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 Bulk Crustal Properties and Layered Velocity Structure in Fujian, SE China: Constraints From P and S Receiver Functions

2021 ◽  
Vol 9 ◽  
Author(s):  
Ayush Goyal ◽  
Shu-Huei Hung
Keyword(s):  
Velocity Structure ◽  
Crustal Thickness ◽  
Receiver Functions ◽  
Nonlinear Inversion ◽  
Low Velocity ◽  
Metamorphic Belt ◽  
Crustal Layer ◽  
Late Mesozoic ◽  
Geological Features ◽  
Lower Crustal

Multiple tectonic events since the Neoproterozoic era have framed the present-day lithosphere in the Fujian province affiliated with the eastern part of the South China Block. Comprehensive information of the crustal structure and bulk properties can aid to understand the geological features and tectonic processes of still much debate in this region. An attempt is made in this study to explore crustal thickness and internal velocities across Fujian using the teleseismic receiver functions (RFs). The H-V stacking of joint P and S RFs improves to simultaneously estimate crustal thickness, average Vp and Vs, and derived Vp/Vs ratio and bulk sound speed in three backazimuth sectors for each of 17 stations. Furthermore, a Neighborhood Algorithm nonlinear inversion of P RFs is employed to determine the layered structures of Vs and Vp/Vs beneath all the stations. Results indicate the crustal thickness varies from at most ∼35 km in northwest Fujian to 30–35 km in the inland mountains and 27–30 km in the southeastern coasts. The inferred Moho geometry is nonplanar or inclined across the Zhenghe-Dapu (ZD) and Changle-Zhaoan (CZ) fault zones, especially in the southern ZD fault area. The average Vp/Vs suggests that the crust is predominantly felsic in the Wuyi-Yunkai orogen and intermediate-to-mafic in the Cretaceous magmatic and metamorphic zones. A high-velocity upper crust along the coastline is revealed, which attributes to the Pingtan-Dongshan metamorphic belt. At the sites near the ZD fault zone, the intracrustal negative discontinuity occurs at a shallower depth of ∼15 km marking an abrupt Vs decrease into the low-velocity mid-to-lower crustal layer, probably linked to the closed paleo-rift basin remnants. The lower crust across the Fujian is generally characterized by relatively lower Vs and higher Vp/Vs (1.80–1.84) consistent with those of the mafic-ultramafic rocks, which do not support the proposed extensive magmatic underplating in the Late Mesozoic.

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 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.

scholarly journals Body-wave amplitude and travel-time correlations across North America

1983 ◽  
Vol 73 (4) ◽  
pp. 1063-1076
Author(s):  
Thorne Lay ◽  
Donald V. Helmberger
Keyword(s):  
North America ◽  
Travel Time ◽  
Body Wave ◽  
Rocky Mountain ◽  
S Wave ◽  
Arrival Times ◽  
S Waves ◽  
Long Period ◽  
Amplitude Data ◽  
Short Period

abstract Relationships between travel-time and amplitude station anomalies are examined for short- and long-period SH waves and short-period P waves recorded at North American WWSSN and Canadian Seismic Network stations. Data for two azimuths of approach to North America are analyzed. To facilitate intercomparison of the data, the S-wave travel times and amplitudes are measured from the same records, and the amplitude data processing is similar for both P and S waves. Short-period P- and S-wave amplitudes have similar regional variations, being relatively low in the western tectonic region and enhanced in the shield and mid-continental regions. The east coast has intermediate amplitude anomalies and systematic, large azimuthal travel-time variations. There is a general correlation between diminished short-period amplitudes and late S-wave arrival times, and enhanced amplitudes and early arrivals. However, this correlation is not obvious within the eastern and western provinces separately, and the data are consistent with a step-like shift in amplitude level across the Rocky Mountain front. Long-period S waves show no overall correlation between amplitude and travel-time anomalies.

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.

Gravity evidence against a high-angle fault crossing the Rocky Mountain Trench near Radium, British Columbia

1977 ◽  
Vol 14 (1) ◽  
pp. 25-31 ◽  
Author(s):  
G. D. Spence ◽  
R. M. Ellis ◽  
R. M. Clowes
Keyword(s):  
British Columbia ◽  
Seismic Refraction ◽  
Rocky Mountain ◽  
High Angle ◽  
Low Velocity ◽  
Low Velocity Zone ◽  
The North ◽  
Resultant Data ◽  
Alternative Hypotheses ◽  
Velocity Zone

