2018 Vol. 29, No. 1
The large magnitude of the dimensionless Rayleigh number (Ra ~108) for Earth's ~3 000 km thick mantle is considered evidence of whole mantle convection. However, the current formulation assumes behavior characteristic of gases and liquids and also assumes Cartesian geometry. Issues arising from neglecting physical properties unique to solids and ignoring the spherical shapes for planets include: (1) Planet radius must be incorporated into Ra, in addition to layer thickness, to conserve mass during radial displacements. (2) The vastly different rates for heat and mass diffusion in solids, which result from their decoupled transport mechanisms, promote stability. (3) Unlike liquids, substantial stress is needed to deform solids, which independently promotes stability. (4) High interior compression stabilizes the mantle in additional minor ways. Therefore, representing conditions for convection in solid, self-gravitating spheroids, requires modifying formulae developed for bottomheated fluids near ambient conditions under an invariant gravitational field. To derive stability criteria appropriate to solid spheres, we use dimensional analysis, and consider the effects of geometry, force competition, and microscopic behavior. We show that internal heating has been improperly accounted for in the Ra. We conclude that the lower mantle is stable for two independent reasons: heat diffusion far outpaces mass diffusion (creep) and yield strength of solids at high pressure exceeds the effective deviatoric stress. We discuss the role of partial melt in lubricating plate motion, and explain why the Ra is not applicable to the multi-component upper mantle. When conduction is insufficient to transport heat in the Earth, melt production and ascent are expected, not convection of solid rock.
As a minor phase, kyanite has been repeatedly shown to have experienced ultrahigh pressure (UHP) metamorphism together with its host eclogites. Thus, it could play some role in transporting water into the deep earth. Here we present a detailed investigation of water concentrations of kyanite, and for reference, of garnet and omphacite from four Maobei eclogites in the Sulu orogenic belt, eastern China. Fourier transform infrared (FTIR) measurements show that kyanites, garnets, and omphacites all have distinct hydroxyl absorption bands due to OH groups bound in their crystal structure. The FTIR profile analyses on ten grains from different samples reveal a homogeneous distribution of water across kyanite, suggesting insignificant water loss during exhumation. The calculated water concentrations in kyanite (21 wt ppm–41 wt ppm) are comparable to those reported previously for kyanite from various geological occurrences when using the most recent calibration. They are however much lower compared with those in garnet (46 wt ppm–83 wt ppm) and omphacite (302 wt ppm–548 wt ppm) from the Maobei eclogites. This implies that kyanite is not a major water carrier in eclogites considering its low volume fraction and contributes negligibly to transport water into the deep mantle accompanying subducted oceanic crust until its possible transformation to AlSiO3OH.
The plate affiliation of the North Dabie terrane (NDT) has been controversial. To address this fundamental question, an integrated study of internal structure, in-situ U-Pb dating and trace element analysis in zircons and field investigation for migmatite in the NDT was carried out, which reveals participation of crustal rocks of the North China craton (NCC) in the protolith in addition to the more common crustal rocks of the Yangtze craton (YC). The evidence of an NCC affinity for protolith of migmatite in the NDT is the ~2.5 Ga (2 486±14 and 2 406±26 Ma) magamtic age and ~1.8 Ga (1 717±79 Ma) metamorphic age in the relict zircon domains because these two age groups are characteristic for the evolution of the NCC. The evidence of a YC affinity for protolith of migmatite in the NDT is the more common 0.7–0.8 Ga (e.g., 787±12 Ma) magamtic zircon age. Mid-Neoproterozoic magmatic age (0.7–0.8 Ga) is a symbol of YC basement rocks. In view of the widely exposed YC crustal components in the NDT, we suggest that the protolith of the NDT is mainly crustal rocks from the YC with minor crustal components from the NCC. The zircon rim domains and new growth grains from all the migmatite samples are characterized by anatectic zircons and have a widely concordant ages ranging from 112.2±2.8 to 159.6±4.3 Ma with several peak values, suggesting a long lasting multistage anatexis. In conclusion, the NDT has a mixed protolith origin of both the YC and the NCC crustal rocks were strongly remoulded by anatexis during orogenic collapse.
