新生代柴达木盆地与周缘造山带的构造耦合

新生代以来,中国西部的一系列古老造山带和盆地在印-亚板块汇聚作用下重新复活,在青藏高原外围形成了现今全球最大的陆内挤压构造域,被称为环青藏高原盆山体系,其形成过程与机制对深入认识陆-陆碰撞如何影响大陆内部变形有重要意义。柴达木盆地是中国西部重要的新生代沉积盆地,四周均被巨型造山带所围限,共同构成了环青藏高原盆山体系北东段的主体。本文利用最新的石油地震勘探数据、地表地质和已发表的深反射地震数据,将上地壳变形与岩石圈深部变形有机结合,系统刻画了柴达木盆地与周缘三大造山带之间岩石圈尺度的构造耦合关系,在此基础上探讨环青藏高原盆山体系北东段的盆山汇聚过程与机制。柴达木盆地与南侧祁曼塔格—东昆仑山、北东侧南祁连山之间在上地壳尺度发育一系列倾向造山带的基底卷入高角度逆断裂体系,自新生代早期就开始活动,以垂直的基底抬升为主,水平缩短量有限;在下地壳和岩石圈地幔深度则发育倾向盆地一侧的深大断裂,使得柴达木盆地与周缘造山带之间发生截然的莫霍面错断。这些变形特征揭示柴达木盆地与南侧祁曼塔格—东昆仑山、北东侧南祁连山之间发育岩石圈尺度的构造楔,即盆地的岩石圈楔入至增厚的造山带下地壳,其发育主要受盆地与造山带...     

利用石榴橄榄岩重建大陆俯冲带的古动力学环境及其演化过程

本文提出一种对于造山带石榴橄榄岩的简单概念分类模型。该模型将造山带石榴橄榄岩划分为地幔楔型和俯冲带型两种类型,根据所记录温压条件及P-T轨迹的差异,地幔楔型石榴橄榄岩进而被划分为A-D四个亚类,分别对应不同厚度的克拉通型岩石圈地幔及普通大陆岩石圈地幔环境中冷的/古老的/稳定的地幔环境及与之对应的相对热的/年轻的/活动地幔环境。我们选择研究程度较高的斯堪的纳维亚加里东期造山带及我国柴北缘造山带中出露的石榴橄榄岩来检验该模型的适用性。出露于挪威西片麻岩地区的Mg-Cr型及Fe-Ti型石榴橄榄岩分别属于地幔楔型的A亚类(来自于古老的/冷的/厚的/亏损的克拉通岩石圈地幔)及壳源俯冲带石榴橄榄岩,而来自Seve,Troms和Linds地体的石榴橄榄岩则属于地幔楔型的C亚类(来自于古老的/冷的/薄的/亏损的大陆岩石圈地幔)后经历俯冲过程变为俯冲带石榴橄榄岩。对我国柴北缘超高压带绿梁山石榴橄榄岩的检验结果相对复杂。前人的研究表明绿梁山石榴橄榄岩存在三种不同的成因方式,对应分类模型中的:1)D亚类橄榄岩(阿拉斯加型岩浆岩堆晶体);2)壳源俯冲带型石榴橄榄岩;3)A亚类来自克拉通岩石圈地幔的太古代石榴橄榄岩。检验结果表明,该分类模型能较好的解释斯堪的纳维亚加里东期造山带中的石榴橄榄岩的出露规律,岩石学及矿物学特征及不同期次矿物之间的年代学关系,但对于我国绿梁山石榴橄榄岩的成因分类仍需进一步研究。假设该分类模型能适用于大多研究程度较高的造山带石榴橄榄岩,起到决定性作用的两个分类参数为地幔楔形成初期的平均温度(T)及造山带岩石圈地幔的厚度(P)。熔融作用发生的时间及深度同样是区分不同类型石榴橄榄岩的重要因素。     

Geology of the Kokchetav UHP-HP metamorphic belt, Northern Kazakhstan

Abstract Ultrahigh-pressure metamorphic (UHPM) rocks of the Kokchetav Massif of Kazakhstan contain metamorphic microdiamond and coesite inclusions inside rigid capsules such as garnet and zircon. Precambrian protoliths of the UHPM rocks were metamorphosed at around 530 Ma, at pressures of about 7 GPa, which suggests that crustal protoliths were subducted to depths of over 200 km. Primary UHPM minerals are poorly preserved due to partial obliteration by subsequent Barrovian overprint during exhumation and later collision events in Caledonian times. We report the results of detailed mapping of the Kokchetav Massif and use structural data to propose intrusion and exhumation mechanisms for the UHPM rocks. Detailed mapping revealed that many subvertical structures in the ultrahigh-pressure–high-pressure (UHP–HP) units were formed due to later folding. The primary structure appears to be subhorizontal and the total thickness of the UHP rocks is estimated at around 2 km. The first order structure is sandwich-like; that is, the UHP–HP units are separated from underlying low-P metamorphic rocks of the Daulet Series and from feebly metamorphosed to unmetamorphosed sedimentary strata on the top by subhorizontal faults. Kinematic indicators show top-to-the-south sense of shear along the top, and top-to-the-north displacement along the bottom boundaries. These shear senses, together with the observed metamorphic gradients, suggest that the thin UHPM sheet was extruded toward the north. We consider wedge extrusion to have been the most effective mechanism for the exhumation of the UHPM rocks.     

