Subduction Shear Zone Research Articles

Effective Bulk Rheology of a Two‐Phase Subduction Shear Zone: Insights From Micromechanics‐Based Modeling and Implications for Subduction Interface Slow Slip Events

AbstractSubduction interfaces exhibit various slip styles, including slow slip events (SSEs). We use a micromechanics‐based approach to calculate the effective rheology of a shear zone containing ellipsoidal amphibolite clasts deforming by dislocation creep within an interconnected linear‐viscous phyllosilicate‐dominated matrix. Frictional failure occurs if local stress exceeds Mohr‐Coulomb yield strength. At moderate fluid overpressure, mixed‐frictional‐viscous behavior emerges at 350–560C, consistent with a broad zone of mixed fault slip behavior without requiring extreme fluid overpressures. Increasing stress in this transition zone promotes local frictional failure and raises bulk strain rate. If, however, the bulk strain rate increases by more than one order of magnitude, system‐wide frictional sliding becomes preferable. This strain rate increase is insufficient to explain the slip rates observed in geophysically detectable SSEs. Therefore, viscous matrix flow as modeled here cannot explain SSEs without either invoking dynamic weakening within a frictional‐viscous flow or a mechanism switch to dominantly frictional sliding.

Strength of viscous subduction interfaces: A global compilation

The long- and short-term dynamics of subduction zones are strongly affected by interface rheology. Current estimates of interface viscosity are based on endmember flow laws or simple mixing models, but the variation of possible materials on the subduction interface theoretically permits viscosity variations of up to five orders of magnitude. To better constrain the range of strength along deep/viscous subduction interfaces, we compiled a global database of rock types and block-and-matrix distributions in exhumed deep subduction interface mélange shear zones. We applied numerical shear experiments to each shear zone to quantify their bulk shear strengths/viscosities. Our results show that natural subduction shear zones have viscosities ranging from ∼1018 to 1020 Pa⋅s and shear strengths of ∼1−100 MPa. The variation among them is dominantly controlled by temperature, but even for shear zones deformed at the same temperature, variations of up to a factor of 50 are observed. These variations are attributed to differences in matrix composition, shear zone width, and block distributions. Our results are consistent with paleopiezometric and force-balance−derived estimates of interface strength, and they confirm the importance of variation in geological inputs and their spatial distribution along the interface for subduction dynamics.

Path and Slip Dependent Behavior of Shallow Subduction Shear Zones During Fluid Overpressure

AbstractElevated pore fluid pressure is proposed to contribute to slow earthquakes along shallow subduction plate boundaries. However, the processes that create high fluid pressure, disequilibrium compaction and dehydration reactions, lead to different effective stress paths in fault rocks. These paths are predicted by granular mechanics frameworks to lead to different strengths and deformation modes, yet granular mechanics do not predict their effects on fault stability. To evaluate the role of fluid overpressure on shallow megathrust strength and slip behavior, we conducted triaxial shear experiments on chlorite and celadonite rich gouge layers. Experiments were conducted at constant temperature (130 and 100°C), confining pressure (130 and 140 MPa), and pore fluid pressures (between 10 and 120 MPa). Fluid overpressure due to disequilibrium compaction was simulated by increasing confining and pore fluid pressure synchronously without exceeding the target effective pressure, whereas overpressure due to dehydration reactions was simulated by first loading the sample to a target isotropic effective pressure and then increasing pore fluid pressure to reduce the effective pressure. We find that the effects of fluid pressure and stress path on the mechanical behavior of the chlorite and celadonite gouges can generally be described using the critical state soil mechanics (CSSM) framework. However, path effects are more pronounced and persist to greater displacements in chlorite because its microstructure is more influenced by stress path. Due to its effects on microstructure, the stress path also imparts greater control on the rate‐dependence of chlorite strength, which is not predicted by CSSM.

