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Maximum shear stress along an active deformation zone marking the subduction plate interface is important for understanding earthquake phenomena and is an important input parameter in subduction zone thermomechanical modeling. However, such maximum shear stress is difficult to measure directly at depths more than a few kilometers and is generally estimated by simulation using a range of input parameters with large associated uncertainties. In addition, estimated values generally represent maximum shear stress conditions over short observation timescales, which may not be directly applicable to long-timescale subduction zone modeling. Rocks originally located deep in subduction zones can record information about deformation processes, including maximum shear stress conditions, occurring in regions that cannot be directly accessed. The estimated maximum shear stress is likely to be representative of maximum shear stress experienced over geological timescales and be suitable to use in subduction zone modeling over timescales of millions to tens of millions of years. In this study, we estimated maximum shear stress along a subduction plate interface by using samples from the Sanbagawa metamorphic belt of southwestern (SW) Japan, in which slivers of mantle-wedge-derived serpentinite are widely distributed and in direct contact with metasedimentary rocks derived from the subducted oceanic plate. These areas can be related to the zone of active deformation along the subduction plate interface. To obtain estimates of maximum shear stress at the subduction interface, we focused on the microstructure of quartz-rich metamorphic rocks – quartz is the main component of the rocks we collected and its deformation stress is assumed to be roughly representative of the stress experienced by the surrounding rock and plate interface deformation zone. Maximum shear stress was calculated by applying deformation temperatures estimated by the crystallographic orientation of quartz (the quartz c -axis fabric opening-angle thermometer) and the apparent grain size of dynamically recrystallized quartz in a thin section to an appropriate piezometer. Combined with information on sample deformation depth, estimated from the P – T (pressure–temperature) path and deformation temperatures, it is suggested that there was nearly constant maximum shear stress of 15–41 MPa in the depth range of about 15–30 km, assuming plane stress conditions even when uncertainties related to the measurement direction of thin section and piezometer differences are included. The Sanbagawa belt formed in a warm subduction zone. Deep slow earthquakes are commonly observed in modern-day warm subduction zones such as SW Japan, which has a similar thermal structure to the Sanbagawa belt. In addition, deep slow earthquakes are commonly observed to be concentrated in a domain under the shallow part of the mantle wedge. Samples showed the depth conditions near the mantle wedge, suggesting that these samples were formed in a region with features similar to the deep slow earthquake domain. Estimated maximum shear stress may not only be useful for long-timescale subduction zone modeling but also represent the initial conditions from which slow earthquakes in the same domain nucleated. [ABSTRACT FROM AUTHOR]

A compilation of the thicknesses of contact metamorphic aureoles (CMAs) developed around intermediate to felsic plutons shows many CMAs are far broader than expected by commonly used thermal models for pluton emplacement. Shortfalls in the amount of heat potentially provided based on pluton size, compared to that needed to form the observed CMA, can be accounted for if some hot magma has been lost by volcanic eruption after passing through the pluton domain and replaced by new hot magma. A high ambient temperature may also contribute to broad CMA formation. However, the presence of coeval pairs of both narrow and broad CMAs in the same area requires contrasting types of pluton growth history. Our thermal modeling, constrained by the peak metamorphic temperature, shows the broad CMA of a well-developed pair of CMAs in the Hongusan area of Japan is due to a magmatic history, including magma tapping and replenishment. A global compilation of CMAs suggests more than 30% of plutons are associated with broad CMAs and fed contemporaneous volcanic eruption. [ABSTRACT FROM AUTHOR]

