南极中山站-昆仑站间地壳厚度分布

自第4个国际极地年2007/2008开始至2013年,中国南极内陆冰盖科考队相继在自南极大陆边缘的中山站至东南极地形最高点昆仑站(Dome A)一线进行了低温甚宽频地震观测。本文对7个天然地震台站数据进行了分析,提取了这些台站的S波接收函数,据此反演获得了这些台站下的地壳厚度分布。结果显示:随着纬度的升高,地壳厚度由大陆边缘的中山站下的约38 km逐渐增加至CHNB台下的58 km,随后又于CHNA台站下方减薄至47 km,然后快速增大到南极地形最高点昆仑站(Dome A)下的62 km。昆仑站或Dome A是南极大陆地壳最厚的地方。从中山站至昆仑站之间地壳厚度的变化与冰下地貌变化存在明显的相关性,它们都说明了从中山站至CHNB之间地壳构造相对均匀。在距昆仑站约200 km的CHNA台下的地壳厚度(约47 km)明显比临近台站地壳偏薄,这可能说明了甘伯采夫山脉地壳侧向变化较大,即其形成时所遭受的构造作用较复杂。     

金沙江金江街段河流阶地年代及对河谷水系演化历史的启示

100多年来,关于金沙江独特水系格局的形成历史一直是地学界争论的重要话题之一。多数学者认为,现代金沙江水系是古长江袭夺古红河上游发展过来的。红河海底扇5.5 Ma泥沙供给中断被认为与这一袭夺事件有关。然而,长期以来人们一直没有找到与这一时代相匹配的地貌证据。最近在金沙江金江街段找到了多达8级的河流阶地序列,ESR测年结果显示这些阶地的形成年代为1.07 Ma、0.70 Ma、0.65 Ma、0.51 Ma、0.47 Ma、0.44 Ma、0.30 Ma和0.18 Ma,结合GPS高程测量数据,推算最近1.0 Ma以来的河谷平均下切速率为147 mm/ka。以填充河谷地形为主要手段的古地形恢复结果（基于DEM数据）显示,古长江袭夺古红河上游形成现代金沙江水系发生在这一区域内海拔2000 m左右的古地形面解体之后,依照河谷平均下切速率外推,古地形面解体时代为5.5 Ma,即现代金沙江水系形成于5.5 Ma之后。我们的研究结果与红河海底扇的资料形成一个相互呼应的证据链,为重建现代金沙江水系格局形成历史提供重要依据。     

希拉穆仁苏木一带中古太古界兴和岩群特征

希拉穆仁苏木古太古界兴和岩群为一套麻粒岩相的高级变质岩系;原岩恢复主要为中基性、中酸性火山岩夹沉积岩。其变质程度达麻粒岩相—高角闪岩相。1∶5万区域地质调查在该岩群二辉麻粒岩进行同位素测年,获得锆石U-Pb表面年龄值为3,049±1.4Ma。结合区域对比资料暂将其置于古太古代。     

南极绕极流的表层环流结构和能量分布特征

利用历史的表面漂流浮标资料研究了南极绕极流的平均结构、变化规律和能量特征。强劲的南极绕极流沿着纬向从西向东流动,并伴随着复杂多变的海洋锋面,主导着整个南大洋的海洋环流系统。首先,标准方差椭圆表明,阿加勒斯回流和东澳大利亚暖流是南极绕极流重要的流量补充。在南半球的冬季,南极绕极流的流速随着西风的增强而增强。在南极绕极流经向摆动的影响下,流核上的经向流速比纬向流速具有更加不稳定的特征。另外,南极绕极流从非洲南部开始自西向东逐渐减弱。其次,从能量的角度讨论了南极绕极流的细节特征。平均动能、扰动动能及其均方根在流核区域都很强,扰动动能和平均动能比值的趋势恰恰相反,强(弱)能量分布区域的涡旋耗散反而比较弱(强)。除了扰动动能和平均动能的比值之外,所有的能量形式都呈现了从西向东减弱的趋势。涡动耗散对南大洋北部的平均动能影响更大。     

