Stratigraphic and structural relations between the trans-Mexican volcanic belt and the Sierra Madre Occidental in the Guadalajara region, Jalisco, Mexico

Abstract

Field studies in a key area located north of Guadalajara, Mexico, straddling the boundary between the trans-Mexican Volcanic Belt and the Sierra Madre Occidental, allowed us to define the regional stratigraphy, to establish the relations among the main Tertiary volcanic units, and to outline a model of the geological evolution of the region. In the Sierra Madre Occidental succession we recognized a basal unit of Eocene age composed of andesitic lavas, followed, after a hiatus in the volcanic activity, by an Oligocene to lower Miocene sequence characterized by thick and widespread ignimbrites. The thinning and the decreasing degree of welding of the ignimbrite sequence to the south suggest that its depositional limit was close to lat 21 degrees N. At the end of the Sierra Madre Occidental volcanic phase, extension along N10 degrees E-N30 degrees E striking faults created a basin and range regional topography, beautifully expressed by a system of two parallel horsts Sierra Morones and Sierra Nochistlan and grabens Jalpa and Tlaltenango. Polyphase activity of the basin and range faults is proven by the presence of tilted late Miocene lacustrine sediments in the Jalpa graben, representing the early fill of the tectonic depression. The geometric relationships among the main stratigraphic units indicate that the southern portion of the Sierra Madre Occidental was subsequently downfaulted along east-west-striking normal faults. This event marked the onset of the regional tectonic regime that governed the Neogene volcanic activity. The products of the upper Miocene Intermediate Volcanic Sequence onlap the southern margin of the Sierra Madre Occidental fault blocks; there is a gap in volcanism between the two sequences of about 10 m.y. The andesitic-basaltic lavas of the Intermediate Volcanic Sequence form an extensive east-west-trending plateau that constitutes the dominant morphological feature of the western trans-Mexican Volcanic Belt. The Pliocene-Quaternary volcanoes of this belt are constructed on this plateau.

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