The Paleoproterozoic Birimian granitoids of the West African Craton (WAC) in the northwestern part of Ghana, have been studied for their zircon trace elements concentrations to infer the source characteristics, origin, and magmatic evolution. The zircons in the granitoids have Th/U ratios ranging from 0.03 to 1.55, and display depleted light rare earth elements (LREE) and enriched heavy rare earth elements (HREE) contents, characterized by pronounced positive to negative anomalies of Eu (Eu/Eu*=0.14–0.98 and 1.01–6.06, respectively) and Ce (Ce/Ce*=0.08–0.98 and 1.02–116, respectively), which may imply that they were derived from both magmatic and hydrothermal sources. The geochemical plots of U/Yb vs. Y and Hf, the positive correlation between Hf and the other high field strength elements (HFSE) and high rare earth elements (REE) contents, with enrichment in Ce and depletion in Eu, indicate that the granitoids possibly formed from partial melting of the crust. The trace elements characteristics (i.e., wide range of Hf, Ce/Ce*, Th/U and Zr/Hf values) of the zircons suggest that crystallization of the magma occurred under variable oxidation states, which spanned over a longer period, implying that our data corroborate interpretations from studies of whole-rock geochemistry and geochronology on the granitoids of northwestern Ghana. This further indicates that the evolution of the Birimian granitoids in this part of the WAC occurred earlier than what had been reported in the literature.
Several Cretaceous Carlin-like or hydrothermal gold deposits along the Garze-Litang suture zone and Early Cretaceous hydrothermal copper mineralization along the southeastern margin of the Songpan-Garze fold belt were presumed to have a magmatic heat source. However, no actual coeval magmatic events nearby were discovered. Here, we report zircon SIMS U-Pb age, whole-rock geochemical and Sr-Nd isotopic data of the Xiqiu basalts in the southern end of the Yidun terrane, eastern Tibetan Plateau. New zircon U-Pb ages yield weighted mean 206Pb/238U age of 117.7±1.6 Ma. The basalts are classified as calc-alkaline to alkaline and have relatively high MgO (4.77 wt.%–10.84 wt.%) and Mg number values (Mg#=(100×Mg/(Mg+Fe2+)); 45.35–67.28) and positive εNd(t) (t=118 Ma) values (+1.86 to +3.2), suggesting a OIB-like mantle source that is consistent with the normalized patterns of trace elements and rare earth elements (REEs). Geochemical data suggest that the primary basaltic magma was generated by low degree partial melting of a peridotite-dominated mantle source with a minor component of garnet-eclogite or pyroxenite and experienced olivine+clinopyroxene dominated fractional crystallization. The primary melt compositions calculated from the high MgO samples, in turn, suggest that the Xiqiu basalts were generated at 1.6–2.9 GPa with abnormally hot mantle potential temperatures from 1 465 to 1 540 ℃. The melting temperatures are similar to the abnormally hot mantle underneath the Colorado Plateau and hotter than the mid-ocean range basalt (MORB) mantle and normal intra-continental mantle. Combined with previous studies, the Cretaceous Xiqiu basalts allow us to reconstruct a tectonic and geodynamic evolutionary model responsible for the Late Jurassic to Late Cretaceous geological records (magmatism, ore deposits and enhanced exhumation) in the Yidun terrane and southern Songpan-Garze fold belt.
