1030 D. Submarine Geology and Geophysics OLR (1985) 32 (12)

segment to the northeast during margin formation. Pacific Geosci. Centre, Dept. of Energy, Mines and Res., Box 6000, Sidney, BC V8L 4B2, Canada.

D250. Plate and global tectonics

85:7160 Hynes, A. and J. Mott, 1985. On the causes of

bagk-are spreading. Geology, geol. Soc. Am., 13(6):387-389.

The characteristics of plate motion within the actively spreading Lau Basin-Havre Trough and Mariana Trough are reviewed in an effort to evaluate the relative importance of several proposed plate-kinematic mechanism theories tied to motions of the upper plate and lower plate and changes in the profile of the lower plate. It is concluded that adjustment of the descending slab dip as a result of changes in subduction rate provides an 'attractive explanation' for trench migration and the recent onset of spreading behind the two arcs and for the fact that spreading in the Lau Basin outpaces spreading in the Havre Trough. Dept. of Geol. Sci., McGill Univ., Montreal, H3A 2A7, Canada. (hbf)

85:7161 Lawver, L.A., J.G. Sclater and Linda Meinke, 1985.

Mesozoic and Cenozoic reconstructions of the South Atlantic. Tectonophysics, 114(1-4): 233 - 254.

Recently published poles of rotation used to recon- struct the configurations of Antarctica, Africa and South America with respect to each other indicate that the opening between the Antarctic Peninsula and the tip of South America began before 35 Ma and that the peninsula moved away from South America at least 10 Ma before the opening of Drake's Passage, if there was no movement between East and West Antarctica at that time. Results also support the hypothesis that the triple junction between Africa, Antarctica and South America underwent a major reorganization 60-65 Ma, chang- ing from ridge-ridge-ridge to ridge-fault-fault. Mad- agascar, India, Sri Lanka and the Somali Basin are also discussed. Inst. for Geophys., Univ. of Texas, Austin, TX 78751, USA. (msg)

85:7162 Mutter, J.C., 1985. Seaward dipping reflectors and the

e o n t i n e n t ~ boundary at passive ~ t a l nmr~ns. Tectonophysics, 114(1-4):117-131.

Multichannel seismic reflection profiles on many passive margins have revealed the presence of remarkable suites of arcuate reflectors, dipping

seaward to form a wedge-shaped structure. Their general characteristics and velocity structure suggest they may be largely volcanic, but there is no agreed upon model for their origin. Nevertheless it is generally thought that they lie at or close to the boundary between continent and ocean, and as such they are extremely important structural markers that may yield important evidence concerning the struc- ture and evolution of passive margins. Lamont- Doherty Geol. Observ., Palisades, NY 10964, USA.

85:7163 Mutter, J.C., K.A. Hegarty, S.C. Cande and J.K.

Weissel, 1985. Breakup between Australia and Antarctica: a brief review in the light of new data. Tectonophysics, 114(1-4):255-279.

The age of breakup was revised from anomaly 22 time to anomaly 34 time. The reconstruction of Broken Ridge and Kerguelen Plateau at anomaly 34 time shows overlap of these two features, but the problem is nearly resolved by anomaly 18 time. Subsidence patterns are interpreted as more consis- tent with the revised age of Australia-Antarctic breakup. Rapid subsidence was associated with the rift phase of margin development followed by much slower thermally-controlled subsidence during the drift phase, the transition coinciding with the age of breakup (~60 to 110 MyBP). Subsidence curves indicate a west-to-east propagation of breakup along the southern margin, consistent with magnetic anomaly patterns and stratigraphic observations. Lamont-Doherty Geol. Observ., Palisades, NY 10964, USA.

D280. Volcanism, magmatism 85:7164

Fornari, D.J., W.B.F. Ryan and P.J. Fox, 1985. Sea-floor lava fields on the East Pacific Rise. Geology, geol. Soe. Am., 13(6):413-416.

SeaBeam and SeaMARC I data west of the East Pacific Rise define a relatively young submarine lava field, which extends over the faulted, eroded flanks of the EPR and laps up against the base of a large seamount with a large caldera. It is suggested that the lava field was produced by seamount magma, which flowed along pre-existing faults, draining shallow magma chambers beneath the seamount, deflating its summit and leaving the caldera. Lamont-Doherty Geol. Observ., Palisades, NY 10964, USA. (hbf)

85:7165 Fujii, Toshitsugu and C.M. Scarfe, 1985. Compo-

sition of liquids coexisting with spinel lherzoHte at