Although information regarding the initiation processes of debris flows is important for the development of mitigation measures,field data regarding these processes are scarce.We conducted field observations of debris...Although information regarding the initiation processes of debris flows is important for the development of mitigation measures,field data regarding these processes are scarce.We conducted field observations of debris-flow initiation processes in the upper Ichinosawa catchment of the Ohya landslide,central Japan.On 19 June 2012,our videocamera monitoring systems recorded the moment of debris-flow initiation on channel deposits(nine surges) and talus slopes(eight surges).The initiation mechanisms of these surges were classified into three types by analyzing the video images: erosion by the surface flow,movement of deposits as a mass,and upward development of the fluid area.The first type was associated with the progress of surface flow from the upper stream on unsaturated channel deposits.The second type was likely caused by an increase in the pore water pressure associated with the rising in the groundwater level in channel deposits;a continuous water supply from the upper stream by the surface flow might have induced this saturation.The third type was associated with changes in the downstream topography caused by erosion.The flow velocity of most surges was less than 3 m s^(-1) and they usually stopped within 100 m from the initiation point.Surges with abundant pore fluid had a higher flow velocity(about 3- 5 m s^(-1)) and could travel for alonger duration.Our observations indicate that the surface flow plays an important role in the initiation of debris flows on channel deposits and talus slopes.展开更多
The Oman ophiolite is regarded as best proxy for accreted oceanic crust from typical fast-spreading ridge systems on land. However, the Oman ophiolite is influenced by initial subduction zone initiation, and the natur...The Oman ophiolite is regarded as best proxy for accreted oceanic crust from typical fast-spreading ridge systems on land. However, the Oman ophiolite is influenced by initial subduction zone initiation, and the nature of the details of the subduction zone setting is still under controversial debate. While a first magmatic phase shows features of magmatic accretion very similar to those known from the East Pacific Rise, except that the primary melts were slightly water-enriched, a second type of magmatism is characterized by an apparent subductionzone related imprint, producing rocks like FAB basalts and boninites in the upper crust, as well as cross-cutting gabbronorites and wehrlites in the deeper crust. In this paper, we apply diverse experimental studies in wet tholeiitic and peridotitic systems performed at lower pressures(100 to 500 MPa) in the experimental lab of the University Hannover, in order to constrain the details of the magmatic processes proceeded at the Oman ophiolite paleoridge during the Cretaceous, with special focus on the influence of water on the phase stabilities and phase relations. The experiments were performed in vertically oriented internally heated pressure vessels(IHPV)(see Berndt et al., 2002;Fig. 1). This facility uses as pressure medium mixtures of Ar and H2 in order to adjust the required fH2 in the vessel, enabling us to control the redox conditions. The fH2 prevailing in the IHPV at high P and T was measured with a Shaw-membrane made of platinum. The overall variation in fO2 in all experimental series was in the range between ~FMQ-1 and ~FMQ+3.2, thus covering the range of oxygen fugacities prevailing in natural MORB magmas(Bezos and Humler, 2005). For understanding the magmatic processes during the Oman ophiolite paleoridge accretion, transects through the lower(GT1) and middle(GT2) crust have been drilled in the frame of ICDP(International Continental Scientific Drilling Program). Drill sites have been selected in the Wadi Tayin massif, which is known that the influence of magmatic phase 2 characterized by subduction-related primary melts is minimal. Details and progress obtained in the Oman Drilling Project(OmanDP) can be found here:(https://www.omandrilling.ac.uk/). Regarding the first magmatic phase of the processes at the Oman ophiolite paleoridge, a characteristic observation made during the description of the drilled cores GT1(lower crust) and GT2(midcurst) was that quite often layers in the layered gabbro series occur which show the presence of clinopyroxene joining olivine instead of plagioclase(under near liquidus conditions). In terms of lithologies this could be interpreted as presence of wehrlitic crystal mushes as early cumulates instead of troctolitic, which are the typical ones for primary magmatism at typical fastspreading ridges. This situation could be experimentally simulated by adding a moderate to high water activity to primitive MORB at pressures ≥ 200 MPa, resulting in a shift of the clinopyroxene-in curve to higher temperatures above the plagioclase-in curve(Feig et al. 2006;see Fig. 2). Regarding the second, late-stage magmatic phase, the formation of typical Oman high-Ca-boninites could be experimentally simulated by water-saturated partial melting of Oman harzburgite at 200 MPa and relatively low temperatures between 1100 and 1200℃. Depleted gabbronorites crosscutting layered gabbros of phase 1 magmatism can be regarded as cumulates formed in these boninitic melts. Late wehrlites crosscutting layered gabbro could be produced by accumulation of olivine and clinopyroxene at temperatures between 1040 and 1080℃ in a hydrous gabbroic system at pressures > 100 MPa with bulk water content of 2–3 wt%.展开更多
基金supported by the Grant-in-Aid for Scientific Research of Japan Society for the Promotion of Science (JSPS KAKENHI) (Grant Nos.80378918,26292077)
文摘Although information regarding the initiation processes of debris flows is important for the development of mitigation measures,field data regarding these processes are scarce.We conducted field observations of debris-flow initiation processes in the upper Ichinosawa catchment of the Ohya landslide,central Japan.On 19 June 2012,our videocamera monitoring systems recorded the moment of debris-flow initiation on channel deposits(nine surges) and talus slopes(eight surges).The initiation mechanisms of these surges were classified into three types by analyzing the video images: erosion by the surface flow,movement of deposits as a mass,and upward development of the fluid area.The first type was associated with the progress of surface flow from the upper stream on unsaturated channel deposits.The second type was likely caused by an increase in the pore water pressure associated with the rising in the groundwater level in channel deposits;a continuous water supply from the upper stream by the surface flow might have induced this saturation.The third type was associated with changes in the downstream topography caused by erosion.The flow velocity of most surges was less than 3 m s^(-1) and they usually stopped within 100 m from the initiation point.Surges with abundant pore fluid had a higher flow velocity(about 3- 5 m s^(-1)) and could travel for alonger duration.Our observations indicate that the surface flow plays an important role in the initiation of debris flows on channel deposits and talus slopes.
