Complex seismic anisotropy at the border of a very low velocity province at the base of the Earth's mantle

2007 ◽  
Vol 112 (B9) ◽  
Author(s):  
Yi Wang ◽  
Lianxing Wen
Keyword(s):  
Seismic Anisotropy ◽  
Low Velocity ◽  
Earth’S Mantle

Crustal anisotropy beneath northeastern Tibetan Plateau from the harmonic decomposition of receiver functions

2019 ◽  
Vol 220 (3) ◽  
pp. 1585-1603
Author(s):  
Zhenxin Xie ◽  
Vadim Levin ◽  
Qingju Wu
Keyword(s):  
Tibetan Plateau ◽  
Seismic Anisotropy ◽  
Receiver Function ◽  
Flow Direction ◽  
Receiver Functions ◽  
Low Velocity ◽  
Northeastern Tibetan Plateau ◽  
Harmonic Decomposition ◽  
Anisotropic Layers ◽  
Qilian Orogen

SUMMARY A uniformly spaced linear transect through the northeastern Tibetan Plateau was constructed using 54 stations from ChinaArray Phase II. We used a set of colocated earthquakes to form receiver function beams that were then used to construct a 2-D image of main converting boundaries in our region and to investigate lateral changes in main impedance contrasts along the transect. The image revealed obvious mid-crustal low-velocity zones beneath the Qilian Orogen and the Alxa Block. We developed a new procedure that uses harmonically decomposed receiver functions to characterize seismic anisotropy, and that can determine both the orientations of symmetry axes and their type (fast or slow). We tested our technique on a number of synthetic models, and subsequently applied it to the data from the transect. We found that: (1) within the upper crust the orientations of slow symmetry axes are nearly orthogonal to the strike directions of faults, and thus anisotropy is likely caused by the shape preferred orientation of fluid-saturated cracks or fractures and (2) together with the low-velocity zones revealed from receiver functions stacks, anisotropic layers in the middle-to-lower crust could be explained by the crustal channel flow that was proposed for this region by previous studies. The shear within the boundary layers of crustal flow forms anisotropy with symmetry axes parallel to the flow direction.

An investigation of seismic anisotropy in the lowermost mantle beneath Iceland

2019 ◽  
Vol 219 (Supplement_1) ◽  
pp. S152-S166 ◽  
Author(s):  
Jonathan Wolf ◽  
Neala Creasy ◽  
Angelo Pisconti ◽  
Maureen D Long ◽  
Christine Thomas
Keyword(s):  
Seismic Anisotropy ◽  
Horizontal Flow ◽  
Mantle Flow ◽  
Small Data ◽  
Vertical Flow ◽  
Low Velocity ◽  
Data Set ◽  
Study Region ◽  
Deep Mantle ◽  
Lowermost Mantle

SUMMARY Iceland represents one of the most well-known examples of hotspot volcanism, but the details of how surface volcanism connects to geodynamic processes in the deep mantle remain poorly understood. Recent work has identified evidence for an ultra-low velocity zone in the lowermost mantle beneath Iceland and argued for a cylindrically symmetric upwelling at the base of a deep mantle plume. This scenario makes a specific prediction about flow and deformation in the lowermost mantle, which can potentially be tested with observations of seismic anisotropy. Here we present an investigation of seismic anisotropy in the lowermost mantle beneath Iceland, using differential shear wave splitting measurements of S–ScS and SKS–SKKS phases. We apply our techniques to waves propagating at multiple azimuths, with the goal of gaining good geographical and azimuthal coverage of the region. Practical limitations imposed by the suboptimal distribution of global seismicity at the relevant distance ranges resulted in a relatively small data set, particularly for S–ScS. Despite this, however, our measurements of ScS splitting due to lowermost mantle anisotropy clearly show a rotation of the fast splitting direction from nearly horizontal for two sets of paths that sample away from the low velocity region (implying VSH > VSV) to nearly vertical for a set of paths that sample directly beneath Iceland (implying VSV > VSH). We also find evidence for sporadic SKS–SKKS discrepancies beneath our study region; while the geographic distribution of discrepant pairs is scattered, those pairs that sample closest to the base of the Iceland plume tend to be discrepant. Our measurements do not uniquely constrain the pattern of mantle flow. However, we carried out simple ray-theoretical forward modelling for a suite of plausible anisotropy mechanisms, including those based on single-crystal elastic tensors, those obtained via effective medium modelling for partial melt scenarios, and those derived from global or regional models of flow and texture development in the deep mantle. These simplified models do not take into account details such as possible transitions in anisotropy mechanism or deformation regime, and test a simplified flow field (vertical flow beneath the plume and horizontal flow outside it) rather than more detailed flow scenarios. Nevertheless, our modelling results demonstrate that our ScS splitting observations are generally consistent with a flow scenario that invokes nearly vertical flow directly beneath the Iceland hotspot, with horizontal flow just outside this region.

