Frictional Characteristics of Oceanic Transform Faults: Progressive Deformation and Alteration Controls Seismic Style. Issue 24 (17th December 2021)

• Record Type: Journal Article
• Authors: Cox, Sophie; Ikari, Matt J.; MacLeod, Christopher J.; Fagereng, Åke
• Is Part Of: Geophysical research letters. Volume 48:Issue 24(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-12-17
• Subjects: structural geology -- rate‐and‐state friction -- fault rheology -- transform fault -- friction experiments -- oceanic crust; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2021GL096292
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: Oceanic transform faults are inferred to be weak relative to surrounding oceanic crust and primarily slip aseismically. Neither their weakness nor tendency to creep are well‐explained. We test the effects of fault‐rock evolution on oceanic transform fault frictional strength and stability using direct‐shear experiments (at room temperature, 10 MPa normal stress, and fluid‐saturated conditions) on dolerite from the East Pacific Rise and natural fault rocks from the exhumed Southern Troodos Transform, Cyprus. Dolerites and cemented breccias are frictionally strong ( μ = 0.52–0.85) and velocity‐weakening (strength decreases with increasing slip velocity, characteristic of earthquakes). In contrast, matrix‐rich chlorite‐bearing fault breccias and gouges are frictionally weak ( μ = 0.25–0.48) and velocity‐strengthening (characteristic of stable creep). This transition implies that seismic behavior is controlled by degree of damage and alteration, such that earthquakes can nucleate within relatively intact oceanic crust, whereas fault segments of increased damage and chlorite content tend to slip aseismically. Plain Language Summary: Oceanic transform faults are plate boundary faults where motion of oceanic lithosphere is dominantly horizontal and parallel to tectonic motion. Fewer and smaller earthquakes than expected occur along these faults, which are also considered weak structures. The reasons for their weakness and lack of large earthquakes are puzzling. To understand these characteristics, we conducted laboratory deformation experiments using rocks collected from the ocean floor (near Hess Deep in the Pacific) and from an ancient transform fault (in Cyprus). We sheared cylindrical samples, holding one half in place and sliding the other over it, creating laboratory equivalents of geological faults. We find that dolerite, one of the primary rock‐types of the oceanic crust, is strong and capable of starting earthquakes. In contrast, we find that already damaged and altered rocks, found within natural faults (containing an increased proportion of the mineral chlorite), are weak. The same damage and alteration responsible for the weakness also prevents earthquakes. Our findings suggest that variations in the size and number of earthquakes on oceanic transform faults is controlled most of all by how damaged the existing rock is and how much alteration to weak, water‐bearing secondary minerals such as chlorite has occurred along fault planes. Key Points: Dolerite hydrothermally cemented at greenschist facies is strong and velocity‐weakening at 10 MPa normal stress and room temperature Natural fault rocks comprising chlorite‐rich, fractured, and comminuted metadolerite are frictionally weak and velocity‐strengthening Along oceanic transform faults, in the mafic oceanic crust, increased damage and alteration can explain fault weakness and aseismic slip