Fission Track Analysis of Detrital Apatites from Sites 859, 860, and 862, Chile Triple Junction

Abstract

Fission track analysis of detrital apatite grains recovered from sediment samples of upper Pliocene to Quaternary age from the Chile Triple Junction (Sites 859, 860, and 862), shows that the apparent ages of individual grains are older than the stratigraphic ages. The apatite fission track data indicate that there has been no significant postdepositional heating (i.e., maximum paleotemperatures have never exceeded ~60°C). The apatite grains, therefore, derive their fission track characteristics from the source area. The mineral assemblages are similar in all samples and indicate that this source area was dominated by crystalline basement and a volcanic terrain. The track-length distributions characterized by relatively long means (13-14 µm) are a significant feature of the fission track data, given that the apatite grains were eroded from mixed sources of a known broad age range. These data record moderately rapid cooling through ~ 110°C, which we attribute to moderately rapid uplift and erosion of the source area, followed by transportation and deposition of the sediment.Two distinct age populations are recognized in the combined samples from each of the three sites. The younger population is represented by a fission track age of -7 ± 2 Ma, whereas the older population gives an age of -25 ± 5 Ma. The younger age group is interpreted as revealing the time of moderately rapid cooling, produced by uplift and erosion of the source area. Recognition of this late Miocene (~7 Ma) event in the fission track data coincides with several major processes, including magmatism and tectonism, which have been documented already in the area. Erosion of the source (arc-forearc) region at this time is most likely a direct result of tectonic uplift caused by collision and subduction of one or two segments of the Chile Ridge. The older age (-25 Ma) identified in the fission track data records a discrete cooling event which we interpret as denudation and/or deformation of the forearc-arc region as a consequence of oblique convergence at the Chile margin.

Source-to-sink relationships along the South-Central Chilean margin: evidence from detrital apatite fission-track analysis

“…4, data from Thomson et al, 2001). Note also that the same age signal is also found within Pliocene sediments of the trench (George & Hegarty, 1995). Therefore, erosional e¥ux is highest for units exposed along the high topography of the hinterland.…”

“…These relatively small areas most likely provided the majority of apatites to the trench ¢ll, due to focused glacial erosion. Such an interpretation is supported by the onland dataset, which show a rather high variation away from the LOFZ (Thomson et al, 2001) and the fact that track-length data from the trench apatites also show minor numbers of short tracks (George & Hegarty, 1995). Note also that the same age signal is also found within Pliocene sediments of the trench (George & Hegarty, 1995).…”

“…Chilean trench sediments were the subject of a single AFT study on ODP141 samples from the vicinity of the CTJ (George & Hegarty, 1995). These authors distinguished between two age groups of c. 7 AE 2 and 25 AE 5 Ma.The younger age peak was explained by exhumation due to collision and subduction of one or two segments of the Chile Ridge.…”

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

“…The continuous uplift of the Andes during the past 7 million years (and especially the area around the Chile triple junction) has been related to crustal thickening as a result of plate-scale compressional tectonics (Howell 1989) and magmatism (Pankhurst et al 1992;Bourgois et al 1996). Evidence for this from geological processes on land was found from various studies: (1) uplift of upper Tertiary marine basins (Flint et al 1994) and the underlying arc and forearc basement , (2) large-scale deformation and size reduction of the forearc by subduction erosion (Behrmann et al 1994;Bourgois et al 1996), (3) distinct uplift, cooling and exhumation on land dated by apatite fission track analysis (George and Hegarty 1995), (4) critical depth of intrusion of young plutons (Pankhurst et al 1992), and (5) the age distribution pattern of flood basalt fields (Ramos and Kay 1992;Lagabrielle et al 1994).…”

“…There are several additional arguments to support this low sedimentation rate estimate: First, most of the southern Andean margin and its continental hinterland was submerged or formed part of lowlands at the time (Pankhurst et al, 1988), probably leading to low rates of terrigenous mass wasting from the continent. As shown from fission track studies on apatite (George and Hegarty 1995), the major uplift of the southern Andes (and thus a related increase in sediment input into the southern Chile trench) took place in the latest Miocene~7 Ma before present. Second,`empty trench scenarios' in the vicinity of modest-sized accretionary prisms are known.…”