•High and low stress subduction recognized in the metamorphic record.•Bounds on apparent coefficients of friction and ancient subduction boundary shear stresses.•Modelling suggests subduction rate ...and lithology control stress state of subduction boundary.
It is generally accepted that the main strength of subduction boundaries occurs in the shallower region where frictional deformation is dominant. However, estimates of absolute values—commonly expressed as apparent coefficient of friction, μ′—show great variation. Frictional shear heating is closely related to μ′, and estimates of the extra thermal energy supplied by shearing can in principle be used to estimate subduction zone strength. One such approach is based on surface heat flow measurements. However, heat flow in convergent margins shows large local scatter and even in the same area, different studies using this method show large variations in estimates of μ′ indicating large uncertainties. The thermal record of subduction conditions preserved in subduction-type metamorphic rocks is developed over geological time scales that average out local complexities in heat flow and therefore has good potential as an alternative indicator of the amount of shear heating and, hence, shear strength along subduction boundaries. Thermal models that incorporate shear heating were developed for two contrasting and well-known subduction-type metamorphic belts: the relatively warm Sanbagawa belt of SW Japan and the relatively cold Franciscan belt of western USA. High-grade rocks of the Sanbagawa belt show strongly curved P–T paths that display increasing P/T to about 2 GPa. Information on the rate of plate movement and the age of the subducting slab at the time of metamorphism can be combined with modelling results to show that relatively high shear stresses, equivalent to μ′∼0.13 are required to account for the observed curved P–T paths. In contrast, the high-grade rocks of the Franciscan belt show relatively cool P–T conditions that do not allow for strong shear heating with an appropriate upper bound for μ′ of ∼0.03. Modelling suggests subduction rate and lithology are potentially important controls on the development of high- versus low-stress subduction zones. High-stress subduction zones are likely to be associated with high aseismic/seismic slip ratios possibly related to slab roughness.
The distribution of different stress states in the subducting slab indicated by centroid moment tensor solutions for intra-slab earthquakes can help constrain the rheological properties of the slabs. ...A comparison of slabs in the western and eastern Pacific realms shows contrasting patterns in the stress states down to depths of ~ 350 km. The majority of slabs in the western Pacific show a pair of down-dip compression (DC) and down-dip tension (DT) domains in the upper and lower parts of the slab reflecting the effects of the slab unbending during progressive subduction. In contrast, slabs in the eastern Pacific show predominantly in-plane DT stress irrespective of slab geometry. Two-dimensional numerical simulations assuming constant slab thickness and viscosity indicate that the development of slabs with in-plane DT stress at depths of 100–300 km requires the slabs to be thin and have a low viscosity (10
23
Pa s). Weak slabs bend easily and tend to fold when they encounter increased resistance to downward movement at the 660-km boundary. The associated DC stresses are not transmitted up the slab so negative buoyancy of the slab and DT stress dominates at intermediate depths for this type of slab. Most experimentally derived rheological parameters predict a high viscosity (> 10
24
Pa s) for such slabs. However, two-dimensional numerical simulations using temperature- and pressure-dependent viscosity show that a relatively low activation energy (~ 110 kJ/mol) for diffusion creep is a possible explanation for the observed distribution of stresses in the slabs. Such low activation energies are compatible with recent experimental work on diffusion creep of polyphase mantle materials in which a low effective activation energy for creep results from a slow grain growth due to pinning effect of the secondary phase. The simulations provide a mechanical explanation for the observed dominantly DT stress state at 100–300 km depths for young slabs and paired DT and DC stress states at the same depth range for old slabs.
Graphical Abstract
The basement geology of Japan from A to Z Wallis, Simon R.; Yamaoka, Ken; Mori, Hiroshi ...
