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.     

陕西南秦岭地质组成及其演化

综合分析南秦岭地质组成与构造特征，显示出区内经历了太古－早、中元古代基底形成阶段，晚元古－早古生代统一扬子大陆北缘演化阶段，古生代勉略－石泉海槽的形成、闭合造山阶段和燕山以来的陆内造山4个大的演化阶段，并形成了相应的岩石建造与构造变形组合。     

陕西勉略宁三角区基本地质组成及演化

陕西勉略宁三角区发育４大套不同构造背景下所形成的岩石组合，表明区内经历了晚太古结晶基底与早中元古浅变质火山岩过渡基底形成、晚元古一早古生代扬子大陆边缘演化、勉略海槽演化及陆内造山４个大的复杂演化阶段，最终形成今日之基本构造格局与地质面貌．     

白银厂铜多金属矿床——壳幔交代中的地幔流体成矿探讨

依据深部构造、火山岩建造与基性和钙碱性、中酸性、酸性火山岩的双峰式火山作用，确定白银厂矿田、矿床形成于切割硅铝层的古裂谷构造环境。其中小铁山、铜厂沟等矿床和折腰山、火焰山矿床两次成矿事件与双峰式火山活动同时发生，成矿作用与酸性火山岩具有时空上的一致性。矿石中CO2流体包裹体除H2O含量高外，与上地幔CO2流体包裹体成分一致；矿石富含大离子亲石元素和具富Mg、K的成矿蚀变元素组合，硫和铅来自下地壳或上地幔。喷气硅质岩锶同位素初始值表明，矿质来源于不均一地幔，S/Se、Co/Ni、Se/Te值亦指示矿质来源于深部。 本文提出矿床形成于强烈扩张的裂谷期，断裂的延深生长使地幔深部产生去挥发作用，释放大量CO2和含金属元素的地幔流体与上地幔基性岩浆向地壳侵入，导致下地壳熔融，形成酸性岩浆。与酸性岩浆喷发的同时，富含挥发份和金属元素的高盐度热卤水，沿断裂上升流入海底沉积成矿。     

盆地演化认识中的几个问题

本文论述了盆地演化认识中的几个主要问题。一是陆缘裂解过程，华南边缘传统上认为是被动裂谷边缘的一个例子，其东部沉积物记录了压实的历史，它明显不同于被动裂谷作用模式，而是显示早第三纪大规模上隆和侵蚀的特征；而且，该地区覆盖在下地壳楔形体上面，具有比周围地壳明显要高的地震速度和密度，指示岩浆底侵作用。这些证据说明，与被动裂谷作用模式相比，华南边缘与主动裂谷作用模式较为一致，它是由早第三纪华南边缘东部底下地幔柱形成的主动裂谷。另一个问题是关于页岩的浅部破裂作用机理。大多数研究者认为，微裂隙的形成需要高孔隙压力，它限于深埋、低渗透率的岩石。然而，最近从北海观察表明，在第三纪页岩中，整个盆地范围内的破裂型式从未达到大于几百米的埋藏深度。这说明泥质源岩经历了水压破裂和浅部脱水，在破裂前可能达到深埋后生作用所需要的深度，实质上渗透率的改变是由于源岩的浅部破裂作用可能根本上改变了烃类的初次运移时间和效率。     

Influence of syn-sedimentary faults on orogenic structure: examples from the Neoproterozoic–Mesozoic east Siberian passive margin

The east margin of the Siberian craton is a typical passive margin with a thick succession of sedimentary rocks ranging in age from Mesoproterozoic to Tertiary. Several zones with distinct structural styles are recognized and reflect an eastward-migrating depocenter. Mesozoic orogeny was preceded by several Mesoproterozoic to Paleozoic tectonic events. In the South Verkhoyansk, the most intense pre-Mesozoic event, 1000–950 Ma rifting, affected the margin of the Siberian craton and formed half-graben basins, bounded by listric normal faults. Neoproterozoic compressional structures occurred locally, whereas extensional structures, related to latest Neoproterozoic–early Paleozoic rifting events, have yet to be identified. Devonian rifting is recognized throughout the eastern margin of the Siberian craton and is represented by numerous normal faults and local half-graben basins.Estimated shortening associated with Mesozoic compression shows that the inner parts of ancient rifts are now hidden beneath late Paleozoic–Mesozoic siliciclastics of the Verkhoyansk Complex and that only the outer parts are exposed in frontal ranges of the Verkhoyansk thrust-and-fold belt. Mesoproterozoic to Paleozoic structures had various impacts on the Mesozoic compressional structures. Rifting at 1000–950 Ma formed extensional detachment and normal faults that were reactivated as thrusts characteristic of the Verkhoyansk foreland. Younger Neoproterozoic compressional structures do not display any evidence for Mesozoic reactivation. Several initially east-dipping Late Devonian normal faults were passively rotated during Mesozoic orogenesis and are now recognized as west-dipping thrusts, but without significant reactivation displacement along fault surfaces.     

