ANOMALOUS OPENING OF THE EQUATORIAL ATLANTIC DUE TO AN EQUATORIAL MANTLE THERMAL MINIMUM

The geology of the Equatorial Atlantic is dominated by a broad east-we st megashear belt where a cluster of large fracture zones offsets anom alously deep segments of the Mid-Atlantic Ridge (MAR). The origin and evolution of this megashear region may lie ultimately in an equatorial mantle thermal minimum. The notion of a mantle thermal minimum in the Equatorial Atlantic is supported by an equatorial minimum of zero-age topography, a maximum in mantle shear waves seismic velocity and a mi nimum in the degree of melting, indicated by the chemistry of MAR basa lts and peridotites. This thermal minimum has probably been a stable f eature since before the Cretaceous separation of Africa from South Ame rica; it caused a pre-opening equatorial continental lithosphere thick er and colder than normal. The Cretaceous Benue Trough in western Afri ca and the Amazon depression in South America are interpreted as morph ostructural depressions created or rejuvenated by strike-slip, transpr essional and transtensional tectonics during extension of the cold/thi ck equatorial lithosphere, The oceanic rift propagating northward from the South Atlantic impinged against the equatorial thicker, colder an d, therefore, stronger than normal continental, lithosphere that conse quently acted as a 'locked zone'. This, and a low magmatic budget due to the cold upper mantle, caused a lower than normal rate of propagati on of the oceanic rift into the equatorial belt, with diffuse deformat ion during mostly amagmatic extension. The thick/cold lithosphere prev ented major Cretaceous igneous activity from the St, Helena plume. Eve ntually initial 'weak' isolated nuclei of oceanic lithosphere were emp laced, separated by E-W continent/continent transforms. Opening occurr ed largely by strike-slip motion along these initial transforms. The c onsequences were that the Equatorial Atlantic opened prevalently along an E-W direction, in contrast to the N-S opening of the North and Sou th Atlantic, and that sheared continental margins are particularly wel l developed in the Equatorial Atlantic. After further continental sepa ration the cold equatorial mantle caused a low degree of melting (with Na-rich MORE and alkali basalt rather than normal MORE and with undep leted mantle peridotites), thin crust, depressed ridge segments and a prevalence of amagmatic extension. Similar conditions still exist toda y. Long transforms offsetting short ridge segments kept sea floor spre ading unstable and dominated by transform tectonics, with transform mi gration, transpression, and transtension causing strong vertical motio n, emersion and subsidence of lithospheric blocks, development of deep pull-apart basins, and preservation of relict slivers of old lithosph ere (occasionally even of continental lithosphere) within younger crus t. The equatorial transforms are caused ultimately by a long lived the rmal minimum in the upper mantle and not vice versa; however, they the n create second-order 'rebound' thermal effects that help maintain the thermal minimum in the upper mantle. It can be speculated that mantle thermal minima at the Earth's equator might be related to true polar wander triggered by subduction of dense masses into the mantle.