Page 178 - Caldera Volcanism Analysis, Modelling and Response

The Sierra Madre Occidental, Mexico 153

subalkalic andesitic lavas, which were grouped and named as the Southern
Cordillera Basaltic Andesitic Suite (SCORBA) by Cameron et al. (1989), with a
subduction tectonic regime geochemical signature combined with the Basin and
Range extension. These rocks range in age from 28 to 12 Ma. However, most of
them occur within 24–20 Ma and are here considered as a separate volcanic group,
named the Miocene Mafic Lavas of the SMO.

4. Graben Calderas of the Sierra Madre Occidental

Aguirre-Dı ´az and Labarthe-Herna ´ndez (2003) proposed that a large
proportion of the ignimbrites of the SMO were related to fissure vents associated
with regional fault systems and grabens of the Basin and Range province, basing
their proposal on geologic observations and on the fact that the SMO volcanic
province and the Basin and Range extensional province overlap in space and time
(Figure 1). However, this fissural eruption style does not completely replace the
vents related to classic semi-circular calderas. There is evidence for both classic
calderas and for fissures as vents for the ignimbrites in the region, but we believe
that most of the volume was erupted from fissure type vents related to Basin and
Range grabens. Swanson et al. (2006) report new caldera sources for ignimbrites of
the northern sector of the SMO, but as new mapping is done in this province, more
evidence of fissure type vents related to grabens are documented. In some cases, the
mapping has been done, and in a few places with great detail; for instance, in the
mining districts that exist throughout the SMO, or the systematic mapping done in
the State of San Luis Potosı ´ (e.g. Labarthe-Herna ´ndez et al., 1982). However, the
interpretation of the geologic maps with respect to the vents and volcanic units
related to these vents was not performed until the late 1990s and early 2000s
(Aguirre-Dı ´az et al., 1998; Aguirre-Dı ´az and Labarthe-Herna ´ndez, 1999, 2003).
In most of the cases that we have studied, we conclude that the ignimbrites were
related to fissure type vents and graben collapse volcanic structures that we have
named ‘‘graben calderas’’ (Aguirre-Dı ´az et al., 2005, 2007). Thus, a graben caldera
can be defined as a collapse-explosive-volcano-tectonic structure from which
ignimbrite-forming pyroclastic flows were erupted through the graben’s faults
during collapse of the roof of a shallow magma chamber that was structurally
controlled by the graben domain. In other words, a collapse caldera with the
form of a graben, or a graben that collapsed as a caldera. There are several types
that we have documented, including piece-meal graben calderas, single-block
graben calderas, half-graben calderas (trap-door), and scattered fissure vents along
regional fault systems that do not concentrate within a single part of the graben or
half-graben.
Geologic observations that support the graben caldera model include five lines
of evidence: (1) pyroclastic dikes that used the graben faults as conduits, (2) co-
ignimbrite lithic-lag breccias that occur next to the graben faults, (3) post-collapse
silicic lava domes and lava diques that occur aligned along the graben master faults,