Palaeomagnetic Results Research Articles

Late Miocene transcurrent tectonics in NW Turkey: evidence from palaeomagnetism and 40Ar–39Ar dating of alkaline volcanic rocks

A number of intra-continental alkaline volcanic sequences in NW Turkey were emplaced along localized extensional gaps within dextral strike-slip fault zones prior to the initiation of the North Anatolian Fault Zone. This study presents new palaeomagnetic and 40Ar–39Ar geochronological results from the lava flows of NW Turkey as a contribution towards understanding the Neogene–Quaternary tectonic evolution of the region and possible roles of block rotations in the kinematic history of the region. 40Ar–39Ar analyses of basalt groundmass indicate that the major volume of alkaline lavas of NW Turkey spans about 4 million years of episodic volcanic activity. Palaeomagnetic results reveal clockwise rotations as high as 73° in Thrace and 33° anticlockwise rotations in the Biga Peninsula. Movement of some of the faults delimiting the areas of lava flows and the timing of volcanic eruptions are both older than the initiation age of the North Anatolian Fault Zone, implying that the region experienced transcurrent tectonics during Late Miocene to Pliocene times and that some of the presently active faults in the region are reactivated pre-existing structures.

Palaeomagnetic Results from the Sarmatian/Pannonian Boundary in North-Eastern Croatia (Vranovi? Section, Našice Quarry)

The Sarmatian/Pannonian boundary in the Central Paratethys basin is marked by a major regressive event, which isolated the basin from the open sea and resulted in a palaeoenvironmental change from restricted marine to brackish water ecosystems. The exact age of this environmental change is still ambiguous since direct age control on the boundary interval is lacking, mainly due to the scarcity of suitable sections. The Vranovi? section in the Našice Quarry in Croatia, however, is relatively long and continuously exposed. A detailed sedimentological and biostratigraphic study indicates that it contains the Sarmatian/Pannonian boundary and that it reflects the same palaeoenvironmental trend as other Paratethyan sequences. Here, we present palaeomagnetic and rock magnetic results from the Vranovi? section, based on 183 sampled levels distributed along 55 m of cyclically bedded limestones and marls. Rock magnetic data indicate the presence of maghemite or haematite in the Sarmatian deposits and low contents of magnetite in the Pannonian rocks. Thermal demagnetization results indicate dominantly normal polarities, and the mean direction closely coincides with the present-day field direction at Našice. We conclude that magnetostratigraphic age control cannot be derived for the Vranovi? section because of a dominant secondary (post-tilt) magnetization. Consequently, a firm numerical age based on magnetostratigraphy cannot be assigned to Sarmatian/Pannonian boundary events from this section.

Palaeomagnetism of 935 Ma mafic dykes in southern Sweden and implications for the Sveconorwegian Loop

New palaeomagnetic results for the 935 Ma Göteborg-Slussen mafic dykes in southern Sweden provide a well-dated high-quality palaeomagnetic pole for Early Neoproterozoic Baltica. New U-Pb geochronological data for several palaeomagnetically studied mafic intrusions yield three additional well-dated palaeopoles and one virtual geomagnetic pole. This set of dated poles suggests minimal drift of Baltica in moderate latitudes between ∼965 and 915 Ma. They also support the hypothesis of a post-900 Ma regional remagnetization event in SW Sweden and SW Norway. The positions of three distinct clusters of ∼1100 to 850 Ma palaeopoles suggest a clockwise time progression of the Baltica apparent polar wander path (the Sveconorwegian Loop) during this time interval. New well-dated palaeomagnetic poles for ∼970 to 900 Ma from Laurentia are required to verify the palaeogeographic reconstructions of Baltica and Laurentia.

Crustal structure across Sikkim, NE Himalaya from new gravity and magnetic data

The new gravity and magnetic data recorded along a profile in the Sikkim, NE Himalaya are combined with the existing data from Tibet, Bangladesh and India, to delineate the crustal structure in this part of Himalaya. Modelling of gravity data, constrained from seismic results suggests that long wavelength gravity anomalies arise due to variations in the depth of Moho (36 to 74 km), which are caused by flexed lithosphere of effective elastic thickness of ∼ 50 ± 10 km. Simultaneous modelling of magnetic anomalies and short wave-length gravity anomalies reveals that (a) the magnetic anomalies observed over the Lesser Himalaya and the Higher Himalaya Crystalline rocks might be caused by remnant magnetisation with inclination I = − 18° ± 8° and declination D = 147° ± 10°, which is in conformity with the palaeomagnetic results. These magnetic parameters correspond to ~ 35 ± 10 Ma age of magnetic direction and suggest that the rocks might have acquired magnetisation during cooling period of metamorphism, (b) low grade meta-sediments of the Lesser Himalaya extend up to 12 km depth and thins on either sides forming a bowl shaped geometry and (c) relative gravity high in the Bengal basin might be caused by intrusion of the Rajmahal volcanics. Modelling has also provided constraint on the geometry of the north dipping thrusts.

