Geochemical indications for the Paleocene-Eocene Thermal Maximum (PETM) and Eocene Thermal Maximum 2 (ETM-2) hyperthermals in terrestrial sediments of the Canadian Arctic

During the late Paleocene to early Eocene, clastic fluvial sediments and coals were deposited in northern high latitudes as part of the Marga­ret Formation at Stenkul Fiord (Ellesmere Island, Nunavut, Canada). Syn-sedimentary tectonic movements of the Eurekan deformation continu­ously affected these terrestrial sediments. Different volcanic ash layers occur, and unconformities subdivide the deposits into four sedimentary units. Rare vertebrate fossils indicate an early Eocene (Graybullian) age for the upper part of the Stenkul Fiord outcrop. Here, we present carbon isotope data of bulk coal, related organic-rich mud and siltstones, a plant leaf wax-derived alkane, and additional plant remains. These data provide a complete carbon isotope record of one stratigraphic section with defined unconformity positions and in relation to other Eurekan deformation features. A previously dated ash layer MA-1 provided a U-Pb zircon age of 53.7 Ma and is used as a stratigraphic tie point, together with a discrete negative carbon isotope excursion found above MA-1 in a closely sampled coal seam. The excursion is identified as the likely expression of the I-1 hyperthermal event. Based on our isotope data that reflect the early Eocene dynamics of the carbon cycle, this tie point, and previous paleontological constraints from vertebrate fossils, the locations of the Paleocene-Eocene Thermal Maximum (PETM) and Eocene Thermal Maximum 2 (ETM-2) hyperthermals and their extent along the complete section are herein identified. Within the intervals of the PETM and ETM-2 hyperthermal events, increasing amounts of clastic sediments reached the site toward the respective end of the event. This is interpreted as a response of the fluvial depositional system to an intensified hydrological system during the hyperthermal events. Our study establishes an enhanced stratigraphic framework allowing for the calcula­tion of average sedimentation rates of different intervals and considerations on the completeness of the stratigraphic record. As one of the few high-latitude outcrops of early Eocene terrestrial sediments, the Stenkul Fiord location offers further possibilities to study the effects of extreme warming events in the Paleogene.

Formation, preservation and extinction of high-pressure minerals in meteorites: temperature effects in shock metamorphism and shock classification

The goal of classifying shock metamorphic features in meteorites is to estimate the corresponding shock pressure conditions. However, the temperature variability of shock metamorphism is equally important and can result in a diverse and heterogeneous set of shock features in samples with a common overall shock pressure. In particular, high-pressure (HP) minerals, which were previously used as a solid indicator of high shock pressure in meteorites, require complex pressure–temperature–time (P–T–t) histories to form and survive. First, parts of the sample must be heated to melting temperatures, at high pressure, to enable rapid formation of HP minerals before pressure release. Second, the HP minerals must be rapidly cooled to below a critical temperature, before the pressure returns to ambient conditions, to avoid retrograde transformation to their low-pressure polymorphs. These two constraints require the sample to contain large temperature heterogeneities, e.g. melt veins in a cooler groundmass, during shock. In this study, we calculated shock temperatures and possible P–T paths of chondritic and differentiated mafic–ultramafic rocks for various shock pressures. These P–T conditions and paths, combined with observations from shocked meteorites, are used to constrain shock conditions and P–T–t histories of HP-mineral bearing samples. The need for rapid thermal quench of HP phases requires a relatively low bulk-shock temperature and therefore moderate shock pressures below ~ 30 GPa, which matches the stabilities of these HP minerals. The low-temperature moderate-pressure host rock generally shows moderate shock-deformation features consistent with S4 and, less commonly, S5 shock stages. Shock pressures in excess of 50 GPa in meteorites result in melt breccias with high overall post-shock temperatures that anneal out HP-mineral signatures. The presence of ringwoodite, which is commonly considered an indicator of the S6 shock stage, is inconsistent with pressures in excess of 30 GPa and does not represent shock conditions different from S4 shock conditions. Indeed, ringwoodite and coexisting HP minerals should be considered as robust evidence for moderate shock pressures (S4) rather than extreme shock (S6) near whole-rock melting.

