Tectonic evolution of the ‘Liguride’ accretionary wedge in the Cilento area,southern Italy: A record of early Apennine geodynamics

The early stages of southern Apennine development have been unraveled by integrating the available stratigraphic record provided by synorogenic strata (of both foredeep and wedge-top basin environments) with new structural data on the Liguride accretionary wedge cropping out in the Cilento area, southern Italy. Our results indicate that the final oceanic subduction stages and early deformation of the distal part of the Apulian continental margin were controlled by dominant NW–SE shortening. Early Miocene subduction-accretion, subsequent wedge emplacement on top of the Apulian continental margin and onset of footwall imbrication involving detached Apulian continental margin carbonate successions were followed by extensional deformation of the previously ‘obducted’ accretionary wedge. Wedge thinning also enhanced the development of accommodation space, filled by the dominantly siliciclastic Cilento Group deposits. The accretionary wedge units and the unconformably overlying wedge-top basin sediments experienced renewed NW–SE shortening immediately following the deposition of the Cilento Group (reaching the early Tortonian), confirming that the preceding wedge thinning represented an episode of synorogenic extension occurring within the general framework of NW–SE convergence. The documented Early to the Late Miocene steps of southern Apennine development are clearly distinct with respect to the subsequent (late Tortonian-Quaternary) stages of fold and thrust belt evolution coeval with Tyrrhenian back-arc extension, which were characterized by NE-directed thrusting in the southern Apennines.     

Strymon and Strymonikos Gulf basins (Northern Greece): Implications on their formation and evolution from faulting

In Central and Eastern Macedonia of Northern Greece large NW–SE trending basins filled up mainly with terrestrial sediments developed during the Neogene over the Alpine basement rocks. Among them, the Strymon basin was established along the NNW–SSE trending Strouma/Strymon Lineament which formed over the tectonic boundary of the Serbomacedonian and Rhodope massifs, both representing the hinterland of the Hellenic orogen. The present study suggests that the Strymon basin was not formed as a syn-detachment basin over the Strymon Valley Detachment Fault, considered to have caused exhumation of the Rhodope massif metamorphic complex. Instead, transpressional s.l. tectonics dominated the region in the Late Oligocene-Early Miocene and it progressively changed into a wrench tectonics under which the Strymon basin has been initiated in the Middle Miocene. The basin continued to develop further under a short-lived NW–SE extension in the Middle-Late Miocene. The whole deformation is attributed to the late-stage collisional processes between the Apulia and Eurasia plates. The prevalent NE–SW extension has been constrained later on in the Late Miocene and Pliocene times activating both low-angle and high-angle NW–SE trending faults and causing the regional tilting towards the SW of the mountain fault blocks (i.e., mountain chains). From Quaternary onwards, the Strymon basin has been separated from the Strymonikos Gulf basin due to an N–S extension that mainly activates E–W striking normal faults.     

The interaction between regional and local tectonics during resurgent doming: the case of the island of Ischia, Italy

The volcanic island of Ischia is located on the Tyrrhenian margin of Central Italy, characterized by Plio-Quaternary NW–SE- and NE–SW-trending extensional fractures. Ischia displays a resurgent dome uplifted by at least 800 m in the last 33 ka. Remote sensing and field data have been collected to study the structural setting of the island, the deformation pattern associated with resurgence and the superimposition of the regional and the resurgence-induced stress fields. NW–SE and NE–SW extensional fracture systems predominate throughout the island and around the resurgent block, suggesting a relationship with the regional extensional structures. These systems were formed before resurgence and were partly reactivated during resurgence. The reactivation of pre-existing regional systems during resurgence confined the extent of the uplifted area. N–S- and E–W-trending systems have been found exclusively at the borders of the dome and are interpreted as being induced by resurgence. The topmost resurgent block shows an octagonal shape in map view and is tilted at an angle of 15° around a NE–SW-trending horizontal axis; the block is partly bordered by high-angle, inward-dipping regional faults. More than 90% in volume of the volcanic products coeval with resurgence on Ischia have been erupted outside the resurgent block area, suggesting that the resurgence process locally replaced volcanic activity in the last 33 ka.     

