Incremental collapse and stratocone growth within a funnel-shaped caldera, Guayabo, Costa Rica

Correlations between the surface geology and detailed borehole stratigraphy/density logs, particularly in the geothermally active area, illustrate that the present morphology and structure of the Guayabo caldera is the product of at least four subsidence events. Much of the sub-caldera tuff-lava sequence, also exposed in the topographic caldera walls, is the product of several ca. 2-Ma-old overlapping stratocones formerly present in the area, but later subjected to subsidence and edifice collapse. The V-shaped gravity anomaly across the caldera is due to a funnel-shaped cross section, developed as follows. The earliest and innermost (now concealed) down-faulted zone was filled by a low-density pumice unit which extended southwest along an early arc-normal graben. At least two subsequent subsidence events produced the present scarps and outflow tuff-breccias, but the erupted volume is markedly lower than the subsidence volume. The missing magma may have been diverted to the east, where, at the same time, further stratocone buildup was occurring. The gravity signature over this eastern caldera margin suggests the presence of a large, dense sub-volcanic intrusive complex. Further edifice collapse and graben formation, due to a combination of gravitational failure and arc-normal strain, are relatively recent and continuing processes. olution of the Guayabo caldera therefore involved a series of subsidence events punctuated by stratocone buildup and collapse. The resolution of concentric, ring-faulted block structures does not support a chaotic collapse model but, instead, indicates the incremental development by widening of a funnel-shaped caldera.