SHALLOW STRUCTURE OF OCEANIC-CRUST IN THE WESTERN NORTH-ATLANTIC FROM SEISMIC WAVE-FORM INVERSION AND MODELING

Seismic reflection data from Mesozoic oceanic crust in the western North Atlantic, in the vicinity of the Blake Spur Fracture Zone, have imaged a number of short, subhorizontal events in the upper 1 km of the crystalline crust. Wide-angle expanding spread profile (ESP) data from the same region indicate the presence of first-order velocity discontinuities in this depth interval. Using ESP data reprocessed to enhance precritical reflection events, we investigate the relationship between the precritical and postcritical reflections with three different approaches: amplitude analysis, forward modeling, and waveform inversion. The precritical reflections have high apparent reflection coefficients (approximately 0.1) and may be divided into two types: those which can be traced out to critical ranges (5-6 km), representing velocity steps, and t,hose which fade out at ranges of 3-4 km, representing shorter-wavelength velocity variations. Synthetic seismogram modeling indicates that the velocity steps occur within a 50 m depth interval, while the shorter-wavelength features may correspond to high- or low-velocity zones with a thickness of 20 m (10% of the dominant seismic wavelength) or less. We investigate the velocity structure in detail by full waveform inversion of a selected portion of ESP data, transformed into the intercept time-slowness domain. The inversion method minimizes the sample-by-sample misfit between the data and reflectivity synthetics in a least squares sense. A conjugate gradient algorithm was used, starting from a velocity structure derived from forward modeling of wide-angle ESP data. The inversion tightly constrains short-wavelength components of the velocity structure, and indicates that intracrustal reflections result both from major velocity steps and from zones of alternating high and low velocity. Synthetic seismograms generated from sonic log dat a from Deep Sea Drilling Project hole 418A show reflections of similar amplitude and character to those seen in the ESP data. These reflections probably originate from rather subtle features of the velocity-depth profile, related primarily to changes in porosity in the pillow lava sequence. A deeper reflector, 800 ms below oceanic basement in the Blake Spur Fracture Zone, may correspond to the top of a zone of serpentinized upper mantle.