Guowei Zhang

Geologic framework and tectonic evolution of the Qinling orogen, central China

Abstract The geologic framework of the Qinling orogen was built up through interplay of three blocks, the North China block (including the North Qinling), the South Qinling, and the South China block, separated by the Shangdan and Mianlue sutures. The Shangdan suture resulted from Middle Paleozoic collision of the North China block and the South Qinling. The Mianlue suture resulted from Late Triassic collision of the South Qinling and the South China block. Present upper crust of the Qinling is structured dominantly by thrust–fold systems. The North Qinling displays thick-skinned deformation with crystalline basement involved, whilst the South Qinling is characterized by thin-skinned thrusts and folds detached above the Lower Sinian. Two types of Precambrian basement, crystalline and transitional, are defined according to lithology and metamorphic grade and different in age. Stratigraphic and sedimentary architecture is characterized by distinct zonation. The Qinling orogen experienced a prolonged continental divergence and convergence between blocks. During the period from Late Neoproterozoic to Early Paleozoic times, the South Qinling was the northern margin of the South China block, and the North Qinling was the southern margin of the North China block, separated by a Proto-Tethyan Qinling Ocean. The North Qinling evolved into an active margin when the Proto-Tethyan Qinling Ocean subducted northward during Ordovician time. Collision of the South and North Qinling took place in Middle Paleozoic along the Shangdan suture. Synchronous with the collision, rifting occurred at the southern rim of the South Qinling and was followed by the opening of the Paleo-Tethyan Qinling Ocean during the Late Paleozoic, resulting in the splitting of the South China block from the South Qinling. Collision of the South Qinling and the South China block came about in the Late Triassic along the Mianlue suture. The Late Triassic collisional orogeny caused extensive fold-and-thrust deformation and granitoid intrusions throughout the Qinling, and led to final amalgamation of the North and South China blocks.

TIMING OF COLLISION OF THE NORTH AND SOUTH CHINA BLOCKS : CONTROVERSY AND RECONCILIATION

The Qinling orogen was formed by the joining of the North and South China blocks, but the timing of their integration has been debated for more than a decade. The controversies obviously stem from different approaches to reconstruction of the integration history. Two contrasting lines of evidence yield two different ages for collision of the North and South China blocks—middle Paleozoic and Late Triassic. The Shangdan suture within the Qinling was regarded in previous studies as the trace along which the North and South China blocks collided. Our studies, however, demonstrate that there are two sutures within the Qinling: the well-documented Shangdan suture and the newly discovered Mianlue suture. We show in this paper that the Late Proterozoic to early Mesozoic evolution of the Qinling involved interactions between the North China block, the North and South Qinling orogens, and the South China block. The middle Paleozoic collision along the Shangdan suture, as constrained by some evidence, accreted only the South Qinling orogen to the southern part (i.e., the North Qinling) of the North China block. Contemporaneous rifting of the South China block and subsequent drifting separated the South Qinling from the South China block during the middle to late Paleozoic. The separation of the South from the North China blocks is supported by other evidence, in particular, geomagnetic data. Evidently it was the Late Triassic collision of the South China block with the South Qinling orogen along the Mianlue suture that led to final integration of the North and South China blocks.

Tectonics of South China continent and its implications

This paper aims at exploring the tectonic characteristics of the South China Continent (SCC) and extracting the universal tectonic rules from these characteristics, to help enrich the plate tectonic theory and better understand the continental dynamic system. For this purpose, here we conduct a multi-disciplinary investigation and combine it with the previous studies to reassess the tectonics and evolution of SCC and propose that the tectonic framework of the continent comprises two blocks, three types of tectonic units, four deformation systems, and four evolutionary stages with distinctive mechanism and tectonic characteristics since the Neoproterozoic. The four evolutionary stages are: (1) The amalgamation and break-up of the Neoproterozoic plates, typically the intracontinental rifting. (2) The early Paleozoic and Mesozoic intracontinental orogeny confined by plate tectonics, forming two composite tectonic domains. (3) The parallel operation of the Yangtze cratonization and intracontinental orogeny, and multi-phase reactivation of the Yangtze craton. (4) The association and differentiation evolution of plate tectonics and intracontinental tectonics, and the dynamic characteristics under the Meso-Cenozoic modern global plate tectonic regime.

