Underplating in the Himalaya-Tibet Collision Zone Revealed by the Hi-CLIMB Experiment
J. NábělekGyörgy HetényiJérôme VergneSoma Nath SapkotaB. KafleJiang MeiHeping SuJohn ChenBor‐Shouh Huangthe Hi-CLIMB Team
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Abstract:
We studied the formation of the Himalayan mountain range and the Tibetan Plateau by investigating their lithospheric structure. Using an 800-kilometer-long, densely spaced seismic array, we have constructed an image of the crust and upper mantle beneath the Himalayas and the southern Tibetan Plateau. The image reveals in a continuous fashion the Main Himalayan thrust fault as it extends from a shallow depth under Nepal to the mid-crust under southern Tibet. Indian crust can be traced to 31 degrees N. The crust/mantle interface beneath Tibet is anisotropic, indicating shearing during its formation. The dipping mantle fabric suggests that the Indian mantle is subducting in a diffuse fashion along several evolving subparallel structures.Keywords:
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We present a regional crustal model of the East Greenland Fjord Region between 69°N and 74°N which spans the Caledonian fold belt and the adjoining Devonian and Mesozoic basins. The model is a compilation of existing seismic models that were partly reinterpreted and newly derived results from different modelling approaches. Remodelling of 33 stations on three deep seismic lines in the southern area yielded consistent P-wave velocity models for the entire Fjord Region. Seismic velocities, between 5.5 km s-1 near the surface and 6.9 km s-1 in the lower crust are typical for regions of Palaeozoic age. Moho depths up to 48 km in the seismic models suggest the existence of a crustal root beneath the Caledonian orogen. Shear wave modelling of 51 stations on six refraction seismic profiles resulted in S-wave velocities between 3.2 km s-1 in the upper crust, 4.0 km s-1 at the crust–mantle boundary and 4.2 km s-1 in the partial magmatic underplating of the lower crust in the northern area. Calculation of Poisson's ratio portrays a fairly homogeneous crust with only slight variations in Poisson's ratio of 0.26–0.30 for the uppermost crystalline crust and 0.22–0.24 for the middle crust. These values cannot be linked to lithological variations because they are either small-scale or span several tectonic provinces. Finite-difference modelling and amplitude analysis confirm models without magmatic underplating in the lower crust of the southern Hall Bredning, Scoresby Sund area.
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Quick-eruptive sanukites of 115Ma and subjacent basalts in Fuxin area,west Liaoning province were studied in this work.Facieological researches discovered in the rocks high-Mg and high-Si glasses forming in different stages and occurring in different modes,analysis of which revealed the intermingling processes of crust- and mantle-melts.With these results the environment for the melts intermingling and its tectonic settings of underplating were further investigated.
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Maps showing depth to the Moho, the 6 km/s and 7 km/s isovelocity surfaces and the thickness of the crust with a velocity greater than 7.0 km/s for the UK and surrounding continental crust have been generated from a compilation of wide-angle/refraction data. The data show that the crust beneath northwestern Scotland is thinner and of higher velocity than that beneath southern Britain. The lower crust beneath the East Irish Sea and parts of the southern North Sea is formed from thick layers of high velocity rock. The lateral extent of these layers cross-cuts the downward projection of major structures mapped at the surface. This suggests that the major structures do not bound regions of lower crust with contrasting properties at depth. Instead these structures may be overprinted by modification of the lower crust, for example, by magmatic underplating, which is not observed directly at the surface. Mapped variations in crustal thickness do not mirror the variations in surface topography, which appears to contradict the view that the crust is in Airy isostatic equilibrium.
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The reactivation of craton is a crucial issue for the understanding of continent lithosphere. Although the partial loss of the thick lithospheric root of the North China Craton (NCC) was documented, its mechanism remains a contentious topic in the study of cratonic evolution. In this paper, we imaged the crust‐mantle structures by using the data from 36 permanent stations with fine spatial coverage in the Yanshan tectonic belt (YSTB) and the Taihangshan region (TSR) in the northern NCC. Two tectonic units with distinct crust‐mantle structures were recognized and their junction zone was identified. The results reveal distinct features of the crust‐mantle boundaries of the YSTB and the TSR which are characterized by a sharp interface and a thick transition zone, respectively. Based on the analysis of the seismic structures of the crust‐mantle boundary and the upper mantle, we propose that the NCC reactivation was possibly derived by a slow secular ascension of hydrated asthenospheric material atop the stagnant Pacific slab, which influenced the formation of crust‐mantle transition zone in the eastern NCC. On the other hand, regional delamination of the deep crust after orogenic thickened and/or magmatic underplating beneath the YSTB caused the spatial variation of the crust‐mantle boundary. Both of the mantle processes possibly changed the nature of the lithospheric mantle.
