In situ characterization of forearc serpentinized peridotite from the Sulu ultrahigh-pressure terrane: Behavior of fluid-mobile elements in continental subduction zone

Serpentinized peridotites in the Yangkou(YK),Suoluoshu(SLS) and Hujialin(HJL) areas in the Sulu ultrahighpressure terrane represent the relic of ancient subcontinental lithospheric mantle below the North China Craton.Their protoliths,harzburgite and dunite,were variably hydrated by aqueous fluids released from subducting Yangtze continent.The rocks are enriched in fluid-mobile elements(FME) including Sb(42–333 times the depleted mantle value) and Pb(30–476 times).The degrees of the FME enrichment are comparable to that of the Himalayan forearc serpentinites,and greater than forearc mantle serpentinites from Marianas,suggesting that the degrees of FME enrichment in the forearc serpentinites are greater in continental subduction zones than those in the oceanic subduction zones.Lizardite after olivine in the SLS serpentinite shows higher degrees of enrichment in Sb and As than those for antigorite after both olivine and orthopyroxene in the YK area.The antigorite has highly enriched in Pb,U,Cs,and LREE,but not for the lizardite.The abundance of FME in two different species of serpentine reflects the different temperature of hydration.At temperature lower than 300 ℃,formed lizardite at shallow depths of the mantle wedge incorporates elements that are fluid mobile at low temperatures,such as Sb and As.When the temperature greater than 300 ℃,formed antigorite at a relatively deep mantle wedge incorporate more FME from the subducting continental slab(or fragments),including Pb,U,Cs,LREE as well as Sb and As.The eventual breakdown of antigorite(600–700 ℃) in prograde metamorphism would discharge water as well as FME into the subducting channel and/or the overlying mantle.

Evaluation of the Relative Tectonic Activity from the Faults in the Gengma-Lancang-Menghai Seismic Belt(Southwestern China)

The Gengma-Lancang-Menghai seismic belt,the southernmost part of the North-South seismic belt,is controlled by four active faults:the Nantinghe fault(NTHF),the Sanjianshan fault(SJSF),the Hanmuba-Lancang fault(HMB-LCF),and the Heihe fault(HHF),from northwest to southeast.However,the tectonic activity of the faults in the Gengma-Lancang-Menghai seismic belt has not been fully studied yet.In the present work,we conducted tectonic geomorphic indices to analyze the relative tectonic activity along the faults in the seismic belt based on the digital elevation model.We interpreted asymmetric factor,index of drainage basin shape,hypsometric integral,normalized streamlength gradient,valley floor-to-width to height ratio,and longitudinal profiles to show that the relative tectonic activity is higher in the north and lower in the south,and is strong in the region from Shanjia to Huimin in the center of the seismic belt.

Erratum to: Evaluation of the Relative Tectonic Activity from the Faults in the Gengma-Lancang-Menghai Seismic Belt (Southwestern China)

The original version of this article unfortunately contained a mistake.The presentation of an author's name was incorrect.The corrected oneis given below.

Cenozoic Uplift of the Central Yunnan Fragment, Southwestern China, Revealed by Apatite(U-Th)/He Dating

The age of central Yunnan fragment uplift has long been debated, with estimates ranging from the Late Eocene to about 1 Ma. To determine the central Yunnan fragment uplift time in the Cenozoic, apatite(U-Th)/He(AHe) was used to analyze the low-temperature thermochronology of samples from the Jiaozi Mountain area of the eastern central Yunnan fragment. The sampling area is located in the Dongchuan District of Kunming, Yunnan Province, near the Xiaojiang fault zone. The results show that AHe ages from the eastern part of central Yunnan fragment were mainly concentrated around 25.7–37.9 Ma, and intensive uplift had happened before 36.5 Ma. Together with previous low-temperature thermochronology research on the western and eastern central Yunnan fragment, we concluded that the Yunnan Plateau uplifted prior to 36.5 Ma, in a west to east sequence. The uplift caused a change in paleo-geographical terrain, which may have altered the ancient river systems of the southeast Tibetan Plateau.

New Conceptual Fold-Fracture Model Including Prefolding Fractures,Based on Fuyang-Lin'an Anticline,Zhejiang Province

Field observations of the crossing relationships of fractures have been used to explain the sequence of fractures. Based on field observations from Fuyang-Lin＇an anticline, located near Hangzhou, Zhejiang Province, this paper proposes that the formation of synfolding fractures was influenced dominantly by one fracture set, which developed prior to folding and the orientation was nearly parallel to the bedding. The length of the prefolding fractures is longer than the synfolding fractures. These prefolding fractures cut thicker strata into small pieces and form a dense network of fractures in thicker strata. Most synfolding fractures, which are oblique to the bedding, are truncated by prefolding fractures in thicker strata. The synfolding fractures, which result from local stress, are inferred to form during folding. Here, the mechanism of truncation was analyzed using finite-element models. The approach was based on the idea that natural fractures can be interpreted or inferred from stress distribution. The presence or absence of prefolding fractures is shown to strongly control the distribution of stress, and this control has an important implication for interpreting the fracture truncation mechanism from geomechanical models.

3D Seismic Attributes for Structural Analysis in Compressional Context: A Case Study from Western Sichuan Basin

Compressional region usually forms complex thrust faults system, which is difficult to identify using traditional migration profiles. The successful application of three-dimensional（3D） seismic attributes analysis greatly reduces the difficulty, and improves the accuracy and efficiency of seismic interpretation and structural analysis. In this paper, we took Qiongxi area in the compressional region of western Sichuan as an example, using two 3D seismic attributes, coherence and instantaneous phase, to identify fault assemblages and variations both vertically and laterally. The results show that the study area mainly consists of NS-, NE- and NEE-trending faults. The NS-trending faults are the largest and have a component of sinistral slip controlling the formation of NEE-trending faults, while the NE-trending faults are intermediate in scale, formed earlier and were cut by the NS-trending faults. Our results demonstrate that using seismic attributes for structural analysis have the following advantages：（1） more details of major fault zones,（2） highlighting minor faults which are hardly traced in seismic migration cube, and（3） easier acquisition of accurate fault systems. The application of seismic attributes provides a new idea for deciphering fine and complicated structures, and will significantly contribute to the development of objective and precise geological interpretation in the future.