Bending within the axial tectonic belt of New Zealand in the last 9 Myr from paleomagnetic data

Tongaporutuan Stage (Late Miocene, 11-6 Myr) sedimentary rocks at two localities within the axial tectonic belt of New Zealand have a direction of magnetisation in which the declination is 020 ± 11° and 030 ± 8° (D ± Δ₉₅). The directions of magnetisation in rocks of similar age outside the belt have a declination like that of the earth's present axial dipole, 360°. The clockwise deviation of the declination is inferred to indicate bending of the rocks within the zone of deformation between the Pacific and Indian plates due to the oblique convergence of these plates.     

Decade fluctuations in the rotation rate of the earth in the last 200 years

Maximum Entropy Spectral Analysis and Multiple Regression Analysis of the 200 year series (1781–1980) of the change in L.O.D. (Length of Day) revealed significant peaks atT=21, 32, 50, 63 and possibly 120 years. The periodsT=22 and 44 years seem to be stable over 200 years.     

Quaternary tectonic rotation in central Japan: Paleomagnetism of the Eboshidake volcanic rocks

Abstract   Neotectonic crustal deformation in central Japan near a triple-junction of plates is investigated on the basis of paleomagnetic data. The progressive thermal demagnetization test isolated characteristic remanent magnetization from 18 sites of the early Quaternary Eboshidake volcanic rocks erupted around the termination of active strike-slip faults. The site-mean directions show considerably large scatter in declinations, and easterly deflection in average (D_m = −161.7°). On the basis of inclination statistics, measured inclinations (I_m = −48.9°, δI = 6.6°) are concordant with an expected value from latitude of the study area. Because the sampling was planned to cover a wide stratigraphic range and eliminate the effect of geomagnetic secular variation, an easterly deflection is attributed to clockwise rotation around vertical axis. Together with previous paleomagnetic data, the present study indicates that clockwise-rotated areas in central Japan are aligned on a northeast–southwest recent shear zone delineated through geodetic survey. Deflection and scatter of paleomagnetic declinations of the Eboshidake volcanic rocks are much greater than those extrapolated from a recent strain rate, and might be explained by complicated motion anticipated at fault terminations and/or enhanced crustal rotation under elevated temperatures around a Quaternary volcanic province.     

Large landslides and their effect on sediment flux in South Westland,New Zealand

Landslides and runoff are dominant erosional agents in the tectonically active alpine South Westland area of New Zealand, characterized by high uplift rates and extreme orographic precipitation. Despite a high density of shallow debris slides and flows, the geomorphic imprints of deep‐seated bedrock failures are dominant and persistent. Over 50 large (>1 km²) landslides comprising rock slide[sol ]avalanches, complex rotational and rock‐block slides, wedge failures, and deep‐seated gravitational slope deformation were detected on air photos and shaded‐relief images. Major long‐term impacts on alpine rivers include (1) forced alluviation upstream of landslide dams, (2) occlusion of gorges and triggering of secondary riparian landslides, and (3) diversion of channels around deposits to form incised meandering gorges. Remnants of large prehistoric (i.e. pre‐1840) landslide deposits possibly represent the low‐frequency (in terms of total area affected yet dominant) end of the spectrum of mass wasting in the western Southern Alps. This is at odds with high erosion rates in an active erosional landscape. Large landslides appear to have dual roles of supplying and retaining sediment. The implications of these roles are that (1) previous models of (shallow) landslide‐derived sediment flux need to be recalibrated, and (2) geomorphic effects of earthquake‐induced landsliding may persist for at least 10² years. Copyright © 2005 John Wiley & Sons, Ltd.     

