巽他弧北部掸邦高原地壳变形、长期块体运动和发震过程:来自大地测量研究结果的约束

本文通过约束大地测量研究来探索掸邦高原及其周围地区现今的地壳变形和长期块体运动,以期提供该地区地球动力学和相关地震危险状况的最新状态。掸邦高原在横向上由西侧的萨干(Sagaing)断裂和东侧的红河断裂这两条主要断裂包围。其中,青藏高原地壳的韧性流挤压被认为是该夹层变形单元变形的主要因素。大地测量清楚地表明,萨干断裂和红河断裂段分别具有约18 mm/a和约45 mm/a右旋运动走滑速率。此外,掸邦高原内部断层体系大地滑移累积表现为1213 mm/a的整体左旋运动速率。我们认为相对于刚性巽他古陆,研究区域的形变分布和长期块体运动主要受区域书架型断层作用控制,其原因是掸邦高原两侧的主断裂(萨干断裂和红河断裂)存在差异性断裂活动。     

Present-day crustal motion around the Pamir Plateau from GPS measurements

We have used geodetic techniques to improve constraints on the crustal motion of the Pamir Plateau. Three campaigns of Global Position System data acquisition between 2011 and 2015 demonstrate that, in association with the India–Asia collision, a complex pattern of crustal motion exists in the Pamir Plateau. In a north–south direction from the Indian Plate to the Hazak Block, the crust has absorbed ~ 35 mm/yr of shortening, of which ~ 35% is distributed around the Hindu Kush region (~ 12 mm/yr), and another ~ 35% is taken up around the Alai Valley (also ~ 12 mm/yr). Global Position System measurements also show ~ 5 mm/yr of shortening between the Pamir Plateau and the Tajik Basin, whereas between the Pamir and the Tarim Basin, an ~ 10 mm/yr extension rate is observed. With respect to the stable Eurasian Plate, the Pamir rotates counterclockwise at a rate of ~ 1.822°Myr^− 1, with an Euler pole positioned about the west end of the Tajik Basin (37.03 ± 0.74°N, 65.89 ± 0.12°E). The strain rate field calculated from Global Position System velocities reveals that the crustal motion is consistent with localized deformation around the Hindu Kush and the Alai Valley, the latter representing a zone with strong shallow seismic activity.     

Seamounts in the Central Indian Ocean Basin: indicators of the Indian plate movement

The study of seamount parameters in the tectonically most-complicated and least-understood Indian Ocean assumes importance since their properties vary as a function of tectonic setting, physics of lithosphere, conduit geometry and chemical composition of magma. More than 100 such seamounts ranging in summit height (h) from 300 to 2870 m, are indentified in the oceanic crust between Indian continent and Mid-Indian Ridge (MIR) and South-East Indian Ridge (SEIR). Most of the minor seamounts (h > 1000) are found in the southern part of the study area. Major seamounts (h < 1000 m) are roughly distributed in two groups—the northern group on Cretaceous Oceanic Crust and southern group on Pliocene-Miocene Oceanic Crust. On an average northern group seamounts (SM 1 to 6) are taller, wider and flatter than those from the southern group. These seamounts appear to be the result of continuous growth from tapped, moving magma chamber while stress depleted magma and inconsistent Indian Plate movement during Mid-Tertiary are attributed to the origin of southern group of smaller seamounts. Distribution and morphology of seamounts as a whole indicate their formation either from Reunion hotspot or from two separate hotspots in the geological past.     

