GNGTS 2013 - Atti del 32° Convegno Nazionale

Shear heating is likely to increase the temperature on the fault planes. The resulting heat flow adds to that generated by radiogenic sources in the upper continental crust. The available data (Della Vedova et al. , 2001; Viganò et al. , 2008, 2011; Clark, 1961) indicate that the total heat flow observed on the surface does not exceed 60 to 80 mW/m 2 , although in middle Jurassic values as high as 85 to 105 mW/m 2 have been reported by Carminati et al. (2010) using organic matter maturity data from outcropping sediments. Following Turcotte and Schubert (2002) we model the surface heat flow and temperature increase on the fault planes as due to a sudden increase of heat flow caused by shear heating on the fault plane. It is reasonable to assume that the total temperature (for a nominal thermal gradient plus shear heating) on the fault plane is in the range 600-800 °C (Vosteen et al. , 2006, for the TRANSALP profile), and that the heat flow on the Earth surface, when added to the radiogenic heat (of the order of 50 mW/m 2 ), does not exceed 60-80 mW/m 2 . The model then implies that the time of initiation of heat production by shear in the half space must be relatively recent, for shear stresses in the range 100-300 MPa. We infer that such epoch should be somewhere in the Plio-Pleistocene, hence more recent than the late Oligocene to Miocene collision of the Adria indenter with the stable European foreland. According to the analog models of Ratschbacher et al. (1991), this collision is responsible the fold and fault structure. Hence it can be concluded that the collision was ‘head on’ in the first 15-18 Myr, and has been accommodated by slip on inclined fault planes only in the past 5-7 Myr, since a longer slipping phase would imply an exceedingly large amount of frictionally generated heat to reach the Earth surface (Fig.3). Specific assumptions on the local geotherm are clearly needed to make these concepts more quantitative. Fig. 3 – (Left) Heat flux on the Earth surface due to shear heating on a rectangular fault at depth. The continuous curves correspond to a dip of 80°, and the dotted curves to a dip of 45°. Different colors are assigned to different shear stresses. (Right) Temperature increase on the fault surface, due to shear heating, for various values of the shear stress. 160 GNGTS 2013 S essione 1.2