Magnetic polarity sequences from downhole measurements in ODP holes 998B and 1001A,leg 165, Caribbean Sea

The magnetostratigraphy of sediments drilled at two sites (998 and 1001) in the Caribbean Sea during the Ocean Drilling Program (ODP) Leg 165 is established directly from downhole measurements. For the first time in the ODP, the magnetostratigraphy of sediments is determined from both logging magnetic data and paleomagnetic analysis of core material for an age interval of about 16 m.y. across the Cretaceous/Tertiary boundary. The total magnetic induction and the susceptibility of the surrounding sediment were recorded in situ and continuously by the GHMT (Geological High-sensitivity Magnetic Tool). The aim of processing these data is to determine the magnetic polarity. At Site 998 (Cayman Rise), no paleomagnetic results are available from the recovered cores given a hard secondary overprint. Nonetheless, a magnetostratigraphic sequence from early Miocene to late Oligocene is proposed from the analysis of the GHMT data. The comparison with biostratigraphic results shows the accuracy of the resulting magnetic polarity. At Site 1001 (Hess escarpment), cores were recovered in two holes thus allowing the comparison of the magnetostratigraphic results obtained from the GHMT data and from paleomagnetic analysis of core samples over a length of 200 m representing about 16 m.y. from early Eocene to late Cretaceous times. The sedimentation rates computed from the determined magnetochrons are very similar and correspond to the ones obtained from shipboard biostratigraphic results. One of the most striking result obtained from the magnetostratigraphic results, especially the GHMT data, at Site 1001, is the abrupt decrease in the sedimentation rate across the K/T boundary. This suppressed sedimentation lasted at least 4 Ma after the K/T event.     

Volcanic rocks from Rosemary Bank (Rockall Trough,NE Atlantic)

A collection of basalts dredged from the southern flank of Rosemary Bank in the Rockall Trough is described. All samples are highly altered, the introduction of large amounts of CaCO₃ into the rocks being particularly notable. Normative calculations suggest that the basalts are transitional or mildly alkaline in character. Several trace elements appear to have been immobile during the process of secondary alteration and are used to infer the tectonic setting of Rosemary Bank at the time of its formation. Employing the discrimination techniques of Pearce and Cann, the basalts are shown to have oceanic affinities. However, caution has to be exercised in interpreting the trace element results in terms of sea-floor spreading in the Rockall Trough since some Scottish basalts extruded on to continental crust have similar compositions. A minimum age of Eocene has been obtained for Rosemary Bank.     

南海海盆及其周缘岩浆岩地球化学特征对岩浆过程及地幔结构的指示意义

本文通过对昆嵩高原,三水盆地以及玳瑁海山岩浆岩的地球化学研究来揭示南海海盆以及周边地区的岩浆过程以及地幔源区的性质。昆嵩,三水及玳瑁海山玄武质岩浆的地球化学特征均反应这些岩浆并非来源于一个起源于核幔边界的深部地幔柱,而是来源于较浅的岩浆源区,例如岩石圈下地幔或者软流圈地幔。此外,南海海盆岩浆岩的地幔源区较昆嵩高原和三水盆地岩浆岩的地幔源区更加亏损并且表现出较低的地幔温度。这些差异可能表明三水盆地内的岩浆活动被南海的开张所抑制,而之后又在昆嵩高原复苏。研究区的地幔表现出不均一性,其中的富集地幔端元表现出EM2的特征。这个EM2富集端员是由于伴随着大陆沉积物的中生代古南海板块俯冲到研究区地幔中所形成的。     

冲绳海槽中部伊平屋脊火山岩地球化学和Sr-Nd-Pb同位素组成：对岩石成因和地幔源区的指示

As an active back-arc basin, the Okinawa Trough is located in the southeastern region of the East China Sea shelf and is strongly influenced by the subduction of the Philippine Sea Plate. Major element, trace element and Sr-NdPb isotopic composition data are presented for volcanic rocks from the Iheya Ridge(IR), the middle Okinawa Trough. The IR rocks record large variations in major elements and range from basalts to rhyolites. Similar trace element distribution characteristics together with small variations in ~(87)Sr/~(86)Sr(0.703 862–0.704 884), ~(144)Nd/~(143)Nd(0.512 763–0.512 880) and Pb isotopic ratios, demonstrate that the IR rocks are derived from a similar magma source. The fractional crystallization of olivine, clinopyroxene, plagioclase, and amphibole, as well as accessory minerals, can reasonably explain the compositional variations of these IR rocks. The simulations suggest that approximately 60% and 75% fractionation of an evolved basaltic magma can produce trace element compositions similar to those of the intermediate rocks and acid rocks, respectively. The analysis of their Sr-Nd-Pb isotopic content ratios suggest that the source of the rocks from the IR is close to the depleted mantle(DM) but extends to the enriched mantle(EMII), indicating that the mantle source of these rocks is a mixture between the DM and EMII end members. The simulations show that the source of the IR volcanic rocks can be best interpreted as the result of the mixing of approximately 0.8%–2.0% subduction sediment components and 98.0%–99.2% mantlederived melts.     

