Emplacement of Proterozoic massif-type anorthosite during regional shortening: evidence from the Bolangir anorthosite complex (Eastern Ghats Province, India)

A study of the 933±32-Ma-old Bolangir massif-type anorthosite complex (Eastern Ghats Province, India) yielded strong evidence for anorthosite emplacement during regional shortening, and thereby new insights in massif-type anorthosite formation. Several lines of evidence strongly suggest synchronism of plutonism and regional deformation. First, structures in the country rocks, which imply N–S-directed shortening accompanied by E–W extension, are mirrored by a E–W trending post-magmatic foliation and N–S trending shear zones in the anorthosite complex. Near the intrusion, the foliation in the country rocks becomes parallel to the contact and an internal marginal foliation, and foliation triple points occur in the country rocks. Second, synshortening dikes inside and outside the anorthosite complex are filled with pluton-related melts. Third, ferrodiorites, which are considered late-stage differentiates of the anorthositic pluton, concentrate in tectonic voids at the pluton margin. Some of these occurrences have been affected by the last increments of the regional deformation, but others transect the same structures. Ascent mechanism and significance of the adjacent terrane boundary of the Eastern Ghats Belt for ascent and emplacement of the Bolangir anorthosite complex are discussed. The results of this study imply that emplacement of Proterozoic massif-type anorthosite is not restricted to extensional settings.     

Heat-flow regimes and the hydrate stability zone of a transient,thermogenic, fault-controlled hydrate system (Woolsey Mound northern Gulf of Mexico)

This study aims to constrain the base of the hydrates stability field in structurally complexsites using the case of Woolsey Mound, a fault-controlled, transient, thermogenic hydrates system, in Mississippi Canyon Block 118, northern Gulf of Mexico. We have computed the base of the hydrates stability field integrating results from a recent heat-flow survey, designed to investigate geothermal anomalies along fault zones which exhibit different fluid flux regimes. An advanced “compositional” simulator was used to model hydrate formation and dissociation at Woolsey Mound and addresses the following hypotheses:

• 1.Migrating thermogenic fluids alter thermal conditions of the Hydrate Stability Zone (HSZ), so heat-flow reflects fault activity;
• 2.Gas hydrate formation and dissociation vary temporally at active faults, temporarily sealing conduits for migration of thermogenic fluids;
• 3.High salinity and inclusion of thermogenic gases with higher molecular weight than methane produce opposite effects on the depth to the bottom of the hydrate stability zone.

Applications of results include identifying and quantifying hydrate deposits in shallow sediments using an interdisciplinary approach that includes multiple resolution seismic data evaluation, geological and geochemical groundtruthing and heat-flow analyses as a proxy for activity along faults.     

3D gravity modeling of the Triassic salt diapirs of the Cubeta Alavesa (northern Spain)

Up to now subsurface information permitted the delineation of the top of the Triassic salt, all throughout the Cantabro–Navarro domain, although little was known on the location and geometry of its base and thus on the estimation of the total salt thickness. A 3D-gravity inversion scheme combined with a 3D analytic method has been conducted to map out the geometry of the main salt structures of the basin. The gravity modeling results have been constrained by well log information and available geological and reflection seismic data. The combined 3D scheme integrated with available geological and geophysical data has allowed us to obtain the geometry of the main diapirs that characterize the central and marginal regions of the basin.From our interpretation, the Salinas de Añana diapir has almost vertical flanks and can be divided into two different parts, one of them forming a lateral overhang of the main body.The Salinas de Oro diapir has near vertical flanks and a main axis in the N–S direction. Also, the anomaly is rather more extensive than the outcrop of the diapir, which implies an important expansion of non-outcropping salt in this area. Like the Hoz-Sobrón diapir, the Salinas de Ollo diapir is long and narrow. stretching in the NW–SE direction, which includes three important highs, plus an intense zone of salt migration.The Estella and Alloz diapirs crop out individually in spite of being connected at depth. Also two non-outcropping salt domes have been detected to the south of Atauri that, like the Estella diapir, are related to the thrust front. We point out the gravity signature of the Murguia diapir, which shows an intense gravity high probably due to the presence of high-density rocks in the cap rock or more probably due to the existence of Triassic volcanites of ophitic texture pinched-off into the diapir.     

