Ductile Flow Vs. Elastic Flexure: How the Mechanism and Rate of Isostatic Subsidence can Influence a Fold and Thrust Belts Architecture

The propagation of a fold and thrust belt applies a load on the underlying crust and mantle. In warmer conditions, the lithosphere responds to this load through ductile flow, enabling Airy-like isostasy to be more prevalent in these domains. In contrast, when the fold and thrust belt evolves in cooler conditions, the lithosphere responds through elastic flexure. Here, we run a suite of high spatial resolution (80 m) geodynamic models to explore how the mechanism of isostasy and the rate of isostatic subsidence influence the architecture and structural styles of a fold and thrust belts on the 10s of km scale. Additionally, we present two numerical models to compare our results to the Subandean zone.

We find that when Airy-like isostasy is the dominant mechanism of isostasy - and with an increasing subsidence rate - fold and thrust belts are narrower and involve more complex internal architectures, including a set of sub-vertical faults that accommodate the isostatic subsidence. When the lithosphere responds through elastic flexure, fold and thrust belts are wider, higher in elevation, and involve a set of sub-horizontal faults. An increase in flexural rate does not significantly influence the first-order evolution of fold and thrust belts. Our findings are echoed in the Subandean zone, where an increase in the subsidence rate is associated with a narrower fold and thrust belt.