A budget for modern‐Holocene sediment on the South Otago continental shelf

A sediment budget for the South Otago continental shelf and coast, between Nugget Point and Otago Peninsula, reveals modern (post 6500 y) sediment input is dominated by the Clutha River (total 3.14 Mt y^‐1; Mt = 10⁶ tonnes). Contributions from the Taieri River (0.6 Mt y^‐1), the adjacent Southland shelf (0.4 Mt y^‐1), and the biogenic production of calcareous shell debris (0.25 Mt y^‐1) account for only 28% of the input. About half of the bedload (sand and gravel) reaching the Otago shelf is stored within a large nearshore sand wedge in the protected waters of Molyneux Bay, off the Clutha River. Bedload that escapes storage (1.1 Mt y^‐1) is transported north‐eastwards to be deposited on beach and inner shelf environments just north of Otago Peninsula. Suspended load (mud) accounts for over half of the sediment input (2.33 Mt y^‐1) and is nearly all transported from the study area to accumulate in north‐easterly shelf and slope depocentres.     

页岩气楔状模型储层的地震响应特征

中国南方古生界页岩气储层分布面积广,碳质含量高,但是储层保存条件较差,受构造运动影响较大。为此设计了5种不同楔形模型研究其地震响应特征——地震子波主频对波场特征的影响;页岩体速度对波场特征的影响;页岩体顶、底界面坡度变化对波场特征的影响;地层夹角对波场特征的影响;顶底界面岩性组合对波场特征的影响。研究表明:地震子波主频增加,分辨能力提高;页岩体速度与围岩差异变大,反射同相轴振幅增强;楔状页岩体顶、底界面坡度变大,横向分辨率增大;对于梯变型页岩体,在厚度为四分之一波长处,相消干涉,反射能量最弱,振幅达到极小值。     

http://www.sciencedirect.com/science/article/pii/S1674987113000820

The combined use of Hf,Nd and Sr isotopes is more useful in understanding the supercontinent cycle than the use of only Hf isotopic data from detrital zircons.Sr and Nd seawater isotopes,although not as precise as εNd and εHf distributions,also record input from ocean ridge systems.Unlike detrital zircons where sources cannot be precisely located because of crustal recycling,both the location and tectonic setting often can be constrained for whole-rock Nd isotopic data.Furthermore,primary zircon sources may not reside on the same continent as derivative detrital zircons due to supercontinent breakup and assembly.Common to all of the isotopic studies are geographic sampling biases reflecting outcrop distributions,river system sampling,or geologists,and these may be responsible for most of the decorrelation observed between isotopic systems.Distributions between 3.5 and 2 Ga based on εHf median values of four detrital zircon databases as well as our compiled εNd database are noisy but uniformly distributed in time,whereas data between 2 and 1 Ga data are more tightly clustered with smaller variations.Grouped age peaks suggest that both isotopic systems are sampling similar types of orogens.Only after 1 Ga and before 3.5 Ga do we see wide variations and significant disagreement between databases,which may partially reflect variations in both the number of sample locations and the number of samples per location.External and internal orogens show similar patterns in εNd and εHfwith age suggesting that both juvenile and reworked crustal components are produced in both types of orogens with similar proportions.However,both types of orogens clearly produce more juvenile isotopic signatures in retreating mode than in advancing mode.Many secular changes in εHf and εNd distributions correlate with the supercontinent cycle.Although supercontinent breakup is correlated with short-lived decreasing εHf and εNd （≤ 100 Myr） for most supercontinents,there is no isotopic evidence for the breakup of the Paleoproterozoic supercontinent Nuna.Assembly of supercontinents by extroversion is recorded by decreasing εNd in granitoids and metasediments and decreasing εHf in zircons,attesting to the role of crustal reworking in external orogens in advancing mode.As expected,seawater Sr isotopes increase and seawater Nd isotopes decrease during supercontinent assembly by extroversion.Pangea is the only supercontinent that has a clear isotopic record of introversion assembly,during which median εNd and εHf rise rapidly for ≤ 100 Myr.Although expected to increase,radiogenic seawater Sr decreases （and seawater Nd increases） during assembly of Pangea,a feature that may be caused by juvenile input into the oceans from new ocean ridges and external orogens in retreating mode.The fact that a probable onset of plate tectonics around 3 Ga is not recorded in isotopic distributions may be due the existence of widespread felsic crust formed prior to the onset of plate tectonics in a stagnant lid tectonic regime,as supported by Nd and Hf model ages.     