南海北部深水区渐新-中新世沉积地球化学特征及物源演变

Geochemical and detrital zircon U-Pb dating data for drilled sediments from the Baiyun deepwater area of the northern South China Sea demonstrate a change of sedimentary sources from the Oligocene to the Miocene.Zircon ages of the pre-rift Eocene sequences are dominated by Yanshanian ages with various peak values(110–115 Ma for U1435 and L21; 150 Ma for H1), indicating local sediment supply from the pre-existing Mesozoic magmatic belt. For the Oligocene sediments in the northern part of the basin, the rare earth elements show different distribution characteristics, indicating sediment supply from the paleo-Zhujiang River(Pearl River), as also confirmed by the multimodal zircon age spectra of the Lower Oligocene strata in Well X28. By contrast, a positive Eu anomaly characterizes sediments from the western and southern parts of the basin, indicating potential provenances from intermediate to basic volcanic rock materials. The Baiyun Movement at the end of the Oligocene contributed to a large-scale subsidence in the deepwater area and also a northward retreat of continental shelf break, leading to deepening depositional environment in the basin. As a result, all the detrital zircon ages of the Upper Oligocene strata from Wells X28, L13, and L21 share a similar distribution, implying the possible control of a common source like the Zhujiang River. During the Miocene, whereas sediments in the northern area were mainly sourced from the Zhujiang River Delta, and those in the southern deepwater area continued to be affected by basic volcanic activities, the Dongsha Uplift could have contributed as the main source to the eastern area.     

南海南沙海槽深部热状态及其构造意义

南沙海槽是古南海俯冲消亡、南沙地块与婆罗洲碰撞的关键区域, 其构造演化史记录了南海前世今生的重要信息。为深入认识对其构造变形有重要影响的南沙海槽深部热状态, 本文首先利用热传输方程分析了滑坡体快速堆积的热披覆效应对海槽底部深水区海底观测热流的影响, 然后利用磁异常的频谱分析技术获取南沙海槽及其邻区的居里面深度。结果表明, 受沉积物快速堆积的影响, 南沙海槽底部深水区文莱滑坡范围内现今海底热流测量值仅为深部背景热流的60%~77%, 推测该区深部背景热流约为77~98mW·m^-2; 南沙海槽内居里面深度一般小于16km, 比位于其北侧的南沙岛礁区居里面深度(18~24km)小。现今南沙海槽区深部具有较高的背景热流, 该区较高的热状态与其地壳强烈减薄特征对应, 是华南陆缘裂陷和南海形成演化的结果。     

A 23 Myr magnetostratigraphic time framework for Site 1148, ODP Leg 184 in South China Sea and its geological implications

The Ocean Drilling Program (ODP) Leg 184 recovered a continuous sedimentary sequence since Miocene at Site 1148 in the northern South China Sea (SCS), which provided a unique record for better understanding the tectonic, environmental and climatic evolution of the SCS. In this study, we report a detailed paleomagnetic study on 881 discrete samples from the upper 460 m cores of Hole 1148A through stepwise alternating field demagnetization (AF) or thermal demagnetization (TD). Rock magnetic results from isothermal remanent magnetization (IRM) acquisition, temperature dependence magnetic susceptibility (κ-T) and hysteresis loop parameters of representative samples indicate that the main magnetic mineral is low coercivity pseudo-single domain magnetite. The characteristic remanent magnetization (ChRM) with moderate downward or upward inclinations was identified after removing a low-stability drilling-induced remanence with steep downward inclinations. A new 23 Myr magnetostratigraphic time framework was established by correlating the magnetic polarity sequence to the Geomagnetic Polarity Time Scale 2012 (GPTS 2012; Ogg, 2012) in combination with biostratigraphic data. The mean sediment accumulation rates for lithostratigraphic Units V and IV, Unit III and lower Unit II, upper Unit II, Unit IB and Unit IA are calculated as ∼20.8 m/Myr, ∼15–17 m/Myr, 24.18 m/Myr, 17.07 m/Myr and 49.01 m/Myr, respectively.     

