Recognition of shear heating on a long‐lived major fault using Raman carbonaceous material thermometry: implications for strength and displacement history of the MTL,SW Japan

The extent to which movement on major faults causes long term shear heating is a contentious issue and an important aspect in the debate about the strength of major faults in the crust. Comparing the results of experimental work on the kinetics of crystallization of carbonaceous material with results of thermal modeling show that the Raman carbonaceous material (CM) geothermometer is well suited to studying shear heating on geological time scales in suitable lithologies exposed around exhumed major fault zones. The Median Tectonic Line (MTL), SW Japan, is the largest on‐land fault in Japan with a length of > 800 km. Application of Raman CM thermometry to pelitic schist adjacent to the fault reveals the presence of a rise in peak temperature of around 60 °C over a distance of around 150 m perpendicular to the MTL fault plane. The spatial association of this thermal anomaly with the fault implies it is due to shear heating. Thermal modeling shows the recorded thermal anomaly and steep temperature gradient is compatible with very high rates of displacement over time scales of a few thousand years. However, the implied displacement rates lie outside those generally observed. An alternative explanation is that an originally broader thermal anomaly that developed during strike slip faulting was shortened due to the effects of normal faulting. Constraints on displacement rate, width of the original anomaly, duration of heating and peak temperature imply a coefficient of friction, μ, greater than 0.4.     

Cosmogenic 3He exposure dating of the Quaternary basalts from Fogo,Cape Verdes: Implications for rift zone and magmatic reorganisation

We have used cosmogenic ³He to date pre- and post-collapse lava flows from southwestern Fogo, Cape Verdes, in order to date rift zone magmatic reorganisation following the lateral collapse of the flank of the Monte Amarelo volcano. The post-collapse flows have exposure ages ranging from 62 to 11 ka. The analysis of multiple flow tops on each lava flows, often at different elevations, provides an internal check for age consistency and the exposures ages conform with stratigraphic level. The exposure ages suggest that volcanic activity along the western branch of the triple-armed rift zone was more or less continuous from before 62 ka to approximately 11 ka. The absence of magmatic activity for the last 11 kyr reflects a structural reconfiguration of the volcano and may be related to renewed flank instability. This volcanic hiatus is similar in duration to that observed in the Canary Islands. Replicate ³He exposure ages of a pre-collapse flow (123.0 ± 5.2 ka) brackets the time of the Monte Amarelo collapse between 62 ka and 123 ka. Reproducible cosmogenic ³He exposure ages of less than 123 ka from flows away from major erosion features demonstrates that the technique is a viable alternative to the radiocarbon, K/Ar and ⁴⁰Ar/³⁹Ar chronometers for dating recent volcanism in arid climate zones.     

Apparent dynamic stability of the southeast African coast despite sea level rise

The coast of southeast Africa is dominated by sandy beaches that tend to be confined within log‐spiral or headland‐bound embayments. Investigations using serendipitous air imagery data set have been previously undertaken and conclusions drawn about the stability of the coast. We show that conclusions drawn from this data, with respect to the high water mark (HWM) position are fraught with errors, which include tidal state, pressure regime, beach slope, high‐swell erosion, seasonal and multi‐annual changes. We highlight and discuss these sources of error, together with their magnitudes. The most significant of these are the high‐swell, seasonal and multi‐annual variations. From case studies we show that the seasonal beach rotation and long‐term beach width variation are responsible for tens of metres of unaccounted HWM variation, 30 to 50 m is common, with maximums reaching 60 to 100 m. Overall the southeast African coastline appears to be in a state of long‐term dynamic equilibrium. There is no evidence of any sea‐level rise‐forced transgression in the coastal sediment budget, despite sea‐level rise (SLR). If such a signal is, in fact present, it is lost within the beach width variation. Some southeast African coastal reaches are suffering chronic erosion, but these are related to anthropogenic impacts. The extreme difficulty of placing a HWM, with any temporal validity on this coast precludes the routine use of the Bruun Rule. Although no transgressive signature is found, there is evidence of a decreasing coastal sand budget as a result of anthropogenic or natural climate change, or both. This decrease in the coastal sand volume is likely to result in increased future erosion. Copyright © 2016 John Wiley & Sons, Ltd.     

