Page 186 - Caldera Volcanism Analysis, Modelling and Response

The Sierra Madre Occidental, Mexico 161

Figure 10 Ignimbrite feeder dikes of Panalillo Inferior ignimbrite (Tpa) of the San Luis Potos|¤
volcanic ¢eld along the NW-oriented Juach|¤ n Fault System. (a) Margin of a lithic-rich, welded,
ignimbrite dike near El Peaje dam that follow the major normal fault of El Peaje; note the
high-angle character of the dike. (b) Same dike margin of photograph (a) a¡ected by normal
faults (white lines) with slicken-sides with the same orientation than the dike and the El Peaje
fault, indicating that after emplacement of the dike faulting continued on the same fault used
as conduit. (c) Plan view of anotherTpa dike only a few cm wide; note the sheared margins.
(d) Dike at Pozuelos is the widest and largest associated with the Juach|¤ n Fault System. Dike is
110 m wide at this site, but it reaches up to 115 m at the widest part. Photograph shows the two
welded margins of the dike, which form narrow walls, less than 1m thick, outstanding from the
ground level, which eventually join along strike forming a lense-shape dike. (e) Marginal zones
of Pozuelos dike were indurated because they are formed of densely weldedTpa ignimbrite that
was silici¢ed along the margins.The internal part of the dike is composed of partly welded,
white, pumiceous pyroclastic material similar to that of Tpa ignimbrite. Note pen for scale
(in circle). (f) Detail of photograph (e) showing the densely welded, dark, silici¢ed ignimbrite
of the dike margin.