Dating long thrust systems on Mercury:New clues on the thermal evolution of the planet

The global tectonics of Mercury is dominated by contractional features mainly represented by lobate scarps,high relief ridges,and wrinkle ridges.These structures are the expression of thrust faults and are linear or arcuate features widely distributed on Mercury.Locally,these structures are arranged in long systems characterized by a preferential orientation and non-random spatial distribution.In this work we identified five thrust systems,generally longer than 1000 km.They were named after the main structure or crater encompassed by the system as:Thakur,Victoria,Villa Lobos,Al-Hamadhani,and Enterprise.In order to gain clues about their formation,we dated them using the buffered crater counting technique,which can be applied to derive the ages of linear landforms such as faults,ridges and channels.To estimate the absolute age for the end of the thrust system's activity,we applied both Le Feuvre and Wieczorek Production Function and Neukum Production Functions.Moreover,to further confirm the results obtained with the buffered crater counting method,the classic stratigraphic approach has been adopted,in which a faulted and an unfaulted craters were dated for each system.The results gave consistent ages and suggested that the most movements along major structures all over Mercury most likely ended at about 3.6-3.8 Ga.This gives new clues to better understand the tectonics of the planet and,therefore,its thermal evolution.Indeed,the early occurrence of tectonic activity in the planet's history,well before than predicted by the thermophysical models,coupled with the orientation and spatial distribution of the thrust systems,suggests that other processes beside global contraction,like mantle downwelling or tidal despinning,could have contributed to the first stage of the planet's history.     

The role of tectonic flow of crustal material in the formation of the Sea of Japan and the Sea of Okhotsk

Based on the concept of tectonic delamination of the lithosphere, we revealed that the Sea of Japan and the Sea of Okhotsk were formed as a result of the tectonic flow of crustal material. The intermittent southward movement of southwestern Japan (Late Cretaceous–Cenozoic) along the eastern Japanese leftlateral strike-slip fault zone resulted in the formation of paired structures: back-arc extensional (Central Japan rift) and frontal compressional (South Japan imbricate–thrust belt) structures. The Sea of Okhotsk was formed in a similar tectonic setting: South Okhotsk rift (back-arc extensional structure) and Kamuikotan–Susunai compressional belt (frontal imbricate-thrust structure). Synchronous extension, compression, and strike-slip movements suggest that the tectonic flow of crustal material played a critical role in the formation of the Sea of Japan and the Sea of Okhotsk.     

Changes of Late Mesozoic Tectonic Regimes around the Ordos Basin (North China) and their Geodynamic Implications

A synthesis is given in this paper on late Mesozoic deformation pattern in the zones around the Ordos Basin based on lithostratigraphic and structural analyses. A relative chronology of the late Mesozoic tectonic stress evolution was established from the field analyses of fault kinematics and constrained by stratigraphic contact relationships. The results show alternation of tectonic compressional and extensional regimes. The Ordos Basin and its surroundings were in weak N-S to NNE-SSW extension during the Early to Middle Jurassic, which reactivated E-W-trending basement fractures. The tectonic regime changed to a multi-directional compressional one during the Late Jurassic, which resulted in crustal shortening deformation along the marginal zones of the Ordos Basin. Then it changed to an extensional one during the Early Cretaceous, which rifted the western, northwestern and southeastern margins of the Ordos Basin. A NW-SE compression occurred during the Late Cretaceous and caused the termination of sedimentation and uplift of the Ordos Basin. This phased evolution of the late Mesozoic tectonic stress regimes and associated deformation pattern around the Ordos Basin best records the changes in regional geodynamic settings in East Asia, from the Early to Middle Jurassic post-orogenic extension following the Triassic collision between the North and South China Blocks, to the Late Jurassic multi-directional compressions produced by synchronous convergence of the three plates (the Siberian Plate to the north, Paleo-Pacific Plate to the east and Lhasa Block to the west) towards the East Asian continent. Early Cretaceous extension might be the response to collapse and lithospheric thinning of the North China Craton.     