Episodic fluid flow in an eclogite-facies shear zone: Insights from Li isotope zoning in garnet

Abstract Episodic fluid overpressure and escape is invoked as a cause or consequence of many subduction-zone seismic phenomena but can be challenging to constrain in exhumed high-pressure metamorphic rocks. In situ measurements of lithium isotopes in garnet reveal evidence of episodic fluid transport in a subduction shear zone now exposed in the Monviso ophiolite (Western Alps). Garnet from an eclogite block and associated metasomatic reaction rind was analyzed by secondary ion mass spectrometry (SIMS). All analyzed garnet preserves core-rim zoning in δ7Li and large negative δ7Li excursions (NEs) in mantles. These excursions cannot be explained by instrumental mass fractionation during analysis, equilibrium fractionation, or intracrystalline diffusion of Li within garnet. Instead, NEs were produced by kinetic fractionation of Li isotopes during bulk diffusion through a pore fluid, and the fractionated isotopic compositions were incorporated into garnet by syn-diffusion growth. Disequilibrium garnet growth textures associated with negative δ7Li support this interpretation and suggest metasomatism drove rapid garnet growth. Four distinct NEs were identified requiring that at least four pulses of fluid were transported within the adjacent shear zone. This evidence of episodic fluid transport along a subduction shear zone at eclogite facies supports models of intermediate-depth seismicity that rely on cyclic fluid overpressure and escape.

Structural and Thermal Evolution of an Infant Subduction Shear Zone: Insights From Sub-Ophiolite Metamorphic Rocks Recovered From Oman Drilling Project Site BT-1B.

Subduction interface thermal structure changes drastically within the first few million years of underthrusting (i.e., subduction infancy). Metamorphic soles beneath ophiolites record snapshots of dynamic conditions and mechanical coupling during subduction infancy. Beneath the Samail Ophiolite (Oman), the sole comprises structurally higher high‐temperature (HT) and lower low‐temperature (LT) units. This inverted metamorphic gradient has been attributed to evolving metamorphic Pressure‐Temperature (P‐T) conditions during infancy; however, peak P‐T and timing of LT sole subduction are poorly constrained. Oman Drilling Project core BT‐1B sampled the base of the ophiolite in a location lacking the HT sole. Metasedimentary and meta‐mafic samples collected from 104 m of core reveal that the LT sole subducted to similar peak P as HT rocks preserved elsewhere in Oman, but experienced ∼300°C lower peak T. Prograde fabrics record Si‐in‐phengite and amphibole chemistries consistent with peak P‐T of ∼7–10 kbar and ∼450–550°C in the epidote‐amphibolite facies. Retrograde fabrics record a transition from near‐pervasive ductile to localized brittle strain under greenschist facies conditions. Titanite U‐Pb ages (n = 2) constrain timing of peak LT sole subduction to ∼91 Ma (post‐dating initial HT sole subduction by ∼12–13 Myr) and dynamic retrogression through ∼90 Ma. Combined with existing geo/thermo‐chronology, our results support a model of protracted subduction and accretion while the infant subduction zone experienced multi‐phase, slow‐fast‐slow cooling. Temporal overlap of HT sole cooling (rehydration?) and ophiolite formation suggests that cooling may lead to interface weakening, facilitating upper‐plate extension and spreading. The LT sole formed in a rapidly‐refrigerating forearc after ophiolite formation and may reflect the transition to self‐sustaining subduction.

Effective rheology of a two-phase subduction shear zone: Insights from numerical simple shear experiments and implications for subduction zone interfaces

Inspired by exhumed subduction shear zones that commonly exhibit block-in-matrix characteristics (mélanges), we create synthetic models with different proportions of strong clasts within a weak matrix and compare them to natural mélange outcrops. Using 2D Finite Element visco-plastic numerical simulations in simple shear kinematic conditions, we determine the effective rheological parameters of such a two-phase medium, comprising blocks of basalt embedded within a wet quartzitic matrix. We treat our models and their structures as scale-independent and self-similar and upscale published field geometries to km-scale models, compatible with large-scale far-field observations. Like in the laboratory deformation experiments, we vary confining pressures, temperatures and strain rates to evaluate effective rheological estimates for a natural subduction interface. As expected, the block-in-matrix ratio affects deformation and strain localization, with the effective dislocation creep parameters (A, n, and Q) varying between the values of the strong and the weak phase, in cases where both materials deform by viscous creep. At conditions where the blocks are frictional and the matrix viscous, the mélange deforms effectively by dislocation creep but the locally frictional deformation of the blocks leads to higher stresses. This results in an effective value of the stress exponent, n, greater than that of both pure phases, as well as an effective rate-dependent viscosity lower than that of the weak phase. In combination with an appropriate evolution law for the block concentration of a mélange, our effective rheology parameters may be used in large scale geodynamic models as a proxy for a heterogeneous subduction interface.

