Heterogeneous localisation of plastic flow in the deepest ...

Heterogeneous localisation of plastic flow in the deepest part of a

seismogenic fault: insight from the Hatagawa Fault Zone, NE

Japan

Norio Shigematsu (GSJ, AIST)

Distribution of hypocenters of mainshock and aftershocks for the Noto Earthquake 2007

(From the web-site of the ERI of the University of Tokyo)

Occurrence of inland earthquakes

Rock deformation under the brittle-plastic transition is important to understand the earthquake process.

Exploring exhumed fault zones Exploring exhumed fault zones for whichfor which crustal sections including the brittleincluding the brittle––plastic plastic transition are exposed at the transition are exposed at the surface is an important surface is an important strategy in understanding fault strategy in understanding fault behaviour.behaviour.

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Takase R iver

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Mylonite with sinistral sense of shearThe area where small shear zones are developed

Cataclasite

km0 2

Central Area

Abukumabelt

SouthKitakami belt

Fault rocks (in the seismogenic zone)

Earthquake

Hypocenter of large earthquake along active faults

Cataclasite (cohesive：MTL)

Fault gouge (incohesive：MTL)

Fault rocks (under the temperatures above 300℃)

Mylonite Zone (Deformed plastically)

Deformed continuously

Plastically deformed rocks (Mylonite)Plastically deformed rocks (Mylonite)

Optical microphotograph of undeformed granite

Optical micrograph of Granitic mylonite

Small mylonite zone

To understand inland earthquakes Boundary depth of cataclasite and mylonite

Cataclasite along the HFZCataclasite along the HFZ

50 km

SENDAISENDAI

HFZ HFZ

K-Ar 98.1±2.5 Ma (Hornblende)

The activity had ceased by

98.1±2.5 Ma ．

A repeating of earthquakes

along the cataclasite zone.

• The assemblage of altered minerals in the cataclasite indicates that the cataclasite zone was formed at temperatures above 220 °C

Two types of Mylonite Microstructures A and B• Microstructure A

Fault Rocks along the HFZ (Mylonite)Fault Rocks along the HFZ (Mylonite)

Fine-Grained Matrix

Pole Figures of quartz <c>

Lower temperature, faster strain rate,

or lower water activity.

K-f Pl

Q

(Lineation)

(Pole to Foliation)

(Lineation)

(Pole to Foliation)

Two feldspar thermometry (Whitney &

Stormer, 1977)

300～360 ºC

• Microstructure B

Higher temperature, slower strain rate,

or higher water activity.

Two feldspar thermometry

360 ～500 ºCPole Figures of quartz <c>

(Lineation)

(Pole to Foliation)

Outcrop extent of two types mylonite

The outcrop extent of microstructure A

Northern Area

Southern Area

Central Area

Association between fracturing and plastic deformation

1 cm1 cm1 cm

Crush zone

subsequently plastically deformed fragment

Outcrop extent of microstructure A

Deformed under the condition of brittle-plastic

transiton

Final localised zones of plastic flow

T=350～500℃ Brittle-plastic transition Brittle Regime

History of displacement along the HFZ

Cross-section of deformation styles

Deformation in the outcrop extent of microstructure A (Dynamic recrystallization of feldspar)

11mm

0.5 0.5 mm

Localisation of deformation to the outcrop extent of microstructure AStrain weakening

QzQz

PlPlKfKf

Crystallographic Orientation of Feldspar

{100} Y0

X0

{010} {001}Pole Figures

[ noy 900. cpr ]Low albite (-1)Complete data set201 data pointsEqual Area projectionUpper hemispheres

Half width:10 °

Cluster size:0 °

Densities (mud):Min= 0.00, Max= 9.65

{100}Fx=0.329

Fy=0.333

Fz=0.338

Y0

X0

{010}Fx=0.338

Fy=0.382

Fz=0.280

{001}Fx=0.330

Fy=0.264

Fz=0.406

Pole Figures

[noy900.cpr]

Low albite (-1)

Complete data set

201 data points

Equal Area projection

Upper hemispheres

Crystallographic Orientations are random

Deformation by superplasticity(Lineation)

(Pole to Foliation)

• Ductile FractureFracturing following the subjection of material to large plastic strain

Nucleation, growth and interlinkage of cavities

Association between fracturing and plastic deformation

1 cm1 cm1 cm

Crush zone

Cavitation during superplastic deformation of alumina (Kottada and Chokshi, 2000; Chokshi, 2005)

Plastic deformation and fracturing of fine-grained feldspar (Shigematsu et al., 2004)

Shear zone including a crush zone along a shear band

Fractures were nucleated during plastic deformation in the outcrop extent of microstructure A

Fine grained feldspar within the shear band

Secondary electron image of fine-grained feldspar．Cavities along grain boundaries are connected to form an intergranular

fracture (arrows).

Fracturing during plastic flow of fine-grained feldspar

Ductile fracture of fine-grained feldspar experimentally reproduced

Rybacki, et al., 2008 GRL35, L04304, doi:10.1029/2007GL032478

Large earthquakes along the HFZ

Outcrop extent of Microstructure A: Nucleation of fracture due to ductile fracture

Nuclei of fractures

Stress concentration due to the different displacement

Large Earthquakes

Restricted plastic displacement in the outcrop extent of Microstructure A: Stress concentration

Summary

Fault rocks formed in the B-D-T was exposed in a limited region along the HFZ, with a length of 6 km. Displacement by plastic flow occurred only in this restricted regions at the depth in the crust where P-T conditions were those of the brittle–plastic transition.The localisation of plastic flow to the region with a length of 6 km possibly resulted from strain weakening accompanied by the dynamic recrystallization of feldspar.The extreme strain localisation led to ductile fracturing of highly deformed fine-grained feldspar. It is likely that numerous fractures were nucleated in these rocks due to ductile fracture.Heterogeneous plastic displacement resulted in a significant stress concentration. Interaction between this stress concentration and fractures nucleated via ductile fracture possibly promoted the nucleation of large earthquakes.