Kinematic Decoupling and Orogenic Trend Variations in Arcuate Fold-and-Thrust Belts: Exploring Possible Deep Controls

First-order orogenic arcs are often divided into second-order curves, termed salients and recesses (convex and concave to the transport direction, respectively). Although several studies have analysed the supracrustal factors controlling this festooned geometry, the potential role of deep-seated mechanisms has received little attention.

In the northern branch of the Gibraltar Arc, the orogenic grain of the central and western Betics external fold and trust belt (FTB) draws two secondary arcs, connected by a salient-recces transition segment, whose southernmost limit is the Torcal shear zone (TSZ). The central FTB salient consists of WSW-ENE to W-E thin-skinned shortening structures involving post-Burdigalian, syn-orogenic sequences in its deformation front. Thrust surfaces are dominantly SE to S-ward dipping and slickenlines suggest NNW-SSE to N-S transport directions. At the SW end of this salient, just east of the TSZ, the shortening structures trend becomes N-S. The westernmost FTB salient, within the Gibraltar Arc hinge, is defined by NW to W-ward verging, shortening structures with radial transport direction. Arc-parallel extension occurred coeval with arc-orthogonal shortening. Both salients are connected by the aforementioned transitional domain, an E-W to ENE-WSE transpressive band, dominated by dextral strike-slip deformation. This transpressive zone is significantly segmented into scattered topographic highs due to  orogen-paralell extension, mainly  accommodated by NW-SE  normal and dextral faults.

These three tectonic domains seem to have been differentiating since the upper Miocene to Holocene suggesting a decoupling between the W-ward migrating hinge of the Gibraltar Arc and the rest of the arcuate chain. Such decoupling would fit well with the existence of a W-E trending STEP fault, whose easternmost tip were located under the transition between the central and western Betics. Thus, the dominantly dextral, significantly stretched TSZ, located just north of the betic FTB/hinterland boundary, would be the expression in the FTB of such deep STEP fault. In this context, the recent FTB deformation in the central Betics would respond mainly to the current NW-SE shortening undergone by the Iberian Peninsula, whereas the kinematic features of both the transitional transpressive band and the westernmost FTB are consistent with a WNW-ESE directed far field vector associated with the arc westward migration. Interestingly, the recent intraplate deformation in the Betics foreland has produced greater relative uplifts in front of the central Betics, mostly accommodated in overall WSW-ENE faults, than in westernmost sectors. Additionally, the kinematics of reactivated structures in the westernmost sector of the foreland is compatible with a WNW-ENE convergence. Assuming some amount of mechanical plates coupling along the northern branch of the Betics, these foreland deformation features would agree with the proposed difference in the convergence angle along the central and western Betics FTB.

This work is supported by projects PID2024-159481NB-I00 and by ERDF/EU.