张裂陆缘地壳结构特征与张裂模式

张裂陆缘作为威尔逊旋回中关键的一环,是研究地球板块构造及其演化过程的重要构造单元.本文阐述了3种类型张裂陆缘(富岩浆型、贫岩浆型和中间型)的地壳结构特征,总结了它们的演化过程与机制,分析表明构造作用、岩浆活动程度、先存结构等是形成不同类型张裂陆缘的主要控制因素.针对南海北部陆缘复杂的构造属性与演化机制问题,提出了今后重点研究方向:南海北部陆缘是否同时具有贫岩浆型与富岩浆型的部分特征;南海北部陆缘丰富的岩浆活动与蛇纹石化地幔剥露能否共存.在南海北部陆缘同时开展三维深地震探测、物理模拟和数值模拟几种手段联合研究,相互约束,共同验证,是建立科学可信的张裂-破裂机制地质模型的必要途径.      

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.      

南海西南次海盆地壳岩石圈伸展破裂过程的构造、沉积和岩浆作用记录

基于跨南海西南次海盆V型尖端区共轭被动大陆边缘的1 000 km长的深反射地震剖面的解释和分析,深入研究南海临界破裂区地壳结构、盆地构造地层格架和岩浆作用特征,阐明地壳岩石圈的伸展破裂过程.结果表明,在南海西南次海盆共轭陆缘可以识别出3条一级界面,即海底、基底（Tg/Tb）和Moho面,根据这些界面可将共轭边缘划分为箱型域、细颈域、楔型域和薄箱型域等多个地壳基底结构构造特征显著不同的构造单元;在西南次海盆V型尖端陆缘盆地充填序列中识别出岩石圈裂解分离的响应界面,即裂后不整合界面T50/Tm,该界面与基底面Tg/Tb限定了陆缘盆地的同裂陷沉积充填序列,其内发育了T70、T60两条重要的幕式构造响应界面,控制了早期高角度正断层控制的断陷盆地系（Tg-T70）、中期拆离断层控制的拆离盆地系（T70-T60）和晚期断坳转换作用控制的坳陷盆地（T60-T50）;西南次海盆V型尖端薄箱型域属于原洋洋壳域,为岩浆型地壳,代表了岩石圈临近裂解分离、但支撑稳定态海底扩张的地幔对流系统还未完全建立起来之前的“临界破裂”状态,发育于23.5~15.5 Ma（T60-Tm）期间.综合考虑地壳初始厚度、断裂活动性和不同时期的盆地原型等,运用平衡剖面技术重建了西南次海盆V型尖端共轭边缘的发育演化过程,建立了南海西南次海盆临界破裂区构造-地层-岩浆相互作用模式,揭示了南海陆缘岩石圈伸展破裂机制.本研究具有重要的理论意义,并对南海的深水油气勘探具有重大的实际应用价值.      

南海北部洋-陆过渡带深部结构与岩石圈破裂过程

洋-陆过渡带是理解大陆岩石圈破裂和海底初始扩张的关键位置,但是在南海北部地区仍然存在关于相关地质过程的诸多疑问.通过近年开展的国际大洋发现计划航次以及深部地质地球物理探测,取得以下4个方面的认识.（1）南海北部的洋-陆边界一般与自由空间重力异常的正-负值过渡位置对应,而更加准确地限定需要结合反射、折射地震资料.稳定大洋岩石圈生成与大陆岩石圈最终破裂之间的洋-陆过渡边界的位置比以往认为的还应往深海盆方向移动.（2）洋-陆过渡带代表了远端带构造作用减弱和岩浆作用逐渐增强的区域.陆坡地壳发育扩张后岩浆底侵、洋-陆过渡带发育同破裂期岩浆喷出结构和侵入反射体.（3）在中生代的古俯冲带弧前区域,新生代的断裂沿着早期的构造开始活动,岩石圈多处发生强烈的共轭韧性剪切作用.随着大陆岩石圈的进一步拉伸减薄,部分靠陆一侧的裂谷中心停止张裂,成为夭折裂谷,以台西南盆地南部凹陷、白云凹陷、西沙海槽为代表,而南海陆缘异常伸展和最终破裂的地方集中在南侧裂谷中心.夭折裂谷下亦发现地幔蛇纹石化,进一步反映了较弱的同破裂岩浆活动.（4）南海初始洋壳的增生沿着大陆边缘走向具有显著的变化,南海东北部洋-陆过渡带下伏地幔明显抬升和部分蛇纹石化,地震纵、横波速度以及折射波衰减特征都支持此观点,反映南海东北部是一个贫岩浆型大陆边缘.未来,南海北部洋-陆过渡带有望成为南海“莫霍钻”的理想备选钻探区.      

