J. Dyment

40Ar/39Ar geochronology and structural data from the giant Okavango and related mafic dyke swarms, Karoo igneous province, northern Botswana

Abstract In NE Botswana, the Karoo dykes include a major N110° dyke swarm known as the Okavango giant dyke swarm (ODS/N110°) and a second smaller set of N70° dykes belonging to the Sabi-Limpopo dyke swarm (SLDS/N70°). New 40Ar/39Ar plagioclase dating of Karoo dolerites of the giant ODS/N110° and the SLDS/N70° in NE Botswana yield plateau ages between 179.6±1.2 and 178.4±1.1 Ma. Our data are concordant with previous 40Ar/39Ar ages for Northern Karoo dykes and lava flows exposed in western Zimbabwe. The data are tightly clustered, indicating a short-lived (179–181 Ma) flood basalt magmatism in this region. The new radiometric dates allow the definition of a diachronous Jurassic flood basalt activity in southern Africa. A significant south to north younging at the scale of the Karoo igneous province correlates with a chemical zonation from low-Ti (south) to high-Ti (north) mafic rocks. Structural measurements on the ODS/N110° and SLDS/N70° Karoo dykes of NE Botswana suggest that: (1) most of the host fractures are inherited Precambrian structures; (2) dyke emplacement occurred under unidirectional tensional stresses; (3) significant syn- and post-volcanic extensional tectonics are lacking. Combined with regional geology, these geochronological and structural data do not confirm unambiguously the triple-junction hypothesis usually put forward to support a mantle plume model for the evolution of the Karoo igneous province, prior to Gondwana breakup.

Evolution of the Carlsberg Ridge between 60 and 45 Ma: Ridge propagation, spreading asymmetry, and the Deccan‐Reunion hotspot

Despite the availability of numerous geophysical data, no convincing magnetic anomaly identification and structural map of a major part of the Arabian and eastern Somali Basins have been published to date. I propose a new interpretation of the available magnetic data based on the identification of ”tiny wiggles,” i.e., singular second-order patterns of the magnetic signal. Only one fracture zone and a set of propagating rifts existed between the Owen Fracture Zone and the Chagos-Laccadive Ridge between anomalies 26 and 20. At anomalies 26–25, at least two eastward propagating rifts are identified, while at anomalies 24–20, seven westward propagating rifts are confidently recognized. The major consequence of this systematic ridge propagation is a tremendous regional asymmetry: between anomalies 26 and 25, about 65% of the crust formed at the Carlsberg Ridge was accreted to the African Plate, while at anomalies 24–20, more than 75% benefited to the Indian Plate. I suggest that these asymmetries result from the relative position of the Carlsberg Ridge and the nearby Deccan-Reunion hotspot, the ridge tending to remain near the hotspot.

Paleogene plate tectonic evolution of the Arabian and Eastern Somali basins

Abstract We review previous models for the Paleogene tectonic evolution of the Arabian and Eastern Somali basins and present a model based on a new compilation of magnetic and gravity data. Using plate reconstructions, we derive a self-consistent set of isochrons for Chron 27 to Chron 21 (61–46 Ma). The new isochrons account for the development of successive ridge propagation events along the Carlsberg Ridge, leading to an important spreading asymmetry between the conjugate basins. Our model predicts the growth of the outer and inner pseudofaults associated with the ridge propagation events. The location of outer pseudo-faults appears to remain very stable despite a drastic change in the direction of ridge propagation before Chron 24 (c. 54 Ma). The motion of the Indian plate relative to the Somalian plate is stable in direction through Paleogene time; spreading velocities decrease from 6 to 3 cm a−1. Our reconstructions also confirm that the Arabia-India plate boundary was located west of the Owen Ridge along the Oman margin during Paleogene time; some compression is predicted at about Chron 21 (47 Ma) between the Indian and Arabian plates.

Contribution of lithospheric remanent magnetization to satellite magnetic anomalies over the world's oceans

