Paper No. 4
Presentation Time: 1:30 PM-5:30 PM
EXHUMATION HISTORY AND BASIN DEVELOPMENT ALONG THE EASTERN MARGIN OF THE CENTRAL ANDEAN PLATEAU, BOLIVIA
25 mica 40Ar/39Ar ages and 15 apatite fission track (AFT) ages from the Eastern Cordillera (EC) of Bolivia define the cooling history of the Cordillera Real (CR) and Quimsa Cruz (QC) mountain ranges at 17-18° S. Provenance analyses and biotite 40Ar/39Ar ages for measured stratigraphic sections within mid-Cz basin fill along the E. margin of the Altiplano (AP) constrain the sedimentation history directly adjacent to the W. flanks of the CR and QC. These datasets demonstrate that the EC was a topographic high supplying sediment to the AP basin as early as 32 Ma. Biotite (bt) and muscovite (mu) Ar/Ar ages for samples collected along a SW-NE transect across the CR indicate a NWward decrease in ages and relatively rapid cooling between ~43 and 38.3 Ma. Mu age distributions display a positive age-elevation correlation for 192.9 - 42.9 Ma with a probable inflection point at ~45 Ma. These data are interpreted as cooling within the mu partial retention zone prior to onset of rapid cooling. Bt ages exhibit a positive age-elevation correlation for 107.2 - 38.3 Ma with a well-defined inflection point at ~43 Ma indicative of an onset of rapid cooling. AFT ages from the same samples range from 18.2±1.1 - 8.6±0.7 Ma with a poorly defined age-elev relationship. Preliminary track-length modeling for individual samples suggest variable cooling histories but generally rapid cooling since ~15 Ma. Bt Ar/Ar ages for samples collected along a transect across the QC range from 24.1 - 23.3 Ma and are nearly indistinguishable within error. New U-Pb zircon ages of 26.0 Ma from the QC suggest the Ar ages represent rapid cooling during batholith emplacement. AFT ages from the same samples yield an age-elev relationship for 16.2±2.4 - 5.8±0.5 Ma and a possible inflection point suggestive of rapid cooling after ~6.8 Ma. Interbedded tuffs from the Peñas and Aranjuaz Fms along the W. flank of the CR yield bt Ar/Ar ages between 32 and 25 Ma. Paleocurrent data from the Aranjuaz Fm. and pre-29 Ma Luribay Fm. exhibit upsection changes from Nward- to Wward-directed sediment dispersal. Conglomerate clast compositions are dominated by quartzite attributed to mid- Pz strata of the EC to the east. Similar depositional ages, lithofacies, provenance, and basal strat contacts with Devonian strata suggest that the Peñas, Aranjuaz, and Luribay deposits originated within a single integrated basin system that recorded unroofing of the EC during Oligocene uplift.