Geological mapping of the East Pacific Rise axis (10 degrees 19' 11 degrees 53'N) using the ARGO and ANGUS imaging systems

The Acoustically Navigated Geological Underwater Survey (ANGUS) and Argo Systems were used to map the neovolcanic zone of a segment of the East Pacific Rise from just north of the Clipperton fracture zone (CFZ) at 10 degrees 19 minutes N-11 degrees 53 minutes N, including the overlapping spreading centres and overlap basin from 11 degrees 39 minutes N to 11 degrees 51 minutes N. The neovolcanic zone on the ridge segment east of the overlap basin is dominated by at least two generations of relatively recent volcanic sheet-flow units, with associated collapse structures and hydrothermal deposits, superimposed on an older sediment-laden pillow terrain. Recent volcanic activity along the ridge north of the CFZ and west of the overlap basin is restricted to a narrow zone on two axial topographic highs centred at 10 degrees 55 minutes N and 11 degrees 26 minutes N, extending as far south as 10 degrees 37 minutes N and as far north as 11 degrees 51 minutes N. North of approx 10 degrees 52 minutes N, this recent volcanic activity is restricted to an axial graben 10-100 m wide and 20 m deep. South of 10 degrees 50 minutes N to approx 10 degrees 38 minutes N, recent volcanic activity occurred along a constructional high. The floor of the overlap basin consists mainly of old sedimented pillows. On the E ridge of the overlapping spreading centres, hydrothermal vents occur within collapse structures. On the ridge segment north of the CFZ and west of the overlap basin, hydrothermal vents occur along fault complexes that flank the ridge crest and axial graben. These observations support the concept that magmatic activity along spreading centres is episodic and that such activity is not continuous throughout the ridge segment at any given time. It is suggested that the magmatism follows a cyclical pattern, progressing from initial pillow-lava emplacement, to extrusion of sheet flows, to a waning phase of pillow extrusion, and finally to pelagic sedimentation.