Deep Geodynamic Processes Research Articles

The Congo Basin: Stratigraphy and subsurface structure defined by regional seismic reflection, refraction and well data

The Congo basin (CB) occupies a large part of the Congo craton, which derived from the amalgamation of different cratonic pieces. As intracratonic basin, it initiated, possibly as a failed rift, in late Mesoproterozoic and evolved during the Neoproterozoic and Phanerozoic in an intraplate setting. For this reason, the CB can be considered a natural laboratory for investigating the processes that govern the long–term evolution of continental interiors. In this study, we reconstructed the stratigraphy and tectonic evolution of the CB using all available and geological seismic data (reflection and refraction seismics, borehole and field data). We interpreted almost 2600 km of seismic reflection profiles and well log data located inside the central area of the CB (the “Cuvette Centrale”). The obtained model will be further constrained by aeromagnetic and satellite gravity data and density measurements from rock samples in a next paper. Results show that the depth to the basement varies quite significantly, defining a series of structural highs and depocenters that developed throughout the history of the basin. The major controlling factors for the development of the CB are, besides the deep geodynamic processes, the inherited heterogeneity of the pre-Neoproterozoic basement, the tectonic evolution of Rodinia, Gondwana and Pangea amalgamation and breakup, and environmental conditions influenced by the drifting through the South Pole towards its present-day equatorial position and global climatic fluctuations between icehouse and greenhouse conditions. • Formation - evolution of Congo Basin using available geological - seismic data • Spatio-temporal evolution during the last 1 GA in the African continent • Intracontinental sag basin initiated probably as failed rift in late Mesoproterozoic. • Several deep basins (locally up to 11 km deep) separated by basement highs. • Registered Rodinia and Gondwana global glacial and geodynamic events

Earth's surface responses during geodynamic evolution: Numerical insight from the southern East China Sea Continental Shelf Basin, West Pacific

The interaction between the Earth's surface and deep geodynamic processes has a great influence on sedimentary patterns and characteristics of sedimentary systems, especially in the continental margin, such as the southern part of the East China Sea Continental Shelf Basin (ECSCSB). However, the exact processes and dynamics are poorly understood. The numerical tool (Badlands) was adopted to simulate the geological evolution of the southern ECSCSB during the Mesozoic transitional regime of the West Pacific continental margin, between 200 and 100 Ma. The numerical model involving four-dimensional (4D) sequence stratigraphy was validated by comparing with the tectono-morphology, sedimentary environments and sediment distribution. According to our findings, the transition was mainly controlled by three structural stages. The first transitional stage of the southern ECSCSB was mainly controlled by lithospheric-scale tectonic evolution. In the second stage, the dynamic topography contributed to slow regional uplift, while the last stage showed mantle-induced dynamic subsidence. The model results on 4D geomorphic characteristics, sediment distribution of basin evolution and deep dynamic topography, provided a quantitative image for geological evolution of the West Pacific continental margin in the Mesozoic.

Molybdenum and Boron Isotopic Compositions of Porphyry Cu Mineralization‐Related Adakitic Rocks in Central‐Eastern China: New Insights Into their Petrogenesis and Crust‐Mantle Interaction

AbstractThe voluminous Late Mesozoic porphyry ore‐related adakitic magmatism in central‐eastern China is crucial to understand the deep geodynamic process of the region and controls on porphyry deposit formation globally. Here, we present a novel study of whole‐rock Mo‐B isotopes on the mineralization‐related adakitic rocks from the Dexing (located in the interior of South China and associated with giant porphyry Cu deposits), and Middle‐Lower Yangtze River (MLYR) (host to one of the largest porphyry Cu‐Au deposit belts) areas in central‐eastern China. The Dexing adakitic rocks have δ98/95Mo ranging from −0.31‰ to −0.01‰ and δ11B from −17.65‰ to −11.49‰. The adakitic rocks from the MLYR area have δ98/95Mo values of −0.5‰ to 0.2‰ and δ11B values of −14.48‰ to −9.53‰. The δ98/95Mo values of both the Dexing and MLYR samples negatively correlate with the La/Yb and La/Sm ratios. Together with the high Mg#, Cr, and Ni contents, these results indicate a process of melt‐mantle interaction resulting in hybridization of initial adakitic melts with low δ98/95Mo and mantle‐derived melts/components with high δ98/95Mo. These adakitic rocks have distinct Mo isotope systematics compared with those of oceanic slab‐derived adakites and MORB‐type eclogites. Their relatively low δ11B values and the lack of correlation between δ11B and B contents support a mafic lower crust source for the initial adakitic melts. We thus suggest that these adakitic rocks were generated by partial melting of delaminated lower crust followed by interaction with the deep mantle, and that melt‐mantle interaction may facilitate Cu‐Mo‐Au mineralization.

