Crustal Architecture And Tectonic-sedimentary Evolution Of The Transform Continental Margin

Transform continental margin acts as one of important components of the continental margin in the global plate tectonic system.It's formed along the transform fault due to the strike-slip movement between two divergent lithospheric plates.Limited by the development of the transform faults,the global inventory of transform continental margins only represents 16%of continental margins.Strike-slip systems of the transform faults have been recognized in transform continental margins,however the crustal structure,tectonic-sedimentary evolution and rifting-breakup dynamic evolution are poorly known.Worldwide investigations of transform continental margins are mainly focus on the Equatorial Atlantic Gulf of Guinea,Newfoundland continental margin,and Gulf of California.Lacking of the geology and geophysical data in the Mozambique continental margin at East Africa,only few transform continental margins have been recognised in the Northern Mozambique margin.However,the scale and dynamic evolution of the transform continental margin at East Africa remain unclear.The East Africa continental margin was formed by strike-slip movements and the break-up of the Gondwana along three large transform faults,i.e.Davie Fracture Zone(DFZ),Mozambique Fracture Zone(MFZ)and Agulhas-Falkland Fracture Zone(AFFZ).These three N-S trending transform faults divide the Mozambique continental margin into three parts:northern,central and southern margin.Based on the Multi-channel and refraction seismic investigation,previous studies focused on the northern Mozambique margin,the relationship between the DFZ and the tectonic-sedimentary evolution of the transform continental margin was firstly revealed in this area.However,due to the lack of seismic survey at the southern Mozambique margin,there are some great unclear on its deep crustal structure,tectonic-sedimentary evolution and margin rifting-breakup dynamic evolution along the MFZ.To study the dynamic development and structural architecture at the southern Mozambique margin associate with the MFZ,this paper reconstructs its crustal structure and tectonic sedimentary evolution.This study can not only increase our understanding of evolution of the Mozambique continental margin,but also has a great significance to explore the controlling factors and deep dynamic mechanism of the global transform continental margin.Thus,based on the data collected by Second Institute of Oceanography,MNR in2016 and the deep seismic results of the crustal structure from French Research Institute for Exploitation of the Oceans(IFREMER),we conducted the multichannel seismic data interpretation and the Gravity-seismic joint inversion at southern Mozambique margin.We created a systematic marine multichannel seismic data collection-processing-interpretation-analysis process to investigate the crustal structure,rifting-breakup dynamic mechanism and tectonic-sedimentary evolution of the southern Mozambique margin.Integrated seismic data with obtained or interpreted drilling,geological and geophysical data,all seismic profiles,stratigraphy units and sequence boundaries are interpreted to established precise sequence stratigraphic framework.Based on seismic and sedimentary facies analysis,we reconstructed the sedimentary evolution at the southern Mozambique margin.With the combination of the multichannel seismic and OBS refraction results,we analyzed the Gravity-seismic joint inversion results to reveal the deep crustal structure and the architecture of the ocean-continent transition zone.The results show as follows:(1)Six major seismic sequence boundaries have been identified at the southern Mozambique margin.From bottom to top:T₅(?157Ma,Late Jurassic unconformity),T₄(129Ma,base of the Early Cretaceous Barremian),T₃(100.5Ma,base of the Late Cretaceous),T₂(66Ma,base of the Paleocene),T₁(23Ma,Oligocene unconformity unconformity)and T₀(present-day seafloor).These sequence boundaries are closely related to the tectonic events in the study area and respectively corresponding to the end of intra-continental rifting,termination of strike-slip movements along the transform faults,first uplift deformation of African continent,strong regression at Late Cretaceous,and second uplift deformation of African continent.(2)One basement unit and five post-rift sedimentary units have been identified at the southern Mozambique margin.From bottom to top:Ub(Jurassic),U1(Late Jurassic-Early Cretaceous unit),U2(Early Cretaceous),U3(Late Cretaceous),U4(Paleogene),U5(Neogene-Quaternary).Ub represents the volcanic-sedimentary sequence(VSS)formed at the rifting stage.Seaward Dipping Reflectors(SDRs)was emplaced near surface or under sub-aerial condition during continental rifting stage.U1 overlies on the basement unconformity.The sedimentary sequence is dipping eastward and thinning gradually with onlap and downlap facies.It appears to be delta deposit at the continental margin in shallow water environment.U2 sedimentary sequence contains parallel horizontal facies developed in the deep sea environments.U3 sequence contains mounded drift formed by contour current and mainly contains migrating wavy facies.As a result of stable ocean current between the oceans,it was formed in the abyssal deposit sedimentary environment.U4 sequence was formed by both turbidity current and contour current as a result of intensive erosion.The corresponding sedimentary facies contain submarine canyons,levees and sediment waves.U5 sequence contains chaotic and blank facies that was formed in the deep oceanic sedimentary environment.(3)The southern Mozambique margin is formed by the extension and thinning of continental crust eastward,which can be divided into thinning continental crust zone,ocean continent transition zone and ocean crust zone.Gravity-seismic joint inversion results indicate that the crustal thickness in the study area is significantly thinning oceanward.Under the thinning continental crust and ocean continent transition zone,high-density bodies in the lower crust are widely developed,which may be related to the activity of Karoo mantle plume;Large scale volcanic sedimentary sequence(VSS)and seaward dipping reflectors(SDRs)are identified in the ocean continent transition(COT)zone.The ocean continent transition zone extends from north to south along the southern Mozambique margin,with a width of 70?90km.The MFZ is the boundary between ocean and continent crust along the southern Mozambique margin,and the basement uplift area with a N-S trending on the continental side represents the continental boundary of COT zone.(4)The tectonic evolution of southern Mozambique margin is closely related to development of transform faults and can be divided into three stages:intra-continental transform rifting stage,active transform stage and passive transform stage.As a result of intensive magmatic activity of Karoo mantle plume,the initial break-up of the Gondwana started with a NW-SE direction,where the southern Mozambique margin obliquely rifting along the N-S-trending MFZ and initiated the intra-continental lithosphere transform rifting stage.Since the spreading direction of the Gondwana rotted clockwise from NW-SE to N-S,the oblique rifting stopped and the onset the strike-slip movement along MFZ.The break-up of Gondwana along the active transform margin onsets the southward strike-slip displacement.After the end of strike-slip displacement,the margin starts rapid subsidence at the passive transform margin stage.(5)Combining with the crustal architecture and tectonic-sedimentary evolution,we propose for the first time that the southern Mozambique margin is a typical transform continental margin.During the rifting and seafloor spreading along the southern Mozambique margin,oblique extension and strike slip movement along the transform fault are the main factors for the development of the transform margin.Before the active transform stage,the transform margin experiences a magma-rich intra-continental rifting stage with intense magmatic activities such as magmatic underplating and dyke intrusion.