Achim J Kopf

Drilling reveals fluid control on architecture and rupture of the Alpine fault, New Zealand

Rock damage during earthquake slip affects fluid migration within the fault core and the surrounding damage zone, and consequently coseismic and postseismic strength evolution. Results from the first two boreholes (Deep Fault Drilling Project DFDP-1) drilled through the Alpine fault, New Zealand, which is late in its 200–400 yr earthquake cycle, reveal a >50-m-thick “alteration zone” formed by fluid-rock interaction and mineralization above background regional levels. The alteration zone comprises cemented low-permeability cataclasite and ultramylonite dissected by clay-filled fractures, and obscures the boundary between the damage zone and fault core. The fault core contains a <0.5-m-thick principal slip zone (PSZ) of low electrical resistivity and high spontaneous potential within a 2-m-thick layer of gouge and ultracataclasite. A 0.53 MPa step in fluid pressure measured across this zone confirms a hydraulic seal, and is consistent with laboratory permeability measurements on the order of 10?20 m2. Slug tests in the upper part of the boreholes yield a permeability within the distal damage zone of ?10?14 m2, implying a six-orders-of-magnitude reduction in permeability within the alteration zone. Low permeability within 20 m of the PSZ is confirmed by a subhydrostatic pressure gradient, pressure relaxation times, and laboratory measurements. The low-permeability rocks suggest that dynamic pressurization likely promotes earthquake slip, and motivates the hypothesis that fault zones may be regional barriers to fluid flow and sites of high fluid pressure gradient. We suggest that hydrogeological processes within the alteration zone modify the permeability, strength, and seismic properties of major faults throughout their earthquake cycles.

Friction experiments on saturated sediments and their implications for the stress state of the Nankai and Barbados subduction thrusts

The Nankai and Barbados forearcs have low-stress subduction thrusts. The sediments entering the subduction zone, and namely the material in the decollement zones, have been well characterized by numerous deep-sea drilling legs and studies of the recovered cores. Nankai has high heat flow and significant amounts of illite, while Barbados is a smectite-dominated system. Based on results from ring shear (<2 MPa normal stress) and direct shear (<30 MPa) tests on marine sediments and mineral standards, this translates into a residual frictional resistance of μr=∼0.25 and μr=∼0.11 in clay horizons, respectively. Such values agree with theoretical estimates from critical wedge theory (Nankai: μb=∼0.16–0.26 and Barbados: μb=∼0.06–0.09) and fault spacing geometries from seismic profiles (Nankai μb=∼0.12–0.23 and Barbados: μb=∼0.11–0.19). Maximum pore pressure ratios of λ*=0.85 and 0.73 for Nankai and Barbados, respectively, allow us to estimate effective shear stresses as a function of friction coefficient and density of the sediment gouge to reach only ∼10 MPa or less in the frontal ∼50 km of the decollement zone, respectively. Our data support the contention that fluid pressure transients and sediment composition contribute equally to the weakness along plate boundary faults down to the seismogenic zone, with the first probably dominating the shallow decollement. Shear velocity stepping tests show that the clay-dominated gouges strengthen velocity irrespective of the clay mineralogy, and hence suggest that clay transformation does not affect the onset of seismogenesis.

Present-day principal horizontal stress orientations in the Kumano forearc basin of the southwest Japan subduction zone determined from IODP NanTroSEIZE drilling Site C0009

A 1.6 km riser borehole was drilled at site C0009 of the NanTroSEIZE, in the center of the Kumano forearc basin, as a landward extension of previous drilling in the southwest Japan Nankai subduction zone. We determined principal horizontal stress orientations from analyses of borehole breakouts and drilling-induced tensile fractures by using wireline logging formation microresistivity images and caliper data. The maximum horizontal stress orientation at C0009 is approximately parallel to the convergence vector between the Philippine Sea plate and Japan, showing a slight difference with the stress orientation which is perpendicular to the plate boundary at previous NanTroSEIZE sites C0001, C0004 and C0006 but orthogonal to the stress orientation at site C0002, which is also in the Kumano forearc basin. These data show that horizontal stress orientations are not uniform in the forearc basin within the surveyed depth range and suggest that oblique plate motion is being partitioned into strike-slip and thrusting. In addition, the stress orientations at site C0009 rotate clockwise from basin sediments into the underlying accretionary prism.

