Structural architecture and micromechanics of the carbonate-hosted low-angle normal fault, Agri Valley, southern Apennines, Italy

Abstract

Slip along low-angle normal faults (LANFs) is theoretically inhibited by standard rock mechanics; however, they are well-documented in several orogenic belts. Frequently, LANFs originate along the brittle-ductile transition, where mylonitic textures form and promote strain localization during fault exhumation. Conversely, understanding how LANFs nucleate and grow in the purely brittle regime remains a crucial topic. To investigate this process, we study a carbonate-hosted LANF exhumed from shallow depths, and currently exposed in the Agri valley of southern Italy. The Marsicovetere-LANF (MLANF) is documented through field observations and microscale analysis. The fault exhibits a complex architecture, characterized by a thick fault zone including a low-angle principal slip surface (PSS), multiple sub-parallel slip surfaces (sss's) and R-shear structures. Field data suggest that low-angle slip surfaces result from the partially shear activation of bedding interfaces, while steeper R-shear structures, progressively growing adjacent to low-angle slip surfaces, facilitate stress concentration along well-oriented surfaces. The resulting fault architecture, defined by a ramp-flat-ramp geometry, arises from the interaction between steeper R-shear structures and gently dipping flat segments. At the microscale, fluidized ultracataclatic layers are frequently observed. We propose that rupture propagation, likely nucleating along steeper R-shear structures, occurs due to fluidization processes activated along low-angle surfaces. This model provides a potential explanation for the nucleation and growth of LANFs in a purely brittle regime.