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1.
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.  相似文献   

2.
A bimodal volcanic suite with KAr ages of 0.05–1.40 Ma was collected from the Sumisu Rift using alvin. These rocks are contemporaneous with island arc tholeiite lavas of the Izu-Ogasawara arc 20 km to the east, and provide a present day example of volcanism associated with arc rifting and back-arc basin initiation. Major element geochemistry of the basalts is most similar to that of basalts found in other, more mature back-arc basins, which indicates that back-arc basins need not begin their magmatic evolution with lavas bearing strong arc signatures.Volatile concentrations distinguish Sumisu Rift basalts from island arc basalts and MORB. H2O contents, which are at least four times greater than in MORB, suppress plagioclase crystallization. This suppression results in a more mafic fractionating assemblage, which prevents Al2O3 depletion and delays the initiation of Fe2O3(tot) and TiO2 enrichment. However, unlike arc basalts,Fe3+/ΣFe ratios are only slightly higher than in MORB and are insufficient to cause magnetite saturation early enough to suppress Fe2O3(tot) and TiO2 enrichment. Thus, major element trends are more similar to those of MORB than arcs.H2O, CO2 and S are undersaturated relative to pure phase solubility curves, indicating exsolution of an H2O-rich mixed gas phase. HighH2O/S, highδD, and low (MORB-like)δ34S ratios are considered primary and distinctive of the back-arc basin setting.  相似文献   

3.
The back-arc region of the Izu-Bonin arc has complex bathymetric and structural features, which, due to repeated back-arc rifting and resumption of arc volcanism, have prevented us from understanding the volcano-tectonic history of the arc after 15 Ma. The laser-heating 40Ar/39Ar dating technique combined with high density sampling of volcanic rocks from the back-arc region of this arc successfully revealed the detailed temporal variation of volcanism related to the back-arc rifting. Based on the new 40Ar/39Ar dating results: (1) Back-arc rifting initiated at around 2.8 Ma in the middle part of the Izu-Bonin arc (30°30′N–32°30′N). Volcanism at the earliest stage of rifting is characterized by the basaltic volcanism from north–south-trending fissures and/or lines of vents. (2) Following this earliest stage of volcanism, at ca. 2.5 Ma, compositionally bimodal volcanism occurred and formed small cones in the wide area. This volcanism and rifting continued until about 1 Ma in the region west of the currently active rift zone. (3) After 1 Ma, active volcanism ceased in the area west of the currently active rift zone, and volcanism and rifting were confined to the currently active rift zone. The volcano-tectonic history of the back-arc region of the Izu-Bonin arc is an example of the earliest stage of back-arc rifting in the oceanic island arc. Age data on volcanics clearly indicate that volcanism changed its mode of activity, composition and locus along with a progress of rifting.  相似文献   

4.
Asymmetric rifting of the northern Mariana Trough   总被引：3，自引：0，他引：3  
The evolution of rifting in the northern Mariana Trough was studied, based on single-channel seismic reflection profiles and heat flow. The rift showed structural asymmetry. The northernmost part of the Mariana Trough at 24°N, just south of Minami-Iwojima Island, is now in an incipient rifting stage and shows a half-graben structure. The arc crust just behind the volcanic front is cut by a few major east-dipping normal faults. The major faults extend southward behind the Hiyoshi seamounts around 23°30'N. The rift develops to a full-graben stage at ∼ 23°N, where the width of the trough increases to 80 km. The trough is comprised of several faulted and tilted blocks of island-arc crust. Maximum subsidence occurs along a row of small grabens on the eastern margin of the trough. These grabens are separated by arc volcanoes, and their depths increase southward from 2500 m at 23°20'N to 4500 m at 22°N. The strike of each graben is north-northwest–south-southeast, which is close to the trend of the remnant West Mariana Ridge, but oblique to the active Mariana arc. Crustal extension becomes concentrated along the eastern margin of the trough as rifting progresses. The transition from rifting to sea floor spreading may occur at ∼ 22°N, where the width of the trough is ∼ 120 km. The possible spreading center lies along the southern extension of the grabens on the eastern margin. The period of back-arc rifting before spreading begins is estimated to be less than 3 million years. Heat flow is asymmetric in the rift. High heat flow was observed only in or close to the row of grabens along the eastern margin of the trough. The asymmetric pure shear extension model fits the observed heat flow distribution better than the simple shear extension model.  相似文献   

