Normal faulting, block tilting, and decollement in a stretched crust
Résumé
Extensional tectonics cover a wide range of crustal structures from narrow continental rifts to large continental areas. If we except continental and oceanic rifts, stretched portions of the crust exhibit complex faulting patterns at upper levels with high‐angle and low‐angle normal faults. Very little is known concerning the geometry of structures and physical processes at depth. It is therefore difficult to extrapolate surface observations to the crustal scale. Five general models are proposed that combine continuous and discontinuous deformation and geometrical and geological tests are proposed. Two natural examples, the West Armorican Atlantic margin and Eldorado Mountains (Basin and Range Province), are presented. They are characterized by an association of high‐angle normal faults, tilted blocks, and low‐angle normal faults. Field and geometrical arguments are given to demonstrate that low‐angle faults are controlled by décollement surfaces along preexisting interfaces. Geometrical implications of the tilted block pattern are presented and the gravity gliding hypothesis is proposed as a possible explanation of block tilting when associated with low angle normal faults. In such cases, the amount of stretching cannot be related to bulk thinning of the crust.
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