A SEISMOLOGICAL STUDY OF THE EASTERN NEW-GUINEA AND THE WESTERN SOLOMON SEA REGIONS AND ITS TECTONIC IMPLICATIONS

We use new and pre-existing seismological data to study the neotectonics and the state of stress in the seismic zones of eastern Papua New Guinea and the western Solomon Sea regions. Earthquake hypocentres for the period 1964-90 were relocated, using the Joint Hypocentre Determination (JHD) technique. We show that the T axes of earthquakes in the 100-250 km depth range beneath eastern Papua New Guinea are aligned parallel to the seismic zone, with no seismicity below approximate to 250 km. This intermediate-depth seismic zone forms an inverted U-shaped zone, with limbs dipping to the north and south, and is interpreted to be seismicity along the subducted Solomon Sea plate. The northern and southern limbs of the seismic zone are connected by a flattened zone of seismicity which is marked by predominantly strike-slip, and a few normal, fault mechanisms. We suggest that the Solomon Sea plate at depth beneath the Finisterre mountains is no longer influenced by the tectonic forces acting at the surface, but breaking up and sinking under its own gravitational forces. A significant north-dipping seismic zone is imaged above the deeper seismic zone. This feature, characterized by thrust mechanisms, extrapolates to the surface along the Ramu Markham Valley and is attributed to the Ramu Markham Fault (RMF). The RMF has an average dip of about 40 degrees to the north-east but its detailed geometry would appear to be complex. The RMF is interpreted to ramp steeply in the uppermost 10-20 km, flattening out sharply to a dip of approximate to 15 degrees at a depth of about 20 km. The RMF marks the suture between the Finisterre Terrane and the Australia-New Guinea plate and may extend to a depth of 90 km beneath the western limit of the Finisterre mountains. There is a clear gap in seismicity between the RMF and the deeper Solomon Sea plate seismic zone beneath the Finisterre mountains, which, together with the alignment of the T axes of earthquakes along the seismic zone at depths >100 km, indicates that these are indeed two distinct features. The seismicity in the depth range 50-80 km, along the New Britain Are, follows the trend of the 2000 m bathymetric contour of the New Britain Trench and the Finsch Deep far more closely than it does the trend of the surface plate boundary, indicating that the increase in the distance between the Solomon Sea/South Bismarck plate boundary and the active volcanos along the New Britain Are, west of (6.8 degrees S, 150.0 degrees E) is only a shallow phenomenon, and does not reflect the geometry of the Solomon Sea plate at depths greater than 50 km.