Exhumation of the Argentine Andes, 32°- 34° S: detrital (U-Th)/He and U-Pb zircon double dating

Abstract:

The central Argentine Andes exhibit variable morphology along-strike due to some combination of slab subduction angle, inherited crustal structure, and climate. Structural mapping and thermochronology has suggested minimal exhumation (~1 km), except perhaps along some major river valleys. However, few thermochronologic results have been reported. In order to better understand the variability of exhumation along-strike, we sampled 3 rivers for detrital zircon and measured both (U-Th)/He and U-Pb ages using laser ablation methods. The sample sites span the southern edge of flat-slab subduction. The northern- and southernmost watersheds each cover ~2000 km² and include elements of both the Principal and Frontal Cordilleras. The central watershed (~200 km²) is largely restricted to the Frontal Cordillera. The geology of the Principal Cordillera is diverse, but the Frontal Cordillera is composed primarily of Permo-Triassic volcanic rocks of the Choiyoi Formation. Preliminary results (n = 27) from the smaller, central Arroyo Grande watershed show that zircons are almost always reset, in the sense that a given zircon’s (U-Th)/He cooling age is significantly younger than its U-Pb crystallization age. Of these, 2 grains have cooling ages <5 Ma, and 7 have ages <25 Ma. To the south, in the larger Rio Tunuyan (n = 27) watershed, 13 reset zircons have cooling ages <5 Ma, and 6 of these have U-Pb ages that are characteristic of the Frontal Cordillera (ranging from 220 to 272 Ma). To the north, in the upper Río de los Patos catchment (n = 30), only 3 reset zircons have ages <25 Ma, and these zircons lack Frontal Cordilleran U-Pb ages (17, 43, and 323 Ma). The abundance of reset (U-Th)/He ages is notable in all 3 watersheds. Laser (U-Th)/He closure temperatures vary according to the geometry of the zircon, the geometry of the laser spot, and the cooling rate. For an average zircon among our samples, at an assumed cooling rate of 10° C/Myr, the closure temperature is approximately 200° C. This implies at least several kilometers of exhumation, even if the geothermal gradient is very steep. Sections of the watersheds appear to have been exhumed very rapidly. These preliminary results suggest that the Frontal Cordillera, particularly in the southern and central watersheds, have accommodated much more exhumation than previously thought.