拉萨地块中部晚白垩世火山岩Ar-Ar年代学和古地磁研究结果及其构造意义

为进一步确定拉萨地块白垩纪-古近纪的古地理位置,我们对青藏高原拉萨地块措勤地区林子宗火山岩18个采点进行了古地磁研究.结果表明高温(高场)特征剩磁分量主要为亚铁磁性的磁铁矿所携带,特征剩磁分量在95%置信水平下通过了褶皱检验. 倾斜校正后采点平均的特征剩磁方向为D/I=16.2°/17.7°, α₉₅=5.6°,对应古地磁极位置为63.1°N,224.6°E,A₉₅=5.1°. 另一方面,Ar-Ar年代学结果表明采样剖面的林子宗火山岩形成年龄为~99-93 Ma, 与拉萨地块林周盆地的林子宗群火山岩的形成年龄存在较大差异.由此我们得到晚白垩世拉萨地块中部措勤地区的古纬度为8.5°±6.9°N,与林周盆地古近纪林子宗群典中组和年波组所揭示出的古纬度相当,进一步表明亚洲大陆最南缘的拉萨地块在晚白垩世-古近世期间位于北半球~10°N的低纬度地区.结合最新的特提斯海相地层古地磁结果,晚白垩世-古近世拉萨地块的古地理位置限定了印度与欧亚大陆的初始碰撞时间不晚于60.5 Ma;~93 Ma以来,拉萨地块和单一刚性欧亚大陆之间存在~1900 km的构造缩短.     

西峡盆地含恐龙蛋化石剖面磁性地层学结果及其构造地质意义

通过对河南南阳西峡盆地含恐龙蛋化石的阳域-丹水剖面的岩石磁学与古地磁学研究表明,该陆相沉积地层的特征剩磁载体以赤铁矿与磁铁矿为主,特征剩磁通过了倒转检验,表明其很可能为岩石形成时期获得的原生剩磁.结合该剖面古生物资料,磁性地层学结果表明,剖面上部2973~3023 m处年代为83~79 Ma;该剖面蛋化石所在最高层位年龄不晚于83 Ma,暗示该地区大型爬行动物的绝灭可能与西峡地区当时的气候、环境变化有关.此外,自晚白垩世以来,该剖面所处位置相对华北和华南地块发生了约18°的顺时针旋转,可能由该地区的局部构造活动引起.     

The palaeorotation of the troodos microplate,cyprus, in the late mesozoic-early cenozoic plate tectonic framework of the Eastern Mediterranean

The Troodos ophiolite formed during the Late Cretaceous (Turonian) in a spreading setting above a northerly-dipping subduction zone. Palaeomagnetic data establish that the ophiolite experienced a 90° anticlockwise palaeorotation that occurred during the Late Cretaceous-Early Eocene interval During this period, the ophiolite was stranded adjacent to a generally south-facing continental margin, the remnants of which are preserved in south-west Cyprus (Mamonia Complex), southern Cyprus (Moni melange) and in northern Cyprus (Kyrenia Range). A review of field evidence from these Mesozoic basin margin units shows that strike-slip played a critical role in their Late Cretaceous-Early Tertiary deformation while magnetic and gravity surveys support the existence of major lineaments preserved at depth beneath these areas. Thus, in agreement with available palaeomagnetic data from mainland Turkey and Africa, the rotated crustal unit is believed to be relatively small with its boundaries probably located in the vicinity of Cyprus. In the favoured palaeorotation model, a small supra-subduction zone oceanic crustal unit was rotated about a pole located close to the exposed ophiolite complex. Fragments of the northern continental margin became attached to the rotating microplate along strike-slip lineaments and were then carried southward to their present position. In the light of the regional tectonic setting, it was probably the oblique consumption of crust beneath the ophiolite and/or a collision outside the present area of Cyprus that provided the necessary driving force for the palaeorotation.     