Magmatic expression of tectonic transition from oceanic subduction to continental collision: Insights from the Middle Triassic rhyolites of the North Qiangtang Block

The tectonic transition from subduction to collision is a fundamental process during orogenesis, yet the magmatic expression of this transition and related deep geodynamic processes remain unclear. This study focuses on a newly identified volcanic belt within the Moyun–Zaduo–Sulu area of the North Qiangtang Block and presents new zircon U-Pb data that indicate that this belt formed during the Middle Triassic (247–241 Ma), a time characterized by a regional transition from subduction to collisional tectonism. The volcanic belt is located to the south of a Permian to Early Triassic arc and is dominated by high-K calc-alkaline and peraluminous rhyolites. These rhyolites have low Mg#, Nb/Ta, and δEu values, contain low contents of Sr, have high Rb/Sr and whole-rock εNd(t) values, and show positive zircon εHf(t) values, all of which suggest that they formed from magmas generated by the dehydration melting of juvenile crustal material. The migration of Middle Triassic volcanism in this region was most likely caused by rollback of the subducting Longmucuo–Shuanghu Tethyan oceanic slab. Combining our new data with previously published results of numerical modeling of subduction–collisional processes and regional data from north-central Tibet yields insights into the magmatic expressions and related deep geodynamics of the transition from oceanic subduction to continental collision. This combination of data also suggests that variations in oxygen fugacity can be used as a proxy for the discrimination of magmatism related to subduction, the transition from subduction to collision, and collisional tectonism.

Zircon Hf-O-Li isotopes of granitoids from the Central Asian Orogenic Belt: Implications for supercontinent evolution

Deep lithospheric processes associated with the formation of major orogens, including the removal of lower crust and underlying mantle through delamination associated with orogen building are poorly constrained. With a view to evaluate the potential link between deep geodynamic processes and magmatic events, we performed in situ zircon Hf-O-Li isotopic analyses of granitoids from the Eastern and Western Junggar, Altai and Beishan orogens, within the Central Asian Orogenic Belt (CAOB). The εHf(t) and δ18O values of magmatic zircons crystallized during 443 Ma and 252 Ma indicate diverse and heterogeneous magma sources. The corresponding δ7Li peaks at ~440 Ma and ~250 Ma, suggesting two distinct high-temperature magmatic events. Based on a comparison with global data, we argue that large-scale delamination formed through Gondwana and Pangaea supercontinent assembly, may have occurred at Ordovician to Silurian boundary (OSB) and Permian to Triassic boundary (PTB), which we term as super-delamination. The subsequent widespread magmatism and volcanism might have made significant impact on the Earth surface ecosystems, ultimately leading to the OSB and PTB mass extinction events. We propose super-delamination as a potential mechanism to explain the link between Earth's internal and external processes, thus providing novel insights into the trigger for mass extinction events.

Determining the Plio-Quaternary uplift of the southern French Massif Central; a new insight for intraplate orogen dynamics

