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

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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Erosion of the Laurentide Region of North America by Glacial and Glaciofluvial Processes

Quaternary Research ◽

10.1016/0033-5894(85)90026-2 ◽

1985 ◽

Vol 23 (2) ◽

pp. 154-174 ◽

Cited By ~ 60

Author(s):

M. Bell ◽

E. P. Laine

Keyword(s):

North America ◽

Seismic Reflection ◽

Continental Margins ◽

Seismic Reflection Data ◽

Ice Caps ◽

Reflection Data ◽

Sediment Loads ◽

Order Of Magnitude ◽

Minimum Estimate ◽

Continental Glaciation

Collection of seismic reflection data from continental margins and ocean basins surrounding North America makes it possible to estimate the amount of material eroded from the area formerly covered by Laurentide ice sheets since major glaciation began in North America. A minimum estimate is made of 1.62 × 106 km3, or an average 120 m of rock physically eroded from the Laurentide region. This figure is an order of magnitude higher than earlier estimates based on the volume of glacial drift, Cenozoic marine sediments, and modern sediment loads of rivers. Most of the sediment produced during Laurentide glaciation has already been transported to the oceans. The importance of continental glaciation as a geomorphic agency in North America may have to be reevaluated. Evidence from sedimentation rates in ocean basins surrounding Greenland and Antarctica suggests that sediment production, sediment transport, and possibly denudation by permanent ice caps may be substantially lower than by periodic ice caps, such as the Laurentide. Low rates of sediment survival from the time of the Permo-Carboniferous and Precambrian glaciations suggest that predominance of marine deposition during some glacial epochs results in shorter lived sediment because of preferential tectonism and cycling of oceanic crust versus continental 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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Lithoprobe onshore seismic reflection transects across the Newfoundland Appalachians

Canadian Journal of Earth Sciences ◽

10.1139/e92-146 ◽

1992 ◽

Vol 29 (9) ◽

pp. 1865-1877 ◽

Cited By ~ 33

Author(s):

Garry M. Quinlan ◽

Jeremy Hall ◽

Harold Williams ◽

James A. Wright ◽

Stephen P. Colman-Sadd ◽

...

Keyword(s):

Lower Crust ◽

Seismic Reflection ◽

Vertical Section ◽

Upper Crust ◽

Seismic Reflection Data ◽

Spatially Correlated ◽

Reflection Data ◽

Iapetus Ocean

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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