Tectonics of the East Pacific Rise, 5⁰ to 12⁰S

Graduation date: 1975The portion of the East Pacific Rise (EPR) between 5° and 12°S\ud can be subdivided into three areas by fracture zones that offset the\ud rise axis at about 6.5°S and 9°S. In all three regions, the spreading\ud axis of the EPR is defined by a topographic block 300 to 350 m high\ud and about 15 km wide. The axial block commonly has subordinate\ud peaks or shoulders on each flank that vary up to 5 km wide.\ud Analysis of linear magnetic anomalies in the southern of the\ud three areas suggests that sea-floor spreading has been occurring at\ud rates of 80 mm/y to the west and 77 mm/y to the east during the\ud last 1.7 million years (my). The rise axis is offset in two places\ud about 10 km. These displacements are interpreted to be the result of\ud discrete jumps of the site of spreading activity. Reconnaissance data\ud from the east flank of the EPR indicate that spreading activity on this\ud portion of the rise began just over 8 my ago when the site of crustal\ud accretion jumped westward from the now dormant Galapagos Rise. About 4.6 my ago the section of rise crest between approximately\ud 10.5°S and the 9°S fracture zone jumped westward a distance of at\ud least 10 km, isolating the pre-existing axial block. By 1.7 my ago,\ud this axial offset no longer existed, implying a relatively westward\ud migration of the rise axis south of 10.5° during the period from 4.6\ud to 1.7 my ago. The two offsets of the present axis occurred between\ud 1.6 and 0.9 my ago and 0.7 my ago and the present in the north and\ud south, respectively.\ud The formation of the axial block, its associated shoulders and\ud the abyssal topography, apparently consisting of tilted fault blocks\ud with steep sides facing the axis, can be explained by an empirical\ud model. This model combines extrusion of basalt along the spreading\ud axis to form the axial block and rotational faulting to form the\ud shoulders of the block and then lower the shoulders to merge with the\ud abyssal topography of the uppermost rise flanks.\ud In the region north of the 6.5°S fracture zone, seismic activity\ud on the rise crest, combined with topographic information, suggests\ud that the EPR axis is unstable here and that it is in the process of reorienting\ud to a more stable regional trend. Identification and mapping\ud of magnetic anomalies in the vicinity of 6°S reveal a spreading rate\ud of about 77 mm/y, an episode of oblique spreading that occurred 1.7\ud my ago, and a 10 km westward jump of the axis which occurred since\ud 0.7 my ago. The fracture zone at 6.5°S offsets the EPR axis 55 km right laterally. Both the bathymetric and magnetic expression of\ud this fracture zone are markedly reduced a few tens of kilometers\ud away from its active portion, implying that some change in the processes\ud forming the fracture zone occurred about 0.3 my ago.\ud Between the fracture zones at 6.5° and 9°S, two deactivated\ud segments of the rise axis, each about 40 km long, occur 90 and 65\ud km west of the currently active axis. These inactive axial segments\ud were isolated by eastward jumps of the spreading center. The more\ud northerly jump, along 6.5°S, occurred 0.3 my ago and spanned 68\ud km. The other, along 7°S, covered a similar distance and is either\ud just completed or still going on. Reconnaissance data from this part\ud of the EPR suggests that sea-floor spreading began here about 6.5\ud my ago and has been continuing at a rate of approximately 78 mm/y