عنوان مقاله [English]
Tectonics play an important role in the evolution of large-scale gravitational phenomenon (Galadini, 2006), mainly through the formation of steep slopes. Competing tectonic and surface processes build and destroy topography in active orogens, hence, thrusting, crustal thickening and isostatic response result in rock uplift and relief production (Agliardi et al., 2013). In some cases, the faults play a primary role in increasing the local relief and their activity is an important geomorphic factor conditioning the gravitational movements (Galadini, 2006). We have studied this kind of gravitational movements and slope instabilities termed “Deep-Seated Gravitational Slope Deformation (DSGSD)”. This paper focuses on a study aimed at defining the role of structural setting, local uplift and morpho-structural evolution on the onset and development of a DSGSD that affects the western parts of the Siahcheshmeh pull-apart basin (SPAB) in a releasing bend of the Gailatu-Siah Cheshmeh-Khoy fault.
DSGSDs are gravity-induced and large to extremely large mass movements generally affecting the entire length of high-relief slopes, extending up to 200–300 m in depth, which can frequently extend beyond the slope ridge and evolving over long periods of time. (Crosta et al., 2013). DSGSDs are not considered hazardous phenomena because they evolve very slowly. Despite their slow deformation rates, they can cause damage to surface and underground man-made structures (Soldati, 2013). The main feature that distinguishes DSGSDs from landslides is the absence of a continuous or well-defined sliding surface (Soldati, 2013) and discontinuous or poorly defined boundaries, both laterally and at their lower ends (Crosta et al., 2013).
Gailatu-Siah Cheshmeh-Khoy fault:
The 200 km long (Karakhanian et al., 2004; Berberian, 1977) Gailatu-Siah Cheshmeh-Khoy fault, with the same trend as the North Tabriz, Chaldiran, Nakhichevan and Pambak-Sevan-Syunik faults, is regarded as a part of the active strike-slip fault system in the Arabian and Eurasian collision zone, which extends from 42Ë E to 48Ë E with the Tutak and North-Tabriz faults in the west and east, respectively (Selçuk et al., 2016). This system includes a series of right-lateral strike-slip faults between the southern front of the Lesser Caucasus to the northeast and Bitlis-Zagros suture zone to the southwest. The available literature, fault plane solutions, offsets of various geomorphological and man-made features indicate the right-lateral strike-slip nature of the Gailatu-Siah Cheshmeh-Khoy fault. The trace of this fault is very obvious and displays a series of well-developed and preserved morphologic structures indicating recent activity of the fault, such as fault scarps and horizontal deflection in the Quaternary features, pull-apart basins, hot water springs and uplifted terrace deposits.
Discussion and results:
Our geological and structural survey along the Gailatu-Siah Cheshmeh-Khoy fault confirms the presence of a large slope instability in the western flank of SPAB. In order to understand the relationship between the nucleation and evolving DSGSDs with structural aspects of this fault, we focused on slip rate of this fault in two segments, the Gailatu-Siahcheshmeh (northwestern sector of Gailatu-Siah Cheshmeh-Khoy fault) and the Siahcheshmeh-Khoy fault segments (Southeastern sector of Gailatu-Siah Cheshmeh-Khoy fault), which overlap at a right step-over in the SPAB. Along the Gailatu-Siahcheshmeh fault segment, Quaternary lavas, known as Maku basalts, form a few ridges that are elongated parallel to the strike of the fault and displaced as a result of this fault activity by ~ 725±50. Using the about 400 kyr published age of these basalts (Pb206/U238 and Ar40/Ar39 dating methods, Allen et al., 2011; Lechmann et al., 2018), a mean slip rate has been calculated 1/65 ± 0.1 mm/yr. On the Siahcheshmeh-Khoy fault segment, we excavated a trench to determine the fault geometry and its rake, and to assess recent offsets. Radiocarbon dating of the youngest deposits in the stream wall that displaced right-lateral by 42±4 m, yield 6764±283 calBC, suggest a horizontal slip rate of 4.6±0.3 mm/yr. The northwestern and southeastern terminations of Siahcheshmeh-Khoy fault segments form the eastern and western flanks of SPAB, respectively. Hence, the higher slip rate of Gailatu-Siahcheshmeh fault compared to Siahcheshmeh-Khoy fault, causes uplift of the western SPAB sectors. This is accompanied by thrust faulting in a general northwest-southeast direction as a splay configuration at the termination of Siahcheshmeh-Khoy fault. Consequently, local uplift has been taken place in the western flank of SPAB that is readily obvious from higher altitude of this flank relative to the eastern side. Therefore, DSGSDs have been occurring almost along the entire slopes facing the pull-apart basin. On the other hand, decreasing altitude in the SPAB in the releasing bend and normal faults are additional controlling and intensifying factors for DSGSD. As a result, most of the expected structural features, especially splay strands of Siahcheshmeh-Khoy fault and normal faults at the margin of SPAB, have been covered by DSGSD phenomena. Therefore, except at a small part of the southwest of SPAB, we could not find exposure of normal faults along the western flank.