南海及其周缘地区全新世海平面遗迹的构造含义

通过对海南岛海岸带野外考察,在文昌市铜鼓岭石头公园观察到高程为30m的海平面遗迹,在东方市黄流镇观察到高程为25~35m的海平面遗迹,在三亚市鹿回头观察到高程为2.5m的海平面遗迹。综合研究前人在该区关于海平面变化的研究成果,得出如下几点初步认识:(1)南海及其周缘地区中全新世以来海平面呈下降趋势,海平面最高位置出现在7000a左右、高于现代海平面3m左右的位置;(2)南海中全新世以来海平面遗迹所揭示的古海平面高程的差异性是古海平面本身持续下降和该区地壳差异性垂直运动共同作用的结果;(3)南海及其周缘地区中全新世期间中央海盆及其西缘的珠江口、红河、湄公河和昭披耶河4个河流三角洲地区为构造沉降区,而南海周缘的台湾岛、雷州半岛、印支半岛、南马来半岛、爪哇岛和苏拉威西岛6个地区为构造抬升区,抬升幅度最大的地方在台湾岛。     

南海北部ODP 1148站沉积物特征及其来源

南海北部ODP 1148站沉积物中的黏土矿物、地球化学以及孢粉学数据反映了沉积物物源及其变化的信息。在23Ma之前,沉积物中黏土矿物以蒙脱石为主,平均含量可达80％,而且具较高的H。Nd／144Nd比值（平均值0．512095）,反映了1148站沉积物23Ma以前可能主要来源于华南陆缘中生代火成岩;而在23～15．5Ma,沉积物中蒙脱石含量（54．1％）明显降低,而且23Ma以后143 Nd／144 Nd值也大幅减小（平均值0．512002）,暗示了1148站物源区远离陆缘向陆内的扩展。ODP1148站23Ma前沉积物同华南陆缘火山岩的主、微量元素特征以及Nd同位素比值均具较好的对比性,说明ODP1148站物源与华南沿海地区存在密切联系,沉积物中渐新世孢粉组合以及古海洋学方面的研究结果也支持该结论。     

Three-stage tectonic subsidence and its implications for the evolution of conjugate margins of the southwest subbasin,South China Sea

To reveal the tectonic characteristics of the continental margins in the southwest subbasin (SWB) of the South China Sea, a long high-resolution seismic profile...     

南海及其周缘地区全新世海平面遗迹的构造含义南海及其周缘地区全新世海平面遗迹的构造含义

通过对海南岛海岸带野外考察,在文昌市铜鼓岭石头公园观察到高程为30 m的海平面遗迹,在东方市黄流镇观察到高程为25～35 m的海平面遗迹,在三亚市鹿回头观察到高程为25 m的海平面遗迹。综合研究前人在该区关于海平面变化的研究成果,得出如下几点初步认识：（1）南海及其周缘地区中全新世以来海平面呈下降趋势,海平面最高位置出现在7 000 a左右、高于现代海平面3 m左右的位置;（2）南海中全新世以来海平面遗迹所揭示的古海平面高程的差异性是古海平面本身持续下降和该区地壳差异性垂直运动共同作用的结果;（3）南海及其周缘地区中全新世期间中央海盆及其西缘的珠江口、红河、湄公河和昭披耶河4个河流三角洲地区为构造沉降区,而南海周缘的台湾岛、雷州半岛、印支半岛、南马来半岛、爪哇岛和苏拉威西岛6个地区为构造抬升区,抬升幅度最大的地方在台湾岛。     

南大西洋两岸盆地海相烃源岩特征与控制因素

南大西洋两岸盆地漂移期层系油气资源十分丰富,通过对两岸重点盆地漂移期主力烃源岩发育层位、生烃潜力、识别特征和分布范围对比分析,明确两岸发育早、晚两套主力海相烃源岩。其中,晚期的赛诺曼—土仑阶海相烃源岩分布范围较广。整体上,两岸北段和赤道段盆地漂移期主力烃源岩地化指标优于中段和南段盆地,西非一侧海域盆地烃源岩地化指标优于南美一侧盆地。在对重点钻井的矿物元素化验分析、沉积相和古地理分析基础之上,总结了局限海湾的古地理背景和沉积相带类型控制优质海相烃源岩发育程度和分布范围的规律,并进一步指出南大西洋两岸北段和赤道段盆地漂移期成熟生烃灶平面分布范围内的深水沉积砂体是主要有利勘探方向,以找油为主,中段含盐盆地内紧邻盐微盆的漂移期深水沉积砂体勘探潜力次之。     

