Boulder, Colo., USA: The Geological Society of America regularly publishes articles online ahead of print. GSA Bulletin topics studied this month include the nature and dynamics of China and Tibet; guidance regarding the publication and interpretation of luminescence results; the Columbia River Basalt Group; the southern Great Basin of the United States; and arsenic and platinum-group minerals in East Dover, Vermont. You can find these articles at https://bulletin.geoscienceworld.org/content/early/recent .
Mid−late Neoproterozoic tectonic evolution of the northern margin of the Yangtze Block, South China, as constrained from the detrital zircon record
Ruirui Wang; Zhiqin Xu; M. Santosh
Abstract: The northern margin of the Yangtze Block (NYB) experienced distinct tectono-magmatic evolution during the mid- and late-Neoproterozoic, as compared with the northwestern margins of the Yangtze Block (NWYB), although the geodynamic mechanism remains controversial. The South Qinling Belt (SQB), which forms the northernmost NYB, is a significant target for studying the Neoproterozoic tectonic evolution of the NYB. Here we present U-Pb geochronology and Lu-Hf isotopes of detrital zircons from clastic rocks belonging to the Yaolinghe and Wudangshan Groups in the SQB. The U-Pb ages reveal that the maximum depositional ages are respectively ca. 630 Ma and ca. 680 Ma for the Yaolinghe and Wudangshan Groups in the Ankang and Wudang Uplifts. Based on the compilation of U-Pb ages and Hf−Nd−O isotopic data as well as multidimensional scaling (MDS) analysis, clastic rocks of the Yaolinghe Group are suggested to be sourced mainly from the NWYB, whereas those of the Wudangshan Group were derived mostly from the NWYB and the NYB. The data compiled also indicate ca. 800−680 Ma northeastward migration of magmatism in response to slab rollback and ca. 680−600 Ma eastward migration of magmatism resulting from slab tearing. A tectonic model is proposed that envisages an arcuate subduction system around the Yangtze Block during the mid−late Neoproterozoic. The continuous slab rollback and slab tearing underneath the NYB resulted in an extensional environment at that time. The rocks in the NWYB underwent uplift and erosion, and the detritus was transported to the NYB. The new model can well account for the two distinct groups of volcanic rocks in the Yaolinghe Group.
Guide for interpreting and reporting luminescence dating results
Shannon A. Mahan; Tammy M. Rittenour; Michelle S. Nelson; Nina Ataee; Nathan Brown …
Abstract: The development and application of luminescence dating and dosimetry techniques have grown exponentially in the last several decades. Luminescence methods provide age control for a broad range of geological and archaeological contexts and can characterize mineral and glass properties linked to geologic origin, Earth-surface processes, and past exposure to light, heat, and ionizing radiation. The applicable age range for luminescence methods spans the last 500,000 years or more, which covers the period of modern human evolution, and provides context for rates and magnitudes of geological processes, hazards, and climate change. Given the growth in applications and publications of luminescence data, there is a need for unified, community-driven guidance regarding the publication and interpretation of luminescence results. This paper presents a guide to the essential information necessary for publishing and archiving luminescence ages as well as supporting data that is transportable and expandable for different research objectives and publication outlets. We outline the information needed for the interpretation of luminescence data sets, including data associated with equivalent dose, dose rate, age models, and stratigraphic context. A brief review of the fundamentals of luminescence techniques and applications, including guidance on sample collection and insight into laboratory processing and analysis steps, is presented to provide context for publishing and data archiving.
