Crustal structure of the Paleozoic Kunlun orogeny from an active-source seismic profile between Moba and Guide in East Tibet, China

We herein present a new seismic refraction/wide-angle reflection profile that crosses the Songpan–Ganzi terrane, the Animaqing suture zone and the eastern Kunlun mountains (comprised of the South Kunlun and Middle Kunlun blocks separated by the Middle Kunlun fault). The profile is 380 km long and extends from Moba to Guide in eastern Tibet. The crustal thickness is about 62 km under the Songpan–Ganzi terrane, 62–64 km under the South Kunlun, and 60 km under the Middle Kunlun block. The Songpan–Ganzi flysch seems to be present up to a depth of 15 km south of the Animaqing suture zone, and up to a depth of 10 km in the Middle Kunlun block, with thicknesses elsewhere that depend on assumptions about the likely lithologies. The profile exhibits clear lateral variations both in the upper and lower crust, which are indicative of different crustal blocks juxtaposed by the Kunlun fault system. Whether or not the Songpan–Ganzi flysch was originally deposited on oceanic crust, at the longitude of our profile (100°E) it is now underlain by continental crust, and the presence of continental crust beneath the Songpan–Ganzi terrane and of a continental arc under the South Kunlun block suggest Paleozoic continent–continent arc collision in the eastern Kunlun Mountains. Comparison of crustal velocity columns from all wide-angle seismic profiles across the eastern Kunlun mountains indicates a remarkable west-to-east change in the Moho topography across the Kunlun fault system (15–20 km Moho step at 95°E, but only 2–5 km along our profile at 100°E). Lower-crustal thickness of the Kunlun terranes is rather uniform, about 35 km, from 80°–95°E, which suggests that similar thrust-thickening processes have played a role where the Qaidam Basin abuts the Kunlun fault, but thins to 20–25 km at 100°E, east of the Qaidam Basin. The increased crustal thickness from 93° to 98°E compared to that at 100°E may be due to the differences in the thickness of the crust of the two plates before their collision, and/or largely achieved by thickening of the lower crust, perhaps indicating a crustal flow mechanism operating more strongly in the western region.     

南海西北部重磁场及深部构造特征

通过对南海重磁数据的重新处理,得到南海西北部自由空间重力异常图、布格重力异常图、磁异常图和化极磁异常图,并对所反映的地球物理场特征加以分析。根据重力场资料对研究区的地壳结构进行了反演计算,结果表明地壳厚度在10～38km之间,总的趋势由陆向洋逐渐减薄,对应于地壳类型从陆壳、过渡壳到洋壳的分布特征。根据磁力资料计算了居里面深度,其埋深变化于11～27km之间,在陆区居里面是下地壳顶界面和莫霍面之间的另一个物性界面,而在海区则接近于莫霍面埋深。     

Crustal structure in an onshore–offshore transitional zone near Hong Kong,northern South China Sea

To study the crustal structure beneath the onshore–offshore transitional zone, a wide-angle onshore–offshore seismic experiment was carried out in northern South China Sea near Hong Kong, using large volume airgun sources at sea and seismic stations on land. The crustal velocity model constructed from traveltime fitting shows that the sedimentary thickness abruptly increases seaward of the Dangan Islands based on the characteristics of Pg and Multiple Pg, and the crustal structure beneath the sedimentary layer is relatively simple. The Moho depth is about 25–28 km along the profile and the P-wave velocity increases gradually with depth. The velocities in the upper crust range from 5.5 to 6.4 km/s, while that in the lower crust is 6.4–6.9 km/s. It also reveals a low velocity zone with a width of more than 10 km crossing the crust at about 75–90 km distance, which suggests that the Littoral Fault Zone (LFZ) exists beneath the onshore–offshore transitional zone. The magnetism anomalies, bouguer gravity anomalies and active seismic zone along the coastline imply the LFZ is a main tectonic fault in the onshore–offshore area. Combined with two previously published profiles in the continental South China (L–G profile) and in the northern margin of South China Sea (OBS1993) respectively, we constructed a land-sea super cross-section about 1000 km long. The results show the onshore–offshore transitional zone is a border separating the unstretched and the stretched continental crust. The low velocity layer (LVL) in the middle crust was imaged along L–G profile. However, the high velocity layer (HVL) in the lower crust was detected along OBS1993. By analyzing the mechanisms of the LVL in the middle crust and HVL in the base of crust, we believe the crustal structures had distinctly different attributes in the continental South China and in the northern SCS, which indicates that the LFZ could be the boundary fault between them.     

