Assessing the 4-D evolution along and across the Insubric Line in the European Central Alps using a multi-method geo- and thermochronological approach

During orogenesis the initial asymmetry of subduction induces asymmetry of continental collision regarding collisional structure, slab geometries, partitioning of crustal shortening and eventually indentation of a stiffer and cooler continent into a relatively warmer and softer continent. After collision and indentation, extrusion and exhumation of deep metamorphic and plutonic rocks are diagnostic processes to evaluate the extent of asymmetry and the long-term structural evolution along and across the continental suture.

An excellent place to study such highly asymmetric patterns are the distinctly non-cylindrical European Alps, an archetypal example of indentation. There, indentation of relatively stiff Adriatic lower crust and upper mantle into the weaker continental Eurasian plate led to unroofing of the Penninic Lepontine dome, as well as strike-slip motion along the Insubric Line. Late-stage collision led to a highly asymmetric exhumation pattern with relative vertical displacement across the fault in the range of 15 (±5) km. The brittle faulting and exhumation history has so far received only little attention, and particularly S of the Insubric Line, large-scale interpretations of cooling and exhumation are based on very little quantitative knowledge. Exploring the faulting and exhumation history of this suture by applying multiple geo- and thermochronometers spanning temperatures from ca. 50 to 450 °C on both sides of the fault is the focus of this project.

(U-Th)/He apatite and zircon dating on more than 50 samples and fission track dating on 25 samples was applied along densely spaced horizontal as well as vertical transects across the Insubric Line. (U-Th)/He apatite ages, which monitor cooling below ca. 80 °C, from north of the fault line prominently cluster around 8-12 Ma. Apatite fission track (closure temperature of ca. 110 °C) as well as zircon (U-Th)/He ages (ca. 210 °C) are only slightly older. Modelling these thermochronological data point to a Late Miocene phase of more pronounced cooling and exhumation of the Lepontine dome than previously assumed. Thermochronological data of Southalpine samples from the immediate vicinity of the fault line record a similar cooling pulse, indicating either joint late-stage exhumation or a heating pulse invoking resetting of Southalpine units due to Lepontine updoming. U-Pb apatite data, recording higher temperature cooling below ca. 450 °C clearly diverge, yielding Permian ages in the south but Oligocene to Early Miocene ages in the north.

Additionally, the seismotectonic evolution of the Insubric fault is targeted by U-Pb dating on pseudotachylites and mylonites. This methodically new approach yields ages clustering at 30 and 16 Ma for a Southalpine pseudotachylite. The signal was measured for a fine-grained mineral assemblage containing U-bearing phases such as apatite, epidote and titanite. The older age cluster corresponds to the phase of major Lepontine updoming, which we confirmed by mylonite dating. The younger age is in line with published Ar-Ar pseudotachylite data (Müller et al., 2001). These initial data suggest that this method could be a valuable tool for dating palaeoseismic events.

Müller, W., et al. (2001). Geochronological constraints on the evolution of the Periadriatic Fault System (Alps). Int J Earth Sci, 90(3), 623-653.