Abstract:
Countless seamounts occur on Earth that can provide important constraints on intraplate volcanism and plate
tectonics in the oceans, yet their nature and origin remain poorly known due to difficulties in investigating the deep
ocean. We present here new lithostratigraphic, age and geochemical data from Lower/Middle Jurassic and Lower
Cretaceous sequences in the Santa Rosa accretionary complex, Costa Rica, which offer a valuable opportunity to
study a small-sized seamount from a subducted plate segment of the Pacific basin. The seamount is characterized
by very unusual lithostratigraphic sequences with sills of potassic alkaline basalt emplaced within thick beds of
radiolarite, basaltic breccia and hyaloclastite. An integration of new geochemical, biochronological and geochronological
data with lithostratigraphic observations suggests that the seamount formed ~175Ma ago on thick oceanic
crust away from subduction zones and mid-ocean ridges. This seamount traveled ~65Ma in the Pacific before
accretion. It resembles lithologically and compositionally “petit-spot” volcanoes found off Japan, which form in
response to plate flexure near subduction zones. Also, the composition of the sills and lava flows in the accreted
seamount closely resembles that of potassic alkaline basalts produced by lithosphere cracking along the Line Islands
chain. We hypothesize based on these observations, petrological constraints and formation of the accreted seamount
coeval with the early stages of development of the Pacific plate that the seamount formed by extraction of small
volumes of melt from the base of the lithosphere in response to propagating fractures at the scale of the Pacific basin.
