712 Kim Korean Geology

8/9/2019 712 Kim Korean Geology

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Continental Margin Geology of Korea :

Review and constraints on the opening of

the East Sea (Japan Sea)

Han-Joon KimMarine Satell ite & Observation Tech.

Korea Ocean Research and Development Institute

Ansan, Korea


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The East Sea / Japan Seaa back-arc sea in the WPformed in a region of

complex pate boundaries

Marginal basins:

a commonfeature of the subductionzone in the western Pacific


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Models for the opening of the East Sea

various models for its formation close in on

(1) FanFan--shaped openingshaped opening

(Otofuji et al., 1983, 1985; Otofuji, 1996)(Otofuji et al., 1983, 1985; Otofuji, 1996)

(2) Pull-apart opening

(Lallemand & Jolivet, 1985; Tamaki et al., 1992; Jolivet et al.,(Lallemand & Jolivet, 1985; Tamaki et al., 1992; Jolivet et al.,

1994)1994)


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The mode of opening (fanThe mode of opening (fan--shaped opening)shaped opening)

Clockwise deflection in declination was foundin pre-middle Miocene rocks in SW Japan

Counter-clockwise deflection in declination wasfound in early Miocene rocks in NE Japan

SW Japan rotated clockwise away from Asia by 45

o

NE Japan rotated counter-clockwise by 40

o

The differential rotation occurred concurrentlyat about 15 Ma mostly during the period of 2 Ma

A very fast spreading rate of 21 cm/yr

(Otofuji et al., 1983, 1985; Otofuji, 1996)(Otofuji et al., 1983, 1985; Otofuji, 1996)


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The mode of opening (pullThe mode of opening (pull--apart opening)apart opening)

Lallemand and Jolivet (1985), Tamaki etal. (1992), Jolivet et al. (1994), Fournier etal. (1994)

The pull-apart opening model relates theopening of the East Sea to the India-Asiacollision.

Indentation tectonics Tapponier and Molnar,

1976; Fournier, 1994)

Dextral shear zones are produced parallelto the eastern free boundary.

An additional component of extension

induces the formation of basins controlled

by those dextral shear zones


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The mode of opening (pullThe mode of opening (pull--apart opening)apart opening)

The Japan Arc translated southward along strike-slip

faults on the eastern and western sides in the early

Miocene.

The strike-slip faults were produced by indentationtectonics

(Lallemand & Jolivet, 1985; Tamaki et al., 1992;Jolivet et al.,(Lallemand & Jolivet, 1985; Tamaki et al., 1992;Jolivet et al., 1994)1994)


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Why is the Korean margin important ?Why is the Korean margin important ?

Transition from continental to backTransition from continental to back--arc crustarc crust

Rift architecture (rift basin, uplifted rift flank, slope)Rift architecture (rift basin, uplifted rift flank, slope)

Pronounced volcanic phase associated with margin tectonicsPronounced volcanic phase associated with margin tectonics

Constraints on the opening of the East Sea/ Japan SeaConstraints on the opening of the East Sea/ Japan Sea


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Geophysical study of the Korean margin

eophysical study of the Korean margin

MaterialMaterial

OBS data from 28 sites (1991, 1998)OBS data from 28 sites (1991, 1998)

> 1500 km MCS data, swath bath and magnetic data (1994> 1500 km MCS data, swath bath and magnetic data (1994--1999)1999)

ObjectivesObjectives

Investigation of crustal structureInvestigation of crustal structureExplanation of (1) basement structure & igneous stratigraphy,Explanation of (1) basement structure & igneous stratigraphy,

(2) processes of continental rifting to sp(2) processes of continental rifting to spreadingreading

(3) separation of SW Japan from Korea(3) separation of SW Japan from Korea


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Deep Seismic Sounding using OBSs at the Korean Margin

1991 1998


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MCS data collection (96 and 56 channels)


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Magmatic underplating (?) and extrusive volcanism at the Korean margin(Kim et al., 2003)


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* Well-imaged velocities

typical of 2B, 2C, and 3

* Presence of 2C

* Acoustic basement at

6 km depth

Kim et al. 1994)

