SE Margin Research Articles

The early Oligocene establishment of modern topography and plant diversity on the southeastern margin of the Tibetan Plateau

Quantifying the interactions between topography, climate and plant diversity within one of the world's biodiversity hotspots, the southeastern margin of the Tibetan Plateau, remains elusive due to few reliable quantitative paleoelevation reconstructions, precise geological age constraints and well-preserved plant fossils. The Lühe Basin, on the southeastern margin of Tibetan Plateau has yielded abundant plant fossils with a U-Pb age of 33–32 Ma, providing an opportunity to estimate the elevation of this region and plant diversity at that time. Fossil leaf physiognomy was used to reconstruct the paleoclimate and the paleoelevation of the basin was derived from moist enthalpy. The results show that the Lühe Basin, had attained it's present elevation (1.7 ± 0.9 km) by the early Oligocene and, compared to now, experienced a humid subtropical climate with a wetter dry season and lower precipitation seasonality in an overall wetter precipitation regime (1748.5 ± 606 mm). This was accompanied by a greater seasonal range in temperature, although the mean annual temperature (14.9 ± 2.3 °C) was similar to that of today (15.6 °C). Combined with previous studies, we conclude that the appearance of the modern flora across the southeastern margin of the Tibetan Plateau had started by the early Oligocene, corresponding with the establishment of modern topography at that time. • The Lühe flora dominated by Fagaceae and Betulaceae is similar to modern vegetation around the fossil locality. • The Lühe Basin had attained its present elevation (1.7 ± 0.9 km) by the early Oligocene. • The topography and modern flora in the SE margin of Tibetan Plateau had been established by the early Oligocene.

Investigation of stochastic-threshold incision models across a climatic and morphological gradient

Abstract. Long-term landscape evolution is controlled by tectonic and climatic forcing acting through surface processes. Rivers are the main drivers of continental denudation because they set the base level of most hillslopes. The mechanisms of fluvial incision are thus a key focus in geomorphological research and require accurate representation and models. River incision is often modeled with a stream power model (SPM) based on the along-stream evolution of drainage area and channel elevation gradient but can also incorporate more complex processes such as threshold effects and statistical discharge distributions, which are fundamental features of river dynamics. Despite their importance in quantitative geomorphology, such model formulations have been confronted with field data only in a limited number of cases. Here we investigate the behavior of stochastic-threshold incision models across the southeastern margin of the French Massif Central, which is characterized by significant relief and the regular occurrence of high-discharge events. Our study is based on a new dataset combining measurements of discharge variability from gauging stations, denudation rates from 34 basins from 10Be cosmogenic radionuclide (CRN) concentration measurements in river sediments, morphometric analysis of river long profiles, and field observations. This new dataset is used for a systematic investigation of various formulations of the SPM and to discuss the importance of incision thresholds. Denudation rates across the SE margin of the Massif Central are in the 20–120 mm kyr−1 (equivalent to mm/ka in the figures) range, and they positively correlate with slope and precipitation. However, the relationship with the steepness index is complex and supports the importance of taking into account spatial variations in parameters (D50, discharge variability k, runoff) controlling the SPM. Overall, the range of denudation rate across the margin can mainly be explained using a simple version of the SPM accounting for spatially heterogeneous runoff. More complex formulations including stochastic discharge and incision thresholds yield poorer performances unless the spatial variations in bedload characteristics controlling incision thresholds are taken into account. Our results highlight the importance of the hypotheses used for such a threshold in SPM application to field studies and notably the impact of actual constraints on bedload size.

