Uplifted crust in parts of western India

During northward movement of the Indian sub-continent, after its breakup from the Gondwanaland in the Late Cretaceous, the western part of India traversed over the Reunion plume. The Saurashtra peninsula and the Cambay Basin are two important geological regions in this part. Two and half dimensional density models, based on the crustal seismic structure, were generated to establish a relationship between these two regions. These models indicate that the crust is 32–33 km thick in the eastern Saurashtra and the northern part of the Cambay Basin. The shallower crust is in a triangular region formed by the extension of the western limb of the Proterozoic Aravalli trend in Saurashtra, its eastern limb and the Narmada fault in the south. Compared to 36–37 km thick crust to the west and 38–40 km to the east of this region the crust in the above triangular region is uplifted by 4 to 6 km. This uplift took place either after the deposition of Mesozoic sediments or was concomitant with the rise of Reunion plume prior to the extrusion of the Deccan volcanics as the region was close to the axis of the plume.     

Role of melt during deformation in the deep crust

Deformation in the deep crust is strongly influenced by the presence of melt. Injected melt (or magma) weakens the crust because strain will tend to localize where melt is present. The amount of strain a pluton may accommodate is dependent on the length of time it takes for a pluton to crystallize and the strain rate. For plutons that intrude into rocks which are near the solidus temperature of the melt, crystallization times can be quite long (> 1Myr).
Partial melting of deep crustal rocks can lead to melt-enhanced embrittlement. This occurs because the volume change for most melting reactions is positive. Therefore, when the rate of melt production outpaces the rate at which melt can leave the system, the melt pressure increases. Eventually, the melt pressure may become sufficiently high that the melting rocks behave in a brittle fashion and fracture.
Conjugate sets of dilatant shear fractures filled with melt occur in migmatite from the Central Gneiss belt (Canada); this suggests that melt-enhanced embrittlement occurred in these rocks. An expression which relates the magnitude of differential stress to the angle between conjugate dilatant shear fractures is derived. Assuming that migmatite has a small tensile strength, differential stresses are ≤ 20 MPa in migmatitic rocks at the time melt-enhanced embrittlement occurs. The occurrence of melt-enhanced embrittlement shows that a switch in deformation mechanism from plastic flow to cataclasis is possible in the deep crust during melting. Furthermore, repeated episodes of melt-enhanced embrittlement in migmatitic rocks may be an efficient mechanism for extracting melt from partially melted terrains.     

Mylonitic deformation of gabbro in the lower crust: A case study from the Pankenushi gabbro in the Hidaka metamorphic belt of central Hokkaido, Japan

The mylonitization of the Pankenushi gabbro in the Hidaka metamorphic belt of central Hokkaido, Japan, occurred along its western margin at ≈600 MPa and 660–700 °C through dynamic recrystallization of plagioclase and a retrograde reaction from granulite facies to amphibolite facies (orthopyroxene + clinopyroxene + plagioclase + H₂O = hornblende + quartz). The reaction produced a fine-grained (≤100 μm) polymineralic aggregate composed of orthopyroxene, clinopyroxene, quartz, hornblende, biotite and ilmenite, into which strain is localized. The dynamic recrystallization of plagioclase occurred by grain boundary migration, and produced a monomineralic aggregate of grains whose crystallographic orientations are mostly unrelated to those of porphyroclasts. The monomineralic plagioclase aggregates and the fine-grained polymineralic aggregates are interlayered and define the mylonitic foliation, while the latter is also mixed into the former by grain boundary sliding to form a rather homogeneous polymineralic matrix in ultramylonites. However in both mylonite and ultramylonite, plagioclase aggregates form a stress-supporting framework, and therefore controlled the rock rheology. Crystal plastic deformation of pyroxenes and plagioclase with dominant (100)[001] and (001)1/2 slip systems, respectively, produced distinct shape- and crystallographic-preferred orientations of pyroxene porphyroclasts and dynamically recrystallized plagioclase grains in both mylonite and ultramylonite. Euhedral to subhedral growth of hornblende in pyroxene porphyroclast tails during the reaction and its subsequent rigid rotation in the fine-grained polymineralic aggregate or matrix produced clear shape- and crystallographic-preferred orientations of hornblende grains in both mylonite and ultramylonite. In contrast, the dominant grain boundary sliding of pyroxene and quartz grains in the fine-grained polymineralic aggregate of the mylonite resulted in their very weak shape- and crystallographic-preferred orientations. In the fine-grained polymineralic matrix of the ultramylonite, however, pyroxene and quartz grains became scattered and isolated in the plagioclase aggregate so that they were crystal-plastically deformed leading to stronger shape- and crystallographic-preferred orientations than those seen in the mylonite.     