Age estimation of the deposition of the cretaceous Upper Sandstones and a later remagnetization event,from Hirna area,SE Ethiopia

Outcrops of the Cretaceous Upper sandstone formation some 375 km to the East of Addis Ababa on the motor Highway to Harar was paleomagnetically investigated. About seventy core samples were collected at various stratigraphic levels from 250–300 meters thick sedimentary formation. After standard sample preparations in the laboratory the resulting specimens were subjected to routine paleomagnetic demagnetization protocol. In the first steps of demagnetizations process the recent and viscous magnetizations were removed by heating until a temperature of level of 300 °C. Further demagnetization of the samples resulted in the isolation of the final magnetization with stable line segments that is directed towards the origin, which is interpreted as Characteristic Remanent Magnetization (ChRM). Rock – magnetic experiments have identified goethite (αFeOOH), hematite (αFe₂O₃), detritial hematite, and magnetite as the magnetic mineral phases carrying the remanence. The ChRM identified resulted in an average value of (D_s = 0.5°, I_s = ?0.7°, α₉₅ = 4.3°, N = 34) for the red sandstones while an average value of (D_s = 335.8°, I_s = ?31.8°, α₉₅ = 4.7°, N = 14) for the limestone intercalations. The former ChRM in the red sandstone is determined to be secondary while the latter ChRM is known to be primary. Comparison of these directional results and their pole equivalents with the African plate Apparent Polar Wander Path curve established by Besse and Courtillot (2003) give ages of between 115–130 Million years for the limestone intercalation and ages of 30 million years for red sandstone unit. These are interpreted respectively as estimates of the age of deposition and a later remagnetization respectively.     

印欧大陆碰撞前亚洲大陆南缘古位置再研究:林周盆地上白垩统红层的古地磁新结果

拉萨地块林周盆地白垩系红层的古地磁数据一直都有较大争议.过去认为磁倾角变浅可能是造成这些分歧的主要原因.我们在林周盆地设兴组背斜两翼进行了系统的古地磁采样,15个采样点的特征剩磁分量在倾斜校正和倾伏褶皱校正后平均方向为D=339.3°,I=22.9°(α_(95)=5.1°).特征剩磁分量在大约69%展开时获得最大集中,表明其为同褶皱重磁化;此时平均方向为D=339.1°,I=27.3°(α_(95)=4.1°),对应的古地磁极为65.4°N,327.5°E(A_(95)=3.5°),参考点29.3°N/88.5°E的古纬度为15.0°N±3.5°.薄片镜下分析显示赤铁矿为次生矿物,岩石磁组构(AMS)也表现为过渡型构造变形组构.样品的特征剩磁方向应为重磁化的结果,E/I(elongation vs inclination)校正法显示特征剩磁方向并没有发生倾角变浅.根据区域构造,重磁化时代约为72.4±1.8 Ma到64.4±0.6 Ma.综合考虑拉萨地块东西部的古地磁数据以及地震层析成像资料后我们认为,碰撞前拉萨地块大约呈NW-SE向准线性分布,并处于~10°N-15.0°N;自~70 Ma以来,拉萨地块与稳定欧亚大陆之间至少存在1200±400 km(11.1°±3.5°)的南北向构造缩短量;印度大陆与欧亚大陆的碰撞不应晚于55 Ma.     

鲁科一井上白垩统沉积岩岩石磁学和天然剩磁分析:兼论利用剩磁方向恢复钻孔岩芯原始方位的可行性

本文对"鲁科一井"(CCSD-LK-Ⅰ)768.9~1112.3m之间的上白垩统沉积岩样品进行了岩石磁学、磁化率各向异性(AMS)以及天然剩磁组分的研究.在此基础上,分析了利用特征剩磁(ChRM)和黏滞剩磁(VRM)方向恢复岩芯原始方位的可行性.三轴等温剩磁热退磁曲线、磁滞回线、反向场退磁曲线、一阶反转曲线等岩石磁学测量结果表明,沉积岩的主要载磁矿物为磁铁矿和赤铁矿.335块样品的AMS测量结果表明磁化率椭球主轴的最大轴K1和中间轴K2与水平面夹角较小,最小轴K3接近垂直于水平面分布,说明沉积岩保留了原始沉积磁组构特征.系统热退磁实验表明,多数样品在25~350℃和500~690℃温度段分别获得VRM和ChRM分量.利用ChRM偏角方向,并考虑构造旋转量校正,对VRM偏角方向进行恢复,Fisher统计得到DVRM=-1.3°,IVRM=59.6°,与当地现代地磁场方向(D=-6.7°,I=53.9°)基本一致.用ChRM偏角方向对磁化率主轴K1偏角方向进行校正,校正的结果为:D_(ch_K1)=349.2°,I_(ch_K1)=-0.7°.本文研究结果对于地质勘探中利用古地磁学方法恢复钻孔岩芯原始方位具有一定参考意义.     

