Lower Continental Crusts Involved in the Alpine Orogeny:New Insight from the Santa Lucia Nappe Metasediments(Corsica,France)

In the Alpine Corsica(France),the Santa Lucia Nappe represents a peculiar unit preserving the unique relicts of Paleozoic lower to medium continental crust.It consists of composite basement affected by Permian granulite facies metamorphic conditions unconformably covered by a Late Cretaceous clastic sequence(Tomboni Conglomerate and Tralonca Flysch)affected by polyphase deformation and low-grade-metamorphism.In this work,we present a new reconstruction of the deformation events registered by the Tralonca Flysch during the Alpine orogeny.The D1 phase was testified by rare isoclinal folds.The D2 phase produced a continuous foliation and a map-scale sheath-fold with a top-to-W sense of shear.The D3 phase produced E-verging non-cylindrical folds and S3 crenulation cleavage that is not associated to metamorphic blastesis.We present the first temperature-pressure-deformation(P-T-d)path for the Tralonca Flysch,demonstrating that the Santa Lucia Nappe underwent accretion and then first stage exhumation in the Alpine wedge during the D1 phase with pressure and temperature peaks both occurred under blueschist metamorphic conditions.The D2 phase occurred at lower pressure-temperature conditions during a second stage exhumation.This pressure-temperaturedeformation path is comparable with those of the Lower Units(i.e.,the subducted continental units of Alpine Corsica)suggesting a common geodynamic history.

Triassic Nappe in the Central Part of the Southern Central Asian Orogenic Belt(Ejinaq,NW China):Evidence from Structural Analysis and Geothermochronology

The thrust nappe played an important role in the Mesozoic tectonic evolution of the middle part of the Central Asian Orogenic Belt(CAOB).However,the timing,structural style and kinematic processes of the thrust nappe remain controversial,particularly the detail of the thrust nappe in the Guaizihu region(110 km east of Ejinaq).In this study,we investigate new field mapping,seismic sections,geochronology and low-temperature thermochronometric dating to provide constraints on the history of this thrust nappe in the Chaheilingashun area(northwestern Guaizihu region).The field mapping,seismic sections and structural analysis reveal that the autochthonous system had developed a series of strong fold structures in the upper Permian strata.The allochthonous system mainly contains Devonian monzogranite(U-Pb age,ranges from 386.7 to 389.0 Ma)and Meso–Neoproterozoic schists(the maximum depositional age,~880 Ma),which were thrust upon the upper Permian strata during Middle to Late Triassic.Based on similar rocks,geochronological dating and the Yagan thrust,we suggest that the postulated root zone of this allochthon might have originated from the Huhetaoergai area(40–60 km northwest of the study area).The geochronological results reveal that the lower age limit of this thrust nappe is constrained by the Lower–Middle Triassic syntectonic sediments(tuffaceous sandstone,~247 Ma),which is the sedimentary response of the fold structure.,The timing of the termination of this thrust nappe is defined by the cooling age(^(40)Ar/^(39)Ar data,217–211 Ma)of the Devonian monzogranite and Meso–Neoproterozoic schists.Thus,we consider this thrust event in the study area to potentially have occurred in the period from 247 Ma to 211 Ma,which may represent the tectonic response to the closure of the Paleo-Asian Ocean.

Early Cretaceous Thrust and Nappe Tectonics in North Qilian Shan,Northern Tibetan Plateau:Evidence from Field Mapping,Geochronology,and Deep Structural Analysis

