Sulitjelma Gabbro,northern Norway: A palaeomagnetic result

The Sulitjelma Gabbro situated at 67.2°N, 15.4°E was intruded close to the Late Ordovician climax of regional metamorphism in the northern Scandinavian Caledonides. Magnetic properties have been examined from samples collected at seven localities in the south western part of this body. Total NRM directions show a tendency to be aligned near the present earth's magnetic field direction in this region. Stability to a.f. demagnetisation is low and commensurate with low Koenigsberger ratios (0.001–0.16) and the presence of unoxidised magnetite as the principal remanence carrier. After cleaning the site mean directions no longer show an alignment near the present earth's field and of six statistically significant sites three are approximately reversed with respect to remainder. The combined mean direction after cleaning isD = 195°,I = 15° (precision parameterk = 6) and the derived virtual geomagnetic pole is at 0°E, 14°S (α₉₅ = 23°). This pole is close to poles of comparable age from the British Isles and suggests that Britain and Norway were part of the same crustal plate in Ordovician times. Discrepancies between Siluro-Devonian results from the two regions may be due to inadequate age coverage of present results.     

On the applicability of oil shale to palaeomagnetic investigations

Samples of oil shales of Autunian age from the Krkono e (Riesengebirge) Piedmont basin, northern Bohemia, which represent rocks with fossil micro-organic matter in the initial stage of carbonification, were subjected to palaeomagnetic investigations. The palaeomagnetic data obtained from these rocks are fully in accordance with those derived previously on rocks of the red-bed facies in the same area. Preliminary results obtained on Lower Carboniferous coal-bearing shales collected from the Upper Silesian basin, Ostrava-Karviná region, indicate also that these rocks, characterized by a higher degree of carbonification, are suitable for palaeomagnetic investigations. These results with those of laboratory carbonification indicate that such rocks as bituminous rocks, black shales, oil shales, coal-bearing shales or generally rocks containing micro-organic matter at different stages of natural carbonification are worthy of being studied palaeomagnetically.     

Magnetic properties of the Lower Old Red Sandstone lavas in the Midland Valley,Scotland; palaeomagnetic and tectonic considerations

Thermal demagnetization studies of lavas in the Strathmore area of the Midland Valley, Scotland, support overall palaeomagnetic data found in previous studies of these rocks. Reduced directional scatter as compared to some earlier studies, is attributed to more effective demagnetization, resolving some of the directional complexity of previous studies. Combined magnetic fabric and directional analysis suggest that at least some deviating directions may be explained by local tectonism. The existence of almost antiparallel directional groups and field tests give supporting evidence for a “primary” (deuteric) origin of the main magnetization of these rocks. Additionally, a second remanence component having shallow reverse directions of magnetization, is attributed to later remagnetization in Old Red Sandstone time. The Midland Valley results are seen in conjunction with other Palaeozoic palaeomagnetic results and possible geodynamic implications are discussed.     

Lower Jurassic palaeomagnetic results from Yorkshire,England, and their implications

Palaeomagnetic study of Middle Liassic siltstones shows a stable magnetization with a mean direction of D = 12.3°, I = 64.6° (N = 60, k = 26, α₉₅ = 3.9°) corresponding to a palaeomagnetic pole at 79.8°N, 125.6°E, similar to that for southern Germany and confirming predictions based on palaeogeographic reconstructions using North American data. Sideritic concretions of Lower Liassic age show a higher magnetic stability with a mean direction of D = 12.6°, I = 61.4° (N = 125, k = 50, α₉₅ = 1.8°) which is not significantly different from the siltstones. This confirms the sedimentological evidence that suggests that such concretions grew very shortly after deposition, i.e. within the Liassic, and suggests that similar concretions of other ages could thus be used for palaeomagnetic studies. Although the Liassic palaeomagnetic pole (76.9°N, 134.7°E), based on this work, appears valid it is still not possible to evaluate a sensible Mesozoic polar wandering curve for the North Atlantic bordering continents.     

