Late Cretaceous tectono-magmatic activity in the Nize region,central Tibet: evidence for lithospheric delamination beneath the Qiangtang–Lhasa collision zone

ABSTRACT

The results of zircon U–Pb age dating and whole-rock geochemistry for the Late Cretaceous Nize granodiorite porphyries, combined with analysis of near-coeval structural deformation of the Lower Cretaceous Langshan Formation, provide new data to better understand the tectonic evolution of the northern Lhasa subterrane, central Tibet. Zircon U–Pb ages of 89.2 ± 0.3 Ma to 87.8 ± 0.3 Ma indicate emplacement during the Late Cretaceous. Granodiorite porphyry intrusions were contemporaneous with the development of a regional angular unconformity, overlain by the Upper Cretaceous Jingzhushan (or Abushan) Formation, within the collision zone between the South Qiangtang and Lhasa terranes. Geochemical data for Nize granodiorite porphyries indicate that they have a calc-alkaline composition enriched in large-ion lithophile elements and light rare earth elements and depleted in high-field-strength elements and heavy rare earth elements. High Al₂O₃ and Sr contents, low Yb and Y contents, and high Sr/Y ratios are similar to adakitic magmas.

Structural analysis indicates two stages of deformation (D₁ and D₂), with D₁ forming the focus of the present study. The D₁ deformation is represented by large-scale faults and records two periods of faulting. These periods are recognized as early compressional thrust faulting and a dominant late stage characterized by normal faulting and extension, with the latter stages of D₁ being near-coeval with the emplacement of the Nize granodiorite porphyries. The combination of zircon ages, geochemical data, and structural analysis indicates that the Nize granodiorite porphyries formed after collision of the South Qiangtang and Lhasa terranes. Adakitic magma derived from partial melting of the thickened lower or middle crust resulted from lithospheric delamination that may have been promoted by the convective removal of deeper lithospheric mantle.     

Development of regional uplift and uplift-related strata in Gunsan Basin,Yellow Sea: implications for Cenozoic crustal extension

ABSTRACT

The Mesozoic–Cenozoic Gunsan Basin is the northeastern part of the Northern South Yellow Sea Basin between eastern China and the Korean Peninsula. On the basis of seismic interpretation, this study presents and interprets geologic features of regionally uplifted structures, the Haema Arch, located in the central western part of the basin. The Haema Arch is defined as dome-shaped uplift complexes, 95 km long and 60 km wide. It is characterized by prominent basement uplifts along its margin and plunging syncline inside the arch. The marginal large-scale uplifts are bounded by outward-dipping faults. The uplift-related strata are identified on the hanging wall block of the bounding faults and within the Haema Arch, which can be divided into pre-, syn-, and post-uplift units. The pre-uplift unit rests on the acoustic basement and shows an upturned stratal pattern near the marginal large-scale uplift. The syn-uplift unit locally occurs on the hanging wall block of the bounding faults along the northern and southern margins. The uplift of the Haema Arch and its coeval fault-controlled subsidence possibly occurred during the late Oligocene. The post-uplift unit initially formed on remnant topographic lows during the early Miocene and subsequently covered the overall area of the Haema Arch and the Gunsan Basin. The late Oligocene uplifting of the Haema Arch can be interpreted as an isostatic response to tectonic unloading by the arch-bounding faults that possibly extend to detachment faults. We suggest that the Gunsan Basin underwent crustal thinning and extensional deformation during the late Oligocene, which accounts for the coeval uplifting and fault-controlled subsidence in the study area.     

利用微裂隙内流体包裹体面研究构造运动——以苏北盆地富民、花庄油田为例

流体包裹体面（FIP）研究是当前的热点研究对象之一,已经取得了较为丰硕的成果,但利用FIP研究构造运动的文献为数不多。以苏北盆地富民、花庄油田为例,利用FIP研究了该油田的构造活动。结果表明：研究区至少经历了4次主要构造运动,主要发育3种类型的FIP。结合构造演化史、埋藏史及热史,确定了构造运动与油气成藏的时间与期次。     