Palaeomagnetic results from the Lopra-1/1A re-entry well, Faroe Islands

The palaeomagnetic dating and evolution of the Faroe Islands are discussed in the context of new density and rock magnetic results from the deepened Lopra-1/1A well. The reversal chronology of the c. 6½ km thick basalt succession is also described. The polarity record of the Faroe Islands may now be correlated in detail with the Geomagnetic Polarity Time Scale. The lowermost (hidden) part of the lower basalt formation correlates with Chron C26r (Selandian age), the top (exposed) part of the lower basalt formation correlates with Chrons C26n, C25r and C25n (Selandian and Thanetian age) and the middle and upper basalt formations correlate with Chron C24n.3r (Ypresian). Inclinations indicate a far-sided position of the palaeomagnetic poles, which is characteristic of results from most Palaeogene volcanics from the northern North Atlantic region. The density, magnetic susceptibility and magnetic remanence of 20 specimens from one solid core (1½ m in length) and 26 sidewall cores from the well between –2219 and –3531 m below sea level (b.s.l.) suggest that the volcanic materials can be divided into two characteristic groups: solid unaltered basalts and altered basalts and tuffs. The magnetic properties are typically log-normally distributed and the carriers of remanence are Ti-poor Ti-magnetites with Curie temperatures close to 580°C. The inclination of the 1½ m core at 2380 m b.s.l. is dominantly negative (two plugs at the very top of the core do show normal polarity, but they are likely to be misoriented as all specimens appear to be from one flow). Magnetic logging (magnetic susceptibility and field intensity) down to 3515 m b.s.l. was made in Lopra-1/1A together with other geophysical logs but did not yield conclusive inclination data.

Anomalously shallow inclination in middle-northern part of the South China block: palaeomagnetic study of Late Cretaceous red beds from Yichang area

SUMMARY We present new palaeomagnetic results from Late Cretaceous red beds of the Paomagang Formation collected at 42 sites in the Yichang area (30.7°N, 111.7°E), middle–northern part of the South China Block. A high unblocking temperature component around 680°C was isolated from 26 sites by stepwise thermal demagnetization. Fold tests are positive at the 95 per cent confidence limit for 14 and 11 sites of the Paomagang and Wangdian areas, respectively. Normal and reversed polarity sequence found in the Wangdian area passed a reversal test at the 95 per cent confidence limit. The tilt-corrected mean direction for Yichang area is D= 3.8°, I= 23.3° (α95= 5.1°, N= 26), corresponding to a palaeopole at 71.5°N, 280.0°E with A95= 4.3°. Comparison with the expected inclination from the 80 Ma Eurasian APWP pole indicates a inclination flattening of 26.3°± 6.4°. Middle Cretaceous to Cenozoic NW–SE extension tectonics within the SCB contributes only 2.0° to the inclination flattening. While the massive samples of the Paomagang Formation show a moderate degree of anisotropy (1.017 < Pj < 1.086) and steeper inclination (I= 29.3°), the laminated samples, which form more than 75 per cent of the studied samples, show a higher degree of anisotropy (1.052 < Pj < 1.227) and shallower inclination value (I= 22.7°). A good correlation between remanence inclination and facies types indicates that depositional processes produced the observed shallow inclination in the Yichang area.

Palaeomagnetic results from the Early Permian Copacabana Group, southern Peru: Implication for Pangaea palaeogeography

Samples collected from folded carbonate rocks of the Early Permian Copacabana Group exposed in the Peruvian Subandean Zone have been subjected to detailed palaeomagnetic analysis. Thermal demagnetisation of most samples yield stable high unblocking temperature directions dominantly carried by titanomagnetite minerals. This remanence, identified in 32 samples (43 specimens), is exclusively of reverse polarity consistent with the Permian–Carboniferous Reversal Superchron (PCRS). The overall directions pass the fold test at the 99% confidence level and are considered as being a pre-folding remanence acquired in Early Permian times. The Copacabana Group yields an overall mean direction of D = 166°, I = +49° ( α 95 = 4.5°, k = 131.5, N = 9 sites) in stratigraphic coordinates and a corresponding palaeosouth pole position situated at λ = 68°S, ϕ = 321°E ( A 95 = 5.2°, K = 100). Combining this pole with the coeval high quality data from South America, Africa and Australia results in a mean pole for Gondwana situated at λ = 34.4°S, ϕ = 065.6°E ( A 95 = 4.9°, K = 73.6, N = 13 studies) in African coordinates. This pole position supports a Pangaea B palaeogeography in Early Permian times. In contrast, the combined pole for Gondwana diverges from the coeval Laurasian mean pole when assuming the Pangaea A-type configuration. Poor quality of the Gondwana dataset and inclination shallowing in sediments seem to play no role in the misfit between the Permian–Triassic poles from Gondwana and Laurasia in Pangaea A reconstruction.

