Crustal structure of the Grenville Province in southeastern Labrador from refraction seismic data: evidence for a high-velocity lower crustal wedge

2001 ◽  
Vol 38 (10) ◽  
pp. 1463-1478 ◽  
Author(s):  
Thomas Funck ◽  
Keith E Louden ◽  
Ian D Reid
Keyword(s):  
Crustal Structure ◽  
High Velocity ◽  
Lower Crust ◽  
Lower Layer ◽  
P Wave ◽  
Grenville Province ◽  
Middle Crust ◽  
Basaltic Magmatism ◽  
Refraction Seismic ◽  
Lower Crustal

The crustal structure of the eastern Grenville and Makkovik provinces was determined using two onshore–offshore refraction seismic lines of the Lithoprobe Eastern Canadian Shield Onshore–Offshore Transect (ECSOOT). A gravity high in the Hawke River terrane correlates with increased P-wave velocities in the upper 30 km of the crust (6.2–6.7 km/s in the upper and middle crust and 6.9–7.1 km/s below) which we interpret as structure inherited from the Labradorian orogen. Velocities in the adjacent Groswater Bay terrane are 6.0–6.55 km/s in the upper and middle crust and 6.6–6.95 km/s in the lower crust. The entire Grenville crust is underlain by a 15–20 km thick high-velocity lower crustal (HVLC) wedge consisting of an upper layer (7.1–7.4 km/s) and a lower layer (7.6–7.8 km/s). The HVLC wedge is interpreted as an underplated layer formed during Iapetan rifting. This interpretation is based on the correlation with the 615 Ma Long Range dykes onshore and the eastward termination of the wedge at the Cartwright Arch. Similar HVLC layers are found offshore western Newfoundland, suggesting that the underplating may be a continuous feature along the passive Grenvillian margin. The Cartwright Arch is characterized by velocities of 6.4 km/s and 4 km thick sediment sequences (4.3–5.7 km/s) in the surrounding basin, interpreted as an extensional basin with basaltic magmatism within the arch. The Grenville front is clearly marked by a decrease of velocities in the Makkovik Province (5.8–6.4 km/s in the upper and middle crust, 6.65–6.85 km/s in the lower crust) and a gradual thickening of the crust (not including the HVLC layer) from 30 km in the Grenville Province to 35 km in the Makkovik Province.

Deep seismic reflection constraints on Paleogene crustal extension in the south-central Intermontane belt, British Columbia

2014 ◽  
Vol 51 (4) ◽  
pp. 393-406 ◽  
Author(s):  
Andrew J. Calvert ◽  
Draga Talinga
Keyword(s):  
British Columbia ◽  
Lower Crust ◽  
Seismic Reflection ◽  
Basaltic Magma ◽  
Shear Zones ◽  
P Wave ◽  
Middle Crust ◽  
South Central ◽  
Ductile Shearing ◽  
Lower Crustal

Following growth of the Canadian Cordillera during the Mesozoic, the southern Cordillera was subject to extension during the Paleocene and Eocene that correlated with widespread volcanic activity in south-central British Columbia, including across much of the Nechako–Chilcotin plateau. In 2008, Geoscience BC acquired 330 km of deep vibroseis reflection profiles on the plateau, mostly over the Stikinia arc terrane, but also over its eastern contact with the oceanic Cache Creek terrane. All seven seismic reflection lines reveal a strongly reflective lower crust that extends from 7 to 9 s down to the Moho, which is defined by the downward termination of reflectivity at 11–12 s. In the uppermost crust, extension occurred by block faulting with faults soling into subhorizontal to shallowly dipping detachments above 10 km depth. Extension in the deeper upper and middle crust, which was partly controlled by antiforms likely related to earlier shortening, was accommodated on a network of anastomosing shear zones that sole out into the top of the reflective lower crust. The lower crustal reflections correlate with seismic P-wave velocities of 6.45–6.98 km/s, indicating that the reflective lower crust has a more mafic composition than the middle crust. As in other extensional settings, we suggest that this pervasive fabric of reflectors arises from the intrusion of mantle-derived basaltic magma into zones of ductile shearing, and that differentiation of these melts resulted in the widespread Paleocene to Eocene volcanism. Reflector dips indicate that extension was approximately east–west, consistent with north-northwest-trending horsts separated by basins filled with Paleocene to Eocene volcanic and volcaniclastic rocks.