青藏高原东南缘构造旋转的古地磁学证据

本文在总结青藏高原东南缘近年来地质研究进展的基础上,从古地磁学的角度讨论其新生代以来的构造运动特征.结果表明:相对稳定的欧亚大陆,新生代以来山泰地块发生了约20°～80°顺时针旋转,局部地区旋转量甚至高达135°,且中部地区的旋转量明显高于南北地区;印支地块经历了～30°的顺时针旋转;川滇地块的顺时针旋转量沿102°E经度线由南向北由30°逐渐减小;另一方面,古地磁数据还揭示出山泰地块新生代以来发生了～8°的南向滑移运动.旋转量随时间的变化表明主要构造旋转发生在始新世与中中新世之间,与哀牢山—红河断裂的左行走滑时间相一致.这表明青藏高原东南缘的新生代构造运动具有差异性和复杂性,现今国际流行的挤出逃逸、地壳缩短增厚及下地壳流模式均有其局限性.值得注意的是,青藏高原东南缘可靠的新生代古地磁数据在时空分布上的严重不足,制约了我们对印度与欧亚大陆碰撞在青藏高原东南缘的运动学响应过程的深入探讨和正确理解.因此,进一步对该地区新生代地层开展深入细致的古地磁学等综合研究,无疑具有重要的科学意义.     

Seismology in New Zealand

New Zealand is a country of moderate seismicity. Its earthquakes are well-located and their magnitudes found with closely-spaced modern seismographs. Historical catalogues have been prepared. Deep and shallow shocks are associated with two active continental margins having the usual geophysical associations. The northern one is orientated towards the Pacific and the southern one towards the Tasman Sea. The structure is complex, and below the continental-type crust there are large lateral inhomogeneities in the upper mantle. There is a marked lack of correlation between all except the most superficial earthquake foci and the geological faults and this persists to the microearthquake level. New Zealand seismologists have made theoretical studies of earthquake mechanism and examined the statistical properties of earthquake occurrence. They have also studied the fine structure of the Earth's core, and made microzoning and other studies with engineering implications.     

The paleoclimate change of Qaidam Basin during the last 2.85 Ma recorded by Gamma-ray logging

The geographical location of Qaidam Basin corresponds to N36°05’-38°10’ and E90°10’-98°05’. The thickness of the Quaternary deposits in the basin reaches 2000-3000 m. This paper deals mainly with the aspect of paleoclimate change of the basin. The approach used for the study consisted of (1) establishing the paleoclimate curve of the basin according to the climate indices, including the spore-pollen statistics, carbonate content and δ¹⁸O, obtained from the samples of the 760 m continuously-cored exploration holes sited in the basin, (2) comparing the paleoclimate curve with the natural gamma-logs of the same exploration holes, and (3) determining the paleoclimatic changes in the last 2.85 Ma on the basis of detailed geochronological calculations. These results are extremely important for comparison with the δ¹⁸O-based climate curve derived from the ODP659 in the Atlantic Ocean and with the cycles of paleosol in the Chinese loess.     

Paleogeographic maps of the Japanese Islands: Plate tectonic synthesis from 750 Ma to the present

Abstract A series of paleogeographic maps of the Japanese Islands, from their birth at ca 750–700 Ma to the present, is newly compiled from the viewpoint of plate tectonics. This series consists of 20 maps that cover all of the major events in the geotectonic evolution of Japan. These include the birth of Japan at the rifted continental margin of the Yangtze craton ( ca 750-700 Ma), the tectonic inversion of the continental margin from passive to active ( ca 500 Ma), the Paleozoic accretionary growth incorporating fragments from seamounts and oceanic plateaux ( ca 480-250 Ma), the collision between Sino-Korea and Yangtze (250–210 Ma), the Mesozoic to Cenozoic accretionary growth (210 Ma-present) including the formation of the Cretaceous paired metamorphic belts (90 Ma), and the Miocene back-arc opening of the Japan Sea that separated Japan as an island arc (25-15 Ma).     

中国西南思茅地体中部白垩纪古地磁结果及陆内地壳变形特征

对我国西南地区思茅地体中部巍山和五印地区白垩纪地层进行了详细的岩石磁学和古地磁研究,获得了两个地区的高温剩磁分量并通过了褶皱检验.巍山剖面特征剩磁方向为Ds=64.3°,Is=48.5°,k=54.6,α₉₅=4.7°;五印剖面特征剩磁方向为Ds=15.4°,Is=44.8°,k=212.0,α₉₅=4.6°.通过思茅地体磁偏角变化与兰坪-思茅褶皱带构造线迹变化的相关性分析,确定思茅地体内部差异性旋转变形受控于思茅地体弧形构造带的形成和演化.通过青藏高原东南缘走滑断裂带活动年代分析,确定兰坪-思茅褶皱带蜂腰构造部位形成于两期构造事件,早期构造变形与东喜马拉雅构造结北北东向挤压缩进有关,后期构造变形与川滇微地块发生顺时针旋转时南向挤出运动有关.以华南板块稳定区白垩纪古地磁极为参考极,计算得出巍山和五印相对于华南板块分别发生了10.5°±6.0°和3.8°±4.9°的南向运移量.通过选取思茅地体内部构造形态较稳定的巍山和普洱地区白垩纪古地磁极为参考极,计算得出五印相对于巍山和普洱分别发生了3.4°±5.0°和3.1°±5.4°的北向纬向运移,表明五印和和巍山之间自印亚碰撞以来经历了较大规模的北向地壳缩短变形作用.     

