Volcanic highlands in the South Atlantic rift zone

The morphostructure of the segment between the Cardno and St. Helen transform fracture zones is studied in the rift zone of the South Atlantic slow-spreading mid-oceanic ridge (SAMOR). It was found that it is atypical of similar ridges because of the absence of an evolved rift valley. The rift zone in the transverse section is a cupola with flat slopes, whose surface is divided by volcanic massifs, plateau-like valleys, and unclear ridges and valleys. The entire morphostructure (a cupola-like regional pedestal and the listed relief forms of the second order) indicates its volcanic origin, and the rift zone in this segment is a volcanic high-land. This conclusion is supported by seismic and magnetic data. Because other (not all) SAMOR segments contain the rift valley, the results of this study indicate alternation of the tectonic and magmatic morphostructures along the entire rift zone and identification of its scales is the most important task of the morphostructural study of the SAMOR rift zone. Determination of geodynamic regimes on the basis of the results of morphostructural studies of the rift zone will arise from the solution of this task.     

Tectonic implications of gravity anomalies associated with some continental rift valleys — a model study

An asthenospheric, diapir-type continental rift-valley model is developed. This model seems to satisfy the gravity anomalies observed over some of the rifts of East Africa without necessitating the assumption of excessively thick sections of low-density sediments or exceedingly low-density volcanics in the valleys. It is suggested that frequently observed large-amplitude Bouguer anomaly minima coinciding with some rift valleys are partially caused by hot creep of material from the lower crust into the mantle material in the rift zone.     

中-晚二叠世上扬子地块(四川盆地区)裂解的沉积响应

中-晚二叠世,上扬子(四川盆地区)碳酸盐岩台地内发育三条北西-南东走向的裂谷,它们是重要的油气储集区带,如普光和元坝气田就产于开江-梁平裂谷两侧。但是对这些裂谷的开始时代和构造成因等关键地质问题,还缺乏详细的沉积学研究,且存在较大争议。本文通过大比例露头剖面和钻井岩芯测量以及详细的沉积相分析对比,对开江-梁平裂谷的构造沉积过程进行了深入细致的研究。调查发现,在开江-梁平裂谷北缘,中二叠统栖霞组上段至茅口组2段发育大规模斜坡相碳酸盐岩滑塌堆积,揭上扬子地块四川盆地区于中二叠世栖霞晚期开始发生伸展裂解。茅口组3段至上二叠统长兴组为超覆于滑塌堆积之上的硅质岩和碳-硅质泥岩等深水盆地相沉积,表明开江-梁平裂谷从中二叠世茅口晚期开始由伸展裂解转变为区域沉降,即开江-梁平裂谷经历了伸展裂解和区域沉降两个构造沉积阶段。伸展裂解阶段,开江-梁平裂谷是一个发育于碳酸盐岩台地内的不对称海相半地堑盆地,北侧为发育大规模碳酸盐岩滑塌堆积的下盘断斜坡带,南侧为发育缓斜坡生物碎屑滩和礁灰岩的上盘缓斜坡带;区域沉降阶段,开江-梁平裂谷表现为东翘西降的不均匀沉降,发育自东向西不断进积的潮坪-潟湖相和高能浅滩相沉积等。本文研究成结果对分析和圈定四川盆地区中-晚二叠世油气储集相带的时空分布具有重要科学意义,也为研究碳酸盐岩滑塌堆积提供了一个新范例。     

扬子北缘南华纪裂谷系统:基于重力异常及沉积学证据

南华纪华南发育一系列裂谷及其相关的沉积盆地,在扬子地块的西部康滇地区、中部四川盆地及东部湘桂地区皆有裂谷盆地的直接证据.扬子地块北缘大巴山地区地形复杂、研究基础薄弱,对裂谷盆地是否存在尚未有清楚的认识.该研究有助于正确了解大巴山地区南华纪裂谷的空间展布规律,并对扬子北缘的地球动力学研究提供一定的启示.针对大巴山地区利用重力异常及野外露头资料开展了综合研究,揭示了扬子北缘大巴山南华裂谷的东西向展布形态,裂谷从南秦岭中部延伸到四川盆地东北部.同时构建出裂谷内与裂谷肩的地层充填序列及空间格架.基于上述盆地格架与沉积填充特征,提出大巴山地区弧后裂谷的构造背景.      

