The sagatu ridge dike swarm, ethiopian rift margin

A swarm of dikes forms the core of the Sagatu Ridge, a 70 km long topographic feature elevated to more than 4000 m above sea level and 1500 m above the level of the Eastern (Somalian) plateau. The ridge trends NNE and lies about 50 km east of the northeasterly trending rift-valley margin. Intrusion of the dikes and buildup of the flood-lava pile, largely hawaiitic but with trachyte preponderant in the final stages, occurred during the late Pliocene-early Pleistocene and may have been contemporaneous with downwarping of the protorift trough to the west. The ensuing faulting that formed the present rift margin, however, bypassed the ridge. The peculiar situation and orientation of the Sagatu Ridge, and its temporary existence as a line of crustal extension and voluminous magmatism, are considered related to a powerful structural control by a major line of Precambrian crustal weakness, well exposed further south. Transverse rift structures of unknown type appear to have limited the development of the ridge to north and south.     

A thermomechanical model of the formation of a rift valley

A numerical model describing the thermomechanical state of the “cold” upper mantle near a mid-oceanic ridge (MOR) spreading at a moderate rate is constructed in the approximation of the boundary layer theory. The condition of rift valley formation leads to a constraint on the temperature and shows what temperature distribution corresponds to the “cold” upper mantle. Taking into account the dependence of mantle rheology on the pressure, temperature, and viscous stresses, the model distributions of the pressure and normal viscous stresses at the base of the lithosphere result in a bend of the heterogeneous lithosphere near the MOR, producing a seafloor topography typical of a rift valley with a depth of a few hundred meters and a spreading rate of ～2.5 cm/yr, characteristic of the Atlantic Ocean. The model width of the rift valley (～10–15 km) agrees with observations fairly well. The model is consistent with the typical heat-flow values observed in the spreading zone.     

The volcanics of the gregory rift valley,east Africa

The paper reviews the stratigraphy, style of activity and some aspects of the petrology of Tertiary to Recent sodic alkaline volcanic rocks in Kenya, eastern Uganda and northern Tanzania. Repeated extrusions of basaltic and nephelinitic volcanics occurred from Miocene times onwards, confirming indications from chemical data that magmas of these compositions were parental. At some central volcanoes, a basalt-trachyte-phonolite series evidently arose by fractional crystallization of basaltic magma, whereas various courses of crystallization from a nephelinitic parent led to the production of phonolites, tephrites and basanites as well as olivine-and melilite-bearing nephelinites and melanephelinites. Phonolitic and trachytic volcanics which dominate an area of repeated upwarping (the Kenya dome) probably originated by processes of partial melting rather than by differentiation of basaltic magma. The basalt-trachyte association which characterizes many central volcanoes north and south of the dome can perhaps best be explained by postulating independent sources for the basic and salic volcanics.     

Volcanism in the western part of the rift valley in Southern Kenya

The stratigraphy, tectonic history, petrography and major oxide petrochemistry of the volcanic sequences in the western part of the rift valley in southern Kenya is summarised. Volcanism and rift faulting began 15 and 7 m.y. ago respectively. A recurrent feature of Miocene and Pliocene volcanism was the tendency for salic magmas to be preferentially erupted in the northern part of the area: the accumulation of a northward-thickening Pliocene trachyte pile is particularly notable. Transitional-mildly alkalic basalts and trachytes were erupted only after the onset of rift faulting, from sites within the rift structure, and so can be considered «rift dependent». Nephelinites, melanephelinites, limburgites and phonolites were erupted before and after onset of rift faulting, from sites within and outside the rift structure, and so can be considered «rift independent».     

