A 3D Electrical Structure Of Gongga Mountain And Its Uplifting Mechanism

Since early Cenozoic,as a collision zone of India Plate and Eurasian,the Tibet Plateaui is still going on significant deformation.Along the eastern margin,there has formed not only steep Longmenshan(LMS)to the western Sichuan Basin,but low gradient topography in southeastern margin.The Gongga Mountain(GGMt.)with elevation of 7556 m is located at the boundary between the LMS and Tibet southeastern margin.Within the area of GGMt.,most of outcrop are Triassic flysch sedimentary,as well as mainly Mesozoic intrusive rocks.To the east is the Xianshuihe sinistral strike-slip faults which separated GGMt.from North-South trending pre-Cambrian Kangding complex belt on the eastern Yangtze side.Nowadays,GGMt.area has the highest vertical uplifting rate in the Tibet eastern margin.How this mountain formed,as well as with so rapid uplifting rate has raised widely attention among the geoscience community.The deep structure beneath this area will provide important evidence to better understand its uplifting mechanism.In this thesis,we obtained a detailed 3D electrical structure of GGMt.area using magnetotelluric(MT),and then proposed a dynamic model to interpret the uplifting mechanism by integrating other geologic observations from previous studies.(1)MT data acquisition and time series processingIn this thesis,we observed 57 new wideband MT points around GGMt.Combining with 70 old wideband MT points distributed at the southern GGMt,the area of our research region is about 200 km in NS direction times 150 km in EW direction.We installed a continue measuring MT points as the remote reference site during the time series processing,and Robust processing technique was applied for reducing the impact of noises in the data.At last,deleting the noise points in the data when editing the apparent resistivity and phase curves at different rotation angles.(2)Comparative study on 2D and 3D inversion of MT single profile dataIn terms of MT single profile data,2D and 3D interpretation which one is better,it is still a controversy question among the MT community.In this thesis,two MT profiles L03 and L04,which across tectonics can be used to study the differences between 2D and 3D inversion results.For 2D inversion,three impedance tensor analysis method: conjugate impedance transfer(CCZ),phase tensor(PT),Groom-Bailey(GB),have been applied simultaneously to investigate the dimensionality and electrical strikes of the two profiles' data.On the base of detailed comparison,these three methods indicated consistent results on dimensionality and electrical strikes of both two MT profiles.Combining the magnetic induction vectors and the geologic tectonics,the optimal strike angle was chosen as N40°E of the profiles.Qualitative analysis showed that upper layer was high resistivity and a conductivity layer below it along the two MT profiles.At last,CCZ was used to recovery the regional impedance tensors.Then we got different 2D resistivity models by inverting different polarization modes and their combinations.Besides,during the 2D inversion,we also made detailed comparison with the MT data before and after impedance tensor decomposition,as well as their corresponding 2D inversion results.We found that inverting the data after impedance tensor decomposition probably effected less by telluric distortion,from which 2D model maybe more reliable.For 3D inversion models,we analyzed the impacts from different cell size,data error floors,model smoothing parameters,as well as the Tippers on the base of L03 data.We found that 3D inversion of a single profile can also obtain high resolution of deep structure if using an appropriate cell size and smoothing parameters.And also,including Tippers in the 3D inversion may helpful to improve the resolution of faults' deep geometry.The prefect model chosen for L03 was inverted from full impedance and tippers,prefect model for L04 was derived from full impedance.Comparison with 2D and 3D models of a profile data,we found they have quite similar features if the MT data mainly 2D,such as L03.While,models look quite different for L04,we suggested that that probably because data in L04 showed more 3D.But comparing to 2D model of L03,the 3D model showed the deep geometry of Yunongxi Faults(YNXF)and Shade Fault(SDF)more clear.This case study probably indicated that 3D inverting a single MT profile can also get high resolution of subsurface structure.