Mesozoic Structural Features And Evolution History Of The Yalu River Fault Zone And Adjacent Area

Active continental margin records the subduction process of oceanic plate,and is commonly the site of significant tectonic and magmatic activity.However,its tectonic evolution,stress state and deformation mechanism remain uncertain and controversial.The NE-SW-striking,ca.700-km-long Yalu River Fault Zone(YRFZ)is a prominent tectonic belt in East China.It underwent a polyphase evolution during the Mesozoic.The YRFZ and Dandong area to the west of the fault zone lie in an overriding plate with respect to the subducting Paleo-Pacific Plate during the Jurassic-Cretaceous,and are ideal for studying the evolution and dynamic process of East China continental margin.In this dissertation,we present structural and geochronological data for the metamorphic basement and Mesozoic structures in the YRFZ and Dandong area.Meta-sediments and metamorphosed plutons were collected from the Dandong area for zircon U-Pb dating.The results show that meta-sediments are scattered in many places of the Dandong area.They have depositional ages of Paleoproterozoic and can be correlated with the Liaohe Group in the Liao-Ji orogenic belt.The Dandong area also experienced extensive and intense regional metamorphism at 1910-1864 Ma,which is comparable to the collision-related metamorphism in the Liao-Ji orogenic belt.Zircon U-Pb dating also indicate that the Dandong area contain a series of Paleoproterozoic plutons dominated by those of 1882-1827 Ma.These facts indicate that component and evolution of the basement rocks in the Dandong area show obvious similarity to those in the Liao-Ji orogenic belt and the basement in the Dandong area belongs to a part of the Liao-Ji orogenic belt,rather than the Nangrim Block as previously considered.It is proposed by comparison with previous studies that the Liao-Ji orogenic belt extends across the entire Liaodong Peninsula,and its southeastern boundary should reach to the Yalu River Fault Zone at least.Our structural analyses show that Dandong area was involved in NW-SE shortening that produced a series of SE-dipping reverse ductile shear zones,pervasive ductile fabrics between the shear zones,and folding and reverse shearing in the Liaohe Group.Microstructures and quartz c-axis fabrics suggest that the shear zones were deformed at temperatures of 500±50?.Zircon U-Pb dating indicates that the NW-SE shortening event occurred during the Late Jurassic(157-146 Ma).These structures are similar to many other NE-SW-striking thrusts in the eastern NCC,which have been attributed to event A of the Yanshan Movement.Detailed field investigation and comprehensive analyses show that the YRFZ was initiated as a sinistral strike-slip fault zone,and numerous NE-SW-striking sinistral faults developed in the Dandong area at the same time.Most of the faults show ductile deformation(ductile shear zones)in the southwest and brittle deformation(brittle faults)in the northeast,the transition between the ductile and brittle parts of each fault is characterized by a zone of brittle-ductile deformation.Microstructures suggest deformation temperatures of 400?-500? for the southwestern part of the ductile shear belts and 300?-400? for the northeast part.The deformation temperatures of the brittle-ductile shear zones in the YRFZ were estimated to be 200?-300?.Zircon U-Pb and mineral 40Ar/49Ar ages from the dutile shear zones constrain the timing of the sinistral faulting to between 146 and 136 Ma.Therefore,it is indicated that the YRFZ originated as a sinistral fault zone in the earliest Cretaceous.This tectonic event is consistent with event B of the Yanshan Movement.Subsequently,during the Early Cretaceous(132-100 Ma),the study area was the site of widespread extension.Extension led to the formation of numerous NE-SW-striking normal faults and reactivation of pre-existing structures.The YRFZ produced NE-SW-striking,SE-dipping basin-bounding normal faults that controlled the development of Early Cretaceous rift basins.Extension was accompanied by widespread magmatism,demonstrated by the emplacement of plutons and dikes,and volcanic eruption in the rift basin.This extensional period is generally referred to as the peak period of the NCC destruction.Sinistral faulting developed again in the earliest Late Cretaceous(100-97 Ma).A series of folds and NE-SW-striking sinistral brittle faults occurred in the Lower Cretaceous sediments,which terminated the development of the Early Cretaceous basins.During the remainder of the Late Cretaceous(97-70 Ma),the dextral normal faulting of the YRFZ was instrumental in forming the Late Cretaceous Gulouzi Basin.Reverse dextral faulting along the YRFZ took place at the end of the Cretaceous,and was responsible for the termination of the Late Cretaceous basin development.We determined the stress field responsible for each phase of Cretaceous deformation from fault-slip data measured on the fault planes.The fault-slip data indicate nearly N-S compression for the sinistral faulting during the earliest Early Cretaceous and earliest Late Cretaceous,respectively;WNW-ESE to NW-SE extension for the normal faulting during the remainder of the Early Cretaceous;N-S extension for the dextral normal faulting during the the remainder of the Late Cretaceous;E-W compression for the reverse dextral faulting at the end of the Cretaceous.Therefore,during the Jurassic-Cretaceous,the stress state in the study area is characterized by alternation of compression-compression-extension-compression-extension-compression.Regional correlations suggest that the entire East China continental margin has a similar Jurassic-Cretaceous evolution to the YRFZ and adjacent area.We propose that the deformation history and temporal dynamic variations of the continental margin during the Jurassic-Cretaceous result from changes in the subduction kinematics of the Paleo-Pacific Plate.In this process,there were longer periods of extension that alternated with shorter periods of compression in the active continental margin.A slab-driven model with relatively long periods of low-velocity subduction alternating with shorter periods of high-velocity subduction could account for the episodicity of stress state.