On the magnetic memory phenomenon in rocks and its bearing on palaeomagnetic measurements

Summary The results of recent extensive studies on the low field hysteresis in basalts from India and East Africa and the memory phenomenon observed in some of the rocks have been found to have a direct bearing on the various parameters such asQ _n,Q _tn, stability by the A.C. field technique etc. which are of great importance in rock-magnetism. It has been found that rocks having low Curie temperature (it _c ) and not exhibiting memory phenomenon are ideally suited for palaeomagnetic studies. Also, rocks having relatively higher it _c often exhibit memory phenomenon, given inconsistent NRM directions and show an irregular behaviour on A.C. field cleaning. In using the latter rock types for palaeomagnetic work one has to take a number of precautions which are clearly brought out by the studies described in this paper.     

A palaeomagnetic and petromagnetic study of Upper-Carboniferous,Permian and Triassic sediments,NW Bulgaria

Summary In order to obtain basic palaeomagnetic data on Upper Carboniferous, Permian and Triassic sediments collected from the NW Bulgaria, laboratory stability tests were extended from A.C. and thermal treatments to studies of mineral phase changes and to investigations of changes of magnetic anisotropy during laboratory procedures. Laboratory criteria were found which permitted to distinguish samples suitable for palaeomagnetic analyses from those representing rocks totally or almost totally chemically reworked during their history. Palaeomagnetic directions and pole positions derived from Stephanian, Lower Permian and Triassic rocks from the southern margin of the Moesian Platform are compatible with the values obtained for the tectonically stable North-European Platform.     

Periodicities of palaeomagnetic intensity and palaeoclimatic variations: a Walsh spectral approach

The Walsh power spectral technique has been applied to the non-sinusoidal palaeoclimatic (theGloborotalia menardii climatic curve) and palaeomagnetic intensity variation records of the last 1.2 Myr. The analysis reveals, in the case of palaeomagnetic variations, statistically significant peaks (at 90% confidence level) centered around 150, 60 and 24 kyr. The palaeoclimatic spectrum, however, indicates only two significant peaks of 60 and 24 kyr. The identical periodicities in both the phenomena may suggest a causal link between the geomagnetic intensity variations and climatic changes.     

The rotation of Italy: Preliminary palaeomagnetic data from the Umbrian sequence,Northern Apennines

A preliminary collection of 43 palaeomagnetic samples (10 sites) from the miogeosynclinal and supposedly autochthonous Umbrian sequence in the Northern Apennines, Italy, was analysed by means of alternating magnetic fields and thermal demagnetization studies. The older group of samples, taken from the upper part of the Calcari Diasprini (Malm), the Fucoid Marls (Albian/Cenomanian) and from the basal part of the Scaglia Bianca (Early Late Cretaceous), all showed normal polarity directions and resulted in a mean site direction:D = 290.5°,I = +51.5°,α₉₅ = 11°,k = 74,N = 4.The younger group of samples, taken throughout the Scaglia Rossa sequence (Latest Cretaceous/Middle Eocene) showed normal and reversed polarity directions. In contrast to the older group, the magnetic analysis of these samples resulted in a considerably less dense grouping of site mean directions. This presumably is due to inaccuracies introduced with the very large bedding tilt corrections that had to be applied to the samples of some sites. A tentative mean site direction for these Scaglia Rossa samples was computed as:D = 351°,I = +52.5°,α₉₅ = 23.5°,k = 11.5,N = 5.Despite the low precision of the Scaglia Rossa result, the significant deviation between this Latest Cretaceous/Early Tertiary direction and the Late Jurassic/Early Late Cretaceous direction indicates a counterclockwise rotation of more than forty degrees. This rotation can be dated as Late Cretaceous.How far these data from the Northern Apennines apply to other parts of the Italian Peninsula has yet to be established. The timing of this rotation is not at variance with the data from other parts of Mediterranean Europe (Southern Alps, Iberian Peninsula) and from Africa. However, taking into account the preliminary nature of the results, the amount of rotation of the Northern Apennines seems to surpass the rotation angle which is deduced from the palaeomagnetic data for Africa.     

