Estratigrafia volcanica de la porcion suroeste del Lago de Cuitzeo y su relacion con la formacion de la caldera tipo graben de Chucandiro

We present the volcanic stratigraphy from the southwestern portion of Lake Cuitzeo, Michoacan located inside the central sector of the Trans-Mexican Volcanic Belt province, where found a graben-type caldera structure. Six volcanic sequences were identified with ages ranging from early Miocene to late Pleistocene that include: 1) Copandaro-Tarimbaro Sequence (CoTS) originated during the early Miocene, contains intercalations of basaltic andesite lavas and ignimbrites with a high degree of hydrothermal alteration, 2) The Tarimbaro Sequence (TS), dated at 18.7 Ma, consists of basaltic andesitic lavas and trachyandesites, 3) The Chucandiro Ignimbrite Sequence (ChIS) with ages of 16.88 +/- 0.34, 16.72 +/- 0.24 and 16.88 +/- 0.22, contains three ignimbritic lithofacies forming a plateau perpendicularly tilted to the NE-SW normal faults. Brecciated deposits (lag breccias) occur at the base of the ignimbrites which follow the trace of the Morelia-Acambay fault system (MAFS). These depositsoverlay the 4) the El Caracol sequence (SEC) dated from 13.73 +/- 0.24 Ma to 3.02 +/- 0.03 Ma, that consists of four volcanic units of basalts, andesites, and basaltic andesites. These rocks interbedded with a late Miocene lacustrine unit dated with an index fossil. The lacustrine deposits are overlaid by the 5) Quinceo-Tetillas Sequence (QT): dated at 1.48 +/- 0.016 Ma, that consists of lahars (flows hyperconcentrated flows) and fall deposits covered by products of Quinceo (1.5 +/- 0.14 Ma) and Tetillas (0.4 +/- 0.08 Ma) volcanoes. Finally, the youngest sequence is represented by the 6) Chucandiro cinder cone (CCh): with an age of 0.161 +/- 0.014 Ma of andesitic composition. The structural and geochemical analysis show that the Copandaro-Tarimbaro (SCoT), the Tarimbaro (ST), and the Chucandiro (SICh) ign-imbritic sequences are closely associated with a transition event between the volcanism of the SMO and the TMVB. Based on the geometry of the Chucandiro Graben, the location of the lag breccia along. the trace of the master faults, the NE-SW direction of secondary faults and the distribu-tion of the ignimbrites, it is proposed that this sequence was originated in the Early Miocene from a graben-type caldera named Chucandiro. Its origin is related to the extensional tectonic setting of the MAFS. In this contribution is reported the onset of the trastension in this central sector of the TMVB during the early Miocene with ages older than 17.1 Ma. Our results indicate that the onset of TMVB in the area occurred dur-ing late Miocene with the emplacement of monogenetic volcanism inside the graben followed by lacustrine sedimentation that continues nowadays.