We determine the features and distribution of fault rocks along the Median Tectonic Line (MTL), SW Japan, to establish the 3D architecture of the fault zone across the brittle–plastic transition. ...Cataclasites exposed close to the lithological boundary fault (the MTL) can be divided into those formed by sinistral faulting at temperatures of ∼300 °C and those formed by dextral faulting at ∼250 °C. Mylonites distributed to the north of the cataclasites were formed by sinistral faulting and can be divided into lower-temperature mylonite (L-T mylonite) close to the MTL and higher-temperature mylonite (H-T mylonite) distant from the MTL, where deformation temperatures were lower and higher than 400 °C, respectively. Structures formed by sinistral faulting are oblique to those formed by dextral faulting, indicating that the former structures are older than the latter. Structures formed by sinistral faulting underwent deformation around the brittle–plastic transition. Thus, the MTL fault zone records deformation through a crustal section. Microstructural observations suggest that the differential stress just below the brittle–plastic transition (L-T mylonite) was ∼200 MPa and that this value may not change substantially in the deep crust (H-T mylonite).
•Generate information for a km-scale 3D model of fault-zone architecture.•Identify structures formed by deformation across the brittle-plastic transition.•The studied fault zone records deformation through a crustal section.•Results suggest considerable differential stress below the seismogenic zone.
This paper reports laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb ages of a suite of high-grade metamorphic rocks collected from Sinnan Rocks, Akebono Rock, Niban ...Rock, Gobanme Rock, Tenmondai Rock, Akarui Point, Cape Omega, and Oku-iwa Rock along the Prince Olav Coast, in the Lützow-Holm Complex (LHC), East Antarctica. The dating results indicate that a newly detected ∼ 990 Ma metamorphism of garnet-sillimanite-biotite gneiss from Niban Rock. A thermal event at 931.7 ± 9.8 Ma is recorded in zircon from staurolite-bearing garnet-gedrite-biotite-chlorite gneiss in Akebono Rock. The metamorphic zircon grains in other analyzed samples provide Ediacaran to Cambrian ages. Their multi-growth textures and age populations are possibly interpreted to exhibit three metamorphic stages during >600-580, 580-550, and 550-500 Ma. Combined with previous reports, the metamorphic rocks in Cape Hinode, Niban-nishi Rock of Niban Rock, and Akebono Rock might have experienced earlier high-temperature metamorphism at ∼ 990-930 Ma without younger overprinting. Extensive high-grade metamorphism during ∼ 650-500 Ma is recorded from not only the granulite-facies zone in the west of the LHC but also the amphibolite-facies zone in its east. The main metamorphic episode in the LHC is likely to be subdivided into a preceding thermal event (either independent single metamorphic event or prograde stage) at pre-580 Ma, near-peak condition stage during 580-560 Ma, and subsequent retrograde stage after 550 Ma. In regional context this indicates that the assembly at the central Gondwana started with the collision of early and late Neoproterozoic terranes prior to 580 Ma, as a part of the East Africa-Antarctic Orogeny. Subsequent collisions took place among late Neoproterozoic igneous terrane, above terranes collided at pre-580 Ma, and Neoarchean terrane, which were probably driven by the Kuunga Orogeny.
Crystals formed during ductile deformation known as the recrystallized grains have been revealed to provide syn-deformational stress and strain information. Despite its importance, current methods of ...distinguishing recrystallized grains are subjective and lack consideration to the evolving microstructures. Using data obtained from electron backscattered diffraction mapping, we provide an unsupervised method to estimate the recrystallized fraction and the recrystallized grain size. The Gaussian mixture model was utilized to upon the grain orientation spread that measures intragranular lattice distortion to separate the grains into clusters. The counterpart of the recrystallized grain, the relict grains which are grains present prior to deformation are identified as grains distributed under the cluster with the largest mean grain orientation spread. Three zones could be separated from the Gaussian mixture model results representing the recrystallized zone, mixture zone and relict zone. Grains located within the mixture zone have an innate probability to be classified as either relict or recrystallized grains. The application of the Monte Carlo approximation enables the evaluation of error for the recrystallized fraction and grain size. Comparison to previously available methods shows that the current method avoids ambiguity in the selection of the cut-off threshold when identifying the recrystallized grains.
•An unsupervised approach to distinguish dynamically recrystallized grains.•The Gaussian mixture model provides physical meaning to the clustering results.•This method is robust and avoids ambiguity in selecting the recrystallized grains.
This paper reports laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) zircon U–Pb ages of a suite of high-grade metamorphic rocks collected from Sinnan Rocks, Akebono Rock, Niban ...Rock, Gobanme Rock, Tenmondai Rock, Akarui Point, Cape Omega, and Oku-iwa Rock along the Prince Olav Coast, in the Lützow-Holm Complex (LHC), East Antarctica. The dating results indicate that a newly detected ~ 990 Ma metamorphism of garnet–sillimanite–biotite gneiss from Niban Rock. A thermal event at 931.7 ± 9.8 Ma is recorded in zircon from staurolite-bearing garnet–gedrite–biotite–chlorite gneiss in Akebono Rock. The metamorphic zircon grains in other analyzed samples provide Ediacaran to Cambrian ages. Their multi-growth textures and age populations are possibly interpreted to exhibit three metamorphic stages during >600–580 Ma, 580–550 Ma, and 550–500 Ma. Combined with previous reports, the metamorphic rocks in Cape Hinode, Niban-nishi Rock of Niban Rock, and Akebono Rock might have experienced earlier high-temperature metamorphism at ~ 990–930 Ma without younger overprinting. Extensive high-grade metamorphism during ~ 650–500 Ma is recorded from not only the granulite-facies zone in the west of the LHC but also the amphibolite-facies zone in its east. The main metamorphic episode in the LHC is likely to be subdivided into a preceding thermal event (either independent single metamorphic event or prograde stage) at pre-580 Ma, near-peak condition stage during 580–560 Ma, and subsequent retrograde stage after 550 Ma. In regional context this indicates that the assembly at the central Gondwana started with the collision of early and late Neoproterozoic terranes prior to 580 Ma, as a part of the East Africa-Antarctic Orogeny. Subsequent collisions took place among late Neoproterozoic igneous terrane, above terranes collided at pre-580 Ma, and Neoarchean terrane, which were probably driven by the Kuunga Orogeny.