Characteristics of gravity and magnetic fields and deep structural responses in the southern part of the Kyushu-Palau Ridge

The southern part of the Kyushu-Palau Ridge (KPR) is located at the conjunction of the West Philippine Basin, the Parece Vela Basin, the Palau Basin, and the Caroline Basin. This area has extremely complex structures and is critical for the research on the tectonic evolution of marginal seas in the Western Pacific Ocean. However, only few studies have been completed on the southern part, and the geophysical fields and deep structures in this part are not well understood. Given this, this study finely depicts the characteristics of the gravity and magnetic anomalies and extracts information on deep structures in the southern part of the KPR based on the gravity and magnetic data obtained from the 11th expedition of the deep-sea geological survey of the Western Pacific Ocean conducted by the Guangzhou Marine Geological Survey, China Geological Survey using the R/V Haiyangdizhi 6. Furthermore, with the data collected on the water depth, sediment thickness, and multichannel seismic transects as constraints, a 3D density model and Moho depths of the study area were obtained using 3D density inversion. The results are as follows. (1) The gravity and magnetic anomalies in the study area show distinct zoning and segmentation. In detail, the gravity and magnetic anomalies to the south of 11°N of the KPR transition from high-amplitude continuous linear positive anomalies into low-amplitude intermittent linear positive anomalies. In contrast, the gravity and magnetic anomalies to the north of 11°N of the KPR are discontinuous and show alternating positive and negative anomalies. These anomalies can be divided into four sections, of which the separation points correspond well to the locations of deep faults, thus, revealing different field-source attributes and tectonic genesis of the KPR. (2) The Moho depth in the basins in the study area is 6–12 km. The Moho depth in the southern part of KPR show segmentation. Specifically, the depth is 10–12 km to the north of 11°N, 12–14 km from 9.5°N to 11°N, 14–16 km from 8.5°N to 9.5°N, and 16–25 km in the Palau Islands. (3) The KPR is a remnant intra-oceanic arc with the oceanic-crust basement.which shows noticeably discontinuous from north to south in geological structure and is intersected by NEE-trending lithospheric-scale deep faults. With large and deep faults F3 and F1 (the Mindanao fault) as boundaries overall, the southern part of the KPR can be divided into three zones. In detail, the portion to the south of 8.5°N (F3) is a tectonically active zone, the KPR portion between 8.5°N and 11°N is a tectonically active transition zone, and the portion to the north of 11°N is a tectonically inactive zone. (4) The oceanic crust in the KPR is slightly thicker than that in the basins on both sides of the ridge, and it is inferred that the KPR formed from the thickening of the oceanic crust induced by the upwelling of deep magma in the process of rifting of remnant arcs during the Middle Oligocene. In addition, it is inferred that the thick oceanic crust under the Palau Islands is related to the constant upwelling of deep magma induced by the continuous northwestward subduction of the Caroline Plate toward the Palau Trench since the Late Oligocene. This study provides a scientific basis for systematically understanding the crustal attributes, deep structures, and evolution of the KPR.©2021 China Geology Editorial Office.     

Thermorheological model for the European Central Alps: brittle–ductile transition and lithospheric strength

A two‐dimensional thermorheological model of the Central Alps along a north–south transect is presented. Thermophysical and rheological parameters of the various lithological units are chosen from seismic and gravity information. The inferred temperature distribution matches surface heat flow and results in Moho temperatures between 500 and 800 °C. Both European and Adriatic lithospheres have a ‘jelly‐sandwich’ structure, with a 15–20 km thick brittle upper crust overlying a ductile lower crust and a mantle lid whose uppermost part is brittle. The total strength of the lithosphere is of the order of 0.5–1.0 × 10¹³ N m⁻¹ if the upper mantle is dry, or slightly less if the upper mantle is wet. In both cases, the higher values correspond to the Adriatic indenter.     

