CONTROLS ON MODERN SEDIMENT PROVENANCE IN AN ACTIVE OROGENIC BELT AND FORELAND BASIN: DETRITAL ZIRCON U-PB AGE DISTRIBUTIONS FOR THE ANDES OF WESTERN ARGENTINA

Sediment provenance studies remain a fundamental tool in reconstructing tectonic, paleogeographic, and paleodrainage histories. Detrital zircon U-Pb age distributions have become increasingly utilized as a proxy for sediment source regions providing robust constraints on sediment provenance and depositional ages in the ancient rock record. However, detrital zircon interpretations involve many unknowns and assumptions including: complete identification of potential sources; zircon source fertility; proximal-to-distal mixing effects; the range of possible erosion and drainage patterns; and sediment recycling. This study presents an actualistic analysis of modern river detrital zircon U-Pb age populations to assess whether modern synorogenic sedimentation reflects (1) proportional contributions from different drainage areas, (2) disproportionate contributions due to variable zircon fertility, or (3) complex contributions due to differences in lithologic erodibility.

The Andean study region in Argentina (27-34˚S) consists of an active contractional orogenic belt and foreland basin system with distinct geologic provinces. Diagnostic bedrock source units, main trunk rivers, and tributaries were sampled throughout the La Rioja, San Juan, and Mendoza provinces of western Argentina. Potential controls on sediment provenance were evaluated by comparing modern river U-Pb age spectra to modeled age spectra weighted on the relative exposure areas of different source units. This study applied several statistical measures (similarity, likeness, and cross-correlation) to compare area-weighted modeled age spectra with actual modern river populations. The modeled age distributions of main trunk river catchments display robust correlations with observed age spectra. In contrast, axial rivers and tributaries tend to yield less robust matches between area-modeled and actual data, suggesting complex controls on sediment composition related to drainage area, drainage order, and disproportionate contributions from erodible sources. Further steps will seek improved understanding of the controls on detrital zircon sediment provenance by better quantifying the erodibility and zircon fertility of source lithologies, and discriminating sampling scale with respect to drainage system order.