Evolution of external shapes of salt extrusions in analogueModels
|Category||Tectonic & Seismotectonic|
|Location||21th symposium on geosciences|
|Holding Date||24 May 2008|
The external shapes of diapirs are very important for potash explorations because passive erosional diapirs are not in the potash programs. The potash is the first material which dissolved in the salt domes.
Like orogenic fold nappes, the external profiles of salt extrusions reflect the interplay between rates of extrusion of ductile rocks squeezed by gravity and lateral forces from deep subhorizontal channels and their rates of degradation by gravity spreading and erosion(Beaumont et al.,2001;Tapponier et al.,2001).The topographies of various generations of these extrusions can be arranged in an evolutionary sequence(see Fig3).This begins with rapier rise hemispherical topographic domes of salt to about 900 m above their vent. After c.1000 years(Talbot et al.,2000,Fig 7) these domes mature and are spread by gravity as viscous fountains(Lister&Kerr,1989)while their extrusion rates 1 ma-1 outpace their erosion rates(Fig3).Evolution of external structures in profiles in a multioverhanging salt
diapirs (base of all experiments) showed in Fig7.
The diapir widened to the top in all stages. The external shape of diapir changed from droplet to fountains in stage 1 to a droplet and pseudofountain-droplet in stage 2 and pseudo-droplet fountain in stage3.The namkiers in each stage reactivated and raised at angle of about 20 degrees to the stems which stems also widened to the upper parts .All the diapirs in all stages are active because the salt supply are more than the rate of deposition, but the activity decreased from stage 1 to 3.The namkiers after reactivation showed thickening and thick reactivated part showed refolding of a primary tanck track folds. This reactivation makes three diapirs with same salt and different pseudostratigraphic layers. The internal pseudostratigraphy in the middle showed a internal salt mushroom between two salt extrusion in two direction(see internal structure in stage two of the multi overhanging diapir of the Fig6.Also the primary flow lines reactivated and changed from each stage. In each stage the reactivated salt pseudostratigraphic refolded again as a vertical fold to subvertical folds but the folds in the terminations are updip. The folds when a salt intruded another salt are downdip (see Aftabi, 2004; e.g. Hormuz salt).
All this evolution formed in experiment O7in this experiment the pull apart was open and the diapirism is syn to post deposition. In experiment O1 a widening diapir formed by opening in the pull apart after deposition of the horizontal formations. The deposits in top of the PDMS sank down to the pull apart about 4 mm. The evolution from a thick droplet also seen in this experiment(Fig8) but the salt covered with 5mm green deposits and the height of the mountain was little more than stage 1 in O7 because of increasing in the height of the rim(Fig8).The namkiers spread downslop from the rims and mountain was asymmetric. The formations in top of the blind salt sank down after opening the pull apart but rised and spread as a droplet of country rock (salt in below) in 17:30 after 15 minutes by 8 Kg load. Then after 30 minutes in 17:45 the PDMS under other deposits came up and extruded in the middle of the droplet country rock and formed a type of fountain with salt only in the center. The evolution after that time is similar to the O7 experiment the mountain is symmetric to asymmetric different sections. In this experiment the time for spreading is little and formed a thick active droplet (Fig9).
The external structure in the profiles in stage 1 and 2 was droplet but in the stage three a widened salt changed from a droplet to pseudofountain when the salt of the main dome isolated from the namakiers by rim. All the evolution which is common in the other multioverhanging models, have also seen in this experiment.
The external shape of the mountains in the experiments of salt overhangs are mainly related to the shape of the pull apart but the rim or collar shape also is very important=