南海南部地壳结构的重力模拟及伸展模式探讨

对南海南部地壳结构研究有助于揭示南海完整的演化历史。本研究对南海南部获取的两条多道地震剖面进行了地震 解释,并对重力数据进行了壳幔密度反演。其中 NH973-1 测线始于南海西南次海盆,覆盖了南沙中部的北段;NH973-2 测 线始于南海东部次海盆,穿越礼乐滩东侧。反演结果显示,莫霍面埋深在海盆区 10~11 km,陆缘区 15~21 km 左右,洋壳向 陆壳莫霍面深度迅速增加。海盆区厚度在 6~7 km,为典型的洋壳;陆缘区地壳厚度在 15~19 km,为减薄型地壳。进一步研 究表明(1)在西南次海盆残余扩张脊之下,莫霍面比两侧略深;(2)在礼乐滩外侧海盆区有高值重力异常体,推测为洋壳与深 部岩浆混合的块体;(3)南沙区域上地壳存在高密度带,且横向上岩性可能变化。南海南部陆缘未发现有下地壳高速层,有 比较一致的构造属性和拉张样式,为非火山型陆缘。我们对两条测线陆缘的伸展因子进行了计算,发现上地壳脆性拉伸因 子与全地壳拉伸因子存在差异,其陆缘的拉张模式在纵向上是不均匀一的。     

华南地壳及上地幔三维速度结构成像

利用国家地震科学数据共享中心的地震目录及临时台网资料,挑选出11 113个区域地震的77 093条P波走时和93 541条S波走时,采用1°×1°的经纬度网格划分,反演获得了深至60km的华南南部地区的地壳及上地幔三维P波和S波的速度结构。研究结果表明,纵波速度结构与横波速度结构从整体来看具有较好的一致性,说明该研究获得的深部速度结果具有较高的可信性,但是在50km的深度纵、横波速度结构的一致性较差,可能是由于该深度的纵横波走时数据存在着较大的差异所导致的。本研究显示了研究区域内的速度结构存在着明显的横向不均匀性,东南沿海地区的地壳中出现了大规模的低速异常,可能与该区地幔物质的上涌有关;而在珠江三角洲、雷州半岛、北部湾及海南岛等地区莫霍面下方出现的低速异常,则与该区的热运动有关。经分析认为,华南南部地壳及上地幔的速度不均匀性和华南板块与扬子地块的相互作用有关,因此开展进一步研究能为探索和分析华南再造以及中国南海北部的构造演化提供重要信息。     

Otway Continental Margin Transect: Crustal architecture from wide‐angle seismic profiling across Australia's southern margin

The Otway Basin in southeastern Australia formed on a triangular‐shaped area of extended continental lithosphere during two extensional episodes in Cretaceous to Miocene times. The extent of the offshore continental margin is highlighted by Seasat/Geosat satellite altimeter data. The crustal architecture and structural features across this southeast Australian margin have been interpreted from offshore‐onshore wide‐angle seismic profiling data along the Otway Continental Margin Transect extending from the onshore Lake Condah High, through the town of Portland, to the deep Southern Ocean. Along the Otway Continental Margin Transect, the onshore half‐graben geometry of Early Cretaceous deposition gives way offshore to a 5 km‐thick slope basin (P‐wave velocity 2.2–4.6 km/s) to at least 60 km from the shoreline. At 120 km from the nearest shore in a water depth of 4220 m, sonobuoy data indicate a 4–5 km sedimentary sequence overlying a 7 km thick basement above the Moho at 15 km depth. Major fault zones affect the thickness of basin sequences in the onshore area (Tartwaup Fault Zone and its southeast continuation) and at the seaward edge of the Mussel Platform (Mussel Fault). Upper crustal basement is interpreted to be attenuated and thinned Palaeozoic rocks of the Delamerian and Lachlan Orogens (intruded with Jurassic volcanics) that thin from 16 km onshore to about 3.5 km at 120 km from the nearest shore. Basement rocks comprise a 3 km section with velocity 5.5–5.7 km/s overlying a deeper basement unit with velocity 6.15–6.35 km/s. The Moho shallows from a depth of 30 km onshore to 15 km depth at 120 km from the nearest shore, and then to about 12 km in the deep ocean at the limits of the transect (water depth 5200 m). The continent‐ocean boundary is interpreted to be at a prominent topographic inflection point 170 km from shore at the bottom of the continental slope in 4800 m of water. P‐wave velocities in the lower crust are 6.4–6.8 km/s, overlying a thin transition zone to an upper mantle velocity of 8.05 km/s beneath the Moho. Outstandingly clear Moho reflections seen in deep‐marine profiling data at about 10.3 s two‐way time under the slope basin and continent‐ocean boundary place further strong controls on crustal thickness. There is no evidence of massive high velocity (>7 km/s) intrusives/underplate material in the lower crust nor any synrift or early post‐rift subaerial volcanics, indicating that the Otway continental margin can be considered a non‐volcanic margin, similar in many respects to some parts of the Atlantic Ocean margins e.g. the Nova Scotia ‐ Newfoundland margin off Canada and the Galicia Bank off the Iberian Peninsula. Using this analogue, the prominent gravity feature trending northwest‐southeast at the continent‐ocean boundary may indicate the presence of highly serpentinised mantle material beneath a thin crust, but this has yet to be tested by detailed work.     

