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African promontory or Adriatic microplate?
OLR(1984)31 (12)
D. SubmarineGeologyand Geophysics
than continued slip on one fault.' Discussing the controversy on the amount of underthrusting of India beneath Tibet the author suggests that 'as India underthrusts the Himalaya, slices of the crust on its leading edge are detached...the remaining underlying mantle lithosphere, possibly with the lowermost crust, plunges back into the asthenosphere,' contributing to the driving forces on the Indian Plate. Includes 4 pages of references. Dept. of Earth and Planetary Sci., MIT, Cambridge, Mass. 02139, USA. (amt)
84:6136 Morelli, Carlo, 1984. Mrlcan promontory or Adriatic microplate? Boll. Oceanol. teor. apply II(2):151168. (In Italian, English abstract.) Geophysical data supporting the existence of an 'African-Adriatic promontory' include: a ~30-km thick continental crust; presence of a granitic layer; a >70-kin thick lithosphere; and a horizontal boundary marked in most areas by 'collided crusts.' In the Ionian bathyal plain, however, the presence of a 14-kin thick oceanic crust, which includes an 8-kin sedimentary cover, suggests the structure may be a microplate or microcontinent. Ist. di Miniere e Geofis. Applicata, Univ. di Trieste, Italy. (hbf) 84:6137 Seeber, L. and J.G. Armbruster, 1984. Some elements of continental subduction along the I-Iimalayan front. Tectonophysics, 105(I/4):263-278. Detachment tectonics, in which the subducting basement is not internally affected by thrusting, and a steady state system, in which the pattern of deformation and topography is invariant, uplift is balanced by erosion and the material can experience subsequent 'phases' of deformation by migrating through the system, are proposed to be the fundamental mechanisms of convergence at the Himalayan arc. The data support these concepts. The belt of intermediate-magnitude thrust-earthquakes, the topographic front at the High Himalaya and the Main Central Thrust all are associated with the basement thrust front (BTF), which traces a small circle in the central portion of the Himalayan arc. Radial convergence at the BTF implies that Tibet is extending laterally at a rate similar to the rate of convergence across the BTF; this extension must be parallel to the BTF. Lamont-Doherty Geol. Observ., Palisades, NY 10964, USA. 84:6138 Stephens, C.D., K.A. Fogleman, J.C. Lahr and R.A. Page, 1984. Wrangell Benioff zone, southern Alaska. Geology, geol. Soc. Am, 12(6):373-376.
877
Microearthquake data have provided 'the first unequivocal evidence for the existence of a Benioff zone that may be related to the Quaternary Wrangell volcanoes.' Out of 86 well-located epicenters, about half occurred at a depth of 25 km or less. Activity between 40 and 85 km depth is subparallel to the strike of the volcanic belt for 115 km. At the western end, the Benioff zone shoals and cannot be distinguished from upper plate seismicity. It remains 'uncertain whether the subducted plate segment that contains the Wrangell Benioff zone is structurally part of the Pacific Plate or the Yakutat block.' USGS, 345 Middlefield Rd., Menlo Park, Calif. 94025, USA. (amt)
84:6139 Walcott, R.I., 1984. Reconstructions of the New Zealand region for the Nengene. Palaeogeogr. Palaeoclimatol. Palaeoecol., 46(1/3):217-231. The major structural elements of the present-day plate boundary zone through New Zealand are a subduction zone under the northern half of the country linked by a wide transform zone to the southward extension of the plate boundary. Using finite rotations of the Pacific/Australian plate pair, a sequence of reconstructions of the relative positions of the plates is presented. Reconstructions based on paleomagnetic data involve essentially the same structural elements as today but with large clockwise changes in their trend with time. Geophys. Div., DSIR, P.O. Box 1320, Wellington, New Zealand.