琼州海峡裂谷成因的火山活动标志

根据雷琼地区存在大规模火山活动以及新生界明显地由断陷的南、北侧向海峡急剧增厚的事实，结合近年来海峡地区的物探成果，论证了琼州海峡的裂谷成因，是上地幔物质上涌的结果。该地区的高热流值、高地热增温率和强震活动都说明，琼州海峡是现今仍具有一定活动性的裂谷构造。     

Manganese geochemistry near high-temperature vents in the Mid-Atlantic Ridge rift valley

In July 1985 the first high-temperature, hydrothermal vents ever discovered on the MAR were found at 26°N (TAG). A transponder-navigated survey of a 4 × 8 km area was begun when on-board measurements of manganese revealed concentrated hydrothermal plumes in the water column comparable in size and intensity to those found over geothermal fields on the EPR. These plumes also possess anomalously high light scattering properties and anomalously warm potential temperatures. Using relay-transponder navigation with a rosette-CTD system, it was possible to sample bottom waters with temperature anomalies of several decidegrees. Samples collected near vent orifices possessed silica anomalies of up to 17 μM and dissolved manganese anomalies as large as 360 nmol/kg. These manganese and silica anomalies are linearly related and predict aMn/Si ratio of 20 mmol/mol for these MAR vent fluids, a value essentially identical to that measured in 350°C fluid from the EPR at 21°N. The distribution of temperature anomalies in the water column over this area requires the presence of more than one active vent site. A rough estimate of the convective heat loss from this section of the MAR based onθ-z profiles is 1.2 × 10⁸ cal/s which is similar to the heat loss estimated for the vent field on the EPR at 21°N.     

Geophysics and geology of an explosion crater in the Ethiopian rift valley

Geophysical and geological studies of an Ethiopian maar, Haro Maja, demonstrate that its eruptive history is more complex than surface geology alone suggests. The crater is 750 m by 1000 m in diameter and varies in depth from 70 m to 110 m. A strong magnetic anomaly is caused by a central basaltic mound, but a broader crater-wide anomaly is best modelled by a 50 m thick frozen lava lake, 30 m below the crater floor. The central mound was not erupted directly onto the lava lake, but was extruded onto top of the sedimentary infill after a quiescent depositional interval. Electrical resistivity measurements further indicate that other basaltic intrusions failed to reach the surface during that eruptive period.     

Massive sulfides in a sedimented rift valley, northern Juan de Fuca Ridge

A number of mounds each several hundred meters across and up to sixty meters high have been observed with SeaMARC II acoustic imagery and Seabeam bathymetry in the sediment-filled axial valley at the northern end of the Juan de Fuca Ridge. The mounds are located a few kilometers west of the eastern valley-bounding normal fault scarp where the local sediment fill is approximately 300 m thick. All of the mounds are believed to be of hydrothermal origin, and one is associated with anomalously high heat flow in excess of 1 W m⁻². A piston core collected from that mound comprises coarse clastic sulfide units interbedded with sulfidic muds. Hydrothermal minerals present in the 2.3 m section include pyrrhotite, pyrite, marcasite, sphalerite, chalcopyrite, iss (intermediate solid solution in the CuFeZnS system), chalcopyrrhotite, galena, talc, barite, and amorphous silica. Mineral fabrics of the clasts indicate that the material was precipitated at or near the sea floor by mixing of hot hydrothermal fluids with cold seawater. Low concentrations of Zn, Cu, Cd, and Ag relative to those found in unsedimented ridge hydrothermal deposits, and the presence of pyrrhotite as an early phase mineral indicates that the vent fluids have been modified by reaction with sediments beneath the mound. Rapid sedimentation in a rift valley is clearly conducive to the formation of large hydrothermal mineral deposits like those believed to be present within and beneath these mounds. The relatively impermeable sediment cover insulates the crust, inhibits groundwater recharge, promotes long-lived discharge at a restricted number of sites, provides a substrate for the efficient subsurface precipitation of minerals, and through continued sedimentation, protects surficial deposits from the corrosive effects of seawater. No reliable estimate of the bulk composition of the mounds can be made with existing data, but their size is comparable to major hydrothermal mineral deposits found on land; ancient settings in which many land deposits formed are in many ways similar to the one in which the features described here are currently forming.     

