Palaeomagnetism of igneous rocks of the Lake District (Caledonian) terrane,Northern England: palaeozoic motions and deformation at a leading edge of Avalonia

The Lake District terrane of northern England comprises Upper Cambrian–Silurian sediments and volcanics accumulated at the northern margin of the Avalonian Plate during growth and demise of the Iapetus Ocean. Ocean closure and suturing resulted in Late Ordovician and Acadian tectonism and were accompanied by emplacement of a large regional batholith. Palaeomagnetic study of intrusive igneous rocks, including application of thermal demagnetization, field tests and principal component analysis, identifies a history of Ordovician to Devonian magnetization. Late plutons (Shap and Skiddaw granites and/or aureoles) record a shallow dipolar (A3) axis (mean declination/inclination (D/I=278/+17°) dating from emplacement in late Early Devonian times (c. 395 Ma). Although this axis is recorded as a sporadic overprint in older rocks, no pervasive remagnetization is attributable to batholith emplacement. Instead, the Carrock Fell Complex Layered Gabbros have a mid- to late Ordovician (A1) remanence (D/I)=17·4/−58·1°, 36 samples, α₉₅=4·8°) predating regional F2 folding. Later events in this igneous complex comprise the Carrock Fell Granophyre with a post-folding Ordovician remanence, and Round Knott Dolerite with a remanence linked to hydrothermal alteration late in the Ordovician magmatic episode. A Late Ordovician (Ashgill) palaeofield is also defined by remanence (A2) in the Threlkeld–St John's Microgranite and aureole (438 Ma, D/I=236·5/63·3°, 41 samples, α₉₅=4·7°). Other intrusions carrying a remanence predating the Acadian deformation include the Great Cockup Picrite (458 Ma, D/I=43·2/−31·8°, 31 samples, α₉₅=7·7°) and basic intrusives in the aureole of the Eskdale Granite (429 Ma, D/I=174·5/25·8°, 32 samples, α₉₅=8·8°). Collectively the palaeomagnetic data from this terrane identify a hairpin in the apparent polar wander path during Late Ordovician (Caradoc–Ashgill) times corresponding to ‘soft’ closure of the Iapetus suture and accompanying deformation. The same motion is recognized in contemporaneous data from the Welsh Caledonides where declinations are rotated by c. 55° relative to contemporaneous results from the Lake District. Adjustment for this (probable late Acadian) rotation beings fold trends of the Paratectonic Caledonides into alignment and identifies a parallel mid- to late Ordovician destructive plate margin comprising forearc (Lake District) and backarc (North Wales). This arc was oriented latitudinally in mid-southerly latitudes during formation and the bulk of the magmatism occurred during a single normal-polarity chron. The relationships between magnetization and folding in both the Lake District and Welsh Borderlands identify the importance of Late Ordovician deformation along this arc during collision of Avalonia and Laurentia. Arc-related volcanism was succeeded in Silurian times by parallel foreland basins embracing the Welsh Basin and southern Lake District as the Laurentian Plate overrode the Avalonian Plate. © 1997 John Wiley & Sons, Ltd.     

A palaeomagnetic study of the Charnian,Caldecote, and Uriconian volcanics and plutons,central England

This palaeomagnetic study has investigated intrusive and extrusive calc-alkaline igneous rocks comprising the pre-Ordovician basement of central England and the Welsh Borderlands and dated ca. 560–530 Ma. Demagnetization studies resolve a range of magnetic components in both the older volcanic successions comprising the eastern Uriconian volcanics, and in the younger intrusive episodes comprising the Charnwood and South Leicestershire outcrops. Post-Ordovician overprinting is largely absent and the remanence record dates from ca. 560–420 Ma. Within this time interval field tests are inadequate to confirm a primary origin and some components may be overprints acquired in immediate post-Uriconian (Cadomian) or during late Ordovician (early Caledonian) thermotectonic events. A first order continuity is recognized on a palaeomagnetic scale between the data from the Midlands-Welsh Borderlands and the terranes exposed in Anglesey and Southwest Wales. The British terranes moved through low latitudes during the interval represented by the Uriconian episode; they crossed the palaeoequator during the folding of these rocks and prior to emplacement of the later intrusive suite. Collectively they had rotated through ca. 170° and moved into intermediate latitudes by mid-Lower Cambrian times. Post-Lower Cambrian movements on the Malvernian lineament are a possible explanation for second order differences between the palaeomagnetic records of the English Midlands and the Welsh Borderlands; the largest possible regional rotation here is a ca. 90° movement of the Charnwood block with respect to the other outcrops. Correlation with the Gondwana APW path places the Anglo-Welsh basement in the vicinity of a comparable calc-alkaline volcanic province linked to a subducting plate margin in the Afro-Arabian area. Subsequent continental breakup led to termination of this volcanic activity and establishment of a passive marine shelf environment later in Lower Cambrian times.     

