Superposed and refolded metamorphic isograds and superposed directions of shear during late orogenic extension in the Alborán Domain, southern Spain

Abstract

The timing, kinematics, and mode of exhumation within the Alpujárride Complex, the largest tectonic unit of the Alborán Domain, are controversial. Alternative tectonic models include synorogenic exhumation during pre-Miocene crustal thickening, intraorogenic exhumation during repeated phases of contraction and extension, and late orogenic exhumation during early Miocene crustal thinning. New structural and petrographic relationships documented in this study support the latter model: subhorizontal foliations, stretching lineations, and recumbent folds are associated with early Miocene high-temperature decompressional metamorphism and vertical condensation of the metamorphic sequence. Relict isograds associated with high-pressure metamorphism and burial are systematically overprinted by isograds associated with high-temperature metamorphism and rapid decompression. During the early stages of extensional exhumation and growth of sillimanite, ENE directed shear (D2) overprinted thrusts and foliations associated with crustal thickening (D1). During the later stages of extensional exhumation and growth of andalusite, from ductile to semibrittle conditions, north directed shear (D3) produced kilometer-scale recumbent folds that refold isograds. Stretching lineations characterize superposed directions of orogen-parallel (L2) and later orogen-perpendicular (L3) shear associated with a rapid temporal transition in the kinematics of extensional exhumation. During the final stages of crustal thinning and cooling, ductile structures (D2/D3) were cut by brittle low-angle normal faults (D4).