In the Dabieshan, the available models for exhumation of ultrahigh-pressure (UHP) rocks are poorly constrained by structural data. A comprehensive structural and kinematic map and a general cross-section of the Dabieshan including its foreland fold belt and the Northern Dabieshan Domain (Foziling and Luzenguang groups) are presented here. South Dabieshan consists from bottom to top of stacked allochtons: (1) an amphibolite facies gneissic unit, devoid of UHP rocks, interpreted here as the relative autochton; (2) an UHP allochton; (3) a HP rock unit (Susong group) mostly retrogressed into greenschist facies micaschists; (4) a weakly metamorphosed Proterozoic slate and sandstone unit; and (5) an unmetamorphosed Cambrian to Early Triassic sedimentary sequence unconformably covered by Jurassic sandstone. All these units exhibit a polyphase ductile deformation characterized by (i) a NW–SE lineation with a top-to-the-NW shearing, and (ii) a southward refolding of early ductile fabrics.
The Central Dabieshan is a 100-km scale migmatitic dome. Newly discovered eclogite xenoliths in a Cretaceous granitoid dated at 102 Ma by the U–Pb method on titanite demonstrate that migmatization post-dates HP–UHP metamorphism. Ductile faults formed in the subsolidus state coeval to migmatization allow us to characterize the structural pattern of doming. Along the dome margins, migmatite is gneissified under post-solidus conditions and mylonitic–ultramylonitic fabrics commonly develop. The north and west boundaries of the Central Dabieshan metamorphics, i.e. the Xiaotian–Mozitan and Macheng faults, are ductile normal faults formed before Late Jurassic–Early Cretaceous. A Cretaceous reworking is recorded by synkinematic plutons.
North of the Xiaotian–Mozitan fault, the North Dabieshan Domain consists of metasediments and orthogneiss (Foziling and Luzenguang groups) metamorphosed under greenschist to amphibolite facies which never experienced UHP metamorphism. A rare N–S-trending lineation with top-to-the-south shearing is dated at 260 Ma by the 40Ar/39Ar method on muscovite. This early structure related to compressional tectonics is reworked by top-to-the-north extensional shear bands.
The main deformation of the Dabieshan consists of a NW–SE-stretching lineation which wraps around the migmatitic dome but exhibits a consistently top-to-the-NW sense of shear. The Central Dabieshan is interpreted as an extensional migmatitic dome bounded by an arched, top-to-the-NW, detachment fault. This structure may account for a part of the UHP rock exhumation. However, the abundance of amphibolite restites in the Central Dabieshan migmatites and the scarcity of eclogites (found only in a few places) argue for an early stage of exhumation and retrogression of UHP rocks before migmatization. This event is coeval to the N–S extensional structures described in the North Dabieshan Domain. Recent radiometric dates suggest that early exhumation and subsequent migmatization occurred in Triassic–Liassic times. The main foliation is deformed by north-verging recumbent folds coeval to the south-verging folds of the South Dabieshan Domain. An intense Cretaceous magmatism accounts for thermal resetting of most of the 40Ar/39Ar dates.
A lithosphere-scale exhumation model, involving continental subduction, synconvergence extension with inversion of southward thrusts into NW-ward normal faults and crustal melting is presented. 相似文献
Pn arrivals from mining-induced earthquakes on the edge of the Witwatersrand basin show that the P wavespeeds in the uppermost mantle are almost constant throughout most of the Kaapvaal craton. The presence of only small wavespeed variations allows the use of a simple method of estimating crustal thicknesses below the stations of the Kaapvaal broad-band network using Pn times that has been compared with results from receiver functions. One thousand three hundred thirty-seven Pn arrivals were used to derive crustal thicknesses at 46 stations on the Kaapvaal craton. The average crustal thicknesses for 19 centrally located stations on each of the northern and southern regions of the craton that yielded well-constrained thicknesses were 50.52±0.88 km and 38.07±0.85 km, respectively. In contrast, the corresponding average thicknesses determined from receiver functions were 43.58±0.57 km and 37.58±0.70 km, respectively. The systematically lower values for receiver functions in the northern part of the Kaapvaal craton that was affected by the Bushveld magmatism at 2.05 Ga, suggest that the receiver functions do not enable the petrological crust mantle boundary to be reliably resolved due to variations in composition and metamorphic grade in a mafic lower crust. The Pn times also suggest pervasive azimuthal anisotropy with maximum wavespeeds of about 8.40 km/s at azimuths of about 15° and 217° in the northern and southern regions of the craton, respectively, and minimum wavespeeds of about 8.25 km/s. 相似文献
