R. D. Hyndman

Serpentinization of the forearc mantle

A wide range of geophysical and geological data indicate that extensive serpentinization in the forearc mantle is both expected and observed. Large volumes of aqueous fluids must be released upwards by dehydration reactions in subducting oceanic crust and sediments. Subduction of oceanic lithosphere cools the overlying forearc such that low temperature hydrous serpentine minerals are stable in the forearc mantle. Over several tens of millions of years estimated fluid fluxes from the subducting plate are sufficient to serpentinize the entire forearc mantle wedge. However, fluid infiltration is probably fracture controlled such that mantle serpentinization is heterogeneous. Geological evidence for hydration of the forearc mantle includes serpentine mud volcanoes in the Mariana forearc and serpentinites present in exposed paleo-forearcs. The serpentinization process dramatically reduces the seismic velocity and density of the mantle while increasing Poisson’s ratio. Serpentinization may generate seismic reflectivity, an increase in magnetization, an increase in electrical conductivity, and a reduction in mechanical strength. Geophysical evidence for serpentinized forearc mantle has been reported for a number of subduction zones including Alaska, Aleutians, central Andes, Cascadia, Izu-Bonin–Mariana, and central Japan. Serpentinization may explain why the forearc mantle is commonly aseismic and in cool subduction zones may control the downdip limit of great subduction thrust earthquakes. Flow in the mantle wedge, induced by the subducting plate, may be modified by the low density, weak serpentinized forearc mantle. Large volumes of H2O may be released from serpentinized forearc mantle by heating during ridge subduction or continent collision.

An inverted continental Moho and serpentinization of the forearc mantle

Volatiles that are transported by subducting lithospheric plates to depths greater than 100 km are thought to induce partial melting in the overlying mantle wedge, resulting in arc magmatism and the addition of significant quantities of material to the overlying lithosphere. Asthenospheric flow and upwelling within the wedge produce increased lithospheric temperatures in this back-arc region, but the forearc mantle (in the corner of the wedge) is thought to be significantly cooler. Here we explore the structure of the mantle wedge in the southern Cascadia subduction zone using scattered teleseismic waves recorded on a dense portable array of broadband seismometers. We find very low shear-wave velocities in the cold forearc mantle indicated by the exceptional occurrence of an ‘inverted’ continental Moho, which reverts to normal polarity seaward of the Cascade arc. This observation provides compelling evidence for a highly hydrated and serpentinized forearc region, consistent with thermal and petrological models of the forearc mantle wedge. This serpentinized material is thought to have low strength and may therefore control the down-dip rupture limit of great thrust earthquakes, as well as the nature of large-scale flow in the mantle wedge.

Thermal constraints on the zone of major thrust earthquake failure: The Cascadia Subduction Zone

Constraints on the seismogenic portion of the subduction thrust zone along the Cascadia margin are provided by the thermal regime. The zone of stick-slip “locked” behavior where earthquakes can nucleate may be limited downdip by a temperature of about 350°C, and the transition stable sliding zone into which coseismic displacement can extend by a temperature of approximately 450°C. The seaward limit of the stick-slip zone may be associated with the dehydration of stable sliding clays at 100 to 150°C and dissipation of high pore pressures in the area of the deformation front. Temperatures on the thrust have been estimated by numerically modelling the thermal regimes along three profiles crossing the margin with constraints provided by surface heat flow and detailed structural information, particularly at southern Vancouver Island. The models that best fit the heat flow data have negligible shear strain heating. The Cascadia subduction margin is unusually hot as a consequence of the very young plate age and the thick insulating sediment section on the incoming plate; the temperature at the top of the oceanic crust at the deformation front is about 250°C. As a result, the modelled zone of stick-slip seismogenic behaviour is restricted to a narrow zone beneath the continental slope and outer shelf, with the transition zone extending to the inner shelf. The seismogenic zone is wider off the Olympic Peninsula compared to off southern Vancouver Island because of the much shallower thrust dip angle and the slightly older incoming plate. The profile off Oregon is found to have intermediate width zones. An important assumption, well justified only off southern Vancouver Island, is that the thrust detachment is located at the top of the downgoing oceanic crust. The same modelling technique shows that more typical subduction zones with older incoming oceanic lithosphere such as central Chile have thermally restricted seismogenic zones that are much wider, commonly extending well beneath the coast. Support for the position of the Cascadia locked zone from the thermal results is provided by a comparison of the horizontal and vertical interseismic deformation predicted by simple dislocation models with the observed rates from tide gauge and geodetic surveys on adjacent coastal regions. The general agreement indicates that any seismic “locked zone” must be located offshore where the subduction thrust fault is less than about 20 km deep and where the contact is between the oceanic crust and the accreted sedimentary wedge, not between the oceanic and continental crusts. The restriction to an offshore zone provides an important limit to the maximum magnitude and to the ground motion and seismic hazard from subduction megathrust earthquakes in southwestern British Columbia, Washington, and Oregon.