Evolution of sedimentary basins from Mesozoic times in Australia's continental slope and shelf

Three basic tectonic styles are described from structural trends and sedimentary sequences within sedimentary basins in the Australian continental slope and shelf. These tectonic styles are related to sea-floor spreading events and plate-tectonic movements within the adjacent ocean floor. The same tectonic styles occur within sedimentary basins of different ages; Mesozoic and early Tertiary basins contain rift valley sequences and late Cainozoic basins contain geosynclinal sedimentary suites.Northwestern, western and southern continental margins reflect spreading events explained by an Atlantic-type model in which there are rift-valley sedimentary sequences. The oldest rift valleys in the northwest and the youngest rifts in the south formed ahead of Gondwanaland break-up. After sea-floor spreading commenced, the rate of continental margin collapse varied from place to place. The eastern and northeastern slopes and shelves border marginal seas and do not contain recognizable rift-valley sequences, except for tensional splays (triple junctions) in the Tasman Sea. Short-lived spreading within marginal seas started in the Late Cretaceous in the south and in the Paleocene in the northeast. The tectonism of the northern margin is mainly recorded on land in Timor, Irian Jaya and Papua New Guinea, where, in the Neogene to Holocene, the Australian continent collided with the Asian Plate at the Banda Arc and the sub-plates of the western Pacific at the Louisiade and Bismarck Arcs.     

新元古代—早古生代华南裂谷系的格局及其演化

本文运用地层、构造与地质事件的综合研究方法,对新元古代—早古生代华南裂谷系的格局及其演化进行了研究。该裂谷系发生于晋宁期造山(约820Ma)后不久的青白口纪晚期(约815Ma),有皖浙赣堑垒带、湘桂台阶式斜坡带和南华裂谷海盆3个部分。皖浙赣堑垒带由"两谷两隆"组成,火山活动强烈,消亡于"青白口纪"末(约780Ma)。湘桂斜坡带拥有武陵(红板溪)、雪峰(灰板溪)和钦杭西段(黑板溪)3个台阶式斜坡。南华裂谷海盆以南岭中央海盆为核心,外侧有武夷、岭南两个水下裂陷块体和闽中裂谷等。海盆于志留纪时由东向西逐渐闭合造山,以"北贴西拼"为主要运动方式,形成了复杂的复合构造格局。由于南岭与岭南块体不同步的向西推移并南北贴合形成了河池-定南挤压转换式拼接带,出现了南岭东西向构造带的雏形。钦杭裂谷是华南裂谷系的主干和主要海水通道,其南端的钦州残留裂谷海槽于中二叠世末才宣告封闭。华南裂谷系的形成发展是华南地史上的一个重大事件,对后期地质构造以及岩浆成矿活动都具有重要的约束作用。     

Spatial distribution and temporal evolution of sediment transport pathway of deltaic deposits in a rift basin: an example from Liaodong Bay sub-basin,Bohai Bay,China

