Achim Kopf

Compositional and fluid pressure controls on the state of stress on the Nankai subduction thrust: A weak plate boundary

Abstract We show that both fault mineralogy and regional excess fluid pressure contribute to low resolved shear stresses on the Nankai subduction plate boundary off southwest Japan. Ring and direct shear tests indicate that saturated clay minerals in the fault possess intrinsically low residual friction coefficients ( μ r ) at stress levels between 1.0 and 40 MPa. The direct shear μ r values for purified smectite are ∼0.14±0.02, for illite ∼0.25±0.01, and for chlorite 0.26±0.02 (for point load velocities of 0.0001 mm/s). These clay minerals dominate the Nankai subduction decollement zone. Illite (plus quartz) is mechanically important in the altered incoming Muroto section and the predicted decollement μ r should lie between 0.2 and 0.32. This low residual strength, together with elevated fluid pressure, limits shear stresses to below ∼4 MPa within the frontal ∼50 km of the subduction system, consistent with the low wedge taper in this region. A higher wedge taper off the Ashizuri peninsula indicates basal shear stresses rise slightly along strike towards this region. Our analysis indicates lower fluid pressures must predominantly be responsible because only small second order along strike variations in μ r are predicted to occur as a result of variations in smectite and total clay content. These variations should be further reduced at depth under the wedge as smectite is diagenetically altered to illite. However, our data suggest the low μ r values of the clay-rich decollement still limit shear stresses to between ∼17 and 29 MPa within the frontal ∼50 km of the wedge, consistent with other estimates of plate boundary weakness. Indeed, we propose that it should be expected that subduction plate boundaries like Nankai will be weak because of the intrinsic presence of clay-rich faults and moderate fluid overpressures. Our data do not support the hypothesis that the smectite-to-illite reaction directly controls the onset of seismogenic behavior deep in the Nankai system because there is already a mechanical dominance of illite (rather than smectite) in the shallow decollement zone, and we find all the clay phases tend to velocity strengthen. However, temperature-activated clay diagenesis and dehydration may cause secondary changes in the fault properties and state of stress across the up-dip limit of the seismogenic zone.

Deformational styles of the eastern Mediterranean Ridge and surroundings from combined swath mapping and seiemic reflection profiling

Abstract Recent swath mapping and seismic reflection profiling across the eastern(Levantine) branch of the Mediterranean Ridge (MR), in the eastern Mediterranean Sea, illustrate a strong variability of the deformational styles that characterize this precollisional accretionary prism. Along a north–south cross-section of the MR, a structural analysis, based on surface mapping, combined with vertical seismic reflection data, reveals two main structural domains. A southern Outer Domain consists of a series of three disconnected distinct fold belts. Folding affects a sedimentary cover which includes an approximately 2-km thick Pliocene and Quaternary wedge in this domain, resting on fairly thick Messinian evaporitic sequences that act as probable decollement layers. The MR Inner Domain includes three regions showing evidences of strong internal deformations and of numerous probable mud cones and mud flows, but no seismic evidence of significant underlying Messinian evaporites can be detected. The inner sub-regions are thrusting northwards over an area made of faulted, and locally uplifted, acoustic basement blocks that constitute the southernmost extension of the Crete continental margin, acting as a continental buttress for the MR. This innermost domain is itself structurally disconnected from the Crete continental margin by the en echelon Pliny troughs system that shows evidence of left lateral displacement. Altogether, the different structural and sedimentary cover patterns reveal a strong contrast between both MR Inner and Outer Domains, and important lateral variations within the Inner MR itself. Strike-slip faulting seams to characterizes both areas, and large-scale mud accumulations potentially exist in the northern one. Our results support a model of imbricated accretionary prisms, including, at least, two stages: (1) a pre-Messinian stage during which the Inner MR probably developed in response to northward subduction of the African lithosphere beneath southern Europe, and (2) a Messinian to post-Messinian period, during which the kinematics of the Aegean–Anatolian microplates and the presence of thick Messinian deposits became prevalent and which led to the creation of the Outer MR folded wedge piling against the previous one. In our interpretation, the occurrence of thick Messinian evaporites in this area induces important local and regional modifications of the structural pattern. Strike-slip activities reflect partitioning related to oblique subduction, and likely lateral escape of the sedimentary cover in the whole studied area.

