Plinian Galápagos volcanism- evidence in ODP/IODP Legs offshore Central America

Drill cores recovered during several ODP and IODP Expeditions offshore Central America contain an extensive Early Cenozoic ash layer record. These ash layers have been deposited by plinian eruptions that originated either at the Central American Volcanic Arc (CAVA) or at the Galápagos Hot Spot. While plinian eruptions are well known from the CAVA, volcanism from the Galápagos region is dominantly recorded in effusive and strombolian deposits from subaerial and submarine eruptions although rare large explosive eruptions of evolved trachytic or dacitic compositions did occur in the Pleistocene (e.g., Geist et al., 1994).We have established a tephrostratigraphy from recent through Miocene times from the unique archive of ODP/IODP sites offhore Central America in which we identify tephra source regions by geochemical compositions of the glass shards. Thus we found numerous CAVA-derived tephra layers characterized by typical arc signatures (e.g., Nb-Ta troughs, LILE enrichments), but more surprisingly also an extensive record of tephra layers mostly of Miocene age featuring ocean island geochemical compositions (e.g., low La/Nb and Ba/La ratios, high Nb/Rb ratios). At this geographical setting the only plausible source for these layers is the Galápagos archipelago. Such Miocene ash layers occur in the cores of ODP Sites 1039, 1241, and 1242. At IODP Site U1381, on the Cocos Ridge offshore Costa Rica, 67 primary Miocene (~8 Ma to ~16.5 Ma) fallout ash layers have been recovered. Inferred transport distances of at least 50to 450 km from their vents imply Plinian eruptions, although two-thirds of the ash beds formed from basaltic magmas and only one-third from rhyolitic magmas that are typically associated with plinian eruptions. Our age model for Site U1381 based on sediment accumulation rates, 40Ar/39Ar dating and biostratigraphic ages, reveals a distinct increase in eruption frequency at around 14 Ma. We interpret this as an increase in magma production rates due to changes in interactions between Galápagos plume and spreading ridge