基金granted by the Deutsche Forschungsgemeinschaft DFG
文摘The Oman ophiolite is regarded as best proxy for accreted oceanic crust from typical fast-spreading ridge systems on land. However, the Oman ophiolite is influenced by initial subduction zone initiation, and the nature of the details of the subduction zone setting is still under controversial debate. While a first magmatic phase shows features of magmatic accretion very similar to those known from the East Pacific Rise, except that the primary melts were slightly water-enriched, a second type of magmatism is characterized by an apparent subductionzone related imprint, producing rocks like FAB basalts and boninites in the upper crust, as well as cross-cutting gabbronorites and wehrlites in the deeper crust. In this paper, we apply diverse experimental studies in wet tholeiitic and peridotitic systems performed at lower pressures(100 to 500 MPa) in the experimental lab of the University Hannover, in order to constrain the details of the magmatic processes proceeded at the Oman ophiolite paleoridge during the Cretaceous, with special focus on the influence of water on the phase stabilities and phase relations. The experiments were performed in vertically oriented internally heated pressure vessels(IHPV)(see Berndt et al., 2002;Fig. 1). This facility uses as pressure medium mixtures of Ar and H2 in order to adjust the required fH2 in the vessel, enabling us to control the redox conditions. The fH2 prevailing in the IHPV at high P and T was measured with a Shaw-membrane made of platinum. The overall variation in fO2 in all experimental series was in the range between ~FMQ-1 and ~FMQ+3.2, thus covering the range of oxygen fugacities prevailing in natural MORB magmas(Bezos and Humler, 2005). For understanding the magmatic processes during the Oman ophiolite paleoridge accretion, transects through the lower(GT1) and middle(GT2) crust have been drilled in the frame of ICDP(International Continental Scientific Drilling Program). Drill sites have been selected in the Wadi Tayin massif, which is known that the influence of magmatic phase 2 characterized by subduction-related primary melts is minimal. Details and progress obtained in the Oman Drilling Project(OmanDP) can be found here:(https://www.omandrilling.ac.uk/). Regarding the first magmatic phase of the processes at the Oman ophiolite paleoridge, a characteristic observation made during the description of the drilled cores GT1(lower crust) and GT2(midcurst) was that quite often layers in the layered gabbro series occur which show the presence of clinopyroxene joining olivine instead of plagioclase(under near liquidus conditions). In terms of lithologies this could be interpreted as presence of wehrlitic crystal mushes as early cumulates instead of troctolitic, which are the typical ones for primary magmatism at typical fastspreading ridges. This situation could be experimentally simulated by adding a moderate to high water activity to primitive MORB at pressures ≥ 200 MPa, resulting in a shift of the clinopyroxene-in curve to higher temperatures above the plagioclase-in curve(Feig et al. 2006;see Fig. 2). Regarding the second, late-stage magmatic phase, the formation of typical Oman high-Ca-boninites could be experimentally simulated by water-saturated partial melting of Oman harzburgite at 200 MPa and relatively low temperatures between 1100 and 1200℃. Depleted gabbronorites crosscutting layered gabbros of phase 1 magmatism can be regarded as cumulates formed in these boninitic melts. Late wehrlites crosscutting layered gabbro could be produced by accumulation of olivine and clinopyroxene at temperatures between 1040 and 1080℃ in a hydrous gabbroic system at pressures > 100 MPa with bulk water content of 2–3 wt%.
