Integrating Spatial-Temporal Scales to Decipher Dynamics of Continental Subduction and Collision in the Alps

Abstract:

The European Alps display a complex record of subduction (Western Alps) and collision (Central Alps). Many of the tectonic units consist of mostly continental crust. Such polycyclic rocks are notoriously difficult to decipher, and countless efforts to analyze and quantify samples of Alpine basement have produced confusing data and controversial interpretations. As a result, the Alps have occasionally been portrayed as a pathological orogen…

Recent progress in petrochronology yields robust in situ mineral ages that can be linked to physical conditions, i.e. quantitative PT-data for samples with a well established record of polyphase deformation. As the structural context is well known, integration of PTDt-data from mm-scale to km-tectonic units is possible.

In the Western Alps, the analysis allows an assessment of dynamics in the continental subduction channel. Major kinematic differences emerge between adjacent slices of km-size; rates of the main orogenic processes are now known. The major diachronicity in the high-pressure imprint (e.g. Sesia: 84-63 Ma) reflects kinematic differences near the subduction interface, during subduction and (generally fast) exhumation – but with pressure cycling of at least one tectonic slice.

The regional metamorphic imprint in the Central Alps is diachronous as well, but the causes are entirely different. High-pressure rocks are confined to two spatially separated units (a lithospheric mélange unit + an accretionary wedge); these are interleaved with several basement thrust sheets. The entire nappe stack experienced a medium pressure imprint. Petrochronology yields heating and cooling rates, and again the structural context is well enough established to understand the reasons of age differences at peak conditions (T_max reached at 32-19 Ma in different parts of the belt).

Overall, the reassessment of the orogenic timetable and processes helps to correct or reconcile many of the supposedly “abnormal” properties of the Alps. Unusual complexities mostly reflect that much detail is known in this chain, not least owing to the geophysically well established structure. Is it time to turn this around and examine in detail how seismic signatures reflect the characteristics (e.g. spatial density contrast) beneath now well understood parts of a “normal” orogen?