Geological Significance of Late Permian Magmatic Rocks in the Middle Section of the Ailaoshan Orogenic Belt, SW China: Constraints from Petrology, Geochemistry and Geochronology

The Ailaoshan orogenic belt, located in the SE margin of the Qinghai–Tibet Plateau, is an important Paleo-Tethys suture zone in the eastern margin of the Sanjiang Tethys tectonic domain. The areas of Mojiang and Zhenyuan, located in the middle part of the Ailaoshan orogenic belt, are the key parts of the Ailaoshan Paleo-Tethys Ocean closure and collision orogeny. The rhyolites outcropped in the Mojiang area, and the granite porphyries outcropped in Zhenyuan area, are systematically studied for petrology, isotope geochemistry and geochronology. The Zircon U-Pb geochronology of rhyolites and granite porphyries give weighted average ages of 253.4 ± 4.2 Ma and 253.3 ± 2.0 Ma, respectively, both of which were formed in the late Permian period. The rhyolites belong to potassic calc-alkaline to subalkaline series. The patterns of the rare earth elements (REE) show a right-inclined seagull-type distribution, and the trace elements plot is right-inclined. The granite porphyries are high potassic calc-alkaline to subalkaline. The REE patterns show a right-inclined distribution, and the trace elements plot is right-inclined, which is consistent with the typical patterns observed in the crust. The peraluminous, highly differentiated and high ASI values suggest that rhyolites and granite porphyries are S-type granites. The zircon εHf(t) of the rhyolites range from −7.22 to −0.72, and two-stage Hf zircon model ages are (TDMC) 1771–2352 Ma, indicating that the magma source area is mainly crust-derived. The zircon εHf(t) of the granite porphyries range from −0.97 to 4.08, and two-stage Hf zircon model ages are (TDMC) 1336–1795 Ma, indicating that the magma is derived from a depleted mantle source and the partial melting of ancient crustal materials. The rhyolites and granite porphyries were possibly formed in the syn-collisional tectonic setting during the late Permian, and their ages limited the time of the final closure of the Ailaoshan Ocean and the initiation of collisional orogeny.

New chronological constraints on the Plio-Pleistocene uplift of the Guizhou Plateau, SE margin of the Tibetan Plateau

The Guizhou Plateau represents a geomorphic transition between the Tibetan Plateau and the Yangtze River Plain. It likely formed in response to the propagation of surface uplift in southeastern Tibet during India-Eurasia continental collision. However, the uplift history of the region is unclear largely due to a lack of datable material. The bedrock geology is dominated by carbonate rocks, which contains numerous multi-level caves in the main river valleys that are linked to the river incision history. Cosmogenic 26 Al and 10 Be burial dating of sediments in caves and river terraces from the northwestern and southern plateau reveals the fluvial chronology and provides the first direct determination of long-term river incision rates. The caves and terraces on the Liuchong River in NW Guizhou yield burial ages of between 0.41 ± 0.12 Ma and 2.85 ± 0.21 Ma, indicating an average incision rate of 57 ± 3 m/Ma. Four level caves at Libo in southern Guizhou yield burial ages of between 0.56 ± 0.16 Ma and 3.54 (+0.25/-0.22) Ma, indicating slightly slower incision rate (47 ± 5 m/Ma). These new results imply that the high elevation of the Guizhou Plateau had developed before the Late Pliocene, and that surface uplift during the Late Cenozoic was largely uniform across the region.

U–Pb geochronology of Upper Triassic – Lower Jurassic detrital sequences from SE margin of the South China Block: implications for Palaeo-Pacific subduction and tectonic evolution

AbstractWe performed U–Pb dating of detrital zircons and conducted petrological and whole-rock geochemical analyses to assess the provenance of the Upper Triassic – Lower Jurassic clastic rocks in the southeastern margin of the South China Block. Detrital zircon U–Pb ages are mainly classified into age groups of 2000–1700, 900–700, 490–390 and 280–210 Ma, consistent with derivation from the Jiangnan orogenic belt, Nanling Belt, as well as Wuyi and Yunkai domains. Lower Jurassic samples yield a special main age population of 200–190 Ma, and these detrital zircon grains have low Th/U and Nb/Hf ratios and high Th/Nb and Hf/Th ratios, showing they are derived from a continental magmatic arc. However, the cross-correlation and likeness coefficients of kernel density estimates of Upper Triassic and Lower Jurassic sandstones are 0.8608 and 0.8403, indicating that their populations are highly similar. Since the tectonic setting is the key factor in controlling the relationship between source and sink, the stable supply of identical provenance suggests that the tectonic setting did not significantly change during Late Triassic – Early Jurassic time. Sandstone petrography, regional facies distribution and the detrital zircon age patterns all reflect a consistent tectonic setting for the South China Block during Late Triassic – Early Jurassic time. The Palaeo-Pacific subduction therefore did not control the tectonic evolution of the South China Block until after the Early Jurassic Epoch.

