FOSSIL MAMMALS AND 40AR/39AR RADIOISOTOPIC DATING FROM LAGUNA DEL LAJA, CHILE; IMPLICATIONS FOR POST-NEOCOMIAN TECTONIC HISTORY

For more than a century, establishment of a secure geochronology for the post-Neocomian units of the Andean Main Range of central Chile has been hindered by monotonous lithology, Neogene structural overprinting, and post-depositional alteration.   Early work frequently relied on tenuous stratigraphic correlations and low-quality ⁴⁰K-⁴⁰Ar dates, which in turn became incorporated into models of Andean evolution. The recent discovery of age diagnostic fossil mammals within the volcanic and volcaniclastic Abanico Formation (a previously poorly dated unit exposed over much of the region) has created a means through which the tectonic history of the range between 33°-35° S may be re-evaluated.

New fossil and radioisotopic data from the Abanico Formation indicate that its age has been greatly overestimated.  This revelation has precipitated an effort to determine if the extent to which the traditional models of the post-Neocomian tectonic evolution of the central Andean Main Range that are particularly sensitive to this age information remain valid. Important parameters of the models in question include the timing and cause of basin subsidence, the timing and degree of deformation, and the relationship between plate convergence rates and upper plate volcanism.

To address these issues, we have extended the range of paleontological and radioisotopic sampling several hundred kilometers south into the Cura-Mallín Formation at Laguna del Laja (37.5° S). Preliminary taxonomic study in conjunction with ⁴⁰Ar/³⁹Ar radioisotopic analyses of the volcanic and volcaniclastic intervals within the Cura-Mallín Formation show this unit to be roughly equivalent in age to the Abanico Formation, indicating a maximum age several million years younger than even the youngest previous estimate. A detailed chronology of a broad transect of the central Main Range using our dual dating method will help to discriminate between competing hypotheses. Discoveries at Laguna del Laja extend the distribution of proven fossiliferous volcaniclastic outcrop across four degrees of latitude, making this area one of the most important repositories of South American mammal fossils known.