Confronting Solid‐State Shear Bias: Magmatic Fabric Contribution to Crustal Seismic Anisotropy

The North Anatolian Fault Zone (NAFZ) is a prominent tectonic structure with a significant impact on the observed active deformation in Türkiye. Detailed knowledge of the seismic anisotropy in the crust and mantle along this nascent shear deformation zone provides insights into the kinematics associated with past and present tectonic events. We employed teleseismic earthquakes observed by the Dense Array North Anatolia seismic network to map 3‐ D variations in crustal and mantle anisotropy in/around the NW segment of the NAFZ. To achieve this, we first performed a harmonic decomposition analysis of P‐receiver functions. The results were then used as a priori information to conduct an anisotropic receiver function inversion with the Neighborhood Algorithm that enabled imaging of the actual orientation and geometry of anisotropic structures. SKS splitting measurements are further used to make a comparison between the anisotropic behavior of crustal and mantle structures. Crustal anisotropy parameters estimated in our analyses/models well identify the signature of deformation caused by accumulated strain in the earthquake cycle through the strike of shallow cutting faults in the brittle crust beneath the NAFZ. Diffuse intense anisotropic energy at lower crustal depths was attributed to lattice preferred orientation of crystals or partially molten lenses elongated along the shear direction. Strong harmonic energy variations beneath the northern part of the Istanbul Zone likely reflect imprints of LPO‐originated frozen fabric at shallow depths (0–20 km) associated with the palaeotectonic Odessa Shelf, Intra‐Pontide Suture Zones or remnants of the Tethys Ocean.

Section: Discussionmentioning

confidence: 99%

Depth‐Dependent Anisotropy Along Northwest Segment of the North Anatolian Fault Zone: Evidence for Paleo‐Tectonic Structures Contributing to Overall Complexity

Keleş,

Eken,

Licciardi

et al. 2024

“…As should be expected, the seismic structure of volcanic ocean islands can be very complex, resulting from pre-existing boundaries, magma reservoirs, and a complex structure that may include structural (i.e., dipping interfaces or faulting) and/or intrinsic (i.e., mineral-controlled) anisotropy. These effects on RF amplitude and arrivals are covered extensively in multiple studies (Frothingham et al, 2023;Schulte-Pelkum & Mahan, 2014;Schulte-Pelkum et al, 2020). This complexity can be observed at station HV.…”

Section: Receiver Functionsmentioning

confidence: 99%

Constraining the Lithospheric Discontinuity Structure Beneath Hawaiʻi Using Teleseismic Receiver Functions

Herr,

Delph

2023

The Hawaiian Islands represent the youngest portion of the long‐lived Hawaiian‐Emperor Seamount chain sourced from hotspot volcanism on the northwesterly moving Pacific plate. Despite being one of the best studied hotspots on Earth, our understanding of the lithospheric discontinuity structure of Hawaiʻi is inhibited by a lack of a long‐term distributed broadband seismic network and lithospheric‐scale imaging of other oceanic hotspots. In this study, we analyze 3,530 teleseismic waveforms from 1,130 events recorded at 47 stations to compute P‐wave radial receiver functions. We then incorporate an elevation correction to adaptive common conversion point stacking to create a 3D discontinuity volume of the lithospheric structure associated with an ocean island hotspot. We find a positive amplitude conversion consistently at ∼12 km below sea level, likely representing the crust‐mantle transition below much of the island. However, this conversion occurs at only ∼6 km below Kīlauea, where local tomography images a zone of high P‐wave velocities (>7 km/s) within the crust and deformation of olivine phenocrysts within a magmatic reservoir is thought to occur. At this location, seismicity also extends vertically to 35 km depth before merging with an interpreted sill complex at ∼40 km. We interpret this anomalously shallow high amplitude discontinuity as an olivine‐dominated magmatic reservoir with an estimated maximum 5% melt over the resolution of our data at crustal depths. This represents the shallow seismic signature of a much more extensive magma system extending to depth. These images provide a better understanding of the magmatic structure associated with oceanic hotspot volcanoes.

“…This suggests that the lower crustal layer exhibits a higher Vp/Vs ratio, which implies the presence of mafic restite generated during a magma underplating process is a more plausible explanation (Figure 8). Additionally, during magma underplating processes, the alignment of minerals like amphibolite can induce moderate to strong (3%-9%) anisotropy (Frothingham et al, 2023), which has been observed at various volcanic regions (Schulte-Pelkum et al, 2020;. To further investigate the anisotropic properties of the lower crustal layer, we conducted an analysis by calculating an azimuthal contrast stack by computing the difference between southern and northern azimuth arrivals to highlight its anisotropic characteristic (Figure S9 in Supporting Information S1).…”

Section: Nature Of the Double Moho Structure Beneath The East Junggarmentioning

confidence: 99%

Impact of Ancient Tectonics on Intracontinental Deformation Partitioning: Insights From Crustal Structures of the East Junggar‐Altai Area

Yang,

Tian,

Wan

et al. 2024

Compressive stress generated at collision fronts can propagate over long distances, inducing deformation within the continent's interior. Nevertheless, the factors governing the partitioning of intracontinental deformation remain enigmatic. The Altai Mountains serve as a type‐example of ongoing intracontinental deformation. Here, we investigate the crustal architecture of the Chinese Altai Mountains, using receiver functions obtained from newly deployed dense seismic nodal arrays. The new seismic results reveal distinct crustal features, including (a) a negative polarity discontinuity beneath Chinese Altai Mountains, suggesting a low‐velocity layer; (b) a north‐dipping mid‐crustal structure beneath the suture zone between East Junggar and Chinese Altai, indicating underthrusting of East Junggar's lower crust beneath the Chinese Altai Mountains; (c) a double Moho structure beneath East Junggar, revealing a high‐velocity lower crustal layer. In conjunction with constraints from previous multi‐disciplinary regional studies, the double Moho structures are interpreted as mafic restite from Late Paleozoic magma underplating. The addition of mafic materials can significantly enhance the rheological strength of East Junggar's crust, causing it to function as an indenter that thrust beneath the Chinese Altai Mountains during the subsequent convergence process. As a consequence, significant deformation occurs in the Chinese Altai region, resulting in the emergence of decollements, as evident by the negative polarity discontinuity. The presence of pre‐existing decollements makes the Altai Mountains region more susceptible to deformation, thereby facilitating the concentration of intracontinental deformation. These findings illuminate the evolution history of the Chinese Altai Mountains and highlight the great impacts of ancient tectonics on intracontinental deformation partitioning.