OLR (1987) 34 (I 1) D. Submarine Geology and Geophysics 971

The rate-of-slip vector on the San Andreas Fault, estimated from geodetic and Holocene geological data, is inconsistent with rigid plate models. The well-known 'San Andreas discrepancy' is diagnostic of plate deformation east of the fault in the Basin and Range and west along the continental margin. Constraints based on model kinematical boundary conditions, neotectonic and paleoseismic deforma- tion rate estimates, ground-based geodetic meas- urements, and VLBI data are constructed. Space- geodetic data on Basin and Range extension over a 4-yr interval are compatible with geological obser- vations averaged for the Holocene; the best estimate of the integrated deformation rate is too small and in the wrong direction to account for the entire discrepancy. Therefore, much deformation west of the Sierra Nevada-Great Valley block must be accommodated on faults of coastal California with significant right-lateral shear parallel to the San Andreas and some compression perpendicular to it. Sci. Horizons, Inc., 710 Encinitas Blvd, Encinitas, CA 92024, USA.

87:6369 Nagumo, Shozaburo, Toru Ouchi, Junzo Kasahara

and Sadayuki Koresawa, 1986. P-wave velocity in the iower-Hthosphere in the western Northwest Pacific Basin observed by an ocean-bottom seismometer long range array. Bull. Earthq. Res. Inst., Univ. Tokyo, 61(3):403-414.

Using earthquakes as sources, a 1981 ocean bottom seismometer array observation revealed that the apparent surface velocity of P-waves refracted from the lower lithosphere was 8.4 km/sec in the epicentral distances of 7o-9 °. True velocities at their turning points, depth level 60--100 km, were ~8.3 km/sec. The 8.4-8.5 km/sec apparent velocity layers at the top of the lower lithosphere appear to be a general feature of both continental shield regions and old ocean basins. Such a P-wave velocity supports the garnet-pyrolite model for the miner- alogical composition of the lower lithosphere. Earth- quake Res. Inst., Univ. of Tokyo, Japan.

87:6370 Nagumo, Shozaburo, Toru Ouchi, Junzo Kasahara

and Sadayuki Koresawa, 1986. Fifteen degree shadow zone of the P-wave travel-times in the western Northwest Pacific Basin observed by an ocean-bottom seismometer long range array. Bull. Earthq. Res. Inst., Univ. Tokyo, 61(3):415-427.

The apparent surface velocity of the P-wave in the distance ranges from 12 ° to 15 ° is about 8.6 km/sec, which seems to be a continuation of the 8.4 km/sec apparent surface velocity travel-times in the range of 6 ° to 12 ° . The off-set of travel times at the shadow

zone is about 1.7-2.0 seconds. The low velocity zone which corresponds to this shadow zone could be a major transition from lithosphere to asthenosphere. A review of regional differences gives an estimate for the depth of the transition beneath the basin in the range of 100-200 km. Earthquake Res. Inst., Univ. of Tokyo, Japan.

87:6371 Ramsay, D.C. and J.B. Colwell et al., 1986. Ker-

guelen Plateau, southern Indian Ocean. Rig Seismic research cruise 2: initial report. BMR Rept, Australia, 270:41pp.

Approximately 5500 km of multichannel seismic data and 15,000 km of gravity and magnetic data, recorded during a 58-day cruise to the Australian 'legal continental shelf' on the central and southern plateau, indicate an asymmetric plateau north of Heard Island bounded by a sediment free slope buttressed by intrusives on the eastern margin, a central portion underlain by adjoining sedimentary basins, and uplifted blocks of magnetic basement on the western margin. On the eastern part of the southern sector is the >30,000 km 2 sedimentary Raggatt Basin, which contains >3000 m of sedi- ment, apparently undisturbed by any significant faulting. Includes numerous maps and seismic records in pocket. (hbf)

87:6372 Robinson, E.M., Barry Parsons and S.F. Daly, 1987.

The effect of a shallow low viscosity zone on the apparent compensation of mid-plate swells. Earth planet. Sci. Letts, 82(3-4):335-348.

A simple model for mid-plate swells is that of convection in a fluid which has a low viscosity layer between a rigid bed and a constant velocity region. Finite element calculations show that as viscosity decreases in the low viscosity zone, the effective local Rayleigh number for the top boundary layer of the convecting cell increases, and that the low viscosity layer produces proportionally greater horizontal flow near the conducting lid, causing the base of the lid to appear like a free boundary. These changes in turn cause the top boundary layer to thin. By increasing the viscosity contrast, the combined effects of thinning boundary layer and the behavior of the response functions allow the apparent depth of compensation to become arbitrarily small; there- fore, shallow depths of compensation cannot be used to argue against dynamic support of mid-plate swells. For all calculations the effective gravitational mass distribution is shown to be more complex than assumed in simple Pratt models. Dept. of Earth, Atmos., and Planetary Sci., MIT, Cambridge, MA 02139, USA.