喜马拉雅中东部镜像均衡重力异常的形成研究

青藏高原南缘处于重力不均衡状态,由北向南可依次分为高原近重力均衡区、喜马拉雅山正均衡异常区和山前盆地负均衡异常区,正、负异常呈现壮观的镜像分布。本文选取喜马拉雅中东部的均衡重力异常数据,结合地貌高程、地壳厚度、降雨量、冰川及山前沉积等的分布状况,探讨地貌分异与均衡重力异常分布的相互关系。由上述资料获得3条跨越喜马拉雅山的综合剖面,结果显示喜马拉雅中东部正均衡重力异常的分布与冰川、河流等代表的地表剥蚀作用存在明显的空间耦合关系,而与降雨量无直接联系,山前盆地负均衡重力异常与沉积厚度的分布也存在很好的耦合。利用数值模型计算得到了喜马拉雅地区的均衡调整时间域在1 Ma左右的时间尺度内。通过与地貌响应时间域相对比,以及对地表剥蚀厚度的估计,认为山脉地区的正均衡异常主要由地壳厚度补偿不足引起(侧重Airy假说),而山前盆地的负均衡异常主要由低密度沉积层的分布引起(侧重Pratt假说),由于地貌响应时间快于均衡调整时间,在大约5～2 Ma以来,地壳的均衡调整始终延迟于山脉的持续剥蚀和山前的持续沉积,使得岩石圈朝着"反均衡"方向演变,最终形成了喜马拉雅现今壮观的镜像均衡重力异常分布。     

前陆盆地的形成机制和充填演化

前陆盆地发育于前陆褶皱冲断带之上。它是在褶皱冲断带负载的作用下,岩石圈通过区域性均衡调整及流变挠曲而形成的。本文从流变学角度讨论了在前陆地区褶皱逆冲作用下,岩石圈流变、盆地形成及沉积充填三者间的内在成因联系,并着重介绍了前陆盆地形成过程中的两种岩石圈流变挠曲模型及前陆盆地充填演化的模拟反演。     

攀西地区重力均衡异常的研究

研究表明,康滇地轴及其东部地区属均衡补偿平衡区,西部属强烈上升的异常区。上升的原动力为板块碰撞的构造力与均衡调整力的综合。     

地壳均衡与海平面变化

提要:在全球海面变化的近期研究中，无论是均衡模式、还是重力模式，写考虑到冰川均衡作用和水力均衡作用。本文认为，绝对均衡表示地壳均街的最终结果，相对均衡表示地壳均衡中一系列中间过程。这两种均衡之间的差别，往往被人们所忽视。
    定量计算的结果表明，冰盖的吸引不仅使周围的海面升高，而且使内核在外核中产生定向运动。由于参加相对均衡的大陆壳数量不同、宕石圈的强度不同，陆壳与陆充、陆克与洋壳之间将产生不同幅度的垂直运动。这为解释全球海面变化不一致性提供了另一个新思路。
        

Relative sea‐level changes,glacial isostatic modelling and ice‐sheet reconstructions from the British Isles since the Last Glacial Maximum

While contributing <1 m equivalent eustatic sea‐level rise the British Isles ice sheet produced glacio‐isostatic rebound in northern Britain of similar magnitude to eustatic sea‐level change, or global meltwater influx, over the last 18 000 years. The resulting spatially variable relative sea‐level changes combine with observations from far‐field locations to produce a rigorous test for quantitative models of glacial isostatic adjustment, local ice‐sheet history and global meltwater influx. After a review of the attributes of relative sea‐level observations significant for constraining large‐scale models of the isostatic adjustment process we summarise long records of relative sea‐level change from the British Isles and far‐field locations. We give an overview of different global theoretical models of the isostatic adjustment process before presenting intercomparisons of observed and predicted relative sea levels at sites in the British Isles and far‐field for a range of Earth and ice model parameters in order to demonstrate model sensitivity and the resolving power available from using evidence from the British Isles. For the first time we show a good degree of fit between relative sea‐level observations and predictions that are based upon global Earth and ice model parameters, independently derived from analysis of far‐field data, with a terrain‐corrected model of the British Isles ice sheet that includes extensive glaciation of the North Sea and western continental shelf, that does not assume isostatic equilibrium at the Last Glacial Maximum and keeps to trimline constraints of ice surface elevation. We do not attempt to identify a unique solution for the model lithosphere thickness parameter or the local‐scale detail of the ice model in order to provide a fit for all sites, but argue that the next stage should be to incorporate an ice‐sheet model that is based on quantitative, glaciological model simulations. We hope that this paper will stimulate this debate and help to integrate research in glacial geomorphology, glaciology, sea‐level change, Earth rheology and quantitative modelling. Copyright © 2006 John Wiley & Sons, Ltd.     

