Exhumation process of the Nago subduction-related metamorphic rocks, Okinawa, Ryukyu island arc

The Kunigami zone in Okinawa is an extension of the Shimanto zone, Japan. The rocks make up the main part of the Nago metamorphic rocks, and such metamorphic rocks are exceptional in the Shimanto zone. The Anne complex, in the older Motobu zone, is also metamorphosed. The reason for why and how this kind of the metamorphism occurred, and especially why and how the metamorphic rocks were exhumed, is yet uncertain and unresolved. To understand the metamorphic and exhumation process in Okinawa, a structural study is undertaken, and its relation to the Eocene ridge subduction is discussed. We believe exhumation was performed by formation of a D2 extrusion wedge, made up of the Nago metamorphic rocks. The base for this wedge is a subduction thrust, and the roof is a detachment fault. Internally, there exists another Kijoka detachment fault, which is a brittle low-angle fault with top to the northwest shear sense, and the D2 major recumbent folds and thrusts show top to the southeast opposite shear sense in the Kunigami zone. This is the first report that finds detachment faults from the typical and ancient accretionary complex. M2 is mostly retrograde related to exhumation, but its medium P/T-type prograde metamorphism, abnormal at subduction zones, represents a high thermal gradient during ridge subduction. As a result, this ridge subduction is responsible for exhumation. At the time of accretion of the Kunigami zone, D1 ductile contraction and constriction exhibited top to the southeast shear sense, but an opposite and extensional shear sense is recognized in the proto-wedge. During D1, the wedge had already been active and begun to exhume. M1 of the Miyagi complex is accretion related and also of medium P/T-type metamorphism, and is a consequence of Cretaceous ridge subduction without any ability to cause much exhumation.     

The Liuchiu Hsu island offshore SW Taiwan: tectonic versus diapiric anticline development and comparisons with onshore structures

A structural and microtectonic analysis performed in the Liuchiu Hsu island demonstrates that its Plio-Pleistocene tectonic evolution was dominated by alternating NW–SE shortening and local radial extension caused by mud diapirism. Previous models based on seismic data considering both the formation of the Liuchiu Hsu island and the fold development in SW Taiwan as mainly driven by mud diapirism, fail to account for both the asymmetry of the west vergent thrust-related anticlines onshore and the elongated character of the ridges formed by diapir alignments offshore, which rather argue in favour of a tectonic origin. To cite this article: O. Lacombe et al., C. R. Geoscience 336 (2004).     

History and modes of Mesozoic accretion in Southeastern Russia

Abstract The history of Mesozoic accretion and growth of the Asia eastern margin, occupied by Southeastern Russia, includes five main events; two main tectonic regimes were responsible for the growth of the continent. In the Triassic-Jurassic, Early Cretaceous and Late Cretaceous-Paleogene, the subduction of the oceanic lithosphere resulted in the formation of wide accretionary wedges of the Mongol-Okhotsk, Khingan-Okhotsk and Eastern Sikhote-Alin active continental margins, respectively. These stages of the comparatively slow growth of the continent were broken by stages of rapid growth and drastic changes in the shape of the continent, since at these stages large terranes of various tectonic nature collided with active continental margins. At the end of the Early-Middle Jurassic, the Bureya terranes collided with the Mongol-Okhotsk active margin, and at the beginning of the Late Cretaceous there was collision of the Central and Southern Sikhote-Alin terranes with the Khingan-Okhotsk active margin.
Collision-related structural styles in all cases are indicative of oblique collision and great strike-slip motions along the main sutures. The peculiarities of the terrane's geological structure show that prior to collision with the Mongol-Okhotsk and Khingan-Okhotsk active margins, they had already accreted to Asia and then migrated along its margins along the strike-slip faults. The Bureya terranes were squeezed out of the compression zone between Siberia and North China. This compression zone originated after the Paleozoic oceans which divided these cratons had closed. The Khanka terranes and Mesozoic accretionary wedge terranes of the Sikhote-Alin shifted along the strike-slip faults subparallel to the Asia Pacific margin. Strike-slip motions resulted in duplication of the primary tectonic zonation.     

The Keepit arc: provenance of sedimentary rocks in the central Tablelands Complex,southern New England Orogen,Australia, as recorded by detrital zircon

Detrital zircon from the Carboniferous Girrakool Beds in the central Tablelands Complex of the southern New England Orogen, Australia, is dominated by ca 350–320 Ma grains with a peak at ca 330 Ma; there are very few Proterozoic or Archean grains. A maximum deposition age for the Girrakool Beds of ca 309 Ma is identified. These data overlap the age of the Carboniferous Keepit arc, a continental volcanic arc along the western margin of the Tamworth Belt. Zircon trace-element and isotopic compositions support petrographic evidence of a volcanic arc provenance for sedimentary and metasedimentary rocks of the central Tablelands Complex. Zircon Hf isotope data for ca 350–320 Ma detrital grains become less radiogenic over the 30 million-year record. This pattern is observed with maturation of continental volcanic arcs but is opposite to the longer-term pattern documented in extensional accretionary orogens, such as the New England Orogen. Volcanic activity in the Keepit arc is inferred to decrease rapidly at ca 320 Ma, based on a major change in the detrital zircon age distribution. Although subduction continues, this decrease is inferred to coincide with the onset of trench retreat, slab rollback and the eastward migration of the magmatic arc that led to the Late Carboniferous to early Permian period of extension, S-type granite production and intrusion into the forearc basin, high-temperature–low-pressure metamorphism, and development of rift basins such as the Sydney–Gunnedah–Bowen system.     

