New Cretaceous palaeointensity data and the constraints on geodynamics

A combined geochronologic (K-Ar) and palaeomagnetic study has been conducted on a basalt lava sequence at Yixian Formation in Liaoning Province, northeastern China. The new K-Ar age obtained from thirteen lava flows is 120.93±0.88 Ma. Detailed rock-magnetic investigations were conducted on each lava flow to determine their remanence carriers. The modified version of the Thellier-Thellier palaeointensity method with systematic partial thermoremanent magnetization (pTRM) checks was used for the palaeointensity determination. Virtual dipole moment (VDM) value is (3.66±0.10)x10²² Am². This low dipole-field intensity value is approximately forty-five percent of the today field VDM. Combined with all of the other published palaeointensity data, possible links between the earth’s interior process and its control on the variation of the earth’s magnetic field during the geological time were tentatively discussed.     

Pollen and spores from Lower Cretaceous in Guyang Basin of Inner Mongolia,China and their palaeoclimatic and paleoecological significance

Research on the climate and vegetation of the Cretaceous is fundamental to understanding the present environment. The present study focuses on the Lower Cretaceous Guyang Formation (Inner Mongolia), which yields unique macrofossils and diverse palynological assemblages that are used to understand the palaeoecology. In total, 56 genera are identified in the examined assemblage, which are dominated by spores and pollen of pteridophytes and gymnosperms. The gymnosperm pollen is dominated by Taxodiaceae, which belong to Coniferales, and a large proportion of pollen attributable to the Cycadophyta and Ginkgopsida. The pteridophyte spores are dominated by taxa attributable to the Lygodiaceae and Cyatheaceae. Quantitative analysis based on the geographical distribution of modern plant communities and the Sporomorph Ecogroup (SEG) model, the palynomorph associations are used to infer the presence of a closed coniferous forest dominated by Podozamitales, Picea and Podocarpus with understory vegetation comprising ferns prevailed. The fossil palyno flora indicates a warm and humid subtropical climate during Early Cretaceous of Inner Mongolia.     

New paleomagnetic and magnetic fabric results for Early Cretaceous rocks from the Turpan intramontane basin,east Tianshan,northwest China

~~New paleomagnetic and magnetic fabric results for Early Cretaceous rocks from the Turpan intramontane basin,east Tianshan,northwest China~~     

Pseudofrenelopsis fossils from Cretaceous gypsum beds in Guixi,Jiangxi Province,China and their geological significance

Compressed fossil plants were found in the gypsum beds of the Zhoujiadian Formation in Guixi City, Jiangxi Province, China. The cuticular features of two species of Pseudofrenelopsis (Cheirolepidiaceae) were examined by scanning electron microscopy in this study: Pseudofrenelopsis papillosa Cao ex Zhou and Pseudofrenelopsis guixiensis Bainian Sun et Dai, sp. nov. A detailed analysis of the internode epidermis, and the adaxial and abaxial leaf cuticles of the two species, was performed, extending our knowledge of the morphological and anatomical characteristics of the extinct Pseudofrenelopsis. A new species, Pseudofrenelopsis guixiensis sp. nov., is proposed based upon comparison of its cuticular features. It differs from previously published species in that it has hairs on the abaxial surfaces of its leaves, sometimes shares subsidiary cells on the surfaces of internodes, lacks a hypodermis, and has no papillae on its epidermal cells. Because the distribution of fossil Pseudofrenelopsis is restricted to the Lower Cretaceous, the Early Cretaceous age of the Zhoujiadian Formation is affirmed. The fossils of Pseudofrenelopsis show obvious xeromorphic characteristics, such as apparent thickening of the cuticles, subsidiary cells with obvious papillae, and anticlinal walls with strong cutinization, and they are found associated with gypsum beds and close to red beds. All of these features indicate an arid climate.     

New Early Cretaceous paleomagnetic results from Qilian orogenic belt and its tectonic implications

Lower Cretaceous red sedimentary rocks from the depositional basin of East Qilian fold belt have been collected for a paleomagnetic study. Stepwise thermal demagnetization reveals two or three components of magnetization from dark red sandstones. Low-temperature magnetic component is consistent with the present Earth Field direction in geographic coordinates. High-temperature magnetic components are mainly carried by hematite. The mean pole of 19 sites for high-temperature magnetic components after tilt-correction is λ=62.2°N, φ=193.4°E, A95=3.2°, and it passes fold tests at 99% confidence level and reversal tests at 95% confidence level. The paleopole is insignificantly different from that of Halim et al. (1998) from the same sampling area at the 95% confidence level. Compared with paleomagnetic results for North China, South China, and Eurasia, our results suggest that no significant relative latitudinal displacement has taken place between Lanzhou region and these blocks since Cretaceous time. Remarkably, the pole of Lanzhou shows a 20° clockwise rotation with respect to those of North China, South China, and Eurasia. Geological information indicates that the crustal shortening in the western part of Qilian is greater than that in eastern part. In this case, the clockwise rotation of sampling area was related to India/Eurasia collision, and this collision resulted in a left-lateral strike-slip motion of the Altun fault in north Tibetan Plateau after the Cretaceous.     

