羌北地块早-中志留世古地磁学初步研究结果及其构造意义

本文报道羌北地块早-中志留世龙木措组灰岩古地磁研究初步结果,据此探讨羌北地块早-中志留世的古位置及其起源问题.在日土县龙木措北岸(34.4°N,80.3°E)龙木措组中采集了10个采点,共125块古地磁独立定向样品.岩相学与岩石磁学结果表明样品中主要载磁矿物为磁铁矿.系统热退磁与交变退磁显示,退磁曲线具有双分量或单分量...     

Magnetostratigraphy of the Miocene Las Arcas Formation,Santa María Valley,northwestern Argentina

The first magnetostratigraphic study of the Las Arcas Formation (Late Miocene) was carried out in Las Totoritas creek (26º12′S; 65º47′W, NW Argentina), a key place in between of two geological provinces: Northwestern Pampean Ranges and Eastern Cordillera, in northwestern Argentina. This was accompanied by isotopic dating (9.01 ± 0.12 Ma, ⁴⁰Ar–³⁹Ar in amphibole) of the unit, obtained from a 3.4 m thick tuff intercalated at ∼45 m above the base. The Las Arcas Formation is 810 m thick at the sampling locality and it is mainly composed of tabular reddish conglomerates, sandstones and siltstones in both coarsening- and thickening-upward arrangements. The exposed section was sampled at 48 sites, 26 of which are interpreted as carrying primary magnetization. The new magnetostratigraphic column was correlated with the Geomagnetic Polarity Time Scale (GPTS), and suggests that deposition of the Las Arcas Formation strata started at around 9.1 Ma and ended around 6.8 Ma. The paleomagnetic pole obtained for this unit (Dec = 8.7° Inc = −43.9° dp = 14.9 dm 9.3) indicates that this area underwent non-significant rotation (11.0° ± 13.6°) since the Late Miocene.     

Palaeomagnetic and structural constraints on 90° anticlockwise rotation in SW Mongolia during the Permo–Triassic: Implications for Altaid oroclinal bending. Preliminary palaeomagnetic results

New and published paleomagnetic measurements from Trans Altai and South Gobi zones in south Mongolia document large tectonic motions in between Late Carboniferous and Triassic. Magnetic inclinations confirm equatorial position of south Mongolian terranes in Late Carboniferous–Permian times. The evolution of magnetic declinations indicates 90° anticlockwise rotation in between latest Carboniferous and Early Triassic of all studied tectonic units around the Eulerian pole located close to axis of Mongolian orocline. The anticlockwise rotation continues in Triassic being accompanied by a major drift to the north. The structural and published geochronological data suggest Carboniferous E–W shortening of the whole region resulting in N–S trend of all continental and oceanic geological units followed by orthogonal N–S shortening during Late Permian to Early Jurassic. Both paleomagnetic and geological data converge in a tectonic model of oroclinal bending of Mongolian ribbon continent, westerly back arc oceanic domain and Mongol–Okhotsk subduction zone to the east. The oroclinal bending model is consistent with the coincidence of the Eulerian pole of rotation with the structural axis of Mongolian orocline. In addition, the Mesozoic collisional tectonics is reflected by late remagnetizations due to formation of wide deformation fronts and hydrothermal activity.     

New paleomagnetic and paleointensity data from Pliocene lava flows from the Lesser Caucasus