南极冰盖地形数据库BEDMAP 2述评

南极冰盖物质收支与不稳定性对全球气候变化和海平面升高有着重要影响,而冰盖厚度和冰下地形则是研究南极冰盖的物质平衡、动力及不稳定性极为重要的参数。自20世纪50年代以来,国际上针对南极冰盖开展了大量的冰雷达以及重、磁测量,这些测量结果被汇集并形成冰厚和冰下地形数据库,进而服务于冰盖模式和地球系统研究,最新推出的成果便是BEDMAP 2(Bedrock Mapping Project 2)。首先介绍了BEDMAP 2的数据来源、结构以及数据处理,并讨论了数据的质量评价,然后分析了BEDMAP 2中展示的整个南极冰盖与冰下地形及其特点。最后,对于BEDMAP 2对中国在南极冰盖考察和研究方面的作用进行了一些讨论与展望。     

联合卫星重力和卫星测高确定南极绕极流

联合基于GRACE重力卫星观测资料恢复的重力场模型(EIGEN-GL04S1)和卫星测高推求的平均海面高模型(KMSS04)来构造南极绕极流区域的平均海面动力地形,并利用小波滤波方法去掉短波及噪声信号,进而推算大、中尺度的绕极流。与非卫星重力场模型、同化资料及海洋水文资料确定相应结果的验证分析表明:基于新的卫星重力场模型推算的南极绕极流区域的海面动力地形、PF、SAF和表层流场等都与海洋学结果相吻合,且局部特征更加清晰。表明卫-卫跟踪重力卫星计划确定的地球重力场模型较之以前存在的重力场模型在中长波部分精度有较大提高,从大地测量(从空间)角度来探测南极绕极流已达到较高的精度。     

近30年中国极地气象科学研究进展

南极和北极是地球上的气候敏感地区,也是研究全球变化的关键区域.中国的南极和北极实地科学考察研究,分别始于20世纪80年代和90年代.作为国家行为,到2010年底,已经组织了27次南极考察、4次北极北冰洋考察和7次北极陆地考察;在南北极建立了4个科学考察站、5个(中澳/中美)合作自动气象站;初步形成了以有人考察站、无人自...     

南极海冰和极涡指数的时空特征及相互关系

本文采用聚类分析方法将南极海冰距平划分成５个变化相似的区域。并计算了对应各区的南半球５００ｈＰａ极涡面积和强度指数。分析了海冰与极涡指数的时空特征及相互关系。结果表明，不同区域的海冰和极涡的气候特征和年际变化的差异十分显著。１区和４区的海冰变化最大。海冰范围和极涡指数在多数区都在２－２．５ａ和５－７ａ的振荡周期。仅发现１区海冰范围是扩张趋势，其他４个区都呈收缩趋势。南极平均海冰范围以１．６个纬度／     

基于改进NCC算法的东南极极记录冰川流速提取研究

南极冰川的流速在南极物质平衡研究中有着重要的作用。将SIFT算子的旋转参数引入传统的基于归一化互相关(NCC)的冰川流速提取方法,使其拥有旋转不变的特性。利用此方法,基于Landsat 8 OLI数据对东南极的极记录冰川流速进行了提取,并通过利用模拟影像和NASA发布的MEa SUREs冰川流速结果对该方法的精度进行了验证。结果表明,改进的NCC算法能够在冰川流速方向改变区域提取更多的流速信息,极记录冰川流速最大可达870m·a~(-1),提取结果与MEa SUREs结果相比,平均偏差为23.9 m·a~(-1)。     

南极中山站附近海域水深地形特征

南极普里兹湾是我国南极中山站建站的重要依托海域 ,是科考船前往中山站进出的必由之路。本文根据普里兹湾近岸的中山站附近海域概略水深 ,研究分析了地形基本概况 ,崎岖不平的中山站近海湾部水下地形 ,沿其湾顶部轴线SN向形成了一条绵延弯曲的水深达 80 0m以浅的海底峡谷 ,通往冰盖底部。峡谷两侧地形复杂 ,水下丘岭嶙峋。从冰盖沿峡谷崩裂下来的冰山 ,最终依其适宜的水深 ,绕过地形障碍 ,沿海底峡谷西侧随风和流往西离去。除中山锚地抛锚区域属于浅水低洼的平缓台地外 ,湾内其它水域可供科考船抛锚的优越水下地形极少     

