Raman spectral analysis of carbonaceous material to detect shear heating on a large fault—example from the Median Tectonic Line, Southwest Japan
Abstract
Recognition of shear heating has potential to help in estimating the shear stresses (τ) that operate on major faults when they move. Surface heat-flow and fission track thermochronology in the vicinity of the major San Andreas Fault show no clear evidence for major shear heating. This is commonly used to infer a much lower shear stress than that expected based on rock deformation experiments. The cause of this discrepancy between experiment and observation remains unresolved. The E-W trending Median Tectonic Line (MTL) separates the subduction type Sanbagawa metamorphic belt from the continental high temperature Ryoke metamorphic belt. It is the largest on-land fault of the Japanese Islands with a movement history from the Cretaceous to the present, and is a suitable candidate for studies of shear heating and to investigate whether a low degree of shear heating is characteristic of regional long-lived faults. A progressive younging of fission track ages from north to south towards the MTL within the Ryoke belt suggests shear heating was important. However, thermal structure in the Sanbagawa belt to the south of the MTL has not been determined and the detailed thermal structure around the fault is not known. Our study aims to fill these gaps in our knowledge by clarifying the peak temperature attained in the Sanbagawa belt that lies on the southern side of the MTL. The strongest thermal anomaly will be observed close to the fault. A detailed study therefore requires detailed sampling in this region. A semi-continuous core passing through the MTL was recently drilled by the Geological Survey of Japan, AIST in the Kii peninsula, as a part of the observation network to predict the forthcoming Tonankai and Nankai earthquakes. The availability of this core enables us to conduct detailed analyses in key samples close to the fault. To study the broader thermal structure, we also sampled on a kilometer scale and studied the regional structure using field mapping techniques. Pelitic rock is the main rock facies of the Sanbagawa belt in the study area. This pelite was metamorphosed at temperatures <400oC and minerals suitable for typical Fe++-Mg exchange thermometers are poorly developed. As an alternative way of estimating peak temperature, we used Raman spectral analysis of carbonaceous material. Results show a consistent regional temperature of 343oC-350oC at distances between 400m and 4km from the MTL. There is a significant rise within 200m from the MTL to temperatures of 366oC-389oC. These results show no evidence for a heat-anomaly on km-scales to the south of the MTL, but do show a clear temperature increase near the MTL. The spatial association of the heat-anomaly with the fault suggests it is the result of shear heating, but the heat-anomaly is only observed in a narrow zone close to the MTL. More detailed modeling of the heat generated in the fault and its transport will require more continuous data from close to the MTL.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFM.T41B2150M
- Keywords:
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- 3660 MINERALOGY AND PETROLOGY / Metamorphic petrology;
- 8004 STRUCTURAL GEOLOGY / Dynamics and mechanics of faulting;
- 8130 TECTONOPHYSICS / Heat generation and transport;
- 8163 TECTONOPHYSICS / Rheology and friction of fault zones