Sections of the Earth's crust in the equatorial Atlantic

Abstract

Crustal sections, in some cases exceeding 5 km in thickness, are exposed where the Mid-Atlantic Ridge is offset by major equatorial fracture zones. Systematic sampling of some of these sections has resulted in the recovery of rocks from over 100 sites, especially concentrated on the Vema and Romanche fracture zones. The vertical and lateral distribution of the various rock types (i.e., basalts, metabasalts, a variety of gabbros and metagabbros including rodingites and amphibolites, serpentinized peridotites, metaserpentinites, serpentinite and basaltic breccias, sedimentary serpentinites, and various mylonites and sedimentary rocks) has been used to construct models of the structure of fracture zones. It is suggested that sections of ‘normal’ oceanic crust can be exposed in fracture zones, as well as sections of ‘anomalous’ crust produced by processes occurring solely within fracture zones. The northern wall of the Vema fracture valley is probably an exposed section of normal oceanic crust, as suggested by several lines of evidence, including the petrology of basalts recovered from it. One of the inferences drawn from the distribution of rocks in this section is that serpentinites are probably a significant component of the oceanic crust, being emplaced as mantle-derived vertical intrusions in deep fault zones parallel to ridge axis. The southern margin of the Vema fracture valley, as well as both margins of the Romanche fracture, is the locus of prominent positive topographic anomalies (transverse ridges) caused by processes of crustal generation restricted to fracture zones. These processes involve (1) crustal uplift related to diapiric intrusions into the fracture zones of mantle-derived serpentinized peridotite, (2) intense tectonization of the rock units, (3) minor alkali basalt volcanism, and (4) hydrothermal activity and related metallogenesis.