Palaeomagnetic constraints on continental break-up processes: observations from the Main Ethiopian Rift

Abstract We report the first palaeomagnetic results from the Main Ethiopian Rift (MER), the northernmost sector of the East African rift system. This part of the MER shows an along-axis tectono-magmatic segmentation pattern similar to that of slow-spreading mid-ocean ridges, which developed during the past 1.9 Ma. The aims of our palaeomagnetic, structural and geochronological studies are to test plate kinematic models for the right-stepping, en echelon 60–80 km-long magmatic segments. Twenty palaeomagnetic sites were sampled on either basalt or ignimbrite outcropping in the region adjacent to, and within, the &lt;1.9 Ma-old tectono-magmatic segments of Gademsa-Koka, Boset and Fentale-Dofan. Five K-Ar age determinations were made to bracket the age of units studied in the palaeomagnetic analyses. The natural remanent magnetization intensity possibly exhibited a unimodal distribution with a value of 6.6 A/m (σ = 5.6 A/m) for the basalts and a bimodal distribution with magnetization intensity of 0.69 A/m (σ = 0.55 A/m) and 0.03 A/m (σ = 0.02 A/m), statistically similar to values from previous studies in the Afar triple junction zone (e.g. Kidane et al. 1999, 2002). Progressive heating, alternating field analysis, and susceptibility vs. temperature measurements indicated unblocking temperature ranging between 300 °C−600 °C for basalts and between 500 °C−660 °C for ignimbrites, suggesting the magnetic mineralogy to be titanomagnetite and magnetite for the former and magnetite and titanohematite for the latter. Palaeomagnetic measurements using both TH and AF technique revealed quasi-single component of magnetization with viscous remanent magnetization (VRM) on a few samples. Principal component analysis and statistical averaging resulted in an overall mean palaeomagnetic direction of (D s = 2.3°, I s = 7.8°, α 95 = 7, K = 26.9, N = 17) which is statistically identical to the expected direction (D = 1.9°, I = 13.5°, α 95 = 2.5, K = 105.6, N = 32) from the Apparent Polar Wander Path reference curve for Africa at 1.5 Ma (Besse &amp; Courtillot 2003). The angular difference between the observed and expected directions above with their uncertainty is calculated to be 0.4° ± 7.5°. These results indicate that the Late Pliocene-Pleistocene rocks of the MER in the studied region do not suffer vertical axis rotation, arguing against transtensional and seafloor-spreading-transform kinematic models. We suggest that magma intrusion, rather than large offset faults, produce the right-stepping, en echelon magmatic segments of the MER, which is at the transition from continental to oceanic extension.

New Precambrian palaeomagnetic constraints on the position of the North China Block in Rodinia

We report new palaeomagnetic results from a ca. 1300 to 800 Ma continental shelf succession on the southern margin of the North China Block. A total of 386 oriented core samples were subjected to stepwise demagnetisation. Two overprint components (‘A’ and ‘B’) were identified, with ‘A’ being a Recent geomagnetic field component and ‘B’ a likely Mesozoic remagnetisation related to collision of the North and South China Blocks. An interpreted primary remanence was isolated from six rock units. The most reliable results are as follow, in the order of stratigraphic ascendance. (1) Purple mudstone, muddy sandstone and andesite of the lower Yunmenshan Formation (Rb–Sr age ca. 1270 Ma) yields a high-temperature component that passes both reversal and fold tests and gives a palaeopole at (60.6°S, 87.0°E, A 95 = 3.7°). (2) Mudstone in the overlying Baicaoping Formation yields a high-temperature component with a palaeopole at (43.0°S, 143.8°E, A 95 = 11.1°). (3) Purple sandstone of the earliest Neoproterozoic Cuizhuang and Sanjiaotang Formations exhibits a high-temperature component that provides a palaeopole at (41.0°S, 44.8°E, A 95 = 11.3°). Based on both our new results and a critical selection of available palaeomagnetic data, we construct a preliminary apparent polar wander path (APWP) for the North China Block between 1300 and 510 Ma. Regardless of alternative polarity options applicable to these poles, North China was located within equatorial latitudes for much of this interval. Comparing the North China poles with coeval poles from Laurentia suggests that the two continents were situated on the same plate between 1200 and 700 Ma. North China was thus likely part of the supercontinent Rodinia. Separation of North China and Laurentia occurred between 650 and 615 Ma.

