Cryogenic wedges on the NE Qinghai-Tibet and Ordos Plateaus: Their characteristics,origin and OSL dating

Cryogenic wedges developed due to very cold, rather arid conditions during the maximum of the last cold event when the drying up of the neighboring China Sea resulted in the failure of the East Asian Monsoon. As the climate ameliorated and the Monsoon rains reappeared, ice-wedges developed. Further warming permitted thawing of the ice infillings accompanied by replacement of the ice by sediments partly from the host ground as well as from the surface by wind or sheet wash. In cases of extreme surface water flow on slopes after 10 ka B.P., small baydjarakhs typically c. 50 cm high developed, only to have the resulting hollows infilled by sediments carried by wind and/or sheet wash. These shallow structures form a network on top of many of the cryogenic wedges. This complex history makes dating the ages of the wedges difficult using OSL methodology. Unfortunately, past field work ignored the problem of the angle of the cut face to the direction of the wedge infilling when sampling the contents of the narrow wedges, resulting in potential contamination of the samples with the host sediment. Sampling of the larger deposits should be alright, but the likelihood of contamination makes the interpretation of the resulting OSL dates from the narrow wedges questionable. Primary wedges consisting of primary mineral infillings should still have similar OSL dates with depth for a given wedge, but the distinction between ice-wedge infillings and soil wedges is difficult since both can exhibit older dates of the infillings with depth. The available data suggests that ice-wedges were significantly more common than sediment-filled primary wedges. A protocol to avoid having to obtain large numbers of OSL dates by more careful field sampling and the use of grain size determinations is provided in the Appendix.     

Late Cretaceous coeval acidic and basic magmatism,Karacaali Magmatic Complex,Central Anatolia,Turkey

The Central Anatolia Crystalline Complex (CACC) is characterized by Late Cretaceous high-temperature metamorphic rocks intruded by S-, I-, and A-type granitoids. Coeval basic plutonic and volcanic rocks also crop out in the complex. The NE–SW-trending Karacaali Magmatic Complex (KMC) represents a clear example of synchronous basic and acidic magmatic associations. We present new data on this coeval magmatism. The KMC plutonic rocks mainly consist of monzonite, granite, and gabbro, whereas the associated volcanic rocks are chiefly of basalt and rhyolite. All of the units have been cut by quartz, quartz-tourmaline, and calcite veins and by porphyritic leucogranite, aplitic, and basaltic dikes. The rhyolitic, basaltic, and gabbroic samples yield well-defined ⁴⁰Ar/³⁹Ar plateau ages of 69.1 ± 1.3, 58 ± 10, and 66.4 ± 1 million years, respectively; these data indicate that a younger multiphase basic magma was injected into a partially crystallized monzonitic magma chamber. The basic intrusions added heat to the system and gave rise to the re-fusion of the already crystallized parts of the monzonitic melt, forming the younger leucogranitic magma. The gradational contacts, cross-cutting relationships, trace element contents, trace element patterns, rare-earth element (REE) patterns, and ⁴⁰Ar/³⁹Ar geochronological data of the studied igneous suite clearly demonstrate that the acidic and basic rocks of the KMC were contemporaneous and are produced by partial melting of distinct sources rather than by fractional crystallization of a single source.     

Crustal structure and local seismicity in western Anatolia

Western Anatolia is one of the most seismically active continental regions in the world and much of it has been undergoing NS-directed extensional deformation since Early Miocene time. In a cooperative study, seismologists from Saint Louis University, USA and Dokuz Eylül University, Turkey, deployed five broad-band and 45 short-period seismic stations in western Anatolia between 2002 November and 2003 October. The present paper uses data collected by this network and the data from five permanent stations operated by the Kandilli Observatory and Earthquake Research Institute to map the hypocentral distribution of local earthquakes and to determine crustal structure of western Anatolia. We obtained a 1-D P -wave crustal velocity model using a generalized scheme for simultaneously obtaining earthquake locations and a crustal velocity model. Our velocity model is characterized by crustal velocities that are significantly lower than average continental values. The low velocities may be associated with high crustal temperatures, a high degree of fracture, or the presence of fluids at high pore pressure in the crust. We located 725 local earthquakes and classified them in three categories. We found that the level of seismic activity in western Anatolia is higher than previously reported. Station delays resulting from the inversion process correlate with near-surface geology and the thickness of sediments throughout the region. The hypocentral distribution of the events indicates that peak seismicity for the region occurs at depths of about 10 km.     