Palaeomagnetism,rock magnetism and magnetic fabrics in Precambrian metamorphic terranes of the Huabei Shield,China

This study has investigated magnetic remanence, rock magnetism and anisotropy of magnetic susceptibility (AMS) in granulite and amphibolite grade metamorphic terranes of the Huabei Shield between Inner Mongolia in the west and the Bohai Sea in the east. Rock magnetic studies identify annealed metamorphic magnetite grains with multidomain properties as the remanence carriers; a widely recorded stable remanence was probably fixed by grain shape effects. Granulite facies terranes are typically between one and two orders more strongly magnetised than amphibolite terranes and AMS fabrics correlate mostly with metamorphic mineral fabrics observed in the country rocks. Progressive thermal demagnetisation identifies a range of two and three component structures resident in magnetite. An important component recognised as a partial or complete remagnetisation by Late Mesozoic–Tertiary tectonic/magmatic activity is present in basement at the southern margin of the outcrop (Miyun terrane) and where extensive granite plutonism has occurred (Zhunhua terrane). These components have directions corresponding to remanence in the Yunmeng Shan Granite (119–114 Ma, D/I=33/58°, 39 samples, a₉₅=3.5°, palaeopole at 201°E, 64°N). Most remanence elsewhere was probably acquired during post-tectonic uplift and cooling of the basement between ∼2200 and 1850 Ma because palaeomagnetic directions are removed from the Phanerozoic palaeofield path and they are distinct from the palaeomagnetic record in the overlying Jixian Supergroup deposited at ∼1840–900 Ma. These latter magnetisations are considered reliable indicators of the palaeofield during Late Palaeoproterozoic times because deformation of overlying supracrustal rocks is mostly slight and no prominent deflection of magnetic remanence by magnetic fabrics is observed. Palaeofield directions and poles attributed to the time of uplift-related cooling are: Qian’an Terrane (D/I=215/71°, a₉₅=9°, 17 samples, pole at 99°E, 10°N) and North Qianxi Terrane (D/I=44/−45°, a₉₅=4°, 41 samples, pole at 79°E, 11°S). In addition, a more widely-preserved shallow northerly component correlates with a NW→E swathe of components recorded by uplift-related cooling within the Datong–Huan’an granulite terrane in the west of the shield. A preliminary Palaeo-Mesoproterozoic apparent polar wander path for the Huabei Shield is defined from the Palaeoproterozoic record in the metamorphic basement rocks and the Meso-Neoproterozoic record in the overlying Jixian Supergroup. It incorporates a loop between ∼2200 and 1850 Ma and exhibits a general east to west trend in subsequent times.     

Palaeomagnetism of the Palaeoproterozoic ultramafic intrusion near Lake Konchozero,Southern Karelia,Russia

New palaeomagnetic results are presented from the recently dated Palaeoproterozoic ultramafic Konchozero sill, and associated basalts (three sites, 38 oriented samples). Three stable components of remanence have been isolated during thermal and alternating field demagnetisation. The component I, with a mean direction of D=103°, I=40°, k=18, α₉₅=11° (N=11 samples), pole position of 14°S, 282°E, has been obtained from the unaltered deeper part of the sill and from baked schists. The study of the baked contact confirms the conclusion that component I is supposed to be primary and corresponds to the Sm–Nd age of the sill of 1974±27 Ma. The palaeopole of component I is not consistent with the accepted Fennoscandian apparent polar wander path (APWP) for the period 2120–1880 Ma, and for that part the Fennoscandian APWP should be revised. Two other components (component II: D=349°, I=39°, k=35, α₉₅=6°, N=19 samples, pole position 49°N, 231°E; and component III: D=17°, I=41°, k=44, α₉₅=5°, N=19 samples, pole position 50°N, 190°E) fit the APWP well, with palaeomagnetically estimated ages of ca. 1860 and 1760 Ma respectively.     