海南岛早白垩世红层磁组构和古地磁新结果

海南岛白垩纪红层是迄今产出古地磁结果最多的地层,但古地磁结果难以在海南岛周边古地磁结果和地质限制条件下作出合理解释.为了更好地认识海南岛白垩纪红层古地磁方向的可靠性,我们对采自前人工作地区的14个采点132个样品开展了古地磁和磁组构的综合研究.磁化率各向异性测试显示14个采点样品平均各向异性度为1.018,线理度为1.014,面理度为1.004.各采点磁化率椭球体最小轴显著偏离地层法线,没有反映出沉积或压实特征.野外调查发现采点区域的节理组交线(代表中间应力轴)与磁化率椭球体中间轴一致,意味着采点磁性矿物的排列方位很可能因构造应力影响发生改变.逐步热退磁显示14个采点分离出的特征剩磁解阻温度高于660℃,方向区别于现代地磁场方向;褶皱检验表明在褶皱展平度为80.4%(95%置信范围内褶皱展平度为77%±12.2%)时精度参数达最大,对应方向为D=359.9°, I=43.4°,κ=70.2,α95=4.8°,与前人的古地磁方向一致.通过与华南地块参考极对比,以及综合分析海南岛围区古地磁和地质限制条件,表明该古地磁方向是不协调的.我们认为前人和本文采样红层剩磁很可能在沉积时获得,但在沉积后受到区域构造应力的影响,致使磁性矿物排列改变,从而导致古地磁方向也发生变化.因此,我们认为这些样品所记录的古地磁方向不能准确反映海南岛白垩纪古纬度.     

Paleomagnetic study on orogenic belt: An example from Early Cretaceous volcanic rocks,Inner Mongolia,China

Since the 1990s, a large number of paleomagneticstudies have been carried out in the North China block(NCB) and Tarim block[1-8], and more and more geo-physicists recently believe that the last collision andconvergence between Siberia and the Mongolia-NorthChina plate happened in the Late Jurassic, which wascontributed to a paleomagnetic study on these areas byZhao and his colleagues[2]. However, we lack paleo-magnetic results obtained directly from the orogenicbelt between Siberia and th…     

Early Cretaceous palynofloral provinces in China: western additions

Two distinct palynofloral provinces have been recognized for the Early Cretaceous of western China, including Xizang (Tibet): one indicated by the presence of the cheirolepidiacean pollen grain Dicheiropollis and the other by abundant representatives of different gymnospermous pollen, especially Araucariacites and Callialasporites. The boundary between them coincides with the Yarlung Zangbo Suture. To the north of the suture, the Xizang–Tarim Dicheiropollis Province occupies a wide region from southern Xinjiang through the central and northern Qinghai–Xizang Plateau to the west of Yunnan. South of the suture, the Southern Xizang Araucariacites‐Callialasporites Province appears to have affinities with the South Gondwanan (floral) Realm. Despite some similarities with the composition of assemblages from other palynofloral provinces, both are distinct and considered to reflect different climates. Hot arid to semi‐arid conditions are indicated by the Dicheiropollis Province, whereas a more humid and somewhat cooler climate is suggested for the Araucariacites‐Callialasporites Province.     

Cretaceous desert cycles,wind direction and hydrologic cycle variations in Ordos Basin: Evidence for Cretaceous climatic unequability

Whether climate under greenhouse condition in geological history, especially in Cretaceous, was eq-uable or not has been acutely debated for a long time. Early studies[1—8] show that the subtropical zones then expanded northwards and southwards to the latitudes about 65S and 65N, while the temperate zones oc-cupied the latitudes of 65—90S and 65—90N. Con-sequently, the two poles were ice-free, equator-to-pole surface temperature gradient was obviously reduced, amplitude of climatic change…     

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.     