Palaeomagnetism of the continental sector of the Cameroon Volcanic Line, West Africa

Measurement of samples from 154 sites in the continental sector of the Cameroon Volcanic Line yielded six palaeomagnetic poles, at 243.6°E, 84.6°N, α ₉₅ = 6.8°; 224.3°E, 81.2°N, α ₉₅ = 8.4°; 176.1°E, 82.0°N, α ₉₅ = 8.5°; 164.3°E, 86.4°N, α ₉₅ = 3.4°; 169.4°E, 82.6°N, α ₉₅ = 4.6° and 174.7°E, 72.8°N, α ₉₅ = 9.5°, belonging to rocks which have been dated by the K–Ar method at 0.4–0.9  Ma, 2.6  Ma, 6.5–11  Ma, 12–17  Ma, 20–24  Ma and 28–31  Ma, respectively. The results are in general agreement with other palaeomagnetic poles from Oligocene to Recent formations in Africa.
  The first three poles for rocks formed between 0.4 and 11  Ma are not significantly different from the present geographical pole. Together with other African poles for the same period, this suggests that the African continent has moved very little relative to the pole since 11  Ma. The other three poles for rocks dated between 12 and 31  Ma are significantly different from the present geographical pole, showing a 5° polar deviation from the present pole in the Miocene and 13° in the Middle Oligocene.     

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.     

Palaeomagnetism and K-Ar Ages of the South-west African Basalts and their Bearing on the Time of Initial Rifting of the South Atlantic Ocean

Summary. Palaeomagnetic and isotopic results from the Kaoko lavas, Hoachanas basalts and dolerite sills of South-West Africa indicate that the Upper Triassic-Lower Jurassic Stormberg flows of South Africa may have extended into SW-Africa and that younger igneous events of Lower Cretaceous age were simultaneous with the Serra Geral volcanism in Brazil. Five analyses on three samples of the Keetmanshoop sills gave K-Ar ages between 178 ± 4 and 199 ± 4 Ma, four analyses of two samples of the Hoachanas basalts gave ages between 161 ± 3 and 173 ± 2 Ma and eight analyses of five samples of Kaoko basalt gave ages between 110±4 and 128 ± 2 Ma.
The components of remanent magnetization (RM) used to compute palaeomagnetic pole positions for the Kaoko lavas (48° N, 93° W, A95 = 3°) and for the Hoachanas basalts (61° N, 106° W, A95 = 7° are stable to alternating field (AF) and thermal demagnetization.
Correlation on a pre-drift map and on a map reconstructed for 112 Ma BP (before present) between the palaeomagnetic poles from the Kaoko and Serra Geral lavas suggests that the South Atlantic had not opened appreciably by 112 Ma BP. Cretaceous pole positions for S. America and Africa on a map reconstructed for 80 Ma BP are also discussed.     

Palaeomagnetic study of Palaeoproterozoic granitoids from the Voronezh Massif, Russia

A palaeomagnetic investigation has been carried out of rocks from the eastern part of the Voronezh Massif, which constitutes, together with the Ukrainian Shield, the Sarmatian segment in the southern part of the East European Craton. The samples were collected in a quarry close to the town of Pavlovsk (50.4°N, 40.1°E), where a syenitic-granitic body intrudes Archaean units. U–Pb (zircon) dating has yielded an age of 2080  Ma for the intrusion.
  Two characteristic magnetic components, A and B, were isolated by thermal and alternating-field demagnetization. Component A was obtained from granites and quartz syenites (11 samples) and has a mean direction of D = 229°, I = 28°, and a pole position at 12°N, 172°E. This pole is close to a contemporary mean pole (9°N, 187°E) for the Ukrainian Shield, which implies that the Voronezh Massif and the Shield constituted a single entity at 2.06  Ga. These poles differ from contemporaneous poles of the Fennoscandian Shield, indicating that the relative positions of the two shields were different from their present configuration about 2100  Myr ago.
  A component B, isolated only in quartz monzonites (five samples), has a mean direction D = 144°, I = 49°, and a pole position at 4°N, 251°E, which is close to late Sveconorwegian (approximately 900  Ma) poles for Baltica. This suggests that the East European Craton was consolidated some time between 2080 and 900  Ma. Comparison with other palaeomagnetic data permit us to narrow this time span to 1770–1340  Ma.     

