Abstract. New marine geophysical data along the Macquarie Ridge Complex, the AustraliaPacific plate boundary south of New Zealand, illuminate regional neotectonics. We identify tectonic spreading fabric and fracture zones and precisely locate the Australia-Pacific plate boundary along the Macquarie Ridge Complex. We interpret a-•5-10 km wide Macquarie Fault Zone between the two plates along a bathymetric high that extends nearly the entire length of the Australia-Pacific plate boundary south of New Zealand. We conclude that this is the active Australia-Pacific strike-slip plate boundary. Arcuate fracture zones become asymptotic as they approach the plate boundary. A broad zone of less intense deformation associated with the plate boundary extends -50 km on either side of the Macquarie Fault Zone. Marine geophysical data suggest that distinct segments of the plate boundary have experienced convergence and strike-slip deformation, although teleseismic evidence overwhelmingly indicates strike-slip motion along the entire surveyed boundary today. The McDougall and southernmost Puysegur segments show no evidence for past underthrusting, whereas data from the Macquarie and Hjort segments strongly suggest past convergence. The present-day strike-slip plate boundary along the Macquarie Ridge Complex coincides with the relict spreading center responsible for Australia-Pacific crest in the region. Our conceptual model for the transition from seafloor spreading to strike-slip motion along the Macquarie Ridge Complex addresses the decreasing length of spreading center segments and spacing between fracture zones, as well as the arcuate bend of the fracture zones that become asymptotic to the current transform plate boundary.

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New seismic reflection, gravity, and magnetic data from offshore East Africa allow the Davie Fracture Zone to be traced from ,-, 11øS to its intersection with the Kenyan coast at ,--2øS, constraining the relative motion of Madagascar and Africa. Seasat-derived free air gravity anomalies and slope/rise positive magnetic anomalies observed in shipboard data help to locate the continent-ocean boundaries (COB) off the shore of East Africa and Madagascar. Seismic reflection data further document a diapir province off Madagascar, presumably conjugate to that observed off Kenya and Somalia. The Dhow and Very Large Crude Carrier (VLCC) basement ridges are complex features and do not appear to be simple fracture zones owing their existence entirely to the separation of Madagascar and Africa. From these data we determine a predrift fit of Madagascar and Africa involving a 14.2 ø rotation of Madagascar to Africa about a pole at 10øN, 150øE. The geometry of the reconstruction adheres to seismic and potential field data indicating the oceanic nature and extent of the Comoros Basin and of the Somali Basin between Kenya and the Seychelles, and it does not conflict with onshore or offshore stratigraphy. Timing of the opening of the Western Somali Basin is constrained by Mesozoic marine magnetic anomalies and extrapolation to the interpreted COB and occurred between approximately 165 and 130 Ma. GEOMETRY OF THE RECONSTRUCTION The Davie Fracture Zone Heirtzler and Burroughs [1971] first proposed that Davie Ridge (Figures 1 and 2) was a transform fault resulting from relative motion between Madagascar and Africa on the basis of seismic and magnetic data acquired between 9øS and 17øS. Additional marine geophysical data confirmed that the Davie Fracture Zone extends to the south and intersects the margin of Madagascar [Delteil et al., 1978; Segoufin et al., 1978; Lort et al., 1979; Segoufin, 1981]. To the north, Rabinowitz [1971] observed a relative free air gravity high trending north to south extending from ,-•5øS to its intersection with the Kenyan margin at ,-•2øS (Figure 1). He suggested that the gravity high represented a buried basement ridge. Scrutton [1978] and Segoufin [1981], employing the gravity map of Talwani and Kahle [1976], traced a free air gravity minimum from ,-• 10øS to connect with the high of Rabinowitz and considered the entire linear feature extending from the Madagascan to the East African margin a fracture zone created by the separation of Madagascar and Africa. The Davie Fracture Zone is defined best between ,-• 11øS and its intersection with the Madagascan marginal plateau at ,-• 22øS. The geophysical character of the feature varies along strike. Seismic transects generally show either a single or a double asymmetric basement high of up to several thousand meters relief that dips sharply to the west and much more gradually to the east [Segoufin, 1981; Mougenot et al., 1986]. The fracture zone ranges from several tens to 100 km in width, and basement is generally at greater depths to the west of the featur...

Abstract. Seismic refraction and gravity-based crustal thickness estimates of the Ontong Java oceanic plateau, the Earth's largest igneous province, differ by as much as 18 km. In an attempt to reconcile this difference we have evaluated available seismic velocity data and developed a layered crustal model which includes (1) a linear increase in velocity with depth in the Cenozoic sediments and the uppermost extrusive basement and (2)

We present a series of preliminary reconstructions for the Kerguelen Plateau region from the Late Jurassic to the Eocene that summarize and review the outstanding questions about its plate tectonic evolution.

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The Kerguelen Plateau and Broken Ridge form a large igneous province (LIP) in the southern Indian Ocean. The main objectives of Ocean Drilling Program Leg 183 were to understand the origin and evolution of this LIP and the impact of its formation on the environment. Igneous basement (33 to 233 m of penetration) has been recovered from 11 drill sites on the LIP, and 7 are Leg 183 sites. Studies of the basement and sediment cores lead to the following conclusions. Although an important caveat is that we have access only to uppermost basement of a thick (~20 km) igneous crust, these results are inconsistent with massive volcanism associated with a single plume head and continental breakup. 2. The uppermost igneous basement is dominantly tholeiitic basalt. Based on the physical characteristics of the lava flows, which indicate subaerial eruption, and the occurrence of overlying terrestrially derived sediments containing wood fragments, fern remains, and terrestrial palynoflora, much of the LIP was above sea level when magmatic output was high. 3. The geochemical characteristics of basalt forming the LIP are unlike mid-ocean-ridge basalt (MORB). There are, however,

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