Structural evolution and deformation kinematics of a subduction-related serpentinite-matrix mélange, Santa Elena peninsula, northwest Costa Rica

Abstract

Detailed structural analysis of the Isla Pelada serpentinite-matrix mélange on Santa Elena Peninsula, Pacific coast of northwest Costa Rica, elucidates for the spatial distribution and temporal progression of a décollement-related deformation. This décollement zone constitutes the fault rock of a plate boundary in a subduction to collision setting. The décollement zone is defined by a thick tectonic mélange that includes blocks and belts of polymictic breccia, turbiditic greywacke, argillite, radiolarite, basalt, gabbro and serpentinized peridotite in a variably foliated serpentinite matrix. The chemical-mineral data of the peridotite blocks in the mélange indicate that they are fragments of abyssal mantle, different of the very depleted forearc mantle peridotites of the overlying Santa Elena ultramafic nappe.The structural evolution of the mélange can be divided into four main events. Early D1 deformation took place during underthrusting in the footwall of the décollement and is characterized by subhorizontal layer-parallel extension and subsequent by heterogeneous shearing of trench-fill deposits, oceanic volcanic rocks and variably serpentinized peridotites, resulting asymmetric boudinage, mesoscopic porphyroclasts and pervasive (S1) S–C fabrics. D1 is followed by layer-parallel contraction with early D2 folding and local development of an S2 pressure-solution cleavage, and late-D2 thrusting and large-scale duplexing. Therefore, D2 deformation evolves from pervasive coaxial subhorizontal contraction to a more localized non-coaxial strain (along relatively narrow thrust surfaces, in the mélange). D2 deformation mainly occurred in the hanging-wall of the décollement and was related to the downward migration of the subduction thrust. The result is the incorporation of slices of the subducted sequence by underplating into the overriding plate, typical of accretionary prisms.Kinematic indicators in the mélange are consistent with a general eastward-directed shearing during D1 and D2 deformations. This kinematics can be correlated with the direction of emplacement of the overlying Santa Elena ultramafic nappe during the early Upper Cretaceous. This also suggests the western-directed subduction of the oceanic lower plate. After the mélange was incorporated into the accretionary prism, the whole tectonic edifice was deformed in pre-late Campanian time by WNW–ESE trending D3 cylindrical upright folding and S-directed D4 narrow brittle thrusting. The recorded evolution of deformation in rocks already lithified, combined with the exotic origin of the oceanic lithologies and the serpentinite nature of the matrix points to disruption and mélange formation mainly during tectonic burial and underthrusting. In this context, the serpentinite matrix of the mélange is the result of the low-T hydration, metamorphism and deformation of mantle peridotites in a relatively shallow subduction setting.