Diapirs in Western Pyrenees and Foreland, Spain1
The diapirs of the western Pyrenees and their foreland have cores mainly of salt and evaporites of Triassic (Keuper) age. Their shapes and tectonic positions differ. They are surrounded or overlain mainly by Cretaceous and Tertiary strata. Groups of diapirs demonstrate distinct alignments. The distribution of the diapirs is believed to be controlled by variation in the thickness of the Upper and Lower Cretaceous strata. These strata, which reach a maximum thickness of at least 8,000 m, exerted the necessary pressure to start the movement of the saliferous beds toward the flanks of the trough. Shifting of the trough axis in Late Cretaceous time separated the salt accumulation into two distinct welts. Diapirism started in Early Cretaceous time and must have reached its maximum during Late Cretaceous time, because most of the diapirs had reached the surface prior to late Tertiary time.
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“Diapir” and “diapirism” come from the Greek diapeirein, which means “to pierce.” Diapirism sensu lato is a process by which earth materials from deeper levels have pierced, or appear to have pierced, shallower materials; it is divided into magmatic intrusion and diapirism sensu stricto on the basis of the temperature at which piercement occurs. Diapirs s.s. are composed of evaporites, argillaceous sediments, coal, peat, ice, serpentine, or other earth materials which have the critical characteristics of low equivalent viscosity and low density. These materials range in age from Precambrian to Recent. Diapirs are found in all parts of the world except the shield areas. They have many forms, ranging from smoothly rounded pillows to complexly injected laminae, are either connected with or disconnected from the “mother” bed, and are present either at the surface, where they form distinctive features, or at considerable depth. Diapirs have well-developed internal structures indicative of an origin by flow. Strata around a diapir may be strongly affected structurally and/or stratigraphically by the diapir, or they may be unaffected. Field and model studies indicate that diapirs have developed as a result of horizontal compression, gravitational instability, or both. Diapiric structures of various types contain large quantities of oil and gas, sulfur, salt, and potash and are important for underground storage and nuclear testing.