scholarly journals Structure of the crust and upper mantle beneath the Parnaíba Basin, Brazil, from wide-angle reflection–refraction data

2018 ◽  
Vol 472 (1) ◽  
pp. 67-82 ◽  
Author(s):  
José E. P. Soares ◽  
Randell Stephenson ◽  
Reinhardt A. Fuck ◽  
Marcus V. A. G. de Lima ◽  
Vitto C. M. de Araújo ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Wide Angle ◽  
Crust And Upper Mantle ◽  
Refraction Data ◽  
Parnaíba Basin

Crust and upper mantle velocity structure of the Intermontane belt, southern Canadian Cordillera

1992 ◽  
Vol 29 (7) ◽  
pp. 1530-1548 ◽  
Author(s):  
B. C. Zelt ◽  
R. M. Ellis ◽  
R. M. Clowes ◽  
E. R. Kanasewich ◽  
I. Asudeh ◽  
...  
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Velocity Structure ◽  
P Wave ◽  
Seismic Reflection Profile ◽  
Wide Angle ◽  
Low Velocity ◽  
Crust And Upper Mantle ◽  
Middle Crust ◽  
Refraction Data

As part of the Lithoprobe Southern Cordillera transect, seismic refraction data were recorded along a 330 km long strike profile in the Intermontane belt. An iterative combination of two-dimensional traveltime inversion and amplitude forward modelling was used to interpret crust and upper mantle P-wave velocity structure. This region is characterized by (i) a thin near-surface layer with large variations in velocity between 2.8 and 5.4 km/s, and low-velocity regions that correlate well with surface expressions of Tertiary sedimentary and volcanic rocks; (ii) an upper and middle crust with low average velocity gradient, possibly a weak low-velocity zone, and lateral velocity variations between 6.0 and 6.4 km/s; (iii) a distinctive lower crust characterized by significantly higher average velocities relative to midcrustal values beginning at 23 km depth, approximately 8 km thick with average velocities of 6.5 and 6.7 km/s at top and base; (iv) a depth to Moho, as defined by wide-angle reflections, that averages 33 km with variations up to 2 km; and (v) a Moho transition zone of depth extent 1–3 km, below which lies the upper mantle with velocities decreasing from 7.9 km/s in the south to 7.7 km/s in the north. Where the refraction line obliquely crosses a Lithoprobe deep seismic-reflection profile, good agreement is obtained between the interpreted reflection section and the derived velocity structure model. In particular, depths to wide-angle reflectors in the upper crust agree with depths to prominent reflection events, and Moho depths agree within 1 km. From this comparison, the upper and middle crust probably comprise the upper part of the Quesnellia terrane. The lower crust from the refraction interpretation does not show the division into two components, parautochthonous and cratonic North America, that is inferred from the reflection data, indicating that their physical properties are not significantly different within the resolution of the refraction data. Based on these interpretations, the lower lithosphere of Quesnellia is absent and presumably was recycled in the mantle. At a depth of ~ 16 km below the Moho, an upper mantle reflector may represent the base of the present lithosphere.

An improved velocity model for the crust and upper mantle along the central mobile belt of the Newfoundland Appalachian orogen and its offshore extension

1998 ◽  
Vol 35 (11) ◽  
pp. 1238-1251 ◽  
Author(s):  
Deping Chian ◽  
François Marillier ◽  
Jeremy Hall ◽  
Garry Quinlan
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Velocity Structure ◽  
Sedimentary Basins ◽  
Velocity Model ◽  
Mobile Belt ◽  
Wide Angle ◽  
Crust And Upper Mantle ◽  
Angle Data ◽  
Appalachian Orogen

New modelling of wide-angle reflection-refraction data of the Canadian Lithoprobe East profile 91-1 along the central mobile belt of the Newfoundland Appalachian orogen reveals new features of the upper mantle, and establishes links in the crust and upper mantle between existing land and marine wide-angle data sets by combining onshore-offshore recordings. The revised model provides detailed velocity structure in the 30-34 km thick crust and the top 30 km of upper mantle. The lower crust is characterized by a velocity of 6.6-6.8 km/s onshore, increasing by 0.2 km/s to the northeast offshore beneath the sedimentary basins. This seaward increase in velocity may be caused by intrusion of about 4 km of basic rocks into the lower crust during the extension that formed the overlying Carboniferous basins. The Moho is found at 34 km depth onshore, rising to 30 km offshore to the northeast with a local minimum of 27 km. The data confirm the absence of deep crustal roots under the central mobile belt of Newfoundland. Our long-range (up to 450 km offset) wide-angle data define a bulk velocity of 8.1-8.3 km/s within the upper 20 km of mantle. The data also contain strong reflective phases that can be correlated with two prominent mantle reflectors. The upper reflector is found at 50 km depth under central Newfoundland, rising abruptly towards the northeast where it reaches a minimum depth of 36 km. This reflector is associated with a thin layer (1-2 km) unlikely to coincide with a discontinuity with a large cross-boundary change in velocity. The lower reflector at 55-65 km depths is much stronger, and may have similar origins to reflections observed below the Appalachians in the Canadian Maritimes which are associated with a velocity increase to 8.5 km/s. Our data are insufficient for discriminating among various interpretations for the origins of these mantle reflectors.