The crustal structure of the southern Nain and Makkovik provinces of Labrador derived from refraction seismic data

2008 ◽  
Vol 45 (4) ◽  
pp. 465-481 ◽  
Author(s):  
Thomas Funck ◽  
Annette K. Hansen ◽  
Ian D. Reid ◽  
Keith E. Louden
Keyword(s):  
Crustal Structure ◽  
Velocity Model ◽  
Seismic Profile ◽  
P Wave ◽  
Moho Depth ◽  
S Wave ◽  
Velocity Models ◽  
Refraction Seismic ◽  
Forward And Inverse Modeling ◽  
Lower Crustal

Data from a refraction seismic profile parallel to the coast of Labrador (Canada) were used to determine the crustal structure across the boundary of the Nain and Makkovik provinces, and to look for evidence for an offshore continuation of the Mesoproterozoic Nain Plutonic Suite (NPS). Seven seismometers recorded airgun shots along the 283 km long line. P- and S-wave velocity models were developed from forward and inverse modeling of travel times. The velocity model distinguishes three distinct zones. In the Saglek block of the Nain Province, the crust is divided into three layers with P-wave velocities between 5.8 and 6.9 km/s. Farther to the south, upper crustal velocities increase to 6.3–6.5 km/s and the Poisson’s ratio increases from 0.24 to 0.27. This zone correlates with a gravity low that is interpreted to outline the offshore continuation of the NPS. The upper crustal velocities are intermediate between anorthositic and granitoid rock samples collected from the NPS. A lower crustal reflector is limited to the area underneath the NPS and may be related to dioritic magmas. Mid-crustal and lower crustal velocities do not vary along the line and no underplating was detected. Within the Makkovik Province, upper crustal velocities of 5.9–6.2 km/s may indicate a dioritic composition similar to the Island Harbour Bay plutonic suite. Moho depth varies between 28 and 36 km with the maximum beneath the NPS. The variations could not be linked to effects of the Makkovikian orogeny but are thought to relate to Mesozoic rifting in Labrador Sea.

Dynamics of the extension in the Fonualei Rift in the northern Lau Basin at 16 °S

2021 ◽  
Author(s):  
Anna Jegen ◽  
Anke Dannowski ◽  
Heidrun Kopp ◽  
Udo Barckhausen ◽  
Ingo Heyde ◽  
...  
Keyword(s):  
Velocity Gradient ◽  
Lower Crust ◽  
Pacific Plate ◽  
P Wave ◽  
Upper Crust ◽  
Lau Basin ◽  
Australian Plate ◽  
The Pacific ◽  
Refraction Seismic ◽  
Roll Back

