塔里木盆地古生代古地磁结果及其构造地质意义

发表了塔里木盆地奥陶纪、志留纪、泥盆纪、石炭纪、二叠纪古地磁新数据，大部分数据通过了剩磁分离或一致性检验、倒转检验、极性对比检验、褶皱检验和烘烤检验．根据新的数据绘制了塔里木板块古生代视磁极游移曲线（ＡＰＷＰ），探讨了塔里木板块构造演化规律，强调了奥陶纪末的加里东构造运动及早海西构造运动的重要意义．     

塔里木陆块在Rodinia超大陆中位置的新认识——来自地层对比和古地磁的制约

塔里木周缘的新元古代地层中均记录了涉及Rodinia聚合和裂解的构造热事件,但塔里木在Rodinia超大陆中位置尚存争议.本文综合地层对比以及古地磁的研究方法,将塔里木陆块在Rodinia超大陆中置于澳洲板块的西北缘,并且塔里木的西南缘(现今位置)和澳洲的西北缘(现今位置)相连.基于塔里木周缘的构造热事件和塔里木、澳洲运动学特征分析,认为塔里木陆块周缘在约800~700 Ma中发生了强烈的裂谷事件,导致塔里木从Rodinia超大陆中裂解,但塔里木并没有完全从澳洲裂离,而是随澳洲一起,加入冈瓦纳大陆.在约450 Ma左右,塔里木与澳洲发生分离,其原因为古特提斯洋的扩张.     

Siberian paleomagnetic data and the problem of rigidity of the Northern Eurasian continent in the post-Paleozoic

The Meso-Cenozoic paleomagnetic poles from the Siberian platform and its folded margins, which comply with the modern technical and methodological standards, are analyzed. The analysis suggests the following conclusions. (1) The geometrical relationship between the Permo-Triassic poles of the Stable Europe and Siberian Platform prohibits the possibility of relative displacements of these platforms in the post-Paleozoic time. (2) The Mesozoic paleomagnetic poles of the Siberian Platform support the hypothesis of rigid Northern Eurasia. (3) The paleolatitudes of the Mesozoic sections located on the folded margins of the Siberian Platform closely agree with the Apparent Polar Wandering Path (APWP) for Europe. (4) The available data indicate that the vertical-axis rotation of separate local blocks within the folded margins of the Siberian Platform was a widespread phenomenon. Therefore, (1) the modern paleomagnetic data quite certainly show that consolidation of the Northern Eurasian continent was completed by the end of Permian, and, since the very beginning of the Mesozoic, the Siberian and East-European platforms have been parts of a single rigid megablock. (2) The Meso-Cenozoic segment of the APWP for Europe can be used as reference for the Siberian platform.     

Evolution of Pangea: paleomagnetic constraints from the Southern Alps, Italy

A new early Late Triassic paleopole for Adria has been obtained from the Val Sabbia Sandstone in the Southern Alps. As Early Permian and Jurassic-Cretaceous paleomagnetic data from para-autochthonous regions of Adria such as the Southern Alps are consistent with ‘African’ APWPs[1–2], paleomagnetic data from this region can be used to bolster the West Gondwana APWP in the poorly known Late Permian-Triassic time interval. The Southern Alpine paleopoles are integrated with the West Gondwana and Laurussia APWPs of Van der Voo [1] and used to generate a tectonic model for the evolution of Pangea. The Early Permian overall mean paleopole for West Gondwana and Adria, in conjunction with the coeval Laurussia paleopole, support Pangea B of Morel and Irving [3]. The Late Permian/Early Triassic and the Middle/Late Triassic paleopoles from Adria and Laurussia support Pangea A-2 of Van der Voo and French [4]. The phase of transcurrent motion between Laurasia and Gondwana[5] that caused the Pangea B to A-2 transition occurred essentially in the Permian (at the end of Variscan orogeny) with an average relative velocity of approximately 10 cm/yr. Finally, the Late Triassic/Early Jurassic paleopoles from West Gondwana and Laurussia agree with Pangea A-1 of Bullard et al. [6], the widely accepted Pangea configuration at the time of the Jurassic breakup.     