中国西南思茅地体中部白垩纪古地磁结果及陆内地壳变形特征

对我国西南地区思茅地体中部巍山和五印地区白垩纪地层进行了详细的岩石磁学和古地磁研究,获得了两个地区的高温剩磁分量并通过了褶皱检验.巍山剖面特征剩磁方向为Ds=64.3°,Is=48.5°,k=54.6,α95=4.7°;五印剖面特征剩磁方向为Ds=15.4°,Is=44.8°,k=212.0,α95=4.6°.通过思茅地体磁偏角变化与兰坪—思茅褶皱带构造线迹变化的相关性分析,确定思茅地体内部差异性旋转变形受控于思茅地体弧形构造带的形成和演化.通过青藏高原东南缘走滑断裂带活动年代分析,确定兰坪—思茅褶皱带蜂腰构造部位形成于两期构造事件,早期构造变形与东喜马拉雅构造结北北东向挤压缩进有关,后期构造变形与川滇微地块发生顺时针旋转时南向挤出运动有关.以华南板块稳定区白垩纪古地磁极为参考极,计算得出巍山和五印相对于华南板块分别发生了10.5°±6.0°和3.8°±4.9°的南向运移量.通过选取思茅地体内部构造形态较稳定的巍山和普洱地区白垩纪古地磁极为参考极,计算得出五印相对于巍山和普洱分别发生了3.4°±5.0°和3.1°±5.4°的北向纬向运移,表明五印和和巍山之间自印亚碰撞以来经历了较大规模的北向地壳缩短变形作用.     

New paleontological evidence on time determination of the east part of the Eastern Kunlun Mélange and its tectonic significance

New paleontological evidence suggests that the composition of the matrix and slices from the middle and the south of the Eastern Kunlun Melange Belt is very complex, ranging from Proterozoic to Mesozoic in geological time. A Cambrian acritarch assemblage has been discovered from the middle part of the Eastern Kunlun Melange, a Neoproterozoic to the early Paleozoic acritarch assemblage has been discovered from the south of the Eastern Kunlun Melange,and so has been an Early Permian radiolarian from the A'nyemaqen Melange in Buqingshan.In addition, some sporopollen has been obtained from the Mesozoic tectonic slices. The above-mentioned paleontological evidence indicates that the Eastern Kunlun Orogenic Belt experienced two episodes from ocean to continent from the Neoproterozoic to the Early Paleozoic and in the Late Paleozoic respectively. In the process of intracontinent development in Mesozoic, because of heavy thrust-nappe, strike-slip sheer and crust shortening, the Mesozoic formation was intercalated in melange by slices and the Eastern Kunlun Orogenic Belt became more complex.     

Entanglement of New Zealand fur seals in man-made debris at Kaikoura, New Zealand

New Zealand fur seals in the Kaikoura region breed near a town with expanding tourist and fishing industries and commonly come ashore entangled in nets and plastic debris. However, the rate at which entanglement occurs was previously unknown. A decade of Department of Conservation seal callout data was analysed to determine the level of entanglement in the region and the most common debris type. Monitoring of adult female fur seals released from entanglement provided information on the potential for serious wounds to heal and survivorship of released individuals. Entanglement rates of pinnipeds in Kaikoura are some of the highest reported world-wide (average range: 0.6-2.8%) with green trawl net (42%), and plastic strapping tape (31%) together contributing the most to debris types. Nearly half of the reported entangled seals are successfully released (43%) and post-release monitoring shows that with appropriate intervention the chance of an individual surviving even with a significant entanglement wound is high. Our study demonstrates that while entanglement in the region is high, a successful intervention protocol may help reduce the potential for entanglement-related mortality in the region.     

