Abstract

With an area exceeding 25 000 km2 and volumes c. 5000 km3, south Peru hosts the Andes’ second largest Neogene ignimbrite field. We document the extent, stratigraphy and chronology of 12 ignimbrite sheets in the Río Ocoña–Cotahuasi–Marán and Colca deep canyons. Based on 74 40Ar/39Ar and U/Pb age determinations, ignimbrite-forming episodes span 25 myr. Prior to 9 Ma, eight large-volume ignimbrites were produced every 2.4 myr. After 9 Ma, average lulls between small- to moderate-volume ignimbrites decreased to 0.85 myr. The refined volcanic stratigraphy reveals three main features. (1) Larger volume ignimbrites were emplaced by punctuated flare-ups between 25 and 9 Ma during uplift of the Western Cordillera. (2) Numerous smaller ignimbrites were emplaced after 9 Ma as the ignimbrite production rate decreased threefold. This decrease may be due to the declining crustal melting rate, decreasing plate convergence rate after 9 Ma, or more magma stagnation in the shallow crust, which promoted the growth of composite cones. (3) Growth of two volcanic arcs has added twice as much volume (c. 53 km3 Ma−1) to the Río Ocoña–Cotahuasi–Marán volcanic field than the ignimbrites after 2.27 Ma. Estimated linear arc magma output has, however, decreased twofold (0.15 – 0.08 km3 km−1 Ma−1) from the Early Quaternary to the Pleistocene–Holocene.

Supplementary materials: Supplementary text, tables 1–3 and figures 1–4 are available at http://doi.org/10.6084/m9.figshare.c.3147100.

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