Multiple veining in a paleo–accretionary wedge: The metamorphic rock record of prograde dehydration and transient high pore-fluid pressures along the subduction interface (Western Series, central Chile)

Abstract

High pressure–low temperature metamorphic rocks from the late Paleozoic accretionary wedge exposed in central Chile (Pichilemu region) are characterized by a greenschist-blueschist lithological association with interbedded metasediments that reached peak burial conditions of ∼400 °C and 0.8 GPa during late Carboniferous times. We herein combine new extensive field observations, structural measurements, and geochemical and petrological data on vein and matrix material from Pichilemu transitional greenschist-blueschist facies rocks. The studied veins were first filled by albite, followed by quartz and calcite as well as glaucophane and winchite. Field, structural, and microscopic zoning patterns show that these rocks underwent a protracted sequence of prograde vein-opening events, which have been largely transposed to the main foliation before and during underplating in the basal accretion site near 25–30 km depth. While some of the earliest albite-filled vein sets may have formed after prograde breakdown of sub–greenschist facies minerals (<250 °C), our thermodynamic modeling shows that relatively minor amounts of fluid are produced in the subducted pile by dehydration reactions between 250 and 400 °C along the estimated geothermal gradient. It also confirms that the formation of interlayered blueschist and greenschist layers in Pichilemu metavolcanics is a consequence of local bulk composition variations, and that greenschists are generally not formed due to selective exhumation-related retrogression of blueschists. The early vein sets are a consequence of prograde internal fluid production followed by sets of hydrofractures formed at near-peak burial that are interpreted as a record of external fluid influx. We postulate that such a fractured sequence represents a close analogue to the high-Vp/Vs regions documented by seismological studies within the base of the seismogenic zone in active subduction settings.

Cyclic Brittle‐Ductile Oscillations Recorded in Exhumed High‐Pressure Continental Units: A Record of Deep Episodic Tremor and Slow Slip Events in the Northern Apennines

“…We thus propose that local dehydration reactions triggered transient and cyclic fluid overpressure, reaching and exceeding lithostatic values (e.g. Condit et al., 2020; Muñoz‐Montecinos et al., 2020; Peacock, 1990; Saffer & Tobin, 2011). As a result, a temporary shift from ductile to brittle deformation occurred (Figure 14b).…”

“…Their seismological signature is characterized by high V p / V s ratios, where pore pressure may transiently reach and even exceed lithostatic values (e.g., Bernaudin & Gueydan, 2018; Saffer & Tobin, 2011). Recently, field‐based studies have proposed broadly coeval brittle and ductile deformation features to be the physical expression of ETS (Angiboust et al., 2015; Barnes et al., 2020; Behr et al., 2018; Compton et al., 2017; Kotowski & Behr, 2019; Molli et al., 2017; Muñoz‐Montecinos et al., 2020; Tarling et al., 2019). Notably, dilational shear veins (referred to as “dilational hydroshears” by Fagereng et al., 2010) in shallow subduction settings are currently considered as one key, diagnostic element of the geological record of episodic tremor and slow slip events (ETS; Fagereng et al., 2011).…”

Along-dip variations of subduction fluids: The 30–80 km depth traverse of the Schistes Lustrés complex (Queyras-Monviso, W. Alps)

“…BS-facies metasediments of the SL complex contain numerous veins (Fig. 4a), as for most former subduction complexes of various grades (Fisher and Byrne, 1990;Vannucchi, 2001;Sadofsky and Bebout, 2004;Meneghini et al, 2009;Raimbourg et al, 2015;Muñoz-Montecinos et al, 2020). Cartwright and Buick (2000) estimated that veins can make up to 30% of outcrops and commonly 5-10% of BS-facies metasediments in Alpine Corsica.…”

What’s down there? The structures, materials and environment of deep-seated slow slip and tremor

“…Evidence for this progression is abundant in the rock record and affects a wide range of rock types (and not just in subduction zones, e.g. [ 269 , 270 ]) [ 271 ]. The progressive growth, alignment, and concentration of micas in low- to medium-grade subduction interface rocks, for example, has been postulated to strongly influence megathrust seismic behaviours (e.g.…”