Timing and style of deformation of the Andes above the normal subduction segment, 34º-35º S, Southern Central Andes: comparison with the flat slab (32º-33º S) and transition (33º-34º S) segments

LAURA BEATRIZ GIAMBIAGI; FLORENCIA BECHIS; VÍCTOR HUGO GARCÍA

Mendoza

Congreso; Backbone of the Americas - From Patagonia to Alaska; 2006

Asociación Geológica Argentina - Geological Society of America

Abstract A new kinematic model, which integrates original detailed structural data and new Ar/Ar dating with previous surface and subsurface data, is proposed for the Malargüe fold and thrust belt (MFTB). This belt extends from 34º to 36º S and forms part of the Cordillera Principal of the Southern Central Andes above the normal subduction segment.  It is characterised by an eastern sector of thin-skinned deformation and a western sector of thick-skinned deformation related to the inversion of the Triassic-Jurassic Atuel half-graben. Cross-cutting relationships, together with emplacement ages, indicate that the main phase of deformation and uplift of the Cordillera Principal occurred between 15 and 9 Ma. Comparison of the timing of deformation in the thick- and thin-skinned deformational areas strongly supports the hypothesis that the reactivation of normal faults was coeval with the activation of shallow detachments in the thrust front of the MFTB. Uplift of the Cordillera Frontal began after deformation in the Cordillera Principal had started to wane (post 7 Ma). The main phase of deformation in the normal subduction segment (15 - 9 Ma) correlates well with the main phase of deformation in the transitional segment (15 - 8.5 Ma). In the flat-slab segment, however, compressional deformation started around 21 - 22 Ma and the main phase of deformation has been constrained between 19 and 8.7 Ma. Although the onset of deformation seem to have migrated from north to south, around 22-21 Ma in the flat slab segment, 18-19 Ma in the transition zone and 16-15 Ma in the normal subduction segment, the waning of deformation started at the same time in all the segments around 9 to 8 Ma. Comparison of the timing of deformation of both Cordillera Principal and Cordillera Frontal in the flat-slab, transitional and normal segments and its relation with the reconstructed Juan Fernandez Ridge track indicates that deformation in the Cordillera Principal cannot be related to the shallow subduction. Instead, there is a good correlation in time between the arrival of the ridge (between 14 and 9 Ma) and the southward migration of deformation in the Cordillera Frontal. This indicates that the geometry of the subducted plate did not influence the timing and style of deformation in the Cordillera Principal, but it strongly influenced the foreland deformation.