Using 3D kinematic models in subduction channels. The case of the Chañaral tectonic mélange, Coastal Cordillera, northern Chile

Abstract The Chanaral tectonic melange (northern Chile) is a local unit within the late Paleozoic accretionary complex formed at the southwestern margin of Gondwana. The structural characteristics of the studied melange were mostly developed during a first deformation phase (D1) and include a block-in-matrix fabric, lineations (L1) and foliations (S1), tight to intrafoliar folds, S-C and S-C-C′ composite planar fabrics, and a conspicuous spatial separation of domains with predominantly linear and linear-planar fabrics. Folding during a second stage (D2) modified the orientation of the previous fabrics and structures. The eastern boundary of the Chanaral melange is N-S to NNW-SSE oriented, moderately dipping to the east. Its western boundary is not exposed. The plane showing the maximum structural asymmetry (the vorticity normal section) is ENE-WSW directed, and sub-vertical. Kinematic criteria consistently reveal top-to-the-WSW displacement. A kinematic model of triclinic transpression with inclined extrusion has been applied to evaluate the D1 structural features of the Chanaral melange. The pitch of the simple shear direction on the deformation plane ranged from 60°N to 90°. The pitch of the estimated extrusion direction was of 30°-40°S. The coaxial component was clearly constrictional (logarithmic K value of 2 to 5). The vorticity number has not been constrained by the model, but its spatial variation can explain the domainal distribution of the fabrics in the melange. The simulated particle paths show the predominance of material displacement parallel to the margin, with low to moderate down-dip displacements, which is in accordance with the low-pressure metamorphic assemblages found in the melange. The convergence direction between the blocks separated by the melange unit was N50°-60°E. Kinematic blocking of the Chanaral melange, probably related to the accretion of an oceanic volcanic domain, allowed the D2 folding of the previous structures, a process that, at least initially, proceeded without a change in the convergence direction.