Investigating slow-moving landslides in the Zhouqu region of China using InSAR time series

In the Zhouqu region (Gansu, China), landslide distribution and activity exploits geological weaknesses in the fault-controlled belt of low-grade metamorphic rocks of the Bailong valley and severely impacts lives and livelihoods in this region. Landslides reactivated by the Wenchuan 2008 earthquake and debris flows triggered by rainfall, such as the 2010 Zhouqu debris flow, have caused more than 1700 casualties and estimated economic losses of some US$0.4 billion. Earthflows presently cover some 79% of the total landslide area and have exerted a strong influence on landscape dynamics and evolution in this region. In this study, we use multi-temporal Advanced Land Observing Satellite and Phased Array type L-band Synthetic Aperture Radar (ALOS PALSAR) data and time series interferometric synthetic aperture radar to investigate slow-moving landslides in a mountainous region with steep topography for the period December 2007–August 2010 using the Small Baseline Subsets (SBAS) technique. This enabled the identification of 11 active earthflows, 19 active landslides with deformation rates exceeding 100 mm/year and 20 new instabilities added into the pre-existing landslide inventory map. The activity of these earthflows and landslides exhibits seasonal variations and accelerated deformation following the Wenchuan earthquake. Time series analysis of the Suoertou earthflow reveals that seasonal velocity changes are characterized by comparatively rapid acceleration and gradual deceleration with distinct kinematic zones with different mean velocities, although velocity changes appear to occur synchronously along the landslide body over seasonal timescales. The observations suggest that the post-seismic effects (acceleration period) on landslide deformation last some 6–7 months.     

Late Devensian deglaciation of the Tyne Gap Palaeo‐Ice Stream,northern England

The deglacial history of the central sector of the last British–Irish Ice Sheet is poorly constrained, particularly along major ice‐stream flow paths. The Tyne Gap Palaeo‐Ice Stream (TGIS) was a major fast‐flow conduit of the British–Irish Ice Sheet during the last glaciation. We reconstruct the pattern and constrain the timing of retreat of this ice stream using cosmogenic radionuclide (¹⁰Be) dating of exposed bedrock surfaces, radiocarbon dating of lake cores and geomorphological mapping of deglacial features. Four of the five ¹⁰Be samples produced minimum ages between 17.8 and 16.5 ka. These were supplemented by a basal radiocarbon date of 15.7 ± 0.1 cal ka BP, in a core recovered from Talkin Tarn in the Brampton Kame Belt. Our new geochronology indicates progressive retreat of the TGIS from 18.7 to 17.1 ka, and becoming ice free before 16.4–15.7 ka. Initial retreat and decoupling of the TGIS from the North Sea Lobe is recorded by a prominent moraine 10–15 km inland of the present‐day coast. This constrains the damming of Glacial Lake Wear to a period before ∼18.7–17.1 ka in the area deglaciated by the contraction of the TGIS. We suggest that retreat of the TGIS was part of a regional collapse of ice‐dispersal centres between 18 and 16 ka.

     

Interpretation of tensor gravity data using an adaptive tilt angle method

Full Tensor Gravity Gradiometry (FTG) data are routinely used in exploration programmes to evaluate and explore geological complexities hosting hydrocarbon and mineral resources. FTG data are typically used to map a host structure and locate target responses of interest using a myriad of imaging techniques. Identified anomalies of interest are then examined using 2D and 3D forward and inverse modelling methods for depth estimation. However, such methods tend to be time consuming and reliant on an independent constraint for clarification. This paper presents a semi‐automatic method to interpret FTG data using an adaptive tilt angle approach. The present method uses only the three vertical tensor components of the FTG data (T_zx, T_zy and T_zz) with a scale value that is related to the nature of the source (point anomaly or linear anomaly). With this adaptation, it is possible to estimate the location and depth of simple buried gravity sources such as point masses, line masses and vertical and horizontal thin sheets, provided that these sources exist in isolation and that the FTG data have been sufficiently filtered to minimize the influence of noise. Computation times are fast, producing plausible results of single solution depth estimates t hat relate directly to anomalies. For thick sheets, the method can resolve the thickness of these layers assuming the depth to the top is known from drilling or other independent geophysical data. We demonstrate the practical utility of the method using examples of FTG data acquired over the Vinton Salt Dome, Louisiana, USA and basalt flows in the Faeroe‐Shetland Basin, UK. A major benefit of the method is the ability to quickly construct depth maps. Such results are used to produce best estimate initial depth to source maps that can act as initial models for any detailed quantitative modelling exercises using 2D/3D forward/inverse modelling techniques.     

