--> Foreland Basin Record of Advancing and Retreating Subduction in the Andes

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Foreland Basin Record of Advancing and Retreating Subduction in the Andes

Abstract

Multi-phase retroarc foreland basins spanning >50–100 Myr are common along ocean-continent convergent margins. New geochronologic and sediment provenance results from the Andean foreland basin of Argentina provide improved resolution to examine the existence, duration, and controls on long-term mixed-mode deformation and an enigmatic hiatus in the retroarc foreland. Detrital zircon U-Pb ages for clastic basin fill in western Argentina (34-36°S) reveal shifts in sedimentation pathways and accumulation rates consistent with (1) local basement sources during Early–Middle Jurassic back-arc extension, (2) variable cratonic and magmatic arc sources during Late Jurassic–Cretaceous postrift thermal subsidence, and (3) Andean arc and thrust belt sources during irregular Late Cretaceous–Cenozoic shortening. Although pulses of flexural subsidence can be linked to brief periods of fault reactivation (inversion), fully developed foreland basin conditions were only achieved in Late Cretaceous and Neogene time, during probable periods of advancing subduction in which the South American plate rapidly advanced westward toward the Pacific ocean basin. Separating these two contractional foreland episodes is a regional unconformity marking a ca. 40–20 Ma depositional hiatus, potentially signifying a neutral to extensional mode during retreating slab conditions (slab rollback) along the western South American margin. We propose that the Andes have always been sensitive to moderate variations in subduction dynamics throughout Mesozoic-Cenozoic time, such that shifts in relative convergence and degree of mechanical coupling along the subduction interface have governed fluctuating contractional, extensional, and neutral conditions.