A MULTICHANNEL SEISMIC INVESTIGATION OF UPPER CRUSTAL STRUCTURE AT 9-DEGREES-N ON THE EAST PACIFIC RISE - IMPLICATIONS FOR CRUSTAL ACCRETION

Reprocessed multichannel seismic profiles from the 9-degrees-N segment of the East Pacific Rise reveal prominent shallow subbasement events. These events are identified as wide-angle reflections from the base o f seismic layer 2A, based upon modeling of expanding spread profile da ta and velocity functions. The layer 2A reflections typically increase from 0.15 s after the seafloor reflection at the rise axis to 0.3-0.4 5 s within 1-2 km of the axis, corresponding to an increase in layer t hickness of 200-600 m. No further systematic increase in layer thickne ss is observed, although lateral variability of the order of a few hun dred meters in thickness is observed at greater offsets from the rise axis. However, the intermittent character of the imaged layer 2A refle ction is attributed to focusing and defocusing of energy by the seaflo or bathymetry rather than necessarily to intrinsic lateral variability at the base of the layer. The base of layer 2A is interpreted as corr esponding to the transition between the extrusive section, pillow basa lts and sheet flows, and a sheeted dike complex. The rapid thickening of the layer near the rise axis is attributed to successive lava flows burying the initially shallow top of the sheeted dike complex as the layer passes through the neovolcanic zone. Lateral variability of laye r 2A can significantly affect the imaging of the underlying axial magm a chambers as average velocities within layer 2A are approximately hal f that of layer 2B. For an along-axis profile, apparent along-axis var iability in the depth of the axial magnma chamber is traced to variabi lity in the thickness of layer 2A caused by wandering of the profile r elative to axis. Within the resolution of the data, the time delay of the magma chamber reflection relative to the base of layer 2A is const ant.