Collaborative Research: Miocene-Pliocene Paleoelevation of the Bolivian Altiplano

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High elevation plateaus are enigmatic features that form in contractional mountain belts.Regional crustal shortening and thickening cause significant amounts of surface uplift within plateaus. However, other crustal and subcrustal lithospheric processes, such as lithospheric thinning, magmatic additions to the crust, tectonic underplating, or lower crustal flow may also play an important role in producing these broad, high elevation regions. Understanding the mechanisms responsible for plateau uplift requires knowledge of the history of surface elevation. Both the magnitude and time scale of surface uplift are controlled by the underlying mechanisms responsible for uplift. The PI's propose to study the uplift history of the Altiplano in Bolivia using oxygen isotope paleoaltimetry techniques in addition to testing a new cosmogenic isotope paleoaltimeter. These paleoelevation data will help resolve current debates over the early versus recent uplift of the Altiplano. In addition, these data will place constraints on the magnitudes and rates of uplift over time, enabling them to determine the importance of certain uplift mechanisms. The Altiplano has an average elevation of 3700 m, second only to the Tibetan plateau in size and lateral extent. This is an ideal setting in which to carry out a systematic study of long term surface uplift for the following reasons: 1) A previous paleoelevation study, based on leaf physiognomy, provides a record by which to compare our paleoelevation data. These data suggest that no more than half of the uplift of the Altiplano took by ~10 Ma (Gregory-Wodzicki, 1998) and also provide paleotemperature estimates required for oxygen isotope paleoaltimetry. 2) Oxygen isotope paleoaltimetry can only be applied in regions that experience minimal evaporation (humid environments), whereas cosmogenic isotope paleoaltimetry requires minimal erosion (arid environment). There is a large range of climatic variability within the Bolivian Altiplano, enabling us carry out oxygen isotope paleoaltimetry in the more humid northern Altiplano and cosmogenic isotope paleoaltimetry in the arid southern Altiplano. 3) There is a long-term history of sedimentation within the Altiplano, producing an extensive carbonate record from which oxygen isotope paleoelevation estimates can be obtained. The ages of sedimentary rocks in the Corque basin, our target section, have been determined to be between ~14 Ma and ~5 Ma by 40 Ar/ 39 Ar dates of interbedded tuffs (Marshall et al., 1992) and magnetostratigraphy (Roperch et al., 1999). 4) Data on the oxygen isotope composition of monthly and yearly rainfall has been collected providing an excellent d 18 O vs. altitude gradient (Gonfiantini et al., 2001). This gradient will be used to estimate the elevation of paleometeoric water from which carbonates were precipitated. 5) Widespread Miocene to recent volcanism within the Altiplano provides abundant datable paleosurfaces containing the mineralogies needed to apply cosmogenic isotope techniques to determining paleoelevation. In the southern Bolivian Altiplano evidence suggests that these paleosurfaces have experienced very little erosion over the past 15 Ma (Horton, 1998). The preliminary oxygen isotope data show a pronounced shift in oxygen isotopic ratios of about -7.5 between late Oligocene and late Miocene time, which suggests more than 3 km of surface uplift. Initial measurements on apatite and sanidine indicate that both cosmogenic 3 He and 38 Ar are preserved in modern low latitude surfaces on the Altiplano. This research is a collaborative effort between the University of Rochester, the Harvard- Smithsonian Center for Astrophysics, and researchers at Berkeley and Caltech. The broader impacts of this project are that it will provide partial support for two Ph.D. students and two or more undergraduate students. Undergraduate research will lead to the completion of senior thesis projects. In addition, this research will directly inform an ongoing study of student conceptual understandingof earth processes. Scientific views of plate tectonic processes continue to evolve in response to research like that proposed here, and these views are the datum to which student ideas must be compared.