Metamorphism and metamorphic K–Ar ages of the Mesozoic accretionary complex in Northland, New Zealand

Abstract A southwest dipping Mesozoic accretionary complex, which consists of tectonically imbricated turbiditic mudstone and sandstone, hemipelagic siliceous mudstone, and bedded cherts and basaltic rocks of pelagic origin, is exposed in northern North Island, New Zealand. Interpillow limestone is sometimes contained in the basaltic rocks. The grade of subduction‐related metamorphism increases from northeast to southwest, indicating an inverted metamorphic gradient dip. Three metamorphic facies are recognized largely on the basis of mineral parageneses in sedimentary and basaltic rocks: zeolite, prehnite‐pumpellyite and pumpellyite‐actinolite. From the apparent interplanar spacing d₀₀₂ data for carbonaceous material, which range from 3.642 to 3.564 Å, the highest grade of metamorphism is considered to have attained only the lowermost grade of the pumpellyite‐actinolite facies for which the highest temperature may be approximately 300°C. Metamorphic white mica K–Ar ages are reported for magnetic separates and <2 µm hydraulic elutriation separates from 27 pelitic and semipelitic samples. The age data obtained from elutriation separates are approximately 8 m.y. younger, on average, than those from magnetic separates. The age difference is attributed to the possible admixture of nonequilibrated detrital white mica in the magnetic separates, and the age of the elutriation separates is considered to be the age of metamorphism. If the concept, based on fossil evidence, of the subdivision of the Northland accretionary complex into north and south units is accepted, then the peak age of metamorphism in the north unit is likely to be 180–130 Ma; that is, earliest Middle Jurassic to early Early Cretaceous, whereas that in the south unit is 150–130 Ma; that is, late Late Jurassic to early Early Cretaceous. The age cluster for the north unit correlates with that of the Chrystalls Beach–Taieri Mouth section (uncertain terrane), while the age cluster for the south unit is older than that of the Younger Torlesse Subterrane in the Wellington area, and may be comparable with that of the Nelson and Marlborough areas (Caples and Waipapa terranes).     

Link between SSZ ophiolite formation, emplacement and arc inception, Northland, New Zealand: U–Pb SHRIMP constraints; Cenozoic SW Pacific tectonic implications

New U–Pb age-data from zircons separated from a Northland ophiolite gabbro yield a mean ²⁰⁶Pb/²³⁸U age of 31.6 ± 0.2 Ma, providing support for a recently determined 28.3 ± 0.2 Ma SHRIMP age of an associated plagiogranite and  29–26 Ma ⁴⁰Ar/³⁹Ar ages (n = 9) of basalts of the ophiolite. Elsewhere, Miocene arc-related calc-alkaline andesite dikes which intrude the ophiolitic rocks contain zircons which yield mean ²⁰⁶Pb/²³⁸U ages of 20.1 ± 0.2 and 19.8 ± 0.2 Ma. The ophiolite gabbro and the andesites both contain rare inherited zircons ranging from 122–104 Ma. The Early Cretaceous zircons in the arc andesites are interpreted as xenocrysts from the Mt. Camel basement terrane through which magmas of the Northland Miocene arc lavas erupted. The inherited zircons in the ophiolite gabbros suggest that a small fraction of this basement was introduced into the suboceanic mantle by subduction and mixed with mantle melts during ophiolite formation.

We postulate that the tholeiitic suite of the ophiolite represents the crustal segment of SSZ lithosphere (SSZL) generated in the southern South Fiji Basin (SFB) at a northeast-dipping subduction zone that was initiated at about 35 Ma. The subduction zone nucleated along a pre-existing transform boundary separating circa 45–20 Ma oceanic lithosphere to the north and west of the Northland Peninsula from nascent back arc basin lithosphere of the SFB. Construction of the SSZL propagated southward along the transform boundary as the SFB continued to unzip to the southeast. After subduction of a large portion of oceanic lithosphere by about 26 Ma and collision of the SSZL with New Zealand, compression between the Australian Plate and the Pacific Plate was taken up along a new southwest-dipping subduction zone behind the SSZL. Renewed volcanism began in the oceanic forearc at 25 Ma producing boninitic-like, SSZ and within-plate alkalic and calc-alkaline rocks. Rocks of these types temporally overlap ophiolite emplacement and subsequent Miocene continental arc construction.     