Advances in the deep tectonics and seismic anisotropy of the Lijiang-Xiaojinhe fault zone in the Sichuan-Yunnan Block, Southwestern China

The Sichuan-Yunnan Block (SYB) is located at the SE margin of the Qinghai-Tibetan Plateau (TP). Under the influence of the southeastward movement of material originated from the TP, intense crustal deformation, frequent seismic activity, and complex geological structures are observed in the SYB. The Lijiang-Xiaojinhe fault (LXF) goes through the central part of the SYB, dividing it into two blocks from north to south, and forming an intersecting fault system with the surrounding faults. This paper firstly introduces the morphology and the nature of the LXF, the distribution of the regional surface displacements and the focal mechanisms, and then analyzes the medium deformation and the effects of faults. Moreover, according to the regional tectonics and geophysical patterns, the paper discusses the characteristics of the north-south blocks of the SYB and the abrupt change of deep structure along the LXF zone. Since seismic anisotropy is an essential property for detecting crustal stress, deep structures and dynamical mechanisms, this paper is dedicated to the advances in seismic anisotropy at different depths and different scales in the study area. There are noteworthy differences in the anisotropic features between the north part and the south part of the SYB, possibly associated with a clear boundary adjacent to the LXF. Such phenomenon suggests some close correlation between anisotropic zoning boundary and the LXF, although this boundary is not consistent with the LXF in strike. The results from the deformation of the crust and the upper mantle elucidate the distribution patterns of the crust-mantle coupling in the north part and the crust-mantle decoupling in the south part, even though this conclusion needs to be further verified by more studies. Presently, the scientific understanding of the deep tectonics and the media deformation around the “generalized” LXF i.e. the LXF with the Jinpingshan fault on its eastern side, is still insufficient, and related equivocal topics deserve more in-depth studies.

Deep Seismogenic Tectonics of Yangbi Ms6.4 on 21 May 2021 in the Se Margin of the Tibetan Plateau from Earthquake Sequence Relocation, Stress Field and Seismic Anisotropy

Deep Seismogenic Tectonics of Yangbi Ms6.4 on 21 May 2021 in the Se Margin of the Tibetan Plateau from Earthquake Sequence Relocation, Stress Field and Seismic Anisotropy

Palaeostress analysis based on multiple inversion in 9-dimensional space in relation to hydrothermal calcite veins in the SE margin of the Elbe Fault Zone (Bohemian Cretaceous Basin, the Czech Republic)

Palaeostress analysis based on multiple inversion in 9-dimensional space in relation to hydrothermal calcite veins in the SE margin of the Elbe Fault Zone (Bohemian Cretaceous Basin, the Czech Republic)

Morphotectonic Analysis along the Northern Margin of Samos Island, Related to the Seismic Activity of October 2020, Aegean Sea, Greece

On 30 October 2020, a strong earthquake of magnitude 7.0 occurred north of Samos Island at the Eastern Aegean Sea, whose earthquake mechanism corresponds to an E-W normal fault dipping to the north. During the aftershock period in December 2020, a hydrographic survey off the northern coastal margin of Samos Island was conducted onboard R/V NAFTILOS. The result was a detailed bathymetric map with 15 m grid interval and 50 m isobaths and a morphological slope map. The morphotectonic analysis showed the E-W fault zone running along the coastal zone with 30–50° of slope, forming a half-graben structure. Numerous landslides and canyons trending N-S, transversal to the main direction of the Samos coastline, are observed between 600 and 100 m water depth. The ENE-WSW oriented western Samos coastline forms the SE margin of the neighboring deeper Ikaria Basin. A hummocky relief was detected at the eastern margin of Samos Basin probably representing volcanic rocks. The active tectonics characterized by N-S extension is very different from the Neogene tectonics of Samos Island characterized by NE-SW compression. The mainshock and most of the aftershocks of the October 2020 seismic activity occur on the prolongation of the north dipping E-W fault zone at about 12 km depth.

