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 separates Earth' surface into rigid plates (McKenzie & Parker, 1967;Morgan, 1968), and deformation or relative motion between plates reveals three different types of oceanic plate boundaries: (a) constructive plate boundaries at mid-ocean ridges (MOR) where new seafloor is created, (b) destructive plate boundaries at subduction zones where the oceanic lithosphere is transferred into the mantle and recycled, and (c) conservative plate boundaries and hence transform faults (TF) where the lithosphere is neither created nor destroyed as plates move past each other (Morgan, 1968). In ocean basins, transform faults

The Role of Crustal Accretion Variations in Determining Slab Hydration at an Atlantic Subduction Zone

“…Velocities as slow as 7.2 km s -1 observed beneath the Moho in such segments indicate extensive hydrous alteration within the incoming plate. The FZs and NTOs themselves do not stand out from the structure of the surrounding crust, but instead have a structure similar to neighbouring segments, consistent with other recent work in the equatorial Atlantic (Growe et al, 2021;Marjanović et al, 2020).…”

“…By comparing the velocity structures from the two VoiLA profiles (Figure 4), we find section 1 (which lies closest to the 15-20 FZ) strongly correlates with the structure of the ends of magmatically robust spreading segments observed on line 2/3, and section 2 as from a central portion of the segment where melt is more focused. Modest crustal thinning towards the segment ends is consistent with trends observed both within the magmatic spreading segments of Line 2/3 and studies of crustal thickness trends within magmatic segments at the MAR (Growe et al, 2021;Lin & Phipps-Morgan, 1992;Lin et al, 1990;Marjanović et al, 2020;Wang et al, 2015). The interpreted boundary between sections 1 and 2 occurs ~ 75 km from the mapped 15-20 FZ, which is further than any of the segment ends in the Line 2/3 survey area, but both the segment width and FZ offsets are significantly larger than observed in the Davy et al ( 2020) study.…”

“…Recent wide‐angle studies from the Atlantic, have found the structure (and thus degree of alteration) at FZs to be highly dependent on the mechanism of crustal accretion in the adjacent ridge segments. Studies at the Marathon (Davy et al., 2020), St. Paul (Growe et al., 2021) and Chain (Marjanović et al., 2020) FZs, flanked by magmatically robust ridge segments found little structural variation and little indication of elevated hydration, while at the Romanche transform fault, flanked to the south by a tectonically accreting ridge segment Gregory et al. (2021) observed up to 30% serpentinization of the upper mantle (and potentially up to 80% within the fault zone itself).…”

Deep hydration and lithospheric thinning at oceanic transform plate boundaries

“…The presence of thin lithosphere would have a significant effect on the geodynamics of the ridge-transform system. It might account for some recent observations of thick magmatic crust at oceanic TF 5 and FZs 31 , 32 in the equatorial Atlantic Ocean. Wide-spread volcanism is generally absent along oceanic TFs due to the cold edge effect 33 .…”

Seismic evidence for uniform crustal accretion along slow-spreading ridges in the equatorial Atlantic Ocean

“…2a ). The St. Paul TF system encompasses four TFs interrupted by three intra-transform ridge segments, generating a total offset of ~600 km 43 . Away from its eastern ridge-transform intersections (RTIs), the traces of the four TFs persist up to ~20 Ma old seafloor and then only two traces of the TFs can be identified on the older seafloor 44 .…”