Dynamic mechanisms controlling the topography of Longmenshan area   总被引：1，自引：0，他引：1  

Xie  Yuan  Li  Yongdong  Xiong  Xiong 《中国科学:地球科学(英文版)》2020,63(1):121-131

The Longmenshan fault, which defines the eastern edge of the Tibetan Plateau, is one of the steepest margins of the plateau with a sharp elevation drop of about 4 km over a distance less than 100 km across the Longmenshan fault. The mechanism which is responsible for controlling and maintaining the elevation difference is highly debated. Using multiple observations including seismic velocity model, Moho depth, effective elastic thickness of the lithosphere, we conducted a quantitative study for elucidating the contributions from crust and lithospheric mantle by an integrated analysis of lithospheric isostasy and flexure. It is shown that the topography of the Longmenshan fault is supported by both lithospheric isostasy and flexure statically, and lower crustal channel flow and mantle convection dynamically. Different mechanisms have different weights for contribution to the topography of the Songpan-Ganzi block and the Sichuan Basin. The static and dynamic support contribute roughly the same to the topographic difference of ~4 km between the two sides of the Longmenshan fault. The static topographic difference of ~2 km is mainly resulted from the lithospheric isostasy, while the dynamic one of ~2 km is contributed by the uprising of the accumulated material in the lower crust beneath the Songpan-Ganzi block and the downward drag force caused by the upper mantle convection under the Sichuan Basin. It is thus suggested that the lower crustal flow and upper mantle convection are dynamic forces which should be taken into account in the studies on the dynamics in the Longmenshan and surrounding regions.  相似文献   

Lithospheric flexural modelling of the seaward and trenchward of the subducting oceanic plates     

Jiangyang Zhang  Zhen Sun 《International Geology Review》2020,62(7-8):908-923

ABSTRACT

Based on a two-segment plate flexural modelling, we investigated the effective elastic thickness of global subducting oceanic lithosphere. Our results show that for the plate age of 0 to 50 Ma, the seaward effective elastic thickness T _e ^M values are located between 600 and 900°C isotherms, and do not track any isotherm, while the majority of the trenchward effective elastic thickness T _e ^m values are located between 300 and 600°C isotherms. For the plate age older than 50 Ma, T _e ^M values basically matches the 600°C isotherm with some fluctuations for the age older than 110 Ma, while T _e ^m values mainly fall between 200 and 400°C isotherms. The reduction in effective elastic thickness (T _e ^M-T _e ^m) varies from 2.6 to 30.1 km, or 11–68% of seaward T _e ^M. Thus, the absolute value of the decrease in the effective elastic thicknesses (T _e ^M-T _e ^m) increases with the plate age, while the percentage reduction in the effective elastic thickness (1-T _e ^m/T _e ^M) has no obvious relationship with the age, but more related to the curvature of bending plate. Almost all bending-related earthquakes occurred above the T _e ^M line, but many normal-faulting earthquakes are deeper than the T _e ^M-T _e ^m line, implying that the plate may still retain some thickness of an elastic property core in the areas (depth) where earthquakes occur.  相似文献   

Mechanisms for the formation of ridge-axis topography atslow-spreading ridges: a lithospheric-plate flexural model     

