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1.
ABSTRACTNew analysis of the geophysical data of the ultraslow-spreading Mohns Ridge and its off-axis structure reveals a distinctive asymmetric structure. We calculate residual bathymetry (RB) and residual mantle Bouguer gravity anomaly (RMBA) and decompose the anomalies into symmetric and asymmetric components between the ridge conjugates. The western flank of the Mohns Ridge at crustal age of ~50–15 Ma is characterized by a broad zone of elevated RB and more negative RMBA, which we term the Vesteris Plateau (VP). The VP anomaly has a surface area of ~1.12 × 105 km2 and an excess crust volume of ~2.33 × 105 km3, making it a significant anomaly comparable to other anomalies such as the Bermuda Rise. Extending north of the Kolbeinsey Ridge for more than 500 km, the VP lies above an anomalous upper mantle region of low shear-wave seismic velocity, indicating that the VP might represent the northernmost reach of the Iceland-Jan Mayen mantle anomaly. In addition, the western ridge flank of the Mohns Ridge at crustal age of 6–0 Ma is associated with higher RB and more positive RMBA relative to the eastern conjugate, indicating tectonic uplift and associated exposure of lower crust and upper mantle near the ridge axis. 相似文献
2.
南海海盆区具有复杂的构造演化史,但目前对其深部结构的不对称性的研究和控制因素的探讨还存在不足.利用南海最新的重力数据和从27条地震剖面上获取的海盆范围沉积物精确数据计算了全海盆的剩余地幔布格重力异常（residual mantle Bouguer anomaly,RMBA）,并反演了海盆的地壳厚度,运用Crust1.0数据进行了相关性分析.研究结果表明,南海海盆的残留扩张脊两翼在地形、RMBA和洋壳厚度上存在明显的不对称性,北翼比南翼有更多的海山分布、更低的RMBA值以及更厚的洋壳.这种明显的南北不对称性表明北侧比南侧有更高的地幔温度和更活跃的岩浆活动,反映了南海深部结构的不对称性.南海深部结构的不对称性可能与洋中脊向南的跃迁有关.洋脊跃迁导致在新老洋脊之间产生部分熔融,使扩张中心北侧产生更高的地幔温度,以及更强烈的岩浆活动,从而显示出更低的RMBA值和更厚的洋壳,并形成更多的后扩张期海山. 相似文献
3.
Kiranmai Samudrala K A Kamesh Raju P Rama Rao 《Journal of Earth System Science》2016,125(8):1523-1542
High-resolution shipboard geophysical investigations along the Indian Ocean ridge system are sparse especially over the Carlsberg and Central Indian ridges. In the present study, the shipboard gravity and multibeam bathymetry data acquired over a 750 km long section of the Central Indian Ridge between 3 °S and 11 °S have been analysed to understand the crustal structure and the ridge segmentation pattern. The mantle Bouguer anomalies (MBA) and the residual mantle Bouguer anomalies (RMBA) computed in the study area have shown significant variations along the ridge segments that are separated by transform and non-transform discontinuities. The MBA lows observed over the linear ridge segments bounded by well-defined transform faults are attributed to the thickening of the crust at the middle portions of the ridge segments. The estimates of crustal thickness from the RMBA shows an average of 5.2 km thick crust in the axial part of the ridge segments. The MBA and relative RMBA highs along the two non-transform discontinuities suggests a thinner crust of up to 4.0 km. The most significant MBA and RMBA highs were observed over the Vema transform fault suggesting thin crust of 4 km in the deepest part of the transform fault where bathymetry is more than 6000 m. The identified megamullion structures have relative MBA highs suggesting thinner crust. Besides MBA lows along the ridge axis, significant off-axis MBA lows have been noticed, suggesting off-axis mantle upwelling zones indicative of thickening of the crust. The rift valley morphology varies from the typical V-shaped valley to the shallow valley floor with undulations on the inner valley floor. Segments with shallow rift valley floor have depicted well-defined circular MBA lows with persistent RMBA low, suggesting modulation of the valley floor morphology due to the variations in crustal thickness and the mantle temperature. These are supported by thicker crust and weaker lithospheric mantle. 相似文献
4.
