Constraints on fluid flow processes in the Hellenic Accretionary Complex (eastern Mediterranean Sea) from numerical modeling

The dynamics of accretionary convergent margins are severely influenced by intense deformation and fluid expulsion. To quantify the fluid pressure and fluid flow velocities in the Hellenic subduction system, we set up 2-D hydrogeological numerical models following two seismic reflection lines across the Mediterranean Ridge. These profiles bracket the along-strike variation in wedge geometry: moderate compression and a >4 km thick underthrust sequence in the west versus enhanced compression and <1 km of downgoing sediment in the center. Input parameters were obtained from preexisting geophysical data, drill cores, and new geotechnical laboratory experiments. A permeability-porosity relationship was determined by a sensitivity analysis, indicating that porosity and intrinsic permeability are small. This hampers the expulsion of fluids and leads to the build up of fluid overpressure in the deeper portion of the wedge and in the underthrust sediment. The loci of maximum fluid pressure are mainly controlled by the compactional fluid source, which generally decreases toward the backstop. However, pore pressure is still high at the decollement level at distances <100 km from the deformation front, either by the incorporation of low permeability evaporites or additional compaction of the wedge sediments in the two profiles. In the west, however, formation of a wide accretionary complex is facilitated by high pore pressure zones. When compared to other large accretionary complexes such as Nankai or Barbados, our results not only show broad similarities but also that near-lithostatic pore pressures may be easier to maintain in the Hellenic Arc because of accentuated collision, some underthrust evaporates, and a thicker underthrust sequence

The role of input materials in shallow seismogenic slip and forearc plateau development: International Ocean Discovery Program Expedition 362 Preliminary Report Sumatra Seismogenic Zone

Drilling the input materials of the north Sumatran subduction zone, part of the 5000 km long Sunda subduction zone system and the origin of the Mw ∼9.2 earthquake and tsunami that devastated coastal communities around the Indian Ocean in 2004, was designed to groundtruth the material properties causing unexpectedly shallow seismogenic slip and a distinctive forearc prism structure. The intriguing seismogenic behavior and forearc structure are not well explained by existing models or by relationships observed at margins where seismogenic slip typically occurs farther landward. The input materials of the north Sumatran subduction zone are a distinctively thick (as thick as 4-5 km) succession of primarily Bengal-Nicobar Fan-related sediments. The correspondence between the 2004 rupture location and the overlying prism plateau, as well as evidence for a strengthened input section, suggest the input materials are key to driving the distinctive slip behavior and long-term forearc structure. During Expedition 362, two sites on the Indian oceanic plate ∼250 km southwest of the subduction zone, Sites U1480 and U1481, were drilled, cored, and logged to a maximum depth of 1500 meters below seafloor. The succession of sediment/rocks that will develop into the plate boundary detachment and will drive growth of the forearc were sampled, and their progressive mechanical, frictional, and hydrogeological property evolution will be analyzed through postcruise experimental and modeling studies. Large penetration depths with good core recovery and successful wireline logging in the challenging submarine fan materials will enable evaluation of the role of thick sedimentar y subduction zone input sections in driving shallow slip and amplifying earthquake and tsunami magnitudes, at the Sunda subduction zone and globally at other subduction zones where submarine fan-influenced sections are being subducted

Release of mineral-bound water prior to subduction tied to shallow seismogenic slip off Sumatra

Plate-boundary fault rupture during the 2004 Sumatra-Andaman subduction earthquake extended closer to the trench than expected, increasing earthquake and tsunami size. International Ocean Discovery Program Expedition 362 sampled incoming sediments offshore northern Sumatra, revealing recent release of fresh water within the deep sediments. Thermal modeling links this freshening to amorphous silica dehydration driven by rapid burial-induced temperature increases in the past 9 million years. Complete dehydration of silicates is expected before plate subduction, contrasting with prevailing models for subduction seismogenesis calling for fluid production during subduction. Shallow slip offshore Sumatra appears driven by diagenetic strengthening of deeply buried fault-forming sediments, contrasting with weakening proposed for the shallow Tohoku-Oki 2011 rupture, but our results are applicable to other thickly sedimented subduction zones including those with limited earthquake records

Slow slip source characterized by lithological and geometric heterogeneity

Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust

International ocean discovery program expedition 375 preliminary report: Hikurangi subduction margin coring and observatories unlocking the secrets of slow slip through drilling to sample and monitor the forearc and subducting plate, 8 March - 5 May 2018

