Summit low‐angle faults in the Central Apennines of Italy: Younger‐on‐older thrusts or rotated normal faults? Constraints for defining the tectonic style of thrust belts

Abstract

Low-angle faults that juxtapose younger rocks over older ones are widely documented in fold-thrust belts, and reconstruction of tectonic style is strictly dependent on their interpretation. Various modes exist for generating hinterland-dipping low-angle faults with younger-on-older relationships. Indeed, in the Central Apennines of Italy, the hinterland-dipping younger-on-older low-angle faults, which rest on the summits of the major anticlines (i.e., summit low-angle faults), have been interpreted variously as younger-on-older thrusts within out-of-sequence thrust systems, postorogenic normal faults, gravity-driven slides, or as rotated prethrusting normal faults. In this study, we provide a new and robust structural-geological data set, corroborated with stratigraphic timing constraints and balanced geological cross sections, bringing an essential contribution to the interpretation of the hinterland-dipping younger-on-older low-angle faults as preexisting normal faults rotated within the shortcut anticlines during the Neogene thrust-related fold emplacement. A detailed geological and structural characterization carried out on four remarkable examples from the Central Apennines allowed reconstructing an inversion tectonics model. The lack or reverse reactivation of the rotated prethrusting normal faults here analyzed is consistent with fault bend and fault propagation folding models associated with break forward in-sequence thrust propagation implying more conservative estimates of shortening for the Central Apennines thrust system, compared to the previous out-of-sequence models. Taking into account the various possible causes for the development of hinterland-dipping younger-on-older low-angle faults, the structural-geological characterization presented in this study and the results achieved could be critically applied when examining similar structures in other thrust belts.

From Submarine to Subaerial Out‐of‐Sequence Thrusting and Gravity‐Driven Extensional Faulting: Gran Sasso Massif, Central Apennines, Italy

“…Despite several structural studies in the Gran Sasso Massif, significant uncertainty remains on the first‐order structural architecture of the area, which is characterized by high sedimentary and structural complexity (e.g., Cardello & Doglioni, , and references therein). Low‐angle fault zones outcropping along the footwall of the Campo Imperatore extensional fault system and juxtaposing Jurassic rocks onto Triassic ones (Figures a and b) have been alternatively interpreted as belonging to (i) a Mesozoic extensional fault system passively rotated during folding and thrusting (Pace et al, ; Pace & Calamita, ), (ii) a Pliocene out‐of‐sequence thrust system (Ghisetti, ; Ghisetti & Vezzani, , ), or (iii) a Pleistocene postorogenic extensional fault system (D'Agostino et al, ; Figure ). Deciphering whether this low‐angle fault system had contractional or extensional kinematics, and the chronological relationships with the underlying Vado di Ferruccio Thrust (Leah et al, ; named Upper Thrust in Cardello & Doglioni, 2015, and in Pace & Calamita, ), has important implications for understanding the Neogene‐Quaternary evolution of the Central Apennines thrust wedge and the environmental conditions of deformation.…”

Syn‐Contractional Overprinting Between Extension and Shortening Along the Montagna Dei Fiori Fault During Plio‐Pleistocene Antiformal Stacking at the Central Apennines Thrust Wedge Toe

“…The recent 2016 central Italy earthquake sequence (Bonini et al, ; Cheloni et al, ; Chiaraluce et al, ) pertains to this seismo‐tectonic framework. Deciphering whether extensional faulting developed in response to gravity during thrusting or to a regional extensional regime postdating contraction, and whether extensional fault zones are newly formed structures or reactivated structures inherited from either the passive margin or the foreland basin system environment, is not straightforward and is a matter of debate (e.g., Calamita et al, ; Coward et al, ; Ghisetti & Vezzani, ; Pace et al, ; Tavarnelli, ).…”

Defining a mid-Holocene earthquake through speleoseismological and independent data: implications for the outer Central Apennines (Italy) seismotectonic framework

“…The Apennines are a Neogene-Quaternary foreland-verging fold-and-thrust belt, showing a complex structural arrangement derived from the interaction between contractional structures and pre-existing extensional faults (e.g., Coward et al, 1999;Scisciani et al, 2002;Tozer et al, 2002;Butler et al, 2006;Calamita et al, 2011;Scisciani, 2009;Di Domenica et al, 2014a, b;Pace et al, 2014;Cardello and Doglioni, 2015). The orogenesis involved Triassic-toMiocene sedimentary successions related to different basin and platform paleogeographic domains of the Adria Mesozoic paleomargin (e.g., Ciarapica and Passeri, 2002;Patacca and Scandone, 2007).…”