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Causes and rate-limiting mechanisms of ridge propagation: a fracture mechanics model
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D. SubmarineGeologyand Geophysics
86:1589 Forsythe, Randall and Eric Nelson, 1985. Geological nmalfes/atioas of ridge collision: evidence from the Golfo de Penas-Taltao Basin, southern Chile. Tectonics, 4(5):477-495. Recent studies near the intersection of the Chile Rise with the Peru-Chile Trench (Antarctic-Nazca-South American triple junction) have revealed several features and a Neogene history unique along the Pacific margin of South America. A dynamic model shows a stress gradient set up because the 'youngest, most buoyant, oceanic lithosphere is being subducted at the triple junction'; it suggests the importance of ridge-trench interactions to orogenesis at active continental margins. Dept. of Geol. Sci., Rutgers Univ., New Brunswick, N J, USA. (row0 86:1590 Grohmann, N., 1985. Drift tectonics---the fundamental rhythm of crustal drift and deformation. Geologische Rdsch., 74(2):267-310. Research on the deformation of the Alps and central European foreland and development of a map projection which provides a means for analyzing paleomagnetic and tectonic data, have led to a new model for the driving mechanism of continental drift and intraplate tectonics that would link horizontal and vertical tectonic hypotheses. Drift fields, the largest structural units of the Earth's surface, 'provide the mechanism for the conversion of upper mantle temperatures into the direction of lithospheric motion .... By redistributing upper mantle isotherms, the drift fields simultaneously control the course of asthenosphere counterflows, whose internal flow patterns can trigger asthenospheric upwellings and epeirogenic uplifts.' The model thus can account for isostatic anomalies, epeirogeny, and, perhaps, orogeny. Inst. f. Allg. u Angew, Geol. d. Univ. Munchen, Lulsenstrasse 37, D-8000 Mnnchen 2, FRG. (hb0 86:1591 McAdoo, D.C. and D.T. Sandwell, 1985. Folding of
oceank lithosphe~ [Indian ~___B]. J. geophys. Res, 90(B 10):8563-8569. In the northeast Indian Ocean the presence of basement undulations (wavelengths 100-300 km and amplitudes approaching 3 kin) has been demonstrated by acoustical and Seasat altimeter data. Although elastic models would require compressive stresses of about 5 GPa to buckle the 40-50 kin-thick lithosphere in the region, an elastic-plastic model would require only 12% of the elastic buckling stress to produce 200-kin wavelengths. The study supports
OLR(1986)33 (3)
the hypothesis that the folding is linked to the Himalayan orogeny. Natl. Geodetic Surv., Charting and Geod. Serv., NOS/NOAA, RockviUe, MD, USA. (hbf) 86:1592 Morgan, J.P. and E.M. Parmentier, 1985. Causes and rate~imiting mechanisms of ridge propagation: a fracture mechanics model. J. geophys. Res., 90(B 10):8603-8612. A simple model for steady continuous rift propagation, in which excess gravity spreading stresses near the tip of the ridge axis conduit extend the ridge axis, is found to be consistent with the axial morphology of the spreading segment of the Galapagos Spreading Center at 95.5°W. Examination of energy conservation considerations for a more general model of a ridge-transform-ridge system reveals that mechanisms which may halt ridge propagation--as when strain energy for transform migration exceeds energy for ridge axis growth or when strain energy absorbed at the dying axis equals that released at the growing axis--may also be exemplified at the ridge segment at 85°W in the Galapagos. Dept. of Geol. Sci., Brown Univ., Providence, RI, USA. (hb 0
D280. Volcanism, magmatism 86:1593 Batiza, Rodey and D.A. Vanko, 1985. Petrologic evolution of large failed rifts in the eastern Pacific: petrology of volcank and p!utonk rocks from the Mathematician Ridge area and the Gnadalupe Trough. J. Petrology, 26(3):564-602. Although it is known that the Mathematician failed rift was a fast-spreading ridge before 6.5 Ma, its morphology is that of a slow-spreading ridge crest. Dredge samples of sepentinizcd peridotite, cumulate and isotropic gabbro, diabase, and MORB lava from the inner rift also have the petrologic characteristics of a slow-spreading ridge, and samples from failed transform faults in the Mathematicians are composed of fresh alkali lava. The evidence suggests that in late stages of abandonment the rift slows and the underlying crustal magma chamber is absent, and that eruptions of alkali basalt occur as the lithosphere thickens in place. Dept. of Earth and Planet. Sci. and McDonnell Center for the Space Sci., Washington Univ., St. Louis, MO 63130, USA. (hbf) 86:19)4 Campbell, I.H., 1985. The diffet~ace between eceank and eoatinemal tholeates: a flnkl d ~ m k e x i t . nation, Contr. Miner. Petrology, 91(1):37-43.
