Far-field strain transmission and contractional step-overs

Abstract

In contractional (subduction/collisional) settings, convergence is accommodated by the formation of thin- and thick-skinned thrust and fold belts. The transmission of such deformation over larger distances into orogenic foreland areas is influenced by the inherited rheological characteristics of continental lithosphere. Lateral rheological variations parallel to the strike of continental foreland areas creates contrasting geometries and sequences of deformation that interact during orogenic build-up. We investigate the far-field transmission of strain within a continental lithosphere characterized by a laterally variable rheology through physical analogue modelling. Rheological weak crustal zones were introduced at distance from an advancing backstop to study the progressive along strike linkage and interference of structures during contraction. The results reveal that rheologically weak crustal zones localise far-field contractional deformation. When the size of weak zones, by means of their horizontal extend to depth ratio, is large, deformation localises at the boundaries of the weak zone where they lead to the formation of large-offset faults. Subsequently the faults migrate along-strike into areas that are rheologically stronger. When the size of the weak zone is reduced, a large-scale contractional step-over forms in orogenic forelands, where rheologically contrasting domains transmit out-of-sequence deformation by a gradual migration of thrust offsets and fold amplitudes along their strike. These results show that crustal scale orogenic step-overs do not always reflect variations in the geometry of the plate boundary (indenter) or along-strike gradients in shortening rates. Such features may also form in response to variations in rheology, as the ones created by inherited extensional basins situated at large distances from plate boundaries in the orogenic foreland.