Andrea Zanchi

Continental arc volcanism and tectonic setting in Central Anatolia, Turkey

Abstract The Neogene and Quaternary volcanism of Central Anatolia represents the central sector of the Anatolian Volcanic Arc. related to continental collision between the Afro-Arabian and Eurasian plates. It is closely associated with a complex system of tectonic depressions related to brittle deformations of transtentional type and which commenced in the Late Miocene. The volcanism here considered can be divided into three main periods of activity, separated by important deformative and erosive events. The first period is represented by a mostly andesitic effusive activity. The second period is characterized by the emplacement of a thick ignimbritic sequence and shows an areal distribution up to 11,000 km 2 . Seven ignimbrite units have been recognized. The three main units were found at a distance of more than 100 km from the presumed source area. Geological and sedimentological data lead us to recognize the Melendiz Dag volcanic complex and the Ciftlik caldera as the probable ignimbrites source vent. During the third period great andesitic-basaltic stratovolcanoes and a number of prevalently acid monogenic centres developed. The relationship between the volcanic activity is clearly conditioned by the main transcurrent fault systems present in Central Anatolia. The Neogene-Quaternary volcanic activity prevalently developed along the ENE-WSW Karaman-Sivas lineament. Most of the great central volcanoes developed at the intersection between the ENE-WSW trends and the Ecemis and Tuz Golu transcurrent faults. The structural interpretation of the Quaternary monogenic centres is more difficult. Probably they are related to the very recent N-S fault swarms which cross the Anatolides and the Taurus Range.

Simple-shearing block resurgence in caldera depressions. A model from Pantelleria and Ischia

Abstract Similar resurgent blocks occur in caldera depressions at the islands of Pantelleria and Ischia. The two blocks, which are tilted, show similar deformation patterns and similar distributions of volcanic vents related to resurgence. The remaining parts of the floors of the two calderas are not deformed. Similarities between the two features are so striking that the resurgence mechanism must have been alike. A dynamic model for resurgence is presented and called simple-shearing block resurgence. The triggering mechanism is an increase of the magmatic pressure in the upper part of a shallow magma chamber, which results in a vertical direction of the maximum stress applied on the roof rocks. The block is first defined by high-angle inland-dipping marginal detachments and then deformed through a simple-shearing mechanism. Reverse faults on its mostly uplifted side determine an horizontal shortening of the block which is balanced on the opposite side by normal faults. Oblique movements take place along the other two sides. Due to this distribution of the stress around the block, magmas can reach the surface only through the extensional faults on the less uplifted side.

The Eo-Cimmerian (Late? Triassic) orogeny in North Iran

Abstract The Eo-Cimmerian orogen results from the Late Triassic collision of Iran, a microplate of Gondwanan affinity, with the southern margin of Eurasia. The orogen is discontinuously exposed along the northern side of the Alborz Mountains of North Iran below the siliciclastic deposits of the Shemshak Group (Late Triassic–Jurassic). A preserved section of the external part of the belt crops out in the Neka Valley (eastern Alborz) south of Gorgan. Here the Mesozoic successions (Shemshak Group–Upper Cretaceous limestones) overlay a pre-Jurassic Eo-Cimmerian thrust stack with a sharp unconformity. The stack includes the Gorgan Schists, an Upper Ordovician–Lower Silurian low-grade metamorphic complex, overthrusted southward above a strongly deformed Late Palaeozoic–Middle Triassic succession belonging to north Iran. In the Talesh Mountains (western Alborz), the Shanderman Complex, previously interpreted as an ophiolitic remnant isolated along the Eo-Cimmerian suture, is considered an allochthonous nappe of deeply subducted continental crust. The new evidence for this is the occurrence of previously unknown eclogites dating to the Carboniferous, and probably related to the Variscan history of Transcaucasia. South of the Shanderman Complex, Upper Palaeozoic slates and carbonates occurring below the Lower Jurassic Shemshak Group also record the occurrence of an Eo-Cimmerian metamorphic event. Based on our new data, the Eo-Cimmerian structures exposed in the Alborz appear to be remnants of a collisional orogen consisting mainly of deformed continental crust where no ophiolites are preserved.

The drift history of Iran from the Ordovician to the Triassic.

Abstract New Late Ordovician and Triassic palaeomagnetic data from Iran are presented. These data, in conjunction with data from the literature, provide insights on the drift history of Iran as part of Cimmeria during the Ordovician–Triassic. A robust agreement of palaeomagnetic poles of Iran and West Gondwana is observed for the Late Ordovician–earliest Carboniferous, indicating that Iran was part of Gondwana during that time. Data for the Late Permian–early Early Triassic indicate that Iran resided on subequatorial palaeolatitudes, clearly disengaged from the parental Gondwanan margin in the southern hemisphere. Since the late Early Triassic, Iran has been located in the northern hemisphere close to the Eurasian margin. This northward drift brought Iran to cover much of the Palaeotethys in approximately 35 Ma, at an average plate speed of c. 7–8 cm year−1, and was in part coeval to the transformation of Pangaea from an Irvingian B to a Wegenerian A-type configuration.