5.
The West African Rift System has, for the last ten years, been thought to consist of five interconnected rifts extending from the Gulf of Guinea deep into the heart of Africa. Careful re-examination of the geophysical evidence makes it quite clear that there are only three interconnected rifts in West Africa; the Lower Benue Rift which extends to the northeast from the Gulf of Guinea to a triple junction near Chum, and the Gongola and Yola Rifts which extend to the north and east, respectively, from the Chum triple junction. These three rifts opened during the earlier part of the Mesozoic and were subsequently filled with Cretaceous sediments. The evidence for two further rifts, the Ati Rift and the Fort Archambault Rift which were thought to extend to the northeast and southeast, respectively, from a triple junction at the eastern end of the Yola Rift, does not stand up to re-examination.The “Ati Rift” was thought to follow a major linear positive gravity anomaly which had been mapped beneath the Quaternary sediments of the Chad Basin. The main gravity anomaly is separated from the Yola Rift by over 300 km and is probably due to a linear body of basic volcanic or volcano-clastic rocks associated with a suture of Pan-African age. Within the gap, between the main anomaly and the Yola Rift, there are three localised positive anomalies which relate to a gabbro of Precambrian age, a band of dense meta-sediments within the Basement Complex and an acid igneous complex of Palaeogene age. The anomaly as a whole is therefore a sequence of unrelated anomalies, none of which are due to features of Mesozoic age.The “Fort Archambault Rift” was thought to follow a major linear negative gravity anomaly which has been mapped beneath the Quaternary sediments of the Chad Basin. To a large extent the negative anomaly overlies the fosse de Baké-Birao (Baké-Birao Basin) which is itself part of a far larger structure that extends, parallel to the southern margin of the West African Rift System, from Douala on the Gulf of Guinea to Birao near the C.A.R. frontier with Sudan. The Douala-Birao Structure may possibly be loosely related to the West African Rift System in that it would appear to be a compressional structure formed at the same time as the Coniacian-Santonian phase of folding which is observed in the West African rifts. However, the two structures are clearly separated from each other and are quite different in character and to a lesser extent in age.  相似文献   

6.
Bimodal volcanism, normal faulting, rapid sedimentation, and hydrothermal circulation characterize the rifting of the Izu-Bonin arc at 31°N. Analysis of the zigzag pattern, in plan view, of the normal faults that bound Sumisu Rift indicates that the extension direction (080° ± 10°) is orthogonal to the regional trend of the volcanic front. Normal faults divide the rift into an inner rift on the arc side, which is the locus for maximum subsidence and sedimentation, and an outer rift further west. Transfer zones that link opposing master faults and/or rift flank uplifts further subdivide the rift into three segments along strike. Volcanism is concentrated along the ENE-trending transfer zone which separates the northern and central rift segments. The differential motion across the zone is accommodated by interdigitating north-trending normal faults rather than by ENE-trending oblique-slip faults. Volcanism in the outer rift has built 50–700 m high edifices without summit craters whereas in the inner rift it has formed two multi-vent en echelon ridges (the largest is 600 m high and 16 km long). The volcanism is dominantly basaltic, with compositions reflecting mantle sources little influenced by arc components. An elongate rhyolite dome and low-temperature hydrothermal deposits occur at the en echelon step in the larger ridge, which is located at the intersection of the transfer zone with the inner rift. The chimneys, veins, and crusts are composed of silica, barite and iron oxide, and are of similar composition to the ferruginous chert that mantles the Kuroko deposits. A 1.2-km transect of seven alvin heat flow measurements at 30°48.5′N showed that the inner-rift-bounding faults may serve as water recharge zones, but that they are not necessarily areas of focussed hydrothermal outflow, which instead occurs through the thick basin sediments. The rift basin and arc margin sediments are probably dominated by permeable rhyolitic pumice and ash erupted from submarine arc calderas such as Sumisu and South Sumisu volcanoes.  相似文献   

