INTERRELATIONSHIP OF SEDIMENTARY AND VOLCANIC DEPOSITS ASSOCIATED WITH TERTIARY EXTENSION IN SONORA, MEXICO

Clastic sedimentary deposits and associated volcanic rocks record the progress of Tertiary extension in the Mexican state of Sonora. These d eposits accumulated within basins located throughout the eastern two-t hirds of the state. The rocks are slightly indurated conglomerates and sandstones; clast types reflect the local highland exposures. Volcani c units that bound and are interlayered with the sedimentary rocks pro vide stratigraphic and time markers of basin evolution and, by inferen ce, of marginal fault development. This record has been examined in an east-west belt across south-central Sonora. The volcanic rocks occur in three distinct associations. At the base and interbedded within the lower portions of the sedimentary sections are lava flows of dominant ly basaltic andesite composition. In most localities the overlying ela stic sedimentary strata are conformable with these lava flows, and we conclude that the magmatism was triggered by early faulting along the basin margins. A second association present near the base of the secti ons comprises massive lava domes and flows of intermediate composition that contain distinctive phenocrysts of dark brown amphibole. The dom es apparently formed when viscous lava welled up along developing marg inal basin faults. In one case a transition was observed over a short distance from a structureless dome to a thick lava flow that is confor mably interbedded with, and sheds clasts laterally into, the sedimenta ry section. A third volcanic association comprises layers of rhyolitic lava flows and ignimbrites that overlie the coarsest and thickest low er portions of the sedimentary sections. K-Ar ages for volcanic rocks in the two oldest of these settings indicate that each Tertiary basin had a distinct history with no apparent regional geographic pattern. I n the Rio Yaqui basin, where exhumation by the major river of Sonora h as exposed a full section of the volcanic and elastic sequences, exten sion was possibly as old as 27 Ma. Elsewhere, deposition of sediments was underway in most basins by 24 Ma, and in all basins by 20 Ma. K-Ar ages of the younger rhyolitic volcanic rocks fall within a narrow ran ge from 12.8 to 10.5 Ma. These ages provide a younger time limit to de position of the coarser and thicker portions of the elastic sedimentar y sections. The mafic lava flows, dominantly basaltic andesite, are si milar in major-element composition to mafic lava flows that cap felsic sections within the Sierra Madre Occidental volcanic field of western Mexico and the mid-Tertiary Datil-Mogollon volcanic field in southwes tern New Mexico. They are higher in silica and lower in total alkalies than younger capping mafic lava flows within the Gulf of California e xtensional province of western Sonora and Neogene basaltic lava flows in the Datil-Mogollon field. They are similarly distinct from basaltic dikes and lava flows that are clearly associated with Basin and Range faulting in Trans-Pecos Texas and in northern Durango state. A conver gent-margin magmatic are was active in the present-day Gulf of Califor nia region between 24 and 11.5 Ma. The earliest extension in south-cen tral Sonora was therefore occurring in a back-are setting. Continued s edimentation and rotation of volcanic strata indicate that extension c ontinued throughout the region after 10 Ma with little coeval volcanis m.