Crustal Structure across the Northwestern Margin of South China Sea: Evidence for Magma‐poor Rifting from a Wide‐angle Seismic Profile

We present results from a 484 km wide-angle seismic profile acquired in the northwest part of the South China Sea (SCS) during OBS2006 cruise. The line that runs along a previously acquired multi-channel seismic line (SO49-18) crosses the continental slope of the northern margin, the Northwest Subbasin (NWSB) of the South China Sea, the Zhongsha Massif and partly the oceanic basin of the South China Sea. Seismic sections recorded on 13 ocean-bottom seismometers were used to identify refracted phases from the crustal layer and also reflected phases from the crust-mantle boundary (Moho). Inversion of the traveltimes using a simple start model reveals crustal images in the study area. The velocity model shows that crustal thickness below the continental slope is between 14 and 23 km. The continental part of the line is characterized by gentle landward mantle uplift and an abrupt oceanward one. The velocities in the lower crust do not exceed 6.9 km/s. With the new data we can exclude a high-velocity lower crustal body (velocities above 7.0 km/s) at the location of the line. We conclude that this part of the South China Sea margin developed by a magma-poor rifting. Both, the NWSB and the Southwest Sub-basin (SWSB) reveal velocities typical for oceanic crust with crustal thickness between 5 and 7 km. The Zhongsha Massif in between is extremely stretched with only 6–10 km continental crust left. Crustal velocity is below 6.5 km/s; possibly indicating the absence of the lower crust. Multi-channel seismic profile shows that the Yitongansha Uplift in the slope area and the Zhongsha Massif are only mildly deformed. We considered them as rigid continent blocks which acted as rift shoulders of the main rift subsequently resulting in the formation of the Northwest Sub-basin. The extension was mainly accommodated by a ductile lower crustal flows, which might have been extremely attenuated and flow into the oceanic basin during the spreading stage. We compared the crustal structures along the northern margin and found an east-west thicken trend of the crust below the continent slope. This might be contributed by the east-west sea-floor spreading along the continental margin.     

喜马拉雅及其邻区的重力异常、岩石圈构造与板块动力模型

本文按统一比例尺编制了印度-青藏地区1°×1°重力异常图和地形高程图,并用滑动平均方法得到了本区5°×5°重力异常图。用地改后的1°×1°重力异常,采用组合体模型人一机联作选择法,计算了横跨印度-青藏-蒙古长达4680km的岩石圈剖面,还给出了一个楔形体重力正演公式。基本结果有:(1)MBT、MCT的倾角为10°±5°,ITS、NS、KS的倾角为75°±5°;(2)地壳滑脱面的深度在青藏之下约20km,向高喜马拉雅、MCT、MBT抬升至15km;(3)青藏高原南、北边缘均为岩石圈结构的斜坡带,界面倾角由上向下而增大。在大、小喜马拉雅之下,壳内界面(Ⅰ、Ⅱ)的倾角约12°,Moho倾角为18°,岩石圈底面倾角约36°。在祁连山带所有界面倾角都小于喜马拉雅带,其中壳内界面倾角仅约1°,Moho倾角约2°,岩石圈底面倾角约12°;(4)岩石圈厚度由印度、蒙古向高喜马拉雅和祁连山带逐渐增加,与青藏岩石圈的边缘上翘形成主动俯冲和相对逆冲势态。印度岩石圈厚度(或上地幔顶部低密层埋深)不超过50km,蒙古高原(南)厚约70km,到高喜马拉雅和祁连山下分别增加至145和122km,青藏中心地带(怒江两侧)岩石圈厚135km,向南,北边缘各减小到120和90～102km,在高喜马拉雅和祁连山下面形成25和10km的断差;(5)在青藏Moho之下厚5km的高密薄层和软流层之间有一密     

