The gigantic Seymareh (Saidmarreh) rock avalanche, Zagros Fold–Thrust Belt, Iran

2016-06-21T11:18:18Z (GMT) by Nicholas J. Roberts Stephen G. Evans
<p>The Seymareh rock avalanche, first described by J. V. Harrison and N. L. Falcon in the 1930s, is the largest known subaerial non-volcanic landslide on Earth. The volume of its debris (44 Gm<sup>3</sup>) is approximately equal to that of the largest known subaerial landslide of any kind, the 100 ka collapse of Mount Shasta volcano (45 Gm<sup>3</sup>) and approaches those of gigantic landslides on Mars. Using satellite imagery, SRTM-3 data, and detailed field investigations, we gain a new understanding of the gigantic rock avalanche. The initial failure (38 Gm<sup>3</sup>), involving a dip slope consisting of a relatively weak interbedded limestone–mudrock sequence capped by resistant Asmari Limestone, had an average thickness of 410 m along a 15.5 km width of the Kabir Kuh anticline. The rockslide transformed into a rock avalanche that travelled 19.0 km with a <em>fahrböschung</em> (the angle between the highest point of the pre-landslide source area topography and the distal limit of the debris) of only 3.6°. Initial failure involved complex planar sliding dictated by four structural elements: (1) bedding-parallel shears and (2) across-bedding break-throughs combined to produce a low-angle (11°) overall sliding surface that (3) broke out through the Asmari carapace at the toe of the failed slope while (4) tectonically weakened bedding-normal joints provided lateral release surfaces. Nine discrete bedding-parallel basal sliding surfaces divided the detached mass into nine stacked plates. The upper plate (23 Gm<sup>3</sup>) included 20.7 Gm<sup>3</sup> of Asmari Limestone. Analysis of accelerator mass spectrometry radiocarbon dates from this study and by others indicates that the rock avalanche occurred between <em>c</em>. 8710 and 9800 <sup>14</sup>C years BP. </p>