新近纪海相生物地层事件年龄新编

介绍新近纪海相生物地层研究的新进展。2004年新的"国际地质年代表"以405ka偏心率长周期为基础来划分新近纪的主要地层界线,标志着轨道地层学时代的到来。新生代包括E1—E162偏心率长周期,底界的年龄为65.5±0.3Ma,其中新近纪有E1—E58周期,底界年龄为23.03Ma,并且根据天文调谐得出一系列新的古地磁年龄和生物事件年龄。由于这些新年龄,特别是接近新近纪底部的年龄与早期结果相差0.5Ma或更大,所以将其汇总介绍,以求与当前国际研究保持同步衔接,增强海洋新近纪高分辨率地层工作的准确性。     

Lake George revisited: New evidence for the origin and evolution of a large closed lake,Southern Tablelands,NSW, Australia

Lake George contains the longest continuous sedimentary record of any Australian lake basin, but previous age models are equivocal, particularly for the oldest (pre-Quaternary) part of the record. We have applied a combination of cosmogenic nuclide burial dating, magnetostratigraphy and biostratigraphy to determine the age of the basal (fluvial) unit in the basin, the Gearys Gap Formation. Within the differing resolutions achievable by the three dating techniques, our results show that (i) the Gearys Gap Formation, began accumulating at ca 4 Ma, in the early Pliocene (Zanclean), and (ii) deposition had ceased by ca 3 Ma, in the mid late Pliocene (Piacenzian). Whether the same age control provides an early Pliocene (Zanclean) age for the formation of the lake basin is uncertain. During the Piacenzian, the vegetation at the core site was a wetland community dominated by members of the coral fern family Gleicheniaceae, while the surrounding dryland vegetation was a mix of sclerophyll and temperate rainforest communities, with the latter including trees and shrubs now endemic to New Guinea–New Caledonia and Tasmania. Mean annual rainfall and temperatures are inferred to have been ～2000–3000 mm, although probably not uniformly distributed throughout the year, and within the mesotherm range (>14°C <20°C), respectively. Unresolved issues are: (1) Does the basal gravel unit predate uplift of the Lake George Range and therefore provide evidence that one of the proposed paleo-spillways of Lake George, that above Geary's Gap, has been elevated up to 100–200 m by neotectonic activity over the past 4 million years? (2) Did a shallow to deepwater lake exist elsewhere in the lake basin during the Pliocene?     

青藏高原东南缘川滇地块古近纪沉积地层古地磁分析及其构造意义

针对中国西部及中亚地区以红色陆相沉积岩为主的白垩纪—新生代地层中普遍存在古地磁数据磁倾角偏低的现象,对青藏高原东南缘川滇地块中部大姚地区古近纪陆相红层进行了磁倾角偏低研究。实验结果表明,川滇地块内部古近纪陆相红层古地磁数据不存在明显的磁倾角偏低的现象。结合前人对川滇地块内部古地磁数据及周缘新生代走滑断裂的活动演化历史研究,确定川滇地块自中中新世以来川滇地块内部表现为刚性旋转,楚雄地区则发生局部构造旋转叠加。     

Secondary remanent magnetization carried by magnetite inclusions in silicates: a comparative study of unremagnetized and remagnetized granites

Magnetic carriers in remagnetized Cretaceous granitic rocks of northeast Japan were studied using paleomagnetism, rock magnetism, optical microscopy and scanning electron microscopy (SEM) by comparison with unremagnetized granitic rocks. The natural remanent magnetization (NRM) of the remagnetized rocks is strong (0.3–1.7 A/m) and shows a northwesterly direction with moderate inclination (NW remanence), whereas the unremagnetized rocks preserve weak NRM (<0.5 A/m) with westerly and shallow direction (W remanence). Although thermal demagnetization shows that both NRMs are carried by magnetite, the remagnetized rocks reveal a higher coercivity with respect to alternating field demagnetization (20 mT相似文献   

Early Miocene rotational process in the eastern part of south-west Japan inferred from paleomagnetic studies

Abstract This paper provides untilted paleomagnetic data obtained from the early Miocene strata around the Kanazawa‐Iozen area, in the eastern part of south‐west Japan. A thick pile of volcaniclastics and marine transgressive sediments underlie the area; they were deposited in the early stage of the Japan Sea opening event. Progressive thermal demagnetization tests isolated stable primary magnetic vectors from eight sites in the upper part of the Iozen Formation. Overall, the tilt‐corrected mean direction of this unit is D = 36.4°, I = 51.6° and α₉₅ = 12.1. Together with a published paleomagnetic and chronological database, the present results suggest that clockwise rotation of south‐west Japan, linked to the back‐arc opening, commenced in the early Miocene and accelerated at the same time as rapid subsidence along the Japan Sea coast. Post‐opening, differential rotation within the eastern part of south‐west Japan is assumed, based on selected paleomagnetic data from the latest Early Miocene.     

