ABSTRACT
The Colima Volcanic
Complex trends in a nearly N-S direction in western Mexico, and one of its structures, Colima volcano, is the
most historically active volcano in the country. Immediately to the N, there is another volcanic center
called El Cántaro volcano, whose activity started around 1.7 Ma in its N
portion and migrated to the S in various episodes. Volcanic activity migrated
further south, from El Cántaro to the Colima Volcanic Complex where the
southernmost manifestation, Hijos del Volcán domes, is located on the south
slope of Fuego volcano. The above date appears to mark initiation of the rather
continuous volcanic activity in the area. It has been noted that these volcanic
manifestations lie on, or near the Rivera-Cocos inland plate boundary. Colima’s
Fuego volcano is also the closest to the Middle America Trench, among the
polygenetic volcanoes in Mexico. We submit that the anomalous location of
volcanism in this area originates in an anomalous subduction process of the
Rivera and Cocos plates and evoke a tectonic model, proposed elsewhere, to
support the idea. Modeling gravimetric and aeromagnetic data we locate the
magma chambers of the Fuego (active) and Nevado (extinct) volcanoes within a 65
mGals negative Bouguer anomaly elongated in a nearly N-S direction. The
corresponding aeromagnetic map displays a magnetic high over the southern
portion of the Fuego volcano edifice. We found two additional, associated
structures whose anomalies have not been previously reported, which appear to
follow the southward magmatic migration pattern. One of them is a collapse
structure with a circular topographic expression, and the southernmost is a
low-density intrusion ~1 km below sea level, associated with a moderate
topographic bulge at the surface that we interpret as a magma body. Five lines
cross the anomalies; gravimetric and magnetic fields are concurrently modeled
along them to locate the magmatic bodies. In addition to the 2-D models we
perform 3-D gravimetric and magnetic inversions. For each field a 3-D mesh is
built under the area occupied by the Colima Volcanic Complex, the volume
elements are then assigned density or magnetic susceptibility values and their
surface contributions in various points are evaluated. The process is iterated
until the difference between the measured and the calculated fields is less
than a predetermined value. The results of each inversion adequately and
independently define the location of the magmatic chambers although they cannot
distinguish between the individual chambers of the Nevado and Fuego volcanoes.
2-D and 3-D results complement each other and consistently show the locations
of potential magmatic regions. Our models support a multiple, complex magmatic
system that appears to continue to spread southwardly, which can pose
additional volcanic risks to an already threatened local population.