南海北部深水区新生代岩浆岩分布规律及其与海底地质灾害的相关性

前人对南海北部新生代陆缘盆地结构、构造样式和期次等做了大量的研究工作,但对于具体体现内动力过程的岩浆活动关注较少,尤其是北部深水区的研究鲜有涉及。本文基于二维地震剖面的解译分析,将南海北部新生代岩浆岩划分为3个集中分布区:琼东南-西沙区、神狐区和东沙区。各区域岩浆岩的产出状态有所差异,其中琼东南-西沙区岩浆岩的平面展布规律性极强,主要受右行右阶的走滑断裂控制;神狐区岩浆岩位于NW向与NE向断裂的交汇处,侵位空间受先存断裂制约;东沙区岩浆岩的产生与东沙运动期间区域伸展过程有直接关系。作为内动力的表征,岩浆作用对于海底地形地貌具有重要的改造作用,与活动断裂、海底滑坡、浅层气等海底地质灾害也具有成因上的关联。     

Petrography,petrology and tectonic implications of Mitre Island,northern Fiji Plateau

Mitre Island samples, Pliocene in age, can be classified together with Anuda samples, of unknown age, as island-arc basaltic andesites with tholeiitic tendencies. The presence of morphologically unusual iron—titanium oxides and of probably xenocrystic plagioclase suggest that at least part of the observed minerals crystallized in a magma chamber underlying the island. The samples apparently represent a mixture of magma fluid, cumulate plagioclase, pyroxene, and iron—titanium oxides which were ponded in a crater lava lake where they were reheated by subsequent eruptions. Many of them show symplectic magnetite formed by high-temperature oxidation of olivine. The morphological complexity and compositional homogeneity of the iron—titanium oxides cannot be explained at present.The presence of sulfide droplets inside olivine, magnetite and ilmenite crystals suggests a formation of an immiscible sulfide liquid in the magma chamber. Droplets of this liquid were overgrown by minerals crystallizing at that time and thus protected against oxidation during and after eruption.Mitre Island is a part of a currently inactive Vitiaz island arc associated in the past with a westward subduction of Pacific plate along the Vitiaz trench. Increased difficulty in subducting the large mass of the Pacific Border Plateau under the northern Fiji Plateau apparently produced counterclockwise rotation of the Vitiaz island arc. Oblique subduction was active until a steep angle was reached between the Vitiaz trench and the motion vector of the Pacific plate. Then a strike-slip fault developed in the Vitiaz trench and the subducted plate was sheared off. Recently the strike-slip zone migrated south from the Vitiaz trench across the northern Fiji Plateau and is presently extending from Aoba Island, in the New Hebrides, northeastward toward the Pacific Border Plateau.     

Structures of the northeasternmost South China Sea continental margin and ocean basin: geophysical constraints and tectonic implications

The northeastern part of the South China Sea is a special region in many aspects of its tectonics. Both recent drilling into the Mesozoic and new reflection seismic surveys in the area provide a huge amount of data, fostering new understanding of the continental margin basins and regional tectonic evolution. At least four half-grabens are developed within the Northern Depression of the Tainan Basin, and all are bounded on their southern edges by northwestward-dipping faults. One of the largest half-grabens is located immediately to the north of the Central Uplift and shows episodic uplift from the late Oligocene to late Miocene. Also during that period, the Central Uplift served in part as a material source to the Southern Depression of the Tainan Basin. The Southern Depression of the Tainan Basin is a trough structure with deep basement (up to 9 km below sealevel or 6 km beneath the sea bottom) and thick Cenozoic sedimentation (>6 km thick). Beneath the Southern Depression we identified a strong landward dipping reflector within the crustal layer that represents a significant crustal fault. This reflector coincides with a sharp boundary in crustal thicknesses and Moho depths. We show that the northeasternmost South China Sea basin, which may have undergone unique evolution since the late Mesozoic, is markedly different from the central South China Sea basin and the Huatung Basin, both geologically and geophysically. The Cenozoic evolution of the region was largely influenced by pre-existing weaknesses due to tectonic inheritance and transition. The South China Sea experienced multiple stages of Cenozoic extension.     