Early Paleozoic magmatism of the Tannuola terrane located in the northern Central Asian Orogenic Belt is important to understanding the transition from subduction to post-collision settings. In this study, we report in situ zircon U-Pb ages, whole rock geochemistry, and Sr-Nd isotopic data from the mafic and granitic rocks of the eastern Tannuola terrane to better characterize their petrogenesis and to investigate changing of the tectonic setting and geodynamic evolution. Zircon U-Pb ages reveal three magmatic episodes for about 60 Ma from ∼510 to ∼450 Ma, that can be divided into the late Cambrian (∼510–490 Ma), the Early Ordovician (∼480–470 Ma) and the Middle-Late Ordovician (∼460–450 Ma) stages. The late Cambrian episode emplaced the mafic, intermediate and granitic rocks with volcanic arc affinity. The late Cambrian mafic rocks of the Tannuola terrane may originate from melting of mantle source that contain asthenosphere and subarc enriched mantle metasomatized by melts derived from sinking oceanic slab. Geochemical and isotopic compositions indicate the late Cambrian intermediate-granitic rocks are most consistent with an origin from a mixed source including fractionation of mantle-derived magmas and crustal-derived components. The Early Ordovician episode reveal bimodal intrusions containing mafic rocks and adakite-like granitic rocks implying the transition from a thinner to a thicker lower crust. The Early Ordovician mafic rocks are formed as a result of high degree melting of mantle source including dominantly depleted mantle and subordinate mantle metasomatized by fluid components while coeval granitic rocks were derived from partial melting of the high Sr/Y mafic rocks. The latest Middle-Late Ordovician magmatic episode emplaced high-K calc-alkaline ferroan granitic rocks that were formed through the partial melting the juvenile Neoproterozoic sources.These three episodes of magmatism identified in the eastern Tannuola terrane are interpreted as reflecting the transition from subduction to post-collision settings during the early Paleozoic. The emplacement of voluminous magmatic rocks was induced by several stages of asthenospheric upwelling in various geodynamic settings. The late Cambrian episode of magmatism was triggered by the slab break-off while subsequent Early Ordovician episode followed the switch to a collisional setting with thickening of the lower crust and the intrusion of mantle-induced bimodal magmatism. During the post-collisional stage, the large-scale lithospheric delamination provides the magma generation for the Middle-Late Ordovician granitic rocks. 相似文献
5.
A. A. Shchipansky 《Geotectonics》2012,46(2):122-141
The diamond-bearing mantle keels underlying Archean cratons are a unique phenomenon of Early Precambrian geology. The common
stable assemblage of the Archean TTG early continental crust and underlying subcontinental lithospheric mantle clearly shows
their coupled tectogenesis, which was not repeated in younger geological epochs. One of the least studied aspects of this
phenomenon is concerned with the eclogitic xenoliths carried up by kimberlite pipes together with mantle-derived nodules.
The eclogitic xenoliths reveal evidence for their subduction-related origin, but the Archean crustal counterparts of such
xenoliths remained unknown for a long time, and the question of their crustal source and relationships to the formation of
early continental crust remained open. The Archean crustal eclogites recently found in the Belomorian Belt of the Baltic Shield
are compared in this paper with eclogitic xenoliths from kimberlites in the context of the formation of both Archean subcontinental
lithospheric mantle (SCLM) and early continental crust. The crustal eclogites from the Belomorian Belt are identical in mineral
and chemical compositions to the eclogite nodules (group B), including their diamond-bearing varieties. The eclogite protoliths
are comparable in composition with the primary melts of the Meso- and Neoarchean oceanic crust, which was formed at a potential
temperature of the upper mantle which exceeded its present-day temperature by 150–250 K. The reconstructed pathways of the
Archean oceanic crust plunging in the upper mantle suggest that the Archean mantle was hotter than in the modern convergence
settings. The proposed geodynamic model assumes coupled formation of the Archean diamond-bearing SCLM and growth of early
continental crust as a phenomenon related to the specific geodynamics of that time controlled by a higher terrestrial heat
flow. 相似文献
6.
