Low‐temperature frictional characteristics of chlorite‐epidote‐amphibole assemblages: implications for strength and seismic style of retrograde fault zones

In retrograde faults exhuming mafic rocks, shearing occurs in metamorphic and/or hydrothermally altered mineral assemblages whose frictional properties are not well known. Here, we present the results of laboratory shearing experiments on chlorite schist, epidotite, and hornblende‐dominated amphibolite and mixtures of these rocks and evaluate their frictional properties and microstructures. The experiments were conducted on powdered rock samples with starting grain size of <125 μm, at room temperature, under fluid‐saturated conditions and applied normal stress of 10 MPa. The results show that chlorite schist is relatively weak (friction coefficient of 0.36), whereas epidotite and amphibolite are strong (friction coefficients of 0.63 and 0.67, respectively). The friction of chlorite schist‐epidotite and chlorite schist‐amphibolite mixtures decreases nearly linearly with increasing chlorite content. Chlorite schist exhibits velocity‐strengthening behavior, epidotite is velocity‐weakening, and the amphibolite shows mostly velocity‐weakening friction. Mixtures show intermediate strength and velocity dependence of friction. Well‐developed striations formed on slip surfaces in samples with ≥50% chlorite schist. The epidotite slip surface exhibits a mixture of very fine particles and coarser crystals. Amphibolite slip surfaces have less very fine grains and are composed of subhedral to euheral needles. Few intragranular fractures are preserved, and we infer wear at contact asperities to be the likely cause of velocity‐weakening in our epidote gouges. Addition of chlorite to epidotite and amphibolite produces a striated slip surface and disrupts contacts between harder grains. Therefore, retrograde chlorite growth is expected to facilitate frictional weakening and stable slip in higher‐grade mineral assemblages exhumed to low‐temperature conditions

Shear behavior of DFDP-1 borehole samples from the Alpine Fault, New Zealand, under a wide range of experimental conditions

The Alpine Fault is a major plate-boundary fault zone that poses a major seismic hazard in southern New Zealand. The initial stage of the Deep Fault Drilling Project has provided sample material from the major lithological constituents of the Alpine Fault from two pilot boreholes. We use laboratory shearing experiments to show that the friction coefficient µ of fault-related rocks and their precursors varies between 0.38 and 0.80 depending on the lithology, presence of pore fluid, effective normal stress, and temperature. Under conditions appropriate for several kilometers depth on the Alpine Fault (100 MPa, 160 °C, fluid-saturated), a gouge sample located very near to the principal slip zone exhibits µ = 0.67, which is high compared with other major fault zones targeted by scientific drilling, and suggests the capacity for large shear stresses at depth. A consistent observation is that every major lithological unit tested exhibits positive and negative values of friction velocity dependence. Critical nucleation patch lengths estimated using representative values of the friction velocity-dependent parameter a−b and the critical slip distance D c , combined with previously documented elastic properties of the wall rock, may be as low as ~3 m. This small value, consistent with a seismic moment M o = ~4 × 1010 for an M w = ~1 earthquake, suggests that events of this size or larger are expected to occur as ordinary earthquakes and that slow or transient slip events are unlikely in the approximate depth range of 3–7 km

Frictional characteristics of oceanic transform faults: progressive deformation and alteration controls seismic style

Oceanic transform faults are inferred to be weak relative to surrounding oceanic crust and primarily slip aseismically. Neither their weakness nor tendency to creep are well-explained. We test the effects of fault-rock evolution on oceanic transform fault frictional strength and stability using direct-shear experiments (at room temperature, 10 MPa normal stress, and fluid-saturated conditions) on dolerite from the East Pacific Rise and natural fault rocks from the exhumed Southern Troodos Transform, Cyprus. Dolerites and cemented breccias are frictionally strong (μ = 0.52–0.85) and velocity-weakening (strength decreases with increasing slip velocity, characteristic of earthquakes). In contrast, matrix-rich chlorite-bearing fault breccias and gouges are frictionally weak (μ = 0.25–0.48) and velocity-strengthening (characteristic of stable creep). This transition implies that seismic behavior is controlled by degree of damage and alteration, such that earthquakes can nucleate within relatively intact oceanic crust, whereas fault segments of increased damage and chlorite content tend to slip aseismically

Mixed brittle and viscous strain localisation in pelagic sediments seaward of the Hikurangi margin, New Zealand