The North Qilian Shan fold and thrust belt,located at the northern Tibetan Plateau and southern margin of the Hexi Corridor,is a key tectonic unit to decode the formation and expansion of the plateau.Previous studies emphasize the Cenozoic deformation due to the far-field response to the Indo-Asian collision,but the Mesozoic deformations are poorly constrained in this area.We conducted detailed field mapping,structural analysis,geochronology,and structural interpretation of deep seismic reflectional profiling and magnetotelluric(MT)sounding,to address the superposed results of the Mesozoic and Cenozoic deformation.The results recognized the North Qilian thrust and nappe system(NQTS),the root and the frontal belt are the North Qilian thrust(NQT),and the Yumu Shan klippe(YK),respectively.The middle belt is located between the NQT and the YK.Monzonitic granite zircon U-Pb dating from the middle belt yields an age of ca.415 Ma,which is similar to south NQT.The thrusting displacement is estimated at ca.48 km by structural interpretation of deep profiles.The timing is constrained in the early stage of the Early Cretaceous by the formation of simultaneous growth strata.We suggest that the NQTS has resulted from the far-field effect of the Lhasa-Qiangtang collision,and the Yumu Shan is uplifted by the superposed Cenozoic deformation.

A Large-scale Tertiary Salt Nappe Complex in the Leading Edge of the Kuqa Foreland Fold-Thrust Belt, the Tarim Basin, Northwest China

The tectono-stratigraphic sequences of the Kuqa foreland fold-thrust belt in the northern Tarim basin, northwest China, can be divided into the Mesozoic sub-salt sequence, the Paleocene-Eocene salt sequence and the Oligocene-Quaternary supra-salt sequence. The salt sequence is composed mainly of light grey halite, gypsum, marl and brown elastics. A variety of salt-related structures have developed in the Kuqa foreland fold belt, in which the most fascinating structures are salt nappe complex. Based on field observation, seismic interpretation and drilling data, a large-scale salt nappe complex has been identified. It trends approximately east-west for over 200 km and occurs along the west Qiulitag Mountains. Its thrusting displacement is over 30 km. The salt nappe complex appears as an arcuate zone projecting southwestwards along the leading edge of the Kuqa foreland fold belt. The major thrust fault is developed along the Paleocene-Eocene salt beds. The allochthonous nappes comprise large north-dipping faulting monoclines which are made up of Paleocene-Pliocene sediments. Geological analysis and cross-section restoration revealed that the salt nappes were mainly formed at the late Himalayan stage (c.a. 1.64 Ma BP) and have been active until the present day. Because of inhomogeneous thrusting, a great difference may exist in thrust displacement, thrust occurrence, superimposition of allochthonous and autochthonous sequences and the development of the salt-related structures, which indicates the segmentation along the salt nappes. Regional compression, gravitational gliding and spreading controlled the formation and evolution of the salt nappe complex in the Kuqa foreland fold belt.

Mechanical genesis of Henan(China) Yima thrust nappe structure

Considering the serious coal and rock dynamic disasters around the main slip plane called F16 in the coal mining area) of Henan Yima(China) thrust nappe structure,the mechanical genesis of the Yima thrust nappe structure was studied comprehensively using geomechanics,fault mechanics,elastic mechanics,and Coulomb's law of friction.First,using the centrifugal inertia force of Earth's rotation as a source,a mechanical model of N-S compression superimposed with W-E reverse torsion was established to explain the formation of the early Yima coal basin and Jurassic Yima Group coal measures.Second,an equation for the ultimate stress in the forming stage of F16 was derived using the plastic slip-line field theory and the parabolic Mohr failure criterion.Moreover,the distribution of ultimate stress and the geometric characteristics of the fault profile were obtained using the field model parameters.Finally,the stress field of F16 and the mechanical genesis of the large-scale reverse thrust sheet were discussed based on elastic mechanics theory and Coulomb's law of friction.The results show that the tectonic framework of the early Yima coal basin and the formation pattern of Jurassic Yima Group coal measures given by the model are consistent with the in-situ explorations.The geometric characteristics of the fault profile obtained by numerical calculation can better reflect the shape of F16 in its forming stage,and the mechanical genesis of the large-scale reverse thrust sheet also concurred with the field situations.Thus,this work can provide a foundation for further studies on the genesis of the thrust nappe structure,the mechanism of rock bursts induced by F16,and the characteristics of the residual stress field in the Yima mining area.

