5:15 PM - 6:30 PM
[SMP25-P07] Frictional melting process during formation of pseudotachylyte, Sarwar-Junia fault zone, India
Keywords:Pseudotachylyte, Frictional melting, Plastic deformation, Biotite
Introduction: Pseudotachylyte (Pst) is dark, aphanitic rock formed by frictional melting of host rock resulting from seismic rapid slip, and it develops along fault plane as vein and network. Therefore, Pst is a significant rock that preserves the information of the past seismic activity. Although Pst are generally formed in brittle deformation zone (or shallow crust), Pst deformed plastically and associated with mylonite are also reported (e.g., Chattopadhyay et al., 2008), and thus it suggests that seismic fault activity can occur in ductile deformation zone (or deep crust). Contributions of slip propagation from brittle deformation zone and plastic instabilities to fault generation mechanism in ductile deformation zone are suggested, but the mechanism is still not completely definite. In this study, it is the purpose that the frictional melting processes and the fault generation mechanism in ductile deformation zone are revealed by the results of microstructural observation and mineral phase identification using the two types of Pst samples within granitic gneiss from Sarwar-Junia Fault Zone, India.
Result: In the field observation, two types of Pst veins were observed in relation to the foliation of the host rock. One vein was parallel to the foliation (“P-Pst”) and another vein cut the foliation at a high angle (“C-Pst”). It is considered that “P-Pst” was formed in the same stress field as the ductile deformation of the host rock.
In the microstructural observation of the Pst veins, both veins have the black margins at both ends (chilled margin), and there were formed by the quench of the melt. The width of chilled margin of “P-Pst” is larger than the one of “C-Pst”. It is caused by the difference of the quench rates and indicates that “P-Pst” had the slower quench rate and “C-Pst” had the faster quench rate, respectively. Comparing the minerals survived as clasts within Pst veins, “P-Pst” mainly preserves quartz and sillimanite, while “C-Pst” mainly preserves quartz, sillimanite, plagioclase and K-feldspar. In addition, biotite grains, contained many in the host rock, don’t survive as clast within both veins, therefore it is considered that these grains were completely melted during Pst formation. On the other hand, especially in “P-Pst”, many acicular biotite grains are formed in the matrix, and it indicates the crystallization from the melt. The melting points of these minerals suggests that the maximum temperatures of melt were 1400-1726 ℃ for the “P-Pst” and 1200-1300 ℃ for the “C-Pst”, respectively. However, considering the non-equilibrium melting and the drop in melting point due to the dehydration of biotite, the actual temperature was probably lower than estimated. From the above, it is considered that “P-Pst” and “C-Pst” were formed in the ductile deformation zone and the brittle deformation zone, respectively, and their frictional melting processes are as follows. First, the host rock was fractured by fault activity, then the frictional melting occurred resulting from rapid slip. At this time, maximum melt temperature of “P-Pst” was higher than the temperature of “C-Pst”. Subsequently, because of slower quench rate of “P-Pst”, wider chilled margins were formed and acicular biotite grains crystallized from the melt.
In addition, it is considered that the formation of “P-Pst” in ductile deformation zone was contributed by the preferred orientation of biotite grains. However (001) plane of biotite grains in the host rock randomly directed, the preferred orientation of the (001) plane parallel to the foliation (or Pst vein) develop as approaching the Pst vein. The plane is bonded by van der Waals forces and has very low strength. From the observation, the (001) plane of biotite grain works as weak plane, and concentration of stress and increasing of strain rate occurred on the plane, then brittle deformation (seismic fault activity) occurred in ductile deformation zone.
Result: In the field observation, two types of Pst veins were observed in relation to the foliation of the host rock. One vein was parallel to the foliation (“P-Pst”) and another vein cut the foliation at a high angle (“C-Pst”). It is considered that “P-Pst” was formed in the same stress field as the ductile deformation of the host rock.
In the microstructural observation of the Pst veins, both veins have the black margins at both ends (chilled margin), and there were formed by the quench of the melt. The width of chilled margin of “P-Pst” is larger than the one of “C-Pst”. It is caused by the difference of the quench rates and indicates that “P-Pst” had the slower quench rate and “C-Pst” had the faster quench rate, respectively. Comparing the minerals survived as clasts within Pst veins, “P-Pst” mainly preserves quartz and sillimanite, while “C-Pst” mainly preserves quartz, sillimanite, plagioclase and K-feldspar. In addition, biotite grains, contained many in the host rock, don’t survive as clast within both veins, therefore it is considered that these grains were completely melted during Pst formation. On the other hand, especially in “P-Pst”, many acicular biotite grains are formed in the matrix, and it indicates the crystallization from the melt. The melting points of these minerals suggests that the maximum temperatures of melt were 1400-1726 ℃ for the “P-Pst” and 1200-1300 ℃ for the “C-Pst”, respectively. However, considering the non-equilibrium melting and the drop in melting point due to the dehydration of biotite, the actual temperature was probably lower than estimated. From the above, it is considered that “P-Pst” and “C-Pst” were formed in the ductile deformation zone and the brittle deformation zone, respectively, and their frictional melting processes are as follows. First, the host rock was fractured by fault activity, then the frictional melting occurred resulting from rapid slip. At this time, maximum melt temperature of “P-Pst” was higher than the temperature of “C-Pst”. Subsequently, because of slower quench rate of “P-Pst”, wider chilled margins were formed and acicular biotite grains crystallized from the melt.
In addition, it is considered that the formation of “P-Pst” in ductile deformation zone was contributed by the preferred orientation of biotite grains. However (001) plane of biotite grains in the host rock randomly directed, the preferred orientation of the (001) plane parallel to the foliation (or Pst vein) develop as approaching the Pst vein. The plane is bonded by van der Waals forces and has very low strength. From the observation, the (001) plane of biotite grain works as weak plane, and concentration of stress and increasing of strain rate occurred on the plane, then brittle deformation (seismic fault activity) occurred in ductile deformation zone.