Deciphering the timescales and thermal evolution of the subduction interface using petrochronology: a case study from the Western Alps

Abstract:

The thermal structure of a subduction zone wields a first-order control on both the location of prograde metamorphic dehydration reactions and the resultant fluid flux from the downgoing slab. Exhumed high-pressure/low-temperature (HP/LT) rocks have been shown to be useful proxies for probing the paleo-temperatures of a subduction zone and thus reconstructing its thermal structure. By coupling thermobarometric estimates from these rocks with high-precision geochronology (i.e. petrochronology), greater detail about the metamorphic evolution of a particular lithology can be obtained. Importantly, assessing pressure-temperature-time (P-T-t) constraints for various localities across a fossil subduction terrane can establish a ‘snapshot’ of the geothermal gradient, which can offer insights into the paleo-thermal structure and the rates and durations of tectonism, with implications for our understanding of seismicity, geochemical fluxing, and the stress distribution of deeper parts of the subduction interface.

We focus on a case study from the Zermatt-Saas Ophiolite in the Western Alps. High-precision zoned garnet geochronology, coupled with mineral chemistry and quantitative petrology, was performed to provide insight on the tectonic rates and thermal structures of both terranes. For the Western Alps, the garnet growth duration (3.4 ± 2.1 Ma), when coupled with P-T constraints, results in burial and heating rates of ~5 km/Ma and 13 °C/Ma, respectively. Geometric calculations varying slab velocity and dip show that the calculated thermal structure is best explained by a narrow range of convergence rates (1.5-2 cm/yr) and slab dip (15-20°). Lastly, the durations and rates derived here (and from other examples) are compared to coupled thermodynamic-geodynamic models to assess likely timescales of metamorphism in modern subduction settings.

This case study, presented with examples from other fossil subduction terranes, highlight the need for accurate and precise assessments of depths, temperatures and time in order to reconstruct the appropriate ‘snapshots’ of subduction tectonism. High-precision petrochronological data from exhumed subduction terranes should thus be considered as first-order constraints for better understanding the deep thermal structure of modern subduction zones.