中国岩石圈的基本特征 总被引：11，自引：2，他引：9
中国及邻区岩石圈结构构造十分复杂,并具有若干明显的特点:中国大陆地壳西厚东薄、南厚北薄,青藏高原地壳平均厚度为60~65 km,最厚达80 km;东部地区一般为30~35 km,南中国海中央海盆平均只有5 km;中国大陆地壳平均厚度为476 km,大大超过全球地壳392 km的平均厚度。中国大陆及邻区岩石圈亦呈西厚东薄、南厚北薄的变化趋势,青藏高原及西北地区岩石圈平均厚度为165 km,塔里木盆地中东部、帕米尔与昌都地区岩石圈厚度可达180~200 km。大兴安岭-太行山-武陵山以东,包括边缘海为岩石圈减薄区,厚度为50~85 km。西部岩石圈、软流圈“层状结构”明显,反映了板块碰撞汇聚的动力学环境;东部岩石圈、软流圈呈“块状镶嵌结构”,岩石圈薄,软流圈厚,反映了地壳拉张、软流圈物质上涌的特点,并在东亚及西太平洋地区85~250 km深处形成一巨型低速异常体。中国东部上、下地壳及地壳、岩石圈地幔之间普遍存在“上老下新”年龄结构。 相似文献
7.
Geochemical fingerprinting of oceanic basalts with applications to ophiolite classification and the search for Archean oceanic crust 总被引：61，自引：0，他引：61
Two geochemical proxies are particularly important for the identification and classification of oceanic basalts: the Th–Nb proxy for crustal input and hence for demonstrating an oceanic, non-subduction setting; and the Ti–Yb proxy for melting depth and hence for indicating mantle temperature and thickness of the conductive lithosphere. For the Th–Nb proxy, a Th/Yb–Nb/Yb projection demonstrates that almost all oceanic basalts lie within a diagonal MORB–OIB array with a principal axis of dispersion along the array. However, basalts erupted at continental margins and in subduction zones are commonly displaced above the MORB–OIB array and/or belong to suites with principal dispersion axes which are oblique to the array. Modelling of magma–crust interaction quantifies the sensitivity of the Th–Nb proxy to process and to magma and crustal compositions. For the Ti–Yb proxy, the equivalent Ti/Yb–Nb/Yb projection features a discriminant boundary between low Ti/Yb MORB and high Ti/Yb OIB that runs almost parallel to the Nb/Yb axis, reflecting the fact that OIB originate by melting beneath thicker lithosphere and hence by less melting and with residual garnet. In the case of volcanic-rifted margins and oceanic plume–ridge interactions (PRI), where hot mantle flows toward progressively thinner lithosphere (often becoming more depleted in the process), basalts follow diagonal trends from the OIB to the MORB field. Modelling of mantle melting quantifies the sensitivity of the Ti–Nb proxy to mantle potential temperature and lithospheric thickness and hence defines the petrogenetic basis by which magmas plot in the OIB or MORB fields. Oceanic plateau basalts lie mostly in the centre of the MORB part of that field, reflecting a high degree of melting of fertile mantle. Application of the proxies to some examples of MORB ophiolites helps them to be further classified as C (contaminated)-MORB, N (normal)-MORB, E (enriched)-MORB and P (plume)-MORB ophiolites, which may add a useful dimension to ophiolite classification. In the Archean, the hotter magmas, higher crustal geotherms and higher Th contents of contaminants all result in widespread crustal input that is easy to detect geochemically with the Th–Nb proxy. Application of this proxy to Archean greenstones demonstrates that almost all exhibit a crustal component even when reputedly oceanic. This indicates, either that some interpretations need to be re-examined or that intra-oceanic crustal input is important in the Archean making the proxy less effective in distinguishing oceanic from continental settings. The Ti–Yb proxy is not effective for fingerprinting Archean settings because higher mantle potential temperatures mean that lithospheric thickness is no longer the critical variable in determining the presence or absence of residual garnet. 相似文献
8.