Slow slip events (SSEs) at the northern Hikurangi subduction margin, New Zealand, are among the best-documented shallow SSEs on Earth. International Ocean Discovery Program Expedition 375 was undertaken to investigate the processes and in situ conditions that underlie subduction zone SSEs at the northern Hikurangi Trough by (1) coring at four sites, including an active fault near the deformation front, the upper plate above the high-slip SSE sourc e region, and the incoming sedimentary succession in the Hikurangi Trough and atop the Tūranganui Knoll Seamount, and (2) installing borehole observatories in an active thrust near the deformation front and in the upper plate overlying the slow slip source region. Logging-while-drilling (LWD) data for this project were acquired as part of Expedition 372 (26 November 2017-4 January 2018; see th e Expedition 372 Preliminary Report for further details on the LWD acquisition program). Northern Hikurangi subduction margin SSEs recur every 1-2 years and thus provide an ideal opportunity to monitor deformation and associated changes in chemical and physical properties throughout the slow slip cycle. Sampling of material from the sedimentary section and oceanic basement of the subducting plate reveals the rock properties, composition, lithology, and structural character of material that is transported downdip into the SSE source region. A recent seafloor geodetic experiment raises the possibility that SSEs at northern Hikurangi may propagate all the way to the trench, indicating that the shallow thrust fault zone targeted during Expedition 375 may also lie in the SSE rupture area. Hence, sampling at this location provides insights into the composition, physical properties, and architecture of a shallow fault that may host slow slip. Expedition 375 (together with the Hikurangi subduction LWD component of Expedition 372) was designed to address three fundamental scientific objectives: (1) characterize the state and composition of the incoming plate and shallow plate boundary fault near the trench, which comprise the protolith and initial conditions for fault zone rock at greater depth and which may itself host shallow slow slip; (2) characterize material properties, thermal regime, and stress conditions in the upper plate above the core of the SSE source region; and (3) install observatories at an active thrust near the deformation front and in the upper plate above the SSE source to measure temporal variations in deformation, temperature, and fluid flow. The observatories will monitor volumetric strain (via pore pressure as a proxy) and the evolution of physical, hydrological, and chemical properties throughout the SSE cycle. Together, the coring, logging, and observatory data will test a suite of hypotheses about the fundamental mechanics and behavior of SSEs and their relationship to great earthquakes along the subduction interface

Expedition 372B/375 summary

Slow slip events (SSEs) at the northern Hikurangi subduction margin, New Zealand, are among the best-documented shallow SSEs on Earth. International Ocean Discovery Program Expeditions 372 and 375 were undertaken to investigate the processes and in situ conditions that underlie subduction zone SSEs at the northern Hikurangi Trough. We accomplished this goal by (1) coring and geophysical logging at four sites, including penetration of an active thrust fault (the Pāpaku fault) near the deformation front, the upper plate above the SSE source region, and the incoming sedimentary succession in the Hikurangi Trough and atop the Tūranganui Knoll seamount; and (2) installing borehole observatories in the Pāpaku fault and in the upper plate overlying the slow slip source region. Logging-while-drilling (LWD) data for this project were acquired as part of Expedition 372, and coring, wireline logging, and observatory installations were conducted during Expedition 375. Northern Hikurangi subduction margin SSEs recur every 1–2 y and thus provide an ideal opportunity to monitor deformation and associated changes in chemical and physical properties throughout the slow slip cycle. In situ measurements and sampling of material from the sedimentary section and oceanic basement of the subducting plate reveal the rock properties, composition, lithology, and structural character of material that is transported downdip into the SSE source region. A recent seafloor geodetic experiment raises the possibility that SSEs at northern Hikurangi may propagate to the trench, indicating that the shallow thrust fault (the Pāpaku fault) targeted during Expeditions 372 and 375 may also lie in the SSE rupture area and host a portion of the slip in these events. Hence, sampling and logging at this location provides insights into the composition, physical properties, and architecture of a shallow fault that may host slow slip. Expeditions 372 and 375 were designed to address three fundamental scientific objectives: Characterize the state and composition of the incoming plate and shallow fault near the trench, which comprise the protolith and initial conditions for fault zone rock at greater depth and which may itself host shallow slow slip; Characterize material properties, thermal regime, and stress conditions in the upper plate directly above the SSE source region; and Install observatories in the Pāpaku fault near the deformation front and in the upper plate above the SSE source to measure temporal variations in deformation, temperature, and fluid flow. The observatories will monitor volumetric strain (via pore pressure as a proxy) and the evolution of physical, hydrological, and chemical properties throughout the SSE cycle. Together, the coring, logging, and observatory data will test a suite of hypotheses about the fundamental mechanics and behavior of SSEs and their relationship to great earthquakes along the subduction interface