Shear structures in the Serra do Azeite shear zone,southeastern Brazil: Transtensional deformation during regional transpression in the central Mantiqueira province (Ribeira belt)

The Serra do Azeite shear zone (SASZ) is a northeast-trending regional structure in southeastern Brazil. Kinematic analysis carried out in the SASZ suggests a ductile sinistral transtensional regime in amphibolite facies conditions. Constrictional, flattened, and simple-shear strain domains occur in mylonites with stretching lineation plunging to the east–southeast. Kinematic indicators suggest oblique top–down-to-the-east–southeast and sinistral strike-slip components along variably oriented shear planes. Results of the simple geometrical construction applied to the shear-zone pattern, coupled with field data and kinematic analysis, suggest that sinistral transtensional shearing resulted from east–northeast-directed crustal extension and sinistral strike-slip displacement, accompanied by north–northeast/south–southwest contraction and vertical thinning. The K/Ar and Ar/Ar cooling ages match the proposed interval for crustal extension in the central Mantiqueira province (0.6–0.58/0.57 Ga) based on ages of alkaline granitoids and volcanic rocks. These data indicate types-I and -S granite magmatism, as well as metamorphism and dextral transpressional deformation along the Ribeira belt. Therefore, we interpret the transtensional regime as a result of southwest-directed lateral extrusion and uplift crustal slices (overall oblique extrusion) during an orogenic-scale partitioned transpressional regime. Our results suggest this regime was coeval with a phase of regional stretching subparallel to the Ribeira belt, which would explain the coexistence of extensional and compressional structures during overall plate convergence.     

河西走廊平山湖盆地早白垩世构造变形期次及其碎屑锆石U-Pb年龄约束

河西走廊北部的平山湖盆地,被围限于龙首山、北大山和合黎山之间,是一个在早白垩世受南北两侧逆冲断层共同控制形成并发展的盆地。笔者通过研究盆地内下白垩统沉积特征、构造变形、生长地层以及碎屑锆石U-Pb年代学特征,划分了平山湖盆地在早白垩世的构造演化期次,并恢复其形成演化过程。盆地内发育一套由下向上总体变细的下白垩统庙沟群沉积序列,盆地内构造变形以NE-SW向挤压和近E-W向伸展为主,庙沟群上岩组的碎屑锆石最小年龄为(129.3±1.8)Ma,可能代表了地层沉积和同期地堑发育的最早时间。由此得出,在早白垩世早期发育挤压构造盆地,同构造生长地层为挤压盆地的形成与构造演化提供了时代约束;在早白垩世晚期发育伸展断陷盆地,由挤压到伸展的转换时间晚于129.3 Ma。     

准噶尔盆地构造动力学过程

准噶尔盆地被古生代缝合线构造包围,周缘山系向盆地方向逆冲推覆,显示了盆地处于长期受压的构造环境;火山喷发由海相转变为大陆相,火山活动由强到弱,表明二叠纪以后,上地幔物质一直处于收缩状态,盆地整体持续下沉接受沉积,地壳增厚。二叠纪进入盆地演化阶段之后,经历了晚海西、印支、燕山和喜马拉雅四期构造应力场的作用,发生了南北向拉张变形、南北向和北西向碰撞挤压变形、南北向和北西向张压交替变形以及南北向压扭冲断、走滑和重力滑覆变形,形成复杂多样构造类型。      

1.9-1.8 Ga old strike-slip megashears in the Baltic Shield, and their plate tectonic implications

The tectonic evolution of the ca. 2.0-1.75 Ga old Svecokarelian fold belt is reviewed, and evidence is presented for large-scale intraplate strike-slip movements along ductile megashears. After the formation of the Kola collision suture and the neighbouring Granulite-Tanaelv thrust belt around 1.9 Ga ago, dextral shearing was initiated along N-S trending megashears. Subsequent anticlockwise rotation of the initially NNE-SSW oriented principal compressive stress caused dextral shearing along a NW-SE trending megashear and reversal in the sense of shearing in the N-S trending ones. Further anticlockwise stress rotation (to a total of about 120°) brought an end to sinistral shearing along the N-S megashears around 1.8 Ga ago and caused reversal to sinistral slip along the NW-SE megashear. Both the older (1.9-1.85 Ga) and younger (1.84-1.8 Ga) parts of this evolution are recorded within the Karelian province and its southwestern margin, where consolidation of the lithosphere took place shortly after 1.9 Ga ago. In the Svecofennian province, where crustal accretion did not start until around 1.9 Ga ago, the older movements may have caused synaccretional crustal folding, but with increasing consolidation, the deformation was concentrated along megashears. Although it is still not possible to interrelate the function of active subduction zones and intraplate megashears. the evolution traced so far provides support for plate tectonic interpretations of the Early Proterozoic geodynamics of the Baltic Shield.     