Regional studies have shown that remanent magnetization of the Cretaceous quiet zones (CQZs), created during the long period of geomagnetic normal polarity at 118–84 Ma, produce well-defined magnetic anomalies at satellite altitudes. We investigate the effects of the remanent magnetization of the oceanic lithosphere on satellite magnetic anomalies on a global scale. We consider entire oceanic areas because oceanic lithosphere formed during periods other than CQZ, but with a predominant polarity, also have appreciable contributions to the satellite anomalies. The magnetic anomalies of the worlds oceans are calculated from a distribution of vertically integrated remanent magnetization that is computed using age map, plate relative motions, and the apparent polar wander path of Africa. Three magnetization models are examined: thermoremanent magnetization confined to extrusive layer 2A, and thermoviscous remanent magnetization of the crust and uppermost mantle down to a maximum depth of 12 km or 30 km. Although all models lead to rather similar anomaly distributions, the amount of magnetization required suggests the need for deeper sources. All models produce the satellite anomalies associated with the CQZ in the North Atlantic and parts of the Indian and Pacific Oceans, with slight differences in location, depending on the models. Low-amplitude observed anomalies associated with areas created at fast and intermediate spreading rates in the Indian and Pacific Oceans are successfully modeled. A major difference between models and observation is the north-south elongated model anomalies associated, for instance, with the CQZs in the South Atlantic and the fast spreading East Pacific Rise. These elongated anomalies are systematically absent on the observed map, probably because they were removed by along-track filtering in the early stages of processing the satellite data. A careful comparison of the model anomalies with observation favors model with thermoviscous magnetization down to 12 km and saturation magnetizations of 4, 0, 1, and 0.6 A/m for the extrusive basalts, intrusive basalts, gabbros, and peridotites, respectively.

Evolution of the Indian Ocean Triple Junction between 65 and 49 Ma (anomalies 28 to 21)

Reinterpretation of newly published geophysical data (Kamesh-Raju and Ramprasad, 1989) and older profiles of the Central Indian Basin, associated with similar studies of the Madagascar and Crozet basins, shows that the Indian Ocean Triple Junction trace on the Indian plate corresponds, at anomalies 23 and 22, to a N38°E offset of the magnetic lineations, oblique to both the Southeast Indian Ridge (SEIR) and Central Indian Ridge (CIR) spreading directions. The conjugate Triple Junction trace on the African plate identified in the Madagascar Basin is associated with a roughly north-south offset, parallel to the Southwest Indian Ridge (SWIR) fracture zones. In order to account for these observations and the velocity triangle of the Indian, African, and Antarctic plates close to the Triple Junction, a ridge-fault-fault mode is proposed, with a propagatorlike SEIR-CIR offset. The Triple Junction jumped between anomalies 24 and 23 and between anomalies 22 and 21, restoring a ridge-ridge-ridge configuration which immediately turned to a pseudo-ridge-ridge-fault and later to a true ridge-fault-fault configuration. After the Triple Junction jump at anomaly 21, the former SEIR-CIR offset was accommodated by a new CIR fracture zone. The lack of such a fracture zone prior to anomaly 21 suggests that either a pseudo-ridge-ridge-fault or an unstable ridge-ridge-ridge configuration prevailed before anomaly 24, in agreement with the velocity triangles which predict more unstable Triple Junction modes. Both modes support the creation of numerous SWIR fracture zones, presently observed between 52°30′E and 59°30′E, as a consequence of the Triple Junction evolution between anomalies 29 and 24. This result suggests that the physiography of the SWIR records the history of the Triple Junction.

Tectonic structure, lithology, and hydrothermal signature of the Rainbow massif (Mid‐Atlantic Ridge 36°14′N)

Rainbow is a dome-shaped massif at the 36°14′N nontransform offset along the Mid-Atlantic Ridge. It hosts three ultramafic-hosted hydrothermal sites: Rainbow is active and high temperature; Clamstone and Ghost City are fossil and low temperature. The MoMARDREAM cruises (2007, 2008) presented here provided extensive rock sampling throughout the massif that constrains the geological setting of hydrothermal activity. The lithology is heterogeneous with abundant serpentinites surrounding gabbros, troctolites, chromitites, plagiogranites, and basalts. We propose that a W dipping detachment fault, now inactive, uplifted the massif and exhumed these deep-seated rocks. Present-day deformation is accommodated by SSW-NNE faults and fissures, consistent with oblique teleseismic focal mechanisms and stress rotation across the discontinuity. Faults localize fluid flow and control the location of fossil and active hydrothermal fields that appear to be ephemeral and lacking in spatiotemporal progression. Markers of high-temperature hydrothermal activity (∼350°C) are restricted to some samples from the active field while a more diffuse, lower temperature hydrothermal activity (<220°C) is inferred at various locations through anomalously high As, Sb, and Pb contents, attributed to element incorporation in serpentines or microscale-sulfide precipitation. Petrographic and geochemical analyses show that the dominant basement alteration is pervasive peridotite serpentinization at ∼160–260°C, attributed to fluids chemically similar to those venting at Rainbow, and controlled by concomitant alteration of mafic-ultramafic units at depth. Rainbow provides a model for fluid circulation, possibly applicable to hydrothermalism at oceanic detachments elsewhere, where both low-temperature serpentinization and magmatic-driven high-temperature outflow develop contemporaneously, channeled by faults in the footwall and not along the detachment fault.