The significance of tectonic-related tertiary remagnetization along the margins of the Valencia trough

The paleomagnetic data from the margins of the Valencia Trough are derived from Mesozoic and Tertiary rocks from the Balearic Islands, Catalan Coastal Ranges and Eastern Iberian Chain. These rocks are affected by a complex structural evolution consisting of an initial compressive stage followed by one of extension. Cenozoic paleomagnetic data indicate that rotations occurred during the Paleogene compression and before the extension started (Lower Miocene) in the Catalan Coastal Ranges. In contrast, in the Balearic Islands the rotations are synchronous to both compressional (Late Oligocene-Middle Miocene) and extensional tectonics (post-Middle Miocene). In both areas the Mesozoic limestones are remagnetized. In the Catalan Coastal Ranges they display the same direction as the Paleogene syn-compressive deposits whereas in the Balearic Islands they conform with Lower Miocene (Burdigalian) syncompressive rocks. It is concluded that the processes of remagnetization that affected eastern Iberia are related to a compressive rather than an extensional tectonics regime     

Termination of intra‐arc rifting at ca 16 Ma in the Southwest Japan arc: The tectonostratigraphy of the Hokutan Group

Miocene intra‐arc rifting associated with the opening of the Japan Sea formed grabens in several areas in Southwest (SW) Japan, but the extensional tectonics of the arc are still not well understood. In this study, we first document the tectonostratigraphy of the Hokutan Group in the northwestern part of the Kinki district, and demonstrate the termination of extensional tectonics at ca 16.5 Ma, as inferred from grabens in the lower part of the group being unconformably overlain by sediments of the upper part. Second, we review early Miocene grabens in SW Japan to suggest that intra‐arc rifting was abandoned at ca 16 Ma, essentially simultaneously with the end of rotation of the SW Japan arc as evidenced by paleomagnetic studies. The lesser numbers of grabens and reduced thicknesses of graben fills suggest that extensional deformation of the SW Japan arc was significantly weaker than that of the Northeast (NE) Japan arc, which was broken into blocks, indicating various degrees of paleomagnetic rotation within NE Japan. The weak deformation has allowed paleomagnetic studies to infer the coherent rotation of the SW Japan arc.     

Late Cenozoic faulting in SW Bulgaria: Fault geometry,kinematics and driving stress regimes. Implications for late orogenic processes in the Hellenic hinterland

We investigate the geometry and kinematics of the faults exposed in basement rocks along the Strouma River in SW Bulgaria as well as the sequence of faulting events in order to place constraints on the Cenozoic kinematic evolution of this structurally complex domain. In order to decipher the successive stress fields that prevailed during the tectonic history, we additionally carried out an analysis of mesoscale striated faults in terms of paleostress with a novel approach. This approach is based on the P–T axes distribution of the fault-slip data, and separates the fault-slip data into different groups which are characterized by kinematic compatibility, i.e., their P and T axes have similar orientations. From these fault groups, stress tensors are resolved and in case these stress tensors define similar stress regimes (i.e., the orientations of the stress axes and the stress shape ratios are similar) then the fault groups are further unified. The merged fault groups after being filled out with those fault-slip data that have not been incorporated into the above described grouping, but which present similar geometric and kinematic features are used for defining the final stress regimes. In addition, the sequence of faulting events was constrained by available tectonostratigraphic data.Five faulting events named D1, D2, D3, D4 and D5 are distinguished since the Late Oligocene. D1 is a pure compression stress regime with σ₁ stress axis trending NNE-SSW that mainly activated the WNW-ESE to ENE-WSW faults as reverse to oblique reverse and the NNW-SSE striking as right-lateral oblique contractional faults during the Latest Oligocene-Earliest Miocene. D2 is a strike-slip − transpression stress regime with σ₁ stress axis trending NNE-SSW that mainly activated the NNW-SSE to N-S striking as right-lateral strike-slip faults and the ENE-WSW striking faults as left-lateral strike-slip ones during the Early-Middle Miocene. D3 extensional event is associated with a NW-SE to WNW-ESE extension causing the activation of mainly low-angle normal faults of NE-SW strike and NNE-SSW to NNW-SSE striking high-angle normal faults. D4 is an extensional event dated from Late Miocene to Late Pliocene. It activated NNW-SSE to NW-SE faults as normal faults and E-W to WNW-ESE faults as right-lateral oblique extensional faults. The latest D5 event is an N-S extensional stress regime that dominates the wider area of SW Bulgaria in Quaternary times. It mainly activated faults that generally strike E-W (ENE-WSW and WNW-ESE) normal faults, along which fault-bounded basins developed. The D1 and D2 events are interpreted as two progressive stages of transpressional tectonics related to the late stages of collision between Apulia and Eurasia plates. These processes gave rise to the lateral extrusion of the Rhodope and Balkan regions toward the SE along the Strouma Lineament. The D3 event is attributed to the latest stage of this collision, and represents the relaxation of the overthickened crust along the direction of the lateral extrusion. The D4 and D5 events are interpreted as post-orogenic extensional events related to the retreat of the Hellenic subduction zone since the Late Miocene and to the widespread back-arc Aegean extension still prevailing today.     