北京房山地区太平山褶皱的变形特征和形成时代:华北克拉通早白垩世挤压构造的意义

北京西山地区广泛发育轴向近E-W向的褶皱,对理解华北克拉通东部构造演化至关重要,但目前对其形成时代和构造动力学背景却没有明确的认识.北京房山地区的太平山褶皱是轴向近E-W向褶皱的典型代表.通过精细的野外构造观察和构造年代学研究,确定太平山褶皱的空间展布特征和构造样式,厘定构造变形的时代,为该区乃至华北克拉通东部的构造演化提供重要依据.通过详细的野外构造观察和系统的β图解研究,表明太平山褶皱由直立倾伏背斜和斜歪倾伏向斜构成.通过对早期卷入褶皱变形的煌斑岩席和晚期切层侵入褶皱的闪长玢岩脉进行锆石U-Pb定年,得到煌斑岩席和闪长玢岩脉的形成时代分别为147.2±2.4Ma和129.0±3.2Ma,表明太平山褶皱的形成时代为燕山运动B幕.另外,北京西山地区还存在NW-SE向的伸展构造样式.代表本区NW-SE向伸展的房山穹窿(约136Ma)晚于N-S向挤压构造,侵入切割太平山褶皱,塑造了研究区现今主体构造格架,进一步限定太平山褶皱的形成时代为147～136 Ma.因此,研究区轴向近E-W向的褶皱为早白垩世近N-S向挤压构造的产物;N-S向挤压和NW-SE向伸展的构造样式,为华北克拉通中生代构造体制的转换和动力学背景提供关键依据.     

THE EXTENSIONAL MOVEMENT AND ACTION IN THE SOUTHERN TIBET SINCE HERCYNIAN

THE EXTENSIONAL MOVEMENT AND ACTION IN THE SOUTHERN TIBET SINCE HERCYNIAN     

晚中生代—新生代构造体制转换与鄂尔多斯盆地改造

鄂尔多斯盆地是叠加在华北古生代克拉通台地之上的中生代大型陆内盆地。晚中生代—新生代是鄂尔多斯盆地重要的改造阶段,区域构造体制经历了重大转换,在盆地周缘形成不同方向和不同样式的构造带。其中发生在中、晚侏罗世时期的燕山运动主幕,对鄂尔多斯盆地的定型和发展具有划时代意义,这期构造变动导致鄂尔多斯盆地周缘挤压逆冲构造带的形成。早白垩世时期,对区域构造应力体制转换的响应,鄂尔多斯盆地处于弱引张构造环境,引张构造变形主要集中在盆地西南缘地带,六盘山古地堑发育。新生代时期,构造变形主要发生在鄂尔多斯盆地周缘,形成一系列地堑盆地。晚中新世或上新世以来的新构造运动时期,受到青藏高原快速隆升和向东构造挤出作用的影响,鄂尔多斯盆地西南缘六盘山褶皱带快速崛起,而在盆地的其他周边地带则发生引张变形和地块差异性升降。最后,笔者论述了不同构造应力体制下盆地的改造作用,讨论了鄂尔多斯盆地研究中的一些基础地质构造问题。     

论鄂尔多斯盆地及其周缘侏罗纪变形

侏罗纪是东亚大地构造发展的重要转折时期,在鄂尔多斯盆地及其周缘可划分为两个性质不同的构造变形阶段。早中侏罗世,盆地处于弱引张应力环境,引张方向近N-S至NNE-SSW向,伸展变形主要发生在盆地周边地带,其发生与晚三叠世华南-华北地块沿秦岭造山带碰撞后的陆内应力场调整作用有关。中晚侏罗世,盆地遭受多向挤压应力作用,挤压方向近W-E、NW-SE和NE-SW,在盆地周缘形成展布方向不一、构造样式不同的边界挤压构造带,盆地轮廓基本定型。西缘近N-S向逆冲-推覆构造带的形成与阿拉善地块和陇西地块向东挤出作用有关;东缘及东南缘总体呈“S”形展布的挤压边界带表现为反向逆冲断裂及其相关褶皱,推测发生在山西台褶带深部滑脱系统的前锋上盘断坡。盆地北侧大青山地区近东西向大型推覆构造和早中侏罗世伸展断陷盆地构造挤压反转,表明阴山造山带在中晚侏罗世时期强烈的N-S向缩短变形和再生造山。鄂尔多斯盆地周缘边界构造带记录了中晚侏罗世强烈的陆内多向内挤压作用和大陆地壳增厚过程,其发生的动力学背景与周邻不同板块(古太平洋、西伯利亚、特提斯)同时向东亚大陆汇聚产生的远程效应有关。中晚侏罗世多向挤压变形加速了鄂尔多斯盆地生烃过程,对多种能源矿产富集和成藏定位产生重要影响。     

