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.

中文翻译：

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南开增生棱柱中变形增强的成岩作用和细菌增殖

了解增生沉积物中的成岩反应对于在增生棱柱中建立流体源和汇的平衡至关重要，而这对于评估流体压力场和断层承载地震滑动的能力也很重要。为此，我们在 IODP 远征 315 期间，通过显微观察、X 射线衍射、以及主要和微量元素分析。变形带不仅比宿主沉积物更致密，而且还富含叶状黄铁矿，如在显微镜下观察到的那样，并通过亲硫元素富集（Fe、S、Cu、As、Sb、Pb）证实。同时，一个剪切带样品显示蒙脱石或伊利石-蒙脱石混合层的不稳定和伊利石相对于其沉积物基质的轻微结晶，另一个样品显示与宿主沉积物相比，剪切带和脉中的 B 和 Li 相关增加，这两种影响表明在变形带中蒙脱石转化为伊利石。硫化物沉淀和蒙脱石到伊利石转化的两种成岩反应可以通过硫酸盐还原菌和产甲烷菌的共同作用来解释，这强烈表明局部厌氧微生物群落的活性增加在变形带中。这种局部细菌增殖可能通过从应变的页硅酸盐中释放氢而得到增强。我们认为变形促进了缺氧细菌的增殖，可能有助于在增生棱柱深处观察到的孔隙水变淡。变形增强的代谢反应也可以解释在增生棱柱的主要断层中观察到的伊利化。因此，在解释伊利蒂化和其他成岩反应作为剪切加热的证据之前需要小心，因为这些可能是生物成因而不是热成因。