Paper No. 3
Presentation Time: 9:00 AM-6:30 PM
POSSIBLE EVIDENCE FOR DECARBONATION REACTIONS FROM CARBONATE CLUMPED-ISOTOPE ANALYSES OF FAULT GOUGES
The mechanisms of slip for large slip events on low-angle faults have puzzled researchers for decades. Recently, numerous researchers have explored the idea that slip might occur as a result of the reduction of friction via decarbonation reactions in carbonates, and other reactions in silicates. These former reactions release carbon dioxide, which could, under pressure, reduce friction and facilitate slipping at low angles. Experiments suggest that this process can occur in the laboratory, but evidence that it occurs in natural settings is limited. In this study, we test this hypothesis by seeking evidence for high-temperature decarbonation reactions predicted to occur in gouges, from a fault and two large landslides, by comparing temperatures and fluid sources in gouges and their host rocks. Preliminary data from fault gouge along the Mormon Peak detachment in Nevada shows carbonate crystallization temperatures varying by more than 200°C, from 240°C along the slip surface, to 110°C 5 mm above the slip surface, to 40°C 1 cm above the slip surface. Unstrained host rocks near the fault plane generally yield temperatures of 70-80°C, and late recrystallized calcites generally yield temperatures of 30°C. These higher temperatures appear to be the result of significant shear heating along the detachment, which would be necessary for decarbonation reactions to occur. Samples from the 1987 Bualtar landslide in Pakistan, where Hewitt (1988, Science) directly observed evidence for decarbonation reactions, yield gouge temperatures of ~340°C, not significantly different from temperatures measured in host marbles. This may indicate that decarbonation reactions are volumetrically minor, or that the back-reaction of CaO is to a mineral other than calcite. We are currently working to confirm these preliminary results, and analyze gouges from numerous areas, to better understand the role of decarbonation and recarbonation in gouges from faults and landslides.