Extreme efficiency of mud volcanism in dewatering accretionary prisms

Drilling results from two mud volcanoes on the Mediterranean Ridge accretionary complex as well as bottom sampling and the wealth of geophysical data acquired recently have provided fundamental knowledge of the 3D geometry of mud extrusions. Mud volcanism is generally related to buoyancy (density inversion), and is triggered by the collision of the African and Eurasian blocks, forcing undercompacted clayey sediments to extrude along faults in the central and hinterlandward parts of the prism. Volumetric estimates of extruded mud in several well-studied areas were based on pre-stack depth-migrated seismic profiles across the entire, up to >150 km wide, prism. The resulting volumes of mud were combined with ages from mud dome drilling, so that rates of mud extrusion were obtained. Subtracting the solid rock mass from the bulk mud volume using physical property data, fluid flux as a function of mud volcanism alone has been quantified for the first time. The volume of fluid extruding with the mud is found to be variable, but reaches up to 15 km3 fluid per km trench length and Ma along cross sections with abundant mud volcanoes. Such large fluid quantities in a region some 50–150 km behind the deformation front exceed estimates from those elsewhere (where undoubtedly the majority of the interstitial fluid is lost due to compaction). Such fluids near the backstop are likely to result predominantly from mineral dehydration and diagenetic reactions at depth, and consequently provide a window to understand deeper processes along the deep decollement. More importantly, the enormous rates with which such fluids and liquified mud escape along the out-of-sequence faults alter fluid budget calculations in subduction zones drastically.

Slumping and mass transport deposition in the Nankai fore arc: Evidence from IODP drilling and 3-D reflection seismic data

Multiple lines of evidence exist for a range of sediment mass movement processes within the shallow megasplay fault zone (MSFZ) area and the adjacent slope basin in the outer fore arc of the Nankai subduction zone, Japan. Diagnostic features observed in three-dimensional reflection seismic data and in cores of the Integrated Ocean Drilling Program (IODP) document a multifarious mass movement history spanning ∼2.87 million years. Various modes and scales of sediment remobilization can be related to the different morphotectonic settings in which they occurred. From this evidence, we decipher the tectonic control on slumping and mass transport deposition in the Nankai fore arc. Three periods of intensified mass wasting coincided with pulses of enhanced activity on the splay fault: (1) an initial phase of juvenile out-of-sequence thrusting ∼1.95 to 1.7 Ma, (2) a reactivation phase between ∼1.55 and 1.24 Ma, and (3) at about 1 Ma, during a phase of uplift of the fore-arc high and motion along the MSFZ. We suggest that slope oversteepening, extensional stress regimes, and lateral transmission of fluid overpressures may have preconditioned the slope sediments to fail. Individual mass-wasting events may have been triggered by dynamic loading from earthquake waves and/or transient pulses of pore pressure along the splay fault. Overall, our results provide insights into the complicated interplay between tectonic and submarine mass movement processes. We demonstrate that detailed knowledge about the spatial and temporal distribution of submarine mass movements can be integrated into a holistic reconstruction of tectonostratigraphic evolution of accretionary margins.

The Mediterranean Ridge: A mass balance across the fastest growing accretionary complex on Earth

Depth migration of seismic reflection profiles across the Mediterranean Ridge accretionary complex between the African and Eurasian blocks illustrates profound variations in the geometry and internal structure along strike. Structural interpretations of four cross sections, together with bathymetric and acoustic surface information and drilling data, are used to volumetrically balance the amount of subduction versus accretion with time. Results suggest the existence of three distinct scenarios, with a jump in decollement in the west, intense backthrusting in the central part between Libya and Crete, and transcurrent tectonism in the east. The onset of accretion coincides with exhumation of thrust sheets (∼19 Ma), followed by rapid sediment accretion with thick, evaporite-bearing incoming successions facilitating outward growth of the wedge. The minimum rate of accretion (20–25% of the total sediment supply) is observed in the central portion where the ridge suffers maximum deformation. Here the indenting leading edge of the African Plate apparently forces the sediment into subduction, or local underplating. In contrast, an estimated 40–60% of the available sedimentary input was accreted in the western domain where collision is less accentuated. The results support the hypothesis that highly destructive forearc collisional events, like slab break off and exhumation of thrust sheets, can be followed by periods of accretion and continuous growth of accretionary wedges.