Determining the origin of volcanic rocks in the mélange complex of Karangsambung based on the electrical resistivity imaging

AbstractAn ENE‐WSW‐trending localized basalt‐diabase outcrop along the SE margin of Luk Ulo Mélange Complex has been suggested as intrusive rocks cut through the Paleogene Totogan and Karangsambung formations. However, the absolute dating of the volcanics is older than the inferred relative age of the sedimentary formations, hence the in‐situ intrusion theory is less likely. A subsurface imaging should delineate the possibility of the in‐situ nature of volcanic rock by looking at the continuation of the rocks to the depth. In this study, we did a subsurface imaging by electrical resistivity method. The electrical resistivity surveys were conducted at 3 (three) lines across the ENE‐WSW trend of the volcanic distribution. From those three measurements, we obtained three inversion models that present the distribution of the resistivity. We could differentiate between the high resistivity of volcanic rocks and the low resistivity of the clay‐dominated sediments. Instead of the deep‐rooted intrusions, the geometry of the volcanic rocks is concordant with the sedimentary strata. Since we do not observe any spatial continuity of the bodies, both laterally and vertically, the volcanic rocks might be part of broken intrusive rocks. Furthermore, the size and the sporadically distributed of the rocks also indicated that they are more likely as fragments during the olistostrome deposition, transported from its original location.

Sedimentary and pedogenic record of seasonal humidity during the Permian-Triassic transition on the SE margin of Central European Basin (Holy Cross Mountains, Poland)

The transition from the late Permian to Early Triassic was associated with one of the biggest biotic crises in the entire Phanerozoic which affected life in both marine and continental settings. Unlike in the marine realm, the environmental dynamics in continental palaeoenvironments across the Permian-Triassic interval are still poorly understood. In order to fill this knowledge gap, we studied the Permo-Triassic continental succession from the SE part of the intracontinental Central European Basin (CEB). High-resolution sedimentological logging of over 1700 m of cores, supplemented by micromorphology of palaeosols, palynofacies, and quantitative spore-pollen analyses, allowed reconstructing major steps in palaeoenvironmental evolution, and assessing the magnitude of extrinsic factors responsible for such development. The studied interval is dominated by redbeds deposited in arid, semi-arid, and sub-humid subenvironments in which palaeosol profiles are important elements of facies architecture. Vertical and lateral facies arrangement indicate three major development stages associated with progradation of a large distributary fluvial system towards the centre of the basin. These stages correspond to the main lithostratigraphic units in the studied area. In the first stage, the saline mudflats were replaced by muddy floodplain facies and then by medial and proximal parts of the ephemeral fluvial system of the Podzamcze Formation. The overlying Siodła Formation represents the stage of environmental stabilisation associated with widespread pedogenesis resulting in the formation of thick cumulative palaeosol profiles. The last stage was marked by the influx of coarse-grained fluvial sediments of the Jaworzna Formation and change in soil character towards less evolved types in overbank facies of the Szczukowice Formation. The change in the sedimentary record and soil types reflects the successive humidification in the SE part of the CEB, supported by increasing contribution of the hygrophytic over mostly xerophytic palynomorphs in the upper Permian palynological assemblage. This humidification trend was likely an environmental response to global warming in the late Permian.

