RICERCA ITALIANA     

THE CIMMERIAN OROGENY IN IRAN: FROM THE GONDWANAN BREAK-UP TO THE COLLISION WITH THE EURASIAN MARGIN

Università degli Studi di Milano-Bicocca

Abstract

In the frame of the evolution of the Alpine-Himalayan orogenic belts, it is of outstanding significance to unravel the role of the Cimmerian Orogeny, which affected the Eurasian margin from Turkey to Thailand. This event, related to the closure of the Palaeotethys ocean east of the Aegean Sea, straddles the Varisian orogeny and the Alpine one, significantly affecting the Mediterranean region during Middle Triassic. The Cimmerian Orogeny was chiefly caused by the collision with the Eurasian margin of several microplates detached from the Northern Gondwana margin during Early Permian due to the opening of the Neotethys Ocean south of them. In this context, Iran represents a key-area where to study the evolution of the Cimmerian blocks, from the opening of the Neotethys to the closure of the Palaeotethys.
This project is this aimed to unravel the timing of break-up, collision and subsequent evolution of the Iranian block. To reach this issue we intend to analyse the geodynamic evolution of North Iran in selected areas (Masuleh, Central Alborz , Eastern Alborz, Aghdarband) in order to study the following features:
-stratigraphic and palaeontological evolution of the Permo-Triassic succession to check the biotic affinity of Iran through a direct comparison with the northern Gondwana margin, eastern Cimmerian blocks and the Eurasian margin. Through this analysis we will assess on a palaeobiogeographical base the timing and the entity of the northward movement of >>>

Principal Investigator

Andrea Marco ZANCHI Università degli Studi di MILANO-BICOCCA

Research Objectives

The complex geodynamic evolution which led to mountain building along the Alpine-Himalayan orogenic girdles has been intensely studied around the Western Mediterranean and more recently in Central Asia between Pakistan and Nepal. In contrast, large segments of the orogen, namely those comprised between Turkey and Afghanistan, remain still quite poorly known. As a consequence, numerous major geodynamic problems remain unsolved, including the precise timing of opening and closure of Tethyan branches during the Late Paleozoic to Mesozoic. The closure of Paleotethys, apparently diachronous from west to east, is one of the principal problems which still await clarification (e.g. Stampfli & Borel, 2002; Zanchi et al. 2003b).
It has long been recognized that the Permian to Triassic evolution of Alpine and Mediterranean areas is strictly related to and largely controlled by the geodynamic evolution of eastern regions (Gaetani et al. 1998; Stampfli & Borel 2002). Now, it is of crucial importance for us to improve on our still patchy knowledge on key areas comprised between Mediterranean regions and Central Asia.
This further step is needed to continue the long tradition of geological studies carried out by the Earth Science Department of the University of Milano throughout the second half of the last century. This research plan, begun under the guidance of Ardito Desio in the late Twenties, was subsequently carried out by his numerous former assistants, including >>>

First Results

The main expected partial results consist in the construction of a database concerning all the collected field-data in the Alborz and Masuleh areas. Structural field data concerning ductile and brittle deformations will be analysed and elaborated. Basing on the field observations, preliminary structural cross-sections will be prepared. Fault populations will be inverted for palaeostress reconstruction. Preliminary mineralogical analyses and comparison with field observations will take to a general characterisation of the analysed metamorphic complex. The field study will help to define the eventual occurrence of a subduction complex in the Talesh Mountains and its extent. The comparison among the structural analysis performed in the pre-Shemshak units and in the Shemshak and post-Shemshak succession will clarify the characters of the Cimmerian deformation. Fault analysis performed in the most recent structures will also help to understand their kinematics and their relationships with the seismotectonic framework of this area. Structural analyses performed in the Central Alborz will help to recognise the occurrence of Eocimmerian tectonics within the internal parts of North Iran and their importance in the recent evolution of the region.
All the stratigraphic data collected in the field will be organised in a database which will be the basis for the following activities. The petrographic analysis of the Triassic and Jurassic sandstones will begin in order to take to a >>>

Timescale

24 months

National and international background

Introduction
Recent works (Gaetani et al., 2000; Vai, 2003; Stampfli & Borel, 2002; Zanchi et al., 2003b) suggest that from Chios (Aegean Sea, Greece) towards Southeast Asia, no collision occurred at the end of the Palaeozoic between Eurasia and Gondwana, where an important sea-way, the Palaeotethys ocean, remained open up to the beginning of the Mesozoic. The Late Palaeozoic palaeogeographic scenario in the Palaeotethys ocean was dominated by the occurrence of several microplates (Cimmerian blocks) which detached from the northern Gondwanan margin due to the opening of the Neotethys Ocean south of them. These microplates progressively migrated northward to collide with the southern Eurasian margin at the beginning of the Mesozoic causing the Cimmerian orogenic event which has been recognized from Turkey to Thailand (Sengor, 1984; Stampfli et al., 2001). Several years of integrated field works performed by our teams have given light to the evolution of the Northern Karakoram. Karakoram represents a Cimmerian microplate belonging to the Mega-Lhasa block, one of the largest Cimmerian microplates which separated from Gondwana during Early Permian, due to the opening of Neotethys between Karakoram and the future Indian Plate. Karakoram was then accreted to the Eurasian margin, at Late Triassic-Early Jurassic time (Gaetani, 1997; Zanchi et al., 2000; Gaetani et al., 2004).
Worth of note is also the knowledge acquired by the research groups along the Gondwanan >>>