塔里木盆地石炭-二叠纪异常热演化及其对深部构造-岩浆活动的指示

本文分析了塔里木盆地大量实测镜质体反射率(Ro)数据,显示盆地西北部石炭系与二叠系之间Ro值演化不连续,记录了石炭-二叠纪间发生的构造-热事件,是地层抬升剥蚀和高温热事件共同作用的结果.热史模拟得出盆地在石炭纪末地温梯度开始升高,至~300 Ma达到峰值,分布在4.8~5.6℃/100m之间,早二叠世迅速降低,随后进入缓慢稳定降低阶段.区域上高温热效应空间分布与塔里木大火成岩省的分布范围存在较好的相关性,但早于大规模玄武岩喷发的时间(290—288Ma),且后者热效应范围有限.因此推断这种高温异常是大规模的深部岩浆活动,即深部岩浆房相对长时间热烘烤的结果.该研究结果为塔里木盆地热演化机制及相关热效应提供了新的线索.     

早二叠世塔里木陆块的经度：来自大火成岩省的制约

经度的确定是板块重建的难点. 塔里木盆地下二叠统大规模溢流玄武岩已被确定为大火成岩省, 提供了一次根据大火成岩省来定量确定塔里木陆块早二叠世经度的机遇. 核幔边界约2800 km深处地震波低速带与全球300 Ma以来喷发的大火成岩省之间的关系已得到建立: 恢复喷发位置后, 大火成岩省全部分布在核幔边界低速带的边缘之上, 其中大部分在非洲LLSVP和太平洋LLSVP边缘, 个别在规模较小的LSVPs边缘. 在使用塔里木陆块早二叠世古地磁数据来限定其纬度的基础上, 本文利用上述理论方法, 并联系前人的地质结论, 发现塔里木大火成岩省约290 Ma喷发时的位置最可能为20°N, 60°E. 本文提出, 塔里木大火成岩省与西伯利亚大火成岩省相似, 其喷发时并不在两大LLSVPs的边缘带上, 而最可能与非洲LLSVP东侧附近一个单独的、范围较小的LSVP(20°N, 60°E)相关联, 暗示重建之前的假设"塔里木大火成岩省源自核幔边界"是合理的. 如果塔里木、峨眉山和西伯利亚大火成岩省都源自核幔边界, 上述(20°N, 60°E)位置的获得说明三者都不是同一幔源.     

新疆塔里木地台晚古生代古地磁极移曲线及其地质构造含义

用热退磁辅以交变退磁方法对采自塔里木盆地阿克苏地区四石厂剖面47个采样点518块标本进行了逐步磁清洗和测试。由本征剩磁方向统计得到塔里木地台晚古生代的古地磁极位置(晚泥盆世φ=10.5°S、λ=151.2°E;晚石炭世φ=52.2°N、λ=179.5°E;早二叠世φ=56.5°N,λ=190.1°E)。古地磁结果表明:塔里木地台在晚古生代是北方大陆的块体之一。从晚石炭世至早二叠世塔里木地台已和北方的哈萨克斯坦板块、西伯利亚地台、俄罗斯地台等连成一片,并且从中生代以来它们之间的相对位置没有发生过大规模的变动     

Better constrained selection of the Paleozoic West Gondwana (South America) paleomagnetic poles for the APWP determination

The reliability of an Apparent Polar Wander Path (APWP) obviously depends on the paleomagnetic poles used to determine it. The APWP of Africa and South America are fairly well defined for the 330–260 Ma interval. However, this study pointed out a moderate shift between these two curves, and an incoherency of the South American data, contrary to the African ones, which are homogeneous. A number of South American pole positions were re-evaluated in an effort to better constrain the APWP for the entire continent. Most of discarded poles correspond to sites at the area of the junction of Cordillera with the stable craton. That could have structural implications for the evolution of the western margin of the Gondwana. A new criterion for the evaluation of paleomagnetic poles reliability for APWP is presented. Based on comparison of data from different continents and labeled “coherence” criterion, it is independent from Van der Voo’s ones.     

Paleomagnetic direction obtained by strain removal in the Pyrenean Permian redbeds at the “Col du Somport” (France)