Absolute maximum shear stress ("shear stress" in this study) along the subduction plate interface is important for understanding earthquake phenomena and is an important input parameter in subduction zone thermomechanical modelling. However, such shear stress is difficult to measure directly at depths more than a few kilometers and is generally estimated by simulation using a range of input parameters with large associated uncertainties. In addition, estimated values generally represent shear stress conditions over short observation timescales, which may not be directly applicable to long-timescale subduction zone modelling. Rocks originally located deep in subduction zones can record information about deformation processes, such as shear stress conditions, occurring in regions that cannot be directly accessed. The estimated shear stress is likely to be representative of shear stress experienced over geological timescales and be suitable to use in subduction zone modelling over time scales of millions to tens of millions of years. In this study, we estimated shear stress along a subduction plate interface by using samples from the Sanbagawa metamorphic belt of Southwest (SW) Japan, in which slivers of mantle wedge-derived serpentinite are widely distributed and in direct contact with metasedimentary rocks derived from the subducted oceanic plate. These areas can be related to the ancient subduction plate interface. To obtain estimates of shear stress at the subduction interface, we focused on the microstructure of quartz-rich metamorphic rocks--quartz is the main component of the rocks we collected and its deformation stress is assumed to be representative of the region. Shear stress was calculated by applying deformation temperatures estimated by the crystallographic orientation of quartz (the quartz c-axis fabric opening-angle thermometer), and the apparent grain size of dynamically recrystallized quartz in a thin section to an appropriate piezometer. Combined with information on sample deformation depth, estimated from P-T path and deformation temperatures, it is suggested that there was nearly constant shear stress of 16-41 MPa in the depth range 17-27 km, assuming plane stress conditions even when uncertainties related to measurement direction of thin section and piezometer differences are included. The Sanbagawa belt formed in a warm subduction zone. Deep slow earthquakes are commonly observed in modern-day warm subduction zones such as SW Japan, which has a similar thermal structure to the Sanbagawa belt. In addition, deep slow earthquakes are commonly observed to be concentrated in a domain under the shallow part of the mantle wedge. Samples showed the depth conditions near the mantle wedge, suggesting that these samples were formed in a region with features similar to the deep slow earthquakes domain. Estimated shear stress may not only be useful to long-timescale subduction zone modelling but also represent the initial conditions from which slow earthquakes in the same domain nucleated. [ABSTRACT FROM AUTHOR]

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., Author(s): David Floess [sup.1] , Luca Caricchi [sup.1] , Guy Simpson [sup.1] , Simon R. Wallis [sup.2] Author Affiliations: (Aff1) 0000 0001 2322 4988, grid.8591.5, Department of Earth Sciences, University [...]

A change in the linear pressure behavior of the chrysotile Raman O-H banal is revealed by an insitu high-pressure Raman study using a diamond anvil cell (DAC) at 0.1-0.4 GPa pressure intervals. The peak of 3701 [cm.sup.-1] can be accurately determined in the pressure range of 0.2-4.5 GPa regardless of peak fitting models. The pressure (P)-wavenumber (v) relationship for the peak is closely approximated by two linear functions with the slopes (dv/dP) of 4.3 [cm.sup.-1]/GPa and 1.7 [cm.sup.1]/GPa at pressures above and below 1.7 GPa, respectively. The spectral resolution given by a peak fitting method (0.5 [cm.sup.-1]) implies that these functions for changes in the positions of the 3701 [cm.sup.-1] peak provide pressure estimates with resolutions of 0.1-0.2 GPa. This method can be used to estimate remnant pressure in natural samples in the form of thin sections. The pressure shift characterized by the change in slope and the associated decrease of the peak width can be explained by a model where the change in compressibility of the tetrahedral layer affects the interaction between the inner O-H and Si atoms forming a six-membered ring. The high-pressure dependence at pressures lower than 1.7 GPa may be a contribution of a dominant layer-parallel (in-plane) compression that compensates a distortion in the tetrahedral layer of chrysotfle. The conclusion that chrysotile changes its compressibility at around 1.7 GPa is significant for understanding of the properties of chrysotile nano-fibers and possibly for thermodynamic consideration on serpentine phase relations. Keywords: High-pressure Raman spectroscopy, chrysotile, pressure scale, O-H bond, compressional behavior of sheet silicate

[1] 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. Citation: Hacker, B. R., S. R. Wallis, L. Ratschbacher, M. Grove, and G. Gehrels (2006), High-temperature geochronology constraints on the tectonic history and architecture of the ultrahigh-pressure Dabie-Sulu Orogen, Tectonics, 25, TC5006, doi: 10.1029/2005TC001937.