Dulaerqiao granite is located at the Xinlin-Xiguitu-Toudaoqiao suture zone between the Erguna massif and the Xing'an massif, northeast of Inner Mongolia. The rocks are mainly composed of K-feldspar, quartz, and plagioclase. Zircon LA-ICP-MS U-Pb data show that this granite was deposited in the Late Carboniferous Period (308.7±2.0 Ma). The samples are rich in alkali, Fe, and Al and low in Mg, Ca, and P. Chondrite-normalized REEs exhibit right-inclined patterns with significant negative Eu anomalies. Additionally, the granite shows high quantities of trace elements such as Zr, Hf, Th, K, and Rb and decreased quantities of Sr, P, and Ti. The chemical characteristics identified herein and a series of diagrams that distinguish different types of granite show that Dulaerqiao alkali-feldspar granites belong to the aluminous A-type granite group. Meanwhile, the initial magma crystallizes under high-temperature, low-pressure conditions resulting from a tectonic extension setting. The formation of Dulaerqiao aluminous A-type granite is related to the rejuvenation of the ancient Xinlin-Xiguitu-Toudaoqiao suture zone, which was activated by the interaction between the combined Erguna-Xing'an massif and the Songnen massif in the Late Paleozoic Era. This aluminous A-type granite was deposited about 30 Ma after the collision.
The olivine-gabbroic rocks located at the Huangling anticline within the Yangtze Craton are dated at circa 857–854 Ma by LA-ICP-MS method. The rocks belong to the sub-alkaline series and consist of pyroxene (35%–40%), plagioclase (40%–45%), olivine (8%–10%) and spinel (3%–5%). Olivine has Fo values of 73–83 that is classified as chrysolite. Pyroxene has relative low contents of FeO (6.60 wt.%–8.23 wt.%) but high CaO (20.23 wt.%–21.25 wt.%) contents, however, plagioclase has high Al2O3 (31.78 wt.%–32.37 wt.%), CaO (16.08 wt.%–16.25 wt.%) and An (79–80) values, but low Na2O contents (1.95 wt.%–2.11 wt.%). Spinel are magnesioferrite with characteristics of high contents of MgO (13.65 wt.%– 13.68 wt.%), FeO (23.27 wt.%–23.40 wt.%) and Al2O3 (62.43 wt.%–62.74 wt.%). Chemical compositions of these minerals are similar to those of gabbro rocks that were formed in the post-orogeny environment. The olivine-gabbro samples have negative zircon εHf values (-16.57±0.47) that resemble the mafic rocks in the same region, indicating that they are derived from the extremely enriched mantle source. On the compilation of documented Neoproterozoic mafic rocks in the Yangtze Craton, it is proposed that the mantle in the northern Yangtze Craton has experienced different degrees enrichment during the Neoproterozoic.
The Niujuanzi ophiolitic mélange represents the remnant of oceanic crust between the Dunhuang massif and Mingshui-Hanshan massif. Greywacke from different tectonic slices in the Niujuanzi ophiolitic mélange were analyzed for detrital zircon U-Pb geochronology, trace elements and whole-rock trace elements to infer their provenance and the evolution of the Niujuanzi Ocean. Sample N-76s containing Carboniferous spores has the youngest zircon age of 323 Ma, while sample N-85s without fossils has the youngest zircon age of 449 Ma. The two samples were deposited no earlier than 323 and 449 Ma, respectively. The greywackes are depleted in large ion lithophile elements, and are relatively enriched in high field strength elements. The age spectra and trace element concentrations indicate that the sediments may have been deposited near the trench. The Hf, U, and Yb contents of zircons from sample N-76s vary widely, whereas those from sample N-85s have a narrow range. Sample N-76s contains both continental and oceanic zircons, while sample N-85s contains only continental zircons. The sediments were derived from the continental arc and accretionary wedge. The Paleozoic oceanic crust zircons have ages of 430–500 Ma, indicating the timing of the expansion of the Hongliuhe-Niujuanzi-Xichangjing Ocean expansion. The oldest Paleozoic continental zircon has an age of 470 Ma, suggesting that the northward subduction of the oceanic crust may have started at that time.