Factors influencing magnetic susceptibility in the southern Pyrenees

We analyse paramagnetic, ferrimagnetic and frequency-dependent susceptibility in the southern Pyrenees and northern Ebro basin. The analyzed rocks show a wide range of values for the three parameters within the different lithological groups. Sandstones, shales and marls exhibit middle to high values of paramagnetic susceptibility while mudstones usually show low values. However, a general correlation can be established for the different rock types with a wide variability for each group (for example, Eocene marls show average values of paramagnetic susceptibility of 82% but within a range between 12 and 99%, or Flysch materials, with mean values of 47–52% within a range between 15 and 90%). These changes in the paramagnetic contribution to the susceptibility are due to the change in the ferrimagnetic susceptibility. This wide variability is independent of the sedimentological characteristics and can be related to the higher ferrimagnetic contribution to the susceptibility with similar ferrimagnetic mineral phase assemblages. The ferrimagnetic and superparamagnetic contributions, independent of site location with respect to individual macrostructures (thrusts and nappes), show a sharp increase at the occurrence of the macroscopic regional Pyrenean cleavage, probably relating both aspects with changes in pressure-temperature (P-T) conditions.     

Geodetic data shed light on ongoing caldera subsidence at Askja, Iceland

Subsidence within the main caldera of Askja volcano in the North of Iceland has been in progress since 1983. Here, we present new ground- and satellite-based deformation data, which we interpret together with new and existing micro-gravity data, to help understand which processes may be responsible for the unrest. From 2003 to 2007, we observe a net micro-gravity decrease combined with subsidence and from 2007 to 2009 we observe a net micro-gravity increase while the subsidence continues. We infer subsidence is caused by a combination of a cooling and contracting magma chamber at a divergent plate boundary. Mass movements at active volcanoes can be caused by several processes, including water table/lake level movements, hydrothermal activity and magma movements. We suggest that, here, magma movement and/or a steam cap in the geothermal system of Askja at depth are responsible for the observed micro-gravity variations. In this respect, we rule out the possibility of a shallow intrusion as an explanation for the observed micro-gravity increase but suggest magma may have flowed into the residing shallow magma chamber at Askja despite continued subsidence. In particular, variable compressibility of magma residing in the magma chamber as well as compressibility of the surrounding rock may be the reason why this additional magma did not create any detectable surface deformation.     

Age,growth, and sexual maturity of two New Zealand endemic skates,Dipturus nasutus and D. innominatus

Abstract

Rough and smooth skates (Dipturus nasutus (Banks 1841) and/), innominatus (Garrick &; Paul 1974)) were aged by counting growth bands on X‐rays of thick sections of vertebral centra. Band counts were imprecise, but there was no between‐reader bias. Age estimates were not validated. The oldest rough skate was 9 years old, but few were more than 6 years old. Females may live longer than males. The combined sexes von Bertalanffy growth curve was L_r = 91.3 (1 ‐ e^?0.16[^{t + 1.20}]). Half the males matured by c. 52 cm pelvic length (PL) and 4 years, and females by 59 cm PL and 6 years. The oldest smooth skate in the sample was 24 years, but longevity probably exceeds that. Females appear to live longer than males. The combined sexes von Bertalanffy growth curve was: L_t = 150.5 (1 ‐ e^{?0.095[t + 1.06]}). Half the males matured by c. 93 cm PL and 8 years, and females by 112 cm PL and 13 years. Smooth skate are late maturing and long‐lived relative to other skates, whereas rough skate are early maturing with a moderate life span.     