Cascadia low frequency earthquakes at the base of an overpressured subduction shear zone

In subduction zones, landward dipping regions of low shear wave velocity and elevated Poisson’s ratio, which can extend to at least 120 km depth, are interpreted to be all or part of the subducting igneous oceanic crust. This crust is considered to be overpressured, because fluids within it are trapped beneath an impermeable seal along the overlying inter-plate boundary. Here we show that during slow slip on the plate boundary beneath southern Vancouver Island, low frequency earthquakes occur immediately below both the landward dipping region of high Poisson’s ratio and a 6–10 km thick shear zone revealed by seismic reflections. The plate boundary here either corresponds to the low frequency earthquakes or to the anomalous elastic properties in the lower 3–5 km of the shear zone immediately above them. This zone of high Poisson’s ratio, which approximately coincides with an electrically conductive layer, can be explained by slab-derived fluids trapped at near-lithostatic pore pressures.

Fracture and Weakening of Jammed Subduction Shear Zones, Leading to the Generation of Slow Slip Events

AbstractGeodetic data have revealed that parts of subduction interfaces creep steadily or transiently. Transient slow slip events (SSEs) are typically interpreted as aseismic frictional sliding. However, SSEs may also occur via mixed viscobrittle deformation, as observed in shear zones containing mixtures (mélange) of strong fractured clasts embedded in a weak viscobrittle matrix. We test the hypothesis that creep in a subduction mélange occurs through distributed matrix deformation, where flow is intermittently impeded by load‐bearing clast networks (jamming). Our numerical models demonstrate that bulk mélange rheology can be dominated by the strong clasts in the absence of fracturing, while at high driving stresses or low frictional strength, clast fracturing redistributes deformation into the matrix, leading to high bulk strain rates. Because mélange stress is heterogeneous, fracturing of clasts occurs throughout jammed mélange when the driving stresses are only ∼20% of the clast yield strength. The effective rheology of jammed mélange can be characterized by a logarithmic dependence of stress on strain rate, which is used to explore strain rate transients caused by temporal variation in clast friction. Clasts must weaken significantly (∼75%) in order to increase strain rate by 8 times. Spring‐block slider models with a more idealized viscobrittle rheology also demonstrate that strain rate transients can be generated when rate‐and‐state friction is incorporated. We outline a model where high bulk strain rates are generated when pervasive fracturing occurs, but further slip is limited by viscous processes. Incorporating such viscous damping into models may widen the conditions under which SSEs can occur while preventing development of seismic slip.

A viscoplastic shear-zone model for deep (15–50 km) slow-slip events at plate convergent margins

A key issue in understanding the physics of deep (15–50 km) slow-slip events (D-SSE) at plate convergent margins is how their initially unstable motion becomes stabilized. Here we address this issue by quantifying a rate-strengthening mechanism using a viscoplastic shear-zone model inspired by recent advances in field observations and laboratory experiments. The well-established segmentation of slip modes in the downdip direction of a subduction shear zone allows discretization of an interseismic forearc system into the (1) frontal segment bounded by an interseismically locked megathrust, (2) middle segment bounded by episodically locked and unlocked viscoplastic shear zone, and (3) interior segment that slips freely. The three segments are assumed to be linked laterally by two springs that tighten with time, and the increasing elastic stress due to spring tightening eventually leads to plastic failure and initial viscous shear. This simplification leads to seven key model parameters that dictate a wide range of mechanical behaviors of an idealized convergent margin. Specifically, the viscoplastic rheology requires the initially unstable sliding to be terminated nearly instantaneously at a characteristic velocity, which is followed by stable sliding (i.e., slow-slip). The characteristic velocity, which is on the order of <10−7 m/s for the convergent margins examined in this study, depends on the (1) effective coefficient of friction, (2) thickness, (3) depth, and (4) viscosity of the viscoplastic shear zone. As viscosity decreases exponentially with temperature, our model predicts faster slow-slip rates, shorter slow-slip durations, more frequent slow-slip occurrences, and larger slow-slip magnitudes at warmer convergent margins.