地幔中不同含量的水会对洋壳的生成产生重要影响,但目前不同含水量下的均匀和局部含水地幔会怎样影响洋壳厚度还不清楚.利用动力学数值模拟的方法,对上地幔均匀含水和局部含水两种情况下洋壳的生成过程展开研究.结果表明:当上地幔均匀含水时,含水量的增加在减小最大熔融分数的同时,会增大初始熔融深度和熔融面积,因而生成的洋壳厚度会增加.当上地幔局部含水时,局部含水地幔熔融后也会增大生成的洋壳厚度,但开始影响洋壳厚度的时间与其含水量有关.结合南海洋壳特征进一步分析认为:南海扩张期间其地幔源含水量具有非均质性.东部次海盆的洋壳比西南次海盆厚1 km,可能是因为前者地幔源含水量整体高于后者(本模型表明约高50×10-6).南海玄武岩中碳酸盐化硅酸盐熔体的存在,可能是由局部高含水量地幔在深部熔融产生的熔体携带上来的.南海洋壳厚度在时间上没有大幅度变化,可能是因为局部高含水量地幔的体积相对较小或体积虽大但其含水量没有显著高于地幔背景含水量. 相似文献
9.
青藏高原东北缘岩石圈密度与磁化强度及动力学含义 总被引：4，自引：0，他引：4
利用横贯柴达木盆地南北的格尔木—花海子剖面岩石圈二维P波速度结构以及地震波速度与介质密度之间的关系,建立了该剖面岩石圈二维密度结构与二维磁化强度的初始模型。依据重磁同源原理,在柴达木盆地重、磁异常的二重约束下完成了重磁联合反演,获得了该剖面岩石圈二维密度结构与二维磁化强度分布。结果表明:柴达木盆地地壳厚度沿测线变化较大,平均厚度约60km。在柴达木盆地南缘地壳厚约50km,达布逊湖附近地壳最厚为63km左右,大柴旦附近地壳较薄,为50km左右。柴达木盆地的地壳纵向上可分为三层,即上地壳、中地壳与下地壳。位于盆地中部的中、下地壳分别发育大范围的壳内低密度体,并处于上地幔隆起的背景之上;横向上可将盆地分成南北两个部分,分界在达布逊湖附近。整个剖面结晶基底埋深变化也很大,在达布逊湖附近为12km,在昆仑山北缘基底几乎出露地表。结晶基底的展布形态与地壳底界,即莫霍面呈近似镜像对称。综合研究认为,柴达木盆地的岩石圈结构存在着明显的南北差异,其分界在达布逊湖的北面。在盆地南部,岩石圈介质横向变化较小,各层介质分布正常;在盆地的北侧,岩石圈结构特别在中、下地壳和上地幔顶部横向上发生了变化。壳内低密度体的存在意味着柴达木盆地具有较热的岩石圈和上地幔,加之基底界面与莫霍面的镜像对称分布,形成与准噶尔盆地和塔里木盆地的构造差异。多种地球物理参数所揭示的地壳上地幔结构及其横向变化特点为柴达木盆地构造演化及青藏高原北部边界的地球动力学研究提供了岩石圈尺度的地球物理证据。 相似文献
10.
