Chapter 26: Crustal structure of the western US. based on reflection seismology

Interpretation of crustal reflection profiles shows contrasting crustal styles and Moho from the craton to the Cordilleran belt. Crustal deformation, however, may be determined from highly reflective ductile fault (mylonite) zones that occur in many geologic settings, and fractures and chemical alteration may cause shallow detachments to reflect, e.g., the Sevier Desert and the Picacho Mountains detachments. The oldest Archean crust in Minnesota consists of a stack of nappes 30 km thick, and the underlying Moho, which is nonreflective, may be gradational. Younger Archean is sutured to this along a complex, moderately dipping zone marked by a mylonite. Proterozoic crust in Kansas and the Colorado Plateau is characterized by arcuate crossing reflections that can be caused by a combination of folding and intrusion. The Mojo is generally nonreflective except in the extended terranes of the Basin and Range, Rio Grande rift, northwest Cordillera, and Mojave-Sonoran Desert. The best crustal reflections are found in the extended terrane of the Basin and Range, where the subhorizontal reflection geometry is probably caused by ductile flow (metamorphism) under simple shear, resulting in a strong compositional layering and lensing of the deep crust and Moho. This young Moho contrasts sharply with the nonreflective Moho under the craton that may represent a gabbro-eclogite phase change. Underplating by gabbroic magma may have taken place in such diverse tectonic settings as the Minnesota Archean, the Oklahoma Proterozoic, and the Basin and Range Cenozoic crust. No evidence for Moho offset has been found in any of the areas studied. A complex crustal reflection pattern and distinct Moho reflection in the northwest Cordillera may be related to moderate extension conditioned by less (than Basin and Range) thermal input to the crust. Compression caused folding and thrust faulting in the Wyoming foreland. Basaltic magma chambers may have been detected beneath the Rio Grande rift and Death Valley; apparently, basaltic magma is more likely to generate reflections than granitic magma. Questions exist about how much of the exposed Precambrian crust in southern California, where a midcrustal detachment has been proposed, is allochthonous. Reflection profiling reveals a crust that is both vertically and horizontally heterogeneous.