Structural style and deformation mechanism of the southern Dabashan foreland fold-and-thrust belt, central China

The Daba Mountains define the southern margin of the East Qinling orogenic belt, and form the boundary of the Sichuan basin in the north and northeast. The Daba Mountains can be divided into two structural belts by the NW-striking Chengkou fault, namely the northern Dabashan thrust-nappe belt and the southern Dabashan foreland fold-and-thrust belt. The southern Dabashan fold-and-thrust belt is a southwestward extruding thin-skinned thrust wedge, showing obvious belted change in deformation style and deformation intensity along the dip direction, and can be divided further into three sub-belts, i.e. the imbricate thrust sub-belt characterized by imbricate stepped-thrust sheets, the thrust-fold sub-belt characterized by the combination of the equally-developed thrusts and related folds, and the detachment-fold sub-belt characterized by box folds and closed overturned-isoclinal folds on the outcrops. Several kinds of structures have been recognized or inferred, including imbricate thrust system, passive-roof duplex (triangle zone), fault-related folds, back-thrust system and pop-up structure. The NE-SW compressive stress from the Qinling orogenic belt and detachment layers in the covering strata are the two most important determinants of deformation style. After the collision between the North China block and Yangtze block at the end of the Middle Triassic, the northward intracontinental subduction along the southern edge of the Qinling orogenic belt was initiated, which led to the corresponding southward thrusting in the upper crust. The thrusting propagated towards the foreland through the Jurassic and extended to the southernmost part of the southern Daba Mountains around the end of the Early Cretaceous, with thrusting deformation to be preferentially developed along major detachment layers and progressing upwards from the Lower Sinian through the Lower Cambrian and Silurian to Middle-Lower Triassic. Translated from Geotectonica et Metallogenia, 2006, 30(3): 294–304 [译自: 大地构造与成矿学]     

Advective heat transfer and fabric development in a shallow crustal intrusive granite — the case of Proterozoic Vellaturu granite,south India

Syntectonic plutons emplaced in shallow crust often contain intermediate-to low-temperature deformation microstructures but lack a high-temperature, subsolidus deformation fabric, although the relict magmatic fabric is preserved. The Proterozoic Vellaturu granite emplaced at the eastern margin of the northern Nallamalai fold belt, south India during the late phase of regional deformation has a common occurrence of intermediate-to low-temperature deformation fabric, superimposed over magmatic fabric with an internally complex pattern. But high-T subsolidus deformation microstructure and fabric are absent in this pluton. The main crystal plastic deformation and fluid enhanced reaction softening was concentrated along the margin of the granite body. Resulting granite mylonites show Y-maximum c-axis fabric in completely recrystallized quartz ribbonds, dynamic recrystallization of perthites, and myrmekite indicative of fabric development under intermediate temperature (∼ 500–400°C). The weakly-deformed interior shows myrmekite, feldspar microfracturing and limited bulging recrystallization of quartz. The abundance of prism subgrain boundaries is indicative of continuing deformation through low-temperature (∼ 300°C). The relative rates of cooling influenced by advective heat transfer and deformation of the pluton seem to control the overall subsolidus fabric development. The rapid advective heat transfer from the interior in the early stages of subsolidus cooling was followed by slow cooling through intermediate temperature window as a well-developed phyllosilicate rich mylonitic skin around the granite body slowed down conductive heat loss. Low-T crystal plastic deformation of quartz was effected at a late stage of cooling and deformation of the shallow crustal granite body emplaced within the greenschist facies Nallamlai rocks.     

青藏高原北部成矿带划分

青藏高原北部地质构造复杂,成矿条件优越,金属矿产资源成群成带分布特征明显,研究程度相对较低。通过对1999年以来,在青藏高原北部空白区相继完成的1∶25万区域地质调查、1∶5万区域地质调查和一批1∶50万、1∶20万区域化探成果资料的综合研究,根据青藏高原北部金属矿产资源的时、空分布特征,结合区域物、化探资料和区域成矿地质背景,将青藏高原北部地区初步划分为3个级成矿带(域),5个级成矿带,9个级成矿带,对以后青藏高原北部地区地质矿产勘查评价工作具有十分重要的意义。     

大井铜-锡多金属矿床地质特征及其成因

通过对大井铜-锡多金属矿床成矿地质环境、矿床地质特征以及对岩（矿）石同位素、稀土元素、成矿温度等地球化学和成矿物理化学环境的研究,认为,大井铜-锡多金属矿床为一典型的岩浆热液充填型矿床。与成矿有关的隐伏岩体受深部隐伏的深大断裂控制,可能与黄岗梁矽卡型铁-锡矿床同属一构造岩浆成矿带上不同成矿环境下形成的矿床。并认为,在该成矿带上及大井矿区深部仍有较大的找矿潜力。     

Andean evolution at the Guañacos and Chos Malal fold and thrust belts (36°30′–37°s)

The Andes between 36°30′ and 37°S represent a Cretaceous fold and thrust belt strongly reactivated in the late Miocene. Most of the features that absorbed Neogene shortening were already uplifted in the late Cretaceous, as revealed by field mapping and confirmed by previous fission track analysis. This Andean section is formed by two sectors: a western-inner sector generated by the closure of the upper Oligocene-lower Miocene intra-arc Cura Mallín basin between the middle and late Miocene (Guañacos fold and thrust belt), and an eastern-outer sector, where late Triassic-early Jurassic extensional depocenters were exhumed in two discrete phases of contraction, in the latest early Cretaceous and late Miocene to the Present, respectively (Chos Malal fold and thrust belt). Late Miocene deformation has not homogeneously reactivated Cretaceous compressive structures, being minimal south of 37°30′S through the eastern-outer sector (southern continuation of the Chos Malal fold and thrust belt). The reason for such an inhomogeneous deformational evolution seems to be related to the development of a late Miocene shallow subduction regime between 34°30′ and 37°45′S, as it was proposed in previous studies. This shallow subduction zone is evidenced by the eastward expansion of the arc that was accompanied by the eastern displacement of the orogenic front at these latitudes. As a result, the Cretaceous fold and thrust belt were strongly reactivated north of 37°30′S producing the major topographic break along the Southern Central Andes.     