<p>The Lau Basin is a young back-arc basin steadily forming at the Indo-Australian-Pacific plate boundary, where the Pacific plate is subducting underneath the Australian plate along the Tonga-Kermadec island arc. Roughly 25 Ma ago, roll-back of the Kermadec-Tonga subduction zone commenced, which lead to break up of the overriding plate and thus the formation of the western Lau Ridge and the eastern Tonga Ridge separated by the emerging Lau Basin.</p><p>As an analogue to the asymmetric roll back of the Pacific plate, the divergence rates decline southwards hence dictating an asymmetric, V-shaped basin opening. Further, the decentralisation of the extensional motion over 11 distinct spreading centres and zones of active rifting has led to the formation of a composite crust formed of a microplate mosaic. A simplified three plate model of the Lau Basin comprises the Tonga plate, the Australian plate and the Niuafo'ou microplate. The northeastern boundary of the Niuafo'ou microplate is given by two overlapping spreading centres (OLSC), the southern tip of the eastern axis of the Mangatolu Triple Junction (MTJ-S) and the northern tip of the Fonualei Rift spreading centre (FRSC) on the eastern side. Slow to ultraslow divergence rates were identified along the FRSC (8-32 mm/a) and slow divergence at the MTJ (27-32 mm/a), both decreasing southwards. However, the manner of divergence has not yet been identified. Additional regional geophysical data are necessary to overcome this gap of knowledge.</p><p>Research vessel RV Sonne (cruise SO267) set out to conduct seismic refraction and wide-angle reflection data along a 185 km long transect crossing the Lau Basin at ~16 °S from the Tonga arc in the east, the overlapping spreading centres, FRSC1 and MTJ-S2, and extending as far as a volcanic ridge in the west. The refraction seismic profile consisted of 30 ocean bottom seismometers. Additionally, 2D MCS reflection seismic data as well as magnetic and gravimetric data were acquired.</p><p>The results of our P-wave traveltime tomography show a crust that varies between 4.5-6 km in thickness. Underneath the OLSC the upper crust is 2-2.5 km thick and the lower crust 2-2.5 km thick. The velocity gradients of the upper and lower crust differ significantly from tomographic models of magmatically dominated oceanic ridges. Compared to such magmatically dominated ridges, our final P-wave velocity model displays a decreased velocity gradient in the upper crust and an increased velocity gradient in the lower crust more comparable to tectonically dominated rifts with a sparse magmatic budget.</p><p>The dominance of crustal stretching in the regional rifting process leads to a tectonical stretching, thus thinning of the crust under the OLSC and therefore increasing the lower crust’s velocity gradient. Due to the limited magmatic budget of the area, neither the magnetic anomaly nor the gravity data indicate a magmatically dominated spreading centre. We conclude that extension in the Lau Basin at the OLSC at 16 °S is dominated by extensional processes with little magmatism, which is supported by the distribution of seismic events concentrated at the northern tip of the FRSC.</p>

scholarly journals Regional variations in the structure of the crust in the central United States from P-wave spectra

1973 ◽  
Vol 63 (5) ◽  
pp. 1663-1687
Author(s):  
Tuneto Kurita
Keyword(s):  
United States ◽  
Gulf Of Mexico ◽  
Crustal Structure ◽  
Coastal Plain ◽  
Lower Crust ◽  
P Wave ◽  
Ratio Method ◽  
Gulf Coastal Plain ◽  
Regional Variations ◽  
Central United States

abstract Regional variations in the crustal structure in the central United States have been inferred by the transfer ratio method from an analysis of long-period P waves recorded at SHA, OXF, FLO and MDS, the stations nearly along 89°W longitude. The crustal structure in this region is approximated by a stack of horizontal parallel layers except possibly in the area around FLO, where the structure is rather complicated. The crustal thickness is predominantly controlled by the thick silicic upper crust, whereas the mafic lower crust is about 10 km thick throughout this region. The P-wave velocity of the lower crust is about 6.9 to 7.0 km/sec except probably in the area around FLO, where 7.4 km/sec velocity is more likely. A sedimentary layer with a velocity of about 3.0 km/sec, having a thickness of about 3 km near the coast of the Gulf of Mexico, tapers out to the north within the Gulf Coastal Plain. Deep discontinuities in the crust may be replaced by transitional layers up to 10 km thick. The Moho is about 33 km deep near the coast of the Gulf of Mexico, deepens to about 41 km near an intersection of the Gulf coastal plain and the interior plain, reaches about 47 km or more in the midst of the interior plain, and rises to about 41 km toward an intersection of the interior plain and the superior upland. As for the midst of the interior plain, however, the depth of the Moho reduces by as much as 5 km, if the velocity in the lower crust is about 7.0 km/sec instead of about 7.4 km/sec. In any case, the general trend of the depth of the Moho may match with the topographic feature from the Gulf of Mexico to Lake Superior.

