with mountain building. The focus of metamorphic petrology today is shifting from a static mode, which reports the mineral assemblages found in the field, to a much more dynamic mode, aimed at working out the processes involved in metamorphism. This shift emphasizes not only the thermodynamic variables but also nonequilibrium aspects and the kinetics of metamorphic processes. In particular, kinetic study has been extended to the generation, motion, and characterization of metamorphic fluids. There are several processes that must be quantified in describing the results of metamorphic reactions between rocks and fluids so as to provide answers to questions about heat transport processes, both conductive and convective; fluid mass transfer processes; solute mass transfer processes—convective and diffusive; mineral surface reaction processes, both dissolution and growth; and nucleation of new minerals.

The P-T-t histories of rocks commonly include extensive expulsion and migration of fluids. These fluids usually move near the surface, but problems emerge with fluid movement much deeper in the Earth. For example, at depths near 6 km the fluid pressure approaches lithostatic pressure. It is difficult to understand convective fluid flow, potential for channelization, or flow rates and volumes at such high-temperatures and pressures.

The chemical reactions taking place involve moving fluids. Progress has been made in obtaining thermodynamic data for minerals and fluids, but the behavior of fluids as a function of composition over a wide range of pressure and temperature conditions and the equilibrium description of solid solutions needs more data. Models need to be developed that account for the dissolution, transport, and precipitation of chemical components during the flow of fluids through a series of rock units. The quantitative treatment of isotope exchange between minerals and fluids needs to be elaborated further to make use of the increasing isotopic data base on reactions. An understanding of the chemical evolution of the crust must be based on knowing the transport properties of the fluid, such as whether flow is diffusive or convective, as well as the complex heterogeneous kinetics taking place at every mineral surface in contact with the fluid.

Extensional Deformation of Continental Lithosphere

Deformation in mountain belts is dominated by contraction. It is possible to simulate the great folds of the Alps, Himalayas, and Rockies by pressing on the leaves of a book or by pushing a napkin along a table. Although the rheology is very different, there are resemblances to what happens in nature, both in the flat layering of the original material and in the detachment at the base of the deforming layers from the material beneath. Analyses of rock deformation in a contractional regime on mega, macro, and micro scales have become very sophisticated, and environments can be satisfactorily modeled, especially where complementary data on metamorphic state are available.

The concept of converging tectonic plates that contract the crust and produce mountains fits well with our understanding of plate tectonic theory. Within the past decade researchers have emphasized another process that actively deforms vast tracts of continental crust. That force is extensional deformation, and its widespread effects have come as a surprise. Contraction and extension are both characterized by detachment of the crust from the underlying mantle; in the case of extensional deformation the detachment occurs in the form of normal faults.

Surface expression of extension commonly occurs in the form of rifts, and active rift zones can be seen in many continental areas. The character of the modern rifts, however, shows considerable variation. The rift of East Africa spans almost the entire continent from north to south. At its northern end, extension and volcanism have been considerable, leading to the formation of the Red Sea basin that now separates the once-connected Africa and Arabian peninsula. Through central Africa, rifting has been less successful in splitting the continent. Volcanism in this area is rarer, and the rift also contains a higher proportion of peculiar alkali-rich lava types than the northern rift. In the United States the Rio Grande rift that splits the Colorado Plateau from eastern New Mexico is only tens of kilometers wide, whereas the Basin and Range province exhibits continental extension and rifting over a zone more than 600 km wide. Volcanism is widespread in each of these rifts but generally consists of isolated, relatively small volume eruptions.

The rifting of continental material can evolve in one of two ways: either the extension in the rift can develop until new ocean floor forms at the rift site (such a rift is called a ''successful" rift), or extension can cease before a new ocean forms and the rift can become inactive within the continent (such a rift is called a "failed" rift). The former course represents a progressive step in the cycle of the opening and closing of the ocean basins, while the latter represents only one more episode within the evolution of



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