Fault zone fabric and fault weakness

Area of Science:

• Geology
• Tectonics
• Rock Mechanics

Background:

• Crustal faults exhibit weaker frictional strength than laboratory tests suggest.
• Existing explanations for fault weakness (e.g., high fluid pressure, dynamic processes) are insufficient.
• Quasi-static weakening mechanisms are needed for aseismic slip and initiating slip on mis-oriented faults.

Purpose of the Study:

• To investigate a brittle, frictional weakening mechanism in fault rocks based on their natural fabric.
• To provide laboratory evidence for fault weakness in intact fault rocks sheared in their in situ geometry.

Main Methods:

• Shearing of intact fault rocks in their in situ geometry.
• Micro- and nano-structural analysis of sheared samples.
• Comparison of frictional strength between foliated and powdered rock samples.

Main Results:

• Intact fault rocks with well-developed foliation are significantly weaker than their powdered counterparts.
• Frictional sliding occurs along fine-grained phyllosilicate-rich foliations (talc and smectite).
• Powdered samples exhibit high frictional strength, consistent with cataclastic deformation.

Conclusions:

• Fault weakness can arise from slip along a network of weak phyllosilicate-rich surfaces within the rock fabric.
• This mechanism explains fault weakness even when weak minerals constitute a small percentage of the fault rock.
• The prevalence of foliated fault rocks suggests this is a viable explanation for brittle crustal fault weakening.