Early Paleoproterozoic–Archean dykes and gneisses in Russian Karelia of the Fennoscandian Shield—New paleomagnetic, isotope age and geochemical investigations

We present here new palaeomagnetic, isotopic age and geochemical data from Archean and Early Palaeoproterozoic rocks in the eastern Fennoscandian Shield. We have studied NE–SW trending gabbronorite dyke sets and their host Archean basement rocks in the Vodlozero block near the 2449 Ma Burakovka layered intrusion in southern Russian Karelia. Both dyke sets are genetically related to the Burakovka intrusion. The other, ca. 25 km long Avdeev dyke, locating a few kilometers south from the Burakovka intrusion, yields a stable single component remanence direction that is in agreement with the direction previously obtained from the Burakovka intrusion. Another NE–SW trending dyke, 0.8 m wide Shalskiy diabase dyke, about 30 km south of the Burakovka intrusion yields a similar remanence direction as the Avdeev dyke. The overall mean remanence direction has a palaeopole at Plat = −12.3°N, Plong = 243.5°E ( A95 = 15.4°, 4 sites, 28 samples). The thin Shalskiy diabase dyke transects a similarly NE–SW trending 500 m wide coarse grained gabbronorite dyke which has now been dated by Sm–Nd method as 2608 ± 56 Ma. Geochemically all the dykes are quite similar showing slight calc-alkaline affinity and low TiO 2 and high SiO 2 with moderate MgO and low Cr and Ni. Furthermore, the dykes are geochemically identical to the 2.45 Ga dyke swarm in the northern Karelian Province. The remanence direction of the thin Shalskiy diabase dyke differs significantly from the high temperature and high coercivity remanence component of the unbaked Archean gabbronorite dyke which yields a palaeopole at Plat = 22.7°N, Plong = 222.1°E (dp = 8.2°, dm = 16.2°, five samples). On the basis of different remanence directions of the diabase dyke and the unbaked Archean gabbronorite dyke, the baked contact test for the diabase dyke is positive. In addition to the high temperature and high coercivity component of the baked and unbaked Archean gabbronorite dyke, in low temperatures and coercivities we isolated a similar component as in the diabase dyke. A comparable remanence component was also obtained from the Archean basement at ca. 8 km from the dykes. We propose that in the studied area, the Archean basement and the Archaean dyke were partly remagnetized due to emplacement and subsequent uplift and cooling of the large Burakovka layered intrusion and related dykes at about 2.40 Ga ago. This interpretation lends support from a new 40Ar/ 39Ar dating of hornblende from another area, Lake Paajarvi area, in northern Karelia. There, a negative baked contact test was previously obtained for the remanence of the dated ca. 2.45 Ga dyke rocks related to the ca. 2.45 Ga Oulanka layered intrusion. The 40Ar/ 39Ar dating of the unbaked Archean basement which yields the same remanence component as the dykes, shows a plateau age of ca. 2.6 Ga, but in addition, it also shows resetting of the basement at ca. 2.4 Ga ago. The dating thus supports reactivation and partial remagnetization of the Archean basement at ca. 2.4 Ga ago. Our new palaeomagnetic results from the Burakovka dykes and the new 40Ar/ 39Ar dating from the Lake Paajarvi area give support to our previous interpretation that at Lake Paajarvi area the remanence component suggested to be 2.4 Ga, despite to negative baked contact test, is indeed of this age. Therefore, it is implied that the results can be used for continental reconstructions.

Counter-clockwise rotation of the eastern part of the Mongolia block: Early Cretaceous palaeomagnetic results from Bikin, Far Eastern Russia

SUMMARY We present palaeomagnetic results from Lower Cretaceous rocks in Bikin area of the Alchan basin (46.5°N, 134.7°E), Sikhote Alin orogenic belt, Far Eastern Russia. A high-temperature magnetization component with maximum unblocking temperatures at about 590°C was isolated from six sites of dacite welded tuffs in the Albian Alchanskaya Formation. The fold tests for these six sites are positive, suggesting that primary magnetization is preserved in the studied rocks. The tilt-corrected mean direction of D= 309.3°, I= 68.7° (α95= 10.1°), with a corresponding palaeopole position at 57.0°N, 76.8°E (A95= 15.1°), indicates a counter-clockwise (CCW) rotation for the studied area. CCW rotation is also indicated from west-directed declinations (D= 249.1°, I= 64.1°, α95= 11.2°) obtained from secondary magnetization of the Berriasian Kultukha Formation. Combining with the previously reported studies, the west-directed Cretaceous palaeomagnetic directions cover widely the eastern part of the Mongolia block. Comparison with 100 Ma palaeomagnetic pole for Eurasia shows that the eastern part of the Mongolia block experienced a CCW rotation of over 36° with respect to the Eurasian continent later than Late Cretaceous. This rotation is ascribed to post-Late Cretaceous extension that affected the continental basins (the Middle Amur, Sanjiang, Razdolnian, Amur-Zeya and Songliao basins) of the northeast Chinese Plain along the eastern margin of the Mongolia block. Contemporaneous with this CCW rotation, similar extension resulted in clockwise rotation of the eastern part of the North China block.

