Kurosegawa Terrane as a Transform Fault Zone in Southwest Japan

The Kurosegawa Terrane intervening in the Jurassic-Early Cretaceous accretionary complexes along the Pacific side of the SW Japanese Islands is a serpentinite mélange zone. It contains various kinds of exotic rocks, for example, granitoids, metamorphic rocks, Siluro-Devonian deposits and is intimately associated with Cretaceous forearc basin deposits. The terrane is regarded as a key to clarify the Mesozoic geotectonic history of the western circum-Pacific orogenic belts. The current model, in which the formation of the Kurosegawa Terrane is attributed to nappe-movement or sinistral strike-slip faulting, can explain neither the mode of occurrence of the Kurosegawa Terrane we observed in eastern Kii Peninsula nor the array of evidence obtained from the Ryoke Terrane southward to the Shimanto Terrane. We suggest a new hypothesis in which the Kurosegawa Terrane was a transform fault zone that originated because of oceanic ridge subduction along the southern margin of the coeval accretionary prism (Butsuzo T.L.) in the late Early Cretaceous. Our model is mainly based on new geological evidence from the Kurosegawa Terrane in eastern Kii Peninsula where the deepest erosion level is exposed due to neotectonic uplift.     

Evolution of an accretionary complex along the north arm of the Island of Sulawesi, Indonesia

Abstract Seismic reflections across the accretionary prism of the North Sulawesi provide excellent images of the various structural domains landward of the frontal thrust. The structural domain in the accretionary prism area of the North Sulawesi Trench can be divided into four zones: (i) trench area; (ii) Zone A; (iii) Zone B; and (iv) Zone C. Zone A is an active imbrication zone where a decollement is well imaged. Zone B is dominated by out‐of‐sequence thrusts and small slope basins. Zone C is structurally high in the forearc basin, overlain by a thick sedimentary sequence. The subducted and accreted sedimentary packages are separated by the decollement. Topography of the oceanic basement is rough, both in the basin and beneath the wedge. The accretionary prism along the North Sulawesi Trench grew because of the collision between eastern Sulawesi and the Bangai–Sula microcontinent along the Sorong Fault in the middle Miocene. This collision produced a large rotation of the north arm of Sulawesi Island. Rotation and northward movement of the north arm of Sulawesi may have resulted in southward subduction and development of the accretionary wedge along North Sulawesi. Lateral variations are wider in the western areas relative to the eastern areas. This is due to greater convergence rates in the western area: 5 km/My for the west and 1.5 km/My for the east. An accretionary prism model indicates that the initiation of growth of the accretionary prism in the North Sulawesi Trench occurred approximately 5 Ma. A comparison between the North Sulawesi accretionary prism and the Nankai accretionary prism of Japan reveals similar internal structures, suggesting similar mechanical processes and structural evolution.     

Fluid flow in the Sicilian accretionary wedge: Primary geochemical signatures or recrystallization mask

Veins in the Sicilian accretionary wedge were studied petrographically and geochemically with the aim to investigate the relation between fluid flow in a décollement horizon and in overlying Mesozoic basinal sediments. Fluids expelled along the décollement horizon precipitated calcite cements that show a broad spread in stable isotope signatures and that generally have rather high Fe and Mn content. The fluids most likely originated from mixing of hot deep metamorphic fluids and dewatering of the clay unit along which the principal overthrusting occurred.Synkinematic veins in the overlying basinal units are cemented with calcite. The trace element content and δ¹³C signatures of these veins are host-rock dependent, pointing to a host-rock buffering effect. Petrographic observations indicate that calcite cements have been recrystallized. Thus the cements could have inherited their geochemical signatures from the host-rock during recrystallization. This is also supported by their δ¹⁸O signature, which is controlled by temperature fractionation.     

The complex age of orthogneiss protoliths exemplified by the Eoarchaean Itsaq Gneiss Complex (Greenland): SHRIMP and old rocks

Field studies integrated with cathodoluminescence petrography and SHRIMP U–Pb dating of zircons from >150 orthogneisses and metatonalites from the Eoarchaean Itsaq Gneiss Complex (southern West Greenland) shows that only a minority contain ≥3840 Ma zircons, whereas the majority carry only younger ones. Rocks containing ≥3840 Ma zircons vary from very rare single-phase metatonalites to more common complexly banded tonalitic migmatites. The former metatonalites have simple oscillatory-zoned ≥3840 Ma zircon with limited recrystallisation and overgrowth, whereas the more common migmatites have much more complicated zircon populations with both ≥3840 Ma and 3650–3600 Ma oscillatory-zoned zircon, more extensive recrystallisation and widespread complex core-rim multiple growth relationships.     

