Back-arc rifting in the Izu-Bonin Island Arc: Structural evolution of Hachijo and Aoga Shima Rifts

Abstract Multi- and single-channel seismic profiles are used to investigate the structural evolution of back-arc rifting in the intra-oceanic Izu-Bonin Arc. Hachijo and Aoga Shima Rifts, located west of the Izu-Bonin frontal arc, are bounded along-strike by structural and volcanic highs west of Kurose Hole, North Aoga Shima Caldera and Myojin Sho arc volcanoes. Zig-zag and curvilinear faults subdivide the rifts longitudinally into an arc margin (AM), inner rift, outer rift and proto-remnant arc margin (PRA). Hachijo Rift is 65 km long and 20–40 km wide. Aoga Shima Rift is 70 km long and up to 45 km wide. Large-offset border fault zones, with convex and concave dip slopes and uplifted rift flanks, occur along the east (AM) side of the Hachijo Rift and along the west (PRA) side of the Aoga Shima Rift. No cross-rift structures are observed at the transfer zone between these two regions; differential strain may be accommodated by interdigitating rift-parallel faults rather than by strike- or oblique-slip faults. In the Aoga Shima Rift, a 12 km long flank uplift, facing the flank uplift of the PRA, extends northeast from beneath the Myojin Knoll Caldera. Fore-arc sedimentary sequences onlap this uplift creating an unconformity that constrains rift onset to ～1-2Ma. Estimates of extension (～3km) and inferred age suggest that these rifts are in the early syn-rift stage of back-arc formation. A two-stage evolution of early back-arc structural evolution is proposed: initially, half-graben form with synthetically faulted, structural rollovers (ramping side of the half-graben) dipping towards zig-zagging large-offset border fault zones. The half-graben asymmetry alternates sides along-strike. The present ‘full-graben’ stage is dominated by rift-parallel hanging wall collapse and by antithetic faulting that concentrates subsidence in an inner rift. Structurally controlled back-arc magmatism occurs within the rift and PRA during both stages. Significant complications to this simple model occur in the Aoga Shima Rift where the east-dipping half-graben dips away from the flank uplift along the PRA. A linear zone of weakness caused by the greater temperatures and crustal thickness along the arc volcanic line controls the initial locus of rifting. Rifts are better developed between the arc edifices; intrusions may be accommodating extensional strain adjacent to the arc volcanoes. Pre-existing structures have little influence on rift evolution; the rifts cut across large structural and volcanic highs west of the North Aoga Shima Caldera and Aoga Shima. Large, rift-elongate volcanic ridges, usually extruded within the most extended inner rift between arc volcanoes, may be the precursors of sea floor spreading. As extension continues, the fissure ridges may become spreading cells and propagate toward the ends of the rifts (adjacent to the arc volcanoes), eventually coalescing with those in adjacent rift basins to form a continuous spreading centre. Analysis of the rift fault patterns suggests an extension direction of N80°E ± 10° that is orthogonal to the trend of the active volcanic arc (N10°W). The zig-zag pattern of border faults may indicate orthorhombic fault formation in response to this extension. Elongation of arc volcanic constructs may also be developed along one set of the possible orthorhombic orientations. Border fault formation may modify the regional stress field locally within the rift basin resulting in the formation of rift-parallel faults and emplacement of rift-parallel volcanic ridges. The border faults dip 45–55° near the surface and the majority of the basin subsidence is accommodated by only a few of these faults. Distinct border fault reflections decreases dips to only 30° at 2.5 km below the sea floor (possibly flattening to near horizontal at 2.8 km although the overlying rollover geometry shows a deeper detachment) suggesting that these rifting structures may be detached at extremely shallow crustal levels.     

The composition of back-arc basin lower crust and upper mantle in the Mariana Trough: A first report

Abstract The Mariana Trough is an active back-arc basin, with the rift propagating northward ahead of spreading. The northern part of the Trough is now rifting, with extension accommodated by combined stretching and igneous intrusion. Deep structural graben are found in a region of low heat flow, and we interpret these to manifest a low-angle normal fault system that defines the extension axis between 19°45' and 21°10'N. A single dredge haul from the deepest (∼5.5 km deep) of these graben recovered a heterogeneous suite of volcanic and plutonic crustal rocks and upper mantle peridotites, providing the first report of the deeper levels of back-arc basin lithosphere. Several lines of evidence indicate that these rocks are similar to typical back-arc basin lithosphere and are not fragments of rifted older arc lithosphere. Hornblende yielded an ⁴⁰Ar/³⁹Ar age of 1.8 ± 0.6 Ma, which is interpreted to approximate the time of crust formation. Harzburgite spinels have moderate Cr# (<40) and coexisting compositions of clinopyroxene (CPX) and plagioclase (PLAB) fall in the field of mid-ocean ridge basalt (MORB) gabbros. Crustal rocks include felsic rocks (70-80% SiO₂) and plutonic rocks that are rich in amphibole. Chemical compositions of crustal rocks show little evidence for a 'subduction component', and radiogenic isotopic compositions correspond to that expected for back-arc basin crust of the Mariana Trough. These data indicate that mechanical extension in this part of the Mariana Trough involves lithosphere that originally formed magmatically. These unique exposures of back-arc basin lithosphere call for careful study using ROVs and manned submersibles, and consideration as an ocean drilling program (ODP) drilling site.     