Y. John Chen  & Jian Lin 《Geophysical Journal International》1999,136(1):8-18

The seafloor topography of a slow-spreading ridge shows a number of well-documented regularities at the ridge segment scale as the result of the complex interplay between ridge-axis magmatic and tectonic processes. This paper describes the results of a detailed analysis of the seafloor topography of the Mid-Atlantic Ridge near the Atlantis transform, where marine gravity data provide independent, although non-unique, constraints on subseafloor density structure. Using a combined topography and gravity data set, we identified the specific contributions of subseafloor density structure to the seafloor topography. We show that the observed along-axis deepening (0.3–0.8 km) from the midpoint of a ridge segment towards the non-transform offsets in the study area can be explained by the vertical deflection of a zero-age plate in response to along-axis crustal thickness variations. However, this effect can only account for 50–60 per cent of the observed 1.5–1.7 km deepening towards the Atlantis transform, suggesting the presence of significant stresses in the lithosphere near a transform. Results of plate flexural calculations also predict a more elevated rift flank at the inside corner of the ridge–transform intersection than at the conjugate outside corner. Such an asymmetry in rift flank topography is calculated to be greatest near a transform fault with a significant volume of deep transform valley and when adjacent plates across the transform fault are mechanically decoupled or only weakly coupled. Together these results illustrate the complex interplay between various tectonic processes at a slow-spreading ridge.  相似文献   

Implications of a ductile crustal layer for the deformation caused by the Fennoscandian ice sheet     

Volker Klemann  & Detlef Wolf  《Geophysical Journal International》1999,139(1):216-226

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The influence of a stratified rheology on the flexural response of the lithosphere to (un)loading by extensional faulting   总被引：2，自引：0，他引：2  

M. ter Voorde  R. T. van Balen  G. Bertotti  & S. A. P. L. Cloetingh 《Geophysical Journal International》1998,134(3):721-735

We present a two-layered finite difference model for the flexural response of the lithosphere to extensional faulting. The model allows for three modes of flexure: (1) fully coupled, with the upper crust and mantle welded together by the lower crust; (2) fully decoupled, with the upper crust and mantle behaving as independent layers; and (3) partly decoupled, signifying that the response of the upper crust to small-wavelength loads is superimposed on the response of the entire lithosphere to long-wavelength loads. Which of these modes of flexure is to be expected depends on the rheology and especially the thermal state of the lithosphere. Coupled behaviour is related to a cold and strong lithosphere. The Baikal Rift Zone provides a typical example for this mode of flexure. A fully decoupled lithosphere is an exceptional case, related to anomalous high temperatures in the lower crust, and is observed in the Basin and Range province. The most common case is a partly decoupled lithosphere, with the degree of decoupling depending on the thickness and viscosity of the lower crust. This is inferred, for example, for the Bay of Biscay margin.  相似文献   

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杨辉  王勇 《大地测量与地球动力学》2007,27(2):25-31

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渤海东部庙北地区古近系层序与沉积特征     

刘朋波  王昕  官大勇  李才  高坤顺  张海义 《地质科技情报》2012,31(3):1-7

综合壁心、薄片、钻井和三维地震等资料分析渤海东部庙北地区古近系层序与沉积特征,并运用地震相、古地貌和地震属性刻画沉积体系平面展布.研究表明,古近系由1个二级层序、4个三级层序组成,经历了一个完整的湖泊初始发育、扩张及萎缩阶段,主要发育扇三角洲、碳酸盐岩浅滩、半深湖-深湖、辫状河三角洲等沉积体系.不同的沉积体系形成于不同的湖泊演化阶段,地震响应特征也不同.扇三角洲和碳酸盐岩浅滩发育于湖泊初始发育阶段,扇三角洲表现为填充型楔形前积反射,沿凸起边缘断坡带呈朵状分布;碳酸盐岩浅滩表现为水进型丘形叠瓦状反射,呈椭圆状分布于水下古隆起带上,椭圆的长轴方向平行于水下古隆起带走向;半深湖-深湖相暗色泥岩发育于湖泊扩张期,表现为平行连续中弱振幅反射,在庙西北洼和渤东凹陷深陷带最为发育;辫状河三角洲发育于湖泊萎缩期,表现为低角度中高频、中弱振幅前积反射,沿凸起边缘挠曲坡折带呈朵叶状分布.庙北地区油气显示普遍,这种沉积体系识别与刻画结果对于该区油气勘探具有重要意义.  相似文献   

