EXHUMATION OF THE CORDILLERA REAL, BOLIVIA, BASED ON NEW GEOLOGIC MAPPING AND 40AR/39AR THERMOCHRONOLOGY

Plutons coring the Cordillera Real in the hinterland of the central Andes have been considered relatively undeformed in comparison to Paleozoic host rocks. New 1:50,000 scale mapping along two transects reveal that the Huayna Potosi, Zongo, Illampu, and Yani plutons have undergone fold-thrust deformation along with surrounding Paleozoic strata. The SW margin of the Huayna Potosi pluton forms part of the hangingwall of a SW-directed thrust containing imbricated Ordovician strata in its footwall. Similarly, the SW margin of the Illampu pluton is involved in structures related to SW-verging thrusts that cut Ordovician strata. One of the principal structures consists of a SW-verging thrust fault containing the Huayna Potosi pluton in its footwall and the Zongo pluton in its hangingwall. This fault branches into two segments at the Rio Zongo valley: the southern branch continues southeastward through the Huayna Potosi pluton, the structurally higher northern branch places Ordovician strata over both granitic and Ordovician rocks. The fault can be traced >25 km to the northwest where the Yani pluton has been thrust over the Illampu pluton. The Yani pluton has been cut by two additional SW-verging thrust faults to the northeast. Previous thermochronological studies on the Huayna Potosi-Zongo transect have not taken into account the regional geologic structure, which is critical for interpretation of cooling ages. New ⁴⁰Ar/³⁹Ar thermochronological analyses of granites from the Illampu-Yani transect have yielded preliminary biotite cooling ages ranging from 107.2 Ma to 38.3 Ma, with possible thermal resetting of some rocks at about 40 Ma. Muscovite cooling ages from the same samples range from 192.9 Ma to 42.9 Ma, and exhibit disturbed age spectra attributed to retention of excess argon. These results are similar to previous reports from the Huayna Potosi-Zongo transect. From east to west, biotite-muscovite pairs show an increased age discordance and cooling ages of both minerals become systematically older. An exception is across the easternmost thrust fault, where a footwall biotite cooling age of 38.3 Ma is younger than hangingwall biotite ages of 44.6 Ma and 40.7 Ma. Collectively, the thermochronological results suggest exhumation-induced cooling during Paleogene slip along a series of SW-verging thrusts faults with possible out-of-sequence motion.