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.

Crustal structure across the northern Cordillera, Yukon Territory, from seismic wide-angle studies: Omineca Belt to Intermontane Belt

2005 ◽  
Vol 42 (6) ◽  
pp. 1187-1203 ◽  
Author(s):  
Brian Creaser ◽  
George Spence
Keyword(s):  
Upper Mantle ◽  
Gravity Data ◽  
Seismic Refraction ◽  
Velocity Model ◽  
High Heat ◽  
Yukon Territory ◽  
Wide Angle ◽  
The North ◽  
Structural Differences ◽  
Lower Crustal

A seismic refraction – wide-angle reflection experiment shot in 1997 in the southern Yukon Territory crosses the Omineca Belt, which includes the strike-slip Tintina Fault, and terminates within the Intermontane Belt of the northern Canadian Cordillera. Lithospheric structure is interpreted from two-dimensional forward and inverse modelling of traveltimes, combined with forward-amplitude modelling, and from 2.5-dimensional modelling of gravity data. Beneath the Cassiar terrane and the North America miogeocline, average velocities in the upper 20 km of crust are < 6.1 km/s. In the west beneath the accreted Cache Creek, Slide Mountain, and Yukon–Tanana terranes, average velocities increase to ∼6.3 km/s. In the upper crust, the velocity model beneath these terranes thus correlates with more mafic accreted material and not with a subsurface extension of the Cassiar terrane. The Tintina Fault is a crustal-scale structure across which significant structural differences occur. A mid-crustal reflector terminates to the east of the Tintina Fault. The crust immediately west of the fault is thicker (∼37 km) than the crust to the east (∼34 km); the thick crust may suggest movement along the fault from a region of thicker crust to the south. Lower crustal velocities range from 6.4 to 6.7 km/s, with the lowest velocities located 25–50 km west of the Tintina Fault, coincident with the location of the thickest crust. A reflector at 28 km depth may correspond to the top of Proterozoic cratonic basement in the lowermost crust. Upper mantle velocities just below the Moho range from 7.8 to 7.9 km/s, consistent with the high heat flow in the region.

Crustal structure beneath the southern Grand Banks: seismic-refraction results and their implications

1988 ◽  
Vol 25 (5) ◽  
pp. 760-772 ◽  
Author(s):  
I. Reid
Keyword(s):  
Velocity Structure ◽  
Seismic Velocity ◽  
Seismic Refraction ◽  
Ocean Bottom ◽  
Crustal Layer ◽  
Ocean Bottom Seismograph ◽  
Grand Banks ◽  
Reflection Data ◽  
Air Gun ◽  
Lower Crustal

A seismic-refraction profile was shot on the southern Grand Banks using large air-gun sources and an array of ocean-bottom seismograph receivers. A sediment column 1–2 km thick directly overlies Paleozoic basement with velocity structure similar to that of the Meguma Zone of Nova Scotia. The main crustal layer is 27 km thick, with seismic velocity of 6.3 km/s increasing to about 6.5 km/s in the lowest few kilometres. Complexity is apparent in the crust–mantle transition around 32 km depth. Comparison with deep multichannel reflection data suggests that the increased velocity in the lower part of the crust may be associated with a reflective zone and shows the Mohorovičić discontinuity to be delineated by a well-defined reflection. The absence of a major lower crustal layer of intermediate velocity (> 7 km/s) is consistent with observations elsewhere in the region.

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.

Deep crustal structure and tectonic origin of the Tobago-Barbados ridge

2018 ◽  
Vol 6 (2) ◽  
pp. T471-T484 ◽  
Author(s):  
Shenelle Gomez ◽  
Dale Bird ◽  
Paul Mann
Keyword(s):  
Crustal Structure ◽  
Gravity Data ◽  
Seismic Refraction ◽  
Accretionary Prism ◽  
Gravity Models ◽  
Thrust Belt ◽  
Metamorphic Belt ◽  
Seismic Reflection Data ◽  
The North ◽  
Tectonic Origin