The age and stratigraphic context of the Easington Raised Beach, County Durham, UK

The Easington Raised Beach, in Shippersea Bay, County Durham, is the most northerly known interglacial beach deposit in England. It lies directly on Magnesian Limestone bedrock at 33 m O.D. and is covered by glacial sediments attributed to the Devensian. Detailed sedimentological analysis suggests that it is an interglacial beach, which is supported by the presence of pebbles bored by marine organisms and littoral, temperate-climate, marine macro- and micro-fossils. It comprises beds of unconsolidated, bedded, imbricated, well-rounded sands and gravels, overlain by similar, but calcreted, deposits. The gravel fraction is dominated by Magnesian and Carboniferous limestone, with orthoquartzite, flint, and porphyries also present; these are far-travelled erratics that must have derived from the erosion of older glacially transported sediments. Previous workers have described erratics derived from the Oslofjord region of Norway in the raised beach gravel, although rocks diagnostic of a Scandinavian origin have not been recovered as part of this study. The heavy-mineral suite is rich in epidote, dolomite, clinopyroxenes, garnet, tourmaline, and micas. The beach was dated previously by conventional amino acid analysis of the shells, which suggested a Marine Isotope Stage (MIS) 7 age, albeit with a reworked component from MIS 9. This has been confirmed by new optically stimulated luminescence (OSL) dates, which indicate that the beach formed between 240 and 200 ka BP. New amino acid racemisation analyses, using a modified technique, broadly support this interpretation but must await more comparative data before they can be assessed fully. The strong indication of an MIS 7 age for the formation of the beach has implications for the uplift history of northeastern England during the Pleistocene, and indicates an uplift rate of 0.19 mm a⁻¹. The stable isotope geochemistry indicates that the cementation occurred during an interglacial period, whilst U-Series dating of the cement indicates that cementation occurred mostly during the Holocene, and is genetically related to the overlying Devensian till. This work has formed part of a full re-appraisal of the glacial sequence in eastern County Durham, the results of which suggest that the Warren House Formation pre-dates the raised beach, and that the Devensian Horden Till overlies the raised beach.     

The deglaciation of the western sector of the Irish Ice Sheet from the inner continental shelf to its terrestrial margin

This paper provides a new deglacial chronology for retreat of the Irish Ice Sheet from the continental shelf of western Ireland to the adjoining coastline, a region where the timing and drivers of ice recession have never been fully constrained. Previous work suggests maximum ice-sheet extent on the outer western continental shelf occurred at ~26–24 cal. ka BP with the initial retreat of the ice marked by the production of grounding-zone wedges between 23–21.1 cal. ka BP. However, the timing and rate of ice-sheet retreat from the inner continental shelf to the present coast are largely unknown. This paper reports 31 new terrestrial cosmogenic nuclide (TCN) ages from erratics and ice-moulded bedrock and three new optically stimulated luminescence (OSL) ages on deglacial outwash. The TCN data constrain deglaciation of the near coast (Aran Islands) to ~19.5–18.5 ka. This infers ice retreated rapidly from the mid-shelf after 21 ka, but the combined effects of bathymetric shallowing and pinning acted to stabilize the ice at the Aran Islands. However, marginal stability was short-lived, with multiple coastal sites along the Connemara/Galway coasts demonstrating ice recession under terrestrial conditions by 18.2–17. ka. This pattern of retreat continued as ice retreated eastward through inner Galway Bay by 16.5 ka. South of Galway, the Kilkee–Kilrush Moraine Complex and Scattery Island moraines point to late stage re-advances of the ice sheet into southern County Clare ~14.1–13.3 ka, but the large errors associated with the OSL ages make correlation with other regional re-advances difficult. It seems more likely that these moraines are the product of regional ice lobes adjusting to internal ice-sheet dynamics during deglaciation in the time window 17–16 ka.     

DEPENDENCE OF ROTATION VELOCITY OF INDIVIDUAL SUNSPOTS ON THEIR LIFETIME

By using a large number (452) of individual sunspots or individual sunspots with small spots around them, taken from the Greenwich Photoheliographic Results (GPR) for the years 1964–1976 that cover solar cycle No. 20, it is shown that the rotation velocity of the sunspots varies with their lifetimes. This investigation indicates that at the equator, the rotation rate for the last three days (of the lifetime) is about 1.3% slower than that over the whole lifetime and about 0.5% slower than during the first three days, but this is reversed at high and low latitudes, and the difference is much larger in the northern hemisphere than in the southern hemisphere. These results confirm to the fact that the rotation rate of the solar layers increases with depth.