Shallow bias in Neogene palaeomagnetic directions from the Guide Basin, NE Tibet, caused by inclination error

SUMMARY Too-shallow inclinations have frequently been observed in Cenozoic sedimentary strata in central Asia, and new palaeomagnetic results obtained by us from the Guide Basin in NE Tibet are no exception. We use a statistical analysis technique developed by Tauxe and Kent (TK03.GAD), which is based on a geomagnetic field model that predicts distributions of palaeomagnetic directions, and show that the too-shallow Neogene mean inclination (44 °) from 627 sites can be corrected to a value of 58 °, which closely matches the inclination predicted for the area. We conclude that syn- to post-depositional flattening is the most likely cause for the widely observed inclination bias in central Asia.

Palaeomagnetism and40Ar/39Ar geochronology from the South Taimyr igneous complex, Arctic Russia: a Middle-Late Triassic magmatic pulse after Siberian flood-basalt volcanism

SUMMARY New palaeomagnetic results and 40 Ar/ 39 Ar ages from mafic sills and extrusive rocks from the South Taimyr igneous complex (75 ◦ N, 100 ◦ E) document late Middle‐Early Late Triassic igneous activity in Arctic Siberia. The palaeomagnetic pole determined from the sills (47 ◦ N, 122 ◦ E, dp/dm = 5/5) plots on the 230‐220 Ma portion of the apparent polar wander path for Euramerica, and is statistically different from the mean 250 Ma Siberian Traps pole. 40 Ar/ 39 Ar ages from three of the sills yield crystallization ages of ca. 229‐227 Ma and confirm the pole age. The Taimyr igneous rocks are folded together with Carboniferous to Lower Triassic continental sedimentary rocks of the northern Siberia margin and are unconformably overlain by Early Jurassic sedimentary units that place an upper limit on the cessation of the Late Triassic folding event. In contrast to the sills, the palaeomagnetic pole obtained from the extrusive volcanic rocks (59 ◦ N, 146 ◦ E, dp/dm = 14/15) overlaps within uncertainty with previous Siberian Traps results, but is consistent with any age between 250 and 220 Ma. The 40 Ar/ 39 Ar signatures of the extrusive rocks were largely very disturbed and only one sample yielded a precise and interpretable age of 248 Ma. The intrusive rocks in the South Taimyr igneous complex post-date eruption of the Siberian Traps flood basalts by about 20‐25 Myr and are contemporaneous with reported ages from granitic rocks on Novaya Zemlya and late intrusive and pyroclastic rocks from the margins of the Siberian Traps igneous province. The extrusive rocks in South Taimyr may be agecorrelative with the extensive Siberian Traps flood basalts with which they have previously been linked; however, our data alone cannot confirm the comagmatic nature of the extrusive rocks within South Taimyr or their genetic link, if any, to the igneous activity that produced the Siberian Traps flood basalts.

Late Jurassic-Early Cretaceous closure of the Mongol-Okhotsk Ocean demonstrated by new Mesozoic palaeomagnetic results from the Trans-Baïkal area (SE Siberia)

SUMMARY We present new palaeomagnetic results from the Transba¨ ikal area (SE Siberia), from the Mongol-Okhotsk suture zone, the boundary between the Amuria and Siberia blocks. In order to better constrain the time of closure of the Mongol-Okhotsk Ocean in the Mesozoic, we col- lected 532 rock samples at 68 sites in six localities of basalts, trachy-basalts and andesites, from both sides of the Mongol-Okhotsk suture: at Unda river (J 3; 51.7 ◦ N, 117.4 ◦ E), Kremljevka peak (K1; 51.8 ◦ N, 117.5 ◦ E) and Torey lakes (K 1; 50.1 ◦ N, 115.9 ◦ E) on the southern side of the suture, and at Monostoy river (J 1; 51.1 ◦ N, 106.8 ◦ E), Ingoda river (K 1; 51.2 ◦ N, 112.2 ◦ E) and Bichura town (K 1; 50.6 ◦ N, 107.6 ◦ E) on the northern side. Progressive thermal demagnetization enabled us to resolve low (LTC) and high (HTC) temperature components of magnetization at most sites. Jurassic palaeopoles computed from the HTCs show a large discrepancy with respect to the Apparent Polar Wander Path of Eurasia, which we interpret in terms of 1700-2700 km of post-Late Jurassic northward movement of Amuria with respect to Siberia. Although ge- ological data suggest a middle Jurassic closure of the Mongol-Okhotsk Ocean in the west Trans-Baikal region, our data give evidence of a large remaining palaeolatitude difference between the Amuria and Siberia blocks. In contrast, Early Cretaceous sites cluster remarkably well along a small-circle, which is centred on the average site location. This implies the absence of post-Early Cretaceous northward motion of Amuria relative to Siberia, and demonstrates the pre-Early Cretaceous closure of the Mongol-Okhotsk Ocean. Finally, we interpret the very large tectonic rotations about local vertical axes, evidenced by the small-circle distribution of poles, as arising both from collision processes and from left-lateral shear movement along the suture zone, due to the eastward extrusion of Amuria under the effect of the collision of India into Asia.

