THE FOLD-THRUST BELT STRESS CYCLE: SUPERPOSITION OF NORMAL, STRIKE-SLIP, AND THRUST FAULTING DEFORMATION REGIMES (Invited Presentation)

Mesoscale deformation (mm to 10s of m) of sedimentary strata in fold-thrust belts results from the interaction of evolving stress regimes and mechanical stratigraphy, resulting in a mix of failure modes and orientations superimposed through the progressive deformation sequence. Tensile, hybrid, shear, compactive shear, and compactive failure may all occur, with different deformation behaviors developing simultaneously in adjacent mechanical stratigraphic layers. Mesostructural studies in fold-thrust belts typically identify suites of structures that appear inconsistent with a simple regional thrust faulting stress regime, for example the common occurrence of vertical, shortening-parallel, opening-mode fractures.

We present a unifying conceptual model consisting of a five-stage stress regime sequence: (i) early burial and normal faulting regime (vertical maximum principal stress), (ii) early horizontal compression and strike-slip stress regime (vertical intermediate principal stress), (iii) thrust belt compression with increasing minimum horizontal stress that exceeds the overburden stress, causing a switch to a thrust faulting stress regime (vertical minimum principal stress), (iv) late horizontal compression and initial relaxation with reduction in minimum horizontal stress and return to a strike-slip regime, and (v) extensional relaxation and orogenic collapse in a normal faulting stress regime.

Widespread layer-parallel shortening, which is prevalent in sedimentary fold-thrust belts and their forelands, occurs during strike-slip then thrust faulting stress regimes superimposed on burial deformation (normal faulting stress regime). This deformation sequence is commonly developed in subhorizontal strata, then reoriented during larger scale contractional folding and thrust faulting. This model not only explains common suites of mesostructures and their sequential development, but can also be used to (i) predict mesostructural suites in different regional tectonic positions (distal foreland basin, proximal foreland basin, active, inactive or collapsing fold-thrust belts), (ii) explain apparently anomalous, but in fact inherited, structures, and (iii) infer paleostress history from observable mesostructural data.