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2 Major Scientific Problems A fundamental goal of the Transects Program has been the identification and description of the major problems in understanding the tectonic evolution of North American continent-ocean transitions. That goal is addressed in Part II of this report, in which 12 general problems of continent-ocean transitions are discussed: processes of active margins, processes at passive margins, tectonic heredity, tectonic significance of magma types, identifica- tion and processes of terrane boundaries, kinematics of orogenic belts, implications of high-grade metamorphic rocks in orogenic belts, event dating, foreland-margin tec- tonic coupling, diagnostic geophysical expressions of tectonic units, deep continental structure and origin, and Phanerozoic changes in Proterozoic North America. Consideration of these general problems led to identification of four specific prob- lems that are most fundamental to an improved understanding of continent-ocean transitions: 1. Underplating and tectonic attrition: The transfer of sediment, rock, and fluid from a downgoing plate to the base of a forearc and/or crystalline lithosphere of North America at active margins. 2. Origin of the Moho and lower crust: The nature, age, and origin of protoliths of the lower continental crust and mantle immediately below the continental Moho, the processes affecting those regions, and the times over which their evolution occurs. 3. Absolute chronology of tectonic events: The absolute timing of stages of kinematic and tectonic activity in orogenic events at the North American margin, such as durations of steeply versus shallowly dipping subduction, foreland versus hinterland contraction, collision, and major unroofing. 4. Strike-slip components of terrane displacement paths: The components of terrane displacement and velocity that parallel the margins of the continent during oblique convergence that may not be recorded in the contemporaneous structures of the adjacent continental foreland. 10
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