Revised Depositional Chronology for Altiplano Basin Stratigraphy of Northern Bolivia
Abstract
The locus, timing, and pace of vertical movement of Earth's crust is a sensitive recorder of geodynamic processes. Downward movement of the crust (subsidence) is documented by the history of sediment accumulation. Limitations in the chronology of deposition therefore impede development of accurate geodynamic models. The Altiplano Basin of the Bolivian Central Andean Plateau has long been hailed as an archetypal retroarc foreland basin. We reassess this model using updated depositional chronology and provenance information based on detrital zircon U-Pb geochronology. Samples come from the Chuquichambi section, which is the longest published section through Paleogene stratigraphy.
The Altiplano Basin is bounded to the east and west by the north-south trending Eastern and Western cordilleras. The cordilleras formed as a result of crustal shortening in response to ongoing subduction of the Nazca plate beneath the South American plate. Outstanding questions include the relative timing of deposition in the Altiplano Basin and exhumation of the Eastern Cordillera. The youngest zircon populations become systematically younger upsection, indicating that the section is conformable. Zircon geochronology indicates that the base of the section is ~40 Ma. This is younger than previously recognized and post-dates the onset of exhumation in the adjacent Eastern Cordillera. Deposition proceeded rapidly (750 m/Myr) until the late Eocene before tapering off (450 m/Myr) between the late Eocene and early Miocene. Whereas a traditional foreland basin model predicts that there will be an increase in sedimentation through the basins development, in this study the sedimentation rate decreases with time. These observations are inconsistent with a foreland basin model and raise questions about the subsidence mechanism. We propose a new model in which subsidence is driven by loading by both the Eastern and Western cordilleras, suggesting that the Altiplano Basin existed as a hinterland (or perhaps "Laramide-style" basin) throughout its history.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.T14C..02R
- Keywords:
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- 5475 Tectonics;
- PLANETARY SCIENCES: SOLID SURFACE PLANETS;
- 8102 Continental contractional orogenic belts and inversion tectonics;
- TECTONOPHYSICS;
- 8108 Continental tectonics: compressional;
- TECTONOPHYSICS;
- 8118 Dynamics and mechanics of faulting;
- TECTONOPHYSICS