Page 181 - Caldera Volcanism Analysis, Modelling and Response

156 Gerardo J. Aguirre-Dı ´az et al.

Figure 5 Digital shaded elevation model of the southern sector of the SMO showing the loca-
tion and extent of several graben in this sector, and of the San Luis Potos|¤ Volcanic Field
(SLPVF) and the Santa Mar|¤adel R|¤oVolcanic Field (SMRVF).Towns shown: Bol, Bolan ì os;
SMB, San Mart|¤ n de Bolan ì os;Tla,Tlaltenango; Ju, Juchipila; AGS, Aguascalientes; SLP, San
Luis Potos|¤;SMR, Santa Mar|¤adel R|¤o.

dikes (Aguirre-Dı ´az and Labarthe-Herna ´ndez, 2003; Torres-Herna ´ndez et al.,
2006). In Santa Marı ´a del Rı ´o there is a series of relatively small grabens in the
range of 0.5 by 3 km to 3 by 12 km, and a cluster of half-grabens, all oriented
NW-SE (Figure 7). Along the master faults of these grabens there are pyroclastic
dikes, which were the vents for a major ignimbrite in the area, the Panalillo Inferior
ignimbrite, which is a poorly-welded, white deposit, composed mostly of pumice
shards, small pumice fragments, sanidine, quartz and biotite. This ignimbrite is
dated at 27 Ma (Labarthe-Herna ´ndez et al., 1982) and is the product of the last large
ignimbrite event in this region. The graben apparently collapsed as single-
block graben calderas, since structurally they consist of a downdropped block
(the graben) that is bounded by the corresponding horsts (Figure 8). There are also
pyroclastic dikes that fed the Panalillo Inferior Ignimbrite along the faults of several
small half-graben near Santa Marı ´a del Rı ´o(Figure 7). The dikes and faults used as
conduits generally have a high angle, in the range of 70–901 (Figure 9a and b). The
dikes have lenticular shapes in plan view rather than continuous tabular shapes
(Figure 9c), and range in width from a few cm to about 30 m. They have the same
aspect and composition as the Panalillo Inferior ignimbrite and thus, they are
mostly composed of pumice shards and phenocrysts of quartz, sanidine, and biotite,
with small pumice lumps occasionally observed (Figure 9d). The dikes intruded
country rock made of older, densely welded ignimbrites and dacitic lavas. The
contacts with the country rock show signs of heating such as cooking and/or
hydothermal alteration (Figure 9e). At the dike’s margins the pyroclastic rock is