摘要
The Damxung—Jiali shear zone is a shear zone in Southern Tibet that trends W—E for>180km.It appears to have a varied history of both displacement sense and deformation conditions.We have examined microscopic and field data (including fault plane populations) in the westernmost 40km of the shear zone (on the northern margin of the Damxung graben near to the route of the INDEPTH II deep seismic line). The shear zone outcrops along the southern flank of an W—E trending chain of hills in which all valleys run N—S, providing a series of sections of exposure through the shear zone. The shear zone is 3～5km wide and has varying degrees of deformation. It cuts across the main trends of, and does not seem to be restricted to any particular lithology. The principal fabric in the shear zone is steeply to moderately south dipping. The bulk of the shear zone is phyllite and lower grade schist with a consistent degree of both ductile and brittle strain. A mylonitized carbonate unit that is sometimes present within the phyllites may have locally provided a mechanical instability as indicated by the higher strain. Elsewhere a conglomerate unit contains carbonate clasts that are stretched (prolate) up to 30∶1.The most spectacular unit forms lozenges of quartzifeldspathic rich rock (several 100m wide & 2～5km long) whose microfabric indicates deformation mechanisms (e.g., wholesale grainsize reduction) that operated around 400～500℃; the highest grade of the deformation fabrics. The lozenges are remnant evidence of higher temperature strain (at higher strain rate?) that is preserved only in the quartzifeldspathic rock. They are interpreted to be mega\|scale boudins resulting from their strength contrast with the weaker phyllites and schists during later lower temperature deformation. All the presently preserved sense of shear indicators are observed to be left\|lateral, and bulk shortening directions resolved using slip data from fault plane populations are consistent with this sinistral displacement.
The Damxung—Jiali shear zone is a shear zone in Southern Tibet that trends W—E for>180km.It appears to have a varied history of both displacement sense and deformation conditions.We have examined microscopic and field data (including fault plane populations) in the westernmost 40km of the shear zone (on the northern margin of the Damxung graben near to the route of the INDEPTH II deep seismic line). The shear zone outcrops along the southern flank of an W—E trending chain of hills in which all valleys run N—S, providing a series of sections of exposure through the shear zone. The shear zone is 3～5km wide and has varying degrees of deformation. It cuts across the main trends of, and does not seem to be restricted to any particular lithology. The principal fabric in the shear zone is steeply to moderately south dipping. The bulk of the shear zone is phyllite and lower grade schist with a consistent degree of both ductile and brittle strain. A mylonitized carbonate unit that is sometimes present within the phyllites may have locally provided a mechanical instability as indicated by the higher strain. Elsewhere a conglomerate unit contains carbonate clasts that are stretched (prolate) up to 30∶1.The most spectacular unit forms lozenges of quartzifeldspathic rich rock (several 100m wide & 2～5km long) whose microfabric indicates deformation mechanisms (e.g., wholesale grainsize reduction) that operated around 400～500℃; the highest grade of the deformation fabrics. The lozenges are remnant evidence of higher temperature strain (at higher strain rate?) that is preserved only in the quartzifeldspathic rock. They are interpreted to be mega\|scale boudins resulting from their strength contrast with the weaker phyllites and schists during later lower temperature deformation. All the presently preserved sense of shear indicators are observed to be left\|lateral, and bulk shortening directions resolved using slip data from fault plane populations are consistent with this sinistral displacement.
出处
《地学前缘》
EI
CAS
CSCD
2000年第S1期155-156,共2页
Earth Science Frontiers