Regional deformational features of the South China Block inferred from Middle Triassic palaeomagnetic data

SUMMARY Middle Triassic red-purple sandstones were sampled at four localities in the northeastern part of the South China Block (SCB) to detect regional deformational features. Most of the samples gave a characteristic remanent magnetization with unblocking temperatures up to 680 ◦ C. The primary nature of this magnetization was ascertained by a positive fold test and a reversal test. The tilt-corrected mean direction from the Yueshan area (30.6 ◦ N, 116.8 ◦ E), which is located at a distance of 25 km from the eastern part of the Tan-Lu Fault, shows a counter-clockwise deflection of 67.3 ◦ with respect to north. However, mean directions from the Nanzhang (31.5 ◦ N, 111.7 ◦ E), Tongling (30.9 ◦ N, 117.9 ◦ E) and Nanjing (32.0 ◦ N, 118.9 ◦ E) areas, which are located between 75 and 90 km from the northeastern boundary of the SCB, indicate a northeasterly deflection in declination, such as D = 38.2 ◦ , I = 25.1 ◦ (α 95 = 6.4 ◦ ), D = 32.1 ◦ , I = 25.2 ◦ (α 95 = 9.7 ◦ ) and D = 34.9 ◦ , I = 27.8 ◦ (α 95 = 5.4 ◦ ), corresponding to palaeopoles of λ = 50.5 ◦ N, φ = 220.5 ◦ E,dp/dm = 3.7 ◦ /6.9 ◦ , λ = 55.6 ◦ N, φ = 231.8 ◦ E,dp/dm = 5.6 ◦ /10.4 ◦ and λ = 53.9 ◦ N, φ = 229.1 ◦ E, dp/dm = 3.2 ◦ /5.9 ◦ , respectively. Comparison with the previously reported Permo-Triassic palaeomagnetic results from the SCB suggests that very consistent palaeopoles are observed within a stable belt up to 1800 km along the northern margin of the SCB (covering the Changxing, Nanjing, Tongling, Nanzhang and Emeishan areas). This type of behaviour implies that an area located between 70 and 225 km from northern margin of the SCB forms a stable platform where very little relative rotation has been detected. However, the inner region of this block, especially the Mesozoic fold-thrust belt, experienced a clockwise rotation of 11.5 ◦ ± 5.0 ◦ to 30.3 ◦ ± 9.2 ◦ with respect to the stable outer region later than the Middle Triassic. Areas close to the Tan-Lu Fault were subjected to post-Triassic counter-clockwise rotation by 90 ◦ with respect to the stable tectonic belt. We thus conclude that the northern stable belt of the SCB suffered no oroclinal bending during its suturing with the North China Block.

A synthesis of Cretaceous palaeomagnetic data from South Korea: tectonic implications in East Asia

SUMMARY Tectonic provinces in South Korea include the Precambrian Gyeonggi Massif, Palaeozoic Okcheon Belt, Precambrian Yeongnam Massif and Cretaceous Gyeongsang Basin, from northwest to southeast. The Cretaceous strata mainly occur in the Gyeongsang Basin as well as in several small basins along the boundaries of the Okcheon Belt and on the Gyeonggi Massif. This paper reviews and analyses previously published palaeomagnetic data from the Cretaceous successions in South Korea. A total of 23 Cretaceous palaeomagnetic poles, from 18 studies, satisfy more than four standard reliability criteria. The palaeomagnetic pole positions, from the Gyeongsang Basin and from small basins in the Gyeonggi Massif and the Okcheon Belt, are consistent with one another, indicating that the Korean Peninsula has been a single terrane since the Cretaceous. However, based on the comparison of the palaeomagnetic pole positions and the palaeolatitudes of blocks in the Gyeongsang Basin, it was observed that the Gyeongsang Basin had experienced tectonic adjustments during the Cretaceous Period. Within the Gyeongsang Basin, the geographically northern area (Yeongyang block) was rotated counterclockwise by 16.3°± 4.6° with respect to the southern area (Milyang and Uiseong blocks), based on the comparison of palaeomagnetic pole positions between the two areas. This palaeomagnetic result and some geological features in the Gyeongsang Basin collectively indicate that the Yeongyang block underwent counterclockwise rotations, accompanied by northwestward protrusion, into the Yeongnam Massif during the Late Cretaceous. These relative tectonic movements within the Gyeongsang Basin were probably due to the northwestward subduction of the proto-Pacific oceanic plate during the Late Cretaceous. The gradual eastward displacement of the Korea's Cretaceous palaeopoles, from the Eurasian pole, increases with stratigraphic age, indicating that the Korean Peninsula underwent progressive clockwise rotations, with respect to Eurasia, during the Cretaceous Period. The clockwise rotation of Southwest Japan, with respect to Eurasia, is ascribed to the Miocene opening of the East Sea, and has no connection with the Cretaceous clockwise rotations of the Korean Peninsula, indicating that the Korean Peninsula and Southwest Japan may have been independent terranes during the Cretaceous Period.

