David Völker

Latitudinal variation in sedimentary processes in the Peru-Chile Trench off Central Chile

Four cruises of the German research vessel RV SONNE (cruises SO101, SO103, SO104 and SO161) surveyed the Chilean continental margin and oceanic plate using seismic measurements across the Peru-Chile Trench, swath-mapping bathymetry, sediment echosounding, dredges and gravity-core sampling. In this paper, we present data from cruise SO161 derived from the sediment-filled sector of the trench between 35 and 44° S. South of 33°10′ S, sediment fill in the trench ranges from 2200 to 3500 m thickness. The sediment volume decreases northwards, as the trench width narrows from 80 km at 41° S to 25 km at 33° S. Turbidity currents enter the trench mainly via nine canyon systems that are deeply incised into the continental slope. Reflection patterns from the trench fill exhibit a cyclicity that can be linked to Milankovic cycles. Turbiditic deposits at elevated positions within the trench indicate Pleistocene mass-wasting events that were able to overcome a height difference of some hundred meters. Within the trench, a fraction of the turbidity currents is channelled by a northward-dipping, axial channel. This axial channel has eroded up to 200 m into the trench fill and from 42° S, it extends northwards some 1000 km, terminating at the foot of the Juan Fernandez Ridge. The channel has no continuous precursor and might have evolved its present-day form during the last glaciation.

Morphology, seismic characterization and sediment dynamics of the Baiyun Slide Complex on the northern South China Sea margin

A large submarine slope failure, the Baiyun Slide Complex, has been discovered in the northern South China Sea. We describe the slide complex morphology, the seismic character of its structural elements and the slide evolution based on high-quality seismic reflection and multi-beam bathymetry data. The Baiyun Slide Complex has three major slide scars that show differences in headwall and sidewall geometry, the nature of the basal shear surfaces and the internal architecture of the deposits. From these observations, we propose a four-phase emplacement model. An extrapolation of the post-slide drape thickness (60 m) gives a rough age estimate for the mass transport events of 0.3 Ma. Pore pressure models for the unfailed continental slope in the vicinity of the Baiyun Slide are based on porosity measurements at nearby Ocean Drilling Program Site 1146. They show that excess pore pressure in slope sediments is anomalously high at a depth around 93 m, most probably as a consequence of a dramatic increase in sedimentation rates over the past 1.8 Ma. This excess pore pressure is proposed to be the major preconditioning factor for the slide initiation, possibly aided by volcano-tectonic activity and gas hydrate dissociation. The unfailed slope is stable under static conditions. However, a near-field earthquake of Mw 5 would suffice to induce a slope instability at c. 93 m depth.

Sedimentary fill of the Chile Trench (32-46°S): volumetric distribution and causal factors

The Chile Trench of the convergent continental margin of Central Chile is a sediment-filled basin that stretches over 1500 km in a north–south direction. The sediment fill reflects latitudinal variations in climate as well as in the morphology and geology of Chile, but also of sediment transport processes to the trench and within the trench. We try to untangle these signals by calculating the total volume and the latitudinal volume distribution of trench sediments and by relating this distribution to a number of factors that affect this pattern. The volume calculation is based on a model geometry of the top of the subducting oceanic plate that is buried beneath trench sediments and the sea floor as measured by swath bathymetry. We obtain the model geometry of the subducting plate by interpolating between depth-converted seismic reflection profiles that cross the trench. The total volume of the trench fill between 32 and 46°S is calculated to be 46000 ± 500 km3. The resulting latitudinal volume distribution is best explained by a sedimentation model that alternates between (1) glacial phases of high sediment flux from Southern Chile combined with active latitudinal sediment transport within the trench and (2) interglacial phases over which sediment input is dominated by local factors. Supplementary material: Top of the oceanic basement (TOB) grid is available as ascii raw data files (xyz) at www.geolsoc.org.uk/SUP18664.

Constraining input and output fluxes of the southern-central Chile subduction zone: water, chlorine and sulfur

In this paper, we constrain the input and output fluxes of H2O, Cl and S into the southern-central Chilean subduction zone (31°S–46°S). We determine the input flux by calculating the amounts of water, chlorine and sulfur that are carried into the subduction zone in subducted sediments, igneous crust and hydrated lithosphericxa0mantle. The applied models take into account that latitudinal variations in the subducting Nazca plate impact the crustal porosity and the degree of upper mantle serpentinization and thus water storage in the crust and mantle. In another step, we constrain the output fluxes of the subduction zone both to the subcontinental lithospheric mantle and to the atmosphere–geosphere–ocean by the combined use of gas flux determinations at the volcanic arc, volume calculations of volcanic rocks and the combination of mineralogical and geothermal models of the subduction zone. The calculations indicate that about 68 Tg/m/Ma of water enters the subduction zone, as averaged over its total length of 1,480xa0km. The volcanic output on the other hand accounts for 2 Tg/m/Ma or 3xa0% of that input. We presume that a large fraction of the volatiles that are captured within the subducting sediments (which accounts for roughly one-third of the input) are cycled back into the oceanxa0through the forearc. This assumption is however questioned by the present lack of evidence for major venting systems of the submarine forearc. The largest part of the water that is carried into the subduction zone in the crust and hydrated mantle (accounting for two-thirds of the input) appears to be transported beyond the volcanic arc.

