A BASE-LINE FOR UPPER CRUSTAL VELOCITY VARIATIONS ALONG THE EAST PACIFIC RISE AT 13-DEGREES-N

A wide aperture profile of the East Pacific Rise at 13 degrees N provides data necessary to make a high-resolution seismic velocity profile of the uppermost crust along a 52-km segment of ridge crest. Automated and objective processing steps, including tau - p analysis and waveform inversion, allow the construction of models in a consistent way so that comparisons are meaningful. A continuous profile is synthesized from 70 independent one-dimensional models spaced at 750-m intervals along the ridge. The resulting seismic velocity structure of the top 500 m of crust is remarkable in its lack of variability. The main features are a thin low-velocity layer 2A at the top with a steep gradient to layer 2B. The seafloor velocity is nearly constant at 2.45 km/s +/- 3% along the entire ridge. The velocity at the top of layer 2B is 5.0 km/s +/- 10%. The depth to the 4 km/s isovelocity contour within layer 2A is 130+/-20 m from 13 degrees to 13 degrees 20'N, north of which it increases to 180 m. The increase in thickness is coincident with a deviation from axial linearity (DEVAL) rioted by both a slight change in axis depth and orientation and in geochemistry. The waveform inversion, providing more details plus velocity gradient information, shows a layer 2A with about 80 m of constant-velocity material underlain by 150 m of high velocity gradient material, putting the base of layer 2A at approximately 230 m depth south of 13 degrees 20'N and about 50 m thicker north of the DEVAL. The overall lack of variability, combined with other recent measurements of layer 2A thickness along and near the axis, indicates that the thickness of volcanic extrusives is controlled not by levels of volcanic productivity, but the dynamics of emplacement. The homogeneity along axis also provides a baseline of inherent variability in crustal structure of about 10% against which other observed variations in similar regimes can be compared.