Evidence for ice-free summers in the late Miocene central Arctic Ocean

Although the permanently to seasonally ice-covered Arctic Ocean is a unique and sensitive component in the Earths climate system, the knowledge of its long-term climate history remains very limited due to the restricted number of pre-Quaternary sedimentary records. During Polarstern Expedition PS87/2014, we discovered multiple submarine landslides along Lomonosov Ridge. Removal of younger sediments from steep headwalls has led to exhumation of Miocene sediments close to the seafloor. Here we document the presence of IP25 as a proxy for spring sea-ice cover and alkenone-based summer sea-surface temperatures >4 °C that support a seasonal sea-ice cover with an ice-free summer season being predominant during the late Miocene in the central Arctic Ocean. A comparison of our proxy data with Miocene climate simulations seems to favour either relatively high late Miocene atmospheric CO2 concentrations and/or a weak sensitivity of the model to simulate the magnitude of high-latitude warming in a warmer than modern climate.

Recurring and triggered slow-slip events near the trench at the Nankai Trough subduction megathrust

Eight slow-slip events over 6 years accommodated up to 50% of the fault slip on the Nankai megathrust. Silently taking up the slack Megathrust earthquakes occur when locked subduction zone faults suddenly slip, unleashing shaking and causing tsunamis. However, seismically silent slow earthquakes also relieve slip on these dangerous faults. Araki et al. present data from ocean boreholes with which they analyze eight slow-slip events near the Nankai trench off the coast of Japan. These events accommodated up to half of the plate convergence over 6 years. The events appear to occur regularly, which has a long-term impact on hazard assessment for the region. Science, this issue p. 1157 The discovery of slow earthquakes has revolutionized the field of earthquake seismology. Defining the locations of these events and the conditions that favor their occurrence provides important insights into the slip behavior of tectonic faults. We report on a family of recurring slow-slip events (SSEs) on the plate interface immediately seaward of repeated historical moment magnitude (Mw) 8 earthquake rupture areas offshore of Japan. The SSEs continue for days to several weeks, include both spontaneous and triggered slip, recur every 8 to 15 months, and are accompanied by swarms of low-frequency tremors. We can explain the SSEs with 1 to 4 centimeters of slip along the megathrust, centered 25 to 35 kilometers (km) from the trench (4 to 10 km depth). The SSEs accommodate 30 to 55% of the plate motion, indicating frequent release of accumulated strain near the trench.

Identification of Weak Layers and Their Role for the Stability of Slopes at Finneidfjord, Northern Norway

The 1996 Finneidfjord landslide, which took four human lives in northern Norway, initiated along a weak layer in the fjord-marine sediments before developing retrogressively across the shoreline. The integration of results from sediment cores, free-fall cone penetrometer tests and high-resolution 3D seismic data indicates that the slide-prone layer is a regional bed likely sourced from clay-slide activity in the catchment of the fjord. The sediments in this regional layer are softer and more sensitive than the typical bioturbated, fjord-marine deposits, which explains their role in slope instability. In addition, biogenic gas in the stratified event bed may further affect its geotechnical properties. Similar, fine-grained, stratified beds with comparable origin and properties occur in other Norwegian fjords. They are presumably also present along coastlines of other previously glaciated margins, where they could contribute to mass movements.

Deep fluids and ancient pore waters at the backstop: Stable isotope systematics (B, C, O) of mud-volcano deposits on the Mediterranean Ridge accretionary wedge

Products of two mud volcanoes from the distal part of the Mediterranean Ridge accretionary complex have been investigated regarding their B, C, and O stable isotope signatures. The mud breccias have been divided into mud matrix, lithified clasts, biogenic deposits, and authigenic cements and crusts related to fluid flow and cementation. Isotope geochemistry is used to evaluate the depth of mobilization of each phase in the subduction zone. B contents and isotope ratios of the mud and mud clasts show a general trend of B enrichment and decreasing δ 11 B values with increasing consolidation (i.e., depth). However, the majority of the clast and matrix samples relate to moderate depths of mobilization within the wedge (1–2 km below seafloor). The carbonate cements of most of these clasts as well as the authigenic crusts, however, provide evidence for a deep fluid influence, probably associated with the decollement at 5–6 km depth. This interpretation is supported by δ 13 C ratios of the crust, which indicate precipitation of C from thermogenic methane, and by the δ 11 B ratios of pore-water samples of mud-breccia drill cores. Clams ( Vesicomya sp.) living adjacent to fluid vents have δ 11 B and δ 18 O values corresponding to brines known in the area, which acted as the parent solution for shell precipitation. Such brines are most likely Miocene pore waters trapped at deep levels within the backstop to the accretionary prism, probably prior to desiccation of the Mediterranean in the Messinian (6–5 Ma). Combining all results, deep fluid circulation and expulsion are identified as the main processes triggering mud liquefaction and extrusion, whereas brines contribute only locally. Given the high B contents, mud extrusion has to be considered a major backflux mechanism of B into the hydrosphere.