Convergent Belts Research Articles

Seismically imaged lithospheric delamination and its controls on the Mesozoic Magmatic Province in South China

The current lithospheric root of the South China Block has been partly removed, yet what mechanisms modified the lithospheric structure remain highly controversial. Here we use a new joint seismic inversion algorithm to image tabular high-velocity anomalies at depths of ~90–150 km in the asthenosphere beneath the convergent belt between the Yangtze and Cathaysia blocks that remain weakly connected with the stable Yangtze lithosphere. Based on obtained seismic images and available geochemical data, we interpret these detached fast anomalies as partially destabilized lower lithosphere that initially delaminated at 180–170 Ma and has relaminated to their original position after warming up in the mantle by now. We conclude that delamination is the most plausible mechanism for the lithospheric modification and the formation of a Mesozoic Basin and Range-style magmatic province in South China by triggering adiabatic upwelling of the asthenosphere and consequent lithospheric extension and extensive melting of the overlying crust.

Late Cretaceous Transition From Calc-Alkaline to Alkaline Magmatism in the Eastern Anatolian Plateau: Implications for Microblock Collision Timing

Abstract In convergent belts, a collision between two blocks can reshape upper mantle geometry and processes responsible for a change in mechanisms of magma generation with distinct geochemical compositions. Therefore, identifying the turning point of a magma compositional shift can provide key constraints on collision timing, which is decisive in building a framework of regional tectonic evolution. The Eastern Anatolian Plateau is composed of a mosaic of rifted blocks assembled through successive collisional events, culminating with the terminal collision of Arabia with Eurasia and the closure of the southern branch of the Neotethys in the Cenozoic. The timing of the microblock collision of the Bitlis–Pütürge Massifs with the Eastern Taurides Block, the southern Eastern Anatolian Plateau, is a matter of debate due to limited constraints on the timing and petrogenesis of the (post-)collision-related magmatism during the Late Cretaceous. This study identifies three compositionally distinct intrusive suites aged from ~87 to ~69 Ma in the Eastern Taurides Block, the southern Eastern Anatolian Plateau. Group 1 intrusive rocks were emplaced in the southern Eastern Taurides Block at ~87–77 Ma and are characterized by high-K calc-alkaline compositions with predominantly depleted Hf and Nd isotope compositions [εHf(t) = 0.9 to +16.5 and εNd(t) = −2.3 to +6.9]. In contrast, the younger (~77–69 Ma) Group 2A comprises nepheline (Ne)-normative alkaline compositions, and Group 2B consists of shoshonitic compositions; both groups are mainly distributed in the central and northern Eastern Taurides Block. Groups 2A and 2B overlap in age between ~77 and 69 Ma and show relatively enriched Hf–Nd isotope compositions [Group 2A: εHf(t) = −1.0 to +4.7 and εNd(t) = −2.3 to +1.2; Group 2B: εHf(t) = −4.0 to +4.0 and εNd(t) = −6.5 to −1.3]. Group 1 intrusive rocks relate to the sub-arc asthenosphere-derived melts that differentiated toward the granite minimum by fractional crystallization. Parental melts of Group 2A intrusive rocks are interpreted to be derived from metasomatized subcontinental lithospheric mantle and differentiated toward the phonolite minimum. Crustal assimilation during magma ascent pushed the initially silica-undersaturated magma (Group 2A) into silica-(over)saturated compositions (Group 2B) through fractional crystallization that ultimately evolved toward the granite minimum. We propose that the Group 1 magmatism was related to northward subduction and closure of the Berit oceanic lithosphere, whereas the Group 2 magmatism results from collision-induced lithospheric delamination ± slab rollback. Together with the P–T–t evolution of the high-pressure metamorphic rocks from the Bitlis Massif and the spatiotemporal and geochemical variations of the Late Cretaceous magmatism in the Eastern Taurides Block, this study suggests that microblock collision of the Bitlis–Pütürge Massifs with the Eastern Taurides Block (Eurasia) most likely occurred at ~84–77 Ma. This study provides an example to constrain collision timing through a perspective of magmatic transition from calc-alkaline to alkaline series.

