中国东部及邻区晚中生代伸展拆离构造——综述与新认识

伸展构造是大陆岩石圈减薄破坏在上部浅层的一种重要表现。其中, 低角度伸展拆离构造为中国东部以及邻区晚中生代地壳伸展变形的显著构造变形样式, 是研究区域岩石圈浅层构造变形与深部减薄过程关系的重要窗口。结合相关的几何学、运动学、年代学等资料的综述分析, 本文对中国东部及邻区低角度伸展拆离构造时空展布、运动学极性、应变机制进行了系统论述。在空间展布上, 这些伸展拆离构造在东亚大陆的分布不均一, 指示岩石圈的不均匀伸展减薄作用。在形成时代上, 伸展拆离活动主要集中在两个时代区间: 140–125 Ma为伸展拆离构造在中下地壳初始形成期, 在水平伸展作用主导下, 以简单剪切为主; 125–110 Ma为伸展拆离构造快速隆升剥露期, 在垂向挤压的主导下, 以纯剪切为主。相关的几何学和运动学标志表明这些伸展构造总体具有在NW–SE方向上单向低角度伸展拆离运动学特征, 反映它们总体受NW–SE向伸展应力场作用控制。以大兴安岭—太行山—武陵山重力梯度带为界, 东、西两侧区域伸展活动均具有从NW向SE传播的趋势, 且这种趋势西部较东部明显, 表现为不同的动力学背景特征。东部主要受到古太平洋板块后撤影响, 而西部可能与蒙古—鄂霍茨克洋闭合之后, 增厚地壳发生重力崩塌而引发区域大规模伸展活动以及同时发生的秦岭—大别构造带陆内造山后应力伸展的联合作用相关。     

大陆伸展构造综述

简述了大陆伸展构造的研究历史,并从基本概念、构造样式、变形机制和动力背景等方面对大陆伸展构造进行了综述.伸展是大陆构造一种主要类型,并以正断层为主形成多种构造样式,如地堑、裂谷、拆离断层和变质核杂岩等.大陆伸展的变形机制包括纯剪切、简单剪切及分层剪切模式,并由此产生对称与非对称构造.大陆伸展构造的地表表现形式主要为裂谷或变质核杂岩,两者的形成主要取决于岩石圈的流变学结构.大陆伸展的动力学背景主要包括地幔柱上涌、俯冲板片反转与俯冲带后撤、增厚地壳的重力垮塌以及走滑体系的派生拉张等.     

南海北部珠江口盆地中段伸展构造模型及其动力学

位于南海北部大陆边缘上的珠江口盆地发育NNE向、NE向、NW向、近EW向等多组基底断裂,盆地结构复杂,并表现出明显的时空差异性。本文基于珠江口盆地中段地震资料解释的构造样式的变化推断地壳中存在一条向南缓倾斜、呈坡坪式形态的拆离断层,古近系构造属于这条拆离断层上盘的伸展构造系统。北部的西江凹陷属于拆离断层伸展构造系统的头部,凹陷边界正断层铲式断层面形态向深层延伸并收敛在拆离断层面上,凹陷表现为半地堑“断陷”样式;中部的番禺低隆起对应于拆离断层的低角度断坪部位,拆离断层上盘断块的伸展位移导致两侧的恩平组超覆在低隆起上;南部的白云凹陷位于拆离断层的深部断坡部位,充填的文昌组和恩平组表现为“断坳”或“坳断”样式;南部隆起位于拆离断层深部断坪部位,其上盘发育的分支断层控制着荔湾凹陷古近系、新近系的发育并使之表现为复杂的断陷断坳构造样式。该模型强调拆离断层上盘与下盘、不同地壳结构层均发生不同程度的伸展变形,且伸展变形方式、应变量等存在时空差异,而拆离断层正是不同构造单元、不同地壳构造层之间的调节性构造面。总体上,拆离断层上盘以脆性伸展构造变形为主,分支断层控制不同构造单元古近纪的构造演化,下盘则是以韧性伸展变形为主,并拖曳上盘发生不均一的伸展应变;西江凹陷的伸展应变量大于拆离断层下盘的伸展应变量,白云凹陷的伸展应变量则小于拆离断层下盘的伸展应变量。以西江凹陷北部边缘的NE向铲式正断层为头部的拆离断层控制了文昌组沉积,但在恩平组沉积期被近EW向高角度正断层切割破坏而被遗弃,拆离断层系统的头部由西江凹陷北部边缘迁移至番禺低隆起。盆地结构及断裂系统的时空差异性受盆地基底先存构造、地壳与岩石圈结构及伸展量等多方面因素的影响,但主要是对软流圈流动及岩石圈热结构变化的响应。用软流圈由北西向南东流动拖曳上覆岩石圈发生伸展变形的动力学模型能合理地解释珠江口盆地中段古近系构造的形成和演化。     