Tethyan oceanic currents and climate gradients 300 m.y. ago

We reconstruct the oceanic circulation pattern of the Tethys Ocean 300 m.y. ago by placing Late Carboniferous–Early Permian climate-sensitive biotic associations from Gondwana and Laurasia on a Pangea paleogeography constrained by selected paleomagnetic data. Warm-climate fossils and facies from Iran, located at that time along the Gondwanan margin of Arabia, are compatible with the existence in the Tethys Ocean of a warm subtropical surface current gyre, whereas cold surface currents swept the glaciated Gondwanan margin at higher southern latitudes, redistributing cold biota toward the tropics. This Tethyan surface current system and the associated narrow zonal barrier show similarities to recent glacial climate patterns. When placed on a large-scale paleogeographic reconstruction of Pangea of the B type, it neatly explains the otherwise problematic observation that the Carboniferous–Permian biota of Iran and northern Arabia is dominated by warm Euramerican and/or Russian taxa that are strikingly different from typical cold Gondwanan associations.

3D reconstruction of complex geological bodies: Examples from the Alps

Cartographic geological and structural data collected in the field and managed by Geographic Information Systems (GIS) technology can be used for 3D reconstruction of complex geological bodies. Using a link between GIS tools and gOcad, stratigraphic and tectonic surfaces can be reconstructed taking into account any geometrical constraint derived from field observations. Complex surfaces can be reconstructed using large data sets analysed by suitable geometrical techniques. Three main typologies of geometric features and related attributes are exported from a GIS-geodatabase: (1) topographic data as points from a digital elevation model; (2) stratigraphic and tectonic boundaries, and linear features as 2D polylines; (3) structural data as points. After having imported the available information into gOcad, the following steps should be performed: (1) construction of the topographic surface by interpolation of points; (2) 3D mapping of the linear geological boundaries and linear features by vertical projection on the reconstructed topographic surface; (3) definition of geometrical constraints from planar and linear outcrop data; (4) construction of a network of cross-sections based on field observations and geometrical constraints; (5) creation of 3D surfaces, closed volumes and grids from the constructed objects. Three examples of the reconstruction of complex geological bodies from the Italian Alps are presented here. The methodology demonstrates that although only outcrop data were available, 3D modelling has allows the checking of the geometrical consistency of the interpretative 2D sections and of the field geology, through a 3D visualisation of geometrical models. Application of a 3D geometrical model to the case studies can be very useful in geomechanical modelling for slope-stability or resource evaluation.

Seismotectonics of western Anatolia: Regional stress orientation from geophysical and geological data

Available fault plane solutions of shallow earthquakes occurring in the central part of western Anatolia were reviewed in order to determine the stress directions active in this region. Data on 66 shocks were used for numerical analysis with the P- and T-dihedra method, some of these data being used in further determination of stress tensors through numerical methods. Despite uncertainties on depth determinations, results were found consistent, and in agreement with those of geological analyses independently carried out on recent and active faults. Selections from the data were adopted on the basis of areal distribution, magnitude and depth of earthquakes. Variations in stress axes orientation with depth seem significant and related to increasing influence of crustal anisotropy near the surface. The dominant trend of the present day extension in this region, as indicated by the analysis of available earthquake mechanisms, is NNE-SSW with a corresponding azimuth of the σ3 axis N25°E. The stress regime is extensional with dominant normal fault motions and increasing importance of strike-slip faults from south to north. This pattern of stress in western Turkey is consistent with kinematic reconstructions indicating a westward and southwestward motion of Anatolia and Aegea relative to Eurasia.

