A Late Cretaceous pole for the Pacific plate: implications for apparent and true polar wander and the drift of hotspots

We present a Late Cretaceous (81 Ma) pole position for the Pacific plate derived from paleomagnetic analyses of basalt samples from Detroit Seamount (of the Hawaiian–Emperor seamounts) that were oriented using Brunhes-age overprints. This pole is at much higher latitudes than the previously published Late Cretaceous pole positions based on the modeling of magnetic anomalies observed during marine surveys over seamounts. Our new pole suggests that the Pacific plate would have moved rapidly between 95 and 81 Ma at speeds as high as 19.8 (−10.8/+11.2) cm/year. The Pacific plate at this time was smaller than the present-day plate and had a substantial subducting boundary. The high-velocity estimates are comparable with those of other paleoplates having similar characteristics. Therefore, plate tectonic driving forces can explain the motion and there is no need to invoke true polar wander. Decreases in mantle drag associated with vigorous Late Cretaceous volcanism in the Pacific, however, may have contributed to the rapid plate speed. The new pole position, together with other reliable paleomagnetic indicators of Pacific apparent polar wander, further supports the notion of drift of the Hawaiian hotspot during the Late Cretaceous.     

Absolute velocity of North America during the Mesozoic from paleomagnetic data

We use paleomagnetic data to map Mesozoic absolute motion of North America, using paleomagnetic Euler poles (PEP). First, we address two important questions: (1) How much clockwise rotation has been experienced by crustal blocks within and adjacent to the Colorado Plateau? (2) Why is there disagreement between the apparent polar wander (APW) path constructed using poles from southwestern North America and the alternative path based on poles from eastern North America? Regarding (1), a 10.5° clockwise rotation of the Colorado Plateau about a pole located near 35°N, 102°W seems to fit the evidence best. Regarding (2), it appears that some rock units from the Appalachian region retain a hard overprint acquired during the mid-Cretaceous, when the geomagnetic field had constant normal polarity and APW was negligible.We found three well-defined small-circle APW tracks: 245–200 Ma (PEP at 39.2°N, 245.2°E, R=81.1°, root mean square error (RMS)=1.82°), 200–160 Ma (38.5°N, 270.1°E, R=80.4°, RMS=1.06°), 160 to 125 Ma (45.1°N, 48.5°E, R=60.7°, RMS=1.84°). Intersections of these tracks (the “cusps” of Gordon et al. [Tectonics 3 (1984) 499]) are located at 59.6°N, 69.5°E (the 200 Ma or “J1” cusp) and 48.9°N, 144.0°E (the 160 Ma or “J2” cusp). At these times, the absolute velocity of North America appears to have changed abruptly.North America absolute motion also changed abruptly at the beginning and end of the Cretaceous APW stillstand, currently dated at about 125 and 88 Ma (J. Geophys. Res. 97 (1992b) 19651). During this interval, the APW path degenerates into a single point, implying rotation about an Euler pole coincident with the spin axis.Using our PEP and cusp locations, we calculate the absolute motion of seven points on the North American continent. Our intention is to provide a chronological framework for the analysis of Mesozoic tectonics. Clearly, if APW is caused by plate motion, abrupt changes in absolute motion should correlate with major tectonic events. This follows because large accelerations reflect important changes in the balance of forces acting on the plate, the most important of which are edge effects (subduction, terrane accretion, etc.). Some tectonic interpretations: (1) The J1 cusp may be associated with the inception of rifting of North America away from land masses to the east; the J2 cusp seems to mark the beginning of rapid spreading in the North Atlantic. (2) The J2 cusp signals the beginning of a period of rapid northwestward absolute motion of western North America; motion of tectonostratigraphic terranes in the westernmost Cordillera seems likely to have been directed toward the south during this interval. (3) The interval 88 to 80 Ma saw a rapid decrease in the paleolatitude of North America; unless this represents a period of true polar wander, terrane motion during this time should have been relatively northward.     