Regional PM2.5 pollution confined by atmospheric internal boundaries in the North China Plain: boundary layer structures and numerical simulation

Abstract. This study reveals mesoscale planetary boundary layer (PBL) structures under various pollution categories during autumn and winter in the North China Plain (NCP). The role of the atmospheric internal boundaries (AIBs, referring to the discontinuity of meteorological conditions in the lateral direction) in regulating PBL structures and shaping the PM2.5 pollution patterns is emphasized. The Weather Research and Forecast (WRF) model is used to display the three-dimensional meteorological fields, and its performance is evaluated by surface observations and intensive soundings. The evaluation demonstrates that the model reasonably captures the mesoscale processes and the corresponding PBL structures. Based on the reliable simulations, three typical pollution cases are analyzed. Case 1 and case 2 represent the two main modes of the wind shear category pollution, which is featured with airflow convergence line/zone as AIB, and thus is dominated by dynamical effect. Case 1 presents the west–southwest wind shear mode associated with a trough convergence belt. The convergent airflow layer is comparable to the vertical scale of the PBL, allowing PM2.5 transport to form a high pollution area. Case 2 exhibits another mode with south–north wind shear. A “lying Y-shaped” convergence zone is formed with a thickness of about 3000 m, extending beyond the PBL. It defines a clear edge between the southern polluted air mass and the clean air in the north. Case 3 represents the topographic obstruction category, which is characterized by a cold-air damming AIB in front of the mountains. The PBL at the foothills is thermally stable and dynamically stagnant due to the capping inversion and the convergent winds. It is in sharp contrast to the well-mixed/ventilated PBL in the southern plains, especially in the afternoon. At night, this meteorological discontinuity becomes less pronounced. The diurnal variation of the PBL thermal and dynamical structure causes the pollutants to concentrate at the foot of the mountains during the daytime and locally accumulate throughout the entire plain in the evening. These results provide a more complete mesoscale view of the PBL structure and highlight its spatial heterogeneity, which promotes the understanding of air pollution at the regional scale.

A Numerical Simulation of the “1907” Kaiyuan Tornado Weather Process in Liaoning, Northeast China

From 17:00 to 18:00 local standard time (LST) on 3 July 2019, a rare strong tornado occurred in Kaiyuan, Liaoning Province, northeast China. NCEP/NCAR 0.25° × 0.25° reanalysis data and WRF4.0 numerical prediction models were used to carry out the numerical simulation. Double nesting was adopted, and the horizontal grid distance was 9 km by 3 km. Based on the observation data of China meteorological observation stations, surface and upper charts, Doppler radar data, Himawari(HMW)-8 satellite images and numerical simulation results, the mesoscale structure and mechanism of the tornado were studied. The results show that: (1) At the northwest edge of the subtropical high, and the northeast cold vortex located in Northeast China, when the transverse trough moves southward, cold air is supplied continuously. Under the joint influence of the surface northeast cyclone, these are the main synoptic features of the tornado; (2) The northeast cold vortex cloud system was located at the junction of Heilongjiang and Jilin Provinces, and a squall line cloud system is formed. The tornado occurred at the tail of the squall line, and the strongest echo reached 65 dBZ. A mesocyclone, a 20 km northwest–southeast convergence belt, V-shaped gap, echo overhang structure and tornado vortex feature (TVS) were detected by the Doppler radar; (3) Before the tornado occurred, dry and cold air intruded from the northwest of the cold vortex, and a water vapor convergence zone appeared south of the squall line. The water vapor saturation zone with 80% relative humidity in northeast China was concentrated at 700 hPa, and the 20% dry column dropped down to 500 hPa between 115 and 124° E from the west. On the 850 hPa physical fields, there was a −20 × 10−5 s−1 convergence zone, and a 16 × 10−5 s−1 divergence belt appeared south and north of the squall line. A negative vorticity belt and a positive vorticity belt appeared south and north of the squall line, respectively. Kaiyuan is located at the smallest vertical shear, which is the junction place of three large vertical shear belts; (4) After 10:00 LST, the westerly wind 20 (10) m·s−1 dropped to 400 (800) hPa between 126 and 127° E. The northerly gale at 300 hPa north of 45° N moved southward. The rising center of the low level at 17:00 LST at approximately 45° N moved southward, and a sinking center appeared above it; (5) Several pairs of positive and negative vorticity columns formed between the lower troposphere and the place where the tornado occurred. There was convective instability at the lower level. CAPE increased, 0–3 km vertical wind shear increased, and LCL decreased remarkably during the afternoon.

Provenance of Carboniferous strata (the Meishan Group) on the northern margin of the Dabie orogenic belt and implications of oblique convergence between the North and South China blocks

