科研之友
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摘要
With an area exceeding 25 000 km(2) and volumes c. 5000 km(3), south Peru hosts the Andes' second largest Neogene ignimbrite field. We document the extent, stratigraphy and chronology of 12 ignimbrite sheets in the Rio Ocona-Cotahuasi-Maran and Colca deep canyons. Based on 74 Ar-40/Ar-39 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 km(3) Ma(-1)) to the Rio Ocona-Cotahuasi-Maran volcanic field than the ignimbrites after 2.27 Ma. Estimated linear arc magma output has, however, decreased twofold (0.15 - 0.08 km(3) km(-1) Ma(-1)) from the Early Quaternary to the Pleistocene-Holocene.
- 出版日期2016-9