Morphology and geology of the continental shelf and upper slope of southern Central Chile (33°S–43°S)

The continental shelf and slope of southern Central Chile have been subject to a number of international as well as Chilean research campaigns over the last 30xa0years. This work summarizes the geologic setting of the southern Central Chilean Continental shelf (33°S–43°S) using recently published geophysical, seismological, sedimentological and bio-geochemical data. Additionally, unpublished data such as reflection seismic profiles, swath bathymetry and observations on biota that allow further insights into the evolution of this continental platform are integrated. The outcome is an overview of the current knowledge about the geology of the southern Central Chilean shelf and upper slope. We observe both patches of reduced as well as high recent sedimentation on the shelf and upper slope, due to local redistribution of fluvial input, mainly governed by bottom currents and submarine canyons and highly productive upwelling zones. Shelf basins show highly variable thickness of Oligocene-Quaternary sedimentary units that are dissected by the marine continuations of upper plate faults known from land. Seismic velocity studies indicate that a paleo-accretionary complex that is sandwiched between the present, relatively small active accretionary prism and the continental crust forms the bulk of the continental margin of southern Central Chile.

Water input and water release from the subducting Nazca Plate along southern Central Chile (33°S–46°S)

The age of the subducting Nazca Plate off Chile increases northward from 0 Ma at the Chile Triple Junction (46°S) to 37 Ma at the latitude of Valparaiso (32°S). Age-related variations in the thermal state of the subducting plate impact on (a) the water influx to the subduction zone, as well as on (b) the volumes of water that are released under the continental fore arc or, alternatively, carried beyond the arc. Southern Central Chile is an ideal setting to study this effect, because other factors for the subduction zone water budget appear constant. We determine the water influx by calculating the crustal water uptake and by modeling the upper mantle serpentinization at the outer rise of the Chile Trench. The water release under fore arc and arc is determined by coupling FEM thermal models of the subducting plate with stability fields of water-releasing mineral reactions for upper and lower crust and hydrated mantle. Results show that both the influx of water stored in, and the outflux of water released from upper crust, lower crust, and mantle vary drastically over segment boundaries. In particular, the oldest and coldest segments carry roughly twice as much water into the subduction zone as the youngest and hottest segments, but their release flux to the fore arc is only about one fourth of the latter. This high variability over a subduction zone of <1500 km length shows that it is insufficient to consider subduction zones as uniform entities in global estimates of subduction zone fluxes.

Geophysical evidence and inferred triggering factors of submarine landslides on the western continental margin of the Ulleung Basin, East Sea

Submarine landslides form very complex depositional and erosional features on the seafloor, and their dynamics and triggering processes are yet to be understood completely. Numerous studies are being undertaken both because of the scientific significance but also for their potential harm to seafloor infrastructure and coastal areas. This study investigates the styles and causes of landsliding along the western margin of the Ulleung Basin in the East Sea, based on multiple sparker, subbottom profiler, multibeam echosounder and sediment core datasets collected in 2015. The bathymetric analyses indicate that the southern slope of the Ulleung Basin has experienced at least seven submarine failures. These failures left clear arcuate-shaped scarps that initiated at water depths of ~600 m. The observed headwall scarps have heights that exceed 60 m and appear to be the result of retrogressive-type failures. Seismic reflection data clearly image the basal sliding surface that is characterized by a prominent high-amplitude reflector. Chaotic-to-transparent seismic facies occur immediately downslope of the headwall scarps; these represent ~20 m thick landslide deposits. Gravity cores taken from areas adjacent to the scars suggest that these slides are older than ca. 97 ka. Interpretation of the present data shows that faults appear to cut recent sediments upslope of scarps, and that the slope may still be in an active phase of failure. Seismic data also image various overpressurized gases and/or gas fluids, as evidenced by the occurrence of pockmarks and seismic chimneys in upslope or adjacent areas of the scarps. Hence, earthquakes associated with tectonic activity and development of fluid overpressure may have acted as the main conditioning factor for destabilizing the slope sediments. Geotechnical stability analyses indicate that the sampled slope sediments are exceptionally stable under present-day conditions, even under seismic loading. This finding points to additional forces such as excess pore pressure caused by gas fluids at the times of slide emplacement.