ABSTRACT The newly defined Carboniferous Meishan Group, along the northern margin of the Dabie orogenic belt, provides unique opportunities to document the poorly understood Paleozoic tectonic evolution of the Dabie orogenic belt and the Paleozoic convergence between the North and South China blocks. We apply sandstone petrology, geochemistry, and U-Pb detrital-zircon geochronology to constrain the provenance of the Carboniferous Meishan Group and to document its potential tectonic significance. We conclude that the Meishan Group received most sediment directly from early Paleozoic continental island arc rocks that are currently missing in the Dabie orogenic belt, with minor contributions from middle Neoproterozoic magmatic rocks of the South China Block and recycling of Archean to Proterozoic basement rocks of both the North and South China blocks. Compilation and comparison of detrital zircons and geochemistry data of the Silurian–Devonian and Carboniferous units suggests that all of them share similar source areas, but that individual contributions from each source were different. These results support the hypothesis that the Dabie orogenic belt developed a similar Paleozoic accretionary system, and shares a similar tectonic history, with the Qinling orogenic belt. These provenance patterns can be explained by a model of oblique convergence between the North and South China blocks during the Paleozoic. The South China Block was obliquely subducted beneath the North China Block with its opening to the east, forming an eastward-widening sedimentary basin. As a result, the eastern part of the basin received more sediment from the northern passive margin of the South China Block, while the western part of the basin received more material from the southern active margin of the North China Block.

Imaging Neoarchean crustal structures: An integrated geologic-seismic-magnetotelluric study in the western Wabigoon and Winnipeg River terranes, Superior craton

The Neoarchean is considered as a pivotal era during the Earth’s evolution in terms of cratonization and development of plate tectonics. The supporting crustal processes, however, remain elusive due in part to limited comprehensive imaging of crustal structures underlying Neoarchean terranes. This study uses new seismic, magnetotelluric, and geologic data from the Metal Earth Sturgeon transect (70 km) and existing seismic and aeromagnetic data to investigate the Neoarchean crustal architecture underlying the greenstone-dominated western Wabigoon terrane (WWT) and the tonalite-trondhjemite-granodiorite-dominated Winnipeg River terrane (WRT) of the Superior craton. The results suggest that (1) the Sturgeon Lake greenstone belt of the WWT is 5–10 km thick and separated by a thrust fault from the underlying basement, (2) the basement is characterized by gneissic fabrics from the mid- to lower-crust of probable Paleo to Mesoarchean WRT rocks, and that (3) the crust was extensively reworked by Neoarchean post-tectonic magmatism that was characterized by carbon- and/or hydrogen-rich silicates and likely caused by partial melting of mantle/crustal rocks triggered by subduction-related fluids/melts. With the reprocessed Lithoprobe seismic data from the same region, a 3D crustal architecture is reconstructed and reveals a convergent belt between the main WRT to the north and a continental margin promontory of the WRT to the south. This Neoarchean convergent belt is characterized by an allochthonous greenstone belt as a thrust sheet in the upper crust, a collision zone in the mid- to lower-crust, an apparent crustal root of 3–5 km relief, and subcreted crustal rocks beneath a mantle wedge. Published data suggest that an outboard, north-dipping subduction zone associated with the Wawa terrane provided the fluids/melts for the post-tectonic magmatism in the WRT-WWT crust. This study favors a model of subduction followed by collision over a collision-only model, which indicates that multiple subduction zones were operative synchronously in a periodduring the assembly of the western Superior craton. Cratonization by accretionary orogenesis due to plate tectonics in the Neoarchean, therefore, is imaged and characterized in this study using a 3D crustal architecture.

Anomaly-based synoptic analysis and model product application for 2020 summer southern China rainfall events

Nine rainfall events affecting southern China in the 2020 summer were studied by a novel anomaly-based synoptic analysis approach, and the results were compared to those of standard synoptic analysis. These rainfall events occurred in the Dragon-boat rainy season and Meiyu rainy season from the Pearl River to the Yangtze and Huaihe rivers and did not entirely follow the climatic seasonal march of the summer monsoon. The first rainfall event occurred within the Dragon-boat rainy season and was associated with a broad rainfall area with scattered precipitation centers related to a broad convergence area of anomalous flows with scattered moist vorticity anomaly (MVA) centers located between two anomalous anticyclonic systems at 925 hPa. The other eight rainfall events appeared in the Meiyu rainy season and were generally associated with only a narrow convergence belt of anomalous flows and a positive MVA belt, sometimes with an anomalous cyclone moving eastward in the lower troposphere. The ECMWF model successfully predicted these anomalous convergence areas and MVA belts with lead times of 2–7 days. The advantages of this anomaly-based analysis approach include its ability to visually identify the location of a rainfall event, pinpoint possible meteorological causes and extend the model forecast length by the earlier detection of predictors.

