Lateral variations of crustal rheology and surface heat flow in the northern Apennines: correlations with tectonic inversion.

The role of lateral variations of crustal rheology and surface heat flow in the Northern Apennines is explored in order to investigate the geodynamic conditions correlated with the formation of a new E-dipping normal faults in the belt. The sharp variation of the heat flow at the surface along SW-NE profiles marks the transition from a western or Tyrrhenian domain, where extensional deformation has destroyed the preexisting compressional belt, and an eastern or Adriatic domain, where the compressional structures are preserved. The present extension-compression boundary approximately coincides with the youngest and easternmost expression of extension of the Northern Apennines: the Alto Tiberina E-dipping normal fault and associated antithetic faults (cf. e.g Barchi et al., 1999; Boncio e Lavecchia 2000; Collettini and Barchi, 2002; Piccinini et al., 2003; Chiaraluce et al., 2007; Mirabella et al., 2011). The transition between the two domains is approximately marked by the zero gravimetric anomaly (Marson et al., 1998) and by an increase in the depth of the Moho towards the east (from about 20 km to about 40 km, cf. e.g. De Franco et al., 1998). In the transition area, a decrease of surface heat flow is observed, from values of 150 mW/m2 or higher in the Tyrrhenian domain to 70-40 mW/m2 in the Adriatic domain However, the change in surface heat flow occurs in an area where the measuring points are very few (CNR, 1994). To reconstruct the thermal state of the subsurface, we review and analyze the available geothermal measurements from 192 deep boreholes present in this sector of the Northern Apennines and drilled for geothermal and oil exploration purposes, available from the Istituto di Geoscienze e Georisorse (IGG) of the Italian National Research council (CNR) which, has updated the Italian National Geothermal Database (Trumpy and Manzella, 2017). The borehole temperatures have been corrected for drilling, inclination, and topographic effects. The corrected temperature data, combined with petrophysical parameters for each individual formation, have been used to derive shallow geotherms (down to a maximum depth of 8 km), which have yielded site-specific heat flow values. These values, once corrected for palaeoclimatic and erosion/ sedimentation effects, have been contoured by a kriging procedure to obtain the heat flow map. The heat flow map shows clearly two areas, a western one characterized by high and spatially inhomogeneous heat flow, and an eastern one characterized by lower and spatially more homogeneous heat flow (extending approximately to the Adriatic coast). The surface heat flow isoline of 70 mW/m2 separates these two areas, and correlates well with a significant tectonic transition, separating a rheologically weak, hot lithosphere in the western sector and a stronger, cooler lithosphere in the eastern sector, as shown by rheological profiles. Surface heat flow increases (to values larger than 100 mW m2) approaching the Adriatic coast and especially offshore; the latter part was not covered in previous maps. The temperature distribution thus obtained was used to estimate the rheological characteristics of the subsurface. Two-dimensional rheological profiles across the ATF show that lateral rheological crustal variations have played an important role in the formation of the ATF and similar previously active extensional faults to the west. Lithospheric delamination and mantle degassing in the Tyrrhenian domain resulted in an easterly-migrating extension-compression boundary, characterized by the following properties: (i) the thickness of the upper crust brittle layer reaches a maximum; (ii) the critical stress difference required to initiate faulting at the base of the brittle layer is at a minimum; and (iii) the total strengths of both the brittle layer and the whole lithosphere are at a minimum. Our conclusions are independent of any specific geodynamic models, and their reliability is a function only of the estimated crustal temperature distribution and rheological parameters.With respect to the latter, we have explored a range of values in the brittle and ductile fields, and the results are not critically parameter-dependent.