Seismic structure of the St. Paul Fracture Zone and Late Cretaceous to
Mid Eocene oceanic crust in the equatorial Atlantic Ocean near 18°W
Abstract
Plate tectonics characterize transform faults as conservative plate
boundaries where the lithosphere is neither created nor destroyed. In
the Atlantic, both transform faults and their inactive traces, fracture
zones, are interpreted to be structurally heterogeneous, representing
thin, intensely fractured, and hydrothermally altered basaltic crust
overlying serpentinized mantle. This view, however, has recently been
challenged. Instead, transform zone crust might be magmatically
augmented at ridge-transform intersections before becoming a fracture
zone. Here, we present constraints on the structure of oceanic crust
from seismic refraction and wide-angle data obtained along and across
the St. Paul fracture zone near 18°W in the equatorial Atlantic Ocean.
Most notably, both crust along the fracture zone and away from it shows
an almost uniform thickness of 5-6 km, closely resembling normal oceanic
crust. Further, a well-defined upper mantle refraction branch supports a
normal mantle velocity of 8 km/s along the fracture zone valley.
Therefore, the St. Paul fracture zone reflects magmatically accreted
crust instead of the anomalous hydrated lithosphere. Little variation in
crustal thickness and velocity structure along a 200 km long section
across the fracture zone suggests that distance to a transform fault had
negligible impact on crustal accretion. Alternatively, it could also
indicate that a second phase of magmatic accretion at the proximal
ridge-transform intersection overprinted features of starved magma
supply occurring along transform faults.