In many mountain belts, deformation concentrates along mechanically weak fault zones inherited from earlier tectonic events. This work investigates the effects of two modes of structural weakening on the orientation of rifting and later tectonic inversion with respect to the imposed divergence/convergence direction in a high-resolution 3D finite difference model with a viscous-frictional rheology. In the first set of experiments, weakening consists in a decrease in frictional strength with increasing shear strain. The generated normal faults strike orthogonal to the imposed divergence direction. These faults are reactivated during tectonic inversion and absorb 50 to 70 percent of accumulated strain. In the second set of experiments, frictional strength is a decreasing function of shear strain rate. The generated faults are oblique to the divergence direction, implying oblique fault slip. Fault reactivation depends on the obliquity of the inverted rift to convergence direction, where larger obliquity leads to more intense fault reactivation. These new numerical results are compared to previous analogue and numerical models on the one hand, and natural examples of intracontinental mountain ranges due to tectonic inversion on the other hand. These comparisons demonstrate that both modes of frictional weakening should be taken into account when seeking to understand large-scale rifting and inversion tectonics.

Although previous studies demonstrate the photochemical reduction of uranyl (UO22+) in the presence of various organic compounds, the actual roles of organic molecules as ligands and electron donors during the photoreaction are poorly understood. In this study, photochemical reduction of uranyl is examined with respect to organic ligands as electron donors and complexing agents, the role of titanium oxide (TiO2) nanoparticles as a photocatalyst, and the influence of UV light irradiation with emission peaks in the UV-A, UV-B, and UV-C ranges. Organic compounds with different binding affinities to uranyl such as acetate, ethylenediaminetetracetate (EDTA), oxalate, and hydroquinone were selected. Uranyl solutions prepared with one organic compound in a 1:8 molar ratio were irradiated under anoxic and acidic conditions (O2

Oceanic anoxia—including euxinic settings defined by the presence of water column hydrogen sulfide (H2S)—is minor in the ocean today. Such conditions, however, were common or even dominant in the past, particularly during the Precambrian and Phanerozoic oceanic anoxic events. The latter are associated with massive petroleum and mineral reserves and many of the major extinction events in the paleontological record. Our ability to recognize ancient oxygen deficiencies relies strongly on paleontological data viewed in combination with geochemical tracers, and geochemistry is typically our only window onto ancient marine redox during the Precambrian when diagnostic skeletal and behaviorial traces of oxygen-dependent animals are mostly missing. So far no approach has gained wider acceptance than the iron proxies, which rely generally on quantification of the extent to which reactive iron (as oxides principally) is converted to pyrite. The promise of these approaches lies in part with the relative ease of measurement, but it is this ease and the corresponding widespread use that has also led to misuses. Much of the recent confidence in the iron paleoredox proxies lies with sophisticated deconstruction of the reactive Fe pool via mineral-calibrated wet chemical speciation. These validations and calibrations, mostly in the modern ocean, expose the challenges, while at the same time opening other doors of opportunity as the catalog of controlling factors extends beyond water column redox to include sedimentation rate, sedimentary Fe remobilization, signals of oscillatory redox, and hydrothermal versus other primary Fe inputs to the ocean, among other factors. Also key is a deep understanding of the limitations imposed—or at least the due diligence required—as linked to mineral transformations during burial and metamorphism. This review seeks to highlight many of the key issues, including appropriate sample choices, as a roadmap for those keen to apply Fe proxies in their studies of ancient oceans and their relationships to co-evolving life. Among the critical messages to take away is the value of robust Fe-based measures of local redox that, when combined with elemental mass balances and isotopic proxies dependent on those local conditions, can shed light on the global redox state of the oceans through time and related implications for the history of life on Earth.

Neoproterozoic glacial diamictites and rift-related volcanics are preserved throughout the North American Cordillera, yet the nature and timing of both glaciation and rifting are poorly constrained. New geochronological, geochemical, and stratigraphic data from the Cryogenian Gataga volcanics and bounding units at Gataga Mountain, in the Kechika Trough of northern British Columbia, better constrain the age of these rift-related volcanics and suggest that they erupted during glaciation. At Gataga Mountain, three informal sequences are exposed; a basal quartzite, the Gataga volcanics, and an overlying mixed carbonate-siliciclastic succession. The basal quartzite is dominated by cross-bedded sandstone with an intertidal facies assemblage including bidirectional cross-stratification and mud-cracks, indicative of non-glacial deposition. The overlying Gataga volcanics are over one kilometer thick, comprising both mafic and felsic units, with volcaniclastic breccia and interbedded sedimentary units including iron formation and matrix-supported diamictite with exotic clasts. Magmatic ages in the upper Gataga volcanics span 696.2 ± 0.2 to 690.1 ± 0.2 Ma, and detrital zircon from the underlying non-glacial quartzite provide a maximum age constraint on the onset of glaciation