Spectral study of the Bouguer anomaly map of a rift valley and adjacent areas in Central India

The spectral study of the Bouguer anomally map of Central India suggests an uplifted crust-mantle interface under the Mahandi graben. This study has delineated three subsurface levels of anomalous masses at the respective depths of 23 km, 8 km, and 2 km apparently representing the Moho, an intermediate discontinuity in the sialic part of the crust and the basement, respectively. Model study of the Bouguer anomaly along a profile suggests a typical continental graben type subsurface structure with a low density depression in the sialic part of the crust between 8 and 18 km supported by an elevated upper mantle of intermediate density (3.4 g/cm³) varying in depth from 25 km to 55 km. The depths of the inferred interfaces in case of Bundelkhand granite are 32 km, 11 km, and 1.5 km, which might represent the Moho, the base of intruded granite massif, and some shallow compositional variation. Similar studies in case of Vindhyan basin have brought out three discontinuities at the respective depths of 16 km, 6–4.5 km, and 2.4 km. The first horizon at the depth of 16 km probably represents the interface between the granitic and the basaltic part of the crust. The 6–4.5 km is the depth of the basement, with the 2.4 km interface separating Bijawar rocks from Vindhyans wherever they are present. A generalized inversion of a profile across a positive belt of Bouguer anomaly representing the subsurface Bijawar rocks support the above result.     

Non-axial breaching of a rift valley: Evidence from the Lord Howe Rise and the southeastern Australian margin

Present models of continental breakup envisage the formation of a rift valley which undergoes a protracted period of tectonism and eventual seafloor spreading in the axial part of the rift valley. This results in evidence of pre-breakup tectonism on most Atlantic-type margins in the form of normal blockfaults beneath the continental slope. The southeastern margin of the Australian continent has an unusually steep continental slope and shows little evidence of tectonism associated with the rift valley stage of development. The margin was formed by separation of the Lord Howe Rise and Australia during a phase of seafloor spreading in the Tasman Sea which lasted from about 80 to 60 m.y. B.P. Marine geophysical data over the central Lord Howe Rise indicate a contrast between the western and eastern part of of this structure. The western part shows faulted, rough basement topography, disturbed overlying sediments, and a relatively quiet magnetic field. The eastern part shows a smooth basement surface, undisturbed overlying sediments, and a high-amplitude, high-frequency magnetic field. It is suggested that the whole of the pre-breakup rift valley remained attached to the Lord Howe Rise. This explains the absence of rift valley structures within the eastern continental margin of Australia and implies non-axial breaching along the western boundary fault of a pre-Tasman Sea rift valley.     

Compositional changes during crystallization of some peralkaline silicic lavas of the Kenya rift valley

Comparison of crystalline and glassy comendites shows that compositional changes during solidification involve losses of Na, Cs, Cl, F, gain of Sr, and gains and losses of REE at near-solidus temperatures. Variation in a thick trachyte lava suggests deuteric mobility of the same elements, while variation in a trachyte-comendite suite shows that Fe loss can also occur. Al, Si, Zr, Hf, Nb, Ta, Th, U, and Rb are not significantly affected, nor are the proportions of the REE, and these elements can be relied upon in petrochemical studies of crystalline peralkaline silicic rocks.     

近几年山西断裂带水平形变的初步分析

以沿山西断裂带布设的GPS监测网的6期复测资料(1999~2004)所计算的近期趋势性运动速率为基础,获得了2005、2006年山西断裂带相对于近期趋势性运动的偏离结果。结果显示:(1)趋势性差异运动信息不显著,运动信息明显的点数只占十分之一,它们分别是S21、S20、S30和S33(位于太原以南),量级最大约6mm/a;(2)尽管单站趋势性运动信息不显著,但整体模拟计算表明,1999~2004年似乎有南北相对运动的迹象,相对变化最大约2.0mm/a;(3)2005年有偏离趋势运动的迹象,且背离趋势运动方向,最大量级约7mm;(4)截至2006年的累计偏离量最大约6mm;这表明2006年基本未发生累计偏离。由于变化相对平稳,这可能预示着山西断裂带内近期地震活动强度不太可能明显提高。     