沿107.6°E南北向剖面鄂尔多斯地块及周缘地区地壳结构

鄂尔多斯地块紧邻青藏高原东北缘,位于华北克拉通的西部,在我国中生代、新生代以来东部地区的构造活动中起到了重要作用.对鄂尔多斯及其周缘地区的研究可以提供有关华北克拉通的形成、演化和破坏过程的重要信息.本文选取了纵贯鄂尔多斯的107.6°E附近南北剖面上的44个流动地震台站进行分析,采用接收函数方法,进行Kirchhoff偏移成像,并且结合在该区域内前人的地震面波频散进行联合反演,获得剖面下方的地壳内部精细结构.研究结果显示:(1)莫霍面在鄂尔多斯北部较平缓,约45km深;在鄂尔多斯南部有所加深,达到50km;其北边的河套盆地的地壳厚度约为50km;南边的渭河盆地到秦岭地区及四川盆地的地壳厚度从约为40km增厚到47～50km.(2)河套盆地下方存在大规模的低速异常,最深可达25km,反映了其显著的拉张构造和沉积历史.(3)秦岭造山带下方的低速异常对应于其主要为长英质的地壳组分,可能是由于中生代的拆沉作用导致的地壳下部基性岩石层的缺失.(4)以38°N为界的鄂尔多斯地块,南北部地壳速度结构存在差异,可能表明了这两部分经历的构造历史不同.     

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.     

南北地震带北段近年地壳水平运动

采用中国地震局陆态网络数据,解算并对比南北地震带北段速度场、速度投影、基线时间序列。对比发现：①各点速度矢量呈顺时针旋转;②相对于鄂尔多斯块体,2011-2015年银川地堑各站（银川、盐池、中卫、海原等）沿NNW方向扩张;③祁连断裂表现为左旋剪切和逆冲挤压,2013年起闭锁加强;海原断裂表现为稳定的左旋和逆冲,2013年起剪切加速;六盘山断裂表现为持续并放缓的逆冲和较弱的右旋;西秦岭北缘断裂带西段呈高速且放缓的逆冲以及高速但低应变累积的左旋,东段保持高速稳定的左旋和持续逆冲。     

Structural features of the shallow plumbing system of Vulcano Island Italy

In this study, we integrate information gathered from surface geology and tectonics with the results of a shallow (0–2 km b.s.l.) seismic tomography of Vulcano Island (Italy), obtained from the analysis of local earthquakes. The observed low Vp regions correspond to caldera filling products, mainly consisting of pyroclastics, tuffs, lava flows and hyaloclastites. High-velocity anomalies represent intrusive bodies. The striking correspondence between the stratigraphy from deep wells and the calculated velocity structure allows us to reconstruct the geometry and distribution of a main intrusion and to recognize some intra-caldera depressions. The shape and location of the high and low Vp anomalies are consistent with NW–SE and N–S strikes. Eruptive centres younger than 42 kyr, as well as the structural depressions of Vulcano and of the neighbour Lipari Island, align along a N–S direction. The combined interpretation of the available structural data and of the results from the tomography suggests that magmatic reservoirs of Vulcano at shallow depth (>0.5 km) align along a NW–SE strike but their shape is controlled by N–S striking normal faults and/or cracks that accommodate the right-lateral movements of the NW–SE strike-slip fault system.Editorial responsibility: T. Druitt     

