Crustal and Uppermost Mantle Isotropic and Anisotropic P-wave Velocity Variations Beneath Turkey 

2021 ◽  
Author(s):  
Tuna Eken ◽  
Haibo Wang ◽  
Zhouchuan Huang ◽  
Derya Keleş ◽  
Tulay Kaya-Eken ◽  
...  
Keyword(s):  
Lower Crust ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Uppermost Mantle ◽  
Western Turkey ◽  
Velocity Models ◽  
South Eastern ◽  
The North ◽  
Eastern Turkey ◽  
Major Surface

<p>Turkey has been undergoing compressional and extensional tectonics that greatly influences the major surface features following northward plate convergences since the Miocene. Despite increasing efforts in last few decades aiming to elucidate the current architecture of the crust and mantle beneath Turkey, several issues regarding the depth extent of the deformation zones, crust‐mantle interaction (e.g., coupling and decoupling) in relation to the deformation, and stress transmission in the lithosphere remain elusive. Inversion of 204,531 P wave arrival times from 8,103 local crustal earthquakes yields high‐resolution 3‐D P wave isotropic and azimuthal anisotropic velocity models of the crust and uppermost mantle beneath Turkey. Major outcomes of the present work are low‐velocity anomalies or velocity contrasts down to the uppermost mantle along the North and East Anatolian Fault Zones. We observe the fast velocity directions (FVDs) of azimuthal anisotropy in the lower crust and uppermost mantle parallel to the regional maximum extensional directions in western Turkey, whereas parallel to the surface structures in the crust and uppermost mantle beneath south-eastern Turkey. Our isotropic/anisotropic images strongly imply vertically coherent deformation between the crust and uppermost mantle in western and south-eastern Turkey. However, in central northern Turkey, the FVDs in the uppermost mantle are oblique to both the FVDs in the lower crust and the maximum shear directions derived from GPS measurements, suggesting that the crust and lithospheric mantle are decoupled.</p>

scholarly journals Workshop report on hard-rock drilling into mid-Cretaceous Pacific oceanic crust on the Hawaiian North Arch

2019 ◽  
Vol 26 ◽  
pp. 47-58
Author(s):  
Tomoaki Morishita ◽  
Susumu Umino ◽  
Jun-Ichi Kimura ◽  
Mikiya Yamashita ◽  
Shigeaki Ono ◽  
...  
Keyword(s):  
Oceanic Crust ◽  
Lower Crust ◽  
Velocity Structure ◽  
Spreading Rate ◽  
Azimuthal Anisotropy ◽  
P Wave ◽  
Uppermost Mantle ◽  
Magma Intrusion ◽  
Workshop Report ◽  
Geophysical Surveys

Abstract. The architecture, formation, and modification of oceanic plates are fundamental to our understanding of key geologic processes of the Earth. Geophysical surveys were conducted around a site near the Hawaiian Islands (northeastern Hawaiian North Arch region; Hawaiian North Arch hereafter), which is one of three potential sites for an International Ocean Discovery Program mantle drilling proposal for the Pacific plate that was submitted in 2012. The Hawaiian North Arch site is located in 78–81 Ma Cretaceous crust, which had an estimated full spreading rate of 7–8 cm yr−1. This site fills a major gap in our understanding of oceanic crust. Previously drilling has been skewed to young or older crust (<15 or >110 Ma) and slow-spread crust. P-wave velocity structure in the uppermost mantle of the Hawaiian North Arch shows a strong azimuthal anisotropy, whereas Moho reflections below the basement are variable: strong and continuous, weak, diffuse, or unclear. We assume that the strength of the Moho reflection is related to the aging of the oceanic plate. The Hawaiian volcanic chain (200 km to the southwest of the proposed drill site) and the nearby North Arch magmatism on the proposed Hawaiian North Arch sites might also have affected recognition of the Moho via deformation and/or magma intrusion into the lower crust of the uppermost mantle. This workshop report describes scientific targets for 2 km deep-ocean drilling in the Hawaiian North Arch region in order to provide information about the lower crust from unrecovered age and spreading rate gaps from previous ocean drillings. Other scientific objectives to be achieved by drilling cores before reaching the target depth of the project are also described in this report.

