The 1979 Abbotsford Landslide, Dunedin, New Zealand: a retrospective look at its nature and causes

The Abbotsford Landslide of 8 August 1979 occurred in an urban area of Dunedin, New Zealand, causing much damage to houses and urban infrastructure. Rapid failure occurred after weeks of preliminary movements, resulting in the formation of a approximately 5 million m³ block slide. It caused the loss of 69 houses, with an overall cost of about NZ $10–13 million. After several months of investigations, a commission of inquiry found that unfavorable geology (weak clay layers in a 7°-dip slope) was the underlying cause of the landslide. An old sand quarry at the toe of the slope and a leaking water main above the slide area were found to be man-made factors that contributed to the failure. Slope stability analysis showed that after sand excavation (approximately 300,000 m³), the water table had to rise 0.3 m less for failure to occur. Because the quarry closed 10 years before the landslide occurred, it is concluded that a long-term rise in groundwater levels because of the increased rainfall over the previous decade and leakage from the water main controlled the timing of the failure and, in this sense, are considered to have triggered the landslide.     

一种GPS IIR-M型卫星超快星历钟差预报的高精度修正方法

针对IGS超快星历钟差预报产品(IGU-P)精度较低及无法满足高精度实时PPP定位精度的问题,提出了一种GPS IIR-M型卫星超快星历钟差预报的高精度修正方法。该方法对预报值的第一个数据与IGU观测部分(IGU-O)数据的最后一个历元做差,根据差值对整个IGU-O差分序列的影响程度来确定精度修正的大小和方向,从而实现IIR-M型卫星高精度预报的效果。经过IGU实测数据的测试结果表明,在短期预报6 h范围内,本文提出的精度修正方法可使3种预报方案在原有预报精度基础上分别提升6.13%、3.9%和3。48%,预报精度分别控制在0.599 ns、0.570 ns和0.531 ns,且均优于IGU-P产品预报精度。     

Carboniferous palaeomagnetism of the Rocky Creek Block,northern Tamworth Belt,and the New England pole path

Palaeomagnetic, rock magnetic and magnetic fabric results are presented for a Carboniferous (Visean to Westphalian) succession of felsic, mainly ignimbritic, volcanic and volcaniclastic rocks from the Rocky Creek Block of the northern Tamworth Belt, southern New England Orogen. Detailed thermal demagnetisation of 734 samples from 64 sites show three groups of magnetic components with low (<300°C), intermediate (300–600°C) and high (500–680°C) unblocking temperature ranges. Well‐defined primary magnetisations have been determined for 28 sites with evidence of four overprint phases. The overprints arise from a mid‐Tertiary weathering event (or possibly recent viscous origin), and from fluid movements associated with the Late Cretaceous opening of the Tasman Sea, thrusting during the Middle Triassic main phase of the Hunter‐Bowen Orogeny, and latest Carboniferous — Early Permian formation of the Bowen‐Gunnedah‐Sydney Basin system. Rock magnetic tests establish that the primary magnetisation carriers in the volcanic rocks are mainly magnetite (predominantly single domain, or pseudo‐single domain, and little or no multidomain) and hematite. Optimal magnetic cleaning is achieved at high to very high temperatures, with subtle, but systematic, directional and statistical differences between primary components derived from the mainly hematite fraction and pseudo‐components derived from the mainly magnetite fraction. The 28 primary magnetisation results are presented as six mean‐site results, summarised below and representing 25 sites, and three single‐site results. Fold tests could be applied to five mean‐site results. These are all positive, but one of these results may represent a secondary magnetisation. The primary magnetisation results define a Visean to Westphalian pole path. This long pole path indi cates extensive latitudinal and rotational movement for the Rocky Creek Block, and potentially for the New England Orogen, as follows: (i) Yuendoo Rhyolite Member (Caroda Formation, Visean) pole 235.8°E, 27.7°S, ED₉₅ = 9.0°, n = 3; (ii) Peri Rhyolite Member/Boomi Rhyolite Member (Clifden Formation, Namurian, 318.0 ± 3.4 Ma) pole 177.4°E, 63.4°S, ED₉₅ = 5.2°, n = 3; (iii) tuffaceous beds above Boomi Rhyolite Member (Clifden Formation?, Namurian) pole 162.2°E, 59.1°S, ED₉₅ = 10.2°, n = 3; ((iv) upper Clifden Formation/lower Rocky Creek Conglomerate (Namurian/Westphalian) pole 95.3°E, 49.6°S, ED₉₅ = 8.1°, n = 3 (possible overprint)); (v) Rocky Creek Conglomerate (Westphalian) pole 136.5°E, 57.6°S, ED₉₅ = 5.3°, n = 5; (vi) Lark Hill Formation (Westphalian) pole 127.0°E, 50.4°S, ED₉₅ = 4.8°, n = 8.     