南海地球物理场特征及基底断裂体系研究

南海海域主体可划分为南海北缘、中西沙、南沙南海海盆四块，各块具有明显不同的重磁场特征。反演得到的莫霍面总体趋势由陆向洋抬升，反映陆壳、拉伸陆壳、过渡壳、洋壳的分布。东沙高磁异常含一定的高频成份，与新生代玄武岩及中生代岩浆岩有关，而其低频成份可能反映了发育的下地壳高速层，南海海域断裂极为发育，可分为北东向断裂组、东西向断裂组、北西向断裂组和南北向断裂组，南海北缘、南缘均以北东向张性断裂与北西向张剪性、剪性断裂为主要格架，形成了、南北分带、东西分块”构造格局。     

Late Quaternary deformation on the island on Pantelleria: New constraints for the recent tectonic evolution of the Sicily Channel Rift (southern Italy)

Structural observations carried out on the volcanic Island of Pantelleria show that the tectonic setting is dominated by NNE trending normal faults and by NW-striking right-lateral strike-slip faults with normal component of motion controlled by a ≈N 100°E oriented extension. This mode of deformation also controls the development of the eruptive fissures, dykes and eruptive centres along NNE–SSW belts that may thus represent the surface response to crustal cracking with associated magma intrusions. Magmatic intrusions are also responsible for the impressive vertical deformations that affect during the Late Quaternary the south-eastern segment of the island and producing a large dome within the Pantelleria caldera complex. The results of the structural analysis carried out on the Island of Pantelleria also improves the general knowledge on the Late Quaternary tectonics of the entire Sicily Channel. ESE–WNW directed extension, responsible for both the tectonic and volcano-tectonic features of the Pantelleria Island, also characterizes, at a greater scale, the entire channel as shown by available geodetic and seismological data. This mode of extension reactivates the older NW–SE trending fault segments bounding the tectonic troughs of the Channel as right-lateral strike-slip faults and produces new NNE trending pure extensional features (normal faulting and cracking) that preferentially develop at the tip of the major strike-slip fault zones. We thus relate the Late Quaternary volcanism of the Pelagian Block magmatism to dilatational strain on the NNE-striking extensional features that develop on the pre-existing stretched area and propagate throughout the entire continental crust linking the already up-welled mantle with the surface.     

Research on the dynamics of the South China Sea opening:Evidence from analogue modeling

Independent of Indochina extrusion, the South China Sea experienced a process from passive continental rifting to marginal sea drifting. According to the fault patterns in the Beibu Gulf basin and the Pearl River Mouth basin, the continental rifting and early spreading stage from 32 to 26 Ma were controlled by extensional stress field, which shifted clockwise from southeastward to south southeastward. From 24 Ma on, the sea spread in NW-SE direction and ceased spreading at around 15.5 Ma. Integrated geological information with the assumption that the South China Sea developed along a pre-Cenozoic weakness zone, we did analogue experiments on the South China Sea evolu- tion. Experiments revealed that the pre-existing weakness zone goes roughly along the uplift zone between the present Zhu-1 and Zhu-2 depression. The pre-existing weakness zone is composed of three segments trending NNE, roughly EW and NEE, respectively. The early opening of the South China Sea is accompanied with roughly 15° clockwise rotation, while the SE sub-sea basin opened with SE extension. Tinjar fault was the western boundary of the Nansha block (Dangerous Ground), while Lupar fault was the eastern boundary of the Indochina, NW-trending rift belt known as Zengmu basin developed between above two faults due to block divergent of Indochina from Nansha. In the experiment, transtensional flower structures along NW-trending faults are seen, and slight inversion occurs along some NE-dipping faults. The existence of rigid massifs changed the orientations of some faults and rift belt, and also led to deformation concentrate around the massifs. The rifting and drifting of the South China Sea might be caused by slab pull from the proto South China Sea subducting toward Borneo and/or mantle flow caused by India-Asia collision.     