One of three explanations of a prominent time delay in the 6.5 km/s branch of a recent seismic refraction survey in the Rocky Mountain Trench suggested a high-angle crustal fault crossing the trench near Radium, British Columbia. If the density contrast between basement and cover rocks is 0.1 g/cm3, a gravity anomaly of approximately 18 mGal should be observed. To test the fault hypothesis, a gravity survey has been carried out in and adjacent to the trench in the Radium area. The resultant data are not consistent with the proposed fault model. The principal feature of the data is a pronounced low, which coincides with the trench throughout the survey area. The low is due to Cenozoic fill and interpretation by two-dimensional modelling indicates the thickness of fill is about 550 m to the north and 420 m to the south of Radium. As a result of this survey, the two alternative hypotheses to explain the seismic data must be reconsidered. These are (1) the existence of a crustal low velocity zone, and (2) a major deformation of the basement and overlying rocks due to the trench being an ancient zone of weakness, which coincides with the western limit of the continental Precambrian craton. As reflections from the top of a low velocity zone are not observed, the second alternative is preferred.

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.

Lithospheric structure across the craton-Cordilleran transition of northeastern British Columbia

2001 ◽  
Vol 38 (8) ◽  
pp. 1169-1189 ◽  
Author(s):  
J Kim Welford ◽  
Ron M Clowes ◽  
Robert M Ellis ◽  
George D Spence ◽  
Isa Asudeh ◽  
...  
Keyword(s):  
British Columbia ◽  
North America ◽  
Gravity Data ◽  
Seismic Refraction ◽  
Seismic Profile ◽  
Passive Margin ◽  
Lithospheric Structure ◽  
Crustal Layer ◽  
Near Surface ◽  
Rifted Margin

The lithospheric structure of the transition from the craton to the Cordillera in northeastern British Columbia is interpreted from inversion of seismic refraction – wide-angle reflection data along a 460-km profile, and from 3-d (3-dimensional) inversion and 2.5-d forward modelling of Bouguer gravity data. The seismic profile extends westward from the sediment-covered edge of cratonic North America across the Foreland and Omineca morphogeological belts to the eastern boundary of accreted terranes, beyond the Tintina Fault. Across the ancient cratonic margin, the resultant models reveal a westward-thickening package of low upper crustal velocities (6.2 km/s and less) and low densities to almost 20 km depth below the Western Canada Sedimentary Basin, overlying a west-facing ramp of higher velocities and densities in the middle and lower crust. These features are inferred to represent passive-margin sediments deposited on the ancient rifted margin during the mid-to-late Proterozoic and early Paleozoic. A wedge-shaped high-velocity (7.3 km/s) crustal layer at the base of the crust beneath the edge of cratonic North America is interpreted to be the result of magmatic underplating during rifting. In the Cordilleran Foreland Belt, high velocities (6.4 km/s) in the upper 5 km of the crust indicate rocks upthrust from the middle crust. A narrow trench of low velocities in the near-surface, which is imaged ~20 km to the west of the inferred location of the Tintina Fault, is interpreted to represent the actual location of the fault or a major splay. From east to west, the Moho decreases in depth from ~40 km to ~34 km below the rifted margin of ancestral North America, then defines a small root at ~38 km depth below the high topography and upper crustal velocities of the eastern Foreland Belt, and gradually shallows to ~34 km beneath the Omineca belt. An enigmatic laterally heterogeneous upper mantle has anomalously high velocities (up to 8.3 km/s) beneath the Foreland Belt, flanked by regions of low velocities (7.7–7.8 km/s). Results indicate that the location of the Cordilleran deformation front west of the ramped cratonic margin directly affected the tectonic evolution of the region.

Experiments on Oral Secretion of the Rocky Mountain Wood Tick, Dermacentor andersoni Stiles (Acarina: Ixodidae)

1957 ◽  
Vol 89 (1) ◽  
pp. 1-5 ◽  
Author(s):  
J. D. Gregson
Keyword(s):  
British Columbia ◽  
Pacific Northwest ◽  
Rocky Mountain ◽  
Dermacentor Variabilis ◽  
Oral Secretion ◽  
Amblyomma Maculatum ◽  
Dermacentor Andersoni ◽  
The Pacific ◽  
Tick Paralysis ◽  
Rocky Mountain Wood Tick

Tick paralysis continues to be one of the most baffling and fascinating tickborne diseases in Canada. It was first reported in this country by Todd in 1912. Since then about 250 human cases, including 28 deaths, have been recorded from British Columbia. Outbreaks in cattle have affected up to 400 animals at a time, with losses in a herd as high as 65 head. Although the disease is most common in the Pacific northwest, where it is caused by the Rocky Mountain wood tick, Dermacentor andersoni Stiles, it has lately been reported as far south as Florida and has been produced by Dermacentor variabilis Say, Amblyomma maculatum Koch, and A. americanum (L.) (Gregson, 1953). The symptoms include a gradual ascending symmetrical flaccid paralysis. Apparently only man, sheep, cattle, dogs, and buffalo (one known instance) are susceptible, but even these may not necessarily be paralysed.

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