Seismic reflection results from Precambrian crust.

Results of crustal reflection profiling in the Precambrian can provide decisive information on such features as faulting, extension vs compression and crustal scale deformation in general, vertical vs horizontal movements, intrusions, mafic differentiates, underplating and plate tectonics, even though targets in the Precambrian are complex. Mylonite zones are probably the best crustal reflectors so that a moderately-dipping reflector separating the ancient Archaean gneiss terrain from younger Archaean greenstone belts in Minnesota is probably a thrust fault marking the suture zone between the two terrains. This structure implies the operation of plate tectonics by at least the late Archaean. The 3600 m.y. Minnesota gneiss terrain is underlain by a thick (10 km) sequence of layered deformed rocks, whose presence restricts early Archaean events. A Proterozoic suture in SE Wyoming is marked by a thick mylonite zone and a change in crustal structure that has persisted through time. The Adirondack anorthosite and granulite terrain is underlain by a thick layered sequence in the lower crust. Formation of a late Proterozoic basin filled with 10-12 km of interlayered sedimentary rocks in Texas and Oklahoma may have been accompanied by crustal underplating. (Authors' abstract)-J.M.H.