Accretionary complex heterogeneity controls the faulting style of upper plate thrusts

A relevant portion of seismic activity in subduction zones takes place along splay faults and other subsidiary structures of the subduction interface that cut across the upper plate accretionary complex. The heterogeneous lithology of accretionary complexes, reflecting the stratigraphy of the incoming ocean plate, exerts a first-order control on the seismic behaviour. Thus, investigating accretionary complexes exhumed from the seismogenic zone is relevant to understand upper plate seismicity.

The Inuyama Sequence, part of the Jurassic Accretionary Complex of central Japan, is the ideal natural laboratory to investigate the lithological control on faulting style and seismic/aseismic behaviour in the shallow, sediment-dominated portion of an accretionary prism. It consists of a coherent chert-clastic complex with ocean-floor stratigraphy (in ascending order: siliceous claystone unit, ribbon chert unit, siliceous mudstone unit, and a clastic unit composed of lower mudstone, sandstone and upper mudstone) repeated six times by out-of-sequence thrusts that delimit the thrusts sheets [1].

Here we focus on three of the out-of-sequence thrusts (T1, T2, T3 in ascending structural order) that are well exposed along the Kiso River: T1 separates siliceous mudstones of sheet 1 from black and grey cherts of sheet 2; T2 separates upper mudstones of sheet 2 from siliceous claystones and cherts of sheet 3; T3 separates upper mudstones of sheet 3 from siliceous mudstones and cherts of sheet 4 whose stratigraphic topping direction is overturned compared to the other sheets.

Fault zones are 10–50 metres in thickness and mostly accommodate strain in the weaker clay-rich lithologies (siliceous mudstones and siliceous claystones), typically localizing deformation along carbonaceous-material-rich layers. A pervasive foliation in the siliceous mudstones of T1 and 50–100 cm thick slip zones with scaly fabric in siliceous claystones and siliceous mudstone suggest predominant deformation by aseismic creep. The stiffer cherts are also involved in the fault zones. In T1, the hanging-wall derived brecciated cherts host a mm-thick pseudotachylyte fault vein recording earthquake slip [2]. In T3 a localized fault core in the hanging-wall cherts is rich in quartz clasts with pervasive 2-5 µm spaced deformation lamellae, recording high stress pulses.

Chlorite geothermometry applied on syn-kinematic chlorite and chlorite-quartz veins abundant in the fault rocks provide temperatures in the range 170–210 °C, in line with peak condition estimates for the area, confirming that the investigated structures were developed during accretion. Lower chlorite temperatures, down to 100°C, have also been measured in a scaly fabric fault zone, suggesting later reactivations of the fault at colder (shallower) conditions during exhumation.

These preliminary results highlight the importance of the heterogeneous stratigraphy of accretionary complexes in controlling faulting style: while weak mudstones accommodated most of slip by aseismic creep, the stiffer cherts hosted occasional high-stress pulses associated with seismic ruptures. Further questions to answer include how slip is partitioned and what factors promote seismic ruptures in the stiffer lithologies.

 

[1] Kimura, K., Hori, R. (1993) Journal of Structural Geology, 15(2), 145-161.

[2] Ujiie, K., et al. (2021) Earth and Planetary Science letters, 554, 11638