Revisiting the structure, age, and evolution of the Wharton Basin to better understand subduction under Indonesia

Understanding the subduction processes along the Sunda Trench requires detailed constraints on the subducting lithosphere. We build a detailed tectonic map of the Wharton Basin based on reinterpretation of satellite-derived gravity anomalies and marine magnetic anomalies. The Wharton Basin is characterized by a fossil ridge, dated ~36.5 Ma, offset by N-S fracture zones. Magnetic anomalies 18 to 34 (38–84 Ma) are identified on both flanks, although a large part of the basin has been subducted. We analyze the past plate kinematic evolution of the Wharton Basin by two-plate (India-Australia) and three-plate (India-Australia-Antarctica) reconstructions. Despite the diffuse plate boundaries within the Indo-Australian plate for the last 20 Ma, we obtain finite rotation parameters that we apply to reconstruct the subducted Wharton Basin and constrain the thickness, buoyancy, and rheology of the subducting plate. The lower subductability of younger lithosphere off Sumatra has important consequences on the morphology, with a shallower trench, forearc islands, and a significant inward deviation of the subduction system. This deviation decreases in the youngest area, where the Wharton fossil spreading center enters subduction: The discontinuous magmatic crust and serpentinized upper mantle, consequences of the slow spreading rates at which this area was formed, weaken the mechanical resistance to subduction and facilitate the restoration of the accretionary prism. Deeper effects include the possible creation of asthenospheric windows beneath the Andaman Sea, in relation to the long-offset fracture zones, and east of 105°E, as a result of subduction of the spreading center.

Satellite magnetic anomalies related to seafloor spreading in the South Atlantic Ocean

Oceanic magnetic anomalies have been observed from satellite. The strongest anomalies are the long-wavelength components of the sea-floor spreading signature. Unfortunately, because of technical issues involving the treatment of satellite magnetic data, these signals are obscured in the South Atlantic Ocean because they trend north-south. However, a map does exist in which such features are observed, essentially because of a better data processing technique. Further, this map agrees with a physically motivated model based on non-satellite magnetic input. Hence, with properly treated data, the magnetic anomaly maps should be useful for saying something about the geology, rather than vice-versa. This situation will be considerably improved by ongoing advances in methods and new data sets. The amplitude of the observations, a factor of two larger than previous estimates, confirms that the extrusive basaltic layer alone is inadequate to produce the signal and that deeper oceanic sources are required.

Skewness of marine magnetic anomalies created between 85 and 40 Ma in the Indian Ocean

We have performed a detailed analysis of the skewness of marine magnetic anomalies in Indian Ocean basins created between 85 and 40 Ma as a result of the northward motion of India. Visual and semiautomated methods of skewness determination were applied to the data. Both provide consistent results, but the visual method is prefered for its ability to deal with noisy data. Plots of apparent effective remanent inclination (or skewness corrected for present geomagnetic field inclination) versus time for conjugate basins display the combination of three effects: a gradual increase with time, related to the northward motion of the ridges attached to India in the geomagnetic reference frame; a gap between conjugate curves, which represents anomalous skewness senso stricto; and short-period fluctuations, which represent the sequence effect, i.e., the effect of neighboring magnetic sources on the skewness of a given anomaly. The anomalous skewness decreases with faster spreading rate and completely disappears above 50 km/m.y., an observation which negates geomagnetic field behavior as a possible cause of the observed anomalous skewness.

Fossil evidence for serpentinization fluids fueling chemosynthetic assemblages

Among the deep-sea hydrothermal vent sites discovered in the past 30 years, Lost City on the Mid-Atlantic Ridge (MAR) is remarkable both for its alkaline fluids derived from mantle rock serpentinization and the spectacular seafloor carbonate chimneys precipitated from these fluids. Despite high concentrations of reduced chemicals in the fluids, this unique example of a serpentinite-hosted hydrothermal system currently lacks chemosynthetic assemblages dominated by large animals typical of high-temperature vent sites. Here we report abundant specimens of chemosymbiotic mussels, associated with gastropods and chemosymbiotic clams, in approximately 100 kyr old Lost City-like carbonates from the MAR close to the Rainbow site (36 °N). Our finding shows that serpentinization-related fluids, unaffected by high-temperature hydrothermal circulation, can occur on-axis and are able to sustain high-biomass communities. The widespread occurrence of seafloor ultramafic rocks linked to likely long-range dispersion of vent species therefore offers considerably more ecospace for chemosynthetic fauna in the oceans than previously supposed.