Nonspreading crustal blocks at the mid-atlantic ridge.

Transverse ridges consisting of protrusions into crustal fractures of ultramafic bodies derived from the upper mantle exist at the intersection of the Mid-Atlantic Ridge with equatorial fracture zones. Shallow-water limestones containing detrital grains of quartz, microcline, and orthoclase 1 millimeter in diameter were found on the summit of one such transverse ultramafic body at the Vema Fracture Zone; these findings are explained on the assumption that the limestones were deposited within a narrow, shallow proto-Atlantic and were left behind during the further opening of the Atlantic. Transverse ultramafic bodies from the offset zones of the Mid-Atlantic Ridge behave as nonspreading blocks plastered between spreading crustal plates.