THERMAL EFFECTS OF NORMAL FAULTING DURING RIFTED BASIN FORMATION .2. THE LUGANO-VAL GRANDE NORMAL-FAULT AND THE ROLE OF PREEXISTING THERMAL ANOMALIES

We investigate the thermal consequences of rift-related normal faulting and compare the results with a well-studied natural example, the Lugano-Val Grande normal fault (Southern Alps). Only limited heating of the crust is caused by lithospheric thinning. In the simple but realistic situation where heat conduction is substantially faster than heat advection, no major thermal disturbance is associated with the downward movement of the hanging wall. Radiometric ages and fault rocks associated with the Lugano-Val Grande normal fault demonstrate that cooling rather than heating affected the crust during normal faulting. This pattern is not compatible with such a simple numerical model and is explained by a waning thermal anomaly induced by a magmatic intrusion immediately preceding or overlapping with the first stages of normal faulting. The magmatic body must have been emplaced at depths greater than 15-18 km, and probably started to cool in the Carnian, i.e. few million years before the onset of normal faulting along the Lugano-Val Grande fault.