Basin filling evolution of the central basins of Mallorca since the Pliocene

A new compilation of data from 436 drill cores using decompaction and backstripping techniques was used to reconstruct the basin filling history from the Pliocene until the present day in the Palma, Inca and Sa Pobla Basins on the island of Mallorca (Spain). Calcareous rocks dominate the source area and provide a limited amount of clastic input to the basins that has resulted in an average accumulation rate of between 5 and 20 m/Ma during the last 5.3 Ma. Carbonate sediment production dominated the basin filling history during early‐mid Pliocene, but during the Quaternary, the sedimentation processes in the Palma Basin were probably enhanced by an evolution in the drainage network that increased the sediment supply and the accumulated thickness caused by stream capture. However, the maximum sedimentation rate filling the depocentres of the three basins has been decreasing since the Pliocene, showing that not only the catchment transport efficiency but also the relative sea level have been controlling the sediment accumulation in these carbonate basins. The isopach cross‐sections support the idea that a palaeorelief was generated during the Messinian sea level drop and that heterogeneities were filled in from the Pliocene to the Quaternary. We conclude that the central basins of Mallorca were filled heterogeneously due to tectonic and geomorphic processes that controlled sediment transport and production, resulting in different average sedimentation thicknesses that decreased since the Pliocene as the accommodation space became filled and the relative sea level dropped.     

A Maastrichtian palaeomagnetic pole for the Pacific plate from a skewness analysis of marine magnetic anomaly 32

The asymmetry (skewness) of marine magnetic anomaly 32 (72.1–73.3  Ma) on the Pacific plate has been analysed in order to estimate a new palaeomagnetic pole. Apparent effective remanent inclinations of the seafloor magnetization were calculated from skewness estimates of 108 crossings of anomaly 32 distributed over the entire Pacific plate and spanning a great-circle distance of ~12  000  km. The data were inverted to obtain a palaeomagnetic pole at 72.1°N, 26.8°E with a 95 per cent confidence ellipse having a 4.0° major semi-axis oriented 98° clockwise of north and a 1.8° minor semi-axis; the anomalous skewness is 14.2° ± 3.7°. The possible dependence of anomalous skewness on spreading rate was investigated with two empirical models and found to have a negligible effect on our palaeopole analysis over the range of relevant spreading half-rates, ~25 to ~90  mm  yr⁻¹ . The new pole is consistent with the northward motion for the Pacific plate indicated by coeval palaeocolatitude and palaeoequatorial data, but differs significantly from, and lies to the northeast of, coeval seamount poles. We attribute the difference to unmodelled errors in the seamount poles, mainly in the declinations. Comparison with the northward motion inferred from dated volcanoes along the Hawaiian–Emperor seamount chain indicates 13° of southward motion of the Hawaiian hotspot since 73  Ma. When the pole is reconstructed with the Pacific plate relative to the Pacific hotspots, it differs by 14°–18° from the position of the pole relative to the Indo–Atlantic hotspots. This has several possible explanations including bias in one or more of the palaeomagnetic poles, motion between the Pacific and Indo–Atlantic hotspots, and errors in plate reconstructions relative to the hotspots.     