Magnetic characteristics of sediment grains concurrently contaminated with TBT and metals near a shipyard in Busan,Korea

Bottom sediments near shipyards are often susceptible to receiving accidental spills of TBT and metals or their degradation products from hull scraping of antifouling system paints applied prior to 2008, when the AFS Convention 2001 was not in force. We investigated TBT and metal contamination of sediments near the shipyards of a small marina located in Busan, Korea and found that they were highly contaminated with TBT, Cu, and Zn. To better understand the environmental impacts and to make an optimal remediation plan, we characterized individual antifouling fragments in terms of metal and TBT contents, magnetic properties, and grain-size. Coarse-sized individual antifouling fragments exhibited simultaneously high levels of TBT, metals and high magnetic susceptibility, and appeared to be a major source of pollution in the sediment. Therefore, magnetic separation in combination with size-separation appears to be a cost-effective remediation method to remove the TBT and metals from contaminated shipyard sediments.     

Superimposed Recent,Permo-Carboniferous and Ordovician palaeomagnetic remanence in the Builth Volcanic Series,Wales

The Builth Volcanic Series of Llanvirnian age in Llanelwedd Quarries, mid-Wales, carries three components of natural remanent magnetisation. Component P, regarded as primary, is a thermochemical remanence directed at D = 181.7°, I = +54.5°, α₉₅ = 4.4° relative to bedding. Component S is a secondary component with in situ D = 178.7°, I = ?6.7°, α₉₅ = 5.4° and is believed to be a low-temperature chemical remanence (CRM) of Permo-Carboniferous age. Component R is directed close to the present geomagnetic field and is believed to be a recent viscous remanence (VRM).The results are of interest for three reasons. First, they are an unusually good example of multi-component NRM analysis, the three components being so clearly discriminated by thermal demagnetisation because they have almost completely separate blocking temperature ranges. Second, they provide evidence of a Permo-Carboniferous event (possibly a mild thermal or hydrothermal pulse promoting CRM acquisition) some 40 km north of the Hercynian orogenic front. Third, they illustrate very clearly the importance of detailed demagnetisation: this work revises the pole position for these rocks by ～ 10° and removes an obstacle to the palaeomagnetic recognition of the ～ 1000 km wide Iapetus Ocean cutting Britain in Ordovician time.     

A suburban sediment budget: Coarse-grained sediment flux through hillslopes,stormwater systems and streams

Sediment in urban stormwater systems creates a significant maintenance burden, while a lack of coarse-grained bed sediment in streams limits their ecological value and geomorphic resilience. Gravel substrates, for example, provide benthic habitat yet are often scoured from the channel bed only to end up in a detention basin or treatment wetland. This dual problem of both ‘too much’ and ‘too little’ coarse-grained sediment reflects a watershed sediment budget that is profoundly altered. We developed a conceptual urban coarse-grained (>0.5 mm) sediment budget across three domains: hillslopes (urban land surfaces), the built stormwater network and stream channels. We then quantified key sources, sinks and storages for a suburban case study, using a combination of hillslope and in-channel monitoring, and interrogation of local government records. Around 36% of the sediment supplied to the stormwater network reached the catchment outlet, a level of sediment delivery much higher than observed in similar-sized natural catchments. The remainder was deposited in the sediment cascade and either stored, or extracted and removed from the catchment (e.g. material deposited in sediment ponds and gross pollutant traps). Conventional urban drainage networks are characterized by high hillslope sediment supply and low storage, resulting in efficient sediment delivery. Channel erosion, deposition in (and extraction from) pipes and channels, and floodplain deposition are small compared to sediment transport through the cascade. An understanding of the sediment budget of urban headwater catchments can provide stormwater and waterway managers with the information they need to address specific sediment problems such as sedimentation in stormwater assets and geomorphic recovery of urban streams. © 2019 John Wiley & Sons, Ltd. © 2019 John Wiley & Sons, Ltd.     

