9 Chapter 9 Latitudinal Variation in Sedimentary Processes in the Peru-Chile Trench off Central Chile David Völker · Michael Wiedicke · Stefan Ladage · Christoph Gaedicke · Christian Reichert · Klaus Rauch Wolfgang Kramer · Christoph Heubeck trench forms a narrow depression with steep walls, ranges in depth from 6 100 m to more than 7 000 m and contains little sediment (Schweller et al. 1981; von Huene et al. 1997; Flueh et al. 1998; Laursen et al. 2002). South of the JFR, the trench is partly, or completely, filled with sedi- ment. This article is about this sediment fill in the trench sector between the JFR and the more southerly located Chile Ridge. We discuss the processes of sediment input and redistribution within the trench. While it is known that south of 45° S, accretion ceased with the subduction of the Chile Ridge (Herron et al. 1981; Cande et al. 1987, Behrmann et al. 1994), and that the northern Chilean margin is clearly an erosive margin (e.g. von Huene and Scholl 1991; Laursen et al. 2002; Adam and Reuther 2000), the nature of the transition between these two regimes is poorly understood. We consider knowl- edge about this sedimentary basin to be essential for the following reasons: 1. As large quantities of sediment are potentially sub- ducted into the mantle, the flux rates and physical prop- erties of the sediments delivered into the trench are likely to be major controlling parameters on the strength of the interface between the Nazca and the South Amer- ican Plates (e.g. Lamb and Davies 2003). 2. Accordingly, variations in the sediment supply and, as a consequence, variations in the friction coeficcient of the plate interface (“lubrification”) are used to explain spatial and lateral variations in sediment accretion, tectonic fore-arc erosion, and upper-plate shortening along the Andean active margin (Adam and Reuther 2000; Vietor and Echtler 2006, Chap. 18 of this volume). 3. Knowledge of sedimentation rates is basic for any at- tempt to calculate mass fluxes between the plates and for establishing the accretionary history of this sector of the continental margin. 4. There is strong potential for the sedimentary record in this area to bear evidence of mass-wasting events, such as submarine landslides, because of the high sedi- mentary input and the high seismicity at the plate con- tact. These conditions pose a potential tsunami risk for the greater region. Abstract. Four cruises of the German research vessel RV SONNE (cruises SO101, SO103, SO104 and SO161) surveyed the Chilean con- tinental margin and oceanic plate using seismic measurements across the Peru-Chile Trench, swath-mapping bathymetry, sediment echosounding, dredges and gravity-core sampling. In this paper, we present data from cruise SO161 derived from the sediment-filled sector of the trench between 35 and 44° S. South of 33°10' S, sedi- ment fill in the trench ranges from 2 200 to 3 500 m thickness. The sediment volume decreases northwards, as the trench width nar- rows from 80 km at 41° S to 25 km at 33° S. Turbidity currents enter the trench mainly via nine canyon systems that are deeply incised into the continental slope. Reflection patterns from the trench fill exhibit a cyclicity that can be linked to Milankovic cycles. Turbiditic deposits at elevated positions within the trench indicate Pleistocene mass-wasting events that were able to overcome a height differ- ence of some hundred meters. Within the trench, a fraction of the turbidity currents is channelled by a northward-dipping, axial chan- nel. This axial channel has eroded up to 200 m into the trench fill and from 42° S, it extends northwards some 1 000 km, terminating at the foot of the Juan Fernandez Ridge. The channel has no con- tinuous precursor and might have evolved its present-day form during the last glaciation. 9.1 Introduction The continental margin of Chile is characterized by the oblique subduction of the Nazca Plate beneath the South American Plate. It is presently subducting at a rate of 6.6 cm yr –1 with a convergence angle of 77° in relation to the trend of the trench (Angermann et al. 1999, Fig. 9.1). The subducted plate ages northwards: the crust is only 10 million years old at 44° S, but is 20 million years old at 41° S and is 33 million years old at 35° S (Müller et al. 1997, Fig. 9.1). The Nazca Plate subducts at angles of 9–25° (Patzwahl et al. 1999) and 15–20° (Lüth and Wigger 2003) at the subduction front. This produces the Peru-Chile trench which deepens from 4 100 m near southern Chile (44° S) to 6 400 m off the coast of central Chile, and deep- ens further to 8 200 m off northern Chile (Lindquist et al. 2004). The Juan Fernandez Ridge (JFR) enters the subduc- tion zone off the Valparaiso coast (33° S), whereas the Chile Ridge is subducting at 45° S (Fig. 9.1). The JFR forms a topographic high within the trench and separates two distinct sedimentary domains. North of the JFR, the