GPS测量误差分析

GPS测量以其显著的定位特点,得到了空前的应用和发展。它取代了常规的大地测量和工程控制测量,现在已渗入到工程测量、地籍测量、交通管理、导航、地理信息系统、海洋、石油、气象和地球科学等许多研究领域。同时GPS还涉及数学、天文学、数字通讯、计算机科学、测绘学等多学科领域。GPS测量应用的广泛开展,使观测精度的影响因素和解决方法成为科学研究工作中的重点。由于GPS测量涉及多学科、多应用领域,给研究的方向、方法提出了更高的要求。本文介绍了影响GPS测量精度的因素和常用的解决方法。     

Post-2004 mega-earthquake temporal velocity variation at Andaman Islands from GPS measurements

Post-mega-earthquake velocity adjustment at Andaman region was found to vary with time. Coordinate repeatability for 5-year span shows changeover from post-seismic to inter-seismic period in between 900 and 1,000 days from the mega-earthquake. Excluding Havlock Island, all sites move from south to north and from east to west. Velocity vectors gradually rotated from the nearly perpendicular orientation after the mega-earthquake to parallel orientation with the subduction interface in later phase. Velocities in India 2005 reference frame indicate the presence of a structural discontinuity between Bedonabad and Chidiatapu at south Andaman, between Padmanavapuram and Kaushalyanagar at middle Andaman and between Aerial Bay and Radhanagar at north Andaman.     

Flexure of the Indian plate and intraplate earthquakes

The flexural bulge in central India resulting from India's collision with Tibet has a wavelength of approximately 670 km. It is manifest topographically and in the free-air gravity anomaly and the geoid. Calculations of the stress distribution within a flexed Indian plate reveal spatial variations throughout the depth of the plate and also a function of distance from the Himalaya. The wavelength (and therefore local gradient) of stress variation is a function of the effective elastic thickness of the plate, estimates of which have been proposed to lie in the range 40–120 km. The imposition of this stress field on the northward moving Indian plate appears fundamental to explaining the current distribution of intraplate earthquakes and their mechanisms. The current study highlights an outer trough south of the flexural bulge in central India where surface stresses are double the contiguous compressional stresses to the north and south. The Bhuj, Latur and Koyna earthquakes and numerous other recent reverse faulting events occurred in this compressional setting. The N/S spatial gradient of stress exceeds 2 bars/km near the flexural bulge. The overall flexural stress distribution provides a physical basis for earthquake hazard mapping and suggests that areas of central India where no historic earthquakes are recorded may yet be the locus of future damaging events.     

The western border of the Indian plate: implications for Himalayan geology

The study of regions situated beyond the western margin of the present-day Indian plate (Afghanistan principally) point to the following facts:

1. (1) During the Late Precambrian—Early Paleozoic, stratigraphical continuity existed between western and central Iran, Central Afghanistan, Salt Range and western Pakistan.

2. (2) During the Paleozoic a similar epicontinental cover existed in central Afghanistan, Kashmir and Tibet, with Gondwana tillites and associated cold fauna, such as in India (Umaria); however, a so-called Hercynian zone exists also in northern Iran—Hindu Kush and northern Pamir: it exibits a Middle Paleozoic unconformity (Upper DevonianCarboniferous) on metamorphic Early Paleozoic.

3. (3) The end of the Paleozoic, is marked by: a fracturation of the basement of the Hercynian zone, with powerful volcanic eruptions at the northern part of Hindu Kush, Kashmir (Panjal trap) and also Nepal (Nar valley) the formation of a geosynclinal zone at the southern part of Hercynian zone (Turkman, Penjaw).

4. (4) During the Jurassic: the geosynclinal evolution of the Turkman—Penjaw furrow accelerated, with the accumulation of flysch, radiolarites, ophiolites, olistolites and incipient HP metamorphism. A general subduction took place followed by a Neocimmerian orogenic phase with overthrusting of the central Afghanistan ranges on the scar of the geosynclinal furrows.

5. (5) During the Cretaceous: the geosynclinal evolution ended: Lower Cretaceous lies unconformably on the folded Jurassic flysch. In eastern Afghanistan and northern Pakistan, during the Middle (?) or Upper Cretaceous, a new geosynclinal zone was created.