Aspects of the structural and late thermal evolution of the Redbank Thrust system,central Australia: Constraints from the Speares Metamorphics

We present new data on the field geology and late thermal evolution of the Redbank Thrust system in the Arunta Block of central Australia. Geochronological and field data from the Speares Metamorphics are also used to relate the thermal evolution of the Redbank Thrust system to the structural evolution of the region. We show that several stages in the evolution might be discerned. An originally sedimentary sequence was intruded by mafic intrusions and then deformed during partial melting to form the principal foliation observed in the region (D1). This sequence was then folded during D2 into upright folds with north‐ to northeast‐plunging fold axes. These events are likely to correlate with the Strangways and/or Argilke and Chewings Orogenies known from previous studies. Subsequently, the Redbank Thrust was initiated during D3. This event is recognised by deflection of the host rocks into the shear zone and might therefore have been associated with a component of strike‐slip motion. It occurred probably at or before 1500–1400 Ma. Subsequent north‐over‐south thrust motion in the Redbank Thrust formed the intense mylonitic fabric and folded the mylonitic fabric during D4 into asymmetric folds with shallow fold axes. New ⁴⁰Ar/³⁹Ar K‐feldspar ages from three samples collected from variably deformed branches of the Redbank Thrust and undeformed rocks in the Speares Metamorphics suggest that most parts of the Redbank Thrust system cooled relatively slowly after metamorphism and deformation in the Mesoproterozoic so that the D4 thrusting might have been very long‐lived. Minimum ages of the K‐feldspar age spectra show that the entire region cooled below 200°C by approximately 300 Ma. Apatite fission track ages from nine samples show that cooling through the apatite partial annealing zone occurred during Cretaceous time (ca 150–70 Ma) and modelled cooling histories are consistent with the cooling rates obtained from the K‐feldspar data. They indicate that final exhumation of the Redbank Thrust system occurred probably in response to erosion, possibly driven by rifting around the margins of Australia.     

Geochemical,Sr–Nd–Pb isotope,and zircon U–Pb geochronological constraints on the origin of Early Permian mafic dikes,northern North China Craton

Dolerite dike swarms are widespread across the North China Craton (NCC) of Hebei Province (China) and Inner Mongolia. Here, we report new geochemical, Sr–Nd–Pb isotope, and U–Pb zircon ages for representative samples of these dikes. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb analysis yielded consistent Permian ages of 274.8 ± 2.9 and 275.0 ± 4.5 Ma for zircons extracted from two dikes. The dolerites have highly variable compositions (SiO₂ = 46.99–56.18 wt.%, TiO₂ = 1.27–2.39 wt.%, Al₂O₃ = 14.42–16.20 wt.%, MgO = 5.18–7.75 wt.%, Fe₂O₃ = 8.03–13.52 wt.%, CaO = 5.18–9.75 wt.%, Na₂O = 2.46–3.79 wt.%, K₂O = 0.26–2.35 wt.%, and P₂O₅ = 0.18–0.37 wt.%) and are light rare earth element (LREE) and large ion lithophile element (LILE, e.g. Rb, Ba, and K, and Pb in sample SXG1-9) enriched, and Th and high field strength element (HFSE, e.g. Nb and Ta in sample SXG1-9, and Ti) depleted. The mafic dikes have relatively uniform (⁸⁷Sr/⁸⁶Sr)_i values from 0.7031 to 0.7048, (²⁰⁶Pb/²⁰⁴Pb)_i from 17.77 to 17.976, (²⁰⁷Pb/²⁰⁴Pb)_i from 15.50 to 15.52, (²⁰⁸Pb/²⁰⁴Pb)_i from 37.95 to 38.03, and positive ?_Nd(t) (3.6–7.3), and variable neodymium model ages (T_DM1 = 0.75–0.99 Ga, T_DM2 = 0.34–0.74 Ga). These data suggest that the dike magmas were derived from partial melting of a depleted region of the asthenospheric mantle, and that they fractionated olivine, pyroxene, plagioclase, K-feldspar, and Ti-bearing phases without undergoing significant crustal contamination. These mafic dikes within the NCC formed during a period of crustal thinning in response to extension after Permian collision between the NCC and the Siberian Block.     