Back to the roots : boron geochemistry of mud volcanoes and its implications for mobilization depth and global B cycling

At convergent margins, fluid flux through active mud volcanoes (MVs) has been estimated to exceed that of the frontal accretionary prism, and may have done so in earlier earth history. Often, however, it is largely unknown how deep mud volcanoes root within accretionary wedges and orogenic belts. We report results from a systematic geochemical study of fluids and clays/claystones from mud volcanoes around the world using the mobile element boron as depth indicator for progressive diagenesis. Boron shows maximum enrichment in the fluid phase, owing to desorption in the mud, when faulting roots deepest and deformation is strongest. Deep-seated, B-rich fluids liquefy clay-bearing strata to facilitate extrusion, allowing the clay to re-adsorb and incorporate large quantities of B in the process. Given the abundance and high discharge rates of mud volcanoes along subduction zones, we propose that this process has to be considered a major backflux mechanism in global B cycling from the lithosphere into the hydrosphere.

Extrusion dynamics of mud volcanoes on the Mediterranean Ridge accretionary complex

Abstract Drilling of two submarine mud domes situated in the Olimpi field on the northern flank of the Mediterranean Ridge accretionary complex has documented episodic eruptive activity over the last 1 to >1.5 Ma. Mud extrusion is related to plate convergence between Africa and Eurasia, having caused backthrust faulting of accreted strata containing overpressured mud at depth. The domes mainly consist of mud breccia with up to 65% of polymictic clasts embedded in a clayey matrix. On the basis of modifications of Poiseuille’s and Stokes’ laws, mud extrusion rates were calculated for Milano and Napoli mud domes. Mud ascent velocities are estimated to be up to 60–300 km a−1, and are comparable with those of silicate magmas. Using physical property, structural and flux data of the mud breccias, and compiled data from mud domes on land, the diameter of the feeder channel and the depth of origin for the overpressured muds could be reliably estimated for the first time. Feeder channels are likely to be only a few metres wide. Gas efflux estimates constrain a source depth to c. 1700 ± 50 m below sea floor in the Olimpi field, which is considerably shallower than estimations made from the thermal maturity of solid organic carbon in the mud breccias. The efflux data suggest that the overpressured muds were not mobilized at décollement depth, but at a shallower level within the accretionary prism. A comparison of mud ascent rates (as determined from Poiseuille flow) and the total volumes of mud extruded indicate that only a fraction of the time span constrained from biostratigraphic data (c. 1 Ma) is needed to build up the Milano and Napoli mud domes. Durations of 12–58 ka of extrusive activity suggest mud volcanism here to have been a highly episodic phenomenon.

Strong B enrichment and anomalous δ11B in pore fluids from the Japan Trench forearc

Abstract Interstitial pore waters from Ocean Drilling Program (ODP) Sites 1150 and 1151, both penetrating ∼1200 m of sub-seafloor sediment from the upper Japan Trench forearc, were analyzed for B concentrations and δ 11 B isotopes. B concentrations cover a wide range from 329 to 3920 μM (0.8–9.3× seawater) and vary isotopically from +20 to +46‰ (relative to SRM 951). In general, B concentrations increase more or less steadily with depth. At both sites, the B enrichment and isotopic changes seem to be related to a deep fluid influence, possibly coupled with alteration of volcanic products. Silica diagenesis does not affect the B geochemistry of the waters, as is evident from intact diatom tests and high opal-A contents throughout the holes. The B enrichment is more pronounced at Site 1150 in the seismic portion of the forearc. Here, a large positive anomaly in δ 11 B values is observed, which corresponds with abundant fracturing and two prominent shear zones. At about the same depth range at Site 1151, the δ 11 B values show a negative shift, attesting to an enrichment in 10 B in the fluid as a result of B desorption from clay due to burial and tectonic stress. We conclude that B release from clay minerals is the major contributor to pore fluid anomalies, and that fluid flow through permeable out-of-sequence thrust (like the shear zones at Site 1150) may be an efficient mechanism for B backflux from the forearc into the hydrosphere.