The updip and downdip limits to great subduction earthquakes: Thermal and structural models of Cascadia, south Alaska, SW Japan, and Chile

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A mechanism for the formation of methane hydrate and seafloor bottom‐simulating reflectors by vertical fluid expulsion

Bottom-simulating reflectors (BSR) are observed commonly at a depth of several hundred meters below the seafloor in continental margin sedimentary sections that have undergone recent tectonic consolidation or rapid accumulation. They are believed to correspond to the deepest level at which methane hydrate (clathrate) is stable. We present a model in which BSR hydrate layers are formed through the removal of methane from upward moving pore fluids as they pass into the hydrate stability field. In this model, most of the methane is generated below the level of hydrate stability, but not at depths sufficient for significant thermogenic production; the methane is primarily biogenic in origin. The model requires either a mechanism to remove dissolved methane from the pore fluids or disseminated free gas carried upward with the pore fluid. The model accounts for the evidence that the hydrate is concentrated in a layer at the base of the stability field, for the source of the large amount of methane contained in the hydrate, and for BSRs being common only in special environments. Strong upward fluid expulsion into the hydrate stability field does not occur in normal sediment depositional regimes, so BSRs are uncommon. Upward fluid expulsion does occur as a result of tectonic thickening and loading in subduction zone accretionary wedges and in areas where rapid deposition results in initial undercconsolidation. In these areas hydrate BSRs are common. The most poorly quantified aspect of the model is the efficiency with which methane is removed and hydrate is formed as pore fluids pass into the hydrate stability field. The critical boundary in the phase diagram between the fluid-plus-hydrate and fluid-only fields is not well constrained. However, the amount of methane required to form the hydrate and limited data on methane concentrations in pore fluids from deep-sea boreholes suggest very efficient removal of methane from rising fluid that may contain less than the amount required for free gas production. In most fluid expulsion regimes, the quantity of fluid moved upward to the seafloor is great enough to continually remove the excess chloride and the residue of isotope fractionation resulting from hydrate formation. Thus, as observed in borehole data, there are no large chloride or isotope anomalies remaining in the local pore fluids. The differences in the concentration of methane and probably of CO2 in the pore fluid above and below the base of the stability field may have a significant influence on early sediment diagenetic reactions.

Thermal constraints on the seismogenic portion of the southwestern Japan subduction thrust

For coastal cities an important factor in earthquake hazard from subduction zone earthquakes is the landward extent of the seismogenic portion of the subduction thrust fault. In this study we test the hypothesis that the maximum downdip extent is defined by a critical temperature. We have developed a transient thermal model for the Nankai subduction zone of southwest Japan to allow comparison of the thermally estimated downdip extent of the seismogenic zone with that from (1) seismicity and tsunami data for two great subduction earthquakes, (2) the coseismic faulting extent of these events estimated from geodetic deformation data, and (3) the interseismic locked zone determined from interseismic geodetic data. The Nankai margin has extensive heat flow and heat production data to control thermal models and thus crustal temperatures. It has earthquake, tsunami, and geodetic data that constrain the coseismic rupture portion of the subduction thrust fault for past great earthquakes and the portion of the thrust fault that is locked and storing interseismic elastic strain. On the Nankaido margin off Shikoku Island, the thermal model indicates that a temperature of 350°C (taken to be the limit for seismic initiation from laboratory and field data) is reached on the subduction thrust fault 150 km from the trench. A transition zone into which rupture may extend with decreasing offset (taken to be 450°C) extends an additional 45 km downdip. The thermal model results are in excellent agreement with the maximum downdip extent of coseismic displacement for the 1946 Nankaido Ms = 8.2 earthquake off Shikoku Island and with the downdip extent of the present locked zone. In the region of the 1944 Tonankai Ms = 8.2 earthquake to the northeast, the subduction angle is much steeper and the thermal models indicate a narrower downdip seismogenic extent. The seismogenic-locked zone from earthquake and geodetic data is also narrower. Thus our analysis of the southwest Japan margin indicates that all three constraints on the downdip extent of the seismogenic zone, thermal, coseismic and interseismic geodetic data, are in general agreement. The study also supports the hypothesis that the seismogenic portion of subduction thrust faults is limited primarily by temperature. The thermal control implies that subduction thrust faults with shallow dip have wider seismogenic zones compared to those with steep dip. The subducting plate age and thus heat flow, and the thickness of the insulating sediments on the incoming plate, are also very important to the thermal regime and thus to the seismogenic width. The relation of the maximum seismic rupture area to the interseismic locked zone is particularly important for earthquake hazard estimation on subduction margins such as Cascadia where there have been no great historical events.