Contribution of global potential field data to the tectonic reconstruction of the Rio Grande Rise in the South Atlantic

The application of advanced enhancement techniques for geophysical anomalies to global gravity (WGM2012) and magnetic (EMAG2) models sheds light on the complex tectonic evolution of the Rio Grande Rise (RGR) in the southern South Atlantic. Long wavelength Bouguer gravity lows indicate a thicker crust beneath of the ridge, whose nature can be related to a microcontinent or an excess of volcanism within the oceanic realm. Recently dredged continental rocks reinforce the hypothesis of a microcontinent or, at least, slivers of continental crust. However, the reserval magnetic pattern of the processed magnetic anomalies provide no evidence of aborted spreading center similar to the well-studied Jan Mayen microcontinent and the surrounding (inactive) Aegir and (active) Kolbeinsey ridges in the North Atlantic Ocean. The reversal magnetic anomalies show a series N-S trending parallel stripes roughly follow the current South American coastline and segmented by E-W oriented oceanic fracture zones (FZs). The magnetic stripes are bended eastwards at the RGR, showing a more complex magnetic pattern similar to that in the Iceland. The aborted Cruzeiro do Sul Rift (CSR) and the Jean Charcot Chain (JCC) are structures that cross the RGR and contribute to the understanding of the tectonic evolution of the South Atlantic Ocean. NW-SE oriented extensive gravity lows and bathymetric valleys, which mark the CSR, are segmented by E-W trending magnetic lineaments related to FZs. This structural configuration suggests that the extensional event, which formed the rift and the seamounts chain, was replaced by strike-slip movements along the FZs. In addition, we constructed a plate kinematic model for the evolution of the RGR based on bathymetric, free-air and Bouguer gravity and magnetic data. Our model comprises five main stages of the RGR formation and evolution between late Cretaceous and Paleocene (ca. 95 - 60 Ma), separated by published seafloor isochrones. The proposed model suggests that the RGR was built at the mid-Atlantic ridge by increased magmatism probably related to the Tristan da Cunha hotspot.     

Seismic stratigraphy of the western South Korea Plateau,East Sea: implications for tectonic history and sequence development during back-arc evolution

The western South Korea Plateau in the East Sea (Sea of Japan) is occupied by rifted continental fragments formed in association with the early phase of back-arc opening. The present study focuses on the seismic stratigraphy of the sedimentary succession and the underlying acoustic basement in this region, based on closely spaced multichannel seismic reflection profiles. The sedimentary succession occurs mainly within a series of subparallel basement troughs (grabens or half grabens) bounded by faulted continental blocks (horsts) or volcanic ridges, and commonly floored by extrusive volcanic rocks showing hyperbolic reflectors. These features are strongly suggestive of continental rifting accompanied by normal faulting, volcanic activity and high rates of basin subsidence. The sedimentary succession can be subdivided into four seismic units. Unit 1 is characterized by short and irregular high-amplitude reflectors and interpreted as a syn-rift deposit consisting of a non-marine volcanics/sediment complex in topographic lows. Units 2 and 3 formed in an open marine environment during the Middle Miocene to Early Pliocene, characterized by an onlap-fill and later draping marine sedimentary succession dominantly composed of hemipelagic sediments and turbidites with frequent intercalation of mass-flow deposits. Along the western margin of the plateau, these units were deformed under a compressional regime in the Early Pliocene, associated with the back-arc closing phase. Unit 4 (deposited since the Early Pliocene) comprises hemipelagic sediments and turbidites with evidence of sporadic slides/slumps.     

Mesozoic break-up of SW Gondwana: implications for regional hydrocarbon potential of the southern South Atlantic