The Uralide orogen, in Central Russia, is the focus of intense geoscientific investigations during recent years. The international research is motivated by some unusual lithospheric features compared with other collisional belts including the preservation of (a) a collisional architecture with an orogenic root and a crustal thickness of 55–58 km, and (b) large volumes of very low-grade and non-metamorphic oceanic crust and island arc rocks in the upper crust of a low–relief mountain belt. The latter cause anomalous gravity highs along the thickened crust and the isostatic equilibrium inside the Uralides lithosphere as well as the overthrust high-metamorphic rocks. The integrated URSEIS '95 seismic experiment provides fundamentally new data revealing the lithospheric architecture of an intact Paleozoic collisional orogen that allows the construction of density models. In the Urals' lithosphere different velocity structures resolved by wide-angle seismic experiments along both the URSEIS '95- and the Troitsk profile. They can be used to constrain lithospheric density models: a first model consists of a deep subducted continental lower crust which has been highly eclogitized at depths of 60–90 km to a density of 3550 kg/m3. The second model shows a slightly eclogitized lower crust underlying the Uralide orogen with a crustal thickness of 60 km. The eclogitized lower crust causes a too-small impedance contrast to the lithospheric mantle resulting in a lack of reflectors in the area of the largest crustal thickness. Both models fit the measured gravity field. Analyzing the isostatic state of the southern Urals' lithosphere, both density models are in isostatic equilibrium. 相似文献
11.
利用中国地震台网和ISC台站记录的P波到时数据,采用球坐标系有限差分地震层析成像方法反演了南海东北部及其邻近地区壳幔三维P波速度结构,并分析了不同地质单元的构造差异及其深部特征。结果表明:南海东北部表现出陆架地区的岩石层特性,属于华南大陆向海区的延伸,岩石层厚度较大,现今不存在大规模的地幔热流活动,推测大陆边缘张裂作用仅限于地壳内部而没有延伸进入上地幔,具有非火山型大陆边缘的深部特点。中央海盆附近上地幔P波速度明显降低,与海盆下方地幔热流活动密切相关。不同的速度异常特征表明:华南大陆暨台湾地区属于欧亚大陆的正常地壳或是与菲律宾海板块相互作用产生的增厚型地壳,冲绳海槽则是弧后扩张产生的减薄型地壳。滨海断裂带作为华南大陆高速异常和南海北部高速异常的分界,代表了一定地质时期华南地块和南海地块的拼合边界。断裂附近的上地幔低速异常揭示了闽粤沿海岩浆作用的深层动力机制。吕宋岛弧、马尼拉海沟、东吕宋海槽的速度异常与其所处的特殊构造位置有密切的关系,清晰地反映出岛弧俯冲带的地壳结构差异;台湾南部至吕宋岛弧的上地幔低速异常揭示了两个重要火山链的深部构造特征,北吕宋海脊下方100 km深度的条带状高速异常有可能代表了俯冲下沉的岩石层板片。 相似文献
12.
天然地震S波和大地电磁测深给出了两种不同的藏北岩石圈厚度模型,两种测量结果的地质含义至今还不十分清楚。通过对地表高程与地壳厚度回归关系的研究,以回归直线的斜率和截距作为地壳和岩石圈地幔平均密度取值的约束,并考虑相变因素对软流圈密度的影响,采用均衡理论对藏北岩石圈厚度进行了计算。计算结果表明,在可能的软流圈温度取值范围内藏北岩石圈的平均厚度约为106～120km,地壳增厚前的岩石圈平均厚度约80km。藏北新生代火山作用和岩浆起源-分凝深度分析表明,藏北现今岩石圈厚度主要受金云母脱水深度所控制。增厚前岩石圈地幔底部温度高于橄榄岩湿固相线温度,并受闪石和金云母高压脱水作用的影响。加厚岩石圈地幔因其底部不断发生脱水低程度熔融而进入软流圈小尺度对流体系,使岩石圈加厚过程中伴随有底部的脉动减薄作用。 相似文献
13.