Seismological and GPS constraints on Sinai sub-plate motion along the Suez rift

This work presents new seismological and Global Positioning System (GPS) results aimed at understanding the nature and rate of strain associated with the opening of the Suez rift that separates the Sinai sub-plate from the African plate. The Sinai sub-plate has played a significant role in the tectonic evolution of the northern Red Sea and the Eastern Mediterranean region. Most small, moderate and large earthquakes occur within belts associated with the geologically documented borders of this sub-plate including the Dead Sea fault (DSF) system in the east, the Cyprian arc (CA) in the north, and the Suez rift (SR) to the southwest. The DSF and CA are well defined; however, the SR is only partially defined. Earthquake foci distribution supports the idea that the SR is seismically active, and this earthquake activity cannot be ignored throughout the kinematics evaluation of northern Red Sea region. The earthquake activity is relatively higher in the southern part of the SR and gradually decreases northward. The high seismicity is mainly attributed to the presence of the Sinai triple junction. Earthquake focal mechanisms in the SR are dominated by oblique normal faulting with left-lateral strike-slip components on NW trending fault planes consistent with regional kinematics. The extensional semi-principal stress axes derived from fault plane solutions are oriented NNE-SSW in good agreement with the current stress field obtained from borehole breakouts along the SR as well as results from GPS surveying. Recent survey-mode GPS observations provide evidence for coherent northerly motion of the Sinai sub-plate that varies between 2 and 5 mm/yr. Moreover, strain analysis indicates that the southern SR is dominated by extension while its northern segment is characterized by constriction, inconsistent with earthquake focal mechanisms and regional tectonic models. The overall northward motion of the Sinai sub-plate indicates that slab-pull rather than ridge-push is the dominant force controlling regional kinematics. Based on the low rate of extension and lack of oceanic crust, the SR can be considered an incipient plate boundary between the Sinai sub-plate and the Nubian plate.     

Hydrocarbon accumulation in network and its application in the continental rift basin

The concept of hydrocarbon accumulation in network was presented on basis of the overall analysis of the formation and evolution characteristics of the continental faulted basin and of the systemic re-search on the major controlling factors on the hydrocarbon accumulation. The hydrocarbon accumu-lation in network can be defined as hydrocarbon accumulation in a three-dimensional network system which is constituted by the hydrocarbon migration passages under multiple dynamics,following the hydrocarbon generation from source rocks. The research shows that the hydrocarbon accumulation in network is composed of four elements,i.e.,hydrocarbon source (source rock kitchen),hydrocarbon accumulation terminal (trap),network pathway connecting source and terminal (transporting system),and network potential driving hydrocarbon migration in the network pathway (migration dynamics). Compared with other networks,hydrocarbon accumulation in network has three basic characteristics: the irreversible geological process of material and information flow in the network; the loss of material and information in the flow process in the network; the multiple dynamics in the flow process. Interac-tion of all the elements in the geological process can be called hydrocarbon accumulation in network. There are three basic models for hydrocarbon accumulation in network,that is,hydrocarbon accumu-lation in the network source area,hydrocarbon accumulation in the network pathway,and hydrocarbon accumulation in the network terminal. The key in the application of the hydrocarbon accumulation models in network in practice is to confirm the major accumulation stage and the function range of the four elements controlling the hydrocarbon firstly,to predict the profitable accumulation region by su-perposition of the favorable areas confirmed by four elements consequently,and to evaluate the oil-bearing property of the trap as well as confirm drilling targets. This paper takes the Dongying De-pression in the Bohai Bay Basin as an application example.     

