Seismic velocity structure of the Queen Charlotte Basin beneath Hecate Strait

1993 ◽  
Vol 30 (4) ◽  
pp. 787-805 ◽  
Author(s):  
G. D. Spence ◽  
I. Asudeh
Keyword(s):  
Lower Crust ◽  
Constant Velocity ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Moho Depth ◽  
Moderate Increase ◽  
Central Basin ◽  
Velocity Models ◽  
Brittle Ductile Transition ◽  
Numerical Predictions

Seismic refraction data across Hecate Strait in the northern Queen Charlotte Basin were collected in a coincident reflection and refraction survey. Crustal velocity models provide a framework to help understand the formation of the sedimentary basin and the processes occurring near the Queen Charlotte Fault, a major ocean–continent transform fault. Beneath the sediments, which have a maximum thickness of 6 km, a velocity gradient extends from about 5 to 8 km depth, within which velocities increase typically from 6.3 to 6.4 km∙s−1. A thick constant-velocity region was found down to a depth varying from 14 to 22 km, with the smallest depths located beneath the central basin. The base of the constant-velocity layer was marked by a distinct mid-crustal interface, across which velocities increased from 6.4–6.5 km∙s−1 to approximately 6.8–6.9 km∙s−1. Moho was interpreted to be at a near-uniform depth of 26–28 km beneath Hecate Strait and the eastern Queen Charlotte Islands. The associated variation in crustal thickness beneath the basin implies crustal thinning, perhaps caused by extension, of 30% or more.The mid-crustal interface may mark the change to a more mafic and perhaps ductile lower crust. The interface appears to be about 1–4 km deeper than the brittle–ductile transition, as indicated by the estimated depth to the 450 °C isotherm and by the moderate increase in reflectivity on the seismic reflection sections. Ductile flow may also occur in the lower crust near the Queen Charlotte Fault, where the relative motion of the oceanic plate induces lithospheric flow and thinning beneath both the ocean and the continent. The observed decrease in Moho depth from 28 to 21 km near the fault is consistent with recent (1989) numerical predictions of I. Reid for lithospheric flow near ocean–continent transforms.

Transition from oceanic to continental crustal structure: seismic and gravity models at the Queen Charlotte transform margin

1995 ◽  
Vol 32 (6) ◽  
pp. 699-717 ◽  
Author(s):  
G. D. Spence ◽  
D. T. Long
Keyword(s):  
Continental Crust ◽  
Triple Junction ◽  
Lower Crust ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Gravity Data ◽  
Gravity Models ◽  
Moho Depth ◽  
Horizontal Distance ◽  
Velocity Models

Seismic refraction data have been interpreted along a line crossing the Queen Charlotte transform, just north of the triple junction where the Explorer Ridge intersects the continental margin. These data, observed at three onshore sites, help to define the structure of the continental crust beneath the Queen Charlotte sedimentary basin. Sediment thicknesses of up to 4 km were determined from a coincident multichannel reflection line. Beneath the sediments, velocities increase from about 5.5 to 6.3 km·s−1 at 8 km depth, then increase from 6.5 to 6.7 km·s−1 at 18 km depth. Below this depth, the lower crust is partly constrained by Moho wide-angle reflections at the three receiving sites, which indicate a lower crust velocity of 6.8–6.9 km·s−1 and a Moho depth of 26–28 km. The crustal velocity structure is generally similar to that in southern Queen Charlotte Sound. It is in contrast to the velocity structure across Hecate Strait to the north, where a prominent mid-crust interface at ~15 km depth was observed. Seismic velocity models of the continental crust provide constraints that can be used in modelling gravity data to extend structures across the ocean–continent boundary. Along the profile just north of the Queen Charlotte triple junction, the gravity "edge effect" is very subdued, with maximum anomalies of < mGal (1 mGal = 10−3 cm·s−2). To satisfy the gravity data along this profile, the modelled crustal thickness must decrease to oceanic values (5–6 km) over a horizontal distance of 75 (±10) km, which gives a Moho dip of about 14°. Farther north, refraction models across Hecate Strait provide similar constraints for gravity modelling; the gravity data indicate horizontal transition distances from thick to thin crust of 45 (±10) km, comparable with, but slightly smaller than, those nearer the triple junction, and Moho dips at an angle of 18–22°. The greater thinning near the triple junction is consistent with mass flux models in which ductile flow in the lithosphere is induced by the relative motion between oceanic and continental plates.