从地壳上地幔构造看大陆岩石圈伸展与裂解

本篇讨论大陆岩石圈拆沉、伸展与裂解作用过程。由于大陆岩石圈厚度大而且很不均匀,产生裂谷的机制比较复杂。大陆碰撞远程效应的触发,岩石圈拆沉,以及板块运动的不规则性和地球应力场方向转折,都可能产生岩石圈断裂和大陆裂谷。岩石圈拆沉为在重力作用下"去陆根"的作用过程,演化过程可分为大陆根拆离、地壳伸展和岩石圈地幔整体破裂三个阶段。大陆碰撞带、俯冲的大陆和大洋板块、克拉通区域岩石圈,都可能产生岩石圈拆沉。大陆岩石圈调查表明,拉张区可见地壳伸展、岩石圈拆离、软流圈上拱和热沉降;它们是大陆岩石圈伸展与裂解早期的主要表现。从初始拉张的盆岭省到成熟的张裂省,拆离后地壳伸展成复式地堑,下地壳幔源玄武岩浆侵位,断裂带贯通并切穿整个岩石圈,表明地壳伸展进入成熟阶段。中国东北松辽盆地和西欧北海盆地曾处于成熟的张裂省。岩石圈破裂为岩浆侵位提供了阻力很小的通道网。岩浆侵位作用伴随岩石圈破裂和热流体上涌,成熟的张裂省可发展成大陆裂谷。多数的大陆裂谷带并没有发展成威尔逊裂谷带和洋中脊,普通的大陆裂谷要演化为威尔逊裂谷带,必须有来自软流圈的长期和持续的热流和玄武质岩浆的供应。威尔逊裂谷带岩石圈地幔和软流圈为地震低速带,其根源可能与来自地幔底部的地幔热羽流有关。     

造山后伸展构造研究的最新进展

本文综合介绍和比较分析了全球范围内不同时期造山带中的伸展构造样式；地壳尺度的拆离带或低角度正断层的性质、几何学和运动学；拆离带下盘变质核杂岩的抬升机理及其中各种韧性组构的成因和发展演变；后造山伸展塌陷过程中的岩浆活动和热演化；伸展平行褶皱的成因和构造几何学；后造山伸展盆地的形成过程；造山带地球物理剖面解释和岩石圈动力学；后造山伸展构造的物理和数字模拟等方面，当前开展的主要研究内容、研究方法和趋势。     

华北克拉通及邻区晚中生代伸展构造及其动力学背景的讨论

欧亚大陆东部晚中生代伸展构造十分显著,表现为大量发育的变质核杂岩、同构造岩浆岩、韧性拆离断层带等伸展成因的穹隆和地堑-半地堑盆地.通过对这些伸展构造进行系统分析、归纳和总结,将欧亚大陆东部晚中生代伸展构造发育区划分为:泛贝加尔-鄂霍次克带、华北西部带、华北东部带、华北南缘及秦岭-大别带和华南内陆带.这些伸展构造记录了大区域上的NW-SE方向伸展,构成了全球最大的陆壳伸展地区.这些伸展构造使地壳深部的岩石沿拆离断层折返至地表,从而使中下地壳结构发生了强烈的改造.除华北东部带给出了一个较为宽泛的伸展时段外,各个研究区所涉及的伸展穹隆及其相关的拆离断层所表现的伸展峰期时间均十分相近:位于130 ～ 126Ma之间.岩石圈根部的拆沉可能是这个巨型伸展构造带形成的动力学机制.这个模型为探讨华北克拉通破坏和减薄的时限、机制、模式及深部动力学背景提供直接的构造证据.     