Tectonic implications of new age data for the Meratus Complex of south Kalimantan, Indonesia

Cretaceous subduction complexes surround the southeastern margin of Sundaland in Indonesia. They are widely exposed in several localities, such as Bantimala (South Sulawesi), Karangsambung (Central Java) and Meratus (South Kalimantan).
The Meratus Complex of South Kalimantan consists mainly of mélange, chert, siliceous shale, limestone, basalt, ultramafic rocks and schists. The complex is uncomformably covered with Late Cretaceous sedimentary-volcanic formations, such as the Pitap and Haruyan Formations.
Well-preserved radiolarians were extracted from 14 samples of siliceous sedimentary rocks, and K–Ar age dating was performed on muscovite from 6 samples of schist of the Meratus Complex. The radiolarian assemblage from the chert of the complex is assigned to the early Middle Jurassic to early Late Cretaceous. The K–Ar age data from schist range from 110 Ma to 180 Ma. Three samples from the Pitap Formation, which unconformably covers the Meratus Complex, yield Cretaceous radiolarians of Cenomanian or older.
These chronological data as well as field observation and petrology yield the following constraints on the tectonic setting of the Meratus Complex.
(1) The mélange of the Meratus Complex was caused by the subduction of an oceanic plate covered by radiolarian chert ranging in age from early Middle Jurassic to late Early Cretaceous.
(2) The Haruyan Schist of 110–119 Ma was affected by metamorphism of a high pressure–low temperature type caused by oceanic plate subduction. Some of the protoliths were high alluminous continental cover or margin sediments. Intermediate pressure type metamorphic rocks of 165 and 180 Ma were discovered for the first time along the northern margin of the Haruyan Schist.
(3) The Haruyan Formation, a product of submarine volcanism in an immature island arc setting, is locally contemporaneous with the formation of the mélange of the Meratus Complex.     

Sedimentary records of the late Early Cretaceous compressional setting in the South China block: A case study of the Xingning Basin,Guangdong Province

Recently, some scholars have proposed that the South China Block (SCB) was controlled by a compressive tectonic regime in the middle–late Early Cretaceous, challenging the belief that the SCB was under an extensional setting during the Cretaceous. The Early Cretaceous tectonic setting constraint in the SCB can offer vital insight to clarify the Mesozoic subduction history of the Paleo-Pacific. Therefore, to determine the SCB tectonic regime during the Early Cretaceous, this study investigated sedimentary rocks from the Lower Cretaceous Heshui Formation in the Xingning Basin, a foreland basin located in the southeastern SCB. Provenance analysis was performed using sandstone modal analysis, sandstone geochemical characteristics, and detrital zircon geochronology. Based on the results, we discussed basin sediment sources and the SCB tectonic regime during the Early Cretaceous. The results showed that the maximum Heshui Formation depositional age was 103 Ma ± 1.6 Ma in the Early Cretaceous Albian. Detrital framework modes and geochemical characteristics of sandstone indicated that Heshui Formation's source rocks were granites and sedimentary rocks. The detrital zircon U–Pb ages could be classified into two major and four subordinate age populations. The Wuyi Terrane to the north and southeast coastal regions to the east were the primary potential Heshui Formation source areas. However, the lower and upper sandstones are different in the peak ages, ~437 and ~146 to 104 Ma, respectively, indicating that the major source area shifted from the Wuyi Terrane to the southeastern coastal regions during the late Early Cretaceous. The sandstone modal analysis results indicated that the source area comprised mainly collisional–orogenic material. The SCB was under a compressive tectonic regime during the late Early Cretaceous and this compression action continued until at least 103 Ma ± 1.6 Ma.     

Geochemical evidence for the origin of Late Triassic melange units in the Oman Mountains as a small ocean basin formed by continental rifting

A useful tool to elucidate past tectonic environments is the geochemistry of volcanic and sedimentary rocks when used together.The regional structural setting of the Oman Mountains indicates that deep-water sediments and volcanic rocks formed adjacent to the rifted Arabian margin in the Late Triassic near the axis of a narrow ocean basin of Red Sea-type. Tholeiitic to trachytic extrusives formed seamounts associated with Late Triassic reefal build-ups. “Immobile” trace element compositions point to a within-plate origin. The interbedded and overlying Late Triassic deep-sea sedimentary cover comprises ribbon radiolarites and both distal siliclastic and calcareous turbidites that accumulated on an abyssal plain at least ca. 180 km northeast of the Arabian continent. Associated ferromanganiferous oxide-sediments are interpreted as chemical precipitates derived from high-temperature vents in the spreading axis of the young ocean basin. Pervasive regional subsidence took place during end Triassic/Early Jurassic time.Later, in the Cretaceous, oceanic crust was consumed in a northeast-dipping subduction zone. MORB-type crust was subducted while Late Triassic volcanic edifices and sedimentary cover were accreted. During eventual trench-margin collision the Semail ophiolite split into blocks allowing sub-ophiolite melange rocks to be expelled upwards through corridors, creating the Batinah Melange. As the ophiolite nappe ploughed inboard over already thrust-assembled abyssal plain sediments (Hawasina Complex), some duplexes were uplifted, oversteepened, overturned and then slid backwards onto the ophiolite to form the Batinah Sheets.     