The Liaodong Bay sub-basin is a classic non-marine rift sub-basin in the Bohai Bay, northeastern China. The study area is located on the east side of Liaoxi uplift in the west slope of Liaodong Bay sub-basin. It sits on a draped anticline zone above the paleo-uplift and contains the second biggest offshore hydrocarbon field found in China to date. The sub-basin is bound to the west by the TanLu fault zone, the most active and largest fault active zone in eastern of China, and has been active from the Mesozoic to present. The spatial distribution and temporal evolution of the depositional systems in the lacustrine rift basin were significantly controlled by topography of paleo-uplift and the distribution of sediment transport pathways. Using 3D seismic and densely spaced well data, we systematically analysed the spatial distribution and temporal evolution of sediment transport pathway of the deltaic deposits in the SZ36-1 oilfield in the eastern slope of the Liaozhong sag. Two types of sediment transport pathway, including the fault relay ramps and erosional valleys, were recognised: (i) the fault relay ramps between two sub-parallel faults; and (ii) a series erosional valleys on the uplift, with ‘V-’, ‘U-’ or ‘W-'shaped cross-sectional patterns. Seismic stratal slices reveal that the erosional valleys branch and converge from upstream to downstream. The paleomorphology of the Liaoxi uplift (e.g., erosional valleys and sub-uplifts) and their evolution control the depositional systems and the pattern of sediment dispersal in the rift lacustrine basin area. The research indicates that paleogeomorphology controlled the direction of sediment transport, the capacity and position of sediment accommodation, influenced the type of sedimentary micro-facies and the spatial distribution pattern of the sediments. Seismic stratal slices and paleogeography maps reveal the erosional valleys shrunk progressively with sedimentary fills, resulting in decreasing gradients of the depositional slope, and provide a gentle geomorphology for a large-scale fluvial-delta depositional system to develop.     

Rifted arch basins and post-breakup rim basins on passive continental margins

In its evolution by plate divergence to a passive continental margin, a continental arch marked by narrow rift valleys (intra-arch basins) and flanked by broad basins (inter- and extra-arch basins) is most likely to break up along a rift valley boundary fault. The resulting dismembered arch at the continental margin is a rim that constitutes the oceanward flank of a rim basin, and the rim basin succeeds one or other of the basins related to the previous arch. In offshore Western Australia, the juxtaposition of Mesozoic reservoir rock at a rift shoulder and source rock of the succeeding rim basin provide a mechanism for concentrating a large gas deposit.     

新疆北部及邻区地质构造单元与地质发展史

新疆北部及邻区在区域构造上分属西伯利亚、哈萨克斯坦—准噶尔及塔里木三大板块，它们被两条时代不同（石炭—二叠纪、志留—泥盆纪）、规模宏伟的板块缝合构造带所分隔，缝合构造带与两侧板块有着密切的亲缘关系，却无隶属的“父子”关系，三大板块自元古宙发生解体裂移后，增生作用主要发生在早古生代，主要是依据各陆缘的不同特征分别形成岛弧或边缘海盆；进入晚古生代后，哈萨克斯坦—准噶尔板块与培里木板块已率先完成碰撞缝合，此时除斋桑—额尔齐斯—线尚有古亚洲洋残余外，绝大部分地区均进入陆内构造发展阶段，其主要构造活动是“手风琴式’的开合构造，形成一些裂陷槽、裂谷和上叠盆地。研究区内共分出１５个二级构造单元和４４个三级单元。     

Syn‐rift sequence development in a fault‐controlled embayment (Early Permian Irwin River Coal Measures,Northern Perth Basin,Western Australia)

Stratigraphic patterns and sequence development in tectonically active extensional basins remain poorly documented in comparison with passive‐margin settings. Rift basin fills are generally characterized by coarsening‐upward trends in response to the rapid creation of accommodation by extensional faulting, and the progressive filling of graben during more quiescent periods. The Early Permian Irwin River Coal Measures in the Northern Perth Basin (Western Australia) record a complex stratigraphic arrangement of conglomerate, sandstone, mudstone and coal, and have been attributed to delta plain depositional environments that developed in a cool–temperate climatic setting during syn‐rift activity. Sedimentary analysis of outcrop and core data from the fault‐bounded Irwin Terrace is used to distinguish nine facies associations reflecting deposition in braided rivers, fixed‐anastomosed channel belts, tide‐influenced coastal environments and storm‐affected distal bays. The broader depositional system is interpreted as a morphologically asymmetrical tide‐dominated embayment with a fluvial and wave influence. The stratigraphic architecture of the Irwin River Coal Measures was strongly influenced by the evolving rift basin margin. Fault reactivation of the major basin‐bounding Darling Fault in the early syn‐rift phase caused footwall uplift and the inception of transverse palaeo‐valleys occupied by braided fluvial systems. Fault block subsidence during the subsequent balanced, backstepping and drowning phases resulted in a dominantly retrogradational stacking pattern indicating progressive flooding of marginal‐marine areas and culminating in deposition of distal marine elements. In the active rift basin, it is proposed that preservation of a shallow‐marine syn‐rift sequence was promoted by the geomorphological confinement of the embayed system increasing tidal current acceleration and hampering transgressive ravinement. The proposed sequence model demonstrates that transgressive successions can develop in the early syn‐rift phase in response to footwall uplift and tectonic subsidence. The syn‐rift sequence recording the filling of an embayment on a rift basin margin may be applied in similar tectonic and/or depositional contexts worldwide.     