Crustal velocity structure in the southern Coast Belt, British Columbia

1993 ◽  
Vol 30 (12) ◽  
pp. 2389-2403 ◽  
Author(s):  
D. M. O'Leary ◽  
R. M. Clowes ◽  
R. M. Ellis
Keyword(s):  
Upper Mantle ◽  
Velocity Structure ◽  
Seismic Refraction ◽  
Velocity Model ◽  
Structural Features ◽  
P Wave ◽  
Crust And Upper Mantle ◽  
Near Surface ◽  
Collision Zone ◽  
Refraction Data

We applied an iterative combination of two-dimensional traveltime inversion and amplitude forward modelling to seismic refraction data along a 350 km along-strike profile in the Coast Belt of the southern Canadian Cordillera to determine crust and upper mantle P-wave velocity structure. The crustal model features a thin (0.5–3.0 km) near-surface layer with an average velocity of 4.4 km/s, and upper-, middle-, and lower-crustal strata which are each approximately 10 km thick and have velocities ranging from 6.2 to 6.7 km/s. The Moho appears as a 2 km thick transitional layer with an average depth of 35 km and overlies an upper mantle with a poorly constrained velocity of over 8 km/s. Other interpretations indicate that this profile lies within a collision zone between the Insular superterrane and the ancient North American margin and propose two collision-zone models: (i) crustal delamination, whereby the Insular superterrane was displaced along east-vergent faults over the terranes below; and (ii) crustal wedging, in which interfingering of Insular rocks occurs throughout the crust. The latter model involves thick layers of Insular material beneath the Coast Belt profile, but crustal velocities indicate predominantly non-Insular material, thereby favoring the crustal delamination model. Comparisons of the velocity model with data from the proximate reflection lines show that the top of the Moho transition zone corresponds with the reflection Moho. Comparisons with other studies suggest that likely sources for intracrustal wide-angle reflections observed in the refraction data are structural features, lithological contrasts, and transition zones surrounding a region of layered porosity in the crust.

Crust and upper mantle Q from seismic refraction data: Peace River region

1990 ◽  
Vol 27 (8) ◽  
pp. 1040-1047 ◽  
Author(s):  
C. A. Zelt ◽  
R. M. Ellis
Keyword(s):  
Upper Mantle ◽  
Seismic Refraction ◽  
Spectral Ratio ◽  
Ratio Method ◽  
Spectral Ratios ◽  
P Waves ◽  
Crust And Upper Mantle ◽  
Peace River ◽  
River Region ◽  
Refraction Data

Crustal refraction data from the Peace River region of Alberta, Canada, have been analyzed using the spectral ratio method to obtain Q. A total of 1205 first and later arrivals corresponding to turning and reflected P-waves within the crust and upper mantle were studied. Source spectra were estimated from near-offset traces assuming typical sedimentary Q values. The large scatter of measured spectral ratios restricted the resolution to a three-layer model of the crust and upper mantle with Q constant in each layer. This model was obtained using a linear inverse method since the measured spectral ratios and known traveltimes in each layer are linearly related through the attentuation (Q−1) in each layer. A weighted L1 norm was minimized using linear programming, the weights being a measure of the certainty of each spectral ratio. The inversion was performed using the 25% most certain spectral ratios, regardless of magnitude or sign. Model bounds taking account of the scattered data were estimated. The results suggest that Q is between 200 and 500 in the upper crust and greater than 600 in the lower crust and upper mantle. This model is generally consistent with Q obtained from studies on nearby crust.

scholarly journals Cratonic basins and the Wilson cycle: a perspective from the Parnaíba Basin, Brazil

2018 ◽  
Vol 470 (1) ◽  
pp. 463-477 ◽  
Author(s):  
M. C. Daly ◽  
B. Tozer ◽  
A. B. Watts
Keyword(s):  
Upper Mantle ◽  
Lower Crust ◽  
Large Area ◽  
Crust And Upper Mantle ◽  
Radiogenic Heating ◽  
Terrestrial Sediments ◽  
Extensional Strain ◽  
Wilson Cycle ◽  
Vertical Subsidence ◽  
Parnaíba Basin

AbstractCratonic basins appear to occupy a specific place in the Wilson cycle, initiating after continental collision and supercontinent development, but before rifting and continental break-up. They do not result directly from the horizontal plate motions characteristic of the Wilson cycle, but from localized, long-lived subsidence. Covering c. 10% of the Earth's continental crust, most of the preserved cratonic basins developed in the Early Paleozoic after the formation of Gondwana and Laurentia. Recent investigation of the Parnaíba cratonic basin of Brazil has shown that this basin, and potentially cratonic basins in general, are characterized by six features: (1) formation on thickened lithosphere (>150 km); (2) a pronounced basal unconformity; (3) a sub-circular outline and large area of 0.5 × 105 to 2 × 106 km2; (4) long-lived (100–300 myr) quasi-exponential tectonic subsidence of shallow marine and terrestrial sediments; (5) no major extensional strain features, such as rifts, crustal or lithospheric thinning or Moho elevation; and (6) dense, high velocity and conductive lower crust and upper mantle. These characteristics indicate basin initiation and development by purely vertical subsidence of the lithosphere, either thermally or mechanically driven. Thermal subsidence may be related to orogenic thickening, radiogenic heating and erosion associated with supercontinent assembly, whereas mechanical subsidence may be a result of the emplacement in the lower crust or upper mantle of a dense igneous body related to plume activity during the lifetime of a supercontinent.

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