Course and origin of the Fennoscandian uplift: the case for two separate mechanisms

The classical glacial isostatic uplift paradigm of Fennoscandia is in need of a thorough revision as to its course and origin. The total uplift amounts to 830 m (not 300 m). The centre of uplift started to rise 13,000 BP (not 9300 BP). The uplift appears to be composed of two separate mechanisms; one true glacial isostatic rebound, and one linear uplift factor possibly representing a phase boundary adjustment. Calculations and modelling must treat these two factors independently (which has not been the case).     

Subcrustal density inhomogeneities of Northern Eurasia as derived from the gravity data and isostatic models of the lithosphere

For the territory of Northern Eurasia (6°E–165°W; 30–75°N) the distribution of anomalous masses in the lithosphere has been estimated in accordance with the lithosphere isostatic model. The method of model construction is based on the admittance technique. The experimental admittance presents a relation between the part of the outer load uncompensated by the Moho undulations and the residual gravity field and is used to select the best model. The 1 × 1° averaged values of topography elevations, basement and Moho depths, sedimentary cover density and gravity anomalies have been used as initial data. According to the correlation equation relating the outer load and Moho depths, the mean density contrast between the lower crust and the subcrustal lithosphere is 0.43 g/cm³, but the Moho undulation can not provide complete isostatic equilibrium. In some areas, the part of the outer load uncompensated by Moho undulations may be as large as 10⁷ kg/m² and the residual gravity field is as intensive as + 260 mGal. Assuming that for loads of wavelength > 200 km, local isostatic compensation is valid, in accordance with the admittance analysis, the anomalous masses compensating for the part of the outer load, which is not compensated by Moho undulations, have to be located partly in the lower crust and in the subcrustal layer. The regional trend of anomalous compensating masses is negative under Western Europe, the Mediterranean, Eastern Asia and adjacent marginal seas, and positive under the East European Platform and Western and Central Asia. The local compensating masses correspond to particular tectonic units. The isostatic gravity anomalies of Northern Eurasia have been determined and the long-wave component of the field reflecting anomalous masses under the isostatic compensation level has been evaluated.     

前陆盆地沉降机理和地层模型

前陆盆地形成的主要原因是造山带负载导致的岩石圈挠曲。逆冲作用造成地壳增厚,造山带的巨大质量又导致下部岩石圈的区域均衡沉降,从而临近和平行于造山带发育了凹陷。另外,前陆盆地的演化也受到沉积物供应、盆地内沉积物扩散能力、岩石圈强度、造山带逆冲速率、全球海平面变化、和俯冲有关的动力沉降及俯冲负载等众多其它因素的影响。本文阐述了这些因素与前陆盆地沉降的关系,介绍了与幕式逆冲有关的地层模型和欠补偿-过补偿地层模型。希望本文能够对中国西北地区前陆盆地的研究起到一定的借鉴意义。     

Global earth isostatic model using smoothed Airy-Heiskanen and Vening Meinesz hypotheses

