The Influence of the Closure of the East Paleo-Tethys Ocean on Southern South China:Evidences from Kinematics and^(40)Ar/^(39)Ar Geochronology of the Rongxian Ductile Shear Zone in Southeastern Guangxi

The Triassic was a crucial period in the tectonic evolution of the South China Block.Research on tectonic deformation during this period provides information on intracontinental orogenic mechanisms in South China.In this study,alongside thermochronological analyses,we examine the macroscopic and microscopic structural features of the Rongxian ductile shear zone,located south of the Darongshan granite in the southeastern part of Guangxi Province,on the southern margin of South China.Sinistral shear is indicated by the characteristics of rotatedσ-type feldspar porphyroclasts,stretching lineations defined by elongated quartz grains and the orientations of quartz c-axes.LA-ICP-MS U-Pb dating of zircons from two samples of granitic mylonite and one of granite yielded ages of ca.256 Ma.Furthermore,two samples of granitic mylonite yield muscovite^(40)Ar/^(39)Ar plateau ages of 249-246 Ma.These results indicate that the Rongxian ductile shear zone resulted from Early Triassic deformation of the late Permian Darongshan granite.This deformation was likely related to the closure of the eastern Paleo-Tethys Ocean and the subsequent collision of the South China and Indochina blocks,during the early stage of the Indosinian orogeny.

Observing eddy dye patches induced by shear instabilities in the surf zone on a plane beach

The effects of surf zone eddy generated by alongshore currents on the deformation and transport of dye are still poorly understood,and related tracer release experiments are lacking.Therefore,a tracer release laboratory experiment was conducted under monochromatic,unidirectional incident waves with a large incident angle(30°)on a plane beach with a 1:100 slope in a large wave basin.A charge-coupled device suspended above the basin recorded the dye patch image.The evolution of eddy dye patch was observed and the transport and diffusion were analyzed based on the collected images.Subsequently,a linear instability numerical model was adopted to calculate the perturbation velocity field at the initial stage.The observation and image processing results show that surf zone eddy patches occurred and were separated from the original dye patches.Our numerical analysis results demonstrate that the structure of the perturbation velocity field is consistent with the experimental observations,and that the ejection of eddy patches shoreward or offshore may be ascribed to the double vortex.

Structural,^(40)Ar/^(39)Ar Geochronological and Rheological Feature Analysis of the Guoxuepu Shear Zone:Indications for the Jitang Metamorphic Complex in the Northern Lancangjiang Zone

The Jitang metamorphic complex is key to studying the tectonic evolution of the Northern Lancangjiang zone.Through structural-lithological mapping,structural analysis and laboratory testing,the composition of the Jitang metamorphic complex was determined.The macro-and microstructural analyses of the ductile detachment shear zone(Guoxuepu ductile shear zone,2–4 km wide)between the metamorphic complex and the overlying sedimentary cap show that the shear sense of the ductile shear zones is top-to-the-southeast.The presence of various deformation features and quartz C-axis electron backscatter diffraction(EBSD)fabric analysis suggests multiple deformation events occurring at different temperatures.The average stress is 25.68 MPa,with the strain rates(έ)ranging from 9.77×10^(−14)s^(−1)to 6.52×10^(−16)s^(−1).The finite strain of the Guoxuepu ductile shear zone indicates an elongated strain pattern.The average kinematic vorticity of the Guoxuepu ductile shear zone is 0.88,implying that the shear zone is dominated by simple shear.The muscovite selected from the protomylonite samples in the Guoxuepu ductile shear zone yields a 40Ar-39Ar age of 60.09±0.38 Ma.It is suggested that,coeval with the initial Indo–Eurasian collision,the development of strike-slip faults led to a weak and unstable crust,upwelling of lower crust magma,then induced the detachment of the Jitang metamorphic complex in the Eocene.