Rapid Exhumation in the Alpine Belt of the Betic-Rif (W Mediterranean): Tectonic Extrusion

—Extreme cooling rates (500 °C/m.y.) during the late stage, 22–18 Ma, orogenic evolution of the Alpine Betic-Rif belt are suggested to result from rapid exhumation caused by tectonic extrusion and concomitant extensional tectonics. The extrusional/extensional tectonic setting is controlled by the SW-NE trending break-off scar left in the lithosphere of the Alborán Sea and SE Spain after detachment of a lithospheric slab. The extruded material represents the collisional crustal nappe pile (together with fragments of underlying mantle, such as the Ronda peridotites) and the cause of the extrusion is the thermal softening within the crustal section during and after collision. The extrusion/extension took place under the influence of a NW-SE directed compressive regime, perpendicular to the collisional belt. At the same time the sub-lithospheric mantle still showed the E-W compressive regime of the collisional stage. The Alpine tectono-metamorphic evolution of the Betic-Rif belt in the W Mediterranean thus comprises two main stages: (1) continental collision with formation of primary nappes and high-pressure metamorphic parageneses, (2) tectonic extrusion with vertically directed tectonics (high pressure, very rapid decompression) and extensional tectonics with roughly horizontal, lateral transport and final emplacement of the extruded mélange in the form of a stack of detachment sheets (low pressure, very rapid cooling). This model for the Betic-Rif may offer important constraints to all rapidly exhumed convergent terranes.     

鄂尔多斯地块北部及邻区Pn波速度结构与各向异性

利用鄂尔多斯地块北部及其邻区2008—2018年期间固定台网的地震波形记录,手动拾取出高质量的Pn波到时资料,反演获得了研究区上地幔顶部的Pn波速度结构及各向异性。结果表明:鄂尔多斯地块北部及其邻区上地幔顶部的Pn波速度存在明显的横向不均匀性,与区域地质构造和地震活动相关;研究区内平均Pn波速为8.18 km/s,鄂尔多斯地块内部表现出大范围的高速异常,阿拉善地块的高速异常体中存在低速异常现象,河套断陷带、阴山—燕山造山带、银川—吉兰泰断陷带和海原—六盘山弧形断裂带区域均表现为显著的低速异常,河套断陷带下方存在向鄂尔多斯地块内部延伸的明显低速异常条带,大同火山群下方存在强低速异常;多数历史强震均发生在低速异常区或高低速异常过渡带上;鄂尔多斯地块内部Pn波各向异性快波方向西部为近NE?SW向,而东部为近NW?SE向,河套断陷带和鄂尔多斯地块西缘、青藏高原东北缘与阿拉善地块的交界带以及阴山—燕山造山带的各向异性快波方向总体均呈现为NW?SE向,而阴山—燕山造山带东部则呈NE?SW向。      

No significant post-Eocene rotation of the Moesian Platform and Rhodope (Bulgaria): Implications for the kinematic evolution of the Carpathian and Aegean arcs

The region located between the Carpathian–Balkan and Aegean arcs, the Moesian Platform and Bulgarian Rhodope, is generally assumed to have been stably attached to the East European craton during the Cenozoic evolution of these arcs. The kinematic evolution of this region is, however, poorly constrained by paleomagnetic analysis. In this paper we provide new paleomagnetic data (800 volcanic and sedimentary samples from 12 localities) showing no significant post-Eocene rotation of the Moesian platform and Rhodope with respect to Eurasia, therefore confirming the stability of this region. We compare this result to a provided review of paleomagnetic data from the South Carpathians (Tisza block) and the Aegean region. The Tisza block underwent 68.4 ± 16.7° of middle Miocene ( 15–10 Ma) clockwise rotation with respect to the Moesian Platform, in line with previous rotation estimates based on structural geology. The stability of the Moesian platform during middle Miocene eastward emplacement of the Tisza block into the Carpathian back-arc supports dextral shear along the Southern Carpathians recorded by 13–6 Ma clockwise strike-slip related rotations in foreland deposits. The new reference direction for the Moesian platform and Rhodope allows accurate quantification of the rotation difference with the west Aegean domain at 38.0 ± 7.2° occurring between 15 and 8 Ma. To accommodate this rotation, we propose that the pivot point of the west-Aegean rotation was located approximately in the middle of the rotating domain rather than at the northern tip as previously proposed. This new scenario predicts less extension southeast of the pivot point, in good agreement with estimates from Aegean structural geology. Northwest of the pivot point, the model requires contraction or extrusion that can be accommodated by the coeval motion of the Tisza Block around the northwestern edge of the Moesian platform.     