D250. Plate and global tectonics 84:6140 Johansen, Bard, P.R. Vogt and Olav Eldholm, 1984. Reykjanes Ridge: further analysis of crustal subsidence and time-trans~r~sive basement topography. Earth planet. Sci. Letts, 68(2):249-258. Initial depth at zero age and the rate of increase of depth with age as a result of thermal contraction are examined for a series of profiles across the Reykjanes Ridge. In general, the initial depth is less than that of normal oceanic crust and increases with distance from Iceland. Short wavelength variations may be related to the presence of magma chambers. The rate of subsidence is also anomalously low. The previously observed time-transgressive features may represent thermally induced fracturing; if they are instead flow features, they 'must originate in a manner more complicated than previously suggested.' Norsk Hydro A/S, Harstad, Norway. (amt)
D. SubmarineGeologyand Geophysics
than continued slip on one fault.' Discussing the controversy on the amount of underthrusting of India beneath Tibet the author suggests that 'as India underthrusts the Himalaya, slices of the crust on its leading edge are detached...the remaining underlying mantle lithosphere, possibly with the lowermost crust, plunges back into the asthenosphere,' contributing to the driving forces on the Indian Plate. Includes 4 pages of references. Dept. of Earth and Planetary Sci., MIT, Cambridge, Mass. 02139, USA. (amt)
84:6136 Morelli, Carlo, 1984. Mrlcan promontory or Adriatic microplate? Boll. Oceanol. teor. apply II(2):151168. (In Italian, English abstract.) Geophysical data supporting the existence of an 'African-Adriatic promontory' include: a ~30-km thick continental crust; presence of a granitic layer; a >70-kin thick lithosphere; and a horizontal boundary marked in most areas by 'collided crusts.' In the Ionian bathyal plain, however, the presence of a 14-kin thick oceanic crust, which includes an 8-kin sedimentary cover, suggests the structure may be a microplate or microcontinent. Ist. di Miniere e Geofis. Applicata, Univ. di Trieste, Italy. (hbf) 84:6137 Seeber, L. and J.G. Armbruster, 1984. Some elements of continental subduction along the I-Iimalayan front. Tectonophysics, 105(I/4):263-278. Detachment tectonics, in which the subducting basement is not internally affected by thrusting, and a steady state system, in which the pattern of deformation and topography is invariant, uplift is balanced by erosion and the material can experience subsequent 'phases' of deformation by migrating through the system, are proposed to be the fundamental mechanisms of convergence at the Himalayan arc. The data support these concepts. The belt of intermediate-magnitude thrust-earthquakes, the topographic front at the High Himalaya and the Main Central Thrust all are associated with the basement thrust front (BTF), which traces a small circle in the central portion of the Himalayan arc. Radial convergence at the BTF implies that Tibet is extending laterally at a rate similar to the rate of convergence across the BTF; this extension must be parallel to the BTF. Lamont-Doherty Geol. Observ., Palisades, NY 10964, USA. 84:6138 Stephens, C.D., K.A. Fogleman, J.C. Lahr and R.A. Page, 1984. Wrangell Benioff zone, southern Alaska. Geology, geol. Soc. Am, 12(6):373-376.
877
Microearthquake data have provided 'the first unequivocal evidence for the existence of a Benioff zone that may be related to the Quaternary Wrangell volcanoes.' Out of 86 well-located epicenters, about half occurred at a depth of 25 km or less. Activity between 40 and 85 km depth is subparallel to the strike of the volcanic belt for 115 km. At the western end, the Benioff zone shoals and cannot be distinguished from upper plate seismicity. It remains 'uncertain whether the subducted plate segment that contains the Wrangell Benioff zone is structurally part of the Pacific Plate or the Yakutat block.' USGS, 345 Middlefield Rd., Menlo Park, Calif. 94025, USA. (amt)
84:6139 Walcott, R.I., 1984. Reconstructions of the New Zealand region for the Nengene. Palaeogeogr. Palaeoclimatol. Palaeoecol., 46(1/3):217-231. The major structural elements of the present-day plate boundary zone through New Zealand are a subduction zone under the northern half of the country linked by a wide transform zone to the southward extension of the plate boundary. Using finite rotations of the Pacific/Australian plate pair, a sequence of reconstructions of the relative positions of the plates is presented. Reconstructions based on paleomagnetic data involve essentially the same structural elements as today but with large clockwise changes in their trend with time. Geophys. Div., DSIR, P.O. Box 1320, Wellington, New Zealand.
D250. Plate and global tectonics 84:6140 Johansen, Bard, P.R. Vogt and Olav Eldholm, 1984. Reykjanes Ridge: further analysis of crustal subsidence and time-trans~r~sive basement topography. Earth planet. Sci. Letts, 68(2):249-258. Initial depth at zero age and the rate of increase of depth with age as a result of thermal contraction are examined for a series of profiles across the Reykjanes Ridge. In general, the initial depth is less than that of normal oceanic crust and increases with distance from Iceland. Short wavelength variations may be related to the presence of magma chambers. The rate of subsidence is also anomalously low. The previously observed time-transgressive features may represent thermally induced fracturing; if they are instead flow features, they 'must originate in a manner more complicated than previously suggested.' Norsk Hydro A/S, Harstad, Norway. (amt)