The Origin of the 2008 Wenchuan Earthquake Determined by the Analysis on the Active Longmenshan Nappe in Terms of Rockmass Mechanics

On 12 May 2008, the magnitude 8.0 Wenchuan Earthquake occurred along the Longmen Shan nappe, Sichuan, China. This devastating earthquake led to a heavy death toll of greater than 80,000. The seismic origin of this earthquake is currently hotly debated. We suppose that it is a special type of intraplate earthquake called an active-nappe-type earthquake. Using a holistic methodology, incorporating rockmass structure cybernetics and Byerlee's law, we present a comprehensive study on the geological origin of macroseisms in the Longmen Shan area and the seismic origin of the 2008 Wenchuan earthquake. Previous studies of neotectonic activity indicate that the Longmen Shan nappe moves at a rate of 1~3 mm/yr, due to horizontal compressive stress from the Tibetan Plateau. The difference between movement rates in the Bayankala block, Longmen Shan nappe and Sichuan Basin cause slow shear stress and strain accumulation in the Longmen Shan nappe. It is exhibited a relatively simple linear relations for the shear strength and the buried depth of the structural planes, and the detachment layer of the nappe has a higher shearing-sliding strength compared to the overlying fault planes and the underlying ductile shear belts, thus making it more prone to stick-slip deformation. Therefore, the detachment layer was the main section responsible for the Wenchuan earthquake. The initial rupture burst in the detachment layer under the Yingxiu-Beichuan fault, the rupture area nearly 1.4454 × 104 km2,encompassed the cross point of the Yingxiu and the Anxian-Guanxian faults with the detachment layer, then caused the Yingxiu-Beichuan and Anxian-Guanxian faults took an active part in this earthquake, so this earthquake might consist of three chain-like earthquake stages, totally increasing the duration of this earthquake an unusually large amount, to 120 s. The focal depth spanned range of 10-20km,consistent with the observed result of this focal depth by several agencies.

THE THRUST AND NAPPE TECTONIC ZONE ALONG THE SOUTHERN MARGIN OF THE COAL-FORMING REGION OF NORTH CHINA

A thrust and nappe tectonic zone with imbricate branch thrusts is developed along the southern margin of the coal-forming region of North China. This tectonic zone is tightly related to the Qinling-Dabie collision orogen in genesis and belongs to the frontal zone of a huge thrust system developed during Yanshanian episode at the northern foot of the orogen. It is pointed out that thrusting had distorted the original depositional margin of the coal-forming region and some new coal-bearing blocks would be found out in the frontal sheets and under the undulate sole thrust.

The Arcuate Nappe Structure on the Northern Margin of the Wuliang Moutnains in Western Yunnan

The arcuate nappe structure on the north edge of the Wuliang Mountains in westernYunnan Province is a complex nappe structural system with multiple superimposed structures.The autochthonous system is a WNW-trending arcuate fold belt consisting of the Jurassic andCretaceous and the allochthonous system is mainly composed of Upper Triassic rocks. Generally,the nappe structure moved from south to north, with the hanging wall thrusting in a WNW direc-tion for a distance of over 10km. The deep nappe structural system was formed at depths ofabout 5-10km in an environment not exceeding the greenschist facies. It occurred in theOligocene (about 40-20 Ma).

Zircon age depth-profiling sheds light on the early Caledonian evolution of the Seve Nappe Complex in west-central J?mtland

