- Email: [email protected]
Evidence for transform margin evolution from the Ivory Coast-Ghana continental margin
766
D. Submarine Geology and Geophysics
discontinuities vary within 490-560 km and 630-680 km, respectively. This implies that these discontinuities are relatively elevated in the subducting slab, in general agreement with Ringwood and others. The velocity models considered in the present study also indicate lithospheric slab penetration well below the 600 km seismic discontinuity in the Tonga-Kermadec region. Natl. Geophys. Res. Inst., Hyderabad 500007, India. 87:5094 Mascle, Jean and Emmanuel Blarez, 1987. Evidence for transform margin evolution from the Ivory Coast-Ghana continental margin. Nature, Lond., 326(6111):378-381. Here we present the main conclusions derived from a recent study of the northern Gulf of Guinea margins, particularly off the eastern Ivory Coast and Ghana, where the continental margin is one of the best-preserved examples of an extinct transform margin. Our observations support a four-stage model for transform margin evolution. Tectonically active transform contacts, first between normal continental crusts and then between thinned margins, induce characteristic structures such as pull-apart grabens and shear folds. The next stage, in which thermal exchange between oceanic and continental lithosphere controls a complex subsidence, is followed by the transition to a true intra-oceanic fracture zone. Lab. de Geodynamique sous-mar., CEROV, BP 48-06230 Villefranche-sur-mer, France. 87:5095 Srikantia, S.V., 1987. Himalaya--the collided orogen: a plate tectonic evolution on geological evidences. Tectonophysics, 134(1-3):75-90. The Tethys Himalayan zone has all the elements of a microcontinent. During the upper CarboniferousPermian, the Proto-Tethys was sutured to the Proto-Karakorum-Tibet Plate. During the Early Cretaceous, Gondwanic India collided with the Proto-Tethys, opening the suture boundary. Simultaneously, Gondwanic India became tensile and there was reactivation along the Narmada-Son zone leading to extensive Deccan volcanism. As compression increased, the Sangeluma and Shyok seas closed, the newly formed oceanic crust was subducted, dismembered, and obducted, and the deltaic Indus Basin formed. Along the southern margin Proto--Lesser Himalaya Tertiary basins evolved. Widespread regression and uplift was accompanied by overlap of the Tethys zone on to the Gondwanic India-Lesser Himalaya and the Indus ophiolite belt-Ladakh batholith, forming the Himalaya. A.M.S.E. Wing, Geol. Survey of India, No. 2, Church St., Bangalore 560001, India.
OLR (1987) 34 (9)
87:5096 Verma, R.K. and G.V.R. Krishna Kumar, 1987. Seismieity and the nature of plate movement along the Himalayan arc, northeast India and Arakan-Yoma: a review. Tectonophysics, 134(13):153-175. Seismicity along the entire Himalaya and northern Burma has been studied in detail. Besides the main boundary fault and the main central thrust several transverse features are also very active, some behaving like steeply dipping fracture zones. Although thrust movements are predominant, normal and strike-slip faulting is taking place along some of the transverse features. Orientation of P-axes for all thrust solutions show a sharp change from predominantly E-W along the Burmese Arc to N-S and NE-SW along the Himalaya, to NW-SE along the Baluchistan Arc. It appears that the Indian lithosphere is under compression from practically all sides. No simple model appears to be applicable for the entire Himalaya. Indian School of Mines, Dhanbad, India.
D280. Volcanism, magmatism 87:5097 Bailey, J.C., O. Larsen and T.I. Frolova, 1987. Strontium isotope variations in Lower TertiaryQuaternary volcanic rocks from the Kurile Island Arc. Contr. Miner. Petrology, 95(2): 155-165. Inst. for Petrology, Univ. of Copenhagen, Oster Voldgade 10, DK-1350 Copenhagen K, Denmark. 87:5098 Boillot, Gilbert et al., 1986. Basaltic and ultramafic seafloors along a passive continental margin. Preliminary results of the Galinaute cruise (diving to the west of Spain by the submersible Nautile). C. r. Acad. Sci., Paris, (S6r. 1I)303(19): 1719-1724. (In French, English abstract.) Three types of seafloor were investigated at depths of 3500-5500 m (North Atlantic, west of Galicia, Spain): Galicia passive margin, where continental basement and Jurassic-Cenozoic sediments outcrop on the seafloor; basaltic seafloor, probably Cretaceous oceanic crust from the Bay of Biscay, incorporated into the Galicia Bank by Eocene tectonics; and ultramafic seafloor of serpentinized peridotite cut by dolerite dikes. The ultramafic rocks extend as a 100-kin long belt between the North Atlantic oceanic crust on the west and the thinned continental crust on the east. Lab. de Geodynamique, B.P. 48, 06230 Villefranche-sur-Mer, France.