Crustal structure using receiver function in the east part of A＇nyemaqen suture belt

Twenty broadband seismographs were deployed along Hongyuan, Sichuan to Wuwei, Gansu. 81 teleseismic events were recorded in one year. We computed receiver functions from teleseismic waveform data and obtained S wave velocity structure beneath each station along the profile by using receiver function inversion method. The results revealed that the crustal structure is very complex and crustal average S wave velocity is to be on the low side. Low velocity structure generally exists in the depth range of 10～40 km in the crust between Aba arc fault and northern edge fault of Qinling earth＇s axis and it is a tectonic feature of complex geological process such as ancient A＇nyemaqen Tethys ocean from closing and side colliding to subducted plate exhumed or thrust rock slice lifted. The Moho is about 50 km depth along the profile and is slightly deeper in the south than in the north.     

Geology and geochemistry of the Bingdaban ophiolitic mélange in the boundary fault zone on the northern Central Tianshan Belt,and its tectonic implications

The properties and tectonic significance of the fault bound zone on the northern margin of the Central Tianshan belt are key issues to understand the tectonic framework and evolutionary history of the Tianshan Orogenic Belt. Based on the geological and geochemical studies in the Tianshan orogenic belt, it is suggested that the ophiolitic slices found in the Bingdaban area represent the remaining oceanic crust of the Early Paleozoic ocean between the Hazakstan and Zhungaer blocks. Mainly composed of basalts, gabbros and diabases, the ophiolites were overthrust onto the boundary fault between the Northern Tianshan and Central Tianshan belts. The major element geochemistry is characterized by high TiO₂ (1.50%–2.25%) and MgO (6.64%–9.35%), low K₂O (0.06%–0.41%) and P₂O₅ (0.1%–0.2%), and Na₂O>K₂O as well. Low ΣREE and depletion in LREE indicate that the original magma was derived from a depleted mantle source. Compared with a primitive mantle, the geochemistry of the basalts from the Bingdaban area is featureded by depletion in Th, U, Nb, La, Ce and Pr, and unfractionated in HFS elements. The ratios of Zr/Nb, Nb/La, Hf/Ta, Th/Yb and Hf/Th are similar to those of the typical N-MORB. It can be interpreted that the basalts in the Bingdaban area were derived from a depleted mantle source, and formed in a matured mid-oceanic ridge setting during the matured evolutionary stage of the Northern Tianshan ocean. In comparison with the basalts, the diabases from the Bingdaban area show higher contents of Al₂O₃, ΣREE and HFS elements as well as unfractionated incompatible elements except Cs, Rb and Ba, and about 10 times the values of the primitive mantle. Thus, the diabases are thought to be derived from a primitive mantle and similar to the typical E-MORB. The diabases also have slight Nb depletion accompanying no apparent Th enrichment compared with N-MORB. From studies of the regional geology and all above evidence, it can be suggested that the diabases from the Bingdaban area were formed in the mid-oceanic ridge of the Northern Tianshan ocean during the initial spreading stage. Supported by the Major State Research Program of PRC (Grant No. 2001CB409801), the National Natural Science Foundation of China (Grant Nos. 40472115 and 40234041) and the State Research Program of China Geological Survey (Grant No. 2001130000-22)     

Zircon SHRIMP U-Pb ages and trace element geochemistry of the Kuhai gabbro and the Dur''''ngoi diorite in the southern east Kunlun tectonic belt, Qinghai, Western China and their geological implications

Timing of the intermediate-basic igneous rocks developed in the area of Kuhai-A'nyêmaqên along the southern east Kunlun tectonic belt is a controversial issue. This paper presents new zircon SHRIMP U-Pb dating data for igneous zircons from the Kuhai gabbro and the Dur'ngoi diorite in the Kuhai-A'nyemaqen tectonic belt, which are 555±9 Ma and 493±6 Ma, respectively. The trace element geochemical features of the Kuhai gabbro and the Dur'ngoi diorite are similar to those of ocean island basalts (OIB) and island arc basalts (IAB), respectively. Thus, the Kuhai gabbro with the age of 555±9 Ma and OIB geochemical features is similar to the Yushigou oceanic ophiolite in the North Qilian orogen, whereas the Dur'ngoi diorite with the age of 493±6 Ma and IAB geochemical features is similar to the island arc volcanic rocks developed in the north Qaidam. The Late Neoproterozoic to Early Ordovician ophiolite complex in the area of Kuhai-A'nyêmaqên suggests that the southern margin of the "Qilian-Qaidam-Kunlun" archipelagic ocean in this period was located in the southern east Kunlun tectonic belt. Therefore, the southern east Kunlun tectonic belt in the early Paleozoic is not comparable to the Mianlüe tectonic belt in the Qinling orogenic belt.