行星构造:寻求地球演化的踪迹

地质构造是记录地球内、外动力地质作用过程的标志.和地球相似,太阳系其他天体上也发育丰富的地质构造.以研究天体表面的地质构造及其动力学机制为目的的"行星构造学"是建立在构造地质学、遥感地质学和地球物理学等学科基础上的一门新兴前沿学科.由于天体的大小、组分和轨道位置不同,表面构造特征及其形成机制各异.对比研究地球和其他天体...     

Multi-proxy isotopic tracing of magmatic sources and crustal recycling in the Palaeozoic to Early Jurassic active margin of North-Western Gondwana

We trace source variations of active margin granitoids which crystallised intermittently over ~300 Ma in varying kinematic regimes, by combining zircon Lu-Hf isotopic data from Early Palaeozoic to Early Jurassic igneous and metaigneous rocks in the Mérida Andes, Venezuela and the Santander Massif, Colombia, with new whole rock Rb/Sr and Sm-Nd isotopic data, and quartz O isotopic data. These new data are unique in South America because they were obtained from discrete magmatic and metamorphic zircon populations, providing a high temporal resolution dataset, and compare several isotopic systems on the same samples. Collectively, these data provide valuable insight into the evolution of the isotopic structure of the continental crust in long-lived active margins.Phanerozoic active margin-related granitoids in the Mérida Andes and the Santander Massif yield zircon Lu-Hf model ages ranging between 0.77 Ga and 1.57 Ga which clearly define temporal trends that can be correlated with changes in tectonic regimes. The oldest Lu-Hf model ages of >1.3 Ga are restricted to granitoids which formed during Barrovian metamorphism and crustal thickening between ~499 Ma and ~473 Ma. These granitoids yield high initial ⁸⁷Sr/⁸⁶Sr ratios, suggesting that evolved, Rb-rich middle to upper crust was the major source of melt. Granitoids and rhyolites which crystallised during subsequent extension between ~472 Ma and ~452 Ma yield younger Lu-Hf model ages of 0.80 Ga–1.3 Ga and low initial ⁸⁷Sr/⁸⁶Sr ratios, suggesting that they were derived from much more juvenile, Rb-poor sources such as mafic lower crust and mantle-derived melts. The rapid change in magmatic sources at ~472 Ma can be attributed either to reduced crustal assimilation during extension, or a short pulse of crustal growth by addition of juvenile material to the continental crust. Between ~472 Ma and ~196 Ma Lu-Hf model ages remain mostly constant between ~1.0 and ~1.2 Ga. The large scatter and the absence of definite trends in initial ⁸⁷Sr/⁸⁶Sr ratios suggest that both mafic, Rb-poor, and evolved Rb-rich sources were important precursors of active margin magmas in Colombia and Venezuela throughout the Palaeozoic to the Early Jurassic.Previous studies have shown that the genesis of arc magmas may be stimulated by heat advection to the crust during the underplating of mantle derived melt, but the absence of permanent younging trends in Lu-Hf model ages from ~472 Ma to ~196 Ma suggests that very little new crust was generated during this period in the studied region. An overwhelming majority of the analysed igneous rocks yield zircon Lu-Hf model ages of >1 Ga which may be accounted for by documented local crustal end members of 1 Ga–1.6 Ga, and do not require contributions from the depleted mantle. Therefore, recycling of ~1 Ga and older crust was a dominant process in the north-western corner of Gondwana between ~472 Ma and ~196 Ma.This study shows that whole rock Sm-Nd and zircon Lu-Hf data can be interpreted similarly regarding the age of the source regions, whereas Rb-Sr and O isotope data from the same rocks yield valuable information regarding the geochemical nature of the source.     