青藏高原壳幔形变数值模拟研究

现有数值模拟研究已在很大程度上较合理地给出青藏高原演化运动学和动力学过程的图像。利用连续介质快速拉格朗日分析方法,笔者进行了青藏高原壳幔形变数值模拟研究。据此得到的青藏高原三维壳幔形变特征反映纬向上主碰撞带远、近程效应的差异和经向上地壳物质“逃逸”的存在,印证了青藏高原形成过程中南北双向挤压、而且南部作用大于北部作用的可能应力场特征。青藏高原壳幔形变不仅强烈依赖于随深度变化的岩石力学性质及其距离挤压作用前锋带的远近,而且存在强烈的横向不均一性。同时,强应变(剪切)带的存在对高原岩石圈形变具有重要影响,高原形变过程中地壳尺度的耦合流及壳-幔解耦共存。但是,常规数值模拟研究尚存在很大局限性:(1)物理-力学模型单一;(2)几何模型简单;(3)边界形态与条件理想化;(4)模型内部块体划分粗糙;(5)不连续体介质处理困难。借助具有可处理大形变能力的4-D数值模拟方法,将观测资料与数值模拟相互补充是深入研究青藏高原壳幔形变的关键。     

Granulite xenoliths from western Saudi Arabia: the lower crust of the late Precambrian Arbian-Nubian Shield

Mafic and intermediate granulite xenoliths, collected from Cenozoic alkali basalts, provide samples of the lower crust in western Saudi Arabia. The xenoliths are metaigneous two-pyroxene and garnet granulites. Mineral and whole rock compositions are inconsistent with origin from Red Sea rift-related basalts, and are compatible with origin from island arc calc-alkaline and low-potassium tholeiitic basalts. Most of the samples are either cumulates from mafic magmas or are restites remaining after partial melting of intermediate rocks and extraction of a felsic liquid. Initial⁸⁷Sr/⁸⁶Sr ratios are less than 0.7032, except for two samples at 0.7049. The Sm-Nd data yield T_DM model ages of 0.64 to 1.02 Ga, similar to typical Arabian-Nubian Shield upper continental crust. The isotopic data indicate that the granulites formed from mantle-derived magmas with little or no contamination by older continent crust. Calculated temperatures and pressures of last reequilibration of the xenoliths show that they are derived from the lower crust. Calculated depths of origin and calculated seismic velocities for the xenoliths are in excellent agreement with the crustal structure model of Gettings et al. (1986) based on geophysical data from western Saudi Arabia. Estimation of mean lower crustal composition, using the granulite xenoliths and the Gettings et al. (1986) crustal model, suggests a remarkably homogeneous mafic lower crust, and an andesite or basaltic andesite bulk composition for Pan-African juvenile continental crust.     