A paleomagnetic, rock-magnetic and paleointensity study has been carried out on 14 basaltic lava flows from two Pliocene (K–Ar age between 3.09 ± 0.10 Ma and 4.00 ± 0.15 Ma) sequences (Apnia and Korxi) from the eastern Djhavakheti Highland in southern Georgia (Caucasus).Measurement of strong-field magnetisation versus temperature curves yielded three types of thermomagnetic curves: (i) Reversible curves with magnetite as only remanence carrier (type H); (ii) irreversible curves with magnetite as only carrier of remanence (type H⁻) and (iii) irreversible curves showing a low Curie-temperature phase and magnetite (type L). Analysis of hysteresis curves showed that samples were characterised by a mixture of single-domain and multi-domain grains.Paleomagnetic experiments allowed determining characteristic components for all flows and normal polarities (6 flows), reversed polarities (7 flows) and intermediate polarities (1 flow) were observed.. Paleomagnetic poles were calculated using only those sites unequivocally showing normal or reversed polarities. The paleomagnetic pole obtained from flows of both combined sequences (latitude λ = 77.9°N, longitude ϕ = 152.1°E, n = 13, A₉₅ = 11.8°, k = 13.4) showed a good agreement with the 5 Ma window of the European synthetic apparent polar wander path of Besse and Courtillot (2002). The paleomagnetic direction of the combined Apnia-Korxi flows agrees well with the expected one, showing no significant tectonic rotation. The latter cannot be however, completely excluded in the Korxi section. In that section, analysis of the angular dispersion of virtual geomagnetic poles yields a much higher value than expected.Paleointensity experiments using the Coe method were performed on 31 specimens from 10 flows. After application of specific selection criteria, 19 samples from 8 flows were observed to provide successful determinations, with mean flow values showing a wide scatter. If only flows with more than one successful paleointensity determination are taken into account, virtual dipole moments (VDMs) vary between 3.5 × 10²² A m² and 8.3 × 10²² A m². In intermediate polarity site AP2 no weak transitional paleostrength values were observed.     

晚古生代冈瓦纳的演化长逾13500 km的基墨利超级地体的裂离及其向亚洲的漂移

The Cimmerian terrane forms an almost unbroken chain stretching >13,500 km, from central southern Europe to western Indonesia, via SE Europe, the Middle East, Afghanistan, Tibet, SW China and Myanmar. Ar-guably, it is Earth’s most spectacular example of a “sliver” terrane, dwarfing in size more recently devel-oped examples, for instance the Palawan Block in the western Philippines, and the Lord Howe Rise in the Tasman Sea. The presentation will first outline the in-triguing geological features associated with this unique tectonic entity. Following that, recently obtained results following paleomagnetic investigations of two lower Permian rift-related basalt suites will be summarized (Abor Volcanics in northeastern India and Woniusi Ba-salts in Yunnan, China). The two studies are part of a larger programme of ongoing research aimed at deducing (I) the geodynamic configuration that generated the un-usual rifting system, and (II) exactly how Cimmeria fit-ted against Gondwana prior to its dispersal in the Early Permian. The critical unit is Baoshan, which we fit against Gondwana within a narrow longitudinal belt close to where northern India and northwestern Australia were once in close proximity (Fig. 1). Furthermore, we suggest that Sibumasu lay to directly the east, offshore of Australia; Qiangtang and Lhasa almost certainly sat to the west (off northern Greater India-SE Arabia), but we are uncertain as to their exact configuration. Our findings will be compared with several rather different models that have been published in recent years. The new pa-leomagnetic constraint highlights the flexibility authors currently have in reconstructing the region, principally because of the overall lack of similar high-quality data from the various blocks. We explain how new data could resolve these ambiguities, thereby offering more robust explanations for eastern Gondwana’s late Paleozoic de-velopment.     

New paleomagnetic data on the Anabar Uplift and the Uchur-Maya region and their implications for the paleogeography and geological correlation of the Riphean of the Siberian Platform

The results of paleomagnetic studies of the reference sections of the Riphean and Late Proterozoic intrusive bodies of two remote areas of the Siberian Platform are presented. Within the limits of the Uchur-Maya region the sedimentary rocks of the Gonam, Omakhta, Ennin and Konder formations were studied; and the Riphean sedimentary rocks of the Burdur and Kotuykan formations on the northern and western slopes of the Anabar Uplift and, also, the Late Proterozoic intrusive complexes, located in the basins of the Fomich, Magan, Dzhogdzho and Kotuykan Rivers were studied. The paleomagnetic poles obtained in the course of this work and the present-day geochronological data give grounds to assume that: (1) the accumulation of the Riphean of the Anabar Uplift occurred after the formation of the Uchurskaya series of the Uchur-Maya region and was completed in approximately 1.5 Ga; (2) the Konder layers, compared according to the correlation pattern accepted at the present time [Semikhatov and Serebryakov, 1983] with the bottoms of the Totta formation, can be related to the appreciably more ancient stratigraphic level; (3) the intrusion of the studied intrusive bodies of the northern and western slopes of the Anabar Uplift occurred nonsimultaneously, although within close time intervals of approximately 1.5 Ga. The estimates of the kinematic parameters of the drift of the Siberian Platform within an interval of 1.7–1.0 Ga is carried out.     

