Is lower crustal layering related to extension?

Hobbs, Richard; Peddy, Carolyn P.

doi:10.1111/j.1365-246x.1987.tb04414.x

Cited by 15 publications

(6 citation statements)

References 2 publications

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“…However, profiling towards the ocean on rifted margins has not shown the expected increase in lower crustal reflectivity as the degree of extension increases (e.g. Cheadle et al 1987;Hobbs et al 1987;de Voggd & Keen 1987), although reflectivity continues to the continent-ocean transition. In addition, the relative importance of horizontal extension and contraction is difficult to determine because most of these areas have been subject to both types of deformation.…”

Section: Geological Environment Of Lower Crustal Reflectorsmentioning

confidence: 99%

Water in the lower continental crust: modelling magnetotelluric and seismic reflection results

Hyndman

Shearer

1989

Geophysical Journal International

331

216

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S U M M A R YMagnetotelluric and multichannel seismic reflection measurements indicate that the Phanerozoic lower continental crust is commonly electrically conductive and reflective, in contrast to a more resistive and transparent middle to upper crust. A few per cent free saline water can provide an explanation for both results along with the apparent requirement that neither the conductive nor the reflective properties are retained when lower crustal rocks are brought to the upper crust. Common 10 km thick and 20-30 Qm resistivity layers can be explained with 0.5-3 per cent pore water, if there are equilibrium pore geometries and the salinity is close to that of sea-water as suggested by lower crust fluid inclusions. Seismic velocities and impedances must be affected if such porosity exists. Seismic reflectors with reflection coefficients of 5-10 per cent can be explained by layers or lamellae with porosity contrasts of 1-4 per cent and reasonable effective pore aspect ratios of 0.1-0.03. A minimum temperature of 350°C is estimated from a correlation between heat flow and depth to the top of conductive and reflective layers. The upward limit in the crust may occur at an impermeable boundary formed by hydration reactions at the top of greenschist facies conditions or by precipitation of silica. It also may be associated with the minimum temperature for ductile behaviour and equilibrium grain boundary pore configurations. The maximum temperature is about 700 "C according to the evidence indicating that there is no free water in granulite facies conditions. Areas that have been subject to such high temperature conditions without the subsequent addition of water, i.e. the lower crust of shields, are generally non-reflective and electrically resistive.

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Section: Geological Environment Of Lower Crustal Reflectorsmentioning

confidence: 99%

Water in the lower continental crust: modelling magnetotelluric and seismic reflection results

Hyndman

Shearer

1989

Geophysical Journal International

331

216

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“…This suggests that a fruitful approach may be to look deeper in the crust and upper mantle and also to examine a series of transects within a relatively limited area so that common elements may be identified. There have been several recent attempts to study the continentocean boundary with deep seismic reflection profiling [e.g., LASE Study Group, 1986;Hobbs et al, 1987]. In 1984 and 1985, new deep seismic reflection data were collected across the rifted margins, north and east of the Grand Banks, off eastern Canada.…”

Section: Introductionmentioning

confidence: 99%

The continent‐ocean boundary at the rifted margin off eastern Canada: New results from deep seismic reflection studies

Keen¹,

Voogd²

1988

Tectonics

121

100

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Seismic reflection data were collected and processed to 20 s two-way travel time along four lines which cross the rifted continent-ocean boundary off the Grand Banks region of eastern Canada specifically to examine the origin, age, and nature of this fundamental boundary. This represents the first regional study of its kind. The most important result is the presence of landward dipping reflectors near the foot of the continental slope. These occur where oceanic crust appears to terminate against the continent. We suggest that the dipping reflectors mark the continent-ocean boundary and that they may represent magmatic material which has underplated or intruded the rifted and thinned lower continental crust adjacent to the boundary. Sedimentary basins lie just landward of the continent-ocean boundary. Their subsidence history suggests significant heating and thinning of the lower lithosphere during rifting, and this may be an important stage leading to continental breakup. Rift basins formed further landward on the Grand Banks do not exhibit this thinning. Other significant seismic results include the presence of strongly reflective zones in the lower continental crust near the continent-ocean boundary.

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“…The question of whether highly reflective and laminated lower crust is caused by extension, enhanced by extension, or independent of extension has been a subject of much study [e.g., Klemperer et al, 1987;Hobbs et al, 1987;McCarthy and Thompson, 1988], particularly because of the abundant deep reflection profiles and varied lower crustal reflectivity observed on the (extended) continental shelves around Great Britain [McGeary, 1987]. Some studies suggest that reflectivity and extension are directly related [Matthews and Cheadle, 1986;Bois et al, 1987;Reston, 1988] and that reflectivity increases as extension increases .…”

Section: Relationship Between Extension and Reflectivitymentioning

confidence: 99%

Variations in the reflectivity of the moho transition zone beneath the Midcontinent Rift System of North America: Results from true amplitude analysis of GLIMPCE data

Hutchinson¹,

Lee²,

Behrendt³

et al. 1992

J. Geophys. Res.

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True amplitude processing of The Great Lakes International Multidisciplinary Program on Crustal Evolution seismic reflection data from the Midcontinent Rift System of North America shows large differences in the reflectivity of the Moho transition zone beneath the axial rift, beneath the rift flanks, and outside of the rift. The Moho reflection from the axial rift has a discontinuous, diffractive character marginally stronger (several decibels) than an otherwise transparent lower crust and upper mantle. Beneath the axial rift, Moho is interpreted to be a synrift igneous feature. Beneath the rift flanks, the reflectivity of the Moho transition is generally well developed with two identifiable boundaries, although in places it is weakly reflective to nonreflective, similar to Moho outside the rift. The two boundaries are interpreted as the base of essentially intact, although stretched, prerift Archean crust (upper boundary) and new synrift Moho 1–2 s (6–7 km) deeper (lower boundary). Beneath the rift flanks, the layered reflection Moho transition results from the preexisting crustal composition and fabric modified by synrift igneous processes and extensional tectonic/metamorphic processes. The geologic evidence for extensive basaltic magmatism in the rift is the basis for interpreting the Moho signature as a Keweenawan structure that has been preserved for 1.1 b.y. Extension and magmatism appear to enhance reflectivity in the lower crust and Moho transition zone only where stretching factors are moderate (rift flanks) and not where they are extreme (axial rift). This leads to the prediction that the reflectivity across analogous volcanic passive continental margins should be greatest beneath the moderately stretched continental shelves and should decrease towards the ocean‐continent boundary.

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Is lower crustal layering related to extension?

Cited by 15 publications

References 2 publications

Water in the lower continental crust: modelling magnetotelluric and seismic reflection results

Water in the lower continental crust: modelling magnetotelluric and seismic reflection results

The continent‐ocean boundary at the rifted margin off eastern Canada: New results from deep seismic reflection studies

Variations in the reflectivity of the moho transition zone beneath the Midcontinent Rift System of North America: Results from true amplitude analysis of GLIMPCE data

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