The continent-to-ocean transition in the Iberia Abyssal Plain

Marta Neres,Cesar R Ranero,Rafael Bartolomé,Luis Batista,Manel Prada,Valenti Sallares,Ingo Grevemeyer,Cord Papenberg

doi:10.1130/g49753.1

Abstract

Abstract Conceptual models of magma-poor rifting are strongly based on studies of the nature of the basement in the continent-to-ocean transition of the Iberia Abyssal Plain, and suggest that exhumed mantle abuts extended continental crust. Yet, basement has only been sampled at a few sites, and its regional nature and the transition to seafloor spreading inferred from relatively low-resolution geophysical data are inadequately constrained. This uncertainty has led to a debate about the subcontinental or seafloor-spreading origin of exhumed mantle and the rift-related or oceanic nature of magmatic crust causing the magnetic J anomaly. Different interpretations change the locus of break-up by &gt;100 km and lead to debate of the causative processes. We present the tomographic velocity structure along a 360-km-long seismic profile centered at the J anomaly in the Iberia Abyssal Plain. Rather than delineating an excessive outpouring of magma, the J anomaly occurs over subdued basement. Furthermore, its thin crust shows the characteristic layering of oceanic crust and is juxtaposed to exhumed mantle, marking the onset of magma-starved seafloor spreading, which yields the westward limit of an ~160-km-wide continent–ocean transition zone where continental mantle has been unroofed. This zone is profoundly asymmetric with respect to its conjugate margin, suggesting that the majority of mantle exhumation occurs off Iberia. Because the J anomaly is related to the final break-up and emplacement of oceanic crust, it neither represents synrift magmatism nor defines an isochron, and hence it poorly constrains plate tectonic reconstructions.

Highlights

THE WEST IBERIA RIFTED MARGIN The structure of the West Iberia Margin has influenced the creation and development of conceptual models of continental rifting
At the Deep Galicia Bank Margin to the north of the Iberia Abyssal Plain, where the magnetic J anomaly is not present (Fig. S1), modern seismic data are interpreted to indicate that the oldest igneous oceanic crust is a 0.5–1.0-km-thick carapace, only locally present, overlying serpentinized mantle (Davy et al, 2016), but normal oceanic seafloor spreading crust composed of basalts, sheeted dikes, and gabbroic rocks has not been detected
The FRAME-p3 model indicates that exhumed mantle extends for ∼160 km to approximately the J anomaly basement, which disagrees with published studies (Dean et al, 2000; Minshull et al, 2014) of the Iberia Abyssal Plain structure ∼20 km to the north based on seismic line IAM-9, but the difference may be related to data resolution

Summary

Introduction

THE WEST IBERIA RIFTED MARGIN The structure of the West Iberia Margin has influenced the creation and development of conceptual models of continental rifting. In the Iberia Abyssal Plain, seismic data were used to propose mantle exhumation across an ∼80-kmwide region (Dean et al, 2000) followed by several magnetic lineations that were inferred to originate from seafloor spreading prior to the Cretaceous Magnetic Quiet Zone (Russell and Whitmarsh, 2003).

Results

Conclusion