CRETACEOUS-CENOZOIC GROWTH OF THE PATAGONIAN BROKEN FORELAND BASIN, ARGENTINA: CHRONOSTRATIGRAPHIC FRAMEWORK AND PROVENANCE VARIATIONS DURING TRANSITIONS IN ANDEAN SUBDUCTION DYNAMICS

The Cretaceous-Cenozoic evolution of the Patagonian broken foreland basin system at 42-43°S in the northern Chubut province of Argentina is associated with variable retroarc phases of fold-thrust belt shortening, extension, and basement uplift during changes in the dynamics of oceanic slab subduction. Basement inheritance and progressive shallowing of an east-dipping subducting slab are important mechanisms of foreland partitioning, as dictated by the preexisting (pre-Andean) structural architecture and forelandward (eastward) advance of Late Cretaceous arc magmatism. Previously recognized growth strata help define the timing of fold-thrust belt shortening and retroarc basement-involved uplift, but the precise consequences for sediment routing remain poorly understood, with uncertainties in patterns of basin evolution before, during, and after shallowing and resteepening of the subducting slab.

We utilize detrital zircon U-Pb geochronology and Lu-Hf isotope analyses to demonstrate that Aptian-Cenomanian retroarc basin fill was derived principally from the basement massif and intraplate volcanic units to the east, followed by Late Cretaceous flat slab subduction, an associated reversal in sedimentary polarity and subsequent exclusive derivation from the Andean arc and orogenic belt to the west. Late Cretaceous-Paleocene slab shallowing and arc cessation was succeeded by late Eocene–earliest Miocene extension during slab rollback and renewal of arc magmatism. Thereafter, Miocene sedimentation was closely linked to shortening in the Andean fold-thrust belt. The Mesozoic-Cenozoic history of variable contractional and extensional tectonic regimes is largely reflected in εHf signatures that recorded contrasting evolved and juvenile signatures during crustal thickening and thinning, respectively.