The role of lower continental crust and lithospheric mantle in the genesis of Plio–Pleistocene volcanic rocks from Sardinia (Italy)

The first comprehensive chemical and Sr–Nd–Pb isotopic data set of Plio–Pleistocene tholeiitic and alkaline volcanic rocks cropping out in Sardinia (Italy) is presented here. These rocks are alkali basalts, hawaiites, basanites, tholeiitic basalts and basaltic andesites, and were divided into two groups with distinct isotopic compositions. The vast majority of lavas have relatively high ⁸⁷Sr/⁸⁶Sr (0.7043–0.7051), low ¹⁴³Nd/¹⁴⁴Nd (0.5124–0.5126), and are characterised by the least radiogenic Pb isotopic composition so far recorded in Italian (and European) Neogene-to-Recent mafic volcanic rocks (²⁰⁶Pb/²⁰⁴Pb=17.55–18.01) (unradiogenic Pb volcanic rocks, UPV); these rocks crop out in central and northern Sardinia. Lavas of more limited areal extent have chemical and Sr–Nd–Pb isotopic ratios indicative of a markedly different source (⁸⁷Sr/⁸⁶Sr=0.7031–0.7040; ¹⁴³Nd/¹⁴⁴Nd=0.5127–0.5129; ²⁰⁶Pb/²⁰⁴Pb=18.8–19.4) (radiogenic Pb volcanic rocks, RPV), and crop out only in the southern part of the island. The isotopic ratios of these latter rocks match the values found in the roughly coeval anorogenic (i.e. not related to recent subduction events in space and time) mafic volcanic rocks of Italy (i.e. Mt. Etna, Hyblean Mts., Pantelleria, Linosa), and Cenozoic European volcanic rocks. The mafic rocks of the two Sardinian rock groups also show distinct trace element contents and ratios (e.g. Ba/Nb>14, Ce/Pb=8–25 and Nb/U=29–38 for the UPV; Ba/Nb<9, Ce/Pb=24–28 and Nb/U=46–54 for the RPV). The sources of the UPV could have been stabilised in the Precambrian after low amounts of lower crustal input (about 3%), or later, during the Hercynian Orogeny, after input of Precambrian lower crust in the source region, whereas the sources of the RPV could be related to processes that occurred in the late Palaeozoic–early Mesozoic, possibly via recycling of proto-Tethys oceanic lithosphere by subduction.     

Deep geodynamics of far field interconti- nental back-arc extension: Formation of Cenozoic volcanoes in northeastern China

Introduction Northeastem China has the most strong Cenozoic volcanism in China (Liu, 1999), where dis-tributes more than 500 Cenozoic volcanoes, including sleeping volcanoes of Tianchi Lake (Celes-tial Pond) of Changbai Mountain, and Wudalianchi (Five linked Lakes) (LIU, 1999). Vo lcano ofTianchi Lake of Changbai Mountain consists of basaltic rocks of shield-forming stage andtrachytes and pantellerites in cone-forming stage. It is suggested by study of REE, incompatibleelements a…     

Deep geodynamics of far field intercontinental back-arc extension: Formation of Cenozoic volcanoes in northeastern China

There are three cases of variation of trench location possible to occur during subduction: trench fixed, trench advancing, and trench retreating. Retreat of trench may lead to back-arc extension. The Pacific plate subducts at low angle beneath the Eurasia plate, tomographic results indicate that the subducted Pacific slab does not penetrate the 670 km discontinuity, instead, it is lying flat above the interface. The flattening occurred about 28 Ma ago. Geodynamic computation suggests: when the frontier of the subducted slab reaches the phase boundary of lower and upper mantle, it may be hindered and turn flat lying above the boundary, facilitates the retreat of trench and back-arc extension. Volcanism in northeastern China is likely a product of such retreat of subduction, far field back-arc extension, and melting due to reduce of pressure while mantle upwelling. Foundation item: National Natural Science Foundation of China (40234042 and 40174027).     

