THE CRUST OF THE EARTH UNDER HUDSON BAY

1967 ◽  
Vol 4 (5) ◽  
pp. 949-960 ◽  
Author(s):  
J. A. Hunter ◽  
R. F. Mereu
Keyword(s):  
Crustal Thickening ◽  
Hudson Bay ◽  
Near Surface ◽  
Crustal Velocity ◽  
Surface Data ◽  
First Arrival ◽  
Minimum Depth ◽  
Time Term ◽  
Surface Geology ◽  
Mohorovičić Discontinuity

The Hudson Bay crustal experiment of 1965 involved 41 shots placed on two lines, E–W and NW–SE, in the Bay. The first arrival data of eight land stations situated around the bay were utilized in the time–term analysis. The preferred crustal velocity was found to be 6.32 ± .06 km/s, and the velocity of the upper mantle to be 8.23 ± .03 km/s. Depth calculations from time–terms and employing Geological Survey of Canada near-surface data, show the Mohorovicic discontinuity to be undulatory in nature throughout the bay. An overall rise of this interface occurs from a depth at Churchill of approximately 41 km to a minimum depth of approximately 27 km towards Gilmour Island. Crustal thickening occurs again on the east side of the bay, with a depth of 41 km at Povungnituk. As well, the crust thins towards the NW from an approximate depth of 37 km in the center to a depth of 26 km near Chesterfield Inlet. The correlation between existing surface geology and the Mohorovicic discontinuity undulations is discussed.

A TIME-TERM ANALYSIS OF THE FIRST ARRIVAL DATA FROM THE SEISMIC EXPERIMENT IN HUDSON BAY, 1965

1967 ◽  
Vol 4 (5) ◽  
pp. 901-928 ◽  
Author(s):  
Alan Ruffman ◽  
M. J. Keen
Keyword(s):  
Wave Velocity ◽  
Three Dimensional ◽  
Compressional Wave ◽  
Hudson Bay ◽  
Compressional Wave Velocity ◽  
Seismic Experiment ◽  
First Arrival ◽  
Time Term ◽  
Mohorovičić Discontinuity ◽  
Term Analysis

A time-term analysis is made of the first arrival data from the 41 shots of the1965 Hudson Bay seismic experiment. An investigation of the water-wave data is made to determine which of three possible series of navigation is most consistent. A single-layered crust with a compressional wave velocity of 6.33 km/s and an upper mantle compressional wave velocity of 8.27 km/s are proposed for Hudson Bay. The Mohorovičić discontinuity is found to have considerable topography with depths ranging from 42.7 km to less than 26 km. The Churchill–Superior boundary is proposed to be a three-dimensional crustal feature and is extended offshore from Cape Smith and extended westward to the north of the Ottawa Islands through approximately 59° 40′ N and 82° 00′ W. The Mohorovičić discontinuity rises to depths of about 26 km beneath Chesterfield Inlet and Baker Lake. The mantle is about 40 km deep at Churchill, Manitoba and rises to about30 km some 130 km west of Gilmour Island, then drops to almost 42 km farther east. The sudden drop is related to the Churchill–Superior boundary.

CRUSTAL STRUCTURE UNDER HUDSON BAY

1967 ◽  
Vol 4 (5) ◽  
pp. 929-947 ◽  
Author(s):  
George D. Hobson ◽  
A. Overton ◽  
D. N. Clay ◽  
W. Thatcher
Keyword(s):  
Crustal Structure ◽  
Quality Data ◽  
Hudson Bay ◽  
Sufficient Data ◽  
Crustal Model ◽  
Time Term ◽  
Mohorovičić Discontinuity ◽  
Good Quality Data

A crystal refraction experiment was conducted in August 1965 in Hudson Bay in which nine university and government crews participated. Good quality data were obtained from 41 shots providing sufficient data to permit time-term analyses of both Pn and P1 arrivals. No consistent intermediate arrivals were identified leading to the consideration of a two-layer crustal model. Pn time-terms indicate that the depth of the Mohorovicic discontinuity is between 26 and 41 km.