Hydrothermal upgrading as an important tool for the REE mineralization in the Miaoya carbonatite-syenite complex, Central China

Abstract Secondary hydrothermal reworking of REEs has been widely documented in carbonatites/alkaline rocks, but its potential role in the REE mineralization associated with these rocks is currently poorly understood. This study conducted a combined textural and in situ chemical investigation on the REE mineralization in the ~430 Ma Miaoya carbonatite-syenite complex, central China. Our study shows that the REE mineralization, dated at ~220 Ma, is characterized by a close association of REE minerals (monazite and/or bastnäsite) with pervasive carbonatization overprinting the carbonatites and syenites. In these carbonatites and syenites, both the apatite and calcite, which are the dominant magmatic REE-bearing minerals, exhibit complicated internal textures that are generally composed of BSE-bright and BSE-dark domains. Under BSE imaging, the former domains are homogeneous and free of pores or mineral inclusions, whereas the latter have a high porosity and inclusions of monazite and/or bastnäsite. In situ chemical analyses show that the BSE-dark domains of the apatite and calcite have light REE concentrations and (La/Yb)N values much lower than the BSE-bright areas. These features are similar to those observed in metasomatized apatite from mineral-fluid reaction experiments, thus indicating that the BSE-dark domains formed from primary precursors (i.e., represented by the BSE-bright domains) through a fluid-aided, dissolution-reprecipitation process during which the primary light REEs are hydrothermally remobilized. New, in situ Sr-Nd isotopic results of apatite and various REE minerals, in combination with mass balance calculations, strongly suggest that the remobilized REEs are responsible for the subsequent hydrothermal REE mineralization in the Miaoya complex. Investigations of fluid inclusions show that the fluids responsible for the REE mobilization and mineralization are CO2-rich, with medium temperatures (227–340 °C) and low salinities (1.42–8.82 wt‰). Such a feature, in combination with C-O isotopic data, indicates that the causative fluids are likely co-genetic with fluids from coeval orogenic Au-Ag deposits (220–200 Ma) in the same tectonic unit. Our new findings provide strong evidence that the late hydrothermal upgrading of early cumulated REEs under certain conditions could also be an important tool for REE mineralization in carbonatites, particularly for those present in convergent belts where faults (facilitating fluid migration) and hydrothermal fluids are extensively developed.

The heavy rainfall during the warm season over the Pearl River Delta region: Movements and early signals

AbstractThe environments and movement, as well as the surface characteristics of heavy rainfall (HR), were examined based on 5‐year HR episodes collected from the surface observations during the warm season (April–September) from 2015 to 2019. The results showed that the HR generally exhibited eastward movement with average moving‐speed of 59 km⋅hr−1 over South China. The daytime HR exhibited more episodes and faster moving‐speed than the nocturnal HR. Both nocturnal HR and daytime HR were strongly influenced by the southerly wind during April–June over the Pearl River Delta (PRD) region. It showed a cyclonic wind pattern during the occurrence‐time of nocturnal HR and a strong early convergence belt at −3 hr of daytime HR between the southeasterly winds and the northeasterly winds over the north part of the PRD region, which might be related to the cold outflow from the upstream east‐moving convections. Another key characterization parameter of HR was the strong decrease of surface potential temperature over the PRD region, especially during July–September.

Role of Tropical Variability in Driving Decadal Shifts in the Southern Hemisphere Summertime Eddy-Driven Jet

AbstractThe Southern Hemisphere summertime eddy-driven jet and storm tracks have shifted poleward over the recent few decades. In previous studies, explanations have mainly stressed the influence of external forcing in driving this trend. Here we examine the role of internal tropical SST variability in controlling the austral summer jet’s poleward migration, with a focus on interdecadal time scales. The role of external forcing and internal variability are isolated by using a hierarchy of Community Earth System Model version 1 (CESM1) simulations, including the pre-industrial control, large ensemble, and pacemaker runs. Model simulations suggest that in the early twenty-first century, both external forcing and internal tropical Pacific SST variability are important in driving a positive southern annular mode (SAM) phase and a poleward migration of the eddy-driven jet. Tropical Pacific SST variability, associated with the negative phase of the interdecadal Pacific oscillation (IPO), acts to shift the jet poleward over the southern Indian and southwestern Pacific Oceans and intensify the jet in the southeastern Pacific basin, while external forcing drives a significant poleward jet shift in the South Atlantic basin. In response to both external forcing and decadal Pacific SST variability, the transient eddy momentum flux convergence belt in the middle latitudes experiences a poleward migration due to the enhanced meridional temperature gradient, leading to a zonally symmetric southward migration of the eddy-driven jet. This mechanism distinguishes the influence of the IPO on the midlatitude circulation from the dynamical impact of ENSO, with the latter mainly promoting the subtropical wave-breaking critical latitude poleward and pushing the midlatitude jet to higher latitudes.

