Lithospheric transition from the Variscan Iberian Massif to the Jurassic oceanic crust of the Central Atlantic

M. Fernàndez*, I. Marzán, M. Torne

*Corresponding author for this work

Research output: Contribution to journalArticle

45 Citations (Scopus)

Abstract

A 1000-km-long lithospheric transect running from the Variscan Iberian Massif (VIM) to the oceanic domain of the Northwest African margin is investigated. The main goal of the study is to image the lateral changes in crustal and lithospheric structure from a complete section of an old and stable orogenic bel - the Variscan Iberian Massi - to the adjacent Jurassic passive margin of SW Iberia, and across the transpressive and seismically active Africa-Eurasia plate boundary. The modelling approach incorporates available seismic data and integrates elevation, gravity, geoid and heat flow data under the assumptions of thermal steady state and local isostasy. The results s how that the Variscan Iberian crust has a roughly constant thickness of ∼30 km, in opposition to previous works that propose a prominent thickening beneath the South Portuguese Zone (SPZ). The three layers forming the Variscan crust show noticeable thickness variations along the profile. The upper crust thins from central Iberia (about 20 km thick) to the Ossa Morena Zone (OMZ) and the NE region of the South Portuguese Zone where locally the thickness of the upper crust is <8 km. Conversely, there is a clear thickening of the middle crust (up to 17 km thick) under the Ossa Morena Zone, whereas the thickness of the lower crust remains quite constant (∼6km). Under the margin, the thinning of the continental crust is quite gentle and occurs over distances of ∼200 km, resembling the crustal attitude observed further north along the West Iberian margins. In the oceanic domain, there is a 160-km-wide Ocean Transition Zone located between the thinned continental crust of the continental shelf and slope and the true oceanic crust of the Seine Abyssal Plain. The total lithospheric thickness varies from about 120 km at the ends of the model profile to less than 100 km below the Ossa Morena and the South Portuguese zones. An outstanding result is the mass deficit at deep lithospheric mantle levels required to fit the observed geoid, gravity and elevation over the Ossa Morena and South Portuguese zones. Such mass deficit can be interpreted either as a lithospheric thinning of 20-25 km or as an anomalous density reduction of ∼25 kg m-3 affecting the lower lithospheric levels. Whereas the first hypothesis is consistent with a possible thermal anomaly related to recent geodynamics affecting the nearby Betic-Rif arc, the second is consistent with mantle depletion related to ancient magmatic episodes that occurred during the Hercynian orogeny.

Original languageEnglish
Pages (from-to)97-115
Number of pages19
JournalTectonophysics
Volume386
Issue number1-2
DOIs
Publication statusPublished - 2 Aug 2004
Externally publishedYes

Keywords

  • Depleted mantle
  • Geoid
  • Gravity
  • Heat flow
  • Plate boundary
  • SW Iberia

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