THE TECTONIC HISTORY OF THE SAHUARIPA BASIN, EAST-CENTRAL SONORA, MEXICO

Combining what is known about the plate tectonic setting with the stratigraphy and structural relationships near Sahuaripa, Sonora, we add more detail to the tectonic history of northwestern Mexico from the Late Cretaceous to the mid-Miocene. The larger region experienced similar prolonged Mesozoic subduction with corresponding magmatism and contractional deformation followed by Basin and Range extension as found across the western U.S., but there is less data documenting the detailed progression or transitions of the tectonic changes in or across northwest Mexico, especially inboard from the plate boundaries. Most of the data pertaining to the tectonics of this region comes from studies which focus largely on the geophysics and marine and surface geology near the plate boundaries, but we lack a full understanding of the distal affects and products of the tectonic changes.

The superimposed Cretaceous through Tertiary basins near Sahuaripa record episodic deformation, magmatism, and sedimentation. In this area alluvial conglomerates were deposited immediately prior to late Campanian lavas and volcaniclastic deposits. This was a syntectonic basin associated local thrusting which was then surrounded by late Cretaceous volcanism (related to the Tarahumara sequence of central Sonora) and later filled with lacustrine deposits punctuated with tuffs. Laramide deformation gently folded and tilted the 5 km thick sequence during the Paleocene and was followed by significant incision. A large river system cut and filled channels with well-rounded cobble sequences from which paleocurrent data indicate northeast flow. The rivers probably drained an uplifted region of central Sonora indicated by the variety of clasts. A tuff found interbedded with the gravels has an age of 41.5 Ma making the deposits Middle Eocene and roughly coeval with the Rim Gravels found in Arizona. The gravels were tilted eastward possibly via normal faulting before voluminous ignimbrites and andesitic lavas from the western edge of the Sierra Madre Occidental spread across the area from ~37 to ~25 Ma. Basin and Range normal faulting and accumulation of clastic basin fill with minor lavas dominated the local geologic history following the volcanism until the Middle Miocene. Extension stopped here before subduction of the Farallon Plate ceased.