Reconnaissance seismic refraction-reflection surveys in northwestern New Mexico

1984 ◽  
Vol 74 (4) ◽  
pp. 1263-1274
Author(s):  
Lawrence H. Jaksha ◽  
David H. Evans
Keyword(s):  
New Mexico ◽  
Wave Velocity ◽  
Lower Crust ◽  
Seismic Waves ◽  
Seismic Refraction ◽  
Velocity Model ◽  
P Wave ◽  
Uppermost Mantle ◽  
P Wave Velocity ◽  
Crustal Thinning

Abstract A velocity model of the crust in northwestern New Mexico has been constructed from an interpretation of direct, refracted, and reflected seismic waves. The model suggests a sedimentary section about 3 km thick with an average P-wave velocity of 3.6 km/sec. The crystalline upper crust is 28 km thick and has a P-wave velocity of 6.1 km/sec. The lower crust below the Conrad discontinuity has an average P-wave velocity of about 7.0 km/sec and a thickness near 17 km. Some evidence suggests that velocity in both the upper and lower crust increases with depth. The P-wave velocity in the uppermost mantle is 7.95 ± 0.15 km/sec. The total crustal thickness near Farmington, New Mexico, is about 48 km (datum = 1.6 km above sea level), and there is evidence for crustal thinning to the southeast.

The azimuth-dependent dispersion curve inversion method to extract 3D anisotropic structure and its application to the eastern Tibetan Plateau

2020 ◽  
Author(s):  
Chuntao Liang
Keyword(s):  
Tibetan Plateau ◽  
Dispersion Curve ◽  
Large Scale ◽  
Azimuthal Anisotropy ◽  
Inversion Method ◽  
Eastern Tibetan Plateau ◽  
Uppermost Mantle ◽  
Low Velocity ◽  
The North ◽  
Lateral Extrusion

&lt;p&gt;&lt;span&gt;An azimuth-dependent dispersion curve inversion (ADDCI) method is applied to Rayleigh waves to extract 3D velocity and azimuthal anisotropy. The synthetic tests show that the ADDCI method is able to extract azimuthal anisotropy &lt;/span&gt;&lt;span&gt;at different depths. The errors of the fast propagation direction (FPD) and the magnitude of the anisotropy (MOA) are less than 10&amp;#176; and 1-2%, respectively. The 3D anisotropic model shows large variations in the FPDs and MOAs with depth and blocks; strong contrasts are observed across major faults, and the average MOA in the crust is approximately 3%. The FPDs are positively correlated with the GPS velocity direction and the strikes of regional faults in most of the blocks. The low-velocity zones (LVZs) in the middle to lower crust are widely observed in the Songpan Ganze Terrence, the north Chuan-Dian block, and surprisingly in the Huayingshan thrust and fold belt. The LVZs in the middle crust are also positively correlated with the low-velocity belt in the uppermost mantle. These observations may suggest that large-scale deformation is coupled vertically from the surface to the uppermost mantle. Crust shortening by the pure shearing process, which involves the thrusting and folding of the upper crust and the lateral extrusion of blocks, may be the major mechanism causing the growth of the eastern Tibetan Plateau.&lt;/span&gt;&lt;/p&gt;

scholarly journals Seismic investigation of an active ocean–continent transform margin: the interaction between the Swan Islands Fault Zone and the ultraslow-spreading Mid-Cayman Spreading Centre

2019 ◽  
Vol 219 (1) ◽  
pp. 159-184 ◽  
Author(s):  
C Peirce ◽  
A H Robinson ◽  
A M Campbell ◽  
M J Funnell ◽  
I Grevemeyer ◽  
...  
Keyword(s):  
Continental Crust ◽  
Oceanic Crust ◽  
Fault Zone ◽  
Lower Crust ◽  
P Wave ◽  
Fault System ◽  
Transform Fault ◽  
Apparent Lack ◽  
The North ◽  
Transform Margin

SUMMARY The Swan Islands Transform Fault (SITF) marks the southern boundary of the Cayman Trough and the ocean–continent transition of the North American–Caribbean Plate boundary offshore Honduras. The CAYSEIS experiment acquired a 180-km-long seismic refraction and gravity profile across this transform margin, ∼70 km to the west of the Mid-Cayman Spreading Centre (MCSC). This profile shows the crustal structure across a transform fault system that juxtaposes Mesozoic-age continental crust to the south against the ∼10-Myr-old ultraslow spread oceanic crust to the north. Ocean-bottom seismographs were deployed along-profile, and inverse and forward traveltime modelling, supported by gravity analysis, reveals ∼23-km-thick continental crust that has been thinned over a distance of ∼70 km to ∼10 km-thick at the SITF, juxtaposed against ∼4-km-thick oceanic crust. This thinning is primarily accommodated within the lower crust. Since Moho reflections are not widely observed, the 7.0 km s−1 velocity contour is used to define the Moho along-profile. The apparent lack of reflections to the north of the SITF suggests that the Moho is more likely a transition zone between crust and mantle. Where the profile traverses bathymetric highs in the off-axis oceanic crust, higher P-wave velocity is observed at shallow crustal depths. S-wave arrival modelling also reveals elevated velocities at shallow depths, except for crust adjacent to the SITF that would have occupied the inside corner high of the ridge-transform intersection when on axis. We use a Vp/Vs ratio of 1.9 to mark where lithologies of the lower crust and uppermost mantle may be exhumed, and also to locate the upper-to-lower crustal transition, identify relict oceanic core complexes and regions of magmatically formed crust. An elevated Vp/Vs ratio suggests not only that serpentinized peridotite may be exposed at the seafloor in places, but also that seawater has been able to flow deep into the crust and upper mantle over 20–30-km-wide regions which may explain the lack of a distinct Moho. The SITF has higher velocities at shallower depths than observed in the oceanic crust to the north and, at the seabed, it is a relatively wide feature. However, the velocity–depth model subseabed suggests a fault zone no wider than ∼5–10 km, that is mirrored by a narrow seabed depression ∼7500 m deep. Gravity modelling shows that the SITF is also underlain, at &gt;2 km subseabed, by a ∼20-km-wide region of density &gt;3000 kg m−3 that may reflect a broad region of metamorphism. The residual mantle Bouguer anomaly across the survey region, when compared with the bathymetry, suggests that the transform may also have a component of left-lateral trans-tensional displacement that accounts for its apparently broad seabed appearance, and that the focus of magma supply may currently be displaced to the north of the MCSC segment centre. Our results suggest that Swan Islands margin development caused thinning of the adjacent continental crust, and that the adjacent oceanic crust formed in a cool ridge setting, either as a result of reduced mantle upwelling and/or due to fracture enhanced fluid flow.