Isostatic hypotheses are used for different purposes in geophysics and geodesy. The Erath crustal thickness modelling is more complicated than the classical isostatic models. In this study we try to modify Airy-Hesiskanen model, utilizing a smoothing factor, to a model with regional or global isostatic model through a modern solution of the gravimetric-isostatic Vening Meinesz model and CRUST.0. In Airy-Hesiskanen’s theory there is no correlation between neighbouring crustal columns, while this must be the case in reality due to the elasticity of the Earth. The idea is to keep the simplicity of the Airy-Heiskanen model, because it needs only the topographic data, and change the model which becomes to a model with regional/global isostatic model. The isostatic assumption for compensating the topographic potential is incomplete, as there are other geophysical phenomena which should be considered. Using the isostatic hypothesis for determining the depth of crust causes some disturbing signals in the gravity anomaly (approximately 285 mGal), which influence the crustal thickness determination. In this paper a simple method use for removing these effects. Spherical harmonic potential coefficients of the topographic compensation masses are used for modifying Airy-Heiskanen’s model in a least-square adjustment procedure by estimating smoothing factor. The numerical analysis shows that below degree 10, the modified Airy-Hesiskanen and Vening Meinesz models are close together. Smoothing factors for modifying the Airy-Hesiskanen model vary from 0.75 to 0.64 between degrees 200 and 2159.     

New evidence from the southeastern U.S. for eustatic components in the late Holocene Sea levels

Changes in Holocene sea level through time have been attributed to eustatic, isostatic, and neotectonic processes. Eustatic changes imply global expression through linkage in world climate, or changes in ocean water or basin volume, while isostatic adjustment and neotectonic distortion involve regional or local geophysical parameters which must be ascertained. A Holocene sea level curve is being developed for the southeastern United States through a study of marsh stratigraphy and archaeologic sites located in marsh and interriverine areas. The curve is evaluated in light of intensive investigations of regional and local tectonic elements. Both major and some minor trends in sea level change are shown to include significant eustatic components.     

龙门山晚新生代均衡反弹隆升的定量研究

龙门山位于青藏高原东缘与四川盆地的交接部位,是青藏高原周边山脉中地形梯度变化最大的山脉,其隆升过程和机制一直是国际地学界关注的焦点。晚新生代经过大量的滑坡、泥石流等快速剥蚀作用,龙门山的高程却不断升高。讨论了龙门山构造隆升的3种地球动力学机制,即下地壳通道流机制、地壳挤压缩短变形机制、地壳均衡反弹机制。晚新生代龙门山的隆升与剥蚀引起的均衡反弹作用相关,剥蚀作用使得地壳岩石逐步被移去,剥蚀区重力损失,岩石圈或地壳卸载作用导致山脉顶峰的隆升。结合数字高程模型数据研究表明,巨大地震的长期同震构造变形以及滑坡、泥石流等引起的快速剥蚀所导致的地壳均衡反弹,可能是龙门山晚新生代构造隆升的地球动力学新机制。龙门山地区现今高程受构造作用与剥蚀引起的均衡反弹作用的共同影响,其中剥蚀引起的均衡反弹作用对龙门山隆升的影响贡献率约占30%。     

Geomorphological evidence from the Lleyn Peninsula constraining models of the magnitude and rate of isostatic rebound during deglaciation of the Irish Sea Basin

A stepped series of sand and gravel terraces on the Lleyn peninsula of North Wales is used to test the magnitude and rate of isostatic depression required by the recently proposed glacimarine model of deglaciation of the Irish Sea Basin. A relative sea-level fall of 70 m is required while the ice remained pinned at the north Lleyn coast. Even taking the maximum known rate of isostatic uplift, the margin would have to remain stationary for 1400 years. It seems more reasonable to interpret the Lleyn terraces, and similar features around the Irish Sea Basin, as glacifluvial and glacilacustrine.     

Exhumation patterns along shallow low‐angle normal faults: an example from the Altotiberina active fault system (Northern Apennines,Italy)

A multi‐method approach (palaeothermal and thermochronological analyses; thermal modelling) is applied to reconstruct the exhumation history of the Altotiberina Fault (ATF), a representative example of crustal‐scale active low‐angle normal faulting in the Northern Apennines (Italy). Thermal maturity and thermochronological data yield similar burial histories but different exhumation patterns for the sedimentary successions in the hangingwall and the footwall of the ATF. Since 3.8 Ma, the ATF footwall has exhumed at rates of 0.90 mm a^?1. Exhumation led to bending and deactivation of the ATF uppermost portion as a result of tectonic unloading and isostatic adjustment, followed by migration of extension and the development of a set of domino‐like, east‐dipping normal faults, rooting on the buried portion of the ATF. ATF activity and isostatic rebound exhumed Triassic rock units from depths of about 4 km. We suggest that isostatic instability is accommodated at shallow crustal levels, in a similar way to what is observed on larger structures at mid‐low crustal levels.     