Age of Grenville Belt magnetisation: Rb-Sr and palaeomagnetic evidence from Swedish dolerites

Rb-Sr mineral ages and palaeomagnetic data are presented for nine dolerite dykes and sills in central and southern Sweden. The intrusions are representative of widespread dolerites, mainly dykes, which cut pre-Sveconorwegian (Svecofennian) crust and trend parallel to the Sveconorwegian Front Zone (or Schistosity Zone) over a band up to 150 km wide along most of its 700-km length. The Rb-Sr ages suggest emplacement through the approximate interval 1000?900 m.y. Cooling ages on country rocks show little evidence of significant Sveconorwegian (e.g. 1000 m.y.) heating effects, and the new isotopic and palaeomagnetic results are taken to represent the time of dolerite intrusion.The dolerites were emplaced in response to net tensional stresses in the Svecofennian crust: it is proposed that these were caused by marginal upwarping related to post-orogenic uplift of the adjacent Sveconorwegian region.The palaeomagnetic poles for the dolerites are similar to poles from the Sveconorwegian Province and the Front Zone, indicating that these also have a ca. 1000?900-m.y. age. The similar cooling histories of the Sveconorwegian Province and the Grenville Province of Canada allow this age to be assigned also to the Grenville poles. If so, it is not necessary to postulate plate motions during Grenville-Sveconorwegian magnetisation, and the agreement between dolerite poles and Sveconorwegian poles suggests that the orogenic belt cooled essentially in place against its bounding cratons. A possible continental fit shows Scandinavia and North America contiguous in late Grenville time, though relative motions before 1000 m.y. are not ruled out.     

A palaeomagnetic study of Svecofennian basic rocks: Middle Proterozoic configuration of the Fennoscandian,Laurentian and Siberian shields

The Svecofennian mobile zone occupies the bulk of the Fennoscandian shield and represents terrain subjected to profound tectonic activity and granite intrusion at ～1800 My. This study covers the palaeomagnetism of basic rocks within this belt in Sweden between 65.5 and 67.5°N (gabbros and diorites of Kallax, Niemisel, Sangis, Stora Lulevatten and the Gällivare, Jokkmokk and Voullerim regions) magnetised during uplift and cooling of this belt at ～1750-1550 My. AF and thermal demagnetisation define a consistent sequence of high to low blocking-temperature components identifying a migration of the geomagnetic field during part of this interval. Together with the Rådmansö gabbro-diorite of central-east Sweden (palaeomagnetic pole 201°E, 36°N) these components yield a comparable sequence of palaeopoles to those derived from uplift magnetisations of the contemporaneous Svecokarelian terrain of Finland. The post-tectonic, Uppsala metabasite suite possesses a magnetite-held (“A”) remanence comparable to Svecofennian uplift magnetisations from elsewhere; within the aureole of the Almunge alkaline complex this has been largely displaced by a low blocking temperature (“B”) remanence, possibly related to a late stage in the Svecofennian uplift cycle. The Hälleforsnäs giant dyke possesses a magnetite-held remanence attributed to initial cooling at 1518 My (palaeomagnetic pole 167°E, 27°N) and at least two high blocking-temperature components. One of these is correlated with the ～1000-800 My Sveconorwegian mobile activity of southwest Sweden; this latter component is represented as the univectorial remanence in dolerite dykes of this age, and sporadically as a secondary component in the adjacent Svecofennian terrain.The results are compiled with other palaeomagnetic poles from the Fennoscandian shield to derive a generalised apparent polar wandering path for the interval ～1750-1550 My. They define segments of a large loop which agrees closely with uplift magnetisations from the contemporaneous Hudsonian mobile terrain of the Laurentian shield on a single reconstruction derived from Upper Proterozoic (1450-1200 My) palaeomagnetic data. The two shields thus appear to have formed an integral continental unit during the interval 1750-1200 My. A geological reconstruction of the Siberian and Laurentian shields is also tested and found to yield general agreement with the palaeomagnetic evidence. The major geological implications of the collective reconstruction are an alignment of major tectonic trends and a gradual restriction of anorthosite-Rapakivi magmatism between the termination of the ～1800 and ～1100 My mobile episodes.     

