Experimental Determination Of The Effect Of Carbonaceous Structure And Partial Melting On The Electrical Conductivity Of The Crustal Orogenic Belts

After subduction and collision between the Indian and Eurasian continents,the Tibetan plateau preformed N-S crustal shortening,thickening and E-W extension.The Longmenshan fault zones were formed on the surface in the local regions of the eastern of Tibetan plateau,while the Himalayan orogen were also grew in the southern of Tibetan plateau.Magnetotelluric data showed that high electrical anomalies were identified in the upper crust on these two regions,and they might be caused by carbonaceous structure and partial melting,respectivly.In order to probing the effects of them on the electrical conductivity of crustal orogenic belts,i.e.the Longmenshan fault zones and the Himalayan orogen,alternating current impedance spectroscopy was applied in laboratory electrical conductivity measurements of carbonaceous fault gouges and hydrated leucogranitic melts,so that we can explain the highly electrical conductive zones of crustal orogenic belts,analyze their compositions or structures and speculate the geodynamic model of crustal tectonics movements.Carbon is a crucial factor influencing rock electrical conductivity and its enrichment in the fault rocks might be one of the key mechanisms responsible for the anomalously-high electrical conductivity observed in the Longmenshan fault zone.To investigate the effects of the content,grain size and crystal structure of the carbon present in natural fault zones,in this study,electrical conductivity measurements have been performed on simulated fault gouges that were prepared from both synthetic?mixture of carbon and quartz?and natural fault rocks at room temperature and 0.2³00 MPa lithostatic pressure condition.The synthetic samples show a sharp increase in electrical conductivity when the volumetric fraction of carbon??_c?reaches a critical value.This observation is consistent with the prediction from the percolation theory.Our results also show that the grain size of less conductive component?quartz?can affect the electrical conductivity,but in the manners that are different between connected and unconnected samples.Microstructural analysis further revealed the presence of carbon films at the grain boundaries in natural samples.Furthermore,the natural samples have lower electrical conductivities?<9×10^-44 S/m?than the synthetic samples that have similar?_c-values.The measured values are also lower than those determined from the magnetotelluric survey in the study area?0.03⁰.1 S/m?.This discrepancy suggests the necessity to measure the natural samples under in-situ,dynamic conditions.Abundant granitic rocks in ancient mountain belts suggest that crustal melting plays a major role in orogenic processes.However,complex field relations and the frequent superposition of multiple tectonic/orogenic events impede the definition of the role of melting in the construction of mountains belts.Geophysical measurements,on the other hand,image present-day conditions beneath large mountain ranges,but cannot unambiguously discriminate geological scenarios involving melting.Here,we address this problem by measuring the electrical conductivities of hydrous?with 0-8 wt%H₂O?leucogranitic melts at high pressure?0.5-2 GPa?and temperature?750-1400°C?.The conductivity results were fitted using an Arrhenius equation and we developed an empirical a P-T-H₂O model predicting the conductivity of leucogranitic melts.We infer that sodium is the dominant charge carrier in the melt.Decreasing pressure and increasing water concentration can significantly enhance the electrical conductivity of the melt.Connecting pressure-temperature paths of Miocene crustal rocks from the Himalaya,the effective electrical conductivity of the partially molten metapelites,i.e.the two phase?solid rocks and melts?mixtures,were calculated by thermomechanical models,corresponded to the present-day conditions beneath the Tibetan plateau imaged with geophysical data.We showed that fluid-absent melting produces 4-16 vol%melts in the Greater Himalayan Sequence?GHS?required to explain the observed magnetotelluric data?0.03-0.2 S/m?in the middle of upper crust?15-25 km?of the north-western Himalaya,while the southern Tibet,more conductive?⁰.33 S/m?,requires melt fractions>30%reflecting a crust that is either fluid-enriched?+1%H₂O?or hotter?+100°C?compared to the Miocene crust.In addition,shallow regional?10-13 km?electrical anomalies?0.05-0.1 S/m?observed in MT campaigns of Himalaya are explained as the result of cooling and crystallizing leucogranitic magmas?melt fraction is 20±10 vol%?derived by diking and the slow amalgamation of small melt batches which removed from partial molten rocks in the middle of upper crust.The seismic and electrical anomalies are spatially coincident but contain different melt contents in the crust of Tibetan plateau.It suggested that the seismic and MT surveys are sensitive to structures with very different spatial resolutions.During India-Eurasia subduction and collision,fluid-absent and fluid-present melting has been operating in a closed system of the Himalaya for the past 20 Ma.The warming and softening of the Himalayan upper crust contribute to the long-lived weak regions production.With the extrusion of the GHS,exhumed partial melting weak metapelites transformed to migmatites under metamorphic reactions,while the granitic plutons emplaced beneath the South Tibetan Detachment and in the Tethyan Himalayan Sequence crystalized and solidified as High Himalayan leucogranites and Tethyan Himalayan leucogranites,respectively.In the same time,massive amounts of water involved in the partial melting process at the base the upper crust transferred by the melts as dykes and released by solidifying granitic bodies forming the plutonic rocks to contribute the numerous hot springs that feature the South Tibet.