Splay fault branching from the Hikurangi subduction shear zone: Implications for slow slip and fluid flow

AbstractPrestack depth migration data across the Hikurangi margin, East Coast of the North Island, New Zealand, are used to derive subducting slab geometry, upper crustal structure, and seismic velocities resolved to ∼14 km depth. We investigate the potential relationship between the crustal architecture, fluid migration, and short‐term geodetically determined slow slip events. The subduction interface is a shallow dipping thrust at &lt;7 km depth near the trench and steps down to 14 km depth along an ∼18 km long ramp, beneath Porangahau Ridge. This apparent step in the décollement is associated with splay fault branching and coincides with a zone of maximum slip (90 mm) inferred on the subduction interface during slow slip events in June and July 2011. A low‐velocity zone beneath the plate interface, updip of the plate interface ramp, is interpreted as fluid‐rich overpressured sediments capped with a low permeability condensed layer of chalk and interbedded mudstones. Fluid‐rich sediments have been imbricated by splay faults in a region that coincides with the step down in the décollement from the top of subducting sediments to the oceanic crust and contribute to spatial variation in frictional properties of the plate interface that may promote slow slip behavior in the region. Further, transient fluid migration along splay faults at Porangahau Ridge may signify stress changes during slow slip.

Cascadia subducting plate fluids channelled to fore‐arc mantle corner: ETS and silica deposition

AbstractIn this study we first summarize the constraints that on the Cascadia subduction thrust, there is a 70 km gap downdip between the megathrust seismogenic zone and the Episodic Tremor and Slip (ETS) that lies further landward; there is not a continuous transition from unstable to conditionally stable sliding. Seismic rupture occurs mainly offshore for this hot subduction zone. ETS lies onshore. We then suggest what does control the downdip position of ETS. We conclude that fluids from dehydration of the downgoing plate, focused to rise above the fore‐arc mantle corner, are responsible for ETS. There is a remarkable correspondence between the position of ETS and this corner along the whole margin. Hydrated mineral assemblages in the subducting oceanic crust and uppermost mantle are dehydrated with downdip increasing temperature, and seismic tomography data indicate that these fluids have strongly serpentinized the overlying fore‐arc mantle. Laboratory data show that such fore‐arc mantle serpentinite has low permeability and likely blocks vertical expulsion and restricts flow updip within the underlying permeable oceanic crust and subduction shear zone. At the fore‐arc mantle corner these fluids are released upward into the more permeable overlying fore‐arc crust. An indication of this fluid flux comes from low Poisson's Ratios (and Vp/Vs) found above the corner that may be explained by a concentration of quartz which has exceptionally low Poisson's Ratio. The rising fluids should be silica saturated and precipitate quartz with decreasing temperature and pressure as they rise above the corner.

Channelized Fluid Flow and Eclogite-facies Metasomatism along the Subduction Shear Zone

Channelized Fluid Flow and Eclogite-facies Metasomatism along the Subduction Shear Zone

Numerical modeling of frontal and basal accretion at collisional margins

We investigate the deformation of orogenic wedges that form in the early stages of continent‐continent collisions using a two‐dimensional numerical model limited to the upper lithosphere. Our models show that deformation at the plate margins is influenced by rheology, surface processes, and the balance between inward mass flux and outward subduction flux, as controlled by the subduction load (which represents the effects of slab pull and resistive forces) and flexural downbending. We find three characteristic deformation modes: (1) near‐pure subduction with little or no accretion; (2) frontal accretion with development of an accretionary wedge built up by offscraping of the sediment layer at shallow depth; and (3) independent frontal and basal accretion where a retrothrust allows stacking of basement nappes at crustal to mantle depths. Near‐pure subduction is enabled for “ordinary‐rheology” materials, characterized by brittle and viscous material behavior (approximating a “Christmas tree‐type” depth profile), and almost zero friction along the subduction shear zone. Frontal accretion occurs when slightly increased friction along the subduction shear zone allows offscraping of the sediment layer from the subducting plate. Independent frontal and basal accretion develops in strong‐rheology models with an almost fully brittle material behavior. Major surface erosion or a reduction of the subduction load promote the development of large basement nappes. Frontal accretion is favored by major sedimentation during convergence, a large backstop, and in the case of a lateral transition from a “strong‐rheology” to an “ordinary‐rheology” subducting plate. Our numerical models develop first‐order characteristics as observed in natural orogenic wedges, for example upper crustal nappe stacks, frontal and basal accretion, or extension in the core of an orogen. Frontal and basal accretion are interdependent, and tend to stabilize the subduction system.