Discussion on the southeastern boundary location of the Liao-Ji orogenic belt

The basement of the North China Craton formed through collision of the Eastern and Western blocks along the Trans- orth China Orogen at ca. 1.85 Ga. The Liao-Ji orogenic belt in the Liaodong Peninsula and southern Jilin represents a Paleoproterozoic convergent belt between the Longgang and Nangrim blocks in the Eastern Block, and has been considered as the northern segment of the entire NNE- to NE-trending Jiao-Liao-Ji Orogen across the whole Eastern Block from south to north. The Liao-Ji orogenic belt is characterized by the presence of sedimentary and volcanic successions, metamorphism, deformation and magmatism of the Paleoproterozoic ages whereas the Archean blocks show the dominance of Archean TTG gneisses and the absence of the Paleoproterozoic sedimentation, metamorphism and magmatism. However, the southeastern boundary location of the Liao-Ji orogenic belt remains uncertain and controversial, and the southeastern Liaodong Peninsula was considered as an Archean block by many authors. Two samples of meta-sediments and seven samples of meta-igneous rocks were collected from the Dandong area for zircon U-Pb dating in this study. Detrital zircon dating for the two meta-sediment samples, muscovite schist and meta-sandstone, constrains their depositional ages between 2013 (youngest magmatic zircon age) and 1864 Ma (metamorphic age), 2064 and 1910 Ma respectively, indicating Paleoproterozoic ages for their deposition. The Paleoproterozoic meta-sediments, also called the Liaohe Group, are scattered in many places of the Dandong area, and show similar lithology and metamorphic grade to those in the Liao-Ji orogenic belt. These show that the Paleoproterozoic meta-sediments in the Dandong area can be correlated with the Liaohe Group in the Liao-Ji orogenic belt. The basement rocks in the Dandong area, including meta-sediments and meta-plutons, exhibit extensive medium-grade metamorphism. Our zircon dating results suggest that the regional metamorphism experienced by the basement rocks in the Dandong area took place at 1910–1864 Ma (Paleoproterozoic). The obtained metamorphic ages in the Dandong area are synchronous with those recognized in the Liao-Ji orogenic belt, which have been interpreted as time of the collision-related metamorphism. The zircon U-Pb dating results for seven igneous samples in the basement, including granitic pegmatite, weakly deformed granite, monzonitic granitic gneiss and granitic gneiss, give late Paleoproterozoic emplacement time (1882–1827 Ma) for six samples and a 2470 Ma emplacement age for one sample, indicating extensive magmatism of the late Paleoproterozoic. Age spectra of magmatic zircons obtained in this study exhibit that the basement in the Dandong area experienced multi-stage magmatic activities, with age peaks at ca. 2665, 2500, 2200, 2095, 2030, and 1865 Ma respectively. They are similar to those obtained from Liao-Ji orogenic belt, but different from those from the Longgang Block. Previous detrital zircon dating also proved the presence of the Paleoproterozoic sediments and metamorphism in the Dandong area. These facts demonstrate that component and evolution of the basement rocks in the Dandong area show obvious similarity to those in the Liao-Ji orogenic belt, and the basement in the Dandong area belongs to a part of the Liao-Ji orogenic belt, rather than an Archean block as previously considered. Previous zircon dating results have shown that the Paleoproterozoic sediments and metamorphism occur extensively in the Changhai area, just east of the Liaodong Peninsula. The absence of the Paleoproterozoic sediments in the southern Liaodong Peninsula might be related to exhumation and erosion caused by the Liaonan and Wanfu metamorphic core complexes of the Early Cretaceous. It is proposed therefore that the Liao-Ji orogenic belt extends across the entire Liaodong Peninsula, and its southeastern boundary should reach to the NE-striking Yalujiang Fault Zone at least.

Possible connection between the East Asian summer monsoon and a swing of the haze-fog-prone area in eastern China

The summer monsoon has recently been hypothesized to influence haze-fog events over China, but the detailed processes involved have yet to be determined. In the present study, we found that the haze-fog-prone area swings over eastern China during boreal summer (May to September), coinciding with the movement of the subtropical monsoon convergence belt (hereinafter referred to simply as the “convergence belt”). Further investigation showed that the convergence belt modulates the spatial distribution of the haze-fog-prone area by altering the regional atmospheric conditions. When the warm and wet summer monsoon air mass pushes northwards and meets with cold air, a frontal zone (namely, the convergence belt) forms. The ascent of warm and wet air along the front strengthens the atmospheric stability ahead of the frontal zone, while the descent of cold and dry air weakens the vertical diffusion at the same place. These processes result in an asymmetric distribution of haze-fog along the convergence belt. Based on the criterion of absolute stability and downdraft, these atmospheric conditions favorable for haze-fog are able to identify 57–79% of haze-fog-prone stations, and the anticipation accuracy is 61–71%. After considering the influence of air pollutants on haze-fog occurrence, the anticipation accuracy rises to 78–79%. Our study reveals a connection between local haze-fog weather phenomena and regional atmospheric conditions and large-scale circulation, and demonstrates one possible mechanism for how the summer monsoon influences the distribution of haze-fog in eastern China.

