Abstract

Three groups (Miocene, Pliocene, and Pleistocene) off 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 first group, the San Cristobal plateau basalts, has a minimum volume of 1,800 km3 of predominantly alkali olivine basalt and lesser basaltic andesite and fills a pre-existing extensional basin of unknown configuration ∼10 m.y. ago. The relatively rapid eruption of these basalts along with their volume, structural association, and chemistry suggest an upwelling mantle plume beneath the Guadalajara region. This region may be the earliest indication of the separation of the Jalisco block from North America.

The second basalt group, the Guadalajara basalts, consists of small volumes of porphyritic basalts and basaltic andesites, 3.3-5.0 Ma in age, that outcrop near the northern outskirts of Guadalajara and near the town of Hostotip-aquillo to the northwest. The youngest basalts in the Guadalajara area 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. Tequila; these either flowed from nearby cinder cones toward the Rio Santiago canyon or erupted in the canyon to dam the river.

The flat-lying Tertiary (23-27 Ma) ash flows of the Sierra Madre volcanic province that lie to the north of Guadalajara are faulted (north-northeast) 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- to 300-m displacement) cut the Miocene plateau basalts 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 the Tepic-Zacoalco graben system. Faulting of this zone is well displayed in the region of the Santa Rosa dam in the Santiago canyon, northwest 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 south-west, and a currently active (Nieto-Obregon and others, 1985) dextral strike-slip style. The trend of this faulting suggests, along with northwest-southeast lines of cinder cones in the area, that the direction of least principal stress, σ3, has been approximately north-northeast for at least 4 m.y., characteristic of the Mexican Basin and Range province.

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