闽中裂谷系的格局演化及成矿系统时空结构

闽中裂谷系为转换-伸展型裂谷盆地系,其形成发展和复合-成矿作用是福建地史上的一个重大事件。研究认为:闽中裂谷系在古元古代结晶基底上形成于新元古代-早古生代(850～500Ma),盆山反转于奥陶-志留纪(447～412Ma);沉降中心迁移、伸展轴向改变并复活发展于中石炭世-中三叠世(315～223Ma),构造动力反转复合于侏罗纪(188～137Ma)-白垩纪(134～90Ma);构造格局演化驱动了喷流沉积-改(再)造成矿系统与斑岩(矽卡岩)-浅成热液成矿系统之间的复合交融。研究提出:(1)裂谷体内“麻源群”(部分改划古元古代迪口组变质核杂岩系)与马面山群、万全群同归南华纪;(2)裂谷成矿系统时空结构所昭示的闽省找矿主攻方向:在伸展体制主导的裂谷纵向(NNW、NE)“先张后压”型断层带特定层位找中大型喷流沉积-改(再)造型矿床,在挤压体制主导的裂谷横向(NEE)“先压后张”型断裂带特定岩系找大中型斑岩(矽卡岩)-浅成热液型矿床。     

中国海相烃源岩发育控制因素

烃源岩的形成取决于生烃母质生物的生存环境和有机质良好的保存条件,这两大因素从根本上来说又取决于生物繁殖时和埋藏时的古气候、古洋流、古构造及古环境等各要素的良好匹配。海相高有机质丰度烃源岩的形成与大气中的中等含氧量、干热的气候、冰期-冰后期之交的气温快速转暖、冰川迅速融化所导致的海平面快速上升等密切相关。大洋环流的形成和演变也是控制海相高有机质丰度烃源岩形成的主要因素,上升洋流富磷、富硅、富铁族元素等营养盐和富绿硫细菌极大地促进了有机质生产力、埋藏率的激增。通过研究,建立了中国华南和塔里木盆地寒武系上升洋流分布模式、塔里木盆地上奥陶统良里塔格组台缘斜坡灰泥丘相源岩形成时的反气旋洋流分布模式。研究发现,优质烃源岩仅发育于被动大陆边缘背景下的裂谷、克拉通内裂谷、克拉通内坳陷盆地和克拉通边缘坳陷盆地。欠补偿盆地、蒸发潟湖、台缘斜坡(灰泥丘)和半闭塞—闭塞欠补偿海湾,是高丰度烃源岩发育的有利环境;低的无机物输入和低的沉积速率,有利于高有机质丰度烃源岩形成;呈斜坡的海底地貌,有利于烃源岩形成。     

Late Quaternary fluvial activity in the dry valleys (mekgacha) of the Middle and Southern Kalahari,southern Africa

