Quantitative provenance analysis of modern sands: bulk petrography, heavy minerals apatite fission tracks

Sediment petrography and heavy mineral analysis represent two key techniques to decipher the pieces of information stored in the sedimentary record, shedding light on source rocks compositon, weathering conditions and paleodrainages. Traditionally, even though orogenic detritus has always received large attention, orogenic provenance has been recognized as “composite”, thus preventing clear classification. Notwithstanding the composite nature of collision orogens, the detrital fingerprints of neometamorphic axial belts, largely experiencing strong exhumation and erosion, can be regarded as the diagnostic signature of orogenic detritus. We thus decided to focus our attention on the analysis of modern river sands from the Alpine belt, getting full quantitative information both on bulk compositions and heavy mineral assemblages. 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 Colour 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, and appropriate statistical techniques provides fundamental help to decrypt the information stored in the sedimentary record, and thus to identify and reconstruct subduction events of the past. Besides the fundamental contribution that modern sediment analysis can bring in understanding ancient sandstone successions, it can also give insights on short-term erosion distribution over wide areas and readily detect potential area of focused erosion. We tested a new quantitative approach based on the integration of compositional data and detrital apatite fission-tracks in two valleys of the western Alps (Arc and Dora Baltea basins). Samples for bulk-petrography and fission-track analysis were collected at different closure sections along the trunk, in order to investigate how the detrital signal evolves when detritus from different sub-basins is progressively added to the system. Fission-track analysis is a powerful integration tool to quantify sediments mixing if source areas experienced contrasting exhumation paths. Fission-track grain-age distributions provide not only information on long-term exhumation patterns, based on the age of the peaks, but also provide first-order constraints on short-term erosion rates by comparison between the size of the peaks and the size of potential source areas. In the Dora Baltea catchment, the apatite load derives from two major fault-bounded blocks, the Western one yielding 43% of the total amount of apatite, and the Eastern one yielding the remaining 57%. In the Arc catchment, contribution is 29% from the Eastern Block, 14% from the Houiller-Subbriançonnais units and 57% from the Belledonne-Dauphinois units. We assessed apatite fertility in source-rocks by measuring apatite content in processed sediments, after checking for anomalous hydraulic concentrations by geochemical analyses. The lack of compositional anomalies for elements between Y and Cr, largely hosted in ultradense minerals, grants that source-rocks distribution in the basin is faithfully reflected by detrital assemblages. Results demonstrate that erosional processes were focused in different areas of the Western Alps at long-term and short-term timescales, and are now concentrated in the External Massifs. No clear relationship between erosion and climate or relief is observed in this sector of the belt, where endogenic forces may represent the main controlling factor on both long-term and short-term erosion rates.