Multicomponent magnetization in western Dolpo (Tethyan Himalaya, Nepal): tectonic implications

A palaeomagnetic study has been carried out in the Tethyan Himalaya (TH; the northern margin of Greater India). Twenty-six palaeomagnetic sites have been sampled in Triassic low-grade metasediments of western Dolpo. Two remanent components have been identified. A pyrrhotite component, characterized by unblocking temperatures of 270–335 °C, yields an in situ mean direction of D=191.7°, I=−30.9° (k=29.5, α₉₅=5.7°, N=23 sites). The component fails the fold test at the 99% confidence level (k_{in situ}/k_bed=6.9) and is therefore of postfolding origin. For reason of the low metamorphic grade, this pyrrhotite magnetization is believed to be of thermo-chemical origin. Geochronological data and inclination matching indicate an acquisition age around 35 Ma. The second remanence component has higher unblocking temperatures (>400 °C and up to 500–580 °C range) and resides in magnetite. A positive fold test and comparison with expected Triassic palaeomagnetic directions suggest a primary origin.The postfolding character of the pyrrhotite component, and its interpreted age of remanence acquisition, implies that the main Himalayan folding is older than 35 Ma in the western Dolpo area. This study also suggests that the second metamorphic event (Neo-Himalayan) was more significant in the Dolpo area than the first (Eo-Himalayan) one.A clockwise rotation of 10–15° is inferred from the pyrrhotite component, which is compatible with oroclinal bending and/or rotational underthrusting models. This rotation is also supported by the magnetite component, indicating that no rotation of the Tethyan Himalaya relative to India took place before 35 Ma.     

Palaeomagnetic study of the Nagssugtoqidian mobile belt in Central-West Greenland

This paper reports a palaeomagnetic study of amphibolite and granulite facies basement rocks across a 60 km wide north-south traverse through the Nagssugtoqidian mobile belt in West Greenland, which incorporates the c. 2500 Ma Itivdleq shear belt, the c. 1800 Ma Ikertôq shear belt, and adjoining terrains. It also integrates the results with earlier investigations of the belt to produce a composite record of the magnetic field during uplift and cooling of the terrain through c. 600–500°C at c. 1750–1600 Ma. Progressive a.f. and thermal cleaning of samples from 81 sites identifies a range of magnetic components. Lowest blocking temperature remanences in Nagssugtoqidian (c. 1700 Ma) amphibolite facies rocks yield steep W to N positive components residing in magnetite. Granulite facies rocks formed at c. 1800 Ma generally have a dual record incorporating steep N to NW positive components unblocked by 580°C and shallow E-W directed components of dual polarity only unblocked close to the Curie point of haematite. A common feature of relict c. 2500 Ma amphibolite and granulite facies terrains is a progressive movement of sample directions back along a palaeofield migration path with treatment and is interpreted to represent the recovery of a discrete spectrum acquired during a.p.w. Areas with a distributed spectra through the critical range of blocking temperatures, altitude-controlled sections, and blocking temperature relationships all record consistent a.p.w. during the interval of uplift-related cooling. The collective data define a clockwise palaeofield migration path between D = 174, I = 32°, and D = 348, I = 66° referred to as the ‘A’ magnetisation. The equivalent palaeopoles define one limb of an a.p.w. loop conforming to other Laurentian poles assigned to the interval 1750–1650 Ma and widely represented as a post-Hudsonian overprinting or cooling remanence. Shallow E-W directions, designated ‘B’, predominate in late tectonic diorites and are therefore younger than 1700 Ma; they appear to represent a localised reheating event near the northern margin of the Ikertôq shear belt, possibly related to this magmatism, which produced the hemoilmenite phases. The equivalent palaeopoles are provisionally interpreted to represent part of the return path of the APW loop because they are coincident with other poles dated c. 1630 Ma. Collectively the palaeofield migration data from all studies define a thermal dome broadly coincident with the Ikertôq shear belt, but it is not possible to isolate the contribution of components of vertical movement along faults running through the belt to this regional effect.     