Shadong deposit is the first large-scale tungsten deposit found in the East Tianshan orogenic belt, and the geologic characteristics of the deposit indicate that the deeply concealed granite body is genetically related with the mineralization. The LA-ICPMS U-Pb age of zircons from the Shadong concealed granite obtained in this research is 239±2.0 Ma, belonging to the Middle Triassic. The whole rock samples are metaluminous to slightly peraluminous (A/CNK=0.95–1.02) with low contents of SiO2 (64.0 wt.%–68.5 wt.%) and low K2O/Na2O ratios (0.73–0.96). The samples reveal enrichment of K, Rb, Th and depletion of Nb, Ta, P, Ti and have a negative slope from La to Lu (LaN/YbN=16.29–36.8) with weak negative Eu anomaly (Eu/Eu*=0.71–0.82). Initial 87Sr/86Sr ratios of whole rock range of 0.706 59–0.707 75, εNd(t) values range from -1.77 to -2.53 and εHf(t) values of zircon are between 2.54 and 4.90. The lithogeochemistry and Sr-Nd-Hf isotopic characteristics revealed that the concealed granite in Shadong tungsten deposit is Ⅰ-type granite, and occurs in an intraplate tectonic setting. The magma mixing during intraplating of mantle derived magma intruding into the crust in Indosinian Period is the major formation mechanism of the granite. Of which, the proportion of mantle derived magma ranges from 58% to 60%, and the crustal materials are mainly the metamorphic basement of Xingxingxia Group of Mesoproterozoic Changcheng System, which may provide the main source of ore forming metals of Shadong tungsten deposit.
The extensive Eshan granites of Yunnan are made up of three intrusive units distinguished by their field contact relations; in descending order of age they are the Pojiao Unit, the Lüzicun Unit and the Mokela Unit. The Pojiao Unit and Lüzicun Unit contain petrographically and geochemically similar rocks but contact relationships show that the latter is younger. The Mokela Unit mainly consists of dykes intruding the other two and has petrographic and geochemical differences. Zircon U/Pb dating and zircon crystallization temperature measurements confirm the sequence of intrusions. Major and trace element analyses suggest that the magmas of the Pojiao Unit granites derived by partial melting of a clay-poor source from the upper crust; the magmas of the Lüzicun Unit granites derived by partial melting of upper crust with a small proportion of middle crust accompanied by crystallization of albite which triggered strength reduction. Both magmas mixed and underwent with crustal contamination, assimilation and fractional crystallization. The magmas of the Mokela Unit derived from residual melts and assimilation of argillaceous rocks. A time sequence of melting, intrusion and deformation events is derived from these results and compared with other published tectonic models for the evolution of the SW margin of the Yangtze Plate. Magmatism was initiated by exhumation of upper continental crust during which strongly peraluminous porphyritic biotite monzogranite granites were produced at ca. 854–852 Ma, and the genesis of two-mica granite reflected a later batch of exhumed melts with crustal contamination, assimilation and fractional crystallization at ca. 842 Ma. Finally biotite alkali-feldspar granite and tourmaline granite magmas experienced strong fractional crystallization, emplaced in the cooling stage at ca. 823 Ma, indicating the end of exhumation.
The radial sand ridge system (RSRS) located at Jiangsu coast of China attracts much attention on its origin and mechanic of formation for its special structure and potential land resource. Due to complicated hydrodynamic condition, the Jiangsu RSRS is a hot debated on its potential sources, Yangtze River or Yellow River? We collected ten sand samples from surface sediments along the west coast of Bohai Sea and Yellow Sea from the modern Yellow River estuary to Yangtze River estuary in summer, 2013. The samples are analyzed by method of detrital zircon age for source identification of the RSRS sediments. The U-Pb age spectra of detrital zircon grains of the samples show a wide range from Cenozoic to Late Archean with several age peaks. Comparing the age spectra between the Yangtze River and the Yellow River, the detrital zircons have younger age (< 100 Ma) group in the Yangtze River. These age distribution of the Jiangsu coastal RSRS sediments are similar to that of the Yangtze River, but different from the Yellow River. The samples located adjacent to the old Yellow River Delta show more wide-range age distribution, implying a compounded origination from the both rivers. Based on these findings it is proposed that, contrary to common opinion, the main sediment source of the Jiangsu RSRS is the Yangtze River, rather than the Yellow River. By implication, there should be evidence of hydrodynamic mechanics of oceanic currents and tidal motion. This aspect awaits confirmation in future research.
