Age of the rhyolites of the Lord Howe Rise and the evolution of the southwest Pacific Ocean

Drilling at site 207 (DSDP Leg 21), located on the broad summit of the Lord Howe Rise, bottomed in rhyolitic rocks. Sanidine concentrates from four samples of the rhyolite were dated by the⁴⁰Ar/³⁹Ar total fusion method and conventional K-Ar method, and yielded concordant ages of 93.7 ± 1.1my, equivalent to the early part of the Upper Cretaceous. At this time the Lord Howe Rise, which has continental-type structure, is thought to have been emergent and adjacent to the eastern margin of the Australian-antarctic continent. Subsequent to 94 my ago and prior to deposition of Maastrichtian (70-65 myBP) marine sediments on top of the rhyolitic basement of the Lord Howe Rise, rifting occurred and the formation of the Tasman Basin began by sea-floor spreading with rotation of the Rise away from the margin of Australia. Subsidence of the Rise continued until Early Eocene (about 50 myBP), probably marking the end of sea-floor spreading in the Tasman Basin. These large scale movements relate to the breakup of this part of Gondwanaland in the Upper Cretaceous.     

Non-axial breaching of a rift valley: Evidence from the Lord Howe Rise and the southeastern Australian margin

Present models of continental breakup envisage the formation of a rift valley which undergoes a protracted period of tectonism and eventual seafloor spreading in the axial part of the rift valley. This results in evidence of pre-breakup tectonism on most Atlantic-type margins in the form of normal blockfaults beneath the continental slope. The southeastern margin of the Australian continent has an unusually steep continental slope and shows little evidence of tectonism associated with the rift valley stage of development. The margin was formed by separation of the Lord Howe Rise and Australia during a phase of seafloor spreading in the Tasman Sea which lasted from about 80 to 60 m.y. B.P. Marine geophysical data over the central Lord Howe Rise indicate a contrast between the western and eastern part of of this structure. The western part shows faulted, rough basement topography, disturbed overlying sediments, and a relatively quiet magnetic field. The eastern part shows a smooth basement surface, undisturbed overlying sediments, and a high-amplitude, high-frequency magnetic field. It is suggested that the whole of the pre-breakup rift valley remained attached to the Lord Howe Rise. This explains the absence of rift valley structures within the eastern continental margin of Australia and implies non-axial breaching along the western boundary fault of a pre-Tasman Sea rift valley.     

The influence of rock resistance on coastal morphology around Lord Howe Island,southwest Pacific

An Erratum has been published for this article in Earth Surface Processes and Landforms 27(7) 2004, 931. Lord Howe Island, in the northern Tasman Sea, is a remnant of a much larger Late Miocene basaltic shield volcano. Much of the island's coastline is exposed to waves that have unlimited fetch, but a marked contrast is provided by a fringing coral reef and lagoon that very effectively attenuate wave energy along a portion of the western coastline. The geology of the island is varied, with hard and resistant basalt lavas, breccias and tuffs of intermediate resistance, and highly erodible eolianites. This variability provides an excellent opportunity to examine the in?uence of rock resistance on the development of the spectacular rock coast landforms that occur around the island. The hardness of rocks and the extent of weathering around the coastline were assessed using a Schmidt hammer, and statistical analysis was undertaken to remove outlying values. On all but one occasion, higher mean rebound values were returned from fresh surfaces than weathered surfaces, but only half of these differences were statistically signi?cant. Shore platforms with two distinct levels are juxtaposed along two stretches of coastline and Schmidt hammer results lend support to hypotheses that the raised surfaces may be inherited features. Relative rock resistance was assessed through a combination of Schmidt hammer data and measurements of joint density, and constrained on the basis of morphological data. This approach formed a basis for examining threshold conditions for sea‐cliff erosion at Lord Howe Island in the context of the distribution of resistant plunging cliffs and erosional shore platforms. Copyright © 2004 John Wiley & Sons, Ltd.     

