Lithoprobe onshore seismic reflection transects across the Newfoundland Appalachians

Vibroseis seismic reflection data have been recorded to 18 s two-way traveltime along three transects across the island of Newfoundland. The upper crust has both steep and subhorizontal reflectors consistent with a ramp–flat style of deformation, whereas the middle and lower crust are largely free of regional flats. Reflectors descend through ca. 20 km of vertical section in the middle and lower crust to flatten into the Moho or perhaps cut through it in places. The Moho is interpreted to be no younger than the dipping reflectors. Reflection fabrics, interpreted to be indicators of dominantly Mid-Ordovician to Mid-Silurian strain, show consistent orientations among the transects and divide the crust into two blocks. A northwestern block is characterized by upper and middle crustal reflectors dipping mostly southeast at variable angles. This block is underlain to the southeast by supposedly younger and dominantly northwesterly dipping reflectors that define a northwest-tapering, wedge-shaped block floored by the Moho. This latter block is cut by isolated southeast-dipping, upper crustal reflectors near the southeast ends of the seismic transects. One of these reflectors is spatially correlated with the Bay d'Est Fault, on which the last ductile motion was south over north thrusting of Mid-Silurian age. The two crustal blocks are proposed to represent the Laurentian and Gondwanan plates juxtaposed during closure of the Iapetus Ocean. The Gondwanan plate appears to be underthrust westward beneath the Laurentian plate, perhaps by as much as 200 km.

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METHOD USING ELLIPSES TO INTERPRET SEISMIC REFLECTION DATA

Geophysics ◽

10.1190/1.1439445 ◽

1964 ◽

Vol 29 (6) ◽

pp. 926-934

Author(s):

Gary S. Gassaway

Keyword(s):

Seismic Reflection ◽

Seismic Velocity ◽

Vertical Section ◽

Reflection Point ◽

Seismic Reflection Data ◽

Reflection Data ◽

The One ◽

Time Of Travel ◽

Reflecting Surfaces ◽

Reflecting Surface

The properties of an ellipse can be used to interpret seismic reflection data by using the positions in a vertical section of a shot and a geophone as the foci of an ellipse. With the shot and geophone as the foci, the total time of travel of a reflected seismic wave serves as the constant necessary to define the ellipse. The reflecting surface then is tangent to this ellipse. Therefore, if many ellipses are plotted, the reflecting surfaces may be found by drawing smooth curves that are tangent in common to closely intersecting families of arcs. This basic principle is extended to the interpretation of complex structures that are not perpendicular to the line of traverse and to areas where the seismic velocity changes with depth by the following steps: The shots and geophones are plotted on a graph where the units along both the ordinate and the abscissa are virtual seismic traveltimes. These positions of the shots and geophones are then used as the foci of the ellipses as above. The reflecting surfaces are then drawn tangent to the dark bands of closely intersecting elliptical arcs. From this graph the one‐way time from a shot to a point of reflection, and from the point of reflection to a geophone may be scaled off; this is done by drawing the elliptical radii from the shot and geophone to the point of tangency between the ellipse and reflecting surface. The lengths of these radii are the one‐way times at the time scale of the graph. With the attitude of the wavefront as it returned to the surface at a geophone determined by a spread of three parallel geophone lines, and the one‐way time from the reflection point, one has the necessary and sufficient data to find the point of reflection in space coordinates for the assumed velocity function. Using the ray paths from the shot and geophone to this reflection point, the dip and strike of the reflecting surface at this point are found. This process is then repeated for every shot‐geophone combination for each reflecting surface.

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A physical model of the lower crust from North America based on seismic reflection data

Geological Society London Special Publications ◽

10.1144/gsl.sp.1986.024.01.04 ◽

1986 ◽

Vol 24 (1) ◽

pp. 23-34 ◽

Cited By ~ 4

Author(s):

S. B. Smithson

Keyword(s):

North America ◽

Physical Model ◽

Lower Crust ◽

Seismic Reflection ◽

Seismic Reflection Data ◽

Reflection Data

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3-D seismic images of an extensive igneous sill in the lower crust

Geology ◽

10.1130/g46150.1 ◽

2019 ◽

Vol 47 (8) ◽

pp. 729-733 ◽

Cited By ~ 10

Author(s):

T. Wrona ◽

C. Magee ◽

H. Fossen ◽

R.L. Gawthorpe ◽

R.E. Bell ◽

...