X-ray Spectromicroscopy of Mineral Intergrowths in the Santa Catharina Meteorite

This work describes the application of microfocus X-ray absorption spectroscopy (XAS) and X-ray photo-emission electron microscopy (XPEEM) to the study of the complex mineralogical intergrowths within the Santa Catharina meteorite. The Santa Catharina meteorite of this study (BM52283 from the meteorite collection of the Natural History Museum, London, UK) primarily comprises a taenite bulk host phase (Fe:Ni ratio = 70.9 ± 0.8%:29.1 ± 0.8%) with a set of oxide-bearing cloudy zone textured regions (Fe:Ni:O ratio = 40.4 ± 0.3%:49.0 ± 0.7%:10.6 ± 0.8% at the core and Fe:Ni:O ratio = 34.4 ± 1.5%:42.7 ± 0.6%:22.9 ± 1.8% towards the rims) and numerous schreibersite (Fe:Ni:P ratio = 38.6 ± 1.6%:38.4 ± 0.9%:23.0 ± 0.5%) inclusions. Between the schreibersite and the taenite are rims up to 50 μm across of Ni-rich kamacite (Fe:Ni ratio = 93.4 ± 0.4%:6.6 ± 0.5%). No chemical zoning or spatial variations in the Fe and Ni speciation was observed within either the schreibersite or the kamacite phases. The oxide-bearing cloudy zone textured regions mostly comprise metallic Fe–Ni alloy, predominantly tetrataenite. Within the oxide phases, the Fe is predominantly, but not entirely, tetrahedrally co-ordinated Fe³⁺ and the Ni is octahedrally co-ordinated Ni²⁺. Structural analysis supports the suggestion that non-stoichiometric Fe₂NiO₄ trevorite is the oxide phase. The trevorite:tetrataenite ratio increases at the edges of the oxide-bearing cloudy zone textured regions indicating increased oxidation at the edges of these zones. The spatial resolution of the XPEEM achieved was between 110 and 150 nm, which precluded the study of either the previously reported ∼ 10 nm precipitates of tetrataenite within the bulk taenite or any antitaenite.     

Pb isotopic composition,colour, and microstructure of monazites from a polymetamorphic rock in Antarctica

A moderate- to high-grade regionally metamorphosed paragneiss from Antarctica contains monazites of several different colours — brown, yellow and grey. Each colour type has a distinctive U-Pb isotopic composition which appears to result from different proportions of radiogenic Pb loss. Isotopic differences are neither related to La, Nd, Ce, P, Ca, Ti (and/or Ba), nor to U or Th content. All colour types have similar structures at the submicron scale, as determined by both conventional and high-resolution transmission electron microscopy (TEM). These show that the grains are essenttially non-metamict but are composed of 100 Å crystalline domains misoriented from each other by no more than 2× 10^–3 radians, and separated by narrow confused boundary regions where misorientation is probably accommodated by imperfect atomic arrangements. These regions of mismatch form potential zones of high permeability/diffusivity which are believed to be fundamental to the isotopic and colour differences between grains. Colour type is apparently related to the capacity of different minerals to shield included monazite grains from fluids circulating in the rock system.The well aligned monazite U — Pb analyses produce concordia intercepts of 2429 _–16 ⁺¹⁷ Ma and 1087±29 Ma. Both ages are comparable to those of major geological events in this part of Antarctica. They are interpreted in terms of isotopic resetting through Pb loss, and original monazite crystallisation is thought to have occurred somewhat earlier, possibly at the time this terrain first underwent granulite-facies metamorphism, about 3070 Ma ago.     

GPS rapid static and kinematic positioning based on GPS active network

This paper presents a data processing strategy for GPS kinematic Positioning by using a GPS active network to model the GPS errors in double difference observable.Firstly,the double difference residuals are estimated between the reference stations in the active network.Then the errors at a user station are predicted as the network corrctions to user measurements,based on the location of the user,Finally conventional kinematic positioning algorithms can be applied to determine the position of the user station.As an example,continuous 24-hour GPS data in March 2001 has been processed by this method.It clearly demonstrates that,after applying these corrections to a user within the network,both the success rate for ambiguity resolution and the positioning accuracy have been significantly improved.     

