Page 187 - Caldera Volcanism Analysis, Modelling and Response

162 Gerardo J. Aguirre-Dı ´az et al.

Figure 11 Co-ignimbrite lithic-lag breccias associated with Panalillo Inferior ignimbrite in
the Juach|¤ n Fault System. (a) Coarse lithic-lag breccias at the vent site, along a major normal
fault next to El Juach|¤ n creek.The lithics are mostly of rhyolitic and dacitic lavas of older units
in a pumiceous ash matrix. Note person for scale (in circle). (b) Lithics within breccias at (a)
can be up to 3 m in diameter, this particular one shown in photograph next to geologist is
2.5 m across. (c) Angular clasts in the near-vent facies of the lithic-lag breccias at the vent facies.
The deposit is a layered sequence of several co-ignimbrite lithic-lag breccia units indicating
that it was formed from several eruptive pulses. (d) Detail of lithic-lag breccias supported by
pumice-ash matrix.

in the two grabens that bound this depression to the East (Aguascalientes graben)
and to the West (Calvillo graben), which have more typical normal faults (in the
range of 50–701) and blocks downdropped with a domino style. These complex
structural characteristics are reflected on the actual morphology of the Malpaso
graben caldera; thus, there is a rough and irregular topography within this
depression, and a more flat or regular morphology outside the depression, with two
well defined limits along two main scarps that bound the depression (Figure 13).
These two main scarps are slightly curved and concave towards the interior of the
depression, forming a NW scarp with a NE to ENE orientation, and a SE scarp
(Sierra El Laurel scarp) with an ENE to NNE orientation (Figure 13). These scarps
mark the limits of the graben caldera. The chaotic blocks within the depression
apparently resulted from the piece-meal collapse of these blocks during the graben
caldera formation on top of the sub-caldera magma chamber. This may explain
the chaotic arrangement of the blocks and inter-block faults, in contrast with the
non-chaotic and more regular structures outside the graben caldera.