Strain partitioning across the outer boundary of the Alboran Domain crystalline thrust sheet. Implications on the kinematic pattern of the Western Gibraltar Arc

Abstract

Within orogenic belts, large thrust sheets commonly present a crystalline nature. Their thickness represents a significant portion of the total crust section and they are displaced along several tens of km or more. Due to their size and particular rheological properties, strain decoupling is expected to occur between these crystalline thrust sheets and the footwall units. In this work we characterize the structure of the Alboran crystalline thrust sheet (ATS), which forms the inner core of the Western Gibraltar Arc, a Neogene collisional arc that close the Mediterranean alpine belt to the west. In this setting, we explore the modes of strain partitioning occurring during thrusting both within the ATS and across the mountain front through the underlying units. This allows us to evaluate, within the geodynamic frame of the Western Mediterranean, the contribution of the ATS kinematics in the shaping of the entire Western Gibraltar Arc. Our results point out that the main structure of the ATS in its western part is a large-scale frontal culmination wall developed over a 25 km wide hanging wall ramp. Above this ramp, the rock column (nowadays up to 11–12 km thick) is affected by ca. N-S accommodation folds, mostly vergent to the west, which are likely produced by thrust-parallel simple shear. The inferred tectonic transport direction (nearly E-W) is parallel to those of the neighbouring fold-and-thrust belt units in the western front of the ATS but significantly differs from those in the northern boundary (>20º), suggesting a transpressive regime developed in the lateral parts of the thrust sheet. Moreover, strong evidence of fold-axis parallel extension is recorded within the ATS and in the underlying units. This likely indicates the deformation within the ATS was pure shear dominated and strongly partitioned, producing a large-scale thrust at its base (localized simple shear) coupled with vertical flattening and arc-parallel stretching evenly distributed within the hanging wall. We interpret this generalized extension along the orogenic grain, identified from the fold-and-thrust belt to the inner orogenic core, as kinematically linked to the outward fanning pattern of the tectonic transport directions around the Western Gibraltar Arc and in agreement with the progressive nature of the orogenic curvature. Our results highlight that the kinematics of the Western Gibraltar Arc is largely independent of the Europa-Africa convergence, and agree well with the proposed models of westward retreat –and associated slab tearing- followed by collision in the westernmost Mediterranean.