OLR (1988) 35 (6) D. Submarine Geology and Geophysics 553

proposed by earlier research. Inst. of Geomagnetism, the Ionosphere and Radio Wave Propagation, Acad. of Sci., Leningrad, USSR. (hbf)

88:3464 Kissel, C., C. Laj, A.M.C. Sengrr and A. Poisson,

1987. Palcomagnetic evidence for rotation in opposite senses of adjacent blocks in northeastern Aegea and western Anatolia. Geophys. Res. Letts, 14(9):907-910. Centre des Faibles Radio- activites, Lab. CEA/CNRS, Gif-sur-Yvette, France.

88:3465 Lamb, S.H. and P. Vella, 1987. The last million years

of deformation in part of the New Zealand plate-boundary zone. J. struct. Geol., 9(7)'877- 891.

Tectonic deformation of Pleistocene sediments is occurring within the plate-boundary zone in the North Island. Folds, associated with an active fault, are forming mainly by flexural-slip; folding started ca. 1.0 Ma. Simple geometric models indicate the fold structure has shortened over time, though with an increase in the last 100 Ka of about 1.8 times the average rate (compatible with contemporary obser- vations). Strain rates across the plate-boundary zone vary with position and time, with negative corre- lation between the uplift pattern and inferred geological shortening rates. This can be explained by large semi-rigid blocks within the plate-boundary zone which are back-tilting and overriding weaker more rapidly deforming areas. Available evidence suggests a larger proportion of the deformation is occurring seismically. Res. School of Earth Sci., Victoria Univ., Private Bag, Wellington, New Zea- land.

88:3466 Le Suavr, Raymond et al., 1987. Evidence of

intra-plate erosion and recent tectonic activity in the north equatorial Pacific Ocean: examples from data acquired with the deep-towed acoustic system S . A . R . C . r . Acad. Sci., Paris, (Srr. II)305(14):1211-1216. (In French, English ab- stract.)

A series of advanced high technical mapping of several nodule rich-areas in the Clarion-Clipperton fracture zone was made by IFREMER in 1986 using S.A.R., a new acoustic deep sea towed system. Two examples of recent tectonic and erosion activity are presented. IFREMER, Centre de Brest, B.P. No. 337, 29273, Brest Cedex, France.

88:3467 Ocean Drilling Program (Leg 116 shipboard scien-

tific party), James Cochran, D.A.V. Stow and Christian Auroux (co-chief scientists), 1987. Ocean Drilling Program. Collisions in the Indian Ocean. [Report.] Nature, Lond., 330(6148):519- 521.

On Leg 116 to the central Indian Ocean evidence was found for a 'soft' collision between India and an island arc off Asia prior to the 'hard' continent- to-continent collision that resulted in the first major uplift of the Himalaya in the Early Miocene, an earlier date than previously assumed. The supporting evidence for the soft collision comes primarily from findings of 100-300 km wavelength folds in the ocean crust between the Chagos-Laccadive Ridge and the Ninetyeast Ridge, which are broken by high-angle faults, 5-20 km apart, that offset the upper crustal layers by as much as 500 m. Other findings include new data from sites 717, 718, and 719, which penetrated into Miocene turbidites, on the timing and nature of intraplate deformation and the sedimentary history of the distal portion of the Bengal Fan. Lamont-Doherty Geol. Observ., Pali- sades, NY 10964, USA. (hbf)

88:3468 Pushcharovskiy, Yu.M. and Yu.N. Raznitsin, 1986.

Tectonics of the Caroline oceanic step (southwest Pacific Ocean). Geotectonics (a translation of Geotektonika), 20(6):480-490.

The development of the step took place as a result of the relative subsidence of the East Marianas abyssal plate at the end of the Eocene and beginning of the Oligocene. The separation of major oceanic steps, distinguished by their bathymetric levels, allows new possibilities for clarifying the mechanism of deep- ening of the oceans during the Late Mesozoic and Cenozoic. Original data are presented on the structure and material composition of the Mussau Ridge. Geol. Inst., Acad. of Sci., USSR.

88:3469 Reches, Ze'ev and Gerald Schubert, 1987. Models of

post-Miocene deformation of the Arabian Plate. Tectonics, 6(6):707-725.