6. (6) During the Cenozoic, central Afghanistan was emerged; northwards, sedimentary basins were created along the Herat fault, with volcanic and magmatic activity. A southeastern geosynclinal furrow evolved with accumulation of flysch, ophiolites and finally molasse deposits (Katawas—Soleimans). Its western border began overthrusting, but this movement changed into a left lateral fault i.e., the presentday Chaman Arghandeh fault.

Conclusion: Two major phases of dislocation took place during the geological history of Gondwana: the first one began during the Permian and ended in the Jurassic; the second one began during the Cretaceous and is still active. The important Eocimmerian orogenic phenomena, existing in the Central Afghanistan and northern Pakistan, took place at the edge of a Gondwana continental fragment, which was larger than the presentday Indian plate. Coeval phenomena may exist in the Himalayan region and perhaps in one of the ophiolitic sutures of Tibet.     

Present-day Upper-crustal Strain Rate Field in Southeastern Tibet and its Geodynamic Implications: Constraints from GPS Measurements with ABIC Method

The Earth’s surface kinematics and deformation are fundamental to understanding crustal evolution. An effective research approach is to estimate regional motion field and deformation fields based on modern geodetic networks. If the discrete observed velocity field is obtained, the velocity related fields, such as dilatation rate and maximum shear strain rate, can be estimated by applying varied mathematical approaches. This study applied Akaike''s Bayesian Information Criterion (ABIC) method to calculate strain rate fields constrained by GPS observations in the southeast Tibetan Plateau. Comparison with results derived from other three methods revealed that our ABIC-derived strain rate fields were more precise. The maximum shear strain rate highlighted the Xianshuihe–Xiaojiang fault system as the main boundary for the outward migration of material in southeastern Tibet, indicating rotation of eastern Tibet material around the eastern Himalaya rather than whole extrusion along a fixed channel. Additionally, distinct dilatation rate patterns in the northeast and southwest regions of the fault system were observed. The northeast region, represented by the Longmenshan area, exhibited negative dilatational anomalies; while the southwest region, represented by the Jinsha River area north of 29°N, displayed positive dilatational anomalies. This indicates compression in the former and extension in the latter. Combined with deep geophysical observations, we believe that the upper and lower crusts of the Jinsha River area north of 29°N are in an entire expanding state, probably caused by the escape-drag effect of material. The presence of a large, low-viscosity region south of 29°N may not enable the entire escape of the crust, but instead result in a differential escape of the lower crust faster than the upper crust.     

Atmospheric phenomena deduced from radiosonde and GPS occultation measurements for various application related studies

The tropopause height and tropopause temperature are sensitive to temperature changes in troposphere and stratosphere. These are the measures of global climatic variability. Atmospheric profiles of temperature, refractivity and water vapour are always needed for communication, navigation and atmospheric modeling studies. The tropopause characteristics over the Indian region have been studied using radio occultation measurements (CHAMP) on the basis of cold point criterion. Tropopause height shows large variation in the latitude range ∼30°–40°N during winter. Tropopause temperature less than −82°C, assumed to facilitate troposphere to stratosphere air transport, is observed at a number of tropical Indian locations and no seasonal pattern is observed in its occurrence. The bias in temperature and refractivity deduced from radiosonde and radio occultation measurements is also presented.     

Crustal shortening in convergent orogens: Insights from global positioning system (GPS) measurements in northeast India

Deformation in active mountain belts like the Himalaya is manifested over several spatial and temporal scales and collation of information across these scales is crucial to an integrated understanding of the overall deformation process in mountain belts. Computation and integration of geological shortening rates from retrodeformable balanced cross-sections and present-day convergent rates from deforming mountain belts is one way of integrating information across time-scales. The results from GPS measurements carried out in NE India indicate that about 15–20 mm/yr of convergence is being accommodated there. Balanced-cross sections from the NE Himalaya indicate about 350–500 km of shortening south of the South Tibet Detachment (STD). Geothermobarometry suggest that the rocks south of the STD deformed under peak metamorphic conditions at ∼ 22 Ma. This indicates a geological convergence rate of ∼ 16–22 mm/yr which appears to be fairly consistent with the GPS derived convergence rates. Approximately 1.5 to 3.5 mm/yr (∼ 10–20 %) of the total N-S of the present-day convergence in the NE Himalaya is accommodated in the Shillong Plateau. In addition, ∼ 8–9 mm/yr of E-W convergence is observed in the eastern and central parts of the Shillong Plateau relative to the Indo-Burman fold-thrust belt. Balanced cross-sections in the Indo-Burman wedge together with higher resolution GPS measurements are required in the future to build on the first-order results presented here.     