Deformation characteristics of granitic rocks in Erguna ductile shear zone,NE China

The Erguna ductile shear zone is situated in the Erguna Massif,which has been exposed along the eastern bank of the Erguna River in northeastern China.The authors present comprehensive study results on the macro-and micro-structures,finite strain and kinematic vorticity,quartz electron backscatter diffraction(EBSD)fabrics,and geochronology of granitic rocks in the Erguna ductile shear zone.The deformed granitic rocks have experienced significant SE-trending dextral strike-slip shearing.Finite strain and kinematic vorticity in all deformed granitic rocks indicate that the deformation is characterized by simple sheardominated general shearing with S-L tectonites.Mineral deformation behaviors and quartz C-axis textures demonstrate that the deformed granitic rocks developed under greenschist to amphibolite facies conditions at deformation temperatures ranging from 450 to 550℃.New LA-ICP-MS zircon U-Pb ages indicate that these granitic rocks were formed in Early Triassic(~248.6 Ma)and Early Cretaceous(~136.7 Ma).All the evidence indicates that this deformation may have occurred in Early Cretaceous and was related to the compression resulting from the final closure of the Mongol-Okhotsk Ocean.

Characterizing large-scale weak interlayer shear zones using conditional random field theory

The shear behavior of large-scale weak intercalation shear zones(WISZs)often governs the stability of foundations,rock slopes,and underground structures.However,due to their wide distribution,undulating morphology,complex fabrics,and varying degrees of contact states,characterizing the shear behavior of natural and complex large-scale WISZs precisely is challenging.This study proposes an analytical method to address this issue,based on geological fieldwork and relevant experimental results.The analytical method utilizes the random field theory and Kriging interpolation technique to simplify the spatial uncertainties of the structural and fabric features for WISZs into the spatial correlation and variability of their mechanical parameters.The Kriging conditional random field of the friction angle of WISZs is embedded in the discrete element software 3DEC,enabling activation analysis of WISZ C2 in the underground caverns of the Baihetan hydropower station.The results indicate that the activation scope of WISZ C2 induced by the excavation of underground caverns is approximately 0.5e1 times the main powerhouse span,showing local activation.Furthermore,the overall safety factor of WISZ C2 follows a normal distribution with an average value of 3.697.

Petrology, Age and Geodynamic Implication of the Panafrican Granitoids Associated with the Glito-Kpatala Shear Zone (South-East Togo)

The granitic plutons associated with the Glito-Kpatala shear zone are composed of biotite and amphibole granodiorites, biotite granites, two-mica granites and aplitic granites, which are very poorly represented. The chemical and mineralogical compositions of these facies indicate that they are I type and belong to high-K calc-alkaline series, with a chemical metaluminous character displayed by the granodiorites relative to the biotite and two-mica facies whose chemical compositions vary between metaluminous and peraluminous caracter. The Th/Ta (14.04 - 43.82 ppm, mean = 26.05), Th/U (2.58 to 15.05 ppm, mean = 5.85 ppm), Zr/Hf (25.27 to 37.21, mean = 30.67 ppm) and Rb/Sr (0.16 to 4.32;mean = 1.67 ppm) ratios of these granitoids reveal a strong crustal involvement in their magmatogenesis. Variations in CaO/Na2O (0.47 - 1.44 ppm), Rb/Sr (0.14 - 0.27 ppm), Rb/Ba (0.07 - 0.14 ppm) and Sr/Y (38.21 - 174.42 ppm) ratios indicate that biotite and amphibole granodiorites with their excessive Ni (135.37 - 139.51 ppm) and Cr (395.73 - 447.74 ppm) were derived from a mafic to intermediate lower continental crust where garnet and/or amphibole were stable residual assemblage minerals. The moderate Sr/Y ratios (1.81 - 9.47 ppm) and low transition elements Ni (1 - 6.44 ppm) and Cr (7.89 - 13.47 ppm) contents in both the two-mica and biotite granites are consistent with their emplacement at relatively shallow depths in the upper to mean continental crust, at pressures below 10 Kbar. In the two-mica granites, moderate CaO/Na2O (0.20 - 0.57 ppm, mean = 0.38 ppm) and Rb/Ba (0.39 - 1.37, mean = 0.84 ppm) ratios and quite varied Rb/Sr (1.53 - 4.23 ppm, mean = 2.85 ppm) ratios indicate a predominant derivation from psammitic and pelitic metasediments rather than metagreywackes. These low ratios (0.25 ≤ CaO/Na2O ≤ 0.32, mean = 0.28 ppm;0.31 ≤ Rb/Ba ≤ 0.44, mean = 0.39 ppm;1.11 ≤ Rb/Sr ≤ 1.78, mean = 0.39 ppm) in biotite granites are more consistent with melting from a metagreywacke-derived source. Evidence for the contribution of mantle-derived mafic magma with granitic magma in the plutons studied is materialized by the presence of magmatic enclaves in both granodiorites and two-mica granites, the volcanic arc geochemical signatures displayed by the plutons in geotectonic diagrams and Nb/Ta ratios (14.14 - 34.61 ppm) closer to mantle estimates. Geochemical data and radiometric dating elements suggest that the granitoids studied can be integrated into the pan-African late magmatic episode, which corresponds between 606 and 583 Ma, to the activity of transcurrent ductile strike-slips and to the synchronous emplacement of high K calc-alkaline plutons in a post-collisional context.

