Description: 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 ~175 Ma ago on thick oceanic crust away from subduction zones and mid-ocean ridges. This seamount travelled ~65 Ma 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.

Description: The Central American forearc allows insight into the long-term evolution of the Middle American margin and possible shifts between accretionary and erosive periods of subduction. We present a revised tectonostratigraphic subdivision of the Azuero area based on new field observations and biochronologic data, and a synthesis of previous age, geochemical and stratigraphic data. The basement of the area is composed of an autochtonous oceanic plateau, the early Central American arc and accreted seamounts, which are unconformably overlain by forearc sediments. The nature and spatial arrangement of basement units combined with patterns of uplift and subsidence recorded in overlapping sediments allow reconstruction of the local evolution of subduction tectonics between the Upper Cretaceous and Miocene. Comparison of this evolution with that formerly proposed for the south Costa Rican margin based on a similar approach (Buchs et al., 2009, 2010) provides an insight into temporal and along-strike changes of subduction tectonics along a ~ 500 km-long segment of the Middle American margin. We find that subduction erosion (or non-accretion), punctuated by seamount accretion, was the dominant process along the margin between the late Campanian and Middle Eocene. In the Middle Eocene, uplift of the Central American forearc, initiation of a volcanic front retreat in Panama and a pulse of seamount accretion between south Costa Rica and west Panama are likely to relate to a reorganization of plate tectonics in the Pacific. A contrasted evolution occurred in south Costa Rica and Panama afterwards, with continued subduction erosion in the Azuero area and net accretion of olistostromal and hemipelagic sediments in south Costa Rica at least until the Middle Miocene. Our results show that tectononstratigraphic observations in the forearc may represent a valuable complement to offshore drilling and geophysical studies to understand modern subduction tectonics along the Middle American margin.

Description: The oldest known, intact sedimentary record of a nascent intraoceanic arc was recovered in a ∼100-m-thick unit (IV) above ca. 49 Ma basaltic basement at International Ocean Discovery Program Site U1438 in the Amami Sankaku Basin. During deposition of Unit IV the site was located ∼250 km from the plate edge, where Izu-Bonin-Mariana subduction initiated at 52 Ma. Basement basalts are overlain by a mudstone-dominated subunit (IVC) with a thin basal layer of dark brown metalliferous mudstone followed by mudstone with sparse, graded laminae of amphibole- and biotite-bearing tuffaceous sandstone and siltstone. Amphibole and zircon ages from these laminae suggest that the intermediate subduction-related magmatism that sourced them initiated at ca. 47 Ma soon after basement formation. Overlying volcaniclastic, sandy, gravity-flow deposits (subunit IVB) have a different provenance; shallow water fauna and tachylitic glass fragments indicate a source volcanic edifice that rose above the carbonate compensation depth and may have been emergent. Basaltic andesite intervals in upper subunit IVB have textures suggesting emplacement as intrusions into unconsolidated sediment on a volcanic center with geochemical and petrological characteristics of mafic, differentiated island arc magmatism. Distinctive Hf-Nd isotope characteristics similar to the least-radiogenic Izu-Bonin-Mariana boninites support a relatively old age for the basaltic andesites similar to detrital amphibole dated at 47 Ma. The absence of boninites at that time may have resulted from the position of Site U1438 at a greater distance from the plate edge. The upper interval of mudstone with tuffaceous beds (subunit IVA) progresses upsection into Unit III, part of a wedge of sediment fed by growing arc-axis volcanoes to the east. At Site U1438, in what was to become a reararc position, the succession of early extensional basaltic magmatism associated with spontaneous subduction initiation is followed by a rapid transition into potentially widespread subduction-related magmatism and sedimentation prior to the onset of focused magmatism and major arc building.