LONG-LIVED OCEANIC TRANSFORM BOUNDARIES FORMED ABOVE MANTLE THERMAL MINIMA

The causes of segmentation of mid-ocean ridges by long-lived transform boundaries are poorly understood. Large (>200 km) offsets of the Mid- Atlantic Ridge are distributed neither regularly nor randomly, but are mostly clustered in the equatorial region, where they can be traced o utside the active transform to South American and African sheared cont inental margins. This permanent cluster of transforms is located above a long-lived upper mantle thermal minimum. Permanent transforms at 50 degrees-55 degrees N and perhaps at 45 degrees-50 degrees S are assoc iated with mantle secondary thermal minima. A cluster of transforms di srupts the mid-ocean ridge above a mantle thermal minimum at the Austr alian-Antarctic discordance. Conversely, ''hot'' stretches of the Mid- Atlantic Ridge are devoid of major transforms. Low magmatic budget and high-strength rheology of cold, thick lithosphere above cold mantle c aused the formation of weak and unstable oceanic rift segments during the opening of the equatorial Atlantic and Australian-Antarctic oceans , favoring the development of initial transform clusters and maintaini ng them as permanent structural and/or geochemical boundaries. Thus, s ome long-lived transform clusters may be caused ultimately by upper ma ntle thermal minima, and not vice versa; thermal anomalies created in the mantle by these transforms (''transform cold edge effect'') are se cond-order ''rebound'' effects.