West African and Brazilian Conjugate Margins: Crustal Types, Architecture, and Plate Configurations
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A combination of seismic reflection and gravimetric imagery has been used to map four sectors of proto-oceanic crust along conjugate segments of the West African and Brazilian margins. These form corridors isolating oceanic crust, produced about the post-118 Ma pole of rotation, from continental crust. Seaward of the proto-oceanic crust/oceanic crust boundary, relatively uniform, thin oceanic crust (4.2–6.5 km thick) has been generated at the paleo-Mid-Atlantic Ridge. Structural variability is limited largely to fracture zones. Proto-oceanic crust in the northern sectors (i.e., Kribi, Mbini, and Ogooue) is up to 10 km thick, block-faulted, compartmentalized, and seismically layered. These sectors of proto-oceanic crust likely were generated by slow spreading, as the relative plate motions evolved from left-lateral dislocation along theThe has been highly successful as an analog to explain seismic velocities in the ocean crust. The elements of the ophiolite model have all been observed on the seafloor where tectonic processes have exposed the crust. For the Pacific Ocean crust, or that which forms at non-rifted mid-ocean ridges, the development of the ophiolite model appears to resolve the contentious issues of the composition of the lower oceanic crust, and the nature of the Moho; however, studies show that the seismic structure of the crust evolves as it ages, and the depths to boundaries between different layers change. Therefore, these zones cannot represent lithological boundaries, at least in older crust. In this sense, a strict use of the ophiolite model is incorrect.
Seafloor Spreading
Convergent boundary
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A combination of seismic reflection and gravimetric imagery has been used to map four sectors of proto-oceanic crust along conjugate segments of the West African and Brazilian margins. These form corridors isolating oceanic crust, produced about the post-118 Ma pole of rotation, from continental crust. Seaward of the proto-oceanic crust/oceanic crust boundary, relatively uniform, thin oceanic crust (4.2–6.5 km thick) has been generated at the paleo-Mid-Atlantic Ridge. Structural variability is limited largely to fracture zones. Proto-oceanic crust in the northern sectors (i.e., Kribi, Mbini, and Ogooue) is up to 10 km thick, block-faulted, compartmentalized, and seismically layered. These sectors of proto-oceanic crust likely were generated by slow spreading, as the relative plate motions evolved from left-lateral dislocation along the
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Ultra-deep reflection seismic lines (American COCORP, diverse European projects such as ECORS, some industrial surveys such as those from ION-GXT) have conspicuously shown that the lower part of the continental crust is highly reflective. Strong, short, discontinuous, sub-horizontal, wavy reflections are characteristic, imparting an undulating highly reflective pattern to the lower continental crust. Most of the times, the Moho is interpreted at the base of such reflections, at the boundary with the seismically transparent upper mantle. The other important crustal discontinuity, the Conrad, is usually interpreted at the top of such reflectors, at the boundary with the seismically transparent upper crust. In this manner, ultra deep seismic sections usually display the lower continental crust as a strongly reflective wavy layer of varying thickness sandwiched between the transparent upper crust and the transparent sub-continental upper mantle. The reasons for such high reflectivity include the development of abundant sub-horizontal ductile shear zones and the dominant sub-horizontal foliation so characteristic of exposed high-grade metamorphic rocks of the lower crust.
Reflection
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