the continental lithosphere. Whether a rift system "succeeds" or "fails" is presumed to be controlled by the disposition and sum of the plate-driving forces at some critical time in rift evolution.

The "successful" course of rifting is typically represented around the shores of the Atlantic Ocean and other similar margins attributable to continental rupture. A considerable research effort is developing on what has happened in the earliest stages of ocean formation in these areas. Stimuli include the lessons learned in the Basin and Range province and the recognition that the typical dog-leg shape of the shorelines of the Atlantic resembles the shape of the East African rifts. As in East Africa, nodal (hotspot) volcanic areas are concentrated at the dog-leg bends in the Atlantic shore. Marine geophysics calibrated by the drilling of research holes of the Ocean Drilling Project is revealing that enormous amounts of basalt were erupted onto the young ocean floor at the dog-leg bends. In some cases much smaller volumes of lava continued to be erupted as the ocean continued to grow, forming hot-spot tracks that lead to active hot-spots such as Iceland, Jan Mayen, Reunion, and Tristan da Cunha.

More than 100 ancient rift systems have been recognized within the continents. The oldest go back to nearly 3-billion-years ago, indicating that ancient continents behaved rather like their modern counterparts. Regional extension of ancient continents is also indicated by the preservation of parallel swarms of hundreds of narrow vertical dikes occupied by basalt, all emplaced at the same time, that extend for distances of 1,000 km or more across the ancient continents.

The cause of the variations in volcanic output and degree of crustal extension in continental rifts is not understood at present. Significant advances can be made on this topic through detailed field studies employing modern techniques of remote sensing, chemical and chronological analyses, and paleomagnetism, in addition to traditional field mapping. Scientifically oriented drilling into continental rifts can do much to illuminate their structure and the sequence of volcanism and sedimentation that records their evolution. Combined with the increasing resolution of geophysical techniques capable of providing images of subsurface structure, computer modeling and laboratory simulation of the response of rigid crust to extensional stress may illuminate the underlying causes of continental rifting. The Basin and Range province is proving a marvelous area in which to study extension. Detachment systems in that region are the subject of integrated geological, geophysical, and geochemical study.

The detachment systems accommodate extension by the separation of relatively undeformed crustal blocks, with individual faults transecting the upper 15 km or so of the crust and having displacements typically measured in tens of kilometers. The end products of extension are often exposures of wide tracts of middle crustal rocks veneered with patches of upper crustal rocks and subsequently eroded material. Such extended domains have been identified throughout the Basin and Range province. They are on the order of 100 to 200 km wide and are interspersed with relatively unextended upper crustal blocks (Figure 2.13).

Paradoxically, the topography of these tracts, as well as gravity anomalies and reflection seismic data, indicate that despite the vast area and substantial heterogeneity of upper crustal strain-large enough in some cases to form an ocean basin-the crust maintains a near-constant thickness across the boundaries. These observations have led to the recent suggestion that the upper crust may be floating on a deeper layer within the crust. This differs from the usual assumption that the thickening and thinning observed in the upper crust are accommodated at depth by flow in the upper mantle. It appears that the upper crust and upper mantle are probably strong, while the lower crust is generally weak and ductile in this environment. Multidisciplinary studies of the lithosphere in these regions promise major advances in understanding deformational processes in the continental lithosphere over the next decade. Particular advances are anticipated in understanding the bulk rheology, strain field, and deformation mechanisms of the deep crust. These advances will construct a framework with the potential to unify a broad range of disciplines, including petrologic and isotopic thermobarometry, geophysical imaging, geochemical studies, physical modeling, and field geology.

Evolution of the Continents

The plate structure of the lithosphere was accepted in the 1960s. Soon afterward came the realization that plate tectonic processes represented the dominant way in which the Earth dissipates its internal heat. The next question addressed the length of time that plate tectonics has played a major role in earth processes.

There is no strong evidence, even in the oldest rocks, of any processes that were radically different from those of modern plate tectonics. There is



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