The north–south-trending Tobago-Barbados ridge (TBR) extends 250 km from its southern end at the island of Tobago to its northern end at the island of Barbados. On Tobago, exposed metasedimentary and metaigneous rocks have been identified as fragments of a Mesozoic primitive island arc, whereas on Barbados, exposed sedimentary rocks record Paleogene development of the Barbados accretionary prism (BAP). We integrate gravity data with seismic refraction data, well constraints, and seismic reflection data to improve our understanding of the TBR’s crustal structure, uplift mechanism, along-strike compositional variations in the crust, and tectonic origin. Three 2D gravity models suggest that the TBR is underlain by a “pop-up” crustal block uplifted in the trench between the overriding Caribbean plate and the westwardly subducting South American plate. At approximately 11.75° N, the character of the TBR changes over a distance of 60 km from a symmetrical and more elevated, crystalline, thrust fault-bounded structure to a west-verging thrust belt that is less elevated. The symmetrical pop-up and asymmetrical, west-verging thrust belt accommodate east–west, subduction-related shortening that deforms the westernmost edge of the BAP. We think that the crystalline basement of the southern and central TBR is the buried, northeastern continuation of Mesozoic intraoceanic-arc crust and metamorphic belt of Tobago that accreted along the eastern margin of the Great Arc of the Caribbean during its subduction polarity reversal in the early Cretaceous.

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.

A seismic-based cross-section of the Grenville Orogen in southern Ontario and western Quebec

2000 ◽  
Vol 37 (2-3) ◽  
pp. 183-192 ◽  
Author(s):  
D J White ◽  
D A Forsyth ◽  
I Asudeh ◽  
S D Carr ◽  
H Wu ◽  
...  
Keyword(s):  
Cross Section ◽  
Seismic Refraction ◽  
Shear Zones ◽  
Tectonic Zone ◽  
Grenville Province ◽  
Crustal Layer ◽  
Seismic Reflection Data ◽  
Velocity Models ◽  
Laurentian Margin ◽  
Structural Boundary

A schematic crustal cross-section is presented for the southwestern Grenville Province based on reprocessed Lithoprobe near-vertical incidence seismic reflection data and compiled seismic refraction - wide-angle velocity models interpreted with geological constraints. The schematic crustal architecture of the southwest Grenville Province from southeast to northwest comprises allochthonous crustal elements (Frontenac-Adirondack Belt and Composite Arc Belt) that were assembled prior to ca. 1160 Ma, and then deformed and transported northwest over reworked rocks of pre-Grenvillian Laurentia and the Laurentian margin primarily between 1120 and 980 Ma. Reworked pre-Grenvillian Laurentia and Laurentian margin rocks are interpreted to extend at least 350 km southeast of the Grenville Front beneath all of the Composite Arc Belt. Three major structural boundary zones (the Grenville Front and adjacent Grenville Front Tectonic Zone, the Central Metasedimentary Belt boundary thrust zone, and the Elzevir-Frontenac boundary zone) have been identified across the region of the cross-section based on their prominent geophysical signatures comprising broad zones of southeast-dipping reflections and shallowing of mid-crustal velocity contours by 12-15 km. The structural boundary zones accommodated southeast over northwest crustal stacking at successively earlier times during orogeny (ca. 1010-980 Ma, 1080-1060 Ma, and 1170-1160 Ma, respectively). These shear zones root within an interpreted gently southeast-dipping regional décollement at a depth of 25-30 km corresponding to the top of a high-velocity lower crustal layer.

scholarly journals Current knowledge on the crustal properties of Italy

1994 ◽  
Vol 37 (5 Sup.) ◽  
Author(s):  
C. Morelli
Keyword(s):  
Adriatic Sea ◽  
Current Knowledge ◽  
Gravity Data ◽  
Maximum Depth ◽  
Gravity Models ◽  
Tyrrhenian Sea ◽  
Po Plain ◽  
Reflection Seismic ◽  
Adriatic Plate ◽  
The North