U-Pb isotopic thermochronometry of rutile, apatite and titanite from kimberlite-borne lower crustal granulite xenoliths has been used to constrain the thermal evolution of Archean cratonic and Proterozoic off-craton continental lithosphere beneath southern Africa. The relatively low closure temperature of the U-Pb rutile thermochronometer (~400-450 °C) allows its use as a particularly sensitive recorder of the establishment of "cratonic" lithospheric geotherms, as well as subsequent thermal perturbations to the lithosphere. Contrasting lower crustal thermal histories are revealed between intracratonic and craton margin regions. Discordant Proterozoic (1.8 to 1.0 Ga) rutile ages in Archean (2.9 to 2.7 Ga) granulites from within the craton are indicative of isotopic resetting by marginal orogenic thermal perturbations influencing the deep crust of the cratonic nucleus. In Proterozoic (1.1 to 1.0 Ga) granulite xenoliths from the craton-bounding orogenic belts, rutiles define discordia arrays with Neoproterozoic (0.8 to 0.6 Ga) upper intercepts and lower intercepts equivalent to Mesozoic exhumation upon kimberlite entrainment. In combination with coexisting titanite and apatite dates, these results are interpreted as a record of postorogenic cooling at an integrated rate of approximately 1 °C/Ma, and subsequent variable Pb loss in the apatite and rutile systems during a Mesozoic thermal perturbation to the deep lithosphere. Closure of the rutile thermochronometer signals temperatures of °C in the lower crust during attainment of cratonic lithospheric conductive geotherms, and such closure in the examined portions of the "off-craton" Proterozoic domains of southern Africa indicates that their lithospheric thermal profiles were essentially cratonic from the Neoproterozoic through to the Late Jurassic. These results suggest similar lithospheric thickness and potential for diamond stability beneath both Proterozoic and Archean domains of southern Africa. Subsequent partial resetting of U-Pb rutile and apatite systematics in the cratonic margin lower crust records a transient Mesozoic thermal modification of the lithosphere, and modeling of the diffusive Pb loss from lower crustal rutile constrains the temperature and duration of Mesozoic heating to °C for ka. This result indicates that the thermal perturbation is not simply a kimberlite-related magmatic phenomenon, but is rather a more protracted manifestation of lithospheric heating, likely related to mantle upwelling and rifting of Gondwana during the Late Jurassic to Cretaceous. The manifestation of this thermal pulse in the lower crust is spatially and temporally correlated with anomalously elevated and/or kinked Cretaceous mantle paleogeotherms, and evidence for metasomatic modification in cratonic mantle peridotite suites. It is argued that most of the geographic differences in lithospheric thermal structure inferred from mantle xenolith thermobarometry are likewise due to the heterogeneous propagation of this broad upper mantle thermal anomaly. The differential manifestation of heating between cratonic margin and cratonic interior indicates the importance of advective heat transport along pre-existing lithosphere-scale discontinuities. Within this model, kimberlite magmatism was a similarly complex, space- and time-dependent response to Late Mesozoic lithospheric thermal perturbation. 相似文献
14.
《International Geology Review》2012,54(4):492-507
Numerous Triassic granitoids in the Qinling orogenic belt related to the Late Triassic collision between the North China Craton (NCC) and the Yangtze Block (YB) are important for determining the crustal composition at depth and the geodynamic processes by which the orogen formed. Most of the Triassic plutons in the Qinling orogen were emplaced between 205 and 225 Ma. The granitoid rocks from the southern margin of the NCC, North Qinling, South Qinling, and the northern margin of the YB that were emplaced during this interval have two-stage Hf model ages of 0.60–2.52 Ga (average 2.19 Ga), 0.90–2.66 Ga (average 1.29 Ga), 0.41–3.04 Ga (average 1.48 Ga), and 1.00–1.84 Ga (average 1.34 Ga), respectively, and mean εHf(t) values of ?14.5, ?0.32, ?1.36, and ?3.98, respectively. The Hf isotope compositions of the granitoids in different tectonic units differ significantly, mirroring the diverse history of crustal growth of the four units.The temporal and spatial distribution and Hf isotope compositions of the granitoids suggest that there was a unified geodynamic process that triggered the magmatism. Formation of the Triassic granitoid plutons at 225–205 Ma was a consequence of slab break-off or E–W-striking slab tearing, related to slab rollback in the west part of the Qinling orogen and oblique continental collision in the east. Upwelling of the asthenospheric mantle led to partial melting of the subcontinental lithospheric mantle and the lower crust, and mixing and/or mingling of the resulting magmas resulted in the formation of granitoids with diverse geological and geochemical characteristics. 相似文献
15.