7.
杨攀新  任金卫  陈正位  张俊 《地震》2010,30(3):81-89
利用ETM遥感影像与30 m分辨率的ASTER DEM数据, 构建申扎—定结地堑系北段遥感影像三维数字高程模型, 结合野外地质地貌调查结果, 对地堑系地貌特征进行总体分析。并根据对主要断裂的产状和活动性野外调查与活动断裂展布的遥感解译结果, 认为地堑系由一条东倾的铲式主要断裂和一系列同倾向和反倾向的次级断裂组成的半地堑构造。基于半地堑系模式, 将地堑系简化成书斜式模型, 根据地堑的沉降量和地堑宽度值, 计算地堑系的滑脱面深度在3.6～9.7 km之间, 因此地堑中地震震源深度也多集中于滑脱面附近。最后探讨地堑的形成机制, 表明在印度板块的挤压作用下, 高原上地壳与下地壳发生应力解耦, 上地壳在重力作用下, 向南向东发生滑脱, 形成东倾的半地堑构造。  相似文献   

8.
Cenozoic extensional stress evolution in North China   总被引：14，自引：0，他引：14  
Since the beginning of the Cenozoic, north China has been fragmented by intensive intracontinental rifting and extensional tectonics, which resulted in the formation of two extensional domains: the graben systems around the Ordos block in the west and North China Plain in the east. How to link this Cenozoic extensional tectonics to plate kinematics has long been an issue of debate. This paper presents updated results of fault slip data sets collected in different zones in north China and addresses the changes in the direction of extensional stresses over the Cenozoic. A chronology of three successive extensions has been established and provides evidence for constraining the timing and location of either subduction-induced back-arc tectonics along the western Pacific or collision-related extrusion tectonics in Tibet. The oldest NW–SE trending extension occurred concomitantly with the early Tertiary rifting phase, which was initiated in a back-arc setting associated with westward subduction of the Pacific plate under the Asia continent. North China had been subjected, during the Miocene, to regional subsidence with widespread basalt flow, and the direction of extension changed to NE–SW to NNE–SSW, consistent with the spreading direction of the Japan Sea. The dynamic origin of this extension is poorly understood. Since the latest Miocene or earliest Pliocene, north China has been dominated by NW–SE extension resulting in the formation and development of the elongate graben systems around the rigid Ordos block. This extensional phase is accompanied by counterclockwise rotation of blocks such as Ordos, Taihangshan Massif etc., which are bounded to south by the left-lateral strike-slip Qinling fault system. The overall Pliocene-Quaternary deformation in north China accommodates an ESE-ward extrusion of the south China block relative to the Gobi-Mongolia plateau, as the consequence of late-stage India–Eurasia convergence.  相似文献   

9.
西藏高原南北向裂谷研究意义   总被引：8，自引：1，他引：8  
强烈遭受南北向挤压下的西藏高原上却发育了大量的正在活动着的东西向伸展构造，即裂谷系。特别是在挤压力最为集中的喜马拉雅碰撞弧的前方，拉萨地体内发育了大规模、有规律排列的近南北向裂谷系。目前，在拉萨地体内，开展了大量的地球物理探测和地质研究工作，如亚东——格尔木地学断面，INDETPH，中法合作项目等。鉴于当时的认识和科学研究目标，这些成果并没有把所有的裂谷系所发育的环境作为一个整体去研究。因此，裂谷系的深部过程及其原由还是知之甚少。本文在总结前人研究成果的基础上认为，从整个岩石圈流变学结构去研究藏南近南北向裂谷系将有助于去认识其产生这些裂谷系的深部动力学过程，进而能够更好地去认识西藏高原隆升的地球动力学过程。  相似文献   