Strontium isotopes in high- and low-Ge coals from the Shengli Coalfield, Inner Mongolia, northern China: New indicators for Ge source

The coal-hosted Ge deposit (Early Cretaceous) at Wulantuga, Inner Mongolia, northern China, is one of the largest Ge deposits in the world. Although the origin, modes of occurrence, and spatial distribution of Ge in this deposit have been investigated by several studies, the provenance and pathways of Ge from the source to the deposit are largely unknown. In this study, we investigated the 87Sr/86Sr values in the high-Ge coal of the Wulantuga deposit and the stratigraphically-equivalent low-Ge coal of the No. 1 Pit, both from the Shengli Coalfield, Inner Mongolia, northern China. In addition to an organic association, Sr and Ca in the high-Ge and low-Ge coals also occur in gypsum. The water-soluble Sr (i.e., in gypsum) in the high-Ge coal (Ge concentrations: 45–1170 ppm, 273 ppm on average, whole coal basis) has 87Sr/86Sr values in the range of 0.710411 to 0.710633 (with an average of 0.710478). In contrast, the low-Ge coal (Ge concentrations: up to 4.23 ppm, 0.45 ppm on average, whole coal basis) has lower 87Sr/86Sr values in two sub-groups, a low range of 0.709896–0.71005 (with an average of 0.709736) and a high range of 0.710173–0.710547 (with an average of 0.710329 and dominantly in the range of 0.710173–0.710406). The higher 87Sr/86Sr values in the high-Ge coals indicate that the highly-elevated concentration of Ge is associated with a local source of radiogenic Sr. The enrichment of Ge in the coal is due to several consecutive processes. Surface waters penetrated deeply into granite and dissolved at least partially more soluble phases of the granite. The Ge-rich solution flowed over ground or underground into the Shengli Coalfield and Ge precipitated in the peat, the precursor of the coal investigated in the coalfield. The 87Sr/86Sr values of the water-soluble fraction, the elemental compositions of high-Ge and low-Ge coals, and the geomorphological setting of the coalfield point to the Hercynian granodiorite intrusion on the nearby SE margin of the Shengli Coalfield as the primary source of Ge in the Wulantuga coal-hosted Ge deposit. In addition to serving as an indicator of the mode of formation of the coal-hosted Ge deposit, the 87Sr/86Sr values and their spatial distribution can provide an indication for the hydrological pathways of the mineralising fluid.

Triassic–Jurassic evolution of the eastern North China Craton: Insights from the Lushun-Dalian area, South Liaodong Peninsula, NE China

AbstractThe Lushun-Dalian area of the South Liaodong Peninsula, in NE China, located in the SE margin of the North China Craton (NCC) exposes a suite of Middle-Late Proterozoic low-grade metamorphic sedimentary rocks which can be divided into a lower competent layer, a middle incompetent layer, and an upper competent layer on the basis of lithology and deformation style. Two stages of deformation recorded both in the metasedimentary rocks and a magmatic complex intruded in them indicate that the Lushun-Dalian area is a key region to decipher the Triassic–Jurassic tectonic evolution of the eastern NCC. The earliest D1 deformation mylonitized the magmatic complex and thrusted it northeastward over the low-grade metasedimentary rocks, in which a series of NE-verging folds and NE-directed brittle thrust faults developed. The D2 deformation erased the D1 fabrics in the incompetent layer by a top-to-the-NW ductile shearing and refolded the D1 fabrics in the lower and upper competent units, producing a series of km-scale SW-plunging folds. New zircon secondary ion mass spectrometry and laser ablation–inductively coupled plasma–mass spectrometry U-Pb ages from the magmatic complex and the granite porphyry dikes intruded in it, combined with the unconformity between the low-grade metasedimentary rocks and the Early Cretaceous volcanic rocks, indicate that D1 and D2 occurred after 211 Ma and before the Early Cretaceous. The decrease of the deformation intensity of D1 and D2 from the Lushun-Dalian area toward the interior of the NCC in the NE and NW directions suggests that D1 was the structural response in the overriding plate to the NCC-South China Block convergence during the Late Triassic to Early Jurassic, and D2 was the structural response to the northwestward subduction of the Paleo–Pacific plate beneath the NCC in the Middle-Late Jurassic. The superimposition of D2 on D1 recorded a significant tectonic transformation from the nearly E-W–trending Tethysian domain to the NE-SW–trending Pacific domain.