Abstract. The evolution of intraplate orogens is still poorly understood. Yet, it is of major importance for understanding the Earth and plate dynamics, as well as the link between surface and deep geodynamic processes. The French Massif Central is an intraplate orogen with a mean elevation of 1000 m, with the highest peak elevations ranging from 1500 to 1885 m. However, active deformation of the region is still debated due to scarce evidence either from geomorphological or geodetic and seismologic data. We focus our study on the southern part of the Massif Central, known as the Cévennes and Grands Causses, which is a key area to study the relationship between the recent geological deformation and landscape evolution. This can be done through the study of numerous karst systems with trapped sediments combined with the analysis of a high-resolution digital elevation model (DEM). Using the ability of karst to durably record morphological evolution, we first quantify the incision rates. We then investigate tilting of geomorphological benchmarks by means of a high-resolution DEM. We finally use the newly quantified incision rates to constrain numerical models and compare the results with the geomorphometric study. We show that absolute burial age (10Be∕26Al on quartz cobbles) and the paleomagnetic analysis of karstic clay deposits for multiple cave system over a large elevation range correlate consistently. This correlation indicates a regional incision rate of 83 +17/-5 m Ma−1 during the last ca. 4 Myr (Pliocene–Quaternary). Moreover, we point out through the analysis of 55 morphological benchmarks that the studied region has undergone a regional southward tilting. This tilting is expected as being due to a differential vertical motion between the northern and southern part of the studied area. Numerical models show that erosion-induced isostatic rebound can explain up to two-thirds of the regional uplift deduced from the geochronological results and are consistent with the southward tilting derived from morphological analysis. We presume that the remaining unexplained uplift is related to dynamic topography or thermal isostasy due to the Massif Central Pliocene–Quaternary magmatism. Integrating both geochronology and morphometrical results into lithospheric-scale numerical models allows a better understanding of this intraplate–orogen evolution and dynamic. We assume that the main conclusions are true to the general case of intraplate deformation. That is to say, once the topography has been generated by a triggering process, rock uplift is then enhanced by erosion and isostatic adjustment leading to a significant accumulation of mainly vertical deformation.

Terrestrial heat flow and lithospheric thermal structure in the Chagan Depression of the Yingen‐Ejinaqi Basin, north central China

AbstractThe Chagan Depression in the Yingen‐Ejinaqi Basin, located at the intersection of the Paleo‐Asian Ocean and the Tethys Ocean domains is an important region to gain insights on terrestrial heat flow, lithospheric thermal structure and deep geodynamic processes. Here, we compute terrestrial heat flow values in the Chagan Depression using a large set of system steady‐state temperature data from four representative wells and rock thermal conductivity. We also estimate the “thermal” lithospheric thickness, mantle heat flow, ratio of mantle heat flow to surface heat flow and Moho temperature to evaluate the regional tectonic framework and deep dynamics. The results show that the heat flow in the Chagan Depression ranges from 66.5 to 69.8 mW/m2, with an average value of 68.3 ± 1.2 mW/m2. The Chagan Depression is characterized by a thin “thermal” lithosphere, high mantle heat flow, and high Moho temperature, corresponding to the lithospheric thermal structure of “cold mantle and hot crust” type. We correlate the formation of the Yingen‐Ejinaqi Basin to the Early Cretaceous and Cenozoic subduction of the western Pacific Plate and the Cenozoic multiple extrusions. Our results provide new insights into the thermal structure and dynamics of the lithospheric evolution in central China.

Natural endogenous factors of geoecological transformation of the mountain part of North Ossetia

Geoecological transformation of the biocenosis habitat is an evolutionary process, predetermined by the interaction of the lithosphere with the hydrosphere, atmosphere and the solar system as a whole, under the condition of anthropogenic impact absence. Geoecological transformation occurs under the influence of many natural factors of an endogenous and exogenous nature, the effects of which in the mountainous areas are especially intensive and damageable, due to the sharp and intense exposure forms of exogenous factors of the erosive destruction, the presence of a large gravitational potential for a spatial removal of the destroyed material, poor protection of the bedrocks by biogeocenosis, low strength of terrigenous rocks, etc. The main factors of the intensive geoecological transformation of the mountain landscape are endogenous geodynamic processes. The Greater Caucasus, as the geosynclinal region of young Alpine orogenesis, is characterized by the active deep geodynamics, the focal zones of which can be activated in time and migrate in the lithosphere, as evidenced by the interchange of the periods of seismic activity and relative passivity of deep geodynamic processes (fault tectonics, seismicity and magmatic manifestations). A high density of the tectonic faults contributes to the weakening of rock strength in the zone of a faulting crop out due to jointing and claying along the friction planes.