scholarly journals Synthesizing EarthScope data to constrain the thermal evolution of the continental U.S. lithosphere

Geosphere ◽  
2019 ◽  
Vol 15 (6) ◽  
pp. 1722-1737
Author(s):  
Ryan C. Porter ◽  
Suzan van der Lee ◽  
Steven J. Whitmeyer
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Seismic Velocity ◽  
Thermal Evolution ◽  
Thermal Stabilization ◽  
Earth Model ◽  
Continental Lithosphere ◽  
Uppermost Mantle ◽  
Velocity Models ◽  
The Relationship

Abstract In this work, we compile several seismic velocity models publicly available from the Incorporated Research Institute for Seismology (IRIS) Earth Model Collaboration (EMC) and compare subcrustal mantle velocities in the models to each other and to the timing of tectonism across the continent. This work allows us to assess the relationship between the time elapsed since the most recent thermotectonic event and uppermost mantle temperatures. We apply mineral- and physics-based models of velocity-temperature relationships to calculate upper-mantle temperatures in order to determine cooling rates for the lower-crust and uppermost mantle following thermotectonic activity. Results show that most of the cooling occurs in the ∼300–500 million years following orogeny. This work summarizes current estimates of upper-mantle shear velocities and provides insights on the thermal stabilization of continental lithosphere through time.

The seismic velocity and Poisson's ratio structure of the Kapuskasing uplift from laboratory measurements

1994 ◽  
Vol 31 (7) ◽  
pp. 1052-1063 ◽  
Author(s):  
Matthew H. Salisbury ◽  
David M. Fountain
Keyword(s):  
Lower Crust ◽  
Poisson’S Ratio ◽  
Seismic Anisotropy ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Laboratory Data ◽  
Transversely Isotropic ◽  
P Wave ◽  
Poisson's Ratio ◽  
Velocity Models

The compressional (Vp) and shear (Vs) wave velocity structure of the Kapuskasing uplift have been determined as a function of depth, propagation direction, and polarization from laboratory velocity measurements to confining pressures of 600 MPa on oriented samples from known structural levels of the complex. Based on the relative field abundances of the lithologies measured, the three principal terranes exposed in the uplift are characterized at depth by the following average values of Vp, Vs, and apparent Poisson's ratio, σa: (i) Michipicoten greenstone bell (greenschist, depth 0–6 km, Vp = 6.6 km/s, Vs = 3.9 km/s, σa = 0.235); (ii) Wawa gneiss terrane (amphibolite, depth 6–17 km, Vp = 6.5 km/s, Vs = 3.8 km/s, σa = 0.24); and (iii) Kapuskasing structural zone (granulite, depth 17–23 km, Vp = 6.9 km/s, Vs = 3.9 km/s, σa = 0.27). Although anisotropic lithologies such as paragneiss or mafic gneiss are present at all levels and tend to increase in abundance with depth, only in the deepest level (the Kapuskasing zone) are they sufficiently abundant and oriented to produce significant regional seismic anisotropy (transversely isotropic with Vp and Vs fast in the horizontal plane) and detectable shear wave splitting (ΔVs = 0.1 km/s).A comparison between the laboratory data and velocity models determined for the same crustal section from Lithoprobe refraction studies shows excellent agreement, confirming that the lithologies exposed in the Kapuskasing uplift can be projected downdip to the upper–lower crust transition, or Conrad discontinuity, at about 25 km. Below this depth, high P-wave velocities (7.0–7.6 km/s) suggest that the lower crust is more mafic or garnet rich. Similarities between the velocity structure of the Kapuskasing uplift and other sites in the Canadian Shield suggest that the observed crustal section is fairly typical of Archean continental crust.

