The evolution of slate microfabrics during progressive accretion of foreland basin sediments

Ismay Vénice Akker,Alfons Berger,Christoph E Schrank,Michael W.M Jones,Cameron M Kewish,Jop Klaver,Marco Herwegh

doi:10.1016/j.jsg.2021.104404

Abstract

Here, we study slate microfabrics from the exhumed accretionary wedge of the central European Alps and focus on the development of foliation. High-resolution micrographs from novel BIB-SEM imaging and Synchrotron X-ray Fluorescence Microscopy are analysed with 2D auto-correlation functions to quantify the geometry and spacing of slate microfabrics along a metamorphic gradient covering the outer and inner wedge (200–330 °C). The sedimentary layering primarily controls the morphology of the slate microfabrics. However, from outer to inner wedge, a fabric evolution is observed where diagenetic foliations gradually transform to secondary continuous and spaced foliations. With increasing metamorphic grade, the amount of recrystallized phyllosilicate grains and their interconnectivity increase, as does clast/microlithon elongation (aspect ratios up to 11), while foliation spacing decreases to <20 μm. This foliation evolution under non-coaxial deformation involves a combination of mechanical rotation of phyllosilicates, fracturing, and fluid-assisted pressure-dissolution-precipitation creep. The latter is the dominant deformation mechanism at T > 230 °C and accommodates background strain in the inner wedge. The evolving microstructural anisotropy is interpreted to lead to strain weakening by structural softening and may provide preferential fluid pathways parallel to the foliation, enabling the dehydration of large rock volumes in accretionary sediment wedges undergoing prograde metamorphism.

Highlights

Studies from active and exhumed accretionary wedges show that the deformational style, intensity, and structures in the wedge are a function of lithification and burial (e.g., Dielforder et al, 2016a; Ditullio and Byrne, 1990; Kimura et al, 2007)
We only examined hand specimens from slates, excluding as much as possible any other lithologies, over a north-south transect covering a metamorphic gradient from 200 ◦C to 330 ◦C (Fig. 1)
Our observations demonstrate that the high-grade foliations mainly consist of neo- and re-crystallised phyllosilicates formed by pressuredissolution-precipitation creep, a process requiring fluids

Summary

Introduction

Studies from active and exhumed accretionary wedges show that the deformational style, intensity, and structures in the wedge are a function of lithification and burial (e.g., Dielforder et al, 2016a; Ditullio and Byrne, 1990; Kimura et al, 2007). Conse quently, their mechanical and transport properties change dramatically (Donath, 1961, 1964; McLamore and Gray, 1967) These changes in material properties define the deformational style and structures in the wedge and have important implications for fluid flow and seismic behaviour (Moore and Saffer, 2001; Moore and Vrolijk, 1992; Oleske vich et al, 1999; Saffer and Tobin, 2011; Ujiie and Kimura, 2014). With respect to the progressive deformation in the accretionary wedge, one well-studied mechanical consequence in such sediments is the formation of foliations made of aligned phyllosilicates (Ditullio and Byrne, 1990; Norris and Bishop, 1990; Palazzin et al, 2016; Raimbourg et al, 2009). Anisotropic growth of newly formed phyllosilicates may facilitate foliation development (Knipe, 1981)

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Conclusion