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2014/6/3 Lecture 2014 1

7. Crustal Structures

7.1 Historical Review

7.2 Oceanic Crust

7.3 Continental Crust

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The earthquake of Oct.8, 1909

7.1 Historical Review

2014/6/3 Lecture 2014 3Bonini & Bonini (1979) 2014/6/3 Lecture 2014 4

Travel time curve by Mohorovicic (1910)

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2014/6/3 Lecture 2014 5Bonini & Bonini (1979) 2014/6/3 Lecture 2014 6Litak & Brown (1989)

Conrad Discontinuity

2014/6/3 Lecture 2014 7Litak & Brown (1989) 2014/6/3 Lecture 2014 8

Freq. of Inland Earthquakes

Dep

th(k

m)

JMA(1983-1992)

Upper Crust (Granitic)

Lower Crust (Basaltic)

Uppermantle

Classical Model of Crust and Uppermantle

6 km/s

6.6 km/s

8 km/s

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P-Wave Velocity (km/s)

Dep

th (k

m) granitic

mafic

ultramafic

Kanamori (1965)

Comparison of seismic velocities from field experimentsand laboratory measurements

2014/6/3 Lecture 2014 10Litak & Brown (1989)

Complexity of crustal structure

2014/6/3 Lecture 2014 11Litak & Brown (1989) 2014/6/3 Lecture 2014 12

Mooney and Meissner (1992)

Crustal Image from Seismic Reflection Method

transparent upper crust

reflectivelower crust

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2014/6/3 Lecture 2014 13Litak & Brown (1989)

reflective upper crust

Moho

reflective lower crust

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7.2. Oceanic Crust

7.2.1 Classical Model of Oceanic Crust Oceanic layers 1-3

7.2.2 Traveltime and AmplitudeModelling for Oceanic Crust

New picture of the oceanic crust Petrological model

7.2.3 Oceanic Uppermantle Anisotropy Deep structure

2014/6/3 Lecture 2014 15 2014/6/3 Lecture 2014 16Shor et al. (1970)

7.2.1 Classical Model of Oceanic Crust

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2014/6/3 Lecture 2014 17Shor et al. (1970) 2014/6/3 Lecture 2014 18

7.2.2 Traveltime and AmplitudeModelling for Oceanic Crust

Limitation of classical model

Sputich & Orcutt (1980)

2014/6/3 Lecture 2014 19White et al. (1992) 2014/6/3 Lecture 2014 20

Oceanic layer 2 high velocity gradient Crack or metamorphic grade ?

Oceanic layer 3 gentle velocity gradient high basal layer (7.2-7.8 km/s) low velocity zone (age-dependent) transition zone (1-3 km)

Sputich & Orcutt (1980)

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2014/6/3 Lecture 2014 21Sputich & Orcutt (1980) 2014/6/3 Lecture 2014 22Meisnner (1986)

2014/6/3 Lecture 2014 23Sputich & Orcutt (1980) 2014/6/3 Lecture 2014 24

Petrological model

Sputich & Orcutt (1980)

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2014/6/3 Lecture 2014 25Sputich & Orcutt (1980) 2014/6/3 Lecture 2014 26Kennett (1982)

2014/6/3 Lecture 2014 27Shimamura et al. (1983)

7.2.3 Oceanic Uppermantle (Anisotropy)

2014/6/3 Lecture 2014 28Shimamura et al. (1983)

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P wave velocity (km/s) P wave velocity (km/s)

Dep

th (k

m)

Dep

th (k

m)

2014/6/3 Lecture 2014 30The LADLE Study Group (1983)

Deeper Structure

2014/6/3 Lecture 2014 31The LADLE Study Group (1983)

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7.3. Continental Crust

7.3.1 Compressional and shear wave velocities from seismic refraction/wide-angle reflection experiments

7.3.2 Petrological model of continental crust

7.3.3 Crustal image from seismic reflection experiments

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2014/6/3 Lecture 2014 33Holbrook et al. (1992)