南海中部和北部海域重力异常特征与地壳构造关系

1976年,中国科学院南海海洋研究所与国家海洋局南海分局共同协作使用“向阳红”五号海洋调查船,利用西德GSS-2型海洋重力仪和我国的CHHK-1型核子旋进式磁力仪,在南海珠江口外海域(北纬22°—17°、东经113°50′—115°10′),进行约3000公里的海洋重力、磁力和测深。设计的测线方向为南北向,测线距为10海里。     

Crustal density structure in the Spanish Central System derived from gravity data analysis (Central Spain)

Shallow and deep sources generate a gravity low in the central Iberian Peninsula. Long-wavelength shallow sources are two continental sedimentary basins, the Duero and the Tajo Basins, separated by a narrow mountainous chain called the Spanish Central System. To investigate the crustal density structure, a multitaper spectral analysis of gravity data was applied. To minimise biases due to misleading shallow and deep anomaly sources of similar wavelength, first an estimation of gravity anomaly due to Cenozoic sedimentary infill was made. Power spectral analysis indicates two crustal discontinuities at mean depths of 31.1 ± 3.6 and 11.6 ± 0.2 km, respectively. Comparisons with seismic data reveal that the shallow density discontinuity is related to the upper crust lower limit and the deeper source corresponds to the Moho discontinuity. A 3D-depth model for the Moho was obtained by inverse modelling of regional gravity anomalies in the Fourier domain. The Moho depth varies between a mean depth of 31 km and 34 km. Maximum depth is located in a NW–SE trough. Gravity modelling points to lateral density variations in the upper crust. The Central System structure is described as a crustal block uplifted by NE–SW reverse faults. The formation of the system involves displacement along an intracrustal detachment in the middle crust. This detachment would split into several high-angle reverse faults verging both NW and SE. The direction of transport is northwards, the detachment probably being rooted at the Moho.     

Crustal thickening processes in the Central Andes and the different natures of the Moho-discontinuity

New seismic data from the Central Andes allow us to clarify the crustal structure of this mountain chain and to address the problem of crustal thickening. Evidence for the deep crustal root can be observed in both gravimetric and seismological data. Crustal structure and composition change significantly from east to west. In the eastern part of the backarc the Moho discontinuity is clearly recognisable. However only poor Moho arrivals are observed by active seismic measurements beneath the Altiplano and the Western Cordillera where broad-band seismology data indicate such a discontinuity. In the Precordillera, a pronounced discontinuity is detected at a depth of 70 km. Along the coast, the oceanic Moho is developed at a depth of 40 km. There are several processes which can change the petrological and petrophysical properties of the rocks forming the crust. Variations of the classical Moho discontinuity are presented which do not correspond to the petrological crust/mantle boundary. Tectonic shortening in the backarc is the dominant process contributing to at least 50–55% to the root formation along 21°S. In the forearc and arc, hydration of the mantle wedge produced ≈15–20% of crustal thickening. Magmatic thickening and tectonic erosion contributed only ≈5%. The other ≈25% is not yet explained.     

The Diamantina River ring feature,Winton region,western Queensland

The Diamantina ～120 km-diameter ring feature, a unique feature in western Queensland, is manifested by a near-360° circular drainage pattern, radial creeks and a coincident radiometric K–Th–U pattern. The structure has been studied in the context of an investigation of the nature and origin of Australian circular structures. Geophysical signatures, including total magnetic intensity (TMI), gravity and seismic reflection transect data from the region of the ring feature are examined to help test the origin of the structure. A western subdued TMI arc with a ～110 km diameter is offset by ～30 km eastward from the western rim of the drainage ring. Bouguer anomaly data show a gravity low near the centre of the ring structure, but no outer circular pattern. Two recent seismic transects indicate a moderately reflective to weakly reflective crust below flat lying strata of the Jurassic–Cretaceous Eromanga and Permian–Triassic Galilee basins, and above a usually well-defined ～39–45 km-deep Moho. An approximately ～100 km-wide seismically non-reflective to weakly reflective zone overlapping the Diamantina ring feature separates crust of different seismic reflection character to either side. The nature of the seismic non-reflective crust is unknown. A potential interpretation of the ring structure in terms of asteroid impact cannot be confirmed or rejected given the present state of knowledge, owing to (1) the near-30 km depth of the seismically non-reflective zone along the transects; and (2) the shift of the TMI part ring zone relative to the geomorphic expression of the Diamantina ring feature. A test of the nature and origin of the Diamantina ring feature requires a cored drill hole near the centre of the TMI ring structure.     