Uplifted marine terraces of the Akamas Peninsula (Cyprus): evidence of climatic conditions during the Late Quaternary highstands

An outcrop study of uplifted marine terraces provides information on the climate of past sea‐level maxima, supplementing existing palaeoclimatological archives. The western coast of the Akamas Peninsula shows several uplifted and intricately stacked Quaternary marine terraces. This study focuses on the sedimentology, petrography and sequence stratigraphy of the last three recent terraces and provides palaeoclimatological reconstruction and chronological framing. All three terraces display basically the same stratigraphic succession, which consists of regressive sequences ranging from bioturbated, storm‐influenced, subtidal high‐energy sands to subaerial exposure and aeolian deposition. Each sequence differs in petrography, reflecting contemporaneous climatic conditions. The first studied sequence (Marine Isotope Stage 9 or 11?) was deposited in a warm, arid climate, with oligotrophic water favouring ooid formation. The second sequence (Marine Isotope Stage 7) displays colder but humid conditions, with lower carbonate production and strong detrital input. The third sequence (Marine Isotope Stage 5e) records warm, humid conditions, with high carbonate production combined with significant detrital input. These littoral terraces offer high‐quality outcrops of glacioeustatic‐dominated, littoral sedimentology, together with evidence indicative of the regional climate during the late Pleistocene.     

The Triassic of the Thakkhola (Nepal). II: Paleolatitudes and comparison with other Eastern Tethyan Margins of Gondwana

During the Triassic, the Thakkhola region of the Nepal Himalaya was part of the broad continental shelf of Gondwana facing a wide Eastern Tethys ocean. This margin was continuous from Arabia to Northwest Australia and spanned tropical and temperate latitudes.A compilation of Permian, Triassic and early Jurassic paleomagnetic data from the reconstructed Gondwana blocks indicates that the margin was progressively shifting northward into more tropical latitudes. The Thakkhola region was approximately 55° S during Late Permian, 40° S during Early Triassic, 30° S during Middle Triassic and 25° S during Late Triassic. This paleolatitude change produced a general increase in the relative importance of carbonate deposition through the Triassic on the Himalaya and Australian margins. Regional tectonics were important in governing local subsidence rates and influx of terrigenous clastics to these Gondwana margins; but eustatic sea-level changes provide a regional and global correlation of major marine transgressions, prograding margin deposits and shallowing-upward successions. A general mega-cycle characterizes the Triassic beginning with a major transgression at the base of the Triassic, followed by a general shallowing-upward of facies during Middle and Late Triassic, and climaxing with a regression in the latest Triassic.     

青藏高原西部喀喇昆仑断裂活动构造研究进展综述

喀喇昆仑断裂系(KF)位于青藏高原西缘,具有右旋走滑性质,从帕米尔高原至尼泊尔西部延绵1000多km.长期以来,对于喀喇昆仑断裂活动的起始时间、总位移量、在不同时间尺度上的滑移速率以及断层两端的精确位置等问题,都存在较大争议.为了更好的了解喀喇昆仑断裂现今的运动学特征及其与喜马拉雅—青藏高原陆内碰撞造山带的关系,确定喀喇昆仑断裂的滑移速率历史以及它随时间和/或空间的变化规律是极其重要的.目前研究表明,从现今的大地测量学尺度到几个百万年的地质学尺度,喀喇昆仑断裂走滑速率的变化范围为3~10 mm/yr.本论文对断裂各段的分布情况进行了详细描述,阐述了获得晚第四纪以来走滑速率的方法,回顾了喀喇昆仑断裂在大地测量学、晚第四纪以及地质学等不同时间尺度的走滑速率,并重点讨论了晚第四纪以来断裂的走滑速率.然后,确定了喀喇昆仑断裂北端的精确位置、讨论了其运动学意义和地震灾害效应.鉴于喀喇昆仑断裂具有长期的活动历史、规模巨大、运动速率较高,我们认为即使板块内部小尺度的似连续变形非常发育,板块模型依然可以很好的解释由于印度-亚洲板块碰撞造成的喜马拉雅北部的岩石圈变形.喀喇昆仑断裂、阿尔金断裂、昆仑断裂及龙木错—郭扎错断裂等青藏高原周缘的主要走滑断裂对青藏高原向东的挤出起着重要的调节作用.     