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Wide angle recordings at offsets between 20 and 40 km in the Ruby Mountains consistently show 5 strong reflections between 4 and 11 s with enough moveout to estimate velocities to the base of the crust. The uppermost “layer” with a temperature corrected velocity of 6.2 km/s and thickness of 9 km corresponds to quartzofeldspathic rocks such as metasedimentary rocks, migmatites and deformed granites and is underlain by a 7‐km‐thick “layer” with velocities of 6.4 km/s which corresponds to quartzofeldspathic material interlayered with amphibolites. The lower crust consists of a heterogeneous, 9 km‐thick zone with velocities of 6.7 to 6.8 km/s corresponding to mafic rocks interlayered with smaller amounts of quartzofeldspathic rock. The lowermost crust is marked by the “X” reflection overlying a 3‐km‐thick “layer” with velocities between 7.4‐7.8 km/s corresponding to anomolous material, possibly layered cumulates. The subhorizontal, layered structure of the crust is caused primarily by ductile extension, which may well be superposed on material added to the crust by underplating. At most, approximately one third of the present crust could have been added by underplating in the Cenozoic.
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Qaidam basin-Qilian mountain is situated on the north margin of Qinghai-Tibet plateau. Like the main part of the plateau, there commonly exist a muti layer crustal structure and the low velocity layers. The crust thickness of the region is twice more than that of northern and southern China. It may be caused by the transverse compressive shortening and the upper mantle underplating .With the crust underplating getting strong, the thickness of the crust increase , and the lithospheric thickness tend to decrease. A tension condition in the upper crust and the deep melting or the mantle derived fluid replacement in the lower crust exist, which bring on the low velocity layers,terrestrial heat and shallow earthquake in the crust . It is also the reason showing hot crust and cold mantle in Qinghai-Tibet plateau region.
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Abstract: In our study we collected the teleseismic record of 31 broadband stations and 9 PASSCAL stations in West Yunnan, as well as extracted more than a million receiver functions. Using the waveform model and stacking techniques, we calculated the earth crust thicknesses and V p /V s ratios below the stations and obtained 35 valid data points. At the same time, we evenly stacked the receiver functions at the same station and superimposed the two profiles' cross sections of the main tectonic units. The results show a clear difference between the crust thicknesses of different tectonic units. Because of the magma underplatting and delimanition of the lower crust in the role of deep process, the West Yunnan's crust can be divided two kinds—mafic‐ultramafic and feldspathic crusts. The research also shows that the mafic‐ultramafic crust corresponds to a good background of mineralization. The delamination of the lower crust is one of the leading causes for moderate to strong earthquake prone in central Yunnan. The thinner crust and high velocity ratio as well as the multimodal structure of P s in the Tengchong volcanic area confirms existence of a deep process of the strong magma underplating. Due to the basic crust structure and nature, it is believed that the Honghe fault is a main suture of the Gondwana and Eurasia continents.
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Deviation from isostasy is commonly believed to be caused by the strength of the Earth's lithosphere. An analysis of crustal compensation dynamics suggests that the deviation may have a dynamic origin. The analysis is based on analytic models that assume that (1) the medium is incompressible and has a layered and linear viscoelastic rheology and (2) the amplitude of topography is small compared with its wavelength. The models can describe topographic relaxation of different density interfaces at both small (e.g., postglacial rebound) and large time‐scales. The models show that for a simple crust‐mantle system with topography at the Earth's surface and Moho representing the only mass anomalies, while the crust always approaches the isostatic state at long wavelengths (>800 km), crustal isostasy may not be an asymptotic limit at short wavelengths, depending on crustal and lithospheric rheology. For a crust with viscosity smaller than lithospheric viscosity, at wavelengths comparable with widths of orogenic belts (i.e., <300 km), the crust tends to approach a state with significant overcompensation (i.e., excess topography at the Moho) within a timescale of about 10 7 years, and this characteristic time depends on wavelengths and crustal viscosity. This overcompensation is greater for weaker crust and stronger lithosphere. A thicker crust or lithosphere also enhances this overcompensation. If crustal and lithospheric viscosities are both large and comparable, the asymptotic state for the crust displays a slight undercompensation. For an elastic and rigid upper crust, the crust eventually becomes undercompensated after a characteristic decay time of topography at the Moho. The characteristic time is dependent on viscosity and thickness of the lower crust. The deviation from isostasy arises because these viscosity structures result in a ratio of vertical velocity at the surface to vertical velocity at the Moho which in the asymptotic state for short wavelengths differs from the ratio of density contrast at the Moho to that at the surface.
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