DETRITAL ZIRCON GEOCHRONOLOGY OF THE TALARA BASIN, NORTHWEST PERU

The Eocene stratigraphy of the Talara Basin, in northwest Peru, records fluvial to deep-marine deposition into the Andean forearc. Significant deepening of the basin in the lower and middle Eocene corresponds to high convergence rates between the Nazca and South American plates, and a dramatic subsidence event in the middle Lutetian may signal a change in convergence direction at the beginning of Incaic II Phase. Volcanic activity during these events is not well recorded in the basin, however, and the timing of uplift of the Amotape Mountains related to the nature of convergence is also unconstrained. Detrital zircon geochronology of sandstones from key formations throughout the sedimentary section helps to constrain both problems and to characterize non-Andean or non-Amotape sources that may have contributed to the Talara Basin. Sandstones analyzed for detrital zircon U-Pb ages were sampled from the following formations, from base to top of section: Mogollon, fluvial and marginal marine; Pale Greda, marginal marine to outer shelf; Cabo Blanco, deltaic; Helico, bathyal to middle bathyal turbidites; and Verdun, bathyal turbidites. With the exception of the Cabo Blanco Formation, which may have tapped a more local source, all formations show a progressively younger suite of zircons, indicating fairly regular volcanic activity in that part of the Andes during the Eocene. There is a consistent volcanic/plutonic gap between 65-70 Ma, and post-Jurassic magmatism appears to begin at 90-95 Ma in the region sourcing this basin. In the case of the Pale Greda and Verdun Formations, established depositional ages may need to be revised based on the maximum depositional ages constrained by their detrital zircon populations. All formations tap source terranes that contain abundant Paleozoic and Proterozoic zircons. This older population of zircons progressively increases its oldest ages upsection through the Helico Formation, and the Cabo Blanco Formation shows a distinct abundance of pre-Cretaceous zircon ages. The older zircon population is likely to originate from old granitoids and recycled pre-Cretaceous sedimentary rocks of the Amotape Mountains, and their predominance in the Cabo Blanco may represent early stages of Amotape uplift.