Active or passive continental margin? Geochemical and Nd isotope constraints of metasediments in the backstop of a pre-Andean accretionary wedge in southernmost Chile (46°30′–48°30′S)
C. Augustsson, H. Bahlburg, 2003. "Active or passive continental margin? Geochemical and Nd isotope constraints of metasediments in the backstop of a pre-Andean accretionary wedge in southernmost Chile (46°30′–48°30′S)", Tracing Tectonic Deformation Using the Sedimentary Record, T. McCann, A. Saintot
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Provenance analysis of siliciclastic sedimentary rocks gives indications of the tectonic evolution and setting of source regions and the rocks contained in them. The composition of sedimentary rocks ideally reflects the nature of these regions, and only indirectly the tectonic setting of the basin where the erosional debris is deposited. This makes it possible to interpret Late Devonian to Early Carboniferous metasedimentary basement rocks of the Andes in southernmost Chile as having been deposited at a passive margin, despite geochemical indications of an active margin setting for the source rocks, and the position of the metasediments in the backstop of an accretionary wedge. Major and trace elements point to felsic source rocks from an active margin environment. The Nd model ages of 1170–1490 Ma indicate that the source rocks were part of an old continental crust in the Late Palaeozoic. The εNd(T) values range between −7 and −2. These characteristics, in combination with the regional geology, suggest that the geochemical signal is dominated by rocks formed at an active margin, which later acted as feeders for the sediments deposited in a passive-margin environment. If corroborated by research in progress this emphasizes the problem of deducing the tectonic setting of a depositional basin from provenance data.
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The study of sediments and sedimentary basins in terms of their tectonic environment requires a multidisciplinary approach and has increasingly drawn both techniques and objectives from fields outside sedimentology. The application of different theoretical, experimental and empirical resources provided by structural geology, geochemistry, geophysics, scale modelling, and field geology, complement sedimentological methods, with the combined aim of achieving a deeper understanding of the origins, evolution and significance of sedimentary sequences in terms of their tectonic history.
Studies presented in this volume range across a wide spectrum from the analysis of sedimentary sequence architecture at basin scale down to the chemical properties of individual grains, and include studies from a range of tectonic settings.
The volume will be of interest to those involved with, or contemplating, studies involving the linkages between tectonics and sedimentation, as well as a wider audience to whom the results of such studies may provide fresh insight.