The island arc,
January/December 2020, 2020-01-00, 20200101, Letnik:
29, Številka:
1
Journal Article
Recenzirano
Odprti dostop
The Precambrian and lower Paleozoic units of the Japanese basement such as the Hida Oki and South Kitakami terranes have geological affinities with the eastern Asia continent and particularly strong ...correlation with units of the South China block. There are also indications from units such as the Hitachi metamorphics of the Abukuma terrane and blocks in the Maizuru terrane that some material may have been derived from the North China block. In addition to magmatism, the Japanese region has seen substantial growth due to tectonic accretion. The accreted units dominantly consist of mudstone and sandstone derived from the continental margin with lesser amounts of basaltic rocks associated with siliceous deep ocean sediments and local limestone. Two main phases of accretionary activity and related metamorphism are recorded in the Jurassic Mino–Tanba–Ashio, Chichibu, and North Kitakami terranes and in the Cretaceous to Neogene Shimanto and Sanbagawa terranes. Other accreted material includes ophiolitic sequences, e.g. the Yakuno ophiolite of the Maizuru terrane, the Oeyama ophiolite of the Sangun terrane, and the Hayachine–Miyamori ophiolite of the South Kitakami terrane, and limestone‐capped ocean plateaus such as the Akiyoshi terrane. The ophiolitic units are likely derived from arc and back‐arc basin settings. There has been no continental collision in Japan, meaning the oceanic subduction record is more complete than in convergent orogens seen in intracontinental settings making this a good place to study the geological record of accretion. Hokkaido lacks most of the Paleozoic history recognized in Honshu, Shikoku, Kyushu, and the Ryukyu Islands to the south and its geology reflects the Cenozoic development of two convergent domains with volcanic arcs, their approach, and eventual collision. The Hidaka terrane reveals a cross section through a volcanic arc and the main accretionary complex of the convergent system is represented by the Sorachi–Yezo terrane.
We present an up to date summary of the basement geological terranes of Japan including a new revised summary map showing terrane distribution and bathymetry of the surrounding ocean areas.
Application of Raman spectroscopy of carbonaceous material thermometry to samples of metasedimentary rock from the low‐grade Sanbagawa belt in the central Kii peninsula reveals a progressive decrease ...in temperature from ~390 °C close to the northern boundary, a major continental shear zone—the median tectonic line (MTL)—to ~270 °C and ~7 km to the south and roughly constant temperature distribution thereafter. Within the Sanbagawa belt, the thermal structure is not significantly modified by slip‐on fault boundaries between different geological units or folding. Meso‐ and microstructural observations combined with strain analysis using detrital grains in meta‐mudstone indicate a similar deformation history throughout the area and no correlation between ductile strain and temperature gradients. These observations suggest the observed thermal structure was developed after the main stages of ductile deformation of the Sanbagawa belt were complete and are not due to localized preferential exhumation along with the MTL. The observations also require a heat source along with the MTL. Order of magnitude estimates suggest the influx of warm fluid along the MTL are viable causes of the observed thermal anomaly. Although shear heating would be another possible explanation, thermal calculations require anomalous fast slip rates along the MTL and much greater frictional strength than generally considered reasonable. For these reasons, fluid infiltration is our preferred model.
Hydration/dehydration of mantle peridotite significantly influences the mechanical properties at subduction zone interfaces. In particular, brucite is mechanically weak, antigorite + brucite occurs ...in shallow mantle wedges, and dehydration of this assemblage to form olivine is often observed. However, the bulk-rock strength of brucite-bearing serpentinite is poorly constrained due to the difficulty of crystallographic preferred orientation (CPO) measurements of brucite, and the dehydration of brucite + antigorite has not been investigated experimentally. Within the hydrated mantle in subduction zones, brucite disappears at lower temperatures than antigorite. Therefore, previous studies using natural brucite-bearing serpentinites have been limited despite the potential importance of brucite in subduction zones. Here, we succeeded in conducting electron backscatter diffraction (EBSD) analyses of brucite within mantle wedge-derived serpentinite samples (Shiraga body) from the subduction-type Sanbagawa metamorphic belt and, for the first time, conducted experiments on the dehydration of antigorite + brucite using natural samples of brucite-bearing serpentinite subjected to experimental conditions of 500 °C and 0.9 GPa using a Griggs-type apparatus. By directly comparing the texture using scanning electron microscopy (SEM)-EBSD before and after the dehydration reaction, we reveal that grains of metamorphic olivine were nucleated within existing brucite grains and show the importance of olivine topotactic growth from brucite. As a similar texture is observed in naturally formed metamorphic olivine in the Shiraga body, the occurrence of pre-existing brucite and its topotaxy with olivine when it reacts with antigorite to form olivine may be an important feature of the development of olivine CPO within the mantle wedge corner.