Nonlithostatic pressure during sediment subduction and the development and exhumation of high pressure metamorphic rocks

A subduction shear zone can be modeled as a long narrow channel, with the flow of subducted metasedimentary rocks in the channel driven by two sets of forces: the downward shearing force exerted by the subducting slab and the gradient in the hydraulic potential, which combines the effect of both pressure and buoyancy. If the channel walls are effectively rigid, very slight narrowing or broadening of the channel (convergence angles &lt;1°) can result in very dramatic changes in the (nonlithostatic) pressure distribution along the channel. The geometry of the subducting plate, which is forced to bend under the overriding plate, suggests that the channel should initially narrow downward and then gradually broaden. A model assuming this geometry, with initial channel width 1500 m, minimum width ∼600 m and width at 100 km depth of again ∼1500 m, a maximum viscosity of 1019 Pa s, and a convergence rate of 8 cm/yr reaches pressures &gt;2 GPa in the channel at only 40 km depth. The model is consistent with a horizontal balance of forces across the plates and with a reasonable value for the thickness of subducted sediment (∼650 m). The practical limit for overpressures attainable in subduction zones is determined by the strength and permeability of the channel walls. At 40 km depth the channel is effectively confined on both sides by cold lithospheric mantle, which should be strong enough to support a significant tectonic overpressure. Episodic failure of the upper plate to produce great earthquakes at 30–40 km focal depth could vent overpressured fluid from the channel, allowing a cyclical buildup and release of both rock and fluid pressure. Topography on the subducting plate (e.g., seamounts and thinned continental crust) may lead to an anvil‐like jamming of the channel and local high overpressures. Tectonic erosion by topography on the lower plate of slivers from overlying continental crust and the compression of these slivers between the topography and the narrowing channel walls could produce high overpressures in continental rocks. A decrease in the convergence rate or cessation of subduction, with a consequent general warming within the channel and associated viscosity decrease, promotes exhumation by buoyant reverse flow. The most rapid reverse flow occurs in the region of previously greatest overpressure. Since the exhumation distance is shorter than for a simple lithostatic pressure distribution and any increase in temperature is coupled with a strong increase in the rate of exhumation, preservation of high‐pressure assemblages at the surface in fossil subduction zones is promoted for such a model.

Blueschist‐facies metamorphism, shear heating, and P‐T‐t paths in subduction shear zones

The low temperatures recorded by blueschist‐facies metamorphic rocks place upper bounds on the magnitude of shear stresses in subduction zones at depths of 15–50 km. In this paper, steady state and transient pressure‐temperature‐time (P‐T‐t) paths followed by subducted material are calculated using analytical expressions presented by Molnar and England (1990), supplemented by two‐dimensional numerical calculations. In the absence of shear heating, calculated steady state P‐T‐t paths are very cold, intersecting 1 GPa (10 kbar) at T &lt; 200°C for a wide range of subduction parameters. For the case of constant shear stress (τ) in the subduction shear zone and typical thermal parameters, steady state P‐T‐t paths intersect the blueschist facies for τ = 10–60 MPa and 0–100 MPa for convergence rates of 10 and 3 cm yr−1, respectively. Blueschist‐facies metamorphism will not occur in rapid subduction zones if shear stresses exceed 60 MPa. For the case of shear stress increasing linearly with depth, steady state P‐T‐t paths intersect the blueschist facies when τ = 1–9% P and 0–14% P for convergence rates of 10 and 3 cm yr−1, respectively. Subduction zone shear stresses may be considerably lower, and possibly zero, if blueschist terrains represent material metamorphosed during the early stages of subduction (first 5–10 m.y.) while the thermal structure is still evolving or if maximum temperatures in blueschist terrains are achieved during exhumation. Subduction zone shear stresses of several tens of MPa (at 35 km depth) or a few percent of lithostatic pressure are consistent with a steady state model of Franciscan metamorphism, a progressive underplating model for the formation of the Pelona Schist inverted metamorphic gradient, and surface heat flow measurements in the NE Japan subduction zone.