Paleomagnetic age of ferruginous weathering beneath the Hamersley Surface,Pilbara, Western Australia,and the Cenozoic apparent polar wander path

During the Mesozoic and Paleogene, the Precambrian rocks in the Pilbara, Western Australia, underwent erosion and deep weathering that produced an undulating landform now represented by the duricrusted and partly eroded Hamersley Surface. A reddened, ferruginous weathering zone occurs immediately beneath this duricrusted surface. Oriented block samples of ferruginised strata of the Neoarchean–early Paleoproterozoic Hamersley Group exposed within approximately 15 m below the duricrust were collected at 20 sites in roadcuts along the Great Northern Highway between Munjina and Newman and exposures along the adjoining Karijini Drive. Stepwise thermal demagnetisation of cored specimens revealed a stable, high-temperature (680°C) component carried by hematite, with a mean direction (n = 55 specimens) of declination D = 182.0°, inclination I = 52.9° (α₉₅ = 3.6°), indicating a pole position at latitude λ_p = 77.6°S, longitude ?_p = 113.2°E (A₉₅ = 4.3°) and a paleolatitude λ = 33.5 +3.6/–3.3°S. Both normal and reversed polarities are present, indicating that the remanent magnetism was acquired over an interval of at least two polarity chrons (say 10⁵–10⁶ years). Chi-square tests on the determined pole position and three different sets of Cenozoic poles, namely those for dated volcanic rocks in eastern Australia supplemented by poles for Australian Cenozoic weathering horizons, and inferred poles from Pacific Ocean and Indian Ocean hotspot analyses and North American Cenozoic poles rotated to Australian coordinates, yielded a mean age of ca 24 ± 3 Ma, i.e. late Oligocene to early Miocene, interpreted as the time of formation of hematite in the sampled ferruginous zone. The ferruginous weathering occurred under globally warm conditions and was followed during the early to middle Miocene climatic optimum by the deposition of channel iron deposits, which incorporated detrital hematitic material derived from erosion of the ferruginous weathering zone beneath the Hamersley Surface.     

渤海湾盆地辽河西部凹陷的走滑构造作用

渤海湾盆地辽河西部凹陷由于其丰富的油气资源和特殊的构造位置（郯庐断裂穿过此凹陷）,其走滑构造的深入研究,对于揭示凹陷的构造演化和对油气成藏的控制作用以及郯庐断裂北段新生代的活动历史有重要意义。本文通过辽河西部凹陷三维地震资料系统的构造解析,并结合砂箱模拟实验,揭示了辽河西部凹陷存在多种与右行走滑相关的构造形迹,包括雁列正断层带、“梳状”构造、花状构造、“显形”和“隐伏”走滑断层等;确定了辽河西部凹陷的走滑活动开始于渐新世初期,活动强度开始时期（E3s1－2）较弱,逐渐加强,到渐新世末期（E3d1）达到最强,中新世有少量活动,上新世以来活动再次增强,活动比较显著,并持续到现今,推测整个郯庐断裂北段渐新世以来也有类似的演化历史;利用砂箱模拟实验确定在纯走滑拉分的条件下,约三分之二的走滑位移量可以转化为伸展量,并估算出辽河西部凹陷渐新世以来两侧基底累计的右行走滑位移量约为18 km,其中渐新世（E3s1－2—E3d）、中新世（N1g）、和上新世以来（N2m—Q）的走滑位移量分别为10.5 km、1.8 km和5.7 km,对应的平均走滑速度分别为0.71 mm/a、 0.10 mm/a和 1.11 mm/a。     

青藏高原西部喀喇昆仑断裂活动构造研究进展综述（英文）

喀喇昆仑断裂系(KF)位于青藏高原西缘,具有右旋走滑性质,从帕米尔高原至尼泊尔西部延绵1 000多km。长期以来,对于喀喇昆仑断裂活动的起始时间、总位移量、在不同时间尺度上的滑移速率以及断层两端的精确位置等问题,都存在较大争议。为了更好的了解喀喇昆仑断裂现今的运动学特征及其与喜马拉雅—青藏高原陆内碰撞造山带的关系,确定喀喇昆仑断裂的滑移速率历史以及它随时间和/或空间的变化规律是极其重要的。目前研究表明,从现今的大地测量学尺度到几个百万年的地质学尺度,喀喇昆仑断裂走滑速率的变化范围为3～10 mm/yr。本论文对断裂各段的分布情况进行了详细描述,阐述了获得晚第四纪以来走滑速率的方法,回顾了喀喇昆仑断裂在大地测量学、晚第四纪以及地质学等不同时间尺度的走滑速率,并重点讨论了晚第四纪以来断裂的走滑速率。然后,确定了喀喇昆仑断裂北端的精确位置、讨论了其运动学意义和地震灾害效应。鉴于喀喇昆仑断裂具有长期的活动历史、规模巨大、运动速率较高,我们认为即使板块内部小尺度的似连续变形非常发育,板块模型依然可以很好的解释由于印度-亚洲板块碰撞造成的喜马拉雅北部的岩石圈变形。喀喇昆仑断裂、阿尔金断裂、昆仑断裂及龙木错—郭扎错断裂等青藏高原周缘的主要走滑断裂对青藏高原向东的挤出起着重要的调节作用。     