Inferences on Flow at the Base of Earth's Mantle Based on Seismic Anisotropy

Science ◽  
2004 ◽  
Vol 303 (5656) ◽  
pp. 351-353 ◽  
Author(s):  
M. Panning
Keyword(s):  
Seismic Anisotropy ◽  
Earth’S Mantle

Crustal structures of southern Mongolia from seismic anisotropy and gravity analysis

2021 ◽  
Author(s):  
Alexandra Guy ◽  
Christel Tiberi ◽  
Saandar Mijiddorj
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Seismic Anisotropy ◽  
Homogeneous Layer ◽  
Low Density ◽  
Gravity Modelling ◽  
Low Velocity ◽  
Seismic Stations ◽  
Lower Crustal ◽  
Density Zone

<p>This study integrates gravity modelling and analysis with seismic constraints through the prism of seismic anisotropy to characterize the structures of southern Mongolia, in particular at the lower crustal but also the upper mantle levels. Recently, gravity signal analysis and forward modelling combined with magmatic geochemistry and thermodynamic modelling demonstrate that relamination of allochtonous felsic to intermediate lower crust played a major role in southern Mongolia structure. Relamination of material induces a homogeneous layer in the lower crust, which contrasts with the highly heterogeneous upper crustal part composed of different lithotectonic domains. The seismic signals of the seven southernmost stations of the MOBAL2003 experiment were analyzed to get the receiver functions. The data treatment was performed following a new protocol, which reduces the noise on the different components. This treatment reveals the variation of the crustal thickness of cca. 10 km along the first 450 km of the profile. In addition, some seismic stations display significant signals related to the occurrence of a low velocity zone (LVZ) at lower crustal and upper mantle levels. The depth of the Moho discontinuity and the dips of the seismic interfaces obtained from the seismic inversions as well as the boundaries of the different tectonic zones constitute the starting points from the 2D forward gravity modelling along the southern part of the MOBAL 2003 profile. Moreover, the density values applied to the different blocks were determined according to the global lithological composition of the different units and the vergences of the tectonic contacts were constrained by the geodynamic studies. The gravity modelling reveals the occurrence of a low density zone in the lower crust beneath the four southernmost seismic stations, which corresponds to the LVZ observed with the receiver function analysis. The combination of the independent methods enhances the occurrence of a low velocity and a low density zone (LVLDZ) at lower crustal level beneath the southernmost part of the MOBAL 2003 seismic profile. These LVLDZ may demonstrate the existence of the relamination of a hydrous material in southern Mongolia.</p>

Geodynamics of Lithosphere and Earth’s Mantle: Seismic Anisotropy as a Record of the Past and Present Dynamic Processes

1998 ◽  
Vol 151 (4) ◽  
pp. 213 ◽  
Author(s):  
Jaroslava Plomerová ◽  
Robert C. Liebermann ◽  
Vladislav Babuška
Keyword(s):  
Seismic Anisotropy ◽  
Dynamic Processes ◽  
Earth’S Mantle ◽  
The Past
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