Porcellanite and the urban geology of Darwin,Northern Territory

Porcellanite is a siliceous duricrust which has developed within altered Cretaceous mudrocks in the Darwin city area and is well exposed in coastal cliffs to the north. Its distinctive geotechnical properties include low bulk density, variable strength that is highly dependent on moisture content, and relatively high (but inconsistent) durability. The porcellanite rock mass is inhomogeneous and anisotropic; it is unrippable at the surface, but becomes weaker with depth. It is also highly permeable in places, even karst‐like, due to solution cavities. These characteristics are common to all duricrusts and result from processes of solution, replacement and redeposition by silica‐laden groundwater. Such processes may have been intermittently active through most of the Cenozoic and there is evidence that they continue to the present. The upper 2–4 m of the porcellanite profile is made up of a brittle, high‐strength rock with a silica content approaching that of silcrete. The underlying altered and porous siltstone is much weaker and deforms plastically under loading, due to a cellular microfabric composed largely of opaline silica replacing clay minerals. Porcellanite has long been used as the main building stone in Darwin and is now quarried for shoreline filling. Although some of it is of select fill or road sub‐base quality, crushed porcellanite contains an excess of plastic fines making it unsuitable for basecourse. Porcellanite has been a failure as breakwater stone, despite producing blocks of adequate size.     

藏南马拉山高钙二云母花岗岩的年代学特征及其形成机制

马拉山片麻岩穹窿位于特提斯喜马拉雅带内,由马拉山二云母花岗岩、错布二云母花岗岩和派枯错复合淡色花岗岩组成。马拉山二云母花岗岩东西展布约10km,锆石U-Pb分析表明,马拉山二云母花岗岩的结晶时间较长,从17.6Ma到16.9Ma,或者至少是两次深熔作用的产物,分别发生在17.6Ma和16.9Ma。全岩主量元素、微量元素和Sr、Nd、Hf同位素分析表明马拉山二云母花岗岩是一个较均一岩体,具有以下特征:(1)高SiO2,Al2O3和相对较高的CaO(1.2%～2.0%);(2)较高的Sr,较低的Rb和Rb/Sr比值(<1.3),且随着Ba浓度的增加,Rb/Sr比值保持不变;(3)高度变化的Zr/Hf比值(25.9～39.9);(4)富集轻稀土,亏损重稀土,几乎无或弱的负Eu异常;(5)较一致的Sr和Nd同位素组成;(6)锆石岩浆增生边和继承性锆石的Hf同位素比值高度变化,εHf(t)分别为-20.4～-8.0和-27.2～-9.5。这些特征暗示马拉山二云母花岗岩是变泥质岩在较高温压条件下水致白云母部分熔融的产物,与藏南裂谷系的东西向伸展作用密切相关。     

Tectono-sedimentary evolution of the External Liguride units (Northern Apennines,Italy): insights in the pre-collisional history of a fossil ocean-continent transition zone

Abstract

In the Northern Apennines, the External Liguride (EL) units are interpreted as derived from the domain that joined the Ligure–Piemontese oceanic basin to the Adriatic plate continental margin. The EL units can be divided into two different groups according to the lithostratigraphic features of the basal complexes underlying the Upper Cretaceous–Lower Tertiary carbonate flysch (e.g. Helminthoid flysch). The first group includes the western successions characterized by Santonian–Campanian sedimentary melanges where slide blocks of lherzolitic mantle, gabbros, basalts, granulites, continental granitoids are represented. The second group is represented by the eastern successions where the Cenomanian–Campanian basal complexes mainly consist of sandstones and conglomerates where the mafic and ultramafic rocks are scarce or completely lacking. Their original substrate is represented by the Middle Triassic to Lower Cretaceous, mainly platform carbonate deposits, found as slices at the base of the eastern successions.

