Deformation partitioning, foliation successions and their significance for orogenesis: hiding lengthy deformation histories in mylonites

Abstract

AbstractAlthough orogenesis commonly lasts 100 million years, a maximum of three foliations are usually preserved within any outcrop. They record little of the total history due to the effects of preferential partitioning of progressive shearing along S0. Such reactivation eventually destroys or rotates multiple successive foliations into parallelism with any layering that is present. Plate motion subjects a collisional orogen to spatially partitioned, episodic, but non-stop deformation. The bulk forces operating are horizontal and intermittently vertical (associated with gravitational load due to crustal thickening) forming sub-vertical and sub-horizontal foliations. These orientations are preserved in the strain shadows of anything competent that hinders reactivation of the compositional layering. Deformation partitioning results in portions of rock remaining unaffected for many events. Gravitational collapse of over-thickened orogen cores leads to significant extrusion in orogen rims and a dominance of sub-horizontal foliations in the latter and episodically continues throughout orogenesis. Reactivation causes decrenulation and/or rotation of foliations into parallelism with S0, destroying evidence that multiple sub-vertical and sub-horizontal foliations have developed except where preserved as inclusion trails in porphyroblasts and multiply truncated foliations in mylonites. However, the same processes operate in both the cores of orogens and ductile portions of their margins.