Deformation processes at the down-dip limit of the seismogenic zone: The example of Shimanto accretionary complex

Abstract

International audienceIn order to constrain deformation processes close to the brittle-ductile transition in seismogenic zone, we have carried out a microstructural study in the Shimanto accretionary complex (Japan), the fossil equivalent of modern Nankai accretionary prisms. The Hyuga Tectonic Mélange was sheared along the plate interface at mean temperatures of 245 °C ± 30 °C, as estimated by Raman spectroscopy of carbonaceous material (RSCM). It contains strongly elongated quartz ribbons, characterized by very high fluid inclusions density, as well as micro-veins of quartz. Both fluid inclusion planes and micro-veins are preferentially developed orthogonal to the stretching direction. Furthermore, crystallographic preferred orientation (CPO) of quartz c-axes in the ribbons has maxima parallel to the stretching direction. Recrystallization to a small grain size is restricted to rare deformation bands cutting across the ribbons. In such recrystallized quartz domains, CPO of quartz c-axes are orthogonal to foliation plane. The evolution of deformation micro-processes with increasing temperature can be further analyzed using the Foliated Morotsuka, a slightly higher-grade metamorphic unit (342 ± 30 °C by RSCM) from the Shimanto accretionary complex. In this unit, in contrast to Hyuga Tectonic Mélange, recrystallization of quartz veins is penetrative. CPO of quartz c-axes is concentrated perpendicularly to foliation plane. These variations in microstructures and quartz crystallographic fabric reflect a change in the dominant deformation mechanism with increasing temperatures: above ~300 °C, dislocation creep is dominant and results in intense quartz dynamic recrystallization. In contrast, below ~300 °C, quartz plasticity is not totally activated and pressure solution is the major deformation process responsible for quartz ribbons growth. In addition, the geometry of the quartz ribbons with respect to the phyllosilicate-rich shear zones shows that bulk rheology is controlled by quartz behavior. Consequently, below 300 °C, the application of quartz pressure-solution laws, based on realistic geometry derived from Hyuga microstructures, results in strongly lowering the overall strength of the plate interface with respect to the classical brittle envelop

Organic matter cracking: A source of fluid overpressure in subducting sediments

“…This maturity is in agreement with paleotemperatures as large as ~250°C experienced during burial (Kondo et al, 2005;Mukoyoshi et al, 2009;Palazzin et al, 2016).…”

Tertiary evolution of the Shimanto belt (Japan): A large‐scale collision in Early Miocene

“…As in western Shikoku (Figure ), the highest paleotemperatures in eastern Shikoku are concentrated within the southern, Tertiary portion of the belt [ Laughland and Underwood , ; Ohmori et al , ] which is also the case in Kii [ Chijiwa , ]. This thermal anomaly is not observed in Kyushu, where the Tertiary Hyuga and Nichinan Groups experienced much lower temperatures than the Cretaceous portion of the belt (Figure a′) [ Palazzin et al , ; Sampei and Kaji , ].…”

“…These rocks have undergone a large amount of deformation and heating; hence, they have presumably a relatively large density. For comparison, accreted metasediments from the Morotsuka or Hyuga Groups in Kyushu, which have experienced deformation under metamorphic condition of ~250–350°C [ Palazzin et al , ; Raimbourg et al , ], have a density of 2.6–2.7 g/cm 3 [ Tsuji et al , ]. The density of the body responsible for the anomaly off Kii is therefore of the order of 2.9–3.1 g/cm 3 , i.e., similar to basaltic composition.…”

Fluid properties and dynamics along the seismogenic plate interface

“…To study the evolution with depth of the fluid along the plate interface, we have already studied in detail the example of the Shimanto Belt in Japan (Raimbourg et al, 2014b;Raimbourg et al, 2015;Palazzin et al, 2016), and we describe hereafter the principal results ( Fig. 1).…”