SEISMIC RESPONSE AND CONSTRUCTION OF SEAWARD DIPPING WEDGES OF FLOOD BASALTS - VORING VOLCANIC MARGIN

Geological and geophysical data from the > 900 m of volcanic basement drilled at Ocean Drilling Program Site 642 provide the framework for s tudying seismic properties of huge extrusive constructions on volcanic margins. The main part of the drilled section, corresponding to a pro minent seaward dipping reflector sequence, consists of subaerially emp laced tholeiitic basalt flows and thin interbedded sediments. The basa lts exhibit a characteristic velocity and density lava flow distributi on reflecting changing porosity, pore aspect ratio distribution and al teration. Stacks of laterally continuous basalt flows appear to have t hin-layer transverse isotropic properties for typical wavelengths in m ultichannel seismic data. Vertical seismic profiling and average sonic log velocities are similar, 3.77 km/s and 3.88 km/s, respectively, wh ile comparable refraction velocities are 10-20% higher. Synthetic seis mogram 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 refle ctors are related to thick flows. Seismic models based on the characte ristic velocity and density basalt flow distributions and Site 642 str atigraphy show that reflector truncation and onlap may be caused by se ismic interference phenomena in a sequence of landward thinning flows. The base of the dipping reflector sequence, reflector K, correlates w ith flows in the lower part of upper series basalts, while the transit ion from basaltic to underlying dacitic/andesitic lavas correlates wit h a locally defined reflector. A model for the breakup related volcani sm includes (1) prebreakup dacitic/andesitic volcanism, (2) early brea kup basaltic volcanism infilling the prebreakup relief, (3) main break up stage with intense, focused volcanism, large subsidence and lava po unding, and (4) late breakup volcanism during a period of decreased su bsidence and local off-axis activity.