10.
Satoru  Honda  Takeyoshi  Yoshida  Kan  Aoike 《Island Arc》2007,16(2):214-223
Abstract   Arc volcanism of the past 10 my in the northeast Honshu and Izu-Bonin Arcs shows several notable features. In the northeast Honshu Arc, the spatial distribution of volcanism exhibits several clusters elongated nearly perpendicular to the arc and the possible migration of volcanism from the back-arc side to the volcanic front side, at least, during the past 5 my. The pattern of clusters seems to have flip-flopped around 5 Ma. In the Izu-Bonin Arc, there are a series of across-arc seamount chains, in which volcanic activity occurred from ca 17 Ma to ca 3 Ma, similar to the clusters of the northeast Honshu Arc, although the recent active rifting occurs almost parallel to the arc. On the basis of studies of numerical modeling, these features might be explained, at least qualitatively, by the small-scale convection under the island arc. Several inferences can be made from our modeling results for the tectonics of the Izu-Bonin Arc. The angle of dip of subducting plate in the Izu-Bonin Arc might have increased. This can explain the disappearance of volcanism along the seamount chains and the recent along-arc volcanism with narrow rifting. The trend of seamount chains, which is oblique to the arc, might not be their intrinsic feature but rather a result of the lateral movement of the back-arc region after their formation. These inferences can be tested by the future detailed morphological and chronological studies of the Izu-Bonin Arc.  相似文献   

11.
The Izu–Ogasawara arc contains, from east to west, a volcanic front, a back-arc extensional zone (back-arc knolls zone), and a series of across-arc seamount chains that cross the extensional zone in an east-northeast and west-southwest direction and extend into the Shikoku Basin. K–Ar ages of dredged volcanic rocks from these across-arc seamount chains and extension-related edifices in the back-arc region of the Izu–Ogasawara arc were measured to constrain the volcanic and tectonic history of the arc since the termination of spreading in the Shikoku Basin. K–Ar ages range between 12.5 and 1 Ma. Andesitic to dacitic rocks of 12.5–2.9 Ma occur mainly on the western part of the chains. The western part of the chains are the locus of volcanism behind the front which erupted mainly calc-alkaline andesitic lavas. The youngest rocks (< 2.8 Ma), characterized by cpx-ol basalt, occur along the western margin of the back-arc knolls zone. Basaltic rocks of 12.5–2.9 Ma have relatively high concentrations of Na2O (> 2.0 wt%), Zr (> 50 p.p.m.) and Y (> 20 p.p.m.) and low CaO (< 12 wt%). On the other hand, basalts of 2.8–1 Ma have lower Na2O (< 1.8 wt%), Zr (< 50 p.p.m.) and Y (< 20 p.p.m.), but significantly higher CaO (> 12 wt%). The age inferred for the initiation of back-arc rifting (∼ 2.35–2.9 Ma: Taylor 1992 ) behind the current volcanic arc coincides with the time that basalt chemistry changed drastically (eruption of the low-Na2O and high-CaO basalt). This implies that post-2.8 Ma volcanism in the back-arc knolls zone is associated with rifting. Similarly, the change in chemical composition might be explained by a different type of source mantle following rift initiation. Volcanism in the western seamounts ceased after the onset of rifting at ∼ 2.8 Ma.  相似文献   