Stable Eocene Magnetization Carried by Magnetite and Iron Sulphides in Marine Marls (Pamplona-Arguis Formation,Southern Pyrenees,Northern Spain)

In order to establish the magnetic carriers and assess the reliability of previous paleomagnetic results obtained for Eocene marine marls from the south Pyrenean basin, we carried out a combined paleo- and rock-magnetic study of the Pamplona-Arguis Formation, which crops out in the western sector of the southern Pyrenees (N Spain). The unblocking temperatures suggest that the characteristic remanent magnetization (ChRM) is carried by magnetite and iron sulphides. The ChRM has both normal and reversed polarities regardless of whether it resides in magnetite or iron sulphides, and represents a primary Eocene magnetization acquired before folding. Rock magnetic results confirm the presence of magnetite and smaller amounts of magnetic iron sulphides, most likely pyrrhotite, in all the studied samples. Framboidal pyrite is ubiquitous in the marls and suggests that iron sulphides formed during early diagenesis under sulphate-reducing conditions. ChRM directions carried by magnetic iron sulphides are consistent with those recorded by magnetite. These observations suggest that magnetic iron sulphides carry a chemical remanent magnetization that coexists with a remanence residing in detrital magnetite. We suggest that the south Pyrenean Eocene marls are suitable for magnetostratigraphic and tectonic purposes but not for studies of polarity transitions, secular variations and geomagnetic excursions, because it is difficult to test for short time differences in remanence lock-in time for the two minerals. The presence of iron sulphide minerals contributing to the primary magnetization in Eocene marine marls reinforces the idea that these minerals can persist over long periods of time in the geological record.     

古生代中国中西部三大陆块古地理位置重建与演变

针对古生代中国中西部华北、华南和塔里木三大陆块在全球洋-陆格局中的古地理位置还存在的争议问题,本论文以国际最新的古地理位置重建研究方法和思路,在对中国三大陆块盆地(鄂尔多斯、四川和塔里木盆地)古生界钻井岩心的古地磁实测研究、全球古生代古地磁数据收集与有效性筛选处理、全球主要地质事件约束等多参数融合分析的基础上,采用最新的 GPlates 板块重建方法,对中国华北、华南和塔里木三大陆块在全球洋-陆格局中的古地理位置进行了重建和定位。研究结果表明:古生代三大陆块主要在全球 ±30° 之间的南北中低纬度之间迁移;三大陆块在古生代至少发生了 3 次不同的顺时针旋转和方位角转换;三大陆块运移速率至少经历了 3 次以上不同高、低速度间的转换与变化过程;响应于古生代全球洋-陆形成与演化,中国三大陆块古构造格局总体上经历了洋盆扩张下的“多岛洋”离散、俯冲碰撞下的离散-汇聚并存、俯冲消减下的差异汇聚隆升、新旧洋盆转换下的差异汇聚-离散、拼合与地幔柱控制下的差异汇聚-离散内部拉张的差异性演变过程。古生代中国三大陆块在全球洋-陆格局中的位置与差异性演变,奠定了中国三大陆块古生代不同性质盆地的形成与演化、不同层系油气烃源、储集原始物质差异性发育的基础。     

Paleomagnetic studies of the Late Mesozoic deformation stage in the southern Primorye region

The analysis of paleomagnetic data available for the southern Primiorye region revealed that the studied objects were magnetized under regional remagnetization presumably during the Late Mesozoic folding and this magnetization can be interpreted as being synfolding. The interpretation is based on the parameter that characterizes the folding completion degree immediately before regional remagnetization. It is shown that the relaxation of Late Mesozoic horizontal stresses was irregular. The obtained estimates of the degree of folding completion are consistent with the available geological data and Talitskii’s model for tectonic deformations.     

中国中部全新世古地磁长期变和相对古强度研究

高分辨率的古地磁场长期变（Paleomagnetic Secular Variation,简称PSV）和相对古强度（Relative Paleomagnetic Intensity,简称RPI）研究是解决全新世湖泊研究定年难题的有效途径之一,也能为认识区域古地磁场的变化提供不可或缺的基础数据。然而,目前我国中部地区仍缺少年代序列可靠、时间分辨率在百年尺度的全新世PSV和RPI记录。先前的研究显示山西宁武公海全新世岩芯具有稳定的粉砂质粘土湖泊沉积,并通过25个（本文在全新世时段用到18个）陆生植物残体AMS ¹⁴C测年建立了可靠的高分辨率年代序列,为重建高质量的全新世PSV和RPI记录提供了良好条件。本文通过特征剩磁磁倾角的变化建立了PSV序列,利用磁化率（χ）、非磁滞剩磁（ARM）、饱和等温剩磁（SIRM）等环境磁学参数对特征剩磁强度（NRM_25mT）进行归一化,估算了RPI变化序列。结果表明,公海全新世湖泊沉积物的PSV和RPI记录与北半球其他湖泊和海洋记录具有较好的一致性,同时存在着显著的区域差异,可以作为我国中部地区湖泊沉积的对比曲线。