The odyssey of the Cache Creek terrane,Canadian Cordillera: Implications for accretionary orogens,tectonic setting of Panthalassa,the Pacific superwell,and break-up of Pangea

The Cache Creek terrane (CCT) of the Canadian Cordillera consists of accreted seamounts that originated adjacent to the Tethys Ocean in the Permian. We utilize Potential Translation Path plots to place quantitative constraints on the location of the CCT seamounts through time, including limiting the regions within which accretion events occurred. We assume a starting point for the CCT seamounts in the easternmost Tethys at 280 Ma. Using reasonable translation rates (11 cm/a), accretion to the Stikinia–Quesnellia oceanic arc, which occurred at about 230 Ma, took place in western Panthalassa, consistent with the mixed Tethyan fauna of the arc. Subsequent collision with a continental terrane, which occurred at about 180 Ma, took place in central Panthalassa, > 4000 km west of North America yielding a composite ribbon continent. Westward subduction of oceanic lithosphere continuous with the North American continent from 180 to 150 Ma facilitated docking of the ribbon continent with the North American plate.The paleogeographic constraints provided by the CCT indicate that much of the Canadian Cordilleran accretionary orogen is exotic. The accreting crustal block, a composite ribbon continent, grew through repeated collisional events within Panthalassa prior to docking with the North American plate. CCT's odyssey requires the presence of subduction zones within Panthalassa and indicates that the tectonic setting of the Panthalassa superocean differed substantially from the current Pacific basin, with its central spreading ridge and marginal outward dipping subduction zones. A substantial volume of oceanic lithosphere was subducted during CCT's transit of Panthalassa. Blanketing of the core by these cold oceanic slabs enhanced heat transfer out of the core into the lowermost mantle, and may have been responsible for the Cretaceous Normal Superchron, the coeval Pacific-centred mid-Cretaceous superplume event, and its lingering progeny, the Pacific Superswell. Far field tensile stress attributable to the pull of the slab subducting beneath the ribbon continent from 180 to 150 Ma instigated the opening of the Atlantic, initiating the dispersal phase of the supercontinent cycle by breaking apart Pangea. Docking of the ribbon continent with the North American plate at 150 Ma terminated the slab pull induced stress, resulting in a drastic reduction in the rate of spreading within the growing Atlantic Ocean.     

The basement geology of Japan from A to Z

The Precambrian and lower Paleozoic units of the Japanese basement such as the Hida Oki and South Kitakami terranes have geological affinities with the eastern Asia continent and particularly strong correlation with units of the South China block. There are also indications from units such as the Hitachi metamorphics of the Abukuma terrane and blocks in the Maizuru terrane that some material may have been derived from the North China block. In addition to magmatism, the Japanese region has seen substantial growth due to tectonic accretion. The accreted units dominantly consist of mudstone and sandstone derived from the continental margin with lesser amounts of basaltic rocks associated with siliceous deep ocean sediments and local limestone. Two main phases of accretionary activity and related metamorphism are recorded in the Jurassic Mino–Tanba–Ashio, Chichibu, and North Kitakami terranes and in the Cretaceous to Neogene Shimanto and Sanbagawa terranes. Other accreted material includes ophiolitic sequences, e.g. the Yakuno ophiolite of the Maizuru terrane, the Oeyama ophiolite of the Sangun terrane, and the Hayachine–Miyamori ophiolite of the South Kitakami terrane, and limestone‐capped ocean plateaus such as the Akiyoshi terrane. The ophiolitic units are likely derived from arc and back‐arc basin settings. There has been no continental collision in Japan, meaning the oceanic subduction record is more complete than in convergent orogens seen in intracontinental settings making this a good place to study the geological record of accretion. Hokkaido lacks most of the Paleozoic history recognized in Honshu, Shikoku, Kyushu, and the Ryukyu Islands to the south and its geology reflects the Cenozoic development of two convergent domains with volcanic arcs, their approach, and eventual collision. The Hidaka terrane reveals a cross section through a volcanic arc and the main accretionary complex of the convergent system is represented by the Sorachi–Yezo terrane.     

