Transpressional deformation has played an important role in the late Neoproterozoic evolution of the ArabianNubian Shield including the Central Eastern Desert of Egypt.The Ghadir Shear Belt is a 35 km-long,NW-oriented...Transpressional deformation has played an important role in the late Neoproterozoic evolution of the ArabianNubian Shield including the Central Eastern Desert of Egypt.The Ghadir Shear Belt is a 35 km-long,NW-oriented brittleductile shear zone that underwent overall sinistral transpression during the Late Neoproterozoic.Within this shear belt,strain is highly partitioned into shortening,oblique,extensional and strike-slip structures at multiple scales.Moreover,strain partitioning is heterogeneous along-strike giving rise to three distinct structural domains.In the East Ghadir and Ambaut shear belts,the strain is pure-shear dominated whereas the narrow sectors parallel to the shear walls in the West Ghadir Shear Zone are simple-shear dominated.These domains are comparable to splay-dominated and thrust-dominated strike-slip shear zones.The kinematic transition along the Ghadir shear belt is consistent with separate strike-slip and thrustsense shear zones.The earlier fabric(S1),is locally recognized in low strain areas and SW-ward thrusts.S2 is associated with a shallowly plunging stretching lineation(L2),and defines^NW-SE major upright macroscopic folds in the East Ghadir shear belt.F2 folds are superimposed by^NNW–SSE tight-minor and major F3 folds that are kinematically compatible with sinistral transpressional deformation along the West Ghadir Shear Zone and may represent strain partitioning during deformation.F2 and F3 folds are superimposed by ENE–WSW gentle F4 folds in the Ambaut shear belt.The sub-parallelism of F3 and F4 fold axes with the shear zones may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation in fold zones.Dextral ENEstriking shear zones were subsequently active at ca.595 Ma,coeval with sinistral shearing along NW-to NNW-striking shear zones.The occurrence of upright folds and folds with vertical axes suggests that transpression plays a significant role in the tectonic evolution of the Ghadir shear belt.Oblique convergence may have been provoked by the buckling of the Hafafit gneiss-cored domes and relative rotations between its segments.Upright folds,fold with vertical axes and sinistral strike-slip shear zones developed in response to strain partitioning.The West Ghadir Shear Zone contains thrusts and strikeslip shear zones that resulted from lateral escape tectonics associated with lateral imbrication and transpression in response to oblique squeezing of the Arabian-Nubian Shield during agglutination of East and West Gondwana.展开更多
The Louzidian normal fault occurs as the eastern detachment fault of the Kalaqin metamorphic core complex. Field observations and microstructural analyses reveal that the Louzidian-Dachengzi ductile shear zone develop...The Louzidian normal fault occurs as the eastern detachment fault of the Kalaqin metamorphic core complex. Field observations and microstructural analyses reveal that the Louzidian-Dachengzi ductile shear zone developed in its lower-plate was genetically related to sinistral strike-slips and extensional faulting. Two samples from this ductile shear zone yield 40Ar-39Ar plateau ages of 133 Ma (Bi) and 126 Ma (Kp), which are concordant with their isochron ages. The plateau age of 133 Ma (Bi) records the formation age of the ductile shear zone. The inconsistent relationship between the earlier strike-slip ductile shear zone and the later normal fault makes the Kalaqin Quasi-metamorphic core complex distinctive from Cordil-leran nietamorphic core complex. These ages provide important geochronological data for putting constraints on the formation age and genesis of such ductile shear zones.展开更多
A continental-scale strike-slip shear zone frequently presents a long-lasting deformation and physical expression of strain localization in a middle to lower crustal level.However,the deformation evolution of strain l...A continental-scale strike-slip shear zone frequently presents a long-lasting deformation and physical expression of strain localization in a middle to lower crustal level.However,the deformation evolution of strain localization at a small-scale remains unclear.This study investigated<10 cm wide shear zones developing in undeformed granodiorites exposed at the boundary of the continental-scale Gaoligong strike-slip shear zone.The small-scale ductile shear zones exhibit a typical transition from protomylonite,mylonite to extremely deformed ultramylonite,and decreasing mineral size from coarse-grained aggregates to extremely fine-grained mixed phases.Shearing sense indicators such as hornblende and feldspar porphyroclasts in the shear zone are the more significantly low-strain zone of mylonite.The microstructure and EBSD results revealed that the small-scale shear zone experienced ductile deformation under medium-high temperature conditions.Quartz aggregates suggested a consistent temperature with an irregular feature,exhibiting a dominated high-temperature prism <a> slip system.Additionally,coarse-grained aggregates in the mylonite of the shear zone were deformed predominantly by dislocation creep,while ultra-plastic flow by viscous grain boundary sliding was an essential deformation process in the extremely fine-grained(~50μm)mixed-phases in the ultramylonite.Microstructural-derived strain rates calculated from quartz paleopiezometry were on the order of 10^(-15) to 10^(-13) s^(-1)from low-strain mylonite to high strained ultramylonite.The localization and strain ratelimited process was fluid-assisted precipitation presenting transitions of compositions as hydrous retrogression of hornblende to mica during increasing deformation and exhumation.Furthermore,the potential occurrence of the small-scale shear zone was initiated at a middle-deep crust seated crustal condition dominated by the temperature-controlled formation and rheological weakening.展开更多
