Tectonic evolution of a low‐angle extensional fault system from restored cross‐sections in the Northern Apennines (Italy)

Abstract

[1] Broad geological and geophysical documentation is available on regional extensional systems driven by low-angle normal faults. However, little information exists about the three-dimensional geometry and the offset distribution of such extensional structures. We present a new set of balanced geological sections across the extensional fault system driven by the Altotiberina low-angle normal fault in the Northern Apennines of Italy. We document this extensional system throughout a large set of surface (field surveys and geological maps) and subsurface data (seismic reflection profiles and boreholes). The subsurface data allowed us to define the fault deep geometry and to obtain its structural contours. The fault geometry is characterized by both along-dip and along-strike irregularities. In cross-section, the fault displays a staircase trajectory with the shallowest part being dome-shaped and flattened to horizontal. This bending could be due to the footwall uplift triggered by a footwall uploading greater than about 115 MPa. The sequential restoration of five geological cross-sections yields a maximum extension of about 10 km accumulated over approximately 3 Ma. The resulting long-term slip-rate is about 3 mm/yr, which is of the same order as the present-day extensional rate measured by GPS (2.5-3.0 mm/yr), suggesting an almost steady state extension over the last 3 Ma. The distribution of the extension values along the fault strike is bell-shaped, as expected for a continuous surface.

On the mechanical behaviour of a low-angle normal fault: the Alto Tiberina fault (Northern Apennines, Italy) system case study

“…This theory is supported by the evidence that no moderate-to-large magnitude earthquakes have been documented worldwide nucleating on LANFs using positively discriminated slip planes from the focal mechanisms (Jackson and White, 1989;Collettini and Sibson, 2001). On the contrary, observations of large displacements (Lister and Davis, 1989;John and Foster, 1993;Hayman et al, 2003;Collettini and Holdsworth, 2004;Jolivet et al, 2010;Mirabella et al, 2011) and the well-identified microseismic activity (Chiaraluce et al, 2007;Rietbrock et al, 1996) associated with these structures suggest that the LANFs are tectonically active, accommodating crustal extension and possibly formed at low angle. For these reasons, the LANFs cannot be excluded from the assessment of seismic hazard, although their inclusion still represents a debated issue.…”

“…The beach ball is the mechanism solution of the M w 5.1 1984 Gubbio earthquake (Westaway et al, 1989;Collettini et al, 2003). The red lines represent the projection at surface of the Alto Tiberina fault system (after Mirabella et al, 2011). The yellow arrows represent GPS velocities vectors in the last 10 years .…”

“…Blue line represents the cross section used for the 2-D models. (b) Cross section used for the numerical simulations on the basis of seismic profiles (after Mirabella et al, 2011). The grey points represent the microseismicity in a range of 10 km from the cross section (dashed blue line in a).…”

The shallow boreholes at The AltotiBerina near fault Observatory (TABOO; northern Apennines of Italy)

“…Civitello; 5.6 km depth) that is one of the deepest wells drilled by the Italian National organisation for Hydrocarbons (ENI). Borehole and laboratory P wave velocities (Trippetta et al, 2010(Trippetta et al, , 2013 have been integrated with P wave velocities obtained from best migration analysis of the seismic reflection profiles (Mirabella et al, 2011) and earthquake data collected by TABOO (Latorre et al, 2014) to develop a detailed one-dimensional velocity model for earthquake locations (Table 1).…”

Ten Years of the Central Italy Electromagnetic Network (CIEN) Continuous Monitoring

“…They covered the area over the Alto Tiberina Fault, a low angle normal fault that acts as a basal seismic detachment and accommodates extensional deformation together with a complex normal fault system that is located in its hanging wall [9]. The structure bounds the western side of the upper Tiber Quaternary basin, and it is about 70 km long, dipping from 15˚ to 20˚ ENE [10]. It is clearly evident in the CROP03-NVR seismic reflection profiles [11] [12] [13].…”