Deep Origin And Genetic Mechanism Of Ground Fissures In The Kenya Rift Valley,East Africa

The infrastructure connectivity proposed by the "the Belt and Road" strategy has become an important way of cooperation between China and countries along the route.However,as the linear project crosses the Great Rift Valley in East Africa,the severe geological environment has brought unprecedented challenges to the project construction.As an important achievement of the "the Belt and Road" strategy in the African continent,the "Nairobi-Malaba Railway" in the Kenyan section of the the Great Rift Valley is facing serious ground fissure disasters,which has become a crucial problem in railway construction.As the most famous intraplate rift belt and the largest rift belt on earth,the unique geographical location and complex geological background of the Great Rift Valley in East Africa pose new challenges to the study of ground fissures.The systematic investigation and study of the spatial distribution and formation mechanism of ground fissures can not only help to fill the gap in the research of ground fissures,but also escort the steady advancement of the “Belt and Road Initiative” on the African continent.This paper is based on the frontier highlands of geoscience research in the Great Rift Valley of East Africa,and the research is funded by the key international(regional)cooperative research projects of the National Natural Science Foundation of China "East African Rift Valley Disaster and Prevention and Control",the China Road Bridge Corporation(CRBC)scientific research project "Research on the distribution,formation mechanism and engineering measures of ground fissures on the NairobiMalaba Railway" and the Shaanxi Provincial Innovation Capability Support Program Project "Overseas Geotechnical Engineering Innovation Team".Taking the ground fissures in the Kenya Rift Valley as the research object,through detailed data collation,field investigation,engineering investigation,sampling analysis,numerical simulation and theoretical analysis,this paper systematically constructed the continental dynamic background of the formation of ground fissures,comprehensively summarized the spatial characteristics of ground fissures,and deeply studied the coupling and mutual feedback mechanism between ground fissures and deep tectonic thermal activity,as well as the multiple response mechanisms of ground fissures to plate activity,innovatively proposed a multi-scale and multi-factor coupling mechanism for the formation of ground fissures.The main research results of this paper are as follows:(1)Summarized the distribution characteristics,spatial characteristics and activity characteristics of ground fissures.There are 83 ground fissures in the study area,and the distribution area of ground fissures is divided into three zones(belts),namely: the ground fissure development zone in the plain,the ground fissure development zone in the plain-volcanic transition zone and the ground fissure development belt along the active fault.According to the dynamic causes and main controlling factors of the formation of ground fissures,the ground fissures in the study area are divided into two categories: tectonic ground fissures and non-tectonic ground fissures.Summarized the scale and direction,plane shape,profile shape,failure mode,motion characteristics,and disaster causing characteristics of these ground fissures.(2)Revealed the deep origin of ground fissures and the control effect of deep thermal activity on ground fissures.The characteristics of gas emitted from ground fissures are high carbon dioxide,low oxygen nitrogen ratio,containing methane and sulfides,and high helium isotope ratio,which are typical mantle characteristics and reveal the deep origin of ground fissures.Revealed the crustal tectonic fracture system composed of ground fissures,faults,and rock fractures,and based on this,the deep thermal material transport process was reconstructed,and the control role of deep thermal activity on ground fissures was clarified.(3)Revealed the mechanism of ground fissure development under magmatic activity.The geochemical characteristics of volcanic rocks in the study area reflect typical extensional magmatism in continental rift areas,characterized by slight plate expansion,rapid magma rise,and strong magmatism.The mechanism of crust fracturing includes magma chamber uplift and doming action.The magma uplift action generates the crustal tectonic fracture system,and the doming action generates extensive uneven displacement and shear strain on the surface.These two mechanisms provide a deep vertical source of power for the breeding of ground fissures,which are associated products of magmatism.(4)Revealed the dual response mechanism of ground fissures to plate activity.The dual response mechanism of surface rupture to plate activity can be summarized as direct and indirect driving mechanisms.The direct driving mechanism is crustal extension and cracking,while the indirect driving mechanism is fault creep induced cracking.These two driving mechanisms together provide a horizontal driving force for the generation of ground fissures.In addition,the unique "S" shaped bending pattern of the Kenya Rift promotes the deformation and accumulation of tensile stress generated by plate tension in the crust,thereby strengthening the response of ground fissures to plate activity.(5)Revealed the coupling and mutual feedback mechanism between ground fissures and deep tectonic thermal activity.The mode of plate movement in the study area is passive extension and deep tectonic thermal activity extension.The coupling and mutual feedback mechanism between ground fissures and deep tectonic thermal activity is mutual promotion,that is,the existence of ground fissures causes the crust to warm up,enhances volcanic activity,and the enhancement of volcanic activity leads to stronger deformation and rupture of the crust,resulting in more ground fissures.This mutual promotion will be more pronounced under strong plate tension.Plate tension has an excitation effect on deep tectonic thermal activity,which can be summarized as a mechanism of the increasing production of crustal melt and temperature rise.(6)A mechanism for the formation of ground fissures under the control of deep tectonic thermal dynamics has been proposed.The intense and extensive magmatism driven by mantle plume provides a good dynamic background for the formation of ground fissures.The uplift of magma chambers and the dome action have produced large number of tectonic fractures(faults and fractures)in the crust.These tectonic fractures,as prototypes of ground fissures,control the occurrence of surface fractures.Plate separation and divergency not only strengthen the deep tectonic thermal activity in the study area,but also generate tensile stress in the crust,driving tectonic fractures to continuously extend upwards and eventually enter surface sediments.Heavy rainfall forms surface runoff erosion,causing surface fractures to expand and crack,resulting in serious ground fissure disasters.The formation mechanism of ground fissures under the control of deep tectonic thermal dynamics can be summarized as follows: magma determined fracturing,tectonic controlled fracturing,stress induced fracturing,and surface water enlarged fracturing.