Late-stage exhumation history of the Lepontine Dome, Central European Alps: Revelation from low-temperature thermochronology and numerical modeling

Several recent studies proposed a strong increase in exhumation rates for the Central European Alps since ~5 Ma. The triggers of this event and the contribution of the Lepontine Dome to the Alpine-wide sediment budget still remain unclear. In this study the late-stage exhumation history of the Lepontine Dome is investigated with pre-existing apatite fission track (AFT) and apatite (U-Th-Sm)/He (AHe) analyses along two elevation profiles situated in key regions of the Lepontine Dome – the Ticino subdome and the Maggia Steep Zone. The latter separates the Ticino subdome in the east from the Toce subdome in the west. Age-elevation relationships and subsequent application of 1D thermal history inversions and 3D thermal-kinematic model inversions suggest episodic exhumation along both elevation profiles but imply different exhumation histories on a small-scale in the Lepontine Dome. Thermal-kinematic modelling results indicate decreasing exhumation rates from ~1.2 to ~0.2 km/Ma at 12 ± 4 Ma for the northern part of the Ticino subdome. This slow-down coincides with decreasing sedimentation in the circum-Alpine foreland basins and is interpreted to be related to an Alpine-wide deceleration of lateral extrusion and normal faulting activity along the Simplon normal fault and is, therefore, structurally controlled. The decrease in exhumation rates in the northern Ticino subdome is contrasting to the modelled exhumation history of the Maggia Steep Zone. Along the Maggia elevation profile, thermal-kinematic models suggest an exhumation rate of ~0.6 km/Ma until 4 ± 3 Ma subsequently increasing to ~1.0 km/Ma. This increase also correlates with the Alpine sediment budget and coincides with increasing exhumation along the Simplon fault and other areas of the Toce subdome. It is probably related to an overall reorganisation of the Alpine stressfield from orogen- perpendicular to orogen-parallel extension between ~6 – 3 Ma. After ~3 Ma, cyclic glaciations of the European Alps explain long-lasting high exhumation rates in the Lepontine Dome