GPS monitoring of temporal deformation of the Xianshuihe fault

Highly precise (σ ~1 mm) temporal deformation measurements are taken across the Xianshuihe fault from two pairs of continuous GPS stations straddling the fault. Baseline vector changes of the two pairs of stations show clearly the difference in deformation behavior between the Qianning and Daofu segments of the fault: the former deforms steadily, and the latter deforms with a strong transient component. The transient deformation across the Daofu segment is possibly related to its irregular geometry, where the fault splits into two branches, that is, the east and west branches. An attempt is made to interpret the baseline vector changes using a kinematic fault model composed of a brittle layer in the upper crust, a ductile layer in the lower crust, and a transition zone in between. The slip in the transition zone of the south segment of the Xianshuihe fault is steady. The slips in the transition zones of the north and Daofu segments of the Xianshuihe fault, however, are not steady, and the average slip rates there are higher than that of the south segment. The difference in deformation behavior is probably associated with the rheological properties of the fault interface, suggesting that the overall fault strength of the south segment is greater than those of the north and Daofu segments, corresponding to longer earthquake recurrence time.     

Geology and geochemistry of the Ol Doinyo Nyokie trachyte ignimbrite vent complex,south Kenya rift valley

The Ol Doinyo Nyokie complex is of late Pleistocene age and occurs in the floor of the south Kenya rift valley. It consists of a shallow depression 5 km long and 3 km wide occupied by ash-flows, surrounded by a zone of trachyte dykes, and with a dome-shaped ignimbrite vent at its eastern end. The complex began to form approximately 0.7 m.y. ago with eruption of ash-flows from fissures accompanied by subsidence, followed by emplacement of dykes in the fissures and the growth of a steep-sided ignimbrite tuff-ring. The rocks are all of quartz trachyte compositions similar to those of the flood lavas upon which the complex is built. Detailed geochemical evidence indicates that the ignimbrite magma was derived from the flood lava magma by alkali feldspar fractionation.     

The deformation pattern and fault rate in the Tianshan Mountains inferred from GPS observations

Based on GPS measurements conducted from 1992 to 2006, we present the current crustal movement velocity field for approximately 400 sites in the Tianshan Mountains and their adjacent areas, and estimate slip rates on the major faults using a 2-D elastic dislocation model. Our studies show slip rates within the range of 1―4 mm/a on the NW-SE trending strike-slip faults (such as Talas-Fergana fault) in the Tianshan Mountains. We also found the slip rates on the approximately WE-SN trending gently-dipping detachment fault vary from 10―13 mm/a for the southwest Tianshan Mountains to 2―5 mm/a for the eastern Tianshan Mountains, and to 6―12 mm/a for the Kyrgrz Tianshan. The GPS velocity field reveals that the total convergence is not uniformly distributed across the Tianshan Mountains, with 80%―90% of the N-S shortening absorbed along the southern and northern edges, and relatively little deformation accommodated within the interior. This first-order feature of strain pattern is explained best by underthrusting of adjacent blocks beneath the Tianshan Mountains along a basal detachment fault. We found the occurrence of historical M7―8 earthquakes somewhere in the locked ramp that connects the creeping and locking segments of the detachment, thereby resulting in elastic strain concentration and accumulation around it. The elastic strain confined in the upper crustal layer above the detachment ultimately releases through infrequent great earthquakes in the Tianshan Mountains, resulting in considerable folding and faulting at their margins. The Tianshan Mountains propagated outward and rose progressively as a wedge-shaped block.     

腾冲火山区的GPS形变特征

利用腾冲火山区2003和2004年2次测量所得的GPS数据,计算了59条测线的水平长度变化.对任意相邻的3个测点和3条测线组成的51个三角形,分别进行了地壳形变的反演计算,得出每个三角形的主应变和主方向,进而确定每个三角形的面膨胀量和最大剪应变.对整个测量区域作等值线图,可以看出有3个最大值的区域,分别位于马站－曲石(G05)附近;腾冲(G20)－热海(G13)附近,以及五合(G15)－团田(G18)附近.这3个区域与3个具有最大相对地热梯度的区域相一致,也与低速异常体所在的区域相一致.因此可能就是地下岩浆房存在的区域.为了确定岩浆活动性,利用Mogi模型的计算公式,对腾冲－热海附近的区域进行了反演,认为等效源的位置在腾冲的西南方向,岩浆活动量约为8×105m3/年,与以前用精密水准测量的垂直形变所反演的结果处于同样的量级.