Magnesium and Lithium Isotopic Evidence for a Remnant Oceanic Slab Beneath Central Tibet

AbstractThe voluminous Eocene postcollisional potassium‐rich magmatism in central Tibet is crucial in understanding the deep geodynamic process and related uplift history of the Tibetan Plateau. Here, we present an integrated study of whole‐rock Sr‐Nd‐Mg‐Li isotopes and zircon U‐Pb ages on two suites of lavas from Duocaima and Nariniya in the northern Qiangtang block. The Duocaima high‐K calc‐alkaline trachyandesites were formed at ~35 Ma and are enriched in light rare earth elements and large ion lithophile elements. They have slightly lower δ26Mg (−0.41‰ to −0.33‰) and δ7Li (+1.0‰ to +2.6‰) relative to the mantle. Together with their high 87Sr/86Sr(i) (0.7066–0.7067) and low ɛNd(t) (−1.96 to −1.61) values, they most likely originated from the lithospheric mantle previously metasomatized by melts from subducted carbonate‐bearing sediments. Model calculation suggests that ~7–11% sediments were added into the mantle via melting. By contrast, the contemporaneous Nariniya adakitic trachytes have higher δ26Mg (−0.13‰ to −0.02‰) and δ7Li (+3.3‰ to +5.4‰) values, as well as higher Pb/Ce and Ba/La ratios, which most likely resulted from the thickened lower crust that was metasomatized by fluids released from downgoing Songpan‐Ganzi oceanic slab. Our combined isotopic study suggests the existence of a remnant oceanic slab beneath central Tibetan Plateau during the late Eocene and implies that the initial uplift of central Tibet was before ~38 Ma.

Late Mesozoic and Cenozoic tectono-thermal history and geodynamic implications of the Great Xing’an Range, NE China

The Mesozoic–Cenozoic tectonics of Northeastern China involves complex multistage interaction of different tectonic domains and has long been controversial. The tectono-thermal evolutionary history recorded by the widespread outcropped granitoids in the Great Xing’an Range (GXR) may provide significant constraints on the tectonics, illuminating the deep geodynamic process. Here, we present new low-temperature thermochronology data from the GXR, and thus, reconstruct the detailed thermal history. Zircon fission-track dating yielded a group of central ages ranging from 80 ± 4 to 185 ± 10 Ma, while apatite fission-track ages range from 55 ± 3 to 75 ± 5 Ma. The modeled time–temperature paths of all the samples reveal two relatively rapid cooling events at ~100–60 Ma and ~50–0 Ma, and one intervening reheating episode at ~60–50 Ma. The initial uplift, exhumation, and induced cooling events, which represent strong orogenic activity, are related mainly to closure of the Mongol–Okhotsk Ocean in the north and were subsequently influenced by the Paleo-Pacific plate subduction in the east. The evolution of the basin–mountain system consisting of the Songliao Basin and GXR, as well as the accretion, metamorphism, and magmatism at the Northeast Asian continental margin, are all dominated or indirectly influenced by the Paleo-Pacific subduction. This indicates that the Paleo-Pacific subduction had affected the evolution of the GXR to the west since the Jurassic. The significant reheating episode during the Late Paleocene to Eocene may be related to the heat flux derived from asthenosphere upwelling and igneous activity.

Process of lithospheric delamination beneath the Lhasa–Qiangtang collision orogen: Constraints from the geochronology and geochemistry of Late Cretaceous volcanic rocks in the Lhasa terrane, central Tibet

The geochemistry of post-collisional granitoids can be used to reconstruct crustal thickness and constrain deep geodynamic processes. Two series of volcanic rocks of the Jiangba Formation in the Xiongmei area, central Tibet, record the process of lithospheric delamination in the Lhasa–Qiangtang collisional orogen. The andesites and dacites crystallized at c. 87 and 85 Ma, respectively, and have positive zircon εHf(t) values (2.7–5.4 and 2.7–7.1, respectively). They have calc-alkaline compositions, with relatively high Mg# (100 × molar Mg2+/[Mg2+ + Fe2+]; 39–66), Sr contents, and Sr/Y and (La/Yb)N ratios (subscript N signifies normalization to chondrite), low Y and Yb contents, and no clear negative Eu, Sr, or Ti anomalies, similar to adakites. The andesite–dacite series likely formed by partial melting of foundering juvenile mafic lower crust with residual garnet, followed by the interaction of this melt with peridotite. The rhyolite series was emplaced at c. 81 Ma, and has positive zircon εHf(t) values (1.0–3.5). The rhyolites are characterized by non-adakitic signatures and have high-K calc-alkaline to shoshonite compositions, with low MgO contents (Mg# = 21–34) and Sr/Y and (La/Yb)N ratios, coupled with flat HREE patterns and marked negative Eu, Sr, and Ti anomalies. Geochemical data indicate that these rhyolites were generated by partial melting of a much shallower crustal root, with a plagioclase-rich residue. Zircon U–Pb ages and different compositional trends for the two series indicate that the lithospheric mantle keel was partially removed by small-scale mantle convection that triggered the Late Cretaceous high-Mg# adakitic magmatism in the Lhasa–Qiangtang collisional orogen. Partial lithospheric delamination increased heat flow at the base of the crust and caused the thinned crust to melt, producing the low-Mg# non-adakitic rocks.