New palaeomagnetic results from the Whin Sill complex: evidence for a multiple intrusion event and revised virtual geomagnetic poles for the late Carboniferous for the British Isles

A large-scale palaeomagnetic study (125 specimens from 88 sampling localities) has been carried out on the Whin Sill complex and its associated dykes in northern England. The dominant carrier of remanent magnetization of the doleritic rocks is relatively pure magnetite of pseudo-single domain size, and the characteristic remanent magnetizations are typically well defined and unidirectional. Regionally consistent palaeomagnetic directions are obtained from three geographically distinct parts of the Whin Sill complex, here named as the Holy Island Sill (Declination/Inclination ( D / I )=189.5/−2.8, α 95 =6.3), the Alnwick Sill ( D / I =194.6/−25.6, α 95 =8.1) and the Hadrian's Wall–Pennines Sill ( D / I =189.2/3.3, α 95 =3.5). Although the Holy Island Sill and the Hadrian's Wall–Pennines Sill are of similar age ( c . 296 Ma) and indistinguishable on palaeomagnetic grounds, the difference in palaeomagnetic directions between them and the Alnwick Sill demonstrates the multi-component nature of the complex; magma-flow indicators suggest that the Holy Island Sill and the Hadrian's Wall–Pennines Sill may also represent separate intrusion events. Dykes local to the three sills have remanence directions corresponding to their respective sills and can tentatively be identified as their feeder dykes. Virtual geomagnetic poles for the three palaeomagnetic sites are: Holy Island Sill and Holy Island Dyke, 346.8E, 35.4S; Alnwick Sill, High Green Dyke and St. Oswald's Chapel Dyke, 337.1E, 47.1S; Hadrian's Wall–Pennines Sill and Hett Dyke, 347.1E, 32.9S.

Palaeomagnetic results from the Palaeozoic basement of the southern Drummond Basin, central Queensland, Australia

SUMMARY Palaeomagnetic results from the Theresa Creek Volcanics and the Middle Devonian (∼380 Ma) Retreat Batholith, located in the southern Drummond Basin, central Queensland, Australia, reveal up to three components of magnetization. Component 1 is characteristically of reversed polarity with a mean palaeopole at 64.0 ◦ S, 79.7 ◦ E( N = 7, A95 = 11.5 ◦ ), possibly reflecting a remagnetization event associated with the Middle Carboniferous Alice Springs Orogeny, is found in both units. A shallow to very shallow roughly north‐south direction, Component 2, was isolated in seven sites, predominantly within the Retreat Batholith, D = 345.8 ◦ , I =− 6.7 ◦ (α 95 = 10.1 ◦ , k = 36.8). The corresponding palaeomagnetic pole lies at 66.0 ◦ S, 290.7 ◦ E( A95 = 9.1 ◦ ). This pole is consistent with the Middle Devonian segment of Australia’s apparent polar wander path (APWP) and is thought to reflect a primary magnetization related to the emplacement and cooling of the batholith. Component 3 is found exclusively within the Theresa Creek Volcanics and is characterized by east‐west directions with moderate to steep inclinations, D = 62.4 ◦ , I = 59.5 ◦ (α 95 = 10.5 ◦ , k = 25.0). A consistent set of site level virtual geomagnetic pole (VGPs) yield a palaeopole at 4.1 ◦ N, 188.9 ◦ E( N = 9, A95 = 14.0 ◦ ). The presumed Middle Devonian age for the volcanics is at odds with this direction, with respect to the Australian Early to Middle Palaeozoic APWP, suggesting the characteristic direction found in the volcanics reflects an Ordovician or possibly Early Silurian magnetization age. A pre-Middle Devonian magnetization age for the Theresa Creek Volcanics is supported by a positive contact test between the volcanics and the hornfelsing Retreat Batholith as both units are characterized by stable magnetizations whose palaeopoles are dissimilar from each other and younger segments of Australia’s APWP.

