Reconstructing Thick-skin Deformation Patterns in the Southern Colombian Andes by Using Combined Thermobarometry and Thermochronology
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abstract
In active compressive orogens, after prolonged thick-skin deformation and exhumation, the sedimentary cover is often removed resulting in the exposure of basement rocks older than the deformational phases. In this scenario, the absence of structural markers and sedimentary strata deposited coeval with deformation difficult the reconstruction of the time and magnitude of deformation. Low-temperature thermochronology is a powerful tool to reconstruct basement rock uplift and exhumation in multiple tectonic settings, e.g. extensional basins and compressive orogens. However, to convert cooling into exhumation, multiple assumptions about the crust thermal architecture need to be made, which increases the uncertainties on the amount and rates of exhumation obtained from these techniques. Given that plutonic rocks are emplaced at depth and exposed through one or several unroofing events, magma crystallization pressures can be used to estimate their emplacement depths and thus provide an independent constraint on the total amount of exhumation. The abundant plutonic rocks exposed in the southern Colombian Andes present an opportunity to combine granitoid barometry and thermochronology to understand this province’s deformation patterns and thick-skin mechanics. As a part of a research project funded by ANH-MinCiencias, this study presents “in situ” chemical analyses in amphibole-plagioclase pairs combined with zircon and apatite thermochronology (fission tracks and U-Th-Sm/He) in ~20 localities in Permian to Jurassic granitoids in the southern Colombian Andes. Amphibole geobarometry indicates crystallization pressures between ~0.5 and 5 Kbar, suggesting emplacement depths between ~1 to 20 km. These results are consistent with low-temperature thermochronology that suggests higher and faster Cretaceous and Cenozoic cooling for the deeper plutons compared to the shallowly emplaced plutons. Pluton unroofing is interpreted as the result of Lower Cretaceous extension, Late Cretaceous plateau-continent collision, and Miocene strike-slip transpression. Our results represent an example of how disorganized deformation and exhumation patterns in long-lived thick-skin orogens are the results of multiple deformation phases promoted by contrasting geological settings.