Palaeomagnetic results from the ca. 1130 Ma Borgmassivet intrusions in the Ahlmannryggen region of Dronning Maud Land, Antarctica, and tectonic implications

Detailed palaeomagnetic and rock magnetic analyses provide improved palaeomagnetic results from 23 sites in the Borgmassivet intrusions in the Ahlmannryggen region of Dronning Maud Land, East Antarctica. These intrusions are of similar age to their host, the ca. 1130 Ma Ritscherflya Supergroup (RSG). A mean direction of D=235.4°, I=−7.6° with k=45.9 and α₉₅=4.5° was obtained from this study. When combined with previously reported results from 11 sites in the same region, including sites from the Ritscherflya Supergroup, it gives an overall mean direction for 34 sites from the igneous suite with D=236.5°, I=−3.6°, k=27.9 and α₉₅=4.8°. Isothermal remanent magnetization (IRM) experiments on several specimens suggest magnetite or titanomagnetite as the primary remanence carrier, while high temperature magnetic susceptibility experiments indicate the presence of single domain particles. These observations, together with field evidence and the high coercivities and unblocking temperatures, support a primary origin for the observed characteristic remanence. The Borgmassivet palaeomagnetic pole lies at 54.5°E, 8.3°N with A₉₅=3.3°. If Antarctica is moved to its Gondwanan position adjacent to southeast Africa, the Borgmassivet pole (BM) coincides with that of the African well-established, well-dated (1100 Ma) Umkondo Large Igneous Province pole, supporting the hypothesis that the Grunehogna craton of Dronning Maud Land was part of the Kalahari craton of southern Africa at ca. 1100 Ma.     

Late Cretaceous mafic dykes in the Dharwar craton

Palaeomagnetic, geochemical and geochronological studies have been conducted on a set of dolerite dykes intruding the Peninsular gneisses near Huliyurdurga town, Karnataka, as a reconnaissance survey indicated a Cretaceous age for them. The dykes are mainly tholeiitic in composition with their 87Sr/86Sr ratios tightly clustered around 0·7045. Their palaeomagnetic data (D _m =329°,I _m =−55°) and the corresponding palaeopole coordinates (λ _p = 34°S,L _p =108°E) are strikingly close to those of the Deccan Traps to the north. Whole rock K-Ar ages of these dykes ranging between 69 and 84 Ma are also similar to the range of K-Ar ages of the Deccan basalts. The chemical, palaeomagnetic and temporal coherence between the dykes and the Deccan basalts indicate that they may indeed be tectonically related events.     

Palaeomagnetic investigation of Cenozoic volcanic rocks bordering the North Anatolian Fault Zone,Reşadiye and Koyulhisar Districts,central-east Anatolia,Turkey

A palaeomagnetic study is reported from the lavas of Eocene, Miocene and Pliocene age cropping out immediately to the north of the North Anatolian Fault Zone (NAFZ) in the Re?adiye–Mesudiye region of central-eastern Anatolia. Rock magnetic investigations identify a high percentage of multi-domained magnetite as the dominant ferromagnet in these rocks and this probably accounts for a relatively poor response to alternating field and thermal demagnetisation. Thirty of 37 units yielded acceptable groupings of characteristic magnetisation directions. An earlier study indicated small anticlockwise crustal block rotation in this region since Upper Cretaceous times (D/I?=?347/50°), and our study indicates that this was overtaken by clockwise rotation in Eocene times (D/I?=?40/47°), although sample size control from the Palaeogene is poor. Results from later Miocene (D/I?=?2/62°) and Pliocene (D/I?=?0/53°) volcanic rocks indicate that no significant tectonic rotation has occurred in the north of the NAFZ in Neogene times. This contrasts with rotations in the weaker crust comprising the Anatolian collage south of the NAFZ, where differential and sometimes large anticlockwise rotations occurred during the latter part of the Neogene.     