珠江口盆地浅水陆架区拆离断陷的构造变形与沉积充填响应:以恩平凹陷为例

珠江口盆地新生代发育跨越浅水区至深水区、多类型的拆离断陷,其中浅水陆架区拆离断陷是探究拆离构造变形与沉积充填响应的重要窗口.基于地震资料和钻井资料详细解析,对恩平凹陷低角度拆离断层特征进行了研究,探讨了拆离构造变形的控制因素、变形过程和沉积充填响应机制.研究表明,恩平凹陷低角度边界正断层为壳间拆离断层,长度约50 km,倾角平均17.5°,断面最深达中下地壳,裂陷期经历了中低角度到低角度的转变.拆离断层形成于中生代先存逆冲断层基础之上,与中下地壳韧性剪切穹隆和边界调节性走滑断裂相伴生,联合控制形成均衡深拆离、前展式宽拆离和迁移型复式拆离3种凹陷结构类型.垂向上可划分出三期裂陷幕,因差异构造变形而具有不同的沉积充填响应:（1）早文昌期均一裂陷幕,快速的裂陷沉降发育于先存逆冲断层弱面,形成具厚层中深湖的均一窄深箕状半地堑;（2）晚文昌期拆离伸展幕,凹陷受差异构造变形而发育成东、西沉积充填差异化的宽浅断陷,西部韧性剪切穹隆弱隆升与边界走滑断层强走滑控制了恩平17洼自迁移型中深湖和大型辫状河三角洲-重力流沉积,而东部韧性剪切穹隆强隆升控制了恩平12-18洼沉积中心跳跃式异迁移的沉积充填,过渡带发育大型走向斜坡扇三角洲;（3）恩平期拆离-断拗联控幕,构造变形由简单剪切向纯剪切转变,沉积向外流水系主导的浅水辫状河三角洲-浅湖相充填转变.低角度拆离断陷具有与高角度脆性断陷不同的构造变形和沉积充填响应,烃源岩与砂岩储层的时空分布非均质性强.研究成果对恩平凹陷的烃源岩和深部储层预测,以及南海北部陆缘同类拆离断陷的构造-沉积充填研究具有良好的指导意义.      

南海北部深水区靖海凹陷结构和构造演化分析

靖海凹陷为南海北部深水区油气勘探的重点盆地之一,系统深入研究盆地结构特征及演化过程有助于深化对南海北部深水盆地的认识,为深水油气勘探提供新思路。本次研究以靖海凹陷2D地震资料的地质构造解释为基础,描述新发现的拆离断层的几何学特征,对比靖海凹陷东、西部结构样式差异性;通过沉降史模拟结果和典型拆离断层生长发育过程分析,恢复了靖海凹陷自新生代以来的沉降和构造演化过程;结合研究区深部资料与国外深水盆地最新研究进展,探讨了盆地结构和构造演化过程与南海北部陆缘地壳和岩石圈的伸展、减薄以及破裂作用的关系。研究结果表明,靖海凹陷东部与西部的盆地结构样式存在差异性,西部发育大型拆离断层及其所控制的拆离盆地系统,东部则呈现为由多个简单的南断北超的半地堑盆地所构成的复式地堑结构。盆地的构造演化划分为断陷期和坳陷期,其中断陷又可细分为断陷Ⅰ幕和断陷Ⅱ幕。盆地断陷Ⅰ幕对应于南海北部陆缘伸展阶段,高角度断层普遍发育;盆地断陷Ⅱ幕对应于南海北部陆缘减薄阶段—剥露阶段,拆离断层发育;当南海北部岩石圈完全破裂之后,盆地进入到坳陷期。     