Authigenic Clays Versus Carbonate Formation as Products of Marine Silicate Weathering in the Input Sequence to the Sumatra Subduction Zone

Abstract We use geochemical and petrographic data from anoxic sequences of the Nicobar Fan to document extensive marine silicate weathering (MSiW) in the input sediment of the Sumatra subduction zone and the conditions that result in authigenic minerals originating from this reaction: precipitation of authigenic carbonate—which sequesters carbon—and formation of authigenic clay—which releases CO2. Increase in 87Sr/86Sr in pore fluids from International Ocean Discovery Program Expedition 362 (Site U1480 to 0.71376 and Site U1481 to 0.71296) reveals a radiogenic strontium contribution from alteration of the Himalayan continental sediment that dominates the Nicobar Fan. Peaks in the dissolved strontium isotope data coincide with zones of methane presence, consistent with MSiW reactions driven by CO2 generation during methanogenesis. Later‐stage fan sequences from 24 to 400 mbsf (meters below seafloor) contain only minor carbonate with 87Sr/86Sr ratios that deviate only slightly from the co‐eval seawater values (0.70920–0.70930); geochemical data in this zone point to a contribution of authigenic clay formation. In contrast, microscopy and elemental mapping of the carbonate‐cemented zones in the earliest fan deposits (>780 mbsf) show replacement of feldspars and dense minerals by carbonate, which ranges in volume from a few percent of the grain to near total grain obliteration. This deeper authigenic carbonate is significantly enriched in radiogenic 87Sr (0.71136–0.71328). Thus, MSiW leads to distinct products, likely in response to a weathering‐derived supply of silica in the younger setting versus calcium enrichment via diffusion from oceanic basement in the older sequence

Using a Ladder of Seeps With Computer Decision Processes to Explore for and Evaluate Cold Seeps on the Costa Rica Active Margin

Natural seeps occur at the seafloor as loci of fluid flow where the flux of chemical compounds into the ocean supports unique biologic communities and provides access to proxy samples of deep subsurface processes. Cold seeps accomplish this with minimal heat flux. While individual expertize is applied to locate seeps, such knowledge is nowhere consolidated in the literature, nor are there explicit approaches for identifying specific seep types to address discrete scientific questions. Moreover, autonomous exploration for seeps lacks any clear framework for efficient seep identification and classification. To address these shortcomings, we developed a Ladder of Seeps applied within new decision-assistance algorithms (Spock) to assist in seep exploration on the Costa Rica margin during the R/V Falkor 181210 cruise in December, 2018. This Ladder of Seeps [derived from analogous astrobiology criteria proposed by Neveu et al. (2018)] was used to help guide human and computer decision processes for ROV mission planning. The Ladder of Seeps provides a methodical query structure to identify what information is required to confirm a seep either: 1) supports seafloor life under extreme conditions, 2) supports that community with active seepage (possible fluid sample), or 3) taps fluids that reflect deep, subsurface geologic processes, but the top rung may be modified to address other scientific questions. Moreover, this framework allows us to identify higher likelihood seep targets based on existing incomplete or easily acquired data, including MBES (Multi-beam echo sounder) water column data. The Ladder of Seeps framework is based on information about the instruments used to collect seep information (e.g., are seeps detectable by the instrument with little chance of false positives?) and contextual criteria about the environment in which the data are collected (e.g., temporal variability of seep flux). Finally, the assembled data are considered in light of a Last-Resort interpretation, which is only satisfied once all other plausible data interpretations are excluded by observation. When coupled with decision-making algorithms that incorporate expert opinion with data acquired during the Costa Rica experiment, the Ladder of Seeps proved useful for identifying seeps with deep-sourced fluids, as evidenced by results of geochemistry analyses performed following the expedition. © Copyright © 2021 Vrolijk, Summa, Ayton, Nomikou, Hüpers, Kinnaman, Sylva, Valentine and Camilli