Non-scaled analogue modelling of AMS development during viscous flow: A simulation on diapir-like structures

Development of magnetic fabric within a diapirically ascending columnar body was investigated using non-scaled analogue model made of plaster of Paris containing small amount of fine-grained homogeneously mixed magnetite. The apparatus for the modelling consists of a manual squeezer with calibrated spring and a Perspex container. Set of weak coloured layers at the bottom of the container was forced to intrude overlying fine-grained sand through a hole in a board attached to the squeezer. The development of AMS fabric is correlated with complex flow pattern indicated by coloured and originally horizontal plaster layers. Strongly constrictional and vertical fabric in the base and in the lower domain of the diapir resulting from convergent and upwards flows is overprinted by subhorizontal oblate fabrics due to vertical flattening and initial divergent flow in the apical parts. The measured AMS fabrics are compared with natural examples of magmatic stocks and dykes.     

南海北部陆架南北卫浅滩的成因及油气地质意义

世界范围内的陆架调查已经清楚,陆架浅滩是普遍发育于陆架区的一种海底地貌类型.在中国陆架上发育多个知名浅滩,这些浅滩和现今动力条件处于动态平衡状态,为现今潮流沉积体系.然而,南海北部陆架的南北卫浅滩在地貌上被称为陆架浅滩,但其成因却和其他陆架浅滩有着根本不同.根据地震剖面解释结果,本文指出,南海北部珠江口东南陆架上发育的南卫滩、北卫滩和惠州滩都属于构造地形,其下部各对应一个底辟构造.这3个底辟构造并不是孤立的,而是在深部属于同一个较大的背冲构造.这个背冲构造位于南北卫滩的下方,其上几个突出的底辟,则和本文讨论的南卫滩、北卫滩和惠州滩相对应.南卫滩、北卫滩和惠州滩在成因上具有统一的深部背景,从而在平面上构成一个统一的底辟系统,本文称为南北卫滩底辟系统.该底辟系统以北卫滩为中心,包括南卫滩、惠州滩和陆丰滩,形成一个大致呈圆形,直径约50 km的区域.在构造上,该底辟系统位于珠一坳陷和东沙隆起之间.南北卫滩底辟系统的发育起因于东沙隆起后在南海东北部形成的挤压应力环境.底辟系统的发育活化了惠陆-东沙含油气系统的输导层,并重新调整了油气运移的势差和势梯度,在环绕南北卫滩底辟系统一定距离的圆周上形成一个有利于油气聚集的环带.已知的油气田基本分布于这个环带上.下一步的油气勘探方向应该考虑围绕这个环带重点展开.同时,应该考虑在南北卫滩底辟系统的中心带内开展气藏的勘探工作.     

关于大陆地壳消亡与再生循环过程的新认识——大别山新的区域地质资料和榴辉岩专题研究成果提出的启示

越来越多的区域地质资料显示，大别山UHP榴辉岩相变质的大陆岩石以及在片麻岩中发现的含有柯石英包裹体的锆石和含有绿辉石包裹体的石榴子石等重矿物可能却是一个已经熔融了的榴辉岩相大陆岩片遗留下来的残留体，它们被包裹在由变形、变质深成侵入体构成的正片麻岩之中。这些深成侵入体底辟在扬子陆块北缘新元古代低一中级变质的火山一沉积地层之下。依据上述区域地质事实，本文认为，大别山含有榴辉岩残留体的高级片麻岩不是一个“UHP变质地体”或“含有榴辉岩外来构造块体的混杂岩”，其原岩应该是一套由榴辉岩化大陆岩片熔融形成的深成岩浆底辟体。由此本文提出一个新的理论假说和构造模型，即深俯冲到地幔软流圈之中的大陆边缘在不同的深度发生了相应的HP-UHP榴辉岩相变质作用，随着与俯冲洋壳的断离，其在浮力的作用下快速底垫在仰；中大陆边缘之下并发生部分熔融，熔融体底辟并置换了造山带原有的根，后经过同结晶过程的变形和变质作用形成一套含有榴辉岩相残留体的，高级片麻岩本文称之为再生地壳。     

