Page 183 - Caldera Volcanism Analysis, Modelling and Response

158 Gerardo J. Aguirre-Dı ´az et al.

Figure 7 Index map of the Santa Mar|¤adel R|¤o Fault System, showing faults, graben and
half-graben, all with a NW--SE orientation, which were formed during the Basin and Range
extension. Faults in thicker lines indicate those faults used as vents for the Panalillo Inferior
Ignimbrite (27 Ma).Vents mostly occur along the master faults of graben or half-graben; for
instance, at the Paso Blanco, Enramadas, and El Arriate graben, and at the master faults of
the half-graben cluster in the vicinity of Santa Mar|¤adel R|¤o. Shaded areas show extent of
mid-Tertiary ignimbrites and lavas.

System (Figure 6). The margins of these dikes are composed of densely welded
orange-brown ignimbrite-like material, with fiamme aligned vertically (Figure 10e
and f). These welded margins are only a few tens of cm wide, and change to a
poorly welded ignimbrite-like material inside the dike (Figure 10f). Some dikes are
sheared by post-dike displacements along the fault that they occupied (Figure 10b).