Kink-band Kinematic Analysis and its Implications for Late-stage Deformation in the Internal Parts of the Zagros Collision(Sanandaj–Sirjan Zone) in West Iran

In the internal parts of the Zagros collision zone, several deformation phases have been superimposed. The early deformation phase caused the development of a penetrative foliation. The late-stage deformation phase was preferentially accommodated within shear zones and caused the generation of shear bands, implying a non-coaxial component of deformation, the end of this stage deformation was marked by the development of kink-bands. In the vicinity of Zagros suture zone, the kink angle increased from 40° to 60°, and the kink-bands was converted to chevron folds. In this region, the external(α) and internal(β) angular ratio is α/β ≠ 1 and kink angle increased, and deformation occurred with 10% to 30% volume loss. Farther from the suture zone in the east, α/β = 1;and total volume was constant or increased by 5% to 10%. Kink-bands kinematic analysis in the study area revealed this structures were sensitive to deformation conditions and components such that, with decreasing distance to the Zagros suture zone, shearing and rotation increased, a high kinematic vorticity dominated, and volume loss occurred during deformation.

The Early Mesozoic NE-SW Extensional Model and Exhumation Processes at the Southeastern Margin of the Central Asian Orogenic Belt:Insights from the Strain and Kinematic Vorticity Analysis of the Sonid Zuoqi Ductile Detachment Zone

The Sonid Zuoqi ductile detachment zone is located at the southeastern margin of the Central Asian orogenic belt(CAOB),striking EW and dipping to the S.The major rock type of the Sonid Zuoqi ductile detachment zone is mylonite derived from granite.The sequence of mylonite features is:(1)S and C foliations of mylonite,and(2)extensional crenulation cleavage(ecc)or C′and the kinematic vorticity(Wk)value changed from 0.70 to 0.95 and from 0.37 to 0.69,respectively;the strain type of the mylonites within the Sonid Zuoqi ductile detachment zone is compressional to planar strain.The strong deformation mylonite and Halatu plutons yielded a zircon U-Pb age of 244 Ma and a zircon(U-Th)/He age of 214 Ma,respectively.Based on the strain and kinematic vorticity analysis,together with the zircon U-Pb and zircon(U-Th)/He ages and the regional tectonic background,the study area experienced three stage evolution:tangential simpleshear(244 Ma),simple-shear-dominated general shear represented by upper crustal extension(224 Ma)and pure-shear-dominated general shear represented by the Halatu pluton doming(214 Ma),which constrained the early Mesozoic NE-SW crustal extension at the southeastern margin of the CAOB.This NE-SW extension probably originated from the postorogenic extensional collapse of the CAOB,subsequent exhumation being controlled by the far afield effects of the closure of the Mongol-Okhotsk belt.

