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Oceanic Transform Fault Seismicity and Slip Mode Influenced by Seawater Infiltration

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Oceanic Transform Fault Seismicity and Slip Mode Influenced by Seawater Infiltration ARTICLES https://doi.org/10.1038/s41561-021-00778-1 Oceanic transform fault seismicity and slip mode influenced by seawater infiltration Arjun Kohli 1 ✉ , Monica Wolfson-Schwehr 2, Cécile Prigent3 and Jessica M. Warren 4 ✉ Oceanic transform faults that offset mid-ocean ridges slip through earthquakes and aseismic creep. The mode of slip var- ies with depth and along strike, with some fault patches that rupture in large, quasi-periodic earthquakes at temperatures <600 °C, and others that slip through creep and microearthquakes at temperatures up to 1,000 °C. Rocks from both fast- and slow-slipping transforms show evidence of interactions with seawater up to temperatures of at least 900 °C. Here we present a model for the mechanical structure of oceanic transform faults based on fault thermal structure and the impacts of hydration and metamorphic reactions on mantle rheology. Deep fluid circulation is accounted for in a modified friction-effective pressure law and in ductile flow laws for olivine and serpentine. Combined with observations of grain size reduction and hydrous min- eralogy from high-strain mylonites, our model shows that brittle and ductile deformation can occur over a broad temperature range, 300–1,000 °C. The ability of seawater to penetrate faults determines whether slip is accommodated at depth by seismic asperities or by aseismic creep in weak, hydrous shear zones. Our results suggest that seawater infiltration into ocean trans- form faults controls the extent of seismicity and spatiotemporal variations in the mode of slip. lobal studies of seismicity1–3 and deformation experiments OTF mylonites on olivine4 suggest that 600–700 °C is the thermal limit for Mantle mylonites have been dredged from numerous OTFs Gearthquake nucleation on oceanic transform faults (OTFs). (Fig. 1a). The presence of syn-deformational hydrous phases However, recent ocean-bottom seismometer (OBS) deployments in fine-grained shear zones within mylonites indicates that on fast- and intermediate-slipping transforms have located micro- they formed under hydrous conditions (Fig. 1c). We used the earthquakes in the mantle at temperatures up to 1,000 °C in mod- hydrous mineralogy to classify mylonites in terms of the tem- elled thermal structures5–8. The mode of slip is also observed to vary perature of deformation13. Low-temperature (LT) mylonites along-strike, with some fault patches hosting large, quasi-periodic contain amphibole, chlorite and serpentine. Medium-temperature earthquakes while others arrest the propagation of large ruptures and (MT) mylonites contain both amphibole and chlorite. slip through intense swarms of deep microseismicity5. Rupture barrier High-temperature (HT) mylonites contain amphibole as the only zones show low seismic velocities and high ratios of compressional- hydrous phase. LT and HT mylonites with similar characteristics to shear-wave velocities indicative of high porosity, which suggests (syn-deformational hydrous phases and very fine grain size com- a causal link between hydrological properties and the seismogenic pared to that of abyssal peridotites) have been recovered from the behaviour of the lithosphere9,10. Earthquake cycle models demon- fastest- (Garrett) and slowest-slipping (Shaka) transform faults strate that increased dilatancy in these regions can account for obser- (Supplementary Table 1). The high chlorine content of hydrous vations of slow slip and arrest of large ruptures, but do not explain minerals in SWIR mylonites (up to 1 wt%) indicates that the fluid variations in the vertical extent of microseismicity or along-strike source was seawater (Fig. 1e). variations in the mode of slip at temperatures >600 °C (ref. 11). The temperature during deformation of LT, MT and HT Deformed mantle rocks have been dredged from OTFs that span mylonites can be inferred from the stability fields of hydrous min- a wide range of slip rates (Fig. 1a) and provide constraints on the erals. However, the depth of deformation cannot be directly esti- conditions and mechanisms of fault slip. In particular, high-strain mated from mineralogy, as none of the mineral compositions are mylonites contain syn-deformational hydrous phases, which signify pressure sensitive. We therefore used fault thermal models to con- that fluids were present during ductile deformation12,13. Analysis of vert temperature constraints into pressure (depth) on the fault (Fig. mylonites from ultra-slow slipping faults on the Southwest Indian 2a,b and Extended Table 1). Three-dimensional thermal models Ridge (SWIR) indicates that fluids are derived from seawater and were solved for the flow field and thermal structure of the Shaka that fluid–rock interactions occur up to at least 900 °C. In this study, and