In fact DSGS is still not well undertsood, has described by many authors in different ways buy generally we may say that this is a very slow mass movements, occurring usually in high relief hillslope, without well-defined failure surface and one of the main factors causing DSGS is gravity forces and the others factors are not clearly established yet. Part of my work is focusing on establishing this factors. To solve DSGS issue I'm using code ADELI based on Finite Element Method. FEM is numerical method designed to solving problems in engineering and mathematical physics. General principal for this method is subdivision a body into simpler parts. Dividing the body into an equivalent system of smaller bodies interconnected at points (called nodes) equation are calculated for each finite element and subsequently combine them to obtain the solution for the whole body. ADELI is a fortran 77 code developed by Jean Chery and Riad Hassani to solve a problem with termo-mechanical behavior of the crust and the lithosphere at geologic time scale. For any kind of modelling first you have to have a hypothesis. In case of my modelling I had two hypotheses of the instability. In first one I assumed that the triggering mechanism of the instability is destructive activity of the valley glacier combined with activation of pre-existing faults, joints, circulation of the water. These factors may cause decreasing effective friction angle and rock mass strength what in consequence leads to losing stability. In second assumption determining factor except erosional activity of the glacier are geological discontinuities, causing anisotropy in the rock mass. Presence of layering with different lower strength parameters, decreasing effective friction angle and the rock mass strength. (Fig. 2 and 3).

Fig. 2 a In first stage the orogen is in elastic equilibrium, valleys on both side are filled by a glacier. Slowly destructive activity of the glacial water penetrates the microcracks and fractures at the toe of the orogen. b In next stage during glacier retreat the fractures grow, also new discontinuities appears and increase in size. Glacial water penetrates the rock mass causing disintegration of the particles and therefore decreases rock mass strength and friction angle. In the central part of the orogen as a results of changing strain regime combined with the receding glacier, pre-existing faults are re-activated. Vertical surfaces of the rupture appears at the upper part of the orogen, as a results of changing the strain regime. Slowly profile of the valley becomes more the U-shape. At the toe of the orogen material from the glacier and from the slope is deposited. c At this stage valley has U-shaped, a large portion of the material deposited at toe of the slope, glacier formed characteristic for the glaciated valleys triangular faces and trimeline. Accumulation of the strain inside the orogen increases as well as reactivated failure surface and combined with vertical rupture surface creating curved deep-seated failure surface inside the orogen. d Gravitational stability is compromised and the orogen becomes unstable under the gravity. New profile of the slope is too steep relative to their strength parameters, also destructive activity of the glacial water decreased strength parameters and angle of internal friction. In the last stage the triggering mechanism of gravitational instability is activated and characteristic for deep-seated gravitational spreading morphostructures like uphill-facing scarps and crestal graben appeared. Size of displacements is small but the height is comparable with the whole slope.

Fig. 3 a The ridge is stable under the gravity. Microcracks present at the toe of the slope are exposed on erosional activity of the glacial water. Small portion of strain are accumulated at the toe of weak layer and in the middle part of the orogen. b At this stage during glacier retreat accumulation of strain in the middle part of the orogen and at the toe layer highly increased, also new fractures appeared in the upper part of the slope. Plastic strain accumulate in the weak layers. c Here there is significant chaotic accumulation of strain in the weaks layers and inside the orogen with the evolved vertical rupture surface. Layers are deflected without interruption of the continuity, also failure surface is not clearly marked. All deformation focuses inside the orogen without morphological changes at the surface. d After glacier retreated, profile of the slope has changed considerably and it is not stable under the gravity. At the last stage occurs morphological changes of the surface.

In both modellings, failure surface are generated inside the model just below the crest, generated failure surfaces in both cases are almost vertical and the depth corresponds to the mountain height. Our modelling is realistic and expected crucial factors for triggering mechanism of DSGS in Martian condition with lower value of gravity acceleration are first slope inclination, effective friction angle and the cohesion around or below 1.5 MPa favorable factor but surprisingly with the minor meaning in this modelling is deglaciation of the valleys. Whereas presences of the weak layers decreases RMS and therefore slopes are less stable than in this same condition for slopes without layers, this is also an expected effect (Figs. 4 and 5).