Cenozoic cooling and denudation in the North Pennines (northern England,UK) constrained by apatite fission-track analysis of cuttings from the Eastgate Borehole

The Cenozoic landscape development of Britain remains relatively poorly understood. On the one hand, ‘plumists’ have tried to explain the present-day topography as a consequence of effects of the Iceland mantle plume during the Palaeocene-Eocene British Tertiary Igneous Province (BTIP) magmatism, with little or no subsequent modification. On the other hand, abundant evidence exists from fluvial and marine terraces and superimposed karstic levels for significant vertical crustal motions during the Quaternary, which clearly has nothing to do with any mantle plume. To shed light on this issue, we present the first publication of data that constrain the Cenozoic thermal history of the North Pennine uplands of northern England, from apatite fission-track analysis of drill cuttings from the Eastgate Borehole in Weardale, in the western part of County Durham. Our results indicate ～650 m of regional denudation since the latest Oligocene/Early Miocene, plus the ～400 m of localized entrenchment that has created the modern Weardale valley. Before the latest Oligocene/Early Miocene, but following the BTIP magmatism, the crust in this region experienced significant cooling, mainly due to a decrease in the geothermal gradient from ～55 to 61 °C km^?1 to the present 38 °C km^?1, along with ～300 ± 200 m of denudation. Although significant BTIP magmatism occurred in northern England, it thus had only a limited net effect; the crust experienced dramatic heating, but cooled back to its original thermal state within, at most, a few tens of millions of years. We suggest that this rapid cooling effect resulted from westward flow of relatively cold material within the mobile lower-crustal layer, driven by the lateral pressure gradient induced by earlier heating effects and effects of surface processes. Whatever topography developed during the Palaeogene, as a direct result of these heating effects, underplating at the base of the crust, and the associated modest denudation, was presumably also short-lived; significant changes to the crustal thickness, and thus to the topography, can be envisaged as a consequence of subsequent lower-crustal flow.     

Numerical modeling assessment of three-gate structures for capturing contaminated groundwater

This modeling study evaluated the capability of alternative funnel-and-gate structures with three gates for capturing contaminated groundwater in a hypothetical unconfined aquifer. Simulated interceptor structures were linear and 45 m wide, consisting of three gates and two funnels (walls). One gate occupied the center and two gates occupied the ends of the interceptor structures. The structures, positioned perpendicular to regional groundwater flow, traversed the entire thickness of the aquifer. A total of four structures were evaluated (numbers designate widths of end, center, and end gates, respectively, in meters): 3-3-3, 2-5-2, 1-7-1, and 4-1-4. Particle tracking and zonal water budgets identified shapes of capture zones and discharge patterns for each interceptor structure. A mass transport model, accounting for advection and hydrodynamic dispersion, tested the capability of each structure for capturing a contaminant plume. Results suggest that: time-dependent capture zones underestimate the amount of time to capture a contaminant plume, wide center gates facilitate plume capture, and wide end gates facilitate lateral containment of contaminants. Of the structures simulated, the 2-5-2 configuration was relatively efficient at processing and containing the simulated contaminant plume.     

Examining potential genetic links between Jurassic porphyry Cu–Au ± Mo and epithermal Au ± Ag mineralization in the Toodoggone district of North-Central British Columbia,Canada