New insights into partial oxidation model of magnetites and thermal alteration of magnetic mineralogy of the Chinese loess in air

SUMMARY New rock magnetic data including low- and high-temperature thermomagnetic runs and hysteresis loops, as well as X-ray diffraction measurements, are presented from the Chinese loess/palaeosols at JiuZhoutai, on the northwestern margin of the Chinese loess plateau, to provide sound constraints on the partial oxidation model of magnetites and variations in magnetic mineralogy, grain size and oxidation states of loess samples during thermal treatments from room temperature up to 700 °C. The results show that the partially oxidized magnetites in samples are first reduced, probably as a result of the burning of organic matter below 300 °C, and then reoxidized above that. However, even after heating to 700 °C the highly diminished (in size) magnetite core still remains stoichiometric as revealed by the 120 K Verwey transition. We conclude that the diffusion of Fe2+ from the magnetite core to the surrounding maghemite rim is confined to the oxidation front (magnetite-core/maghemite-rim boundary), and has less effect on the stoichiometry of the magnetite core. The new results also reveal that the absolute number of superparamagnetic grains (magnetite/maghemite) remains relatively stable during heating. Furthermore, the left-downward shift in the data points of Day plots (mainly between 25 and 150 °C) is caused by a decrease in the oxidation gradient. In contrast, the right-upward (mainly between 500 and 650 °C) shift of the Day plots corresponds to a much higher degree of oxidation, resulting in the increase of a new oxidation gradient between the magnetite core (or the residual maghemite mantle) and the newly developed haematite outer rim, which is transformed from maghemite by heating. These new results place strong constraints on the fidelity of palaeomagnetic results from the Chinese loess constructed by thermal demagnetization, and provide a more accurate interpretation of the Day plot and low-temperature magnetic behaviours for these partially oxidized magnetites. Furthermore, this study provides robust lines of evidence for the core–rim coupling model between different magnetic phases (magnetite, maghemite and haematite).

Return to Black Mountain: palaeomagnetic reassessment of the Chatsworth and Ninmaroo formations, western Queensland, Australia

SUMMARY Palaeomagnetic results from late Middle Cambrian‐Early Ordovician carbonate sequences sampled at Black Mountain (Mt Unbunmaroo), Mt Datson and near Chatsworth Station (southeastern Georgina Basin) are presented. A palaeomagnetic reassessment of these carbonates was designed in an effort to constrain regional magnetization ages as results from an earlier study, conducted at Mt Unbunmaroo, play a pivotal role in a proposed Cambrian inertial interchange true polar wander (IITPW) event. Remanent magnetizations within these carbonates were found to be variably developed with most specimens displaying two of the five isolated components. Component PF, for which goethite is the identified remanence carrier, is thought to reflect a chemical remanent magnetization of recent origin. Component TR, held by haematite, has a palaeomagnetic pole consistent with the Tertiary segment of Australia’s apparent polar wander path (APWP) and most probably was acquired as a consequence of prolonged weathering during this period. The A component has a palaeomagnetic pole at 54.7 ◦ S, 262.3 ◦ E( dp = 2.3 ◦ , dm = 4.5 ◦ ) after unfolding. This direction, constrained by positive fold and reversal test statistics, is consistent with Australia’s Early Devonian APWP, perhaps reflecting a remagnetization event associated with the intracratonic Alice Springs Orogeny. A Late Ordovician‐Early Silurian remanence, component B, is also described; with 100 per cent unfolding the associated palaeopole lies at 8.0 ◦ S, 216.8 ◦ E( dp = 2.6 ◦ , dm = 5.1 ◦ ). A third Palaeozoic, and presumed primary or early diagenetic, component, C, also passes applied fold and reversal tests and has a palaeomagnetic pole at 48.6 ◦ N, 186.0 ◦ E( dp= 2.1 ◦ S, dm = 4.0 ◦ ). This palaeopole is dissimilar from younger magnetizations, is consistent with Cambrian poles from other parts of cratonic Australia and falls within a cluster of Middle‐Late Cambrian (515‐500 Ma) palaeopoles from other Gondwanan continents. The age attributed to the palaeopole associated with the C component, ∼510 Ma, provides a tight constraint on the younger boundary of the proposed Cambrian IITPW event and its agreement with other Gondwanan palaeopoles is incompatible with the IITPW hypothesis. Components A, B and C are analogous to palaeomagnetic results reported in the earlier investigation of this region, and a comparison of results from the two studies, coupled with rigorous statistical analyses of the new findings, is presented.