Anomalous circulation patterns in association with two types of daily precipitation extremes over southeastern China during boreal summer

Based on the daily rainfall data from China Meteorological Administration, the tropical cyclone (TC) best track data from Japan Meteorological Agency, and the NCEP-NCAR reanalysis data from NOAA, regional mean daily precipitation extreme (RDPE) events over southeastern China (specifically, the Fujian-Jiangxi region (FJR)) and the associated circulation anomalies are investigated. For the summers of 1979–2011, a total of 105 RDPE events are identified, among which 35 are TC-influenced (TCIn-RDPE) and 70 are TC-free events (TCFr-RDPE). Distinct differences between these two types of RDPEs are found in both their statistical features and the related circulation patterns. TCFr-RDPEs usually occur in June, while TCIn-RDPEs mainly take place during July–August. When TCFr-RDPEs happen, a center of the anomalous cyclonic circulation is observed over the FJR, with an anomalous anticyclonic circulation to the south of this region. The warm/moist air flows from the South China Sea (SCS) and western Pacific meet with colder air from the north, forming a narrow convergent belt of water vapor over the FJR. Simultaneously, positive diabatic forcing anomalies are observed over the FJR, whereas negative anomalies appear over both its south and north sides, facilitating the formation and maintenance of the cyclonic circulation anomaly, as well as the upward motion of the atmosphere, over the FJR. When TCIn-RDPEs occur, southeastern China is dominated by a TC-related stronger anomalous cyclonic circulation. An anomalous anticyclonic circulation in the mid and high latitudes north of the FJR exists in the mid and upper troposphere, opposite to the situation during TCFr-RDPE events. Abundant warm/wet air is carried into the FJR from both the Indian Ocean and the SCS, leading to a large amount of latent heat release over the FJR and inducing strong ascending motion there. Furthermore, large differences are also found in the manifestation of Rossby wave energy propagation between these two types of RDPE events. The results of this study are helpful to deepen our understanding of the mechanisms behind these two types of RDPE events.

Foreland-directed propagation of high-grade tectonism in the deep roots of a Paleoproterozoic collisional orogen, SW Montana, USA

The study of deeply exhumed ancient collisional belts offers important constraints on geologic processes and properties complementary to inaccessible portions of the crustal column in active orogens. The ca. 1.8−1.7 Ga Big Sky orogeny in southwest Montana is a major convergent belt associated with the Proterozoic amalgamation of Laurentia. New structural, petrologic, and geochronologic data from the Northern Madison Range, crossing the NE-SW trend of the belt, record key information about the internal dynamics of the orogen. At least two phases of Big Sky−related deformation are preserved, both nearly coeval with peak metamorphic conditions of ∼0.9−0.8 GPa and >700 °C. Metamorphic zircon grains from a deformed mafic dike yield a weighted mean ion probe U-Pb date of 1737 ± 28 Ma (2σ). Monazite grains from a metapelite yield electron microprobe U-Th total-Pb dates of ca. 1750−1705 Ma, spanning prograde, peak, and retrograde intervals. Exposed Proterozoic paleodepths range from deeper levels (∼45−40 km; 1.2 GPa) in the northwestern end of the range to shallower levels (∼30−25 km) in the central-southeast area. The age of high-grade tectonism appears to become younger southeastward away from the core of the orogen, from ca. 1810−1780 Ma in the Highland Mountains, to ca. 1780−1750 Ma in the Ruby Range, Tobacco Root Mountains, and northwesternmost Northern Madison Range, and 1750−1720 Ma in the central Northern Madison Range. These spatial and temporal patterns of lateral growth and propagation of the orogen are similar to those observed in other collisional orogenic systems, and they may reflect multiple collision phases, protracted collision, and/or postcollisional collapse.

Present‐Day Crustal Movement and Focal Mechanism Solutions, and Plate Interaction Since Late‐Miocene in the Eastern South China Sea