Keyword(s):

North Sea ◽

Lower Crust ◽

Seismic Reflection ◽

Three Dimensional ◽

Seismic Reflection Data ◽

Reflection Data ◽

Igneous Intrusions ◽

Northern North Sea ◽

Seismic Images ◽

East West

Abstract When continents rift, magmatism can produce large volumes of melt that migrate upwards from deep below the Earth’s surface. To understand how magmatism impacts rifting, it is critical to understand how much melt is generated and how it transits the crust. Estimating melt volumes and pathways is difficult, however, particularly in the lower crust where the resolution of geophysical techniques is limited. New broadband seismic reflection data allow us to image the three-dimensional (3-D) geometry of magma crystallized in the lower crust (17.5–22 km depth) of the northern North Sea, in an area previously considered a magma-poor rift. The subhorizontal igneous sill is ∼97 km long (north-south), ∼62 km wide (east-west), and 180 ± 40 m thick. We estimate that 472 ± 161 km3 of magma was emplaced within this intrusion, suggesting that the northern North Sea contains a higher volume of igneous intrusions than previously thought. The significant areal extent of the intrusion (∼2700 km2), as well as the presence of intrusive steps, indicate that sills can facilitate widespread lateral magma transport in the lower crust.

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Deep crustal structure beneath a rifted basin: results from seismic refraction measurements across the Jeanne d'Arc Basin, offshore eastern Canada

Canadian Journal of Earth Sciences ◽

10.1139/e90-155 ◽

1990 ◽

Vol 27 (11) ◽

pp. 1462-1471 ◽

Cited By ~ 19

Author(s):

I. D. Reid ◽

C. E. Keen

Keyword(s):

Crustal Structure ◽

Lower Crust ◽

Seismic Reflection ◽

Seismic Refraction ◽

Eastern Canada ◽

Seismic Reflection Data ◽

Deep Seismic Reflection ◽

Jeanne D'arc Basin ◽

Reflection Data ◽

Jeanne D'arc

A crustal seismic refraction experiment was conducted across the south Jeanne d'Arc Basin, one of the rifted sedimentary basins on the Grand Banks, offshore eastern Canada, that developed in Mesozoic time in response to extension and rifting between the North American plate and the African, Iberian, and European plates. The primary objective of this experiment, which was carried out to correlate with an existing deep seismic reflection profile, was to delineate the deep crustal geometry below the basin. Ten ocean-bottom seismometers were deployed across the basin and recorded signals from a large air-gun array. The results show that the crust is primarily composed of two layers, with velocities of 5.8–6.1 and 7.2 km/s, respectively. There is very little relief on the Moho across the basin, with only a 2 km step, from a depth of 37 to 35 km, occurring west of the basin. There is, however, considerable complexity of crustal structure, particularly near Moho depths. These results are valuable when used in conjunction with other data in the region, in particular gravity and deep seismic reflection data. The seismic reflection and refraction data sets together give a fairly complete picture of crustal geometry in the crust. The flat Moho below the basin is compatible with the detachment of the major basin-bounding fault in the lower crust or at the Moho, as seen on the reflection data. The 7.2 km/s layer is not restricted to the zone of Mesozoic crustal extension below the basin, but occurs also below relatively unextended parts of the crust. This layer may represent basaltic intrusion or underplating during a rifting event. It may also correspond to the reflective lower crust observed on the deep seismic reflection data. These results provide strong constraints on models describing the origin and evolution of this and other rifted basins.

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Gravity modelling along a Lithoprobe seismic traverse, northern Grenville Province, western Quebec

Canadian Journal of Earth Sciences ◽

10.1139/e17-010 ◽

1997 ◽

Vol 34 (2) ◽

pp. 127-134 ◽

Cited By ~ 3

Author(s):

Hamid Telmat ◽

Caroline N. Antonuk ◽

Jean-Claude Mareschal

Keyword(s):

Lower Crust ◽

Seismic Reflection ◽

Gravity Data ◽

Regional Scale ◽

Gravity Anomalies ◽

Gravity Modelling ◽

Grenville Province ◽

Seismic Reflection Data ◽

Mafic Intrusions ◽

Reflection Data

High-precision gravity data were collected along Lithoprobe seismic reflection lines in the northern part of the Grenville Province, in western Quebec. An interpretation is presented for line 52, which starts some 60 km southeast of the Grenville Front, traverses the parautochthonous Reservoir Dozois Terrane including the allochthonous slice of the Réservoir Cabonga Terrane, and ends near the town of Mont-Laurier, in the allochthonous Mont-Laurier Terrane. On the regional scale, the Bouguer gravity anomaly is consistent with the interpretation of the seismic reflection data. It supports crustal thinning southward of the Grenville Front, under the Cabonga allochthon. This thinning may be related to postorogenic extension. The gravity modelling shows dramatic thinning of the lower crust and suggests that extension was accommodated by extrusion of the lower crust. The gravity modelling also requires a steep boundary between the Réservoir Cabonga and the Réservoir Dozois terranes extending to ~ 15 km. The geometry of the Baskatong ramp derived from gravity data is also consistent with the seismic interpretation. This supports the suggestion that the Baskatong ramp is a major discontinuity along which Proterozoic terranes were accreted. In the Réservoir Cabonga Terrane in the northern part of the profile, the residual gravity anomalies (short wavelength variations) are related to outcropping mafic intrusions. Modelling of these anomalies complements the seismic reflection data, which did not image the base of the intrusions. The interpretation calls for three small distinct gabbroic bodies that extend no deeper than 3 km. The total volume of the intrusions is ~ 3000 km3.

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