Description: Highlights • Subplinian to Plinian eruptions from Cocos Island • Tectonically controlled melt ascent • Ocean island evolution without passing typical growth stages Abstract We report a series of fourteen marine tephra layers that are the products of large explosive eruptions of Subplinian to Plinian intensities and magnitudes (VEI 〉 4) from Cocos Island, Costa Rica. Cocos Island is a volcanic island in the eastern Central Pacific Ocean ~ 500 km offshore Costa Rica, and is situated on the northwestern flank of the aseismic Cocos Ridge. Geochemical fingerprinting of Pleistocene (~ 2.4–1.4 Ma) marine tephra layers from Ocean Drilling Project (ODP) Leg 202 Site 1241 using major and trace element compositions of volcanic glass shards demonstrates unequivocally their origin from Cocos Island rather than the Galápagos Archipelago or the Central American Volcanic Arc (CAVA). Cocos Island and the adjacent seamounts of the Cocos Island Province have alkalic compositions and formed on young (≤ 3 Ma) oceanic crust from an extinct spreading ridge bounded by a transform fault against the older and thicker crust of the aseismic Cocos Ridge. Cocos Island has six times the average volume of the adjacent seamounts although all appear to have formed during the 3–1.4 Ma time period. Cocos Island lies closest to the transform fault and we explain its excessive growth by melts rising from garnet-bearing mantle being deflected from the thick Cocos Ridge lithosphere toward the thinner lithosphere on the other side of the transform, thus enlarging the melt catchment area for Cocos Island compared to the seamounts farther away from the transform. This special setting favored growth above sea level and subaerial explosive eruptions even though the absence of appropriate compositions suggests that the entirely alkalic Cocos Island (and seamounts) never evolved through the productive tholeiitic shield stage typical of other Pacific Ocean islands, possibly because melt production rates remained too small. Conditions of magma generation and ascent resembled Hawaiian pre-shield volcanoes but persisted for much longer (〈 1 m.y.) and formed evolved, trachytic magmas. Therefore Cocos Island may be a unique example for a volcanic ocean island that did not pass through the typical growth stages.

Description: The active continental margin off south-central Chile (36° to 40°S) is transitional between the tectonically erosive, empty-trench margin north of Juan Fernandez Ridge and the accretionary, trench-filled margin south of the Chile Triple Junction. The small width of the presently active accretionary wedge (maximum width of 25 to 50 km) argues for past phases of tectonic erosion. At present, this sector shows indications of contemporaneous accretion, subduction, and underplating of sediment, as well as readjustment of the slope by various mass-wasting processes. In this context, this study aims to examine the Neogene sedimentary processes on the continental margin from dredge samples recovered during R/V SONNE cruise SO161-5 within this transitional domain using lithology, sandstone petrology, shale mineralogy, and analysis of sedimentary structures. Our results yield that the principal transport of material occurs in high-energy turbidity currents and debris flows via submarine canyons deeply cutting the continental slope, whereas sediment on the shelf is transported by strong coast-parallel bottom currents and trapped by submarine canyons cutting into the shelf. A wide range of mass-wasting processes including slumping, debris flows, evolving to low-density turbidity currents and mud flows, rework the slope sediments. In contrast, thick undisturbed sequences of mostly hemipelagic sediments accumulate in active slope basins, which are largely protected from mass movements. XRD analyses revealed early diagenetic lithification and overall burial depths of up to ∼ 230 mbsf, suggesting a shallow-subsurface cycle of sedimentation, subsidence, diagenesis, uplift, erosion, and resedimentation. The composition of sandstones is dominated by volcanic rock fragments of Andean provenance. Along-strike modal changes reflect a southward increase in glacially denudation and rainfall, the combination of which caused more intense erosion of volcanic rocks and exposure, weathering and, as a result, increased fluvial transport of metamorphic and plutonic rocks to the sea.

Description: The comparison of bathymetric datasets compiled before and after the Mw = 8.8 Maule Earthquake of the 27 February 2010 offshore Central Chile proves that no new submarine landslides on a size scale detectable with hull-mounted bathymetric echosounders (features of a horizontal size of 〉 1 km) formed as a direct consequence of the ground shaking. Gravity coring around a pre-existing slide feature offshore Concepción (BioBio Slide), however, documents that (1) a number of events occurred as retrogressive failures of the BioBio Slide wall, the youngest of which is 700–1000 years old, and that (2) a very recent small scale slide structure resulted from non-destructive imbricate stacking of a thin sediment layer. Pore water geochemical data show that this event post-dates the Maule Earthquake, suggesting that it was triggered by one of the numerous aftershocks. The absence of larger failures and the presence of a small slide let us propose that in contrast to apparent logic, frequent violent earthquakes at convergent margins do not necessarily pose a particular tsunami risk by landslides. The frequent shaking might even limit the slide volume and therefore their tsunami hazard, as instead of rare and large slides, frequent smaller slides are induced.

