Structure and Evolution
Located on the Chile-Bolivia frontier, Guayaques is the youngest vent in a large N-S trending dacite/rhyolite flow dome complex, the Cerros de Guayaques (Figure 24.1). The complex is approximately 10 km long and occupies an interesting structural position immediately east of the Purico ignimbrite shield (q.v.) directly along strike from the resurgent center of the La Pacana caldera (q.v.). Its location may be controlled by the large north-south fault zone, more obvious further south, that forms the western boundary of the uplifted puna block. Field relations, morphological characteristics of the lavas, and evidence of glacial moraines indicate a complex and long evolution for the whole complex perhaps dating back to about 4 Ma. Older topographic scarps on the east side of the volcano suggest that the complex was constructed on a rather irregular basement. Guayaques sensu stricto has an ill-defined summit crater (SC) but, as in the case of many other similar centers, there is little evidence of any pyroclastic activity. Much the largest flow is on the SW side. This is a thick dacitic lava or coulée(L), extending 3 km westwards, with well developed levées and surface ridges (Figure 24.2). A couple of small lava flow/domes (D) to the north of the summit crater of Guayaques appear to be the youngest activity. There are no records of history activity.
No activity has been observed in visits to the area in 1988 and 1989.
The lavas of Guayaques range from glassy, banded rhyolites and crystal-rich dacites with plagioclase, biotite, hornblende, and quartz. Andesites are volumetrically minor.
Marinovic-N, and Lahsen-A. (1984) Hoja Calama. Carta Geologica de Chile, 58. SNGM, Santiago.