Empirical constraints on progressive shock metamorphism of magnetite from the Siljan impact structure, Sweden

Little is known about the microstructural behavior of magnetite during hypervelocity impact events, even though it is a widespread accessory mineral and an important magnetic carrier in terrestrial and extraterrestrial rocks. We report systematic electron backscatter diffraction crystallographic analysis of shock features in magnetite from a transect across the 52-km-diameter ca. 380 Ma Siljan impact structure in Sweden. Magnetite grains in granitoid samples contain brittle fracturing, crystal-plasticity, and lamellar twins. Deformation twins along {111} with shear direction of <112> are consistent with spinel-law twins. Inferred bulk shock pressures for the investigated samples, as constrained by planar deformation features (PDFs) in quartz and shock twins in zircon, range from 0 to 20 GPa; onset of shock-induced twinning in magnetite is observed at >5 GPa. These results highlight the utility of magnetite to record shock deformation in rocks that experience shock pressures >5 GPa, which may be useful in quartz-poor samples. Despite significant hydrothermal alteration and the variable transformation of host magnetite to hematite, shock effects are preserved, which demonstrates that magnetite is a reliable mineral for preserving shock deformation over geologic time.

Mechanical Characteristics and Failure Modes of Low-Strength Rock Samples with Dissimilar Fissure Dip Angles

The mechanical characteristics and failure modes of low-strength rock sample with various fissure dip angles were investigated by conventional uniaxial compression test and three-dimensional (3D) crack reconstruction. The results indicated that compared with high-strength rock masses, cracks had different influences on the low-strength rock mass mechanical deformation features. Thereinto, the dip angle of fissures can cause post-peak failure stage of stress-strain curve change from swift decline to multi-step down, showing obvious ductility deformation and failure characteristics. Peak strength and elastic modulus owned an anti-S-shaped growth tendency with the growth of fissure dip angle, which was positively correlated and greatest subtle to the fissure dip angle α < 21° and α > 66.5°. The axial peak strain reduced first and enlarged rapidly with growing fissure dip angle, suggesting a V-shaped change trend. Increasing the fissure dip angle will change the sample failure mode, experienced complete tensile failure to tensile-shear composite failure, and ultimately to typical shear failure. Also, the crack start angle decreased with enlarging fissure dip angle, larger than that the high-strength rock mass fissure dip angle. The above research findings can complement and improve the study of fissured rock masses.

Mechanical Characteristics and Failure Modes of Low-Strength Rock Samples with Dissimilar Fissure Numbers

The mechanical characteristics and failure modes of low-strength rock sample with various crack dip angles and numbers were investigated by conventional uniaxial compression test and three-dimensional (3D) crack reconstruction. The results indicated that compared with high-strength rock masses, cracks had different influences on the low-strength rock mass mechanical deformation features. Thereinto, the number of fissures can cause post-peak failure stage of stress-strain curve change from swift decline to multi-step down and horizontally extended decline, respectively, showing obvious ductility and ductile-flow deformation and failure characteristics. Due to the structural effect, only under the condition of fissure α < 90°, the modulus of peak strength and elastic modulus lowered with the enhancement of fissure number and had a negative correlation. As the number of fissures increased, the axial peak strain increased first and then decreased, demonstrating a reversed V-shaped change trend. Fissure number can fully affect the crack propagation law only in the case of vertical fissures. The above research findings can complement and improve the study of fissured rock masses.

Application of an improved clustering approach on GPS height time series at CMONOC stations in Southwestern China

The hydrological, geological and meteorological conditions in southwestern China are relatively complex, so that the land surface deformation presents various features. Using 58 Crustal Movement Observation Network of China (CMONOC) stations across four provinces in Southwestern China, we adopt an improved clustering algorithm to classify 49 stations into 12 clusters with different similarity levels. Our results show that the average annual signals of GPS stations within each cluster have strong consistency, while obvious differences exist among the 12 clusters, indicating that clustering algorithm helps to describe surface deformation features more accurately in regions with complex conditions. We then combine other earth observation techniques, such as the Gravity Recovery and Climate Experiment (GRACE) satellite datasets and surface loading models (SLM), and observe that GPS, GRACE and SLM have strong correlation in their monthly displacement series at GPS stations. After excluding non-clustered stations according to our previous clustering results, the correlation coefficients of GPS/GRACE and GPS/SLM are enhanced. Also, the RMS reduction rates of GPS coordinate time series have been improved after deducting displacements obtained from GRACE and SLM, thus the clustering algorithm proves to be effective in improving the consistency of three techniques in joint detection of surface deformation. Moreover, the vertical displacements of four riverside GPS stations in the Three Gorges Reservoir (TGR) area show significant negative correlation with water level of TGR, hence we conclude that the Three Gorges Dam (TGD) may directly affect the consistency of GPS annual signals of its upstream and downstream GPS stations. Graphical Abstract