The role of mud volcanism and deep-seated dewatering processes in the Nankai Trough accretionary prism and Kumano Basin, Japan

Circulation of water at moderate depths in subduction zones is dominantly driven by clay mineral dehydration over distinct pressure and temperature gradients. The signature of these dehydration reactions is found in mud volcano pore waters, however, it is largely unknown, how much of the deep-seated fluids are emitted at mud volcanoes. To unravel this relation for the region off the Kii Peninsula, Japan, we calculated the water volume that is subducted in the Nankai Trough using input data from IODP holes C0011 and C0012 and the correspondent water volume released from the subducted plate under the Kumano Basin, in an area where 13 mud volcanoes are located. According to our model, water released at depth in the mud volcano area is derived almost entirely from basaltic saponite and sedimentary smectite transformation (up to 96%). Nonetheless, the mud volcanoes themselves expel ≪1% of the total volume. To test the contribution of the accreted strata and the Kumano Basin fill to the water budget, we run a second model. Water loss due to compaction of sediments and smectite-illite transition below the basin floor have been calculated. The results were compared with salinity measurements on background cores scattered in the study area to extrapolate the volume of water loss at depth. The comparison of the two methods yielded similar results and led us to conclude that the bulk part of the deep-seated fluid re-enters the hydrosphere via the basin floor, a mechanism rarely taken into account in fluid budgets in the literature.

Size-Frequency Relationship of Submarine Landslides at Convergent Plate Margins: Implications for Hazard and Risk Assessment

We use complete inventories of submarine landslides from the Middle America (MA) and the Central Chile (CC) trench and forearc systems to analyze the size-frequency relationship of such structures on active continental slopes. The MA forearc is characterized by subduction erosion, and the CC forearc has had an accretionary tectonic history since the Late Neogene. Both are end-member types of convergent margins around the world. Both margin segments have been mapped by high-resolution swath bathymetry at strike lengths of about 1,300 km (MA) and 1,000 km (CC). The Middle America forearc has 143 discernible slides with sizes ranging from 0.38 to 1,426 km2. Offshore Central Chile, the 62 mapped slides are 0.9–1,285 km2 in size. Slide localization is markedly different at both margin types. While they also vary strongly along strike of the individual margin, depending on forearc slope gradient, kinematic coupling between plates, or topographic structure of the downgoing plate, the size-frequency relationships are remarkably similar. This allows quantification of the incidence of a submarine slide of given size per margin segment. The relationships hold for slide sizes from 10 to 1,000 km2, with the cut-off defined by slide size (smaller slides) and sample size (larger slides). As slide traces of 100–300 km2 size are obliterated by tectonic deformation after about 200,000 years, recurrence rates for slides of a given size can be estimated. This offers a chance to assess hazard and risk resulting from such events. It is suggested that it takes 20 to 200 plate boundary earthquakes to set off a medium-sized (>10 km2) or larger slump or slide.

Fault zone controlled seafloor methane seepage in the rupture area of the 2010 Maule Earthquake, Central Chile

Seafloor seepage of hydrocarbon-bearing fluids has been identified in a number of marine forearcs. However, temporal variations in seep activity and the structural and tectonic parameters that control the seepage often remain poorly constrained. Subduction-zone earthquakes for example, are often discussed to trigger seafloor seepage but causal links that go beyond theoretical considerations have not yet been fully established. This is mainly due to the inaccessibility of offshore epicentral areas, the infrequent occurrence of large earthquakes, and challenges associated with offshore monitoring of seepage over large areas and sufficient time periods. Here, we report visual, geochemical, geophysical, and modelling results and observations from the Concepcion Methane Seep Area (offshore Central Chile) located in the rupture area of the 2010 Mw. 8.8 Maule earthquake. High methane concentrations in the oceanic water column and a shallow sub-bottom depth of sulfate penetration indicate active methane seepage. The stable carbon isotope signature of the methane and hydrocarbon composition of the released gas indicate a mixture of shallow-sourced biogenic gas and a deeper sourced thermogenic component. Pristine fissures and fractures observed at the seafloor together with seismically imaged large faults in the marine forearc may represent effective pathways for methane migration. Upper-plate fault activity with hydraulic fracturing and dilation is in line with increased normal Coulomb stress during large plate-boundary earthquakes, as exemplarily modelled for the 2010 earthquake. On a global perspective our results point out the possible role of recurring large subduction-zone earthquakes in driving hydrocarbon seepage from marine forearcs over long timescales.