Analyse sismo-stratigraphique du bassin d'Abda (Maroc occidental), exemple de structures inverses pendant le rifting atlantiqueSeismo-stratigraphic analysis of the Abda Basin (West Morocco): a case of reverse structures during the Atlantic rifting

The review of the seismic reflection and well data from the coastal Abda Basin (western Morocco) shows that its Triassic and Jurassic sequences were deposited in a submeridean sag basin, whose eastern margin is characterised by progressive truncations and pinching out of these sequences against a prominent Palaeozoic high. The uplift of this latter is interpreted as a response to an Upper Triassic–Middle Jurassic local compressional event that controlled Triassic–Jurassic sedimentation within the Abda Basin. The present day ‘West Meseta Flexure’ is a surface expression of this uplift. To cite this article: H. Echarfaoui et al., C. R. Geoscience 334 (2002) 371–377.     

An unconformity in the early Miocene syn-rifting succession, northern Noto Peninsula, Japan: Evidence for short-term uplifting precedent to the rapid opening of the Japan Sea

Abstract   The present paper describes the newly discovered early Miocene unconformity in the northern Noto Peninsula, on the Japan Sea side, central Japan. The unconformity marks the boundary between an early Miocene non-marine to marine succession and a more extensive, late early to early middle Miocene marine succession, and contains a time gap of an order of 1 million years or less from 18 Ma or earlier to 17 Ma. The early Miocene succession likely represents an early phase of marine transgression and initial slow rifting. The overlying early to early middle Miocene succession records the climax of the opening of the Japan Sea at ca  16 Ma with widespread, rapid subsidence of the Japan Arc. The unconformity between the two transgressive successions may represent a global sealevel fall or, more likely, crustal uplifting because no upward-shallowing or regressive facies remains between the two successions. Early Miocene unconformities that are thought to be correlative with this unconformity in the northern Noto Peninsula occur in places along the Japan Sea coast of Sakhalin and Japan. They are likely to have been produced during rifting in response to upwelling of asthenospheric mantle, although more accurate age constraints are necessary to evaluate this idea.     

华北东部盆山耦合与内生成矿作用

盆-山耦合关系是当今地学研究的前沿课题。但是,在几何学上,成对盆山耦合论述较多。而华北东部裂陷盆地则西与太行造山带、北与燕山造山带、东与胶辽山地、南与大别造山带均构成盆-山耦合,即中心裂陷盆地与外围各造山带均为耦合关系。研究表明:中心裂陷与华北地幔亚热柱的形成密切相关,由于地幔亚热柱强烈上隆,在岩石圈底部受阻,使华北东部岩石圈发生热减薄-断陷的同时,向外拆离的地幔岩在盆地外围形成一系列次级隆起(太行、燕山、胶辽、大别造山带),即地幔热柱多级演化的第三级单元———幔枝构造。各幔枝构造(造山带)间具有明显的可比性,并共同与中心裂陷盆地构成盆-山耦合关系。与此同时,随地幔热柱-亚热柱-幔枝构造向上迁移的成矿元素,亦以气态-气液混合态-含矿流体的形式向上迁移,并最终在幔枝构造的有利聚集部位成矿,形成张宣、冀东、辽东、胶东、鲁西、小秦岭、阜平等幔枝构造成矿集中区。     

Sedimentary and Tectonic History of the Terranes in the Corridor of the GolmudEjin Qi Geotransect and Its Adjacent Areas

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