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This summary characterizes 28 known or suspected mid‐Tertiary ash flow tuff (ignimbrite) cauldrons, up to 40 km in diameter, in southwestern New Mexico. A combination of postcauldron block faulting and erosion has resulted in widely ranging levels of three‐dimensional exposures, down to plutonic roots. The evolution of the largest resurgent cauldrons followed the course determined by other workers for the Valles caldera (New Mexico) and Lake City cauldron (Colorado), with local variations. Five principal stages can be recognized: (1) precursor, (2) caldera collapse, (3) early post‐collapse volcanism, (4) major ringfracture volcanism, and (5) hydrothermal activity. Development can be terminated at any stage; stages can also be repeated. Resurgent doming can occur during stage 3 or later; doming can also be enhanced by reactivation of cauldron structures by postvolcanic basin and range faulting. Even where the surface of caldera‐fill tuff is domed, the base may have quite a different configuration. Recognition of resurgent doming may be difficult in deeply eroded cauldrons. A proposed modification in the definition of “resurgence” places emphasis on one or more magma pulses after a caldera‐forming eruption; doming is one of several effects that may or may not occur when magma invades a previously subsided cauldron. In some cauldrons, postcollapse lavas can be divided into (1) “cauldron lavas,” i.e., relatively mafic crystal‐rich disequilibrium assemblages derived from shallow defluidized residues of caldera‐forming ash flow tuff eruptions, and (2) “framework lavas,” i.e., relatively siliceous crystal‐poor near‐equilibrium assemblages derived from an evolving siliceous pluton at deeper levels, below the cauldron complex. Special features of New Mexico cauldrons include zoned outflow sheets; zoning may be normal (most siliceous at the bottom) or reversed. Posteruption potassium metasomatism may mask some chemical trends. Several styles of eruption can be recognized among mid‐Tertiary cauldrons of southwestern New Mexico. The largest ones tend to occur in clusters, interpreted as surface expressions of buried composite plutons (e.g., Mogollon Plateau). In an early stage of development, or on the fringe of a cluster, cauldrons tend to be asymmetrical trapdoor structures, formed by repeated small eruptions separated by quiescent periods. During the height of activity, cauldrons formed by eviscerating eruptions, commonly followed by resurgence. The youngest known cauldron is shallow and resulted from asymmetrical subsidence and collapse of caldera walls.

An “extensional orogeny” deformed the Basin and Range province, probably beginning in the late Eocene (about 40 ± 3 Ma). Its characteristics include partial melting of the continental lithosphere during the “ignimbrite flareup,” massive ductile extension (including detachment faulting), and rise of metamorphic core complexes. The affected zone became about 1200 km wide, possibly double its original width. It rose an average of 1–2 km, despite crustal thinning. Locally, some of the highest mountains of North America, up to 4.3 km high, rose through resurgence of ignimbrite cauldrons and isostatic uplift of underlying plutons. The climax of extension occurred prior to the development of the present basin and range topography. Modeling of major and trace elements and Sr and Pb isotopes strongly suggests that mid‐Tertiary volcanic magmas equilibrated, and probably originated, in the continental lithosphere. Components attributable to subducted oceanic lithosphere have not yet been identified. The rocks seem to belong to two provinces, separated by the quartz diorite boundary line of Moore (1959), which also marks the western limit of North America at the end of the late Paleozoic Sonoman orogeny. To the west, low‐K rocks rest on a basement of predominantly oceanic accreted terranes; to the east, high‐K rocks rest on an autochthonous sialic basement. Within the high‐K province, potassium variations can be correlated with crustal thickness; there is no need to invoke a K‐h relationship. Conventional models of plate convergence and back arc extension which involve subduction of old, rigid, cool, and dense oceanic lithosphere may not apply to the mid‐Tertiary Basin and Range province. The overridden Farallon plate is more likely to have been young, hot, ductile, buoyant, and no denser than continental asthenosphere, having been generated in a spreading center close to North America. Under these conditions, motion of the subducting plate slows and slab‐pull is likely to approach zero. Even prior to ridge‐trench collision, overridden oceanic lithosphere may have become underplated beneath the continental lithosphere and ruptured by rising mantle diapirs. Subducted oceanic lithosphere no longer acted as a heat sink, which could partly account for the great width of the affected zone and the anomalous thermal gradients required for partial melting, extension, and metamorphism. Had these processes not died down, after ridge‐trench collision, the western segment of the Cordillera might have separated from North America to form a Japanlike archipelago, while the Basin and Range province foundered into an analog to the Sea of Japan. Instead of rupturing completely, the Basin and Range province fractured into fault blocks.

The mostly 2.1‐ to 0.3‐m.y.‐old Springerville volcanic field is the southernmost of the Colorado Plateau‐margin fields and encompasses −3000 km2, with a volume of −300 km3. Mapping documents 409 flow units, most emanating from one of the −400 predominantly pyroclastic cones. Alkali olivine basalt constitutes about one‐half the outcrop area, with hawaiite and tholeiite each composing about a quarter of the area, and mugearite and benmoreite each covering less than 1% of the area. The composition of the lavas shows a progression away from tholeiitic and toward alkalic affinities with decreasing age, in marked contrast to that found in both the southeast and western margins of the Colorado Plateau. Estimates of volume effusion rates show an early eruptive episode dominated by large volumes of tholeiitic lavas, a middle period of alkali olivine basalt eruption, and a late pulse of evolved alkalic rocks. Changes in chemistry [Na2O + K2O, MgO/(MgO + FeO*), K/P] with respect to location of lava sources that might reflect thermal and/or chemical differences appear to be minimal. Eastward migration of alkali olivine basalt volcanism within the Springerville volcanic field is essentially identical to that seen in the San Francisco volcanic field for the same time period, supporting the suggestion of Tanaka et al. (1986) that it may reflect the westward motion of the North American continent above relatively fixed mantle sources. The late Cenozoic migration of volcanic fields toward the Colorado Plateau suggested by Best and Hamblin (1978) and by Luedke and Smith (1978) does not seem to be reflected by vent migrations from 2.1 Ma to the present within the Springerville and San Francisco fields.

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