Tectonic evolution and understanding fold-and-thrust belts
In this blog Guest Editor Professor Olivier Lacombe discusses the latest thematic issue from Geological Magazine on Tectonic evolution and mechanics of basement-involved fold-and-thrust belts.
“Defining the structural style of fold-and-thrust belts is an important step for understanding the factors that control their long and short-term dynamics, for comprehending seismic hazards associated with them, and for assessing their economic potential. While the thin-skinned model (no basement involvement) has long been the driving methodology for cross section construction and restoration of foreland fold-and-thrust belts, a wealth of new geological and geophysical studies have shown that they are often thick-skinned, that is, basement-involved.
One of the key processes by which basement becomes involved in fold-and-thrust belts is the inversion of pre-existing extensional faults. Inversion tectonics is a widespread process that exerts a strong control on the structural and mechanical evolution of fold-and-thrust belts. Basement fault reactivation may induce localization of thrusts and folds, result in the incorporation of crystalline thrust sheets into the fold-and-thrust belt, cause out-of-sequence thrusting and folding of shallow thrust sheets, and lead to the development of accommodation structures such as lateral ramps.
However, extensional faults may sometimes not get reactivated despite being suitably oriented. In fact, inversion tectonics is a highly selective process that depends not only on the orientation of the faults with respect to the applied stress field and on the friction along the fault planes, but also on the contrast of strength between the inherited fault zones and the surrounding crustal rocks. This points toward the importance of rheology : not only is the rheology of a structurally homogenous crust markedly different from that of a crust in which pre-existing fault zones are suitably oriented for reactivation, but even in a pre-fractured crust, brittle thickness and strength are strongly dependent on the nature of the geotherm. Basement involvement in shortening requires an overall specific thermo-mechanical behavior of the lithosphere, e.g, a young thermo-tectonic age, so the lithosphere is generally rather hot, hence mechanically weak.
Basement-involvement in fold-and-thrust belts also raises the questions of the way the orogen is mechanically coupled to the foreland and how orogenic stresses are transmitted far into the pro or the retro-foreland through the crust and mantle lithosphere. Specific boundary conditions sometimes appear to be necessary, like those encountered in flat-slab subduction setting (e.g., Sierras Pampeanas, Laramide uplifts).
This special issue illustrates some of the diverse research that is being carried out on basement-involved fold-and-thrust belts. It aims at contributing to a better understanding of fold-and-thrust belts in particular, but also of orogenic processes and of the rheology of the continental lithosphere in general.”
All articles in the thematic issue Tectonic evolution and mechanics of basement-involved fold-and-thrust belts are freely available until 1st January 2017.