Late Eocene to Early Miocene Andean uplift inferred from detrital zircon fission track and UePb dating of Cenozoic forearc sediments (15e 18  S) A. Decou a, * , H. von Eynatten a , I. Dunkl a , D. Frei b , G. Wörner a a Geowissenschaftliches Zentrum der Universität Göttingen, Goldschmidtstr. 3, D-37077 Göttingen, Germany b Department of Earth Sciences, Stellenbosch University, Private Bag X1, 7602 Matieland, South Africa article info Article history: Received 2 March 2012 Accepted 13 February 2013 Keywords: Central Andes Tertiary Crustal thickening Provenance analysis Detrital zircon Fission track thermochronology UePb dating abstract Timing, amount, and mechanisms of uplift in the Central Andes have been a matter of debate in the last decade. Our study is based on the Cenozoic Moquegua Group deposited in the forearc basin between the Western Cordillera and the Coastal Cordillera in southern Peru from w50 to w4 Ma. The Moquegua Group consists mainly of mud-flat to fluvial siliciclastic sediments with upsection increasing grain size and volcanic intercalations. Detrital zircon UePb dating and fission track thermochronology allow us to refine previous sediment provenance models and to constrain the timing of Late Eocene to Early Miocene Andean uplift. Uplift-related provenance and facies changes started around 35 Ma and thus predate major voluminous ignimbrite eruptions that started at w25 by up to 10 Ma. Therefore magmatic addition to the crust cannot be an important driving factor for crustal thickening and uplift at Late Eocene to Early Oligocene time. Changes in subduction regime and the subducting plate geometry are suggested to control the formation of significant relief in the area of the future Western Cordillera which acts as an efficient large-scale drainage divide between Altiplano and forearc from at least 15.5 to 19  S already at w35 Ma. The model integrates the coincidence of (i) onset of provenance change no later than 35 Ma, (ii) drastic decrease in convergence rates at w40, (iii) a flat-subduction period at around w40 to w30 Ma leading to strong interplate coupling, and (iv) strong decrease in volcanic activity between 45 and 30 Ma. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction The Central Andes presently form the largest mountain chain built by subduction processes in one of the most intensely thick- ened region of the South American continent (Sempere et al., 2008). Since Jurassic time the oceanic Nazca plate is being sub- ducted beneath the South American continent. Ongoing subduc- tion, crustal shortening (Isacks, 1988; Allmendinger et al., 1997) and e to a minor extend e magma addition have lead to the con- struction of the Central Andes edifice with its up to 70 km thick continental crust. However, despite long lasting subduction, the uplift is generally considered to have not started before middle Eocene time (Isacks, 1988; Allmendinger et al., 1997; Sempere et al., 2008). The elevation reached during this first pulse of uplift (w45e20 Ma; Anders et al., 2002; Gillis et al., 2006) is strongly debated (Gregory-Wodzicki, 2000; Sempere et al., 2008). A second pulse of uplift has been recognized to be late Miocene in age starting at w10 Ma (Lamb and Hoke, 1997; Schildgen et al., 2007; Thouret et al., 2007; Garzione et al., 2008; Sempere et al., 2008). Despite its fundamental role for surface uplift, processes which lead to w70 km thick crust are strongly debated. Tectonic shortening is widely accepted to be responsible for initial crustal thickening (Oncken et al., 2006). However, nearly no shortening occurred in the western Andean margin of the Central Andes since more than 10 Ma (Isacks, 1988; Wörner et al., 2000b, 2002; Oncken et al., 2006; Sempere and Jacay, 2007). Molnar and Garzione (2007) and (Garzione et al., 2007 , 2008) proposed that delamination of dense lithospheric material into the mantle was responsible for uplift. However, delamination should be a consequence of thickening (Kay and Mahlburg-Kay, 1991) and cannot be its cause. Moreover, no magmatic products typical of this process are actually known from the region (Kay and Mahlburg Kay, 1993; Kay and Coira, 2009; Mamani et al., 2010). Alternatively, various authors (e.g. Meissner and Mooney, 1998; Babeyko et al., 2002; Beck and Zandt, 2002; Wörner et al., 2002; Husson and Sempere, 2003; Oncken et al., 2006) suggested that large-scale lateral flow of ductile lower crust may have contributed significantly to crustal thickening in the west with regional tilt on the Western Andean margin and limited upper crustal shortening. * Corresponding author. Present address: CASP, West Building,181a Huntingdon Road, Cambridge CB3 0DH, UK. E-mail address: audrey.decou@casp.cam.ac.uk (A. Decou). Contents lists available at SciVerse ScienceDirect Journal of South American Earth Sciences journal homepage: www.elsevier.com/locate/jsames 0895-9811/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jsames.2013.02.003 Journal of South American Earth Sciences 45 (2013) 6e23