D. SubmarineGeologyand Geophysics
86:1589 Forsythe, Randall and Eric Nelson, 1985. Geological nmalfes/atioas of ridge collision: evidence from the Golfo de Penas-Taltao Basin, southern Chile. Tectonics, 4(5):477-495. Recent studies near the intersection of the Chile Rise with the Peru-Chile Trench (Antarctic-Nazca-South American triple junction) have revealed several features and a Neogene history unique along the Pacific margin of South America. A dynamic model shows a stress gradient set up because the 'youngest, most buoyant, oceanic lithosphere is being subducted at the triple junction'; it suggests the importance of ridge-trench interactions to orogenesis at active continental margins. Dept. of Geol. Sci., Rutgers Univ., New Brunswick, N J, USA. (row0 86:1590 Grohmann, N., 1985. Drift tectonics---the fundamental rhythm of crustal drift and deformation. Geologische Rdsch., 74(2):267-310. Research on the deformation of the Alps and central European foreland and development of a map projection which provides a means for analyzing paleomagnetic and tectonic data, have led to a new model for the driving mechanism of continental drift and intraplate tectonics that would link horizontal and vertical tectonic hypotheses. Drift fields, the largest structural units of the Earth's surface, 'provide the mechanism for the conversion of upper mantle temperatures into the direction of lithospheric motion .... By redistributing upper mantle isotherms, the drift fields simultaneously control the course of asthenosphere counterflows, whose internal flow patterns can trigger asthenospheric upwellings and epeirogenic uplifts.' The model thus can account for isostatic anomalies, epeirogeny, and, perhaps, orogeny. Inst. f. Allg. u Angew, Geol. d. Univ. Munchen, Lulsenstrasse 37, D-8000 Mnnchen 2, FRG. (hb0 86:1591 McAdoo, D.C. and D.T. Sandwell, 1985. Folding of
oceank lithosphe~ [Indian ~___B]. J. geophys. Res, 90(B 10):8563-8569. In the northeast Indian Ocean the presence of basement undulations (wavelengths 100-300 km and amplitudes approaching 3 kin) has been demonstrated by acoustical and Seasat altimeter data. Although elastic models would require compressive stresses of about 5 GPa to buckle the 40-50 kin-thick lithosphere in the region, an elastic-plastic model would require only 12% of the elastic buckling stress to produce 200-kin wavelengths. The study supports
OLR(1986)33 (3)
the hypothesis that the folding is linked to the Himalayan orogeny. Natl. Geodetic Surv., Charting and Geod. Serv., NOS/NOAA, RockviUe, MD, USA. (hbf) 86:1592 Morgan, J.P. and E.M. Parmentier, 1985. Causes and rate~imiting mechanisms of ridge propagation: a fracture mechanics model. J. geophys. Res., 90(B 10):8603-8612. A simple model for steady continuous rift propagation, in which excess gravity spreading stresses near the tip of the ridge axis conduit extend the ridge axis, is found to be consistent with the axial morphology of the spreading segment of the Galapagos Spreading Center at 95.5°W. Examination of energy conservation considerations for a more general model of a ridge-transform-ridge system reveals that mechanisms which may halt ridge propagation--as when strain energy for transform migration exceeds energy for ridge axis growth or when strain energy absorbed at the dying axis equals that released at the growing axis--may also be exemplified at the ridge segment at 85°W in the Galapagos. Dept. of Geol. Sci., Brown Univ., Providence, RI, USA. (hb 0
D280. Volcanism, magmatism 86:1593 Batiza, Rodey and D.A. Vanko, 1985. Petrologic evolution of large failed rifts in the eastern Pacific: petrology of volcank and p!utonk rocks from the Mathematician Ridge area and the Gnadalupe Trough. J. Petrology, 26(3):564-602. Although it is known that the Mathematician failed rift was a fast-spreading ridge before 6.5 Ma, its morphology is that of a slow-spreading ridge crest. Dredge samples of sepentinizcd peridotite, cumulate and isotropic gabbro, diabase, and MORB lava from the inner rift also have the petrologic characteristics of a slow-spreading ridge, and samples from failed transform faults in the Mathematicians are composed of fresh alkali lava. The evidence suggests that in late stages of abandonment the rift slows and the underlying crustal magma chamber is absent, and that eruptions of alkali basalt occur as the lithosphere thickens in place. Dept. of Earth and Planet. Sci. and McDonnell Center for the Space Sci., Washington Univ., St. Louis, MO 63130, USA. (hbf) 86:19)4 Campbell, I.H., 1985. The diffet~ace between eceank and eoatinemal tholeates: a flnkl d ~ m k e x i t . nation, Contr. Miner. Petrology, 91(1):37-43.