An interpretation of the first-arrival data of the Lake Superior Experiment by the time-term method

1966 ◽  
Vol 56 (1) ◽  
pp. 141-171
Author(s):  
M. J. Berry ◽  
G. F. West
Keyword(s):  
Lake Superior ◽  
Igneous Rocks ◽  
Low Velocity ◽  
The West ◽  
Head Waves ◽  
First Arrival ◽  
Keweenaw Peninsula ◽  
Time Term ◽  
Similar Basis ◽  
Mohorovičić Discontinuity

abstract The first-arrival data of the Lake Superior Experiment of 1963 have been interpreted by the time-term method. The analysis has shown the method to be well suited to this type of survey, and the results appear to be consistent and meaningful. Approximately 500 first-arrivals from head waves generated at the Mohorovičić discontinuity, have been reduced to estimates of crustal time-terms at over 100 locations. A much shallower refracting surface (here called the Upper Refractor) furnished nearly 1,000 observations to yield upper crustal time-terms at the same locations. The analysis reveals the material beneath the UR and beneath the M to have velocities of 6.63 and 8.10 km/sec respectively. The surface of the Upper Refractor, on the basis of a simple interpretation of the time-terms, is revealed as undulating, coming close to the surface at the edges of the lake and reaching maximum depths of approxmately 15 km to the east and west of the Keweenaw Peninsula. On a similar basis the Mohorovičić discontinuity is revealed as an easterly dipping surface, having a depth of approximately 35 km at the west end of the lake and reaching a maximum depth of about 60 km in the region just west of the Keweenaw Peninsula. Eastwards, the time-term values fluctuate but do not increase or decrease systematically. The velocity of the material lying above the Upper Refractor is not well determined, but appears to be roughly 5.5 km/sec. A perusal of geological literature suggests that this low velocity material is mostly sedimentary, filling a well-known synclincal basin whose axis bends around the Keweenaw Peninsula. This mainly sedimentary section is known to be underlain by a great thickness of igneous rocks, which in all probability corresponds to the Upper Refractor mapped by the seismic studies.

Shear and compressive wave data acquisition using multicomponent vibrating source and landstreamer

2021 ◽  
Author(s):  
Andre Pugin ◽  
Barbara Dietiker ◽  
Kevin Brewer ◽  
Timothy Cartwright
Keyword(s):  
Leading Edge ◽  
Data Sets ◽  
Compressive Wave ◽  
Near Surface ◽  
Source Type ◽  
Receiver Array ◽  
Surface Data ◽  
Source Orientation ◽  
Refracted Waves ◽  
Wave Modes

<p>In the vicinity of Ottawa, Ontario, Canada, we have recorded many multicomponent seismic data sets using an in-house multicom­ponent vibrator source named Microvibe and a landstreamer receiver array with 48 3-C 28-Hz geophones at 0.75-m intervals. The receiver spread length was 35.25 m, and the near-offset was 1.50 m. We used one, two or three source and three receiver orientations — vertical (V), inline-horizontal (H1), and transverse-horizontal (H2). We identified several reflection wave modes in the field records — PP, PS, SP, and SS, in addition to refracted waves, and Rayleigh-mode and Love-mode surface waves. We computed the semblance spectra of the selected shot records and ascertained the wave modes based on the semblance peaks. We then performed CMP stacking of each of the 9-C data sets using the PP and SS stacking velocities to compute PP and SS reflection profiles.</p><p>Despite the fact that any source type can generate any combination of wave modes — PP, PS, SP, and SS, partitioning of the source energy depends on the source orientation and VP/VS ratio. Our examples demonstrate that the most prominent PP reflection energy is recorded by the VV source-receiver orientation, whereas the most prominent SS reflection energy is recorded by the H2H2 source-receiver orientation with possibility to obtain decent shear wave near surface data in all other vibrating and receiving directions.</p><p>Pugin, Andre and Yilmaz, Öz, 2019. Optimum source-receiver orientations to capture PP, PS, SP, and SS reflected wave modes. The Leading Edge, vol. 38/1, p. 45-52. https://doi.org/10.1190/tle38010045.1</p>