Analysis of palaeomagnetic time series — Techniques and applications

This review is intended to provide an introduction for the nonspecialist to concepts and techniques which are useful for analysing palaeomagnetic time series. Emphasis is placed on analysis in the frequency domain, particularly the periodogram and maximum entropy methods. The review consists of two parts dealing with techniques and applications respectively.     

Correction: Guidelines and pitfalls of refraction microtremor surveys

Journal of Seismology -     

Signal crayfish burrowing,bank retreat and sediment supply to rivers: A biophysical sediment budget

Burrowing into riverbanks by animals transfers sediment directly into river channels and has been hypothesised to accelerate bank erosion and promote mass failure. A field monitoring study on two UK rivers invaded by signal crayfish (Pacifastacus leniusculus) assessed the impact of burrowing on bank erosion processes. Erosion pins were installed in 17 riverbanks across a gradient of crayfish burrow densities and monitored for 22 months. Bank retreat increased significantly with crayfish burrow density. At the bank scale (<6 m river length), high crayfish burrow densities were associated with accelerated bank retreat of up to 253% and more than a doubling of the area of bank collapse compared with banks without burrows. Direct sediment supply by burrowing activity contributed 0.2% and 0.6% of total sediment at the reach (1.1 km) and local bank (<6 m) scales. However, accelerated bank retreat caused by burrows contributed 12.2% and 29.8% of the total sediment supply at the reach and bank scales. Together, burrowing and the associated acceleration of retreat and collapse supplied an additional 25.4 t km⁻¹ a⁻¹ of floodplain sediments at one site, demonstrating the substantial impact that signal crayfish can have on fine sediment supply. For the first time, an empirical relation linking animal burrow characteristics to riverbank retreat is presented. The study adds to a small number of sediment budget studies that compare sediment fluxes driven by biotic and abiotic energy but is unique in isolating and measuring the substantial interactive effect of the acceleration of abiotic bank erosion facilitated by biotic activity. Biotic energy expended through burrowing represents an energy surcharge to the river system that can augment sediment erosion by geophysical mechanisms.     

A new direction for shallow refraction seismology: integrating amplitudes and traveltimes with the refraction convolution section

The refraction convolution section (RCS) is a new method for imaging shallow seismic refraction data. It is a simple and efficient approach to full‐trace processing which generates a time cross‐section similar to the familiar reflection cross‐section. The RCS advances the interpretation of shallow seismic refraction data through the inclusion of time structure and amplitudes within a single presentation. The RCS is generated by the convolution of forward and reverse shot records. The convolution operation effectively adds the first‐arrival traveltimes of each pair of forward and reverse traces and produces a measure of the depth to the refracting interface in units of time which is equivalent to the time‐depth function of the generalized reciprocal method (GRM). Convolution also multiplies the amplitudes of first‐arrival signals. To a good approximation, this operation compensates for the large effects of geometrical spreading, with the result that the convolved amplitude is essentially proportional to the square of the head coefficient. The signal‐to‐noise (S/N) ratios of the RCS show much less variation than those on the original shot records. The head coefficient is approximately proportional to the ratio of the specific acoustic impedances in the upper layer and in the refractor. The convolved amplitudes or the equivalent shot amplitude products can be useful in resolving ambiguities in the determination of wave speeds. The RCS can also include a separation between each pair of forward and reverse traces in order to accommodate the offset distance in a manner similar to the XY spacing of the GRM. The use of finite XY values improves the resolution of lateral variations in both amplitudes and time‐depths. The use of amplitudes with 3D data effectively improves the spatial resolution of wave speeds by almost an order of magnitude. Amplitudes provide a measure of refractor wave speeds at each detector, whereas the analysis of traveltimes provides a measure over several detectors, commonly a minimum of six. The ratio of amplitudes obtained with different shot azimuths provides a detailed qualitative measure of azimuthal anisotropy and, in turn, of rock fabric. The RCS facilitates the stacking of refraction data in a manner similar to the common‐midpoint methods of reflection seismology. It can significantly improve S/N ratios.Most of the data processing with the RCS, as with the GRM, is carried out in the time domain, rather than in the depth domain. This is a significant advantage because the realities of undetected layers, incomplete sampling of the detected layers and inappropriate sampling in the horizontal rather than the vertical direction result in traveltime data that are neither a complete, an accurate nor a representative portrayal of the wave‐speed stratification. The RCS facilitates the advancement of shallow refraction seismology through the application of current seismic reflection acquisition, processing and interpretation technology.     