Paleomagnetic results of the Cretaceous marine sediments in Tongyouluke, southwest Tarim

Southwest Tarim (hereafter SW Tarim) is one of afew areas that well developed Cretaceous marinesedimentary rocks in China [1]. The Cretaceous marinesediments are stretched in front area along the Tian-shan and Kunlun Mountains. Toward the center ofTarim Basin, the Cretaceous sediments are buried bygreat thickness of Tertiary and Quaternary sedimentswith little exposure. Compared with the Cretaceousterrestrial strata of north Tarim, the Cretaceous marinestrata of SW Tarim continue and d…     

Strike–slip origin of Cretaceous Mazhan Basin, Tan-Lu Fault Zone, Shandong, east China

The Mazhan Basin, Shandong Province, China, is located between the main faults, F₃ and F₄, of the Tan-Lu Fault Zone. It is an elongated basin more than 60 km in length and 8 km in width and contains a series of typical continental sediments (the Upper Cretaceous Wangshi Group). This series was divided into three sedimentary facies associations: conglomerate facies association; sandstone facies association of alluvial fan to lake margin environment; and siltstone facies association of lacustrine origins. Their zonal distribution pattern may represent a contemporaneous heterotopic facies due to a lateral facies change from margins to axis of the basin. Their stratigraphic sequence becomes younger northward along the boundary faults. This suggests that the depocenter of the fan–lake system tends to migrate northward along F₃. From the asymmetric features (i.e. basin shape, lithofacies distribution, facies change) the Mazhan Basin can be explained by progressive subsidence at the Tangwu releasing bend of F₃ with sinistral strike–slip movement. Judging from the fission track (FT) ages from the Wangshi Group, it was concluded that a sinistral strike–slip movement along the main fault, F₃ of the Tan-Lu Fault in Shandong, has lasted until the Late Cretaceous. Its displacement is estimated to be larger than the migrated distance, 60 km, of the depocenter of the Mazhan Basin.     

Geochemical characteristics of Cretaceous basaltic rocks in South China and constraints on Pacific plate subduction

The South China, including Yangzi Craton and the Cathaysian Block, belongs to the southern part of East Asia continent. It borders Pacific plate on the east side and Qinling-Dabie Orogen on the north side. During the middle-late Cretaceous, a number of downfaulted red basins (the terrestrial sedimentary basins in Fig. 1) and volcanic- sedimentary basins had been developed in South China[1,2], in which the con-temporaneous basaltic rocks were generally distributed (Fig. 1). Although the …     

黑龙江东部白垩纪-古近纪古地磁初步结果及其构造意义

陆内块体旋转是周边构造环境和深部构造活动相互作用的结果.前人研究表明华北东部和俄罗斯远东地区晚中生代以来的块体旋转样式,很可能以牡丹江断裂为界发生了显著变化.进一步对牡丹江断裂两侧块体晚中生代以来的块体旋转样式的限定,有助于正确理解这一差异旋转的机制.对采自黑龙江省东部白垩纪和古近纪岩石的(51个采点)古地磁学研究表明,相对于稳定欧亚大陆,牡丹江断裂东侧的佳木斯地块内部的穆棱、鸡西、七台河和桦南地区旋转样式一致,整体发生了30°~40°的逆时针旋转,逆时针旋转很可能发生在晚白垩世末之后.华北东部及俄罗斯远东地区的差异性相对旋转很可能与白垩纪以来太平洋板块的俯冲作用和作为深俯冲带的牡丹江断裂的重新活化有关.     

内蒙古和林格尔6.3级地震前地震的空段异常

1976年4月6日内蒙古和林格尔6．3级地震是华北北部地区1次重要的地震，该地震前出现了明显的地震空段异常。介绍了该地震空段的时空特征，分析了地震空段的形成至解体与中强地震成组活动的关系。认为，地震空段可作为华北北部地区中强地震预测的中期判据，这对该地区地震预测有帮助意义。     

Petrogenesis and tectonic implications of Early Cretaceous high-K calc-alkaline volcanic rocks in the Laiyang Basin of the Sulu Belt, eastern China