D. Submarine Geology and Geophysics
discontinuities vary within 490-560 km and 630-680 km, respectively. This implies that these discontinuities are relatively elevated in the subducting slab, in general agreement with Ringwood and others. The velocity models considered in the present study also indicate lithospheric slab penetration well below the 600 km seismic discontinuity in the Tonga-Kermadec region. Natl. Geophys. Res. Inst., Hyderabad 500007, India. 87:5094 Mascle, Jean and Emmanuel Blarez, 1987. Evidence for transform margin evolution from the Ivory Coast-Ghana continental margin. Nature, Lond., 326(6111):378-381. Here we present the main conclusions derived from a recent study of the northern Gulf of Guinea margins, particularly off the eastern Ivory Coast and Ghana, where the continental margin is one of the best-preserved examples of an extinct transform margin. Our observations support a four-stage model for transform margin evolution. Tectonically active transform contacts, first between normal continental crusts and then between thinned margins, induce characteristic structures such as pull-apart grabens and shear folds. The next stage, in which thermal exchange between oceanic and continental lithosphere controls a complex subsidence, is followed by the transition to a true intra-oceanic fracture zone. Lab. de Geodynamique sous-mar., CEROV, BP 48-06230 Villefranche-sur-mer, France. 87:5095 Srikantia, S.V., 1987. Himalaya--the collided orogen: a plate tectonic evolution on geological evidences. Tectonophysics, 134(1-3):75-90. The Tethys Himalayan zone has all the elements of a microcontinent. During the upper CarboniferousPermian, the Proto-Tethys was sutured to the Proto-Karakorum-Tibet Plate. During the Early Cretaceous, Gondwanic India collided with the Proto-Tethys, opening the suture boundary. Simultaneously, Gondwanic India became tensile and there was reactivation along the Narmada-Son zone leading to extensive Deccan volcanism. As compression increased, the Sangeluma and Shyok seas closed, the newly formed oceanic crust was subducted, dismembered, and obducted, and the deltaic Indus Basin formed. Along the southern margin Proto--Lesser Himalaya Tertiary basins evolved. Widespread regression and uplift was accompanied by overlap of the Tethys zone on to the Gondwanic India-Lesser Himalaya and the Indus ophiolite belt-Ladakh batholith, forming the Himalaya. A.M.S.E. Wing, Geol. Survey of India, No. 2, Church St., Bangalore 560001, India.
OLR (1987) 34 (9)
87:5096 Verma, R.K. and G.V.R. Krishna Kumar, 1987. Seismieity and the nature of plate movement along the Himalayan arc, northeast India and Arakan-Yoma: a review. Tectonophysics, 134(13):153-175. Seismicity along the entire Himalaya and northern Burma has been studied in detail. Besides the main boundary fault and the main central thrust several transverse features are also very active, some behaving like steeply dipping fracture zones. Although thrust movements are predominant, normal and strike-slip faulting is taking place along some of the transverse features. Orientation of P-axes for all thrust solutions show a sharp change from predominantly E-W along the Burmese Arc to N-S and NE-SW along the Himalaya, to NW-SE along the Baluchistan Arc. It appears that the Indian lithosphere is under compression from practically all sides. No simple model appears to be applicable for the entire Himalaya. Indian School of Mines, Dhanbad, India.
D280. Volcanism, magmatism 87:5097 Bailey, J.C., O. Larsen and T.I. Frolova, 1987. Strontium isotope variations in Lower TertiaryQuaternary volcanic rocks from the Kurile Island Arc. Contr. Miner. Petrology, 95(2): 155-165. Inst. for Petrology, Univ. of Copenhagen, Oster Voldgade 10, DK-1350 Copenhagen K, Denmark. 87:5098 Boillot, Gilbert et al., 1986. Basaltic and ultramafic seafloors along a passive continental margin. Preliminary results of the Galinaute cruise (diving to the west of Spain by the submersible Nautile). C. r. Acad. Sci., Paris, (S6r. 1I)303(19): 1719-1724. (In French, English abstract.) Three types of seafloor were investigated at depths of 3500-5500 m (North Atlantic, west of Galicia, Spain): Galicia passive margin, where continental basement and Jurassic-Cenozoic sediments outcrop on the seafloor; basaltic seafloor, probably Cretaceous oceanic crust from the Bay of Biscay, incorporated into the Galicia Bank by Eocene tectonics; and ultramafic seafloor of serpentinized peridotite cut by dolerite dikes. The ultramafic rocks extend as a 100-kin long belt between the North Atlantic oceanic crust on the west and the thinned continental crust on the east. Lab. de Geodynamique, B.P. 48, 06230 Villefranche-sur-Mer, France.