含金磷酸盐矿物的发现及其地质意义 ——以山东牟平腊子沟金矿为例

山东牟平腊子沟金矿位于华北板块苏鲁造山带胶南-威海隆起区,牟平-乳山金矿成矿带中段,区内岩浆活动频繁,构造活动强烈,成矿地质条件优越。本文通过镜下显微结构观察、电子探针测试分析对腊子沟金矿载金矿物的组成、成分及金的赋存状态进行了研究,首次在该矿区发现含金独居石和含金磷灰石,金含量达到0.283%～0.456%,推测金可能通过类质同象形式替代磷灰石中的Ca2+或独居石中的稀土元素(Ce2+、 La2+)以晶格金的形式存在。前人研究认为成矿流体中金主要以Au[HS]2－和AuCl2－络合物形式进行迁移,本次含金磷酸盐矿物的发现表明：可能部分金以磷酸根络合物形式进行迁移。     

甘肃北山马鬃山俯冲增生杂岩带中特殊"古海山"的发现及其沉积序列解析

古海山是缝合带的关键组成部分,中亚造山带西段的天山造山带内已发现多处古海山,而北山地区却鲜有报道,古海山的发现可以弥补该地区海山研究的不足.通过在北山中部野马泉开展地质调查、测制剖面,发现了一套原始层序完整、以玄武岩、玄武质凝灰岩和大理岩为主的地层,具有火山岩基座和碳酸盐岩顶盖的结构,二者原始接触关系为整合接触,符合海山的沉积特征.依据地层中的岩石组合和沉积构造,确定其形成于海山斜坡相.野马泉古海山残骸呈NWW-SEE向延伸,东南部更接近海山顶.该海山中的玄武岩富集大离子亲石元素、亏损高场强元素,具有岛弧玄武岩的特征.该海山为洋内弧型海山,野马泉一带位于海山斜坡,其被构造肢解后呈NWW-SEE向分布.      

Geological relationship between Anyui Metamorphic Complex and Samarka terrane,Far East Russia

The Anyui Metamorphic Complex (AMC) of Cretaceous age is composed of metachert, schist, gneiss, migmatite and ultramafic rocks, and forms a dome structure within the northernmost part of the Jurassic accretionary complex of the Samarka terrane. The two adjacent geological units are bounded by a fault, but the gradual changes of grain size and crystallinity index of quartz in chert and metachert of the Samarka terrane and the AMC, together with the gradual lithological change, indicate that at least parts of the AMC are metamorphic equivalents of the Samarka rocks. Radiolarian fossils from siliceous mudstone of the Samarka terrane indicates Tithonian age (uppermost Jurassic), and hence, form a slightly later accretion. This signifies that the accretionary complex in the study area is one of the youngest tectonostratigraphic units of the Samarka terrane. The relationship between the Samarka terrane and AMC, as well as their ages and lithologies, are similar to those of the Tamba–Mino–Ashio terrane and Ryoke Metamorphic Complex in southwest Japan. In both areas the lower (younger) part of the Jurassic accretionary complexes were intruded and metamorphosed by Late Cretaceous granitic magma. Crustal development of the Pacific‐type orogen has been achieved by the cycle of: (i) accretion of oceanic materials and turbidites derived from the continent; and (ii) granitic intrusion by the next subduction and accretion events, accompanied by formation of high T/P metamorphic complexes.     

阿拉伯海北部莫克兰增生楔天然气水合物浅表层识别标志

阿拉伯海北部的莫克兰增生楔是阿拉伯板块以低速、低角度俯冲到欧亚板块之下形成的主动陆缘构造,蕴藏着丰富的天然气水合物资源。依据2019年中国在莫克兰增生楔海域采集的高分辨率多道地震资料、浅地层剖面及多波束测深数据,并结合以往的调查成果,探讨莫克兰海域天然气水合物存在的浅表层识别标志。地震识别标志主要有似海底反射层(BSR)和振幅空白带2种标志,地形地貌标志包括海底麻坑、海底滑塌、丘状体、泥火山、冷泉系统等,水体标志主要为羽状流。在水深1000m和2900m的站位已分别钻获水合物样品。莫克兰增生楔丰富的水合物识别标志可能与低速、低角度的俯冲地质背景有关,使该区水合物存在指示兼具主动大陆边缘和被动大陆边缘的特征。综合研究区的异常标志分布特征,推测增生楔中部和西部的背斜脊及其附近区域是天然气水合物远景区。     

Metamorphic imprint of accretion and ridge subduction in the Pan‐African Damara Belt,Namibia

Ridge subduction is an inescapable plate tectonic process, but has only been documented in modern circum‐Pacific environments and not yet been recognized from suture zones associated with supercontinent assembly, likely because its imprint is obliterated by later collision. The formation of the Pan‐African Damara Belt of central Namibia involved northward subduction of the Khomas Sea underneath the Congo Craton, prior to final suturing of the Congo and Kalahari Cratons. The accretionary history of the Belt is preserved in the Southern and Southern Marginal Zones, which consist of turbiditic metasedimentary and intercalcated mafic rocks with MORB affinity. Two localities in the Kuiseb and Gaub canyons reveal that aluminous metapelites contain a fabric‐defining assemblage of fine‐grained muscovite, chlorite, biotite, quartz and graphite that is overprinted by randomly oriented porphyroblasts and poikiloblasts of garnet, staurolite, kyanite and biotite. Associated metamafic rocks consist of hornblende, chlorite, epidote, rutile and quartz, with actinolite cores preserved in amphibole porphyroblasts. Metamorphic conditions for the fabric‐defining assemblage are estimated at ～10 kbar and 540–560 C, whereas peak metamorphism likely occurred at 10–10.5 kbar and 600 C. Consequently, these rocks preserve a two‐stage prograde metamorphic history, where initial tectonic burial was followed by relatively rapid, near‐isobaric heating without attendant deformation to peak metamorphic conditions. We propose that initial burial occurred through subduction and underplating to the accretionary prism, before ridge subduction and opening of a slab window heated the rocks to peak metamorphic conditions. The exceptional preservation of the tectono‐thermal imprint of the accretionary orogenic stage is due to the relatively soft, largely aborted collision that characterized the Damara orogeny, which can be attributed to the confined extent of the Khomas Sea.