Symmetry of the South China Sea conjugate margins in a rifting, drifting and collision context

The symmetry of conjugate rifted margins is a first-order observable feature reflecting the geodynamic processes acting during and after continental rifting. The symmetry of the South China Sea (SCS) rifted margins can be deduced from comprehensive geophysical datasets on a continental-margin scale. Here, we combine three key approaches: (1) lateral fault distributions are delineated according to free-air gravity anomalies, (2) crustal stretching styles are mapped based on gravity inversion constrained by deep seismic profiles, and (3) total extension of conjugate margins is estimated according to margin restorations and tectonic settings. We infer a predominantly symmetric rifting style caused by pure shear extension, with only narrow domains of asymmetric deformation in continent-ocean transition (COT) regions that have undergone simple shear and where the lower crust of the highly thinned distal margin is embrittled before continental breakup. However, our analysis also suggests that this symmetry has been modified by post-rift geodynamic processes. Southward lower crustal flow, which occurred only on the southern margin due to the low viscosity of the lower crust and the slab pull induced by the subducting Proto-SCS plate during seafloor spreading, shifted the crustal stretching styles from symmetric to asymmetric. The collision between the southern margin and Borneo thickened the lower crust more than the upper crust at the southern end of the southern margin and shortened the southern margin. This event had a large impact on the SE margin but a small impact on the SW margin. We conclude that (1) for the SE and NE margins, the crustal stretching styles shifted from asymmetric to approximately symmetric, and the total extension shifted from symmetric to asymmetric; (2) for the SW and NW margins, the crustal stretching and total extension remained asymmetric and symmetric, respectively. • The total extensions are asymmetric in the eastern margins, but symmetric in the western margins. • The crustal stretching styles of the eastern margins are approximately symmetric, but asymmetric in the western margins. • Post-rifting lower crust flow and collision modified margin symmetry.

The Permian–Triassic boundary in continental sedimentary succession at the SE margin of the Central European Basin (Holy Cross Mountains, Poland)

AbstractWe studied the upper Permian and Lower Triassic deposits from the northern and northwestern marginal part of the Holy Cross Mountains (SE part of the Central European Basin or CEB, Poland) to examine stratigraphic continuity between these two systems, and to revise the currently existing stratigraphic framework. A previously existing informal lithostratigraphic scheme has been revisited and placed in a broader chronostratigraphic and palaeoenvironmental context. Sedimentary continuity across the Permian–Triassic (P/T) boundary has been confirmed by the presence of Lueckisporites virkkiae Bc morphological norm and Lundbladispora obsoleta–Protohaploxypinus pantii palynomorphs. Facies development reflects an interplay between climatic variations and tectonism during late Permian – Early Triassic time. The P/T boundary was placed between the Siodła Formation and overlying Szczukowice and Jaworzna formations, which coincides with the classical Zechstein–Buntsandstein boundary in the SE part of the CEB. The facies changes recorded in the studied terrestrial succession of the P/T boundary shed light on the environmental dynamic prior, during and after one of the biggest biotic crises in Earth’s history.

Spatiotemporal evolution and causes of marine euxinia in the early Cambrian Nanhua Basin (South China)

