Crustal thermal regime prior to, during, and after rifting: A geochronological and modeling study of the Mesozoic South Alpine rifted margin

An approximately 15-km-thick segment of the Mesozoic passive continental margin of Adria was involved in Alpine shortening and is now exposed in the Lake Come-Lake Lugano area (southern Alps, northern Italy and southern Switzerland). The segment comprises the Monte Generoso basin, a deep (similar to 9 km) extensional basin controlled by the activity of the Lugano-Val Grande normal fault. The paleosurface and Mesozoic geometric relations are well preserved so that the kinematics of fault blocks prior to, during, and after extension are well constrained. Rb/Sr, 40Ar/39Ar and zircon fission track data from basement rocks underlying at various the Generoso basin depths, provide independent estimates on the thermal evolution of the crustal segment during and after rifting. Most Rb/Sr and 40Ar/39Ar ages are contemporaneous with the initial stages of rifting (Late Triassic to Early Jurassic) documenting (1) strongly altered thermal conditions before the onset of extension and (2) cooling throughout rifting. Thermal gradients decreased from >60 degrees C km(-1) at the onset of extension to similar to 20 degrees C km(-1) shortly after breakup. Zircon fission track ages are Jurassic to Early Cretaceous providing evidence for continued cooling until the beginning of Alpine shortening. Thermokinematic numerical modeling of the considered crustal segment provides a full, two-dimensional description of synrift and postrift evolution. Synthetic time versus temperature curves are extracted from the model and compared with those derived from absolute ages.