Cooperative Institute for Research in Environmental Sciences

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Wiki Royden

Abstract: Long-Distance Flow in the Lower Continental Crust

Exposures of ductilely deformed rocks are common in the cores of many orogenic systems, ancient and modern. Recently, earth scientists have begun recognize that the lower continental crust can flow laterally for hundreds of kilometers within deep "crustal channels". This process occurs in extensional and convergent deformational zones, probably being most unambiguously documented in the Basin and Range Province of the US and in Tibet and surrounding regions. Data from these regions shows that the lateral transfer of crustal material by lower crustal flow can be extremely large and that flow may occur without significant shortening or extensional deformation of the overlying crust. For example, uplift of most of eastern Tibet from near sea level to its current elevation of nearly 5 km seems to have occurred almost exclusively by eastward flow of deep crustal material from beneath the high plateau, with approximately 10⁸ km³ of lower crustal material having been translated eastward over distances of more than 100 km. Where lower crustal flow occurs over such great distances it is driven by lateral pressure gradients within the crust and associated with regional topographic gradients that drive flow from areas of higher to lower topographic elevation.

The behavior of the deep crust probably depends on its temperature, composition and water content. Thus regions that have higher temperatures within the deep crust are more likely to undergo regional scale flow than colder regions, but rock-type must play an important role as adjacent blocks of continental crust can show very different types of behavior within the deep crust. Because deep crustal flow can produce large changes in crustal thickness without coincident deformation at the surface, regional-scale topography in conjunction with regional patterns of deformation remains one of the best tools available for studying the behavior of the deep crust. Numerical modeling of crustal deformation shows that the presence or absence of a weak lower crust should be associated with systematic differences in regional topography and strain partitioning in deforming areas, consistent with observations from a variety of orogenic belts.

The presence of very weak lower crust precludes the transmission of significant shear stresses from the mantle to the overlying crust, producing effective decoupling between upper crust and mantle (although crust and mantle may be strongly coupled at the edges of the region of weak lower crust). The sobering implication for such regions is that surface deformation does not provide information about the behavior of the underlying mantle.