The Toodoggone district comprises Upper Triassic to Lower Jurassic Hazelton Group Toodoggone Formation volcanic and sedimentary rocks, which unconformably overlie submarine island-arc volcanic and sedimentary rocks of the Lower Permian Asitka Group and Middle Triassic Takla Group, some of which are intruded by Upper Triassic to Lower Jurassic plutons and dikes of the Black Lake suite. Although plutonism occurred episodically from ca. 218 to 191 Ma, the largest porphyry Cu–Au ± Mo systems formed from ca. 202 to 197 Ma, with minor mineralization occurring from ca. 197 to 194 Ma. Porphyry-style mineralization is hosted by small-volume (<1 km³), single-phase, porphyritic igneous stocks or dikes that have high-K calc-alkaline compositions and are comparable with volcanic-arc granites. The Fin porphyry Cu–Au–Mo deposit is anomalous in that it is 16 m.y. older than any other porphyry Cu–Au ± Mo occurrence in the district and has lower REEs. All porphyry systems are spatially restricted to exposed Asitka and Takla Group basement rocks, and rarely, the lowest member of the Hazelton Group (i.e., the ca. 201 Ma Duncan Member). The basement rocks to intrusions are best exposed in the southern half of the district, where high rates of erosion and uplift have resulted in their preferential exposure. In contrast, low- and high-sulfidation epithermal systems are more numerous in the northern half of the district, where the overlying Hazelton Group rocks dominate exposures. Cogenetic porphyry systems might also exist in the northern areas; however, if they are present, they are likely to be buried deeply beneath Hazelton Group rocks. High-sulfidation epithermal systems formed at ca. 201 to 182 Ma, whereas low-sulfidation systems were active at ca. 192 to 162 Ma. Amongst the studied epithermal systems, the Baker low-sulfidation epithermal deposit displays the strongest demonstrable genetic link with magmatic fluids; fluid inclusion studies demonstrate that its ore fluids were hot (>468°C), saline, and deposited metals at deep crustal depths (>2 km). Sulfur, C, O, and Pb isotope data confirm the involvement of a magmatic fluid, but also suggest that the ore fluid interacted with Asitka and Takla Group country rocks prior to metal deposition. In contrast, in the Shasta, Lawyers, and Griz-Sickle low-sulfidation epithermal systems, there is no clear association with magmatic fluids. Instead, their fluid inclusion data indicate the involvement of low-temperature (175 to 335°C), low-salinity (1 to 11 equiv. wt.% NaCl) fluids that deposited metals at shallow depths (<850 m). Their isotope (i.e., O, H, Pb) data suggest interaction between meteoric and/or metamorphic ore fluids with basement country rocks.     

Sources, distribution and storage of heavy metals in the Río Pilcomayo, Bolivia

The Río Pilcomayo rises on the Cerro Rico de Potosí precious metal-polymetallic tin deposits of Bolivia, and flows in a southeasterly direction for ca. 600 km to Bolivia's southern border with Argentina. Mining of the Potosí deposits has occurred continuously since 1545, generating large quantities of waste materials in the headwater of the basin. In addition, a tailings dam breach at the Porco mine in 1996 released an estimated 235 000 m³ of tailings and fluid into the upper reaches of the Río Pilaya, the largest tributary to the Pilcomayo.Concentrations of As, Sb, Cd, Cu, Pb, Hg, Ag, Tl and Zn in contemporary channel sediments upstream of the Pilaya confluence are significantly elevated above background values. Elevated levels appear to be associated with pyrite- and other sulphide mineral-bearing tailings materials transported more than 200 km downstream of the Potosí mines. Significant downstream declines in elemental concentrations occur within 15 km, and again between 150 and 200 km, from the mines. The initial decrease in concentrations is due to the rapid dilution of nearly pure tailings effluent released to the river from milling facilities near Potosí. The latter decrease results from a combination of geomorphic processes including the storage of sediment-borne metals within the channel bed and the influx of ‘clean’ sediment from several large tributaries. Downstream of the Pilaya confluence, concentrations of Cu, Pb, Hg and Zn are only slightly elevated above background values, and Ag, Cd, Sb and Tl cannot be distinguished from background levels. These data suggest that while the Porco tailings spill may have had a significant short-term impact on sediment and water quality along the lower reaches of the Río Pilcomayo, its longer-term impacts were limited. Metals stored and eroded from alluvial deposits of historical age in upstream reaches appear to be an important source of metals to the river today. An additional, and perhaps more significant source, is the release of tailings effluent to the river from modern milling operations. The transport of these contaminants downstream of Icla (203 km from Potosí) appears to be restricted by aggradational processes occurring in the vicinity of Puente Sucre. In addition, downstream of the confluence of the Río Pilaya, inputs of large amounts of ‘clean’ sediment have caused dilution of the metal contaminants. Data from other studies where similar geomorphic processes have occurred suggest that the metals in the upper Pilcomayo may eventually be moved downvalley as the aggradational processes are reversed and channel stabilisation occurs. Thus, the most significant impacts of metal contamination may not be realised in downstream areas for decades.     

