Crustal Structure and Continent‐Ocean Boundary Along the Galicia Continental Margin (NW Iberia): Insights From Combined Gravity and Seismic Interpretation

Abstract

The magma-poor rifted continental margin of Galicia has an extremely complex structure. Its formation involved several rifting episodes that occurred ultimately during the early Cretaceous near a ridge triple junction, which produced a change in the orientation of the main structures in its transition to the north Iberia margin. In addition, there is a superimposed partial tectonic inversion along its northwest and northern border which developed from the Late Cretaceous to at least Oligocene times. The present study integrates a large volume of new geophysical information (mainly marine gravity data and 2-D seismic reflection profiles) to provide insights on the formation of this rift system and on the development of its later inversion. The combined interpretation and modeling of this data enable the presentation of a new crustal and structural domain map for the whole Galicia margin. This includes the rift domains related to the extreme thinning of the crust and the lithospheric mantle (stretched, necking, and hyperextension and mantle exhumation domains), as well as a domain of intense compressional deformation. New constraints arise on the origin, the deep structure, and the characterization of the along-and across-strike variation of the continent-ocean transition of the margin, where a progressive change from hyperextension to partial inversion is observed. The development of both rifting and later partial tectonic inversion is influenced by the existence of former first-order tectonic features. Most of the tectonic inversion is focused on the hyperextension and mantle exhumation domain, which in some areas of the northwestern margin is completely overprinted by compressional deformation.

Bouguer anomalies of the NW Iberian continental margin and the adjacent abyssal plains

“…), the Alpine compressional stress field led to a partial tectonic inversion of the north Iberian margin (De Vicente et al, 2008;Thinon, Fidalgo-González, Réhault, & Olivet, 2001;Tugend, Manatschal, & Kusznir, 2015). The effects of this later compressional episode clearly extend to the northern sector of the west margin (Grimaud et al, 1982;Murillas et al, 1990), partly overriding the former extensional deformation domain succession (Druet et al, 2018).…”

“…Murillas et al, 1990), including a westward rift axis jump (Manatschal & Bernoulli, 1999), in the vicinity of a triple ridge junction connecting the west and north Iberian margins. As a result of this staged rifting, there is an along-strike segmentation of the margin, as well as an across-strike succession of the different deformation domains related to the rift process (Druet et al, 2018). Since Late Cretaceous (∼85 M.a.)…”

“…Whitmarsh, White, et al, 1996;Lymer et al, 2019); (4) a marginal platforms region to the northwest of Galicia, resulting from the tectonic inversion of former half-graben basins (Druet et al, 2018;Murillas et al, 1990); (5) the Iberia abyssal plain, surrounding the margin to the west, and (6) the Biscay abyssal plain, to the north and northwest. These deformation domains are related to different crustal thickness and crustal type distribution, having distinctive gravimetric signal along the margin (Druet et al, 2018). Thus, the detailed Bouguer anomaly chart that we present here is a good first approximation to address a regional study of the whole margin and to characterize its deep structure.…”

“…Álvarez-Marrón et al, 1996;Fernández-Viejo, Gallastegui, Pulgar, & Gallart, 2011). However, despite this, the transition between the west (extensional) and the north (compressional) margins has been scarcely studied to date (Druet et al, 2018).…”

“…Only then, the Bouguer gravimetric anomalies can be interpreted in terms of density, geometry and depth variations of the different anomaly sources below the water sheet (Ball, Eagles, Ebinger, McClay, & Totterdell, 2013;Carbó, Muñoz, Llanes, & Álvarez, 2003). Recently, Druet et al (2018) made a detailed gravimetric analysis from the Bouguer anomaly data of the NW Iberian margin, including spectral analysis, Bouguer anomaly grid filtering, and gravity modeling.…”