Investigation On The Rheology And Anisotropy Of Himalayan Amphibolites

Understanding the rheology of continental mid-lower crust is critical for investigations pertaining to crustal structure,composition evolution,water budget,the crust and mantle coupling,the focal mechanism of deep earthquakes,and magmatism.Amphibole is one of the dominant constituent minerals in the water saturated middle-lower crust,whose rheology dominates the structure and evolution of the continental mid-lower crust.There are few studies on amphibole rheology,which largely limits our understanding of the deformation and evolution of amphibole in the deep crust and related geodynamic processes.In order to explore the deformation mechanism of amphibolite,the rheological strength and its anisotropy,this dissertation aims to decipher the strength,deformation mechanism and anisotropy of amphibole in continental crust using amphibolite samples from the Namcha Barwa region of the eastern Himalayan tectonic syntaxis.We have studied the deformation in natural samples,conducted experimental investigations at high pressure and high temperature conditions,simulated fabric development with the VPSC methods,and compared amphibole fabric with magnetic susceptibility anisotropy.The new discoveries were summarized as follows.?1?We propose that there is a thick layer of amphibolite underneath the Lasha terrane of southern Tibet based on the comparison of seismic anisotropies of Himalayan amphibolites and geophysical observations.We conducted a systematic study on amphibolites from exposed deep crust in the eastern Himalayan tectonic syntaxis,including major element anlaysis,EBSD analysis,microstructure observation,and calculation of seismic anisotropy.The compositions of constituent minerals and their formation conditions were determined.Amphibole did not show obvious wavy extinction or twisted band and other plastic deformation characteristics in the crystal,while plagioclase can be seen mechanical twinning and wavy extinction.The dissolution in plagioclase indicate late stage transformation.The EBSD results show a strong fabric of amphibole with[001]forming a well-defined point maximum parallel to lineation and[100]and[010]forming girdles normal to the lineation,and a weak or near random fabric of quartz and plagioclase.The seismic anisotropies of amphibolites are AV_P=6.7-11.7%,AV_S=5.7-9.6%.The low GOS value of amphibole and lacking evidence of intracrystalline deformation or compositional banding,indicate that amphibole may not be dominated by intracrystalline plastic deformation,or dissolution-precipitation process.We propose that the amphiboles were deformed by cataclastic flow,except in TOO-38,which is deformed by pressure solution.The fastest P wave direction and fast S wave polarization of amphibolite are subparallel to lineation,with the anisotropy?AV_P?of 6.4%to 6.7%,and the maximum shear wave anisotropy?AV_S?of 5.67%to 9.63%.The amphibolite anisotropy are higher than basic granulite and deep crustal rocks such as eclogite.We propose that the amphibolite of Indian crust were subducted to the Tibetan plateau and squeezed to be vertical foliated.Comparing to a deep crust composed of nearly isotropic mafic granulite and weakly anisotropic eclogite,a thick amphibolitic deep crust would better account for the 0.19-0.53 s shear wave delay times,the strong Vp and Vs anisotropies and their polarization directions,as well as the widespread postcollisional adakite-like potassic rocks in south and central Tibet.Therefore,we propose that the Tibetan deep crust is likely composed of deformed amphibolite rather than granulite or eclogite.?2?The constitutive flow law of polycrystalline amphiboles was determined for the first time(?=10^-5.87?^5.03exp[-?280kJ/mol?/RT]).To investigate the rheological strength of hornblende in the middle and lower crust,we have conducted a systematic axial compression experiments on hornblende aggregates using the modified 5 GPa Griggs-type deformation apparatus.The experimental pressure,temperature and strain rate is 1.5 GPa,750-880?and 10^-4-10^-5/s,respectively.The flow law of hornblende is obtained.Rheological experiments showed that the deformation of amphibole was dominated by semi-brittle deformation in the high-pressure stable region,with partial dehydration process at elevated temperatures,which was characterized by the development of micro-cracks,dehydrated pores and cleavages,and thus led to strain weakening.In addition,dehydration process is accompanied by the dissolution and diffusion of elements,but decomposition and phase transition of amphibole was not found.The EBSD results show that pole to?100?form a girdle subnormal to foliation plane,and the rotation axis indicates that the sliding system of?100?[001]is also involved in the deformation process of amphibole.There results suggest that amphibole reached the plastic limit due to high deformation strain rate,and the facilitation of dehydration and fracture development.With the increase of temperature,the strength of amphibole successively exceeds that of wet anorthite and omphacite,and is obviously weaker than that of clinopyroxene,dry anorthite and olivine in the temperature range of lower than 900?.?3?The characteristic amphibole fabrics resulted from dislocation creep were elaborated with VPSC simulation.We simulated dislocation creep and resulting fabrics of amphibole aggregates using the VPSC program.Under the dislocation creep mechanism,the steady state deformation of amphibole needs at least five slip systems:?100?[001],?010?[001],{110}[001],{110}<110>,?-101?[-10-1].These five slip systems are the necessary conditions for the plastic deformation of amphibole.Under the axial compression deformation,amphibole show pole to?100?normal to foliation plane,while[010]and[001]axes forming girdle in foliation.Under the simple shear deformation,amphibole develop strong fabric similar to natural samples,with a preferred alignment of[001]axes along the lineation,whereas[100]axes subnormal to foliation plane.?4?The anisotropy of magnetic susceptibility of rocks is mainly controlled by paramagnetic amphibole minerals while its magnetism is mainly contributed by isotropic?titanium?magnetite.The bulk magnetic susceptibility varies between7.06×10^-3 and 33.1×10^-33 SI,with the Jelínek anisotropy values?P_j?ranging from 1.221to 1.463.The maximum susceptibility is approximately parallel to lineation and the minimum susceptibility is subnormal to foliation plane.We suggest that the magnetic fabrics of the studied amphibolites are largely controlled by the CPOs of amphibole.Amphibolite has high magnetic susceptibility,and its magnetism is mainly contributed by isotropic?titanium?magnetite.Combining the simulated AMS with the measured AMS,it shows that the anisotropy of magnetic susceptibility of rocks is mainly controlled by paramagnetic amphibole minerals.Amphibolite has stronger magnetism than ophiolite and granulite,indicating candidate for the source of strong magnetic negative anomaly in eastern Himalayan syntaxis.