The crustal evolution of South America from a zircon Hf‐isotope perspective

Hf‐isotope data of >1100 detrital zircon grains from the Palaeozoic, south‐central Andean Gondwana margin record the complete crustal evolution of South America, which was the predominant source. The oldest grains, with crustal residence ages of 3.8–4.0 Ga, are consistent with complete recycling of existing continental crust around 4 Ga. We confirm three major Archaean, Palaeoproterozoic (Transamazonian) and late Mesoproterozoic to early Neoproterozoic crust‐addition phases as well as six igneous phases during Proterozoic to Palaeozoic time involving mixing of juvenile and crustally reworked material. A late Mesoproterozoic to early Neoproterozoic, Grenville‐age igneous belt can be postulated along the palaeo‐margin of South America. This belt was the basement for later magmatic arcs and accreted allochthonous microcontinents as recorded by similar crustal residence ages. Crustal reworking likely dominated over juvenile addition during the Palaeozoic era, and Proterozoic and Archaean zircon was mainly crustally reworked from the eroding, thickened Ordovician Famatinian arc.     

黄河口河流沙碎屑沉积物锆石U-Pb年龄及地质意义

陆缘碎屑沉积（物）岩是来自物源区物质的天然混合物,保存了源区不同地质作用过程形成产物的重要信息,可以为揭示盆地沉积、区域构造和地壳增长和演化以及古大陆再造等方面提供重要证据。对采集自山东东营黄河口河流沙的碎屑锆石进行了U Pb年龄分析,并结合河流沙碎屑矿物粒度分析及重砂矿物组成特征,确定黄河口河流沙主体粒度为10~280 μm,重砂矿物主要为角闪石、赤-褐铁矿、石榴子石、绿帘石、磁铁矿等。黄河口碎屑锆石以岩浆成因锆石为主,其U-Pb年龄呈多峰特征,最大U-Pb谐和年龄为3.65 Ga,意味着华北克拉通其他地区也有＞3.6 Ga的地壳物质存在;近2.7 Ga的碎屑锆石反映鲁西地区该期岩浆活动产物的贡献。锆石U-Pb年龄主体分布于206~440 Ma、843~1 239 Ma、1 476~2 714 Ma等几个阶段,并以~2.5 Ga、~1.8 Ga和400 Ma为峰期,其中~2.5 Ga与~1.8 Ga的峰期反映华北板块的物质贡献,而400 Ma峰期与1 000~800 Ma的年龄代表源自苏鲁造山带的物质。结合已有的研究资料,认为黄河流域2.5~1.8 Ga为主要的大陆地壳增生阶段,该阶段形成地壳应占当今流域地壳的60%以上。     

Neoarchean (2.5-2.8 Ga) crustal growth of the North China Craton revealed by zircon Hf isotope: A synthesis

The crustal growth of the North China Craton(NCC) during the Neoarchean time(2.5—2.8 Ga) is a hotly controversial topic,with some proposing thai the main crustal growth occurred in the late Neoarchean (2.5—2.6 Ga),in agreement with the time of the magmatism,whereas others suggest that the main crustal accretion took place during early Neoarchean time(2.7—2.8 Ga),consistent with the time of crustalformation of other cratons in the world.Zircon U-Pb ages and Hf isotope compositions can provide rigorous constraints on the time of crustal growth and the evolution and tectonic division of the NCC.In this contribution, we make a comprehensive review of zircon Hf isotope data in combination with zircon U-Pb geochronology and some geochemistry data from various divisions of the NCC with an aim to constrain the Neoarchean crustal growth of the NCC.The results suggest that both 2.7—2.8 Ga and 2.5—2.6 Ga crustal growth are distributed over the NCC and the former is much wider than previously suggested.The Eastern block is characterized by the main 2.7—2.8 Ga crustal growth with local new crustal-formation at 2.5—2.6 Ga,and the Yinshan block is characterized by～2.7 Ga crustal accretion as revealed by Hf-isotope data of detrital zircons from the Zhaertai Group.Detrital zircon data of the Khondalite Belt indicate that the main crustal growth period of the Western block is Paleoproterozoic involving some～2.6 Ga and minor Early- to Middle-Archean crustal components,and the crustal accretion in the Trans-North China Orogen(TNCO) has a wide age range from 2.5 Ga to 2.9 Ga with a notable regional discrepancy.Zircon Hf isotope compositions,coupled with zircon ages and other geochemical data suggest that the southern margin may not be an extension of the TNCO,and the evolution and tectonic division of the NCC is more complex than previously proposed,probably involving multi-stage crustal growth and subduction processes.However, there is no doubt that 2.7—2.8 Ga magmatism and crustal-formation are more widely distributed than previously considered,which is further supported by the data of zircons from Precambrian lower crustal rocks, overlying sedimentary cover,modern river sediments and Late Neoarchean syenogranites.     