岩石圈伸展的壳/幔拆离模型(Parallel Extension Tectonics):华北克拉通东部早白垩世岩石圈减薄与破坏机理

华北克拉通岩石圈减薄和破坏机理长期以来存在争议,基于岩石学、岩石地球的化学分析研究突出强调深部过程的重要性。前人提出了两种重要模式:包括以拆沉作用为代表的top-down tectonics模型和以热-机械侵蚀与化学侵蚀,或地幔置换、交代作用的bottom-up tectonics模型。然而,对于这两种模式而言尚存在许多无法合理解释的问题,比如在此深部过程中,区域性岩石圈伸展有多大的贡献?地壳伸展构造是作为深部过程的响应,还是同为岩石圈伸展的产物?本文基于早白垩世东亚地区(尤其是华北克拉通东部地区)伸展构造与岩浆活动的综合分析,揭示出华北克拉通东部不同地区伸展构造变形与岩浆活动之间的时、空和成因关系有一定的差异。但整体上看,岩石圈伸展起着主导作用,控制着岩浆上侵和就位,在拆离断层下盘侵入形成各种规模的花岗质为主的侵入体,或于上盘喷发形成火山-沉积岩盆地。在伸展构造发育的不同阶段,可以有伸展早期、伸展期及伸展期后的岩浆活动。岩浆活动的强度及岩浆源区特点有显著的时空变化。一方面,在同一地区不同演化阶段其源区有很大的差异。表现为主体上是早期以古老下地壳源为主,随着壳/幔伸展作用演化,逐渐向混合源或独立幔源的演化。同时,不同地区岩浆源区的变化规律也显著不同。以胶辽地区为例,胶东整体上是壳幔混合源区对于岩浆演化有重要贡献;而辽东地区具有显著的源区演化特点:从剪切早期古老下地壳源区为主,并伴有幔源物质加入,剪切期古老下地壳为主,到剪切晚期和剪切期后以新生下地壳为主。本文认为岩石圈伸展的壳/幔拆离模型(Parallel Extension Tectonics),可以合理地解释华北克拉通及邻区早白垩世构造-岩浆活动性。在该模型中,遭受伸展的华北克拉通岩石圈发生壳-幔拆离作用。在岩石圈伸展作用期间,地壳层次的拆离作用与岩石圈地幔层次上的拆离作用可以是耦合的或者是解耦的,从而导致华北克拉通岩石圈减薄过程中在地壳尺度上的拆离作用与变质核杂岩的剥露有三种不同的类型:同岩浆活动型伸展(C型:Co-magmatism mode extension)、无岩浆活动型伸展(A型:Amagmatism mode extension)和多阶段混合型(M型:Multi-mode extension)。     

南海北部陆缘结构及构造-岩浆演化

南海北部陆缘发育独特的远端带结构,以“裂谷宽、基底厚和地貌起伏”为主要特点,显著有别于经典贫岩浆型和富岩浆型张裂陆缘.为了解释陆缘结构的成因,综合已有研究进展和国际大洋发现计划（IODP）的钻探成果,对南海北部陆缘基底性质进行了调研,探讨了拆离断层和岩浆作用的特点以及两者间的相互作用.结果表明,在38 Ma之前南海北部大范围发育核杂岩构造,并伴随大量岩浆侵入到中下地壳;岩浆作用一方面加剧了地壳的韧性变形,导致应变无法集中而在多个地方同时发育大型拆离,另一方面对拆离面和减薄的基底进行了强烈改造.最终提出同张裂期就位的岩浆作用和中下地壳的韧性流动是形成南海北部宽裂谷陆缘的关键,深化了对陆缘结构、变形过程和岩石圈减薄机制的理解.      

Late Mesozoic rift evolution and crustal extension in the central Songliao Basin,northeastern China: constraints from cross-section restoration and implications for lithospheric thinning

The Songliao Basin, the largest oil-producing basin in China, was the centre of late Mesozoic rifting and lithospheric thinning in northeastern China. However, the rifts are still poorly revealed due to a thick cover of subsidence successions. By structural interpretation and sequential restoration of cross-sections based on new 2D seismic data and well data, this study presents the structural style, basin evolution, and horizontal crustal extension of the central Songliao Basin. We have developed a novel method to retrieve the regional extension principal strains. The results enable an assignment of rifting into two episodes. The earlier episode (ca. 157–130 Ma) was dominated by distributed faulting of numerous planar normal faults trending NNE–SSW, NNW–SSE, or near NS, probably reflecting pre-existing basement fabrics; in contrast, the later episode (ca. 130–102 Ma) was controlled by localized extension along several major listric faults. Horizontal crustal extension during rifting is estimated to have been 11–28 km (10.6%–25.5%), with the long-term average rate varying from 0.20 to 0.51 mm yr^–1. Regional horizontal strains show a gradual evolution from biaxial extension at the beginning of rifting to WNW–ESE uniaxial stretching during the later rifting episode. Brittle crustal extension is interpreted to have been associated with vertical strain due to tectonic stretching, which is estimated to have contributed more in thinning the lower crust than the mantle lithosphere. Accordingly, a two-episode dynamic model is proposed to explain rifting in the Songliao Basin. We suggest that the earlier event was dominated by delamination of the thickened continental lithosphere, whereas the later event was probably controlled by regional crustal detachment due to slab subduction and stagnancy of the Izanagi lithospheric plate.     