The stratigraphic features shown by the basal complexes allow the reconstruction of their source area that is assumed to be also representative for the pre-Upper Cretaceous setting. The proposed reconstruction suggests the occurrence in the EL domain of two distinct domains. The eastern domain was characterized by a thinned and faulted continental crust belonging to the Adriatic continental margin. The western domain was instead floored by subcontinental mantle associated with lower and upper continental crust, representing the ocean–continent transition. This setting is interpreted as the result of the opening of the Ligure–Piemontese oceanic basin by passive rifting, mainly developed by simple shear, asymmetric extension of the continental crust. © 2001 Éditions scientifiques et médicales Elsevier SAS     

A multibiomarker approach in barramundi (Lates calcarifer) to measure exposure to contaminants in estuaries of tropical North Queensland

A suite of biomarkers were measured in barramundi (Lates calcarifer) from five North Queensland estuaries along a perceived pollution gradient. The biomarkers selected were 7-ethoxyresorufin-O-deethylase (EROD), cytochrome P450, fluorescent aromatic compounds (FACs), DNA integrity, RNA:DNA ratio, cholinesterase activity (ChE), condition factor and hepatosomatic index. The resulting database was subjected to uni- and multi-variate analyses in order to assess the most suitable biomarkers to assess pollution in North Queensland estuaries and to classify the environmental quality of the sites. Principal components analysis (PCA) on the biochemical markers revealed that EROD, EROD/P450, DNA damage and to a lesser extent ChE and FACs were found to be responsive to contaminants in the environment while cytochrome P450, condition factor and the hepatosomatic index were found to be less responsive biomarkers. This study has demonstrated the utility of applying a multibiomarker approach in conjunction with traditional analysis of contaminants in providing valuable information in environmental risk assessment.     

Short term impact of planktonic mucilage aggregates on macrobenthos along the Istrian rocky coast (Northern Adriatic, Croatia)

During nearly every summer of the last 15 years, massive mucilage aggregations have formed in the water column of the northern Adriatic. The phenomenon also occurred during the summer of 2004, lasting approximately from the middle of June to the end of July. After approximately 20 days of mucilage impact (coverage of the rocky bottom from 40% to 90%), the effects on selected macrobenthic species were assessed at depths of 5-7m. The responses to the mucilage impact were species dependent. Smothering by mucilage caused partial necrosis of the sponge Verongia aerophoba: from 7% to 56% of the colony surface was deteriorated. In contrast, the impact was lethal for the mollusc Arca noae: from 5% to 45% of the molluscs died. Usually, no deleterious effects were ascertained on perennial parts of the thalli (axes) of macroalgae of the genus Cystoseira. However, primary branches showed signs of necrosis, and Cystoseira compressa was more sensitive compared to Cystoseira corniculata and Cystoseira barbata. Apparently, this does not profoundly affect Cystoseira populations, which have been in a phase of expansion along the shallow Istrian rocky coast over the last 15 years.     

Land-use comparison of depression-focussed groundwater recharge in the Canadian prairies

In the cold semiarid Canadian prairies, groundwater recharge is focussed under numerous topographic depressions, in which snowmelt runoff converges. Agricultural land uses on the uplands surrounding the depressions affect snow accumulation, snowmelt infiltration, evapotranspiration (ET) and soil moisture dynamics, thereby influencing snowmelt runoff and depression-focussed recharge. The objective of this study is to compare the differences in hydrological processes under two common land uses in the Canadian prairies, namely grazed grass and annual crop, and examine how they affect groundwater recharge. A short-term (3 years) paired catchment study was used for detailed observation of hydrological processes in two depressions, supplemented by a longer-term (17 years) data set covering a larger scale to quantify the differences in snowmelt runoff between the two land uses. Compared to the grazed grassland, the cropland had a shorter and more intense period of ET, and root water uptake restricted to the shallower (top 0–80 cm) soil zone. The amount of snowmelt runoff was greater in the grazed grassland primarily due to a higher amount of snow accumulation, which was dictated by differences in topography. This finding was contrary to previous studies in the Canadian prairies that indicated substantially smaller snowmelt runoff in ungrazed grassland, but was consistent with the larger-scale remote sensing results, which showed only a marginal difference between grazed grasslands and croplands. Groundwater recharge rates were estimated using the chloride mass balance method for the present condition using “modern” pore water containing tritium. The rates were similar between the grazed grassland and croplands, implying similarity in snowmelt runoff characteristics. These results suggest that groundwater recharge will continue to be focussed under depressions in the future, though the amount and seasonality of recharge may be influenced by warmer winters.