SCREECH SCIENTIFIC OBJECTIVES

Deformation and magmatism during continental rifting and eventual breakup are fundamental yet poorly understood processes. To advance our understanding of these processes significantly, we must characterize with unprecedented detail and accuracy the structure and composition of conjugate rifted margins across the full width of the transition from undeformed continental crust to normal oceanic crust. The Newfoundland - Iberia conjugate margins are an excellent natural laboratory for such a study. Their geological characteristics are favorable to address the fundamental scientific questions, logistically the areas are readily accessible, and a large ODP and geophysical database (currently strongly skewed toward the Iberia margin) already exists to focus inquiries on essential scientific questions.

Our field program is on the Newfoundland side of the Newfoundland - Iberia rift, and our results will be integrated with complementary data on the Iberia margin to give an unequaled overview of a complete, mature rift system. The critical characteristics of the Newfoundland margin that need to be understood are tectonic structure, composition, and thickness of crust in four zones:

• full-thickness, unrifted continental crust beneath the Grand Banks.
• rifted and thinned crust of known continental origin, generally beneath the continental slope and rise
• transitional crust of disputed origin in the deep Newfoundland Basin
• known oceanic crust seaward of the transitional crust.

We are studying these zones with state-of-the-art reflection/refraction seismic experiments, using dense arrays of ocean-bottom seismic instruments together with R/V Ewing's large, tuned airgun array and new, 6-km-long hydrophone streamer. Our focus is on three transacts that extend from known continental crust on the shelf seaward to known oceanic crust; these transacts completely traverse a wide region of thin, transitional crust of enigmatic origin in the deep Newfoundland Basin. Each of these transacts is optimally located with respect to detailed geophysical and drilling studies on the conjugate Iberia margin. This maximizes the utility of each Newfoundland transect in terms of interpreting results at existing ODP drill sites on the Iberia margin, improving the siting of proposed Newfoundland-margin drill sites and eventually interpreting their results, and interpreting the development of the full rift. Our Newfoundland transacts, combined with existing seismic studies on the Iberia margin, constitute the first complete, densely sampled, wide-angle/vertical-incidence transacts across conjugate non-volcanic rifted margins.

Our workwill distinguish among competing hypotheses for the origin of crust in the Newfoundland Basin, which propose that the basin may be floored by thin continental crust, slow-spreading (or even normal) oceanic crust, or by a wide zone of serpentinized upper mantle. Specific research goals are to:

• Characterize the transitional crustal structure of the deep Newfoundland Basin as well as the eastern, unextended edge of the adjacent Grand Banks so as to constrain the origin of the crust in the Newfoundland Basin (continental vs. oceanic) and its tectonic evolution.
• Determine the position and nature of the boundaries between continental and transitional crust, and between transitional and oceanic crust.
• Evaluate the amount of igneous material accreted on this "non-volcanic" margin to provide constraints on the volcanic/non-volcanic paradigm for rifted margins.
• Compare the crustal structure of the Newfoundland margin directly with that of the conjugate Iberian margin to allow well constrained and systematic evaluation of the mechanisms involved in the rifting process.

Our research is a US-Canadian-Danish cooperative effort that includes the Woods Hole Oceanographic Institution (B.E. Tucholke), the University of Wyoming (W.S. Holbrook), the Danish Lithosphere Centre (H.C. Larsen and J.R.Hopper), Dalhousie University (K. Louden) and Memorial University of Newfoundland (J. Hall and C. Hurich), all of whom bring substantial expertise and resources to the project. Our research will also be coordinated with ongoing studies of the conjugate Iberia margin by researchers including R. Whitmarsh (IOS), T. Minshull (Cambridge U.), K. Louden (Dalhousie U.), D. Sawyer (Rice U.) and T. Reston (GEOMAR).