Continent-ocean transition at the western Barents Sea/Svalbard continental margin

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  • OLR (1988) 35 (6) D. Submarine Geology and Geophysics 555

    Clarion Fracture Zone within a belt 50 to 150 km wide. The specimens included both basaltic glasses and basalts, 45-75% crystalline consisting of pla- gioclase and clinopyroxene, covered with iron- manganese oxide films and crusts up to 13 mm thick. The apparently widespread occurrence of the ferro- basalts in association with magnesium-rich basalts within the Clarion and Clipperton fracture zones suggests a genetic relationship with volcanic activity. South Pacific Geol. Expedition, Southern Mar. Geol. Corp., Yuzmorgeologiya, Nakhodka, USSR. (hbf)

    88:3475 Wetzel, Klaus, 1987. New geochemical criteria for

    distinguishing between fractional crystallization and partial melting of igneous rocks. Z. geol. Wiss., 15(5):553-563.

    88:3476 Zhuravlev, D.Z., A.A. Tsvetkov, A.Z. Zhuravlev,

    N.G. Gladkov and I.V. Chernyshev, 1987. 143Nd/144Nd and ~Sr/86Sr ratios in recent mag- matic rocks of the Kurile Island Arc. Chem. Geol., 66(3-4):227-243.

    Results of the first measurements of Nd and Sr isotopic ratios in the Quaternary volcanic rocks show a new type of transverse isotopic zonation; both ~43Nd/144Nd and STSr/S6Sr decrease from the frontal (Pacific) side of the arc toward submarine volcanoes in the marginal Sea of Okhotsk within its back-arc side. Discussion of theoretical models of island arc magmatism based on the Nd and Sr isotopic data leads to the conclusion that present-day isotopic and possibly geochemical zonation in the Kuriles are the result of compositional heterogeneity of the upper mantle beneath the arc. Contamination of magmatic melts by seawater radiogenic Sr is of minor impor- tance. The contribution from subducted oceanic sediments and basement rocks in arc petrogenesis is estimated to be very small. Inst. of Ore Deposit Geol., Petrol., Mineral. and Geochem., Acad. of Sci., Moscow 109 017, USSR.

    88:3477 Zolotarev, B.P. and B.N. Kotenev, 1986. Tectonics

    and magmatism of the W alvis Ridge and the Discovery Seamount (Atlantic Ocean). Geotec- tonics (a translation of Geotektonika), 20(6):491- 501.

    On the basis of the results of magnetometric studies, both structures are characterized by a crust of oceanic type, from 20 km (Discovery Seamount) to 25-30 km (Walvis Ridge) thick. A determining role in their geologic structure and composition is played by rocks of the subalkalic and alkalic series. The manifestations of volcanism changed in form with

    time from linear to central. The distinctive geologic structure of these elevations, the age of the volcanic rises, and their composition are inconsistent with the hypothesis that they were formed through the action of mantle hotspots. Geol. Inst., Acad. of Sci., USSR.

    D290. Crust, mantle, core

    88:3478 Adamek, Scott, Fumiko Tajima and D.A. Wiens,

    1987. Seismic rupture associated with subduction of the Cocos Ridge. Tectonics, 6(6):757-774.

    The 1983 Costa Rica earthquake (M~=7.3) and its aftershocks, which occurred where the aseismic Cocos Ridge is partially subducted beneath the Caribbean Plate near the southeastern end of the Middle America Trench, are examined to determine the influence of the ridge on the seismicity of the Middle America Trench. Although the low stress drop and low ratio of seismic slip to tectonic slip for the 1983 event would indicate weak coupling between ridge and plate, the occurrence of other large events in that area in 1904 and 1941 suggest a recurrence time of ~40 yr and implies that the Cocos Ridge may cause increased partial coupling in that general area. 649th Engr. Battalion, U.S. Army Europe, APO NY 90981, USA. (hbf)

    88:3479 Eldholm, Olav, J.I. Faleide and A.M. Myhre, 1987.

    Continent-ocean transition at the western Ba- rents Sea/Svalbard continental margin. Geology, geol. Soc. Am., 15(12):1118-1122.

    A grid of multichannel seismic profiles, a few of which extend onto oceanic crust, defines the main structural elements of the margin--two large shear zones and a central rifted margin segment--and supports a model of the stepwise opening of the Greenland Sea in the Paleogene. It is concluded that the continent-ocean boundary lies 'just seaward of the well-defined lineaments south of Svalbard and within a narrow zone farther north.' Dept. of Geol., Univ. of Oslo, P.O. Box 1047, 0316 Blindern, Oslo 3, Norway. (hbf)

    88:3480 Fisher, Donald and Tim Byrne, 1987. Structural

    evolution of underthrusted sediments, Kodiak Islands, Alaska. Tectonics, 6(6):775-793.

    The Late Cretaceous-Early Tertiary Kodiak accre- tionary complex includes two distinct structural terranes----coherent terranes of layered turbidites broken by normal faults, including mineralized

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