The Scandinavian Caledonides comprise nappe stacks of far-travelled allochthons that record closure of the Iapetus Ocean and subsequent continental collision of Baltica and Laurentia.The Seve Nappe Complex(SNC)of the Scandinavian Caledonides includes relics of the outermost Baltoscandian passive margin that were subducted to mantle depths.The earliest of the deep subduction events has been dated to ca.500-480 Ma.Evidence of this event has been reported from the northern exposures of the SNC.Farther south in the central and southern segments of the SNC,(ultra)high-pressure rocks have yielded younger ages in the range of ca.470-440 Ma.This study provides the first record of the early Caledonian event in the southern SNC.The evidence has been obtained by depth profiling of zircon grains that were extracted from the Tv?r?klumparna microdiamond-bearing gneiss.These zircon grains preserve eclogite facies overgrowths that crystallized at 482.6±3.8 Ma.A second,chemically-distinct zircon overgrowth records granulite facies metamorphism at 439.3±3.6 Ma,which corroborates previous geochronological evidence for granulite facies metamorphism at this time.Based on these results,we propose that the entire outer margin of Baltica was subducted in the late Cambrian to early Ordovician,but the record of this event may be almost entirely eradicated in the vast majority of lithologies by pervasive late Ordovician to early Silurian metamorphism.

Characteristics of Nappes and Segmentation of the Longmen Mountains Thrust Belt, Western Sichuan Basin, China

In order to reveal the nature of the segmentation of Longmen Mountains Thrust Belt caused by the three nappes (Jiaoziding, Jiudingshan, and Baoxing Nappe), several methods are applied in this paper, including field investigation, seismic explanation and balanced crossed section, etc. Results of research reveal that nappes in Longmen Mountains vary in geometry, kinematics, and dynamics. Jiaoziding Nappe has generally behaved in a ductile manner, whereas Jiudingshan Nappe has been rigid, and the rheology of Baoxing Nappe has been intermediate between that of the other two nappes. The development of nappes has resulted in tectonic segmentation of Longmen Mountains: the main structural style of the northern segment is thrust faulting, with Jiaoziding Nappe representing a giant syncline. Given its ductility, it absorbed lots of stress, with the least amount of tectonic shortening in the SE part of the nappe. In the middle segment, the deformation is controlled by the rigid Jiudingshan Nappe, whose frontal area records lots of tectonic shortening. Deformation in the southern segment is intermediate in character between that of the other two segments, characterized by horizontal zonation, as demonstrated by fault development, and vertical stratification, which indicates that fault development was controlled by lithology.

TRANSPRESSIVE DEFORMATION OF THE LONGXINGGOU NAPPE AND ITS SIGNIFICANCE,SOUTH OF TONGBAI MOUNTAINS, CENTRAL CHINA

The Longxinggou Nappe which is located in Guangshui county, north of Hubei province, has been first identified by our detail mapping. It is composed of lowgrade metamorphosed rocks of middle Proterozoic Shuixian group, late Proterozoic Yaolinghe group, Sinian Liantuo,Doushantuo and Dengying formations. It is quite different from the lower beds, middle.Proterozoic Hong’an epidote-amphibole facies metamorphose metapelite and metagranite. They are different not only in composition, metamorphic degree, but also in deformation forms. The Longxinggou nappe has characteristics both of ductile thrusting and ductile dextral strike-sliping, illustrating a transpressive deformation regime of middle crustal in the orogenic belt due to the oblique collision, between the Shuiying terrain and the Tongbai terrain during Caledonian period.

THRUST NAPPE STRUTRUE IN THE JINGGANG MOUNTAIN: CHARACTERISTICS AND SIGNIFICANCE FOR PROSPECTION OF ORE DEPOSITS

Nappe structure, as was first discovered by the authors during the regional geological survey at the scale of 1:50,000 in The Jinggang Mountain, is mainly comprised of a series of NNE-NE-striking thrust fault zones and thrust sheets among them. Sinian, Cambrian, Ordovician, Devonian, Carboniferous,Triassic, Jurassic and Cretaceous strata are involved in the thrust nappe system. The nappe structure is of the type of duplex structures formed as a result of the earlier stage migration from SE to NW and late stage migration from E to W of sedimentary cover or basement strata. Formation of the nappe structure in the studied area involves two main epochs: Early Yanshanian and Late Yanshanian to Early Himalayan. The mineral deposits and the buried coalfields in the area, especially the latter, are extensively controlled by the nappe structure.

