Guman Fault-bending Folding And Bending-moment Normal Faulting And Related Strong Earthquake In The Western Kunlun Foreland Basin

The occurrence of Pishan Mw 6.4 in Xinjiang in 2015,Gorkha Mw 7.8 in Nepal and Lushan Ms 7.0 in 2013,when the seismogenic faults did not rupture the surface,highlighted the hazards of blind thrust fault and the urgency of research on them.On 3 July 2015,the Mw 6.4 Pishan earthquake occurred on the southward blind fault slope about 9-16 km deep beneath the Guman fold in the foreland thrust-fold of West Kunlun,resulting in nearly 10 cm coseismic uplift of the fold on the surface.However,based on seismic profiles in front of the Western Kunlun Mountain,previous studies have different interpretations of the Guman fold,including fault-propagation fold,fault-bend fold,and wedge-thrust fold.These variable results hinder us from choosing an appropriate kinematic fold model to estimate the deformation of Guman fold.The fold belt remains active,manifested by the extensive presence of fault scarps on the land surface and the occurrence of moderately strong earthquakes along the fold trace.However,GPS data to the north and south of the Guman fold shows nearly constant northward velocity.How to quantify the shortening rate across the range and understand how the deformation occurs in such areas?There are dozens of large bending-moment normal fault scarps developed in the study area,which did not rupture in the Mw 6.4 Pishan event.Can these scarps be used as "indicators" to represent the larger earthquakes of underground blind thrust faults?What are the formation conditions,spatial distribution,and deformation characteristics of these scarps?What is the activity history of these scarps?It is a great challenge to the traditional method of trench and dating for dozens of fault scarps.To address these issues,we investigated the active deformation of the Guman anticline and bending-moment fault activity along the West Kunlun Mountains.Along the valleys crossing the anticline,ongoing folding has uplifted flights of fluvial terraces above the modern riverbed and a great number of bending-moment fault scarps are preserved on the terrace surface.We collected 1.0 m resolution Google Earth satellite images,12.5 m ALOS DEM,and 30 m SRTM DEM data.Combined with several field geological surveys,detailed large-scale structural geomorphology mapping was carried out along the Kuashtag River,Pixina Township valley and their adjacent zones.The main terrace surfaces and bending-moment fault scarp topography were measured by differential GPS.Two cosmogenic nuclide depth profiles were carried out to determine the terrace exposure ages in the study area.UAV images were collected for bending moment fault scarps with more than 8730 images,covering an area of 17.6 km2 on the terrace west side of Pixina Valley.We collected and interpreted a 60 km-long of seismic reflection profile across the Guman fold zone.We explored the nonlinear scarp degradation model,established the morphological age determination scheme,and wrote the Matlab script.Through these works,the following important advances and new understandings were obtained:(1)Dividing the geomorphic surfaces across the Guman anticline and determining the chronological framework in the study area.The terraces in the study area are much older than expected,with high terraces of 3.7-5.2 Ma and low terraces of 413-673 ka.In the study area,the Keliyang River originating in the West Kunlun Mountains crosses the Guman anticline and forms three major river valleys,which are the kuoshtagh River,Pixina,and Basilangan valley from east to west.Along these valleys,fold deformation has uplifted the terrace surface above the present bed.We divided geomorphologic surfaces into four major terrace groups from the highest terrace to the lowest and one fan stage(T0)using field observations,geomorphic mapping,and dating.Terraces T3,T2 and T1 can be divided into several sublevels.The luminescence signals of the collected sediment samples were saturated.Dating samples collected by Guilbaud et al.(2017)from the high terrace on the west bank of the Pixina valley were saturated with 10Be concentration.We used the cosmogenic nuclide 10Be depth profile method to determine the exposure ages of the lower terrace surface on the eastern side of the valley as 413-673 ka.Using the cosmogenic nuclide 21Ne dating method,the exposure ages of the eastern high terrace surface of the valley were successfully defined as 3.7-5.2 Ma.These results indicate that the terrace surface of the study area is much older than expected.