The recent advances in experimental petrography together with the information derived from the super-deep drilling projects have provided additional constraints for the interpretation of refraction and reflection seismic data. These constraints can also be used in the interpretation of magnetic and gravity data to resolve nonuniqueness. In this study, we re-interpret the magnetic and gravity data of the Italian peninsula and neighbouring areas. In view of the constraints mentioned above, it is now possible to find an agreement between the seismic and gravity models of the Central Alps. By taking into account the overall crustal thickness, we have recognized the existence of three types of Moho: 1) European which extends to the north and west of the peninsula and in the Corsican-Sardinian block. Its margin was the foreland in the Alpine Orogeny and it was the ramp on which European and Adriatic mantle and crustal slices were overthrusted. This additional load caused bending and deepening and the Moho which now lies beneath the Adriatic plate reaching a maximum depth of approximately 75 km. 2) Adriatic (or African) which lies beneath the Po plain, the Apennines and the Adriatic Sea. The average depth of the Moho is about 30-35 km below the Po plain and the Adriatic Sea and it increases toward the Alps and the Tyrrhenian Sea (acting as foreland along this margin). The maximum depth (50 km) is reached in Calabria. 3) Pery-Tyrrhenian. This is an oceanic or thinned continental crust type of Moho. It borders the oceanic Moho of the Tyrrhenian Sea and it acquires a transitional character in the Ligurian and Provençal basins (<15 km thickness) while further thickening occurs toward the East where the Adriatic plate is overthrusted. In addition, the interpretation of the heat flow data appears to confirm the origin of this Moho and its geodynamic allocation.

Crustal velocity structure from SAREX, the Southern Alberta Refraction Experiment

2002 ◽  
Vol 39 (3) ◽  
pp. 351-373 ◽  
Author(s):  
Ron M Clowes ◽  
Michael JA Burianyk ◽  
Andrew R Gorman ◽  
Ernest R Kanasewich
Keyword(s):  
Velocity Structure ◽  
Data Interpretation ◽  
Crustal Thickening ◽  
Tectonic Zone ◽  
Crustal Layer ◽  
Crustal Velocity ◽  
Crustal Velocity Structure ◽  
Southern Alberta ◽  
Magmatic Underplating ◽  
Lower Crustal

Lithoprobe's Southern Alberta Refraction Experiment, SAREX, extends 800 km from east-central Alberta to central Montana. It was designed to investigate crustal velocity structure of the Archean domains underlying the Western Canada Sedimentary Basin. From north to south, SAREX crosses the Loverna domain of the Hearne Province, the Vulcan structure, the Medicine Hat block (previously considered part of the Hearne Province), the Great Falls tectonic zone, and the northern Wyoming Province. Ten shot points along the profile in Canada were recorded on 521 seismographs deployed at 1 km intervals. To extend the line, an additional 140 seismographs were deployed at intervals of 1.25–2.50 km in Montana. Data interpretation used an iterative application of damped least-squares inversion of traveltime picks and forward modeling. Results show different velocity structures for the major blocks (Loverna, Medicine Hat, and Wyoming), indicating that each is distinct. Wavy undulations in the velocity structure of the Loverna block may be associated with internal crustal deformation. The most prominent feature of the model is a thick (10–25 km) lower crustal layer with high velocities (7.5–7.9 km/s) underlying the Medicine Hat and Wyoming blocks. Based on data from lower crustal xenoliths in the region, this layer is interpreted to be the result of Paleoproterozoic magmatic underplating. Crustal thickness varies from 40 km in the north to almost 60 km in the south, where the high-velocity layer is thickest. Uppermost mantle velocities range from 8.05 to 8.2 km/s, with the higher values below the thicker crust. Results from SAREX and other recent studies are synthesized to develop a schematic representation of Archean to Paleoproterozoic tectonic development for the region encompassing the profile. Tectonic processes associated with this development include collisions of continental blocks, subduction, crustal thickening, and magmatic underplating.