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.

The transition of the East European cratonic lithosphere to that of the Palaeozoic collage of the Trans-European Suture Zone as depicted on the TTZ-South deep seismic profile (SE Poland to NW Ukraine)

2020 ◽  
Author(s):  
Tomasz Janik ◽  
Vitaly Starostenko ◽  
Paweł Aleksandrowski ◽  
Tamara Yegorova ◽  
Wojciech Czuba ◽  
...  
Keyword(s):  
Crustal Structure ◽  
Lower Crust ◽  
Seismic Velocity ◽  
Sedimentary Cover ◽  
Suture Zone ◽  
Crystalline Basement ◽  
Moho Depth ◽  
Crustal Layer ◽  
Velocity Inversion ◽  
East European

&lt;p&gt;Crustal and uppermost mantle structure along the Teisseyre-Tornquist Zone (TTZ)&amp;#160; was explored along the ~550 km long, NW-SE-trending TTZ-South profile, using seismic wide-angle reflection/refraction (WARR) method. The profile line was intended to follow the border between the East European Craton (EEC) and the so called Palaeozoic Platform (PP) of north-central Europe, believed to contain a number of crustal blocks that were accreted to the craton during pre-late Carboniferous times, defining the Trans-European Suture Zone (TESZ).&lt;/p&gt;&lt;p&gt;The seismic velocity model of the TTZ-South profile shows lateral variations in crustal structure. Its Ukrainian segment crosses the interior of the Sarmatian segment of the EEC, where the crystalline basement gradually dips from ~2 km depth in the SE to ~12 km at the Ukrainian-Polish border. This part of the model shows a four-layered crustal structure, with an up to 15 km-thick sedimentary cover, an underlying crystalline upper crust, a 10-15 km-thick middle crust and a ~15 km thick lower crust. In Poland, the profile passes along the TESZ/EEC transition zone of complex crustal structure. The crystalline basement, whose top occurs at depths of 10-17 km, separates the sedimentary cover from the ~10 km thick mid-crustal layer (Vp=6.5-6.6 km/s), which, in turn, overlies a block of 10-15 km thickness with upper crustal velocities (Vp~6.2 km/s). The latter is underlain by a ~10-15 km-thick lower crust. Along most of the model one can see conspicuous velocity inversion zones occuring at various depths. At intersections of the TTZ-South profile with some previous deep seismic profiles (e.g. CEL02, CEL05, CEL14, PANCAKE) such inversions document complex wedging relationships between the EEC and PP crustal units. These may have resulted from tectonic compression and thick-skinned thrusting due to either Neoproterozoic EEC collision with accreting terranes or intense Variscan orogenic events. Five high velocity bodies (HVB; V&lt;sub&gt;p&lt;/sub&gt; = 6.85-7.2 km/s) were detected in the middle and lower crust at 15-37 km depth. The Moho depth varies substantially along the profile. It is at ~42 km depth in the NW and deepens SE-ward to ~50 km at ~685 km. Subsequently, it rises abruptly to ~43 km at the border of the Sarmatian segment of the EEC and sinks again to ~50 km beneath the Lviv Paleozoic trough at ~785 km. From this point until the SE end of the profile, the Moho gently shallows, up to a depth of ~37 km, including a step-like jump of 2 km at ~875 km. Such abrupt Moho steps may be related to crust-scale strike-slip faults. Along the whole profile, sub-Moho velocities are ~8.05-8.1 km/s, and at depths of 57-63 km Vp values reach 8.2-8.25 km/s. Four reflectors/refractors were modelled in the upper mantle at ~57-65 km and ~80 km depths.&lt;/p&gt;

scholarly journals Seismic velocity model of the crust in the northern Canadian Cordillera from Rayleigh wave dispersion data

2017 ◽  
Vol 54 (2) ◽  
pp. 163-172 ◽  
Author(s):  
Shutian Ma ◽  
Pascal Audet
Keyword(s):  
Rayleigh Wave ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Group Velocity Dispersion ◽  
Velocity Model ◽  
Canadian Cordillera ◽  
Seismic Velocities ◽  
Crustal Layer ◽  
Velocity Models ◽  
Moderate Earthquake