Palaeoproterozoic A-type magmatism in northern Wuyishan terrane,Southeast China: petrogenesis and tectonic implications

ABSTRACT

In this article we present zircon U–Pb ages, Hf isotopes, and whole-rock geochemistry of the Longzhu rhyolite porphyry from the Cathaysia Block, Southeast China to constrain its petrogenesis and provide insights into the early Precambrian tectonic evolution of the Cathaysia Block. LA-ICP-MS zircon U–Pb dating of a representative sample yields a weighted mean ²⁰⁶Pb/²⁰⁷Pb age of 1819 ± 16 Ma, interpreted as the crystallization age of the Longzhu rhyolite porphyry. Zircons from this sample have ε_Hf(t) values ranging from – 8.4 to – 2.2 and T^Hf_DM2 model ages from 2.76 to 2.46 Ga. The whole-rock Nd isotopic data from the Longzhu rhyolite porphyries yield ε_Nd(t) values spanning – 6.3 to – 4.7 and T^Nd_DM2 model ages from 2.81 to 2.69 Ga. The rhyolite porphyries have geochemical features similar to those of the typical A-type granites (rhyolites), with high SiO₂, total alkali contents and FeO^t/MgO ratios, and low CaO and MgO contents. Additionally, the rhyolite porphyries have high total rare earth element concentrations (627 ~ 760 ppm), high (La/Yb)_N values (14.5 ~ 26.9), strongly negative Eu anomalies (δEu = 0.28 ~ 0.41), and display enrichments of Rb, Ga, Th, and U and depletions of Sr, Nb, Ta, Eu, and Ti. The geochemical and Nd-Hf isotopic features suggest that the Palaeoproterozoic Longzhu rhyolite porphyries were generated by partial melting of source rocks similar to those of the Badu Complex in an intra-plate extensional setting. The results from this study, when combined with existing geochronological data, further demonstrate that the Palaeoproterozoic rocks of Wuyishan terrane probably represent a remnant of the Columbia supercontinent.     

Tectonic evolution of the western Ordos Basin during the Palaeozoic-Mesozoic time as constrained by detrital zircon ages

ABSTRACT

The Ordos Basin has experienced a complicated tectonic evolution since the Palaeozoic. Its multi-stage evolution was closely related to the tectonic events that occurred along plate boundaries. The detrital zircon ages and crystallization age (CA)-deposition age (DA)/cumulative proportion curves obtained from Palaeozoic-Mesozoic strata from different tectonic units in and around the western Ordos Basin demonstrate that during the early Palaeozoic, the so-called Helan Aulacogen did not develop along the western Ordos Basin, the Alxa Block was an independent unit from the North China Craton, and the southern Ordos Basin was a foreland basin of the North Qinling Orogenic Belt. During the early Palaeozoic, the western Ordos Basin and its vicinity belonged to three different tectonic units (i.e. the North China Craton, the Alxa Block, and the North Qilian Orogenic Belt). At the end of the early Palaeozoic, the Alxa Block amalgamated with the Ordos Basin. From the Silurian to the Middle Devonian, the southern Alxa Block was a foreland basin of the North Qilian Orogenic Belt and underwent regional extension during the Late Devonian. During the late Palaeozoic, the western Ordos Basin and its vicinities were located in a back-arc extensional setting of the western Qinling Orogenic Belt. The southern part of the western Ordos Basin may have been a retro-arc foreland basin of the western Qinling Orogenic Belt during the Late Triassic, and the northern part of the western Ordos Basin experienced large-scale left-lateral strike-slip at the same time. The CA-DA/cumulative proportion curves can adequately explain the evolution of the western Ordos Basin during the Palaeozoic; however, the settings indicated by the CA-DA/cumulative proportion curves in intraplate evolutions are different from those proposed in other studies, which may be due to the number and distribution of samples and rapid lateral changes in sedimentary facies.     