In this paper we review processes responsible for the formation of granites in subduction systems. While there is robust evidence that strongly peraluminous, so-called S-type granites are formed by melting of metasediments in the (lower) crust, the processes that form metaluminous, so-called I-Type granites are less certain. We review four lines of evidence that have been presented in support of a dominantly crustal origin for these granites: 1) field observations; 2) isotopic compositions; 3) the missing cumulate problem; and 4) the bimodal distribution of igneous rock compositions. We show that in most cases the evidence is at least equally permissive of a process dominated by crystallization-differentiation. Moreover, we show that this mechanism is more efficient than re-melting and avoids additional difficulties associated with the source of heat and or fluids necessary for productive partial melting. Lastly, we show with a simple stochastic model that crystallization-differentiation also explains the first-order compositional and density stratification of arc crust and hence continental crust. We conclude that the structure of arc crust is comparable to oceanic crust at fast spreading ridges and may be controlled by analogous melt transport and stagnation mechanisms.

Besides depth and temperature, CO2 and H2O, are the two most important variables in stabilizing partial melts in the Earth9s mantle. However, despite decades of experimental studies on the roles of these two volatile species in affecting mantle melting, ambiguity remains in terms of the stability, composition, and proportion of volatile-bearing partial melts at depths. Furthermore, the difference in the influence of H2O versus CO2 in production of mantle melts is often inadequately discussed. Here I first discuss how as a function of depth and concentration of volatiles, the peridotite + H2O versus peridotite + CO2 near-solidus melting conditions differ - discussing specifically the concepts of saturation of volatile-bearing phases and how the mode of storage of ‘water’ and carbon affects the near solidus melting relations. This analysis shows that for the Earth9s mantle beneath oceans and continents, deep, volatile-induced melting is influenced mostly by carbon, with water-bearing carbonated silicate melt being the key agent. A quantitative framework that uses the existing experimental data, allows calculation of the loci, extent of melting, and major element compositions of volatile-bearing partial melts beneath oceans and continents. How the domains of volatile-bearing melt stability are affected when possible oxygen fugacity variation at depths in the mantle is taken into account is also discussed. I show that trace amount hydrous carbonated silicate melt is likely stabilized at two or more distinct depths in the continental lithospheric mantle, at depths ranges similar to where mid-lithospheric discontinuity (MLD) and lithosphere-asthenosphere boundary (LAB) have been estimated from seismology. Whereas beneath oceans, hydrous carbonated silicate melt likely remain continuously stable from the base of the thermal boundary layer to at least 200 km or deeper depending on the prevailing oxygen fugacity at depths. Hotter mantles, such as those beneath oceans, prevent sampling strongly silica-undersaturated, carbonated melts such as kimberlites as shallower basaltic melt generation dominates. Thick thermal boundary layers, such as those in cratonic regions, on the other hand allow production of kimberlitic to carbonatitic melt only. Therefore, the increasing frequency of occurrence of kimberlites starting at the Proterozoic may be causally linked to cooling and growth of sub-continental mantles through time.

The present work aims to contribute to the Cenozoic tectonic setting of the western part of the Makran Accretionary Wedge in SE Iran. We determine the provenance of both deep marine turbiditic and deltaic-shelf Late Cretaceous-Miocene sandstones, describe the sandstone modal framework and heavy minerals and report a new geochronological and isotopic study including 2307 detrital zircons U-Pb ages and 204 in-situ Hf isotopic analyses. Modal sandstone framework compositions indicate that a magmatic arc and recycled accreted sediments were the main sources of Eocene-Oligocene and Miocene sandstones. Cr-spinel and heavy mineral assemblages indicate ultramafic rocks, likely ophiolitic mantle, as a subsidiary source. Detrital zircon U-Pb ages cluster in five main age groups: (1) Neoproterozoic grains suggesting a continental crust provenance within the Central Iran blocks, (2) Jurassic grains with Hf isotopic compositions of continental crust, suggesting a rifting related magmatic provenance, (3) Late Cretaceous and (4) Eocene grains, with Hf isotopic compositions typical of continental crust and non-depleted mantle, suggesting a continental magmatic arc provenance, and (5) Early Miocene grains. The new U-Pb age and Hf isotopic ratios correspond to those obtained in the east Iranian Makran. They fit tectonic reconstructions with Middle Jurassic intracontinental rift, Early Cretaceous to Eocene subduction below Central Iran forming a continental arc to the north of Makran and closure of the related oceanic in the Paleogene. Erosional products of the corresponding magmatic arc are found in the Makran Basin. Our data disprove that provenance characteristics of the Makran sedimentary rocks are consistent with derivation from the Himalayan sources.