Vacuum-induced lateral deformation around a vertical drain in dredged slurry

The horizontal strain in the vacuum preloading/dewatering of dredged slurry is significant to the apparent clogging effect and estimation of surface settlement around a drain; however, it has seldom been investigated in previous studies. In this study, a vacuum consolidation model test assisted with the particle image velocimetry (PIV) technology was conducted. The displacement vector field was obtained through PIV analysis and image processing; it was used to visually observe the deformation features around a drain. Based on the displacement field, the vertical/horizontal strains at varied radial distances were calculated to explain the “soil pile” and apparent clogging effect. From the strain distribution with radial distances, a significant lateral compression zone near the drain and an extension zone at farther areas were confirmed. Furthermore, a simple explicit model was established to evaluate the horizontal strain within a prefabricated vertical drain unit cell considering a horizontal attenuated vacuum and compression/extension zone. Finally, this method was applied to predict the horizontal displacement in the model test. The results showed that the proposed method can estimate the lateral displacement of soft clay slurry fairly well.

KINNEYIA-TYPE WRINKLE STRUCTURES ON SANDSTONE BEDS: NOT MICROBIALLY INDUCED BUT DEFORMATION FEATURES CAUSED BY SYNSEDIMENTARY EARTHQUAKES

ABSTRACT A category of wrinkle structures, often termed Kinneyia structure or Runzel marks, comprises bedding plane features consisting typically of anastomosing, low-relief, flat-topped ridges with intervening depressions. Topographic relief is usually less than a millimeter. They are locally common on the upper surfaces of fine- to medium-grained sandstone beds interbedded with mudstone deposited in offshore settings, especially in Precambrian and lower Paleozoic strata but as young as Cretaceous. For more than the last two decades these wrinkle structures have been widely regarded as due to microbial mats, and have been taken as evidence for dominance in the Proterozoic of microbially stabilized sediment and, in the Phanerozoic, a matground marine benthic ecology which gradually gave way to a mixground ecology. The detailed morphology and cross-cutting relationships demonstrated by a range of specimens of Proterozoic, Cambrian, and Silurian age, however, cast this interpretation into doubt. The relationship between the wrinkled surface and bioclasts such as shells and both prior- and later-formed scour surfaces, and horizontal and vertical burrows show that these wrinkles did not develop due to the surface topography of microbial mats or compaction of microbial mats during burial, but instead formed at the top of a sand bed at the interface with an overlying layer of mud. Deformation is ascribed to vibration from low-magnitude earthquakes. The presence in some units of small-scale sedimentary dikelets and crack arrays that formed later after some stiffening, along with locally associated seismites and other evidence for nearby faulting, show that syndepositional tectonic activity was not unexpected and support the interpretation that this category of wrinkle structures is a type of seismite.

Holocene surface-rupturing earthquakes on the Dinaric Fault System, western Slovenia

The Dinaric Fault System in western Slovenia, consisting of NW–SE-trending, right-lateral strike-slip faults, accommodates the northward motion of Adria with respect to Eurasia. These active faults show a clear imprint in the morphology, and some of them hosted moderate instrumental earthquakes. However, it is largely unknown if the faults also had strong earthquakes in the late Quaternary. This hampers our understanding of the regional tectonics and the seismic hazard. Geological evidence of co-seismic surface ruptures only exists for one historical event, the 1511 Idrija earthquake with a magnitude of ∼ M 6.8, but the causative fault is still disputed. Here we use geomorphological data, near-surface geophysical surveys, and paleoseismological trenching to study two of these faults: the Predjama Fault and the Idrija Fault. In a paleoseismological trench across the Predjama Fault we found deformation features that may have been caused by an earthquake between 13–0.7 ka, very likely not earlier than 8.4 ka. At the Idrija Fault, a surface-rupturing earthquake happened around 2.5 ka. We show that instrumental and historical seismicity data do not capture the strongest events in this area.

Constitutive Model and Deformation Characteristics of a Throat Insert Made of the C/C Composite

Axial braided C/C composite materials are widely applied in the throat insert of solid rocket motors. It is the key for the development and design of solid rocket motors to characterize accurately the macromechanical properties of the materials. The Jones-Nelson model is employed to characterize the constitutive relation of the axial braided C/C composite material in this paper. The Jones-Nelson model is expanded and modified to characterize the C/C composite under complex load conditions. The typical external load tests were carried out to verify the accuracy. On this basis, the deformation features of the axial braided C/C composite throat insert are investigated during the working process of motors, and the strain of the throat insert during the working process is obtained by FEM. The strain and temperature in the outer surface of the throat insert are measured in the ground test of motors and are compared with the numerical results by FEM to verify the accuracy of the model. The results show that the result calculated based on the modified Jones-Nelson model by FEM is in a good agreement with the test result. It shows that the modified Jones-Nelson model can better describe the constitutive relation of C/C composite materials, and it can be promoted to the engineering application of the throat insert of solid rocket motors.