Geodynamic transition from subduction to extension: evidence from the geochronology and geochemistry of granitoids in the Sangsang area, southern Lhasa Terrane, Tibet

The northward subduction of the Neo-Tethys oceanic lithosphere and subsequent collision between India and Asia continents gave rise to the Tibetan Plateau. However, how and when oceanic subduction started to transform to an intraplate environment are still open questions. The granitoids distributed in Lhasa Terrane of south Tibet offer a unique chance for us to investigate the deep geodynamic processes. Here, we present zircon U–Pb–Hf isotope, whole-rock major and trace element and Sr–Nd isotope data of granitoid intrusions in the Sangsang area of the southern Lhasa Terrane. The Sangsang granodiorites and granites were crystallized at ca. 54 Ma, while the emplacement age of the quartz monzonites is ca. 47 Ma. The granodiorites are characterized by relatively high Mg# values (35.3–41.1) and Fe2O3t (5.16–6.26 wt%) contents, and low Na2O + K2O contents (6.4–6.9 wt%) and A/CNK values (0.91–0.99), which are similar to the geochemical characteristics of I-type, high-K calc-alkaline rocks. They have high 87Sr/86Sri ratios (0.706455–0.706490), and low eNd(t) (− 3.58 to − 2.96) and zircon eHf(t) (− 3.4 to 0.3) values, indicating they were derived from a hybrid source of ancient mafic crust and juvenile lower crust. The coeval granites have lower Mg# values (22.5–27.25) and similar zircon eHf(t) values (− 2.6 to 1.1), suggesting they were probably differentiation productions of the granodiorites. The quartz monzonites have higher Na2O + K2O contents (9.18–9.59 wt%) and A/CNK values (0.98–1.03), higher zircon eHf(t) values (− 2.2 to 2.6) and more depleted Sr and Nd isotopes than the granodiorites and granites. The quartz monzonites were probably produced by melting of mixed juvenile crustal materials and metagreywacke. The new geochronological and geochemical data help constrain the geodynamic processes in the Lhasa Terrane during the Early Cenozoic, as the Sangsang granitoids represent the change from subduction termination to intraplate extension at the southern margin of the Lhasa Terrane.

A thermobaric mechanism for the formation of low velocity zones in the crystalline crust of the northwestern Black Sea shelf: a new type of traps for abiogenic methane

Seismic studies of the NW Black Sea shelf have delineated low-velocity zones (LVZs) in the crystalline crust at depths of 6—16 km. For the first time, this paper presents a novel thermodynamic mechanism for their formation. The active temperature regime and deep geodynamic processes of the region provide evidence in favour of this mechanism. The LVZs are mainly associated with the thermal decompaction of rocks, which is not compensated by deep pressures. The rocks of such zones are characterized by an increase in fracturing and porosity and decrease in density, elastic parameters and thermal conductivity. As the rocks are permeable and hygroscopic they are able to more freely transform, absorb and localize mantle hydrocarbon-bearing fluids which, in turn, further destroy rock matter. Within the LVZs the existence of hydrocarbons is confirmed by the occurrence about 3200 activegas seeps and 8 gas and gas condensate fields. For the first time, the spatial coexistence was revealed between LVZs and a degassing tube to which hydrocarbon fields are related. LVZs zones should be considered as a new search criteria for thermobaric traps. Favou­rable conditions exist for the accumulation of abiogenic methane at a depth of 6—16 km that can substantially expand the hydrocarbon potential of the shelf in usage modern drilling technology.

