Mid-Ocean Ridges: Fine Scale Tectonic, Volcanic and Hydrothermal Processes Within the Plate Boundary Zone

A first order model of spreading centers as idealized linear boundaries of crustal and lithospheric generation provides only a gross understanding of global scale plate kinematics. As we attempt to understand the complexity of crustal and lithospheric structure of two thirds of the earth's surface, it is becoming increasingly necessary to study the tectonic, volcanic, and hydro­ thermal processes within the spreading center plate boundary zone. All ocean­ ic crust bears the imprint of these processes . This review focuses on a few selected topics concernining the fine scale tectonics and geophysics of the active axial zone of mid-ocean ridges with reference to associated volcanic and hydrothermal processes. It draws heavily on recent studies that use deeply towed instrument packages, multi-beam bathymetric mapping, ocean bottom instruments , and ALVIN (e.g. the Famous, AMAR, RISE, and Galapagos expeditions). We begin with a review of the large-scale structure of spreading centers. We then take a close look at the axial neovolcanic zone and progress away from the axis through the active tectonic zones. Next we consider the characteristics of the axial magma chamber and associated hydrothermal actvity , as well as the generation of magnetic anomaly stripes and their implications for crustal generation. One of our findings is that the initial two-dimensional model of volcanic and tectonic zones must be expanded upon to allow for variations