Distinct upper mantle deformation of cratons in response to subduction: Constraints from SKS wave splitting measurements in eastern China

Interaction between the subducting slab, the overriding continental lithosphere and mantle flow are fundamental geodynamic processes of subduction systems. Eastern China is an ideal natural laboratory to investigate the behavior and evolution of cratonic blocks within a subduction system. In this study, we investigate deformation of the upper mantle beneath eastern China. We present seismic shear wave splitting measurements from three networks consisting of over 483 broadband stations, with 157 stations giving a total of 516 results. The splitting parameters exhibit complex regional patterns but are relatively coherent within individual tectonic units. Tectonic blocks exhibited distinctive fast directions relative to regional features. The dominant attitude of fast directions for the North China Craton was subparallel to the direction of subduction, whereas fast directions for Southeastern China were perpendicular to the direction of subduction. The shear wave splitting measurements were interpreted according to a high resolution tomographic body-wave velocity model. Combining these two datasets showed that the predominant geodynamic models for the region (mantle plume, mantle wedge and flat-slab subduction models) are incompatible with the observations presented here. We suggest that the North China Craton, Yangtze Craton and the Cathaysia block have undergone different deformational events due to differing mantle flow patterns, and distinct spatial and temporal subduction histories of the Pacific and Philippine Sea plates.     

Constraints on the geodynamic evolution of the AfricaeIberia plate margin across the Gibraltar Strait from seismic tomography

Geophysical studies point to a complex tectonic and geodynamic evolution of the Alboran Basin and Gulf of Cadiz. Tomographic images show strong seismic waves velocity contrasts in the upper mantle. The...     

Variable behavior of the Dead Sea Fault along the southern Arava segment from GPS measurements

The Dead Sea Fault is a major strike-slip fault bounding the Arabia plate and the Sinai subplate. On the basis of three GPS campaign measurements, 12 years apart, at 19 sites distributed in Israel and Jordan, complemented by Israeli permanent stations, we compute the present-day deformation across the Wadi Arava fault, the southern segment of the Dead Sea Fault. Elastic locked-fault modelling of fault-parallel velocities provides a slip rate of 4.7 ± 0.7 mm/yr and a locking depth of 11.6 ± 5.3 km in its central part. Along its northern part, south of the Dead Sea, the simple model proposed for the central profile does not fit the velocity field well. To fit the data, two faults have to be taken into account, on both sides of the sedimentary basin of the Dead Sea, each fault accommodating ∼ 2 mm/yr. Locking depths are small (less than 2 km on the western branch, ∼ 6 km on the eastern branch). Along the southern profile, we are once again unable to fit the data using the simple model, similar to the central profile. It is very difficult to propose a velocity greater than 4 mm/yr, i.e. smaller than that along the central profile. This leads us to propose that a part of the relative movement from Sinai to Arabia is accommodated along faults located west of our profiles.     