台湾南部恒春海脊地质地球物理特征及其大地构造属性

恒春海脊的地质、地球物理特征与其相邻的南海海盆、吕宋火山弧和北吕宋海槽等地质构造单元所反映的特征明显不同 ,主要表现为低密度、弱磁性。推测其地壳性质为陆壳 ,是恒春半岛的南延部分。海脊西侧缓坡为陆缘增生楔 ,可见刺穿现象 ,这些刺穿构造是由逆冲挤压引起的泥底辟。海脊东部受弧陆碰撞的影响而急剧抬升。东部的弧陆碰撞是海脊抬升和增生楔形成的主要原因     

Wave coupling between the Tianshan orogen and the deformations in its south and north foreland basins in Cenozoic

Attention has been paid to the deformations in the south and north foreland basins of the Tianshan Mountains and their relations to the tectonic movements of the mountains. Based on the seismic data interpretation and field work,the tectonic features,tectonic styles,controlling factors of deformation,deformation time,and their relationships in the three tectonic units were comparably studied. The Ce-nozoic deformations in the two foreland basins were characterized by zonation from south to north,segmentation from west to east,and layering from deep to shallow. The tectonic styles are compres-sive ones,including both basement-involved and cover-detaching. The two foreland basins underwent several times of deformation in Cenozoic and the deformations were transmitted from the Tianshan orogen to the inner basin. The deformation dynamics of the south and north foreland basins of the Tianshan Mountains can be represented by dynamics of orogenic wedges. In terms of the deformations of the wedges with wave-like features,the concept of a wave orogenic wedge was put forward,and a double wave orogenic wedge model was established.     

带有楔形轨道的滑移支座力学性能数值模拟研究

针对结构隔震装置在较大的倾覆力作用下抗拉能力不足的问题,提出了一种带有楔形分离式轨道的新型滑移支座。新支座能够承受上部结构传递的荷载以及适应较大水平位移,同时利用楔形轨道与楔形滑块之间的相互作用提供抗拉能力。采用通用有限元程序ABAQUS建立了支座的三维实体有限元模型,对其抗压、抗拉、压剪和拉剪力学性能进行数值模拟研究,同时考虑了楔形角度的改变对支座力学性能的影响。通过分析表明：在竖向新支座具有较好的抗压和抗拉能力,抗拉性能受楔形角度的影响较大,角度大于60°时,支座的抗拉能力随着角度的减小不断提升;而当角度减小到60°时,楔形滑块与上部连接部分出现薄弱点,导致支座的抗拉能力开始降低。在压剪与拉剪的作用下,支座水平向的滞回曲线比较饱满,具有较好的水平向摩擦耗能能力且能够适应较大的水平位移。     

OCEANIC DEPOSITS IN THE YARLUNG—TSANGPO SUTURE ZONE: STRUCTURAL SETTING, RADIOLARIAN AGES AND THEIR TECTONIC IMPLICATIONS

OCEANIC DEPOSITS IN THE YARLUNG—TSANGPO SUTURE ZONE: STRUCTURAL SETTING, RADIOLARIAN AGES AND THEIR TECTONIC IMPLICATIONS     

楔形物体在静水与波浪中自由入水砰击试验研究

通过自由落体的入水方式,分别在静水和规则波中开展了两种不同横剖面的曲面楔形体入水砰击问题试验研究。使用高速摄像系统记录楔形体入水过程流场演变和运动特性,采用加速度传感器和压力传感器进行数据的动态采集。试验结果表明,在静水中入水时,外凸剖面楔形体入水砰击后模型两侧的射流飞溅比反曲剖面更剧烈,而在楔形体前端的水面以下部分形成的气腔更小;在规则波中入水时,对于相同模型,在波峰和上跨零点相位下模型入水砰击后两侧的射流飞溅比在波谷相位更剧烈。相同工况时,反曲剖面模型所受砰击的加速度峰值和压力峰值更小;在相同的入水速度下,对于相同模型,波浪载荷和砰击载荷的共同作用会使模型所受砰击压力显著增大。     

水库对海岸带海水入侵影响的数值模拟研究

修建海岸水库是满足滨海地区淡水资源的保障的一个重要方式,为了进一步分析海岸水库的建立对滨海含水层的海水入侵的影响,采用SUTRA(saturated-unsaturatedtransport)模型建立具有代表性的二维海岸模型开展定量和定性研究,结果表明:潮汐作用抑制底层盐水楔的入侵,加快海陆物质交换;实例中计算表明建坝后,海岸水库坝体的建立加剧了海水入侵;水库水位在平均海平面以下时,潮汐动力因素占主导,海水入侵严重且随着水位抬高,海水入侵程度基本保持不变,应注意海水入侵的防治;库区水位在平均海平面以上时,海水入侵程度随着库区水位的抬高而减少,可以调节水库水位进行滨海含水层的海水入侵的防控;海岸水库在建设中存在一个控制水位,当水库水位在控制水位以上,库区水体盐度和含水层淡水储量能得到保证;库区高水位时内陆物质运移时间较长,运移区域较广,氮类污染物易在库底含水层堆积,需重视含水层的地下水污染防治。