Rotational overthrusting of the northwestern Himalaya: further palaeomagnetic evidence from the Riasi thrust sheet, Jammu foothills, India

Thermal demagnetization results (316 samples) are presented for the Tertiary succession of the Riasi thrust sheet (Jammu foothills, northwestern Himalaya). Primary and secondary magnetization directions of Murree Group red beds (Miocene to Upper Eocene) sampled northeast of Jammu indicate, for this part of the Riasi thrust sheet, a clockwise rotation over about 45° with respect to the Indian shield since Late Eocene/Early Miocene time. This accords with clockwise rotations of similar magnitude observed in the Panjal Nappe and the Krol Belt, and is interpreted as representative for the northwestern Himalaya. Results from the western part of the Kalakot inlier, sampled northwest of Jammu, i.e. basal Murree claystone (Middle Eocene) and carbonate from the Subathu Group (lower Middle to Lower Eocene), indicate an aberrant 20–25° counterclockwise rotation which is of local importance only. Available observations on rotation of Himalayan thrust sheets with respect to the Indian shield, indicate that the Himalayan Arc has formed through oroclinal bending. This supports Powell and Conaghan's and Veevers et al.'s model of Greater India with large-scale intracontinental underthrusting along the Main Central Thrust beneath the Tibetan Plateau. Minimal magnitudes of underthrusting of 550 km in the Krol Belt and 650 km in the Thakkhola region are concluded. Palaeolatitude observations (herein and in [1[) agree with absolute positioning of the Indian plate based on India-Africa relative movement data fixed to a hotspot frame in the Atlantic Ocean, and with palaeolatitude observations from DSDP cores on the Indian plate. Collision-related secondary magnetic components observed both to the north and to the south of the Indus-Tsangpo Suture zone show palaeolatitudes between the equator and 7°N. Comparison of both datasets indicates that initial contact between Greater India and south-central Asia had been established in the Hindu Kush—Karakorum region by about 60 Ma ago whereas eastwards progressive suturing had advanced to the Lhasa Block segment of the Indus-Tsangpo Suture zone before 50 Ma ago.     

A multicomponent,dual-polarity palaeomagnetic regional overprint from the Moine of northwest Scotland

Palaeomagnetic remanence may be reset by orogeny in a manner analogous to isotopic systems. The heavy mineral seams and associated psammite in the deformed and metamorphosed Lower Morar Division of the Moine Assemblage of northwest Scotland have a palaeomagnetic signature that is characteristic over a wide area. The various components of the magnetization are resolved by detailed thermal demagnetization. These components are post-tectonic, as they are not affected by any structural fabric. They may be classified according to blocking temperature and, when statistically distinguishable, show a systematic change of direction from high to lower temperature possibly associated with apparent polar wander. Comparison to the European palaeomagnetic record suggests a late Silurian to early Devonian age of magnetization for the localities south of the Strath Conon Fault. The distribution of remanance directions and blocking temperatures is not consistent with the pattern of potassium-argon ages in this part of the British Caledonides. If the magnetization is purely of thermal origin, the palaeomagnetic record indicates higher temperatures at later time than does the potassium-argon record. This remagnetization event is related to the closing stages of the Caledonian orogeny.