Kalahari dry ‘fossil’ valleys (mekgacha) have been little used in palaeoenvironmental reconstruction, partly because the origins and functions of the valleys are a subject of debate, and partly because they contain few datable sediments. In the Middle Kalahari the valleys are endoreic, and, following a decline in ground-water tables owing to human interference over the past 150 years, rarely contain water. Evidence from three sites suggests that the valleys contained standing water between 15000 and 12000 yr BP, and that the main Okwa-Mmone system flowed into palaeolake Makgadikgadi at 920 m a.s.l. during this period. The Southern Kalahari contains the exoreic Molopo network which is hydrologically more active, being prone to surface flows in response to extreme rainfall events. The spring—fed Kuruman valley has two terraces, the lower composed of a series of flood deposits of late Holocene age. The flood of February 1988 provides a useful analogue for the conditions under which these deposits were formed, and of the temporal regime of the Kuruman River. Comparison with adjacent sites suggests that evidence put forward to indicate cycles of climatic amelioration in the last 4000 yr may, in part, represents the effect of extreme precipitation events. Historical floods show a strong correlation to Southern Oscillation high-phase (cold event) episodes, suggesting possibilities for extension of the Southern Oscillation record through palaeoflood studies.     

Depleted and enriched mantle processes under the Rio Grande rift: spinel peridotite xenoliths

Upper mantle xenoliths from the southern Rio Grande rift axis (Potrillo and Elephant Butte) and flank (Adam’s Diggings) have been investigated to determine chemical depletion and enrichment processes. The variation of modal, whole rock, and mineral compositions reflect melt extraction. Fractional melting is the likely process. Fractional melting calculations show that most spinel peridotites from rift axis locations have undergone <5% melting versus 7–14% melting for xenoliths from the rift shoulder, although the total range of fractional melting overlaps at all three locations. In the rift axis, deformed (equigranular and porphyroclastic texture) spinel peridotites are generally characterized by significantly less fractional melting (2–5%) than undeformed (protogranular) xenoliths (up to 16%). This difference may reflect undeformed xenoliths being derived from greater depths and higher temperatures than deformed rocks. Spinel peridotites from the axis and shoulder of the Rio Grande rift have undergone mantle metasomatism subsequent to melt extraction. Under the rift shoulder spinel peridotites have undergone both cryptic and patent (modal) metasomatism, possibly during separate events, whereas the upper mantle under the rift axis has undergone only cryptic metasomatism by alkali basaltic magma.     

On the origin of tunnel valleys of the Saginaw Lobe of the Laurentide Ice Sheet; Michigan,USA

Tunnel valleys are common throughout the terrain of the Saginaw Lobe of the Laurentide Ice Sheet in southern Michigan. The set of valleys described in this paper is regularly spaced in a radial pattern behind the Kalamazoo Moraine, an ice‐marginal position formed during retreat from the Last Glacial Maximum. These valleys are divided into proximal and distal groups lying north and south, respectively, of a major river valley that cross‐cuts the tunnel valleys at right angles. Based on a series of rotasonic borings and core analysis, the proximal valleys are shallow, contain minimal sediment fill, and overlie fine‐grained diamicton and glaciolacustrine sediment, whereas the distal valleys are deeply incised into the substrate and are partially filled with coarse sediment. The distal valleys terminate within a broad zone of high‐relief, hummocky topography representing stagnation and collapse behind the Kalamazoo ice margin. The proximal valleys occur within a more subdued landscape located farther from the ice margin. Although some elements of existing genetic models are consistent with these valleys, none appears to be completely compatible with their stratigraphy and morphology. Initial incision of the valleys could have involved short‐lived moderate‐ to high‐discharge flows, followed by deposition during or after the events. The deep incision and thick, coarse sediment in distal valleys in the stagnant marginal zone probably involved supraglacial meltwater draining to the bed as the margin downwasted. Fining‐upward eskers inset into the valleys were formed by flows of declining energy in small late‐stage conduits.     

The Baikal system of rift valleys

The Baikal system of rift valleys has no evident structural connections with the World Rift System. The peculiar features of its structure, morphology and volcanicity reflect this isolation. The spatial position and major structural features of the system are determined where the central segment (the South Baikal depression) is confined to the junction of two major lithospheric plates, the Precambrian Siberian platform and the heterogenous folded framework of Sayan—Baikal. The contrasting structures and thermodynamic conditions of these two plates, and the deep nature of the suture zone developed between them, have been responsible for the location of crustal extension and proto-rift formation within the Baikal depression proper, first initiated not later than Eocene and then propagating to zones both west and northeastwards.     

