Page 179 - Caldera Volcanism Analysis, Modelling and Response

154 Gerardo J. Aguirre-Dı ´az et al.

(4) inward and/or chaotic tilting of downdropped blocks inside the graben
observed only in segments of the graben that collapsed as a caldera (piece-meal
collapse), (5) drastic thickness variations of the intra-graben ignimbrite (relatively
much thicker) with respect the out-flow ignimbrite (relatively much thinner).
Our studies have focused on the southern SMO, in a project that we have called
the Southern Transect of the SMO, a geologic transect that our group has been
mapping systematically since the 1980s (Figure 1). We have also carried out studies
in the central sector of the SMO, which include sites in the states of Durango and
Zacatecas (e.g., Labarthe-Herna ´ndez et al., 1985; Aguirre-Dı ´az and McDowell,
1991, 1993; Trista ´n-Gonza ´lez et al., 1994; Labarthe-Herna ´ndez et al., 1996). In the
southern sector of the SMO there are a series of large grabens with a general
orientation from NW to NE, with lengths of about 80–150 km by 25–40 km wide,
and some about 2 km (topographically) deep (Figure 4). Faulting and subsidence
continued for several millions of years after the graben caldera collapse and
ignimbrite emplacement, displacing the intra-graben caldera products downward
into the tectonic depression, but preserving and even enhancing the chaotic
arrangement of the collapsed blocks for the case of piece-meal graben calderas.
Fluvial red beds and lacustrine deposits indicate that in most cases alluvial fans and
lakes existed before the graben caldera collapse, suggesting that graben subsidence
started before the caldera formation and ignimbrite emplacement. Subsidence
and faulting continued after caldera collapse, producing relatively thick sequences
of post-ignimbrite lacustrine beds that filled the graben and were later faulted and
tilted (Aguirre-Dı ´az and Carranza-Castan ˜eda, 2000). These pre- and post-ignimbrite
fluvio-lacustrine deposits suggest differences in the subsidence rate during graben
development, with apparently fast rates during graben caldera collapse and ignimbrite
emplacement, and relatively slow rates before and after graben caldera collapse. In
many cases, such as Bolan ˜os (Scheubel et al., 1988; Lyons, 1988), the graben caldera
vents are related to gold and silver hydrothermal mineralisation, and thus,
understanding the relationship between the graben tectonics, the graben caldera
vents, the hydrothermal processes that occurs around them, and the water input from
the intra-graben lakes, will be important for economic purposes.
The graben of the southern SMO are, from east to west, the Santa Marı ´a del
Rı ´o and the Juachı ´n half-graben system, and the Aguascalientes, Malpaso, Calvillo,
Juchipila, Tlaltenango, Bolan ˜os, Tuxpan de Bolan ˜os, and the Huejumic graben
(Figures 4 and 5). There are also the Jerez, Atengo, and Jesu ´s Marı ´a graben, and
other less well known graben in the central SMO, such as Ventanas, El Rodeo,
Santiaguillo, and San Juan Papasquiaro graben. All of these are in the Durango state.
Here we only describe the cases of Santa Marı ´a del Rı ´o, Juachı ´n, Malpaso,
Juchipila, and Bolan ˜os as representative examples (Figure 5).

4.1. Graben calderas and fissure vents of San Luis potosı ´

In the central-southern portion of the State of San Luis Potosı ´ (Figure 6) there are
two volcanic areas that, although have some similarities, contain different volcanic
sequences; one is the San Luis Potosı ´ volcanic field, and the other the Santa Marı ´a
del Rı ´o volcanic field (Labarthe-Herna ´ndez et al., 1982; Trista ´n-Gonza ´lez, 1986).