Deformation of the Crust and Upper Mantle beneath the North China Craton and Its Adjacent Areas Constrained by Rayleigh Wave Phase Velocity and Azimuthal Anisotropy

The North China Craton (NCC) has experienced strong tectonic deformation and lithospheric thinning since the Cenozoic. To better constrain the geodynamic processes and mechanisms of the lithospheric deformation, we used a linear damped least squares method to invert simultaneously Rayleigh wave phase velocity and azimuthal anisotropy at periods of 10–80 s with teleseismic data recorded by 388 permanent stations in the NCC and its adjacent areas. The results reveal that the anomalies of Rayleigh wave phase velocity and azimuthal anisotropy are in good agreement with the tectonic domains in the study area. Low-phase velocities appear in the rift grabens and sedimentary basins at short periods. A rotation pattern of the fast axis direction of the Rayleigh wave together with a distinct low-velocity anomaly occurs around the Datong volcano. A NW–SE trending azimuthal anisotropy and a low-velocity anomaly at periods of 60–80 s are observed subparallel to the Zhangbo fault zone. The whole lithosphere domain of the Ordos block shows a high-phase velocity and counterclockwise rotated fast axis. The northeastern margin of the Tibetan plateau is dominated by a low-velocity and coherent NW–SE fast axis direction. We infer that the subduction of the Paleo-Pacific plate and eastward material escape of the Tibetan plateau mainly contribute to the deformation of the crust and upper mantle in the NCC.

Resolving Seismic Anisotropy of the Lithosphere–Asthenosphere in the Central/Eastern Alps Beneath the SWATH-D Network

The Alpine orogeny is characterized by tectonic sequences of subduction and collision accompanied by break-off events and possibly preceded by a flip of subduction polarity. The tectonic evolution of the transition to the Eastern Alps has thus been under debate. The dense SWATH-D seismic network as a complementary experiment to the AlpArray seismic network provides unprecedented lateral resolution to address this ongoing discussion. We analyze the shear-wave splitting of this data set including stations of the AlpArray backbone in the region to obtain new insights into the deformation at depth from seismic anisotropy. Previous studies indicate two-layer anisotropy in the Eastern Alps. This is supported by the azimuthal pattern of the measured fast axis direction across all analyzed stations. However, the temporary character of the deployment requires a joint analysis of multiple stations to increase the number of events adding complementary information of the anisotropic properties of the mantle. We, therefore, perform a cluster analysis based on a correlation of energy tensors between all stations. The energy tensors are assembled from the remaining transverse energy after the trial correction of the splitting effect from two consecutive anisotropic layers. This leads to two main groups of different two-layer properties, separated approximately at 13°E. We identify a layer with a constant fast axis direction (measured clockwise with respect to north) of about 60° over the whole area, with a possible dip from west to east. The lower layer in the west shows N–S fast direction and the upper layer in the east shows a fast axis of about 115°. We propose two likely scenarios, both accompanied by a slab break-off in the eastern part. The continuous layer can either be interpreted as frozen-in anisotropy with a lithospheric origin or as an asthenospheric flow evading the retreat of the European slab that would precede the break-off event. In both scenarios, the upper layer in the east is a result of a flow through the gap formed in the slab break-off. The N–S direction can be interpreted as an asthenospheric flow driven by the retreating European slab but might also result from a deep-reaching fault-related anisotropy.

Caribbean slab dynamics beneath northwest South America from SKS and Local S splitting

