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 basa lts associated with normal faulting have been identified in an area su rrounding the city of Guadalajara, Mexico. Although most basalts withi n these groups have compositional characteristics of an asthenospheric source, each group is also associated with lavas that have subduction -related traits. The first group, the San Cristobal plateau basalts, h as a minimum volume of 1,800 km3 of predominantly alkali olivine basal t and lesser basaltic andesite and fills a pre-existing extensional ba sin of unknown configuration approximately 10 m.y. ago. The relatively rapid eruption of these basalts along with their volume, structural a ssociation, and chemistry suggest an upwelling mantle plume beneath th e Guadalajara region. This region may be the earliest indication of th e separation of the Jalisco block from North America. The second basal t group, the Guadalajara basalts, consists of small volumes of porphyr itic basalts and basaltic andesites, 3.3-5.0 Ma in age, that outcrop n ear the northern outskirts of Guadalajara and near the town of Hostoti paquillo to the northwest. The youngest basalts in the Guadalajara are a are the 0.4-1.4 Ma Santa Rosa suite of basalts-hawaiites-mugearites close to the contemporary (>0.2 Ma) andesitic central volcano, V. Tequ ila; these either flowed from nearby cinder cones toward the Rio Santi ago canyon or erupted in the canyon to dam the river. The flat-lying T ertiary (23-27 Ma) ash flows of the Sierra Madre volcanic province tha t lie to the north of Guadalajara are faulted (north-northeast) into h orsts 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- to 300-m displacement) cut the Miocene plateau ba salts but are hidden by a cover of younger volcanic rocks close to the city. Also hidden by the young volcanic succession there is the north -west-southeast fault zone that extends to the Gulf of California as t he Tepic-Zacoalco graben system. Faulting of this zone is well display ed in the region of the Santa Rosa dam in the Santiago canyon, northwe st of Guadalajara. Two styles of faulting are found there: an older (> 1 Ma, < 5.5 Ma) episode of normal faulting, with blocks downthrown to the southwest, and a currently active (Nieto-Obregon and others, 1985 ) dextral strike-slip style. The trend of this faulting suggests, alon g with northwest-southeast lines of cinder cones in the area, that the direction of least principal stress, sigma3, has been approximately n orth-northeast for at least 4 m.y., characteristic of the Mexican Basi n and Range province.