Crust and upper mantle velocity structure of the Intermontane belt, southern Canadian Cordillera

1992 ◽  
Vol 29 (7) ◽  
pp. 1530-1548 ◽  
Author(s):  
B. C. Zelt ◽  
R. M. Ellis ◽  
R. M. Clowes ◽  
E. R. Kanasewich ◽  
I. Asudeh ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Velocity Structure ◽  
P Wave ◽  
Seismic Reflection Profile ◽  
Wide Angle ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Middle Crust ◽  
Refraction Data

As part of the Lithoprobe Southern Cordillera transect, seismic refraction data were recorded along a 330 km long strike profile in the Intermontane belt. An iterative combination of two-dimensional traveltime inversion and amplitude forward modelling was used to interpret crust and upper mantle P-wave velocity structure. This region is characterized by (i) a thin near-surface layer with large variations in velocity between 2.8 and 5.4 km/s, and low-velocity regions that correlate well with surface expressions of Tertiary sedimentary and volcanic rocks; (ii) an upper and middle crust with low average velocity gradient, possibly a weak low-velocity zone, and lateral velocity variations between 6.0 and 6.4 km/s; (iii) a distinctive lower crust characterized by significantly higher average velocities relative to midcrustal values beginning at 23 km depth, approximately 8 km thick with average velocities of 6.5 and 6.7 km/s at top and base; (iv) a depth to Moho, as defined by wide-angle reflections, that averages 33 km with variations up to 2 km; and (v) a Moho transition zone of depth extent 1–3 km, below which lies the upper mantle with velocities decreasing from 7.9 km/s in the south to 7.7 km/s in the north. Where the refraction line obliquely crosses a Lithoprobe deep seismic-reflection profile, good agreement is obtained between the interpreted reflection section and the derived velocity structure model. In particular, depths to wide-angle reflectors in the upper crust agree with depths to prominent reflection events, and Moho depths agree within 1 km. From this comparison, the upper and middle crust probably comprise the upper part of the Quesnellia terrane. The lower crust from the refraction interpretation does not show the division into two components, parautochthonous and cratonic North America, that is inferred from the reflection data, indicating that their physical properties are not significantly different within the resolution of the refraction data. Based on these interpretations, the lower lithosphere of Quesnellia is absent and presumably was recycled in the mantle. At a depth of ~ 16 km below the Moho, an upper mantle reflector may represent the base of the present lithosphere.

scholarly journals Vestiges of underplating and assembly in the central North China Craton based on S-wave velocities

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Haoyu Tian ◽  
Chuansong He
Keyword(s):  
Group Velocity ◽  
Wave Velocity ◽  
Crustal Structure ◽  
North China Craton ◽  
Lower Crust ◽  
North China ◽  
Velocity Structure ◽  
S Wave ◽  
Middle Crust ◽  
Wave Velocities

AbstractThe destruction of the North China Craton (NCC) is a controversial topic among researchers. In particular, the crustal structure associated with the craton’s destruction remains unclear, even though a large number of seismic studies have been carried out in this area. To investigate the crustal structure and its dynamic implications, we perform noise tomography in the central part of the NCC. In this study, continuous vertical-component waveforms spanning one year from 112 broadband seismic stations are used to obtain the group velocity dispersion curves of Rayleigh waves at different periods, and surface wave tomography is employed to extract the Rayleigh wave group velocity distributions at 9–40 s. Finally, the S-wave velocity structure at depths of 0–60 km is determined by the inversion of pure-path dispersion data. The results show obvious differences in the crustal structure among the Western Block (WB), the Trans-North China Orogen (TNCO) and the Eastern Block (EB). The lower crust of the northern part of the EB exhibits a high-velocity S-wave anomaly, which may be related to magmatic underplating in the lower crust induced by an upwelling mantle plume. The S-wave velocity of the WB is lower than that of the TNCO in the upper and middle crust and is lower than that of both the TNCO and the EB in the lower crust. The crust of the TNCO shows higher S-wave velocities than the WB and EB in the upper and middle crust, and its overall S-wave velocity structure is clearly different from those of the WB and EB, implying that the crustal structure of the TNCO may contain vestiges of the Paleoproterozoic collision between the WB and EB and their subsequent assembly. This study marks the first time these findings are identified for the NCC.