Models of the seismic velocity structure of the crust in the seismically active northern Canadian Cordillera remain poorly constrained, despite their importance in the accurate location and characterization of regional earthquakes. On 29 August 2014, a moderate earthquake with magnitude 5.0, which generated high-quality Rayleigh wave data, occurred in the Northwest Territories, Canada, ∼100 km to the east of the Cordilleran Deformation Front. We carefully selected 23 seismic stations that recorded the Rayleigh waves and divided them into 13 groups according to the azimuth angle between the earthquake and the stations; these groups mostly sample the Cordillera. In each group, we measured Rayleigh wave group velocity dispersion, which we inverted for one-dimensional shear-wave velocity models of the crust. We thus obtained 13 models that consistently show low seismic velocities with respect to reference models, with a slow upper and lower crust surrounding a relatively fast mid crustal layer. The average of the 13 models is consistent with receiver function data in the central portion of the Cordillera. Finally, we compared earthquake locations determined by the Geological Survey of Canada using a simple homogenous crust over a mantle half space with those estimated using the new crustal velocity model, and show that estimates can differ by as much as 10 km.

scholarly journals Origin of the seismic belt in the San-in district, southwest Japan, inferred from the seismic velocity structure of the lower crust

2019 ◽  
Vol 71 (1) ◽  
Author(s):  
Hiroo Tsuda ◽  
Yoshihisa Iio ◽  
Takuo Shibutani
Keyword(s):  
Lower Crust ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Seismic Belt ◽  
Low Velocity ◽  
Intraplate Earthquakes ◽  
Linear Distribution ◽  
Low Velocity Zone ◽  
Velocity Anomaly ◽  
Velocity Zone

Abstract A long linear distribution of epicenters is seen along the Japan Sea coast in the San-in district located in southwestern Japan. This linear distribution of epicenters is called the seismic belt in the San-in district. The localization of intraplate earthquakes in the San-in district, far from plate boundaries, is not well understood. To answer this question, we look at the seismic velocity structure of the lower crust beneath the San-in district using seismic travel-time tomography. Our results show the existence of a low-velocity anomaly in the lower crust beneath the seismic belt. We infer that the deformation was concentrated in the low-velocity zone due to compressive stress caused by the subduction of oceanic plates, that stress concentration occurred just above the low-velocity zone, and that the seismic belt was therefore formed there. We also calculated the cutoff depths of shallow intraplate earthquakes in the San-in district. Based on the results, we consider the possible causes of the low-velocity anomaly in the lower crust beneath the seismic belt. We found that the cutoff depths of the intraplate earthquakes were shallower in the eastern part of the low-velocity zone in the lower crust beneath the seismic belt and deeper in the western part. Thus, the eastern part is likely to be hotter than the western part. We inferred that the eastern part was hot because a hot mantle upwelling approaches the Moho discontinuity below it and the resulting high temperature may be the main cause of the low-velocity anomaly. On the other hand, in the western part, we inferred that the temperature is not high because the mantle upwelling may not exist at shallow depth, and water dehydrated from the Philippine Sea plate reaches the lower crust, and the existence of this water may be the main cause of the low-velocity anomaly.

scholarly journals Seismic velocity structure of Unzen Volcano, Japan, and relationship to the magma ascent route during eruptions in 1990–1995

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Kanta Miyano ◽  
Koki Aizawa ◽  
Takeshi Matsushima ◽  
Azusa Shito ◽  
Hiroshi Shimizu
Keyword(s):  
Velocity Structure ◽  
Seismic Velocity ◽  
P Wave ◽  
Pyroclastic Flows ◽  
Magma Ascent ◽  
Unzen Volcano ◽  
Subsurface Structures ◽  
Brittle Ductile Transition ◽  
P Wave Velocity ◽  
Tectonic Earthquakes

AbstractSubsurface structures may control the migration of magma beneath a volcano. We used high-resolution seismic tomography to image a low- P-wave velocity (Vp) zone beneath Unzen Volcano, Japan, at depths of 3–16 km beneath sea level. The top of this low-Vp zone is located beneath Mt. Fugendake of Unzen volcano, which emitted 0.21 km3 of dacitic magma as lava domes and pyroclastic flows during eruptions in 1990–1995. Based on hypocenter migrations prior to the 1990–1995 eruptions and modeled pressure source locations for recorded crustal deformation, we conclude that the magma for the 1990–1995 eruptions migrated obliquely upward along the top of the low-Vp zone. As tectonic earthquakes occurred above the deeper part of the low-Vp zone, the deep low-Vp zone is interpreted to be a high-temperature region (> 400 °C) overlying the brittle–ductile transition. By further considering Vs and Vp/Vs structures, we suggest that the deeper part of the low-Vp zone constitutes a highly crystalized magma-mush reservoir, and the shallower part a volatile-rich zone.