AbstractThe plate convergent belt in the eastern South China Sea (SCS) is an important window for studying the tectonic evolution of the SCS and the Philippine Sea (PhS) since late‐Miocene. We studied the plate interactions of this area since the late‐Miocene based on block kinematics, morphology of the subducted SCS slab and focal mechanism solutions (FMS). At the late stage of the late‐Miocene, the westward motion of the PhS plate was first impeded by its collision with the Palawan micro continental plate in the south, and then by the collision of the Luzon arc with the South China continental margin in the north. Consequently, the middle part between the two collision zones became a passage where westward movement of the PhS plate continued relatively smoothly. Because of these two impediments, the rate of westward motion increased northwards to its maximum in the northern Luzon and then decreased northwards, leading to the westward migration of the Manila trench. The subducted SCS slab, however, did not roll back accordingly due to the SE‐directed mantle flow. As a result, the subducted slab suffered strong push and slightly reverse bending of the subducted slab occurred in the contact face between PhS and SCS lithosphere. According to this model, morphology of the Manila trench was shaped by the differential westward motion rates from south to north of the northern Luzon, and the width of the subducted slab played little role. Furthermore, the double arc in the north Luzon arc was also induced by a differential westward moving velocity along the arc. A shear zone whose location coincides with the east arc might have facilitated its formation.

Evaluation of CMIP5 coupled atmosphere-ocean general circulation models and projection of the Southeast Asian winter monsoon in the 21st century

The Southeast Asian (SEA) winter monsoon (December–February) precipitation simulation and the potential future climate changes may bring about under different representative concentration pathways (RCPs) were evaluated using ten coupled atmosphere–ocean general circulation models (AOGCMs) in the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Although all the models simulated the broad features of winter monsoon precipitation spatial patterns, the spread of the bias magnitudes was very large across the AOGCMs. All the models simulated the relationship between the regional rainfall and circulation associated with El Niño-Southern Oscillation (ENSO). However, the strength of the association tended to be weaker in the simulations, ensuing a generally weaker winter monsoon rainfall interannual variability. Three AOGCMs, namely CNRM-CM5, NorESM1-M and IPSL-CM5A-MR, that simulated the most realistic present-day climate, were used for examination of future climate projection based on the RCP 2.5, RCP 4.5 and RCP 8.5 scenarios. The projections showed a gradual increment of SEA winter monsoon rainfall under all the scenarios with the highest changes in the RCP 8.5 scenario. The increment was manifested as a zonal band of positive anomalies located at ∼10°N and ∼5°S, indicating northward and southward expansion of the intertropical convergence belt. Between these bands of increasing rainfall, the equatorial South China Sea (SCS) area was projected to be slightly drier. The projected increase of rainfall appeared to be associated with the prevailing of cyclonic anomalies over the centre SCS, which promote enhance moisture convergence over this region in the warmer climate.

A review on the numerical geodynamic modeling of continental subduction, collision and exhumation

Continental subduction and collision normally follows oceanic subduction, with the remarkable event of formation and exhumation of high-to ultra-high-pressure (HP-UHP) metamorphic rocks. Based on the summary of numerical geodynamic models, six modes of continental convergence have been identified: pure shear thickening, folding and buckling, one-sided steep subduction, flat subduction, two-sided subduction, and subducting slab break-off. In addition, the exhumation of HP-UHP rocks can be formulated into eight modes: thrust fault exhumation, buckling exhumation, material circulation, overpressure model, exhumation of a coherent crustal slice, episodic ductile extrusion, slab break-off induced eduction, and exhumation through fractured overriding lithosphere. During the transition from subduction to exhumation, the weakening and detachment of subducted continental crust are prerequisites. However, the dominant weakening mechanisms and their roles in the subduction channel are poorly constrained. To a first degree approximation, the mechanism of continental subduction and exhumation can be treated as a subduction channel flow model, which incorporates the competing effects of downward Couette (subduction) flow and upward Poiseuille (exhumation) flow in the subduction channel. However, the (de-)hydration effect plays significant roles in the deformation of subduction channel and overriding lithosphere, which thereby result in very different modes from the simple subduction channel flow. Three-dimensionality is another important issue with highlighting the along-strike differential modes of continental subduction, collision and exhumation in the same continental convergence belt.

Time-lagged impact of spring sensible heat over the Tibetan Plateau on the summer rainfall anomaly in East China: case studies using the WRF model

This study explores the time-lagged impact of the spring sensible heat (SH) source over the Tibetan Plateau (TP) on the summer rainfall anomaly in East China using the Weather Research and Forecasting model. Numerical experiments for 2003 indicate that a spring SH anomaly over the TP can maintain its impact until summer and lead to a strong atmospheric heat source, characterized both by the enhanced SH over the western TP and enhanced latent heat of condensation to the east. Wave activity diagnosis reveals that the enhanced TP heating forces a Rossby wave train over the downstream regions. A cyclonic response over the northeast TP brings about a low-level northerly anomaly over northern China, while an anticyclonic response over the western Pacific enhances the subtropical high and the low-level southerly on its western flank. As a result, cold and dry airflow from mid-high latitudes, and warm and wet airflow from tropical oceans converge around the Huaihe River basin. In addition, warm advection originating from the TP induces vigorous ascending motion over the convergence belt. Under these favorable circulation conditions the eastward-propagating vortexes initiated over the TP intensify the torrential rainfall processes over the Huaihe River basin. In contrast, additional experiments considering the year 2001 with weak spring SH over the TP and an overall southward retreat of the summer rainfall belt in East China further demonstrate the role of spring SH over the TP in regulating the interannual variability of EASM in terms of wave activity and synoptic disturbances.

