Geological and geophysical data from the >900 m of volcanic basement drilled at Ocean Drilling Program Site 642 provide the framework for studying seismic properties of huge extrusive constructions on volcanic margins. The main part of the drilled section, corresponding to a prominent seaward dipping reflector sequence, consists of subaerially emplaced tholeiitic basalt flows and thin interbedded sediments. The basalts exhibit a characteristic velocity and density lava flow distribution reflecting changing porosity, pore aspect ratio distribution, and alteration. Stacks of laterally continuous basalt flows appear to have thin-layer transverse isotropic properties for typical wavelengths in multichannel seismic data. Vertical seismic profiling and average sonic log velocities are similar, 3.77 km/s and 3.88 km/s, respectively, while comparable refraction velocities are 10--20% higher. Synthetic seismogram modeling based on downhole logs shows that basement reflectors originate from interference and tuning effects of numerous basalt flow and interbedded sediment interfaces, though the most continuous reflectors are related to thick flows. Seismic models based on the characteristic velocity and density basalt flow distributrions and Site 642 stratigraphy show that reflector truncation and onlap may be caused by seismic interference phenomena in a sequence of landward thinning flows. The base of the dipping reflector sequence, reflector K, correlates with flows in the lower part of upper series basalts, while the transition from basaltic to underlying dacitic/andesitic lavas correlates with a locally defined reflector. A model for the breakup related volcanism includes (1) prebreakup dacitic/andesitic volcanism, (2) early breakup basaltic volcanism infilling the prebreakup relief, (3) main breakup stage with intense, focused volcanism, large subsidence and lava pounding, and (4) late breakup volcanism during a period of decreased subsidence and local off-axis activity. ¿ American Geophysical Union 1994