Detrital Fingerprints of Fossil Continental-Subduction Zones (Axial Belt Provenance, European Alps)

Abstract

Collision orogens such as the Alps or the Himalayas are generated by plate convergence, culminating in attempted subduction of a thinned continental margin. Massive amounts of metamorphic rocks displaying high-pressure parageneses are produced during such relatively brief tectonic events and then rapidly exhumed to form the axial backbone of the new orogen. Sediment composition provides a fundamental key to identify past events of continental subduction, although coupled detrital-geochronology techniques are needed to discriminate neometamorphic and paleometamorphic sources of detritus. Within the Austroalpine Cretaceous and Penninic Eocene axial belts of the Alps, we ideally distinguish three structural levels, each characterized by diagnostic detrital fingerprints. The shallow level chiefly consists of offscraped remnant-ocean turbidites and unmetamorphosed continental-margin sediments and mostly produces lithic to quartzolithic sedimentaclastic sands yielding very poor heavy mineral suites including ultrastable minerals. The intermediate level includes low-grade metasediments and polymetamorphic basements and sheds quartzolithic to feldspatholithoquartzose metamorphiclastic sands yielding moderately rich epidote-amphibole suites with chloritoid or garnet. The deep level contains eclogitic remnants of continent-ocean transitions and supplies feldspatholithoquartzose/feldspathoquartzose high-rank metamorphiclastic to lithic ultramaficlastic sands yielding rich to extremely rich suites dominated by garnet, hornblende, or epidote, depending on protoliths (continental vs. oceanic) and pressure/temperature paths during exhumation. Although widely overprinted under greenschist-facies or amphibolite-facies conditions, occurrence of ultradense eclogite in source areas is readily revealed by the heavy mineral concentration (HMC) index, which mirrors the average density of source rocks in the absence of hydraulic-sorting effects. Rather than the pressure peak reached at depth, the metamorphic index (MI) and hornblende color index (HCI) reflect peak temperatures reached at later stages, when subduction is throttled by arrival of thicker continental crust and geothermal gradients increase, as documented in detritus derived from the Tauern window and Lepontine Dome. Experience gained from modern sediments provides fundamental help to decrypt the information stored in the sedimentary record and thus to identify and reconstruct subduction events of the past.