This is a proposal for two field programs to study the tectonics of two regions of the mid-ocean ridge system. The principal tool is SeaMARC II, and these data will be complemented by gravity and magnetics data. The first region of study is the Mid-Atlantic Ridge from 24 to 29 degrees North latitude. Continuous coverage of the axial discontinuities and the morphotectonic variations along the rift valley over a distance of at least 100 km will be obtained. The main objective is to understand the variations in volcanic and tectonic activity along a slow spreading center, the nature of small axial discontinuities that offset the MAR, and the mantle processes that lead to segmentation of the MAR. The second project focuses on the Australian-Antarctic Discordance (AAD). This is a unique and poorly understood region of the mid-ocean ridge system, characterized by rugged topography and a wide range of axial morphologies. The geophysical data will be complemented by a dredging program. The objectives are (1) to map the region and determine the tectonic structure and the temporal evolution of the region, (2) to determine the nature of geochemical discontinuities in the region, and the nature of the marked boundaries of the AAD, and (3) to determine the interrelationships among geochemistry, tectonic structure, gravity, and magnetics within and outside of the AAD in an effort to understand its nature, origin and evolution. These projects use an important, relatively new tool for exploring the structure of the sea floor along mid-ocean ridges. The data obtained from these studies should provide important information about the mid- ocean ridge system and its tectonic and volcanic evolution.