Near-salt stress-induced seismic velocity changes and seismic anisotropy and their impacts on salt imaging: A case study in the Kuqa depression, Tarim Basin, China

2020 ◽  
Vol 8 (3) ◽  
pp. T487-T499
Author(s):  
Yunqiang Sun ◽  
Gang Luo ◽  
Yaxing Li ◽  
Mingwen Wang ◽  
Xiaofeng Jia ◽  
...  
Keyword(s):  
Salt Stress ◽  
Seismic Anisotropy ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Seismic Velocities ◽  
Kuqa Depression ◽  
Velocity Models ◽  
Salt Structure ◽  
Seismic Velocity Changes ◽  
Velocity Changes

It has been recognized that stress perturbations in sediments induced by salt bodies can cause elastic-wave velocity (seismic velocity) changes and seismic anisotropy through changing their elastic parameters, thus leading to difficulties in salt imaging. To investigate seismic velocity changes and seismic anisotropy by near-salt stress perturbations and their impacts on salt imaging, taking the Kuqa depression as an example, we have applied a 2D plane-strain static geomechanical finite-element model to simulate stress perturbations and calculate the associated seismic velocity changes and seismic anisotropy; then we used the reverse time migration and imaging method to image the salt structure by excluding and including the stress-induced seismic velocity changes. Our model results indicate that near-salt stresses are largely perturbed due to salt stress relaxation, and the stress perturbations lead to significant changes of the seismic velocities and seismic anisotropy near the salt structure: The maximum seismic velocity changes can reach approximately 20% and the maximum seismic anisotropy can reach approximately 10%. The significant changes of seismic velocities due to stress perturbations largely impact salt imaging: The salt imaging is unclear, distorted, or even failed if we exclude near-salt seismic velocity changes from the preliminary velocity structure, but the salt can be better imaged if the preliminary velocity structure is modified by near-salt seismic velocity changes. We find that the locations where salt imaging tends to fail usually occur where large seismic velocity changes happen, and these locations are clearly related to the geometric characteristics of salt bodies. To accurately image the salt, people need to integrate results of geomechanical models and stress-induced seismic velocity changes into the imaging approach. The results provide petroleum geologists with scientific insights into the link between near-salt stress perturbations and their induced seismic velocity changes and help exploration geophysicists build better seismic velocity models in salt basins and image salt accurately.

Crustal structure across the Nain – Makkovik boundary on the continental shelf off Labrador from seismic refraction data

1996 ◽  
Vol 33 (3) ◽  
pp. 460-471 ◽  
Author(s):  
Ian Reid
Keyword(s):  
Continental Shelf ◽  
Wave Velocity ◽  
Crustal Structure ◽  
Lower Crust ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Seismic Refraction ◽  
P Wave ◽  
Air Gun ◽  
P Wave Velocity

A detailed seismic refraction profile was shot along the continental shelf off Labrador, across the boundary between the Archean Nain Province to the north and the Proterozoic Makkovik orogenic zone to the south. A large air-gun source was used, with five ocean-bottom seismometers as receivers. The data were analysed by forward modelling of traveltimes and amplitudes and provided a well-determined seismic velocity structure of the crust along the profile. Within the Nain province, thin postrift sediments are underlain by crust with a P-wave velocity of 6.1 km/s, which increases with depth and reaches 6.6 km/s at about 8 km. Moho is at around 28 km, and there is no evidence for a high-velocity (>7 km/s) lower crust. The P- and S-wave velocity structure is consistent with a gneissic composition for the Archean upper crust, and with granulites becoming gradually more mafic with depth for the intermediate and lower crust. In the Makkovik zone, the sediments are thicker, and a basement layer of P-wave velocity 5.5–5.7 km/s is present, probably due to reworking of the crust and the presence of Early Proterozoic volcanics and metasediments. Upper crustal velocities are lower than in the Nain Province. The crustal thickness, at 23 km, is less, possibly due in part to greater crustal stretching during the Mesozoic rifting of the Labrador Sea. The crustal structure across the Nain–Makkovik boundary is similar to that across the corresponding Archean–Ketilidian boundary off southwest Greenland.

