Structural setting
The outer portion of the Northern Apennines is bound by the Pliocene Olevano-Antrodoco-Sibillini (OAS) thrust, cropping out along the mountainous front between the Umbria-Marche mountain ridge and the Marche-Abruzzo pede-Apennines area (Fig. 3). The thrust plane has an irregular structural pattern roughly defined by frontal NW-SE and oblique NNE-SSW trending thrust ramps to the northwest and southwest of the apical zone, respectively (Fig. 4).
Figure 3. Schematic geologic map of the Northern Apennines.
Bold lines indicate the locations of photographs in Fig. 4. Bold squares indicate the location of Fig. 8.
Figure 4. OAS thrust front
Photographs of the OAS thrust front, showing the frontal NW-SE trending (a) and the oblique NNE-SSW trending (b) thrust ramps, located north and south of the apical zone, respectively (for location, see Fig. 3).
The OAS oblique thrust ramp (NNE-SSW trending sector) reactivated the Ancona-Anzio line (Fig. 5) that separated the Umbria-Marche pelagic domain and the Lazio-Abruzzo carbonate platform (Calamita et al., 2011; Di Domenica et al., 2012, and references therein). The Neogene kinematics of the OAS has been long debated, with special concern being given to the presence of a dextral component of shear (Castellarin et al., 1978; Salvini and Vittori, 1978; Coli, 1981; Koopman, 1983; Satolli and Calamita, 2008). Fold profiles are controlled by structural heritage. In fact, the NNE–SSW trending anticlines show a fault-bend reactivation mechanism, whereas the NW–SE trending anticlines develop with a fault-propagation shortcut (Calamita et al, 2012). The OAS thrust shows shear zones characterized by foliated fault rocks, which are produced by pressure-solution, cataclastic, and slip deformation mechanisms (Koopman, 1983;). In the NW-SE trending sector of the OAS frontal thrust ramp, SC tectonites are developed in centimeter-thick bands and show N60° tectonic transport within a simple shear-dominant deformation. Here, the S fabric is parallel to the XY plane of the strain ellipsoid. In the NNE-SSW trending oblique ramp of the OAS thrust, S-tectonites parallel to the thrust plane are characterized by a slip vector, oriented ca. N65°, within a pure shear-dominant deformation (Fig. 6). This fabric is compatible with the kinematics of the NW-SE trending sector of the OAS thrust (Calamita et al., in press).
Figure 5. N-S trending Jurassic normal faults
Sketch illustrating the N-S trending Jurassic normal faults between the Lazio-Abruzzo carbonate platform and the Umbria-Marche slope-to-pelagic domains (A). These faults were reactivated during the Neogene Period compressive event (B). The vertical scale is exaggerated.
Figure 6. Geometric and kinematic analyses of the OAS thrust shear zones.
Kinematic compatibility between the frontal (a-c) NW-SE- and oblique (d-e) NNE-SSW-trending ramps; photographs of the shear zones (a, d); equal angle projections in the lower hemisphere of all structural features (b, e); and equal angle projections of the lower hemisphere of averaged structural features (c, f).