Calcareous‐pelagic input sediments are present at several subduction zones and deform differently to their siliciclastic counterparts. We investigate deformation in calcareous‐pelagic sediments drilled ~20 km seaward of the Hikurangi megathrust toe at Site U1520 during IODP Expeditions 372 and 375. Clusters of normal faults and subhorizontal stylolites in the sediments indicate both brittle faulting and viscous pressure solution operated at 150°C where frictional (possibly seismic) slip likely predominates

Spectrum of slip behaviour in Tohoku fault zone samples at plate tectonic slip rates

During the 2011 Tohoku-oki earthquake, extremely extensive coseismic slip ruptured shallow parts of the Japan Trench subduction zone and breached the sea floor1, 2. This part of the subduction zone also hosts slow slip events (SSE)3, 4. The fault thus seems to have a propensity for slip instability or quasi-instability that is unexpected on the shallow portions of important fault zones. Here we use laboratory experiments to slowly shear samples of rock recovered from the Tohoku-oki earthquake fault zone as part of the Japan Trench Fast Drilling Project. We find that infrequent perturbations in rock strength appear spontaneously as long-term SSE when the samples are sheared at a constant rate of about 8.5 cm yr−1, equivalent to the plate-convergence rate. The shear strength of the rock drops by 3 to 6%, or 50 kPa to 120 kPa, over about 2 to 4 h. Slip during these events reaches peak velocities of up to 25 cm yr−1, similar to SSE observed in several circum-Pacific subduction zones. Furthermore, the sheared samples exhibit the full spectrum of fault-slip behaviours, from fast unstable slip to slow steady creep, which can explain the wide range of slip styles observed in the Japan Trench. We suggest that the occurrence of SSE at shallow depths may help identify fault segments that are frictionally unstable and susceptible to large coseismic slip propagation

Frictional Behavior of Input Sediments to the Hikurangi Trench, New Zealand

AbstractThe Hikurangi subduction zone hosts shallow slow‐slip events, possibly extending to the seafloor. The mechanisms allowing for this behavior are poorly understood but are likely a function of the frictional properties of the downgoing seafloor sediments. We conducted friction experiments at a large range of effective stresses, temperatures, and velocities on incoming sediment to the Hikurangi subduction zone to explore the possible connection of frictional properties to slow‐slip events. These experiments were conducted on multiple apparatuses, allowing us to access a wider range of deformation conditions than is available on any one machine. We find that the material frictionally weakens and becomes less velocity strengthening with increasing effective stress, whereas temperature has only a small effect on both friction and frictional stability. When driven at the plate convergence rate, the sediment exhibits velocity‐weakening behavior. These results imply that the frictional properties of the sediment package subducting at Hikurangi could promote slow‐slip events at the pressures, temperatures, and strain rates expected along the plate boundary thrust up to 10‐km depth without requiring elevated pore fluid pressures. The transition to velocity‐strengthening behavior at faster slip rates could provide a mechanism for limiting unstable slip to slow‐sliding velocities, rather than accommodating deformation through ordinary earthquakes

Evidence of seismic slip on a large splay fault in the Hikurangi subduction zone

The Hikurangi subduction zone is capable of producing moderate to large earthquakes as well as regularly repeating slow slip events. However, it is unclear what structures host these different slip styles along the margin. Here we address whether splay faults can host seismic slip at shallow (1 m as observed in the 1947 Poverty and Tolaga Bay earthquakes

Shear behavior of DFDP-1 borehole samples from the Alpine Fault, New Zealand, under a wide range of experimental conditions

The Alpine Fault is a major plate-boundary fault zone that poses a major seismic hazard in southern New Zealand. The initial stage of the Deep Fault Drilling Project has provided sample material from the major lithological constituents of the Alpine Fault from two pilot boreholes. We use laboratory shearing experiments to show that the friction coefficient µ of fault-related rocks and their precursors varies between 0.38 and 0.80 depending on the lithology, presence of pore fluid, effective normal stress, and temperature. Under conditions appropriate for several kilometers depth on the Alpine Fault (100 MPa, 160 °C, fluid-saturated), a gouge sample located very near to the principal slip zone exhibits µ = 0.67, which is high compared with other major fault zones targeted by scientific drilling, and suggests the capacity for large shear stresses at depth. A consistent observation is that every major lithological unit tested exhibits positive and negative values of friction velocity dependence. Critical nucleation patch lengths estimated using representative values of the friction velocity-dependent parameter a−b and the critical slip distance D c , combined with previously documented elastic properties of the wall rock, may be as low as ~3 m. This small value, consistent with a seismic moment M o = ~4 × 1010 for an M w = ~1 earthquake, suggests that events of this size or larger are expected to occur as ordinary earthquakes and that slow or transient slip events are unlikely in the approximate depth range of 3–7 km

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