Recent advances in the understanding of mid-ocean ridge tectonics and volcanism using swath-mapping tools



New swath mapping sonar systems have revealed the structure of the mid-ocean ridge (MOR) and sunounding deep ocean floor with unprecedent clarity. These images show that the MOR is partitioned into segments by a variety of offsets such as transform faults, overlapping spreading centers and very fine scale discontinuities which are barely detectable. The smallest scale segments are the fundamental building blocks for creation of new oceanic crust. They are only 2-20 km long and are active, distinct units for only =100-10,000 years. At fast spreading centers, the axial neovolcanic zone is a persistent 300-400 m high elevation produced by the bouyancy of hot rock and magma; it is not a volcanic construction (most of the time and in most plac e ~ )so there is almost no vestige of it off-axis. Along ~60%of its length, the spreading center is characterized by an axial summit caldera produced by volcanic inflation and collapse. The size and shape of the axial high are very sensitive indicators of a relatively steady and robust magma supply at fast-spreading ndges, and have been used successfuliy to predict the location of magma chambers and to forecast recent volcanic emptions including one witnessed from ALVIN in March-Apnl 1991. At intermediate spreading rates, the axial region cools sufficiently for a volcanic constructional edifice to develop episodically and for normal faulting to occur along an axial graben. Under these conditions axial volcanoes are split in two by the axial graben and rernnants can be found on the flanks of spreading centers. At slow-spreading ridges, the magma budget is relatively starved as indicated by a persistent axial nft valley and a highly discontinuous neovolcanic zone, and strong asymmetry in profiles along and across the strike of the ridge.

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