青藏高原近南北向裂谷的时空分布特征及动力学机制

青藏高原南部发育的一系列近南北向裂谷是印度-欧亚大陆持续挤压作用下的大型伸展构造,也是揭示高原后碰撞构造演化过程的重要对象。目前,关于南北向裂谷的形成机制存在多种假说模型,并对裂谷时空分布特征做出了不同的预测,这成为约束裂谷成因机制的关键条件。综合关于裂谷启动时间的已有研究成果,进一步梳理了南北向裂谷的时空分布特征,结果表明近南北向裂谷的启动时间似乎具有自西向东逐步减小的趋势,这与拉萨地体广泛出露的后碰撞岩浆作用演化过程一致。在此基础上,结合地球物理观测,推断近南北向裂谷的动力学机制与印度板片向东拆离假说最为契合。印度板片自西向东的拆离建立了向东传播的重力势能梯度,从而驱动岩石圈向东流动,最终导致南北向裂谷依次向东发育。      

Continental strike-slip rifts and their stratigraphic signature: application to the Bangong/Nujiang zone (Tibet) and the South Penninic zone (Alps)

Our literature studies show that the thermal regime along continental strike-slip rifts is inconspicuous and that they are "low-volcanicity rifts" at best. Along with that, young continental strike-slip rifts exhibit no signs of major thermally controlled doming. We suggest that the larger the strike-slip component of a rift is, the less likely major thermal doming is causally associated with the rift zone. Since vertical lithosphere movements are reflected in the stratigraphic record of a rifted area, different rift modes (strike-slip, dip-slip) may be distinguished by analyzing the relevant sequences. Two ancient and especially suitable strike-slip rift margins in Tethyan mountain belts, the Bangong/Nujiang zone of Tibet and the South Penninic zone of the Alps, were analyzed with regard to their uplift history. The results confirm recent regional rift models which indicate in both cases that rifting was dominated by strike-slip. The stratigraphic approach may provide significant clues as to the mode of paleorifting when structural data are unavailable.     

Structural Variability and Rifting Process of the Segmented Cenozoic Pearl River Mouth Basin, Northern Continental Margin of the South China Sea

The Pearl River Mouth Basin (PRMB) is an important area for studying the evolution of continental marginal basins in the northern South China Sea (SCS), but the structural variability and spatiotemporal rifting process remains poorly understood. This study investigates the differential structural features of the eastern, middle and western PRMB, as well as the extensional deformation laws in operation during the rifting stage, according to an integrated analysis of geometric characteristics and kinematic parameters, i.e., horizontal displacement and stretching factors of basin and crust. The PRMB underwent at least three phases of intense extension, which varied in time and space. (1) During the middle Eocene, most sags in the PRMB were intensely stretched and high-angle planar to listric boundary faults controlled the wedge-shaped stratigraphic geometry. (2) During the late Eocene-to-early Oligocene, the stratigraphic geometry of the sags was slightly wedge-shaped and continuously controlled by boundary faults, however, the extensional strength decreased relatively in the Northern depression zone, but increased in the Southern depression zone. (3) During the late Oligocene, the extension was extremely weak in the northeast PRMB, but relatively strong in the southwest PRMB, leading to tabular stratigraphic geometry in the northeast PRMB, but localized slightly wedge-shaped stratigraphic geometry in the southwest. The southwest PRMB still underwent relatively strong extension during the early Miocene. The southwest PRMB that was induced by a small-scale localized mantle convection system constantly rifted during the late Oligocene, controlled by the weak lithosphere, westward (southwestward) diachronous opening and southward jump of the ocean ridge. The applied quantitative parameters and spatiotemporal rifting process may be used as a reference with which to study the segmented continental margin rifts.     

陆缘扩张型地洼盆地系构造-岩浆作用的独特性──以南海北部陆缘盆地系为例

南海北部陆缘扩张型地洼盆地系既显示了裂谷构造的某些一般性特征,又以其位于大陆壳体与大洋壳体相互作用的东亚陆缘地带;具有复杂的动力场和应力场环境;张裂发生于华夏型地洼余动期;发育由陆变海、陆海相交替的沉积建造;出现由钙碱性岩系、双峰式岩系到拉斑玄武岩和碱性玄武岩系的岩浆演化序列;形成宽阔而弥散的拉伸变形带,具条块状分割的构造格局,总体表现为由大陆盆岭型构造带（地壳张裂）发展到陆缘海盆地系（岩石圈张裂）的演变过程;强烈而持久的地壳运动,发生多幕式拉伸-造盆作用,晚期并在局部出现挤压（反转）构造;以及含丰富油气等矿产资源而展示特色。比较学研究进一步表明,地洼区的裂谷构造可以分出两种基本类型：①华夏型,其中包括东亚陆缘式和里奥格兰德陆内式两种亚型,它们是在地洼型挤压造山阶段之后发生拉伸裂陷;②东非型,它们是在古老克拉通（地台）基础上发生张裂,形成裂谷型地洼区。     

