Thesis #405

In the Rio Tinguiririca valley in the Main Cordillera of the Andes of central Chile,
35 ° south, parts of a stratigraphic section ranging from the late Jurassic to the
Quaternary are exposed. Fission track analysis was carried out on samples from
all the stratigraphic units exposed in the area in order to gain information on the
low–grade metamorphic history of the Central Andes and to test older models for
the metamorphic and tectonic evolution of the area. The sequence exposed in the
Rio Tinguiririca valley is characterised by several distinctive features not found in
other localities in the Central Andes; the deposits of the middle Cretaceous (Aptian
– Albian) Colimapu Formation and of the middle to late Micoene Farellones
Formation are completely missing in the study area. Instead, a volcanic tuff layer,
the White Tuff, and a unit consisting of fan deposits and alluvial plane deposits,
the Brownish–Red Clastic Unit, unconformably overlie the Late Jurassic deposits
of the Ba˜nos del Flaco Formation. The fission track data give some indications
on the style and timing of metamorphic events in the study area, enable more
accurate constraint of the age of the Brownish–Red Clastic Unit and allow some
statements on the tectonic evolution of the study area from the Late Jurassic to
present. Burial metamorphism has been proposed by various authors as the main
mechanism to produce large suites of rocks altered at low grades in the Central
Andes. The results of this study indicate that, on the contrary, hydrothermal
alteration connected to magmatic and/or volcanic activity was the main cause
of alteration of the rocks and that burial metamorphism played at most a very
minor role. Pulses of hydrothermal activity appear to have occurred from Cretaceous
to almost recent times and led to alteration of the rocks at slightly varying
metamorphic conditions at different times in different parts of the study area. A
new model is proposed for the tectonic evolution of the study area. Fission track
analysis of detrital zircons from the Brownish–Red Clastic Unit shows that the
unit must have been deposited during the latest Cretaceous (Maastrichtian) and
that it is certainly younger than the White Tuff. Thermal modelling shows that
considerable exhumation of the lower part of the Rio Damas Formation occurred
during the Late Cretaceous to Early Tertiary. This exhumation is thought to be
connected to tilting and erosion of the Mesozoic units in the area prior to the
formation of an extensional basin in the Late Eocene. Data from the Eocene to
Miocene Coya Machali Formation imply that sedimentation within the Tertiary
basin continued somewhat longer than hitherto supposed.