Deformation processes operating in the toe buttress of the Pos Selim landslide

Abstract

 A study of longitudinal topographic profiles in 2003, 2005 and 2010 has been carried out to provide measurements of surface dips of the Pos Selim landslide in Malaysia. The objective of the study is to explore the deformation processes operating within the toe buttress of the landslide, and estimate the basal rupture surface using the new dip data. Data has been obtained from previous studies and by constructing 2D displacement vectors manually at berm edges where longitudinal topographic sections in 2003, 2005 and 2010 are superimposed. Landslide basal rupture surfaces were constructed by the writer along longitudinal sections by Cruden’s method (Cruden 1986), using values derived from the new displacement vectors. Automation of Cruden’s procedures by the writer (with assistance) allows quick construction of multiple loci and provides direct inputs to simulate the 3D geometry of the landslide. The data is presented here in the form of 2D dip data, 2D loci and 3D model of the entire landslide basal rupture surface in ArcGIS. Horizontal shear strain between 2005 and 2010 is calculated by the writer, based on the angular change in features demarcated in shaded relief models produced by Dr Andy Hansen. The pattern suggests indentation of a relatively unyielding Northern Spur into surrounding displaced mass. The data has been interpreted in order to understand the modes of sliding, contribution of landslide slip on geological faults and deformation mechanism within the toe buttress. It is concluded that the lower band of rapid change in dip is not controlled by Fault C but is mainly governed by topography. Change in longitudinal dip across faults is produced by oblique slip of the hanging wall relative to the footwall. Faults B and C do not make a significant contribution to articulation of the compound landslide. A four-zone model for the central part of the landslide has been hypothesized in which the toe buttress comprises two parts: a translational zone and a transitional zone, in addition to the main body and the head graben. Articulation takes place between the downwards movement of the main body and the upwards movement of the translational zone, which is delineated by a lower band of rapidly-changing dip. Distributed shear occurs on an assemblage of slip polygons and slip fans, which may be modelled as a compatible mechanism. Geological features e.g. foliation and joints, accompanied by rock fracturing, are possibly exploited for a kinematically admissible mechanism to form.