(3)3D inverting regional MT data including topographyThe GGMt.area has high relief,in order to reducing topographic impact on regional structure,we implemented detailed analysis on 3D inversion with topography.Before constructing large 3D model with topography,the work of inverting diagonal apparent and phase(RP),diagonal impedance(Z2),full impedance(Z4),and full impedance plus tippers(Z4T2)have been done.According to these models from different components,we found the model from RP is consistent very well with model from Z2.They had very limited difference.Besides,all of the 3D models without topography can fit components of Zxy,yx very well but cannot fit Zxx,yy and Tippers reasonably.We inferred that probably due to the horizontal cells size are too big,and maybe also related to the impact of topography.Therefore,we made the size of core cells of the 3D forward model decreasing from 3.5km to 2.5km,and modeling the topography on the base of digital elevation model from srtm90 with uniform thickness of 90 m.65 uniform layers were used to modeling topography and 70 layers below the topography.This new 3D grid was applied to invert the regional MT dataset again.Three 3D models that from inverting Tippers only(T2),Z4,and Z4T2 were analyzed.First,the model from T2 showed clear lateral resistivity boundaries at which is corresponding well to faults on the surface,and the variation tendency of the resistivity in the horizontal slice is consistent with the features indicated by impedance tensor.Second,even though the overall RMS of Z4T2 is a little bit bigger than Z4,the two resistivity models are quite similar.Except that Z4T2 probably better constrained the lateral location of the faults on the surface.For the reason that why the RMS increased when adding Tippers into the inversion,there likely have two factors: the first main reason we found is that some frequency points of impedance which cannot be fitted well in Z4 will get much bigger RMS values in Z4T2,but the data fitted well in Z4 are able to be fitted well too in Z4T2.The other reason is that the calculated RMS of Tippers are bigger than impedance,which actually partly affected by the error floors signed to them.So,actually,in this thesis,adding Tippers into the inversion may not worse the fitting of impedance.Moreover,the forward response of model from Z4 completely misfit the observed Tippers.Finally,based on those comparisons mentioned above,the model from Z4T2 with topography was chosen as the perfect interpretation electrical resistivity model.(4)the characteristics of regional crustal electrical resistivity structureFrom surface to deep,regional crust is characteristiced by two layers,high resistivity layer in the upper crust underlie which is a lower resistivity layer.The upper crustal high resistivity layer was cut into separated small blocks by strike-slip faults.In the east-west cross-sections,we found the top interface of high conductivity layer in mid-crust is much flat,while there are changes along the north-south cross-sections.Our MT data have weak constrain for the structure of regional low crust.In terms of the lateral variation,the resistivity to the western Xianshuihe-Xiaojinhe faults(XSHF)is much lower than the eastern section in middle crust,which respectively corresponding to Songpan-Ganzhi(SG)terrain to the west and Yangtze block to the east.Within the north section around GGMt.where covered well by MT data,upper crust is a high resistivity layer,its geometry shape in the horizontal slice corresponds to the outline of outcropped granite.Around this high resistivity body is a ‘ring' shape conductor.In the middle crust,right beneath GGMt.,is also a high resistivity body with resistivity of 300 ohm.m and thickness of 10 km,which also surrounded by conductors in the regional middle crust.On the top of this mid-crustal high resistivity is a thin relative conductor layer which connecting to regional mid-crustal conductors in deep.From our electrical structure,the GGMt.area has relatively higher resistivity than its surround area in the middle to lower crust.We suggest that the high temperature partial melting material,if any,probably distributed around GGMt.not right beneath the mountain.Integrating the previous cognition on the deep structure of this area,we explained the conductor layer as hot soft material easily to be deformed,and explained the high resistivity layer in the middle crust beneath GGMt.as relict Yangtze block which is going on thermal erosion,and the thin conductor layer on the top of it is explained as an old detachment which act as channel of fluid and hot in nowadays.