Mid-paleozoic collision in the north Qinling: Sm-Nd, Rb-Sr and 40 Ar/ 39 Ar ages and their tectonic implications

Abstract Different isotopic geochronometers have been used to study the nature of the mid-Paleozoic tectonothermal event in the North Qinling arc system at the southern margin of the North China Block. 40Ar/39Ar age of amphibole from sample QH-3 (426±2xa0Ma), Rb–Sr mineral isochron age (411±5xa0Ma) of sample QSP-10 of the Danfeng Group and Rb–Sr whole rock isochron age (406±22xa0Ma) of a greenschist facies pillow lava sample of the Erlangping Group indicate a widespread Late Silurian tectonothermal event in the North Qinling area. The data also suggest a limited uplift in the North Qinling during the Late Silurian tectonothermal event. This event is restricted to the North Qinling area and the southern margin of North China Block, and not observed in the South Qinling belt and the northern margin of the South China Block. Consequently, it is related to the collision between the North China Block and a North Qinling arc system, which resulted in the closure of the North Qinling back-arc basin(s).

An ophiolitic tectonic melange first discovered in Huashan area, south margin of Qinling Orogenic Belt, and its tectonic implications

On the basis of the synthetic studies of geology and geochemistry, an ophiolitic tectonic melange waa discovered in Sanligang-Sanyang area, the western part of Xiangfan-Guangji fault, the south margin of the Qinling Orogenic Belt. It is composed of different tectonic blocks with different lithological features and ages, mainly including the Huashan ophiolite blocks, Xiaofu Island-arc volcanic blocks, pelagic sediments, fore-arc volcanic-sedimentary system, and the massif of the basement and the covering strata of the Yangtze Block. These massifs were emplaced in the western part of Xiangfan-Guangji fault, the boundary between the Qinling Orogenic Belt and Yangtze Block, contacting each other by a shear zone or chaotic matrix. The characteristics of geochemistry indicate that the bash of the Huashan ophiolite are similar to mid-oceanic ridge basalts (MORB) formed in an initial oceanic baain, and the Xiaofu volcanic rocks are formed in a tectonic setting of island arc. The ophiolitic tectonic melange is the fragments of subduction wedge, which implies that there has been an oceanic basin between Qinling Block and Yangtze Block.

Orogen-Parallel Westward Oblique Uplift of the Qinling Basement Complex in the Core of the Qinling Orogen (China): An Example of Oblique Extrusion of Deep-Seated Metamorphic Rocks in a Collisional Orogen

Both vertical uplift and lateral extrusion of rocks in convergent orogens have been widely documented. This article, using the basement core of the Qinling orogen as an example, demonstrates orogen‐parallel westward uplift with both vertical and horizontal displacement components. The Qinling complex of the Qinling orogen is a tectonic slice of a Precambrian crystalline basement. It is an elongate lens between younger terranes bounded by the Shangdan suture to the south and by the Zhuxia zone to the north. During the middle Paleozoic, the complex underwent large‐scale orogen‐parallel westward oblique ductile uplift. Supporting evidence for this includes the following: (1) opposite shear senses of the two bounding shear zones, i.e., the thrust‐sinistral shear on the south and thrust‐dextral shear on the north; (2) well‐developed eastward plunging (orogen‐parallel) A‐type structures in the complex, such as stretching lineation and sheath folds; (3) strong decompression metamorphism of the complex from 0.66 GPa to 0.45 GPa and eclogites decompressed from 1.2 GPa to 0.5 GPa; and (4) eastward migration in the emplacement of synkinematic granitic plutons. Integrated analysis of PT evolution paths suggests a vertical uplift of the basement complex relative to adjacent Paleozoic supracrustal rocks ranging from 15 to 40 km. Coeval west‐directed horizontal displacement is then estimated to range from 32 to 86 km and total oblique displacement from 36 to 95 km, using an average 25° plunge of mineral stretching lineations in the boundary shear zones. This oblique uplift could be explained by soft oblique extrusion derived from a scissors‐like (or zipped) subduction/collision between the North and South Qinling belts. Because plate boundaries are usually irregular and nonparallel both in the vertical and horizontal, orogen‐parallel oblique uplift (soft oblique extrusion) may be widespread in collisional orogens.

