Geomorphic appraisals of active tectonics associated with uplift of the Gohpur–Ganga section in Itanagar, Arunachal Pradesh, India

The Gohpur–Ganga section is located southwest of Itanagar, India. The study area and its adjacent regions lie between the Main Boundary Thrust (MBT) and the Himalayan Front Fault (HFF) within the Sub-Himalaya of the Eastern Himalaya. The Senkhi stream, draining from the north, passes through the MBT and exhibits local meandering as it approaches the study area. Here, five levels of terraces are observed on the eastern part, whereas only four levels of terraces are observed on the western part. The Senkhi and Dokhoso streams show unpaired terraces consisting of very poorly sorted riverbed materials lacking stratification, indicating tectonic activity during deposition. Crude imbrications are also observed on the terrace deposits. A wind gap from an earlier active channel is observed at latitude 27°04′42.4″ N and longitude 93°35′22.4″ E at the height of about 35 m from the present active channel of Senkhi stream. Linear arrangements of ponds trending northeast–southwest on the western side of the study section may represent the paleochannel of Dokhoso stream meeting the Senkhi stream abruptly through this gap earlier. Major lineament trends are observed along NNE–SSW, NE–SW and ENE–WSW direction. The Gohpur–Ganga section is on Quaternary deposits, resting over the Siwaliks with angular contact. Climatic changes of Pleistocene–Holocene times seem to have affected the sedimentation pattern of this part of the Sub-Himalaya, in association with proximal tectonism associated with active tectonic activities, which uplifted the Quaternary deposits. Older and younger terrace deposits seem to mark the Pleistocene–Holocene boundary in the study area with the older terraces showing a well-oxidized and semi-consolidated nature compared to the unoxidized nature of the younger terraces.     

Tectonometamorphic evolution of the Himalayan metamorphic core between the Annapurna and Dhaulagiri, central Nepal

The metamorphic core of the Himalaya in the Kali Gandaki valley of central Nepal corresponds to a 5-km-thick sequence of upper amphibolite facies metasedimentary rocks. This Greater Himalayan Sequence (GHS) thrusts over the greenschist to lower amphibolite facies Lesser Himalayan Sequence (LHS) along the Lower Miocene Main Central Thrust (MCT), and it is separated from the overlying low-grade Tethyan Zone (TZ) by the Annapurna Detachment. Structural, petrographic, geothermobarometric and thermochronological data demonstrate that two major tectonometamorphic events characterize the evolution of the GHS. The first (Eohimalayan) episode included prograde, kyanite-grade metamorphism, during which the GHS was buried at depths greater than c. 35 km. A nappe structure in the lowermost TZ suggests that the Eohimalayan phase was associated with underthrusting of the GHS below the TZ. A c. 37 Ma ⁴⁰Ar/³⁹Ar hornblende date indicates a Late Eocene age for this phase. The second (Neohimalayan) event corresponded to a retrograde phase of kyanite-grade recrystallization, related to thrust emplacement of the GHS on the LHS. Prograde mineral assemblages in the MCT zone equilibrated at average T =880 K (610 °C) and P =940 MPa (=35 km), probably close to peak of metamorphic conditions. Slightly higher in the GHS, final equilibration of retrograde assemblages occurred at average T =810 K (540 °C) and P=650 MPa (=24 km), indicating re-equilibration during exhumation controlled by thrusting along the MCT and extension along the Annapurna Detachment. These results suggest an earlier equilibration in the MCT zone compared with higher levels, as a consequence of a higher cooling rate in the basal part of the GHS during its thrusting on the colder LHS. The Annapurna Detachment is considered to be a Neohimalayan, synmetamorphic structure, representing extensional reactivation of the Eohimalayan thrust along which the GHS initially underthrust the TZ. Within the upper GHS, a metamorphic discontinuity across a mylonitic shear zone testifies to significant, late- to post-metamorphic, out-of-sequence thrusting. The entire GHS cooled homogeneously below 600–700 K (330–430 °C) between 15 and 13 Ma (Middle Miocene), suggesting a rapid tectonic exhumation by movement on late extensional structures at higher structural levels.     