Isotopic evidence (B, C, O) of deep fluid processes in fault rocks from the active Woodlark Basin detachment zone

We report results from boron, carbon and oxygen stable isotope analyses of faulted and veined rocks recovered by scientific ocean drilling during ODP Leg 180 in the western Woodlark Basin, off Papua New Guinea. In this area of active continental extension, crustal break-up and incipient seafloor spreading, a shallow-dipping, seismically active detachment fault accommodates strain, defining a zone of mylonites and cataclasites, vein formation and fluid infiltration. Syntectonic microstructures and vein-fill mineralogy suggest frictional heating during slip during extension and exhumation of Moresby Seamount. Low carbon and oxygen isotope ratios of calcite veins indicate precipitation from hydrothermal fluids (δ13CPDB down to −17‰; δ18OPDB down to −22‰) formed by both dehydration and decarbonation. Boron contents are low (<7 ppm), indicating high-grade metamorphic source rock for the fluids. Some of the δ11B signatures (17–35‰; parent solutions to calcite vein fills) are low when compared to deep-seated waters in other tectonic environments, likely reflecting preferential loss of 11B during low-grade metamorphism at depth. Pervasive devolatilization and flux of CO2-rich fluids are evident from similar vein cement geochemistry in the detachment fault zone and splays further updip. Multiple rupture-and-healing history of the veins suggests that precipitation may be an important player in fluid pressure evolution and, hence, seismogenic fault movement.

Permeability variation across an active low-angle detachment fault, western Woodlark Basin (ODP Leg 180), and its implication for fault activation

Abstract In the western Woodlark Basin, off Papua New Guinea, the variation from continental rifting to sea-floor spreading has profound effects on the mechanical response of the lithosphere. The extension is well expressed in a seismically active, shallow-dipping detachment fault. Recent Ocean Drilling Program drilling (ODP Leg 180) in the area obtained cores from the hanging wall, detachment fault gouge, and footwall, of which samples underwent permeability testing in the laboratory. Permeability variation was found to be critically dependent on (1) flow direction, i.e. fabric anisotropy of the rocks, and (2) deformational structures in the hanging wall to the fault. Regarding the first, a slight but distinct increase in permeability has been recorded parallel to the fabric (compared with flow normal to this, as indicated by anisotropy indices of khorizontal/kvertical of >1.7). This phenomenon appears most profound directly above fault zones in the hanging-wall block, which are interpreted to represent splays to the main detachment fault plane at depth. Here, shear-enhanced compaction seals fluid flow to the sea floor, so that conductive flow parallel to the fault plane is favoured (in general one order of magnitude higher). The fault gouge, mainly consisting of highly serpentinized basalt and chlorite, exhibits an increase in permeability relative to the clay- and siltstones of the hanging-wall block. In the metamorphic basalt from the tectonic footwall, permeability decreases again by three orders of magnitude (k is c. 6e—17 to 5e—18 m2). Consequently, the detachment and synthetic splays related to it are zones of enhanced fluid migration in the fault plane direction. Fluid overpressure, and hence fault activity, is suggested to be trigered by seal of the top of the fault zone, owing to both shear fabrics and cementation processes.

Deformation fabrics of faulted rocks, and some syntectonic stress estimates from the active Woodlark Basin detachment zone

Abstract Ocean Drilling Program (ODP) Leg 180 investigated, in the western Woodlark Basin off Papua New Guinea, the nature and evolution of continental extension, eventually leading to crustal break-up and sea-floor spreading. At Moresby Seamount, the rift-related extension is localized at a recently active low-angle (30°) detachment fault, partly buried beneath a Pliocene-Pleistocene sedimentary synrift sequence. Data from three drillsites sample the detachment fault itself, secondary faults in its hanging wall and a steep normal fault cutting the footwall. The fault plane itself is manifested as a strongly altered fault gouge. Deformation of turbiditic sediments in several fault zones in the hanging wall is dominated by brittle mechanisms, and accompanied by intensive veining and pervasive diagenetic cementation. The metabasic rocks of the footwall below the detachment show an unusual transition from ductile to brittle deformation fabrics with increasing depth. Many fracture systems show evidence of repeated opening and healing during multistage hydrothermal mineralization. Syn-mylonitic microstructures and vein fill mineralogy suggest exhumation of the detachment footwall from considerable depth in the crust. Two palaeo-piezometers were applied to calcite-filled veins that show evidence of plastic deformation. Differential stress values of similar magnitude and probably close to the rock failure strength are found in both the hanging wall and footwall.