A MECHANICAL ROUTE FROM MODERN TO ANCIENT OROGENS

Investigations of the mechanics of active orogens point toward patterns of strain, metamorphism and fluid flow that are characteristic of specific erosional regimes and that we predict would exist in ancient orogens. These robust patterns arise in two and three dimensions due to the strongly positive feedback between exhumation and deformation. Recognition of these same patterns in ancient orogens provides some constraints on the surficial processes active during orogenesis. For example, in oblique convergence, time-dependent switching from dual structures, that partition lateral and convergent strain, to a single structure that accommodates oblique relative plate motion is characteristic of strongly asymmetric erosional regimes as currently found along the Alpine Fault, New Zealand. Post-orogenic exhumation of the roots of an oblique plate boundary will expose two, parallel shear zones, one dominantly convergent and one dominantly strike slip. Widely reported orogen parallel transport in the late stages of ancient oblique convergence may be the exhumation of the lateral transport zone without any change in plate vector. Under a different erosion regime, that of efficient fluvial incision into a collisional orogen, a domal mineralogical pattern can be produced through similar feedback that results in a tectonic aneurysm and further concentration of flow into a local metamorphic hotspot. In Nanga Parbat of the western Himalaya, a tectonic aneurysm results in the growth of a lozenge of granulite facies stability surrounding a lozenge of partial melt. Passage of rock beneath and through this type of aneurysm, fixed within a crustal reference frame, produces the distinctive petrology and structure of mantled gneiss domes and is recorded within the moving petrological reference frame. Such erosional-rheological coupling can explain the occurrence of some high-grade gneiss domes in ancient collisional belts as well as the presence of active metamorphic massifs at both ends of the Himalayan orogen.