شواهد تغييرات سطح اساس پلاياي ميقان در كواترنري و تاثير آن بر مورفولوژي و توالي مخروط افكنه ها

Evidence base level changes of Mighan playa in the Quaternary and its effects on morphology and alluvial fan sequences

تغييرات اقليمي و زمين ساختي نقش اساسي در تحول و تغييرشكل مخرو ط افكنه ها دارند. اين پژوهش به نقش تغييرات سطح اساس چاله ميقان بر مورفولوژي مخروط افكنه هاي پيرامون چاله پرداخته است. براي انجام اين پژوهش از بررسي هاي ميداني، داده هاي مغزه رسوبي چاه ها، تصاوير ماهواره اي و داده هاي رقومي ارتفاعي استفاده شده است. با مقايسه مغزه رسوبي چاه ها و درونيابي بخش هاي مشترك آنها، حداكثر گسترش درياچه مشخص شد. پس از استخراج مخروط افكنه ها با توجه به تغييرات سطح اساس، مورفولوژي توالي مخروط افكنه ها، موقعيت زمين شناسي و حوضه هاي زهكشي، در بخش شمالي و جنوبي چاله مورد مقايسه و تجزيه و تحليل قرار گرفته است. نتايج نشان مي دهند كه تغييرات سطح اساس چاله ميقان و فرونشست تدريجي چاله همزمان با برخاستگي بخش حاشيه اي چاله كه نشانگر وجود عدم تقارن و مورفوتكتونيك است بر مورفولوژي مخروط افكنه هاي پيرامون اثر گذاشته است. البته اگرچه عامل تغيير اقليم بر تغيير مورفولوژي مخروط افكنه هاي(آشتيان، اراك و ...) نقش اساسي داشته، ولي نقش نوزمين ساخت بر تغييرشكل و تحول مخروط افكنه آشتيان نسبت به ساير مخروط افكنه ها موثر تر بوده است. زيرا در بخش شمالي پنج مخروط قديمي تا جديد با اختلاف ارتفاع حدود 700 متر باعث عدم تقارن در منطقه شده است در حالي كه در مخروط افكنه هاي بخش جنوبي اين توالي ديده نمي شود.

Extended Abstract Introduction Interaction between of internal and external processes of the earth during time epoch have an important role in formation and deformation and landforms transformation, we can identify course of their processes changes with landform survey. In the inner parts of Iran is apparent several geomorphological evidence of dry and rainy climate changes. Playaes and lakes or generally internal holes had an important role in climate balance and ecological changes in the quaternary in its surrounding areas. Because of this their study has been considered always to determine past climate.Morphology and alluvial fan deposits, contains traces of past enviromental changes. The key issue is that be determined changes in alluvial fan processes, isthe result of which of tectonic factors, climatic and or geomorphological conditions of alluvial fans. Materials and methods To conduct this research from fieldwork, are used sediment core data of wells and further satellite imagery and digital elevation data with different scales in different software enviroment.In this research with comparing the sediment core of wells in surrounding area of Mighan hole and interpolation of their common areas was determined maximum spread of the lake.after was tend to extraction of alluvial fans, then according to region morphology, are compared and analyzed geological structure and location, region drainage basins, base level changes and alluvial fans sequence in northern part of pit (Ashtian’s alluvial fan) and the southern part of the pit (alluvial fans of Arak, Tamar Abad, Mehr Abad). Results and discussion: Finding from this research are presented as follows: Investigation of evidence of clay deposits of region and interpolation of core wells of Velashjerd, Taremozd and Mashhad Mighan, show about 57 meter water forasmuch as the same formation area for clay layers 90, 80, 120 meter this cores for Mighan lake in the quaternary. Evidence of sediment of cores and clay sediments abundant in the western and southern proves that maximum deep of lake has been in western and southern slopes of craters and about 8000 to 16000 last year extent of clay sediments indicate wetter climate conditions and reflects the development and progression of the lake at that time. With study of region geological conditions and tectonic it should be state that clearly visible function of Talkhab and Tabrateh faults and also effect of craters subsidence on the drainage basin and alluvial fans surrounding the hole and that has been caused asymmetry on the northern slopes and should be stated that this region has been affected by the impact of Neotectonics. Also with existence of same climate conditions for sedimentary basin Mighan we determined five base level changeson the northern slopes of the well (hole) for the Ashtian cone that 3 base level related to river redirection has been in effect of neotectonics activities and is not sensible for the next two levels detection of impact of climate and neotectonics function. Including evidence of base level changes available evidence on the Ashtian alluvial fans indicates considerable activity of neotectonics processes during quaternary are: 1) Creation of mound hills on the formations of quaternary epoch that are represented tectonic movements. 2) The effect of Talkhab fault line on quaternary sediments and quaternary landforms. 3) Existence of thick alluvial deposits that starts from 50 meter to 100 meter at the bottom of the cone. But base level change in southern slopes has not been caused alluvial fans sequence, existence large mound hills, fraction of alluvial fans and being abandoned cones level. Conclusions: The result of the study show that the sequence of fine-grained sediments of silt and clay and sand and gravel in the core of exploratory wells indicate environmental changes particularly climate changes in the quaternary. Likewise the cause of climate change on morphology change of alluvial fans surrounding Mighan hole (Ashtian, Arak and etc) has been a major role, but was more effective neotectonics rule on deformation and development of Ashtian alluvial fan than the other alluvial fans. Because it is measurable in the northern part of five old cones to new cones with about 700 meters height difference while this sequence is not found in the alluvial fans of southern part. Base level changes of Mighan hole and hole gradual subsidence with rising marginal part of hole that indicate existence of asymmetry and morphotectonic for this hole, are effected on the morphology of around alluvial fans. In other words, neotectonics effects has been more effective than climate change in base level changes of Mighan hole at this region and this effects is caused asymmetry in North domain. According to the geomorphological evidence can be concluded that this Playa has been placed in dynamic tectonically. And climate change, especially neo-tectonic has been caused base level changes in Mighan playa.