四川盆地南部泸州—云锦地区走滑断裂构造变形特征

以多重滑脱褶皱冲断构造为主要变形的泸州—云锦地区同样发育走滑断裂。逆冲构造和走滑断裂的分类、分期厘定对于重新认识泸州—云锦地区的断裂系统具有重要意义。以泸州—云锦地区的高精度三维地震数据、钻测井数据为基础,通过平面—剖面结合精细构造解析和井—震标定分析,获得如下认识：1）北东向、北西向、近南北向等不同走向、不同级别的逆断层和褶皱带叠加、复合变形,构成了泸州—云锦地区网状的多重滑脱冲断褶皱构造变形系统。2）区内主要发育4条走滑断裂带,断层对关键界面的改造呈现出微小伸展地堑的变形特征,在剖面上则主要表现出“Y”字形负花状构造的变形样式。其中,蜀南1号走滑断裂带近南北走向,在左旋剪切作用下形成;蜀南2号、3号走滑断裂带由多条次级断层带或小型正断层左阶斜列构成,形成于右旋剪切背景。蜀南1号、蜀南2号、蜀南3号走滑断裂带均形成于二叠系沉积前（相当于泥盆纪—石炭纪）。蜀南4号走滑断裂带则呈北东走向,主体沿着背斜带的脊部发育,在右旋剪切作用下发育,形成于二叠系沉积后（相当于早-中三叠世）。蜀南4号断裂带、蜀南3号断裂带的局部区段以及主体发育在二叠系—三叠系内的逆冲断层,对志留系、二叠系、三叠系的多套源储组合具有良好的沟通作用;同时,它们对断控缝洞型储集层的形成具有积极影响,利于二叠系栖霞组、茅二段、嘉陵江组等多层系的油气聚集成藏。     

塔里木盆地西北部萨拉姆断裂和阿恰断裂的构造特征与形成演化

萨拉姆断裂和阿恰断裂位于塔里木盆地西北部,前者是新发现的一条边界断裂,构成柯坪凸起和温宿凸起的分界,后者是巴楚凸起与阿瓦提凹陷之间的边界断裂。两者均为高角度基底卷入型逆冲断层,走向NNW-SSE,倾向SWW,向NEE逆冲。萨拉姆断裂呈向NEE凸出的舒缓弧形展布,上盘发育一条规模不大的背冲断裂。断裂雏形形成于泥盆纪晚期—石炭纪早期,二叠纪晚期—三叠纪早期和中新世复活,构成两个冲断加速期,第四纪有一定的变形改造。阿恰断裂的断层线近于平直,其西侧发育一条背冲断裂——乔来买提断裂,两者组成阿恰断裂带。它们在断裂带北段呈“Y”型剖面组合关系,南段组合成一基底卷入型冲断楔。阿恰断裂带形成于中新世,持续演化至第四纪。阿恰断裂带和萨拉姆断裂晚新生代的断裂活动,都受控于印度—亚洲碰撞远程效应。二叠纪晚期—三叠纪早期和泥盆纪晚期—石炭纪早期的断裂活动是研究区新发现的两期构造事件,前者成因与南天山碰撞造山有关,后者成因有待进一步研究。     

大兴安岭西侧盆地群推覆构造的地震学证据

推覆构造是一种和油气有关的重要构造形式,主要产出于造山带前陆,是挤压或压缩作用的结果,对油气的生成、储集、运移、圈闭和保存条件都有着重要的影响.研究区从漠河盆地、海拉尔盆地到二连盆地内的中生代推覆构造,组构了大兴安岭西盆地群推覆构造.本文通过对三个盆地五种推覆构造的地震学研究,将其推覆方向归结为两类：第一类由北西向南东推覆,在三个盆地中表现明显;第二类由南东向北西推覆,在海拉尔盆地和二连盆地内有显示.认为第一类推覆作用来源于蒙古-鄂霍茨克洋自西向东的闭合;第二类推覆作用来源于太平洋板块俯冲的远程应力作用.