12.
The Neoproterozoic Kangdian Rift Basin is a continental rift basin in the western Yangtze Continent. Determining its time of opening and subsequent filling pattern is an important aspect of understanding the formation and evolution of the Yangtze Continent, South China. The Luliang Formation is the early filling in the eastern part of this rift basin, and its deposition age and filling sequence are significant for studies of the regional stratigraphic correlation, opening time, and filling pattern of this basin. Having been correlated to the upper part of the Mesoproterozoic Kunyang Group or to the Neoproterozoic Chengjiang Formation, the age and regional stratigraphic correlation of the Luliang Formation have long been uncertain. This is due to a lack of reliable absolute age constraints. To address this issue, the first zircon SHRIMP U-Pb geochronology has been established for this formation, yielding two high-precision isotopic ages of 818.6±9.2 and 805±14 Ma for the tuff layers at the bottoms of the lower and upper members of the Luliang Formation, respectively. Given the error factor, the bottom age of the lower member of the Luliang Formation can be interpreted as ca. 820 Ma, corresponding to the bottom age of the lower part of the Banxi Group, which is the early filling of the Neoproterozoic Xianggui Rift Basin, a representative basin of the Neoproterozoic rift basins in the Yangtze Continent, South China. The bottom age of the upper member of the Luliang Formation can be interpreted as ca. 800±5 Ma, corresponding to the bottom age of the Chengjiang Formation in the western part of the Neoproterozoic Kangdian Rift Basin and also corresponding to the bottom age of the upper part of the Banxi Group in the Neoproterozoic Xianggui Rift Basin. These ages indicate that the Neoproterozoic Kangdian Rift Basin shares the same opening time and filling sequences as those of the other Neoproterozoic rift basins in South China. Basin analysis shows that the Neoproterozoic Kangdian Rift Basin is a typical half-graben basin, with its main boundary fault in the west and basin center in the east. This basin consisted of mini unidirectional half-graben basins in its juvenile stage and simplified to become a large united half-graben basin in its mature stage, i.e., a supradetachment basin.  相似文献   

13.
More than 5000 km3 of magmatic material was erupted in Pliocene-Pleistocene times in a volcano-tectonic depression, i. e., the Hohi volcanic zone (HVZ) in central Kyushu, Japan. The eruptive deposits consist mainly of andesite lava flows and large-scale pyroclastic-flow deposits. Their eruptions were accompanied by the formation of an EW-oriented graben (70 km × 45 km) under regional NS extensional stress. Pre-Tertiary basement rocks are absent on the surface of the graben but occur at depth, having subsided up to 3 km. Radiometric ages of volcanic rocks on the surface show zoned isochrons from 5 Ma at the margin to 0.3 Ma in the center of the HVZ. The youngest center of age zonation coincides with a 30 mgal negative Bouguer gravity anomaly. Radiometric ages of rocks from drill cores are older toward the bottom of the graben, reaching a maximum of at least 4 Ma. Volcanic activity concentrated over time toward the center of the graben and buried successively erupted material. Areas of active volcanism in the HVZ became smaller and changed in style during the 5-Ma history of activity. Volcanism of the early stage (5-2 Ma) was characterized by voluminous eruptions of andesitic lava flows that formed lava plateaus and were intruded by EW-oriented feeder dikes, perhaps related to fissure eruptions. In contrast, late-stage volcanism (2-0 Ma) resulted primarily in andesitic to dacitic lava domes with features of monogenetic volcanoes produced at low eruption rates. The HVZ shows unimodal volcanism dominated by andesitic and dacitic lavas with a small amount of rhyolite and only traces of basalt; these characteristics differ from those that typify volcanism in most other extensional areas. Erupted material in the HVZ is of the calc-alkali and high-alkali tholeiite series and shows no significant chemical changes over 5 Ma, except for an increase in K2O after 1.6 Ma. The net horizontal displacement along normal faults indicates that the HVZ widened by about 10%–20% across the graben at an average rate of 0.1 cm/yr. I interpret the HVZ to be neither a pull-apart structure of the pre-Tertiary basement nor the result of propagation of the Okinawa Trough, but rather the earliest stage of rifting when vertical subsidence caused by normal faulting is compensated by filling with volcanic material.  相似文献   