[1] The Higashi-akaishi peridotite body in the Sanbagawa metamorphic belt is a unique example of a kilometer-scale body including garnet-lherzolite facies rocks from an oceanic subduction-type orogen. In this body, four distinct deformational phases [D.sub.1], [D.sub.2], [D.sub.3], and [D.sub.4] can be defined. The tectonic significance of [D.sub.1] is unclear. Microstructural observations and application of garnet-orthopyroxene geothermobarometry suggest that [D.sub.2] took place during high-temperature subduction up to depths of ~ 100 km (from 2.3 GPa to above 2.8 GPa at temperature of 700800[degrees]C) or more. [D.sub.3] represents a major phase of exhumation. After exhumation the Higashi-akaishi body became juxtaposed with the adjacent Besshi and Eclogite units at a depth of around 35 km (1 GPa). The common history of the two units after [D.sub.3] is shown by the intertectonic growth of tremolite between [D.sub.3] and [D.sub.4] that corresponds to a stage of intertectonic growth of plagioclase in the Besshi and Eclogite units. Both the spatial distribution and the estimated pressure-temperature (P-T) conditions of this intertectonic phase of mineral growth in the different units are in excellent agreement. The development of an antigorite schistosity during the late stage of [D.sub.2] indicates an anticlockwise P-T path with cooling at depths of around 100 km. The association of the antigorite-bearing fabric and the onset of exhumation after the [D.sub.2] stage suggest that the density decrease due to antigorite formation may have been sufficient to trigger the exhumation of dense ultramafic body from UHP conditions in subduction zones. Syn-[D.sub.2] water introduction and the anticlockwise P-T path are expected when convection in the mantle wedge brings peridotite toward a subduction boundary through mantle wedge or when peridotite is dragged down along a subduction boundary that is progressively cooling immediately after initiation of subduction. Citation: Mizukami, T., and S. R. Wallis (2005), Structural and petrological constraints on the tectonic evolution of the garnet-lherzolite facies Higashi-akaishi peridotite body, Sanbagawa belt, SW Japan, Tectonics, 24, TC6012, doi:10.1029/2004TC001733.

We combine zircon sensitive high-resolution ion microprobe U-Pb spot dating and mica [sup.40][Ar-.sup.39]Ar plateau ages with field-geological and geochemical constraints from the Malashan area of Southern Tibet to show that the deformed granite core of the North Himalayan metamorphic domes in this area is not Indian basement, but was intruded and deformed during the Himalayan orogeny. Microstructural observations reveal that a transition from top-to-the-south thrust-related to top-to-the-north extension-related deformation occurred during granite intrusion and related metamorphism. This suggests that intrusion triggered the onset of extensional tectonics in the Tibetan middle to upper crust. Expected positive feedback mechanisms between decompression melting leading to more intrusion and more extensional deformation suggest that this mechanism may have been important on a regional scale. Keywords: north Himalayan gneiss domes, granite, age dating, extensional tectonics.

Author(s): Tomoyuki Mizukami (corresponding author) [1, 2]; Simon R. Wallis [2]; Junji Yamamoto [3, 4] Tectonic plate motion is thought to cause solid-state plastic flow within the underlying upper mantle [...]

Petrologic studies of the Kotsu eclogite from the Sambagawa high-pressure (P), low-temperature (T) metamorphic belt show that the eclogite has recorded a very steep subduction-stage P-T path reaching 20 kbar and 600 [degrees]C; the dP/dT is >7.1 kbar/100 [degrees]C. Compilation of previously derived subduction-stage P-T paths from the Sambagawa belt and the result of this study show that these P-T paths are not on a straight line from the origin, but define a curve with dP/dT increasing with pressure. A new model incorporating the progressive approach of an oceanic ridge to a subduction zone shows that the Sambagawa P-T paths are best explained by a setting in which a ridge is close to being subducted at a slow rate relative to the plate-subduction rate. Exhumation of high-P, low-T metamorphic rocks in oceanic subduction zones may be associated with the slow approach and subsequent subduction of oceanic ridges. Keywords: pressure-temperature path, subduction, oceanic ridge, modeling, eclogite.

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. [ABSTRACT FROM AUTHOR]

In their recent paper 'Metamorphic olivine after dehydration embrittlement in Serpentinite: Case study from the Shiraga Serpentinite mass in the Sanbagawa high P/T metamorphic belt, central Shikoku, Japan', Fukumura, Okamoto, and Terabayashi () present information on an important relic of the mantle wedge located in the Sanbagawa belt of central Shikoku. The presence of talc and olivine in the Shiragayama body reported by Fukumura et al. () is in agreement with previous studies (e.g. Kawahara et al.,). MINERAL ASSEMBLAGES The authors report orthopyroxene (opx) in 10 of their 21 samples (table 1 in Fukumura et al.,) but provide no chemical analyses or photomicrographs to support the claim. However, such selection criteria are not mentioned in Fukumura et al. () and a comparison of the patterns of sampling locations does not suggest radically different sampling strategies in the two studies of Fukumura et al. () and Kawahara et al. (). [Extracted from the article]