Analysis of Nappes and Glided Nappes in China

ＡｎａｌｙｓｉｓｏｆＮａｐｐｅｓａｎｄＧｌｉｄｅｄＮａｐｐｅｓｉｎＣｈｉｎａ￥ＹａｎｇＫｅｓｈｅｎｇ（ＢｕｒｅａｕｏｆｏｉｌＧｅｏｐｈｙｓｉｃａｌＰｒｏｓｐｅｃｔｉｎｇ）Ｋｅｙｗｏｒｄｓ：Ｎａｐｐｅ．Ｓｔｒｕｃｔｕｒｅ．Ｔｈｒｕｓｔｆａｕｌｔ．Ｔｅｃ...

Experimental Investigation of Discharge Capacity of Labyrinth Weirs with and without Nappe Breakers

Labyrinth weirs provide higher discharge capacity than conventional weirs, with the ability to pass large flows at comparatively low heads. Labyrinth weirs are primarily used as spillways for dams where the spillway width is restricted. In recent years, many research investigations have considered the hydraulic performance of labyrinth weirs, particularly as dependent on the geometric features. The previous work has improved the design basis for such weirs. However, their design still requires experimentally derived and generalized performance curves. It is especially important to observe the behavior of the weir nappe to ensure the design provides hydraulic optimization and to account for pressure fluctuations, possible vibrations, resonance effect, noise and flow surging. In the present study, discharge coefficients were experimentally determined for both circular labyrinth weirs and sharp crested trapezoidal labyrinth weirs of varying side wall angle (α). Additional studies were completed with nappe breakers included to reduce the impact of vibration on the labyrinth weirs. In general, the test data indicated that nappe breakers placed on the trapezoidal labyrinth weirs and circular labyrinth weirs reduced the discharge coefficient by up to 4% of the un-amended weir.

AN ENORMOUS THRUST NAPPE AND EXTENSIONAL METAMORPHIC CORE COMPLEX NEWLY DISCOVERED IN SINO-MONGOLIAN BOUNDARY AREA

An enormous thrust nappe and a metamorphic core complex of extensional origin have been recently discovered within the Hercynian-Indosinian orogen of Inner Mongolia. Both features are of the Mesozoic age, but the former is older. The thrust nappe strikes WNW-ESE and Proterozoic dolomite was thrust southwards atop strata ranging from the Cambrian to the Triassic in age. The visible displacement of the thrust is ca. 70 km and the deduced one is over 140 km. The metamorphic core complex (mainly mylonites) extends in an N-E direction, forming itself into a dome geometry and lying under an extensional detachment fault. The mylonitic foliation dips gently and the stretching mineral lineation, as a whole, plunges SSE. Various shear sense markers show a normal-sense shear movement. Some significant problems atise from the coexistence of nappe and extensional structures at a postoro-genic stage.

Measurement of terrace deformation and crustal shortening of some renascent fold zones within Kalpin nappe structure

The Kalpin nappe structure is a strongest thrust and fold deformation belt in front of the Tianshan Mountains since the Cenozoic time. The tectonic deformation occurred in 5―6 striking Meso-zoic-Cenozoic fold zones, and some renascent folds formed on the recent alluvial-proluvial fans in front of the folded mountains. We used the total station to measure gully terraces along the longitudinal to-pographic profile in the renascent fold zones and collected samples from terrace deposits for age de-termination. Using the obtained formation time and shortening amount of the deformed terraces, we calculated the shortening rate of 4 renascent folds to be 0.1±0.03 mm/a, 0.12±0.04 mm/a, 0.59±0.18 mm/a, and 0.26±0.08 mm/a, respectively. The formation time of the renascent folds is some later than the major tectonic uplift event of the Qinghai-Tibet Plateau 0.14 Ma ago. It may be the long-distance effect of this tectonic event on the Tianshan piedmont fold belt.

