--> Time, An Indispensable Variable To Be Determined within Fold and Thrust Belts. The Sub-Andean System Case: Thermochronology and Geochronology as a Possible Solution

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Time, An Indispensable Variable To Be Determined within Fold and Thrust Belts. The Sub-Andean System Case: Thermochronology and Geochronology as a Possible Solution


The evolution of fold and thrust belts requires chronological data constraints in order to determine the rates related to the interaction of surface and subsurface processes and quantifying the time involved within the components of the petroleum system: reservoir, seal, source rock and trap. The combined application and integrated analysis of low and high temperature dating techniques can lead to solid interpretations and realistic structural and stratigraphic modeling. Within the low temperature thermo-chronometers (< 300 °C), fission-track and (U-Th-Sm)/He dating applied to apatites (AFT-AHe) and zircons (ZFT-ZHe) from surface and drill-hole samples measurably improves calculation of the timing of structural growth (conventional targets) based on the definition of time-temperature paths applicable to petroleum system modeling. AFT and AHe ages from surface and subsurface Devonian-to-Pliocene-in-age samples have permitted to define the chronology of deformation within the sub-Andean fold and thrust belt structures within the time interval 10 to 0 Ma identifying in-sequence, out-of-sequence and simultaneous when structural growing ages are compared. On the other, thermochronological data have contributed to characterize simultaneous or diachronic growth within the same structure while a strike-along sense of analysis is performed. The combination of thermo-chronometers leading to time-temperature curves give place to the definition of the heating/cooling trajectories of the rock volume analyzed which can be translated into subsidence and structural exhumation phenomena. These data are able to be combined with vitrinite reflectance (%Ro max, %Ro mean) and Thermal Alteration Index (TAI) values through the development of numerical modeling, defining the maximum temperature reached. The inverse modeling procedures (iterative mode based on Monte Carlo search method) performed within the southern (Argentina-Bolivia), central and northern (Bolivia) sub-Andean system, testing the geological constraints against the thermochronological data set (fission track and (U-Th-Sm)/He), have allowed the determination of the growing time of fault-related-folds and their exhumation/erosion rate during Miocene-Pliocene time. This is the case of Mayaya-Lliquimuni-Beu structures (northern sub-Andean, Bolivia), La Lluviosa range (Isarsama Area, central sub-Andean, Bolivia), Igüembe-San Antonio-Aguaragüe-Charagua structures (Bolivia) and Bermejo-Pintascayo ranges (Argentina) in the southern part of the sub-Andean system. On the other hand, the results obtained from the numerical modeling have permitted to precise the time-temperature evolution of the samples analyzed from their deposition age until nowadays. This represents a key point while analyzing heating and cooling trajectories related to pre-Cenozoic tectonic processes (eg. Mesozoic and/ or Paleozoic times). The thermochronological data combined with absolute U/Pb ages (SHRIMP and LA-ICPMS techniques applied to zircons) within the Miocene pre-growth and growth strata represent a powerful tool while trying to assess not only the structural timing and growing rates fluctuations but also describing the influence of the Miocene-Pliocene sediments load overlain a particular sample or set of samples.