(5)Discussing the electrical conductivity of faults and activity of earthquakesAt the location where L03 profile across by,SDF is a conductor stripe extending to deep,dipping to Southeastern(SE).It showed YNXF is a subvertical faults or dipping to SE with a high angle.Both faults extended to conductors in the middle crust.At the location where L04 profile across by,the deep geometric extension of these two faults are unclear.We infer that may indicate the YNXF has different activities in North and South sections,which is consistent with the geomorphic trace that YNXF is very clear to the North,but unclear to the South.In the south section of XSHF(from Kangding to Shimian),the faults cutoff the 20 km thick high resistivity layer,significant conductors were below the faults and to its western flank.To its eastern flank is underlay by high resistivity materials corresponding to Kangding complex and crystal Yangtze basement.This result indicate that XSHF nowadays is the boundary between SG terrain and Yangtze block.Along the cross-section of faults plane,we found north section is relatively lower resistivity,while to the south from Lianghekou to Moxi there is a high resistivity body extending to the depth of 25 km where has little earthquakes.We suggest that large elastic strain energy is probably being accumulated in this high resistivity section.Along the North-South trend Anninghe(ANHF)and Xiaojinghe Faults(XJHF),there are apparent lateral changes in electrical resistivity.At the northern section,the conductors to the western faults may already across the XJHF up to the place right beneath ANHF which subvertically cutting through the brittle upper crust.Near Mianning town,we can see the fault plane of ANHF probably dipping to west along the top of high resistivity body from Yangtze block and then meet with XJHF at further west at depth of the bottom of the crust.At the south section near the Anning country,a EW cross-section showed there are high resistivity layer with thickness about 15-20 km between XJHF and ANHF.The model along the plane of ANHF shows that the resistivity much lower in Northern part from Shimian to Tuowuxiang,from Tuowuxiang to Mianning is a higher resistivity body.It is a relatively conductor in Mianning upper crust.To further south to Anning country is characterized by higher resistivity again which may extend to depth of 25 km.The relative high resistivity southern Wutuoxiang correspond to the area of little earthquakes and blocked segment in ANHF.We think that the accumulated strain energy will more likely to be released near Mianning town where characterized by relative lower resistivity indicating relative weaker than its near north and south part.So the place with higher seismic risk may located near Mianning town.Along the faults plane of YNXF,we can see continuous much low resistivity material in the middle crust.Separated by Yunongxi country,upper crust in north section has lower resistivity than south section.We infer that YNXF serve as a fault that harmonizing the vertical and horizontal different motions of small rigid upper crustal blocks in this GGMt region.(6)Discussing the uplifting mechanism of GGMt.The previous models came up to explain the uplifting of GGMt more or less conflict to the deep structures revealed in this thesis.“Dynamic topography” and “Tectonic aneurysm” need high temperature and soft material right beneath the GGMt,it is not supported by our 3D resistivity model in which showing high resistivity body beneath the mountain in middle crust.“Restraining bend” mode needs upper crust of the GGMt is easy to be shorten,but our 3D electrical resistivity model indicates the upper crust characterized by high resistivity implying a relatively strong body.Therefore,we proposed a new dynamic mode to interpret the uplifting of GGMt.We think the uplifting of GGMt probably has experienced three phases:The first phase may happen around 30-25 Ma,the eastern Tibet collide with Yangtze block during this time period.The rigid Yangtze block passively subducted into the soft SG terrain resulted in the uplifting of eastern margin of Tibet Plateau.The early Cenozoic foreland sedimentary basin was formed above the Yangtze crust.The second phase probably occurred around 15 Ma,the front end of the Yangtze subducted plate beneath SG may be broken off due to the eastern push in Mid-lower crust from Tibet Plateau.After that,the elastic force accumulated in the Yangtze subducted plate will be released and the plate beneath the SG will rebound upward.This force may drove the eastern Tibet uplifting further.Meanwhile,the force of Yangtze block obstructing the eastern motion of SG was probably decreased as the subduction angle changing to horizontal.The SG will likely be pushed further east uploading on the Yangtge block and covered the early Cenozoic foreland basin that was absent in the Sichuan basin.The third phase likely occurred after XSHF forming,the huge sinistral strike-slip faults cut off the subducted Yangtze block and leading a relict rigid body in the mid-to-low crust of SG terrain.This rigid body may obstruct the motion of low crust soft material to SSE direction,as a consequence,the crust around this local area may be thicken and then uplifting the surface.Moreover,we think the uplifting of GGMt possibly also associated with the restraining bend along the XSHF,at where a little bit squeeze in the upper crust will make local area horizontal shorting and vertical uplifting.