Geophysical modeling of the northern Appalachian Brompton-Cameron, Central Maine, and Avalon terranes under the New Jersey Coastal Plain

A regional terrane map of the New Jersey Coastal Plain basement was constructed using seismic, drilling, gravity and magnetic data. The Brompton-Cameron and Central Maine terranes were coalesced as one volcanic island arc terrane before obducting onto Laurentian, Grenville age, continental crust in the Taconian orogeny [Rankin, D.W., 1994. Continental margin of the eastern United States: past and present. In: Speed, R.C., (Ed.), Phanerozoic Evolution of North American Continent-Ocean Transitions. DNAG Continent-Ocean Transect Volume. Geological Society of America, Boulder, Colorado, pp. 129–218]. Volcanic island-arc rocks of the Avalon terrane are in contact with Central Maine terrane rocks in southern Connecticut where the latter are overthrust onto the Brompton-Cameron terrane, which is thrust over Laurentian basement. Similarities of these allochthonous island arc terranes (Brompton-Cameron, Central Maine, Avalon) in lithology, fauna and age suggest that they are faulted segments of the margin of one major late Precambrian to early Paleozoic, high latitude peri-Gondwana island arc designated as “Avalonia”, which collided with Laurentia in the early to middle Paleozoic. The Brompton Cameron, Central Maine, and Avalon terranes are projected as the basement under the eastern New Jersey Coastal Plain based on drill core samples of metamorphic rocks of active margin/magmatic arc origin. A seismic reflection profile across the New York Bight traces the gentle dipping (approximately 20 degrees) Cameron's Line Taconian suture southeast beneath allochthonous Avalon and other terranes to a 4 sec TWTT depth (approximately 9 km) where the Avalonian rocks are over Laurentian crust. Gentle up-plunge (approximately 5 degrees) projections to the southwest bring the Laurentian Grenville age basement and the drift-stage early Paleozoic cover rocks to windows in Burlington Co. at approximately 1 km depth and Cape May Co. at approximately 2 km depths. The antiformal Shellburne Falls and Chester domes and Chain Lakes-Pelham dome-Bronson Hill structural trends, and the synformal Connecticut Valley-Gaspe structural trend can be traced southwest into the New Jersey Coastal Plain basement. A Mesozoic rift basin, the “Sandy Hook basin”, and associated eastern boundary fault is identified, based upon gravity modeling, in the vicinity of Sandy Hook, New Jersey. The thickness of the rift-basin sedimentary rocks contained within the “Sandy Hook basin” is approximately 4.7 km, with the basin extending offshore to the east of the New Jersey coast. Gravity modeling indicates a deep rift basin and the magnetic data indicates a shallow magnetic basement caused by magnetic diabase sills and/or basalt flows contained within the rift-basin sedimentary rocks. The igneous sills and/or flows may be the eastward continuation of the Watchung and Palisades bodies.     

Buruanga peninsula and Antique Range: Two contrasting terranes in Northwest Panay,Philippines featuring an arc–continent collision zone

Alternating chert–clastic sequences juxtaposed with limestone blocks, which are units typical of accretionary complexes, constitute the Buruanga peninsula. New lithostratigraphic units are proposed in this study: the Unidos Formation (Jurassic chert sequence), the Saboncogon Formation (Jurassic siliceous mudstone–terrigenous mudstone and quartz‐rich sandstone), the Gibon Formation (Jurassic(?) bedded pelagic limestone), the Libertad Metamorphics (Jurassic–Cretaceous slate, phyllite, and schist) and the Buruanga Formation (Pliocene–Pleistocene reefal limestone). The first three sedimentary sequences in the Buruanga peninsula show close affinity with the ocean plate stratigraphy of the North Palawan terrane in Busuanga Island: Lower–Middle Jurassic chert sequences overlain by Middle–Upper Jurassic clastics, juxtaposed with pelagic limestone. Moreover, the JR5–JR6 (Callovian to Oxfordian) siliceous mudstone of the Saboncogon Formation in the Buruanga peninsula correlates with the JR5–JR6 siliceous mudstone of the Guinlo Formation in the Middle Busuanga Belt. These findings suggest that the Buruanga peninsula may be part of the North Palawan terrane. The rocks of the Buruanga peninsula completely differ from the Middle Miocene basaltic to andesitic pyroclastic and lava flow deposits with reefal limestone and arkosic sandstone of the Antique Range. Thus, the previously suggested boundary between the Palawan microcontinental block and the Philippine Mobile Belt in the central Philippines, which is the suture zone between the Buruanga peninsula and the Antique Range, is confirmed. This boundary is similarly considered as the collision zone between them.     