Spatial and temporal variations in the geochemistry of Cretaceous high-Sr/Y rocks in Central Tibet

Recent studies based on low-temperature chronology and sedimentology have proposed the existence of a proto-Tibetan Plateau (p-TP); however, the timing and mechanisms of its formation and evolution remain ambiguous. High-Sr/Y rocks are an important petrological indicator of thickening. Here, we compile geochemical data of Cretaceous rocks to interpret their petrogenesis and to constrain deep geodynamic processes. Geochemical characteristics, in combination with zircon Hf isotopic compositions, indicate that the high-Sr/Y rocks were derived from the partial melting of thickened juvenile lower crust, with or without contamination by mantle peridotite. Comparing geochronological and geochemical data, we observe a correlation between magma migration and the composition of high-Sr/Y rocks. Based on these observations, we propose a revised tectonomagmatic evolution model for central Tibet, involving crustal thickening, retreating delamination, and breakoff. Our research suggests that the rapid uplift of the p-TP was a consequence of the removal of isostatic load during the Mesozoic.

Postcollisional delamination and partial melting of enriched lithospheric mantle: Evidence from Oligocene (ca. 30 Ma) potassium-rich lavas in the Gemuchaka area of the central Qiangtang Block, Tibet

Postcollisional potassium (K)-rich magmatic rocks have important implications for unveiling the deep geodynamic processes active during the evolution of orogenic belts and the surface uplift of plateaus. However, their petrogenesis remains highly controversial. Here we report on zircon U-Pb and mineral Ar-40-Ar-39 age, zircon Hf-O isotope, mineral composition, whole-rock major and trace element composition, and Nd-Sr isotope data for the postcollisional K-rich lavas in the Gemuchaka area of the central Qiangtang Block (central Tibet). Age dating suggests that these lavas were generated in the late Early Oligocene (ca. 30 Ma). Two series of rocks were recognized: silica-undersaturated and -saturated lavas. Both series are geochemically characterized by continuous variation trends. All rocks are enriched in light rare earth elements (LREEs) relative to heavy REEs (HREEs) and depleted in Ta-Nb-Ti, with negligible Sr and Eu and obviously positive Pb anomalies. The rocks have higher Th/ La (0.2-0.8) and Nb/Ta (18.6-19.8) values than asthenospheric mantle-derived rocks but slightly variable values of epsilon(Nd) (t) (-6.0 to -4.6), (Sr-87/Sr-86)(i) (0.7076-0.7083), and zircon epsilon(HF)(t) (-7.2 to +0.8) and delta O-18 (7.1-8.5 parts per thousand). We suggest that the parental magmas of these K-rich lavas were likely derived by variable partial melting of lithospheric mantle metaso-matized by subducting continental sediment-derived melts with the addition of minor asthenospheric components. Element modeling results suggest that the basic end members were likely generated by fractionation processes dominated by olivine and minor clinopyroxene from parental magmas, and some other evolved rocks were generated by crystal (olivine, clinopyroxene, Fe-Ti oxides) fractionation from basic magmas. Combining geological and Cenozoic magmatic rock data in the Qiangtang Block, we propose that the Gemuchaka magmatism was most likely triggered by the delamination of lithospheric mantle thickened by early (Paleocene- Eocene) intracontinental subduction and subsequent asthenospheric upwelling.

Geothermal data analysis at the high-temperature hydrothermal area in Western Sichuan

