2-D geoelectrical model for the Parnaiba Basin conductivity anomaly of northeast Brazil and tectonic implications

A magnetometer array study in the north-northeast of Brazil has revealed a roughly NE-SW-trending conductive structure in the southeastern part of the intracratonic Parnaíba Basin. The magnetovariational response functions of this structure are numerically modelled to constrain its geometry to facilitate its geological and tectonic interpretation. The 2-D numerical model that incorporates the ocean effect and can account for the spatial and period dependence of the observed response locates the source regions of enhanced conductivity in a graben structure in the basement as well as in a block confined to the central part of the basin with an embedded resistive body. The anomalous electrical character of the sediments in the central part of the basin is consistent with the magnetotelluric data, the graben structure in the basement is corroborated by the aeromagnetic data. The formation of the graben structure is considered to be a manifestation of the extensional tectonics associated either with the Brasiliano orogeny or with the Jurassic–Cretaceous magmatic events. The diabase dikes intruded in the basin in association with the Jurassic-Cretaceous magmatic activity are shown to be accountable for the mapped resistive body entrapped in the conducting Paleozoic sediments. The thermal effects associated with magmatic activities are invoked to produce enhanced conductivity by the generation of carbon through the pyrolysis of hydrocarbon-saturated sediments

On the interpretation of the distinctive pattern of geomagnetic induction observed in northwest India

The geomagnetic variation data from the 1979 Indian array experiment have been reanalyzed and reexamined using the hypothetical event analysis technique. The contour map of the |Z/H ratio replicates distinctive anomaly in northwest India previously delineated in maps of the Fourier coefficients. The anomaly reveals the presence of a significant conductor under the Ganga basin. The contour map has been used to derive a response profile perpendicular to the strike of the anomaly, for comparison with 2-D numerical models. An excellent fit was found for a conductor at a depth of 32 km, with a width of 110 km and a conductivity contrast of 1000. This result places the conductor deep within the lithosphere. In the absence of supporting data the origin of the conductor is difficult to resolve. However, it is thought to be related to pressure-released partial melting, caused by fracturing of the Indian crust during the collision of India with Asia

Mantle structure and tectonic history of SE Asia

Seismic travel-time tomography of the mantle under SE Asia reveals patterns of subduction-related seismic P-wave velocity anomalies that are of great value in helping to understand the region's tectonic development. We discuss tomography and tectonic interpretations of an area centred on Indonesia and including Malaysia, parts of the Philippines, New Guinea and northern Australia. We begin with an explanation of seismic tomography and causes of velocity anomalies in the mantle, and discuss assessment of model quality for tomographic models created from P-wave travel times. We then introduce the global P-wave velocity anomaly model UU-P07 and the tectonic model used in this paper and give an overview of previous interpretations of mantle structure. The slab-related velocity anomalies we identify in the upper and lower mantle based on the UU-P07 model are interpreted in terms of the tectonic model and illustrated with figures and movies. Finally, we discuss where tomographic and tectonic models for SE Asia converge or diverge, and identify the most important conclusions concerning the history of the region. The tomographic images of the mantle record subduction beneath the SE Asian region to depths of approximately 1600. km. In the upper mantle anomalies mainly record subduction during the last 10 to 25. Ma, depending on the region considered. We interpret a vertical slab tear crossing the entire upper mantle north of west Sumatra where there is a strong lateral kink in slab morphology, slab holes between c.200-400. km below East Java and Sumbawa, and offer a new three-slab explanation for subduction in the North Sulawesi region. There is a different structure in the lower mantle compared to the upper mantle and the deep structure changes from west to east. What was imaged in earlier models as a broad and deep anomaly below SE Asia has a clear internal structure and we argue that many features can be identified as older subduction zones. We identify remnants of slabs that detached in the Early Miocene such as the Sula slab, now found in the lower mantle north of Lombok, and the Proto-South China Sea slab now at depths below 700. km curving from northern Borneo to the Philippines. Based on our tectonic model we interpret virtually all features seen in upper mantle and lower mantle to depths of at least 1200. km to be the result of Cenozoic subduction

Another Participants View on Dutch Archaeology in Post War Times

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