The Cimmerian evolution of the Nakhlak–Anarak area, Central Iran, and its bearing for the reconstruction of the history of the Eurasian margin

Abstract New structural, sedimentological, petrological and palaeomagnetic data collected in the region of Nakhlak–Anarak provide important constraints on the Cimmerian evolution of Central Iran. The Olenekian–Upper Ladinian succession of Nakhlak was deposited in a forearc setting, and records the exhumation and erosion of an orogenic wedge, possibly located in the present-day Anarak region. The Triassic succession was deformed after Ladinian times and shows south-vergent folds and thrusts unconformably covered by Upper Cretaceous limestones following the Late Jurassic Neo-Cimmerian deformation. Palaeomagnetic data obtained in the Olenekian succession suggest a palaeoposition of the region close to Eurasia at a latitude around 20°N. In addition, the palaeopoles do not support large anticlockwise rotations around vertical axes for central Iran with respect to Eurasia since the Middle Triassic, as previously suggested. The Anarak Metamorphic Complex (AMC) includes blueschist-facies metabasites associated with discontinuous slivers of serpentinized ultramafic rocks and Carboniferous greenschist-facies ‘Variscan’ metamorphic rocks, including widespread metacarbonates. The AMC was formed, at least partially, in the Triassic. Its erosion is recorded by the Middle Triassic Bāqoroq Formation at Nakhlak, which consists of conglomerates and sandstones rich in metamorphic detritus. The AMC was repeatedly deformed during post-Triassic times, giving origin to a complex structural setting characterized by strong tectonic fragmentation of previously formed tectonic units. Based on these data, we suggest that the Nakhlak–Anarak units represent an arc–trench system developed during the Eo-Cimmerian orogenic cycle. Different tectonic scenarios that can account for the evolution of the region and for the occurrence of this orogenic wedge in its present position within Central Iran are critically discussed, as well as its relationships with a presumed ‘Variscan’ metamorphic event.

Neogene block rotation in central Iran: Evidence from paleomagnetic data

Paleomagnetic results from Oligocene–Miocene sedimentary units in central Iran are used to reconstruct the history of Neogene tectonic deformation of this region. Paleomagnetic data show that in central Iran, crustal blocks bounded by sets of strike-slip faults are rotated to accommodate NNE-SSW shortening related to Arabia-Eurasia convergence. Counterclockwise rotations of 20°–35° have been measured in the Tabas and Anarak areas, south of the Great Kavir fault, characterized by the presence of N-S to NNW-SSE right-lateral strike-slip faults. Conversely, in the Great Kavir and Torud areas, where ENE-WSW left-lateral strike-slip faults have been recognized, paleomagnetic results are less conclusive because the small amount of measured clockwise rotation shows a statistically uncertainty, which also includes the possibility of no rotation. Some of these faults have been active during the Quaternary up to present day, suggesting the possibility that block rotation is still occurring in central Iran.

The geology of the Karakoram range, Pakistan: the new 1:100,000 geological map of Central-Western Karakoram

A new geological map of the central-western part of the Karakoram belt (Northern Areas and North West Frontier Province, Pakistan) is presented with its explanatory notes. The map is printed at a 1:100,000 scale, summarizing original field surveys performed at a 1:25,000 scale, which result from the first systematic reconnaissance of the area. This work represents the synthesis of several years of exploration studies and is mainly based based on original stratigraphic and structural field analyses focused on one of the less known orogenic belts of Central Asia. Original field surveys have been integrated within a GIS using georeferenced Russian topographic maps and grey-tone panchromatic SPOT images. The study area is located along the border between Pakistan and Afghanistan, extending from the top of the Chapursan Valley of the Hunza region to the Yarkhun Valley from the Karambar Pass to Gazin and to the upper part of the Rich Gol, which belong to Chitral. Three major tectonic units are exposed in the study area. From north to south they are: the East Hindu Kush-Wakhan, the Tirich Boundary Zone and the Karakoram Terrane. The first and the last units consist of Gondwana-related terranes showing a Pre-cambrian to earliest Paleozoic basement covered by Paleozoic to Mesozoic sedimentary successions which record their Late Paleozoic rifting from Gondwana, their drifting, and successive accretion to the Eurasian margin. They both show some similarities with the S-Parmir ranges, exposed to the north of the Afghan Wakhan. The Tirich Boundary Zone is a complex assemblage of high grade metabasites and gneiss with small remnants of sub-continental peridotites, which separate East Hindu Kush from the Karakoram. Its emplacement has been related to the possible opening of a basin between the two blocks at the end of the Paleozoic, followed by its deformation during the collision of Karakoram with East Hindu Kush, dating to the end of Triassic or beginning of the Jurassic. Detailed mapping has been carried out in the Karakoram belt, especially along its northern portion, which consists of a complex stack of tectono-stratigraphic units, showing peculiar stratigraphic and structural features. These units were progressively deformed and thrusted during the collision with the Kohistan Paleo-Arc and with India which occurred between the end of the Cretaceous and Paleogene. These collisions were also followed by continuous crustal thickening and by left-lateral shearing, which was especially active along the western margin of the mapped area. Our map also includes parts of the Karakoram Batholith, mainly Cretaceous in age, and of the Darkot-Gazin Metasedimentary Belt, which is exposed to the south of the main intrusive bodies and consists of Permo-Triassic metasediments.