Magnetic anomalies across Campbell Plateau,New Zealand

Magnetic measurements over the Campbell Plateau, New Zealand, show the existence of a linear positive anomaly belt which extends for 900 km along an approximately east-west trend. This anomaly belt is considered to be a continuation of the Stokes Magnetic Anomaly occurring through New Zealand and associated with rocks of the New Zealand Geosyncline. If this is so, a transcurrent fault with a dextral displacement of about 330 km and aligned in a northeast-southwest direction must occur between the eastern end of the Stokes Magnetic Anomaly and the western end of the anomaly belt across Campbell Plateau.     

Radiometric ages for some rocks from Snares and Auckland Islands,Campbell Plateau

On the basis of K/Ar muscovite and biotite ages, and of Rb/Sr whole-rock, muscovite, and feldspar ages, the last cooling of granites from Snares and Auckland Islands on the Campbell Plateau took place in early Late Cretaceous times. The original emplacement is unlikely to have been older than Late Jurassic. These results from the basement rocks of the Campbell Plateau conform with data from Fiordland and southern Stewart Island in southern New Zealand and from Ford Range and other localities in West Antarctica. Two gabbros from Auckland Island yield Miocene K/Ar ages.     

Petrology of the Campbell Island Volcanics, Southwest Pacific Ocean

The disposition and petrology of a fractionated alkali olivine basalt—peralkaline rhyolite suite from subantarctic Campbell Island are discussed. These rocks (Campbell Island Volcanics: new name) are flows and high-level intrusions derived from two centres of igneous activity. Their age is Upper Miocene and they evolved over a period of 5 Ma. A gabbro intrusion pre-dates volcanism by 5 Ma. The ages of the flows and high-level intrusions cannot be separated, although the intrusions are chemically distinct as they contain all the intermediate members of the suite (mugearite and benmoreite). Similar La/Ce and Zr/Nb ratios for flows and high-level intrusions suggest a co-magmatic origin. Chemical variations indicate that the suite resulted from low-pressure mafic then felsic-dominated fractional crystallisation, which is substantiated for intermediate members of the suite by least-squares and Rayleigh fractionation modelling. One flow of alkali olivine basalt clearly pre-dates other volcanic rocks, and is thus regarded as being genetically unrelated.Although chemically similar to alkali olivine basalt and hawaiite, variations in the mineral chemistry and modal mineralogy of gabbro indicates a prolonged period of in-situ fractionation and re-equilibration.     

Crustal structure and origin of the Cape Verde Rise

The Cape Verde Islands are located on a mid-plate topographic swell and are thought to have formed above a deep mantle plume. Wide-angle seismic data have been used to determine the crustal and uppermost mantle structure along a ~ 440 km long transect of the archipelago. Modelling shows that ‘normal’ oceanic crust, ~ 7 km in thickness, exists between the islands and is gently flexed due to volcano loading. There is no direct evidence for high density bodies in the lower crust or for an anomalously low density upper mantle. The observed flexure and free-air gravity anomaly can be explained by volcano loading of a plate with an effective elastic thickness of 30 km and a load and infill density of 2600 kg m^− 3. The origin of the Cape Verde swell is poorly understood. An elastic thickness of 30 km is expected for the ~ 125 Ma old oceanic lithosphere beneath the islands, suggesting that the observed height of the swell and the elevated heat flow cannot be attributed to thermal reheating of the lithosphere. The lack of evidence for high densities and velocities in the lower crust and low densities and velocities in the upper mantle, suggests that neither a crustal underplate or a depleted swell root are the cause of the shallower than expected bathymetry and that, instead, the swell is supported by dynamic uplift associated with the underlying plume.     

