Seismological Analysis of Active Transverse Faults in the rifted Northern Apennines: Insights into Fault Evolution, Linkage and Inheritance

Rifts and rifting orogens display complex three-dimensional fault patterns due to prolonged basin nucleation, propagation, and interaction. During rift development, faults with some amount of strike-slip component, and that range from oblique to orthogonal (transverse) to rift trend, can transfer extension between offset basins. The Northern Apennines exemplify this with an overall left stepping extensional system, developing with a NE-migrating active extensional front which progressively overprints previously shortened domains. While prominent lateral steps often mark transverse structures at various scales, surface evidence is limited, keeping the issue of transverse faults open.

This study tries to understand the geometry, kinematics and role of transverse faults in the Northern Apennines using a seismological approach by precisely relocating dense recent sequences in the adjacent Garfagnana and Lunigiana basins, where 2013-06 Mw=4.8 and 2013-01 Mw=5.1 mainshocks occurred, respectively. Public focal mechanisms suggest involvement of transverse structures, yet the events are rooted in different extensional domains at varying evolutionary stages. The Garfagnana cluster occurred in a high-topography, poorly extended external domain near active contraction, with minor lateral steps. The Lunigiana cluster, in contrast, occurred within a low-topography, highly extended internal domain with prominent pluri-kilometric basin offsets.

Using publicly available INGV phases and waveform data, we performed absolute location with NonLinLoc and precise waveform cross-correlation double-difference relocation using HypoDD and GrowClust codes. Focal mechanisms were re-computed for the identified faults. Relocations highlighted complex fault interactions and confirmed transverse fault slip in both cases. The Garfagnana sequence revealed two deep-seated transverse faults extending to 19 km depth, with shallower faults parallel to the chain and limited surface exposure of transverse structures. We suggest fault system interactions are in an incipient stage here. However, the existence of evolved transverse structures at depth in this young extensional domain indicates rapid extensional overprinting of contractional features. The Lunigiana sequence primarily develops on transverse faults and chain-parallel faults extending from the surface down to 16 km, which are part of a regional horsetail fault system, considered a major crustal-scale transfer zone. We propose alternative kinematics and faults architecture compared to previous studies, with a more accurate solution of the deep fault architecture underlying the observed seismicity.

These two cases illustrate how active extension is partitioned within soft- and hard-linkage configurations in two different sectors of the orogen. We interpret them to be inherited from pre-orogenic or contractional structures and we suggest that the two case studies represent distinct steps of progressive deformation in the evolution of the continental transverse fault zones within the Northern Apennines. We believe the Apennines is a key region for understanding how pre-existing transverse faults persist through an orogen's entire evolution, from undeformed foreland to subduction, the orogenic belt, and proximal to distal extensional domains.