Crustal structure of the Canadian polar margin: results of the 1985 seismic refraction survey

1989 ◽  
Vol 26 (5) ◽  
pp. 853-866 ◽  
Author(s):  
I. Asudeh ◽  
D. A. Forsyth ◽  
R. Stephenson ◽  
A. Embry ◽  
H. R. Jackson ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Seismic Refraction ◽  
Thick Layer ◽  
Parallel Line ◽  
Inner Shelf ◽  
Lower Paleozoic ◽  
Crustal Layer ◽  
Interval Velocity ◽  
Upper Paleozoic ◽  
Lower Crustal

The 1985 refraction survey based on Ice Island covered a northern transition zone along the Canadian polar margin north of Axel Heiberg Island. The refraction survey included a 60 km line along the inner shelf, a 180 km parallel line along the outer shelf, and a 60 km connecting line. Shotpoints offset from the line ends recorded upper mantle observations to a distance of 240 km.Along the inner shelf, the upper 700 m, with an interval velocity of 3.7 km/s, is interpreted as Tertiary–Cretaceous strata. The underlying 4 km thick layer has a starting velocity of 5 km/s and a gradient of 0.2 s−1. It is thought to consist of mainly deformed lower Paleozoic strata capped by upper Paleozoic – Triassic clastics and carbonates and (or) Cretaceous volcanics. Sequentially, the lower unit, with a starting velocity of 5.8 km/s, most likely consists of Proterozoic – lower Paleozoic rocks.Beneath the offshore line, up to 5 km of strata with a starting velocity of 2.2 km/s and a gradient of 0.5 s−1 probably represents Tertiary–Cretaceous elastics. The underlying material with a starting velocity of 4.5 km/s and a gradient of 0.1 s−1 is interpreted as a sedimentary succession of either Cretaceous–Tertiary elastics or upper Paleozoic to Cretaceous strata. Beneath this section, a probable Proterozoic – lower Paleozoic lower crustal layer with a starting velocity of 6.2 km/s extends to about 25 km. Apparent upper mantle velocities in the 8.0–8.2 km/s range are observed.Beneath the transitional onshore–offshore line, a Neogene sedimentary basin is interpreted as being floored by faulted blocks of probably deformed Proterozoic to lower Paleozoic rocks on the landward side and possibly Cretaceous to lower Tertiary rocks on the seaward side.

scholarly journals Crustal structure from a seismic refraction profile across southern British Columbia

1979 ◽  
Vol 16 (5) ◽  
pp. 1024-1040 ◽  
Author(s):  
W. B. Cumming ◽  
R. M. Clowes ◽  
R. M. Ellis
Keyword(s):  
British Columbia ◽  
Seismic Refraction ◽  
Rocky Mountain ◽  
Seismic Structure ◽  
Metamorphic Belt ◽  
Crustal Layer ◽  
Arrival Times ◽  
Short Period ◽  
Mine Blasts ◽  
Lower Crustal

A partially reversed seismic refraction profile utilizing mine blasts as sources was recorded across southern British Columbia from Sparwood to the Highland Valley. The westwardly directed profile consisted of 32 short period seismograms covering 440 km, while the reversed line extended 330 km with 41 seismograms. From a starting model based on first arrival times and previous geological and geophysical data, a seismic structural section is developed using both synthetic seismograms and a program for ray tracing through inhomogeneous media.The refraction data indicate that the M-discontinuity dips to the east from an approximate depth of 30 km east of the Highland Valley to in excess of 40 km beneath the Purcell Anticlinorium. Undulations of about 165 km wavelength and several kilometres amplitude characterize the crust–mantle boundary. The Pn velocity is 7.8 km/s. Above the M-discontinuity, secondary arrivals are interpreted to be from a lower crustal layer of thickness near 12 km and velocity 6.9 km/s. The upper boundary of this layer also dips gently to the east.The seismic structure of the upper crust correlates closely with the regional geology as evidenced by traveltime and amplitude anomalies where the profile crosses the Rocky Mountain Trench and the Interior Plateau – Eastern Metamorphic Belt boundaries. The crustal P and S phases in the Interior Plateau yield a relatively low value of Poisson's ratio of 0.23. The detailed data close to the Highland Valley indicate significant velocity heterogeneity. For the Guichon Creek batholith, the inner Bethlehem phase is found to have a higher velocity than the surrounding Highland Valley phase.

scholarly journals Complex rift patterns, a result of interacting crustal and mantle weaknesses, or multiphase rifting? Insights from analogue models

2021 ◽  
Author(s):  
Frank Zwaan ◽  
Pauline Chenin ◽  
Duncan Erratt ◽  
Gianreto Manatschal ◽  
Guido Schreurs
Keyword(s):  
Upper Mantle ◽  
Upper Crust ◽  
Crustal Layer ◽  
Tectonic History ◽  
Lithospheric Extension ◽  
Analogue Models ◽  
History Of ◽  
Lower Crustal ◽  
Degree Of Coupling ◽  
Over Time

Abstract. During lithospheric extension, localization of deformation often occurs along structural weaknesses inherited from previous tectonic phases. Such weaknesses may occur in both the crust and mantle, but the combined effects of these weaknesses on rift evolution remains poorly understood. Here we present a series of 3D brittle-viscous analogue models to test the interaction between differently oriented weaknesses located in the brittle upper crust and/or upper mantle. We find that crustal weaknesses usually express first at the surface with the formation of graben parallel to their orientation; then, structures parallel to the mantle weakness overprint them and often become dominant. Furthermore, the direction of extension exerts minimal control on rift trends when inherited weaknesses are present, which implies that present-day rift orientations are not always indicative of past extension directions. We also suggest that multiphase extension is not required to explain different structural orientations in natural rift systems. The degree of coupling between the mantle and upper crust affects the relative influence of the crustal and mantle weaknesses: low coupling enhances the influence of crustal weaknesses, whereas high coupling enhances the influence of mantle weaknesses. Such coupling may vary over time due to progressive thinning of the lower crustal layer, as well as due to variations in extension velocity. These findings provide a strong incentive to reassess the tectonic history of various natural examples.