A 50 m.y. melting model for the rare metal−rich Koktokay pegmatite in the Chinese Altai: Implications from a newly identified Jurassic granite
Jinsheng Han; Zhenhua Zhao; Pete Hollings; Huayong Chen
Abstract: The petrogenesis of the world famous Koktokay No. 3 pegmatite in the Chinese Altai Mountains is still enigmatic due to its superenrichment of rare metals and the apparent absence of a parental granite. We present results from a granite apophysis that was recently discovered in the No. 3 pegmatite open pit. Results show that it has low K/Rb (32.35−38.76), Zr/Hf (13.78−23.30), Nb/Ta (1.00−6.02), and extremely low K2O/Na 2O (0.12−0.20) ratios, which, together with its mica composition and the occurrences of garnet, indicate that it is a highly evolved muscovite albite granite. Columbite and apatite from the granite apophysis yielded U-Pb ages of 182.3 ± 1.0 Ma and 184.9 ± 4.3 Ma, respectively, which are younger than the Triassic ages (ca. 210 Ma) of the main magmatic stage but fall into the age range of the No. 3 pegmatite series (220−175 Ma). Both the granite and the apatite grains within it share nearly identical rare earth element patterns with the magmatic stage of the No. 3 pegmatite. The whole-rock εHf(t) values ( t = 183 Ma) range from −0.11 to +1.32, consistent with those of the No. 3 pegmatite and indicating a similar source. We propose that the Jurassic granite represents a late pulse of magma injected as apophyses from a deep-seated magma chamber (perhaps from the lower crust), which overlapped with the early pegmatite and promoted the rare metal mineralization. The No. 3 pegmatite and the Jurassic granite may represent a continuous magmatic system with a 50 m.y. melting process, generating this unusual giant pegmatite intrusion with abundant rare metals.
Timing and spatial variation of deformation along the Kanggur-Huangshan shear zone in the Chinese Tianshan: Implications for regional differential uplift and mineralization
M.N. Muhtar; Chang-Zhi Wu; Matthew J. Brzozowski; M. Santosh; Rong-Song Tian …
Abstract: The Chinese Tianshan experienced large-scale transcurrent tectonics, synkinematic emplacement of ultramafic to felsic intrusions, and the formation of various mineral deposits during late Paleozoic accretionary orogenesis. The relationships among the spatial variation of deformation, the distribution of Permian orogenic Au and magmatic Ni-Cu sulfide deposits, and the kinematic evolution of crustal-scale shear zones, however, remain ambiguous. To address these ambiguities, the spatial variation in the degree of deformation in the Kanggur-Huangshan shear zone in the Chinese Tianshan was characterized using detailed structural measurements and zircon U-Pb and muscovite 40Ar/39Ar age data. The new structural data indicate that a prominent spatial variation exists in the style of deformation throughout the Kanggur-Huangshan shear zone; intense ductile deformation structures are dominant in the east, while brittle structures become progressively more dominant toward to the west. Zircon U-Pb and muscovite 40Ar/ 39Ar age data for syn- and postkinematic intrusions along the Kanggur-Huangshan shear zone indicate that dextral strike-slip shearing occurred between 279 Ma and 249 Ma. The spatial variation in the degree of deformation and exhumation along the Kanggur-Huangshan shear zone was potentially caused by regional differential uplift induced by the collision of the Tianshan and Beishan regions; this was likely responsible for the predominant occurrence of magmatic Ni-Cu sulfide deposits in the eastern portion of the Kanggur-Huangshan shear zone and orogenic Au deposits in the western portion. The identified spatio-temporal relationship between deformation and distribution of orogenic Au and magmatic Ni-Cu sulfide deposits is crucial to the future success of mineral exploration in the Central Asian orogenic belt.
