Experimental Modeling of Role of Gravity and Lateral Shortening in Zagros Mountain Belt

Abstract

Dynamically scaled analogs of the geologic structures of the Zagros mountain belt are used to argue that different parts of the Zagros Mountains of Iran record different combinations of the effects of a gravity-driven overturn and a southwest-northeast lateral shortening superimposed on the Zagros overturn. Partially scaled material models have been used to simulate the Zagros geodynamics, which involve layer-parallel compression of a 6 to 7 km-thick Phanerozoic carbonate cover containing a pattern of preshortening diapirs. The folds in the Zagros form rapidly (1.5 mm/yr in a 20 to 30 km-wide zone), reactive some of the preshortening diapirs, and generate new synshortening listric diapirs. A third set of postshortening diapirs rises from the Hormuz decollement behind the fold-thrust front. Model buckle folds superimposed on diapirs or pillows tend to avoid and curve around preshortening diapirs, which flatten in the synclines. Model profiles show that lateral shortening induces residual salt at depth to flow toward and rise through the anticlinal cores as synshortening or postshortening diapirs. I suggest that any salt pillows in currently diapir-free zones of the Zagros fold-thrust belt may surface as diapirs through the anticlines in the future. In the absence of well data, seismic, and field observations, I have used only three map pattern criteria to recognize the timing of diapirs; these are locally obscured by extrusive salt sheets. Preshortening diapirs are generally small and elongate parallel to fold axes and are restricted to synclines. Synshortening diapirs are elongate perpendicular to the fold axes and are restricted to anticlines. Postshortening diapirs can be the same size as synshortening diapirs or larger and can be circular; they are more common in the anticlines.