Analysis of thrust shear zones in curved-shaped belts: Deformation mode and timing of the Olevano-Antrodoco-Sibillini thrust (Central/Northern Apennines of Italy)

This work reports the results of our analysis of the brittle-ductile shear zone associated with the Olevano-Antrodoco-Sibillini (OAS) curved-shape thrust in the Central/Northern Apennines of Italy. Its southern sector is characterized by NNWeSSE trending footwall anticlines, which also affect the OAS thrust surface. S tectonites in the NNEeSSW trending sector are crenulated by conjugate extensional surfaces (extensional crenulation cleavage). The NWeSE trending segment of the OAS thrust is characterized by SC tectonites developed along the thrust-related shear zone. Both the SC and S tectonites are consistent with a top-to-the N60e70E direction of tectonic transport. The SC tectonites are associated with a simple-shear-dominated deformation (Wn ¼ 0.86e0.98). On the other hand, the S tectonites and their related extensional shear planes document two stages of strain: (i) sub-simple shear (Wn ¼ 0.34e0.50, related to OAS thrusting contemporaneous to growing incipient footwall anticlines, and (ii) pure-shear-dominated deformation (Wn ¼ 0.17e0.00), subsequent to the OAS thrusting and caused by the definitive growth of footwall anticlines within an in-sequence deformation context. The present study proposes the analysis of shear zones to: (i) discriminate in-sequence against out-of-sequence evolution, and (ii) use as a tool to constrain the modes and timing of the curved belt’s development.