Connecting subduction, extension, and shear localization across the Aegean Sea and Anatolia

2021 ◽  
Author(s):  
S Barbot ◽  
J R Weiss
Keyword(s):  
Shear Zone ◽  
Upper Mantle ◽  
Lower Crust ◽  
Eastern Mediterranean ◽  
Kinematic Model ◽  
Aegean Sea ◽  
Strike Slip ◽  
Western Turkey ◽  
The North ◽  
Slip Deformation

Summary The Eastern Mediterranean is the most seismically active region in Europe due to the complex interactions of the Arabian, African and Eurasian tectonic plates. Deformation is achieved by faulting in the brittle crust, distributed flow in the viscoelastic lower-crust and mantle, and Hellenic subduction but the long-term partitioning of these mechanisms is still unknown. We exploit an extensive suite of geodetic observations to build a kinematic model connecting strike-slip deformation, extension, subduction, and shear localization across Anatolia and the Aegean Sea by mapping the distribution of slip and strain accumulation on major active geologic structures. We find that tectonic escape is facilitated by a plate-boundary-like, trans-lithospheric shear zone extending from the Gulf of Evia to the Turkish-Iranian Plateau that underlies the surface trace of the North Anatolian Fault. Additional deformation in Anatolia is taken up by a series of smaller-scale conjugate shear zones that reach the upper mantle, the largest of which is located beneath the East Anatolian Fault. Rapid north-south extension in the western part of the system, driven primarily by Hellenic Trench retreat, is accommodated by rotation and broadening of the North Anatolian mantle shear zone from the Sea of Marmara across the north Aegean Sea, and by a system of distributed transform faults and rifts including the rapidly extending Gulf of Corinth in central Greece and the active grabens of western Turkey. Africa-Eurasia convergence along the Hellenic Arc occurs at a median rate of 49.8 mm/yr in a largely trench-normal direction except near eastern Crete where variably-oriented slip on the megathrust coincides with mixed-mode and strike-slip deformation in the overlying accretionary wedge near the Ptolemy-Pliny-Strabo trenches. Our kinematic model illustrates the competing roles the North Anatolian mantle shear zone, Hellenic Trench, overlying mantle wedge, and active crustal faults play in accommodating tectonic indentation, slab rollback, and associated Aegean extension. Viscoelastic flow in the lower crust and upper mantle dominate the surface velocity field across much of Anatolia and a clear transition to megathrust-related slab pull occurs in western Turkey, the Aegean Sea, and Greece. Crustal scale faults and the Hellenic wedge contribute only a minor amount to the large-scale, regional pattern of Eastern Mediterranean interseismic surface deformation.

The relationship between Vs, Vp, density and depth based on PS-logging data at K-NET and KiK-net sites

2021 ◽  
Author(s):  
Fumiaki Nagashima ◽  
Hiroshi Kawase
Keyword(s):  
Standard Deviation ◽  
Seismic Velocity ◽  
Saturated Soil ◽  
P Wave ◽  
Soil Types ◽  
Depth Range ◽  
Velocity Inversion ◽  
Velocity Models ◽  
Regression Curves ◽  
Ps Logging

Summary P-wave velocity (Vp) is an important parameter for constructing seismic velocity models of the subsurface structures by using microtremors and earthquake ground motions or any other geophysical exploration data. In order to reflect the ground survey information in Japan to the Vp structure, we investigated the relationships among Vs, Vp, and depth by using PS-logging data at all K-NET and KiK-net sites. Vp values are concentrated at around 500 m/s and 1,500 m/s when Vs is lower than 1,000 m/s, where these concentrated areas show two distinctive characteristics of unsaturated and saturated soil, respectively. Many Vp values in the layer shallower than 4 m are around 500 m/s, which suggests the dominance of unsaturated soil, while many Vp values in the layer deeper than 4 m are larger than 1,500 m/s, which suggests the dominance of saturated soil there. We also investigated those relationships for different soil types at K-NET sites. Although each soil type has its own depth range, all soil types show similar relationships among Vs, Vp, and depth. Then, considering the depth profile of Vp, we divided the dataset into two by the depth, which is shallower or deeper than 4 m, and calculated the geometrical mean of Vp and the geometrical standard deviation in every Vs bins of 200 m/s. Finally, we obtained the regression curves for the average and standard deviation of Vp estimated from Vs to get the Vp conversion functions from Vs, which can be applied to a wide Vs range. We also obtained the regression curves for two datasets with Vp lower and higher than 1,200 m/s. These regression curves can be applied when the groundwater level is known. In addition, we obtained the regression curves for density from Vs or Vp. An example of the application for those relationships in the velocity inversion is shown.

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