STRUCTURAL EVOLUTION OF THE SOUTHERN PATAGONIA RETROARC FOLD-THRUST BELT; OVER FOUR DECADES OF RESEARCH IN THE SOUTHERNMOST ANDES INSPIRED BY IAN DALZIEL

In 1974, Ian Dalziel published a landmark Nature paper documenting the inversion of the Rocas Verdes backarc basin (RVB) in the southernmost Andes. We build on this and subsequent work with recent structural studies of a mid-lower crustal shear zone, herein named the Magallanes shear zone, that is kinematically linked with the overlying retroarc fold-thrust belt. We present new geologic mapping, quartz microstructural and crystallographic preferred orientation (CPO) fabric analysis to document the kinematic evolution, temperature, and deformation mechanisms of the shear zone. Mafic schist of the RVB were thrust over garnet bearing chlorite schist and psammite of the Cordillera Darwin Metamorphic Complex (CDMC). Below the contact, the CDMC has a quartz/chlorite composite schistose foliation (S_1-2) that is progressively refolded by two generations of noncylindrical, tight and isoclinal folds (F₃–F₄). Deformation intensifies forming a >5 km thick high strain zone that is defined by the tightening of F₃, pronounced quartz stretching lineations (L₂), and F₃ sheath folds that are subparallel to and fold L₂. Younger kink folds and steeply inclined tight folds (F₄) with both north- and south­dipping axial planes (S₄) overprint D₂ and D₃ structures. Quartz textures from D₂ and D₄ fabrics show subgrain rotation and grain boundary migration recrystallization equivalent to Regime 3, and quartz CPO patterns with mixed <a> and [c] slip systems, indicating deformation temperatures between 500˚-650˚C. Approximately 40 km toward the foreland, the shear zone thins (~1 km thick) and is defined by the tightening of F₂ recumbent isoclinal folds and a prominent southwest plunging quartz stretching lineation (L₂). Quartz textures indicate subgrain rotation recrystallization typical of Regime 2 and quartz CPO patterns suggest dominantly basal <a> slip, consistent with deformation temperatures between 400˚-550˚C. Deformation occurred under greenschist and amphibolite facies conditions in the foreland and hinterland, respectively, indicating that the shear zone dipped shallowly (~4-15˚) toward the hinterland. The Magallanes shear zone is an example of the down-dip end of a major regional décollement that accommodated distributed mid-lower crustal thickening below a retroarc fold-thrust belt in an Andean-style setting.