Crustal seismic velocity structure of southern Poland: preserved memory of a pre-Devonian terrane accretion at the East European Platform margin

2010 ◽  
Vol 148 (2) ◽  
pp. 191-210 ◽  
Author(s):  
M. NARKIEWICZ ◽  
M. GRAD ◽  
A. GUTERCH ◽  
T. JANIK
Keyword(s):  
East European Platform ◽  
Velocity Structure ◽  
Seismic Velocity ◽  
Seismic Refraction ◽  
East European ◽  
Velocity Models ◽  
Se Poland ◽  
Refraction Seismic ◽  
Platform Margin ◽  
The Difference

AbstractThe updated geological and potential fields data on the East European Platform margin in SE Poland confirm the existence of several regional units differing in Ediacaran to Silurian development: the Upper Silesian Block, Małopolska Block and Łysogóry Block. All the blocks are characterized by a distinct crustal structure seen in Vp velocity models obtained from the seismic refraction data of the CELEBRATION 2000 Programme. The first two units are interpreted as exotic terranes initially derived from Avalonia-type crust and ultimately accreted before the late Early Devonian. The Łysogóry Block is probably a proximal terrane displaced dextrally along the Baltica margin. The sutures between the terranes do not precisely match lateral gradients in Vp models. This is partly explained by a limited resolution of refraction seismic data (20 km wide interpretative window). Most of the difference is related, however, to a post-accretionary tectonism, mainly Variscan transtension–transpression. The latter processes took advantage of lithospheric memory recorded earlier as zones of rheological weakness along the former suture zones. The course of the East European Platform margin (= Teisseyre–Tornquist Zone) corresponds most likely to the Nowe Miasto–Zawichost Fault marking the NE boundary of the proximal Łysogóry Terrane.

Crustal velocity models for the Archean Abitibi greenstone belt from seismic refraction data

1995 ◽  
Vol 32 (2) ◽  
pp. 149-166 ◽  
Author(s):  
Gilles Grandjean ◽  
Hua Wu ◽  
Donald White ◽  
Marianne Mareschal ◽  
Claude Hubert
Keyword(s):  
Lower Crust ◽  
Greenstone Belt ◽  
Velocity Structure ◽  
Complex Structure ◽  
Seismic Refraction ◽  
Seismic Energy ◽  
Crustal Layer ◽  
Middle Crust ◽  
Velocity Models ◽  
Abitibi Greenstone Belt

We present velocity models for two seismic wide–angle-refraction profiles across the Archean Abitibi greenstone belt and the Pontiac Subprovince. The seismic profiles are 210 and 220 km long. Traveltime inversion and amplitude forward modelling were used to obtain two-dimensional velocity structure and interface geometry. The main features of the velocity models include (1) three crustal layers; (2) variable velocities (5.6–6.4 km/s) in the upper crust (~0–12 km), with the higher velocities generally associated with mafic metavolcanics and the lower velocities with metasediments and granitic plutons; (3) a relatively uniform middle crust (~12–30 km) with velocities ranging from 6.4 to 6.6 km/s; (4) a velocity increase of 0.3 km/s across the middle crust–lower crust boundary; (5) a lower crust (~30–40 km) with velocities increasing from 6.9 km/s at the top to 7.3 km/s at the base; (6) an average upper mantle velocity of 8.15 km/s; (7) depth to Moho of about 40 km in the north-central Abitibi belt, decreasing southward to 37 km beneath the Pontiac Subprovince; and (8) observed attenuation of seismic energy propagating through the Casa–Berardi deformation zone, suggesting a complex structure in this fault zone. The velocity model is generally consistent with seismic reflection interpretations that suggest that the shallow supracrustal assemblages form an allochthonous veneer, overlying a mid-crustal imbricate sequence of metaplutonic and metasedimentary rocks. The uniform-velocity structure below 12 km depth indicates that the tectonic zones juxtaposing disparate crustal blocks may have limited depth extent. The 40 km thick crust and 10 km thick high-velocity lower crustal layer exceed the thicknesses observed in other studies of Archean crust.

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