Lava Transport and Accumulation Processes on EPR 9 27'N to 10N: Interpretations Based on Recent Near-Bottom Sonar Imaging and Seafloor Observations Using ABE, Alvin and a new Digital Deep Sea Camera

Sonar and digital photographic images of the seafloor on the East Pacific Rise (EPR) crest at 9 50'N and 9 28.5' N were collected during two cruises using the DSL-120A sidescan sonar, autonomous underwater vehicle ABE (Autonomous Benthic Explorer), a new digital towed camera, and the submersible Alvin. High-resolution micro-bathymetry, and sidescan sonar images of the EPR crest were correlated with visual and photographic observations of the seafloor. These data show evidence of channeled lava transport and extensive re-paving by successive volcanic flows consistent with eruptions originating from the axis and flowing onto the upper rise flank out to ~2 km. The EPR crest out to approximately 2 km from the axis, between 9 27'N and 10N, is, in map-view, dominated by enechelon and coalescing scalloped acoustic reflectors that we infer to be interfingered lava flow fronts. These reflectors are a few hundred meters to more than 1 km long, and are convex outward (away from the axis) in form suggesting primary flow patterns are downslope to either side of the ridge axis. Eruption of lava within or proximal to the axial trough and downslope transport is the dominant constructional mode for building the fast spreading crust in this area. ABE microbathymetry (1 m contour interval) shows the fronts to be several meters high, while towed camera digital images indicate that the lava fronts are either bulbous pillows and tubes, or distal lobes of lobate flows. Lava flows within 2 km of the EPR axis have predominantly lobate morphologies. There are numerous indications in the sonar imagery of low amplitude, dendritic flow channels emanating east and west from the axial trough along various sections of the ridge axis in the 9 28'N and 9 50'N region. Towed camera imagery confirmed that the channels are floored by smooth-surfaced sheet flows. Characteristics of the near-bottom magnetic field as measure by ABE correlate positively with interpretations and observations of lava morphology from the sonar, observational and photographic data. The western edges of the Central Anomaly Magnetic High (CAMH) in the 9 50'N and 9 28'N region appear to follow the scalloped flow front edges in the ridge crest morphology whereas the eastern edges of the CAMH are more linear. At 9 28.5' N, the eastern edge of the CAMH follows a set of outward facing scarps that bound a heavily sedimented trough. The CAMH follows the zone of younger volcanic infill where the morphology suggests that lava flowed over the eastern edge and locally filled the trough. In contrast, at 9 50'N, the eastern CAMH edge is extremely sharp and linear. It does not follow the scalloped plan-view shape of the volcanic morphology but lies consistently at 1.7 km from the axis. Sidescan and Alvin observations of a 200-m long recent normal fault suggest the linearity and sharpness of this CAMH edge is due to an active normal growth fault. The fault is mostly buried beneath the recent lava flows in the area, however, immediately south of the area this normal fault is continuous over several kilometers in the side-scan records.