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Modern convergent and strike-slip margins are crossed by six continent/ocean transects (Fig. 1). Along these margins, continental and oceanic crust are separated by a seismically active tectonic contact, which is the boundary between the North American Plate and the Pacific or Cocos Plates. Four transects cross convergent margins where oceanic crust underthrusts continental crust (A-2, B-2, B-3, and H-2). One transect (B-l) is across the Queen Charlotte Transform Fault, a part of the Pacific-North American plate boundary, which is as long as the better-known San Andreas Fault. Another transect (A-3) crosses a collision zone and the controversial transition fault zone, which is interpreted by some as a highly oblique thrust and by others as inactive during the past 5 m.y. or more, and only recently an active fault.

We summarize here the tectonic interpretations of these boundaries made by the transect teams, and results from work completed after the transects were assembled. After transect compilation, the multichannel seismic records across active margins in A-2 and H-2 were reprocessed, and a new record was acquired in the B-2 transect area.

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