早期碰撞造山过程与板块构造：前寒武纪地质研究的机遇和挑战

普遍认为修正后的板块构造模式仍适用于新太古代地质研究，但是早期板块构造过程与后期有明显差异，包括陆块规模、造山带类型、碰撞造山过程等。典型碰撞造山带在地史上的初次形成具有划时代的构造演化意义，涉及典型板块构造初始发生过程、最早超级大陆拼合、威尔逊旋回及板块碰撞造山过程等方面。华北中部保留一条近南北向的碰撞造山带（2 600~2 500 Ma BP），保留特征的巨型复式褶皱、不同层次推覆构造、蛇绿岩混杂带等。围绕华北中部造山带及其25亿年重大构造热事件的研究，对认识华北早期构造演化及其超大陆再造具有重要意义，也是早期板块构造研究的关键突破口之一，开展其不同地壳层次构造变形及其前陆盆地的研究，将深化早期板块边界及其造山过程的科学认识。     

Dominant Lid Tectonics behaviour of continental lithosphere in Precambrian times:Palaeomagnetism confirms prolonged quasi-integrity and absence of supercontinent cycles

Although Plate Tectonics cannot be effectively tested by palaeomagnetism in the Precambrian aeon due to the paucity of high precision poles spanning such a long time period,the possibility of Lid Tectonics is eminently testable because it seeks accordance of the wider dataset over prolonged intervals of time;deficiencies and complexities in the data merely contribute to dispersion.Accordance of palaeomagnetic poles across a quasi-integral continental crust for time periods of up to thousands of millions of years,together with recognition of very long intervals characterised by minimal polar motions(～2.6-2.0,～1.5-1.25 and～0.75-0.6 Ga)has been used to demonstrate that Lid Tectonics dominated this aeon.The new PALEOMAGIA database is used to refine a model for the Precambrian lid incorporating a large quasiintegral crescentric core running from South-Central Africa through Laurentia to Siberia with peripheral cratons subject to reorganisation at～2.1,～1.6 and～1.1 Ga.The model explains low levels of tidal friction,reduced heat balance,unique petrologic and isotopic signatures,and the prolonged crustal stability of Earth's"Middle Age",whilst density concentrations of the palaeomagnetic poles show that the centre of the continental lid was persistently focussed near Earth's rotation axis from～2.8 to 0.6 Ga.The exception was the～2.7-2.2 Ga interval defined by～90°polar movements which translated the periphery of the lid to the rotation pole for this quasi-static period,a time characterised by glaciation and low levels of magmatic activity;the～2.7 Ga shift correlates with key interval of mid-Archaean crustal growth to some 60-70%of the present volume and REE signatures whilst the～2.2 Ga shift correlates with the Lomagundiδ～(13)C and Great Oxygenation events.The palaeomagnetic signature of breakup of the lid at～0.6 Ga is recorded by the world-wide Ediacaran development of passive margins and associated environmental signatures of new ocean basins.This event defined the end of a dominant Lid Tectonic phase in the history of Earth's continental lithosphere recorded by the quasi-integral Precambrian supercontinent Palaeopangaea and the beginning of the comprehensive Plate Tectonics which has characterised the Phanerozoic aeon.Peripheral modifications to the lid achieved a symmetrical and hemispheric shape in Neoproterozoic times comparable to the familiar short-lived supercontinent(Neo)Pangaea(～350-150 Ma)and this appears to be the sole supercontinent cycle recorded by the palaeomagnetic record.Prolonged integrity of a large continental nucleus accompanied by periodic readjustments of peripheral shields can reconcile divergent tectonic analyses of Precambrian times which on the one hand propose multiple Wilson Cycles to explain some signatures of Plate Tectonics,and alternative interpretations which consider that Plate Tectonics did not commence until the end of the Neoproterozoic.     

