In the Rio Grande rift of southern New Mexico, the intrabasinal East Robledo fault in the Mesilla basin and the Jornada fault in the Jornada del Muerto basin experienced hundreds of meters to kilometers of offset during late Miocene to early Pliocene time and tens of meters of offset since middle Pleistocene. Late Pliocene and early Pleistocene activity on the faults is assessed by comparing sedimentological characteristics of the Camp Rice Formation, which is correlated by reversal magnetostratigraphy, on either side of the faults. The Jornada fault is interpreted to have been inactive from approximately 3.4 to 2.5 Ma, because Gauss-age fluvial strata at Rincon Arroyo, located on the hanging wall, have similar sediment accumulation rates, degree of development of calcic paleosols, and relative abundance of fluvial channel lithofacies as coeval footwall strata at Cedar Hill and Lucero Arroyo. In contrast, syndepositional movement on the Jornada fault along the northern flank of the Dona Ana Mountains from 2.5 to 0.7 Ma is suggested by a condensed Matuyama interval characterized by mature stage III and IV calcic paleosols at Lucero Arroyo. The northern segment of the East Robledo fault was active during Gauss time, based on an abundance of relatively thick (59 m), fluvial-channel deposits in the hanging-wall section at Northeast Robledo. The southern segment of the East Robledo fault, however, was probably not active during most of Gauss time, because of the presence on the footwall of a thick (50 m) section of fluvial strata of Gauss age at Picacho Mountain. Major movement on the East Robledo fault near the end of Gauss time terminated sedimentation at Picacho Mountain and in the Corralitos basin, abandoning the upper La Mesa geomorphic surface.

An elongate northwest-trending zone of batholith-size metamorphic core complexes extends some 130 km from the Rincon Mountains to the Picacho Mountains in southeastern Arizona. The complexes are characterized by undeformed to gneissic granitic intrusions, gneissic to phyllonitic xenoliths and wall rocks derived mainly from Precambrian granitic rock, shallow-dipping foliation, and remarkably uniform directions of lineation. Parts of this zone have been recognized and studied extensively for more than 30 yr, but there remains a divergence of opinion about the age, depth of emplacement, and origin of the complexes. Field relations indicate that host rocks as young as or younger than Mesozoic were involved in cataclasis. K-Ar and fission-track ages indicate that the complexes were at temperatures uniformly in excess of400 °C in the middle Tertiary (20 to 30 m.y. ago) and that the bedrock between and very near the complexes was not thermally affected. Tertiary plutons characteristically associated with the high-grade metamorphic rocks are also cataclastically deformed, and stratigraphic depths to the top of metamorphic terranes were no more than 6 km and possibly less than 3 km. These and other data suggest to some that the complexes developed during intrusion of composite batholiths at shallow depth in an anisotropic stress field during the middle Tertiary. On the other hand, Rb-Sr and U-Th-Pb techniques yielded older ages (≥44 m.y.) for some samples. These and additional data suggest to others that major development of cataclasis preceded the middle Tertiary and included regional thrusting.

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.

Reconnaissance mapping indicates that parts of nine mountain ranges previously considered to be Precambrian basement are instead variations of Tertiary metamorphic core complexes. From southeast to northwest, these ranges include the Pinaleno, Picacho, South Mountains, parts of the Buckeye, White Tank, Harquahala, Harcuvar, Buckskin, and Rawhide Mountains. Together with the already recognized Santa Catalina–Rincon–Tortolita complex, these ranges define a broad northwest-trending belt through Arizona. The northeast-trending Buckskin-Harcuvar-Harquahala Mountains are transverse foliation arches, the latest expression of a huge, northwest-elongated metamorphic area herein named the Harcuvar metamorphic core complex. This Tertiary phenomenon is superimposed on an ill-defined center of late Mesozoic metamorphism. Traverses into the complex from its unmetamorphosed southwestern margin reveal progressive Cretaceous conversion of Mesozoic sedimentary rocks into migmatites. Metamorphism just preceded intrusion of the Tank Pass batholith, an Upper Cretaceous pluton which itself became foliated and involved in early Tertiary migmatization and intrusion in the Harcuvar Mountains. A marginal zone of penetrative mylonitization, capped by a more brittlely deformed dislocation surface, flanks the Harcuvar complex on its upper and broadly arcuate northeastern margins. Resting on this tectonic surface are highly tilted, unmetamorphosed, layered rocks (Paleozoic to Tertiary). Geochronologic and geologic data place the time of mylonitization as Tertiary, perhaps as recently as 25 to 20 m.y. B.P. This deformation (flattening and northeast-southeast extension) was closely followed by development of chlorite breccia, the dislocation surface, thick wedges of coarse clastic sediment, and listric faulting. Finally, the core complex was arched and uplifted. A model for this sequence of events is predicated on mobile northeast-directed extension of a flat upper-crustal layer facilitated by intense mid-Tertiary plutonism in an actively tensile stress field. Tectonism in a brittle, surficial upper plate is governed by listric faulting and detachment as the plate fragments and extends “piggyback” style upon subjacent, ductilely stretched layer.