应用综合震源机制解法推断鄂尔多斯块体周缘现今地壳应力场的初步结果

利用2007年8月1日至2013年7月21日发生在鄂尔多斯块体周缘的8499个地震的49844个P波初动符号资料,应用综合震源机制解法获得了鄂尔多斯块体周缘0.25°×0.25°的精细地壳应力场,所得应力场结果基本上覆盖了整个鄂尔多斯周缘地区.研究结果表明鄂尔多斯周缘地壳应力场具有以下特征:(1)在环绕鄂尔多斯周缘的银川—吉兰泰断陷带、河套断陷带、岱海断陷带、山西断陷带和渭河断陷带内,综合震源机制解结果以正断层型为主,且综合震源机制解节面走向大体与控制断陷带边界的主要断裂走向相一致,与鄂尔多斯周缘断陷带现今的拉张状态相一致.(2)在鄂尔多斯西南缘,综合震源机制解类型主要为逆冲、逆冲走滑和走滑型,反映了鄂尔多斯块体在西南缘受到青藏高原北东向挤压作用.鄂尔多斯西南缘的应力场的主压应力方向在远处为东向,源自于青藏高原向东北挤压作用,靠近鄂尔多斯块体表现为北东—南西向.(3)P轴方位在局部地区变化较大,但总体呈现规律性变化.P轴方位在鄂尔多斯块体西缘,从南向北,主压应力轴方位更加偏北;在其北缘,由西向东,主压应力轴方位更加偏东.在其南缘和东缘,主压应力轴方位变化不大,大体上平行于控制各断陷带主要断裂走向.P轴倾角在西南缘为近水平,在其周缘各盆地内P轴倾角近直立.(4)T轴方位总体表现为北西—南东向;在鄂尔多斯周缘各断陷带内,T轴走向大体与控制断陷带主要断裂走向以及断陷盆地走向相垂直.(5)鄂尔多斯块体在其西南角受到来自青藏高原的北东向挤压和其东北角深部物质上涌形成的北西—南东向拉张力联合作用,上述作用使得鄂尔多斯块体周缘地区除西南区为挤压区外,其余区域均为剪切拉张区,与先前研究认为鄂尔多斯周缘地区处于引张应力场作用相符合,较好地解释了环鄂尔多斯周缘的断陷盆地构造,亦符合鄂尔多斯块体东西两侧的右旋剪切拉张带以及南北两侧的左旋剪切拉张带的认识.     

冲绳海槽弧后张裂构造及其动力机制讨论

在总结、讨论冲绳海槽弧后张裂构造的地形地貌、地球物理场和动力背景特征的基础上,归纳出海槽北、中、南三段之间的差异,以及它们在陆坡、槽底、岛坡上的不同表现特征.槽底雁行排列的地堑及断层斜切入陆坡,伴随的断块隆眷作用往南加强,使得钓鱼岛一赤尾屿隆起带区别于其北边的陆架外缘隆起带,吕宋岛向台湾的碰撞挤压引起的旋张活动加强了海槽南段的地壳拉张,从北往南岛坡侧都可以追踪出双列岛孤特性.海槽内部构造现象不能为笼统的构造动力背景所能完全解释,需要对各个典型中央地堑的具体深入探测研究.鉴于地堑、地垒、断层以及它们所界定的构造单元和它们内部发生的岩浆作用各有特点,最后讨论了各个地段中央地堑的构造属性、动力要素的差异及变化规律,以期为海槽构造动力的进一步深入研究提供借鉴参考.     

