Spatial and temporal relationships between mid‐Tertiary magmatism and extension in southwestern Arizona

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.