Structure and tectonic history of the foreland basins of southernmost South America

The common elements and differences of the neighboring Austral (Magallanes), Malvinas and South Malvinas (South Falkland) sedimentary basins are described and analyzed. The tectonic history of these basins involves Triassic to Jurassic crustal stretching, an ensuing Early Cretaceous thermal subsidence in the retroarc, followed by a Late Cretaceous–Paleogene compressional phase, and a Neogene to present-day deactivation of the fold–thrust belt dominated by wrench deformation. A concomitant Late Cretaceous onset of the foreland phase in the three basins and an integrated history during the Late Cretaceous–Cenozoic are proposed. The main lower Paleocene–lower Eocene initial foredeep depocenters were bounding the basement domain and are now deformed into the thin-skinned fold–thrust belts. A few extensional depocenters developed in the Austral and Malvinas basins during late Paleocene–early Eocene times due to a temporary extensional regime resulting from an acceleration in the separation rate between South America and Antarctica preceding the initial opening of the Drake Passage. These extensional depocenters were superimposed to the previous distal foredeep depocenter, postdating the initiation of the foredeep phase and the onset of compressional deformation. Another pervasive set of normal faults of Paleocene to Recent age that can be recognized throughout the basins are interpreted to be a consequence of flexural bending of the lithosphere, in agreement with a previous study from South Malvinas basin. Contractional deformation was replaced by transpressive kinematics during the Oligocene due to a major tectonic plate reorganization. Presently, while the South Malvinas basin is dominated by the transpressive uplift of its active margin with minor sediment supply, the westward basins undergo localized development of pull-apart depocenters and transpressional uplift of previous structures. The effective elastic thickness of the lithosphere for different sections of each basin is calculated using a dynamic finite element numerical model that simulates the lithospheric response to advancing tectonic load with active sedimentation.     

Reconciliation of conflicting tectonic models for Proterozoic rocks of northern New Mexico

Two major Proterozoic tectonic events are documented in the Taos Range of northern New Mexico. Regional structures involving the tectonic interleaving of c.   1.65  Ga granitoids with supracrustal rocks are interpreted to have formed before 1.42  Ga and probably during collisional assembly of island arc crust into new (1.7–1.6  Ga) continental lithosphere. Supracrustal rocks record 650–750  °C, 6–8  kbar metamorphism (M₂); these high temperatures may have been reached during sandwiching between c.   1.65  Ga granitoids. However, the early history has been obscured by renewed tectonism at c.   1.4  Ga that resulted in partial melting, fabric reactivation and new mineral growth at 4  kbar (M₃). Metamorphic temperature variations from uppermost-amphibolite to amphibolite facies rocks may be associated with c.   1.65 and/or 1.4  Ga plutonism, but not to a 1.4  Ga extensional shear zone as previously proposed. Syn- and post-1.4  Ga contraction is suggested by high- and low-temperature microstructures showing top-to-the-south-east thrusting. This work reconciles conflicting models by suggesting that the geometry of the structures was mainly established by c.   1.65  Ga, but that the present fabric also records 1.4  Ga tectonism involving high- T  metamorphism and fabric reactivation.     

