Resumen de Efectos morfo-estructurales de la deflación en diapiros de sal, Montañas Zagros, Irán

Francisco Gutiérrez Santolalla, H. Deirnik, Mehdi Zarei, M. Rahimi

• español

  Se analizan, a partir de trabajos de cartografía en campo, los rasgos morfo-estructurales asociados a diapiros de sal en las Montañas Zagros, Irán. Las estructuras salinas estudiadas incluyen una extrusión de sal (Diapiro Jenah) y dos domos de cobertera generados por diapiros que no han alcanzado la superficie (Gavbast y Kooh Bedoo). Todas ellas están situadas con anticlinales con evidencia de actividad (fallas flexural-slip y direccionales afectando a abanicos cuaternarios). Los rasgos más destacables observados en todos los casos son la presencia de fosas y sistemas poligonales de fallas normales, así como la presencia de grandes deslizamientos favorecidos por la dinámica salina.

• English

  The Zagros Mountains in Iran include a large number of salt diapirs related to the localized rise of the Precambrian-Middle Cambrian Hormuz salt and the piercing of an overburden as much as 7-9 km thick. These salt structures, typically associated with anticlinal ridges, display various stages of development (i.e., active or passive rise, fall). Buried salt diapirs have not reached the surface and are expressed as structural domes in the suprasalt cover. Most diapirs have extruded at the surface and display a wide range of morphologies related to their evolutionary stage: (1) expanding domes with a salt fountain above the feeding vent and laterally flowing salt glaciers (namakiers); (2) retreating domes; (3) deflated domes nested within depressions; and (4) deep alluvium-filled craters. Investigations dealing with the geomorphic and structural features asso- ciated with the dynamics of these salt structures (inflation-expansion, deflation-contraction, dissolution) are rather limited and are mainly focused on growing domes with positive salt budget. This work, mainly based on detailed geological-geo- morphic mapping carried out in the field, explores the morpho-structural features associated with extruded and buried salt domes affected by deflation. The mapped structures include a deflating salt extrusion (Jenah Diapir in Gezeh Anticline) and two structural domes underlain by buried salt diapirs (Gavbast in Gavbast Anticline and Kooh Bedoo in Nakh Anticline).

  Interestingly, the anticlines show evidence of active tectonic compression and growth, including flexural-slip faults and a left-lateral strike-slip fault (Nakh Anticline) that offset Quaternary alluvial fans. Geomorphic effects and evidence of diapir deflation-contraction in the Jenah salt extrusion include: (1) distant erratic blocks of Hormuz; (2) disconnected residual deposits of Hormuz; (3) backward expansion of alluvial fans; (4) higher sinkhole densities than in expanding salt extrusions;

  and (5) polygonal normal faults in the country rock with disrupted drainage (i.e., deflected, defeated, beheaded and hanging streams). The overburden domes with buried salt structures display collapse grabens and polygonal fault blocks with disrup- ted drainage. In Gavbast, collapse mainly affects the NW half of the dome, showing a dominant fold-normal horst and graben structure. Kooh Bedoo is mainly affected by a major fold-normal graben. The areas associated with deflating salt structures show a high density of large landslides, including the ca. 9 km long Emad Deh rock avalanche. Factors that favor slope ins- tability include slope and dip oversteepening, weakening of the rock mass by gravitational faulting, enhanced groundwater recharge, and debuttressing of slopes by the shrinkage of salt extrusions.