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Journal of the Geological Society

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Deformation of country rock by an intrusion in the Sierra de Moreno, northern Chilean Andes

The highest sedimentary cover rocks of the Sierra de Moreno are the Tertiary, Sichal Formation. These are flat lying and undeformed, except at one locality adjacent to an igneous intrusion, where the sediments exhibit a spectacular though minor belt of folds that, from the stratigraphy, can be shown to have formed under a cover of 300–500 m. From the orientation and interaction of fibrous veins within the folds it is possible to unravel the history of deformation. We infer that the lateral compression of these rocks first resulted in minor conjugate thrusts. The major phase of deformation which followed gave rise to folds whose geometry and position can be attributed to the existence of gypsum intercalations which underwent the gypsum/anhydrite reaction as a result of the high geothermal gradient associated with the emplacement of an igneous intrusion. This reaction released large quantities of water which enabled kink folds to develop, coupled with the infilling of the pre-existing barren thrusts to form a complex of fibrous veins. A final phase of extensional deformation can be attributed to subsequent shrinkage of the country rock as the intrusion cooled.