Study of thrust and nappe tectonics in the eastern Jiaodong Peninsula, China

Thrust and nappe tectonics have affected the eastern Jiaodong Peninsula, the easternmost terminal of the Sulu Ultra-high Pressure Metamorphic Belt. Four nappes have been mapped, named respectively the Shidao, Rongcheng, Mishan and Mouping nappes. The methods used included multi-scale struc- tural analysis and structural chronology analysis. These nappes define four deep level slip-thrust shear zones that were mainly active in the Mesozoic. The amount of ductile deformation decreases from the Shidao to Rongcheng to Mouping to Mishan shear zones, and shows an inverse relationship with temperature. 40Ar/39Ar chronological analysis and the chronological results of former workers reveal four movement steps defined by the development of thrusts and nappes in the late Triassic (210-180 Ma), extensional movement from the Jurassic to early Cretaceous (180-130 Ma), slip-thrust movement in the Early Cretaceous (130-120 Ma), and extensional movement since the Late Cretaceous (120 Ma). The order of boundary shear zone motion in the period of slip-thrust movement during the Early Cre- taceous (130-120 Ma) was along the Shidao, Rongcheng, Mouping and finally the Mishan shear zone. This resulted in clockwise rotation of the nappes relative to block west to the Tan-Lu Faults. Because of the similar evolutionary history of the Tan-Lu Faults and the thrust and nappe structure in the eastern Jiaodong Peninsula, slip dislocation along the Tan-Lu Faults might have been absorbed by thrust and nappe tectonics in the Jiaodong area in the Mesozoic era, resulting in much less dislocation on the Tan-Lu faults in North Eastern China than that in south along the Jiaodong Peninsula.

Mesozoic Thrust-Nappe and Extensional Structure Frameworks in the East Segment of Southeast Yangtze Block, Southeast China

Multi-stage Mesozoic thrust-nappe and extensional structures are distributed in the east segment of the Southeast Yangtze Block situated in the junction region of Zhejiang-Jiangxi-Anhui provinces. The features and genetic mechanism of the deformations were analyzed after a detailed field observation of their distribution, geometry, and kinematics. In addition, the time sequences of the thrust and extensional structures were determined by combining the results of the comparative analysis with the chronological evidence of strata and magmatic rocks cut by a fault or formed after a fault according to field facts. This study identified three stages of the nappe structures and at least two stages of the extensional structures during the Mesozoic. The geotectonic setting of the nappe and extensional structures was considered to be related to the different geodynamics in the study area including the Early Mesozoic geological event, i.e., N-S compression, forming Lantian fault, etc.;the Late Mesozoic flat-slab subduction, forming Xiaoxi thrust fault and tectonic window;and the roll-back of the paleoPacific Plate, forming extensional structures like basin marginal fault;the last compression, forming Wucheng-Shenxian fault. These findings provide additional evidence for remodeling the tectonic and geodynamic evolution of Southeast China.

A DISCOVERY OF VERTICAL ZONATION OF THE ROCKS IN THE DEEP-LEVEL NAPPE STRUCTURAL ZONE IN WESTERN GUANGDONG

There are many Sinian-Cambrian (Z^c-∈_b) epimetamorphic blocks either in groups or scatteredly covering the granitoids in the area of Guangning-Zhaoqing, the northwestern part of Wuchuan-Sihui faults in western Guangdong (Fig. 1). It has been shown that the blocks are allochthonous, i. e. klippes of thrust, and the granitoids are of a part of the slip system which consists vertically of mylonites, migmatites and granites from top to bottom in the deep-level nappe structural zone. Based on this discovery, the authors suggest a new model of fractural metamorphism (thermodynamic metamorphism).

XU-HUAI NAPPE

The Xu-Huai nappe extends from its northeastern end Taierzhuangin Shandong Province through Xuzhou in Jiangsu Province to Huaibei in Anhui Province. It has been found to be a complex one and contains two generations of folds, the earlier one of which presents itself as a recumbent or recline fold. Two kinds