前陆盆地陆源沉积序列的特征与成因机制

前陆盆地接受的来自毗邻造山带的陆源碎屑沉积是盆山耦合关系研究的主要对象之一.这套沉积通常具有以下3个方面的特征:①在地层剖面中,前陆盆地陆源沉积序列呈向上变粗的趋势,其成因有多种可能,包括源区构造隆升、侵蚀卸栽导致的盆地基底均衡回弹、气候变化、岩性变化等;②构造抬升导致的粗粒相沉积事件与构造事件之间存在时滞,其大小取决于源区基岩剥露速率、侵蚀与搬运条件、沉积区距造山带构造变形前缘之间的距离等因素;③粗粒相沉积在前陆盆地内的分布具有穿时性.重视这些方面的研究,有助于得出有关前陆盆地及毗邻造山带演化的合理结论.     

Shallow high-resolution seismics and reprocessing of industry profiles in southern Bavaria: The Molasse and the northern Alpine front

The purpose of this seismic investigation in the Upper Bavarian Miesbach area, as part of the international TRANSALP project, was to study the tectonic contact between the autochthonous Foreland Molasse and the allochthonous Folded Molasse marking the tectonic front of the Alpine orogen. Another specific target was the dip of the frontal emerging main thrust of the tectonic units Helveticum/Ultrahelveticum and Rhenodanubian Flysch overriding the Folded Molasse. Twelve seismic profiles obtained from the hydrocarbon industry were reprocessed. From the Foreland Molasse southward to the Autochthonous Molasse in the subsurface of the overthrust Folded Molasse conspicuous features such as steep normal faults at the Molasse base, S-directed thickening of Molasse sediments or sedimentary discordant base of Upper Marine Molasse can be recognized.Shallow high-resolution seismic measurements were conducted along two profiles across the tectonic contact between Foreland Molasse and Folded Molasse, as well as along a profile across the frontal emerging main thrust of the Helveticum/Ultrahelveticum and the Rhenodanubian Flysch. Geological structures could be identified in the top 300–500 ms two-way traveltime interval, which is hardly possible with the usual deep-seismic method. The method thus provides a bridge between deep-reflection seismics and surface geology.In contrast to the western Bavarian Molasse zone, the tectonic boundary between the Foreland Molasse and the Folded Molasse in the investigated area is not characterized by a large blind-thrust triangle zone but by a simple south-dipping thrust plane. Adjacent to the S follow several steeply south-dipping inverse Molasse thrust slices and the Miesbach syncline. The inverse thrust slices are interpreted as the overturned and sheared northern limb of a fault propagation fold, which linked the Folded Molasse to the Foreland Molasse during a final orogenic phase.The main thrust of the Helveticum/Ultrahelveticum and the Rhenodanubian Flysch are well imaged in the near-surface interval of the high-resolution reflection seismic data. In contrast to previously published results, these thrust planes show a gentle dip to the S from the surface down to at least 500–1000 m depth.     

Forebulge dynamics and environmental control in Western Amazonia: The case study of the Arch of Iquitos (Peru)

The Iquitos Arch corresponds to a broad topographic high in the Western Amazonia. Morphostructural and geophysical data and flexural modeling show that the Iquitos Arch is the present-day forebulge of the Northwestern Amazonian foreland basin. A detailed tectono-sedimentary study of the Neogene and Quaternary deposits of the Iquitos area has been carried out in order to circumscribe the timing of the forebulge uplift and its environmental consequences. The Neogene and Quaternary sedimentary succession of the Iquitos Arch consists of six formations that evolved from tidal to fluvial environments. The first three formations exhibit Late Miocene gliding features and synsedimentary normal faults. Such soft-sediment deformations bear witness to tectonic activity ascribed to the growth of the forebulge. Regional erosive surfaces that separate the Neogene and Quaternary formations recorded the progressive forebulge emersion and the evolution of Amazonian drainage system. This uplift is related to an increase in tectonic activity within the Andes, which has provoked the eastern propagation of the orogenic wedge and caused an orogenic loading stage in the Amazonian foreland basin system. The emersion of the forebulge induced the retreat of the Pebas “marine megalake” nearby the Iquitos area and consequently caused important environmental changes in the Amazonian basin. From the end of the Late Miocene to the Pliocene, the forebulge acted as a barrier inducing the deposition of fluvial deposits in the forebulge depozone and the deposition of the “White Sand” deposits in the backbulge depozone. Since about 6 Ma, the forebulge is incised and crossed over by the modern Amazon River. The Iquitos forebulge is still growing as shown by the faulted Holocene terrace deposits.     