被动陆缘洋陆转换带和岩石圈伸展破裂过程分析及其对南海陆缘深水盆地研究的启示

作为伸展陆壳和正常洋壳之间重要的过渡和衔接,洋陆转换带(ocean-continent transition,简写为OCT)蕴含有丰富的地壳岩石圈伸展破裂过程的信息。文中通过系统的资料调研,在总结OCT研究历史、现状和发展趋势的基础上,阐明了OCT的现代概念、类型及其识别标志;详细介绍了以OCT为基础而建立的被动陆缘地壳岩石圈结构构造单元划分方案、表层沉积盆地构造地层格架及重要的构造变革界面特征;分析了大型拆离断层在地壳岩石圈薄化、地幔剥露过程中的控制作用;揭示了陆缘变形集中、迁移和叠合的规律,建立了被动陆缘岩石圈伸展、薄化、剥露和裂解模式。最后,论文对比了国际非岩浆型被动大陆边缘与我国南海OCT的研究,介绍了南海OCT和陆缘深水超深水盆地研究的新发现,提出深入研究南海OCT将为南海陆缘构造演化、洋盆扩张过程和深水超深水盆地的成因机制研究提供新的启示。     

南海大陆边缘动力学:从陆缘破裂到海底扩张

边缘海如何形成是地球科学的基本问题.本研究通过对南海区域深反射地震数据及钻井数据的综合解释,聚焦地壳深部结构和三维全变形机制,在南海陆缘张裂-海盆扩张的构造动力学研究中取得重要进展:(1)“大陆破裂非均一”:拉张过程垂向上分层非均一,受拆离断层系统控制;裂离过程横向上高度变化,中-东侧受岩浆作用主导,西侧受构造作用主导.(2)“海盆扩张非对称”:受周期性地幔对流活动主导,扩张表现为两次洋脊南向跃迁,方向也发生多次转变,导致南海扩张的不连续-非对称性.据此提出西太俯冲背景下周缘受限型海盆高度变化-非均衡扩张模式的新认识,丰富大陆边缘动力学理论.      

南海南部地壳结构的重力模拟及伸展模式探讨

对南海南部地壳结构研究有助于揭示南海完整的演化历史。本研究对南海南部获取的两条多道地震剖面进行了地震 解释,并对重力数据进行了壳幔密度反演。其中 NH973-1 测线始于南海西南次海盆,覆盖了南沙中部的北段;NH973-2 测 线始于南海东部次海盆,穿越礼乐滩东侧。反演结果显示,莫霍面埋深在海盆区 10~11 km,陆缘区 15~21 km 左右,洋壳向 陆壳莫霍面深度迅速增加。海盆区厚度在 6~7 km,为典型的洋壳;陆缘区地壳厚度在 15~19 km,为减薄型地壳。进一步研 究表明(1)在西南次海盆残余扩张脊之下,莫霍面比两侧略深;(2)在礼乐滩外侧海盆区有高值重力异常体,推测为洋壳与深 部岩浆混合的块体;(3)南沙区域上地壳存在高密度带,且横向上岩性可能变化。南海南部陆缘未发现有下地壳高速层,有 比较一致的构造属性和拉张样式,为非火山型陆缘。我们对两条测线陆缘的伸展因子进行了计算,发现上地壳脆性拉伸因 子与全地壳拉伸因子存在差异,其陆缘的拉张模式在纵向上是不均匀一的。     

洋-陆转换带类型与成因机制

洋-陆转换带是被动陆缘的特殊构造,是伸展背景下大陆岩石圈与大洋岩石圈相互作用的关键区域,对于理解和认识大洋和大陆的地球动力过程、机制尤为关键。基于不同的被动陆缘类型,本文总结了不同类型被动陆缘的洋-陆转换带分类及特征,同时探讨其成因机制。根据大陆边缘类型,洋-陆转换带可以划分为四类,火山型被动陆缘中以海倾反射层和下地壳高速体为特征的洋-陆转换带、非火山型被动陆缘蛇纹石化地幔橄榄岩出露的洋-陆转换带、异常减薄洋壳组成的洋-陆转换带、强烈减薄陆壳为主的洋-陆转换带。洋-陆转换带成因模式取决于不同类型被动陆缘的伸展破裂过程,火山型被动陆缘起源于主动的火山裂谷,通过热作用来减薄岩石圈的底部进而发生地幔熔融,产生溢流玄武岩,浅表形成海倾反射体,下地壳表现为高P波速度异常且巨厚的高速体。非火山型陆缘的岩石圈横向伸展与深度相关,岩石圈变形既有均一纯剪切变形(均匀伸展)也有不对称的简单剪切变形(拆离断层)。     