Paleomagnetism and U–Pb geochronology of the Black Range dykes,Pilbara Craton,Western Australia: a Neoarchean crossing of the polar circle

We report a new paleomagnetic pole for the Black Range Dolerite Suite of dykes, Pilbara craton, Western Australia. We replicate previous paleomagnetic results from the Black Range Dyke itself, but find that its magnetic remanence direction lies at the margin of a distribution of nine dyke mean directions. We also report two new minimum ID-TIMS ²⁰⁷Pb/²⁰⁶Pb baddeleyite ages from the swarm, one from the Black Range Dyke itself (>2769 ± 1 Ma) and another from a parallel dyke whose remanence direction lies near the centre of the dataset (>2764 ± 3 Ma). Both ages are slightly younger than a previous combined SHRIMP ²⁰⁷Pb/²⁰⁶Pb baddeleyite weighted mean date from the same swarm, with slight discordance interpreted as being caused by thin metamorphic zircon overgrowths. The updated Black Range suite mean remanence direction (D = 031.5°, I = 78.7°, k = 40, α₉₅ = 8.3°) corresponds to a paleomagnetic pole calculated from the mean of nine virtual geomagnetic poles at 03.8°S, 130.4°E, K = 13 and A₉₅ = 15.0°. The pole's reliability is bolstered by a positive inverse baked-contact test on a younger Round Hummock dyke, a tentatively positive phreatomagmatic conglomerate test, and dissimilarity to all younger paleomagnetic poles from the Pilbara region and contiguous portions of Australia. The Black Range pole is distinct from that of the Mt Roe Basalt (or so-called ‘Package 1’ of the Fortescue Group), which had previously been correlated with the Black Range dykes based on regional stratigraphy and imprecise SHRIMP U–Pb ages. We suggest that the Mt Roe Basalt is penecontemporaneous to the Black Range dykes, but with a slight age difference resolvable by paleomagnetic directions through a time of rapid drift of the Pilbara craton across the Neoarchean polar circle.     

Palaeomagnetism of the Tan y Grisiau granite,North Wales: Evidence for a subvolcanic origin in Late Ordovician times

Progressive thermal demagnetization of samples from the Tan y Grisiau granite defines a coherent easterly positive characteristic remanence (D/I = 124.9/60.3°;, 42 samples, R = 40–51, a₉₅ = 4.8°;) residing in magnetite. An ancient reversal of magnetization is recovered in the highest blocking temperature spectrum of a few samples and suggests that a cooling-related dipolar axis is recorded by this pluton. Only facies of the granite which have been reddened, probably by submagmatic streaming, have recorded a stable remanence. Adjustment for tilt yields a very steep remanence (D/I = 193/88°;) incompatible with any known Early Palaeozoic and younger field direction from Britain. The in situ remanence has a similar declination to the primary magnetization in Late Ordovician dolerites from the Welsh Borderlands and yields a comparable palaeolatitude (41.5°;S). It is concluded that the Tan y Grisiau pluton was magnetized in Late Ordovician times after deformation. Folding in this region is therefore interpreted to be substantially of Taconic (Late Ordovician) origin and not Acadian in age. As both in situ and tilt-adjusted remanence directions are incompatible with Silurian and younger palaeofield directions from Britain, the pluton is interpreted as a subvolcanic component of the North Wales igneous province. Large anticlockwise rotation of Avalonia is identified between Late Ordovician and Late Silurian times.     

An example of multicomponent magnetization in welded tuffs: a case study of Upper Cretaceous welded tuffs of eastern Russia

Four distinct components of natural remanent magnetization were isolated from a single site in welded tuffs in the Upper Cretaceous Kisin Group of the Sikhote Alin mountain range, Russia. In order to contribute toward a basis for an interpretation of multicomponent magnetization, rock magnetic experiments were performed on the welded tuffs. All four magnetization components essentially reside in magnetite. The lowest-temperature component up to 300 °C (component A: D=349.3°, I=60.9°, α₉₅=7.3°, N=7) is a present day viscous magnetization. The third-removed component (component C: D=41.4°, I=51.8°, α₉₅=3.5°, N=8), isolated over the temperature range of 450–560 °C, is a primary remanence. The second- and fourth-demagnetized components (component B: D=174.7°, I=−53.1°, α₉₅=21.2°, N=3 and component D: D=188.1°, I=−64.5°, α₉₅=4.0°, N=8, respectively) are secondary magnetizations related to a thermal event in Sikhote Alin between 66 and 51 Ma. Components B and D were acquired through different remagnetization processes. Component B is ascribed to a thermoviscous remanent magnetization carried by single-domain magnetite, and component D is a chemical remanent magnetization.     

