The role of penetrative deformation in orogenic processes. An example from the Eastern Cordillera of Colombia


The Lower Cretaceous units of the Eastern Cordillera of Colombia have been deposited during a stage when the area where nowadays lies a contractional orogen was an actively deforming rift. Therefore, they are syn-rift sedimentary units. These rocks have a unique feature in the Cretaceous sedimentary column of this mountain chain. This is a widespread planar fabric expressed mostly as a penetrative slaty cleavage. This planar fabric is only macroscopically evident in Cretaceous units older than Barremian. Planar fabric (e.g. slaty cleavage) is one of the products of internal or penetrative deformation (i.e. contractional deformation at microscopical scale in rock units). Pressure solution and even cleavage have been recently reported in areas undergoing only subtle burials (Engelder & Marschak 1985). However, in the Eastern Cordillera, the units where planar fabric is evident are the base of a sequence of at least 5km and they have vitrinite reflectance values up to 2. These data and the formation of clorithoid contemporary with the slaty cleavage, allow us to propose that this planar fabric was developed when the Lower Cretaceous units had an amount of overburden close to the thickness of the entire Cretaceous sequence...
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