扬子褶皱带侏罗纪砂岩古地磁及其褶皱带弧形弯曲的成因

为了更好地认识上扬子褶皱带和中扬子褶皱带走向差异的机制,我们对中、上扬子褶皱带过渡的关键地区重庆市万州和云阳两个地区的中、晚侏罗世砂岩进行了古地磁研究.逐步热退磁分离出两个组分,低温组分（LTC）在所有样品中均分离出来,为现代地磁场的重磁化;中侏罗世样品和万州地区的晚侏罗世样品分离出来的高温组分（HTC）也为现代地磁场的重磁化.云阳地区晚侏罗世样品分离出来的高温组分通过逐步展平褶皱检验显示：在褶皱展平至33.8%时,精度参数达到最大,相应的古地磁方向为D=19.1°,I=48.9°（α₉₅= 6.3°）,古地磁极为73.5°N,198.2°E（dp=5.5°, dm=8.3°）,与白垩纪参考古地磁极对比,此高温组分揭示云阳地区在褶皱变形的后期经历了7.7°±6.1°的顺时针旋转.结合前人的数据,我们认为中扬子褶皱带普遍存在弯山构造（orocline）,这可能与华北板块向华南板块的挤入作用有关;但是中、上扬子褶皱带过渡地区的弧形弯曲总体上不是由弯山构造形成的,很可能是在太平洋板块向北西方向俯冲的宏观板块构造背景下的应变分异作用形成的.     

Magnetostratigraphy and paleoenvironmental significance of sediments from ANT29-P7-09 core in Prydz Bay,Antarctica

Due to the unique geographical location and sensitive response to global climate changes, the Antarctic region plays an important role in paleoclimate researches, and attracts great attentions from various scholars. One 324 cm long sediment core (ANT29-P7-09) was obtained from Prydz Bay, Antarctica, during the 29th Chinese National Antarctic Research Expedition. Based on sediment particle size, TOC, δ¹³C analyses and magnetism data, the authors show that the dominant magnetic minerals are ferrimagnetic pseudo single domain (PSD)-multi domain (MD) magnetite. Variations in the paleoenvironmental records allow us to define 4 zones in the core. These zones outline the climatic variations in the region since the late Early Pleistocene, including a warm period, a transitional period, and a cold period. The magnetic particle assemblage varies with glacial-interglacial cycles. Abrupt changes in particle size, TOC content, and geomagnetism occur at 102–90 cm deep in the core, indicating a sudden warming in the Antarctic region, signaling the onset of the Holocene. The authors identified 3 additional climatic signals in the middle part of the core (232–162 cm) that show unexpected cooling events during the warm period in Prydz Bay, Antarctica.     

The northern Qiangtang Block rapid drift during the Triassic Period: Paleomagnetic evidence

As one of the pivotal Gondwana–derived blocks, the kinematic history of the northern Qiangtang Block (in the Tibetan Plateau) remains unclear, mainly because quantitative paleomagnetic data to determine the paleoposition are sparse. Thus, for this study, we collected 226 samples (17 sites) from Triassic sedimentary rocks in the Raggyorcaka and Tuotuohe areas of the northern Qiangtang Block (NQB). Stepwise demagnetization isolated high temperature/field components from the samples. Both Early and Late Triassic datasets passed field tests at a 99% confidence level and were proved to be primary origins. Paleopoles were calculated to be at 24.9°N and 216.5°E with A₉₅ = 8.2°(N = 8) for the Early Triassic dataset, and at 68.1°N, 179.9°E with A₉₅ = 5.6° (N = 37) for the Late Triassic, the latter being combined with a coeval volcanic dataset published previously. These paleopoles correspond to paleolatitudes of 14.3°S±8.2° and 29.9°N±5.6°, respectively. Combining previously published results, we reconstructed a three-stage northward drift process for the NQB. (1) The northern Qiangtang Block was located in the subtropical part of the southern hemisphere until the Early Triassic; (2) thereafter, the block rapidly drifted northward from southern to northern hemispheres during the Triassic; and (3) the block converged with the Eurasian continent in the Late Triassic. The ∼4800 km northward movement from the Early to Late Triassic corresponded to an average motion rate of ∼11.85 cm/yr. The rapid drift of the NQB after the Early Triassic led to a rapid transformation of the Tethys Ocean.