CARBONIFEROUS EXPANSION OF THE HERCYNIAN OROGENY ACROSS SOUTHERN BOLIVIA: INSIGHTS FROM RSCM THERMOMETRY AND THERMAL MODELING

A widespread unconformity between Ordovician metasedimentary rocks and unmetamorphosed Mesozoic sedimentary rocks in the Eastern Cordillera of Bolivia provides evidence for a significant pre-Late Cretaceous deformational and erosional event in western Gondwana, which has been variably attributed in previous studies to either the Ordovician Ocloyic or Carboniferous Hercynian Orogeny. Using new and published data, we refine the middle-late Paleozoic tectonic history of the western margin of Gondwana at the latitude of southern Bolivia. We combine Raman spectroscopy of carbonaceous material (RSCM) thermometry with semi-quantitative deformation temperature ranges from quartz recrystallization microstructures and published illite crystallinity values from Cambrian-Devonian sedimentary rocks that span the Eastern Cordillera at ~21 °S. Peak and deformation temperatures range primarily between 220 and 400 °C and show a general increase in temperature with depth that defines a metamorphic field gradient of 42 ± 5 °C/km. Reset cooling ages and temperatures obtained from Ordovician rocks that lie directly below the unconformity require removal of >5 km of overburden prior to the Late Cretaceous, which was likely accomplished by Paleozoic folds and thrust faults that are revealed from cross section retro-deformation. The peak temperature data along with published illite K-Ar, zircon (U-Th)/He, and zircon fission-track ages are incorporated into thermal models, which indicate that Paleozoic deformation at this latitude occurred during the Carboniferous Hercynian orogeny. We interpret that deposition of Devonian-Carboniferous sedimentary rocks was driven by flexural subsidence associated with growth of an orogenic wedge along the western margin of Gondwana, and that peak temperature conditions were attained during the eastward migration of a foreland basin across this region between ~420 and ~320 Ma. Erosional exhumation between ~350 and ~295 Ma followed thereafter and was the result of the continued eastward expansion of the orogenic wedge across the Eastern Cordillera. This eastward migration of exhumation coincided with changing subduction conditions along the Gondwanan margin, as indicated by a significant influx of Carboniferous arc-related zircons into the Late Paleozoic foreland basin system.