Crustal structure of the Nova Scotian margin in the Laurentian Channel region

1987 ◽  
Vol 24 (9) ◽  
pp. 1859-1868 ◽  
Author(s):  
I. Reid
Keyword(s):  
Crustal Structure ◽  
Seismic Velocity ◽  
Structural Information ◽  
Seismic Refraction ◽  
Ocean Bottom ◽  
Eastern Canada ◽  
Crustal Layer ◽  
Air Gun ◽  
Lower Crustal

A seismic-refraction study on the outer Scotian Shelf of eastern Canada, carried out using large air-gun sources and ocean bottom seismograph receivers, has provided structural information on the entire crustal column. A thick (about 13 km) sedimentary sequence is characterized by significant lateral variation in this area, and a marked increase in seismic velocity around 8 km depth may delineate the synrift–postrift transition. Beneath the sediments is highly attenuated continental crust, about 11 km thick, with some evidence for a lower crustal layer of velocity around 7 km/s, which may be partly due to under-plating during rifting. Determination of the complete crustal structure, including the tentative delineation of the rift–drift transition, in a region of large crustal extension provides a useful test for models of continental rifting, and a simple uniform extension–subsidence model is found to produce an adequate fit to the interpreted structure.

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 structure in Northwestern Ontario: Regional magnetic anomalies

1969 ◽  
Vol 6 (1) ◽  
pp. 101-107 ◽  
Author(s):  
Peter H. McGrath ◽  
Donald H. Hall
Keyword(s):  
Crustal Structure ◽  
Upper Mantle ◽  
Magnetic Anomalies ◽  
Two Dimensional ◽  
Crustal Layer ◽  
Smoothing Operator ◽  
Northwestern Ontario ◽  
Map Series ◽  
Lower Crustal ◽  
Seismic Discontinuity

A regional aeromagnetic map, portraying the regional magnetic anomaly system in Northwestern Ontario west of longitude 92 °W and south of latitude 55 °N and extending westward into Manitoba to longitude 97 °W (with an additional block bounded by latitudes 54° N and 56 °N and longitudes 97° W and 102 °W) is presented. The map was prepared by multiple application of a two-dimensional smoothing operator applied to data digitized at 3 km intervals from the 1-inch-to-1-mile aeromagnetic map series published by the Geological Survey of Canada. Comparison was made with previous maps overlapping on portions of the area, which had been made by various techniques, including Fourier analysis, fitting of 6th-order polynomials, and photographic reduction. The general features of the anomaly system were found to be similar for all of these techniques. The regional anomaly system is found to be related in some cases to the thickness of the upper crustal layer (defined as lying above the Intermediate seismic discontinuity) and to structure within it, but not to the lower crustal layer or to the upper mantle.

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.

scholarly journals Complex rift patterns, a result of interacting crustal and mantle weaknesses, or multiphase rifting? Insights from analogue models

Solid Earth ◽  
2021 ◽  
Vol 12 (7) ◽  
pp. 1473-1495
Author(s):  
Frank Zwaan ◽  
Pauline Chenin ◽  
Duncan Erratt ◽  
Gianreto Manatschal ◽  
Guido Schreurs
Keyword(s):  
Upper Mantle ◽  
Upper Crust ◽  
Crustal Layer ◽  
Tectonic History ◽  
Lithospheric Extension ◽  
Analogue Models ◽  
History Of ◽  
Lower Crustal ◽  
Degree Of Coupling ◽  
Over Time

Abstract. During lithospheric extension, localization of deformation often occurs along structural weaknesses inherited from previous tectonic phases. Such weaknesses may occur in both the crust and mantle, but the combined effects of these weaknesses on rift evolution remain poorly understood. Here we present a series of 3D brittle–viscous analogue models to test the interaction between differently oriented weaknesses located in the brittle upper crust and/or upper mantle. We find that crustal weaknesses usually express first at the surface, with the formation of grabens parallel to their orientation; then, structures parallel to the mantle weakness overprint them and often become dominant. Furthermore, the direction of extension exerts minimal control on rift trends when inherited weaknesses are present, which implies that present-day rift orientations are not always indicative of past extension directions. We also suggest that multiphase extension is not required to explain different structural orientations in natural rift systems. The degree of coupling between the mantle and upper crust affects the relative influence of the crustal and mantle weaknesses: low coupling enhances the influence of crustal weaknesses, whereas high coupling enhances the influence of mantle weaknesses. Such coupling may vary over time due to progressive thinning of the lower crustal layer, as well as due to variations in extension velocity. These findings provide a strong incentive to reassess the tectonic history of various natural examples.

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.

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