Tectonic Framework and Deep Structure of South China and Their Constraint to Oil‐Gas Field Distribution

South China could be divided into one stable craton, the Yangtze Craton (YzC), and several orogenic belts in the surrounding region, that is the Triassic Qinling-Dabie Orogenic Belt (QDOB) in the north, the Songpan-Garzê Orogenic Belt (SGOB) in the northwest, the Mesozoic-Cenozoic Three-river Orogenic Belt (TOB) in the west, the Youjiang Orogenic Belt (YOB) in the southwest, the Middle Paleozoic Huanan Orogenic Belt (HOB) in the southeast, and the Mesozoic-Cenozoic Maritime Orogenic Belt (MOB) along the coast. Seismic tomographic images reveal that the Moho depth is deeper than 40 km and the lithosphere is about 210 km thick beneath the YzC. The SGOB is characterized by thick crust (>40 km) and thin lithosphere (<150 km). The HOB, YOB and MOB have a thin crust (<40 km) and thin lithosphere (<150 km). Terrestrial heat flow survey revealed a distribution pattern with a low heat flow region in the eastern YzC and western HOB and two high heat flow regions in the TOB and MOB respectively. Such a “high-low-high” heat flow distribution pattern could have resulted from Cenozoic asthenosphere upwelling. All oil-gas fields are concentrated in the central part of the YzC. Remnant oil pools have been discovered along the southern margin of the YzC and its adjacent orogenic belts. From a viewpoint of geological and geophysical structure, regions in South China with thick lithosphere and low heat flow value, as well as weak deformation, might be the ideal region for further petroleum exploration.     

Gravity interpretation along seismic reflection profile DEKORP 9-N (northern Rhine Graben)

A 2-D gravity model, incorporating geophysical and geological data, is presented for a 110 km long transect across the northern Rhine Graben, coinciding with the 92 km long DEKORP 9-N seismic reflection profile. The Upper Rhine Graben is marked by a prominent NNE-striking negative anomaly of 30–40 mgal on Bouguer gravity maps of SW Germany. Surface geological contacts, borehole data and the seismic reflection profile provide boundary constraints during forward modelling.
Short-wavelength (5–10 km) gravity features can be correlated with geologic structures in the upper few km. At deeper levels, the model reflects the asymmetry visible in the seismic profile; a thicker, mostly transparent lower crust in the west and a thinner, reflective lower crust in the east. From west to east Moho depth changes from 31 to 26–28 km. The entire 40 mgal minimum can be accounted for by the 2–3 km of light sedimentary fdl in the graben, which masks the gravitational effects of the elevated Moho. The thickened lower crust in the west partly compensates for the mass deficit from the depressed Moho. A further compensating feature is a relatively low density contrast at the crust-mantle boundary of 0.25 g cm^-3. The Variscan must displays heterogeneity along the profile which cuts at an angle across the strike of Variscan structures. The asymmetry of the integrated crustal model, both at the surface and at depth suggests an asymmetric mechanism of rift development.     

Moho depth determination beneath the Zagros Mountains from 3D inversion of gravity data

Due to its geological and economic importance, the Zagros Mountains have been investigated by many researchers during the last decades. Nevertheless, in spite of all the studies conducted on the region, there are still some controversial problems concerning the structure of the Zagros Mountains, including crustal depths, demanding more insights into understanding the crustal constraints of the region. Accordingly, we have conducted a gravity study to determine Moho depth map of the Zagros Mountains region, including its major structural domains from the coastal plain of the Persian Gulf to central Iran. The employed data are the densest and most accurate terrestrial gravity data set observed until now with the precision of 5 μGal and resolution of 5 arc-minute by 5 arc-minute. To image Moho depth variations, gravity inversion software GROWTH2.0 is used, proposing the possibility to model stratified structures by means of a semi-objective exploratory 3D inversion approach. The obtained results reveal the crustal thickness of ~?30–35 km underneath the southwestern most Zagros Fold-Thrust Belt increasing northeastward to 48 km. The maximum Moho depth is estimated ~?62 km below the Zagros Mountains belt along the Main Zagros Thrust. Northeast of the study area, an average crustal thickness of 46 km is computed beneath Urumieh–Dokhtar magmatic arc and central Iran.     

