SMECTITE TRANSFORMATION IN THE NANKAI TROUGH: EFFECTS ON SUBDUCTION ZONE MECHANICS AND HYDROGEOLOGY

At subduction zones, the transformation of smectite to illite has emerged as a potentially key process in controlling both hydrologic and mechanical behavior of plate boundary megathrusts. Release of bound water during thermally-driven illitization may have significant effects on the development of fluid overpressure, which leads to fault weakening and may partly determine the updip extent of seismic slip. In addition, the mineralogical change and silica cementation that accompanies it may drive changes in intrinsic frictional strength and sliding stability. Further, the fresh water released by clay transformation is a likely explanation for pore water freshening observed in boreholes at numerous margins; constraining the loci of fluid release is needed to understand fluid flow pathways. Here, we combine data from ODP drill sites with numerical models of illitization, to (1) estimate the distribution of smectite transformation and fluid production down-dip of the trench, and (2) evaluate its hydrologic and mechanical implications. We report the results of simulations for the Nankai subduction zone, where abundant constraints are available for model calibration, and also for a suite of generic sensitivity analyses.

At Nankai, our results are in excellent agreement with data from drill sites. High heat flow (~180 mW m^-2) along the axis of the Kinan Seamount Chain (the “Muroto transect”) drives considerable reaction progress outboard of the trench, whereas lower heat flow (70-120 mW m^-2) 100 km to the SW (the “Ashizuri transect”) results in negligible reaction. As a result, considerably more bound fluid is subducted along the Ashizuri transect. Peak fluid sources along the Muroto transect of ~6 x 10^-15 s^-1 occur ~18 km from the trench; those along the Ashizuri transect are ~1.2-1.3 x 10^-14 s^-1 and occur 13-26 km from the trench. For comparison, estimated fluid sources from compaction at 20 km from the trench are 4 x 10^-15 to 1.3 x 10^-14 s^-1. Results of our sensitivity analysis demonstrate that slower plate convergence rate, steeper taper angle, and thicker incoming sediment all drive the reaction to completion closer to the trench. These results provide key insights into links between clay transformation and hydrologic/mechanical processes that may be applied to many of the Earth's subduction zones.