scholarly journals Long-lived Paleoproterozoic eclogitic lower crust

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Sebastian Buntin ◽  
Irina M. Artemieva ◽  
Alireza Malehmir ◽  
Hans Thybo ◽  
Michal Malinowski ◽  
...  
Keyword(s):  
High Velocity ◽  
Lower Crust ◽  
Crustal Layer ◽  
Crustal Rocks ◽  
Long Term Stability ◽  
Eclogite Facies ◽  
Conventional Models ◽  
Lower Crustal

AbstractThe nature of the lower crust and the crust-mantle transition is fundamental to Earth sciences. Transformation of lower crustal rocks into eclogite facies is usually expected to result in lower crustal delamination. Here we provide compelling evidence for long-lasting presence of lower crustal eclogite below the seismic Moho. Our new wide-angle seismic data from the Paleoproterozoic Fennoscandian Shield identify a 6–8 km thick body with extremely high velocity (Vp ~ 8.5–8.6 km/s) and high density (>3.4 g/cm3) immediately beneath equally thinned high-velocity (Vp ~ 7.3–7.4 km/s) lowermost crust, which extends over >350 km distance. We relate this observed structure to partial (50–70%) transformation of part of the mafic lowermost crustal layer into eclogite facies during Paleoproterozoic orogeny without later delamination. Our findings challenge conventional models for the role of lower crustal eclogitization and delamination in lithosphere evolution and for the long-term stability of cratonic crust.

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 structures of Grenville, Makkovik, and southern Nain provinces along the Lithoprobe ECSOOT Transect: regional seismic refraction and gravity models and their tectonic implications

1998 ◽  
Vol 35 (5) ◽  
pp. 583-601 ◽  
Author(s):  
Keith E Louden ◽  
Jianming Fan
Keyword(s):  
Lower Crust ◽  
Velocity Structure ◽  
Gravity Data ◽  
Seismic Refraction ◽  
Gravity Models ◽  
Grenville Province ◽  
Crustal Layer ◽  
The North ◽  
Crustal Structures ◽  
Lower Crustal

Crustal structures of the eastern Grenville, Makkovik, and southern Nain provinces are determined using seismic reflection-refraction and gravity data along the Lithoprobe Eastern Canadian Shield Onshore-Offshore Transect (ECSOOT). Within the Grenville Province, the velocity model contains a 5 km thick upper crust and a variable-thickness middle to lower crust. The total crustal thickness varies from 25 to 43 km, with the thickest crust in the south and thinnest crust in the north. A high-velocity, lower crustal wedge is coincident with a strong band of northward-dipping reflectors. The two-dimensional velocity structure is compatible with modelling of a 60 mGal gravity high over the Hawke River terrane. In the Makkovik Province, the thickness of upper crustal velocities increases to 17 km. The velocity decrease in the upper to middle crust from the Grenville Province to the Makkovik Province is similar to that of refraction models across the Grenville Front in Ontario and Quebec. It is possibly related to a decrease in metamorphic grade from south to north and (or) a larger volume of unmetamorphosed plutons in the Makkovik Province. A lower crustal layer is coincident with a region of increased reflectivity in the lower crust. There are no major crustal discontinuities associated with terrane boundaries within the Makkovik Province. The base of the crust is consistent with a change from north- to south-dipping reflectors beneath the Cape Harrison domain. Alternatively, it may consist of a thick zone of complex velocity variations, consistent with a zone of diffusive reflectivity observed to the north of the Allik domain.

EXHUMATION OF LOWER CRUSTAL UHT ROCKS INTO THE UPPER AMPHIBOLITE FACIES MIDDLE CRUST: THE GRUF COMPLEX, EUROPEAN CENTRAL ALPS

2016 ◽  
Author(s):  
Jeffrey Oalmann ◽  
◽  
Andreas Möller ◽  
Romain Bousquet
Keyword(s):  
Amphibolite Facies ◽  
Central Alps ◽  
Middle Crust ◽  
European Central ◽  
Gruf Complex ◽  
Lower Crustal
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