Creepy Hematite

 

USU Geo has a long and strong history of studying mineralization along faults and in fault gouge and how it affects deformation. The tradition has continued over the last two years with grad student Alex DiMonte, mentored by Dr. Alexis Ault, who has published two manuscripts that highlight the role of hematite in deformation during the earthquake cycle. For her MS research, DiMonte sampled hematite-coated fault surfaces in the Mecca Hills, California, analyzing them with the scanning electron microscope and (U-Th)/He thermochronometry data. She also simulated faulting in the lab by deforming hematite in a rotary-shear apparatus, mentored by collaborator Dr. Greg Hirth at Brown University.

Geodetic data show that today, different strands of the southern San Andreas fault experience episodes of creep (slip without an earthquake). DiMonte reveals in a 2022 paper in Geology that networks of hematite fault surfaces may have formed and deformed in the upper km of the fault zone during creep episodes over the past million years. DiMonte followed this up with a 2024 paper in the Journal of Geophysical Research: Solid Earth, which illustrates that some hematite is frictionally strong and strengthens when slip rates increase, consistent with creep documented in natural fault rocks.

As DiMonte is continuing now towards a PhD, she is building upon this work to investigate how clay – perhaps the most important mineral in the shallowest portion of a fault – forms, deforms, and controls how fault slip and earthquake energy is distributed throughout the seismic cycle.