Identification of the island-arc magmatic zone in the Lianghe-Raofeng-Wuliba area; south Qinling and its tectonic significance

Detailed studies indicate that a typical island-arc magmatic zone exists in the Lianghe-Raofeng-Wuliba area of south Qinling. This is characterized by continental marginal andesite and bimodal volcanic rock association which was formed in a rift environment within an oceanic island-arc. The island-arc magmatic zone is the product of Devonian-Carboniferous oceanic crust sub-duction and rifting of Mianlue ancient oceanic basin. The presence of the island-arc magmatic zone suggests that the Mianxian-Lueyang suture zone had extended to the Bashan arcuate area. The Sunjiahe volcanic rock association of the Xixiang Group formed in a volcanic-arc setting which should have a close relationship with Mianlue suture zone.Detailed studies indicate that a typical island-arc magmatic zone exists in the Lianghe-Raofeng-Wuliba area of south Qinling. This is characterized by continental marginal andesite and bimodal volcanic rock association which was formed in a rift environment within an oceanic island-arc. The island-arc magmatic zone is the product of Devonian-Carboniferous oceanic crust subduction and rifting of Mianlue ancient oceanic basin. The presence of the island-arc magmatic zone suggests that the Mianxian-Lueyang suture zone had extended to the Bashan arcuate area. The Sunjiahe volcanic rock association of the Xixiang Group formed in a volcanic-arc setting which should have a close relationship with Mianlue suture zone.

Geological and geochemical constraints on genesis of the Liziyuan gold-dominated polymetal deposit, western Qinling orogen, central China

The Liziyuan gold deposit, located on the northern margin of the western Qinling orogen (WQO), consists of five mineralized sites hosted by metavolcanic rocks, and one hosted by the Tianzishan monzogranite. Orebodies mainly occur as lenticular veins along NW-striking dextral ductile strike–slip shear zones. Major wall rock alteration includes silicification, pyritization, and carbonation, progressively increasing in intensity towards the orebodies. Ore minerals are dominated by pyrite, galena, and chalcopyrite; the major gangue minerals are quartz and calcite. Native gold is present chiefly as separate phases in sulphide and quartz microfractures. We recognize four stages of mineralization: (I) pyrite-quartz, (II) native gold-chalcopyrite-pyrite-quartz, (III) pyrite-freibergite-galena-quartz-carbonate, and (IV) carbonate stages. Fluid inclusion petrography and microthermometric results suggest that three types of primary fluid inclusions (carbonic, CO2–H2O, and aqueous) are present in the deposit. Microthermometric data and Laser Raman analyses indicate that the ores were deposited from moderate temperature (240–280°C), low salinity (0.5–9.1 wt.% NaCl equiv.) H2O–NaCl–CO2–(CH4) fluids. The common coexistence of aqueous, CO2–H2O, and carbonic inclusions indicates the trapping of heterogeneous immiscible fluids. Trace elements in mineralized quartz and carbonate veins suggest that the ore-forming fluids were mainly produced by prograde metamorphism of the upper crust. The calculated and δD values are 1.8–3.8‰ and –77‰ to –75‰, respectively, for quartz from stage II and –8.5‰ to –6.6‰ and –72‰ to –71‰ for calcite from stage III, implying that the ore-forming fluids were metamorphic in origin and evolved by late-stage mixing with meteoric water. Pyrite and galena have δ34S values of 3.9–8.5‰ with a pronounced mode at 5–8‰. Such relatively homogeneous sulphur isotopic compositions are consistent with orogenic gold deposits throughout the world, indicating that the sulphur was mainly sourced from reduced metamorphic fluids. The Liziyuan gold deposit can be grouped with other orogenic gold deposits that have similar characteristics. Geological observations and available isotopic ages suggest that the Liziyuan lode mineralization was related to the Indosinian orogenesis of the Qinling belt. Triassic continental collision caused crustal thickening, dextral ductile shearing, and greenschist facies metamorphism throughout the orogen. Due to the intensive deformation and metamorphism, crustal rocks released copious fluids that evidently mobilized and extracted ore elements along their flow pathways. The ductile shear zone in the Liziyuan mining area provided regions of low mean stress and high permeability, and thus appeared to be preferential channels for precipitation of the ore-forming fluids. As highly charged fluids migrated into the high permeability shear zones, rapid changes in physicochemical conditions resulted in sulphide precipitation and gold mineralization.