Evolution history of the Hidaka-oki (offshore Hidaka) basin in the southern central Hokkaido, as revealed by seismic interpretation, and related tectonic events in an adjacent collision zone

Off the southern coast of Hokkaido the Hidaka-oki (offshore Hidaka) basin has developed on the western flank of a collision suture under the influence of long-standing compressional plate motion and provoked tectonic stresses around the northwestern Pacific rim throughout the late Cenozoic. The basin forming history of the Japan arc and Kuril arc collision zone is described on the basis of seismic reflection data interpretation. We identify two stages of basin formation: the older (late Oligocene-Miocene) faulted en echelon graben (pull-apart basin) and younger (Plio-Pleistocene) regional downwarping. Paleoenvironmental changes recorded within the fore-arc sediments indicate that the older basin filled up by the late Miocene. We inferred the volumes of the distinctive basins from the depth-conversion of seismic data, which suggest episodic uplifts and massive erosion of the Hidaka Mountains in the middle-late Miocene and the Plio-Pleistocene. Estimated sediment supply rates into the basins have a similar level for the both stages. Cause of an episodic uplift in the older stage is attributed to the delayed opening of the Japan Sea. The eastern Eurasian margin underwent N-S right-lateral faulting at 25 Ma as a result of rifting of the Kuril back-arc basin. Formation of the Japan Sea back-arc basin since the early Miocene (ca. 20 Ma) caused eastward motion of the western Hokkaido block and transpressive regime along the pre-existing N-S shear deformation zone.     

Origin of the Kunlun Mountains by arc-arc and arc-continent collisions

Abstract The Kunlun Mountains were formed by early Mesozoic arc-arc and arc-continent collisions. The Middle Kunlun Are was the outer volcanic arc of the Paleozoic Asiatic continent, and the arc-related magmatic activities from the Proterozoic to Mesozoic are recorded by numerous volcanic and plutonic rocks of the area. Several back-arc basins and relic arcs exist north of the arc and the north Kunlun arc is one of these. The Kudi mélange of Kunlun was formed in a south-dipping subduction zone when the basin between the north and middle Kunlun arcs was consumed by the process of back-arc basin collapse, and the ophiolite mélange marked the suture zone where the two arcs collided. The Mazar mélange was formed in the north-dipping subduction zone under the middle Kunlun arc, and the mélange marks the main Paleotethys suture where the Qogir-Karamilan rocks of the Qangtang block (a fragment of Gondwanaland) is sutured on to Laurentia. The geology of Kunlun emphasizes the importance of arc-arc and arc-continent collisions in mountain-building processes.     

基于地震属性优化组合微断裂识别技术及应用

本文以断裂精细解释技术和地震属性分析技术为基础,通过对断裂具有响应特征的地震属性进行优化组合,有效刻画了研究区大级别断裂和微断裂的平面组合以及交切关系。工作流程首先以基于导向的相干体属性控制断裂展布趋势,然后优选融合凸显局部细节的倾角体、方位角体和曲率体等地震属性体精细刻画三、四级断裂和微断裂的空间展布特征,通过该方法有效识别出研究区断裂的5种平面组合样式(梳状、反梳状、岔Ⅰ型、岔Ⅱ型、帚状),7种剖面组合关系(反Y字、Y字、复Y字、X型、帚状、阶梯状和叉状)。该技术方法对复杂断裂的识别具有一定的借鉴意义。     

Geochronology, geochemistry and tectonic implications of Xiongshan diabasic dike swarm, northern Fujian

Sm/Nd isotopic age determination showed that Xiongshan dike swarm was at 585.7 Ma ± 30 Ma. The trace element geochemistry and Sr/Nd/Pb isotope gemhemistry studies indicate that the dike swarm was products of back-arc basin spreading ridge and the magma originated from the depleted mantle region which was metasomatized by LILE-rich liquids/melts derived from subduction slab. Project supported by the National Natural Science Foundation of China.     

A new model of back-arc spreading in the Parece Vela Basin, northwest Pacific margin

Abstract Swath bathymetric data, single-channel seismic reflection profiles, magnetic and gravity anomalies in the northern part of the Parece Vela (West Mariana) Basin were obtained by comprehensive surveys conducted by the Hydrographic Department of Japan. The central zone of the Parece Vela Basin is characterized by the north-south trending chain of depressions in a right-stepping en echelon alignment. The morphology of these depressions is diamond shaped and bordered by steep escarpments of 1000-1500 m relative height. These fault escarpments extend northeastward and southwestward from the depressions into the surrounding basin floor and then gradually fade out. These escarpments have an S-shaped trend, and their geometry seems to be symmetric about the depressions. Minor ridges and troughs trending orthogonal to these escarpments are recognized. It is concluded that these depressions and escarpments are the topographic expression of extinct spreading axes and S-shaped transform faults, respectively. The age of the central depressions seems to be young, although details of tectonic processes forming them remains unsolved. The western province of the basin floor and basement is extremely rugged and characterized by minor ridges and troughs trending in a north-south direction. Although magnetic anomalies of the basin are very weak, magnetic lineations trending parallel to the topographic trend are recognizable in the central and western parts of the basin. Based on updated geomorpholog-ical features and magnetic anomalies revealed by the present survey, together with the previously published data including drilling results, it is proposed that the evolution of Parece Vela Basin took place in four stages of opening and tectonic activity: rifting, east-west spreading, northeast-southwest spreading with counter-clockwise rotation of spreading axes, and post-spreading deformation and volcanism. This proposed spreading model of the Parece Vela Basin is similar to that of the adjacent Shikoku Basin. The spreading axes of both basins were segmented and gradually rotated counter-clockwise in a later phase of the basin evolution, after the cessation of relatively uniform spreading nearly in an east-west direction.     

