SEA-BEAM, SEAMARC-II AND ALVIN-BASED STUDIES OF FAULTING ON THE EAST PACIFIC RISE 9-DEGREES-20' N 9-DEGREES-50' N

A study of Sea Beam bathymetry and SeaMARC II side-scan sonar allows u s to make quantitative measures of the contribution of faulting to the creation of abyssal hill topography on the East Pacific Rise (EPR) 9 degrees 15' N-9 degrees 50' N. We conclude that fault locations and th rows can be confidently determined with just Sea Beam and SeaMARC II b ased on a number of in situ observations made from the ALVIN submersib le. A compilation of 1026 fault scarp locations and scarp height measu rements shows systematic variations both parallel and perpendicular to the ridge axis. Outward-facing fault scarps (facing away from the rid ge axis), begin to develop within similar to 2 km of the ridge and rea ch their final average height of similar to 60 m at 5-7 km. Beyond the se distances, outward-dipping faults appear to be locked, although the re is some indication of continued lengthening of outward-facing fault scarps out to the edge of the survey area. Inward-facing fault scarps (facing toward the ridge axis), initiate similar to 2 km off axis and increase in height and length out to the edge of our data at 30 km, w here the average height of inward fault scarps is 60-70 m and the leng th is similar to 30 km. Continued slip on inward faults at a greater d istance off axis is probable, but based on fault lengths, similar to 8 0% of the lengthening of inward fault scarps occurs within 30 km of th e axis (> 95% for outward faults). Along-strike propagation and linkag e of these faults are common. Outward-dipping faults accommodate more apparent horizontal strain than inward ones within 10 km of the ridge. The net horizontal extension due to faulting at greater distances is estimated as 4.2-4.3%, and inward and outward faults contribute compar ably. Both inward- and outward-facing fault scarps increase in height from north to south in our study area in the direction of decreasing i nferred magma supply. Average fault spacing is similar to 2 km for bot h inward-dipping and outward-dipping faults. The azimuths of fault sca rps document the direction of ridge spreading, but they are sensitive to local changes in least compressive stress direction near discontinu ities. Both the ridge trend and fault scarp azimuths show a clockwise change in trend of similar to 3-5 degrees from 9 degrees 50' N to 9 de grees 15' N approaching the 9 degrees N overlapping spreading center.