DeSl - The potential dewatering of subduction inputs and its impact on hydrology along the NanTroSEIZE drilling transect
Final Report Abstract
Fluids play a critical role for mechanical and mass transport processes in subduction zones. They enter the subduction zone as pore water or bound in hydrous minerals and are released by consolidation and diagentic to metamorphic processes. The main scientific objective of the DeSI project was to investigate the dewatering in the shallow Nankai Trough subduction zone. To investigate the consolidation behavior of sediments entering the Nankai Trough, consolidation tests were conducted on unlithified clays and sands recovered during Expedition 322 of the Integrated Ocean Drilling Program. The results document for clays a high compressibility with compression indices of Cc= 0.48-0.69. The pre-consolidation data suggest a normal to slightly overconsolidated state for shallow samples and a significant overconsolidation for deeper samples with overconsolidation ratios of 2.06-2.51. Inferred in situ permeabilities for clay samples document a decreasing trend with depth from 1.4·10^-17 m2 at 377.81 mbsf to 8.78·10^-20 m2 at 749.375 mbsf. The tested sands were found to behave significantly different during consolidation tests. Sands are characterized by low compression (Cc= 0.23 to 0.43) and yield at higher pre-consolidation stresses. This is probably to be a result of grain crushing. Permeabilities of sands are higher with values between 4.1·10^-17 and 1.4·10^-15 m2. The consolidation data went into a coupled loading and diffusion model to simulate the mechanical dewatering behavior of the lithologically heterogeneous underthrust sediment sequence of the western Nankai Trough. The simulations show that sand layers are an efficient drainage of the underthrust sediments. Fluid flow localizes along sand beds and allows deep fluids to migrate updip. The fluid loss leads to smaller pore water pressures, which is consistent with a higher taper angle compared to the central Nankai Trough where sands are absent. The simulations also show that sand layers dictate the position of the effective stress minimum in the sediment. It is found to be consistent with the position of the décollement observed from seismic images including the step down into the underthrust sediment at ~25 km from the trench. This suggests that underplating may be caused by a hydrological process. The diagentic dehydration of hydrous smectite minerals is propably one of the most important fluid sources in the shallow subdcution zone. The dehydration process is normally regarded as an temperature driven reaction but recent studies show that also increasing effective stress during burial causes dehydration. Within the project, hydrothermal consolidation tests on a silty smectite-rich claystone were conducted with stresses up to 70 MPa and temperatures of up to 100 °C to quantify the stress induced dehydration. Fluids expelled during the tests document substantial fluid freshening when effective stress exceed 1.38-2.15 MPa. During the tests the smectite interlayer water content decreases from 27 wt-% to ~14 wt-% The released interlayer water accounts for up to 17 % of the total fluid volume released during consolidation. Additional geochemical analysis of major and volatile element geochemistry shows that the dehydration process is accompanied by an uptake of K and Ca while volatiles are released (e.g B, Li) from smectite. The results are applied to the “cold” Barbados subduction zone to investigate the stress induced smectite dehydration under in situ conditions. The results show a decrease of smectite interlayer water content from 27 wt-% at the surface down to ~14 wt-% at 2 km depth which suggests that the the stress driven dehydration is an important aspect of smectite dehydration in subduction zones.
Publications
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(2011), Experimental investigation of smectite dehydration and pore water geochemistry at stress and temperature conditions similar to the shallow subduction zone, 2011 IODP-ICDP Colloquium, Münster, GER., 14-16 Mar
Hüpers, A. and Kopf, A.J.
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(2011), Variation of excess pore pressures in underthrust sediments as a result of lithostratigraphic heterogeneity along strike the Nankai subduction zone, Japan, Abstract T21B-2328 presented at 2010 Fall Meeting, AGU, San Francisco, Calif., 13-17 Dec
Hüpers, A. and Kopf, A.J.