Seismological features of island arc crust as inferred from recent seismic expeditions in Japan

Crustal studies within the Japanese islands have provided important constraints on the physical properties and deformation styles of the island arc crust. The upper crust in the Japanese islands has a significant heterogeneity characterized by large velocity variation (5.5–6.1 km/s) and high seismic attenuation (Qp=100–400 for 5–15 Hz). The lateral velocity change sometimes occurs at major tectonic lines. In many cases of recent refraction/wide-angle reflection profiles, a “middle crust” with a velocity of 6.2–6.5 km/s is found in a depth range of 5–15 km. Most shallow microearthquakes are concentrated in the upper/middle crust. The velocity in the lower crust is estimated to be 6.6–7.0 km/s. The lower crust often involves a highly reflective zone with less seismicity, indicating its ductile rheology. The uppermost mantle is characterized by a low Pn velocity of 7.5–7.9 km/s. Several observations on PmP phase indicate that the Moho is not a sharp boundary with a distinct velocity contrast, but forms a transition zone from the upper mantle to the lower crust. Recent seismic reflection experiments revealed ongoing crustal deformations within the Japanese islands. A clear image of crustal delamination obtained for an arc–arc collision zone in central Hokkaido provides an important key for the evolution process from island arc to more felsic continental crust. In northern Honshu, a major fault system with listric geometry, which was formed by Miocene back arc spreading, was successfully mapped down to 12–15 km.     

川西地区甲基卡穹隆的构造变形特征

甲基卡穹隆因其丰富的锂辉石资源而闻名,在华北、扬子和羌塘3个板块之间的南北向和东西向双向收缩背景下,形成了其复杂的变形特征。穹隆顶部地层平缓,向外逐渐变陡,总体为同心圆状向外倾的复式背斜。顶部发育有A型等斜平卧褶皱、肠状石英脉及对称型压力影;翼部变质矿物多被拉伸旋转而呈不对称状,表现出向穹隆周围的正向剪切运动;穹隆外围地层产状较陡,多发育直立紧闭褶皱及倒转褶皱。总体来说,甲基卡穹隆岩浆底辟作用前,先后经历了南北向及东西向的挤压,成穹后再一次经历南北和东西向挤压的两期构造运动,属于构造变形与岩浆作用的共同产物。     

Continuous cratonic crust between the Congo and Tanzania blocks in western Uganda

Western Uganda is a key region for understanding the development of the western branch of the East African rift system and its interaction with pre-existing cratonic lithosphere. It is also the site of the topographically anomalous Rwenzori Mountains, which attain altitudes of >5000 m within the rift. New structural and geochronological data indicate that western Uganda south and east of the Rwenzori Mountains consists of a WSW to ENE trending fold and thrust belt emplaced by thick-skinned tectonics that thrust several slices of Proterozoic and Archaean units onto the craton from the south. The presence of Archaean units within the thrust stack is supported by new Laser-ICP-MS U–Pb age determinations (2637–2584 Ma) on zircons from the Rwenzori foothills. Repetition of the Paleoproterozoic units is confirmed by mapping the internal stratigraphy where a basal quartzite can be used as marker layer, and discrete thrust units show distinct metamorphic grades. The thrust belt is partially unconformably covered by a Neoproterozoic nappe correlated with the Kibaran orogenic belt. Even though conglomerates mark the bottom of the Kibaran unit, intensive brittle fault zones and pseudotachylites disprove an autochthonous position. The composition of volcanics in the Toro-Ankole field of western Uganda can be explained by the persistence of a cratonic lithosphere root beneath the northwardly thrusted Archaean and Palaeoproterozoic rocks of westernmost Uganda. Volcanic geochemistry indicates thinning of the lithosphere from >140 km beneath Toro-Ankole to ca. 80 km beneath the Virunga volcanic field about 150 km to the south. We conclude that the western branch of the East African rift system was initiated in an area of thinner lithosphere with Palaeoproterozoic cover in the Virunga area and has propagated northwards where it now abuts against thick cratonic lithosphere covered by a thrust belt consisting of gneisses, metasediments and metavolcanics of Neoarchaean to Proterozoic age.     