A combined local damage index for seismic assessment of existing RC structures     

P. E. Mergos  A. J. Kappos 《地震工程与结构动力学》2013,42(6):833-852

A new local damage index for existing RC structures is introduced, wherein deterioration caused by all deformation mechanisms (flexure, shear, anchorage slip) is treated separately for each mechanism. Moreover, the additive character of damage arising from the three response mechanisms, and the increase in degradation rate caused by their interaction, are fully taken into consideration. The proposed local damage index is then applied, in conjunction with a finite element model developed previously by the authors, to assess seismic damage response of several RC column and frame test specimens with substandard detailing. It is concluded that in all cases and independently from the prevailing mode of failure, the new local damage index describes well the damage pattern of the analysed specimens. Copyright © 2012 John Wiley & Sons, Ltd.  相似文献   

Tectonic significance of syn-rift sediment packages across the Gabon-Cabinda continental margin   总被引：1，自引：0，他引：1  

Garry D. Karner  Neal W. Driscoll  John P. McGinnis  William D. Brumbaugh  Nick R. Cameron 《Marine and Petroleum Geology》1997,14(7-8)

The tectonic development of a continental margin is recorded in the stratigraphic successions preserved along and across the margin in terms of stratal relationships (e.g., onlap, downlap, truncation), lithofacies, biostratigraphy, and paleo-water depths. By using these observations coupled to a kinematic and flexural model for the deformation of the lithosphere, we have elucidated the tectonic significance of the preserved stratigraphy that comprises the Gabon-Cabinda margin of west Africa. Two hinge zones, an Eastern and Atlantic, formed along the Gabon-Cabinda margin in response to three discrete extensional events occuring from Berriasian to Aptian time. The Eastern hinge zone demarcates the eastern limit of a broadly distributed Berriasian extension that resulted in the formation of deep anoxic, lacustrine systems as evidenced by the silts and shales of the Sialivakou and lower Djeno Formations and the regressive packages of the upper Djeno Formation. Approximately 1.5 to 2 km of asymmetric footwall uplift was induced across the Eastern hinge zone in response to the mechanical unloading of the lithosphere during this first phase of rifting. In contrast, the Atlantic hinge, located approximately 90 km west of the Eastern hinge, marks the eastern limit of a second phase of extension that began in the Hauterivian. Footwall uplift and rotation exposed earlier syn-rift and pre-rift sediments to at least wavebase causing varying amounts of erosional truncation across the Atlantic hinge zone along much of the Gabon-Cabinda margins. We interpret the thickness variations of reworked clastic sediment of this age (e.g. the Melania Formation) between the hinge zones as indicative of variations in the degree of uplift and erosional truncation of the Atlantic hinge. For example, the absence of Melania Formation across the Congo margin implies that uplift of the Atlantic hinge was relatively minor compared to that across the Cabinda and Gabon margins, the latter being characterized by significant thicknesses of Melania Formation (or equivalent). Material eroded from the Cabinda and Gabon Atlantic hinge zone may in part account for the thick wedge of sediment deposited seaward of the Gabon-Cabinda Atlantic hinge (the Erva Formation). Our modelling suggests that this wedge of reworked elastics represents deposition by along-axis gravity flows within a deep water (≈2 km) environment. A third and final phase of extension in the late Barremian-early Aptian was responsible for breaching the continental lithosphere to form the ocean/continent boundary and thus the installation of open marine conditions. Elsewhere, the environments will tend to be marginal marine to brackish, depending on the efficiency of the Atlantic hinge zone to act as a barrier to marine enchroachment. This third rift phase reactivated both the Eastern and Atlantic hinge0zones thereby creating accomodation for the Marnes Noires Formation (and equivalent) source rock deposition between the hinges and the Falcão source rock equivalent seaward of the Atlantic hinge. Two possible scenarios exist for the lateral distribution of the Marnes Noires Formation. If the reactivated rift flank topography across the Atlantic hinge was significant, then sedimentation would be restricted between the hinge zones within discrete lacustrine settings (e.g., Congo margin). Alternatively, if hinge zone uplift was relatively minor, then a coral-rimmed archipelago may have developed parallel to the margin with restricted communication across the Atlantic hinge zone (e.g., Cabinda margin). In this latter scenario, dilution of the Marnes Noires source rocks by terrigenous input from the eroding Atlantic hinge zone should be relatively minor thereby enhancing source rock quality. Furthermore, potential marine upwelling outboard of the Atlantic hinge zone is likely the cause for the production and accumulation of organic-rich material associated with the Falcão source rock of the Kwanza basin. By late Aptian time, the remaining accomodation between the hinge zones was partially filled by across- and along-axis prograding deltaic systems of the Argilles Vertes and Tchibota Formations. The progradation and interaction of the Argilles Vertes depositional lobes resulted in the formation of residual paleo-relief. Subsequent marine incursions and flooding of this paleo-relief led to the development of basal conglomerates (the Chela ‘lag’ unconformity) grading upward into fine-grained sands and evaporites. Consequently, an inverse relationship should exist betweeb evaporite thickness (in particular, the lower members) and the thickness of the underlying Argilles Vertes and Tchibota Formations. Variations in Loeme evaporite thickness is a consequence of stratigraphic and structural control with salt instability influencing local variability.Our modelling suggests the occurrence of two distinct evaporite sequences on the Congo margin, an earlier evaporite deposited seaward (west) of the Atlantic hinge during the second and third rift phases and the late Aptian Loeme Formation deposited between the hinge zones. An evaporite sequence seaward of the Atlantic hinge is inferred on the basis of extensive diapirs and salt tectonic structures observed in seismic data. In order to match the distribution and thickness of the observed post-salt stratigraphy across the basin, however, we require large paleowater depths west of the Atlantic hinge during the later Aptian. The existence of large paleowater depths precludes the formation of thick evaporite sequences within the outer basin. Consequently, we propose that the evaporites seaward of the Atlantic hinge were formed during the syn-rift development of the margin and are not contemporaneous with the post-rift Loeme salts deposited between the hinge zones. This double salt hypothesis is consistent with observations from the conjugate Brazilian margin.  相似文献   