(2)Combining the river terrace deformation and seismic reflection profile,the Guman fold is best explained as a fault-bend fold with lower flat-ramp-upper flat geometry that grows by kink-band migration.the slip rate along the underlying blind slope since 413-673 ka is 1.8 mm/a,87%of the slip will be transferred forward along the upper fault flat and ultimately absorbed by the Mazatagh Thrust located 160 km away in the hinterland of the Tarim Basin.a)Combining deformation characteristics of the terrace profiles and subsurface structures interpreted from the seismic reflection data,the Guman fold is best explained as a fault-bend fold with lower flat-ramp-upper flat geometry that grows by kink-band migration.Previous studies and the seismic reflection profile show that the blind fault ramp dips 20±4° to the south.b)Using the fault bend fold model,the slip rate along the underlying blind slope since 413-673 ka is 1.8+0.7/-0.2 mm/a based on terrace surface deformation and exposure age of T1b on the east side of the Valley.c)For a lower flat-ramp-upper flat fault-bend fold with a fault dip of 20°,13%of the slip along the underlying fault ramp would be absorbed by the fold above the fault ramp,whereas 87%of the slip will be transferred forward along the upper fault flat.Therefore,the shortening accommodated by the Guman anticline is 0.24+0.09/-0.03 mm/a while the slip rate along the upper fault flat is 1.6+0.6/-0.2 mm/a since the T1b terrace abandonment.This result explains why the shortening of the Guman Anticline is largely hidden from the relative movement of Global Positioning Stations sites to the north and south of the fold.These findings support the view that most of the crustal shortening of the Western Kunlun Mountain is transferred basinward along with the shallow detachment and is likely ultimately absorbed by the Mazatagh Thrust located 160 km away in the hinterland of the Tarim Basin.(3)The bending-moment fault scarps in the study area show obvious regular activity and indicate the existence of strong seismic activity.The bending-moment normal faults themselves do not constitute seismogenic faults.Their scarps may be the products of strong earthquakes of magnitude 7 or above,and the Guman fold belt has the structural conditions of strong earthquakes.We generated a 0.2 m resolution digital elevation model(DEM)from photographs acquired from low altitudes(～250 m)using unmanned aircraft vehicles(UAV).The DEM covers a 17.6 km2 area on the west bank of the Pixina Valley.The standard deviation of elevation difference between the profiles extracted from the DEM and differential GPS measured profile is 0.08 m.the measured profiles by differential GPS.We used topographic profiles extracted from the DEM to estimate the scarp height(h),the maximum slope(φ),and the width(W)of fault scarps,conducted a lot of statistical analysis,and modeled the degradation process of fault scarps using the nonlinear transport model.The following understandings were obtained:a)The bending moment faults in the study area are present along the anticlinal axis.The faults dip to the axial plane,with a dip of 70-80° at the surface.Clear vertical slickenside on the fault surface indicates that the fault should be a stick-slip.More than a dozen north-facing fault scarps were developed to the south,while only 1-2 slopes were south-facing to the north,forming an asymmetrical graben on the axis.From the anticline axial to the southern flank,the vertical separations of the faults increase.On the terrace T3a,the fault separations increase from 3-4 m to about 14 m southward and the height of the southernmost fault scarp(14.4 m)is close to that of the northernmost fault(10.6 m).On the terrace T1b,the fault separations increase from<1 m to about 5 m southward.b)The terraces were cut into several strip blocks with a north-south width of 380-650 m by subparallel normal faults in the vertical direction.These blocks tilt outwards to the fold axis.Blocks cut by the N-dip fault tilt to the south,while those cut by the S-dip fault tilt to the north.The amount of block tilt is approximately proportional to the height of the corresponding fault scarp.c)The ratio between the cumulative displacements of the bending moment faults on the terrace T3a and T1b suggests that the faults did not become significantly younger towards the fold flank.It may be the first to establish the framework of the fault rupture range,and then with the framework of the fault activity,interspersed with some new faults closer to the anticlinal axis.d)The displacement along faults steps down as the terraces become younger.The cumulative displacement along the entire fault zone shows the same phenomenon.The cumulative displacement of faults on the terrace T3a and T1b is 54.5±3.3 m and 19.5±1.1 m,respectively.The total displacement of T3a is 2.8 times that of T1b,indicating that the fault zone has been active.Assuming that the fault activity rate is constant,the exposure age of the T1b terrace surface was 413-673 ka from the corresponding terrace on the eastern bank of the Pixina valley,so the bending-moment faults on the T3a terrace began to be active at 1.16-1.88 Ma.e)The bending-moment faults in the study area are rootless and developed at a shallow depth(0.8 km),so they are not seismogenic faults.In the 2015 Pishan Mw 6.4 earthquake,the moment faults did not rupture the surface.The bending-moment fault scarps in the study area may mainly record the activities of large earthquakes above