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K–Ar dating demonstrates that all but eight of 41 dated porphyry copper and related ore deposits of Mexico were emplaced during the Laramide episode of maximum plate convergence. One older deposit is related to the Jurassic volcanic arc of western North America, one is pre-Laramide Cretaceous, four are Oligocene in age, and two late Cenozoic deposits are within the modern trans-Mexican–Chiapenecan volcanic arc. Thirty-three of the deposits lie within a long narrow belt that continues into Arizona and New Mexico, and widens from 100 km to over 300 km in the region of maximum extension in the southern Basin and Range Province. Eighty-five percent of the deposits were emplaced during the eastward transgression of the Cordilleran volcanic arc in middle Cretaceous through Eocene time.The occurrence of the porphyry copper deposits of Mexico appears to be independent of the terrane intruded and the copper content of the wall rocks where the wall rocks predate the volcanic arc, which is syngenetic with the porphyry stock. However, strontium is significantly more radiogenic where the host porphyry has intruded terrane having a Precambrian crystalline basement. Most frequently, the porphyry pluton can be observed to have intruded penecontemporaneous volcanic rocks or the batholith itself. The porphyries appear to be apophyses of the batholiths. The relationships suggest that the ore components are contained within the calc-alkaline batholiths and concentrated in the subvolcanic porphyries and wall rocks during transport of hydrothermal fluids to the volcanic orifice.The shape of the Cordilleran copper belt is controlled by magma composition, existence of a protective capping of dominantly volcanic rock, uplift, time, and erosion. As the continental volcanic arc that produced the porphyry copper deposits progressed eastward, the associated magma became more alkalic and copper poor. Thus, enrichment to ore grade became increasingly improbable to the east. Uplift and ample time for erosion prior to the return of the continental volcanic arc reduced the probability of ore preservation to the west. Optimum conditions for preservation were present within the belt where burial of calc-alkalic porphyry plutons under a thick volcanic cover occurred before removal of the ore zone by erosion. The broader width of the porphyry belt to the north is probably the result of both more extensive basin-and-range extension and basin-and-range taphrogeny that exposed some of the porphyries to relatively recent denudation and consequently made them available for economic exploitation.

The Toledo caldera was formed at 1.47±0.06 Ma during the catastrophic eruption of the lower member, Bandelier Tuff. The caldera was obscured at 1.12±0.03 Ma during eruption of the equally voluminous upper member of the Bandelier Tuff that led to formation of the Valles caldera. Earlier workers interpreted a 9‐km‐diameter embayment, located NE of the Valles caldera (Toledo embayment), to be a remnant of the Toledo caldera. Drill hole data and new K‐Ar dates of Toledo intracaldera domes redefine the position of Toledo caldera, nearly coincident with and of the same dimensions as the younger Valles caldera. The Toledo embayment may be of tectonic origin or a small Tschicoma volcanic center caldera. This interpretation is consistent with distribution of the lower member of the Bandelier Tuff and with several other field and drilling‐related observations. Explosive activity associated with Cerro Toledo Rhyolite domes is recorded in tuff deposits located between the lower and upper members of the Bandelier Tuff on the northeast flank of the Jemez Mountains. Recorded in the tuff deposits are seven cycles of explosive activity. Most cycles consist of phreatomagmatic tuffs that grade upward into Plinian pumice beds. A separate deposit, of the same age and consisting of pyroclastic surges and flows, is associated with Rabbit Mountain, located on the southeast rim of the Valles‐Toledo caldera complex. These are the surface expression of what may be a thicker, more voluminous intracaldera tuff sequence. The combined deposits of the lower and upper members of the Bandelier Tuff, Toledo and Valles intracaldera sediments, tuffs, and dome lavas form what we interpret to be a wedge‐shaped caldera fill. This sequence is confirmed by deep drill holes and gravity surveys. This fill accumulated in depressions formed during precaldera rifting and episodes of caldera collapse. We interpret the Toledo‐Valles caldera complex to be a pair of nearly coincident trapdoor calderas, with the hinge on the west side and thick caldera fill in the east.

Cenozoic magmatism in southwestern Arizona, which is within the Basin and Range tectonic province, occurred almost entirely between 15 and 25 Ma. Volcanic rocks typically consist, in ascending order, of (1) a thin sequence of mafic to intermediate lava flows, (2) voluminous felsic lava flows and pyroclastic rocks with minor to moderate amounts of intermediate to mafic lava flows, and (3) basalt and andesite. Volcanic rock sequences rest disconformably on pre‐Tertiary bedrock in most areas but locally overlie substantial coarse clastic debris that was deposited immediately before and during earliest magmatism. Prevolcanic clastic debris is interpreted as a consequence of local early normal faulting. In most regions, tilting related to extension began later and occurred during or after eruption of felsic volcanic rocks and before the end of younger mafic volcanism. Extension generally ended before about 17 Ma except in a northwest trending belt adjacent to the relatively unfaulted and topographically elevated Transition Zone tectonic province which is adjacent to the Colorado Plateau. Rapid cooling of metamorphic core complexes and tilting of young basalts and coarse clastic rocks continued in this belt until as recently as 11 Ma. Extension was extreme in this belt, whereas it was generally moderate to slight in other parts of southwestern Arizona. Large‐magnitude extension was not associated with areas of greatest igneous activity, and rapid cooling and exhumation of core complexes postdated local magmatism. These relationships are inconsistent with theories that relate genesis of metamorphic core complexes to magma intrusion in the upper crust. Except for young extension in this northwest trending belt, there are no apparent regional migration trends for either magmatism or extension within southwestern Arizona. Lack of substantial extension before magmatism and general lack of magmatism during youngest extension are inconsistent with the hypothesis that magmatism was the product of decompression melting during lithospheric extension. The long duration and large magnitude of extension adjacent to the Transition Zone tectonic province and within an area of earlier crustal thickening are consistent with the hypothesis that extension was driven by the gravitational potential energy of elevated land mass and crustal roots. Regional magmatic heating apparently weakened the lithosphere and triggered extension but did not control extension locally.

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