The Rio Fortaleza centre is one of four known centred complexes of the Coastal Batholith and is a hypabyssal ring-dyke complex. Subterranean cauldron subsidence is represented by a bell-jar pluton, while a later ring-dyke may have produced subaerial pyroclastics associated with a caldera.
Inclusion screens represent the remnants of spalled-off country rock and ring-fault mylo-nites, tuffisites and gas breccias with limited hydrothermal mineralizations are indications of its high-level nature.
The two ring-dykes, some 22 km in diameter, are structurally controlled with polygonal outcrop patterns related to the regional joint pattern. At depth, the intersections of major wrench fractures may have focussed the magmas and delimited the symmetrical disposition of the four centres.
The centred complexes show a close relation between plutonism and vulcanism and probably represent the basal remnants of caldera volcanoes that supplied some of the extrusive envelope of the batholith.