Middle CrustVp : 6.0-6.8 km/sVs : 3.7-4.0 km/s

Lower CrustVp : 6.4-7.4 km/sVs : 3.5-4.2 km/s

7.3.1 Compressional and shear wave velocities from seismic refraction/wide-angle reflection experiments

2014/6/3 Lecture 2014 34Holbrook et al. (1992)

7.3.2 Petrological model of continental crust

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Holbrook et al. (1992)2014/6/3 Lecture 2014 36

Holbrook et al. (1992)

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Middle Crust Vp : 6.0-6.8 km/s (strong peak : 6.5-6.6 km/s). Vs : 3.3-4.0 km/s (most values : 3.7-4.0 km/s). : 0.24-0.27 (low : 0.15, high : 0.29). quartz-mica schst, gabbro and amphibolite

Lower Crust Vp : 6.4-7.4 km/s (57% : 6.6-7.0 km/s). Vs : 3.5-4.2 km/s (most values : 3.9-4.2 km/s). : 0.24-0.29 (most values : 0.24-0.29).

bimodal distribution 6.7-6.8 km/s … anorthosite or mafic granulite 7.2-7.5 km/s … granulate-facies metapelite or pyroxenite

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Precambrian shield 7.3 km/s and 0.26-0.27 … metapelites or mixture of

high-mafic granulite and low-pyroxenites

Basin and Range 6.9 km/s and 0.28-0.29 … mafic granulites, anorthosite or

amphibolite or these mixture. Paleozoic of SW Germany

from 6.4 km/s and 0.25 (intermediate granulates)to 6.7 km/s and 0.28 (a mix of intermediate and maficgranulites).

Lower Crustal Rocks from Vp and Vs

2014/6/3 Lecture 2014 39Holbrook et al. (1992)

2014/6/3 Lecture 2014 40Holbrook et al. (1992)

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2014/6/3 Lecture 2014 41Mooney and Meissner (1992)

Typical example of crustal image from seismic reflection profiling

(1) Transparent upper crust

(2) Reflective (Laminated)lower crust

seismic layering---> “lamellae (laminae)”

(1)

(2)

Black Forest (Germany)

7.3.3 Crustal image from seismic reflection experiments

2014/6/3 Lecture 2014 42Mooney and Meissner (1992)

Precambrian crust

steeply dipping reflections

consistent with modern tectonic process

collisional orogenscrustal extension andbasin development

operating in Precambrian time

CentralAustralia

(AruntaProvince)

Central North America(Superior

Province)

2014/6/3 Lecture 2014 43Mooney and Meissner (1992)

Baltic Shield

Superior/Grenville

Provinces

Ancient Orogens

(2) evidence ofa suture zone

(1) strong lower crustal reflectivity

(3) distinct dipping reflectivity

evidence of thrusting

---> Proterozoic compressional belt

(Ancient plate tectonics)

(1)

(2)

(1)

(3)

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Precambrian Crust

Dipping reflectivityStrong lower crustal reflectivity and Moho reflections

Lower crust: full range of seismic responses, ranging from transparent to highly reflective laminae like Phanerozoic crust.

Remnant of crustal root formed by ancient tectonic events----> the long-term thermal and tectonic stability

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Phanerozoic crust

Show significant differences in comparison with Proterozoic belts.

Has lost its crustal roots and show well developed, predominantly subhorizontal lamellae in the lower crust (the Caledonian and Variscan orogenic belts of Europe).

These features appear to be due to post-orogenic crustal extension.

Despite of this overprinting by extensional process, some structures remain that are presumably associated with Variscan crustal shortening.

2014/6/3 Lecture 2014 46Mooney and Meissner (1992)

Reflection image west of Ireland across Iapetus suture(formed by the closure of Iapetus Sea

in the early Caledonian orogeny)

(1)

(2)

(1) Northward dipping lower crustal reflections.