南海西南次海盆两侧陆缘中生代晚期构造接触关系

为探究南海西南次海盆两侧陆缘地块在中生代晚期构造接触关系及其对南海形成演化的影响,利用过南海西南次海盆两侧陆缘采集的地球物理资料以及公开发表的数据资料,对两侧陆缘的地壳结构及前新生界构造变形特征进行了研究.研究结果显示,西南次海盆两侧陆缘的地壳结构及物质组成存在差异,属于性质不同的两个微地块;两侧陆缘前新生代地层在晚中生代经历了来自不同方向的挤压作用,且遭受抬升剥蚀.结合南海及邻区中生代花岗岩分布特征及区域构造背景,进一步推测两侧陆缘地块在晚中生代以俯冲碰撞的方式完成拼贴缝合,该俯冲碰撞带是南海北部俯冲带在南海西南方向的延伸,并且新生代南海的扩张可能与该俯冲碰撞带这个先存的软弱带有关,是南海海盆初始破裂的部位.      

构造控制型天然气水合物矿藏及其特征

构造环境是天然气水合物富集成藏的重要控制因素,增生楔、断裂体系、褶皱、(泥)底辟、滑塌等特殊构造体是影响天然气水合物成藏的主要地质载体。通过对这些特殊构造体与天然气水合物成藏关系的研究,结合流体活动对水合物形成的影响,总结出陆缘地区有增生楔型、盆缘斜坡型、埋藏背斜型、断褶型、滑塌型及底辟型等六类构造控制型水合物矿藏。南海位于欧亚板块、太平洋板块及印澳板块的交汇处,早期为活动陆缘,晚期演化为被动陆缘,其构造活动具有早期张裂、后期挤压的特点,这既不同于被动陆缘,也有别于活动陆缘,可视为“复合型”大陆边缘,兼具了“被动陆缘沉积速率高、活动陆缘构造活跃”的优点,从而形成了“增生楔型、断褶型、底辟型、滑塌型、盆缘斜坡型”等多种构造控制型水合物矿藏,是“复合型”大陆边缘水合物成藏地质模式的典型代表。     

Crustal structure of the continental margin of Korea in the East Sea (Japan Sea) from deep seismic sounding data: evidence for rifting affected by the hotter than normal mantle

Despite the various opening models of the southwestern part of the East Sea (Japan Sea) between the Korean Peninsula and the Japan Arc, the continental margin of the Korean Peninsula remains unknown in crustal structure. As a result, continental rifting and subsequent seafloor spreading processes to explain the opening of the East Sea have not been adequately addressed. We investigated crustal and sedimentary velocity structures across the Korean margin into the adjacent Ulleung Basin from multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data. The Ulleung Basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal. The continental margin documents rapid transition from continental to oceanic crust, exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of Moho over a distance of about 50 km. The crustal model of the margin is characterized by a high-velocity (up to 7.4 km/s) lower crustal (HVLC) layer that is thicker than 10 km under the slope base and pinches out seawards. The HVLC layer is interpreted as magmatic underplating emplaced during continental rifting in response to high upper mantle temperature. The acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption. These features suggest that the evolution of the Korean margin can be explained by the processes occurring at volcanic rifted margins. Global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the Korean margin and in the Ulleung Basin.     

The variation of crustal stretching and different modes of rifting along the Australian southern continental margin

The southern margin of Australia is a passive continental margin, formed during a Late Jurassic–Cretaceous rifting phase. The development of this passive margin is mainly associated with extensional processes that caused crustal thinning. In this work, we have measured the amount of extension and the stretching factor (β factor) across seven transect profiles approximately evenly distributed across the margin. The obtained results show that the amount of extension and the β factor along the margin vary from west to east. The lowest amount of extension, low–intermediate β factors and a very narrow margin are observed in the western part with 80 km of extension and is underlain mostly by the Archean Yilgarn Craton and the Albany–Fraser Orogen. The Gawler Craton in the centre of the south Australian margin is another region of low extension and low–intermediate β factor. The largest amount of extension (384 km) and the largest β factor (β = 1.88) are found in the eastern part of the passive margin in an area underlain by Phanerozoic Tasman Orogen units. Our results imply that there is a strong control of the age and thickness of the continental lithosphere on the style of rifting along the Australian passive margin. Rifting of old and cold lithosphere results in a narrow passive margin, with the formation of relatively few faults with relatively wide spacing, while rifting of younger, warmer lithosphere leads to wide rifting that is accommodated by a large number of faults with small spacing.     

