دگرریختی ثقلی ژرف شیب، در حوضه کششی سیه‌چشمه: امتداد گسل گیلاتوسیه‌چشمهخوی

در این مقاله برای اولین بار در ایران، نوعی از ناپایداری شیبی در بخش همپوشان دو پاره گسلی گیلاتوسیه‌چشمهخوی که در یک ترکیب خم رهایی تشکیل حوضه کششی سیه‌چشمه را داده‌اند، تحت عنوان دگرریختی ثقلی ژرف شیب معرفی و مطالعه شده است. گسل گیلاتوسیه چشمهخوی، با سازوکار جنبشی راستالغز راست‌بر و با راستای شمال‌باختریجنوب‌خاوری در بخش میانی پهنه برخوردی صفحه‌های قاره‌ای عربیاوراسیا، در شمال‌باختری ایران واقع شده است. در این پژوهش، عوامل موثر بر این رخدادها و سایر ناپایداری‌های شیبی این منطقه و نیز ارتباط آنها با ساختارهای زایشی حوضه‌های کششی و تاثیر تغییرات آهنگ لغزش در امتداد گسل، بر روی ناپایداری‌های شیبی مورد بررسی قرار گرفته است. بدین منظور، از طریق سن سنجی رادیوکربن نمونه خاک دیواره آبراهه منحرف شده بر روی پاره گسلی سیه‌چشمهخوی ( بخش جنوب‌خاوری گسل گیلاتوسیه چشمهخوی) و همچنین با استناد به سن روانه‌های سنگ‌های بازالتی منطقه در مطالعات پیشین و تشخیص مقدار جابجایی متاثر از اثر عملکرد پاره گسلی گیلاتوسیه‌چشمه (بخش شمال‌باختری گسل گیلاتوسیه چشمهخوی)، به ترتیب آهنگ لغزش 4.6±0.3 mm/yr و 1.65 ± 0.1 mm/yr برای این پاره‌های گسلی محاسبه شده است. این تغییر آهنگ لغزش در امتداد گسل، باعث فراخاست هر چه بیشتر بخش باختری حوضه کششی سیه‌چشمه نسبت به بخش خاوری آن و همچنین تشکیل افشانه‌های گسلی راندگی در پایانه شمال‌باختری پاره گسلی سیه‌چشمهخوی شده است و در نتیجه، پدیده‌های دگرریختی ثقلی ژرف در شیب‌های ارتفاعات محصور کننده این حوضه کششی رخ داده است.

Deepseated gravitational slope deformation in the Siahcheshmeh pullapart basin: along the GailatuSiahcheshmehKhoy fault

Introduction:Tectonics play an important role in the evolution of largescale 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 “DeepSeated Gravitational Slope Deformation (DSGSD)”. This paper focuses on a study aimed at defining the role of structural setting, local uplift and morphostructural evolution on the onset and development of a DSGSD that affects the western parts of the Siahcheshmeh pullapart basin (SPAB) in a releasing bend of the GailatuSiah CheshmehKhoy fault.DSGSD:DSGSDs are gravityinduced and large to extremely large mass movements generally affecting the entire length of highrelief 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 manmade structures (Soldati, 2013). The main feature that distinguishes DSGSDs from landslides is the absence of a continuous or welldefined sliding surface (Soldati, 2013) and discontinuous or poorly defined boundaries, both laterally and at their lower ends (Crosta et al., 2013).GailatuSiah CheshmehKhoy fault:The 200 km long (Karakhanian et al., 2004; Berberian, 1977) GailatuSiah CheshmehKhoy fault, with the same trend as the North Tabriz, Chaldiran, Nakhichevan and PambakSevanSyunik faults, is regarded as a part of the active strikeslip fault system in the Arabian and Eurasian collision zone, which extends from 42˚ E to 48˚ E with the Tutak and NorthTabriz faults in the west and east, respectively (Selçuk et al., 2016). This system includes a series of rightlateral strikeslip faults between the southern front of the Lesser Caucasus to the northeast and BitlisZagros suture zone to the southwest. The available literature, fault plane solutions, offsets of various geomorphological and manmade features indicate the rightlateral strikeslip nature of the GailatuSiah CheshmehKhoy fault. The trace of this fault is very obvious and displays a series of welldeveloped and preserved morphologic structures indicating recent activity of the fault, such as fault scarps and horizontal deflection in the Quaternary features, pullapart basins, hot water springs and uplifted terrace deposits.Discussion and results:Our geological and structural survey along the GailatuSiah CheshmehKhoy 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 GailatuSiahcheshmeh (northwestern sector of GailatuSiah CheshmehKhoy fault) and the SiahcheshmehKhoy fault segments (Southeastern sector of GailatuSiah CheshmehKhoy fault), which overlap at a right stepover in the SPAB. Along the GailatuSiahcheshmeh 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 SiahcheshmehKhoy 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 rightlateral 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 SiahcheshmehKhoy fault segments form the eastern and western flanks of SPAB, respectively. Hence, the higher slip rate of GailatuSiahcheshmeh fault compared to SiahcheshmehKhoy fault, causes uplift of the western SPAB sectors. This is accompanied by thrust faulting in a general northwestsoutheast direction as a splay configuration at the termination of SiahcheshmehKhoy 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 pullapart 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 SiahcheshmehKhoy 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.