柴达木盆地北缘中新生代地层的磁组构特征及其沉积——构造学意义

柴达木盆地沉积地层记载着青藏高原东北部的构造演化信息.对该盆地路乐河地区上中生界—新生界地层系统采样,获得千余块定向岩心样品.岩石磁学研究表明样品中的磁性矿物主要为赤铁矿和磁铁矿;磁组构研究表明为初始沉积磁组构特征.磁组构特征指示了自中侏罗统大煤沟组(J2d)至早中新统下油砂山组(N12y)7个地层单位沉积时期,古水流方向共经历了4次阶段性的变化,表明柴达木块体相应地发生了4次旋转.在中—晚侏罗世块体逆时针旋转约22°;至早白垩世,块体又顺时针旋转约65°;在65.5～32 Ma期间块体旋转方向再次改变,逆时针旋转约63°;到32～13Ma阶段块体又发生约50°的顺时针旋转.柴达木块体的旋转及其方向的转换,可能与其南的羌塘块体、拉萨块体和印度板块阶段性北向碰撞挤压紧密相关.拉张环境与挤压环境的多次转换可能与中特提斯的关闭、新特提斯的张开和闭合、高原快速隆升后其边部松弛相联系.     

Ca. 13 Ma strike-slip deformation in coastal Sonora from a large-scale,en-echelon,brittle-ductile,dextral shear indicator: implications for the evolution of the California rift

The Rancho Nuevo semi-circular structure is a geomorphological structure defined by drainage patterns in coastal Sonora, about 160 km NW of Hermosillo. The structure is about 15 by 30 km, and it is cored by felsic to intermediate plutons (granodiorite, monzogranite, quartz-porphyry) covered by Miocene volcanic rocks. This work is focused on the deformation of the intrusives which cover most of study area.The plutons are a co-magmatic suite dated between 71 ± 1.1 and 67.9 ± 1.0 Ma (U-Pb zircon, LA-ICPMS). The most voluminous unit is a granodiorite characterized by conspicuous sigmoidal fractures at the scale of high resolution satellite images, along which rhyolite dikes were emplaced about 13.2 Ma. Magnetic fabric (AMS) and paleomagnetic data were collected from 27 sites in the granodiorite. Magnetic fabrics are weak but well developed, and are characterized by steep foliation planes with strikes that follow the sigmoidal fracture pattern and suggest NE-SW to NW-SE flattening after emplacement. The characteristic magnetization is of dual polarity, but it is dominantly reverse consistent with emplacement during chron C31r. The prevalent magnetization is southwest and moderately steep negative (ten sites), a discordant direction rotated clockwise about 41 ° ± 11 with respect to the expected Late Cretaceous reference direction, also indicating gentle southward tilt. There is, however, paleomagnetic evidence suggesting that the structure did not rotate as a rigid body, but it deformed internally instead. These data are interpreted to indicate that the Rancho Nuevo semicircular structure is a large-scale, dextral, brittle-ductile shear indicator. The age of the dikes and the fact that they are covered discordantly by rocks assigned to the tuff of San Felipe indicate that northwest, srike-slip, motion of Baja California peninsula (and thus the Pacific plate relative to North America) was accommodated by faults in coastal Sonora about 13 Ma ago.     

New paleomagnetic constraints on the late Cretaceous and early Cenozoic tectonic history of the Eastern Pontides