14.
Forty new K-Ar and 40Ar/39Ar isotopic ages from the northern Main Ethiopian Rift (MER)–southern Afar transition zone provide insights into the volcano-tectonic evolution of this portion of the East African Rift system. The earliest evidence of volcanic activity in this region is manifest as 24–23 Ma pre-rift flood basalts. Transition zone flood basalt activity renewed at approximately 10 Ma, and preceded the initiation of modern rift margin development. Bimodal basalt–rhyolite volcanism in the southern Afar rift floor began at approximately 7 Ma and continued into Recent times. In contrast, post-subsidence volcanic activity in the northern MER is dominated by Mio-Pliocene silicic products from centers now covered by Quaternary volcanic and sedimentary lithologies. Unlike other parts of the MER, Mio-Pliocene silicic volcanism in the MER–Afar transition zone is closely associated with fissural basaltic products. The presence of Pliocene age ignimbrites on the plateaus bounding the northern MER, whose sources are found in the present rift, indicates that subsidence of this region was gradual, and that it attained its present physiography with steep escarpments only in the Plio-Pleistocene. Large 7–5 Ma silicic centers along the southern Afar and northeastern MER margins apparently formed along an E–W-oriented regional structural feature parallel to the already established southern escarpment of the Afar. The Addis Ababa rift embayment and the growth of 4.5–3 Ma silicic centers in the Addis Ababa area are attributed to the formation of a major cross-rift structure and its intersection with the same regional E–W structural trend. This study illustrates the episodic nature of rift development and volcanic activity in the MER–Afar transition zone, and the link between this activity and regional structural and tectonic features.  相似文献   

15.
东海丽水-椒江凹陷构造样式与含油气远景   总被引：4，自引：0，他引：4  
丽水-椒江凹陷位于东海陆架盆地台北坳陷西部,其沉降过程经历了裂陷期、坳陷期、抬升期及区域沉降期4个阶段。盆地构造样式复杂,具有半地堑-地堑式结构并存、频繁变化等特征,断裂构造发育,主要有铲式、坡坪式、多米诺式、"Y"型、阶梯状、地垒和地堑等断裂构造组合。构造活动有效控制油气圈闭的形成,灵峰构造带(潜山构造带)、丽水西次凹中央反转背斜构造带、丽水西次凹屋脊断块构造带、椒江凹陷中部断块半背斜构造带为该区未来勘探的有利区带。  相似文献   

16.
The Angareb Ring Dike Complex is located in Begemder Province in northwestern Ethiopia, 55 kilometers NNW of the provincial capital Gondar. The structure was first identified in 1973 from Landsat 1 satellite imagery analysis of the East African Rift System. The complex is exposed on the floor of the Tana Graben, a feature bearing structural affinity to the East African Rift. A post-early Miocene age is ascribed to the complex. Association with other as yet univestigated circular features may define a volcanic province of early continental rifting in Ethiopia. Contained within the ring structure is a heterogenous tuff breccia of over 100 cubic kilometers in volume, composed of massive agglomorates, ash flows, welded tuffs, and subaqueous tuffs. The tuff is interpreted as filling a subsiding caldron above an eviscerating magma chamber. Mineralogy and chemical analyses depict a fractionating basaltic magma of transitional-mildly alkaline composition. Partial melting of the Precambrian basement is envisioned to explain the subalkaline granites. Comparison of chemical data with regional petrochemical interpretations suggests that the complex represents the beginning of central vent activity in the waning stages of Tertiary volcanism in Ethiopia.  相似文献   

