One particularly controversial conjecture from these studies is that aerially large 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 are misinterpreted and actually form 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 weakalmost frictionlessto 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 abound in the literature in which low-angle detachment faulting is an essential mechanism of large-scale strain accommodation.
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 pressures that are high and near to the fault-normal compressive stress within the fault zone, but that 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.
The continuum of active extensional processes, laterally varying from continental rifting to seafloor spreading in the western Woodlark Basin-Papuan Peninsula region of Papua New Guinea (Fig. 1) provides the opportunity to test these various models. Seafloor spreading magnetic anomalies indicate that during the last 6 m.y. the formerly contiguous, eastward extensions of the Papuan Peninsula (the Woodlark and Pocklington Rises) were separated as a westward propagating spreading center opened the Woodlark Basin about a pole close to Port Moresby (~9.5°S, 147°E). The current spreading tip is at 9.8°S and 151.7°E. Farther west, extension is accommodated by continental rifting, with associated full and half graben metamorphic core complexes and peralkaline rhyolitic volcanism. Earthquake source parameters and seismic reflection data indicate that low-angle normal faulting is active in the region of incipient continental separation (Figs. 2, 3, 4, 5; Abers, 1991; Taylor et al., 1995, 1996; Mutter et al., 1996; Abers et al., 1997). Leg 180 will drill a transect of sites (just ahead of the spreading tip) above, below, and through a low-angle normal fault to determine the vertical motion and horizontal extension history prior to seafloor spreading and to characterize the composition and in situ physical properties of the active fault zone.
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