COMPARISON OF TOP-DOWN AND BOTTOM-UP APPROACHES TO QUANTITATIVE SEDIMENT PROVENANCE MODELLING IN THE PALEOGENE NORTHERN ALTIPLANO BASIN

The southern Peruvian Andes experienced several phases of basin formation and subsequent sediment recycling between the Mesoproterozoic and present, rendering it difficult to uniquely identify sediment sources. However, discriminating between Eastern and Western cordilleran sources provides a critical test for opposing models of orogenic growth, which propose different timing and loci of crustal shortening, exhumation, and topographic growth. Previous provenance studies indicate dominant sourcing from the west based on paleocurrent and quantitative (top-down) modelling of detrital zircon age spectra. However, recent coupled thermo-mechanical modelling points to exhumation in the Eastern Cordillera as early as ~55 Ma.

We apply both top-down and bottom-up mixture modelling approaches to detrital zircon U-Pb ages from sediments in the Paleogene Muñani Formation and Puno Group. The top-down approach uses a Monte Carlo model to fit well-characterized source age distributions to mixed basin samples. In contrast, the bottom-up approach employs non-negative matrix factorization (NMF) to deconvolve source distributions based on multiple measured basin samples. Whereas the top-down approach attributes 40 – 50% to geographically ambiguous sources and points to Eastern Cordilleran contributions within uncertainty of zero, the bottom-up approach reduces this ambiguity and suggests there is up 30% contributed from the Eastern Cordillera. The additional clarity provided by empirical NMF results suggests that sources for the top-down approach are missing or inadequately characterized. This is the first study to use a bottom-up approach to sediment source characterization in the central Andes. The discrepancies between the top-down and bottom-up approaches highlight the difficulty in discriminating sediment sources in the central Andes, but also mark a step forward in provenance analysis within this area and to reconciling the basin history with thermo-kinematic models.