Kinematic Coupling Between Hinterland and Foreland Deformation In Convergent Orogens

The high-grade internal cores of many convergent orogens are characterized by flat-lying pervasive foliations. Integrated 3D strain and quantitative vorticity analyses has demonstrated that flow associated with formation of many of these high strain tectonites / mylonite zones has involved important pure shear components associated with vertical thinning (assuming present day geographic coordinates) and concomitant maximum principal stretching (extension) in the transport direction indicated by grain shape and crystal fabrics, thrust/nappe geometries, etc. Examples include: the Greater Himalayan Slab of the Everest, Annapurna and Sutlej River regions; the European Alps and the Caledonides of the North Atlantic region.

In some of these cases (e.g. Greater Himalayan Slab in the Everest region) high temperature deformation/flow in these flat-lying mid-crustal high strain zones appears to have been close to plane strain with no significant strain along orogenic strike, while in other cases (e.g. Moine thrust zone mylonites of NW Scotland) flow was three dimensional (non plane strain) with locally either stretching or shortening (intermediate principal strain direction) along orogenic strike. In all cases however, these now well documented departures from strict simple shear deformation - with maximum principal stretching always in the transport direction - indicate significant space problems that must be compensated for by processes such as volume loss or extrusion towards the syn-orogenic topographic surface, or by linkage to other structural processes operating at shallower crustal levels. Stated differently, significant components of pure shear deformation at deeper crustal levels may act as drivers for synchronous structural processes operating up transport direction at shallow crustal levels. Such extrusion process must involve a rapid increase in deformation rates traced up-transport from the extrusion source.

We illustrate this review with closely integrated vorticity and 3D strain data from Moine thrust zone mylonites exposed at the Stack of Glencoul in northern Assynt.