This configuration lasted until seafloor spreading ceased in the Coral and Tasman seas at about 52 Ma. Our model implies extension in the Osprey Embayment that might explain small areas of oceanic crust west of the Coral Sea Basin. The western boundary of the Coral Sea was a NE-SW strike-slip fault, active between 58 and 52 Ma.

A preliminary tectonic fabric chart of the Indian Ocean

We present a preliminary tectonic chart of the Indian Ocean based on a joint compilation of bathymetric data, magnetic anomaly data and Geosat altimetry data. Satellite altimeters such as Geosat map the topography of the equipotential sea surface or marine geoid. Our interpretation of the GEOSAT data is based on an analysis of the first derivative of the geoid profiles (i.e. deflection of the vertical profiles). Because of the high correlation between the vertical deflection (at wavelength <200 km) and the seafloor topography, the Geosat profiles can be used to delineate accurately numerous tectonic features of the ocean floor such as fracture zones, seamounts and spreading ridges. The lineations in the Geosat data are compared with bathymetric data and combined with magnetic anomaly identifications to produce a tectonic fabric chart of the Indian Ocean floor.

Tectonic fabric map of the ocean basins from satellite altimetry data

Abstract Satellite altimetry data provide a new source of information on the bathymetry of the ocean floor. The tectonic fabric of the oceans (i.e., the arrangement of fracture zones, ridges, volcanic plateaus and trenches) is revealed by changes in the horizontal gravity gradient as recorded by satellite altimetry measurements. SEASAT and GEOSAT altimetry data have been analyzed and a global map of the horizontal gravity gradient has been produced that can be used to identify a variety of marine tectonic features. The uniformity of the satellite coverage provides greater resolution and continuity than maps based solely on ship-track data. This map is also the first global map to incorporate the results of the GEOSAT mission, and as a result, new tectonic features are revealed at high southerly latitudes. This map permits the extension of many tectonic features well beyond what was previously known. For instance, various fracture zones, such as the Ascension, Tasman, and Udintsev fracture zones, can be extended much closer to adjacent coninental margins. The tectonic fabric map also reveals many features that have not been previously mapped. These features include extinct ridges, minor fracture zone lineations and seamounts. In several areas, especially across aseismic plateaus or along the margins of the continents, the map displays broad gravity anomalies whose origin may be related to basement structures.