Formation and shortening deformation of a back-arc rift basin revealed by deep seismic profiling, central Japan

The northern Fossa Magna (NFM) basin is a Miocene rift system produced in the final stages of the opening of the Sea of Japan. It divides the major structure of Japan into two regions, with north-trending geological structures to the NE of the basin and EW trending structures to the west of the basin. The Itoigawa-Shizuoka Tectonic Line (ISTL) bounds the western part of the northern Fossa Magna and forms an active fault system that displays one of the largest slip rates (4–9 mm/year) in the Japanese islands. Deep seismic reflection and refraction/wide-angle reflection profiling were undertaken in 2002 across the northern part of ISTL in order to delineate structures in the crust, and the deep geometry of the active fault systems. The seismic images are interpreted based on the pattern of reflectors, the surface geology and velocities derived from refraction analysis. The 68-km-long seismic section suggests that the Miocene NFM basin was formed by an east dipping normal fault with a shallow flat segment to 6 km depth and a deeper ramp penetrating to 15 km depth. This low-angle normal fault originated as a comparatively shallow brittle/ductile detachment in a high thermal regime present in the Miocene. The NFM basin was filled by a thick (>6 km) accumulation of sediments. Shortening since the late Neogene is accommodated along NS to NE–SE trending thrust faults that previously accommodated extension and produce fault-related folds on their hanging wall. Based on our balanced geologic cross-section, the total amount of Miocene extension is ca. 42 km and the total amount of late Neogene to Quaternary shortening is ca. 23 km.     

Coupling between fluids and rock deformation in the continental crust:Preface

正1.Introduction Fluids are essential and widespread components of the Earth crust.They regulate the main process of mass and energy transport,conditioning the geochemical and geophysical evolution of the crust (Bredehoeft and Norton.1990) and,consequently,its properties and mechanical behavior.The coupling between fluids,rock deformation,and heat are crucial engines for plate tectonics,playing a leading role in the rheology and evolution of the lithosphere(e.g.Jamtveit et al.,2000;Kennedy and van Soest,2007;Thompson,2010;Miller.2013;Yardley and Bodnar,2014:Hirauchi et al.,2016:     

The problem of the mechanism of deformation of the crust and upper mantle

A tectonic interpretation of data on the mechanism of earthquake foci enables us to clarify the mechanism of deformation of the earth's crust and upper mantle. The association between the stressed state and the physicomechanical properties of the deformed material ts emphasized. The source of the tectonic deformations is considered to be gravitational differentiation of material of the deep parts of the mantle. —Authors.     

A suggestion on the role of tidal forces in the deformation of the Earth's crust

It is proposed that a contribution to orogeny is made by tidal flexing of the Earth's crust. It is suggested that the crust of the Earth is continually expanding horizontally as a result of tidal flexing, and that the pressures caused by this expansion are relieved by folding, faulting and volcanism in zones of weakness which ultimately become zones of granitic accumulation. This expansion also helps account for the concentration of granitic materials of the crust into continental masses.     

Kinematics and deformation of central and southwestern Greece from historical triangulation data and implications for the active tectonics of the Aegean

Changes in the coordinates of 38 stations, 32 of which were common to three historical triangulation surveys (1889–1905, 1927–1930 and 1950–1970) in central Greece were computed based on conventional adjustment techniques and the assumption of a nearly fixed baseline length. Some of the calculated coordinate changes are significant against the a posteriori network adjustment errors and are likely to indicate tectonic motions consistent with those deduced from geological and geophysical data and other geodetic studies (comparison of the first survey network with GPS data).