Rayleigh‐Taylor instability, lithospheric dynamics, surface topography at convergent mountain belts, and gravity anomalies

AbstractSurface topography and associated gravity anomalies above a layer resembling continental lithosphere, whose mantle part is gravitationally unstable, depend strongly on the ratio of viscosities of the lower‐density crustal part to that of the mantle part. For linear stability analysis, growth rates of Rayleigh‐Taylor instabilities depend largely on the wave number, or wavelength, of the perturbation to the base of the lithosphere and weakly on this viscosity ratio, on plausible density differences among crust, mantle lithosphere, and asthenosphere, and on ratios of crustal to total lithospheric thicknesses. For all likely densities, viscosities, and thicknesses, the Moho is drawn down (pushed up) where the base of the lithosphere subsides (rises). For large viscosities of crust compared to mantle lithosphere (ratios > ~30), a sinking and thickening mantle lithosphere also pulls the surface down. For smaller viscosity ratios, crustal thickening overwhelms the descent of the Moho, and the surface rises (subsides) above regions where mantle lithosphere thickens and descends (thins and rises). Ignoring vertical variations of viscosity within the crust and mantle lithosphere, we find that the maximum surface height occurs for approximately equal viscosities of crust and mantle lithosphere. For large crust/mantle lithosphere viscosity ratios, gravity anomalies follow those of surface topography, with negative (positive) free‐air anomalies over regions of descent (ascent). In this case, topography anomalies are smaller than those that would occur if the lithosphere were in isostatic equilibrium. Hence, flow‐induced stresses—dynamic pressure and deviatoric stress—create smaller topography than that expected for an isostatic state. For small crust/mantle viscosity ratios (< ~10), however, calculated surface topography at long wavelengths is greater than it would be if the lithospheric column were in isostatic equilibrium, and at short wavelengths local isostasy predicts surface deflections of the wrong sign. For the range of wavelengths appropriate for convergent mountain belts (~150–600 km), calculated gravity anomalies are negative over regions of lithospheric thickening, especially when allowance for flexural rigidity of a surface layer is included. Correspondingly, calculated values of admittance, the ratio of Fourier transforms of surface topography and free‐air gravity anomalies, are also negative for wave numbers relevant to mountain belts. For essentially all mountain belts, however, measured free‐air anomalies and admittance are positive. Whether gravitational instability of the lithosphere affects the structure of convergent belts or not, its contribution to the topography of mountain belts seems to be small compared to that predicted for isostatic balance of crustal thickness variations.

Cratonic reactivation and orogeny: An example from the northern margin of the North China Craton

Reactivation of cratonic basement involves a number of processes including extension, compression, and/or lithospheric delamination. The northern margin of the North China Craton (NCC), adjacent to the Inner Mongolian Orogenic Belt, was reactivated in the Late Paleozoic to Early Mesozoic. During this period, the northern margin of the NCC underwent magmatism, N–S compression, regional exhumation, and uplift, including the formation of E–W-trending thick-skinned and thin-skinned south-verging folds and south-verging ductile shear zones. zircon U–Pb SHRIMP ages for mylonite protoliths in shear zones which show ages of 310–290Ma (mid Carboniferous–Early Permian), constraining the earliest possible age of deformation. Muscovite within carbonate and quartz–feldspar–muscovite mylonites from the Kangbao–Weichang and Fengning–Longhua shear zones defines a stretching lineation and gives 40Ar/39Ar ages of 270–250Ma, 250–230Ma, 230–210Ma, and 210–190Ma. Deformation developed progressively from north to south between the Late Paleozoic and Triassic. Exhumation of lower crustal gneisses, high-pressure granulites, and granites occurred at the cratonic margin during post-ductile shearing (~220–210Ma). An undeformed Early Jurassic (190–180Ma) conglomerate overlies the deformed rocks and provides an upper age limit for reactivation and orogenesis. Deformation was induced by convergence between the southern Mongolia and North China cratonic blocks, and the location of this convergent belt controlled later deformation in the Yanshan Tectonic Province. This province formed as older E–W-trending Archean–Proterozoic sequences were reactivated along the northern margin of the NCC. This reactivation has features typical of cratonic basement reactivation: compression, crustal thickening, remelting of the mid to lower crust, and subsequent orogenesis adjacent to the orogenic belt.