In order to investigate the possibility and limitations of paleomagnetic works within strained regions, a paleomagnetic study, related with strain analysis has been conducted in the deformed Pyrenean Permian redbeds in the “Col du Somport” area. Paleomagnetic sampling together with strain estimates have been conducted in 6 sites through a fold. The results obtained by measuring the orientation and axial ratios of elliptical reduction spots show that (1) the shale beds have undergone a penetrative strain, (2) the sandy beds can be regarded as tectonically unstrained with reduction spots flattened in the bedding, showing that they recorded the compaction. It is shown that the total strain recorded in the slaty beds probably results from the superimposition of tectonic strain upon the compaction fabric. The paleomagnetic study shows that the primary pretectonic magnetization is widely overprinted by a secondary syn- or post-tectonic magnetic component. As both components appear to be carried by hematite pigment, their separation using classical demagnetization procedures has been difficult. A characteristic remanent magnetization (ChRM) has however been determined, when possible, as the hardest component in demagnetization curves. Then, the ChRM direction distributions are represented in stereographic density plots. Although these ChRM directions exhibit a clear tendency towards SE declinations and shallow inclinations, characteristic of Permian paleomagnetic field direction for the Iberian plate, the tilt correction does not induce a clustering of these directions. Strain is inferred to be responsible for this situation. Assuming that both pretectonic magnetization directions and bedding planes closely follow the material plane and line strain response model of March [1], an attempt has been made to remove the effect of strain upon the remanent magnetization. It is shown that when using a reconstructed tectonic strain tensor (i.e., the total strain tensor as measured in the field, corrected for an estimated compaction) we obtain a significant clustering of ChRM directions. The computation of the relevant VGP, gives a pole position (210.5°E, 42.0°N) compatible with the reference APWP for the Iberian plate. It is therefore inferred that the strain removal technique is a usable tool in order to obtain paleomagnetic results within such strained rocks.     

Paleomagnetism of carboniferous formations of the Russian platform: Reinvestigation of old collections

The collections of Carboniferous rocks from sections of the Russian Platform (Gzhelian, Moscovian, Bashkirian, and Visean stages) are studied. The new mean paleomagnetic poles are obtained from the Gzhelian, Moscovian, and Visean layers of the Carboniferous of the Russian Platform. In the redbed Gzhelian and Moscovian rocks, the natural remanent magnetization (NRM) components with the inclination shallowing are revealed, which is due to the presence of the large hematite particles or particle aggregates associated with the interaction between the magnetic and clay particles. Based on the obtained determinations and the results contained in the World paleomagnetic database, the trajectory of the apparent polar wander path (APWP) for the East European Platform is constructed in the interval from the Devonian to Early Permian. The Carboniferous kinematics of the East European Platform is estimated.     

Paleomagnetism of the Lower Ordovician and Cambrian sedimentary rocks in the section of the Narva River right bank: For the construction of the Baltic Kinematic model in the Early Paleozoic

The paleomagnetic study of the Lower Ordovician and Cambrian sedimentary rocks exposed on the Narva River’s right bank revealed a multicomponent composition of natural remanent magnetization. Among four distinguished medium- and high-temperature magnetization components, the bipolar component, which carries the reversal test, is probably the primary component and reflects the geomagnetic field direction and variations during the Late Cambrian and Early Ordovician. The pole positions corresponding to this component have coordinates 22°N, 87°E (dp/dm = 5°/6°) for the Late Cambrian, and 18°N, 55°E (dp/dm = 5°/7°) for the Early Ordovician (Tremadocian and Arenigian). Together with the recently published paleomagnetic poles for the sections of the Early Ordovician in the Leningrad Region and the series of poles obtained when the Ordovician limestones were studied in Sweden, these poles form new key frameworks for the Upper Cambrian-Middle Ordovician segment of the apparent polar-wander path (APWP) for the Baltica. Based on these data, we propose a renewed version of the APWP segment: the model of the Baltica motion as its clockwise turn by 68° around the remote Euler pole. This motion around the great circle describes (with an error of A₉₅ = 10°) both variations in the Baltic position from 500 to 456 Ma ago in paleolatitude and its turn relative to paleomeridians. According to the monopolar components of natural remanent magnetization detected in the Narva rocks, the South Pole positions are 2°S, 351°E (dp/dm = 5°/9°), 39°S, 327°E, (dp/dm = 4°/7°), and 42°S and 311°E (dp/dm = 9°/13°). It is assumed that these components reflect regional remagnetization events in the Silurian, Late Permian, and Triassic.     

The Devonian - Permian APWP for the West Sudetes,Poland

Studia Geophysica et Geodaetica - The paper presents the Apparent Polar Wander Path (APWP) for the West Sudetes for the period from Devonian to Carboniferous. The data used for construction of the...     