Abstract Early Cretaceous high‐K calc‐alkaline volcanism occurring in the Laiyang Basin north of the Sulu high‐pressure to ultrahigh‐pressure (HP‐UHP) Metamorphic Belt, eastern China, comprises a wide spectrum of rock types, ranging from trachybasalts to trachydacites. The basaltic–andesitic rocks erupted at 107–105 Ma, spanning an SiO₂ range of 50.1–59.6% and an MgO range of 2.6–7.2%, and are characterized by large ion lithophile element (LILE; e.g. Ba and K) and light rare earth element (LREE) enrichment, high field strength element (HFSE) depletion and highly radiogenic Sr but non‐radiogenic Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr(i) = 0.70750–0.70931; ?_Nd(t) = ?17.9 ? ?15.6). The geochemical similarities between these rocks and the earlier Sulu Belt lamprophyres suggest that both types of mafic rocks were derived from similar mantle sources with LILE and LREE enrichment. Thus, the Wulian–Qingdao–Yantai Fault that separates the two terranes at the surface should not be considered as a lithospheric boundary between the North China and Yangtze blocks. The felsic lavas erupted at 93–91 Ma, spanning an SiO₂ range of 61.6–67.0% and an MgO range of 1.1–2.6%, and show a trace element geochemistry similar to the basaltic rocks, but with higher radiogenic Sr and even lower Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr(i) = 0.70957–0.71109; ?_Nd(t) = ?19.1 ? ?17.5), similar to I‐type granitoids in the Sulu Belt. A crustal origin was proposed to explain their compositions (which are comparable to those of experimental slab melts), the >10 Ma eruption interval and the compositional gaps in some elements (e.g. P, Ti and Sr) between them and the older basaltic–andesitic rocks. These melts were derived from predominant metaigneous protoliths containing mafic accumulative counterparts of the basaltic–andesitic and/or lamprophyric magmas. The extensive extrusion of Early Cretaceous high‐K calc‐alkaline rocks in the Laiyang Basin favored an extensional regime in response to the progressive attenuation of the thickened lithosphere and orogenic collapse, as reflected in the development of the basin from a foreland basin (before the end of the Jurassic period) to a fault basin (since the Early Cretaceous period).     

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.     

松辽盆地营城组火山机构相带地震-地质解译

将火山机构按距火山口远近划分为火山口-近火山口、近源和远源三个相带.营城组火山机构相带有6种地震相类型,分别是丘状、透镜状、穹状、池塘状、楔状和席状地震相.丘状、透镜状和穹状均见于火山机构中心相带,但所代表的优势岩相不同,分别与爆发相、喷溢相和侵出相对应.池塘状和楔状均为近源相带,但前者以喷溢相辫状熔岩流为主,而后者代表爆发相火山碎屑岩与喷溢相熔岩互层.席状地震相是以火山沉积相为特征的远源相带为主.火山机构中心相带岩性和岩相多变、地震相复杂,这是初始喷发条件、同生垮塌、后期断裂以及孔隙和流体综合作用的结果;该带是火山岩储层发育最有利的部位.     

Late Cretaceous palynology and paleoclimate change:Evidence from the SK1(South) core,Songliao Basin,NE China

Cretaceous climate was warmer than today.The Songliao Basin contains one of the most important Late Cretaceous non-marine deposits in China for the research of the paleoenvironment and paleoclimate.This research is based on core samples from the SK1(S)borehole.The strata sampled are the upper part of the Quantou Formation to member 2 of the Nenjiang Formation,where spores,pollen,dinoflagellates,and other microfossils are abundantly preserved.Based on analysis of the spores and pollen fossils from the core samples,the following six fossil assemblage zones have been recognized in ascending order:The Cicatricosisporites-Cyathidites-Pinuspollenites,Schizaeoisporites-Cyathidites-Classopollis,Cyathidites-Schizaeoisporites,Schizaeoisporites-Cyathidites-Proteacidites,Proteacidites-Cyathidites-Dictyotriletes,and the Lythraites-Callistipollenites-Schizaeoisporites zones.The six fossil zones range from the late Cenomanian to early Campanian.The Late Cretaceous dinoflagellate cysts in the Songliao Basin are of high abundance and low diversity.Specific phytoplankton types reflect salinity changes of the Songliao Lake.Paleoecology of the dinoflagellates suggests that sediments of members 2 and 3 of the Yaojia Formation(K2y2+3)were deposited in a freshwater environment,whereas members 2 and 3 of the Qingshankou Formation(K2q2+3)and members 1 and 2 of the Nenjiang Formation(K2n1+2)were deposited in freshwater to brackish water environments.Combined with the paleoecology of dinoflagellates and the palynomorph biozones,valuable information of the paleoclimate was provided.The quantitative analyses of spores and pollen fossils,such as vegetation type,climate type,and humidity type,diversity and dominance,indicate a relatively sub-humid,mid-subtropical paleoclimate,with slight climatic fluctuation and/or temporal change.