The Moura Phyllonitic Complex: An Accretionary Complex related with obduction in the Southern Iberia Variscan Suture

The structure of the southernmost domain of the Ossa Morena Zone in Portugal (south sector of the Iberian Autochthonous Terrane) is strongly controlled by earlier deformation events. The first two deformation events correspond to tangential strain regimes, marked by subhorizontal milonitic foliations. These events seem to be directly related with the obduction/subduction process during the Variscan ocean closure and the emplacement of the Beja-Acebuches Oceanic Terrane. In this domain (Évora-Beja Domain), the upper tectono-stratigraphic unit (Moura Phyllonitic Complex) is mainly represented by phyllites and corresponds to a strongly imbricated complex, involving several layers of autochthonous sequence (mainly rocks of a volcano-sedimentary complex), but it also includes dismembered and scattered slices of ophiolites. The widespread greenschists facies overprint an earlier high-pressure metamorphic event (blueschists in the central sector of Évora-Beja Domain and eclogites in the western sector). With regard to its geochemical signature, the Moura Phyllonitic Complex includes amphibolites ranging from N-MORB to T/P-MORB (ophiolitic slices) and mafic alkaline and peralkaline metavolcanics (autochthonous slices). At macroscopic scale, the autochthonous sequence of the Évora-Beja Domain is almost complete in the eastern region, with a stratigraphic sequence ranging from Precambrian to Silurian/Lower Devonian. Towards WSW, the Moura Phyllonitic Complex progressively become tectonically discordant on the sequence below, just near the suture, where it superposes Precambrian levels. The overall evidences (tectonic, metamorphic and geochemical) allow the conclusion that the Moura Phyllonitic Complex is an accretionary complex related with the obduction process during earlier times of the variscan ocean closure.     

近岸强海流切变锋作用下悬浮沉积物的输送和沉积--以山东半岛东端外海为例

利用2007年国家908专项ST02区块（渤海海峡及北黄海）水体专项夏季和冬季两个航次调查的温度、浊度和悬浮体浓度数据,选取了跨越山东半岛东端外海泥质沉积体的B1和B2两个断面,结合该区浅地层剖面资料,辅以研究海域的冬季环流数值模拟,着重分析了山东半岛东端外海强海流切变锋作用下的悬浮体输送和沉积特征。结果表明山东半岛东端外海悬浮体向外海输送存在类似“夏储冬输”的规律:夏季,北黄海海水浊度普遍小,不仅北黄海温跃层抑制了底层悬浮体向中上层水体扩散,而且北黄海冷水团在中底层与山东半岛东部沿岸水也形成了显著的温度和悬浮体锋面,阻碍了悬浮物的纬向输送,故夏季应是悬浮体沉积的主要时节;冬季,温跃层消亡,在强风的作用下,山东半岛东部悬浮体浓度高且垂向混合较好,然而在泥质沉积体顶积层两侧,山东半岛东部沿岸中下层北上的逆风补偿流和北上的黄海暖流与位于顶积层上方表中层南下的沿岸流构成了两道显著的强海流切变锋,使得底积层上大量的再悬浮沉积物不能越过山东半岛东部陆架末梢,从而有助于形成山东半岛东端外海特有的Ω状泥质沉积体。     

米苍山东河地质大剖面的构造几何学与运动学及其对上扬子北部陆内俯冲机制的约束

米苍山位于四川盆地北缘,受秦岭造山带由北向南挤压作用的影响,构造变形特征复杂,深入研究米苍山地区构造变形特征对陆内造山作用以及山前冲断带油气勘探具有积极作用。本文通过对米苍山地区的野外地质考察,对地表产状进行详细观测,建立了米苍山地区东河地表地质大剖面,并且利用等倾角区划与轴面分析,对东河剖面进行构造解析,建立了米苍山造山带构造几何学模型,通过构造楔模型分析了米苍山扇形隆升机制,通过不整合面分析,对四川盆地北部米苍山构造变形时间进行了研究。米苍山南缘基底发育4个构造楔,是影响米苍山南部地层抬升的主要原因,其顶部滑脱层为震旦系,底部滑脱层为元古界变质岩,前缘单斜带由嘉陵江-雷口坡组膏岩层作为滑脱层产生的反冲断层控制,寒武系滑脱层与震旦系滑脱层共同影响了大两会背斜的形成,而米苍山北缘则发育单个构造楔,其顶部滑脱层为震旦系,元古界变质岩为底部滑脱层; 通过对米苍山地区不整合面研究,米苍山造山带自中生代以来,至少经历了印支期与燕山期两次大规模的隆升。因此,米苍山前表现为多期次多层滑脱的构造特征。     

Seismic interpretation of crustal structure in the flinders‐mt lofty ranges and gulf regions,South Australia

There appears to be little correlation of earthquake epicentres with known surface geological features in South Australia. Seismic wave travel‐time residuals are used to derive corrections for the velocity and depth parameters for the simple uniform crustal model which approximates to that in South Australia. Local studies of Moho depth in the seismic zone and analysis of travel‐time station corrections from both local earthquake and teleseismic data suggest that lateral and vertical variations in the South Australian crust are small. Data presented in this paper appear to be consistent with a plate tectonic model derived from focal mechanism studies (Stewart & Mount, 1972) for the active South Australian seismic zones.