Marine magnetic anomalies,palaeomagnetism and the drift history of Gondwanaland

The results of palaeomagnetic surveys of Mesozoic and Tertiary rocks from Gondwanaland can be reconciled with the results of modelling the evolution of oceanic floors from analyses of marine magnetic anomalies. Previous inconsistencies were mainly due to errors apparent in the Australian Cenozoic palaeomagnetic data. An alternative Tertiary apparent polar wander path (APWP) has been constructed from an analysis of all published laterite and weathered profile data. Palaeomagnetic results for Africa, Antarctica, Australia, India, Madagascar and South America are compared for rotations corresponding to marine magnetic anomalies 16, 22, 28, 34 and M1 and for “fit”. India has been selected as the reference continent since it provides the most detailed APWP having drifted about 50° of latitude since breakup.     

Density Structure,Isostatic Balance and Tectonic Models of the Central Tien Shan

A new combined satellite-terrestrial model of the gravity field is used together with seismic data for construction of a density model of the lithosphere of the Central Tien Shan and for estimation of its isostatic balance. The Tien Shan is one of the most active intraplate orogens in the world, located about 1,500 km north of the convergence between Indian and Eurasian plate, and surrounded by stable Kazakh platform to the north and the Tarim block to the south. Although this area was extensively studied during recent decades, several principal problems, related to its structure and tectonics, remain unsolved up to now: (1) various geodynamic scenarios have been discussed so far to explain tectonic evolution, such as direct “crustal shortening,” intracontinental subduction and some others, but no definite evidence for any of them has been found. (2) Still, it is not clear why Tien Shan grows so far from the plate boundary at the Himalayan collision zone. Gravity modeling can provide valuable constraints to resolve these questions. The results of this study show that: (1) there exists a very strong deflection of the Tien Shan lithosphere from isostatic equilibrium. At the same time, the patterns of the isostatic anomalies are very different in the Western and Central Tien Shan. The latter one is characterized by much stronger variations. The best fit of the modeling results is found for the model according to which the Tarim plate partially underthrusts the Central Tien Shan; (2) negative density anomalies in the upper mantle under the central block possibly relate to magmatic underplating during the initial stage of the tectonic evolution. Therefore, the weak lithosphere could be the factor that initiates mountain building far away from the collision zone. Alternatively, this might be a gap after detachment of the eclogised lower crust and lithospheric lid, which is filled with the hot asthenospheric material.     

Spatial variations of flexure parameters over the Tibet–Quinghai plateau

We investigate the Tibet–Quinghai plateau and the Tarim basin in terms of spatial variations of the elastic thickness (T_e) in the frame of the thin plate flexure model. The method of investigation makes use of a convolutive method, which allows high spatial resolution of the flexure properties and overcomes some of the problems tied to the spectral admittance/coherence methodologies. We study the relation between the topographic and subsurface loads and the observed crust–mantle interface (CMI) undulations, the latter having been obtained from gravity inversion. The gravity data used for the inversion are a unique set of high quality data available over the Chinese part of the plateau, and constitute the highest resolution grid today available in this impervious area. The gravity inversion is constrained by results from the study of the propagation of seismic waves. The two extensive sedimentary basins, the Tarim and the Qaidam basins, are modeled by forward gravity modeling. The oscillations of the CMI obtained from the gravity inversion agree well with those expected by loading the thin plate model of spatially variably elastic thickness with the surface and subsurface loads. It is found that the modeling of the sedimentary basins is essential in the flexure analysis. The spatial variations of elastic thickness correlate with the extensions of the different terrains that constitute the plateau. Most of the Tibet plateau has low T_e, varying in the bounds 10–30 km, with lower values in the Qiangtang terrain, where the T_e reaches 8 km. The Tarim and the Qaidam basins, Precambrian platforms overlain by sediments, are rigid and have a T_e of up to 110 km and 70 km, respectively. The flexural analysis distinctly discerns the Tibet plateau, with thick crust, part of which is molten, from the cratonic areas, the Tarim and Qaidam basins, which though of thinner crust, act as undeformable rigid blocks.     

