DETRITAL ZIRCON U-PB GEOCHRONOLOGY OF LATE MIOCENE–EARLY PLIOCENE HINTERLAND BASIN DEVELOPMENT IN THE ANDES OF NORTHERN PERU

Andean hinterland basins preserved within the Marañon fold-thrust belt in the Western Cordillera of northern Peru contain Neogene nonmarine deposits (up to 1 km thick) resting in angular unconformity on principally Jurassic-Cretaceous marine strata. The Cajabamba and San Marcos hinterland basins are positioned along the Cajamarca Deflection, a prominent tectonic feature defining the transition from the central to northern Andes and marked by a sharp change in the regional structural grain. Resolving the poorly constrained timing and mode of hinterland basin genesis, associated structures, and possible tectonic (vertical-axis) rotations is fundamental to understanding the contrasting tectonic histories of the northern and central Andes. We use detrital zircon U-Pb geochronology, basin analysis, and sedimentology to more precisely define the chronostratigraphic and tectonic history of basin development and associated sediment provenance. New detrital zircon U-Pb age distributions from Mesozoic and Cenozoic sandstones show that Neogene basin sedimentation had commenced by the late Miocene (~11 Ma) and was dominated by syndepositional volcanic input that persisted to the early Pliocene (~5.2 Ma). In contrast, the uppermost levels of the basin-fill successions show an increase in Jurassic-Cretaceous age peaks and Proterozoic age peaks (recycled from Mesozoic strata) and an absence of Pliocene-age zircons, suggesting cessation of volcanism, changes in sediment routing, and further unroofing of adjacent Mesozoic units. This shift in detrital zircon age populations is coincident with a change from lacustrine conditions to coarse-grained fluvial to alluvial fan sedimentation in upper stratigraphic levels. Further efforts will evaluate the potential competing roles of previously proposed late Miocene to Quaternary shortening, extension, and strike-slip deformation.