Fine scale morphological and structural analysis of the East Pacific Rise, 21°N (Rita project)

Manned submersible observations on a transect from the axis of the East Pacific Rise (21°N) to the area of the Brunhes-Matuyama magnetic inversion were made during the Cyamex and Rise diving phases of the Rita project. In the northern part of the study area, axial, linear. 400 to 600 m-wide topographic highs are formed by relatively unfaulted fresh pillow-lava-flows (zone 1). Flanking the axis of recent volcanism is a 1,200 to 1.500 m-wide zone of highly fractured volcanic hilly terrain that represents the main region of extension as expressed on the surface (zone 2). In the southern part of the study area, the volcanic highs of zones 1 and 2 are flooded by fluid lava flows forming a gentle topography. In both areas, this central part of the Rise is bordered by outward-tilted flattish blocks that are mainly formed by fluid lavas. This zone. zone 3, could represent a former, large flooding episode of the sea floor at the axis of the Rise. Tectonic activity is particularly evident in zone 2, decreases rapidly across strike, and vanishes 12 km from the axis of the East Pacific Rise. The morphology of the East Pacific Rise is controlled by normal faulting, and by the relative abundance of fluid lavas and pillows-lavas which are emplaced at the axis of the Rise. The differing types of volcanism could reflect important variations in magma chamber size or depth or variations in the instantaneous accretion rates along the axis of the East Pacific Rise. [NOT CONTROLLED OCR]