Filling critical gaps in the space-time record of High Lava Plains and co-Columbia River Basalt Group rhyolite volcanism
Vanessa M. Swenton; Martin J. Streck; Daniel P. Miggins; William C. McIntosh
Abstract: Miocene rhyolitic volcanism of eastern Oregon, USA, can be divided into two main episodes. Mantle plume upwelling is thought to have generated Columbia River Basalt Group (CRBG) lavas and coeval >16.5−15 Ma silicic volcanism trending north−south from northeast Oregon to northern Nevada. Rhyolite volcanism of the 12−0 Ma High Lava Plains province has been ascribed to either buoyancy-driven westward plume spreading or to slab rollback and mantle convection spanning from southeast Oregon to Newberry Volcano to the west. The apparent ca. 15−12 Ma eruptive hiatus suggests that rhyolites of these provinces were a product of separate processes, yet this gap was based on incomplete data. The lack of data on ∼33 of the total ∼50 rhyolitic eruptive centers in the area where the two provinces overlap (117−119°W, 43−44°N) yields only tenuous relationships between these two provinces. We acquired 40Ar/39Ar ages for 29 previously unanalyzed rhyolite centers that confirm the existence of a rhyolitic eruptive episode concurrent with CRBG flood basalt volcanism. Rhyolite eruptions gradually initiated at ca. 17.5 Ma, and our new ages indicate that peak intensity of the first eruptive episode occurred between 16.3 Ma and 14.4 Ma. We refine the ca. 15−12 Ma rhyolitic eruptive hiatus to 14.4−12.1 Ma, where strong recommencement of rhyolite eruptions began with Beatys Butte at 12.05 Ma. We find two prominent fluxes in rhyolitic eruptive activity after 12.1 Ma as opposed to one continuous, age-progressive trend, at 12.1−9.6 Ma and 7.7−5.1 Ma, which are separated by an ∼2 m.y. period of decreased rhyolite volcanism. Rhyolite eruptions were scarce after 5.1 Ma, at which point most eruptions were associated with Newberry Volcano. Periodicity of rhyolite volcanism along the High Lava Plains demands more punctuated basalt inputs than what continuous partial melting from west-spreading plume material should generate. Our new data suggest that regional rhyolite eruptions are a series of episodic events related to the arrival and storage of mafic mantle magmas. Paucity in rhyolite eruptions from 14.4 Ma to 12.1 Ma is related to decreased flux of CRBG flood basalt magmas at ca. 15 Ma. Strong recommencement of rhyolite volcanism at 12.1 Ma is related to continued Northwest Basin and Range extension and a peak rotation rate of Siletzia affecting regional lithosphere weakened by CRBG volcanism. Waning rhyolitic eruptive activity from ca. 9.6 Ma to 7.7 Ma reflects a regional transition in the primary mode of accommodation of extension from Northwest Basin and Range normal faulting to extension and shearing of the Brothers Fault Zone. Rhyolite volcanism between 7.7 Ma and 5.1 Ma was driven by continued regional extension in an area less affected by CRBG magmatism. Post-5.1 Ma rhyolite eruptions occurred within crust not influenced by CRBG magmatism but impacted by both regional extension and the Cascadia subduction zone.
The Ediacaran−Cambrian transition in the southern Great Basin, United States
Emily F. Smith; Lyle L. Nelson; Nizhoni O’Connell; Athena Eyster; Mary C. Lonsdale
Abstract: The Ediacaran−Cambrian boundary strata in the Great Basin of the southwestern United States record biological, geochemical, and tectonic change during the transformative interval of Earth history in which metazoans diversified. Here, we integrate new and compiled chemostratigraphic, paleontological, sedimentological, and stratigraphic data sets from the Death Valley region, the White-Inyo Ranges, and Esmeralda County in Nevada and California and evaluate these data within a regional geologic framework. A large negative carbon isotope (δ 13C) excursion—also known as the Basal Cambrian Excursion, or BACE—is regionally reproducible, despite lateral changes in sedimentary facies and dolomitization across ∼250 km, consistent with a primary marine origin for this perturbation. Across the southern Great Basin, Ediacaran body fossils are preserved in a variety of taphonomic modes, including cast and mold preservation, two-dimensional compressional preservation, two-dimensional and three-dimensional pyritization, and calcification. The stratigraphic framework of these occurrences is used to consider the relationships among taphonomic modes for fossil preservation and paleoenvironmental settings within this basin. In this region, Ediacaran-type fossils occur below the nadir of the BACE, while Cambrian-type trace fossils occur above. Sedimentological features that include giant ooids, stromatolites, and textured organic surfaces are widespread and abundant within the interval that records biotic turnover and coincide with basaltic volcanism and the BACE. We hypothesize that the prevalence of these sedimentological features, the BACE, and the disappearance of some Ediacaran clades were caused by environmental perturbation at the Ediacaran-Cambrian boundary.
