Kefa Zhou

Late Devonian–early Permian accretionary orogenesis along the North Tianshan in the southern Central Asian Orogenic Belt

The Palaeozoic orogenic process in the North Tianshan of the southern Central Asian Orogenic Belt is controversial. Systematic field study indicates that the ophiolitic fragments of the North Tianshan are mainly thrust slices and blocks of a late Palaeozoic accretionary complex, which was intruded by granitoids. U-Pb zircon dating of plagiogranites from the North Tianshan ophiolite yielded a mean age of 343.1 ± 2.7 Ma. These are typical oceanic plagiogranites but with a supra-subduction zone (SSZ) signature. Ophiolitic basalts display N-MORB, E-MORB, and OIB compositions. One gabbro with an age of 301.9 ± 2.2 Ma shows E-MORB geochemistry mixed with N-MORB and OIB. Some andesites show clear island arc characters indicated by enrichment of LILEs relative to HFSEs. Mean ages of 344.9 ± 4.2 and 298.7 ± 2.4 Ma were obtained for a granite porphyry and a mylonitic granite, respectively. The two granitoids display an island arc geochemical signature evidenced by enrichment of LILEs and depletion of HFSEs. Combined with an eastward migration of Late Devonian to Carboniferous arc magmatism and related Cu-Au-Mo deposits, we propose that trench retreat and slab roll-back took place during subduction of the Junggar Ocean spreading ridge beneath the North Tianshan arc, and that the accretion may have lasted into early Permian time, an important late stage of the long-lived accretionary orogenesis in the southern Central Asian Orogenic Belt.

Structures and detrital zircon ages of the Devonian–Permian Tarbagatay accretionary complex in west Junggar, China: imbricated ocean plate stratigraphy and implications for amalgamation of the CAOB

ABSTRACT The Tarbagatay Complex, located in northwest Junggar, is situated tectonically between the Zharma–Saur arc to the north and the Tacheng terrane and the Boshchekol–Chingiz arc to the south. This Complex belt is variably composed of ophiolitic mélange, sedimentary mélange, and coherent units of turbidites and shallow water sediments. These rocks crop out in fault-bound slices with fault-parallel asymmetric folds. Both the lithologies and deformation features of the Tarbagatay Complex suggest an accretionary origin generally with a top-to-the-south tectonic vergence, suggesting N-dipping subduction beneath the Zharma–Saur arc. The presence of a former ocean is indicated by the Ordovician ophiolite mélanges and related marine fossils. The time duration of the Tarbagatay Complex can be bracketed by detrital zircon ages of turbidites and shallow water sediments with a lower limit of major peak ages of 350–370 Ma, and an upper limit of middle Permian indicated by detrital zircon ages of 262.3 Ma. Based on these data, we suggest that the subduction of the Tarbagatay Ocean likely started in the Late Devonian and lasted until the middle Permian. Taking into account the formation of the northern part of the Kazakhstan orocline, which has a similar temporal-spatial framework, we propose a tectonic model for the western CAOB that involves accretion and amalgamation from the Ordovician to the middle Permian.

Oblique wedge extrusion of UHP/HP complexes in the Late Triassic: structural analysis and zircon ages of the Atbashi Complex, South Tianshan, Kyrgyzstan

ABSTRACT The exhumation and tectonic emplacement of eclogites and blueschists takes place in forearc accretionary complexes by either forearc- or backarc-directed extrusion, but few examples have been well analysed in detail. Here we present an example of oblique wedge extrusion of UHP/HP rocks in the Atbashi accretionary complex of the Kyrgyz South Tianshan. Our field mapping and structural analysis demonstrate that the Atbashi Eclogite–Blueschist Complex is situated in a complicated duplex formed by a northerly dextral transpression system and a southerly sinistral transtension system. The two major shear systems suggest that the Atbashi Complex was extruded obliquely southwestwards during eastward penetration of the southern tip of the Yili–Central Tianshan Arc of the Kazakhstan Orocline during the Late Triassic. Also, we report new zircon U–Pb metamorphic ages of four eclogites and one garnet-bearing quartz-schist from the Atbashi complex of 217–221 Ma and 223.9 Ma, respectively, suggesting that the main extrusion was later than previously proposed and that the final orogenesis was not completed until the Late Triassic. The HP/UHP rocks have an oblique plunge to the NE and extrusion took place southwestwards during escape tectonics along the South Tianshan accretionary wedge in the Late Triassic. Our work shows that the movement of HP/UHP rocks had a 3D style with an arc-parallel structure, and sheds light on earlier 2D models with either forearc- or backarc-directed extrusions, which indicates that more systematic structural and geochronological work is needed to characterize the accretionary tectonics of many orogens around the world. Our data on the timing of extrusion and emplacement of the Atbashi Eclogite–Blueschist Complex also help to resolve the long-standing controversy about the time of terminal orogeny of the Central Asian Orogenic Belt.

Application of fuzzy analytical hierarchy process (AHP) and prediction-area (P-A) plot for mineral prospectivity mapping: a case study from the Dananhu metallogenic belt, Xinjiang, NW China

This paper describes a GIS-based application of a fuzzy analytical hierarchy process (AHP) to map porphyry-Cu prospectivity in the Dananhu metallogenic belt, NW China. Based on a model of porphyry-Cu mineralization, evidential layers were derived from geological, geochemical, and geophysical data. These layers were subsequently assigned weights by implementing a fuzzy AHP method using knowledge from three experts in porphyry-Cu exploration. After obtaining normalized weights, different fuzzy operators were tried to combine the weighted evidential layers into potential maps, which were then compared and evaluated by predication-area (P-A) plots. Subsequently, a ternary map was generated by defuzzification of the optimum prospectivity map as selected by the P-A plot; this ternary map shows zones of high, moderate, and low favorability for porphyry-Cu deposits in the study area. To further evaluate the results, potential zones were analyzed for two-dimensional spatial domain. The results demonstrate that the fuzzy AHP method can be effectively applied to mineral prospectivity mapping in vaguely known areas.