Postglacial temperature anomalies and glacial isostasy

The isostatic depression of an ice-free land surface, originally caused by glacial ice, ought to be converted into a change in mean annual temperature according to the usual value for the environmental lapse rate, 6.5°C/1000 m. Calculations are undertaken to show the temperature changes, relative to present values at a site, that can be expected from this effect during the retreat of a major continental ice sheet. It is concluded that when restrained rebound is taken into account temperatures relative to the present could vary from 0°C at the start of deglaciation to about +2.32°C at the final disappearance of the Laurentide Ice Sheet. The decay of the warmer-than-present temperatures towards their current values is controlled, in this model, by isostatic recovery of the land surface. Variance estimates are built into the model and suggest that the anomaly might be as small as 0.75°C or as large as 4.0°C depending on the choice of values for crucial components such as maximum ice thickness, the proportion of isostatic deformation, and the amount of restrained rebound achieved by the time a site become ice free.     

Elastic and isostatic subsidence of the Blåsjø artificial lake, Southern Norway

When a load is applied to the surface of the Earth, there is an immediate elastic deformation followed by a time-dependent isostatic subsidence. The two processes on the Blåsjø artifical lake, Southern Norway are modelled. The isostatic subsidence turns out to be negligible. A comparison of the modelled elastic deformation and the relative land levelling data gives valuable information about the elastic properties of the Earth's upper layers. The elastic parameters inferred from this study are lower than those from seismic investigations, and more in accordance with laboratory experiments.     

Post-glacial relative sea level, isostasy, and glacial history in Icy Strait, Southeast Alaska, USA

We use the radiocarbon ages of marine shells and terrestrial vegetation to reconstruct relative sea level (RSL) history in northern Southeast Alaska. RSL fell below its present level around 13,900 cal yr BP, suggesting regional deglaciation was complete by then. RSL stayed at least several meters below modern levels until the mid-Holocene, when it began a fluctuating rise that probably tracked isostatic depression and rebound caused by varying ice loads in nearby Glacier Bay. This fluctuating RSL rise likely reflects the episodic but progressive advance of ice in Glacier Bay that started around 6000 cal yr BP. After that time, RSL low stands probably signaled minor episodes of glacier retreat/thinning that triggered isostatic rebound and land uplift. Progressive, down-fjord advance of the Glacier Bay glacier during the late Holocene is consistent with the main driver of this glacial system being the dynamics of its terminus rather than climate change directly. Only after the glacier reached an exposed position protruding into Icy Strait ca. AD 1750, did its terminus succumb - a century before the climate changes that marked the end of the Little Ice Age - to the catastrophic retreat that triggered the rapid isostatic rebound and RSL fall occurring today in Icy Strait.     

Sea-level change and paleogeographic reconstructions,southern Vancouver Island,British Columbia,Canada

Forty-eight new and previously published radiocarbon ages constrain deglacial and postglacial sea levels on southern Vancouver Island, British Columbia. Sea level fell rapidly from its high stand of about +75 m elevation just before 14 000 cal BP (12 000 radiocarbon yrs BP) to below the present shoreline by 13 200 cal BP (11 400 radiocarbon years BP). The sea fell below its present level 1000 years later in the central Strait of Georgia and 2000 years later in the northern Strait of Georgia, reflecting regional differences in ice sheet retreat and downwasting. Direct observations only constrain the low stand to be below ?11 m and above ?40 m. Analysis of the crustal isostatic depression with equations utilizing exponential decay functions appropriate to the Cascadia subduction zone, however, places the low stand at ?30 ± 5 m at about 11 200 cal BP (9800 BP). The inferred low stand for southern Vancouver Island, when compared to the sea-level curve previously derived for the central Strait of Georgia to the northwest, generates differential isostatic depression that is consistent with the expected crustal response between the two regions. Morphologic and sub-bottom features previously interpreted to indicate a low stand of ?50 to ?65 m are re-evaluated and found to be consistent with a low stand of ?30 ± 5 m. Submarine banks in eastern Juan de Fuca Strait were emergent at the time of the low stand, but marine passages persisted between southern Vancouver Island and the mainland. The crustal uplift presently occurring in response to the Late Pleistocene collapse of the southwestern sector of the Cordilleran Ice Sheet amounts to about 0.1 mm/yr. The small glacial isostatic adjustment rate is a consequence of low-viscosity mantle in this tectonically active region.     