How headward erosion breaches upstream paleolakes: Insights from dated longitudinal fluvial terrace correlations within the Sanmen Gorge, Yellow River
Hao Liang; Ke Zhang; Zhongyun Li; Jianli Fu; Zhangxin Yu …
Abstract: Headward erosion breaching a formerly closed paleolake has been interpreted as an integration process between individual drainage networks. However, the rarity of well-documented cases of this process makes it difficult to explore the fluvial response or its mechanism. Fortunately, dated deposits from a former paleolake in the Fenwei Basin, coupled with fluvial terraces along the Sanmen Gorge of the Yellow River, provide ample opportunities to replicate the upstream integration process and associated landform response. Herein, we applied terrace correlation and age constraints to construct dated longitudinal profiles throughout the Fenwei Basin and the downstream Sanmen Gorge. We identified two age models in longitudinal profiles: (1) relatively high diachronous terraces (Terrace series A) aged headward from ca. 621 ka to 336 ka, localized in the Sanmen Gorge; and (2) relatively low isochronous terraces (Terrace series B) capped by S 2 paleosol (ca. 245−190 ka) developed throughout the basin and the gorge. This study hypothesized these two terrace series to have emerged as a mid-Pleistocene bottom-up integration event. In this event, headward incision initiated at least at ca. 621 ka, progressed upstream at a rate of 79.8 mm/a, and breached the Fenwei Basin at ca. 245 ka. This event is likely ascribed to tectonic subsidence of the North China Plain, and may be the latest integration process between the middle and lower reach of the Yellow River. We synthesize comprehensive models to describe terrace genesis and responses in tectonic, climatic, and bottom-up integration processes, which could widen our understanding of long-term large river behaviors.
Petrogenesis of arsenic and platinum-group minerals from a partially serpentinized dunite in East Dover, Vermont, USA
John M. Brigham; Suzanne L. Baldwin
Abstract: In the Appalachian Mountains of Vermont, USA, variably serpentinized ultramafic rocks mark the Ordovician Taconic orogenic suture zone. These ultramafic rocks provide evidence for several alteration events that occurred during Appalachian orogenesis. The largest of these ultramafic bodies occurs as a partially serpentinized meta-dunite located in East Dover, Vermont. Whole-rock X-ray fluorescence spectroscopy and electron microprobe data on variably serpentinized meta-dunite samples are interpreted with respect to several processes including fluid/melt-rock interaction in the mantle, serpentinization, and subsequent regional metamorphism. We report the first discovery of nickel arsenide minerals hosted in this meta-dunite, as well as rare occurrences of platinum-group mineral inclusions in chromitite. Although the platinum-group minerals and chromitite are rare, their occurrence and chemistry suggest that they formed by fluid/melt-rock interaction during partial melting events that produced the dunite, likely in a supra-subduction zone setting. Arsenic minerals are rare in un-serpentinized samples but are ubiquitous in highly serpentinized samples, which suggests that most of the arsenic was introduced into the ultramafic rocks during serpentinization. Whole-rock geochemical analyses also indicate that highly serpentinized samples contain the highest concentrations of arsenic. The discovery of arsenic minerals identifies a potential source to explain elevated arsenic in groundwater in Vermont, which is a serious health concern in places where wells have been drilled in serpentinite bedrock.
Plio-Pleistocene cooling of the northeastern Tibetan Plateau due to global climate change and surface uplift
Fabiana Richter; Carmala N. Garzione; Weiguo Liu; Xiaoke Qiang; Hong Chang …
Abstract: It has been proposed that the northeastern Tibetan Plateau (NETP) was the most recent plateau sub-region to gain elevation, leading to profound environmental changes in northern East Asia around the mid-Pliocene (ca. 3.6 Ma). Alternatively, environmental changes in the region have been linked to ongoing global cooling and glacial intensification after 3.3 Ma. Here, we test these hypotheses using Plio-Pleistocene estimates of paleotemperatures and paleoelevations in the NETP derived from oxygen and carbon stable isotopic composition (n = 792) and clumped isotope measurements (n = 32) of carbonates from a Lake Qinghai Basin sediment core, eastern NETP. From 5 to 2 Ma, basinal mean annual air temperatures (MAATs) decreased by 4.9 ± 2.8 °C at rates of 1.6 °C ± 0.5 °C/myr, concurrently with regional and global cooling. However, the largest MAAT decline occurs between ca. 4.8−3.7 Ma (4.1 ± 3.2 °C) and ca. 3.4−2.0 Ma (−0.3 ± 2.8 °C) and may correspond with an elevation change of 1.0 ± 1.0 km at 3.6 Ma, coincident with tectonic activity in the Lake Qinghai Basin and several other basins in the NETP and its foreland. Taken together, these results suggest a combination of global cooling and a small magnitude of surface uplift (View article: https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/doi/10.1130/B36302.1/617714/Plio-Pleistocene-cooling-of-the-northeastern
