Formation And Evolution Of Turkana Depression In The East African Rift System Based On The Analogue Modelling

The East African Rift System(EARS)is a natural modern example of active continental rift and provides ideal opportunities to study the formation and evolution of such tectonic systems.Within the EARS,the Turkana depression,the area of interaction between the Kenyan and Ethiopian rifts,is of special interest since it has experienced a complex and long-lived tectonic history,characterized by multiple stages of rifting.Previous studies generally believe that the East African Rift is controlled by the the N-S trending pre-Cambrian basement pre-existing fabrics,and develops along the paleotectonic belt.However,the faults in some areas intersect with the pre-existing fabrics of the basement on the outcrop at a large angle,showing the contral effect of the pre-existing fabrics is not so obvious.Recent studies have shown that the area has experienced at least two phases of extension.The morphology and evolution of the Cenozoic rift is obviously affected by the reactivation of the Mesozoic rift.Affected by the pre-existing fabrics of the Precambrian basement(metamorphic rock lineage and/or foliation)and Mesozoic rifting,the evolution process of Cenozoic rifts in the study area is complex,and the influence of pre-existing fabrics/faults on the development and evolution of Cenozoic rifts,on the control effect of structural morphology is still unclear.In addition,the mantle plume activity in the study area is also an important factor affecting the formation and evolution of Cenozoic rifts.It is unclear whether and how the mantle activity in the study area affects the development and evolution of Cenozoic rifts.This paper uses a large amount of two-dimensional seismic data and drilling data to conduct a comprehensive analysis of the structural geometry and kinematics of the Cenozoic rift.On this basis,the use of structural physical simulation analysis methods is used to further study the following two issues: First,In the rift activity that has undergone multiple extension stages,the characteristics of the reactivation of pre-existing fabrics(pre-existing faults)and the influence of pre-existing fabrics on the formation and evolution of later rifts;second,how does the mantle activity affect the special geometry and evolution of rifts process? How does the combined effect of mantle activities and pre-existing structures affect the evolution mode and formation mechanism of the rifts?Through the carefully interpretation of a large amount of seismic data,this paper has further deepened the understanding of the tectonic and kinematics of the Turkana Depression in the East African Rift.This study identified basic filling and structural characteristics of the 7 sags in the study area: The study area is filled with OligocenePleistocene as a whole,controlled by boundary faults,and There are five types of sag structures: typical continuous graben,typical half graben,migratory half graben,dish type graben and parallel transformation graben.From south to north,the stratum filling of sags gradually changes from old to new.In terms of structural features,there are changes from "half graben" to "graben",from "east-west parallel" NNW trendingwide rift zone to "south-north belt" narrow rift transition.According to the most recent dating data,combined with a part of the drilling and outcrop data,the rift activity began at ～ 45 Ma in the South Lokichar sag,the central Turkana Depression,and then dispread in a “leap-forward” north-south direction.By analyzing the fault activity rate of the main boundary faults and the fabrics development history of the key section,the evolution of the Cenozoic rift is divided into two periods and four evolution phases.The characteristics of the evolution of the study area are summarized as follows: early stage rift began in the south and late stage in the north,first wide rift and later narrow rift,moving to the eastward.That is,in the early Cenozoic,a series of rifts were NW-SE trends and nearly parallel;in the early Miocene,the rifts were concentrated near South Lokichar and Kerio in the south;after the Miocene,the rifts migrated northward;since the Pliocene,the rift system evolved and migrated eastward.Analogue model experiments with multiple extension phases(different extension directions)show that the number of faults formed in the early phase and the size of the faults(length,vertical fault distance)are the key factors that control fault reactivation:The greater the amount of extension,and the larger the density and scale of the faults in early stage,the easier it is for fault to be reactivated.If the amount of extension in the initial stage is limited,the effect of the pre-existing faults on the newborn faults may be limited.Pre-existing faults can be reactivated locally,but the reactivation effect is not obvious on the whole.Comparison with experimental results,it is found that the reactivation of pre-existing faults in Turkana Depression as a whole is not obvious(the activation only occurs locally).It is speculated that the extension of the Mesozoic rift in the study area is limited in the period of the Mesozoic rift.The number and scale of the existing faults may be limited,and they will be insignificantly reactivated in the Cenozoic rift period.Only the reactivated faults are connected locally with the newborn faults,forming a local complex "Z"-shaped fracture development in the Cenozoic rift.The analysis of small-scale sandbox analogue simulation experiments suggest that:(1)Under the simple shear mode,the sag generally appears as an asymmetric half graben structure,and the depocenter is stably distributed near the unilateral boundary fault.Under the pure shear extension mode,the overall structural style of the sag is a symmetrical graben structure.Together with the strengthening of extension,a new central graben is formed in the depression,and the depocenter is more stable in the center of depression.(2)The transformation of extensional shear mode has an important influence on the transformation of sag structure and the migration of sedimentary centers.The transformation of extensional mode has a obvious influence on the development characteristics of late faults and the migration characteristics of depocenter.(3)The transformation of the extension direction yield the echelon faults and small fault depressions in the depression slope belt.In the late stage of the rift evolution,the conversion of extension direction also has an influence and control on the development of newborn faults and the distribution of depocenter.The overall dynamical model of the Turkana Depression may have converted in the evolution of the rift,that is,the simple shear mode before the Miocene changed to the pure shear mode after the Miocene.The Cenozoic rift in the Turkana Depression is generally developed in the eastern margin of the Tanzania Craton,on the Mozambique orogenic belt formed by the PanAfrican event in the late Neoproterozoic,and along pre-existing fabrics of the preCambrian metamorphic basement.The NW trends and the “pervasive” pre-existing structural weakness zone at the lower crustal scale formed by the activities of the Mesozoic rift controlled the overall “wide rift” feature of the Turkana Depression.The pre-existing faults of “discrete” formed during the Mesozoic rift period affect the local fault characteristics of Cenozoic rift.The complex mantle activity in the study area affects the deep thermal structure,and is controlled by the deep thermal structure,which further affects the dynamic mode of the rift,forming different extensional shear modes,and controlling the evolution of the rift.On the whole,the development characteristics and evolution process of the Turkana Depression are controlled by different types of preexisting structures and changes in mantle activities.