最佳向上延拓高度的估计

提出根据两个相邻高度重力异常向上延拓值相关系数与高度的关系 ,以估计应用向上延拓分离区域及剩余重力异常的最佳向上延拓高度的方法。二维模型计算表明 ,不同高度的观测重力异常向上延拓值和观测面上区域重力异常值的互相关系数与高度的关系曲线 ,存在一个明显的极大值 ;这个极大值对应的高度 ,就是从观测异常中分离出这一区域重力异常所需要的最佳向上延拓高度。两个相邻高度重力异常向上延拓值之间的互相关系数与高度的关系曲线 ,存在一个明显的转折点 ,这个转折点对应的高度 ,就是所求的最佳向上延拓高度。应用本方法处理华南北部地区布格重力异常的结果表明 ,由于引起本区区域重力异常的地质因素 ,除了莫霍面及上地壳底面外 ,还受到本区广泛分布甚至出露的花岗岩的影响 ;所以为了从观测异常中分离这一区域异常所需要的最佳向上延拓高度为 2 0 0km ,小于莫霍面及上地壳底界面的平均深度。为了从观测异常中分离出由莫霍面引起的重力异常所需要的向上延拓高度 ,达到 15 0km。因此 ,应用本方法处理实测重力资料 ,必须首先了解引起区域重力异常的场源情况。     

长江中下游成矿带及邻区地壳速度结构：来自利辛-宜兴宽角地震资料的约束

为了理解长江中下游地区在中生代成矿的深部动力学过程,Sinoprobe-03-02项目于2011年9月至10月,在跨宁芜矿集区和郯庐断裂带实施了从安徽利辛至江苏宜兴450km长的宽角反射/折射地震剖面。速度剖面结果显示,Moho面深度和地壳速度结构在郯庐断裂两侧东西方向存在明显的差异:(1)在东部扬子块体内部,地壳覆盖层厚3~5km,西部的合肥盆地下方,则达到4~7km。(2)剖面平均Moho面深度为30~32km左右,在郯庐断裂下方,Moho面深度在35km左右;在宁芜矿集区下方,Moho面整体深度偏浅,达30~31km左右,但局部范围内,Moho面深度至34km左右。(3)剖面的下地壳平均速度在6.5~6.6km/s左右,在宁芜矿集区下方,下地壳速度偏低,为6.4~6.5km/s左右。剖面上地幔顶部的速度结构平均在8.0~8.2km/s。在宁芜矿集区下方,速度偏低,为7.9~8.1km/s左右。(4)郯庐断裂带的下方,从地表开始,还存在20多千米长的低速异常带,一直延伸到Moho面附近。剖面的宁芜矿集区下方Moho面上隆、下地壳及上地幔的低速异常等壳幔结构特征,预示下地壳不以榴辉岩残体为主,支持燕山期地幔岩浆的上涌和侵入并成矿,是热上涌物质的源地。     

Upper mantle structure from the Trans‐Australia Seismic Survey (TASS) and other seismic refraction data

The recordings made during 1972 from large explosions at Kunanalling (W.A.), Mount Fitton (S.A.). and Bass Strait have added considerably to seismic refraction data measured over distances of 1000 km in continental Australia. Taken together with data from the 1956 Maralinga atomic bomb and 1970–71 Ord Dam explosions they show the existence of a refractor with apparent P‐wave velocity in the range 8.26–8.29 km/s, which is interpreted as the Moho under shield regions, at a depth of 34 km under Kalgoorlie and deepening eastward to 39 km under Maralinga. In northern South Australia and farther north and east this refractor is evident as a sub‐Moho refractor at a depth of about 60 km; the Moho refractor is also evident, with an apparent P velocity of 8.04 ± 0.04 km/s at a depth of 40 km. Two computer models (TASS‐1a and 2a) match the observed data. The subsequent arrivals recorded are consistent with the velocity of 8.53 km/s in a refractor at 165 km depth interpreted from the Ord Dam; there is little conclusive evidence for a low‐velocity zone above this depth.