Boulder-induced form roughness and skin shear stresses in a gravel-bed stream

Boulder spacing in mountain rivers and near-wake flow zones within the boulder array is very useful for fish habitat and growth of aquatic organisms.The present study aims to investigate how the boulder array and spacing influence the near-bed flow structures in a gravel-bed stream.Boulders are staggered over a gravel-bed stream with three different inter-boulder spacing namely(a)large(b)medium and(c)small spacing.An acoustic Doppler velocimeter was used for flow measurements in a rectangular channel and the results were compared with those acquired from numerical simulation.The time-averaged velocity profiles at the near-wake flow zones of boulders experience maximum flow retardation which is an outcome of the boulder-induced form roughness.The ratio of velocity differences associated to form and skin roughness and its positive magnitude reveals the dominance of form roughness closest to the boulders.Form roughness computed is 1.75 to 2 times higher than the skin roughness at the near-wake flow region.In particular,the collective immobile boulders placed at different inter-boulder spacings developed high and low bed shear stresses closest to the boulders.The low bed shear stresses characterised by a secondary peak developed at the trough location of the boulders is attributed to the skin shear stress.Further,the spatial averaging of time-averaged flow quantities gives additional impetus to present an improved illustration of fluid shear stresses.The formation of form-induced shear stress is estimated to be 17%to 23%of doubleaveraged Reynolds shear stress and partially compensates for the damping of time-averaged Reynolds shear stress in the interfacial sub-layer.The quadrant analysis of spatial velocity fluctuations depicts that the form-induced shear stresses are dominant in the interfacial sub-layer and have no significance above the gravel-bed surface.

Seismic anisotropy and upper mantle dynamics in Alaska:A review of shear wave splitting analyses

Shear wave splitting(SWS)is regarded as the most effective geophysical method to delineate mantle flow fields by detecting seismic azimuthal anisotropy in the earth's upper mantle,especially in tectonically active regions such as subduction zones.The Aleutian-Alaska subduction zone has a convergence rate of approximately 50 mm/yr,with a trench length reaching nearly 2800 km.Such a long subduction zone has led to intensive continental deformation and numerous strong earthquakes in southern and central Alaska,while northern Alaska is relatively inactive.The sharp contrast makes Alaska a favorable locale to investigate the impact of subduction on mantle dynamics.Moreover,the uniqueness of this subduction zone,including the unusual subducting type,varying slab geometry,and atypical magmatic activity and composition,has intrigued the curiosity of many geoscientists.To identify different sources of seismic anisotropy beneath the Alaska region and probe the influence of a geometrically varying subducting slab on mantle dynamics,extensive SWS analyses have been conducted in the past decades.However,the insufficient station and azimuthal coverage,especially in early studies,not only led to some conflicting results but also strongly limited the in-depth investigation of layered anisotropy and the estimation of anisotropy depth.With the completion of the Transportable Array project in Alaska,recent studies have revealed more detailed mantle structures and characteristics based on the dense station coverage and newly collected massive seismic data.In this study,we review significant regional-and continental-scale SWS studies in the Alaska region and conclude the mantle flow fields therein,to understand how a geometrically varying subducting slab alters the regional mantle dynamics.The summarized mantle flow mechanisms are believed to be conducive to the understanding of seismic anisotropy patterns in other subduction zones with a complicated tectonic setting.

A method for extracting the preseismic gravity anomalies over the Tibetan Plateau based on the maximum shear strain using GRACE data

The occurrence of earthquakes is closely related to the crustal geotectonic movement and the migration of mass,which consequently cause changes in gravity.The Gravity Recovery And Climate Experiment(GRACE)satellite data can be used to detect gravity changes associated with large earthquakes.However,previous GRACE satellite-based seismic gravity-change studies have focused more on coseismic gravity changes than on preseismic gravity changes.Moreover,the noise of the north–south stripe in GRACE data is difficult to eliminate,thereby resulting in the loss of some gravity information related to tectonic activities.To explore the preseismic gravity anomalies in a more refined way,we first propose a method of characterizing gravity variation based on the maximum shear strain of gravity,inspired by the concept of crustal strain.The offset index method is then adopted to describe the gravity anomalies,and the spatial and temporal characteristics of gravity anomalies before earthquakes are analyzed at the scales of the fault zone and plate,respectively.In this work,experiments are carried out on the Tibetan Plateau and its surrounding areas,and the following findings are obtained:First,from the observation scale of the fault zone,we detect the occurrence of large-area gravity anomalies near the epicenter,oftentimes about half a year before an earthquake,and these anomalies were distributed along the fault zone.Second,from the observation scale of the plate,we find that when an earthquake occurred on the Tibetan Plateau,a large number of gravity anomalies also occurred at the boundary of the Tibetan Plateau and the Indian Plate.Moreover,the aforementioned experiments confirm that the proposed method can successfully capture the preseismic gravity anomalies of large earthquakes with a magnitude of less than 8,which suggests a new idea for the application of gravity satellite data to earthquake research.

