WESTWARD JURASSIC BACK-ARC BASIN MIGRATION IN THE CENTRAL ANDEAN CORDILLERA

Thick (>1km) Jurassic volcanic and sedimentary sequences in southern Peru provide critical constraints on the Mesozoic tectonic evolution of the Central Andean orogenic system. Hildebrand & Whalen (2014) proposed that the Arequipa Massif is an exotic ribbon continent that sutured to the metamorphic Marañon Complex of western South America in the Late Cretaceous. This model rejects the previous interpretation of a tectonically active Mesozoic Central Andes and predicts passive margin sedimentation along the west Peruvian margin with westward subduction beneath an off-board Arequipa Massif until the Late Cretaceous. Furthermore, Hildebrand & Whalen (2014) attribute emplacement of the Peruvian Coastal Batholith to slab break off magmatism and invoke a subsequent subduction polarity reversal to establish modern eastward-dipping subduction. Here we test this model with stratigraphic and geochronological constraints on the Jurassic of southern Peru.

New Early Jurassic U-Pb detrital zircon ages (LA-ICP-MS) from the Ongoro Fm. in the Majes Valley support correlation with the Chocolate Fm. and suggest widespread basin formation and increased sedimentation as early as ~200 Ma in a back-arc setting. New Middle-Jurassic U-Pb zircon ages from the Rio Grande Fm. in the coastal San Fernando Reservation indicate significant arc-volcanism on the Arequipa Massif at ~175 Ma, coinciding with a period of diachronous sub-basin formation and the westward migration of the volcanic arc. Additionally, new geochemical and petrographic data from both field sites indicate a calc-alkaline arc setting and the inheritance of Ordovician basement material that correlates with Famatinian intrusives documented within both the Arequipa Massif and Marañon Complex. These data constrain the adjacency of the Arequipa Massif to the South American continent through the Mesozoic, and are consistent with Jurassic sedimentation in a westward-facing and migrating back-arc basin.