scholarly journals Towards a national 3D geological model of Denmark

2016 ◽  
Vol 35 ◽  
pp. 27-30
Author(s):  
Peter B.E. Sandersen ◽  
Thomas Vangkilde-Pedersen ◽  
Flemming Jørgensen ◽  
Richard Thomsen ◽  
Jørgen Tulstrup ◽  
...  
Keyword(s):  
Subsurface Layer ◽  
Geological Model ◽  
Technical Standards ◽  
Near Surface ◽  
Subsurface Geology ◽  
3D Geological Model ◽  
Combining Data ◽  
National Model ◽  
Surface Geology ◽  
2D To 3D

As part of its strategy, the Geological Survey of Denmark and Greenland (GEUS) is to develop a national, digital 3D geological model of Denmark that can act as a publicly accessible database representing the current, overall interpretation of the subsurface geology. A national model should be under constant development, focusing on meeting the current demands from society. The constant improvements in computer capacity and software capabilities have led to a growing demand for advanced geological models and 3D maps that meet the current technical standards (Berg et al. 2011). As a consequence, the users expect solutions to still more complicated and sophisticated problems related to the subsurface. GEUS has a long tradition of making 2D maps of subsurface layer boundaries and near-surface geology (Fredericia & Gravesen 2014), but in the change from 2D to 3D and when combining data in new ways, new geological knowledge is gained and new challenges of both technical and organisational character will arise. The purpose of this paper is to present the strategy for the national 3D geological model of Denmark and the planned activities for the years ahead. The paper will also reflect on some of the challenges related to making and maintaining a nationwide 3D model. Initially, the model will only include the Danish onshore areas, with the Danish offshore areas and Greenland to be added later using a similar general setup.

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.

The nature and origin of cratons constrained by their surface geology

2021 ◽  
Author(s):  
A.M. Celal Şengör ◽  
Nalan Lom ◽  
Ali Polat
Keyword(s):  
North China ◽  
Plate Boundary ◽  
Crustal Thickening ◽  
Metamorphic Rocks ◽  
Upper Crust ◽  
Subsequent Removal ◽  
Total Removal ◽  
Common Misconception ◽  
Resistance To Deformation ◽  
Surface Geology

To the memory of Nicholas John (Nick) Archibald (1951−2014), master of cratonic geology. Cratons, defined by their resistance to deformation, are guardians of crustal and lithospheric material over billion-year time scales. Archean and Proterozoic rocks can be found in many places on earth, but not all of them represent cratonic areas. Some of these old terrains, inappropriately termed “cratons” by some, have been parts of mobile belts and have experienced widespread deformations in response to mantle-plume-generated thermal weakening, uplift and consequent extension and/or various plate boundary deformations well into the Phanerozoic. It is a common misconception that cratons consist only of metamorphosed crystalline rocks at their surface, as shown by the indiscriminate designation of them by many as “shields.” Our compilation shows that this conviction is not completely true. Some recent models argue that craton formation results from crustal thickening caused by shortening and subsequent removal of the upper crust by erosion. This process would expose a high-grade metamorphic crust at the surface, but greenschist-grade metamorphic rocks and even unmetamorphosed supracrustal sedimentary rocks are widespread on some cratonic surfaces today, showing that craton formation does not require total removal of the upper crust. Instead, the granulitization of the roots of arcs may have been responsible for weighing down the collided and thickened pieces and keeping their top surfaces usually near sea level. In this study, we review the nature and origin of cratons on four well-studied examples. The Superior Province (the Canadian Shield), the Barberton Mountain (Kaapvaal province, South Africa), and the Yilgarn province (Western Australia) show the diversity of rocks with different origin and metamorphic degree at their surface. These fairly extensive examples are chosen because they are typical. It would have been impractical to review the entire extant cratonic surfaces on earth today. We chose the inappropriately named North China “Craton” to discuss the requirements to be classified as a craton.

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