Cretaceous to Neogene cooling and exhumation history of the Oetztal-Stubaibasement complex, eastern Alps: A structural and fission track study

The central Austroalpine Oetztal-Stubai basement complex (OSK) of the easte rn Alps together with its overlying sedimentary (Brenner Mesozoic (BMZ)) an d tectonic units (upper Austroalpine nappes) underwent extension and exhuma tion during the Cretaceous before its incorporation into the hanging wall o f the Neogene Brenner normal fault. During this Late Cretaceous to early Te rtiary deformation the Brenner Mesozoic and the immediately underlying uppe rmost part of the OSK developed a strong stretching lineation within a near -horizontal and often mylonitic foliation related to consistent top-to-ESE directed transport. Quartz and calcite behaved ductilely throughout the are a, whereas dolomite shows a transition from brittle fracture in the north t o plastic flow in the south, consistent with the known Cretaceous metamorph ic temperature gradient. The transition to crystal plastic flow of dolomite occurs at similar to 480 degrees C. Quartz textures indicate an increase i n the contribution of prism glide together with a decrease in basal glide t oward the south, also consistent with the temperature gradient in the OSK a nd BMZ. Fission track analysis of both zircon and apatite along a N-S profi le, in combination with existing data from the literature, allows a continu ous cooling path to be constructed for the OSK. Cooling of the eastern OSK started at 90 Ma (the thermal peak of metamorphism) and ended some 30 Myr l ater at near-surface temperatures. At 60 Ma, the northern region had alread y cooled below the upper limit of the apatite partial annealing zone, where as in the south, cooling related to this exhumation phase stopped at temper atures between 100 degrees and 200 degrees C. The ESE directed shearing is interpreted as Late Cretaceous to early Tertiary normal faulting that exhum ed the OSK and brought the upper Austroalpine (Blaser and Steinacher) nappe s into contact with the underlying Brenner Mesozoic. The pattern of Alpine metamorphic isograds established in the OSK directly reflects the geometry and kinematics of this Late Cretaceous/early Tertiary exhumation due to low -angle normal faulting.