Behrmann, Jan Hinrich

Heberer, B.

Kopf, Achim

Rahn, M.

Roeser, G.

English

OceanRep

Geophysical Research Abstracts, Vol. 9, 05357, 2007SRef-ID: 1607-7962/gra/EGU2007-A-05357© European Geosciences Union 2007Sedimentology, petrography and provenance of modernSouthern Chile Trench sediments (36˚S-47˚S)G. Roeser (1), B. Heberer (1,2), J. H. Behrmann (3), M. Rahn (4), A. Kopf (5)(1) University of Freiburg, Germany, (2) University of Salzburg, Austria, (3) IFM-GEOMARKiel, Germany, (4) HSK, Villigen-HSK, Switzerland, (5) RCOM, University of Bremen,Germany (georg.roeser@geologie.uni-freiburg.de / Phone: +49-761-2036503)Sedimentology, petrography and the provenance of sediments from the Southern ChileTrench (36˚S to 47˚S) that are about to be subducted into the seismogenic zone be-neath the South American Plate, were investigated in an integrated approach combin-ing description of a large collection of gravity cores, quantitative X-ray petrography,modal analysis and fission track age analysis on detrital apatite. The sedimentary en-vironments studied were trench hemipelagics, trench fan deposits, and more distalhemipelagics sedimented on the Nazca Plate north of the Chile Triple Junction (CTJ).The trench is fed by terrigenous turbidity currents from multiple point sources viasubmarine canyons, extending from the shelf break across the slope, and building sub-marine fans at the bottom. The turbidites are interlayered with clay- and silt-sizedhemipelagics. Within the trench, sediment is transported northwards along a slightlyinclined axial channel. The regional depth gradient in the trench floor is caused by thenorthward age increase of the Nazca Plate.Both, grain-size and core physical properties reveal distinct latitude-dependent trends.In the trench and the trench fans, contents of clay-sized particles increase towards Nwith a coeval decrease of the medium and coarse silt fraction due to a combinationof several factors, such as the northward directed sediment transport, changes in hin-terland lithologies, and different erosion patterns as well as greater water depths inthe N. Bulk density of the hemipelagic section reveals a slight but consistent increasetowards the S. This relates to higher sediment input, morphologically expressed by acompletely buried trench, and goes along with an increase in average turbidite thick-ness.Modal analyses of turbidites show a southward increase in sediment maturity. Whilevolcanic lithics represent the most dominant fraction within samples from 36˚ to 43˚S,quartz, and metamorphic and magmatic lithics prevail near the CTJ (at 46.5˚S). Thisreflects the source lithologies, especially the absence of present-day volcanic activityin the hinterland between 47˚ and 49˚S, and is also mirrored in the southward decreas-ing magnetic susceptibility found in the gravity core material. Further north, activevolcanoes in the Main Cordillera partly cause singularities in the provenance signaldue to an overwhelming contribution of highly erodible volcanics. Neither were ma-jor intrasite variations of detrital modes detected nor variations between trench andtrench fans, the latter being ascribed to intense source mixing within the trench.

Sedimentology, petrography and provenance of modern Southern Chile Trench Sediments (36°S-47°S)

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