A comparative predictive analysis of weighted overlay,weighted sum and fuzzy logic for mineral prospectivity mapping of precious and base metal mineralizations at north-east of Gümüşhane city,NE Turkey

The study area located south of the Eastern Pontide metallogenic belt (NE Turkey) has an important potential for valuable and base metallic minerals. Geological, tectonic, geochemical, hydrothermal alteration, mineralogical and fluid inclusion characteristics of ore formations in the region have been studied in detail. The purpose of this study is to combine and analyse the acquired spatial data layers in a geographic information system (GIS) environment using knowledge-driven approaches and to identify prospective areas in terms of mineralization. Moreover, evaluating the performance of different knowledge-driven mineral probability modelling results comparatively and quantitatively constitutes the other goal of the study. As a result, in addition to the known mineralization locations, a number of new prospective and favourable areas have been identified for future detailed studies. In addition, it has been found that the mineral predictive map generated using fuzzy logic-OR method produces the best and successful results compared to others.     

Preservation by basalt of a staircase of latest Pliocene terraces of the River Murat in eastern Turkey: Evidence for rapid uplift of the eastern Anatolian Plateau

Unspiked K–Ar dating makes the age of the Çakmaközü basalt in eastern Turkey 1818 ± 39 ka (± 2σ). This basalt overlies a staircase of four terraces of the River Murat, a Euphrates tributary, each separated vertically by  20 m. We deduce from the relationship with the basalt that these fluvial deposits aggraded during successive  40 ka climate cycles around the Pliocene–Pleistocene boundary (probably MIS 72-66). The incision and rock uplift at  0.5 mm a^− 1, thus indicated, are roughly consistent with the  500 m of entrenchment of this  1.8 Ma Murat palaeo-valley into a former lake basin since the Mid-Pliocene climatic optimum. We infer that the  130 m of incision in this locality since  1.8 Ma dramatically underestimates the associated rock uplift, estimated as  600 m. The  1100 m of rock uplift and  800 m of surface uplift thus estimated since the Mid-Pliocene indicate (assuming Airy isostatic equilibrium)  5 km of thickening of the continental crust, from  37 km to the present 42 km. Eastern Anatolia was thus at a much lower altitude in the Mid-Pliocene than at present, consistent with the low-relief lacustrine palaeo-environment. We infer that the subsequent development of topography and excess crustal thickness are being caused by coupling between surface processes and induced flow in the lower crust: climate change following the Mid-Pliocene climatic optimum resulted in faster erosion that has drawn mobile lower crust beneath the study region.     

Palaeomagnetic evidence for the Gondwanian origin of the Taurides and rotation of the Isparta Angle,southern Turkey

The Taurides, the southernmost of the three major tectonic domains that constitute present‐day Turkey, were emplaced following consumption of the Tethyan Ocean in Late Mesozoic to mid‐Tertiary times. They are generally assigned an origin at the northern perimeter of Gondwana. To refine palaeogeographic control we have investigated the palaeomagnetism of a range of Jurassic rocks. Forty‐nine samples of Upper Jurassic limestones preserve a dual polarity remanence (D/I=303/−9°, α₉₅=6°) interpreted as a primary magnetization acquired close to the equator and rotated during emplacement of the Taurides. Result from mid‐Jurassic dolerites confirm a low palaeolatitude for the Tauride Platform during Jurassic times at the Afro–Arabian sector of Gondwana. Approximately 4000 km of Tethyan closure subsequently occurred between Late Jurassic and Eocene times. Although related Upper Jurassic limestones and Liassic redbeds preserve a sporadic record of similar remanence, the dominant signature in these latter rocks is an overprint of probable mid‐Miocene age, probably acquired during a single polarity chron and imparted by migration of a fluid front during nappe loading. This is now rotated consistently anticlockwise by c. 30° and conforms to results of previous studies recording bulk Neogene rotation of the Isparta region following Lycian nappe emplacement. The regional distribution of this overprint implies that the Isparta Angle (IA) has been subject to only small additional closure (<10°) since Late Miocene time. A smaller amount (c. 6°) of clockwise rotation within the IA since Early Pliocene times is associated with an ongoing extensional regime and reflects an expanding curvature of the Tauride arc produced by southwestward extrusion of the Anatolian collage as a result of continuing northward motion of Afro–Arabia. Copyright © 2002 John Wiley & Sons, Ltd.     