西藏雅鲁藏布江缝合带西段南北亚带蛇绿岩的成因探讨

雅鲁藏布江蛇绿岩带自萨嘎以西分为达巴-休古嘎布(南亚带)和萨嘎-达机翁(北亚带)两个亚带,但两者的关系和构造环境还存在争论。本文在实测剖面的基础上,对比南北亚带蛇绿岩产出和岩石组成,以及对比基性岩脉和围岩方辉橄榄岩的地球化学和年代学特征,探讨两个带的关系和构造环境。北带错不扎、加纳崩和巴尔基性岩脉具有高Si、Al,低K、Ti、P特征,属钙碱性玄武质成分,锆石U-Pb年龄为125~128Ma;南亚带东波、普兰、休古嘎布基性岩具有高Mg、Ti,低Si、K特征,为低钾拉斑玄武质成分,锆石U-Pb年龄为120~130Ma。南北带基性岩具有类似产状和年代学,球粒陨石标准化曲线与NMORB一致;N-MORB标准化蛛网图中显示Nb、Ta、Ti负异常,指示洋内俯冲带弧前或弧后环境。此外,两带基性岩围岩方辉橄榄岩中橄榄石、辉石、尖晶石具有类似的矿物成分特征,地球化学成分兼有弧前和深海地幔橄榄岩的特点,指示南北亚带蛇绿岩形成于俯冲带弧前环境。结合区域对比,南亚带蛇绿岩不发育沟-弧-盆体系,具有从北到南的构造侵位特征,南北亚带之间的仲巴微地体具有特提斯喜马拉雅的亲缘性,指示南北蛇绿岩亚带可能为相同大洋岩石圈的不同残余。     

Geochronology,paleomagnetism and magnetic fabric of metamorphic rocks in the northeast Fraser Belt,Western Australia

The first zircon U–Pb SHRIMP dating on high-grade meta-igneous units in the northernmost parts of the Fraser Belt along the southern margin of the Western Australian Yilgarn Craton, reveal crystallisation ages between 1299 ± 10 and 1250 ± 23 Ma. A small number of older xenocrystic zircons, incorporated in some samples, indicate the presence of Late Paleoproterozoic crust in the region. Zircon that crystallised within a melt accumulated in the neck of a boudinaged mafic unit was dated at 1296 ± 4 Ma, indicating that the emplacement of the igneous protoliths took place syntectonically. The anisotropy of magnetic susceptibility of the granulites indicates minimum axes with a mean inclination of 4° towards 130°, corresponding to a nearly vertical southwest–northeast (50–230°) magnetic foliation. This is very close to the structural trend of the Fraser Belt suggesting that the magnetic fabric was acquired syntectonically, during the collision between the Yilgarn and Gawler Cratons. The paleomagnetic data on the granulites overlap with published poles for various 1.2 Ga units in the Albany Belt and the 1.2 Ga Fraser dykes, possibly suggesting that the remanence was acquired during the second stage of the Fraser tectonism. A younger magnetisation component resembles a pole of uncertain age published for Bremer Bay in the Albany Belt.     

Petrogenesis and tectonic significance of Jurassic IAT magma types in the Hellenide ophiolites as deduced from the Rhodiani ophiolites (Pelagonian zone, Greece)

The Rhodiani ophiolites are represented by two tectonically superimposed ophiolitic units: the “lower” Ultramafic unit and the “upper” Volcanic unit, both bearing calcareous sedimentary covers. The Ultramafic unit consists of mantle harzburgites with dunite pods and chromitite ores, and represents the typical mantle section of supra-subduction zone (SSZ) settings. The Volcanic unit is represented by a sheeted dyke complex overlain by a pillow and massive lava sequence, both including basalts, basaltic andesites, andesites, and dacites. Chemically, the Volcanic unit displays low-Ti affinity typical of island arc tholeiite (IAT) ophiolitic series from SSZ settings, having, as most distinctive chemical features, low Ti/V ratios (< 20) and depletion in high field strength elements and light rare earth elements.The rare earth element and incompatible element composition of the more primitive basaltic andesites from the Rhodiani ophiolites can be successfully reproduced with about 15% non-modal fractional melting of depleted lherzolites, which are very common in the Hellenide ophiolites. The calculated residua correspond to the depleted harzburgites found in the Rhodiani and Othrys ophiolites. Both field and chemical evidence suggest that the whole sequence of the Rhodiani Volcanic unit (from basalt to dacite) originated by low-pressure fractional crystallization under partially open-system conditions. The modelling of mantle source, melt generation, and mantle residua carried out in this paper provides new constraints for the tectono-magmatic evolution of the Mirdita–Pindos oceanic basin.     

