From orogen-scale deformation to valley carving: Deciphering the 4-D evolution of the Insubric Line by thermochronology

The dynamic process of mountain building involves large-scale thrusting and exhumation of rocks. The outer layer of the Earth, the crust, is fragmented into blocks, which move laterally as well as vertically. In the European Alps, the Periadriatic Line forms the most important fault zone, juxtaposing blocks of distinct paleogeographic origin and overprint. In the proposed study area, along the E-W trending Insubric Line, a key segment of the Periadriatic Line, rocks of Apulian Plate affinity south of the fault, are exposed in the immediate vicinity of rocks of European Plate affinity to the north. Subduction of the Eurasian Plate led to temperatures up to 700 C and metamorphic overprint up to anatexis. In contrast, south of the Periadriatic Line metamorphic overprint was minor with maximum temperatures up to 250 C. Thus, rocks exposed at the surface today have undergone distinct post-metamorphic exhumation histories. It is the utmost goal of this project to decipher the long-term temporal and spatial motion history of these blocks opposite of the Insubric Line by applying a combination of established and emergent innovative dating methods on rocks from various profiles crossing the fault zone. Low-temperature thermochronological tools will comprise fission track and (U-Th-Sm)/He dating on zircon and apatite as well as 4He/3He thermochronometry on apatite. These methods are based on the fact, that rocks undergo cooling on their paths towards Earths surface. During cooling specific minerals store information on the timing, when the rock cooled below a specific temperature. Below this temperature defects in the crystal lattice, called fission tracks, as well as isotopes generated by natural nuclear decay, will be retained. Combining the measured time span and temperature information leads to a so called thermochronometer , which allows to reconstruct thermal histories of rocks for the upper 10 km of the Earths crust. Apatite 4He/3He thermochronometry allows to access uniquely low temperatures close to the Earths surface, enabling reconstructions of landscape evolution and valley formation along the Insubric Line. Apart from vertical movements along the Periadriatic fault system, lateral movements play a major role, their extent being controversially discussed in the literature. Fault movements locally lead to calcite precipitation along the fault trace. The formation age of such newly-grown crystals may be dated by a new U-Pb dating method, which shall be applied for the first time along the Insubric Line within the framework of this study. Results shall allow to bracket late lateral fault activity, which shall then be correlated with vertical movements. Integrating findings from various dating methods as well as their thermokinematic modelling shall allow to infer the post-collisional 4-D evolution of the Insubric Line. The derived rates for exhumation and erosion of the mountain range will contribute to a better understanding of short-term processes relevant to society, such as landslides or earthquakes. The project will build upon collaboration with several colleagues from the same department, the Universities of Göttingen, Frankfurt, Freiburg, Berlin, Tübingen, in Germany and the University of California, Berkeley in the USA.

The primary objective of the project was to investigate the long-term spatial and temporal cooling patterns of crustal blocks on either side of the Insubric Line (Ticino, Switzerland), one of the most important fault systems in the Alps. The study area is characterized by markedly stronger exhumation north of the fault, within the Lepontine Dome, where rocks have been uplifted by approximately 15 km more over the past 30 million years than those in the Southern Alps to the south. The study region therefore represents a formerly highly tectonically active segment of the Alps. This dynamic evolution is clearly reflected in the field: on the one hand, very different rock types occur on either side of the fault, on the other hand, rocks along the fault zone preserve evidence of past seismic activity and displacement. During major earthquakes, intense frictional heating along the fault plane causes not only mechanical fragmentation of the rock but also the formation of frictional melts known as pseudotachylytes. These remnants of ancient earthquakes were dated using the U-Pb method. During melt formation, mineral fragments from the surrounding host rock become incorporated into the melt. At sufficiently high temperatures, the original age information of these minerals can be partially or completely reset, as radioactive decay systems restart, in other words, the "geological clock" is set back to zero. The host rock, however, retains its original age. Minerals enclosed within pseudotachylytes thus provide direct chronological information on intense seismic activity, which in this case occurred between 30 and 32 million years ago. To reconstruct the pathway of the rocks from deep crustal levels to the surface, thermochronological analyses were conducted on approximately 60 samples. These samples were collected along profiles oriented perpendicular to the fault and spanning elevation differences of up to 2,000 meters. Minerals such as apatite and zircon record when specific mineral-dependent closure temperatures were crossed during cooling. In this way, the timing of uplift, erosion, and mountain-building processes can be constrained. For the Lepontine Dome, the data reveal a multiphase cooling history. A phase of very rapid cooling around 30 million years ago reflects the intense tectonic activity also indicated by the pseudotachylytes. In addition, the results document a previously unknown phase of accelerated cooling and uplift between approximately 17 and 12 million years. In contrast, the Southalpine block shows evidence of such a young cooling phase only in close proximity to the fault. Farther south, thermochronological ages reach up to 300 million years. This implies that 30 million years ago the Southern Alpine rocks were located at relatively shallow crustal depths of 2-5 km, whereas rocks of the Lepontine Dome experienced temperatures of up to 700 C at depths exceeding 20 km.