Study On The Evolution Law Of Deformation Field Of Fold-Thrust Belt With Double Detachments:Insight From Analogue Modelling

Analogue modelling is an important method to study the fold-thrust zone.Most of the previous studies mostly considered the single detachment,while the study of double detachments is less.Besides the most of them based on qualitative analysis with the experimental photos of different deformation stages.Although few of studies have carried out quantitative studies on the velocity and strain fields based on PIV,the work is not systematic enough,resulting in a limited understanding of this aspect.The author uses physical simulation technology and PIV analysis method to thoroughly understand the evolution law of the deformation field of fold-thrust zone deformation with double detachments.Based on the results of 5 sets of single detachment experiment and 3 sets of double detachments experiments,this paper draws the following main insights:(1)The distribution characteristics and evolution of the deformation field of the lower tectonic layer in the fold-thrust zone with double detachments are consistent with the deformation of the fold-thrust zone with single detachment.The fault activity of the lower tectonic layer influences the fault nucleation of the brittle layer of the upper tectonic layer by affecting the flow of the ductile layer of the upper tectonic layer.And the relative motion of the hanging wall in the lower tectonic layer influences the flow of the upper ductile layer or the fault dip angle in the upper brittle layer,which control the activity of the fault in upper brittle layer.(2)Under the condition of single frictional detachment,the initial deformation stage of the model,before the wedge reaches the critical state,and after the wedge reaches the critical state,the fault nucleation mode is different in the brittle layer.The friction coefficient of the detachment surfaces and the horizontal compression change the direction of the major principal stress(?₁),resulting in the change of the dip angle of the strain localisation band within fault nucleation zone or active fault zone.The dip angle of the fault and the load of the fault control the fault activity.(3)Under the condition of single ductile detachment,the initial?_s/?_d ratio of the model controls the folding of brittle layer,ductile layer flow mode,and fault nucleation mode.The flow of the ductile layer leads to the rotation of the local principal stress(?₁)direction inside the brittle layer,thus controlling the degree of fault nucleation zone or active fault.Besides changes in the dip angle of the active fault,the flow of the ductile layer,and the load of the fault influences the friction of the fault surface,which in turn controls the fault activity.