Description: A ship-based acoustic mapping campaign was conducted at the exit of Ilulissat Ice Fjord of West Greenland and in the sedimentary basin of Disko Bay west of the fjord mouth. Submarine landscape and sediment distribution patterns represented by five acoustic facies types represent glaciomarine sediment facies types that are related to variations in the past position and relative motion of the glacier front. Asymmetric ridges on the shelf that form a curved entity and a large sill at the fjord mouth represent moraines that depict at least two relatively stable positions of the ice front in the Disko Bay and at the fjord mouth. Comparable ice-end features are not observed seaward of the East Greenland Kangerdlugssuaq Glacier, although both glaciers are comparable in their latitudinal position, present size and present role for the ice discharge from the Inland Ice sheet. Apparently, the retreat of the Greenland Inland Ice after the last maximum expansion was a more discontinuous process on the West Greenland Shelf than on the East Greenland Shelf. The Iceberg Bank, a prominent sill at the fjord exit appears to play an important role for the sedimentation after the retreat of the ice front from the shelf was completed. The retreat of the glacier behind the Iceberg Bank into the inner fjord is marked by a reorganization of sediment delivery in Disko Bay, as most of the till is now deposited within the fjord. Two linear clusters of pockmarks in the center of the sedimentary basin seem to be linked to methane release due to dissociation of gas hydrates, a process driven by fast crustal uplift of the Greenland Shelf. The orientation of these clusters appears to reflect a migration path that is defined by a buried structure which we could not resolve.

Description: Volcano edifice volume calculations are presented for 65 volcanoes of the 1400 km long Chilean Southern Volcanic Zone (SVZ) as a basic step in subduction zone mass budgets. Volume calculations are performed in a Geographical Information System that integrates Digital Elevation Models based on of Shuttle Radar Topography Mission as well as ASTER-GDEM topographic data, LANDSAT satellite images and geological maps. The method of volume calculation is straightforward for isolated, morphologically well-defined stratovolcanoes. Uncertainties increase for volcanic edifices that formed on pre-existing rugged terrain, for multi-phase eruptive centers, as well as for eroded edifices. A revised segmentation of the arc is used to describe the spatial volume distribution of extruded magma along the SVZ and to discuss controlling tectonic factors. Peak volumes between arc and back arc are offset by 400 km. The total volcanic extrusion is in the range of 10–13 km3/km/Ma. Major differences between the SVZ and the Central American subduction system are notable with regard to volcano density and maximum volumes.

Description: The submerged section of the North Anatolian fault within the Marmara Sea was investigated using acoustic techniques and submersible dives. Most gas emissions in the water column were found near the surface expression of known active faults. Gas emissions are unevenly distributed. The linear fault segment crossing the Central High and forming a seismic gap – as it has not ruptured since 1766, based on historical seismicity, exhibits relatively less gas emissions than the adjacent segments. In the eastern Sea of Marmara, active gas emissions are also found above a buried transtensional fault zone, which displayed micro-seismic activity after the 1999 events. Remarkably, this zone of gas emission extends westward all along the southern edge of Cinarcik basin, well beyond the zone where 1999 aftershocks were observed. The long term monitoring of gas seeps could hence be highly valuable for the understanding of the evolution of the fluid-fault coupling processes during the earthquake cycle within the Marmara Sea.

Description: Gravity cores obtained from isolated seamounts located within, and rising up to 300 m from the sediment-filled Peru–Chile Trench off Southern Central Chile (36°S–39°S) contain numerous turbidite layers which are much coarser than the hemipelagic background sedimentation. The mineralogical composition of some of the beds indicates a mixed origin from various source terrains while the faunal assemblage of benthic foraminifera in one of the turbidite layers shows a mixed origin from upper shelfal to middle-lower bathyal depths which could indicate a multi-source origin and therefore indicate an earthquake triggering of the causing turbidity currents. The bathymetric setting and the grain size distribution of the sampled layers, together with swath echosounder and sediment echosounder data which monitor the distribution of turbidites on the elevated Nazca Plate allow some estimates on the flow direction, flow velocity and height of the causing turbidity currents. We discuss two alternative models of deposition, both of which imply high (175–450 m) turbidity currents and we suggest a channelized transport process as the general mode of turbidite deposition. Whether these turbidites are suspension fallout products of thick turbiditic flows or bedload deposits from sheet-like turbidity currents overwhelming elevated structures cannot be decided upon using our sedimentological data, but the specific morphology of the seamounts rather argues for the first option. Oxygen isotope stratigraphy of one of the cores indicates that the turbiditic sequences were deposited during the last Glacial period and during the following transition period and turbiditic deposition stopped during the Holocene. This climatic coupling seems to be dominant, while the occurrence of megathrust earthquakes provides a trigger mechanism. This seismic triggering takes effect only during times of very high sediment supply to the shelf and slope.