BUILDING AN ANDES PLATE TECTONIC RECONSTRUCTION BACK TO THE LATE CRETACEOUS ~80MA USING UNFOLDED SLABS FROM SEISMIC TOMOGRAPHY: IMPLICATIONS FOR FLAT SLAB TECTONICS

The Andean margin has long been regarded as a classic example of long-lived, continuous oceanic lithosphere subduction beneath a continental margin. The current continuous subduction phase has apparently endured since the early Jurassic, or early Cenozoic, but uncertainty exists due to incomplete or missing geological records. Within a presumed context of continuous subduction, many studies have attributed episodic and localized changes in Andean deformation, magmatism and topography to repeated cycles of slab flattening and steepening or slab break-off (i.e. flat slabs). Here we present a slab-based, quantitative plate tectonic reconstruction that shows Andean subduction was not always continuous since the Mesozoic and has included episodic phases of strike-slip/transtension or divergence. Our results imply that plate tectonic-scale reorganizations and slab-mantle interactions – factors other than slab dip (i.e. flat slabs) – may have also played a crucial role in controlling the evolution of Andean geology.

In this study we mapped and unfolded (i.e. structurally restored) the Nazca slabs from global seismic tomography to produce a quantitative, slab-constrained plate reconstruction of the Andes back to the late Cretaceous ~80 Ma. Our reconstruction implies a subduction resumption at late Cretaceous following a 100 to 80 Ma regional plate tectonic reorganization. We suggest our predicted plate reorganization event corresponds geologically to the Andean margin-wide compressional event at mid-late Cretaceous. Our plate model also shows the Andes potentially experienced periods of strike-slip and even divergent tectonics until 55 Ma, which roughly fits the magmatic records.

Quantitative output of subducted slab lengths over time along the Andean margin from our model implies that at ~50 Ma the Andean margin had an anchored northern Nazca slab and a free southern slab in the upper mantle. We argue such a difference can explain the distinct geology north (compression) and south (extension) of 33ºS at Andean margin between Eocene to Oligocene, a time period that almost all subduction conditions are equal along the margin. We will also compare other along-strike variations in Andean geology including magmatic patterns and compressional/extensional structures development to our slab-based plate model.