Structural and Geochronological Evidence for Multiple Episodes of Tertiary Deformation along the Ailaoshan-Red River Shear Zone, Southeastern Asia, Since the Paleocene

Structural analyses show that the Ailaoshan-Red River shear zone （ASRRSZ） in Ailao Mountain is composed of three different deformational domains. These domains may represent three episodes of left-lateral slip experienced by the ASRRSZ. The first episode of such deformation occurred throughout the eastern high-grade belt of the ASRRSZ under a transtensional regime and produced L- type tectonites of amphibolite grade. The second episode of left-lateral slip formed high strain zones overprinting the high-grade belt. Its deformational mechanism is similar to simple shear and the deformed rocks are L-S mylonites of greenschist grade. The third episode of left-lateral slip took place chiefly in a western low-grade belt of the ASRRSZ. This deformation occurred in a transpressional regime, formed an overall structure pattern of a sinistral thrust system and produced phyllonites of low-greenschist grade. Geochronological data indicated that the three episodes of left-lateral slip happened before ～58-56 Ma, at least from ～27 Ma to 22 Ma and at ～13-12 Ma respectively. The first episode of slip in the ASRRSZ appeared to correspond to the initial collision of India and Asia at ～60 Ma. The second episode took place almost at the same time as the most intensive compression and uplift in Tibet. The latest event might represent a further eastward material flow in Tibet after ～16-13 Ma. Thus, the ASRRSZ of southeastern Asia probably experienced three main episodes of Tertiary left- lateral slip in the course of intracontinental convergence since the India-Asia collision.

High Temperature Strain Structures and Quartz C-Axis Fabrics from Mylonitic Rocks in the Ailaoshan-Red River Shear Zone,Yunnan,and Their Tectonic Implication

The Ailaoshan-Red River（ASRR） shear zone is one of the major Southeast Asian tectonic discontinuities that have figured the present tectonic framework of the eastern Tibet.Several metamorphic massifs are distributed linearly along the shear zone,e.g.Xuelongshan,Diancangshan, Ailaoshan and Day Nui Con Voi from north to south.They bear a lot of lines of evidence for the tectonic evolution of the eastern Tibetan at different crustal levels in different tectonic stages.Controversy still exists on the deformation structures,microstructures and their relationship with metamorphisms along the ASRR.In this paper detailed microstructural and EBSD（Electron Backscattered Diffraction） fabric analysis of some highly sheared granitic rocks from different massifs along the ASRR are conducted.High temperature structures and microstructures are preserved in unsheared gneisses,in weakly sheared xenoliths or in some parts of the highly sheared rocks（mylonites）.Several types of high temperature quartz c-axis fabrics show symmetrical patterns or transitions from symmetrical to asymmetrical patterns.The former are attributed to coaxial deformation during regional shortening in an early stage of the Indian-Eurasian tectonic interaction and the latter are related to the transitions from coaxial compression to noncoaxial shearing during the post-collisional ASRR left lateral shearing.

New Zircon U-Pb Geochronology of the Post-kinematic Granitic Plutons in the Diancang Shan Metamorphic Massif along the Ailao Shan-Red River Shear Zone and Its Geological Implications

The Ailao Shan-Red River fault zone is the boundary between the Yangtze block to the northeast and the Indochina block to the southwest.It is an important tectonic zone due to its role in the southeastward extrusion of the Indochina block during and subsequent to the Indian-Eurasian collision.Diancang Shan（DCS） high-grade metamorphic complex,located at the northwest extension along the Ailao Shan-Red River（ASRR） shear zone,is a representative metamorphic complex of the ASRR tectonic belt.Structural and microstructural analysis of sheared rocks in the high-grade metamorphic rocks reveals that they are coherent with solid-state high-temperature ductile deformation,which is attributed to left-lateral shearing along the ASRR shear zone.New LA-ICP-MS zircon U-Pb geochronological and microstructural studies of the post-kinematic granitic plutons provide a straightforward time constraint on the termination ductile left-lateral shearing and exhumation of the metamorphic massif in the ASRR shear zone.It is suggested that the left-lateral shearing along the ASRR shear zone ended at ca.21 Ma at relative lower-temperature or decreasing temperature conditions.During or after the emplacement of the young dikes at ca.21 Ma,rapid brittle deformation event occurred,which makes the DCS massif start fast uplift/exhumation and cooling to a shallow crustal level.

