Page 185 - Caldera Volcanism Analysis, Modelling and Response

160 Gerardo J. Aguirre-Dı ´az et al.

Figure 9 Ignimbrite feeder dikes of the Panalillo Inferior ignimbrite (Tpa) at the Santa Mar|¤a
del R|¤o volcanic ¢eld. (a) Western part of a 30 m-wide pyroclastic dike along a half-graben
master fault of the Santa Mar|¤adel R|¤o Fault System intruding dacitic lava. (b) Pyroclastic dike
of Panalillo Inferior ignimbrite (Tpa) along the western master fault of the Enramadas graben.
(c) Pyroclastic dike of Tpa showing the discontinuos and pinching-out shape along strike that
is a common feature in these dikes. (d) Close-up of photograph (a) showing a pyroclastic mate-
rial, mostly composed of a white sintered pumiceous-ash, with sparse lithics. (e) Detail of the
contact between the pyroclastic dike shown in photograph (a) and the dacitic lava that makes
the country rock; note the darker color of the dike along the contact because of welding of pyr-
oclastic material and the alteration of the country rock by leaching. (f) Close-up of photograph
(e) showing the welded (dark) zone of the pyroclastic dike at the contact with the country
rock; light-color inclusions are lithics of lava within the dike material.

blocks and the great structural complexity of the faults among these blocks,
including reverse faults of the downdropped blocks within the graben, and the
high-angle (over 751) fault scarps that bound this structure to the Northwest and to
the Southeast. This structural complexity contrasts with the normal aspect observed