MORPHOLOGICAL AND STRUCTURAL ANALYSIS OF THE CENTRAL SECTOR OF THE TRANSMEXICAN VOLCANIC BELT
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In this paper the post-orogenic evolution of a central sector of the Mexican Volcanic Belt (MVB) in the states of Guanajuato and Michoacán is analyzed. The main structural units are recognized on the basis of a morphological analysis and geological data. Four categories of morphological units were established. These are:
1. Reliefs belonging to geological formations older than MVB and characterized by Laramide orogenic deformation.
2. Pre-MVB planar structural surfaces.
3. Depositional surfaces belonging to sedimentary orvolcanoclastic infilling of post-orogenic tectonic depressions.
4. Surfaces of MVB volcanoes.
In this analysis the main morphogenetic stages were recognized; these were related to tensional tectonic phases and to the main volcanic events. An early tensional tectonic phase is associated with the volcanic activity which formed the pre-Miocene deposits of the Sierra Madre Occidental. A NNW-SSE trending system is dominant during the late Miocene and is related to the "Basin and Range" province of western U. S. A. NW-SE faulting during Pliocene along Californian trends is responsible for the initial volcanic activity of the MVB. Late Pliocene trends are WSW-ENE. The last tensional phase causes N-S and NNE-SSW grabens, as a possible reactivation of "Basin and Range" structures.