Eolian sedimentary records for the evolution of monsoon and westerly circulations of northern China in the last 2.6 Ma

Atmospheric circulation for transportation and deposition of eolian sediments of northern China has been an important palaeoclimatic issue of loess research. The first approach to the issue clarified the role of westerly circulation on transportation and sedimentation of loess[1]. The pa- leomonsoon theory interpreted the loess-paleosol sequences as the products of Asian monsoon circulation[2,3]. The source areas, passage way and deposition areas of eolian dust of northern China are signi…     

A new pollen record of the last 2.8 Ma from the Co Ngoin,central Tibetan Plateau

A new pollen record from the lake of Co Ngoin in the central Tibetan Plateau provides information on the vegetation and climate changes during the last 2.8 Ma. Seven major significant changes in pollen associations indicate the processes of vegetation change and possible tectonic uplifts. The seven changes in vegetation succession include a temperate montane conifer and broad-leaved mixed forest, cold temperate montane dark conifer forest, alpine shrub-meadow and alpine desert, montane dark coniferous forest and alpine shrub meadow, montane dark coniferous forest and alpine shrub meadow, montane dark coniferous forest and alpine meadow, and alpine desert and meadow. The pollen record provides the evidence of at least five times tectonic uplifts occurring at about 2.58 Ma, 1.87 Ma, 1.17 Ma, 0.83 Ma, and 0.3 Ma ago, respectively. Before 0.8 Ma, this region maintained the altitude below 4000 m a.s.l. Larger amplitude of uplift occurring at about 0.8 Ma ago enforced the plateau rising into cryosphere, shaping the basic topographic pattern of modern plateau. The major successions in vegetation of this area were largely controlled by stepwise uplift of the Tibetan Plateau.

     

A study on characteristics of tectonic block motion and tectonic setting of strong earthquakes in northern part of the Shanxi fault depression zone

Introduction Shanxi fault depression zone (SFDZ) is one of important Cenozoic fault basin zones and strong earthquake belts in Chinese mainland. Its northern part has aroused wide research interests due to the complicated tectonics and high activity of strong earthquakes there. Early researches on this depression zone were carried out since 60s of last century (DENG, et al, 1973; DENG, YOU, 1985; LU, DING, 1985; XU, 1990; XU, et al, 1996, 2002). In 90s of last century, the geologica…     

Exploring variations in upper ocean structure for the last 2 Ma of the Nansha area by means of calcareous nannofossils

A great number of calcareous nannofossils have been found in the deep-sea sediments of 2.32 Ma at ODP Site 1143 located in the Nansha area, the southern South China Sea. The number of coccoliths varies from about 0.5×10⁶ up to almost 53×10⁶ coccoliths/g sediment, with an average of 16×10⁶ coccoliths/g sediment. The accumulation rate of total coccoliths varies from 1×10⁶ to 278×10⁶ coccoliths/cm² ka. The nannofossil assemblages are usually dominated by a lower-photic species-Florisphaera profunda, of which the average percentage is about 70% in all samples. The absolute abundance and the accumulation rate of nannofossils as well as the percentage ofF. profunda display significant oscillations on two different time scales. One is the fluctuation coincident with the glacial-interglacial cycle, and the other is the long-term changes on a time scale longer than 100 ka. Six evolutionary stages of calcareous nannofossils could be divided for the last 2.32 Ma, from which we can reconstruct the changes in the depth of nutricline of the Nansha area. In this paper, the possible mechanism resulting in these variations is also discussed.     

Analysis of the seismicity in the central part of the Pyrenees (France), and tectonic implications

Two temporary seismological networks have been set up in 2000 and 2002 in the central part of the Pyrenees, in a region, which appears as a transition between two domains where both the seismic activity and the tectonic regime are different. Together with the permanent networks, they allowed us to obtain precise hypocenter locations for more than 400 events with local magnitudes ranging from 1.5 to 4.6, as well as 30 new focal mechanisms. The seismicity is distributed in several clusters, which are not located along the North Pyrenean Fault, considered as the major tectonic accident resulting from the suture of the Iberian and Eurasian convergent plates when the range formed. Several small fault segments dipping to the north are identified. The maximum focal depth varies from 10 to 20 km, with variations which are roughly parallel to those of the Moho, indicating a thickening of the seismogenic layer to the east of the studied area. The obtained focal solutions reveal a predominance of normal faulting to the West and reverse faulting to the East, with strike-slip motions in between. The largest fault segment to the East, with a length of about 20 km, could possibly be related to a large historical event which occurred in 1660, with intensity IX, close to cities which have become since then important touristic centres.     