断陷湖盆中强制湖退沉积作用及其成因机制

断陷湖盆中相对湖平面下降引起的湖退作用称之为强制湖退作用。强制湖退作用时期若存在沉积物供给盆地,将加速湖退作用,并形成一套具有明显前积结构的强制湖退前积体。强制湖退前积体的顶积层不发育,地震剖面上具斜交前积反射结构。强制湖退前积体的顶界面为层序界面,且伴随有下切谷的形成。气候变化、构造抬升和边界断层的幕式断陷活动引起的盆地缓坡带基底差异沉降运动均可引起断陷湖盆中相对湖平面的下降,也是引起强制湖退作用的三大成因机制。强制湖退前积体具有良好的油气成藏条件,是隐蔽油气藏有利勘探目标。     

塔里木盆地构造演化与构造样式

塔里木盆地的发展演化受不同时期板块构造背景的控制，形成了陆内裂谷、裂陷槽、克拉通内拉张盆地、克拉通内挤压盆地、被动大陆边缘盆地、弧后拉张盆地、弧后前陆和周缘前陆盆地等多种原型盆地并相互叠加和改造。盆地中存在挤压、引张、扭动和叠加构造样式，可以形成良好的圈闭构造，盆地中的大型隆起带是主要的油气聚集带，前陆盆地褶皱－冲断带具有较好的油气前景。     

Pressure and temperature evolution of upper mantle under the Rio Grande Rift

Spinel peridotite xenoliths associated with the Rio Grande Rift axis (Potrillo and Elephant Butte volcanic fields) and the western rift shoulder (Adams Diggings) have been investigated to correlate pre-eruptive pressure and temperature conditions with xenolith deformation textures and rift location. Temperatures of xenolith equilibration at the rift shoulder are 100–250°C cooler for a given pressure than the temperatures at the rift axis. Undeformed xenoliths (protogranular texture) are derived from higher temperature and higher pressure conditions than deformed xenoliths (porphyroclastic and equigranular textures) in the rift axis. Exsolution lamellae in pyroxenes, small decreases in Al contents of orthopyroxenes from core to rim, and small differences in porphyroclastic orthopyroxene compositions versus neoblastic orthopyroxene compositions indicate high temperatures followed by cooling and a larger cooling interval in deformed rocks than in undeformed rocks. These features, along with thermal histories based on calcium zoning in olivine rims, indicate that the upper mantle under Adams Diggings and Elephant Butte has undergone cooling from an initial high temperature state followed by a late heating event, and the upper mantle under Potrillo has undergone cooling, reheating, and late heating events.     

Rifting Attractor Structures in the Baikal Rift System: Location and Effects

The current geodynamics and tectonophysics of the Baikal rift system (BRS) as recorded in lithospheric stress and strain are discussed in the context of self organization of nonlinear dissipative dynamic systems and nonlinear media. The regional strain field inferred from instrumental seismic moment and fault radius data for almost 70,000 M_LH ⩾ 2.0 events of 1968 through 1994 shows a complex pattern with zones of high strain anisotropy in the central part and both flanks of the rift system (the South Baikal, Hovsgöl, and Muya rift basins, respectively). The three zones of local strain anisotropy highs coincide with domains of predominantly vertical stress where earthquakes of different magnitudes are mostly of normal slip geometry. Pulse-like reversals of principal stresses in the high-strain domains appear to be nonlinear responses of the system to subcrustal processes. In this respect, the BRS lithosphere is interpreted in terms of the self organization theory as a geological dissipative system. Correspondingly, the domains of high strain anisotropy and stress change, called rifting attractor structures (RAS), are the driving forces of its evolution. The location and nonlinear dynamics of the rifting attractors have controlled lithospheric stress and strain of the rift system over the period of observations, and the same scenario may have been valid also in the Mesozoic-Cenozoic rifting history. The suggested model of a positive-feedback (fire-like) evolution of nonlinear dynamical systems with rifting attractors opens a new perspective on the current geodynamics and tectonophysics of the Baikal rift system.