INFLUENCES OF OBLIQUITY ANGLE DIFFERENCE ON THE EVOLUTION OF FEN-WEI RIFT: A STUDY FROM SEGMENTED TRANSTENSION CLAY MODEL

The Fen-Wei rift is composed of a series of Cenozoic graben basins, which extends in an S-shape and strikes mainly NNE. Two distinct types of basins are defined in the Fen-Wei rift. The NEE-striking basins(or basin system) are bounded by active faults of mainly normal slip while the NNE-striking basins are characterized by their dextral strike-slip boundary faults. The adjacent NEE-striking basins(or basin systems) are linked by the arrangement of NNE-striking basins and horsts that is called the linking zone in this study. The segmentation of the Fen-Wei rift shows that the geometry and the activity of different rift segments are varied. The southern and northern rift segments strike NEE and are characterized by tensile movement while the central rift segment strikes NNE with transtensional motion. Previous field surveys show that the ages of the Cenozoic basins in the Fen-Wei rift are old in the southern rift segment, medium in the northern rift segment, and young in the central rift segment. The sizes of linking zones are large in the central rift segment, medium in the northern rift segment, and small in the southern rift segment. In addition, the east tip of Xinding Basin propagates towards NEE along the northern rift segment and the west tip of the basin grows towards NNE, while the shape of Linfen Basin is almost antisymmetric with respect to the Xinding Basin. However, the previous laboratory or numerical simulations cannot explain these features because they didn't pay enough attention to the control of the rift segmentation on the evolution of NEE-striking basins and their linking zones. In this study, based on the previous field studies, we study the fracture process of a clay layer under the segmented dextral transtension of the basement. The spatiotemporal evolution of the deformation field of the clay layer is quantitatively analyzed via a digital image correlation method. The experiment reproduced the main architecture of the Fen-Wei rift. The results show that:(1) The chronological order of basin initiation and the different sizes of linking zones in deferent rift segments are caused by the different obliquity angles(the angle between the rift trend and the displacement direction between the opposite sides of the rift) among the southern, northern and central rift segments.(2) The interaction between adjacent NEE-striking basins leads to the formation of NNE-striking linking zones.(3) The interaction between adjacent rift segments may cause the special distribution of Xinding and Linfen Basins. Thus, we propose that the differences of the Fen-Wei rift segments are mainly controlled by the different obliquity angles. The lack of considering the influences of pre-exiting structures leads to the limited simulation of the details within the southern and northern segments of the Fen-Wei rift. Further studies may improve the model if this is taken into account.     

渤海盆地现代构造应力场与强震活动

渤海位于北华北新生代裂陷盆地的东部,是一个晚第四纪形成的内陆海盆. 渤海盆地活动断裂发育,地震活动强烈,交会于渤海中部的NE向营口——潍坊断裂带北段、庙西北——黄河口——临邑断裂带及NW向北京——蓬莱断裂带是主要的活动构造带,将海区分成4个次级新构造区,成为现代应力场作用的构造基础. 综合研究38个震源机制解和75个井区应力场等资料,以及构造应力场二维数值模拟计算结果表明,渤海及其邻区现代构造应力场的压应力方向为NE60～90,张应力为SN——NW30;以水平和近水平应力作用为主;不同构造区主应力方向存在一定的差异. 现今渤海地区地壳发育以NNE——NE和NW——WNW走向的共轭剪切破裂为特征,是控制地震活动的主要构造.     

对门源盆地\"红层\"形成时代的新认识

门源盆地位于青藏高原东北缘北祁连山腹地,为狭长的新生代走滑拉分山间宽谷盆地。其中充填一厚约407~960 m、干旱气候条件下的河湖相沉积\"红层\",长期以来各项研究工作均未能于其中采获任何生物化石。因此,尽管各家均认同其属\"第三纪\"无疑,但仍对其确切时代存在着较大的争论,而对其时代的确定有助于正确认识门源盆地形成时的构造背景,具有较为重要的意义。通过区域地层对比、\"红层\"形成气候条件与南侧西宁盆地及北邻酒泉—张掖盆地古气候环境所作的对比,深入探讨认为其时代属中中新世,并初步分析了门源盆地形成的新构造运动大陆动力学机制背景。