Kinematics of the Iberia–Maghreb plate contact from seismic moment tensors and GPS observations

The Iberian Peninsula and the Maghreb experience moderate earthquake activity and oblique,  NW–SE convergence between Africa and Eurasia at a rate of  5 mm/yr. Coeval extension in the Alboran Basin and a N35°E trending band of active, left-lateral shear deformation in the Alboran–Betic region are not straightforward to understand in the context of regional shortening, and evidence complexity of deformation at the plate contact. We estimate 86 seismic moment tensors (M_W 3.3 to 6.9) from time domain inversion of near-regional waveforms in an intermediate period band. Those and previous moment tensors are used to describe regional faulting style and calculate average stress tensors. The solutions associated to the Trans-Alboran shear zone show predominantly strike-slip faulting, and indicate a clockwise rotation of the largest principal stress orientation compared to the regional convergence direction (σ₁ at N350°E). At the N-Algerian and SW-Iberian margins, reverse faulting solutions dominate, corresponding to N350°E and N310°E compression, respectively. Over most of the Betic range and intraplate Iberia, we observe predominately normal faulting, and WSW–ENE extension (σ₃ at N240°E). From GPS observations we estimate that more than 3 mm/yr of African (Nubian)–Eurasian plate convergence are currently accommodated at the N-Algerian margin,  2 mm/yr in the Moroccan Atlas, and  2 mm/yr at the SW-Iberian margin. 2 mm/yr is a reasonable estimate for convergence within the Alboran region, while Alboran extension can be quantified as  2.5 mm/yr along the stretching direction (N240°E). Superposition of both motions explains the observed left-lateral transtensional regime in the Trans-Alboran shear zone. Two potential driving mechanisms of differential motion of the Alboran–Betic–Gibraltar domain may coexist in the region: a secondary stress source other than plate convergence, related to regional-scale dynamic processes in the upper mantle of the Alboran region, as well as drag from the continental-scale motion of the Nubian plate along the southern limit of the region. In the Atlantic Ocean, the  3.5 mm/yr, westward motion of the Gibraltar Arc relative to intraplate Iberia can be accommodated at the transpressive SW-Iberian margin, while available GPS observations do not support an active subduction process in this area.     

印度板块的北缘在哪里?

通过对INDEPTHII在雅鲁藏布江南的2条南北向深地震反射剖面资料的进一步处理,观察到主喜马拉雅逆冲断裂带(MHT)形成的反射向北逐渐倾没于藏南地壳之下。这一反射一直可延伸至康马穹隆北、浪卡子南,在向北延伸的过程中,断裂带向北倾角逐渐加大,可以看到MHT反射最北端的反射同相轴向北倾斜的角度到达27°30'～29°,最深处的双程走时达到22.5s左右。根据深地震反射资料并结合大地电磁(MT)资料,提出印度板块在雅鲁藏布江南30～40km(大约28°50'N)处沿MHT俯冲到了藏南的地壳之下,即在地壳范围内印度板块的最北部边缘位于雅鲁藏布江南30～40km处。     

印度板块-古亚洲板块碰撞及亚洲恐龙的出现

印度板块在5500万年前的古近纪撞击了亚洲板块,造就了东西走向的雅鲁藏布江水系和喜马拉雅山系。根据近年来的研究,笔者又提出一个新观点：印度板块在约2亿年前与亚洲板块发生碰撞时,将一个发源于非洲、南美洲的“冈瓦纳恐龙动物群”送上亚洲大陆,在中国西南地区形成以禄丰龙、蜀龙、峨眉龙、马门溪龙为特征的恐龙类动物群。这一事件不仅使亚洲首次出现了恐龙动物,而且板块相撞还造就了南亚地区南北走向的横断山脉和伊洛瓦底江、怒江、澜沧江、金沙江等纵向水系,以及四川盆地的形成。     

依据恐龙化石恢复印度板块的运行轨迹

越来越多的事实表明,印度板块曾经连续两次撞击了欧亚板块。第一次发生在近200Ma前,通过印度板块将冈 瓦纳大陆上的恐龙动物群带到了亚洲,使中国首次出现了早侏罗世恐龙。确切地说,中国恐龙起源于印度板块的第一 次撞击。侏罗纪之后,印度板块又被引发起顺时针转动,在其东部出现了与泰国之间的断裂,并最终孕育着孟加拉湾 的形成,为第二次撞击拉开了序幕。55Ma前印度板块再次撞向欧亚板块的西藏地区,造就了喜马拉雅山。