M7,which requires the participation of multiple-segment structures in the Guman fold belt or the root fault of thrust nappe structures.The lateral inhomogeneity of coseismic offset distribution along the fault during an earthquake could complicate the loading process of the accumulated deformation along with the blind thrust.f)The Guman fold belt is mainly composed of Guman anticline,Hexitage anticline,and Slik anticline in the middle fold front.These anticlines share a common detachment and then merge with thrust faults and strike-slip faults in the orogenic belt in the deep.It is not easy to accurately assess a thrust nappe seismogenic body.However,the recurrence periods of earthquakes that could trigger the surface bending-moment fault are relatively longer.The research on bending moment fault and fold kinematics in the study area shows that the West Kunlun Mountains has the structural conditions for strong earthquakes.(4)We have further refined the nonlinear transport coefficient in numerical modeling and established a scheme to determine the best-fit morphological age of scarps.Based on these works,we explored the active history of fault scarps in the study.Building on the previous works,we have further refined the nonlinear transport coefficient in numerical modeling and written a MATLAB script for exploring nonlinear scarp evolution.We tested our model by applying it to a study area in a semi-arid high-altitude Pamir region of northwestern China and obtained reasonable results.We then applied this method to explore the active history of multiple-events scarps in the study area.a)Through modeling multiple-events scarps,we found that for multiple-events scarp,a new faulting event can erase part of the memory of the previous event in the scarp morphology.After five faulting events,the morphologic age was 30-50%of the actual age.The maximum scarp slope of the multiple-event scarp is larger than that of a single-event scarp with the same height,so multiple-event scarps will look "young".This may explain why the terraces in the study area are hundreds of thousands or even millions of years old,and the fault scarps on the surfaces are still clearly visible.b)The fault scarps in the study area show the characteristics of regularly repeated activities.The nonlinear transport model was used to simulate the combinations of different displacements and recurrence periods,and the results obtained from the displacements of 0.3-0.5 m and recurrence periods of 1.5-2.5 m2(morphologic age)were in good agreement with the measured height-slope data.By comparing with the terrace date,the diffusion constant is as low as 0.06+0.02/-0.01 m2/ka.The possible reason is that the fault scarps in the study area are very old.As a result,the degradation rate is very low.According to the diffusion constant,the recurrence cycle of the bending-moment fault activity in the study area is 28+11/-5 ka.Together,the studies demonstrate that based on high-quality topographic data,a large number of scarps profile simulations can scientifically assess the age of scarps and explore the active history of fault scarps.(5)Bending-moment faults are mainly formed in poorly-layered beds and are somewhat independent of the kinematics of fold growth.The generation and asymmetric distribution of bending-moment faults in the study area are related to the extremely thick sedimentary layers and their overall tendency above the blind thrust fault.Combining the Guman fold deformation and the analysis of the geomorphological performance,lithologic conditions,and the relationship between bending moment normal faults,and fold deformation,the bending moment faults in the study area have the following characteristics:a)Bending moment faults are mainly developed in poorly-layered beds and are somewhat independent of the kinematics of fold growth.They are closely related to the bending degree of strata and the thickness of overlying beds.For the Guman anticline,the theoretical shortening rate is only 0.24+0.09/-0.03 mm/a and a large number of bending moment faults are still present on the surface because the thickness of the overlying strata is about 10-13 km,that is to say,a small shortening with a large radius can also produce a large extension at the surface.b)The bending moment fault zone forms a half-graben where most of the fault scarps face north,and only 1-2 scarps in the northernmost face south.The asymmetrical development may be influenced by gravity and formation curvature.Because the terrain and stratum dip to the north along the West Kunlun Mountains front,the northward component of gravity will superimpose on the tensile stress,thus promoting the development of north-dipping normal faults,but inhibiting the development of south-dipping normal faults.The seismic reflection profile reveals that the south flank of the Guman anticline dips 2-5° to the north where dozens of bending-moment faults scatter in a wide range,while the north flank dips 12° to the north where fewer faults are concentrated in a narrow range.