The western Sichuan hydrothermal area is located at the northeastern margin of the eastern syntaxis of the Qinghai-Tibet Plateau, which is also the eastern end of the Mediterranean-Himalayan geothermal activity zone. There are 248 warm or hot springs in this area, and 11 have temperatures beyond the local boiling temperature. Most of these hot springs are distributed along the Jinshajiang, Dege-Xiangcheng, Ganzi-Litang, and Xianshuihe faults, forming a NW-SE hydrothermal belt. A geothermal analysis of this high-temperature hydrothermal area is an important basis for understanding the deep geodynamic process of the eastern syntaxis of the Qinghai-Tibet Plateau. In addition, this study offers an a priori view to utilize geothermal resources, which is important in both scientific research and application. We use gravity, magnetic, seismic, and helium isotope data to analyze the crust-mantle heat flow ratio and deep geothermal structure. The results show that the background terrestrial heat flow descends from southwest to northeast. The crustal heat ratio is not more than 60%. The high temperature hydrothermal active is related to crustal dynamics processes. Along the Batang-Litang-Kangding line, the Moho depth increases eastward, which is consistent with the changing Q c/ Q m (crustal/mantle heat flow) ratio trend. The geoid in the hydrothermal zone is 4–6 km higher than the surroundings, forming a local “platform”. The NW-SE striking local tensile stress zone and uplift structure in the upper and middle crust corresponds with the surface hydrothermal active zone. There is an average Curie Point Depth (CPD) of 19.5–22.5 km in Batang, Litang, and Kangding. The local shear-wave ( S -wave) velocity is relatively low in the middle and lower crust. The S -wave shows a low velocity trap ( V s 3.2 km s‒1) at 15–30 km, which is considered a high-temperature partial melting magma, the crustal source of the hydrothermal active zone. We conclude that the hydrothermal system in this area can be divided into Batang-type and Kangding-type, both of which rely on a crustal heating cycle of atmospheric precipitation and surface water along the fracture zone. The heat is derived from the middle and lower crust: groundwater penetrates the deep faults bringing geothermal energy back to the surface and forming high-temperature springs.

Morphotectonics of the Tunka rift and its bordering mountains in the Baikal rift system, Russia

The Tunka section of the Baikal rift system presents a uniform alternation of the following neostructural forms: tilted horsts and asymmetrical block uplifts on the northern flank; the central system of the rift valleys; and the arched uplift of the southern flank. This is a standard set of morphostructural elements for the Baikal rift system. The main morphological feature of the Tunka rift is the strong inclination of its floor, ranging from 900m to 200km in general elevation above Lake Baikal. Such traits of recent geodynamics as volcanism, thermal activity, and seismicity are also different from other parts of the rift zone. All of these features of the Tunka rift are related to the deep structure of the rift zone.The peculiarities of the neotectonic structure of the Tunka rift, which are clearly expressed morphologically as is typical of the Baikal rift system, as well as its unique features are in accordance with deep geodynamic processes of the region. On the other hand, the development of the rift basin structures of the southwestern area near Baikal is complicated by inversion deformations. Local uplifts followed by deformations of the basin sedimentary cover and inverted morphostructures expressed in relief are fixed against the background of the general subsidence of blocks of the pre-Cenozoic basement grabens. The Tunka rift has repeatedly experienced inversion deformations throughout its history. The last wave of such deformations involved the southwestern region near Baikal in the second half of the late Pleistocene. During the Quaternary, the positive component prevailed in the whole range of vertical movements of the inter-rift and interbasin blocks; since the late Neogene, these structures have experienced a slow but steady uplift, accompanied by their extension at the expense of the bordering basins.The remote influence of the India-Asia collision on the formation of the southwestern section of the Baikal rift system is very significant and well pronounced in the development of the morphostructural elements of the Tunka rift and its bordering mountains.

Meso-Cenozoic Lithosphere Thinning in the Eastern North China Craton: Evidence from Thermal History of the Bohai Bay Basin, North China

The formation and evolution of a rift basin is the shallow response of deep geodynamic processes. The Bohai Bay Basin is at the center of lithosphere destruction and thinning in the eastern North China Craton (NCC). In this article, the Meso-Cenozoic thermal lithospheric thickness of the Bohai Bay Basin was calculated by reconstructing the thermal history of the sedimentary basin using apatite fission track and vitrinite reflectance data. The Meso-Cenozoic thermal modeling results show that the basin experienced four evolutionary stages with two heat flow peaks in the late Early Cretaceous and in the Middle to Late Paleogene, which had heat flow values of 82–86 and 81–88 mW/m2, respectively. Accordingly, the thermal lithospheric thicknesses of the basin, calculated from the thermal histories, experienced two thinning stages in the Cretaceous and the Paleogene. The lithosphere began with an initial thickness of 150 km in the Early Mesozoic and was thinned to approximately 51–61 km in the late Early Cretaceous. The thickness then increased to approximately 80–90 km at the end of the Cretaceous. A second thinning phase occurred in the Middle to Late Paleogene, resulting in a thickness of only 43–55 km. The lithosphere thickened thereafter, and the NCC now has a thermal lithosphere of 75–97 km. The lithospheric thinning in the Cenozoic was dominated by extension rather than thermal erosion. At the same time, there existed some differences in the thermal evolution and lithospheric thinning evolution among different sub-basins, but the time of peak heat flow and peak lithosphere thinning in the Cenozoic is consistent with the migration of the depocenter of the basin. Therefore, the thermal history of sedimentary basins may provide continuous temporal and spatial evidence to reveal deep geodynamic processes. Our research provides new geothermal evidence for the lithospheric thinning of the NCC.

