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Partitioning of transpressive motions within a sigmoidal foldbelt: the Variscan Sierras Australes, Argentina

1991; Elsevier BV; Volume: 13; Issue: 7 Linguagem: Inglês

10.1016/0191-8141(91)90001-y

1873-1201

P.R. Cobbold, Denis Gapais, Eduardo A. Rossello,

Geological formations and processes

The Sierras Australes (Buenos Aires province, Argentina) form a sigmoidal foldbelt, about 150 km long. The last major orogenic event was Variscan, as shown by synsedimentary folds in upper Palaeozoic sediments and by K-Ar or Rb-Sr ages on cleavage-forming minerals of low metamorphic grade. Folds in the Paleozoic cover are associated with cleavage, stretching lineation and kinematic indicators (shear bands and sigmoidal tails around porphyroclasts). Reworking of the granitic Precambrian basement resulted in shear zones and faults. We distinguish three structural domains within the Sierras Australes: a Northwestern Arc, a Southeastern Basin and a Central Belt. The Northwestern Arc is a simple fold-and-thrust belt, verging towards the NE. The Southeastern Basin is a foreland basin, where an underthrust towards the SW is overshadowed by a right-lateral wrench along strike. Finally, the Central Belt has resulted from a combination of right-lateral wrenching and overthrusting, both in a northerly direction. All three domains may have resulted from a uniform state of transpressive stress. Although the deformation is strongly partitioned at outcrop, it may be simpler at depth. This is suggested by experiments in which sandpacks are subjected to transpression, by reactivating a single basement fault in oblique (right-lateral reverse) slip. We describe one such experiment where fault blocks rotate counterclockwise about vertical axes, producing arcuate thrusts at the surface. By analogy, we suggest that the Sierras Australes formed in a transpressive context, as a result of oblique-slip reactivation of a deep fault zone. Strong partitioning in the upper crust was facilitated by upward splaying of faults.