Carbonates of Central Luconia, Malaysia, had been serving as hydrocarbon reservoirs for more than 25 years. However, the relationship and impacts of extensive tectonic events to the growth of Miocene carbonates in Central Luconia have not been revealed sufficiently. In this work, two carbonates platforms in the southern part of Central Luconia were used for detailed interpretation and seismic based structural restoration. This work had provided new insight for the interpretation of carbonate growth in Central Luconia and its association with the surrounding tectonic. This work suggested three possible tectonic evolutions from Late Oligocene to Pliocene are responsible for the growth of carbonates in Central Luconia. These stages are equivalent with pre-carbonate stage (Late Oligocene-Early Miocene), syn-carbonate stage (Middle-Late Miocene) and post-carbonate stage (Pliocene). Rifting of the South China Sea and subduction of proto-South China Sea are believed to be responsible for the development of faulting during pre-carbonate stage, while movement of the ancient Baram Line is thought to control the parallel striking direction of normal faults during syn-carbonate stage. Finally, subsidence and compaction due to the overburden clastic materials from the prograding deltas is considered as the main reason for the impacts of gravitational tectonics in this area, which corresponding to the post-carbonate stage.
This study used 2D seismic profiles to investigate the Cenozoic evolution of faults in the Huizhou Sub-basin. It aims to define the basin structure style through describing the geometric features of the faults and quantitatively analyzing their activities. The results indicate that the boundary faults in the Huizhou Sub-basin display en echelon arrangement in plan view, which indicates that, it was caused by a kind of oblique extension. Calculating the fault slips shows long-term activities of faults occurred in the basin, and some boundary faults kept active after 5.5 Ma. The evolution history of the fault system is reconstructed. Initially, during the Eocene-Early Oligocene, mainly NNE-NE and NW trending faults and NE striking grabens and half-grabens formed in the basin and a series of faults system controlled the deposition. During the Late Oligocene-Early Miocene, the structural activities were relatively weak, the fault activity and the fault growth rate decreased sharply. Finally, in the late stage from Late Miocene to the present, the structure movement was re-activated, and some faults were also reactivated. Our study will help better understand the structural features and evolution of the petroleum-bearing basins in the northern margin of the South China Sea.
The Altyn Tagh fault is the northern boundary of Tibetan Plateau. As one of the most well-known strike-slip fault in the world, great achievements on tectonic deformation and Late Quaternary slip rate have been made. However, there is a long-lasting debate on whether the Altyn Tagh fault extends into the Jinta Basin or even eastward. In this paper, we use satellite image interpretation, field investigation, trench excavation, and optical stimulated luminescence dating to study newly found NS striking scarps in the eastern end of Jinta Nan Shan. The results are as follows: firstly, a group of normal faults develop on terrace T2 of Heihe River, the total length amounts to ~40 km, total scarp height is 30±5 m; secondly, four paleoseismic events have been interpreted from three trenches, approximate ages of events are 79.97±19.14 ka BP, 62.55±13.10~55.41±10.77 ka BP, before 16.89±2.08 ka BP, 8.52±1.49 ka BP, respectively; thirdly, just like NS normal faults in the western end of Altyn Tagh fault, the newly found NS extensional faults are likely the terminating tectonics of the eastern end of Altyn Tagh fault, the large Altyn Tagh fault may end in the eastern end of Jinta Nan Shan.
