Serpentinized peridotite versus thick mafic crust at the Romanche oceanic transform fault

Emma P.M Gregory,Milena Marjanović,Satish C Singh,Zhikai Wang

doi:10.1130/g49097.1

Abstract

AbstractThe crust beneath transform faults at slow-spreading ridges has been considered to be thin, comprising a thin mafic layer overlying serpentinized peridotite. Using wide-angle seismic data, we report the presence of a Moho at ∼6 km depth and a low-velocity anomaly extending down to 9 km beneath the 20-km-wide Romanche transform valley floor in the equatorial Atlantic Ocean. The low crustal velocities above the Moho could be due to either highly serpentinized mantle peridotite or fractured mafic rocks. The existence of clear Moho reflections and the occurrence of a large crustal-depth rupture during the 2016 magnitude 7.1 earthquake suggest that the crust likely consists of fractured mafic material. Furthermore, the presence of low velocities below the Moho advocates for extensive serpentinization of the mantle, indicating that the Moho reflection is unlikely to be produced by a serpentinization front. The crust to the north of the transform fault likely consists of mafic material, but that in the south appears to be more amagmatic, possibly containing serpentinized peridotite. Our results imply that the transform fault structure is complex and highly heterogeneous, and thus would have significant influence on earthquake rupture and alteration processes.

Highlights

Oceanic transform faults (OTFs) comprise over 40% of mid-ocean-ridge (MOR) spreading systems (Bird, 2003), with their inactive traces—fracture zones (FZs)—extending across entire ocean basins
OTFs offsetting slow-spreading ridges are thought to exhibit thinned crust, comprising a thin mafic layer overlying serpentinized mantle rocks (Detrick et al, 1993). This perception is mainly based on the recovery of serpentinized peridotites from OTF valleys and bounding walls (Prinz et al, 1976; Cannat et al, 1991) and early geophysical studies showing an absence of a gabbroic crustal section (White et al, 1984; Whitmarsh and Calvert, 1986; Detrick et al, 1993)
We present the results of a geophysical study across the slow-slipping Romanche OTF in the Atlantic Ocean, the longest active OTF on Earth

Summary

INTRODUCTION

Oceanic transform faults (OTFs) comprise over 40% of mid-ocean-ridge (MOR) spreading systems (Bird, 2003), with their inactive traces—fracture zones (FZs)—extending across entire ocean basins. OTFs offsetting slow-spreading ridges are thought to exhibit thinned crust, comprising a thin mafic layer overlying serpentinized mantle rocks (Detrick et al, 1993). This perception is mainly based on the recovery of serpentinized peridotites from OTF valleys and bounding walls (Prinz et al, 1976; Cannat et al, 1991) and early geophysical studies showing an absence of a gabbroic crustal section (White et al, 1984; Whitmarsh and Calvert, 1986; Detrick et al, 1993).

Findings

DISCUSSION

CONCLUSIONS