The mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics, and thus the understanding of the processes at the lithosphere-asthenosphere boundary (LAB) is fundamental to understand how our planet works. It is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the LAB. Data from borehole broadband ocean bottom seismometers show that the LAB beneath the Pacific and Philippine Sea plates is sharp and age-dependent. The observed large shear wave velocity reduction at the LAB requires a partially molten asthenosphere consisting of horizontal melt-rich layers embedded in meltless mantle, which accounts for the large viscosity contrast at the LAB that facilitates horizontal plate motions.

Volcanism on Earth is known to occur in three tectonic settings: divergent plate boundaries (such as mid-ocean ridges), convergent plate boundaries (such as island arcs), and hot spots. We report volcanism on the 135 million-year-old Pacific Plate not belonging to any of these categories. Small alkalic volcanoes form from small percent melts and originate in the asthenosphere, as implied by their trace element geochemistry and noble gas isotopic compositions. We propose that these small volcanoes erupt along lithospheric fractures in response to plate flexure during subduction. Minor extents of asthenospheric melting and the volcanoes' tectonic alignment and age progression in the direction opposite to that of plate motion provide evidence for the presence of a small percent melt in the asthenosphere.

Fig. 4. An example of the localized structures for u = 0.6 x ohm-' m-' and E = 0.05. The image covers an area of 0.1 7 cm by 0.1 7 cm.ulated, yielding an amplitude that varied only slightly over the image. The spatial average An(t) was then studied separately for the four modes. A 30-min segment of A,(t) for the right-traveling zig and zag rolls for E = 0.01 is shown in Fie. 3. A t times one u

New multichannel seismic reflection data collected over the Nankai Trough image the accretionary complex in two areas: the International Program of Ocean Drilling leg 87 transect area (western area) and the region of upcoming Ocean Drilling Program leg 131 (eastern area). The incoming Shikoku Basin sedimentary section consists of hemipelagic clays and thin terrigenous turbidites. The basin section is overlain by a trench wedge that is 12-16 km wide and 350-750 m thick at the thrust front. Accretionary deformation begins in a protothrust zone that is characterized by thickening and seaward tilting of the trench wedge. The zone in the western area is 4.5 km wide and is characterized by "kink" folds; the zone in the eastern area is only 2.5 km wide and does not exhibit such folds. The frontal thmsts in each area are imaged as fault plane reflections and ramp upward from within the basin hemipelagic section. The overthmsting sediments form fault-bend folds over these ramps. Thrust spacing at the toe of the slope is 1.5-2.5 km. The second thrust cuts up from an inferred dtcollement within the Shikoku Basin sedimentary section. In the eastern area, a reflection marking the top of the basin pelagic sediment section changes from normal to reversed polarity about 6.3 km seaward of the thrust front and underlies the entire protothmst zone. This reflector continues with reversed polarity under the accretionary complex and is at the level of the basal dtcollement. The underlying basin pelagic section is apparently thmst undisturbed beneath the accretionary prism. The reversal of polarity indicates a change in reflection coefficient that is due to a combination of decreasing seismic velocity and density across the interface. This decrease in velocity and density may indicate that the dtcollement is a zone of high porosity due to fluid expulsion from deeper within the accretionary prism. The reflections from the first and second thmsts are also reversed polarity, possibly indicating that they also are pathways of fluid expulsion. The critical wedge taper of the westem area is greater than that of the eastern area, an observation that is consistent with the existence of an overpressured dtcollement in the eastern area. MOOREET AL.: STRU• OFTHEN••OU• 8755 Am., 66, 117-133, 1986.