Structural geology of the Earth's exterior.
Plate tectonics offers an explanation for the present motions and heterogeneity of the rocks that form the external part of the Earth. It explains the origin of the first-order heterogeneity of oceanic and continental lithospheres. Furthermore, it explains the youth and simplicity of the oceanic lithosphere and offers the potential to explain the antiquity, complexity, and evolution of the continental lithosphere. The framework of plate tectonics must be used carefully, because there are geological features within continents, particularly in the more ancient rocks, that may require alternative explanations. The task of understanding lithospheric motions through geologic time must be focused on the continents, where the major evidence for 95% of Earth history resides.IN INTERPRETING EARTH MOTIONS FROM THE GEOLOGIC RECORD, THREE NEEDS SEEM PARAMOUNT: (i) to develop a three-dimensional understanding of the kinematics, dynamics, and thermal structure of modern plate boundary systems and at the same time to recognize those geological and geophysical features that are unrelated to plate interaction; (ii) to use this understanding to reconstruct the extent and evolution of ancient systems that form the major elements of continental crust; and (iii) to determine the dynamics and evolution of systems that have no modern analogs. Decoupling along subhorizontal zones within the lithosphere may be widespread in all types of plate boundary systems. Thus, in order to interpret the motion and dynamics of the mantle correctly, it is important to know if upper lithospheric motion within boundary systems is controlled directly or indirectly by or is independent of deeper mantle motions.
Burchfiel
B C
BC
Massachusetts Institute of Technology, Cambridge, Massachusetts 02139.