More explicitly, the historical geodetic data confirm the rotational character of the deformation in the area, but they show that the strain pattern is likely to have changed after 1930: between 1890 and 1930 strain was changing smoothly, as if no strain discontinuities existed, while the Peloponnesus was under contraction from the northeast, probably reflecting accommodation of strain from the arc. Significant left-lateral shear in the gulf of Corinth and N-S extension in the whole of the study area were observed only after 1930. Estimates of strain are consistent with those deduced from comparison of historical triangulation and GPS data, corrected for the scale error that exists in the terrestrial geodetic data in Greece, and estimates of strain from seismological data. The change in the kinematic pattern after 1930 seems to be confirmed by changes in the shear strain computed directly from changes in observed angles between stations and from changes in the seismicity rates; such changes, although not unusual in the geological record, are not easy to explain, but may reflect elastic rebound effects in area with an extremely complicated tectonic fabric or hysteresis in the accommodation of E-W compression from the arc by N-S stretching.     

The transition from brittle to ductile deformation in the Sambagawa metamorphic belt, Japan

The dominant deformation mechanism during the Sambagawa metamorphism changes from brittle to ductile with increasing metamorphic temperature. The magnitude of plastic strains inferred from the shapes of deformed radiolaria in metachert increases sharply across the boundary between the epidote-pumpellyite-actinolite zone and the epidote-actinolite zone. The synmetamorphic crack density of metachert is an indicator of the contemporaneous brittle strain of rocks, and it decreases sharply as the grade reaches the epidote-actinolite zone. Hence, the ratio of the ductile strain to the brittle strain of metachert decreases rapidly across the transition to the epidote-actinolite zone of the Sambagawa metamorphic belt.
The sharp change of the ductile strain magnitude also takes place at the epidote-actinolite grade in the Shimanto metamorphic belt of Japan, an example of the intermediate pressure facies series of metamorphism. It is concluded that the transition from brittle to ductile deformation takes place at about 300-400°C. and is independent of pressure of metamorphism.     

Faulting on the western flank of Mt Etna and magma intrusions in the shallow crust

Surface deformations on the western flank of Mt Etna volcano, spanning 1980–2004, have been analysed as they pertain to stress interactions between magma intrusions within the shallow crust along the S–SE Rift and faulting sensitivity. During this period, an accurate analysis of strain parameters, computed by inversion of SW electro‐optical distance data, suggested that the observed strong displacements on this flank of the edifice can also be related to dextral shear movements along a roughly NE–SW buried fault crossing the area covered by this network, as supported by seismic observations of the 20–24 April 2001 swarm. Moreover, Coulomb stress change model analysis confirms that the displacement along this fault, heralding the July–August 2001 eruption 2 months earlier can be related to major stresses applied by a dike intrusion at depth along the S–SE Rift, as testified by the microseismicity occurring between November 2000 and 19 April 2001.     

Thicknesses and types of the crust beneath the Sea of Japan inferred from marine and satellite gravimetric investigations

The combined gravimetric, altimetric, and seismic data used for calculating the crust thickness beneath the Sea of Japan revealed its different types in this region: oceanic and continental with different degrees of destruction. It is shown that the crust thickness derived from satellite altimetry is well consistent with that calculated from the data obtained by on-board gravimetric observations.     

磨西断裂变形与运动学特征研究

磨西断裂位于鲜水河活动断裂带的东南侧,北段为松潘-甘孜地槽褶皱系与扬子陆块的分界,南段则伸入扬子地台内部,成为扬子地台西缘拗褶带与康滇地轴的分界,是扬子西缘不同构造单元的控制性断裂构造。通过宏观分析地层分布,磨西断裂北段磨西磨子沟(新近崩塌出露),中段湾东剖面和南段新民腊树岗剖面的构造变形、几何学与运动学特征,收集整理擦痕资料,再结合糜棱岩微观石英C轴组构特征,认为磨西断裂曾经历5次强烈活动:早期为正断性质,西低东高,控制两侧沉积分异(晋宁期);中期为韧性逆冲(印支期),形成糜棱岩;晚期新近纪中新世又一次韧性逆冲,之后又经历了脆性左旋逆冲、脆性左旋逆冲平移两次强烈活动(喜马拉雅期)。