Magnetic field hyperactivity during the early Neoproterozoic: A paleomagnetic and cyclostratigraphic study of the Katav Formation,southern Urals,Russia

We present a detailed magnetostratigraphic and cyclostratigraphic profile through the Riphean (Tonian) Katav Formation in the southern Urals. The study confirms the primary nature of the magnetization in these rocks. The cyclostratigraphic study identified several orbital periods including the 405 ka long eccentricity. This allows us to quantify the reversal frequency in the Katav and our estimates range of 7–12 reversals per million years. Based on our study, we identify an interval of magnetic field reversal hyperactivity in the Neoproterozoic interval. Age estimates for the Katav are contentious and range somewhere between 800 Ma and 900 Ma based on carbonate Pb-Pb ages and stable isotope correlations. The paleomagnetic poles obtained in this study of the Katav (and overlying Inzer) Formation do not fit anywhere on the Baltica apparent polar wander path between 1100 Ma and 900 Ma. Furthermore, they lie 90° away from the 900 Ma segment of the path. We tentatively estimate their age to be closer to 800 Ma and perhaps confirm a previously hypothesized pulse of rapid true polar wander between 825 Ma and 790 Ma.     

库车坳陷西部构造圈闭形成期与烃源岩生烃期匹配关系探讨

根据库车坳陷西部地质剖面分析其构造圈闭类型,利用平衡剖面和生长地层分析构造圈闭形成期次。选取大北1井和一口人工井,应用PRA公司的BasinMod1-D软件,对其进行烃源岩成熟度史模拟,分析烃源岩的主要生烃期。根据构造圈闭形成期与烃源岩主要生烃期的匹配关系,认为西秋里塔格构造带盐下古构造圈闭与拜城凹陷烃源岩生烃期匹配良好,形成的油气藏大部分在后期保存良好,盐上圈闭可能形成油藏,盐层内部圈闭可能形成油藏、气藏或油气藏,在较厚盐层之下的圈闭可能会形成气藏。克拉苏构造带古构造圈闭与克拉苏构造带烃源岩主要生烃期以及拜城凹陷的主要生油期匹配良好,可以形成良好的油气藏,但是库车组沉积末期—第四纪构造破坏严重,油气藏经受构造调整、破坏和再分配形成残余油气藏、次生油气藏,此时正处于侏罗系烃源岩生气期,可以在盐下形成大量气藏。     

Seafloor spreading in the Weddell Sea from magnetic and gravity data

A re-compilation of magnetic data in the Weddell Sea is presented and compared with the gravity field recently derived from retracked satellite altimetry. The previously informally named ‘Anomaly-T,’ an east–west trending linear positive magnetic and gravity anomaly lying at about 69°S, forms the southern boundary of the well-known Weddell Sea gravity herringbone. North of Anomaly-T, three major E–W linear magnetic lows are shown, and identified with anomalies c12r, c21–29(r) and c33r. On the basis of these, and following work by recent investigators, isochrons c13, c18, c20, c21, c30, c33 and c34 are identified and extended into the western Weddell Sea. Similarly, a linear magnetic low lying along the spine of the herringbone is shown and provisionally dated at 93–96 Ma. Anomaly-T is tentatively dated to be M5n, in agreement with recent tectonic models.Although current tectonic models are generally in good agreement to the north of T, to the south interpretations differ. Some plate tectonic models have only proposed essentially north–south spreading in the region, whilst others have suggested that a period of predominantly east–west motion (relative to present Antarctic geographic coordinates) occurred during the mid-Mesozoic spreading between East and West Gondwana. We identify an area immediately to the south of T which appears to be the southerly extent of N–S spreading in the herringbone. Following recent work, the extreme southerly extent of the N–S directed spreading of the herringbone is provisionally dated M9r/M10. In the oldest Weddell Sea, immediately to the north and east of the Antarctic shelf, we see subtle features in both the magnetic and gravity data that are consistent with predominantly N–S spreading in the Weddell Sea during the earliest opening of East and West Gondwana. In between, however, in a small region extending approximately from about 50 km south of T to about 70°S and from approximately 40° to 53°W, the magnetic and gravity data appear to suggest well-correlated linear marine magnetic anomalies (possible isochrons) perpendicular to T, bounded and offset by less well-defined steps and linear lows in the gravity (possible fracture zones). These magnetic and gravity data southwest of T suggest that the crust here may record an E–W spreading episode between the two-plate system of East and West Gondwana prior to the initiation of the three-plate spreading system of South America, Africa and Antarctica. The E–W spreading record to the east of about 35°W would then appear to have been cut off at about M10 time during the establishment of N–S three-plate spreading along the South American–Antarctic Ridge and then subducted under the Scotia Ridge.     

