Along‐strike variability of back‐arc basin collapse and the initiation of sedimentation in the Magallanes foreland basin, southernmost Andes (53–54.5°S)

Abstract

The Patagonian Andes record the Cretaceous demise of the quasi‐oceanic Rocas Verdes back‐arc basin and formation of the Magallanes foreland basin. For >500 km along the strike of the mountains, this tectonic transition is marked by a sandstone‐mudstone package that records the beginning of turbiditic sand deposition and fan growth. Sandstone modal analyses and U‐Pb detrital zircon spectra show changes in rock composition and provenance across the transition on a basin‐wide scale, indicating it has tectonic significance and is related to orogenic uplift and the progressive evolution of the Andean fold‐thrust belt. Spatial variations in transition zone characteristics indicate the foreland basin's central and southern sectors were fed by different sources and probably record separate fans. At Bahía Brookes, on Tierra del Fuego, foreland basin sedimentation began at least after 88–89 Ma, and possibly after ∼85 Ma, several million years after it did ∼700 km away at the northern end of the basin. This event coincided with increased arc volcanism and the partial obduction of the basaltic Rocas Verdes basin floor onto continental crust. By 81–80 Ma, conglomerate deposition and increased compositional and provenance complexity, including the abundance of metamorphic lithic fragments, indicate that the obducted basaltic floor first became emergent and was eroding. The results suggest that the beginning of turbidite sedimentation in the Magallanes foreland basin and the progressive incorporation and exhumation of deeply buried rocks in the Andean fold‐thrust belt, occurred later in southern Patagonia than in the north by a few million years.