Lithospheric evolution of the Andean fold–thrust belt, Bolivia, and the origin of the central Andean plateau

We combine geological and geophysical data to develop a generalized model for the lithospheric evolution of the central Andean plateau between 18° and 20° S from Late Cretaceous to present. By integrating geophysical results of upper mantle structure, crustal thickness, and composition with recently published structural, stratigraphic, and thermochronologic data, we emphasize the importance of both the crust and upper mantle in the evolution of the central Andean plateau. Four key steps in the evolution of the Andean plateau are as follows. 1) Initiation of mountain building by 70 Ma suggested by the associated foreland basin depositional history. 2) Eastward jump of a narrow, early fold–thrust belt at 40 Ma through the eastward propagation of a 200–400-km-long basement thrust sheet. 3) Continued shortening within the Eastern Cordillera from 40 to 15 Ma, which thickened the crust and mantle and established the eastern boundary of the modern central Andean plateau. Removal of excess mantle through lithospheric delamination at the Eastern Cordillera–Altiplano boundary during the early Miocene appears necessary to accommodate underthrusting of the Brazilian shield. Replacement of mantle lithosphere by hot asthenosphere may have provided the heat source for a pulse of mafic volcanism in the Eastern Cordillera and Altiplano at 24–23 Ma, and further volcanism recorded by 12–7 Ma crustal ignimbrites. 4) After 20 Ma, deformation waned in the Eastern Cordillera and Interandean zone and began to be transferred into the Subandean zone. Long-term rates of shortening in the fold–thrust belt indicate that the average shortening rate has remained fairly constant (8–10 mm/year) through time with possible slowing (5–7 mm/year) in the last 15–20 myr. We suggest that Cenozoic deformation within the mantle lithosphere has been focused at the Eastern Cordillera–Altiplano boundary where the mantle most likely continues to be removed through piecemeal delamination.     

Mesozoic structural architecture of the Lang Shan, North-Central China: Intraplate contraction, extension, and synorogenic sedimentation

The Lang Shan, North-Central China, has experienced a complex Mesozoic to recent history of intraplate deformation and sedimentation. Well-exposed cross-cutting relationships document Jurassic right-lateral strike-slip faulting (transtension) followed by several tens of kilometers of Late Jurassic to Early Cretaceous north-northwest–south-southeast crustal shortening and development of an associated foreland basin. Since the Early Cretaceous, the south-central Lang Shan has undergone two phases of extension. The first, which occurred along north–south oriented structures, may represent collapse of an overthickened crust. The youngest deformation is represented by the active Cenozoic mountain-front normal fault system. This compound history may be the result of the complicated far-field effects of plate interactions combined with structural inheritance in a region adjacent to a rigid and undeformed crustal block, the Ordos block.     

天山北缘前陆冲断带形成时间的地层学证据

乌鲁木齐附近天山北缘喀拉扎背斜及其周缘的地层学分析表明，上中新统一上新统昌吉河群(N1-2ch)(相当于独山子组N1-2d)是喀拉扎山地区发育的生长地层，是喀拉扎背斜形成时的同构造沉积层序．这个结论表明、包括喀拉扎背斜在内的天山北缘第一排前陆冲断构造带形成于晚中新一上新世时期．     

大巴山前陆冲断带构造样式和演化过程的数值模拟

由于大巴山地区处于独特的构造背景,拥有丰富的油气资源,正吸引人们开展日益深入的研究。本文通过有限差分方法,利用平面应变的二维弹塑性本构模型,对大巴山前陆冲断带开展了系列数值模拟研究。结果表明,前陆区拥有相对软弱的滑脱层(含膏盐岩的中下三叠统),是大巴山前陆冲断带主滑脱面由后陆向前陆逐渐抬升的主控因素。该滑脱层的分布以及同构造沉积的分布限定了主前缘逆冲断裂(镇巴断裂)的位置,通过该断裂,大巴山后陆的强烈缩短应变由深滑脱层(震旦系下部)向南传递至浅滑脱层(三叠系中下统)。镇巴断裂和城口断裂一起,它们的长期活动性,可能调节了大巴山一半以上的总缩短应变。同构造沉积发生后,前陆区浅层侏罗山式褶皱往前扩展的同时,后陆的先成断裂也继续活动并发生顺时针旋转,构成无序逆冲扩展序列。主要断层的逆冲扩展和经典背驮式不同,缩短应变主要由主断层下盘向后陆深部俯冲来实现。模拟结果和大巴山地质原型的近似程度,说明了利用有限差分软件对地质构造进行模拟的可行性。     

Diagenesis of Upper Carboniferous rocks in the Ouachita foreland shelf in mid-continent USA: an overview of widespread effects of a Variscan-equivalent orogeny