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. 相似文献
16.
We present a method based on the combination of elevation and geoid anomaly data together with thermal field to map crustal and lithospheric thickness. The main assumptions are local isostasy and a four-layered model composed of crust, lithospheric mantle, sea water and the asthenosphere. We consider a linear density gradient for the crust and a temperature dependent density for the lithospheric mantle. We perform sensitivity tests to evaluate the effect of the variation of the model parameters and the influence of RMS error of elevation and geoid anomaly databases. The application of this method to the Gibraltar Arc System, Atlas Mountains and adjacent zones reveals the presence of a lithospheric thinning zone, SW–NE oriented. This zone affects the High and Middle Atlas and extends from the Canary Islands to the eastern Alboran Basin and is probably linked with a similarly trending zone of thick lithosphere constituting the western Betics, eastern Rif, Rharb Basin, and Gulf of Cadiz. A number of different, even mutually opposite, geodynamic models have been proposed to explain the origin and evolution of the study area. Our results suggest that a plausible slab-retreating model should incorporate tear and asymmetric roll-back of the subducting slab to fit the present-day observed lithosphere geometry. In this context, the lithospheric thinning would be caused by lateral asthenospheric flow. An alternative mechanism responsible for lithospheric thinning is the presence of a hot magmatic reservoir derived from a deep ancient plume centred in the Canary Island, and extending as far as Central Europe. 相似文献
17.
南海北部陆缘发育独特的远端带结构,以“裂谷宽、基底厚和地貌起伏”为主要特点,显著有别于经典贫岩浆型和富岩浆型张裂陆缘.为了解释陆缘结构的成因,综合已有研究进展和国际大洋发现计划（IODP）的钻探成果,对南海北部陆缘基底性质进行了调研,探讨了拆离断层和岩浆作用的特点以及两者间的相互作用.结果表明,在38 Ma之前南海北部大范围发育核杂岩构造,并伴随大量岩浆侵入到中下地壳;岩浆作用一方面加剧了地壳的韧性变形,导致应变无法集中而在多个地方同时发育大型拆离,另一方面对拆离面和减薄的基底进行了强烈改造.最终提出同张裂期就位的岩浆作用和中下地壳的韧性流动是形成南海北部宽裂谷陆缘的关键,深化了对陆缘结构、变形过程和岩石圈减薄机制的理解. 相似文献
18.