17.
Mikiya  Yamashita  Tetsuro  Tsuru  Narumi  Takahashi  Kaoru  Takizawa  Yoshiyuki  Kaneda  Kantaro  Fujioka  Keita  Koda 《Island Arc》2007,16(3):338-347
Abstract   The Parece Vela Basin (PVB), which is a currently inactive back-arc basin of the Philippine Sea Plate, was formed by separation between the Izu-Ogasawara Arc (IOA) and the Kyushu-Palau Ridge (KPR). Elucidating the marks of the past back-arc opening and rifting is important for investigation of its crustal structure. To image its fault configurations and crustal deformation, pre-stack depth migration to multichannel seismic reflection was applied and data obtained by the Japan Agency for Marine-Earth Science and Technology and Metal Mining Agency of Japan and Japan National Oil Corporation (Japan Oil, Gas and Metals National Corporation). Salient results for the pre-stack depth-migrated sections are: (i) deep reflectors exist around the eastern margin of KPR and at the western margin of IOA down to 8 km depth; and (ii) normal fault zones distributed at the eastern margin of the KPR (Fault zone A) and the western margin of the IOA (Fault zone B) have a total displacement of greater than 500 m associated with synrift sediments. Additional normal faults (Fault zone C) exist 20 km east of the Fault zone B. They are covered with sediment, which indicates deposition of recent volcanic products in the IOA. According to those results: (i) the fault displacement of more than 500 m with respect to initial rifting was approximately asymmetric at 25 Ma based on PSDM profiles; and (ii) the faults had reactivated after 23 Ma, based on the age of deformed sediments obtained from past ocean drillings. The age of the base sediments corresponds to those of spreading and rotation after rifting in the PVB. Fault zone C is covered with thick and not deformed volcanogenic sediments from the IOA, which suggests that the fault is inactive.  相似文献   

18.
Abstract Extensive subduction-related and intraplate volcanism characterize Cenozoic magmatism in the North Is., New Zealand. Volcanics in the central North Is., predominantly intermediate to felsic, form above the dipping seismic zone and show tectonic/geochemical features common to magmatism in most subduction zones. Basaltic volcanism in Northland, the northern part of the North Is., has chemical characteristics typical of intraplate magmatism and may be caused by the upwelling of asthenospheric materials from deeper parts of the mantle. The rifting just behind the present volcanic front (the Taupo-Rotorua Depression), which follows the trench ward migration of the volcanic front and the gradual steepening of the subducted slab, is also a feature of the North Is. A possible mechanism for the back-arc rifting in the area is injection of asthenospheric materials into the mantle wedge; this asthenospheric flow results from the mantle upwelling beneath Northland and pushes both the rigid fore-arc mantle wedge and the subducted slab trenchwards. This mechanism is also consistent with the stress fields in the North Is.: dilatation in Northland, northwest-southeast tension in the Taupo-Rotorua Depression, and the northeast-southwest compression in the fore-arc region.  相似文献   

19.
PeterD.  Clift & Jongman  Lee 《Island Arc》1998,7(3):496-512
The sedimentary sequences that accumulate around volcanic arcs may be used to reconstruct the history of volcanism provided the degree of along-margin sediment transport is modest, and that reworking of old sedimentary or volcanic sequences does not contribute substantially to the sediment record. In the Mariana arc, the rare earth and trace element compositions of ash layers sampled by Deep Sea Drilling Project (DSDP) site 451 on the West Mariana Ridge, and sites 458 and 459 on the Mariana Forearc, were used to reconstruct the evolution of the arc volcanic front during rifting of the Mariana Trough. Ion microprobe analysis of individual glass shards from the sediments shows that the glasses have slightly light rare earth element (LREE)-enriched compositions, and trace element compositions typical of arc tholeiites. The B/Be ratio is a measure of the involvement of subducted sediment in petrogenesis, and is unaffected by fractional crystallization. This ratio is variable over the period of rifting, increasing up-section at site 451 and reaching a maximum in sediments dated at 3–4 Ma, ∼ 3–4 million years after rifting began. This may reflect increased sediment subduction during early rifting and roll-back of the Pacific lithosphere. Parallel trends are not seen in the enrichment of incompatible high field strength (HFSE), large ion lithophile (LILE) or rare earth elements (REE), suggesting that flux from the subducting slab alone does not control the degree of melting. Re-establishment of arc volcanism on the trench side of the basin at ca 3 Ma resulted in volcanism with relative enrichment in incompatible REE, HFSE and LILE, although these became more depleted with time, possibly due to melt extraction from the mantle source as it passed under the developing back-arc spreading axis, prior to melting under the volcanic front.  相似文献   

20.
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.  相似文献   

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