3. Background and Regional Setting¹

Brian Taylor²

THEMATIC INTRODUCTION

The processes by which continental lithosphere accommodates strain during rifting and the initiation of seafloor spreading are presently known primarily from the study of either (1) passive margins bordering rifted continents where extensional tectonics have long ceased and evidence for active tectonic processes must be reconstructed from a record that is deeply buried in post-rift sediments and thermally equilibrated or (2) regions of intracontinental extension, such as East Africa, the U.S. Basin and Range, and the Aegean, where extension has occurred recently by comparison to most passive margin examples, but has not proceeded to the point of continental breakup.

One particularly controversial conjecture from these studies is that the larger normal detachment faults dip at low angles and accommodate very large amounts of strain through simple shear of the entire lithosphere. The role of low-angle normal detachment faults has been contested strongly, both on observational and theoretical grounds. It has been suggested that intracontinental detachments have been misinterpreted and actually formed by rollover of originally high-angle features, or that they occur at the brittle/ductile boundary in a pure shear system. Theoretically, it has been shown that normal faulting on detachment surfaces would require that the fault be extremely weak--almost frictionless--to allow horizontal stresses to cause failure on low-angle planes. The growing evidence for a weak fault and strong crust associated with motion on the San Andreas transform fault supports the weak normal detachment fault model, and models in which low-angle detachment faulting is an essential mechanism of large-scale strain accommodation abound in the literature.

Nevertheless, the mechanisms by which friction might be effectively reduced on low-angle normal fault surfaces are not understood. One possibility is that active shearing in the fault zone creates a strong permeability contrast with the surrounding crust (by opening cracks more quickly than precipitation can heal them), allowing pore-pressure distributions that are high and near to the fault-normal compressive stress within the fault zone, but decrease with distance into the adjacent crust (Rice, 1992; Axen, 1992). Others have suggested that fluid-rock reactions form phyllosilicates in the fault zone that are particularly weak because of their well-developed fabrics (Wintsch et al., 1995). Alternatively, principal-stress orientations may be rotated into configurations consistent with low-angle faulting, although it has not been demonstrated that the magnitudes of reoriented stresses are sufficient to initiate and promote such slip (Wills and Buck, 1997). Testing for such fault-proximal high permeability and pore pressures, for the presence of weak phyllosilicates, and/or for local rotation of stress axes, requires drilling into an active system. This would also allow determination of the properties of the fault rock at depth (do they exhibit reduced frictional strength at higher slip velocities, consistent with unstable sliding and observed earthquakes?), as well as studies of the mechanisms by which fluid-rock reactions affect deformation (constitutive response, frictional stability, long-term fault strength; see Hickman et al., 1993, and Barton et al., 1995, for extensive discussion of the mechanical involvement of fluids in faulting, and Wernicke, 1995, for a review of low-angle normal faulting).

A primary objective of Leg 180 was to drill into and characterize an active low-angle normal fault--the extreme example of the low-stress fault paradox. Such a fault, dipping 25º-30º, has been imaged north of Moresby Seamount where seafloor spreading in the western Woodlark Basin is breaking into the continental lithosphere of Papua New Guinea (see Fig. F1; also "Introduction" in the "Leg 180 Summary" chapter).

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 sea-floor spreading is unique. This region affords the possibility to definitively tie together the sedimentology, magmatism, and structures of (incipient) conjugate margins before they are separated and buried by a subsequent history of seafloor spreading and sedimentation. Determining these parameters was a second objective of Leg 180 with the intent to use them as local ground truth to be input into regional models for the timing and amount of extension prior to spreading initiation. A precruise description of the region is provided below.