Gofar transform faults (Methods). We estimated the tem- we used temperature constraints from the mylonites in combina- perature on the basis of orthopyroxene composition (Fig. 2c) and tion with numerical models of fault thermal structures to construct compared geotherms from the centre of each fault with the experi- rheological profiles of OTFs that incorporate deep seawater circula- mentally derived upper stability limits of hydrous minerals14–17 to tion and fluid-deformation feedbacks. We then used these models place bounds on the pressure (depth). We designated the LT, MT to interpret the OBS observations5–8 of along-strike variations in the and HT mylonite regions based on the hydrous mineralogy and extent of seismicity and the mode of slip in terms of fault mechani- used these regions as the bounds on the pressure–temperature con- cal properties. ditions of deformation. 1Department of Geophysics, Stanford University, Stanford, CA, USA. 2Center for Coastal and Ocean Mapping, University of New Hampshire, Durham, NH, USA. 3Institut de Physique du Globe de Paris, Université de Paris, CNRS, Paris, France. 4Department of Earth Sciences, University of Delaware, Newark, DE, USA. ✉e-mail: [email protected]; [email protected] 606 NatURE GEoscIENCE | VOL 14 | AUGUST 2021 | 606–611 | www.nature.com/naturegeoscience NATURE GEOSCIENCE ARTICLES a 60 N 60 N AAR EPR JDF Blanco Gibbs 30 N 30 MAR PAR Kane SWIR Quebrada Vema 0 Discovery SPR Chain 0 Gofar Romanche Garrett Terevaka DII Gallieni 30 S Marion 30 S Islas Orcadas Pr. Edward Conrad Shaka Udinstev 60 S 60 S b c d Chr Opx Amph Ol Amph Chl Shear zone Serp Serpentine 5 cm e LT mylonites MT mylonites Amph HT mylonites N = 19; mean = 790 Chl Serp ) –1 g g ) μ N = 2; mean = 340 –1 g LT Amph g μ N = 7; mean = 98 Chl d Amph-rich N = 24; mean = 430 Seawater (20,000 shear zones MT Amph Anhydrous mantle (0.1-0.3 N = 23; mean = 3,900 HT Amph 10–1 100 101 102 103 104 –1 Mineral Cl concentration (μg g ) 1 mm 100 μm Fig. 1 | Map and microstructures of OTF mylonites. a, Global map of OTFs where mylonites have been recovered (circles) and OBS have been deployed (triangles). Bolded faults are discussed in the main text. AAR, American-Antarctic Ridge; MAR, Mid-Atlantic Ridge; PAR, Pacific-Antarctic Ridge. b, Serpentinized peridotite mylonite from the fast-slipping Garrett transform fault, EPR. c, Photomicrographs (plane polarized light) of HT/MT/LT mylonites from the slow-slipping Shaka transform fault12. d, Electron backscatter diffraction phase map of fine-grained, amphibole-rich bands within a HT mylonite12. e, Chlorine concentrations in hydrous minerals in Shaka and Prince Edward mylonites. Amph, amphibole; Chl, chlorite; Ol, olivine; Opx, orthopyroxene; Serp, serpentine. NatURE GEoscIENCE | VOL 14 | AUGUST 2021 | 606–611 | www.nature.com/naturegeoscience 607 ARTICLES NATURE GEOSCIENCE a b SWIR Shaka Transform Fault (12.9 mm yr–1) SWIR EPR Gofar 3 (124.3 mm yr–1) ITSC Gofar 2 T (°C) 0 0 0 Shaka SWIR SWIR 5 5 600 °C 50 km 200 10 10 1,200 °C 400 15 600 °C 15 600 20 20 Depth (km) 800 25 25 1,000 30 30 2 35 35 Gofar 3 1,200 1,200 °C 50 km EPR 40 40 0 50 100 150 200 –50 0 100 150 Distance along profile (km) Distance along profile (km) c 0 0 5 Serp Chl Gofar Amph 0.2 10 LT 15 MT Depth (km) 0.4 Pressure (GPa) 20 HT 25 Shaka 0.6 30 15 10 5 Samples 0 400 500 600 700 800 9000 1,000 1,100 Temperature/Deformation temperature from thermometry (°C) Fig. 2 | Thermal models and hydrous mineralogy of OTFs. a,b, Shaka transform fault (slip rate 12.9 mm yr–1) (a) and Gofar segments 2 and 3 (slip rate 124.3 mm yr–1) (b). Profiles run parallel to the ridge–fault–ridge system (insets) (see Extended Data Table 1 for model parameters). c, Top: fault centre geotherms and hydrous mineral stability limits for amphibole, chlorite and serpentine, which distinguish the LT, MT and HT mylonites. Bottom: thermometry calculations based on the calcium in orthopyroxene (Opx) in the SWIR mylonites13 constrain the LT limit of ductile recrystallization. Fault rheology and fluid-deformation feedbacks and 2). The flow-law equations and parameters are provided in The mechanisms that govern ductile deformation in OTF mylonites Extended Data Table 2. For both faults, the protolith is expected to and their protolith can be interpreted from the mylonite micro- deform by a combination of dislocation creep and grain boundary structures12. Prior to deformation in the fault zone, the mantle pro- sliding at strain rates of ~10−9–10–11 s–1. tolith was assumed to be coarse-grained peridotite formed by melt Once fluids are introduced into the fault zone, weak, fine-grained extraction within the asthenosphere (for example, Fig. 3a insets). shear zones can form via a positive feedback loop. The presence of This starting point corresponds to the pressure (depth) at which the fluids weakens olivine, which increases the strain rate and decreases fault is at the mantle potential temperature in the thermal models the grain size21–24.
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