Salt tectonics at passive margins: Geology versus models

Salt tectonics at passive margins is currently interpreted as a gravity-driven process but according to two different types of models: i) pure spreading only driven by differential sedimentary loading and ii) dominant gliding primarily due to margin tilt (slope instability). A comparative analysis of pure spreading and pure spreading is made using simple mechanics as well as available laboratory experiments and numerical models that consider salt tectonic processes at the whole basin scale. To be effective, pure spreading driven by sedimentary loading requires large differential overburden thicknesses and therefore significant water depths, high sediment density, low frictional angles of the sediments (high fluid pore pressure) and a seaward free boundary of the salt basin (salt not covered by sediments). Dominant gliding does not require any specific condition to be effective apart from the dip on the upper surface of the salt. It can occur for margin tilt angles lower than 1° for basin widths in the range of 200-600 km and initial sedimentary cover thickness up to 1 km, even in the absence of abnormal fluid pressure. In pure spreading, salt resists and sediments drive whereas in dominant gliding both salt and sediments drive. In pure spreading, extension is located inside the prograding sedimentary wedge and contraction at the tip. Both extension and contraction migrate seaward with the sedimentary progradation. Migration of the deformation can create an extensional inversion of previously contractional structures. In pure spreading, extension is located updip and contraction downdip. Extension migrates downdip and contraction updip. Migration of the deformation leads to a contractional inversion of previously extensional structures (e.g. squeezed diapirs). Mechanical analysis and modelling, either analogue or numerical, and comparison with margin-scale examples, such as the south Atlantic margins or northern Gulf of Mexico, indicate that salt tectonics at passive margins is dominated by dominant gliding down the margin dip. On the contrary, salt tectonics driven only by differential sedimentary loading is a process difficult to reconcile with geological evidence.     

The origin of dolomite associated with salt diapirs in central Tunisia: Preliminary investigations of field relationships and geochemistry

Black and white dolomite crystals (mm to cm width) of different isotopic composition are associated with Triassic diapirism in central Tunisia, as well as with evaporite minerals and clays. The white dolomites occur mostly in the Jabal Hadifa diapir near the contact with Cretaceous limestones, whereas the smaller black dolomites occur in the Jabal Hamra diapir. The former dolomite has a narrow range of δ¹⁸O and δ¹³C values (− 3.83‰ to − 6.60‰ VPDB for δ¹⁸O; − 2.11‰ to − 2.83‰ VPDB for δ¹³C), whereas the latter dolomite has a wider range and more depleted values (− 4.92‰ to − 9.97‰ for δ¹⁸O; − 0.55‰ to − 6.08‰ for δ¹³C). However, the ⁸⁷Sr / ⁸⁶Sr ratios of most of the samples are near Triassic seawater values. Dolomite formation is due to at least two different fluids. The main fluid originated from deeper hydrothermal or basinal sources related to the Triassic saliferous rocks and ascended through faults during the diapiric intrusion. The second, less important fluid source is related to meteoric water originating from Cretaceous rocks.     

Magnetic fabric and modeled strain distribution in the head of a nested granite diapir,the Melechov pluton,Bohemian Massif

The Melechov pluton, Bohemian Massif, is interpreted as a mid-crustal nested granitic diapir with an apical part exposed at the present-day erosion level. The diapir head exhibits a concentric structure defined by lithologic zoning and by the anisotropy of magnetic susceptibility (AMS). In concert with theoretical models, outward-dipping margin-parallel magnetic foliations are associated with oblate shapes of the susceptibility ellipsoids and higher degree of anisotropy, passing inward into weaker triaxial to prolate fabric. By contrast, magnetic fabric in an inner granite unit is in places oriented at a high angle to internal contacts and is interpreted as recording an internal diapir circulation. We use inverse modeling to calculate strain variations across the diapir from the AMS data. The magnetic fabric parameters and calculated strains are in agreement with strain distribution in heads of model Newtonian diapirs traveling a distance of two body radii and suggest granitic magma ascent as a crystal-poor suspension followed by crystallization of fabric markers and their response to strain near the final emplacement level. The intrusive fabric thus formed late but, though generally weak, was still capable of recording incremental strain gradient in the granite diapir.