Modern active oceanic margins of North America
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.
Figures & Tables
This volume presents syntheses in twelve chapters of the tectonic evolution of continent-ocean transitions of North America (Canada-Mexico-U.S.A.) since the Precambrian. The syntheses are interpretations based on the 19 continent-ocean transects across North American margins published by GSA as part of its Decade of North American Geology (DNAG) series. The transitional region is the part of North America between the craton, little deformed in Phanerozoic time, and the modern ocean basins. The region developed heterogeneously within plate boundary zones that led to sequences of passive, collisional, and active margins that differ place to place. Nine chapters address individual segments of the transitional region, two consider active and passive margin tectonics topically, and one treats the evolution of Phanerozoic transitions of North America as a whole.