文摘Transpressional deformation has played an important role in the late Neoproterozoic evolution of the ArabianNubian Shield including the Central Eastern Desert of Egypt.The Ghadir Shear Belt is a 35 km-long,NW-oriented brittleductile shear zone that underwent overall sinistral transpression during the Late Neoproterozoic.Within this shear belt,strain is highly partitioned into shortening,oblique,extensional and strike-slip structures at multiple scales.Moreover,strain partitioning is heterogeneous along-strike giving rise to three distinct structural domains.In the East Ghadir and Ambaut shear belts,the strain is pure-shear dominated whereas the narrow sectors parallel to the shear walls in the West Ghadir Shear Zone are simple-shear dominated.These domains are comparable to splay-dominated and thrust-dominated strike-slip shear zones.The kinematic transition along the Ghadir shear belt is consistent with separate strike-slip and thrustsense shear zones.The earlier fabric(S1),is locally recognized in low strain areas and SW-ward thrusts.S2 is associated with a shallowly plunging stretching lineation(L2),and defines^NW-SE major upright macroscopic folds in the East Ghadir shear belt.F2 folds are superimposed by^NNW–SSE tight-minor and major F3 folds that are kinematically compatible with sinistral transpressional deformation along the West Ghadir Shear Zone and may represent strain partitioning during deformation.F2 and F3 folds are superimposed by ENE–WSW gentle F4 folds in the Ambaut shear belt.The sub-parallelism of F3 and F4 fold axes with the shear zones may have resulted from strain partitioning associated with simple shear deformation along narrow mylonite zones and pure shear-dominant deformation in fold zones.Dextral ENEstriking shear zones were subsequently active at ca.595 Ma,coeval with sinistral shearing along NW-to NNW-striking shear zones.The occurrence of upright folds and folds with vertical axes suggests that transpression plays a significant role in the tectonic evolution of the Ghadir shear belt.Oblique convergence may have been provoked by the buckling of the Hafafit gneiss-cored domes and relative rotations between its segments.Upright folds,fold with vertical axes and sinistral strike-slip shear zones developed in response to strain partitioning.The West Ghadir Shear Zone contains thrusts and strikeslip shear zones that resulted from lateral escape tectonics associated with lateral imbrication and transpression in response to oblique squeezing of the Arabian-Nubian Shield during agglutination of East and West Gondwana.
基金This work was supported jointly by the Chinese Academy of Sciences (Grant Nos. KZCX1-07 and KZCX1-Y-03-01-05) the National Key Basic Research Project (Grant No. G1999043302).
文摘The Louzidian normal fault occurs as the eastern detachment fault of the Kalaqin metamorphic core complex. Field observations and microstructural analyses reveal that the Louzidian-Dachengzi ductile shear zone developed in its lower-plate was genetically related to sinistral strike-slips and extensional faulting. Two samples from this ductile shear zone yield 40Ar-39Ar plateau ages of 133 Ma (Bi) and 126 Ma (Kp), which are concordant with their isochron ages. The plateau age of 133 Ma (Bi) records the formation age of the ductile shear zone. The inconsistent relationship between the earlier strike-slip ductile shear zone and the later normal fault makes the Kalaqin Quasi-metamorphic core complex distinctive from Cordil-leran nietamorphic core complex. These ages provide important geochronological data for putting constraints on the formation age and genesis of such ductile shear zones.
基金financially supported by the National Natural Science Foundations of China(Nos.41972220,4188810)the National Key Research and Development Program(No.2017YFC0602401)the Excellent Youth Fund of the National Natural Science Foundation of China(No.41722207)。
文摘A continental-scale strike-slip shear zone frequently presents a long-lasting deformation and physical expression of strain localization in a middle to lower crustal level.However,the deformation evolution of strain localization at a small-scale remains unclear.This study investigated<10 cm wide shear zones developing in undeformed granodiorites exposed at the boundary of the continental-scale Gaoligong strike-slip shear zone.The small-scale ductile shear zones exhibit a typical transition from protomylonite,mylonite to extremely deformed ultramylonite,and decreasing mineral size from coarse-grained aggregates to extremely fine-grained mixed phases.Shearing sense indicators such as hornblende and feldspar porphyroclasts in the shear zone are the more significantly low-strain zone of mylonite.The microstructure and EBSD results revealed that the small-scale shear zone experienced ductile deformation under medium-high temperature conditions.Quartz aggregates suggested a consistent temperature with an irregular feature,exhibiting a dominated high-temperature prism <a> slip system.Additionally,coarse-grained aggregates in the mylonite of the shear zone were deformed predominantly by dislocation creep,while ultra-plastic flow by viscous grain boundary sliding was an essential deformation process in the extremely fine-grained(~50μm)mixed-phases in the ultramylonite.Microstructural-derived strain rates calculated from quartz paleopiezometry were on the order of 10^(-15) to 10^(-13) s^(-1)from low-strain mylonite to high strained ultramylonite.The localization and strain ratelimited process was fluid-assisted precipitation presenting transitions of compositions as hydrous retrogression of hornblende to mica during increasing deformation and exhumation.Furthermore,the potential occurrence of the small-scale shear zone was initiated at a middle-deep crust seated crustal condition dominated by the temperature-controlled formation and rheological weakening.
