Fine-scale seismic structure of young upper crust at 17°20?S on the fast spreading East Pacific Rise
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 The detailed upper crustal structure of the East Pacific Rise (EPR) at 17degrees20’S is examined by applying a genetic algorithm-based waveform inversion to five multichannel seismic lines: one on-axis and two to either side along 42- and 85-kyr-old crust. On-axis, a double-stepped velocity pattern is recorded beneath 70-100 m of low-velocity extrusives (2.1-2.4 km s(-1)). We define the upper velocity contrast as the base of seismic layer 2A due to its severity and continuity along and across axis. The more subdued and intermittent lower-velocity step is not observed off-axis. Material between the two high-gradient intervals is proposed to represent the pillow/dike transition, bounded above by a sharp increase in dike fraction with depth and below by an abrupt change in rheology and/or deformation. Extrusive velocities increase quite rapidly in this area, with velocities similar to3 km s(-1) common in crust less than or equal to85 kyr old. This, plus a rapid (300-400 m) thickening of layer 2A observed within 1-4 km of the rise axis, indicates that this segment is undergoing focused melt delivery (<500-m-wide dike intrusion zone) and elevated hydrothermal activity. These findings demonstrate the ability of single-ship multichannel data to record detailed information on the reflectivity and velocity of the upper crust and the ability of the genetic algorithm to efficiently construct accurate seismic models based on this information.