Talc is widely distributed over the Earth's surface and is predicted to be formed in various tectonic settings. Talc is a very soft and anisotropic sheet silicate showing very low friction behavior. ...Therefore, the formation of talc is expected to weaken the strength of talc-bearing rocks and may be associated with the initiation of subduction, and with a decrease in the coupling coefficient resulting in aseismic movements along faults and shear zones within subduction zones. For these reasons, understanding the crystallographic preferred orientation (CPO) of talc is important to quantify the anisotropy and physical properties of the host rock. However, it is difficult to measure a significant number of talc crystal orientations and to evaluate the accuracy of the measurements using electron-backscattered diffraction (EBSD). Therefore, talc CPO has not been reported, and there is uncertainty regarding the estimation of the strength of deformed talc-bearing rocks. Using methods developed for antigorite, we report the first successful EBSD measurements of talc CPO from a talc schist formed due to Si-metasomatism of ultramafic rocks by subduction zone fluids. We used a combination of W-SEM and FE-SEM measurements to examine domains of various grain sizes of talc. In addition, we used TEM measurements to evaluate the accuracy of the EBSD measurements and discuss the results of talc CPO analysis. Talc CPO in the present study shows a strong concentration of the pole to the (001) plane normal to the foliation. The strongest concentration of the 100 direction is parallel to the lineation. The talc schist produces similar S-wave splitting and P- and S-wave anisotropy as antigorite schist in deeper domains, thus identifying talc-rich layers in subduction zones may require a combination of geophysical surveys, seismic observations, and anisotropy modeling. The presence of strong talc CPO in rocks comprising the slab-mantle interface boundary may promote spatial expansion of the slip area during earthquakes along the base of the mantle wedge.
Igneous rocks associated with the Cretaceous to Paleogene volcanic arc in SW Japan show ages that young from west to east in a direction parallel to the Median Tectonic Line suggesting corresponding ...translation of a heat source traditionally interpreted in terms of oblique subduction of a spreading ridge. However, recent oceanic plate reconstructions suggest ridge subduction may be younger than the main arc activity. Age compilations of 1227 points of felsic to intermediate Cretaceous and Cenozoic igneous rocks from the Japan arc show arc magmatism that can be separated into an early active period 130–60 Ma (stage 1), a subsequent period of quiescence 60–46 Ma (stage 2), which is followed by a resumption of igneous activity from 46 Ma onward (stage 3). In southwest Japan, the orientations of the magmatic arcs of stages 1 and 3 show and angular discordance of about 20°. The lack of active arc magmatism and the occurrence patterns of adakitic and high-Mg andesitic magmas indicate that ridge subduction occurred during stage 2. The arc age distribution pattern of stage 1 is explained by the slab shallowing related to a younging of the subducting slab as the ridge approaches. Furthermore, the obliquity of the arcs formed at stages 1 and 3 is explained by a 20° clockwise rotation of the inner zone of southwest Japan during the ridge-subduction phase. Oceanic plate reconstructions show counterclockwise rotation in the subduction direction after the ridge subduction phase, and coupling of the subducting oceanic plate with the upper plate would support microplate rotation in the inner zone. The new proposed tectonic reconstructions provide a framework to related Paleogene subduction of an active spreading ridge along the east Asia margin not only to the distribution of granitic bodies but also to rift-related basin formation on the eastern margin of the Eurasian continent and to rotation of crustal blocks indicated by paleomagnetic data of Cretaceous terranes.