Drainage initiation, expansion, and channel-head arrest in heterogenous bedrock landscapes of the Colorado Plateau
A.T. Steelquist; M.G.A. Lapôtre; G.E. Hilley
Abstract: The transition between hillslopes and channels defines landscape characteristics such as drainage density, ridge spacing, and hillslope length. Previous studies of the location of this transition have demonstrated that soil advection and diffusion work either individually or in concert to set the limits of channelization in largely soil-mantled landscapes. However, no such model explaining the limits of channelization in bedrock landscapes has been established. In this study, we explore the morphologic signatures and processes of channel initiation, headward elongation, and eventual arrest in the bedrock landscape of the Raplee Ridge monocline, Utah, USA. The monocline provides an opportunity to test whether a threshold shear stress controls the initiation of channels through inspection of slope and upstream drainage area at varyingly incised portions of the landscape. Using a combination of lidar and structure-from-motion data, we find that incised and unincised channels occupy overlapping but separable portions of slope-upstream area space, while geomorphic mapping and field-based observations allow for further distinction between erosional processes with threshold shear stress values that range from ∼60 Pa to 160 Pa. We develop a conceptual model of the initiation and expansion of these bedrock channels using field observations, in which channels first initiate by vertical plucking of blocks, disaggregate limestone by block sliding, elongate upslope by knickpoint retreat in variably erosive lithologies, and eventually arrest at predictable landscape positions due to block buttressing. These results suggest that shear stress-controlled processes play a fundamental role in setting the degree of channelization in bedrock landscapes; however, the thresholds for channel erosion can change in response to local geologic factors such as lithology and structural geometry. This implies that detailed geologic knowledge may be necessary for the interpretation and modeling of fluvial channels in terrestrial bedrock landscapes and those on other planets.
Migration and localization of faulting near the intersection of the Dead Sea Fault and the Carmel−Gilboa−Faria Fault System
Yariv Hamiel; Oded Katz; Yoav Avni
Abstract: Crustal deformation and seismicity in the Levant region are mainly related to the plate-boundary Dead Sea Fault (DSF) and the intraplate Carmel−Gilboa−Faria Fault System (CGFS). The intersection between these two major fault systems is generally treated as a ∼35-km-wide deformation belt stretched between the Faria and Gilboa Faults. Here, we present spatial and temporal analysis of faulting near this intersection. Our analysis is based on new geological mapping, new high-resolution airborne light detection and ranging (LiDAR) data, and seismic reflection profiles and indicates northward migration and localization of the intersection over time since the early Miocene. We discovered and mapped outcrops of Miocene, Pliocene, and Pleistocene rock units as well as faults and reconstructed the evolution of deformation. Three main tectonic phases were identified in this area covering the following periods: the early−middle Miocene, the late Miocene−Pliocene, and the Quaternary. During the first phase, the DSF and the CGFS developed, and the CGFS faulted along a series of subparallel grabens and elongated NW−SE, between the southernmost Faria and the northernmost Gilboa faults, over a belt width of ∼35 km. During the second phase, deformation along the CGFS migrated northward and concentrated at an ∼6-km-wide zone in the northern Faria Anticline. During the third stage, small-scale northward migration and localization of the deformation to a width zone of ∼1−2 km at the southern boundary of the Beit She’an Valley occurred. Faults from the third phase reveal both sinistral and normal faulting. We propose that the currently active intersection between the DSF and the CGFS is located east of this localized deformation zone, near a right step of the DSF and the uplifted area of Tel Al-Qarn in the eastern Jordan Valley. We suggest that the northward migration and localization of this intersection are related to regional tectonic changes, spatial variations in the Sinai-Arabia Euler pole, and the localization of deformation along the DSF.