藏东昌都地区侏罗纪岩石磁组构研究及其构造意义

本文对藏东昌都地区侏罗纪汪布组、东大桥组和小索卡组红层共71个采点开展了磁组构(AMS)研究。磁组构测试结果表明,早侏罗世汪布组岩石磁线理较磁面理发育,磁化率各向异性度较高,磁化率椭球最小轴K3散布于层面缩短方向,代表了与构造成因相关的磁组构;中侏罗世东大桥组和晚侏罗世小索卡组岩石则磁面理较磁线理发育,磁化率各向异性度较低,磁化率椭球最小轴K3与层面近垂直,指示了原生沉积磁组构。早侏罗世汪布组地层的磁组构揭示了其构造应力场方向为NE-SW向。中侏罗世东大桥组的磁组构指示了其沉积时的古水流方向为SE向(138.3°),而晚侏罗世小索卡组磁组构指示了其沉积时的古水流方向为NNW向(328.3°)。古水流方向的明显变化揭示了昌都地区从中侏罗世到晚侏罗世沉积物物源发生了相应的转变,表明昌都地区南早北晚的隆升过程。     

Emplacement of the Ardara pluton (Ireland): new constraints from magnetic fabrics, rock fabrics and age dating

The Ardara pluton as part of the Donegal batholith was intruded into Neoproterozoic metasediments and metadolerites at mid-crustal levels. The emplacement mechanism of the Ardara granite is very controversial, and mechanisms ranging from diapirism, ballooning and stoping followed by nested diapirism have been proposed. Magnetic fabrics, rock fabrics and K/Ar dating of micas are used here to constrain the emplacement history. The compositional zoning of the Ardara pluton is clearly reflected in the different bulk magnetic susceptibilities between the outer quartz monzodiorite and the central granodiorite, whereas the intervening tonalite is of intermediate nature. The magnetic carriers are characterized by the anisotropy of the magnetic susceptibility (AMS), thermomagnetic measurements and through high field analyses (HFA). The separation of the ferrimagnetic and paramagnetic contributions revealed that biotite and magnetite control the AMS in the quartz monzodiorite. Both minerals are oriented in such a way that their summed contribution is constructive and originates from the shape fabric of magnetite and the texture of biotite. Biotite is responsible mainly for the AMS in the tonalite and granodiorite. The magnetic foliation can be directly related to the macroscopic foliation and also to the D4 structures in the country rocks. The foliation is consistent with the geometry of the roughly circular shape and has a mostly steep to vertical dip. Towards the central granodiorite the magnetic foliation dies out, although plagioclase texture measurements indicate a weak magmatic shape fabric. With the exception of the tail, the K_max axes (magnetic lineation) vary from steeply to gently plunging. The so-called lineation factor is approximately 1.01 and therefore points to a less significant axial symmetry. These observations coincide with strain estimates on mafic enclaves that show a very consistent pattern of K ∼0 flattening strain. Texture analyses of biotite and quartz additionally support the observations made by the strain analyses and the magnetic fabric data. Microstructural investigations give evidence that the fabrics are associated with the emplacement over a range of temperatures from truly magmatic to high-temperature solid-state conditions. The age of the intrusion is still under discussion, but a new cooling age was determined by K/Ar dating of biotite at 403.7±8 Ma corresponding to a temperature range between 450 and 300°C. For a mylonite along the southern contact between the Ardara pluton and the country rock a K/Ar muscovite age of 378.8±7 Ma indicates a minimum age for the shear zone when the Ardara pluton must have already been cooled down below 350±50°C. Received: 28 January 1999 / Accepted: 28 December 1999     

Tectonics and Chemical Transport: The Rules of the Resource Game

The most basic solid earth resources for use by man include construction materials, energy resources, metals, ceramics, soils. With the exception of construction materials, the formation of economic mineral deposits and soils involves the thermal or gravity driven flow of various types of geofluids. The search for mineral deposits thus involves the understanding of the exact nature of fluids, their mass and flow patterns. Given the giant thermal disturbances associated with the formation and destruction of Gondwana, it must be no surprise that these regions contain very significant mineral resources on a large scale.