Structures of Syn-deformational Granites in the Longquanguan Shear Zone and Their Monazite Electronic Microprobe Dating

The Longquanguan shear zone is an important structural belt in the North China Craton, separating the underlying Fuping complex from the overlying Wutai complex. This shear zone has experienced three episodes of deformation: the first and main episode is a ductile top-to-ESE shear along the gently northwest-west dipping foliations, while the other two episodes are later collapse sliding. Prolonged granites parallel to the shear foliations make one of the main compositions of the Longquanguan shear zone. These granites experienced deformation to form mylonitic rocks when they emplaced during the first episode of deformation. Structural characters of the granites and their contacts to the country rocks indicate that these granites possibly resulted from in-situ partial remelting by shearing, i.e., they are syn-deformational granites. Monazites in these mylonitic granites are magmatic minerals and their crystallization ages may represent ages of the magmatic events, and also the ages for the main deformation of the Longquanguan shear zone. Monazite electronic microprobe dating were carried on two samples of granite, which gives multiple peak ages, among which 1,846 Ma and 1,877 Ma are the main peak ages for the two samples. These ages represent the main deformation of the Longquanguan shear zone, which is consistent with the main regional geological event at about 1,850 Ma caused by the collision between the Eastern and Western Blocks in North China. The good match between the monazite ages and the corresponding regional tectono-thermal events shows the feasibility and reliability of monazite electronic microprobe dating.

A Large Ductile Sinistral Strike-Slip Shear Zone and Its Movement Timing in the South Qilian Mountains, Western China

There is a large ductile shear zone, 2 km wide and more than 350 km long, inthe South Qilian Mountains, western China. It is composed of volcanic, granitic and calcareousmylonites. The microstructures of the ductile shear zone show nearly E-W extending subverticalfoliation, horizontal and oblique stretching lineations, shearing sense from sinistral to obliquesinistral strike-slip from east to west, 'A' type folds and abundant granitic veins. Measuredlattice preferred orientations (LPOs) of the mylonitic and recrystallized quartz of the graniticmylonite in the west segment suggest a strong LPO characterized by the dominant slip systems (1010) formed at high temperature (>650 deg C). K-feldspar of the mylonite shows an ^(39)Ar/^(40)Arhigh-temperature plateau age of 243.3 +- 1.3 Ma, and biotite, 250.5 +- 0.5 Ma, which represent theformation age of the ductile shear zone. The ^(39)Ar/^(40)Ar plateau ages of 169.7 +- 0.3 Ma and160.6 +- 0.1 Ma and the ^(39)Ar/^(40)Ar isochron ages of 166.99 +- 2.37 Ma and 160.6 +- 0.1 Ma ofbiotites in the mylonite represent the subsequent deformation age. These ages indicate that thisductile shear zone is similar to the Altun and South Kunlun sinistral ductile shear zones in itsages of movement, formation, reactivation and duration.

Fluid-Rock Interaction of Shear Zones in Continental Crust:as Evidenced by Xincheng-Xishui and Hetai Shear Zones

There is a coupling of thermal, mechanical, chemical and fluidal processes in a continental shear zone. Both Xincheng - Xishui and Hetai shear zones are typical continental crust shear zones of greenschist facies environment. The representative mylonite zones of the shear zones are studied with whole rock major and trace element analyses. The chemical compositional variation tendencies in both shear zones are very similar and the gain - loss ratios of various components in the mylonitic rocks are reflected in the mass balance equations. The enrichment of those immobile high - field- strengh elements is considered to be related to the volume loss of the mylonitic rocks in a shear zone. Based on the volume loss expression Cs /Co = 1/(1- V), the fractional volume losses (V) are 37.5% and 36.5%-42.3% respectively for mylonites and ultramylonites in the Xincheng-Xishui shear zone and 11 % and 28% respectively for mylonites and phyllonites in the Hetai shear zone. The high volume loss and large removal of SiO2 from the system imply that there is a large amount of percolating fluids in the shear zones. From the SiO2 loss, the fluid/rock ratios (N) are calculated as Nmyl = 113 - 563, Nultramyl= 133-664 for the Xincheng-Xishui shear zone and Nmyl=42-208, Nphyl=110-550 for the Hetai shear zone. Such a large amount of percolating fluid must have profoundly affected the rheological behavior, chemical behavior and metallogenesis of the shear zones.