The geological results of gravity and magnetic surveys in the Malvern Hills and adjacent districts

Detailed gravity and vertical force magnetic surveys in and around the Malvern Hills are described, and the results of determinations of densities and magnetic properties of local rocks are presented. The geophysical surveys indicate that the Malvern axis is a broad anticline involving basement rocks and that the folded basement extends northwards at shallow depth under the Storridge anticline; it cannot be traced further north under the Abberley Hills. Areas of surface Pre-Cambrian rock are interpreted as comprising intensely folded and faulted axial portions of the major fold, which is assumed to be of Armorican age. On the basis of the shallow depth to basement directly north of the Hills, and by analogy with the situation in the southern Malverns where Llandovery strata overstep the Local Cambrian succession, it is concluded that the Cambrian rocks were largely removed from the Storridge area as well as from the Malvern Hills area following a period of pre-Llandovery tectonism. To the west, the basement surface dips at about 30° to a depth of at least 5,000 ft. Cook and Thirlaway's (1955) suggestion that the Eastern Boundary Fault is part of a step-fault complex is accepted, and their interpretation of the configuration of the Worcestershire Triassic basin is not significantly amended in the present account.     

New structural implications for the central Sahara (Algeria), from the revisited Upper Carboniferous “Hassi Bachir” Formation: Paleomagnetic constraints

Paleomagnetic investigations of the folded Upper Namurian–Lower Moscovian “Hassi Bachir” Formation cropping out in the “Ahnet” basin (Central Sahara, Algeria) yield two magnetic components. A pre-folding primary magnetization (D = 136.1°, I = 22.0°, k = 217, α₉₅ = 2.6°) enables us to define a paleomagnetic pole (32.8°S, 55.7°E, K = 328 and A₉₅ = 2.0°) which better constrains a paleopole that was determined by Daly and Irving [Daly, L., Irving, E., 1983. Paléomagnétisme des roches carbonifères du Sahara central; analyse des aimantations juxtaposées; configurations de la Pangée. Ann. Geophys. 1, 207–216] for the same formation. A secondary component consists in a synfolding remagnetization and shows that post-Permian tectonics account for at least about half of the total folding in the studied area. This indicates that Mesozoic folding noted 150 km to the West in the Reggane basin [Smith, B., Derder, M.E.M., Henry, B., Bayou, B., Amenna, M., Djellit, H., Yelles, A.K., Garces, M., Beamud, E., Callot, J.P., Eschard, R., Chambers, A., Aifa, T., Ait Ouali, R., Gandriche, H., 2006. Relative importance of the Hercynian and post-Jurassic tectonic phases in the Saharan platform: a palaeomagnetic study of Jurassic sills in the Reggane basin (Algeria). Geophys. J. Int. 167, 380–396] is not local and affected at least the entire north-western part of the Hoggar area. This reconfirms that the folding of the Paleozoic cover in the Sahara platform should not be restricted to the Hercynian orogeny.     

Time‐scale of deformation and intertectonic phases revealed by P–T–D–t relationships in the orogenic middle crust of the Orlica‐Śnieżnik Dome,Polish/Czech Central Sudetes