Geographical signatures of middle Atlantic estuaries: Historical layers

Estuaries of the middle Atlantic region can be characterized and viewed broadly against the backdrop of their geomorphologic features. While geomorphology is literally at the base of every estuary, these features do not necessarily yield regional signatures. A conceptual model, with layering in time and space, is proposed as an alternative to simplistic geomorphologic characterization. Humans have altered virtually every physical, chemical, and biological feature of middle Atlantic estuaries. A basic model premise is that middle Atlantic estuaries have a base of fundamental geomorphology features. Layered, in GIS fashion, on this base are the estuaries' components: climate, nutrients, watershed soils and vegetation, producers, and consumers. These components have been so strongly influenced by humans in time and space that the signature is anthropogenic. As a consequence, best management practices, stock assessment, and restoration have replaced concepts such as ecosystem integrity and stability. The focus of the layered model is the Chesapeake Bay watershed, and although middle Atlantic estuaries differ along climatic and latitudinal gradients, all reflect the detrimental effects of a massive human presence. The ability or inability of middle Atlantic estuaries to absorb human perturbation over the last 10,000 years gives them their signatures. From the Hudson-Raritan to the Pamlico-Albemarle estuaries, we have made some progress in curbing our impacts. Nearly everything we do affects our estuaries, and our actions are proportional to the number of humans living in the watersheds. Continued population growth on our coasts and many years of abuse may be irreversible as our estuaries lose their ability to be self-regulating, biological systems.     

Regional modelling of permafrost thicknesses over the past 130 ka: implications for permafrost development in Great Britain

The greatest thicknesses of permafrost in Great Britain most likely occurred during the last glacial–interglacial cycle, as this is when some of the coldest conditions occurred during the last 1 000 000 years. The regional development of permafrost across Great Britain during the last glacial–interglacial cycle was modelled from a ground surface temperature history based on mean annual temperatures and the presence of glacier ice. To quantify the growth and decay of permafrost, modelling was undertaken at six locations across Great Britain that represent upland glaciated, lowland glaciated, upland unglaciated and lowland unglaciated conditions. Maximum predicted permafrost depths derived in this academic study range between several tens of metres to over 100 m depending upon various factors including elevation, glacier ice cover, geothermal heat flux and air temperature. In general, the greatest maximum permafrost thicknesses occur at upland glaciated locations, with minimum thickness at lowland sites. Current direct geological evidence for permafrost is from surface or shallow processes, mainly associated with the active layer. Further research is recommended to identify the imprint of freeze/thaw conditions in permanently frozen porous rocks from beneath the active layer.     

Biostratigraphic and detrital zircon age constraints on the basement of the Himalayan Foreland Basin: Implications for a Proterozoic link to the Lesser Himalaya and cratonic India

The Himalayan Foreland Basin in the Ganga Valley is key to assessing the pre‐collision relationship between cratonic India and the Himalaya – the world's largest mountain chain. The subsurface Ganga Supergroup, representing the sedimentary basement of the Ganga Valley, has been interpreted as a northern extension of the Proterozoic Vindhyan Supergroup in cratonic India. This interpretation is contentious because the depositional age of the Ganga Supergroup is not resolved: whereas the lower Ganga Supergroup is widely regarded as Proterozoic, the upper Ganga Supergroup has been variously inferred to include Neoproterozoic, lower Palaeozoic, or Cretaceous strata. Here, we integrate biostratigraphic and detrital zircon data from drill cores to show that the entire Ganga Supergroup is likely Proterozoic and can be correlated with Proterozoic successions on the northern Indian craton and in the Lesser Himalaya. This helps redefine the first‐order stratigraphic architecture and indicates broad depositional continuity along the northern Indian margin during the Proterozoic.     

中国西北地区早古生代柴达木北缘超高压变质带的大陆俯冲：证据来自此带中发现柯石英

作者使用激光拉曼微光谱在柴达木北缘超高压变质带与一大型榴辉岩体有关的泥质片麻岩的锆石单矿物中发现柯石英。柯石英呈包体产出。此外,还发现一些石墨包体。这发现表明曾发生原地超高压变质作用。地体很可能是在低于金刚石稳定区压力条件下结晶的。     

Tree-ring dated landslide movements and their relationship to seismic events in southwestern Montana, USA

To determine periods of incremental landslide movement and their possible relationship to regional seismic events, the tree-ring records of 32 tilted and damaged conifers at three sites on landslides in the Gravelly Range of southwestern Montana were examined. Several signs of disturbance in the tree-ring record indicating landslide movement were observed. Commonly, the tree-ring record displayed a marked reduction in annual ring width and/or the reaction wood formation. The tree-ring records from the three landslide sites indicate multiple periods of movement during the 20th century. Many of the periods of movement indicated by the strongest signals (most trees) at the sites occurred the year following significant earthquakes in the region. Those seismic events for which evidence in the tree-ring record was found at one or more of the three sites are the 1983 Borah Peak, 1959 Hebgen Lake, 1935 Helena, 1925 Clarkson, and 1908 Virginia City earthquakes. This study suggests that many of the landslide movements were triggered by, or are coincident with, earthquakes as much as 200 km from the study area.