Evolution of the Ordos Plateau and environmental effects

Based on the analysis of temporary-spatial distribution, geomorphic position, contact relationship with underlying strata and grain size of red clay, we studied the formation and environmental background of red clay. During late Miocene-Pliocene, the Ordos Block finished the transformation from the basin to the plateau, which had an obvious environmental effect on the topography, indicated by the formation of highland undergoing wind erosion and lowland receiving red clay deposits. The red clay materials were sourced from dusts carried by wind energy and covered on the initial topography. Unlike Quaternary loess dust covering the overall the Loess Plateau, red clay deposited on the highland would be transported to the lowlands by wind and fluvial process. As a result, there was no continuous “Red Clay Plateau” in the Ordos region and red clay was only preserved in former lowlands. However, red clay was discontinuously distributed through the Loess Plateau and to some extent modified the initial topography. The differential uplift in interior plateau is indicated by the uplift of northern Baiyushan, central Ziwuling and southern Weibeibeishan. The Weibeibeishan Depression formed earlier and became the sedimentary center of red clay resulting in the thicker red clay deposits in Chaona, Lingtai and Xunyi. Since Quaternary the aridity in the northern plateau enhanced and accelerated loess accumulation caused the formation of the Loess Plateau. During the late Pleistocene the rapid uplift led to the enhancement of erosion. Especially after the cut-through of Sanme Lake by the Yellow River, the decline of base level caused the falling of ground water level and at the same time the increase of drainage density resulting in the enhancement of evaporation capacity, which enhanced the aridity tendency of aridity in the Loess Plateau region.     

Evolution of the Ordos Plateau and environmental effects

Based on the analysis of temporary-spatial distribution, geomorphic position, contact relationship with underlying strata and grain size of red clay, we studied the formation and environmental background of red clay. During late Miocene-Pliocene, the Ordos Block finished the transformation from the basin to the plateau, which had an obvious environmental effect on the topography, indicated by the formation of highland undergoing wind erosion and lowland receiving red clay deposits. The red clay materials were sourced from dusts carried by wind energy and covered on the initial topography. Unlike Quaternary loess dust covering the overall the Loess Plateau, red clay deposited on the highland would be transported to the lowlands by wind and fluvial process. As a result, there was no continuous "Red Clay Plateau" in the Ordos region and red clay was only preserved in former lowlands. However, red clay was discontinuously distributed through the Loess Plateau and to some extent modified the initial topography. The differential uplift in interior plateau is indicated by the uplift of northern Baiyushan, central Ziwuling and southern Weibeibeishan. The Weibeibeishan Depression formed earlier and became the sedimentary center of red clay resulting in the thicker red clay deposits in Chaona, Lingtai and Xunyi. Since Quaternary the aridity in the northern plateau enhanced and accelerated loess accumulation caused the formation of the Loess Plateau. During the late Pleistocene the rapid uplift led to the enhancement of erosion. Especially after the cut-through of Sanme Lake by the Yellow River, the decline of base level caused the falling of ground water level and at the same time the increase of drainage density resulting in the enhancement of evaporation capacity, which enhanced the aridity tendency of aridity in the Loess Plateau region.     

Origin of metalliferous sediments from the East Pacific Rise

The distribution of several metals in East Pacific Rise sediments, when normalized to Al₂O₃, exhibit stronger maxima near the rise crest than when simply plotted on a carbonate-free basis. The similarity (1) between the distribution of metals in ridge sediments and previously measured mean heat flow values and (2) between the composition of crestal sediments and terrestrial ore bodies associated with greenstone belts, strongly supports a hydrothermal origin for rise crest sediments.     

Origin of the Willaumez-Manus Rise,Papua New Guinea

The aseismic Willaumez-Manus Rise on the Bismarck Sea floor separates the Manus Basin from the New Guinea Basin. The rise does not appear to be an extinct spreading axis, or a remnant arc, but may be the result of excess magmatism possibly related to an inferred mantle hot spot beneath St. Andrew Strait. A preferred interpretation is that the rise is the raised edge of the New Guinea Basin, formed in response to a thermal anomaly beneath the extensional Manus Basin which formed later than the New Guinea Basin.     

New chronological constraints on the Plio-Pleistocene uplift of the Guizhou Plateau,SE margin of the Tibetan Plateau