AMS 14C dating of deglacial events in the Irish Sea Basin and other sectors of the British–Irish ice sheet

Sedimentary sequences deposited by the decaying marine margin of the British–Irish Ice Sheet (BIIS) record isostatic depression and successive ice sheet retreat towards centres of ice dispersion. Radiocarbon dating by accelerator mass spectrometry (AMS) of in situ marine microfaunas that are commonly associated with these sequences constrain the timing of glacial and sea level fluctuations during the last deglaciation, enabling us to evaluate the dynamics of the BIIS and its response to North Atlantic climate change. Here we use our radiocarbon-dated stratigraphy to define six major glacial and sea level events since the Last Glacial Maximum. (1) Initial deglaciation may have occurred ⩾18.3 kyr ¹⁴C BP along the northwestern Irish coast, in agreement with a deglacial age of ∼22 ³⁶Cl kyr BP for southwestern Ireland. Ice retreated to inland centres and areas of transverse moraine began to form across the north Irish lowlands. (2) Channels cut into glaciomarine deglacial sediments along the western Irish Sea coast are graded to below present sea level, identifying a fall of relative sea level (RSL) in response to isostatic emergence of the coast. (3) Marine mud that rapidly infilled these channels records an abrupt rise in global sea level of 10–15 m ∼16.7 ¹⁴C kyr BP that flooded the Irish Sea coast and may have triggered deglaciation of a marine-based margin in Donegal Bay. (4) Intertidal boulder pavements in Dundalk Bay indicate that RSL ∼15.0 ¹⁴C kyr BP was similar to present. (5) A major readvance of all sectors of the BIIS occurred between 14 and 15 kyr ¹⁴C BP which overprinted subglacial transverse moraines and delivered a substantial sediment flux to tidewater ice sheet margins. This event, the Killard Point Stadial, indicates that the BIIS participated in Heinrich event 1. (6) Subsequent deposition of marine muds on drumlins 12.7 ¹⁴C kyr BP indicates isostatic depression and attendant high RSL resulting from the Killard Point readvance. These events identify a dynamic BIIS during the last deglaciation, as well as significant changes in RSL that reflect a combination of isostatic loading and eustatic changes in global sea level.     

Syn-convergence exhumation of the Central Alps

Abstract

The exhumation of rocks in a plate convergence setting is commonly related to erosion and/or tectonic denudation accompanied by isostatic adjustment. Isostatic compensation is the physical response to denudation. It leads to unroofing of deep levels of the crust. A new model for producing topographic relief is proposed which explains well the rapid exhumation of high-temperature rocks in the Central Alps via erosion and tectonic denudation (i.e. gravitational collapse and normal faulting). It is shown that the forward motion of the cold and rigid Adriatic indenter into the European crust is twofold. Firstly, horizontal compression led to the vertical extrusion of the deepest ductile European basement into shallower levels. This tectonic process induced heat transfer through the southern steep belt as well as heat advection together with the extruded material, resulting in the metamorphic aureole observed in the Central Alps. Secondly, the lower part of the Adriatic crust protruded into the warm European crust as a result of continuous forward motion. Geophysical data suggest that the isostatic response to indentation (i.e. deepening of the alpine root) has been inhibited by the mechanical strength of the cold and rigid Adriatic crust. Then, the indentation process induced a deviation from isostatic equilibrium by creating a tremendous topographic relief. This relief disappeared rapidly, possibly as fast as it forms, by enhanced erosion and tectonic denudation leading to rapid exhumation of the metamorphic dome.