Structural analysis of an accretionary prism in a continental collisional setting, the Late Paleozoic Pulo do Lobo Zone, Southern Iberia

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Models concerning the tectonic evolution of accretionary complexes typically relate outcrop-scale to plate-scale multiphase deformation as a smooth variation of strain on all scales. However, at oblique convergent margins, regional scale brittle faults in the shallow crust are commonly parallel to the main orogenic grain. These faults impose a strong structural anisotropy and can subsequently control deformation at subordinate scales. As a result, finite strain in each domain may not record local kinematics consistent with the overall orogenic-scale motion implying that structural data must be analyzed selectively from a large area in order to relate outcrop-scale kinematics to global plate-scale dynamics. Field mapping and preliminary structural analysis of the Late Devonian Pulo do Lobo (PDL) Formation, and suspect “exotic” South Portuguese Zone (SPZ) in southern Iberia indicate tectonic juxtaposition of diverse deposits such as foreland basin flysch, sedimentary and tectonic mélange, and passive margin sediments showing an overall geometry consistent with an accretionary wedge setting. Variations in finite strain, lithology and regional structure were used as proxies for defining tectonic domains for structural analysis. Numerous local kinematic indicators within the PDL suggest a complex regional deformation with several enigmatic features that can be explained by sequential compartmentalization of strain during the development of the imbricate stack followed by late-stage bulk strain imposed across the entire complex. Structural data produced by local strain partitioning reveals kinematic indicators, which contradict the overall regional structural style (e.g. spatial juxtaposition of sinistral and dextral fabrics). When viewed at larger scales (i.e. regional scale), however, these data indicate that significant sinistral strike–slip movement occurred in conjunction with both an extension and shortening. Outcrop-scale deformation in polydeformed domains is controlled by local conditions resulting from brittle deformation coeval with orogenic-scale bulk strain. The entire Pulo do Lobo Zone is dominated by a pervasive late-stage vertical to sub-vertical E–W cleavage axial planar to chevron folds which overprint earlier deformation in the older passive margin units. This overprinting suggests that in the late stages of the evolution of the accretionary complex, bulk strain was imposed over the entire complex as a result of internal locking of the accretionary complex and reduced strain rates during the waning stages of collision between Gondwana and Laurussia. Stereographic analysis of fabric elements from each distinct tectonic domain, together with regional geological constraints, support this hypothesis and are indicative of progressive deformation imposed on the PDL during the Variscan Orogeny.