Basaltic volcanism and extension near the intersection of the Sierra Madre volcanic province and the Mexican Volcanic Belt

Three groups (Miocene, Pliocene, and Pleistocene) of oceanic-type basalts associated with normal faulting have been identified in an area surrounding the city of Guadalajara, Mexico. Although most basalts within these groups have compositional characteristics of an asthenospheric source, each group is also associated with lavas that have subduction-related traits. The flat-lying Tertiary (23-27 Ma) ash flows of the Sierra Madre volcanic province that lie to the north of Guadalajara are faulted into horsts and grabens that in one case can be dated as being older than 21.8 Ma. This fault trend has been reactivated closer to Guadalajara, as normal faults (200- 300-m displacement) cut the Miocene plateau basalts but are hidden by a cover of younger volcanic rocks close to the city. The trend of this faulting suggests, along with northwest-southeast lines of cinder cones in the area, that the direction of least principal stress, σ ₃ , has been approximately north-northeast for at least 4 m.y., characteristic of the Mexican Basin and Range province. -from Authors