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 of seismic layer 2A, based upon modeling of expanding spread profile data and velocity functions. The layer 2A reflections typically increase from 0.15 s after the seafloor reflection at the rise axis to 0.3-0.45 s within 1-2 km of the axis, corresponding to an increase in layer thickness of 200-600 m. No further systematic increase in layer thickness is observed, although lateral variability of the order of a few hundred meters in thickness is observed at greater offsets from the rise axis. However, the intermittent character of the imaged layer 2A reflection is attributed to focusing and defocusing of energy by the seafloor bathymetry rather than necessarily to intrinsic lateral variability at the base of the layer. The base of layer 2A is interpreted as corresponding to the transition between the extrusive section, pillow basalts 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 layer 2A can significantly affect the imaging of the underlying axial magma chambers as average velocities within layer 2A are approximately half that of layer 2B. For an along-axis profile, apparent along-axis variability in the depth of the axial magnma chamber is traced to variability in the thickness of layer 2A caused by wandering of the profile relative to axis. Within the resolution of the data, the time delay of the magma chamber reflection relative to the base of layer 2A is constant.