Logging While Drilling In The Frontal Part of The Nankai and Barbados Accretionary Prisms - Physical Characteristics of The Decollement Zone Compared Revealed
Abstract
The Nankai Trough and Barbados Ridge accretionary prisms are examples for the different end member situations of sediment accretion on convergent margins. The Nankai Trough, Japan is a type example for convergent margins characterized by a high input of terrigeneous clastic sediment, while the Barbados, Lesser Antilles ac- cretionary complex is dominated by the accretion of hemipelagic sediments. Both settings have been successfully drilled and cored several times by the Ocean Drilling Program (ODP). Sediment physical property data from ODP drilling were recently supplemented by high resolution Logging-While-Drilling data. In both areas the position of major thrust and decollement zones were previously identified through drilling-derived structural and geochemical evidence. Industry standard Logging- While-Drilling measurements however provided density and resistivity data of much higher resolution reflecting the in-situ stress and fluid regime at the toe of both prisms.
Combining core and Logging-While-Drilling data the changes in physical proper- ties during transition from proto- to well developed decollement during approach and frontal thrusting were documented in detail. At Nankai Logging-While-Drilling data showed little evidence for a propagating protodecollement in the reference site in front of the prism. Rapid loading of the section combined with limited dewatering is caus- ing the development of a strong density inversion across the decollement below the frontal thrust. In contrast the Barbados transect shows a gradual transition from a pri- mary density anomaly across the protodecollement at the reference site to a broad low porosity decollement zone at a site several km behind the frontal thrust. Here the base of the decollement is marked by an offset to higher densities, whereas in Nankai the top of the underthrust sediments shows an offset to lower densities. Our comparison shows the influence of sediment properties and loading history for the localization and evolution of both types of decollement.- Publication:
-
EGS General Assembly Conference Abstracts
- Pub Date:
- 2002
- Bibcode:
- 2002EGSGA..27.5436B