The post Miocene tectonic features of the Arabian Plate are described along a profile extending from a region of uplift, volcanism, and extensional faulting along the Red Sea eastward across the relatively featureless interior to the subsidence, folding, and thrusting of the Zagros. Models are derived to explain the change from near-surface horizontal extensional deviatoric stress on the west to com- pressional on the east across a wide zone of small

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

tectonic stresses. These models reveal the 'combined effects of lateral density variations within the mantle, ridge push at the Red Sea, and convective shear stress at the plate base.' Geol. Dept., Hebrew Univ., Jerusalem, 91904, Israel. (hbf)

88:3470 Savostin, L.A., B.V. Baranov, T.Z. Grigoryan and

L.R. Merklin, 1986. Tectonics and origin of the western Bering Sea. Dokl. Earth Sci. Sect. (a translation of DokL Akad. Nauk SSSR), 286(1- 6):88-91.

Results of the 1982 survey of the western Bering Sea, which supplied new information about the sediments and structure of the Komandorskiy Basin, are discussed. These results, coupled with those of other Soviet and U.S. surveys suggest that there were at least two major stages in the tectonic evolution of the area. The first was marked by Early Tertiary compression which led to thrusting, folding, and the collision of a submarine ridge with the Eurasia margin, producing a southward shift of the subduc- tion zone to the Aleutian island arc. The second stage began during Late Oligocene-Early Miocene when rifting and seafloor spreading was initiated in the Komandorskiy Basin. Shirshov Inst. of Oceanol., Acad. of Sci., Moscow, USSR. (hbf)

D250. Plate and global tectonics

88:3471 DeMets, Charles, R.G. Gordon, Seth Stein and D.F.

Argus, 1987. A revised estimate of Pacific-North America motion and implications for western North America Plate boundary zone tectonics. Geophys. Res. Letts, 14(9):911-914.

An analysis of marine magnetic profiles from the Gulf of California indicates that over the past 3 m.y. the spreading rate between the Pacific and North America plates has averaged 48 mm y r ~, 10 mm yr less than previous estimates based on plate motion studies, and in better agreement with determinations for the combined effects of slip along the San Andreas and extension across the Basin and Range Province. The new findings would also require less strike-slip motion on faults west of the San Andreas. Dept. of Geol. Sci., Northwestern Univ., Evanston, IL, USA. (hbf)

88:3472 Dunbar, J.A. and D.S. Sawyer, 1987. Implications of

continental crust extension for plate reconstruc- tion: an example from the Gulf of Mexico. Tectonics, 6(6):739-755.

A method is described for the prerift reconstruction of continental plate margins, which takes into account both seafloor spreading and extension of the continental crust. The approach includes the non- rigid aspects of plate motion and provides an estimate of the amount and direction of plate motion during rifting. The method is applied to the Gulf of Mexico region, where a reconstruction based on the best-fit Euler pole would place the Yucatan block along the Texas-Louisiana shelf and the southern margin of North America against the northern margin of South America. Inst. for Geophys., Univ. of Texas, 8701 Mopac Blvd., Austin, TX 78759, USA. (hbf)

D280. Volcanism, magmatism

88:3473 Barrett, T.J. and H. Friedrichsen, 1987. Oxygen-

isotopic composition of basalts from young spreading axes in the eastern Pacific. Can. J. Earth Sci., 24(11):2105-2117.

Oxygen isotope compositions are reported for young oceanic basalts from two tectonic settings in the eastern Pacific--the 1 Ma uppermost basement on the Galapagos Rift and the <3.5 Ma Gulf of California subbasement. In the Galapagos area whole-rock 8~80 values exhibit a narrow range of 5.7-6.6 ppt, consistent with minimal to slight low-temperature alteration. In the Gulf of California high whole-rock values are found near the Baja margin where the greatest range of ~80 (2.5 to 12.5 ppt) also occurs, indicating considerable variation in temperature and degree of alteration. Analyses of primary magmatic phases at all sites yield 180 fractionations between plagioclase, clinopyroxene, and magnetite consistent with crystallization in a closed system without high-temperature subsolidus reaction with seawater. Sr and oxygen-isotope data for four 6 Ma basalts at Site 504B are also reported. IREM-MERI, Dept. of Geol. Sci., McGill Univ., Montreal, PQ H3A 2A7, Canada.

88:3474 Korsakov, O.D., Yu.A. Martynov, A.V. Mechetin,

V.M. Andreyev, V.V. Malakhov and V.I. Taskayev, 1986. Ferrobasalts of the Clarion Fracture Zone, Pacific Ocean. Dokl. Earth Sci. Sect. (a translation of Dokl. Akad. Nauk SSSR), 286(1-6):75-77.

Ferrobasalt fragments, rubble, and occasional blocks of aphyric basalt were found in bottom dredge and trawl samples from depths of 3500 to 6000 m for more than 350 km along the eastern part of the