Thermal effects of metamorphic fluids in subduction zones

In subduction zones, large volumes of fluid are released from the subducting slab during prograde metamorphism of hydrated oceanic crust. Once released, metamorphic fluids tend to migrate upward owing to buoyancy. Fluid migration can affect the thermal structure of a subduction zone by (1) advecting heat and (2) causing retrograde (hydration, carbonation) reactions in the hanging wall. Under most P - T conditions, the volumetric heat capacities of H 2 O and rocks are similar; therefore, the thermal effects of fluid migration and uplift are comparable. Calculations suggest that advective heat transport by fluid flow causes relatively minor thermal effects. Pervasive upward fluid flow results in thermal effects analogous to regional uplift rates on the order of 0.2 mm/yr. Fluid flow channelized along the subduction shear zone translates the local thermal structure of the shear zone updip at ∼1 mm/yr. A more important thermal effect results from exothermic retrograde reactions in the hanging wall that release ∼30 kJ/mol of H 2 O consumed. Retrograde reactions significantly retard cooling of the hanging wall just as prograde reactions retard warming of the subducting slab.

On the origin of orogenic volcanism

It is widely believed that the explanation of orogenic volcanism lies in the shear heating and subsequent partial melting of the crustal fraction of subducted lithosphere. Careful examination of simple models of the shear-heating process indicates, however, that this process cannot occur in a system with constant strain-rate boundary conditions, for reasonable values of the strain rate and material parameters. Thermal runaway cannot occur because viscous dissipation process is limited by the effect of temperature on viscosity, although shear-heating instability may develop during non-steady-state shear at stress or strain-rate values above certain critical values. Examination of subduction history models indicates that supercritical shear stresses constant over some tens of kilometers may be found in subduction shear zones. Since instability may develop if such conditions are present over even a 1–10 km range, it is still possible that non-steady-state process could explain the origin of orogenic volcanism.

Orogenic volcanism caused by thermal runaways?

Summary. Quartz eclogite as source rock seems capable of explaining the chemistry of calc-alkaline volcanics. This model requires partial melting of quartz eclogite in the depth range 100–200 km. Shear heating of the subducting crust is one of the most debated models. The problem may be treated with two different boundary conditions: constant strain rate or constant shear stress. In the former case an increase in temperature tends to reduce the heat production thus stabilizing the shear flow; the result is a moderate increase in temperature which will remain constant during flow. The latter case may lead to a thermal feedback instability and thus temperature and strain rate may suddenly grow to very high values. This phenomenon is termed a runaway and will be discussed in this paper using an adiabatic approximation. It is shown that for a runaway to occur the local energy density must amount to a common value independent of the rheology. In contrast to the constant strain rate case, shear heating is negligible until just before the instability occurs. When the melting point is reached shear stress will break down but the stored local energy will be set free and supply the latent heat of melting. The possibility of a runaway occurring is strongly dependent on the ambient temperature. In subduction shear zones neither shear stress nor strain rate are likely to be constant throughout, but if the former is constant or changes little on a 1–10 km scale a runaway is liable to occur at a depth of around 150 km thus possibly being the cause of calcalkaline volcanism.

Stress and temperature in subduction shear zones: Tonga and Mariana

Summary. Because there is secondary sea-floor spreading in the Tonga and Mariana subduction systems, the island arcs are separate plates. Horizontal forces on the two sides of the arc must balance, and the maximum force on the back-arc side can be calculated from a lithostatic ridge model. This, in combination with gravity data, allows calculation of the average shear stress in the top 100 km of the subduction shear zone. Stress in Tonga is 220±100 bar, and in the Mariana it is 165±75 bar. These low stresses are probably made possible by a fluid pore pressure almost equal to the least compressive stress. Knowledge of stress allows approximate calculation of temperature in the shear zone by integration of a single differential equation. These temperatures are too low to activate most dehydration reactions in the subducted crust. As it approaches the volcanic line, this crust is at 150–350°C in Tonga and 150–300°C in the Mariana. Shear melting of the crust is ruled out, and conductive melting of the slab by contact with the asthenosphere meets with geochemical objections. Magmas in these systems are probably produced by partial melting of asthenosphere, triggered by a sudden release of water from the slab.