MAGNETOSTRATIGRAPHY OF 5-10 MA SEDIMENTS OF THE CORQUE SYNCLINE, BOLIVIA

A magnetostratigraphic study of the Corque Syncline, Bolivia was used to refine age estimates between two dated tuffs, the Callapa Tuff (~9Ma) and the Toba 76 Tuff (~5Ma). A total of 263 hand samples were collected from 86 sites during two trips to the study area. Following NRM measurement with a superconducting magnetometer, samples were thermally demagnetized in a shielded oven. Temperature steps were widely spaced at low temperatures to monitor the removal of any overprinting magnetizations, and then were closely spaced starting at 525°C to maximize the number of data points representing the Characteristic Remanent Magnetization (ChRM). Isothermal Remanent Magnetization (IRM) analyses were also performed on eight samples to determine the dominant magnetic mineralogies. Together, demagnetization and IRM results suggest that the ChRM is carried primarily by hematite. The direction of ChRM for each sample was picked using the Super-IAPD 2000 program and site means were calculated for those sites with two or more samples per site. Due to poor quality of the samples or demagnetization data, high MAD angles (>15°), or ambiguous polarity, 21 of the original sample sites were removed from the final analysis. A total of 65 sites with 135 samples were used in determining magnetostratigraphic correlations. Based on comparison of site mean paleomagnetic data with the known Geomagnetic Polarity Time Scale, a best-case magnetostratigraphic interpretation was developed for the section, providing new information about the timing of deposition and sediment accumulation rates. These new data suggest that a pronounced decrease in sedimentation rates occurred after 8.7 Ma, during the same time period when the Altiplano experienced rapid surface uplift.