Tremendous change of the earth surface system and tectonic setting of salt-lake formation in Yuncheng Basin since 7.1 Ma

The Yuncheng salt lake has formed under the setting of stepped subsidence of fault-blocks from the north to the south in Yuncheng Basin. In the phase of red clay accumulation during 7.1-3.6 Ma, the size of palaeo-lake was larger than the present salt lake, and palaeo-monsoon had formed. At 3.6 Ma, the northern basement in the basin raised abruptly due to the radiative effect of Qinghai-Tibet Plateau uplifting, and palaeo-lake was contracting southwards. At ca. 2.6 Ma ancient river flowed into the northern part of the basin. During ca. 2.0-1.9 Ma aerolian effect strengthened and loess started to accumulate on the most part of the basin. Since ca. 1.8-1.0 Ma the subsidence of the lake fault-block has been speeding up abruptly. As under the natural hydrogradient the salt lake received enough groundwater supply, and the rate of loess accumulation in the lake area was lower than that of subsidence of the lake fault-block, the lake could be preserved and becomes the only modern lake on Chinese Loess Plateau. Four large strengthening change records of the monsoon were found in the lake sequence of 5.8-1.9 Ma B.P.     

Volcanic ash soils in New Zealand

Ash and lapilli derived from Holocene and Late Pleistocene volcanic cruptions cover approximately half of the North Island of New Zealand. Deposits from 16 separate series of showers are recognised in the surface soils and areas with 3 inches and more of each ash bed have been mapped out during soil surveys. These and other shower deposits are also recognised and described in fossil soils. The effects of the volcanic ashes on soil properties are related to the thickness and frequency of deposition, to mineral composition, and to the period of exposure to soil formation. Soils formed from shower deposits of Holocene age are extensive and have characteristic properties of friability, fine granular structure, deep humic topsoils and slippery non-sticky clays. The clays are chiefly amorphous and responsible for the notable practical properties of free drainage, high moisture retention, low bulk density, and critical limits of stability under pressure. In New Zealand the development of these characteristies increases with time up to about 15,000 years and then decreases with the crystallisation of the clays to halloysite. Soils formed from Late Pleistocene volcanic ashes are high in halloysite clay and with increasing age resemble soils formed from marine sedimentary rocks. Differing degrees of development of the characteristics are shown in the soil classification. The soils from volcanic ash are widely used for crop, pasture or forestry production. More intensive permanent usage depends on more detailed knowledge of the materials which will be obtained through combined volcanological and pedological research.     

Large Occurrence Patterns of New Zealand Deep Earthquakes: Characterization by Use of a Switching Poisson Model

The paper studies the statistical properties of deep earthquakes around North Island, New Zealand. We first evaluate the catalogue coverage and completeness of deep events according to cusum (cumulative sum) statistics and earlier literature. The epicentral, depth, and magnitude distributions of deep earthquakes are then discussed. It is worth noting that strong grouping effects are observed in the epicentral distribution of these deep earthquakes. Also, although the spatial distribution of deep earthquakes does not change, their occurrence frequencies vary from time to time, active in one period, relatively quiescent in another. The depth distribution of deep earthquakes also hardly changes except for events with focal depth less than 100 km. On the basis of spatial concentration we partition deep earthquakes into several groups—the Taupo-Bay of Plenty group, the Taranaki group, and the Cook Strait group. Second-order moment analysis via the two-point correlation function reveals only very small-scale clustering of deep earthquakes, presumably limited to some hot spots only. We also suggest that some models usually used for shallow earthquakes fit deep earthquakes unsatisfactorily. Instead, we propose a switching Poisson model for the occurrence patterns of deep earthquakes. The goodness-of-fit test suggests that the time-varying activity is well characterized by a switching Poisson model. Furthermore, detailed analysis carried out on each deep group by use of switching Poisson models reveals similar time-varying behavior in occurrence frequencies in each group.