ABSTRACT

The Chile Trench study area extends over 2,000 km along the Andean continental margin, from 23°S to 42°S latitude. The QFL composition of Chile Trench sands encompasses the average compositions of modern sands from strike-slip, continental arc, oceanic backarc, and oceanic forearc types of active margins (Valloni and Maynard 1981), and spans the complete spectrum from dissected to undissected magmatic arc settings (Dickinson et al. 1983). Trench sands along North Chile, Central Chile, and the glaciated archipelago of South Chile are derived from a more plutonic source region, or dissected magmatic arc; these sands exhibit abnormally low lithic and calcic plagioclase content, but high quartz and alkali feldspar. In Central Chile, the plutonic provenance is correlated with an ab ence of Quaternary volcanism. Along the Chilean archipelago, Pleistocene glaciation has carved into the magmatic roots of the Andes, effecting arc dissection in spite of active volcanism. In arid North Chile, the volcanic debris of the High Cordillera is trapped onshore in longitudinal forearc basins, allowing an exaggerated contribution from crystalline rocks of the Coast Range.

The light and heavy, mineral and lithic composition of trench sands is described by four ideal petrologic assemblages, isolated by Q-mode factor analysis. The Basic Magmatic Arc petrofacies (volcanic lithics, calcic plagioclase, olivine, clinopyroxene, and orthopyroxene), the most chemically unstable and least differentiated assemblage, is derived exclusively from basalts and basaltic andesites in the Quaternary volcanic arc of South Chile. The Acidic Magmatic Arc petrofacies (volcanic lithics, alkali feldspar, biotite, hornblende, and magnetite) is derived from andesitic to rhyolitic volcanics, and granodioritic plutons emplaced during various Mesozoic and Cenozoic magmatic episodes in the history of the Andean margin. The Metamorphic Magmatic Arc petrofacies (mica, alkali feldspar, hlorite, actinolite, epidote, and blue-green hornblende) is derived from ferromagnesian rocks altered to the greenschist facies in the basal marine section of the Andean eugeosyncline and in the Paleozoic basement. The Cratonic Block petrofacies (quartz and the accessory minerals andalusite, garnet, and tourmaline) is derived from mature platform sediments, late-stage granitic intrusions, and high-grade regional metamorphics associated with the South American craton; it is most prominent along northern Chile where the continental crust is thickest and presumably truncated by subduction erosion.

Actual petrofacies in the Chile Trench are typically mixtures of the four ideal petrofacies. Their compositions can be related to 1) the lithology and erodibility of the source terranes, 2) the distribution and composition of modern arc volcanism, 3) the prevailing climate in the provenance region, and 4) the structure and morphology of the continental margin and trench. In many cases contemporary volcanism inundates and dominates the petrologic assemblage, diluting contributions from all other source-rock types and forming a low-diversity petrofacies. At high latitudes, where glaciation was extensive, a variety of source-rock terranes were actively denuded and petrofacies of high diversity were deposited in the adjacent trench. The linear frequency of sediment-supply points along the base of the continental slope controls the spatial heterogeneity of petrofacies on the depositional surface in the trench basin. The most homogenous petrofacies, derived from major submarine canyon systems and extensive subaerial drainage basins, may extend hundreds of kilometers along-margin due to axial transport processes within the trench.