Architecture of the continental lithosphere

Abstract

The continental lithospheric plates are traversed by numerous large-scale dislocations, many of which date back to late Archean or Proterozoic times. These major dislocations, often extending for 1000 km or more, are commonly characterized not only by the presence of fault rocks and by significant misfits between the adjacent crustal blocks, but also by the presence of localized basins of deposition, or by anomalous igneous suites, or mineral deposits. Although to a first approximation they may be regarded as narrow linear features, most consist of complex arrays of faults, ductile shear zones and fractures; some with associated deformation spread over tens and hundreds of kilometres. Lineaments that originated as within-plate transcurrent shear zones or rift systems are generally steep and many of these probably extend down through the base of the crust into lithospheric mantle. Lineaments that originated as sutures along which formerly separated crustal blocks have been welded together are, on the other hand, sometimes expressions of low-angle dislocations not continuous with mantle structures directly beneath the lineament observed at the surface. Geological records show that displacements or magmatic activity (or both) along many lineaments were repeated over long periods spanning successive changes in the global tectonic regime. Modern lineament movements can be fitted to plate tectonic hypotheses. However pre-Phanerozoic movements are more conjectural: for example, early Proterozoic lineaments appear to be associated with more extensive internal deformation within large crustal blocks. The siting of fracture systems activated in response to regional plate motions has, on occasion, been determined by the occurrence of pre-existing deep dislocations. The scale and longevity of the deep dislocations raise many interesting general questions with respect to the coordination of mantle and crustal activities and to the evolution of the continental lithosphere.