Paleoaltimetric studies have characterized in detail the relationship between carbonate oxygen isotope ratios (δ18Oc) and elevation in orogens with simple, single-moisture-source hydrological systems, and applied this relationship to ancient continental carbonates to provide constraints on their past elevation. However, mixing of different atmospheric moisture sources in low-elevation orogens should affect δ18Oc values, but this effect has not yet been confirmed unequivocally. In the American Southwest, summer monsoonal moisture, sourced in the Equatorial Pacific and the Gulf of Mexico, and winter moisture, sourced in the East Pacific, both contribute to annual rainfall. We present stable isotope results from Quaternary carbonates within the American Southwest to characterize the regional δ18Oc-elevation relationship. We then provide stable isotope results from local Eocene carbonates to reconstruct late Laramide paleoelevations. The Quaternary δ18Oc-elevation relationship in the American Southwest is not as straightforward as in more simple hydrological systems. δ18Oc changes with altitude are non-linear, scattered, and display an apparent isotopic lapse rate inversion above 1200 m of elevation. We speculate that decreasing surface temperatures at high altitudes limit the duration of carbonate growth to the summer months, biasing δ18Oc values toward higher values typical of the summer monsoon and leading to lapse rate inversion. δ18Oc-elevation relationships based on modern water isotope data or distillation models predict paleoelevations that range up to as much as 2 km higher than the modern elevations of 2000 to 2400 m for our late Eocene sites located at the southern edge of the Colorado Plateau. By contrast, our δ18Oc-elevation relationship for the American Southwest yields lower paleoelevation estimates. These alternate estimates nonetheless suggest that significant elevation (at least ∼1 km) had already been attained by the Eocene, but are also compatible with

The hydrologic connectivity between hillslopes and streams impacts the geomorphological evolution of catchments. Here, we propose a conceptual model for hydrogeomorphological evolution of the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO), a first-order catchment developed on shale in central Pennsylvania, U.S.A. At SSHCZO, the majority of available water (the difference between incoming meteoric water and outgoing evapotranspiration) flows laterally to the catchment outlet as interflow, while the rest is transported by regional groundwater flow. Interflow, shallow hillslope flow, is limited to the upper 5 to 8 m of highly fractured bedrock, thought to have formed during periglacial conditions in the late Pleistocene. In contrast, groundwater flowpaths are influenced by the primary permeability of the varying stratigraphic units. Both flowpaths respond to weathering-related secondary permeability. O2-rich interflow mixes with deep O2-poor groundwater under the catchment outlet at depths of 5–8 m. Penetration of this oxygenated interflow under the valley results in pyrite oxidation, release of sulfuric acid, dissolution of minerals, and weakening of bedrock. This is hypothesized to enhance channel incision and, in turn, to promote drainage of deep groundwater from the ridges. Drainage subsequently lowers the catchment water table, advancing the cascade of reactions that produce regolith. Weathering in the catchment is characterized by both sharp and diffuse reaction fronts. Relatively sharp fronts (pyrite, carbonate) mark where vertical, unsaturated flow changes to horizontal, saturated flow, while diffuse fronts (illite, chlorite, feldspar) mark where flow is largely vertical and unsaturated. According to this model, catchment morphology reflects subsurface pyrite reactions due to mixing of interflow and groundwater flow under the valley floor that ultimately results in clay weathering and regolith production nearer the land surface.

The Chukchi Borderland is a large bathymetric high that extends from the Alaskan Chukchi Shelf into the Amerasia Basin of the Arctic Ocean. Widely interpreted to be underlain by continental crust, the Chukchi Borderland has played a pivotal role in plate reconstructions of the Arctic, despite the fact that its geologic nature, origin and evolution are largely unknown. We present new lithologic descriptions, along with U-Pb, 40Ar/39Ar, and apatite fission track analyses, for rocks dredged from the Chukchi Borderland. These new results provide constraints on the Chukchi Borderland9s crustal architecture, Paleozoic evolution, and pre-Cretaceous paleogeographic position prior to the formation of the Amerasia Basin. Based on the location and nature of dredged rocks, the Chukchi Borderland comprises at least two distinct terranes, juxtaposed by the proposed Chukchi Borderland fault zone. Early Paleozoic units recovered from Northwind Ridge in the eastern Chukchi Borderland consist of low-grade metasedimentary rocks with a Laurentian detrital zircon signature that is statistically indistinguishable from that of Cambrian age units of the Franklinian passive margin exposed on Ellesmere Island in the Canadian Arctic Archipelago. In contrast, high-grade gneisses and amphibolites dredged from the Chukchi Plateau in the central Chukchi Borderland yield U-Pb and 40Ar/39Ar ages that suggest an affinity with rock units of the Pearya terrane of Arctic Canada. U-Pb and 40Ar/39Ar ages reveal a Paleozoic tectonic history beginning in Early Cambrian to Early Ordovician time with subduction/arc-related high-T metamorphism. Intrusive relationships indicate that Late Ordovician to Silurian orthogneisses intruded the high-T units within an arc setting. Devonian exhumation of these high-grade units is recorded by U-Pb dating of sphene and 40Ar/39Ar thermochronology of micas and potassium feldspar. The exhumation of the high-grade rocks was coeval with the deposition of coarse-grained sediments bearing detrital zircon compositions similar to Silurian foreland basin sediments deposited in Pearya, as well as with greenschist facies metamorphism of metasedimentary rocks dredged from Northwind Ridge. The new data suggest paleogeographic restoration of the Chukchi Borderland to a position adjacent to Ellesmere and Axel Heiberg islands prior to Amerasia Basin rifting. This pre-rift position supports and refines the widely cited rotational models for opening of the Canada Basin.