Page 184 - Caldera Volcanism Analysis, Modelling and Response

The Sierra Madre Occidental, Mexico 159

Figure 8 Schematic cross-section of the grabens at the Santa Mar|¤adel R|¤o Fault System,
from which ignimbrite-forming eruptions occurred using the grabens master faults as conduits,
now evident as pyroclastic dikes.These eruptions produced the single-block graben calderas
of Paso Blanco, Enramadas and El Arriate.

At the Juachı ´n Fault System we also found evidence of fault-related vents in the
form of co-ignimbrite lithic-lag breccias. These breccias are found next to large
normal faults of this system. The co-ignimbrite breccias are very coarse, with blocks
up to 3 m in diameter of older rocks from the region at the vent facies (Figure 11),
supported by a pumiceous matrix, and forming a stack of several flow units, each
with an ash-rich top. At the vents, the deposit is mostly composed of lithics (mostly
of rhyolitic lava), with little content of pumiceous-ash matrix, but this relationship
changes with distance from the vent, where the deposit is a sequence of lithic-rich
stratified ignimbrites in the proximal facies, to a sequence of ignimbrites with fewer
lithics in the distal facies.

4.2. The piece-meal graben caldera of Malpaso
Just to the west of Aguascalientes City there is a large depression that has been
interpreted as part of the Calvillo graben (Aranda-Go ´mez, 1989; Gonza ´lez Arroyo
et al., 1997; Nieto-Samaniego et al., 1999 — Figures 4 and 5). We have named this
depression as Malpaso, from the largest town that it contains, because it is neither
the Calvillo graben, nor part of the Calvillo graben as the previous works indicate.
Our studies show that the Malpaso depression is an explosive-collapse-volcano-
tectonic structure, thus a graben caldera. This structure occurs between two well
known grabens, the N–S trending Aguascalientes graben to the East and the
NE-trending Calvillo graben to the West (Figure 12). The Malpaso depression has
several structural characteristics that complicate the original idea for a normal
graben; for instance, the chaotic arrangement of the intra-graben downdropped