赣西南下寒武统杂砂岩地球化学特征和沉积环境

赣西南地区早寒武世发育巨厚的砂泥质复理石沉积,砂岩杂基含量普遍大于15%,以杂砂岩为主。赣州市、井冈山市和萍乡市3个地区杂砂岩的主量、微量、稀土元素含量和组合特征表明,赣西南下寒武统杂砂岩具有较高的SiO2含量,高的Al2O3/(CaO+Na2O)和K2O/Na2O比值,较低的Al2O3/SiO2、Fe2O3/MgO比值和CaO含量;微量元素含量接近中国东部上地壳值,富集大离子亲石元素;稀土元素具有LREE富集、Eu亏损明显的特征。综合分析推测,物源剥蚀区为华夏陆块,其北缘显示为被动大陆边缘的地球化学特征,赣西南地区海水较深,且自南向北略微加深。     

拉萨地块东南缘晚白垩世高温变质作用及其地质意义:来自石英出溶金红石的证据和锆石U-Pb定年

文中对拉萨地块东南缘林芝杂岩中的含榴斜长角闪岩进行了详细的岩相学研究和锆石U-Pb定年。岩相学观察表明,含榴斜长角闪岩经历了峰期麻粒岩相变质和角闪岩相退变质作用。峰期麻粒岩相矿物组合为石榴子石+高Ti角闪石+紫苏辉石+斜长石+石英+金红石,其中的石榴子石、石英和角闪石中含有大量金红石出溶体,说明这些矿物的初始成分具有高Ti含量。角闪岩相退变质矿物组合为低Ti角闪石+斜长石+斜黝帘石+石英+金红石。利用Ti在石英中的含量(TitaniQ)温度计计算得到峰期麻粒岩相变质温度为803～924℃,后期角闪岩相退变质温度为555～732℃。样品中的锆石具有明显的核边结构,核部为典型岩浆型锆石,具有高的Th/U值,强烈富集HREE,明显的正Ce异常和负Eu异常,206Pb/238U年龄为(89.3±0.6)Ma,代表含榴斜长角闪岩原岩结晶年龄。锆石边部呈无环带结构,同核部相比,具有低的Th/U值,低的M-HREE和弱的Eu负异常,为变质峰期生长的锆石,206Pb/238U年龄为(81.1±0.8)Ma,代表麻粒岩相峰期变质年龄。我们认为约81Ma高温麻粒岩相变质作用可能与洋中脊俯冲造成的板片窗导致的软流圈上涌有关。     

南苏门答腊盆地Jabung 区块油气成藏特征 与主控因素分析

[摘 要] 本文以油气成藏理论为指导,通过分析区块的构造、生储盖、圈闭和油气分布特征,总结出 区块的成藏主控因素,明确区块下一步的勘探方向。研究发现区块的油气成藏具有构造复杂、烃源岩优 越、储集层丰富、盖层良好和圈闭多样的特征。油气分布具有西油东气,西部下部地层( LTAF 和UTAF 组)为主,东部上部地层(Gumai 组)为主,下油上气的规律。分析认为区块的油气成藏主要受烃源岩、储 集层和断裂的控制。对于区块下一步的勘探,西部地区应以寻找剩余的断层相关圈闭为主;东部地区应 以寻找远离断层的背斜圈闭为主,同时岩性地层圈闭是重要的勘探对象。