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

We investigate the thermal consequences of rift-related normal faultin g and compare the results with a well-studied natural example, the Lug ano-Val Grande normal fault (Southern Alps). Only limited heating of t he crust is caused by lithospheric thinning. In the simple but realist ic situation where heat conduction is substantially faster than heat a dvection, no major thermal disturbance is associated with the downward movement of the hanging wall. Radiometric ages and fault rocks associ ated with the Lugano-Val Grande normal fault demonstrate that cooling rather than heating affected the crust during normal faulting. This pa ttern is not compatible with such a simple numerical model and is expl ained by a waning thermal anomaly induced by a magmatic intrusion imme diately preceding or overlapping with the first stages of normal fault ing. 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 millio n years before the onset of normal faulting along the Lugano-Val Grand e fault.