70-8 Drip Tectonics and Surface Deformation in Mountain Belts: Role of Crustal Flow and Lithospheric Strength

Abstract:

Lithospheric foundering is often hypothesized as an important tectonic process, but crustal deformation and surface uplift patterns during a lithospheric drip are poorly understood. Using 324 visco-elasto-plastic numerical models, we systematically vary the lithosphere's rheology and density structure and develop a framework for understanding the tectonic effects of dripping. Model behaviors form a continuum, in which we recognize four characteristic tectonic responses: upper crustal shortening and surface subsidence (Type 1), upper crustal extension and surface uplift (Type 2), a transitional type from subsidence/shortening to uplift/extension (Type 3), and an intermediate type that combines upper crustal shortening with surface uplift (Type 4).

We argue that the occurrence of the Type 1 vs. Type 2 endmembers depends on two competing timescales: gravitational instability vs. crustal flow. We develop predictive equations for these timescales and define the crustalf flow efficiency (C), a dimensionless paramter that describes how readily a given crust flows in response to a given drip. Sustained subsidence and shortening (Type 1) requires C to be less than approximately 0.2, while uplift and extension (Type 2) requires C > 0.2. This timescale analysis gives a physical basis for the critical crust/mantle viscosity ratio (η' ) that produces surface uplift vs. extension in classic Rayleigh-Taylor instability analyses, and it further predicts a strong dependence of η' on crustal buoyancy.

In practice, the response type can be predicted from lithospheric strength profiles together with the instability timescale. For example, Type 1 requires "crème brûlée" profiles, in which most of the strength resides in the crust, while Type 2 dominates "jelly sandwich'" profiles, which have a strong upper mantle. We find that the mantle lithosphere's rheology exerts the strongest control on drip style, followed by eclogite density and rheology. Processes that modify mantle rheology, such as hydration/metasomatism, heating, melting, and melt extraction, could therefore alter the tectonic expression of dripping through time or along strike in large orogens.

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