Marine redox conditions and their dynamic variations in the early Cambrian are thought to have strongly influenced the evolution of early animals. However, the patterns and mechanisms of contemporaneous marine redox variation remain poorly understood. The Nanhua Basin (South China) was a failed rift basin located between the Yangtze Block (NW margin) and Cathaysia Block (SE margin) in the early Cambrian. In this study, we conducted a comprehensive study of marine redox conditions, sulfur cycling, and chemical weathering intensity of source areas for the lower Cambrian shales of the Qingxi Formation in the basinal Shimensuidao section, which was geographically close to the Cathaysia Block. We integrated these results with previously published data from the nearby Silikou section as well as the coeval inner-shelf Xiaotan, upper-slope Daotuo, lower-slope Longbizui, and basinal Zhalagou sections, where were closer to the Yangtze Block. At Shimensuidao, iron species and redox-sensitive trace element data suggest dominantly euxinic conditions for the organic-rich black shale interval (lower member; LM) and variable redox conditions (oxic-ferruginous-euxinic) for the overlying organic-lean shales (upper member; UM) of the Qingxi Formation. Integrated redox results show a similar lessening of euxinic conditions from LM to UM across the entire basin and that euxinia developed along both the Yangtze and Cathaysia basin margins, but it was mainly in upper-slope areas (e.g., Daotuo) on the Yangtze margin and reached basinal areas (e.g., Silikou and Shimensuidao) on the Cathaysia margin. A decline of δ34Spy from LM to UM was observed at Shimensuidao and other sections, which can be explained by decreasing microbial sulfate reduction rates due to decreasing organic availability and increasing riverine sulfate flux at study sections. Lower δ34Spy values in shallow-marine relative to deep-marine areas suggest increased terrestrial sulfate availability to the shallower sites. Chemical index of alteration (CIA) values at Shimensuidao are uniformly high for both the LM (CIAcorr1: 82.5 ± 0.4, CIAcorr2: 88.6 ± 0.5) and UM (CIAcorr1: 80.2 ± 2.3, CIAcorr2: 86.2 ± 2.5) compared with the low-to moderate values at Daotuo (CIAcorr2: 61.9 ± 3.9 for LM, 71.3 ± 4.8 for UM). This pattern is consistent with intense chemical weathering of source areas on the Cathaysia Block and weaker chemical weathering of source areas on the Yangtze Block. Based on these observations, we propose that strong chemical weathering of the Cathaysia Block led to regionally high terrigenous sulfate fluxes, resulting in expansion of mid-depth euxinia into deep basinal areas on the Cathaysia margin of the Nanhua Basin. In contrast, weak chemical weathering of the Yangtze Block yielded lower terrigenous sulfate fluxes, limiting euxinia to upper slope areas on the Yangtze margin of the basin. Weaker euxinia during UM deposition may have been due to its decreasing organic availability and/or small seawater sulfate reservoir caused by persistent euxinic consumption during LM deposition. Our results highlight the complex controls and the key role of continental weathering and resulting terrigenous sulfate fluxes in marine euxinic variations in early Cambrian marginal seas, providing new insights into the co-evolution of continents, oceans, and early animals.

Passive Adjoint Tomography of the Crustal and Upper Mantle Beneath Eastern Tibet With a W2‐Norm Misfit Function

AbstractFull waveform tomography is an effective method to obtain high‐resolution subsurface velocity structures. It is, however, difficult for the conventional L2‐norm‐based inversion to reach the global minimum because of cycle‐skipping problem. In this study, we investigated the feasibility of using the quadratic Wasserstein metric distance (W2 norm) as the misfit function for passive adjoint tomography. We first derived equations of the Fréchet gradient and adjoint source under W2‐norm misfit function. We then conducted numerical experiments to illustrate the effectiveness of the proposed method in avoiding cycle skipping. We finally applied the adjoint tomography to eastern Tibet and obtained a 3‐D velocity model of the lithosphere. The adjoint tomography revealed two low‐velocity channels beneath the NE and SE margins of the Tibetan plateau, which were also observed by previous studies. Our results are consistent with the lower crust flow model that was proposed to explain the deformation occurring at the two margins.

Magmatic activity at the Silurian/Devonian boundary in the Brunovistulia and Małopolska Terranes (S Poland): possible link with the Rheic Ocean closure and the onset of the Rheno-Hercynian Basin

AbstractThe age of granophyric diorite from the Sosnowiec IG-1 borehole (Brunovistulia Terrane) was studied by means of U–Pb single-grain zircon analysis performed on a SHRIMP (sensitive high-resolution ion microprobe) IIe device. The isotope ages and provenance of zircons from the Emsian tuffs cropping out in the southern part of the Holy Cross Mountains (Małopolska Terrane) were also investigated using the same method. The age of the diorite intrusion (420 ± 2 Ma) is comparable with the combined Ar–Ar/magnetostratigraphic age of the Bardo diabase intrusion from the northern part of the Małopolska Terrane. These intrusions were emplaced during the same event of regional tectonic extension associated with the Rheic Ocean closure and the onset of processes creating the Rheno-Hecynian Basin near the Silurian/Devonian boundary. A negative Nb anomaly characteristic of both intrusions could be linked with the subduction of the Rheic oceanic crust under the SE margin of the Old Red Continent. Emsian magmatic activity in the distant Rheno-Hercynian Zone provided several tuff layers in the northern part of the Małopolska Terrane. As can be inferred from zircon ages, these tuffs were derived from mafic eruptions that cut sedimentary rocks containing detrital zircons transported from Baltica. This interpretation fits the existing models of development of the Rheno-Hercynian Basin in the Emsian.