The Pontides are characterized by a series of Mesozoic-Cenozoic fold belts comprising a N-vergent foreland fold and thrust belt in the Western Pontides and a concave, upward-shaped fold belt in the Eastern Pontides. The curvature of the fold belt follows the Caucasus which may imply a phase of oroclinal bending. In order to test whether the fold curvature represents a phase of oroclinal bending, a paleomagnetic study has been carried out in the Eastern Pontides on late Cretaceous and middle Eocene volcanic and sedimentary rocks from 29 sites. Rock magnetic studies reveal medium-temperature components with an unblocking temperature of 400–580 °C, indicating pseudo-single domain titanomagnetite as the most abundant carrier of magnetic remanence in the middle Eocene rocks studied here. In the upper Cretaceous rocks, a high-temperature component with an unblocking range of 580–650 °C was isolated. Stepwise thermal and alternating field demagnetization isolated two components of remanent magnetization in middle Eocene rocks comprising a low unblocking temperature/coercivity component near the present field direction and a characteristic remanent magnetization (ChRM) component of D_s = 332.3°, I_s = 49.9° (k = 33.3, α₉₅ = 9.2°, N = 15 sites). A positive fold test at a 95% confidence level and a reversal test indicate a primary magnetization. Component analysis of the upper Cretaceous rocks identifies a stable ChRM D_s = 160.3°, I_s = −45.0°, (k =  85.6, α₉₅ = 6.0°, N =  8 sites) following removal of secondary remanence. Their ChRM direction passes fold and reversal tests at a 95% confidence level. Both the upper Cretaceous and middle Eocene paleomagnetic data from the Eastern Pontides and the Lesser Caucasus clearly demonstrate evidence of oroclinal bending that occurred contemporaneouslywith the convergence between Arabia and Eurasia in the Paleocene.     

Geology, magmatic affinity and geotectonic environment of some Caledonian ophiolites in Norway

Ophiolites within the Norwegian Caledonides were generated during at least two distinct periods, i.e. Tremadocian-Arenigian and Ashgillian. The older generation show a long-lived magmatic development (ca 500-470 Ma) with rocks that range from MORB, IAT, boninites, calc-alkaline to alkaline basalt in geochemical affinity. This development is compatible with generation in a subduction-influenced environment, and a plausible modern equivalent may be the ensimatic arc-basin evolution in the western Pacific.The metabasalts of the younger (Ashgill, 443±3 Ma) ophiolite occurrence are characterized by a dominance of N- to E-MORB compositions with subduction zone influence. The volvanics of the complex are intercalated with, and overlain by, continentally-derived sediments. Calc-alkaline as well as alkaline lavas and volcanic-lastics apparantly define late magmatic products. Associated with the youngest ophiolite is a sedimentary melange with olistoliths of mature island arc-derived volcanics, as well as clasts with MORB-IAT affinity, chert and sandstone, the latter probably derived from an accretionary prism. An Andaman Sea model appears most applicable to the youngest ophiolite/associated sediments assemblage.Contemporaneously with the renewed, Ashgillian spreading episode, widespread deposition of clastic sequences of Ashgillian age took place unconformably on the Lower Ordovician ophiolite complexes in fault controlled basins.     

U–Pb zircon geochronology of the Daraban leucogranite,Mawat ophiolite,Northeastern Iraq: A record of the subduction to collision history for the Arabia–Eurasia plates

The Mawat ophiolite is part of the Mesozoic Neo‐Tethyan ophiolite belt of the Middle East and is located in the Zagros Imbricate Zone of Iraq. It represents fossil fragments of the Neo‐Tethyan oceanic lithosphere within the Alpine collisional system between the Arabian and Eurasia Plates. The first U–Pb zircon dating of the Daraban leucogranite from the Mawat ophiolite provides a ²⁰⁷Pb–²⁰⁶Pb age of 96.8 ± 6.0 Ma. The age is 59.0 ± 6.0 m.y. older than the previously published age of the Daraban leucogranite obtained by ⁴⁰Ar–³⁹Ar muscovite dating method. The U–Pb dating of magmatic zircons collected from the Daraban leucogranite, which intrudes into the Mawat ophiolite, reveals that melting of the pelagic sediment beneath the hot Zagros proto‐ophiolite in an intra‐oceanic arc environment led to anatexis at the subduction front and the generation of granitic melts at 96.8 ± 6.0 Ma, which were emplaced in the overlaying mantle wedge. This process was a response to the initial formation of the Neo‐Tethys ophiolite above a northeast‐dipping intra‐oceanic subduction zone at 96.8 ± 6.0 Ma. Published ⁴⁰Ar–³⁹Ar muscovite dating from the same leucogranite dike yields plateau ages of 37.7 ± 0.3 Ma, reflecting that the age was reset during the Arabia–Eurasia continental collision. Therefore, the bimodal age populations from the granitic intrusion in the Mawat ophiolite preserve a record of the subduction to the collision cycle of the Zagros Orogenic Belt. The 59.0 ± 6.0 m.y. age difference from the Daraban leucogranite represents the duration of the subduction‐collision cycle of the Zagros Orogenic Belt in the Kurdistan region of Iraq and the time span for the closure of the Neo‐Tethys Ocean along the northern margin of the Arabian plate.