Palaeomagnetic constraints on formation of the Mianwali reentrant, Trans-Indus and western Salt Range, Pakistan

Successions of Lower to lower Middle Cambrian, Upper Permian to Upper Triassic and Lower Tertiary carbonates and arenites have been sampled in five sections, representative of the three main segments of the Mianwali reentrant in the (Trans-Indus) Salt Range (northern Pakistan), i.e.: the southern Khisor Range, the northern Surghar Range and the western Salt Range. Comparison of primary and secondary magnetization directions with the Indian APWP demonstrates the secondary origin of the Mianwali reentrant and shows a pattern of rotations which varies in sense and magnitude along the reentrant with the main structural trends. Data from the Trans-Indus and western Salt Range and published Early Cambrian, Early Permian and Late Tertiary palaeomagnetic results from the southern Salt Range and the Potwar Plateau show that the Hazara Arc underwent a 20–45° counterclockwise rotation relative to the Indian Shield. A contrasting clockwise rotation over about 45° has recently been established for thrust sheets in the opposing eastern limb of the Western Himalayan Syntaxis, i.e. for the Panjal Nappe [1] and the Riasi thrust sheet [2]. These palaeomagnetically established rotations conform with the about 75° azimuthal change in structural trend along the Syntaxis, and support Crawford's [3] suggestion that the Salt Range was originally in line with the northwestern Himalaya. The Salt Range front prograded and moved southwards as part of the Hazara Arc thrust sheet, detached from basement along the evaporitic Salt Range Formation. The Mianwali reentrant originated through obstruction of the southwards advancing thrust sheet by moulding around basement topography of the northwest oriented Sarghoda Ridge.     

Paleomagnetic constraints on the tectonic history of the major blocks of China duing the Phanerozoic

Paleomagnetic study of China and its constraints on Asia tectonics has been a hot spot. Some new paleomagnetic data from three major blocks of China. North China Block (NCB), Yangtze Block (YZB) and Tarim Block (TRM) are first reported, and then available published Phanerozoic paleomagnetic poles from these blocks with the goal of placing constraints on the drift history and paleocontinental reconstruction are critically reviewed. It was found that all three major blocks were located at the mid-low latitude in the Southern Hemisphere during the Early Paleozoic. The NCB was probably independent in terms of dynamics. its drift history was dominant by latitudinal placement accompanying rotation in the Early Paleozoic. The YZB was close to Gondwanaland in Cambrian, and separated from Gondwanaland during the Late-Middle Ordovician. The TRM was part of Gondwanaland, and might be close to the YZB and Australia in the Early Paleozoic. Paleomagnetic data show that the TRM was separated from Gondwanaland during the Late-Middle Ordovician, and then drifted northward. The TRM was sutured to Siberia and Kazakstan blocks during the Permian, however, the composite Mongolia-NCB block did not collide with Siberia till Late Jurassic. During Late Permian to Late Triassic, the NCB and YZB were characterized by northern latitudinal placement and rotation on the pivot in the Dabie area. The NCB and YZB collided first in the eastern part where they were located at northern latitude of about 6°—8°, and a triangular oceanic basin remained in the Late Permian. The suturing zone was located at northern latitude of 25° where the two blocks collided at the western part in the Late Triassic. The collision between the two blocks propagated westward after the YZB rotated about 70° relative to the NCB during the Late Permian to Middle Jurassic. Then two blocks were northward drifting (about 5°) together with relative rotating and crust shortening. It was such scissors-like collision procedure that produced intensive compression in the eastern part of suturing zone between the NCB and YZB, in which continental crust subducted into the upper mantle in the Late Permian, and then the ultrahigh-pressure rocks extruded in the Late Triassic. Paleomagnetic data also indicate that three major blocks have been together clockwise rotating about 20° relative to present-day rotation axis since the Late Jurassic. It was proposed that Lahsa Block and India subcontinent successively northward subducted and collided with Eurasia or collision between Pacific/Philippines plates and Eurasia might be responsible for this clockwise rotating of Chinese continent.     

华北蓟县中、上元古界古地磁研究

本文论述了华北蓟县中、上元古界地层标准剖面古地磁采样及实验室技术,采样地层的磁性特征,多磁成分的分析和测试结果.主要讨论了蓟县中、上元古界地层所代表的古地磁极性、极移路径和古纬度,并与北美大陆及华南(扬子)地块该时期的极移路径进行对比.     

塔里木盆地南部上地幔各向异性研究

塔里木南缘位于塔里木块体同青藏高原碰撞的前缘,是认识青藏高原同周围块体相互作用的重要位置.横波分裂方法可以获得岩石圈及软流圈地幔的介质各向异性特征,进而探讨岩石圈变形和地幔流动.本文利用横波分裂方法对中国科学院地质与地球物理研究所、北京大学和南方科技大学联合布设在塔里木盆地南部的8个宽频带流动地震台站记录的SKS和SK...