Elsevier

Marine Geology

Volume 407, January 2019, Pages 299-315
Marine Geology

Morphostructure of the Galicia continental margin and adjacent deep ocean floor: From hyperextended rifted to convergent margin styles

https://doi.org/10.1016/j.margeo.2018.11.011Get rights and content

Highlights

  • Multibeam bathymetry and seismic allow mapping morphostructure of the Galicia margin.

  • Galicia margin hosts transition between two end-members of continental margin stages.

  • Western margin related to the Mesozoic hyperextended rift margin

  • Northern margin related the Cenozoic subduction active margin

  • Morphostructure styles characterise main stages of the margin's evolution along the Wilson cycle.

Abstract

A joint study and mapping of the morphological features and architecture of the Galicia margin based on high-resolution multibeam bathymetry data and multichannel seismic profiles, provides new insights of the margin morphostructure. Tectonic processes are the major control of the margin's morphology, where the imprint of the North Atlantic rifting and opening and later subduction of the Bay of Biscay and compressional events are still well preserved. The West Galicia margin, which is up to 350 km wide, is composed from east to the west of inner, transitional and outer sectors. The inner sector shows a graded upper slope strongly dissected by a system of submarine canyons and valleys that flow into the 170 km long and 40–60 km wide NW-SE Valle Inclán Valley. The transitional sector, which is up to 75 km wide, displays a smooth relief dominated by landslides and giant craters. Finally, the outer sector is characterised by several banks, N-S sub-parallel ridges and elongated valleys that project seaward over 200 km (i.e., Galicia Bank, Half-Graben, Deep Galicia margin and Peridotite Ridge domains). The North Galicia margin is characterised by a system of terraces up to 30 km wide, that form a stepped slope followed by an abrupt lower slope affected by large-scale rotational failures.

The Galicia margin constitutes an exceptional area within the Atlantic margins for identifying different morphostructure styles that characterised the main stages of the margin's evolution along the Wilson cycle. The Galicia margin hosts the transition between two end-members of continental margin styles: a western hyperextended rifted margin (being the conjugate with the Newfoundland margin during the Late Jurassic) and a northern convergent margin (now fossilised) associated with the partial subduction of the southern sector of the Bay of Biscay underneath the North Iberian margin, which occurred in Paleocene-Eocene times. Thus, the morphostructure of the West Galicia margin still reflects the detached fault systems and tilted blocks that resulted from the Mesozoic North Atlantic rifting. Furthermore, the Coruña and Jean Charcot seamounts, which are adjacent to the Galicia margin, correspond to spreading ridges formed by the simultaneous opening of North Atlantic Ocean and the Bay of Biscay in the Upper Cretaceous. The rest of the terraced morphology of the North Galicia margin is derived from extensional faults caused by an overriding of the Iberian plate in relation to the subduction of the Bay of Biscay oceanic crust during Cenozoic times. Since the late Neogene, compressional tectonics resulting from the reorganisation of the Eurasia/Iberia and Africa plates is also reflected on the present-day morphology as the following: (i) an elevation of the Biscay Abyssal Plain up to 150 m over the Iberia Abyssal Plain; (ii) deep-seated landslides associated with an uplift of the Galicia Bank; and (iii) an orientation of submarine canyons along NW-SE strike slip faults.

Introduction

The evolution of ocean basins and continental margins according to the “Wilson Cycle” (Wilson, 1966), a paradigmatic concept in plate tectonics, can be described in the following three phases: (1) opening and spreading; (2) foundering of the passive margins and development of new subduction zones; and (3) consumption and closure. The Wilson Cycle is not only registered in the geological record but is also expressed in the sea-floor geomorphology of the margins defining the morphological expression of these tectonic processes as morphotectonics. Therefore, the tectonic processes governing the evolution of ocean basins and continental margins give rise to different geomorphic features corresponding to the different stages of evolution (Harris et al., 2014).

The margins of the Atlantic Ocean are generally described as typical examples of passive margins (e.g., Wilson, 1966; Cloetingh et al., 1982), even though three subduction arcs are reported: the Scotia Arc (e.g., Jabaloy et al., 2003; Maldonado et al., 2014), the Lesser Antilles Arc (e.g. Westbrook et al., 1984) and the Gibraltar Arc (e.g. Maldonado et al., 1999; Gutscher et al., 2002; Duarte et al., 2013). However, the first transition from an Atlantic passive margin into an active one took place along the North Iberian margin during early Cenozoic times (Álvarez-Marrón et al., 1997; Gallastegui and Pulgar, 2002). This subduction resulted from the inversion of a hyperextended rift margin along the Iberian-European plate boundary (e.g., Tugend et al., 2014; Cadenas et al., 2018), when the oceanic lithosphere of the Bay of Biscay underwent southward subduction beneath the Iberian plate during Paleocene-Eocene times (e.g., Boillot et al., 1979; Álvarez-Marrón et al., 1997; Gallastegui and Pulgar, 2002). At the same time, the West Iberian margin remained as a hyperextended rifted margin resulting from the poly-phase extension of the Iberian-Newfoundland conjugate margin during the Upper Jurassic (e.g., Boillot and Malod, 1988; Péron-Pinvidic and Manastchal, 2008).