Seismic tomography and anisotropy of the Helan‐Liupan tectonic belt: Insight into lower crustal flow and seismotectonics

We applied the tomographic methods of Zhao et al. [1, 2] and Wang and Zhao [3] to our data set to study the detailed 3-D Pand S-wave velocity (Vp, Vs) and Poisson’s ratio (σ) images as well as P-wave azimuthal anisotropy in the crust and uppermost mantle beneath the Helan-Liupan tectonic belt (HLTB) and adjacent regions. The data set used in this study consists of 38,880 P-wave and 35,117 S-wave arrival times from 5,028 local earthquakes recorded by 66 seismic stations in the study area during 1980 to 2014.

West Pacific and North Indian Ocean Seafloor and Their Ocean‐Continent Connection Zones: Evolution and Debates

The Indian Ocean and the West Pacific Ocean and their ocean-continent connection zones are the core area of the Belt and Road. Scientific and in-depth recognition to the natural environment, disaster distribution, resources, energy potential of “the Belt and Road” development, is the cut-in point of the current Earth science community to serve urgent national needs. This paper mainly discusses the following key tectonic problems in the West Pacific and North Indian oceans and their ocean-continent connection zones (OCCZs): 1. modern marine geodynamic problems related to the two oceans. Based on the research and development needs to the two oceans and the ocean-continent transition zones, this item includes the following questions. (1) Plate origin, growth, death and evolution in the two oceans, for example, 1) The initial origin and process of the triangle Pacific Plate including causes and difference of the Galapagos and West Shatsky microplates; 2) spatial and temporal process, present status and trends of the plates within the Paleoor Present-day Pacific Ocean to the evolution of the East Asian Continental Domain; 3) origin and evolution of the Indian Ocean and assembly and dispersal of supercontinents. (2) Latest research progress and problems of mid-oceanic ridges: 1) the ridge-hot spot interaction and ridge accretion, how to think about the relationship between vertical accretion behavior of thousands years or tens of thousands years and lateral spreading of millions years at 0 Ma midoceanic ridges; 2) the difference of formation mechanisms between the back-arc basin extension and the normal mid-oceanic ridge spreading; 3) the differentials between ultra-slow Indian Ocean and the rapid Pacific spreading, whether there are active and passive spreading, and a push force in the mid-oceanic ridge; 4) mid-oceanic ridge jumping and termination: causes of the intra-oceanic plate reorganization, termination, and spatial jumps; 5) interaction of mantle plume and mid-oceanic ridge. (3) On the intraoceanic subduction and tectonics: 1) the origin of intra-oceanic arc and subduction, ridge subduction and slab window on continental margins, transform faults and transform-type continental margin; 2) causes of the large igneous provinces, oceanic plateaus and seamount chains. (4) The oceanic core complex and rheology of oceanic crust in the Indian Ocean. (5) Advances on the driving force within oceanic plates, including mantle convection, negative buoyancy, trench suction and mid-oceanic ridge push, is reviewed and discussed. 2. The ocean-continent connection zones near the two oceans, including: (1) Property of continental margin basement: the crusts of the Okinawa Trough, the Okhotsk Sea, and east of New Zealand are the continental crusts or oceanic crusts, and origin of micro-continent within the oceans; (2) the ocean-continent transition and coupling process, revealing from the comparison of the major events between the West Pacific Ocean seamount chains and the continental margins, mantle exhumation and the ocean-continent transition zones, causes of transform fault within back-arc basin, formation and subduction of transform-type continental margin; (3) strike-slip faulting between the West Pacific Ocean and the East Asian Continent and its temporal and spatial range and scale; (4) connection between deep and surface processes within the two ocean and their connection zones, namely the assembly among the Vol. 91 No. 6 pp.2283–2301 ACTA GEOLOGICA SINICA (English Edition) Dec. 2017 * Corresponding author. E-mail: Sanzhong@ouc.edu.cn