Mineral prospectivity mapping with weights of evidence and fuzzy logic methods

Knowledge-and data-driven approaches are two major methods used to integrate various evidential maps for mineral prospectivity mapping (MPM). Geological maps, geochemical samples and data from known gold deposits were collected in the western Junggar area, Xinjiang Province. The geological and a spatial database for geological and mineral occurrences were constructed for the studied region. A weights-of-evidence model and a fuzzy logic model were employed for MPM, and the results were compared. Results indicate that favorable sedimentary rocks, fault density, fault distance and concentration of Au were the primary factors affecting Au mineralization. Arsenic (AS), Stibium (Sb), fault direction, quartz veins and intrusive rocks were secondary factors affecting Au mineralization. Conditional independence exerted a major influence on the weights-of-evidence model. However, posterior probability would be very high if the conditional independence was disregarded, which impaired results. Combining the quantification results provided by weights-of-evidence and the fuzzy membership values determined by expert knowledge, the mineral prospectivity mapping according to the fuzzy logic method was proved to be valid. For the study area, which had a large number of deposits, data-driven approaches for MPM are generally considered to be appropriate. However, if sufficient data are not collected, the knowledge-driven approaches, for example, the fuzzy logic method used in the present study, usually achieves a better result.

Research of GIS-based Spatial Data Mining Model

In this paper, the theories of spatial data mining and geographic information system are described firstly, and the integration model of the spatial data mining is also researched and analyzed in-depth. On the basis of this, a new GIS system structure based on the spatial data mining is presented, which has the advantages of good universality, interaction and easy realization comparing with other structures.

Ground-penetrating radar for monitoring the distribution of near-surface soil water content in the Gurbantünggüt Desert

In the Gurbantünggüt Desert, snowmelt-induced high soil water contents briefly create favorable conditions for the germination and growth of plants every spring. Monitoring the rapidly changing conditions in this time period demands fast and efficient methods for measuring soil water contents at the field scale. For this study, a series of ground-penetrating radar (GPR) measurements were carried out on sites characterized by semi-vegetated dunes both in April 2010 and 2011. We compare water contents calculated from the GPR direct ground wave signal to both point scale validation measurements by time-domain reflectometry (TDR) and gravimetric sampling. Our results show that GPR is an effective method to rapidly obtain a detailed image of the field scale soil water content distribution in the Gurbantünggüt Desert with an accuracy similar to TDR. Observed large scale soil water content variations are dominated by dune topography: During snow melting, melt water was found to trickle slowly from the dune ridges to interdune valleys, increasing the soil water content there while the dune ridges quickly started to dry down. In dune valleys, smaller scale near-surface soil water content changes were dominated by variations in the vegetation coverage, leading to snowmelt funnels at distinct locations: The snowmelt initially occurred around the stems and branches of plants, forming funnel-shaped melt water induced holes through the snow cover and leading to an increasing amount of melt water collected around these plant roots. Our comparison of data from 2010 to 2011 furthermore suggests a temporally stable distribution of near-surface soil water content. This has important ecological significance for controlling desertification and for restoring and reconstructing vegetation in the Gurbantünggüt Desert.

Exploring spatial patterns of soil water content in the Urumqi region with Ground-Penetrating Radar

This study presents current GPR field methods for monitoring spatio-temporal soil water content variations at different representative sites in a highly dynamic regional watershed in Northwestern China. We detail the applicability of current GPR methods at characteristically different sites for assessing multi-scale structures in the respective soil water content fields, and compare the distinct, heterogeneous spatial patterns in soil water content.

Extraction of Alteration Mineral Information from Moderate Remote Sensing Images Using MPS Method

Although alteration minerals related to metallogenesis is very important in mineral exploration, information of alteration mineral is weakly expressed in remote sensing imagery, which is often subject to interfering noise and sometimes limited in spectral and spatial resolutions. Because of easy access, moderate images are the main sources of alteration mineral information. Therefore, it is very important to develop alteration mineral information extraction methods from remote sensing images. In this paper, a combined method based on Mask, principal component analysis (PCA) and support vector machine method (SVM) was used to extract alteration mineral information from Enhanced thematic mapper plus remote sensing data with limited spectral and spatial resolutions. First, a mask image of the remote sensing imagery was created to remove interference information such as vegetation, shadow and water. Then, PCA was employed to collect sample data relating to iron, argillic, and carbonatization alteration. Finally, SVM was used to deal with alteration anomaly and build a feature extraction model of high accuracy. The Mask-PCA-SVM model is used to extract alteration mineral information from remote sensing images of Hatu area, Xinjiang Uygur Autonomous Regions, China. The results show that the new methods proposed in this paper can coincide well with known deposits occurrences, rate reached 86.51%. While, the consistent rate with known deposits of the ratio model, PCA model and Spectral angle mapper model were only 3.37, 65.08 and 69.05% respectively. This suggests that the proposed model can find the actual distribution of mineral deposits more effectively by reducing interference to a greater degree.

Analysis of chemical components and spectral characteristics of rock

The old surface and the fresh surface of the rock samples have been subjected to field spectrum test what should be compared indoor control spectrum test, and the mineral element test of the sample and the observation of the sample under thin film have been carried out. The spectral reflectance of old surface and fresh surface shows that the old effect has little effect on the spectral characteristics of rock minerals which affects the size of the reflectance. The difference are between the mineral elements and the spectral characteristics of the samples. The spectral characteristics and spectral indices of different samples have ex-tracted and the data have extracted for hyper-spectral data extraction of alteration information.