The Fifteen Twenty Fracture Zone and the North American–South American plate boundary

Abstract

The Fifteen Twenty Fracture Zone offsets the Mid-Altantic Ridge over 195 km in a sinistral sense. It was surveyed in 1982 between 30 ° W and 60 ° , i.e. from anomaly 34 on the African plate to anomaly 34 on the American plate where it disappears as the Barracuda Ridge into the Caribbean subduction zone. The fossil limbs of the Fifteen Twenty Fracture Zone show a remarkable symmetry to both sides of the Mid-Atlantic Ridge. Barracuda Ridge appears to have undergone a later deformation. The sections of the Fifteen Twenty FZ between 49 ° W and 54 ° W to the west of the Mid-Atlantic Ridge and between 37.5 ° W and 42.5 ° W to the east do not exhibit the normal characteristics of a fracture zone but have elongate and extremely narrow transverse ridges (dykes) along the fracture zone axis. The present transform domain shows signs of leaking. This is interpreted as a result of a fairly recent (about 7 Ma bp) change in spreading direction of the area. This change in spreading direction seems to have affected only the area south of 17 ° 50′N. It may have resulted from a northward migration of the plate boundary between the North American and South American plates and the coming into existence of a triple point near 16 ° N. From this reconstruction, the location of the differential pole between the North American and South American plates is 16 ° N/53.5 ° W. This location leads to increasing compression from 53.5 ° to 60 ° W, affecting the Barracuda Ridge and Trough with a N–S shortening of about 16 km in 7 Ma. From 53.5 ° to 45 ° W, N–S extension occurred (about 19 km near the Mid-Atlantic Ridge), which led to the origin of the volcano–tectonic complex of Researcher Ridge, Researcher Trough and Royal Trough to the west of the Mid-Atlantic Ridge.