Lateral changes in the crustal architecture of the Eastern Pyrenees: Assessing Moho and Conrad geometries using potential field data

The Eastern Pyrenees mark the transition of the Pyrenean Range towards the Mediterranean Sea. Because of this specific structural location, crustal geometries in this area register a significant along-strike change, mainly resulting from the overprint of Neogene extension on the Late Cretaceous-Cenozoic orogenic structure.  Along-strike crustal changes in the Eastern Pyrenees are mainly marked by the lateral termination of the Iberian lower crust subduction and the progressive eastwards thinning of both the Iberian and European crusts. Although Moho depth studies in the area are abundant and agree on the main crustal architecture, they show significant depth differences that locally reach ca. 10-12 km underneath the Axial Pyrenees. Besides, these previous works rarely evaluate lower crust geometries and their relationship to upper crustal features.

To address both issues (differences in Moho depths and lower crust geometries), we have collected previous crustal data along three cross-sections and modelled them in 2.5D using available gravity and magnetic information. Constructed models (i) are tightly constrained at upper crustal levels by surface geology, exploration wells, petrophysical data and preceding studies on cover and basement units and (ii) compile and test various Moho geometries derived from an extensive compilation of available geophysical data. For lower crustal levels, the modelling of previous Moho surfaces has constrained the geometry of the upper-lower crust boundary (i.e., the Conrad discontinuity).

Models challenge some of the previously proposed Moho surfaces, which provide geologically inconsistent Conrad discontinuities. Besides, they highlight a progressive shallowing and Moho/Conrad topography decrease to the East. In the West, modelled Conrad discontinuities depict a lower crust that thickens significantly from the foreland domains towards the Axial Pyrenees. These lower crust geometries align with upper crust orogenic shortening values from the literature, without requiring the subduction of the Iberian plate. In the East, the modelled lower crust thins moderately underneath the Axial Pyrenees. Obtained geometries indicate a significant lower crustal thinning, consistent with an increased crustal extension eastward.