Langshan, a monoclinic mountain, which started to uplift since Oligocene, bounds the northwest margin of the Hetao Basin. The continuous activity of the active normal Langshan range-front fault forms the typical basin-and-range landform in Langshan area and controls the landform evolution of Langshan. Langshan is an ideal place to study relationship between quantitative geomorphological index and active deformation. According to study on knickpoints, fitting on longitudinal channel profiles and steepness index, we demonstrate that the main controlling factors on distribution of normalized steepness index of channels are not climate (precipitation), lithology, sediment flux, but tectonic factor, or the activity of Langshan range-front fault. The short channels in southeast flank, whose lengths are shorter than 16 km, may be still in the non-steady status. If not considering these short channels, the distribution of normalized steepness index along the Langshan range-front fault appears like M-shape pattern, while the normalized steepness index in the middle section is higher than those at both ends. This pattern is well consistent with geometrical segmentation model of the Langshan range-front fault. Combining previous active tectonic research on Langshan range-front fault, which demonstrates the Langshan range-front fault has been in the stage of linkup, we reasonably infer the Langshan range-front fault now is the result of linkup of both fault which continuously bilaterally extended independently. Our tectonic geomorphological study also supports the conclusion that the Langshan range-front fault has been in the stage of linkup. The formation of several knickpoints due to tectonic factor may have been caused by slip-rate variation because of linkup of both independent faults. Based on cognition above, we also proposed the geological and geomorphological evolutionary model of the Langshan range-front fault since Oligocene.
Radiolarites have an important role in the reconstruction of the paleogeography, bathymetry and their coexistence with ophiolites gives an opportunity to determine the tectonic history of collisional zones. The radiolarite units of the southwestern Neyriz are part of the Zagros accretionary prism and positioned beneath the thick bedded carbonate sediments of Tarbur Formation. The existing various structural elements within this unit give a unique fortunate for investigation on the deformation history and studying of the kinematics of the Zagros collision zone. The numerous evidence show that this unit has experienced different deformation conditions during ongoing evolution, including ductile, brittle-ductile and brittle deformation conditions. The main strike of E-W for axial planes of folds, eastward trend and plunge of fold axes, boudins' neck axes and Type Ⅲ of the fold interference patterns are indicators of formation and evolution of folds during transpressional deformation. Structural evolution of the study area has been affected by an NE-dipping subduction zone of Neo-Tethyan oceanic lithosphere below the Iranian microcontinents. These structural elements suggest that high mechanical anisotropy and two successive generations of folding simultaneously with thrusting and extrusion of this unit had led to formation and evolution of this highly asymmetrical folded unit on top of the subducted oceanic crust.
High-purity N2 was used to increase the mobile phase flow rate during ion purification of ion-exchange resin. This was performed to improve the efficiency of isotope separation and purification, and to meet the efficiency requirements of rapid multiple-collector-inductively coupled plasma mass spectrometry (MC-ICPMS) analysis. For Cu isotope separation, our results indicated that at a gas flow rate > 60 mL/min, the separation chromatographic peaks broadened and the recovery rate decreased to < 99.2%. On the other hand, no significant change in the Cu peaks was observed at a gas flow rate of 20 mL/min and the recovery rate was determined to be > 99.9%. The Cu isotope ratio, measured by the standard-sample bracketing method, agreed with reference data within a ±2 SD error range. The separation time was reduced from the traditional 10 h (without N2) to 4 h (with N2), indicating that the efficiency was more than doubled. Moreover, Sr and Nd isotope separation in AGV-2 (US Geological Survey andesite standard sample) accelerated with a 20 mL/min gas flow, demonstrating that with the passage of N2, the purified liquid comprised Rb/Sr and Sm/Nd ratios of < 0.000 049 and < 0.000 001 5, respectively. This indicated an effective separation of Rb from Sr and Sm from Nd. MC-ICPMS could therefore be applied to accurately determine Sr and Nd isotope ratios. The results afforded were consistent with the reference data within a ±2 SD error range and the total separation time was shortened from 2 d to < 10 h.
This paper presents a comprehensive area expansion prediction index method to apply GNSS for short-impending prediction of earthquakes. Based on continuous GNSS observation data from Yunnan Province, a displacement field was detected after data cycle-slip repair using precision data processing software and geophysical field effect model correction. The Yunnan area was divided into 56 grid cells for displacement field interpolation to obtain a more uniform displacement field and a strain field variation time series. The pre-earthquake response of each grid-cell expansion time series was evaluated and synthesized to extract a short-impending earthquake anomaly identification index. The results show that this index indicated occurrence times and hypocenter for earthquakes of magnitude M≥5. Fourteen earthquakes were predicted accurately, and there were five false reports. This index can therefore be used for the short-impending prediction of earthquakes.