Implications for the thermomechanics of the San Andreas fault zone Wayne Thatcher, Philip C. England

Yihe Huang Ge277 02/10/2011

Ductile shear zone beneath strike-slip faults:

• Outline

What is ductile shear zone (DSZ)?

1-D ductile shear zone

Let’s go to San Andreas Fault Zone.

Conclusion and Discussion

Introduction

Physics

End

Physics

Application

2-D ductile shear zone

•Ductile shear zone is the deep-level equivalent of faults.

•If shear resistance significantly impedes fault motions, there should be substantial generation of heat.

• Ductile Shear Zone

( http://www.see.leeds.ac.uk/structure/shearzones/gallery/picturegallery.htm )

• Ductile Shear Zone

• 1-D Ductile Shear Zone

• η=(T/2B)exp(Q/RT), where T is absolute temperature, Q is the activation energy, R is the gas constant and B is a material constant.

• δT~2(κt)1/2, where κ is thermal diffusivity and t is time since slip began.

• Tmax~log(v0), where Tmax is the steady state temperature at the center of the shear zone, v0 is the slip velocity.

• 1-D Ductile Shear Zone

η=(T/2B)exp(Q/RT)δT~2(κt)1/2

Tmax~log(v0).

• 2-D Ductile Shear ZoneThe differences from 1-D are:

•The shear zone also loses heat toward the land surface.

•The shear stress is no longer constant throughout the deforming medium.

•The temperature before shearing increases with depth (gradient β0).

X

Z

• 2-D Ductile Shear Zone

The shear zone is treated as being buried beneath a conductive lid of thickness H within which no dissipation occurs.

• 2-D Ductile Shear Zone: Model 1

The greatest temperature increase and highest rates of dissipation are concentrated into a small region at the top of the shear zone.

• 2-D Ductile Shear Zone: Model 2

The zone of concentrated shear heating has a blade-like cross-sectional shape and much greater lateral extent than the dissipative zone for model 1.

• 2-D Ductile Shear Zone

The width of shear zone:

δM ~ δTRTmax/Q(1-T0/Tmax)

Where δM is the width within which v<0.8v0 and δT is the width within which (Tmax-T)>0.8(Tmax-T0).

Model 1

Model 2

• Application to San Andreas Fault Zone

• Application to San Andreas Fault Zone

• Conclusion and Discussion

• Heat flow is a useful tool to analyze the deep structure like ductile shear zone.

• However, it is difficult to estimate shear zone property only using heat flow data.

• Is there any other constraint?

• Conclusion and Discussion

http://earthquake.usgs.gov/regional/asl/pubs/Alsina_JGR/fig1.php