Late Permian-Triassic ridge subduction beneath the northeastern margin of the North China Craton: Evidence from the Kaiyuan appinitic to high-Mg intermediate intrusions in northern Liaoning, North China
Lingling Yuan; Xiaohui Zhang; Zhili Yang
Abstract: Recognizing terminal ridge subduction is of first-order importance in chronicling the tectonic timeline of any accretionary orogens, but remains elusive in the final suturing zone of the Paleo-Asian Ocean along the Solonker−Xar Moron−Changchun−Yanji swath. Here we report two generations of ultramafic-mafic to intermediate intrusions from northern Liaoning, North China, linking eastern Central Asian Orogenic Belt and northern North China Craton. The ca. 259 Ma appinites show variable enrichment in large-ion lithophile elements (LILEs) but depletion in high-field strength elements (HFSEs), and variably enriched isotopic compositions with εNd(t) of −11.8 to −2.4 and zircon εHf(t) of −15.7 to +5.0. These petro-geochemical traits argue for their derivation from partial melting of subduction-modified lithospheric mantle. By contrast, the high-Mg diorites of 226−223 Ma display decoupling features between LILEs and HFSEs and juvenile isotopic signatures with εNd(t) of −0.3 to +2.6 and zircon εHf(t) of +5.4 to +11.4. These characters are consistent with a magmatic origin from partial fusion of sediment-fertilized asthenospheric mantle. Synthesizing the two mantle-melting events with regional coeval igneous activities leads to the identification of Late Permian (265−250 Ma) and Middle-Late Triassic (241−218 Ma) magmatic flare-ups with a 10−15 m.y. lull along the northern North China Craton. This magmatic tempo tends to record a salient mantle replacement from hydrous metasomatized lithosphere to anhydrous asthenosphere. In combination with ensuing progressive crustal growth, simultaneous metamorphism, and structural activities, these diverse manifestations make a compelling case of ridge subduction and concomitant slab window formation during the eventual closure of the Paleo-Asian Ocean along the Solonker−Xar Moron−Changchun−Yanji swath.
Tracing remnants of Ediacaran S-type granitoids from beach placers in SE Madagascar
P.G. Athira; K. Sajeev; P.M. George; M.G. Zhai; L.G. Zhou …
Abstract: Southeast Madagascar hosts several major deposits of beach placers, the provenance of which is little understood. To redress this imbalance, we present new laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb ages, and trace elements of zircons and monazites, as well as Hf isotopes of zircons from heavy mineral beach sands at Taolagnaro (Fort Dauphin) in SE Madagascar. The ages of detrital zircon grains range from 650 Ma to 450 Ma and from 1900 Ma to 1700 Ma (one grain has an age of 2633 Ma), and detrital monazite ages range from 574 Ma to 484 Ma. The Ediacaran−Cambrian zircons (650−450 Ma) and monazites (574−484 Ma) record a major Pan-African orogenic event in SE Madagascar. Geologically, the 650−450 Ma ages correlate well with coeval granitic gneisses in southern Madagascar, and a few 1900−1700 Ma ages can be linked to putative Paleoproterozoic protoliths of metasedimentary rocks from the Anosyen Domain and from the Itremo Group of central Madagascar. The zircon mineral chemistry indicates that some of the grains (P>25 μmol g−1) were sourced from S-type granitic rocks. The monazite chemistry points toward a metamorphic provenance from garnet-bearing amphibolite facies rocks, such as migmatitic gneisses in southern Madagascar. The Hf isotopic compositions of detrital zircons indicate reworking of their probable Paleoproterozoic−Archean source rocks. We conclude that the beach sand zircons and monazites were largely sourced from the uplifted and eroded Anosyen and Androyan Domains in SE Madagascar, and with minor contributions from central Madagascar, which were all transported by rivers to the ambient ocean to the southeast.