大陆解体与被动陆缘的演化

火山型被动陆缘是大陆解体过程中形成的一类陆缘类型,其演化过程与活动陆缘一样复杂多变。随着近年来对大陆解体过程与被动陆缘演化的深入研究,对其沉积过程、岩浆活动以及变质作用研究都有了很大的进展。陆壳减薄解体的过程有许多不同的模式,不对称的简单剪切模式可能是火山型被动陆缘的成因,其机制是软流圈隆起的最大位置从剖面上看与地壳减薄最大位置不在一条垂线上,造成软流圈上升的岩浆在解体的大陆一侧形成火山型被动陆缘。被动陆缘的沉积建造由两套沉积物组成,一套是大陆解体的裂谷阶段所形成的陆相沉积物和双模式火山岩组合,另一套是稳定陆缘的复理石组合;岩浆作用中基性岩类反应了物质直接源于上地幔的主要特点,并有部分受到地壳混染的特征;变质作用中高温低压环境主要发生在裂谷作用阶段,其特点反映了大陆解体过程中随着时间的增温和减压过程,而拆离伸展阶段则被脆性变形所代替。     

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.     

汇聚背景下的多幕裂陷作用及其迁移机制:以南海北部珠江口盆地为例

南海的形成演化受控于印-澳、欧亚以及太平洋板块的相互运动,为研究汇聚背景下板块碰撞及其远程效应提供重要窗口。为了揭示该汇聚背景下的多幕裂陷过程,本文选取地质信息丰富的整个珠江口盆地为典型区,利用三条高精度地震剖面,对盆地各地质单元进行断层活动速率和构造沉降速率的定量计算及综合分析。结果表明盆地裂陷期东部、中部和西部主要控凹断层的平均活动速率分别为96 m/Ma、223 m/Ma和124 m/Ma,且其平均沉降速率依次为8.5 m/Ma、34 m/Ma和12.7 m/Ma,盆地整体呈现中部裂陷作用最强,其后向西部和东部逐渐减弱的特征。本文认为这与先存断裂以及初始地壳厚度有关:盆地东部和中部存在NE向先存断裂,并且东部先存断裂更加活跃,因此在新生代拉伸应力下东部更易表现为裂陷作用最强的区域,其次为中部和西部;而受前新生代时期俯冲作用的影响,岩浆的底垫作用引起盆地东部地壳增厚,东部裂陷作用强度急剧降低,造成裂陷作用强度的东西差异。此外,盆地南段凹陷裂陷期的断层活动和沉降速率发生激增,裂陷作用存在向南迁移的现象。本文推测在深度相关的伸展模式的影响下,南段凹陷地壳温度升高,强度减弱,因而在伸展应力下发生快速的拉伸减薄,导致裂陷中心向南迁移及岩浆物质上涌。同时,侵入的岩浆物质导致高角度正断层转换成低角度正断层,进一步促进裂陷中心向南迁移。     

Evolution of the eastern margin of Korea: Constraints on the opening of the East Sea (Japan Sea)

We interpreted marine seismic profiles in conjunction with swath bathymetric and magnetic data to investigate rifting to breakup processes at the eastern Korean margin that led to the separation of the southwestern Japan Arc. The eastern Korean margin is rimmed by fundamental elements of rift architecture comprising a seaward succession of a rift basin and an uplifted rift flank passing into the slope, typical of a passive continental margin. In the northern part, rifting occurred in the Korea Plateau that is a continental fragment extended and partially segmented from the Korean Peninsula. Two distinguished rift basins (Onnuri and Bandal Basins) in the Korea Plateau are bounded by major synthetic and smaller antithetic faults, creating wide and considerably symmetric profiles. The large-offset border fault zones of these basins have convex dip slopes and demonstrate a zig-zag arrangement along strike. In contrast, the southern margin is engraved along its length with a single narrow rift basin (Hupo Basin) that is an elongated asymmetric half-graben. Analysis of rift fault patterns suggests that rifting at the Korean margin was primarily controlled by normal faulting resulting from extension rather than strike-slip deformation. Two extension directions for rifting are recognized: the Onnuri and Hupo Basins were rifted in the east-west direction; the Bandal Basin in the east–west and northwest–southeast directions, suggesting two rift stages. We interpret that the east–west direction represents initial rifting at the inner margin; while the Japan Basin widened, rifting propagated southeastward repeatedly from the Japan Basin toward the Korean margin but could not penetrate the strong continental lithosphere of the Korean Shield and changed the direction to the south, resulting in east–west extension to create the rift basins at the Korean margin. The northwest–southeast direction probably represents the direction of rifting orthogonal to the inferred line of breakup along the base of the slope of the Korea Plateau; after breakup the southwestern Japan Arc separated in the southeast direction, indicating a response to tensional tectonics associated with the subduction of the Pacific Plate in the northwest direction. No significant volcanism was involved in initial rifting. In contrast, the inception of sea floor spreading documents a pronounced volcanic phase which appears to reflect asthenospheric upwelling as well as rift-induced convection particularly in the narrow southern margin. We suggest that structural and igneous evolution of the Korean margin, although it is in a back-arc setting, can be explained by the processes occurring at the passive continental margin with magmatism influenced by asthenospheric upwelling.     