Diagenesis of Upper Carboniferous foreland shelf rocks in southeastern Kansas took place at temperatures as high as 100–150° C at a depth of less than 2 km. High temperatures are the result of the long distance (hundreds of kilometers) advection of groundwater related to collisional orogeny in the Ouachita tectonic belt to the south. Orogenic activity in the Ouachita area was broadly Late Carboniferous, equivalent to the Variscan activity of Europe. Mississippi Valley-type Pb-Zn deposits and oil and gas fields in the US midcontinent and elsewhere are commonly attributed to regional groundwater flow resulting from such collisional events. This paper describes the diagenesis and thermal effects in sandstone and limestone of Upper Carboniferous siliciclastic and limestone-shale cyclothems, the purported confining layer of a supposed regional aquifer. Diagenesis took place in early, intermediate, and late stages. Many intermediate and late stage events in the sandstones have equivalents in the limestones, suggesting that the causes were regional. The sandstone paragenesis includes siderite cement (early stage), quartz overgrowths (intermediate stage), dissolution of feldspar and carbonates, followed by minor Fe calcite, pore-filling kaolinite and sub-poikilotopic Ca ankerite (late stage). The limestone paragenesis includes calcite cement (early stage); megaquartz, chalcedony, and Fe calcite spar (intermediate stage); and dissolution, Ca-Fe dolomite and kaolinite (late stage). The R _m value of vitrinite shows a regional average of 0.6–0.7%; Rock-Eval T _maX suggests a comparable degree of organic maturity. The T _h of aqueous fluid inclusions in late stage Ca-Fe-Mg carbonates ranges from 90 to 160° and T _mice indicates very saline water (>200000 ppm NaCl equivalent); ¹⁸O suggests that the water is of basinal origin. Local warm spots have higher R _m, T _max, and T _h. The results constrain numerical models of regional fluid migration, which is widely viewed as an artesian flow from recharge areas in the Ouachita belt across the foreland basin onto the foreland shelf area. Such models must account for heating effects that extend at least 500 km from the orogenic front and affect both supposed aquifer beds and the overlying supposed confining layer. Warm spots indicate either more rapid or more prolonged flow locally. T _h and T _mice data show the highest temperatures coincided with high salinity fluids.     

造山带研究中有关复理石和磨拉石的几个问题

在造山带研究中,碰撞方式和时限的争议是全球造山带研究中的一个共同现状或面临的难题。利用复理石和磨拉石时代来限定碰撞时限是造山带研究的一项重要内容。本文首先综述了复理石和磨拉石从提出到广泛应用以及不同历史时期被赋予的构造含义;其次,根据其可能发育的构造环境,指出复理石和磨拉石纪录造山带的相关信息及其演变过程。被动陆缘复理石可能纪录了参与碰撞拼贴块体的亲缘性,而且被动陆缘、活动陆缘或者岛弧环境的复理石均形成于碰撞作用之前,因此最年轻的复理石时代可提供碰撞时间的下限;(周缘)前陆盆地的磨拉石形成于碰撞造山作用过程中,因此其最早的磨拉石沉积时代可用于限定碰撞时间的上限。从复理石到磨拉石地层的渐变过程(连续沉积)指示了碰撞构造事件的发生、以及之后的碰撞造山作用是均变过程,会发育不同尺度规模的不整合。因此,造山带中的不整合未必代表有重大构造事件的发生,而连续沉积并不表明没有大的构造事件。     

The biogenic gas potential of the submarine canyon systems of Plio-Peistocene foreland Basin,southwestern Taiwan

Biogenic gas was accidentally discovered and produced from the Plio/Pleistocene formation of the Hsinying gas field in southwestern Taiwan in 1989. A stratigraphic trapping mechanism related to the evolution of submarine canyon systems in the Plio-Peistocene foreland basin has been proposed in a previous study which explained underestimated recoverable gas reserve before drilling. To verify this shallow gas exploration hypothesis and to systematically evaluate the biogenic gas generation and entrapment potential of the submarine canyon systems, seismic interpretation, high-resolution sequence stratigraphic interpretation, seismic attribute analysis and geochemical analysis were performed and integrated in this study. Twenty-nine submarine canyons mapped mainly trend in a NE direction, except the NW trending Eurchungchi submarine canyons located near the Chiali paleo-high. Bright seismic amplitudes were often observed at the incised valley heads of the canyon systems. The shales located near the incised valley heads and deposited during flooding stage possess the highest biogenic gas generation potential, as canyon fill reveals the second highest. Due to the high sediment accumulation rate in the foreland basin, organic matter in such a depositional environment tends to become diluted. A Class III AVO anomaly, inverted impedance lower than 4.7 e + 6 kg/M³*M/S and A/B (the ratio between the target horizon amplitude and the RMS amplitude from the background strata) greater than 1.78 were identified as valid direct gas indicators as sand is buried shallower than 1000 m. Class IV AVO anomaly and A/B greater than 1.4 were concluded to be the indicators of gas sand in the case that sand is buried deeper than 1600 m. Based on the results of sequence stratigraphic interpretation and the consistency between spatial geometries of seismic attributes and those of the submarine canyons, a stratigraphic trap associated with the incised valley heads was concluded to be the original gas entrapment style of the Hsinying and the Kuantian gas fields. Biogenic gas migrated after being trapped startigraphically, hence contributing to the present-day gas field structure. Due to the prevalent erosional features of the submarine canyons on the time structural maps, different types of stratigraphic traps formed in combination with faults and submarine canyons can be recognized easily.