Insights into biaxial extensional tectonics: an examplefrom the Sandıklı Graben,West Anatolia,Turkey

West Anatolia, together with the Aegean Sea and the easternmost part of Europe, is one of the best examples of continental extensional tectonics. It is a complex area bounded by the Aegean–Cyprus Arc to the south and the North Anatolian Fault Zone (NAFZ) to the north. Within this complex and enigmatic framework, the Sandıklı Graben (10 km wide, 30 km long) has formed at the eastern continuation of the Western Anatolian extensional province at the north‐northwestward edge of the Isparta Angle. Recent studies have suggested that the horst–graben structures in West Anatolia formed in two distinct extensional phases. According to this model the first phase of extension commenced in the Early–Middle Miocene and the last, which is accepted as the onset of neotectonic regime, in Early Pliocene. However, it is controversial whether two‐phase extension was separated by a short period of erosion or compression during Late Miocene–Early Pliocene. Both field observations and kinematic analysis imply that the Sandıklı Graben has existed since the Late Pliocene, with biaxial extension on its margins which does not necessarily indicate rotation of regional stress distribution in time. Although the graben formed later in the neotectonic period, the commencement of extension in the area could be Early Pliocene (c. 5 Ma) following a severe but short time of erosion at the end of Late Miocene. The onset of the extensional regime might be due to the initiation of westward motion of Anatolian Platelet along the NAFZ that could be triggered by the higher rate of subduction at the east Aegean–Cyprus Arc in the south of the Aegean Sea. Copyright © 2003 John Wiley & Sons, Ltd.     

Genetic implications of Zn‐ and Mn‐rich Cr‐spinels in serpentinites of the Tidding Suture Zone,eastern Himalaya,NE India

Metamorphosed serpentinites of the Tidding Suture Zone (TSZ), eastern Himalaya, contain variably altered Cr‐spinels that are concentrically zoned from high‐Cr, low‐Fe³⁺ spinel at the core to Cr‐magnetite at the rim. Two types of Cr‐spinel have been recognized, based on back‐scattered electron imaging in conjunction with microprobe analytical profiles. Cr‐spinel type‐I is present in the least metamorphosed serpentinite (Cr# = 0.78–0.85, Mg# = 0.38–0.45) and Cr‐spinel type‐II is present in the most highly metamorphosed serpentinite (Cr# = 0.86–0.94, Mg# = 0.10–0.34). Primary igneous compositions are preserved in the type‐I chromites whereas these compositions have been partly or completely obscured by metamorphism and alteration in type‐II grains. The enrichment of Mn and Zn increases from the type‐I (MnO = 1.86–2.42 wt.%, ZnO = 0.77–1.67 wt.%) to type‐II (MnO = 2.72–4.04 wt.%, ZnO = 1.33–3.22 wt.%) and the strong similarity in their distribution patterns implies that these elements were introduced during low‐grade metamorphism and serpentinization. The abundance of Mg‐rich chlorite and serpentine minerals suggest that olivine was the predominant silicate phase before serpentinization. Zn and Mn enrichment in the core zone of the Cr‐spinel is due to the substitution of Mg²⁺ and in part of Fe²⁺, by Zn and Mn. These elements were probably supplied from olivine upon serpentinization during and after obduction of the ophiolitic mélange along the Tidding Suture Zone in the eastern Himalaya, NE India. Copyright © 2012 John Wiley & Sons, Ltd.