The thermal influence on the consolidation state of underthrust sediments from the Nankai margin and its implications for excess pore pressure

Abstract

The Nankai Trough convergent margin has been the focus of many multi-methodological surveys including half a dozen scientific deep-sea drilling expeditions. The boreholes focused on the smectite-dominated area off Cape Ashizuri and the thermally altered, illite-dominated region off Cape Muroto. On the basis of these surveys a number of studies addressed to the stress state of the underthrust sediments and its implications for the plate boundary thrust. Although the basement temperature has been found to be up to ∼ 110 °C, none of these studies drew close attention to temperature effects on the consolidation state of the sediments. To overcome this shortcoming, we selected end member sediment lithologies from the incoming oceanic plate in the Shikoku Basin and subjected them to elevated stresses and temperatures. We here present results from a series of heated (20 °C, 100 °C, 150 °C) uniaxial consolidation experiments up to effective normal stresses of ca. 70 MPa. The main finding is a positive correlation between temperature and pore space reduction. Based on in-situ temperature information from earlier scientific drilling, our study suggests that temperature has an influence on the consolidation state of underthrust sediments along the Nankai Margin. Together with secondary consolidation, thermal consolidation serves to explain steep log-linear consolidation curves of the incoming Lower Shikoku Basin sediments. The onset of diagenesis in this realm led to the transition of smectite to illite and to a different consolidation behaviour. Estimated in-situ pore pressures based on in-situ temperature data result in up to ∼ 1 MPa smaller overpressures than those previously estimated from drilling data alone. Those values, which imply underconsolidation at drill sites near the frontal Nankai accretionary complex, are further believed to facilitate frictional sliding along the subduction thrust.