鄂尔多斯块体西缘地壳介质各向异性:从银川地堑到海原断裂带

通过收集鄂尔多斯块体西缘固定地震台网2010年6月至2017年8月的近场地震资料,选择符合剪切波分裂分析的14个台站记录的共137个有效事件波形,得到了剪切波分裂参数,即快剪切波（简称快波）偏振方向和慢剪切波（简称慢波）时间延迟.结果表明,研究区的快波偏振方向和慢波时间延迟具有明显的分区特征,快波偏振方向主要与构造应力场方向或者断层走向大体一致.鄂尔多斯西缘紧邻块体边界的台站,快波偏振方向自北向南呈现NS、NNE、NE向的变化,与青藏高原东北缘主压应力方向变化基本一致.银川地堑东西两侧的快波偏振方向有差异,东侧区域主要受青藏高原NNE向挤压和黄河-灵武断裂共同影响,而西侧区域可能受到阿拉善块体与鄂尔多斯块体之间的NW方向的主张应力和阿拉善块体内部应力分布的影响;鄂尔多斯块体、阿拉善块体与青藏高原的交汇区快波优势偏振方向为NE向,与青藏高原东北缘主压应力方向一致;海原断裂带及以南区域快剪切波优势偏振方向为WNW向,与断裂走向基本一致,较好的说明了海原断裂带为活跃的活动断裂.构造与断裂分布都是控制快波偏振方向的主要因素,走滑断裂上的台站快波偏振方向与断裂走向一致,表明这些台站主要受到断裂的强烈影响;走滑断裂附近的个别台站快波偏振方向呈现与构造应力场一致的方向,表明几乎没有受到断裂的影响.鄂尔多斯、阿拉善与青藏高原的交汇区平均时间延迟高于其他地区,反映了青藏高原在NE向运动过程中,受到稳定的鄂尔多斯块体阻挡作用,导致了交汇区地壳介质各向异性程度增加.以海原断裂带到六盘山断裂带为界,其两侧区域的各向异性差异性明显,揭示了应力与介质特性的差异,暗示其邻近区域,特别在海原断裂带东端到六盘山断裂带与鄂尔多斯块体西缘交汇区域,可能有较高的强震危险背景.本研究还对该区域的地壳和上地幔的耦合问题进行了初步讨论.     

Possible Miocene rifting of the Izu–Ogasawara (Bonin) arc deduced from magnetic anomalies

A magnetic anomaly map of the northern part of the Philippine Sea plate shows two conspicuous north–south rows of long-wavelength anomalies over the Izu–Ogasawara (Bonin) arc, which are slightly oblique to the present volcanic front. These anomalies are enhanced on reduced-to-pole and upward-continued anomaly maps. The east row is associated with frontal arc highs (the Shinkurose Ridge), and the west row is accompanied by the Nishi-Shichito Ridge. Another belt of long-wavelength anomalies very similar to the former two occurs over the Kyushu–Palau Ridge. To explain the similarity of the magnetic anomalies, it is proposed that after the spreading of the Shikoku Basin separated the Izu–Ogasawara arc from the Kyushu–Palau Ridge, another rifting event occurred in the Miocene, which divided the Izu–Ogasawara arc into the Nishi-Shichito and Shinkurose ridges. The occurrence of Miocene rifting has also been suggested from the geology of the collision zone of the Izu–Ogasawara arc against the Southwest Japan arc: the Misaka terrain yields peculiar volcanic rocks suggesting back-arc rifting at ～ 15 Ma. The magnetic anomaly belts over the Izu–Ogasawara arc do not extend south beyond the Sofugan Tectonic Line, suggesting a difference in tectonic history between the northern and southern parts of the Izu–Ogasawara arc. It is estimated that the Miocene extension was directed northeast–southwest, utilizing normal faults originally formed during Oligocene rifting. The direction is close to the final stage of the Shikoku Basin spreading. On a gravity anomaly relief map, northeast–southwest lineaments can be recognized in the Shikoku Basin as well as over the Nishi-Shichito Ridge. We thus consider that lines of structural weakness connected transform faults of the Shikoku Basin spreading system and the transfer faults of the Miocene Izu–Ogasawara arc rifting. Volcanism on the Nishi-Shichito Ridge has continued along the lines of weakness, which could have caused the en echelon arrangement of the volcanoes.