Aspects of the structural evolution of the Lusitanian Basin in Portugal and the shelf and slope area offshore Portugal

The study provides a regional seismic interpretation and mapping of the Mesozoic and Cenozoic succession of the Lusitanian Basin and the shelf and slope area off Portugal. The seismic study is compared with previous studies of the Lusitanian Basin. From the Late Triassic to the Cretaceous the study area experienced four rift phases and intermittent periods of tectonic quiescence. The Triassic rifting was concentrated in the central part of the Lusitanian Basin and in the southernmost part of the study area, both as symmetrical grabens and half-grabens. The evolution of half-grabens was particularly prominent in the south. The Triassic fault-controlled subsidence ceased during the latest Late Triassic and was succeeded by regional subsidence during the early Early Jurassic (Hettangian) when deposition of evaporites took place. A second rift phase was initiated in the Early Jurassic, most likely during the Sinemurian–Pliensbachian. This resulted in minor salt movements along the most prominent faults. The second phase was concentrated to the area south of the Nazare Fault Zone and resulted here in the accumulation of a thick Sinemurian–Callovian succession. Following a major hiatus, probably as a result of the opening of the Central Atlantic, resumed deposition occurred during the Late Jurassic. Evidence for Late Jurassic fault-controlled subsidence is widespread over the whole basin. The pattern of Late Jurassic subsidence appears to change across the Nazare Fault Zone. North of the Nazare Fault, fault-controlled subsidence occurred mainly along NNW–SSE-trending faults and to the south of this fault zone a NNE–SSW fault pattern seems to dominate. The Oxfordian rift phase is testified in onlapping of the Oxfordian succession on salt pillows which formed in association with fault activity. The fourth and final rift phase was in the latest Late Jurassic or earliest Early Cretaceous. The Jurassic extensional tectonism resulted in triggering of salt movement and the development of salt structures along fault zones. However, only salt pillow development can be demonstrated. The extensional tectonics ceased during the Early Cretaceous. During most of the Cretaceous, regional subsidence occurred, resulting in the deposition of a uniform Lower and Upper Cretaceous succession. Marked inversion of former normal faults, particularly along NE–SW-trending faults, and development of salt diapirs occurred during the Middle Miocene, probably followed by tectonic pulses during the Late Miocene to present. The inversion was most prominent in the central and southern parts of the study area. In between these two areas affected by structural inversion, fault-controlled subsidence resulted in the formation of the Cenozoic Lower Tagus Basin. Northwest of the Nazare Fault Zone the effect of the compressional tectonic regime quickly dies out and extensional tectonic environment seems to have prevailed. The Miocene compressional stress was mainly oriented NW–SE shifting to more N–S in the southern part.     

Late Paleozoic provenance shift in the south-central North China Craton: Implications for tectonic evolution and crustal growth

U–Pb geochronologic and Hf isotopic results of seven sandstones collected from Late Carboniferous through Early Triassic strata of the south-central part of the North China Craton record a dramatic provenance shift near the end of the Late Carboniferous. Detrital zircons from the Late Carboniferous sandstones are dominated by the Early Paleozoic components with positive ε_Hf(t) values, implying the existence of a significant volume of juvenile crust at this age in the source regions. Moreover, there are also three minor peaks at ca. 2.5 Ga, 1.87 Ga and 1.1–0.9 Ga. Based on our new data, in conjunction with existing zircon ages and Hf isotopic data in the North China Craton (NCC), Central China Orogenic Belt (CCOB) and Central Asian Orogenic Belt (CAOB), it can be concluded that Early Paleozoic and Neoproterozoic detritus in the south-central NCC were derived from the CCOB. Zircons with ages of 1.9–1.7 Ga were derived from the NCC. However, the oldest components can't be distinguished, possibly from either the NCC or the CCOB, or both. In contrast, detrital zircons from the Permian and Triassic sandstones are characterized by three major groups of U–Pb ages (2.6–2.4 Ga, 1.9–1.7 Ga and Late Paleozoic ages). Specially, most of the Late Paleozoic zircons show negative ε_Hf(t) values, similar to the igneous zircons from intrusive rocks of the Inner Mongolia Paleo-Uplift (IMPU), indicating that the Late Paleozoic detritus were derived from the northern part of the NCC. This provenance shift could be approximately constrained at the end of the Late Carboniferous and probably hints that tectonic uplift firstly occurred between the CCOB and the NCC as a result of the collision between the South and North Qinling microcontinental terranes, and then switched to the domain between the CAOB and the NCC. Additionally, on the basis of Lu–Hf isotopic data, we reveal the pre-Triassic crustal growth history for the NCC. In comparison among the three crustal growth curves obtained from modern river sands, our samples, and the Proterozoic sedimentary rocks, we realize that old components are apparently underestimated by zircons from the younger sedimentary rocks and modern river sands. Hence, cautions should be taken when using this method to investigate growth history of continental crust.