The western Woodlark Basin is arguably the best characterized region of active continental breakup. The proximity of a seismogenic low-angle normal fault that has been imaged by seismic reflection data and zero-offset conjugate margins that are about to be penetrated by seafloor spreading is unique. There are two major objectives for drilling in this region. Both are within the broader context of understanding the physical processes and mechanics of lithospheric extension:
1. Characterize the composition and in situ properties (stress, temperature, physical properties, and fluid pressure) of an active low-angle normal-fault zone to understand how such faults slip. Questions to be answered include the following: What are the differences in properties of the active fault compared to the surrounding crust? How is friction effectively reduced? Is there a strong permeability and fluid pressure contrast between the fault zone and its surroundings? Do the fault zone materials exhibit reduced frictional strength at higher slip velocities, consistent with unstable sliding? How do fluid/rock reactions affect the deformation mechanisms and rock fabrics?
Drilling a very deep hole (>2.5 km) through the low-angle normal fault in the seismogenic zone, where both the footwall and hanging wall are composed of basement, remains our long-term objective. However, as Leg 180 may penetrate only ~1200 m across the fault zone, this and further fault zone experiments and monitoring (e.g., CORK) will require a future leg.
2. Determine the vertical motion history of both the down-flexed hanging wall and the unloaded footwall (by backstripping the biostratigraphy, regionalizing the well data using seismic stratigraphy, and by Pressure-Temperature-time [P-T-t] data and petrofabric studies of metamorphic basement), as local groundtruth for input into regional models to determine the timing and amount of extension prior to spreading initiation.
To 180 Drilling Strategy
To 180 Table of Contents