Structure, geochronology, and petrogenesis of Permian peraluminous granite dykes in the southern Chinese Altai as indicators of Altai−East Junggar convergence
Tan Shu; Yingde Jiang; Karel Schulmann; Yang Yu; Chao Yuan …
Abstract: How and at which thermal conditions the convergence between the Chinese Altai and East Junggar operated remain poorly understood. This issue is addressed in the current study by focusing on the timing and petrogenesis of syntectonic granite dykes from the representative areas of Fuyun (convergent front) and Kalasu-Aletai (Chinese Altai interior). It is shown that Fuyun and Kalasu-Aletai dykes are fractionated I- and S-type granites, with zircon and monazite U-Pb ages of 300−291 Ma and 281−265 Ma, respectively. Geochemically, the Fuyun dykes have lower contents of aluminous (ASI: 0.97−1.13) and light rare earth element-enriched features, while the Kalasu-Aletai dykes have ASI = 1.01−2.17 and show overall flat rare earth element patterns with tetrad effects. The Fuyun dykes exhibit less evolved Sr-Nd isotopic characteristics (87Sr/86 Srinitial: 0.7039−0.7048, εNd(t): + 5.7 to + 6.1) with respect to those of the Kalasu-Aletai dykes (87Sr/ 86Srinitial: 0.6978−0.7183, εNd(t): −7.6 to +3.0). The Fuyun and Kalasu-Aletai dykes are geochemically compatible with isotopically less evolved East Junggar arc components and heterogeneous Ordovician wedge sediment of the Chinese Altai, respectively, implying genetic links. We propose that the late Paleozoic Chinese Altai−Junggar convergence created a local perturbation of weak mantle beneath the southern Chinese Altai, causing partial melting of the underthrusting East Junggar and the overriding Altai components successively. The resulting magmas were emplaced along northward propagating syn-tectonic tensional fractures perpendicular to the Chinese Altai−East Junggar deformation front that serves as an excellent indicator of the convergent-shortening process.
Deoxygenation and organic carbon sequestration in the Tethyan realm associated with the middle Eocene climatic optimum
Margot J. Cramwinckel; Robin van der Ploeg; Niels A.G.M. van Helmond; Niels Waarlo; Claudia Agnini …
Abstract: The middle Eocene climatic optimum (ca. 40 Ma) stands out as a transient global warming phase of ∼400 k.y. duration that interrupted long-term Eocene cooling; it has been associated with a rise in atmospheric CO 2 concentrations that has been linked to a flare-up in Arabia-Eurasia continental arc volcanism. Increased organic carbon burial in the Tethys Ocean has been proposed as a carbon sequestration mechanism to bring the middle Eocene climatic optimum to an end. To further test these hypotheses, we assessed the sedimentary and geochemical expression of the middle Eocene climatic optimum in the northern Peri-Tethys, specifically, the organic-rich Kuma Formation of the Belaya River section, located on the edge of the Scythian Platform in the North Caucasus, Russia. We constructed an age-depth model using nannofossil chronobiostratigraphy. Throughout the studied middle Eocene interval (41.2−39.9 Ma), we documented sea-surface temperatures of 32−36 °C based on the tetraether index of tetraethers consisting of 86 carbons (TEX86), depending on proxy calibration, and during the early middle Eocene climatic optimum, we observed sea-surface warming of 2−3 °C. Despite the proximity of the section to the Arabia-Eurasia volcanic arc, the hypothesized source of volcanic CO2, we found no evidence for enhanced regional volcanism in sedimentary mercury concentrations. Sedimentary trace-element concentrations and iron speciation indicate reducing bottom waters throughout the middle Eocene, but the most reducing, even euxinic, conditions were reached during late middle Eocene climatic optimum cooling. This apparent regional decoupling between ocean warming and deoxygenation hints at a role for regional tectonics in causing basin restriction and anoxia. Associated excess organic carbon burial, extrapolated to the entire regional Kuma Formation, may have been ∼8.1 Tg C yr−1, comprising ∼450 Pg C over this ∼55 k.y. interval. Combined with evidence for enhanced organic carbon drawdown in the western Peri-Tethys, this supports a quantitatively significant role for the basin in the termination of the middle Eocene climatic optimum by acting as a large organic carbon sink, and these results collectively illustrate that the closing Tethys Ocean might have affected global Paleogene climate. Moreover, this study highlights the importance of the interplay between global climate and regional oceanic gateway evolution in determining local climate and oceanographic change.
GSA BULLETIN articles published ahead of print are online at https://bulletin.geoscienceworld.org/content/early/recent