T. V. Romanyuk 《Geotectonics》2009,43(4):305-323
The presented model of the Late Cenozoic geodynamic evolution of the central Andes and the complex tectonic, geological, and geophysical model of the Earth’s crust and upper mantle along the Central Andean Transect, which crosses the Andean subduction zone along 21°S, are based on the integration of voluminous and diverse data. The onset of the recent evolution of the central Andes is dated at the late Oligocene (27 Ma ago), when the local fluid-induced rheological attenuation of the continental lithosphere occurred far back of the subduction zone. Tectonic deformation started to develop in thick-skinned style above the attenuated domain in the upper mantle and then in the Earth’s crust, creating the bivergent system of the present-day Eastern Cordillera. The destruction of the continental lithosphere is correlated with ore mineralization in the Bolivian tin belt, which presumably started at 16° S and spread to the north and to the south. Approximately 19 Ma ago, the gently dipping Subandean Thrust Fault was formed beneath the Eastern Cordillera, along which the South American Platform began to thrust under the Andes with rapid thickening of the crust in the eastern Andean Orogen owing to its doubling. The style of deformation in the upper crust above the Subandean Thrust Fault changed from thick- to thin-skinned, and the deformation front migrated to the east inland, forming the Subandean system of folds and thrust faults verging largely eastward. The thickening of the crust was accompanied by flows at the lower and/or middle crustal levels, delamination, and collapse of fragments of the lower crust and lithospheric mantle beneath the Eastern Cordillera and Altiplano-Puna Plateau. As the thickness of the middle and lower crustal layers reached a critical thickness about 10 Ma ago, the viscoplastic flow in the meridional direction became more intense. Extension of the upper brittle crust was realized mainly in gliding and rotation of blocks along a rhombic fault system. Some blocks sank, creating sedimentary basins. The rate of southward migration estimated from the age of these basins is 26 km/Ma. Tectonic deformation was accompanied by diverse magmatic activity (ignimbrite complexes, basaltic flows, shoshonitic volcanism, etc.) within the tract from the Western Cordillera to the western edge of the Eastern Cordillera 27–5 Ma ago with a peak at 7 Ma; after this, it began to recede westward; by 5 Ma ago, the magmatic activity reached only the western part of the Altiplano-Puna Plateau, and it has been concentrated in the volcanic arc of the Western Cordillera during the last 2 Ma. 相似文献
19.
The relatively low elevation and thick crust in the Altiplano, in comparison to the higher elevation, but thinner crust in the Puna plateau, together with geophysical data, suggests that isostatic equillibrium is achieved by cooler and denser lithospheric mantle in the Altiplano. Excess density in the Altiplano mantle could create differential horizontal stress in the order of 25 MPa between both lithospheric columns. Numerical models accounting for pressure and temperature-dependent rheology show that such stress can induce horizontal ductile flow in the lower crust, from the Puna towards the Altiplano. With a minimum viscosity of 1019 Pa s, this flow reaches 1 cm/year, displacing more than 50 km of material within 5 Ma. If the lower crust viscosity is smaller, the amount of orogeny-parallel lower crustal flow can be even greater. Such a mechanism of channel flow may explain that different amounts of crustal material have been accommodated by shortening in the Altiplano and in the Puna. Because of the strength of the elastic-brittle upper crust, this channel flow does not necessitate large amounts of surface deformation (except vertical uplift), making it difficult to detect from the geology. 相似文献
20.
The North Penninic basin was a subbasin in the northern part of the Mesozoic Tethys ocean. Its significance within the framework of this ocean is controversial because it is not clear whether it was underlain by thinned continental or oceanic crust. Remnants of the eastern North Penninic basin are preserved in the Alps of eastern Switzerland (Grisons) as low metamorphic "Bündnerschiefer" sediments and associated basaltic rocks which formed approximately 140–170 Ma ago (Misox Bündnerschiefer zone, Middle Jurassic to Early Cretaceous). Nb/U, Zr/Nb, and Y/Nb ratios, as well as Nd–Sr isotopic and REE data of most of the metabasalts point to a depleted MORB-type mantle origin. They have been contaminated by magmatic assimilation of Bündnerschiefer sediments and by exchange with seawater, but do not prove the existence of a subcontinental lithospheric mantle or continental crust beneath the North Penninic basin. This suggests that the studied part of the North Penninic realm was underlain by oceanic crust. Only the metabasalts from two melange zones (Vals and Grava melanges) show a more important contamination by crustal material. Since this type of contamination cannot be observed in the other tectonic units, we suggest that its occurrence is related to melange formation during the subduction of the North Penninic basin in the Tertiary. The North Penninic basin was probably, despite the occurrence of oceanic crust, smaller than the South Penninic ocean where the presence of oceanic crust is well established. Modern analogues for the North Penninic basin could be the transitional zone of the Red Sea or the pull-apart basins of the southernmost Gulf of California where local patches of oceanic crust with effusive volcanism have been described. 相似文献