Results from DSS using OBSs


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Thicker than normal oceanic crust created above theThicker than normal oceanic crust created above thehotter than normal mantle (hotter than normal mantle (Kim et al., 1998)Kim et al., 1998)


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(6)Mantle structure of NE Asia from global tomography


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MCS profiles from the rifts in the Korea Plateau


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(13)

MCS profiles from the shelf to the center of UB


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Structural interpretation of MCS dara


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Conclusions

onclusions

The Korean marginThe Korean marginStructural elements consist of a seaward succession of riftStructural elements consist of a seaward succession of rift

basin, uplifted rift flank, steep slope, and deep sea basin callbasin, uplifted rift flank, steep slope, and deep sea basin calleded

thethe UlleungUlleung

Basin.Basin.

Crustal structureCrustal structure

Thicker than normal oceanic crust in theThicker than normal oceanic crust in the UlleungUlleung

BasinBasin

Emplacement of oceanic crust above the hotter than normalEmplacement of oceanic crust above the hotter than normalmantlemantle

MagmaticMagmatic underplatingunderplating

& extrusive volcanism at the Korean& extrusive volcanism at the Korean

marginmargin

Continental rifting before breakupContinental rifting before breakup

Rifts show a fundamental unit of rift architectureRifts show a fundamental unit of rift architecture

--

border fault zoneborder fault zone: convex dip slope,: convex dip slope, zigzig--zagzag

arrangementarrangementalong strikealong strike

F lt P ttF lt P tt


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Fault PatternFault Pattern

rifting wasrifting was

controlled by normal faulting resulting fromcontrolled by normal faulting resulting from

extension:extension: no strikeno strike--slip deformationslip deformation (no pull(no pull--apart opening)apart opening)

TwoTwo--stage rifting, breakup, and spreadingstage rifting, breakup, and spreading

Rifting orthogonal to the line of breakup along the slope base:Rifting orthogonal to the line of breakup along the slope base:

some amount of rotation of SW Japan during backsome amount of rotation of SW Japan during back--arcarcspreadingspreading (partly compatible with fan(partly compatible with fan--shaped opening)shaped opening)

Separation of the SW Japan Arc :Separation of the SW Japan Arc : SEwardSEward

in response to the NWin response to the NW

subductionsubduction

of the Pacific Plateof the Pacific Plate

VolcanismVolcanism

pronounced volcanic phase at breakup and onset of sea floorpronounced volcanic phase at breakup and onset of sea floor

spreadingspreading asthenosphericasthenospheric

upwelling and riftupwelling and rift--induced convectioninduced convection

Structure and igneous evolution of the Korean marginStructure and igneous evolution of the Korean margin

explained by the processes occurring at the passive continentalexplained by the processes occurring at the passive continental

marginmargin


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Thanks!Thanks!


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Seismic Traces Spike Reflectivity Acoustic ImpedanceSeismic Traces Spike Reflectivity Acoustic Impedance

Lee et al. 2003)


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Why is magnetization low in the

hy is magnetization low in the

Ulleung

lleung

asin?

asin?

Enclosed backEnclosed back--arc basin with thick sedimentsarc basin with thick sediments* Sediments seal extrusive basalts and* Sediments seal extrusive basalts and

enhance hydrothermal circulation: Feenhance hydrothermal circulation: Fe

from Fefrom Fe--Ti oxides is removed andTi oxides is removed and

precipitated as nonmagneticprecipitated as nonmagnetic FeSFeS

* Slow Cooling or elevated temperatures* Slow Cooling or elevated temperatures

* Long* Long--lived postlived post--spreading volcanismspreading volcanism


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Why was volcanism so vigorous at the

hy was volcanism so vigorous at the

Korean margin ?orean margin ?

Secondary convection in the upper mantleSecondary convection in the upper mantle

* The Korean margin is vary narrow.* The Korean margin is vary narrow.

Mantle plumeMantle plume

* No evidence of a deep* No evidence of a deep--rooted plumerooted plume

* But East Asia is floored by a large* But East Asia is floored by a largecompositional and thermal anomaly in thecompositional and thermal anomaly in the

upper mantle.upper mantle.

AsthenosphericAsthenospheric

upwellingupwelling* induced in a back* induced in a back--arc region by thearc region by the subductedsubducted

slabslab

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712 Kim Korean Geology