The Galicia margin (GM), which is located at the northwestern edge of the Iberian Peninsula (Fig. 1), has been the object of numerous tectonic and geodynamic studies (e.g. Montadert et al., 1974; Mauffret et al., 1978; Boillot et al., 1979, Boillot et al., 1987a, Boillot et al., 1987b; Sibuet and Ryan, 1979; Vanney et al., 1979; Reston et al., 1996; Péron-Pinvidic et al., 2007). The margin hosts the western prolongation of the Mediterranean Alpine-Pyrenean collision belt and the Mesozoic seafloor spreading of the North Atlantic Ocean and Bay of Biscay (e.g., Sibuet and Collette, 1991). The GM represents one of the rare examples where both the hyperextended rift and the convergent margin can be investigated in one and the same area (Fig. 1).

The main objective of this work is twofold as follows: i) mapping and characterisation of the morphological features across the GM; ii) establishing the interrelations between the submarine morphology and the sequence of tectonic events that affected the margin(s). This objective is met through the joint interpretation of an extensive set of high-resolution multibeam bathymetry (166,000 km2) and multichannel seismic lines (2896 km). The multibeam data are used to categorise the morphological elements of the seabed and the multichannel seismic lines to correlate the deep structure of the GM with this seabed morphology. The morphostructural features are then related to successive tectonic events that shaped this complex margin.

Section snippets

Geodynamic evolution of the West Iberian Margin as a hyperextended-rifted margin

The first studies of the GM began during the 1970's and dealt with the tectonic and the geodynamic evolution of the West Iberian margin (e.g., Montadert et al., 1974, Montadert et al., 1979; Mauffret et al., 1978; Groupe Galice, 1979; Sibuet and Ryan, 1979; Malod et al., 1982; Boillot et al., 1987b). The first geomorphological map of the GM was published by Vanney et al. (1979). The map was based on single-beam bathymetry and single-channel seismic reflections profiles and it described the main

Bathymetric data

This study uses a large multibeam bathymetric echosounder (MBES) dataset acquired for the Spanish Economic Exclusive Zone (EEZZ) and Extended Continental Shelf (ECS) mapping programs of the Galicia area (Somoza et al., 2009, Somoza et al., 2017). Data were obtained over seven oceanographic cruises from 2001 to 2008 on board the Spanish research vessel “Hespérides”. MBES data were acquired with a Kongsberg Simrad EM12 system during the older cruises (2001−2003) and with a Kongsberg Simrad EM120

Morphostructure of the West Galicia hyperextended rifted margin

The WGM is composed by an inner and outer sector divided by the so-called Transitional Zone. The inner WGM is constituted from land to sea, by the shelf and upper slope, and by the NNW-SSE trending Valle Inclán Valley, which is the morphological expression of the Galicia Interior Basin (Fig. 2, Fig. 3). The outer WGM is constituted by several geomorphic domains that project seawards over 200 km into the eastern Atlantic Ocean (Fig. 3): (i) the Galicia Bank and Tablemounts domain; and (ii) the

Discussion

The morphology of the GM from the shelf to the deep sea varies substantially in its different sectors (Fig. 3). The NGM is characterised by a rounded shelf break and a system of up to 30 km wide terraces forming a stepped slope ending into an abrupt lower slope. In contrast, the WGM is characterised by outer and inner geomorphic domains developed along a distinctive broad transitional region between the continental shelf and the deep basin. We propose that the difference in the morphostructure

Conclusions

The Galicia margin (NW Iberian margin) represents a complex key area in the North Atlantic Ocean, as it links the North Iberian relict subduction margin and the west Iberian hyperextended margin. This complex tectonic history is reflected in two main morphostructural regions, namely, the North and West Galicia margins. This work contributes to previous studies on the GM by revealing the linkage between the geomorphic forms and processes, through the examination of the geodynamic evolution of

Acknowledgements

The authors wish to thank all those involved in data acquisition along this extensive margin and for making these data available. The Spanish Extended Continental Shelf bathymetric and seismic data in the Galicia area submitted to United Nation Convention for the Law of the Sea (UNCLOS) form the framework of this study. We wish to thanks the crews of the research vessels Hespérides and Sarmiento de Gamboa involved in the cruises and technicians from the Unit of Marine Technology (UTM-CSIC).

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