Summary ¶Rock zones containing a high fracture density and/or soft, low cohesion materials can be highly problematic when encountered during tunnel excavation. For example in the eastern Aar massif of central Switzerland, experiences during the construction of the Gotthard highway tunnel showed that heavily fractured areas within shear zones were responsible for overbreaks in the form of chimneys several metres in height. To understand and estimate the impact of the shear zones on rock mass behaviour, knowledge concerning the rock mass strength and deformation characteristics is fundamental. A series of laboratory triaxial tests, performed on samples from granite- and gneiss-hosted shear zones revealed that with increasing degree of tectonic overprint, sample strength decreases and rock behaviour shows a transition from brittle to ductile deformation. These trends may be explained by increasing fracture densities, increasing foliation intensity, increasing thickness of fine-grained, low cohesion fracture infill, and increasing mica content associated with the increasing degree of tectonic overprint. As fracture density increases and the influence of discrete, persistent discontinuities on rock mass strength decreases, behaviour of the test samples becomes more and more representative of rock mass behaviour, i.e. that of a densely fractured continuum. For the purpose of numerical modeling calculations, the shear zones may be subdivided with respect to an increasing fracture density, foliation intensity and mica content into a strongly foliated zone, a fractured zone and a cohesionless zone, which in turn exhibit brittle, brittle-ductile and ductile rock mass constitutive behaviour, respectively.Received December 17, 2001; accepted January 9, 2003
Published online April 29, 2003 相似文献
Despite the various opening models of the southwestern part of the East Sea (Japan Sea) between the Korean Peninsula and the Japan Arc, the continental margin of the Korean Peninsula remains unknown in crustal structure. As a result, continental rifting and subsequent seafloor spreading processes to explain the opening of the East Sea have not been adequately addressed. We investigated crustal and sedimentary velocity structures across the Korean margin into the adjacent Ulleung Basin from multichannel seismic (MCS) reflection and ocean bottom seismometer (OBS) data. The Ulleung Basin shows crustal velocity structure typical of oceanic although its crustal thickness of about 10 km is greater than normal. The continental margin documents rapid transition from continental to oceanic crust, exhibiting a remarkable decrease in crustal thickness accompanied by shallowing of Moho over a distance of about 50 km. The crustal model of the margin is characterized by a high-velocity (up to 7.4 km/s) lower crustal (HVLC) layer that is thicker than 10 km under the slope base and pinches out seawards. The HVLC layer is interpreted as magmatic underplating emplaced during continental rifting in response to high upper mantle temperature. The acoustic basement of the slope base shows an igneous stratigraphy developed by massive volcanic eruption. These features suggest that the evolution of the Korean margin can be explained by the processes occurring at volcanic rifted margins. Global earthquake tomography supports our interpretation by defining the abnormally hot upper mantle across the Korean margin and in the Ulleung Basin. 相似文献
Predictions from dynamic modelling of the lithospheric deformation are presented for Northern Europe, where several basins underwent inversion during the Late Cretaceous and Early Cenozoic and contemporary uplift and erosion of sediments occurred. In order to analyse the evolution of the continental lithosphere, the equations for the deformation of a continuum are solved numerically under thin sheet assumption for the lithosphere. The most important stress sources are assumed to be the Late Cretaceous Alpine tectonics; localized rheological heterogeneities can also affect local deformation and stress patterns. Present-day observations available in the studied region and coming from seismic structural interpretations and stress measurements have been used to constrain the model. Our modelling results show that lateral variation in lithospheric strength below the basin systems in Central Europe strongly controls the regional deformation and the stress regime. Furthermore, we have demonstrated that the geometry of the boundary between Baltica and Avalonia, together with different rheological characteristics of the two plates, had a crucial role on local crustal deformation and faulting regime resulting in the Baltica–Avalonia transition zone from the S–N Alpine convergence. 相似文献
The Gondwana successions (1–4 km thick) of peninsular India accumulated in a number of discrete basins during Permo-Triassic period. The basins are typically bounded by faults that developed along Precambrian lineaments during deposition, as well as affected by intrabasinal faults indicating fault-controlled synsedimentary subsidence. The patterns of the intrabasinal faults and their relationships with the respective basin-bounding faults represent both extensional and strike-slip regimes. Field evidence suggests that preferential subsidence in locales of differently oriented discontinuities in the Precambrian basement led to development of Gondwana basins with varying, but mutually compatible, kinematics during a bulk motion, grossly along the present-day E–W direction. The kinematic disparity of the individual basins resulted due to different relative orientations of the basement discontinuities and is illustrated with the help of a simple sandbox model. The regional E–W motion was accommodated by strike-slip motion on the transcontinental fault in the north. 相似文献