Marine Geophysical Investigation of the Chain Fracture Zone in the Equatorial Atlantic From the PI-LAB Experiment.

Nicholas Harmon,Saikiran Tharimena,J. Michael Kendall,Catherine Rychert,Matthew Agius,Tim Le Bas,Steven Constable

doi:10.1029/2018jb015982

Abstract

The Chain Fracture Zone is a 300‐km‐long transform fault that offsets the Mid‐Atlantic Ridge. We analyzed new multibeam bathymetry, backscatter, gravity, and magnetic data with 100% multibeam bathymetric data over the active transform valley and adjacent spreading segments as part of the Passive Imaging of the Lithosphere Asthenosphere Boundary (PI‐LAB) Experiment. Analyses of these data sets allow us to determine the history and mode of crustal formation and the tectonic evolution of the transform system and adjacent ridges over the past 20 Myr. We model the total field magnetic anomaly to determine the age of the crust along the northern ridge segment to better establish the timing of the variations in the seafloor fabric and the tectonic‐magmatic history of the region. Within the active transform fault zone, we observe four distinct positive flower structures with several en échelon fault scarps visible in the backscatter data. We find up to −10 mGal residual Mantle Bouguer Anomaly in the region of the largest positive flower structure within the transform zone suggesting crustal thickening relative to the crustal thinning typically observed in fracture zones in the Atlantic. The extensional/compressional features observed in the Chain Transform are less pronounced than those observed further north in the Vema, St. Paul, and Romanche and may be due to local ridge segment adjustments.

Highlights

Oceanic transform faults accommodate differential plate motions along offsets in mid-ocean ridge segments (Sykes, 1967; Wilson, 1965) and represent an important component of plate tectonics
We find up to À10 mGal residual Mantle Bouguer Anomaly in the region of the largest positive flower structure within the transform zone suggesting crustal thickening relative to the crustal thinning typically observed in fracture zones in the Atlantic
The active spreading center in the northern Mid-Atlantic Ridge (MAR) segment is visible in the bathymetry as a median valley, deep bathymetry characterized by high backscatter intensity, flanked by shallow bathymetry and moderate backscatter

Summary

Introduction

Oceanic transform faults accommodate differential plate motions along offsets in mid-ocean ridge segments (Sykes, 1967; Wilson, 1965) and represent an important component of plate tectonics. Romanche, and Chain, which have large offsets (> 200 km), appear to have existed since the opening of the South Atlantic ~81 Ma (Figure 1; Cande et al, 1988) The sense of their ridge offsets traces the coastlines of the Gulf and Guinea of Africa and South America, and these fracture zones have been hypothesized to be initial zones of weakness during continental break up (Wilson, 1965). Understanding the structure and evolution of the active transforms of fracture zones provides insight into the long-term behavior of the fracture zones

Results

Discussion

Conclusion