Seismic Faulting of Dolomite Rocks: Textures and Possible Slip Weakening Mechanisms

Details Seismic Faulting of Dolomite Rocks: Textures and Possible Slip Weakening Mechanisms Seismic Faulting of Dolomite Rocks: Textures and Possible Slip Weakening Mechanisms

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.t54a..04t&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #T54A-04

Physics

7209 Earthquake Dynamics (1242), 8010 Fractures And Faults, 8012 High Strain Deformation Zones, 8034 Rheology And Friction Of Fault Zones (8163)

Scientific paper

We performed high velocity friction experiments on dolomite gouge material for normal stress sn=0.4-2 MPa and slip rates v=1.3 m/s. During the experiments, the CO2 emission rate was monitored, since dolomite decomposes to Mg-calcite+MgO+CO2 for temperatures T>550 C. For sn>0.8 MPa, CO2 emissions have been recorded. XRD analyses of the deformed gouge layer confirmed the presence of small amount (3%) of Mg-calcite and MgO. The shear strength (t)displays an exponential decay trend, over a slip weakening distance Dc, from a peak to a steady state value. For displacement values >Dc, t sharply increases resulting in the attainment of a second steady state. The observed strengthening always occurs at about the same time of a dramatic increase in the rate of the CO2 emissions. We interpret the increase of t, after the first steady state, as due to the increase of the effective normal stress when pressurized CO2 escapes from the slip zone and fluid pressure is reduced. Micro-structural observations show the development of a thin slip zone (50-150 microns) of very to ultra fine-grained material. Thick bands (100-500 microns) of decomposed dolomite gouge developed adjacent to the slip zone. The thermally decomposed dolomite is characterized by rounded and vesiculated clasts (different from clasts of initially intact material which have angular shape) displaying reaction rims and patches of ultra fine-grained decomposed material, enriched in Ca (Mg-Calcite) and Mg (MgO). A thin layer (10-20 microns) of ultra fine-grained decomposed material, made of Mg-Calcite and MgO, has been observed to develop within the slip zone. It displays a sharp contact with the band of decomposed material developed outside the slip zone. The microstructures observed in the areas adjacent/within the slip zone can be used in future studies as new diagnostic features to aid in the recognition of seismic faulting within carbonatic rocks and, potentially, of thermal pressurization slip weakening processes.

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