A seismic study of methane hydrate marine bottom simulating reflectors

Multichannel seismic reflection data have been analyzed from an area of clear bottom simulating reflectors (BSRs) on the northern Cascadia subduction zone margin off Vancouver Island. The reflector at a depth of about 300 m subbottom is interpreted to represent the base of a layer of methane hydrate or clathrate. The shallow water depth of 1300 m and the 3600-m-long hydrophone array have allowed BSR amplitude-versus-offset and high-resolution velocity analysis, as well as modelling of vertical incidence data. The results of all three types of analysis can be best explained by a 10 to 30-m-thick high-velocity layer located immediately above the BSR about 300 m below the seafloor, having a sharp base and transitional top. In the layer, about one third of the sediment pore spaces must be filled with hydrate “ice”. There is no seismically detectable free gas beneath the BSRs. These results put important constraints on models for the distribution and formation of BSR hydrate.

Subduction zone backarcs, mobile belts, and orogenic heat

Two important problems of continental tectonics may be resolved by recognizing that most subduction zone backarcs have hot, thin, and weak lithospheres over considerable widths. These are (1) the origin of long-lived active “mobile belts” contrasted to the stability of cratons and platforms, and (2) the origin of the heat of continental collision orogeny. At many continental margin plate boundaries, there are broad belts with a long history of distributed deformation. These regions are mobile because the lithosphere is sufficiently weak to

Hydrous minerals in the mantle wedge and the maximum depth of subduction thrust earthquakes

In many subduction zones the downdip limit of thrust earthquakes approximately coincides with the intersection of the subduction thrust with the forearc mantle. This limit may be explained by aseismic hydrous minerals present in the mantle wedge. During subduction, fluids released from the subducting slab infiltrate the overlying forearc mantle forming serpentine + brucite, especially in cool subduction zones. At the slab interface itself, talc-rich rocks form in the mantle by the addition of silica transported by rising fluids and by mechanical mixing of mantle and siliceous rocks. In the laboratory, serpentine generally exhibits stable-sliding aseismic behavior. The behavior of talc, a layered hydrous silicate, and brucite, a layered hydroxide, has not been investigated, but their structures also suggest weak stable-sliding behavior. We suggest all three layered hydrous minerals promote aseismic behavior and that their presence controls the downdip limit of thrust earthquakes in many subduction zones.

Case for very low coupling stress on the Cascadia Ssubduction Fault

A fundamental problem in plate tectonics is the shear strength of major plate boundary faults. This translates to the question whether the generally observed small earthquake stress drops of 3–10 MPa on major faults release most of the accumulated stress or only a small fraction of it. There is strong evidence that the San Andreas fault, a major transform plate boundary, is weak (<20 MPa shear resistance). It is not yet clear whether subduction thrust faults are also weak. We present two types of evidence from the northern Cascadia subduction zone that indicate very low coupling shear stress on that plate interface and hence very low strength of the subduction thrust fault, comparable to that estimated for the San Andreas fault. First, the well-defined surface heat flow and heat generation allow negligible frictional heating on the plate interface. The average shear stress on the fault must thus be very low over a time scale of a few million years. Second, focal mechanism solutions for small crustal earthquakes in the southern Vancouver Island area indicate that the horizontal stress in the direction of plate convergence has a similar magnitude to the vertical stress. This inferred stress state requires the present tectonic stress coupled across the subduction thrust fault to be very low. One explanation for the weakness of the fault is the presence of near-lithostatic pore fluid pressure in the region of the fault zone for which there is independent evidence. The conclusion of a weak subduction thrust fault does not conflict with geodetic observations of contemporary surface deformation which indicate that the fault is currently locked, accumulating strain energy toward a future great earthquake. The surface deformation responds to the small (<20 MPa) temporal changes of the stress field associated with the subduction earthquake cycle. This transient stress is superimposed on the larger background regional stress field in which the maximum compression is parallel to the margin. The weakness of the Cascadia subduction thrust fault and the unusual stress state of the forearc region have important implications for earthquake hazards. For example, a subduction earthquake may induce large strike-slip earthquakes in the forearc that affect a large area.