A section of the orogenic middle crust (Orlica‐?nie?nik Dome, Polish/Czech Central Sudetes) was examined to constrain the duration and significance of deformation (D) and intertectonic (I) phases. In the studied metasedimentary synform, three deformation events produced an initial subhorizontal foliation S1 (D₁), a subsequent subvertical foliation S2 (D₂) and a late subhorizontal axial planar cleavage S3 (D₃). The synform was intruded by pre‐, syn‐ and post‐D₂ granitoid sheets. Crystallization–deformation relationships in mica schist samples document I_1–2 garnet–staurolite growth, syn‐D₂ staurolite breakdown to garnet–biotite–sillimanite/andalusite, I_2–3 cordierite blastesis and late‐D₃ chlorite growth. Garnet porphyroblasts show a linear Mn–Ca decrease from the core to the inner rim, a zone of alternating Ca–Y‐ and P‐rich annuli in the inner rim, and a Ca‐poor outer rim. The Ca–Y‐rich annuli probably reflect the occurrence of the allanite‐to‐monazite transition at conditions of the staurolite isograd, whereas the Ca‐poor outer rim is ascribed to staurolite demise. The reconstructed P–T path, obtained by modelling the stability of parageneses and garnet zoning, documents near‐isobaric heating from ~4 kbar/485 °C to ~4.75 kbar/575 °C during I_1–2. This was followed by a progression to 4–5 kbar/580–625 °C and a subsequent pressure decrease to 3–4 kbar during D₂. Pressure decrease below 3 kbar is ascribed to I_2–3, whereas cooling below ~500 °C occurred during D₃. In the dated mica schist sample, garnet rims show strong Lu enrichment, oscillatory Lu zoning and a slight Ca increase. These features are also related to allanite breakdown coeval with staurolite appearance. As Lu‐rich garnet rims dominate the Lu–Hf budget, the 344 ± 3 Ma isochron age is ascribed to garnet crystallization at staurolite grade, near the end of I_1–2. For the dated sample of amphibole–biotite granitoid sheet, a Pb–Pb single zircon evaporation age of 353 ± 1 Ma is related to the onset of plutonic activity. The results suggest a possible Devonian age for D₁, and a Carboniferous burial‐exhumation cycle in mid‐crustal rocks that is broadly coeval with the exhumation of neighbouring HP rocks during D₂. In the light of published ages, a succession of telescoping stages with time spans decreasing from c. 10 to 2–3 Ma is proposed. The initially long period of tectonic quiescence (I_1–2 phase, c. 10 Ma) inferred in the middle crust contrasts with contemporaneous deformation at deeper levels and points to decoupled P–T–D histories within the orogenic wedge. An elevated gradient of ~30 °C km^?1 and assumed high heating rates of c. 20 °C Ma^?1 are explained by the protracted intrusion of granitoid sheets, with or without deformation, whereas fast vertical movements (2–3 Ma, D₂ phase) in the crust require the activity of deformation phases.     

Palaeomagnetic re-examination of the basal Caithness Old Red Sandstone; aspects of local and regional tectonics

A palaeomagnetic re-examination of the basal strata of the Caithness Old Red Sandstone has given results that are fully compatible with previous palaeomagnetic findings in this region. After structural correction the dominant remanence component has D = 205°, I = +3°, α₉₅ = 6.4° (N = 27). The existence of this shallow inclined magnetization in the Middle Devonian strata of Caithness invalidates the model, proposed by Van der Voo and Scotese (1981), involving a ca. 2000 km sinistral offset along the Great Glen Fault in the Carboniferous. However, the available data are in favour of a few hundred kilometres sinistral movement along this fracture zone. However, the possibility of there having been a much larger transcurrent shift between Europe and North America in late/post-Devonian times, accumulated along various fracture zones within the Caledonian fold belt, is discussed. On the basis of an inferred overprinted magnetization, it is tentatively concluded that the tectonic deformation of the Old Red Sandstone of Caithness has a mid-Jurassic or younger age.     

Palaeomagnetism of Cretaceous carbonates from the northwestern part of the Dinaric fold belt

From the island of Cres and the fold belt north of autochthonous Istria 260 light-grey Cretaceous limestone samples were collected at 25 localities. Stepwise thermal demagnetization was employed to clean the remanence up to max. 525 °C. About two-thirds of the samples were successfully cleaned and the locality means were calculated from the characteristic remanences. The locality means significantly deviate from the present field direction both before and after tectonic correction: the mean inclination is about 15° lower than the present inclination and the declination is rotated counterclockwise i.e. it basically fits the pattern so far observed in the Periadriatic region.Unfolding barely influences the scatter of the directions (Cres: D = 308°, I = 45°, k = 26, α₉₅ = 13.3°, before tilt correction, and D = 330°, I = 48°, k = 33, α₉₅ = 11.9° after tilt correction; fold belt north of autochthonous Istria: D = 309°, I = 48°, k = 17, α₉₅ = 11.9°, before tilt correction, and D = 336°, I = 41°, k =13, α₉₅ = 14.0°, after tilt correction. In view of the inconclusive fold test, the magnetization cannot be dated precisely and thus does not yield sufficient information concerning possible relative movements between autochthonous Istria-Gargano and the fold belt studied. Nevertheless, the counterclockwise rotated declinations of the area studied are in contrast with the clockwise rotated declinations of the Ionian Zone of Western Greece. The large angular difference in declination between two parts of the Dalmatian-Ionian Zone with similar tectonic trends casts doubt on the continuity of the fold belt.     

