Page 190 - Caldera Volcanism Analysis, Modelling and Response

The Sierra Madre Occidental, Mexico 165

Besides the structural characteristics briefly mentioned above, there are other
features that confirm that the Malpaso depression is a graben caldera. The first is a
high-grade, rheomorphic ignimbrite that we have named the El Ocote ignimbrite
from the type section locality at El Ocote village on the SE margin of the Malpaso
depression (Figure 13). This ignimbrite is over 400 m thick (with no base exposed)
within the Malpaso depression, and only 20–60 m thick on the NW shoulder of the
depression, and about 70–120 m thick on the SE shoulder along the Sierra El Laurel
(Figure 14a and b). From the NW margin, the ignimbrite gradually thickens to the
North, and once filled an open valley, it now forms the San Jose ´ de Gracia plateau
(Figure 13). This ignimbrite is easily recognised because of its extreme high-
grade welding and rheomorphism that formed overturned and even sheath folds
(Figure 14c and d). Rheomorphic folds only occur within the Malpaso depression,
whereas outside the depression the ignimbrite only shows flat or slightly undulating
flow foliation with extremely flattened pumices (Figure 14e and f). The great
difference in thickness of the ignimbrite inside and outside the depression represents
the intra-caldera and outflow deposits marking the boundary of the graben caldera
precisely where this change occurs, that is, on the graben’s shoulders or graben
caldera margins. The rheomorphic ignimbrite formed folds only in the interior of
the graben caldera, possibly as the piece-meal blocks foundered and accommodated
during and/or after the caldera collapse.
Further evidence indicates that the vents for the El Ocote ignimbrite
are precisely next to the main fault scarps that bound the graben caldera. These
include co-ignimbrite collapse-breccias along the internal graben caldera wall and
co-ignimbrite lithic-lag breccias within a thick sequence of intra-caldera pyroclastic
flow deposits that preceded the El Ocote ignimbrite and that are part of the caldera-
forming sequence (Figure 15). This sequence of lithic-rich pyroclastic flow deposits
may represent the decompression phase of the sub-caldera magma chamber
just before the piece-meal collapse of the graben caldera, which produced the
main catastrophic explosive eruption that formed the El Ocote ignimbrite. These
observations will be described in more detail in other work that is in progress and
that focuses on the stratigraphy, structure, rheomorphism, and volcano-tectonic
evolution of the graben caldera of Malpaso.

4.3. Pyroclastic dikes of the piece-meal graben caldera of Juchipila
To the west of the Malpaso graben caldera is the graben of Calvillo, which is a
northern branch of the larger Juchipila graben (Figures 12 and 13). The Juchipila
graben is a structure about 85 long and 20–32 km wide. To the north it branches to
the Calvillo graben and the Tabasco graben, which connects to the Villanueva
graben that continues farther to the north (Figure 4). Together, the Juchipila and
Tabasco graben have a length of 140 km. At segments of this large graben system
there are several fissure vents that used the master faults of the graben as conduits.
The Juchipila graben in particular contains some of the best examples of these dikes,
from which ignimbrite-forming pyroclastic flows were erupted, producing the
collapse structure named here the Juchipila graben caldera, after the main town of
this area. One is a lithic-rich dike near Juchipila, which is N–S oriented and dips