Abstract

We analyze two-ship multichannel seismic wide aperture profiles (WAP) acquired on the East Pacific Rise (EPR) between 9-degrees 30'N and 10-degrees-N in May and June 1985. For offsets between 2 and 5 km, most of the WAP common depth point (CDP) gathers exhibit a strong retrograde, reflection-like seismic arrival that corresponds to rays turned within a very strong velocity gradient zone underlying the uppermost crustal layer 2A. Stacking of this postcritical arrival for hundreds of CDP gathers along the WAP lines yields remarkably clear, and virtually continuous, images of the bottom of layer 2A for several tens of kilometers along and across the EPR. An innovative combination of this image with others of the seafloor (SF) and of the axial magma chamber (AMC) provides an unusually clear composite picture of young upper crust. We combine these images with detailed seismic velocity information, also obtained from the CDP gathers, to achieve an accurate determination of the layer 2A thickness in the area. Along the EPR axis between 9-degrees 30'N and 10-degrees-N, layer 2A shows a relatively constant thickness between 100 and 150 m (80-120 ms two-way travel time). Across the axis, however, a line near 9-degrees 30'N indicates that layer 2A thickness rapidly increases by approximately a factor of 2 within 2-4 km of the axis and remains nearly constant afterwards. Thus the final thickness of layer 2A is attained before the development of large-scale tectonic faulting, strongly suggesting that the thickening of layer. 2A away from the axis is due to successive episodes of volcanic activity increasing the thickness of the uppermost crustal section from the top, rather than tectonic fracturing lowering the uppermost crustal seismic velocities and pushing the bottom of layer 2A downwards as the crust moves away from the axis.