AbstractsEarth & Environmental Science

A study of Rayleigh wave dispersion from earthquakes within the New Guinea area has revealed two features of shear velocity distribution not examined previously. Variations in depth,to 300 km below the tectonically stable platform of southern New Guinea were determined from recordings of fundamental and higher mode Rayleigh waves. This average profile for paths shorter than 2000 km, contains a low velocity zone reaching 4.2 km/sec. at depths of 120 to 160 km, which is overlain by a relatively thick cap of material with a velocity of 4.4 km/sec. The lateral gross distribution of shear velocities beneath the crust, emerged from examination of fundamental mode dispersion profiles. Many of the phase velocity profiles were resolved from single station data and techniques used to define the correct dispersion in such cases are discussed. Although interpretations of these are strictly limited in structurally heterogeneous regions such as New Guinea, phase velocities at periods near 40 seconds are controlled mainly by shear velocities in the uppermost 50 km of the mantle, in zones of greater homogeneity than the crust. Known geophysical data, existing hypotheses of geotectonic development of the New Guinea region, the concept of sea floor spreading, and observed characteristics of mineral assemblages under high temperatures and pressures, provide a framework within which these data are examined. Upper mantle Velocities are highest, about 4.7 km/sec., beneath the crust of the Solomon Sea, lowest beneath the shield structure of southern New Guinea and appear to have intermediate values beneath the cordilleran region. These differences are large enough to indicate that regional variations exist in the mineralogy of the upper mantle. The relatively low velocities underlying the southern New Guinea crust may also imply a higher heat flow than normally expected in the most ancient pre Cambrian shield regions of the world. In general the results are not sufficiently detailed or comprehensive to test existing hypotheses of geotectonic development of this complex region and the study should rather be treated as a basic contribution to knowledge of the New Guinea area.