The separation of melt from crystals is a fundamental process driving the chemical differentiation of magmas and can lead to the formation of pockets of potentially eruptible magmas in highly ...crystallised magma reservoirs. While geochemical and geophysical evidence exists for the presence of such isolated pockets of eruptible melt, the processes that control their volume and spatial arrangement remain unclear. The Muroto sill in Japan provides an excellent opportunity to study these processes as it is perfectly exposed and shows clear evidence for melt segregation. We collected geochemical and structural data across the sill and performed thermal modelling to quantify extraction timescales and to constrain the range of crystallinity at which melt extraction occurred. Our data and calculations show that the middle–lower portion of the sill experienced melt extraction at crystal fractions between 0.65 and 0.8 over 100–150 years, until magma was too crystalline for further segregation to occur. We propose a new approach that can be used to invert measured geochemical profiles and identify the range of crystallinity at which melt extraction takes place. With this approach, the results we obtain for the Muroto sill can be generalised to magma reservoirs of different sizes and chemistries. Our calculations, and the comparison with natural magmatic systems, show that the volume of melt-rich pockets in a magma reservoir is proportional to the reservoir volume, while their spatial arrangement depends on the physiochemical properties of magmas. The results of this study increase our understanding of the factors controlling the distribution and volume of pockets of eruptible magmas in large magma reservoirs. Our calculations show that eruptible magma in dacitic and rhyolitic magma reservoirs, which are responsible for some of the largest eruptions on Earth, tend to be distributed in lenses of small volume within highly crystallised magma. Such architecture diminishes our capacity of identifying eruptible magma in large magma reservoirs such as Yellowstone using geophysical methods, and jeopardises our capacity of assessing the potential of a reservoir to feed a large eruption.
Active faulting in southern Tibet consists of N–S trending extensional faults and linked strike-slip faults, which are an expression of regional E–W extension. A second type of extensional ...deformation associated with N–S movement is also recognized. This extension is expressed as a series of shear zones and normal faults in the High Himalayas – the Southern Tibetan Detachment System – and mid-crustal rocks exposed in metamorphic domes. Reported constraints on the timing of movements associated with these two phases of extension indicate that N–S extension predates the onset of E–W extension. However, only a few studies have provided clear constraints on the timing of E–W extension and the extent to which the two kinematically distinct domains of extension were contemporaneous is unclear.
The Kung Co fault in southern Tibet is a major N–S trending normal fault. The associated E–W extension is locally expressed as high-strain ductile deformation. Both field and microstructural observations show that this deformation occurred synchronously with granite intrusion. Previously reported U–Pb zircon dating shows granite crystallization took place at around 19Ma, implying that ductile E–W extension in the Kung Co area was also active at around 19Ma. This is the oldest documented example of E–W extension in Tibet and shows that E–W extension was at least locally contemporaneous with N–S extension to the south at shallower crustal levels. Simultaneous mid-crustal N–S extension and upper crustal E–W extension may be explained by southward flow of Tibetan crust with a divergent radial component.
►We document E–W ductile extension in the Kung Co area, southern Tibet. ►E–W deformation and granite intrusion at around 19Ma were in part synchronous. ►E–W extension in southern Tibet was active at 19Ma at the same time as N–S extension. ►Radial south-directed flow of the Tibetan crust can explain both types of extension.
New U/Pb zircon and Th/Pb monazite ages are presented from the giant Sulu ultrahigh‐pressure (UHP) terrane. Combined with Sm/Nd ages, Rb/Sr ages, inclusion relationships, and geologic relationships, ...they help define the timing of peak recrystallization, the timing of subsequent amphibolite‐facies metamorphism, and the architecture of the Dabie‐Sulu suture zone between the collided Sino‐Korean and Yangtze cratons. The data indicate a ∼15 Myr record of UHP recrystallization, the first clearly documented for a giant UHP terrane; this requires that continental subduction in the Dabie‐Sulu orogen involved multiple UHP tectonic or recrystallization events. A 244–236 Ma “precursor” UHP event, seen only in the Dabie Shan, was followed by a second, ∼230–220 Ma “main” UHP event, which was itself terminated by a 220–205 Ma amphibolite‐facies overprint. Older eclogite‐facies events seen in the Qinling segment of this orogenic belt raise the possibility that these rocks have undergone (U)HP metamorphism three or four times, but at present, there is no geochronological evidence in the Dabie‐Sulu area to support this. The subduction of the lower, Yangtze plate did not proceed in a simple fashion: The ages of inherited zircon cores demonstrate that a ribbon continent of Yangtze affinity escaped subduction and became wedged against the Sino‐Korean plate hanging wall.