An innovative application of the Kinect in Earth sciences: quantifying deformation in analogue modelling of volcanoes

Measuring surface deformation is critical in analogue modelling of Earth science phenomena. Here, we present a novel application of the Microsoft Kinect sensor to measure vertical deformation in a scaled analogue model of Nisyros volcano (Greece), simulating two magmatic sources and related surface deformation. The Kinect permits capture of real time, 640 × 480 pixel, true‐colour images (RGB) and a grid of distances to the modelled surface with a horizontal and vertical resolution of ±1 mm. Using recorded distances permits quantification of vertical deformation in terms of maximum and average down‐ and uplift during deflation and inflation phases respectively, which is of crucial importance for defining the kinematics of faults formed during the modelling, determined from interpretation of the RGB images. Although other techniques have demonstrated similar or higher accuracy, our study demonstrates the cost‐effectiveness of the Kinect in analogue modelling of volcanoes.     

The petrogenesis of carbonatite, melnoite and kimberlite from the Eastern Goldfields Province, Yilgarn Craton

We have obtained major and trace element data for a suite of rocks emplaced over an area of 45,000 km² in the Eastern Goldfields Province (EGP), Yilgarn Craton, that are petrographically and mineralogically described as kimberlites, melnoites and carbonatites. Kimberlites dominate the rock types found in the west whereas carbonatites and melnoites are common in the east. Compatible element data from the carbonatites and melnoites tend to lie along trends that imply silicate–carbonate fractionation. The kimberlites exhibit a much greater amount of compatible element scatter as a result of the variable contribution from continental lithospheric mantle (CLM). When compared to southern African kimberlites, the EGP kimberlites have consistently lower MgO and Os concentrations at comparable Ni concentrations. The opposite is true for Ti–Ni variation where the EGP kimberlites have higher Ti than the southern African kimberlites at comparable Ni concentrations. These data are interpreted to suggest that the CLM beneath the province was either melt metasomatised some time prior to kimberlite emplacement, or that the EGP CLM is less refractory (melt depleted) than the Kaaapvaal CLM.

In contrast, the incompatible element ratios and initial Nd values are constant throughout the entire rock suite. Carbonate C and O isotope data show a broad positive correlation, consistent with magmatic-hydrothermal trends found in many carbonatite complexes. These incompatible element and isotope data link all of the rocks within the province to the same mantle source that was similar to modern-day mantle plume sources.

Re–Os data for the various samples, including oxide minerals from all of the petrographic types, xenocryst-poor kimberlites and melnoites yield a precise Re–Os isochron of 2025±10 Ma and an initial γOs of 5.3±3.1 (MSWD=5.8). These data support the conclusion based on incompatible element, stable isotope and Sm–Nd isotope data that the rocks are comagmatic.

Initial Os isotopic compositions and Re/Os ratios for the xenocryst-rich kimberlites are also correlated. However, the correlation does not pass through the calculated initial γOs and Re/Os of the isochron. The Re–Os data show that the incompatible element-enriched melt exhibited very little control on the Re–Os variations of the xenocryst-rich kimberlites.

Correlations between deep mantle seismic velocities, petrology and whole-rock compatible element geochemistry suggest that the rheology and history of the EGP CLM played a significant role in determining the petrographic characteristics of the magmas that were ultimately emplaced into the EGP crust.     

Current meter observations from the Labrador and Newfoundland shelves and comparisons with barotropic model predictions and IIP surface currents

Abstract

Recent current measurements from the southern Labrador and northeastern Newfoundland shelves confirm the presence of inshore and offshore branches of the Labrador Current with high mean currents and low standard deviations. At mid‐shelf weaker and more variable currents occur over the banks, and cross‐shelf flows are found to be associated with the shelf topography. An annual cycle of the inshore branch, in phase with wind forcing, is significant on the NE Newfoundland Shelf but not detectable on Hamilton Bank. The phase of the annual cycle in the offshore branch is consistent with buoyancy, not wind forcing. The observations compare reasonably well with results from a barotropic model for the region and the International Ice Patrol (IIP) surface current map. Differences occur particularly in regions of high bathymetrie curvature or an ill‐defined shelf break. The model location of the Labrador Current lies inshore of that indicated by the data, suggesting the need for better definition of the northern inflow boundary condition and the inclusion of baroclinicity. The HP surface current map agrees well with observations offshore, but shows an unrealistic, broad inshore branch, especially on the Grand Bank These differences have important implications for the drift models.