S-Wave Velocity Images of the Crust in the Southeast Margin of Tibet Revealed by Receiver Functions

The southeast margin of Tibet is the region in clockwise rotation around the Eastern Himalayan Syntaxis due to the India–Eurasia collision and the resistance of the stable Sichuan Basin and South China block. However, the dynamic processes involved in the evolution and deformation of the region remain poorly understood due to a lack of reliable geophysical observations. We collected abundant seismic data recorded by 108 permanent broadband stations deployed in the SE margin of Tibet since 2000, and obtained 4536 pairs of P-wave receiver functions (PRFs) with high signal-to-noise ratio. In this study, we have implemented a novel two-step data inversion procedure that can reduce the dependence of the inversion results on the initial model. We first use low-frequency PRFs obtained by iterative deconvolution in the time domain, and then an initial model consisting of a series of 2-km-thick isotropic layers to fit velocity models, and thus determine an overall statistical solution by means of the bootstrap resampling technique. This statistical solution is then regarded as a new initial model to adjust high-frequency PRFs. Hence, the same resampling process is executed again to estimate the optimal S-wave velocity structure below each station. The results provide an accurate 3D image of the crust and uppermost mantle in the SE margin of Tibet. We infer a wide intra-crustal low-velocity zone that varies laterally and in depth, which is thinner or even absent in the most southern part of Yunnan. Our hypothesis is that this low-velocity zone is the result of the accumulation of lower crustal flow coming from central Tibet. Furthermore, we show that this lower crustal flow extends largely through the Sichuan–Yunnan diamond-shaped block, and that there are significant variations in both crustal velocity structure and deformation mechanism across the great strike-slip faults of the Jinshajiang–Red River and Xiaojiang fault systems.

SHRIMP U-Pb ages of zircons from mafic granulites of the Eastern Ghats Belt, SE India: Implications for the evolution of the palaeoproterozoic arc crust

We determined SHRIMP U-Pb ages of zircons from three mafic granulite samples of the polymetamorphosed, multiply-deformed Kondapalli-Ongole domain of the Eastern Ghats Belt (EGB), SE India. Our goals are: (1) to evaluate the origin of zircons in the high-grade mafic granulites; and (2) to clarify the petrotectonic evolution of magmatic and metamorphic rocks along the SE margin of India. The zircons record three distinct Paleoproterozoic U-Pb age groups: igneous protoliths of the mafic granulites apparently crystallized at ∼1715 Ma, and underwent granulite-facies high-temperature metamorphism at ∼1676 Ma, with a subsequent fabric-defining garnet-granulite grade metamorphic overprint at ∼1610 Ma. Based on textures, cathodoluminescence images, and geochemistry, we identified three zircon types among the ∼1610 Ma zircons: Type I and II zircon grains recrystallized or newly grown in equilibrium with feldspar and garnet, respectively; Type III zircons crystallized from aqueous anatectic melt in quartzofeldspathic veins. Some zircons of all the three types show alteration effects. The ∼1715 Ma event likely represents formation of the Kondapalli-Ongole magmatic arc crust, whereas the ∼1610 Ma high-grade metamorphic event reflects suturing of the arc to proto-India and cratonization of the EGB. The interval between the formation of U-poor, low Th/Yb zircons (equilibrated with garnet) and U-rich, high Th/Yb zircons (equilibrated with fluid-rich melt) is geologically insignificant, suggesting that tectonic accretion of the EGB, its final suturing, and orogenic collapse took place rapidly.