Crust-mantle velocity structure in Shanxi rift, Central North China Craton

2020 ◽  
Author(s):  
Yan Cai ◽  
Jianping Wu
Keyword(s):  
Upper Mantle ◽  
High Velocity ◽  
North China Craton ◽  
Lower Crust ◽  
North China ◽  
Velocity Structure ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
The North ◽  
Shanxi Rift

&lt;p&gt;North China Craton is the oldest craton in the world. It contains the eastern, central and western part. Shanxi rift and Taihang mountain contribute the central part. With strong tectonic deformation and intense seismic activity, its crust-mantle deformation and deep structure have always been highly concerned. In recent years, China Earthquake Administration has deployed a dense temporary seismic array in North China. With the permanent and temporary stations, we obtained the crust-mantle S-wave velocity structure in the central North China Craton by using the joint inversion of receiver function and surface wave dispersion. The results show that the crustal thickness is thick in the north of the Shanxi rift (42km) and thin in the south (35km). Datong basin, located in the north of the rift, exhibits large-scale low-velocity anomalies in the middle-lower crust and upper mantle; the Taiyuan basin and Linfen basin, located in the central part, have high velocities in the lower crust and upper mantle; the Yuncheng basin, in the southern part, has low velocities in the lower crust and upper mantle velocities, but has a high-velocity layer below 80 km. We speculate that an upwelling channel beneath the west of the Datong basin caused the low velocity anomalies there. In the central part of the Shanxi rift, magmatic bottom intrusion occurred before the tension rifting, so that the heated lithosphere has enough time to cool down to form high velocity. Its current lithosphere with high temperature may indicate the future deformation and damage. There may be a hot lithospheric uplift in the south of the Shanxi rift, heating the crust and the lithospheric mantle. The high-velocity layer in its upper mantle suggests that the bottom of the lithosphere after the intrusion of the magma began to cool down.&lt;/p&gt;

The structure of Archean–Ketilidian crust along the continental shelf of southwestern Greenland from a seismic refraction profile

1992 ◽  
Vol 29 (2) ◽  
pp. 301-313 ◽  
Author(s):  
Deping Chian ◽  
Keith Louden
Keyword(s):  
Lower Crust ◽  
Velocity Structure ◽  
Seismic Refraction ◽  
Mobile Belt ◽  
Ocean Bottom ◽  
Upper Crust ◽  
S Wave ◽  
Seismic Line ◽  
Wave Velocities ◽  
Air Gun

The velocity structure of the continental crust on the outer shelf of southwestern Greenland is determined from dense wide-angle reflection–refraction data obtained with large air-gun sources and ocean bottom seismometers along a 230 km seismic line. This line crosses the geological boundary between the Archean block and the Ketilidian mobile belt. Although the data have high noise levels, P- and S-wave arrivals from within the upper, intermediate, and lower crust, and at the Moho boundary, can be consistently identified and correlated with one-dimensional WKBJ synthetic seismograms. In the Archean, P- and S-wave velocities in the upper crust are 6.0 and 3.4 km/s, while in the intermediate crust they are 6.4 and 3.6 km/s. These velocities match for the upper crust a quartz–feldspar gneiss composition and for the intermediate crust an amphibolitized pyroxene granulite. In the Ketilidian mobile belt, P- and S-wave velocities are 5.6 and 3.3 km/s for the upper crust and 6.3 and 3.6 km/s for the intermediate crust. These velocities may represent quartz granite in the upper crust and granite and granitic gneiss in the intermediate crust. The upper crust is ~5 km thick in the Archean block and the Ketilidian mobile belt, and thickens to ~9 km in the southern part of the Archean. This velocity structure supports a Precambrian collisional mechanism between the Archean block and Ketilidian mobile belt. The lower crust has a small vertical velocity gradient from 6.6 km/s at 15 km depth to 6.9 km/s at 30 km depth (Moho) along the refraction line, with a nearly constant S-wave velocity around 3.8 km/s. These velocities likely represent a gabbroic and hornblende granulite composition for the lower crust. This typical (but somewhat thin) Precambrian crustal velocity structure in southwestern Greenland shows no evidence for a high-velocity, lower crustal, underplated layer caused by the Mesozoic opening of the Labrador Sea.

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