Abstract
The Basque-Cantabrian junction corresponds to an inverted rift accommodation zone at the limit between the former hyperextended Pyrenean and Cantabrian rift segments. The recognition of an inherited rift segment boundary allows to investigate the reactivation associated with large-scale rift segmentation in an orogenic system. We use criteria from published field observations and seismic data to propose a new map of rift domains for the Basque-Cantabrian junction. We also provide balanced cross-sections that allow to define the along-strike architecture associated with segmentation during rifting and subsequent Alpine reactivation. Based on these results, this study aims to characterize and identify reactivated and newly formed structures during inversion of two rift segments and its intermitted segment boundary. It also aims to describe the timing of thin-skinned and thick-skinned deformation associated with the inversion of segmented rift systems. During convergence, two phases have been recognized within the rift segment (eastern Mauléon basin). The Late Cretaceous to Paleocene underthrusting/subduction phase was mostly governed by thin-skinned deformation that reactivated the former hyperextended domains and the supra-salt sedimentary cover. The Eocene to Miocene collisional phase, controlled by thick-skinned deformation that took place once necking domains collided and formed an orogenic wedge. At the rift segment boundary, the underthrusting/subduction phase was already controlled by thick-skinned deformation due to the formation of shortcutting thrust faults at the termination of overlapping V-shaped rift segments. This led to the formation of a proto-wedge composed of the Basque massifs. We suggest that this proto-wedge is responsible for the preservation of pre-Alpine structures in the Basque massifs and for the emplacement of subcontinental mantle rocks at a crustal level beneath the western Mauléon basin. These results argue for a first order cylindrical orogenic architecture from the Central Pyrenean segment to the Cantabrian segment (up to the Santander transfer zone) despite rift segmentation. They also highlight the control of 3D rift-inheritance for the initial phase of orogenic evolution and for the local architecture of mountain belts.
Highlights
In most collisional orogens worldwide, the restoration of the pre-collisional stage and the role of rift-inheritance have been investigated in the external, fold-and-thrust belt domains by the help of 2D balanced cross-sections
We aim to study the role of segmentation and alongstrike rift-inheritance on the 3D orogenic architecture using the example of the Western Pyrenees
In the northern branch of the overlapping rift system, the north-vergent WNW-ESE striking North Pyrenean Frontal Thrust (NPFT) transferred the deformation from the western termination of the Pyrenean segment to the Cantabrian segment, whilst in the southern branch, the south-vergent E-W striking Roncesvalles thrust fault transferred the deformation between the eastern termination of the Cantabrian segment to the Pyrenean segment (Figs. 4 and 5)
Summary
In most collisional orogens worldwide, the restoration of the pre-collisional stage and the role of rift-inheritance have been investigated in the external, fold-and-thrust belt domains by the help of 2D balanced cross-sections. Of particular importance to understand the structural evolution of internal parts of orogenic systems are the boundaries between rift domains (Sutra et al, 2013), where changes in mechanical and rheological properties occur (Mohn et al, 2014; Tugend et al, 2015; Chenin et al, 2017) These studies stressed in particular the role of the “coupling point” defined as the boundary between the necking and hyperextended domains, corresponding to the limit between crust thicker than 10 km showing ductile levels in the crust; and crust thinner than 10 km where the residual continental crust is brittle and the top of the mantle is serpentinized and can act as an efficient decoupling horizon during reactivation (Péron-Pinvidic et al, 2008). Such 3D implications might account for some of the complexities encountered when dealing with 2D restorations and explain geological anomalies observed in orogenic systems
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