Strain Localization, Basement Involvement and Interpretation Strategies for Continental Thrust Belts

Unlike their deepwater counterparts, continental thrust systems are notoriously badly imaged on seismic reflection profiles. Consequently structural interpretations are traditionally highly model-dependent and structural styles are regularly applied from a rather small number of “classic” locations or even analogue experiments. For the past 30 years, section balancing has been used to limit the range of perceived plausible explanations, and for much of this time a variety of computer generated solutions have been adopted. The effect has been to artificially restrict the range of thrust-fold relationships used in interpretations with the consequent over-optimistic assessment of uncertainties. Most interpretations fail the test of drilling or, if untested, contribute to potentially unfounded tectonic descriptions of mountain belts. A variety of outcrop, growth strata and seismic/well-based investigations is establishing the importance of buckle folding in general contractional deformation. Although the structural styles can be simplified in terms of trishear, strain localization through multilayers can show considerably greater complexity. Additionally the tacit assumption of regionally extensive basal detachments continues to inhibit understanding of thrust systems developed through previously rifted successions. These issues are discussed using examples from the external Alps and Apennines. Both settings show important basement involvement through reactivation and ramp-seeding, despite regional detachments. Pre-existing normal faults strongly influence fold-thrust growth. Multilayer carbonate-shale sequences are characterised by variations in thrust localization through the stratigraphic pile. Complex fold-thrust alternations are also required to explain growth stratal patterns. While these styles may be simplified in terms of fault-propagation or trishear-type models in order to construct and restore regional profiles across entire thrust systems, such simplifications are inappropriate for understanding individual fold-thrust complexes and their internal deformation structures. General restoration and forward-modelling approaches are needed while existing ones are used to test, rather than build, structural models of fold and thrust belts.