Model Diapirs, Plasticity, and Tension1
A series of model experiments has been conducted to study relationships between diapirism and regional stress. The model ratio of length was in the range of 10−5 to 10−6. Model domes and ridges, much like the salt domes and salt ridges known under the Gulf of Mexico coastal plain, were produced readily in all models in which sedimentation and tensional deformation were nearly simultaneous; the same features did not appear readily in other models. The chief surface deformation visible during sedimentation was the formation of grabens; normal faults were the main type found in the model sediments. In the models, ridges paralleled grabens. The areal distribution of model domes was very similar to the spacing of salt domes in Louisiana.
The model ratio of viscosity was not completely satisfactory. The asphalt used was slightly too fluid to represent halite viscosity as given in published reports.
The models strongly indicate that a field of domes and/or ridges is a corollary of regional tension. They also indicate that source-bed plasticity, rather than density differential (and hence instability), is the critical parameter in determining which beds respond to tension by producing diapiric structures. The difficulty the writers experienced with viscosity does not necessarily vitiate these statements; the ranges of uncertainty are larger than the discrepancy. It is therefore concluded that any sufficiently plastic material (such as halite, clay, sand, or magma) will respond to a predominantly tensional stress by developing diapiric structures. Halite beds of considerable extent which have not produced diapirs constitute evidence that density differential alone does not provide fordevelopment of stress fields conducive to the development of salt domes. Thus the extensive distribution of domes and ridges in the Gulf of Mexico coastal plain is evidence of important regional tension.
Figures & Tables
“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.