Palaeomagnetism of the Loch Doon Granite Complex, Southern Uplands of Scotland: The Late Caledonian palaeomagnetic record and an Early Devonian episode of True Polar Wander

The Southern Uplands terrane is an Ordovician–Silurian back-arc/foreland basin emplaced at the northern margin of the Iapetus Ocean and intruded by granite complexes including Loch Doon (408.3 ± 1.5 Ma) during Early Devonian times. Protracted cooling of this 130 km³ intrusion recorded magnetic remanence comprising a predominant (‘A’) magnetisation linked to initial cooling with dual polarity and mean direction D / I = 237 / 64° (α₉₅ = 4°, palaeopole at 316°E, 21°N). Subsidiary magnetisations include Mesozoic remanence correlating with extensional tectonism in the adjoining Irish Sea Basin (‘B’, D / I = 234/− 59°) and minority populations (‘C’, D / I = 106/− 2° and ‘D’, D / I = 199/1°) recording emplacement of younger ( 395 Ma) granites in adjoining terranes and the Variscan orogenic event. The ‘A’ directions have an arcuate distribution identifying anticlockwise rotation during cooling. A comparable rotation is identified in the Orthotectonic Caledonides to the north and the Paratectonic Caledonides to the south following closure of Iapetus. Continental motion from midsoutherly latitudes ( 40°S) at 408 Ma to equatorial palaeolatitudes by  395 Ma is identified and implies minimum rates of continental movement between 430 and 390 Ma of 30–70 cm/year, more than double maximum rates induced by plate forces and interpreted as a signature of true polar wander. Silurian–Devonian palaeomagnetic data from the British–Scandinavian Caledonides define a 430–385 Ma closed loop comparable to the distributed contemporaneous palaeomagnetic poles from Gondwana. They reconcile pre-430 Ma and post-380 Ma APW from this supercontinent and show that Laurentia–Baltica–Avalonia lay to the west of South America with a relict Rheic Ocean opening to the north which closed to produce Variscan orogeny by a combination of pivotal closure and right lateral transpression.     

华南早三叠世的古地磁学与大地构造

从华南几个具代表性地点的下三叠统灰岩中,获得了有意义的古地磁学初步结果。这些结果支持华南存在分离板块的设想。根据新近获得的地质与古地磁学证据,推断华南存在4个岩石图板块。它们是扬子(D=232.4°,I=-11.8°,K=28.2,α₉₅=5.5°),湘桂(D=198.0°,I=32.2°,α₉₅=12.9°),华夏(D=88.0°,I=13.0°,K=20.1,α₉₅=9.1°)和海南(上二叠统D=338.2°,I=15.9°,K=10.2,α₉₅=14.3°)板块。4个板块的古地磁极位分散,纬度差明显,并存在碰撞后的局部与区域性的旋转作用,尤其是顺时针旋转。华南早中生代的大地构造可理解为4个来自古特提斯和冈瓦纳的岩石圈碎块与欧亚大陆聚合作用的产物。     

Paleomagnetic dating of ironstone nodules (‘nuts’) from the Yowah opal field,central southern Queensland

To constrain the age of Australian opal formation, we have undertaken a paleomagnetic study of oxidised ironstone ‘nuts’ from Yowah, Queensland. Following standard methods, we have calculated a mean direction of declination D = 191.4°, inclination I = 61.7° (α₉₅ = 4.0°), indicating a paleomagnetic pole position at latitude λ_p = 71.3°S, longitude ?_p = 119.4°E (A₉₅ = 5.3°). The direction comprises both normal and reverse polarities that fail a reversal test most probably owing to contamination by small recent/present-day components. The mean direction should not be significantly affected. A chi-square comparison with paleomagnetic poles for dated Cenozoic rocks in eastern Australia, poles derived from the Global Moving Hotspot Reference Frame and the Cenozoic pole path for North America, appropriately transferred to Australian coordinates, yields a mean age estimate of 35 ± 7 Ma, i.e. late Eocene to early Oligocene. This is interpreted as the age of the ironstone formation, which places a maximum age for the formation of precious Yowah opal. This result confirms and tightens the age for the nearby Canaway weathered profile.