THE CORDILLERA BLANCA DETACHMENT, PERU: ACTUALISTIC MODEL FOR INITIATION OF EXTENSION IN A CONVERGENT OROGENIC SETTING

The Cordillera Blanca Detachment (CBD) in the Peruvian Andes is a rare example of active orogen-normal extension within a convergent orogen where recently exhumed tectonites can be correlated to ongoing seismicity and observable geodynamic boundary conditions. Study of the CBD provides insight into processes of syn-convergent extension and may help to elucidate the tectonic controls on extinct orogen-parallel extensional structures such as the Bitterroot and Anaconda core-complex detachments along the Mesozoic convergent margin of Idaho and western Montana. The CBD comprises a ~50-100m thick shear zone of brittle and ductile tectonites including mylonite, ultramylonite, pseudotachylyte, breccia and specular gouge. The detachment, active since about 5 Ma, dips moderately SW and juxtaposes Paleogene to recent sedimentary and volcanic basin-fill strata in the hanging wall against the ~8 Ma Cordillera Blanca batholith and Mesozoic metasedimentary country rocks in the footwall. Microseismicity is limited to the upper 8-10 km of the crust suggesting that plastic creep accommodates most deformation below this level. Microstructural and textural analysis of CBD tectonites provides valuable insight into the role that the thermomechanical state of the crust plays in localizing the initiation of extension in convergent orogens. Grain size paleopiezometry and texture analysis indicates fairly high flow stresses of 30-80 MPa at temperatures ranging from ~300-550º C. Ongoing geothermometry and fluid inclusion work will provide further constraints on deformation conditions.