The Guizhou Plateau represents a geomorphic transition between the Tibetan Plateau and the Yangtze River Plain. It likely formed in response to the propagation of surface uplift in southeastern Tibet during India-Eurasia continental collision. However, the uplift history of the region is unclear largely due to a lack of datable material. The bedrock geology is dominated by carbonate rocks, which contains numerous multi-level caves in the main river valleys that are linked to the river incision history. Cosmogenic ²⁶Al and ¹⁰Be burial dating of sediments in caves and river terraces from the northwestern and southern plateau reveals the fluvial chronology and provides the first direct determination of long-term river incision rates. The caves and terraces on the Liuchong River in NW Guizhou yield burial ages of between 0.41 ± 0.12 Ma and 2.85 ± 0.21 Ma, indicating an average incision rate of 57 ± 3 m/Ma. Four level caves at Libo in southern Guizhou yield burial ages of between 0.56 ± 0.16 Ma and 3.54 (+0.25/-0.22) Ma, indicating slightly slower incision rate (47 ± 5 m/Ma). These new results imply that the high elevation of the Guizhou Plateau had developed before the Late Pliocene, and that surface uplift during the Late Cenozoic was largely uniform across the region.     

青藏高原地表温度的比较分析

利用青藏高原改则、甘孜、拉萨和那曲气象站的地面水银温度计观测的地面0 cm温度和地面向上、大气向下长波辐射观测资料,计算和比较分析了地面水银表温度与地表辐射温度的差异.分析表明,由于地面水银温度计接触式测温方法的局限性,使得气象台站观测场地的地面水银表温度存在较大的误差：在裸露地表,夏季白天的正偏差可达4℃以上,夜间的负偏差在-2℃左右;在稠密植被地表,白天正偏差最高可达16℃以上,夜间的负偏差可达-3℃.由于偏差的不均衡性和离散性,即使对地面水银表温度作简单平均或积分平均处理,其代表性仍然很差.在对地表温度准确性要求较高的定量化研究与应用中,应避免直接采用地面水银表温度资料,即使由于资料缺乏而以其代之,也应对其采取适当的订正补偿等处理措施.随着科学研究对地表温度准确性的更高要求,推广使用地表辐射测温的观测方法是必要的.     

Dredged basalts from the western Nazca plate and the evolution of the East Pacific Rise

Ocean-floor basalts and glasses were recovered from three stations along the western Nazca plate, from a sublinear topographic feature believed to represent the proto-East Pacific Rise (EPR), and include abyssal tholeiites, FeTi-basalts and glasses, as well as transitional and little fractionated compositions. When compared with their coexisting fresh glasses, the FeTi-basalts have higher total alkalies, TiO₂ and MgO, and lower FeO^*, suggesting that they have also been affected by non-oxidative post-magmatic alteration processes. The FeTi-glasses form a remarkably uniform compositional group through space and time. A little fractionated composition having anMg-number= 73, similar to those reported from the Mathematician Ridge, has higher Na₂O and TiO₂, and slightly lower CaO than similar compositions from the slowly accreting Mid-Atlantic Ridge. The basalts and glasses reported here exhibit the compositional diversity expected for propagating rifts and probably represent more than one volcanic episode.Both geochemical and geophysical interpretations support the inference that the EPR grew from Miocene times by the progressive growth and propagation of mantle perturbations, leaving a remnant sublinear zone of rough topography characteristic of slower accretion as the trace of the proto-EPR. Continuing translations and rotations of axial segments are occurring along the EPR, probably in response to self-reorganizations of mantle flow patterns arising from rapid melting and depletion of the source regions. The data allow the inference that the youthful rift systems of the eastern Pacific are far from thermodynamic equilibrium as might be expected if such systems were to drive fundamental life processes.     

Hyperbolic echo zones in the eastern Atlantic and the structure of the southern Madeira Rise

The Madeira Rise is a 450 km northeast-trending structural-sedimentary feature which lies west of the Madeira Islands. Its northern half is controlled by a basement ridge, but its southern section consists of an apparent current-controlled sediment deposit. Its maximum sediment thickness is about 1 km over a relatively level basement. There are two reflectors which can be traced within the sediment pile. A shallow reflector (R1) may mark the termination of a rapid constructional phase of the drift. Sediment cores taken on the southern Madeira Rise have recovered brown marls and chalks with sedimentation rates of only about 1.5 cm/1000 years over the last 225,000 years.A striking zone of hyperbolic reflectors mapped around the flank of the southern Madeira Rise in the 4000–4800 m depth range may be the expression of bedforms created by contour-following currents. Another zone of hyperbolic echoes is found on the continental rise at about 24–26°N in a similar depth range. Trend determinations in this area suggest that the bedforms which give rise to the hyperbolae are oriented north-northeast and may be similar to the abyssal furrows discovered by DEEP-TOW observations on the Blake-Bahama Outer Ridge.The bottom photographs available from both hyperbolic echo zones show a tranquil bottom. This suggests that the hyperbolic bedforms are relict.     

