Balance of tectonically accreted and subducted sediment at the Chile Triple Junction

Abstract

An active oceanic spreading ridge is being subducted beneath the South American continent at the Chile Triple Junction. Mass balance estimations to characterize temporal and spatial variations in the frontal accretion, or underplating and subduction of sediments since the Late Miocene, were made using seismic and drill-hole data. At 200 km north of the triple junction, almost 80% of the sediment on the downgoing Nazca plate are subducted. Sediment subduction rate decreases towards the triple junction because of a low in sedimentation rates as the flank of the spreading ridge approaches the trench. At the triple junction, the forearc is almost completely destroyed by spreading ridge collision and subduction erosion. Less than 12% of the available sedimentary input is accreted. South of the triple junction, where the spreading ridge passed 6 Ma ago, a large fraction (>60%) of the sediment on the incoming Antarctic plate has been scraped off and was frontally accreted to the Chile forearc. Spreading ridge subduction leaves a distinctive geological fingerprint, and has a large impact on the mass balance of the subduction zone. However, the high rates of change in the process may make this fingerprint hard to detect in fossil convergent orogens. In the ridge collision zone the sediment supplied to the trench, and the amount of sediment subducted, show strong and distinctive variations on a 1-to 5-million-year time scale. On a 10-million-year time scale, sediment subduction to the Earth's mantle is reduced by spreading ridge collision, caused by the need of the overriding forearc to regain a low angle of taper by frontal accretion.

Deep lithospheric structures along the southern central Chile margin from wide-angleP-wave modelling

“…However, the latter two lines may be atypical for southern central Chile as they are in the vicinity of the incoming Chile Ridge, with a thin or currently absent trench basin at the deformation front, and most or all sediments here are currently accreted frontally at the toe of the frontal slope (Behrmann & Kopf 2001). Here in the south, the Chile Ridge itself is about to erode the upper plate (Bourgois et al 2000).…”

“…This 30-40 km wide region of low velocities can be interpreted as the active accretionary prism, similar to what is observed a few hundred km further north near Arauco peninsula at TIPTEQ Corridor 1 and the SPOC profiles (Figs 13 and 15;Krawczyk et al 2006;Contreras-Reyes et al 2008b) and also as interpreted on seismic reflection data (Bangs & Cande 1997;von Huene et al 1997). This prism would have been accumulated within the last 1-2 Myr while active accretion has been continuous after episodes of subduction erosion and accretion (Bangs & Cande 1997;Behrmann & Kopf 2001).…”

“…The analysis of forearc basin structures suggests that the change from subduction erosion to accretion occurred about 3 Ma ago . According to the size of the accretionary wedge, accretion has been active for the last 1-2 Myr, but a large part of the incoming sediment is actively subducting (Bangs & Cande 1997;Diaz-Naveas 1999;Behrmann & Kopf 2001;Ranero et al 2006).…”

Holocene sediments from the Southern Chile Trench: a record of active margin magmatism, tectonics and palaeoseismicity

“…It becomes steeper (10-158) towards the trench floor (Scholl et al 1970) and is steepest close to the Chile Triple Junction, where subduction erosion occurs (e.g. Behrmann & Kopf 2001). The continental slope is irregular in shape, showing small plateaux, lineaments, and escarpments ( Fig.…”

Complete recycling of a magmatic arc: evidence from chemical and isotopic composition of Quaternary trench sediments in Chile (36°–40°S)

“…Accumulation of these sediments in an accretionary wedge appears to be minor and subduction of sediments is considered as important (e.g. Behrmann and Kopf 2001;Kukowski and Oncken 2006). The present sediment supply from the shelf (marine forearc basins) and upper slope into the trench is largely controlled by submarine cañon linked to the major E-W drainages (rivers) of the western flank of the Andes (e.g.…”