Upper mantle deformation and seismic anisotropy in continental rifts

We investigate the seismic anisotropy signature of the continental rifting process. Several sources of anisotropy are considered: the lithospheric deformation, the asthenospheric flow, and the occurrence of oriented meltpockets in the asthenospheric mantle. Our results show that contrasted anisotropy patterns should be associated with the various conceptual models of rifting. Thus seismic anisotropy measurements may allow one to discriminate between these models. Anisotropy measurements in the Rio Grande, East-African and Rhine rifts suggest that these rifts formed by a transtensional deformation of the lithospheric mantle rather than by homogeneous extension of the lithosphere. Alignment of melt-lenses in the asthenospheric wedge may also account for a significant part of the seismic anisotropy recorded in the internal domains of these rifts.     

Wilson cycle passive margins: Control of orogenic inheritance on continental breakup

Rifts and passive margins often develop along old suture zones where colliding continents merged during earlier phases of the Wilson cycle. For example, the North Atlantic formed after continental break-up along sutures formed during the Caledonian and Variscan orogenies. Even though such tectonic inheritance is generally appreciated, causative physical mechanisms that affect the localization and evolution of rifts and passive margins are not well understood.We use thermo-mechanical modeling to assess the role of orogenic structures during rifting and continental breakup. Such inherited structures include: 1) Thickened crust, 2) eclogitized oceanic crust emplaced in the mantle lithosphere, and 3) mantle wedge of hydrated peridotite (serpentinite).Our models indicate that the presence of inherited structures not only defines the location of rifting upon extension, but also imposes a control on their structural and magmatic evolution. For example, rifts developing in thin initial crust can preserve large amounts of orogenic serpentinite. This facilitates rapid continental breakup, exhumation of hydrated mantle prior to the onset of magmatism. On the contrary, rifts in thicker crust develop more focused thinning in the mantle lithosphere rather than in the crust, and continental breakup is therefore preceded by magmatism. This implies that whether passive margins become magma-poor or magma-rich, respectively, is a function of pre-rift orogenic properties.The models show that structures of orogenic eclogite and hydrated mantle are partially preserved during rifting and are emplaced either at the base of the thinned crust or within the lithospheric mantle as dipping structures. The former provides an alternative interpretation of numerous observations of ‘lower crustal bodies’ which are often regarded as igneous bodies. The latter is consistent with dipping sub-Moho reflectors often observed in passive margins.     

Crustal thickening in an extensional regime: application to the mid-Norwegian Vøring margin

The structure of the mid-Norwegian volcanic Vøring margin at the onset of the Maastrichtian–Paleocene extension phase reflects the cumulative effect of earlier consecutive rifting events. Lateral structural differences present on the margin at that time are a consequence of migration of the location of maximum extension in time between Norway and Greenland. The most important imprints (Moho depth, thermal structure) of these events on the lithosphere are incorporated in a numerical simulation of the final extension phase. We focus on a possible mechanism of formation of the Vøring Marginal High and address the relationship between spatial and temporal evolution of crustal thinning and thickening, uplift of the surface and strength of the lithosphere.It is found that the Vøring Basin formed the strongest part of the margin which explains why the Maastrichtian–Paleocene rift axis was not located here but instead jumped westward with respect to the earlier rift axes locations. The modeling study predicts that local crustal thickening during extension can be expected when large lateral thermal variations are present in the lithosphere at the onset of extension. Negative buoyancy induced by lateral temperature differences increases downwelling adjacent to the rifting zone; convergence of material at the particular part of the margin is mainly taken up by the lower crust. The model shows that during the final phase of extension, the crust in the Vøring Marginal High area was thickened and the surface uplifted. It is likely that this dynamic process and the effects of magmatic intrusions both acted in concert to form the Marginal High.