The redox geodynamics linking basalts and their mantle sources through space and time

The Earth's mantle redox state regulates the fate and transfer of metals by magmatism, buffers the igneous inputs of volcanic gases in the atmosphere and controls the depth of mantle melting. It therefore strongly affects ore forming processes, biogeochemical cycles and deep geodynamic processes. This paper reviews the current knowledge on the redox state of the upper mantle and of magmas produced by mantle melting. The geochemical processes likely to control and modify it through space and time are discussed.We analyze the link between the redox state of magma and that of their mantle source and we conclude that melts produced in the mantle may well all equilibrate in a narrow range of oxidation state, where the speciation of sulfur in basalts shifts from sulfide to sulfate, that is, FMQ+1±1 (1 log unit below and above the oxygen fugacity buffered by the assemblage fayalite–magnetite–quartz). Subsequently, degassing and partial crystallization of melts can affect their redox states, producing most of the range of redox states observed on magmas reaching Earth's surface. The asthenosphere sourcing basaltic magmas may therefore be more oxidized than the FMQ−1 value generally assumed.We also discuss redox transfers from the mantle to the atmosphere via volcanic degassing and the backward fluxes via subduction processes of the hydrothermalized oceanic lithosphere. Arc-magmas are oxidized (up to FMQ+4) but it is unclear when this feature is acquired since strongly oxidized primary arc-basalts have yet to be found. The oxidizing event may be the assimilation of slab-derived SO3-rich fluids by primary basalt generated by decompression melting in the mantle-wedge.Overall, subduction must result in a transfer of oxygen from the Earth's surface down to the mantle. This must imply that subduction and its initiation can hardly be the trigger of the great oxidation event at the end of the Archaean. In contrast, the cooling of the Earth's interior through time must have impacted on the redox state of basalts, by decreasing the depth of mantle melting. According to the long-established vertical stratification of the Earth's mantle, ancient primary magmas are therefore likely to have been more reduced (i.e. <FMQ−3) than present-day ones. However, geochemical observations on ancient basalts suggest a constant oxidation state since the early Archaean.We conclude that large uncertainties in the calibration of mineralogical oxygen barometer probably explains why we have difficulties in identifying (i) ancient primary basalts being more reduced than recent ones and (ii) primary basalts from subduction zones being as oxidized as arc-lavas reaching the surface. Finally, the degree of mantle melting is certainly a key issue for the interpretation of the mantle oxidation state. Extremely oxidized melts, enriched in C–H–S volatile species, produced by very low degrees of mantle melting may be indicative of an Earth's mantle more oxidized than usually considered.

The shear-wave splitting in the crust and the upper mantle around the Bohai Sea, North China

In order to infer the distribution of local stress and the deep geodynamic process in North China, this study detects seismic anisotropy in the crust and upper mantle beneath the Bohai Sea area. A total of 535 local shear-wave and 721 XKS (including SKS, PKS and SKKS phases) splitting measurements were obtained from stations in permanent regional seismograph networks and a temporary seismic network called ZBnet-E. The dominant fast polarization orientation of local shear-waves in the crust is nearly East-West, suggesting an East-West direction of local maximum compressive stress in the area. Nearly North-South fast orientation was obtained at some stations in the Tan-Lu fault belt and the Zhang-Bo seismic belt. The average fast orientation from XKS splitting analysis is 87.4° measured clockwise from the North. The average time-delays of XKS splitting are range from 0.54s to 1.92s, corresponding to a 60–210km thick layer of anisotropy. The measured results indicate that upper mantle anisotropy beneath Bohai Sea area, even the eastern part of North China, is mainly from asthenospheric mantle flow from the subduction of the Pacific plate. From the complicated anisotropic characteristics in this study, we infer that there might be multiple mechanisms in the crust and upper mantle around the Bohai Sea area that led to the observed anisotropy.