Comments on'The palaeomagnetism of the Central Zone of the Lewisian foreland, north-west Scotland'by J. D. A. Piper

Summary. Piper suggested that the Lewisian has rotated 30° anticlockwise since magnetization, whereas the opposite appears more likely. The main magnetization in the Lewisian recognized by Piper and Beckmann was imposed upon cooling after the Laxfordian metamorphism at about 1750 (± 50) Ma. The palaeomagnetic pole corresponding to this magnetization is at 37.6° N, 273.2° E ( dp = 3.7°, dm = 5.2°).
In Greenland, palaeomagnetic poles similar to each other, with a mean pole at 21.6° N, 280.1° E ( K = 52, A ₉₅= 9.4°), have been determined from five widely separated regions in central West Greenland and from Angmags-salik in East Greenland. The magnetization observed in all these regions was established upon cooling after the Nagssugtoqidian metamorphism, again at about 1750 (± 50) Ma.
The Laxfordian and Nagssugtoqidian metamorphisms were equivalent. It is therefore assumed that the two palaeomagnetic poles quoted above were originally identical. Their present difference can be explained by clockwise rotation of north-west Scotland about a local rotation pole since the Lewisian became magnetized, in addition to opening of the Atlantic assuming conventional reconstructions:
(1) assuming the reconstruction of Bullard, Everett & Smith, the local rotation proposed is 39.5° (± 18.1°) about a pole of rotation at 60.3° N, 354.5° E, or
(2) assuming the reconstruction of Le Pichon, Sibuet & Francheteau, the local rotation is 28.0° (±17.7°) about a pole of rotation at 54.1° N, 354.6° E.
These proposals of local clockwise rotation of north-west Scotland accord with that of Storetvedt based on palaeomagnetic results from Devonian rocks on the north-west side of the Great Glen Fault.     

Cambro-Ordovician palaeomagnetic and geochronologic data from southern Victoria Land, Antarctica: revision of the Gondwana apparent polar wander path

We present new palaeomagnetic and isotopic data from the southern Victoria Land region of the Transantarctic Mountains in East Antarctica that constrain the palaeogeographic position of this region during the Late Cambrian and Early Ordovician. A new pole has been determined from a dioritic intrusion at Killer Ridge (⁴⁰Ar/³⁹Ar biotite age of 499 ± 3 Ma) and hornblende diorite dykes at Mt. Loke (21°E, 7°S, A 95 = 8°, N = 6 VGPs). The new Killer Ridge/Mt. Loke pole is indistinguishable from Gondwana Late Cambrian and Early Ordovician poles. Previously reported palaeomagnetic poles from southern Victoria Land have new isotopic age constraints that place them in the Late Cambrian rather than the Early Ordovician. Based upon the new palaeomagnetic and isotopic data, new Gondwana Late Cambrian and Early Ordovician mean poles have been calculated.     

Palaeomagnetism of the Lower Ordovician Orthoceras Limestone, St. Petersburg, and a revised drift history for Baltica in the early Palaeozoic

Palaeomagnetic investigation of Lower Ordovician limestone in the vicinity of St. Petersburg yields a pole position at latitude 34.7°N, longitude 59.1°E ( dp / dm =5.7°/6.4°). A probable primary remanence origin is supported by the presence of a field reversal. The limestone carries one other remanent magnetization component associated with a Mesozoic remagnetization event.
An apparent polar wander path is compiled for Baltica including the new result, ranging in age from Vendian to Cretaceous. Ages of the published Lower to mid-Palaeozoic palaeomagnetic pole positions are adjusted in accordance with the timescale of Tucker & McKerrow (1995). The new Arenig result is the oldest of a series of Ordovician and Silurian palaeomagnetic pole positions from limestones in the Baltic region. There are no data to constrain apparent polar wander for the Tremadoc, Cambrian and latest Vendian. If the Fen Complex results, previously taken to be Vendian in age ( c . 565 Ma), are reinterpreted as Permian remagnetizations, an Early Ordovician–Cambrian–Vendian cusp in the polar wander path for Baltica is eliminated. The apparent polar wander curve might then traverse directly from poles for Vendian dykes on the Kola peninsula ( c . 580 Ma) towards our new Arenig pole ( c . 480 Ma). The consequence of this change in terms of the motion of Baltica in Cambrian times is to reduce significantly a rotational component of movement.
The new Arenig pole extends knowledge of Ordovician apparent polar wander an increment back in time and confirms the palaeolatitude and orientation of Baltica in some published palaeogeographies. Exclusion of the Fen Complex result places Baltica in mid- to high southerly latitudes at the dawn of the Palaeozoic, consistent with faunal and sedimentological evidence but at variance with some earlier palaeomagnetic reconstructions.