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 ( hearzones/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. δ T ~2( κ t) 1/2, where κ is thermal diffusivity and t is time since slip began. T max ~log(v 0 ), where T max is the steady state temperature at the center of the shear zone, v 0 is the slip velocity.

1-D Ductile Shear Zone η =(T/2B)exp(Q/RT) δ T ~2( κ t) 1/2 T max ~log(v 0 ).

2-D Ductile Shear Zone The 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 β 0 ). 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: δ M ~ δ T RT max /Q(1-T 0 /T max ) Where δ M is the width within which v 0.8(T max -T 0 ). Model 1 Model 2

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 Alsina_JGR/fig1.php