Page 198 - Caldera Volcanism Analysis, Modelling and Response

The Sierra Madre Occidental, Mexico 173

5. Conclusions

The SMO is the largest continuous ignimbrite province in the world, with a
3
minimum estimated rock volume of about 400,000 km .
The Ignimbrite Flare-up in Mexico was a period of intense explosive volcanic
activity that produced enormous volumes of silicic ignimbrite sheets, which took
place mainly between 38 and 23 Ma in Mexico.
This paper summarises evidence that suggest that a large volume of the
ignimbrites of the SMO were erupted from fissural vents related to grabens and
fault systems associated with the Basin and Range extension.
We propose the name of graben caldera for a volcano-tectonic explosive collapse
structure from which large-volume ignimbrites were erupted through the master
faults of the graben and the faults of the intra-graben collapsed blocks. This
implies the existence of a sub-graben caldera magma chamber that was at least
partially controlled by the regional extensional tectonics, in this case, the Basin
and Range extensional regime.
The graben calderas observed in the SMO include three types, single-block
(piston equivalent), piece-meal (the most common), and half-graben caldera
(trap-door equivalent).
The succession associated with the graben calderas of the SMO in general consists
of a red beds sequence followed by a pre-major ignimbrite sequence of
pyroclastic surge deposits and minor ignimbrites, followed by the major
ignimbrite, and then the emplacement of silicic lava domes along the main
trend of the graben. This succession represents the typical graben caldera cycle in
the SMO, from magma chamber depressurisation phase (surge deposits), the
graben caldera collapse and ignimbrite emplacement (major ignimbrite), and the
post-collapse emplacement of degassed magma as lava domes through the main
faults of the collapsed structure. A Plinian fallout phase is not evident in the
graben caldera volcanic eruption cycle.
Ignimbrite-feeder pyroclastic dikes are associated with regional fault systems and
graben caldera master faults, and represent the vents through which large-volume
ignimbrite-forming pyroclastic flows were erupted. Pyroclastic dikes range in size
from a few cm to over 100 m wide, commonly have lense-shapes, and can be
welded to non-welded, lithic-rich or lithic-poor.

ACKNOWLEDGMENTS

We greatly thank reviewers Ray Cas and Stephen Self whose comments substantially improved the
original manuscript. We also thank Martha Godchaux for assisting in the revision of English. We
acknowledge Grupo Me ´xico and the personnel in the Bolan ˜os mine project and to Minera El Pilo ´nof
San Martı ´n de Bolan ˜os for logistical help during our visits to the Bolan ˜os graben. We recognise the
Consejo Nacional de Ciencia y Tecnologı ´a (CONACYT) scholarship for a Master program to Isaac
Gutie ´rrez Palomares, and the Coordinacio ´n de la Investigacio ´n Cientı ´fica of the Universidad Nacional
Auto ´noma de Me ´xico (UNAM) for financial aid within the academic interchange program ‘‘Estancias
de Investigacio ´n en Instituciones Espan ˜olas entre la UNAM y el Consejo Superior de Investigaciones Cientı ´ficas