This work provides new palinspastic palaeofacies reconstructions of SW Gondwana incorporating rotation of a Falkland/Malvinas microplate. We discuss the implications of this for the tectonic evolution of the southern South Atlantic and hence for the regional hydrocarbon potential.Existing Gondwana reconstructions display good fits of major continents but poorly constrained fits of microcontinents. In most continental reconstructions, the Falkland/Malvinas Plateau was assumed to be a rigid fragment of pre-Permian South American crust. However, it has been suggested, on the basis of palaeomagnetic data, that the Falkland/Malvinas Islands were rotated by ∼180° after 190 Ma. This rotation hypothesis has been successfully tested on the basis of Devonian stratigraphy and palaeontology, Permian stratigraphy and sedimentology and Late Palaeozoic and Early Mesozoic structure, making it unlikely that the plateau behaved as a rigid structure during breakup. We have explored the consequences of accepting this hypothesis for the tectonic evolution of SW Gondwana by compiling new palaeogeographic maps for the Permian–Cretaceous of the southern Atlantic area. To achieve a realistic close fit, we have devised a pre-rift proxy for the ocean–continent boundary for the South Atlantic. In order to produce the best fit, it is necessary to subdivide South America into four plates. The consequences of this are far-reaching. Our work suggests that although sedimentary basins were initiated at different times, three major tectonic phases can be recognised; in regional terms these can be thought of as pre-, syn- and post-rift.During the pre-rift time (until the Late Triassic), the area was dominated by compressional tectonism and formed part of the Gondwana foreland. The Falkland/Malvinas Islands lay east of Africa, the Falkland/Malvinas Plateau was ∼33% shorter and Patagonia was displaced east with respect to the rest of South America, in part along the line of the Gastre Fault System. Potential source facies are dominantly post-glacial black shales of Late Permian age deposited in lacustrine or hyposaline marine environments; these rocks would also be an effective regional seal. Sandstones deposited in the Late Permian would be dominantly volcaniclastic with poor reservoir qualities; Triassic sandstones tend to be more mature.There was significant extension from about 210 Ma (end-Triassic) until the South Atlantic opened at about 130 Ma (Early Cretaceous). In the early syn-rift phase, extension was accompanied by strike-slip faulting and block rotation; later extension was accompanied by extrusion of large volumes of lava. Early opening of the South Atlantic was oblique, which created basins at high angle to the trend of the ocean on the Argentine margin, and resulted in microplate rotation in NE Brazil. Intermittent physical barriers controlled deposition of Upper Jurassic–Cretaceous anoxic sediments during breakup; some of these mudrock units are effective seals with likely regional extent. During crustal reorganisation, clastic sediments changed from a uniform volcaniclastic provenance to local derivation, with variable reservoir quality.In the late rift and early post-rift phase, continental extension changed from oblique to normal and basins developed parallel to the continental margins of the South Atlantic. This change coincides with the main rifting in the Equatorial basins of Brazil and the early impact of the Santa Helena Plume. It resulted in widespread development of unconformities, the abandonment of the Recôncavo–Tucano–Jatoba rift and the end of NE Brazil plate rotation, which remained attached to South America. There was extensive deposition of evaporites, concentrated in (but not restricted to) the area north of the Rio Grande Rise/Walvis Ridge.Widespread deposits can be used to define potential regional elements of hydrocarbon systems and to provide a framework for relating more local elements. Our main conclusion is that the regional hydrocarbon potential of the southern South Atlantic has been constrained by the tectonic evolution.     

Rifted margin architecture and crustal rheology: Reviewing Iberia-Newfoundland,Central South Atlantic,and South China Sea

Crustal rheology controls the style of rifting and ultimately the architecture of rifted margins. Here we review the formation of three magma-poor margin pairs, Iberia-Newfoundland, the central segment of the South Atlantic Rift, and the South China Sea by integrating observational data into a numerical forward modelling framework. We utilise a 2D version of the finite element code SLIM3D, which includes nonlinear temperature- and stress-dependent elasto-visco-plastic rheology and is able to reproduce a wide range of rift-related deformation processes such as flexure, lower crustal flow, and faulting.Extension in cold, strong, or thin crust is accommodated by brittle faults and ductile shear zones that facilitate narrow rifts with asymmetric fault geometries. Hot, weak, or thick continental crust is dominated by ductile deformation and often extends symmetrically into a wide rift system. This simple recipe provides the standard framework to understand initial rift geometry, however, it is insufficient to account for the dynamics of intermediate and late rift stages that shape the final margin architecture.Asymmetric conjugate margins where one side is wide and the other narrow can be formed via both wide and narrow rift styles, which we reproduce with weak and strong crustal rheologies, respectively. Exemplified by the Iberia-Newfoundland conjugates and the Central South Atlantic, we define three characteristic rift phases: an initial phase of simultaneous faulting, an intermediate phase of rift migration that involves sequential fault activity, and finally, the breakup phase. Crustal rheology plays an overarching role in governing the dynamics of these asymmetric margins: we illustrate that weak rheologies generally prolong the phase of simultaneous faulting, while rift migration is enabled by initial fault asymmetry as well as relatively weak crust.Formation of the predominantly symmetric and wide margins of the South China Sea was controlled by extraordinarily weak crust that extended the phase of simultaneous faulting until breakup. The weak crustal rheology of this region relates to the South China Sea's pre-rift history where plate convergence lead to crustal thickening and magmatic additions in a back-arc regime shortly before the onset of rifting.