Isostasy and paleotemperatures in the Southern Tyrrhenian Basin,Mediterranean Sea     

V. Pasquale  P. Chiozzi  M. Verdoya 《Marine Geophysical Researches》2007,28(2):139-151

The present-day basement depth of the seafloor in the absence of sediment loading was inferred along a traverse crossing the Southern Tyrrhenian Basin. A correction for sediment loading was proposed on the basis of density, seismic velocity and porosity data from selected deep boreholes. The empirical relation between sediment correction and seismic two-way travel time was extrapolated downward by applying the Nafe–Drake curve and a specific porosity–depth relation. The sediment loading response of the basement calculated for flexural isostasy is on average about one hundred meters lower than results for local isostasy. A pure lithosphere extensional model was then used to predict quantitatively the basement subsidence pattern on the margins of the basin. The basement depth is consistent with uniform extension model predictions only in some parts of the margins. The observed variability in the region of greatest thinning (transition from continental to oceanic crust) is attributable to the weakening effect caused by diffuse igneous intrusions. Subsidence of the volcanic Calabrian–Sicilian margin is partly accounted for by magmatic underplating. The comparison of the calculated subsidence with an oceanic lithosphere cooling model shows that subsidence is variable in some areas, particularly in the Marsili Basin. This argues for a typical back-arc origin for the Tyrrhenian Basin, as a result of subduction processes. By taking into account the geodynamic setting, stratigraphic data from the deepest hole and the terrestrial heat flow, we reconstructed the paleotemperatures of cover sediments. The results suggest that low temperatures generally have prevailed during sediment deposition and that the degree of maturation is expected not to be sufficient for oil generation processes.  相似文献