<p>     The southernmost edge of the Caribbean (CAR) plate, a buoyant large igneous province, subducts shallowly beneath northwestern South America (NWSA) at a trench that lies northwest of Colombia. Recent finite frequency P-wave tomography results show a segmented CAR subducting at a shallow angle under the Santa Marta Massif to the Serrania de Perijá (SdP) before steepening while a detached segment beneath the Mérida Andes (MA) descends into the mantle transition zone. The dynamics of shallow subduction are poorly understood. Plate coupling between the flat subducting CAR and the overriding NWSA is proposed to have driven the uplift of the MA. In this study we analyze SKS shear wave splitting to investigate the seismic anisotropy beneath the slab segments to relate their geometry to mantle dynamics. We also use local S splitting to investigate the seismic anisotropy between the slab segments and the overriding plate. The data were recorded by a 65-element portable broadband seismograph network deployed in NWSA and 40 broadband stations of the Venezuelan and Colombian national seismograph networks.</p><p>     SKS fast polarization axes are measured generally trench-perpendicular (TP) west of the SdP but transition to trench-parallel (TL) at the SdP where the slab was imaged steepening into the mantle, consistent with previous studies. West of the MA the fast axis is again TP but transitions to TL under the MA. This second transition from TP to TL is likely due to mantle material being deflected around a detached slab under the MA. Local S fast polarization axes are dominantly TP throughout the study area west of the Santa Marta Massif and are consistent with slab-entrained flow. Under the Santa Marta Massif the fast axis is TL for reasons we do not yet understand.</p>

Mapping mantle flows underneath the North American and Caribbean Plates

<p>In this talk, I will present a new 3-D azimuthally anisotropic tomographic model, namely US32, for the North American and Caribbean Plates. This model is constrained by using seismic data from USArray and full waveform inversion. The inversion uses data from 180 regional earthquakes recorded by 4,516 seismographic stations, resulting in 586,185 frequency-dependent phase measurements. Three-component short-period body waves and long-period surface waves are combined to simultaneously constrain deep and shallow structures. The current azimuthally anisotropic model US32 is the result of 32 pre-conditioned conjugate-gradient iterations. In the current model, I observe a complex depth-dependent pattern for fast axis directions across the North American and Caribbean Plates.<span>  </span>At shallow depths, these fast axis directions delineate local geological provinces, such as the Snake River Plain, Cascadia subduction zone, Rio Grand Rift, etc. At greater depths, the fast axis directions follow the absolute plate motion trajectories at most places. At depths around 700 km, the fast axis directions are perpendicular to the strikes of the mapped Farallon slab, suggesting the presence of 2-D corner flows induced by this ancient subduction underneath the mantle transition zone. In addition, underneath the Cascadia and Cocos subduction zones at depths from 250 to 500 km, the fast axis directions suggest the presence of toroid-mode mantle flows, following the geometry of fast downwelling materials.</p>

Lithosphere-asthenosphere decoupling in the Central/Eastern Alps from seismic anisotropy beneath the dense SWATH-D network

<p>The Alpine orogeny is characterized by tectonic sequences of subduction and collision accompanied by break-off events and possibly preceded by a flip of subduction polarity. The tectonic evolution of the transition to the Eastern Alps has thus been under debate. The dense Swath-D seismic network as complementary experiment to the AlpArray network provides unprecedented lateral resolution to address this open discussion. We analyze shear wave splitting of this data set to get insights into the deformation at depth by studying seismic anisotropy. Previous studies indicate two-layer anisotropy in the Eastern Alps. This is supported by azimuthal pattern of the measured fast axis direction across all stations of the network. The temporary character of the deployment requires a joint analysis of multiple stations to increase the number of events adding complementary information of the anisotropic property of the mantle. We perform a cluster analysis based on a correlation of the remaining transverse energy between all stations. The energy tensor is calculated in the grid search for the best fitting two-layer splitting parameters to the ensemble of events at each station. This leads to two main groups of different two-layer properties separated at 12.5 degrees Longitude. We identify a layer with constant fast axis direction of 60° over the whole area, with a possible dip from West to East. The lower layer in the West shows N-S direction and upper layer in the East 115° alignment. We propose two likely scenarios, both accompanied by a slab break-off in the Eastern part. The continuous layer can either be interpreted as frozen-in anisotropy with lithospheric origin or an asthenospheric flow evading the retreat of the European slab that would precede the break-off event.  In both scenarios the upper layer in the East is result of a channel flow through the gap formed in the slab break-off. The N-S direction is interpreted as asthenospheric flow mainly driven by the subduction of the European plate below Adria.</p>