Exhumations- und Hebungsgeschichte der zentralen Anden in Südbolivien (21°S)durch Spaltspur-Thermochronologie an Apatit

Abstract

The Tertiary formation of the Altiplano-Puna plateau at the active continental margin of South America resulted in the doubling of the crust and surface uplift by about 4000 m. Uplift leads to erosion, exhumation and cooling of rocks. The aim of this study is to reconstruct the Tertiary cooling and exhumation history by apatite fission track thermochronology (AFT) and hence to characterise the processes which produced plateau uplift. The study area follows a transect at 21°S in Southern Bolivia across the plateau and the eastern flank of the Andes. The plateau with mean elevations >3000 m extends from the recent volcanic arc of the Western Cordillera, across the Altiplano to the eastern margin of the Eastern Cordillera. The plateau is bordered to the east by a foreland fold-thrust belt (Interandean and Subandean). In the Western Cordillera, pre-Tertiary rocks are covered beneath Neogene volcanics. The Altiplano represents an intramontane basin with up to 7 km thick Cenozoic continental sediment fill. Paleozoic units are exposed along a major fault zone in the central part. The Eastern Cordillera forms a bivergent thrust system in Ordovician sedimentary rocks. Apatite fission track results are obtained from 28 samples of Paleozoic-Mesozoic and from 2 samples of Tertiary sedimentary rocks. The variable annealing behaviour of detrital apatites is adressed by measuring the kinetic parameter Dpar. The total annealing temperatures of the dated apatites range from 110 °C to 160 °C. Separation of apatite-populations with different annealing kinetics provides a higher resolution of the temperature history. The majority of samples have mean track lengths between 12 μm and 14 μm. Forward modelling of the thermal history (AFTSolve) shows that the onset of cooling can be considerably older as well as younger than the apparent fission track age. The consideration of the geological context constrains plausible time-temperature paths. Apparent ages in the plateau range from 36 Ma to 20 Ma, with a maximum around 30 Ma in the middle Oligocene. Significantly younger cooling ages with 199 Ma are obtained in the foreland thrust belt. Samples with mixed ages of 29 Ma and 75 Ma in the Subandean cooled after 8 Ma. Based on published geothermal data from the Bolivian Andes, paleo-geothermal gradients from 19 °C/km to 32 °C/km are assumed for the interpretation of the AFT-data in the spatial frame of reference. Changes of the lithospheric thermal conditions during the Tertiary probably had a subordinate influence on the observed cooling rates compared to the exhumational cooling. The maximum temperature and burial depth of a sample in the central Altiplano are used to estimate a paleo-surface heat flow density of 70 ± 20 mW/m for the early Miocene. Late Eocene (ca. 42 Ma) exhumational cooling in the central part of the Eastern Cordillera correlates with Miocene unconformities, the inversion of Cretaceous rift structures and the provenance of EoceneOligocene sediments in the Altiplano and Camargo basins. The middle Oligocene (33-27 Ma) was characterised by a broad onset of exhumation, distributed across the future plateau region. AFT-data of basement pebbles from early Miocene sediments in the Altiplano suggest rapid exhumation in the source areas of the western Altiplano or Western Cordillera. In the central Altiplano, Paleozoic rocks were exhumed along the east-vergent Uyuni-Khenayani fault zone (UKFZ), starting from 31 ± 2 Ma. In the adjacent depocenter west of this fault zone, however, heating by sedimentary burial continued until 18 Ma. These opposite temperature paths indicate that the western thrust of the UKFZ was initially active as a normal fault. In the western part of the Eastern Cordillera, exhumation initiated in the frontal as well as the inner