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

Abstract

The central Austroalpine Oetztal‐Stubai basement complex (OSK) of the eastern Alps together with its overlying sedimentary (Brenner Mesozoic (BMZ)) and tectonic units (upper Austroalpine nappes) underwent extension and exhumation during the Cretaceous before its incorporation into the hanging wall of the Neogene Brenner normal fault. During this Late Cretaceous to early Tertiary deformation the Brenner Mesozoic and the immediately underlying uppermost 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 area, whereas dolomite shows a transition from brittle fracture in the north to plastic flow in the south, consistent with the known Cretaceous metamorphic temperature gradient. The transition to crystal plastic flow of dolomite occurs at ∼480°C. Quartz textures indicate an increase in the contribution of prism glide together with a decrease in basal glide toward the south, also consistent with the temperature gradient in the OSK and BMZ. Fission track analysis of both zircon and apatite along a N‐S profile, in combination with existing data from the literature, allows a continuous 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 later at near‐surface temperatures. At 60 Ma, the northern region had already cooled below the upper limit of the apatite partial annealing zone, whereas in the south, cooling related to this exhumation phase stopped at temperatures between 100° and 200°C. The ESE directed shearing is interpreted as Late Cretaceous to early Tertiary normal faulting that exhumed the OSK and brought the upper Austroalpine (Blaser and Steinacher) nappes 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.