Woodlark Basin Fault Zone

Woodlark synthetic seismograms

Theories of frictional slip on normal faults predict that faults should not be able to slip at angles of 30° or less. Researchers have shown however, that slip is theoretically possible in the presence of fault gouge or fluids, which may lower the normal stress on the fault plane. In 1998, I participated on Ocean Drilling Program (ODP) Leg 180 to the Woodlark Basin of Papua New Guinea to study a major detachment fault that is shown in seismic reflection data to dip near and possibly below the 30° limit. The fault lies within the rifting-spreading transition zone immediately ahead of the westward propagating Woodlark spreading center, which may be weakening the fault zone by driving hydrothermal fluid circulation.

Woodlark Basin fault zone

For this study, published in Floyd et al. (2001), I used a genetic algorithm and constraints provided by drilling data to invert seismic reflection waveforms for the seismic velocity and thickness of the fault zone. The inversion results show that the fault zone consists of a 33.4 ± 5.7 m-thick layer with an average P wave velocity of 4.3 ± 0.22 km/s and isolated sections with an exceptionally low velocity of 1.74 ± 0.24 km/s. Calculations based on physical property measurements of fault gouge recovered from the exposed footwall of the fault during drilling indicate that a porosity of 61% or more is required to explain the decrease in velocity to 1.74 km/s. Amplitude variation with offset (AVO) analysis of the common midpoint (CMP) gathers provide supporting evidence for fluids in the low velocity zones. The high porosity is best explained by the presence of high pore-fluid pressures in the fault zone, which may be weakening the fault and allowing it to slip at a low angle.

woodlark basin paper imagePublication: Floyd, J.S., Mutter, J.C., Goodliffe, A.M., and Taylor, B., 2001, Evidence for fault weakness and fluid flow within an active low-angle normal fault, Nature, 411, 779-783.