青藏高原Pn波研究进展及问题

简要总结了青藏高原地区Pn波速度结构、各向异性研究进展；介绍了Pn波速度结构、各向异性等在岩石圈结构、构造背景反映等方面的应用研究进展。分析了目前青藏高原Pn波研究中存在的一些问题。     

Asymmetric mantle dynamics in the MELT region of the East Pacific Rise

The mantle electromagnetic and tomography (MELT) experiment found a surprising degree of asymmetry in the mantle beneath the fast-spreading, southern East Pacific Rise (MELT Seismic Team, Science 280 (1998) 1215–1218; Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Wolfe and Solomon, Science 280 (1998) 1230–1232; Scheirer et al., Science 280 (1998) 1221–1224; Evans et al., Science 286 (1999) 752–756). Pressure-release melting of the upwelling mantle produces magma that migrates to the surface to form a layer of new crust at the spreading center about 6 km thick (Canales et al., Science 280 (1998) 1218–1221). Seismic and electromagnetic measurements demonstrated that the distribution of this melt in the mantle is asymmetric (Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Evans et al., Science 286 (1999) 752–756) at depths of several tens of kilometers, melt is more abundant beneath the Pacific plate to the west of the axis than beneath the Nazca plate to the east. MELT investigators attributed the asymmetry in melt and geophysical properties to several possible factors: asymmetric flow passively driven by coupling to the faster moving Pacific plate; interactions between the spreading center and hotspots of the south Pacific; an off-axis center of dynamic upwelling; and/or anomalous melting of an embedded compositional heterogeneity (MELT Seismic Team, Science 280 (1998) 1215–1218; Forsyth et al., Science 280 (1998) 1235–1238; Toomey et al., Science 280 (1998) 1224–1227; Wolfe and Solomon, Science 280 (1998) 1230–1232; Evans et al., Science 286 (1999) 752–756). Here we demonstrate that passive flow driven by asymmetric plate motion alone is not a sufficient explanation of the anomalies. Asthenospheric flow from hotspots in the Pacific superswell region back to the migrating ridge axis in conjunction with the asymmetric plate motion can create many of the observed anomalies.     

青藏高原东北缘扩展变形范围与深部结构

青藏高原东北缘的隆升和扩展一直是地学界长期关注的重要科学问题,对于研究整个青藏高原的形成演化过程具有非常重要的意义.青藏高原东北缘是高原向北东方向扩展变形的结果,其进一步向北东方向扩展变形的边界范围长期存在较大争议.本文主要收集了近年来前人在青藏高原东北缘及其邻区开展的浅表地质、地壳-上地幔三维结构以及壳-幔变形的相关...     

Processes and modes of permafrost degradation on the Qinghai-Tibet Plateau

Climate warming must lead the mainly air temperature controlled permafrost to degrade.Based on the numerical simulation,the process of permafrost degradation can be divided into five stages,i.e.,starting stage,temperature rising stage,zero geothermal gradient stage,talic layers stage,and disappearing stage,according to the shape of ground temperature profile.Permafrost on the Qinghai-Tibet Plateau (QTP) is generally considered a relic from late Pleistocene,and has been degenerating as a whole during Holocen...     

Inhomogeneity of the lithosphere of the Tibetan Plateau and implications for geodynamics

This paper discusses inhomogeneity in structure of the present lithosphere underneath the Tibetan Plateau, and deduces the P-T-t paths and deep processes during the orogenic process of post India-Asia collision through revealing the nature and sequence of geological events. A three-stage-evolution model for tectonic phases of the Tibetan Plateau has been presented. It is suggested that the formation of Parmirs-type of cool lithosphere roots represents the early phase; Nianqingtanggula-type of thinned lithosphere roots, the middle phase; and Qiangtang-type of “warm” lithosphere roots (formed by cooling of the asthenosphere), the late phase.