(2) Truncation of dipping events at Moho.

Crustal root formed during the orogeny has been eliminated by post-orogenic extension and lower crustal ductile flow.

Multi-origin of reflectivity: lithospheric collision and later extension

2014/6/3 Lecture 2014 47Mooney and Meissner (1992)

Young (post-Mesozoic) orogens (Eastern Swiss Alps)

Crustal roots are in isostatic equilibrium with their high topographies.

(1) The lower crusts dip toward the center of the root from both sides and shows seismic lamination terminating approximately at the refraction Moho.

The reflective pattern of the lower crust may be the results of deformation along multiple shear zones.

(2) The pronounced crustal shortening suggests at least some lower crustal material might be delaminated beneath the deepest part of the root.

(1)(1)

(2)

(3)

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Mooney and Meissner (1992)

(3) The upper and middle crust show a more complex reflective pattern that is suggestive of crustal wedging, with nappe displacements above these wedges.

(1)(1)

(2)

(3)

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2014/6/3 Lecture 2014 49Mooney and Meissner (1992)

Pyrenees

100 km of north-south shortening between the Iberian and Europian plates.

(1)

(2)

(1) Moho offset of 15 km.(2) The crust within the axial zone of the collision belt has been thickened by at least 15 km

by thick-skinned stacking of whole-crustal flakes.

Thick-skinned crustal shortening and crustal thickning in Cenozoic orogenic belts----> marked contrast to thin-skinned tectonics in the Paleozoic orogens

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Recently Extended Crust

The lower crust is highly reflective, in sharp contrast to the relatively transparent upper crust.

Upper crustal faults are listric at depth and merge into the reflective lower crust.

Reflectivity pattern is variable, sometimes filling the lower crust nearly uniformly, and other times concentraing into two or more narrow zones, or forming a single narrow zone of reflections close to the Moho.

The Moho is remarkably flat, and there are several instances of upper mantle reflections, both dipping and flat-lying.

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The Moho

The reflection Moho has been proved to be a highly variable and, at times, elusive feature.

Mid-crustal reflections are often of higher amplitude than the Moho.

The reflection Moho has come to be defined as the deepest set of reflections before the nearly-complete die out of reflectivity at the top of the upper mantle.

The reflection Moho is generally a 1-2 s-wide zone of reflectivity that can only be traced laterally in a piecewise fashion, and less often as a very narrow, nearly continuous horizon.

Reflection Moho is nearly horizontal over large distances, despite the presence of significant topography and/or structure at surface.

Explanation for the lack of Moho relief is that the Moho has been reformed by igneous intrusions and ductile deformation in the lower crust.

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Mooney and Meissner (1992)

Moho

Seismic reflection offshore of Britain

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2014/6/3 Lecture 2014 53Mooney and Meissner (1992)

A model of crust explaining the characteristicof the reflection Moho

3-5 km wide transition zone

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Multi-genetic origin of lower crustal reflections

Origin of reflectivity : concepts with direct borehole and outcrop evidence

(1) Reflectivity caused by igneous intrusives by drill-hole and field studies

Dolerite sheets (Vp=6.6 km/s) within granitic county rock (Vp=6.0 km/s).The individual sheets ranged in thickness from about 4 to 60 m.

(2) Reflectivity of a high-grade metamorphic terrane

Computer modelling of P-wave velocity and density data determined in the laboratory

Fine-scale layering of metamorphic rocks can produce reflection amplitude typical of lower crust.

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Additional factors

Anisotropy in metamorphic rocks can decrease vertical impedance contrasts at lithologic contacts and reflection amplitudes.

Lower crustal seismic bright spots, which can commonly attributed to high pore pressure or partial melting, can also be induced by fine tuning of thin metamorphic layers.

The seismic properties of laminated, thin layers within the geological units may be more important for generation of deep crustal near-vertical reflections than the contacts between major mapped geologic units.