5.14 Tectonic Aneurysms and Mountain Building

Abstract

Some of the most spectacular topographies on the planet arise as a product of localized interactions between climate, erosion, and tectonics. A notable example emerges when climate and tectonic systems mutually influence their thermal structures. In the atmosphere, this influence can lead to orographic effects that focus precipitation, which is funneled by rivers and glaciers into spatially restricted zones with a high potential for erosion. Focusing of this erosional power onto the solid earth at deforming convergent margins then perturbs stress fields and the material properties of the lithosphere, diverting the three-dimensional tectonic velocity field. The resulting rapid ascent of crustal material produces a hot and correspondingly weak domain in the upper crust. This weak domain further concentrates strain, generating the zone of stable, rapid uplift and exhumation that is referred to as a tectonic aneurysm. The tectonic aneurysm is characterized by extreme relief where rapid exhumation has been sustained (for 3Myr or more) and very young metamorphic assemblages are exposed adjacent to exceptionally powerful fluvial (∼200kWm−1 or more) or glacial (∼40kWm−1) erosional agents. This characteristic ensemble of metamorphic, surface, and tectonic features, which together constitute the tectonic aneurysm, is present at varying degrees in the three examples that are reviewed: Nanga Parbat, northwest Pakistan; Namche Barwa, southeast Tibet; and St Elias, southeast Alaska. Petrologic, geomorphic, and geodynamic investigations of these three extreme regions, summarized here, illuminate the nonlinear interaction and cooperative behavior of tectonics and surface processes, which are likely universal, but are most clearly manifested in these regions where the high process rates produce a high signal-to-noise ratio.