A Uniform Orogen-parallel Extension System of the Shear Zones in the Tongbai-Dabie Orogenic Belt, Central China

Large-scale magmatism affected the Tongbai-Dabie orogenic belt during post-collisional lateral tectonic extension in the Cretaceous, which was suggested to account for the widespread deformation and migmatization in the Tongbai-Dabie complexes. However, it cannot explain the most deformations in the shear zones. The northwest-southeast shear zones are developed around or wrapped the Tongbai-Dabie complexes. They play an important role for the interpretation of the tectonic evolution of the Tongbai-Dabie orogenic belt. By a systematically observation and description of the geometry and kinematics of these shear zones, we found that the shear zones to the north dip NE and have a uniform sinistral shear sense, the shear zone to the south dips SW and has a uniform dextral shear sense, and the shear zones at the core are sub-horizontal and have a uniform top-to-NW sense of shear. Combining with the comparison of previous and our geochronological studies, we interpret these associations as indicating that these shear zones were originally a single, more flat-lying and sub- horizontal shear zone with a uniform top-to-NW shear sense before the folding-doming of the Tongbai- Dabie complexes and suggest that the Tongbai-Dabie orogenic belt experienced a uniform top-to-NW orogen-paraUel extension in the ductile lithosphere before the widespread magmatism in the Cretaceous.

Analysis of the Ore-Controlling Structure of Ductile Shear Zone Type Gold Deposit in Southern Beishan Area, Gansu, Northwest China

The ductile shear zone-type gold deposit is a kind that both the ore-forming mechanism and ore-controlling factors are closely related to the ductile shear zone and its evolution. Ductile shear zone develops in Beishan area, Gansu of Northwest China, and develops especially well in the south belt. The controls of the ductile shear zone on gold deposits are as follows. （1） The regional distribution of gold deposits （and gold spots） is controlled by the ductile shear zone. （2） The ductile-brittle shear zone is formed in the evolution process of ductile shear zone and both are only ore-bearing structures and control the shape, attitude, scale, and distribution of mineralization zones and ore-bodies. （3） Compresso-shear ductile deformation results in that the main kind of gold mineralization is altered mylonite type and the main alteralization is metasomatic. （4） Ore-bearing fracture systems are mainly P-type ones, some D-type and R-type ones, but only individual R＇-type and T-type ones. （5） Dynamic differen- tiation and dynamic metamorphic hydrothermal solution resulting from ductile deformation is one of the sources of ore-forming fluid of gold mineralization, and this is identical with that ore-forming materials are mainly from metamorphic rocks, and ore-forming fluid is mainly composed of metamorphic water, and with the fluid inclusion and geo-chemical characteristics of the deposit. （6） There is a negative correlation between the gold abundance and susceptibility anlsotropy （P） of the altered mylonlte samples from the deposit, which shows that the gold mineralization is slightly later than the structural deformation. All above further expound the ore-forming model of the ductile shear zone type of gold deposits.

Transportation of REE in the Ductile Shear Zone in Hetai Gold Deposit, Guangdong Province

The REE transportation in two sorts of mylonites of ductile shear zone, in Hetai gold deposit, Guangdong province was studied. The results show that there is REE compositional variation regularly in the ductile shear zone. When mica-quartz schist and migmatite are changing into mylonites or ultramylonites, i.e., along with intensifying deformation, the contents of each rare earth element and the total contents of the LREE, HREE and REE increase or decrease regularly, but the REE patterns of the rock are accordant. The isocon diagram of the tectonic rocks before and after deformation shows that the REE compositional variation of the mylonites or ultramylonites are very small, but there is a few REE moving in or out in the rock deformation. The reason of the REE compositional variation is that there are volume change and fluid flow in the rock deformation, and it leads to the LREE relative enrichment and HREE relative loss.