Abstract
Abstract. Understanding diagenetic reactions in accreted sediments is critical for establishing the balance of fluid sources and sinks in accretionary prisms, which is in turn important for assessing the fluid pressure field and the ability for faults to host seismic slip. For this reason, we studied diagenetic reactions in deformation bands (shear zones and veins) within deep mud sediments from the Nankai accretionary prism (SW Japan) drilled at site C0001 during IODP Expedition 315, by means of microscopic observation, X-ray diffraction, and major- and trace-element analyses. Deformation bands are not only more compacted than the host sediment but are also enriched in framboidal pyrite, as observed under microscopy and confirmed by chalcophile-element enrichments (Fe, S, Cu, As, Sb, Pb). In tandem, one shear zone sample displays a destabilization of smectite or illite–smectite mixed layers and a slight crystallization of illite relative to its sediment matrix, and another sample shows correlated increases in B and Li in shear zones and veins compared to the host sediment, both effects suggesting a transformation of smectite into illite in deformation bands. The two diagenetic reactions of sulfide precipitation and smectite-to-illite transformation are explained by a combined action of sulfate-reducing and methanogen bacteria, which strongly suggests an increased activity of anaerobic microbial communities localized in deformation bands. This local bacterial proliferation was possibly enhanced by the liberation of hydrogen from strained phyllosilicates. We suggest that the proliferation of anoxic bacteria, boosted by deformation, may contribute to the pore water freshening observed at depth in accretionary prisms. Deformation-enhanced metabolic reactions may also explain the illitization observed in major faults of accretionary prisms. Care is therefore needed before interpreting illitization, and other diagenetic reactions as well, as evidence of shear heating, as these might be biogenic instead of thermogenic.
Highlights
The shallow seismicity and the stress state of convergent margins is strongly influenced by the distribution of pore fluid pressure in the accretionary prism (e.g., Davis et al, 1983; Moore and Saffer, 1998)
One shear zone sample displays a destabilization of smectite or illite– smectite mixed layers and a slight crystallization of illite relative to its sediment matrix, and another sample shows correlated increases in B and Li in shear zones and veins compared to the host sediment, both effects suggesting a transformation of smectite into illite in deformation bands
Microscopic observation, X-ray diffraction, and major- and trace-element analyses reveal that deformation bands in mud sediments of the Nankai accretionary prism, which are similar to microstructures found worldwide in active and passive margin sediments, have localized some diagenetic reactions in addition to mechanical compaction
Summary
The shallow seismicity and the stress state of convergent margins is strongly influenced by the distribution of pore fluid pressure in the accretionary prism (e.g., Davis et al, 1983; Moore and Saffer, 1998). For this reason, a large amount of work has been devoted to understanding the processes of fluid production, consumption, or migration based on the composition of pore waters in accreted sediments (e.g., Brown et al, 2001; Henry and Bourlange, 2004; Kastner et al, 1991; Pohlmann et al, 2009; Raimbourg et al, 2017). Knowing which diagenetic reactions occur in sediments is critical for interpreting the chemistry of pore waters determined by drilling in accretionary prisms
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