The structure,depositional style and accumulation characteristics of continental margin with diachronous breakup in the northern South China Sea

ABSTRACT

A typical diachronous breakup developed in the South China Sea (SCS); the breakup has been unequivocally identified, and gradually decreases in age from NE to SW. To illustrate the influence of a diachronous breakup on hydrocarbon accumulation, we used seismic profiles from CNOOC, and analysed crustal extension characteristics, sedimentary style and source rock accumulation. The main conclusions are as follows: (1) Because of the diachronous breakup in the SCS, various tectonic units developed in different positions on the northern continental margin with different degrees of extension. The Pearl River Mouth Basin (PRMB) is closer to the central sea basin of the SCS and has been affected by regional tension stress for a longer time. Tectonic units of proximal domain, necking zone, and distal margin types developed, and the crust structure thinned from the land towards the sea. The Qiongdongnan Basin (QB) is closer to the Southwest Sub-basin. There are symmetrical proximal domains on the southern and northern sides of the QB, with a necking zone in the central part. It presents as an abandoned passive continental margin. (2) The PRMB exhibited rifted cells first, followed by canyon type extensional faulting and horizontal axial transportation, and then by a longitudinal shelf-slope sedimentary style. But the QB has only rifted cells and canyon type faulting stages, no longitudinal shelf-slope sedimentary stage. The major source rocks in the northern SCS become younger from east to west, and the sedimentary facies change from lacustrine facies to marine-continental transitional facies. (3) The diachronous breakup process ultimately affecting oil and gas migration and accumulation process. The PRMB is dominated by deep-water shelf break accumulation with deep-water fan reservoir bodies. The QB is dominated by canyon type accumulation with channels.     

Tectonic controls on sediment provenance evolution in rift basins: Detrital zircon U–Pb and Hf isotope analysis from the Perth Basin,Western Australia

The role of tectonics in controlling temporal and spatial variations in sediment provenance during the evolution of extensional basins from initial rifting to continental breakup and passive margin development are not well established. We test the influence of tectonics in a rift basin that has experienced minimal uplift but significant extension throughout its history: the Perth Basin, Western Australia. We use published zircon U–Pb and Hf isotope data from basin inception through to continental drift and complement this with new data from samples deposited synchronously with the continental breakup of eastern Gondwana. Three primary source regions are inferred, namely the Archean Yilgarn Craton to the east, the Paleo- and Mesoproterozoic Albany–Fraser–Wilkes Orogen to the south and east, and the Mesoproterozoic and Ediacaran–Cambrian Pinjarra Orogen underlying the rift basin and comprising the dominant crustal components to the west and southwest. From mid-Paleozoic basin inception to Early Cretaceous breakup of eastern Gondwana, drainage in the Perth Basin was primarily north- to northwest-directed as evidenced by the dominant Mesoproterozoic detrital zircon cargo, paleodrainage patterns and paleocurrent directions. Thus, provenance was primarily parallel to the rift axis and perpendicular to the extension direction, particularly during periods of thermal subsidence. During episodes of mechanical extension, detrital zircon ages are polymodal and consistently dominated by Paleo- and Mesoproterozoic grains derived from the Albany–Fraser–Wilkes Orogen, but with significant Archean and Neoproterozoic inputs from the rift margins. It is inferred that during mechanical extension the rate of subsidence exceeded sediment supply, which generated basin-margin scarps and enhanced direct input from the rift shoulders. Detrital zircon spectra from temporally-equivalent samples at the rift margin and in the rift axis reveal that distinct sedimentary routing operated on the flanks. In summary, sediment provenance in the Perth Basin (and probably other rift basins) is tectonically controlled by: (1) extension direction, (2) episodes of mechanical extension (rift) or thermal subsidence (post-rift), and (3) proximity to rift axis or rift margin.