Length scales and types of heterogeneities along the deep subduction interface: Insights from exhumed rocks on Syros Island, Greece

Abstract

We use structural and microstructural observations from exhumed subduction-related rocks exposed on Syros Island (Cyclades, Greece) to provide constraints on the length scales and types of heterogeneities that occupy the deep subduction interface, with possible implications for episodic tremor and slow slip. We selected three Syros localities that represent different oceanic protoliths and deformation conditions within a subduction interface shear zone, including: (1) prograde subduction of oceanic crust to eclogite facies; (2) exhumation of oceanic crust from eclogite through blueschist-greenschist facies; and (3) exhumation of mixed mafic crust and sediments from eclogite through blueschist-greenschist facies. All three localities preserve rheological heterogeneities that reflect metamorphism of primary lithological, geochemical, and/or textural variations in the subducted protoliths and that take the form of brittle pods and lenses within a viscous matrix. Microstructural observations indicate that the matrix lithologies (blueschists and quartz-rich metasediments) deformed by distributed power-law viscous flow accommodated by dislocation creep in multiple mineral phases. We estimate bulk shear zone viscosities ranging from ∼1018 to 1020 Pa-s, depending on the relative proportion of sediments to (partially eclogitized) oceanic crust. Eclogite and coarse-grained blueschist heterogeneities within the matrix preserve multiple generations of dilational shear fractures and veins formed under high-pressure conditions. The veins commonly show coeval or overprinting viscous shear, suggesting repeated cycles of frictional and viscous strain. These geologic observations are consistent with a mechanical model of episodic tremor and slow slip (ETS), in which the deep subduction interface is a rheologically heterogeneous distributed shear zone comprising transiently brittle (potentially tremor-genic) sub-patches within a larger, viscously creeping interface patch. Based on our observations of outcrop and map areas of heterogeneous patches and the sizes, distributions, and amounts of brittle offset recorded by heterogeneities, we estimate that simultaneous brittle failure of heterogeneities could produce tremor bursts with equivalent seismic moments of 4.5 × 109–4.7 × 1014 N m, consistent with seismic moments estimated from geophysical data at active subduction zones.

Evidence of Localized Failure Along Altered Basaltic Blocks in Tectonic Mélange at the Updip Limit of the Seismogenic Zone: Implications for the Shallow Slow Earthquake Source

“…Previous work on the blocks model have shown shear surfaces at high angles to the predominant foliation of the mélange or limited evidence of cataclasis within the competent blocks (e.g., Hayman & Lavier, 2014; Kotowski & Behr, 2019). Here, we show well‐developed cataclasites along the margins of velocity‐weakening blocks surrounded by the velocity‐strengthening matrix, and repetitiously overprinting shear and tensile fractures, consistent with deformation having occurred at near‐lithostatic fluid pressures.…”

“…The rocks exhumed from downdip of the seismogenic zone have led to two principal hypotheses to explain the occurrence of slow slip and tremor. One hypothesis is that a mélange structure with strong blocks embedded within a weak matrix leads to localized failure in the blocks (LFEs and VLFEs) during distributed deformation in the matrix (Fagereng, 2011; Fagereng et al, 2014; Hayman & Lavier, 2014; Kotowski & Behr, 2019). In some cases, metasomatic reactions between blocks and matrix may lead to locally elevated pore fluid pressure around block margins promoting brittle failure of both blocks and matrix (Tarling et al, 2019).…”

“…If mixing velocity‐weakening altered basalt with velocity‐strengthening shale serves as a mechanism for producing slow earthquake phenomena, then this transitional behavior is restricted to environments less than 250°C (Figure 10) (Phillips et al, 2020). At the downdip limit of the seismogenic zone, subducting sediments likely deform through distributed viscous (and therefore velocity‐strengthening) behavior (e.g., French & Condit, 2019; Hayman & Lavier, 2014; Kotowski & Behr, 2019). The return to strengthening behavior of the matrix at the downdip limit of the seismogenic zone may allow for a common rate‐and‐state framework for slow slip phenomena at the updip and downdip limits of the seismogenic zone (Liu & Rice, 2007).…”

“…From a geological perspective, examining exhumed rocks from transitional zones provides insights into common features between the various settings of slow earthquake phenomena which may match observed geophysical characteristics. To date, most geologic studies have focused on characterizing features from the downdip limit of the seismogenic zone (Behr et al, 2018; Fagereng et al, 2014, 2017; Hayman & Lavier, 2014; Kotowski & Behr, 2019; Ujiie et al, 2018). Almost no studies have focused on the geologic processes controlling slow earthquake phenomena from shallow portions of subduction or strike‐slip plate boundaries or the creeping to locked segments of strike‐slip faults (an exception being Compton et al, 2017 from a hot strike‐slip fault zone).…”

Forced Return Flow Deep in the Subduction Channel, Syros, Greece

“…Because carbonates and quartzites tend to be impermeable for metamorphic fluids (Skelton et al, ), some of the mafic rocks preserve mineral assemblages formed during prograde and peak high‐pressure metamorphism (Skelton et al, ). For this reason, the section at Fabrika is ideally suited as a natural laboratory for the study of the interplay between metamorphism, fluid flow, and structural development in subduction zones (see also Kotowski & Behr, ). Therefore, we have mapped the Fabrika section at the 1:500 scale (Kleine et al, ; Skelton et al, ; this study) (Figure ).…”