structural deformations along the active sagaing fault ... structural deformations along the active
Post on 24-Jun-2020
Embed Size (px)
Structural Deformations Along the Active Sagaing Fault Zone Between Sagaing and Mingun area, Central Myanmar Kaung SITHU1#+ and Saw Ngwe KHAING 2,3
1University of Yangon, Myanmar, 2Hinthada University, Myanmar, 3Myanmar Earthquake Committee, Myanmar # Corresponding author : firstname.lastname@example.org +Presenter
ABSTRACT The Sagaing fault is a transform fault that separates the Burma plate and Sunda plate, parallel to the plate boundary. Earthquake history and tectonic geomorphic features along the fault zone strongly suggest that the area is active segment and potential earthquake source. The fault zone structures are observed based on the meso and microscopic scale. High angle foliation patterns in some exposures with the shear zone suggest that the strongly rock deformation and high strain rate. The brittle deformation of geomorphologically prominent Sagaing fault is closely observed with ductile shear zone along the mountain front that is characterized by faulted veins, shear fractures and brecciation. Shear sense indicators of the fault zone are presented and discussed. The results of this investigation is essential to consider the deformation process, structure style of the area and focuses on the discussion of Sagaing Fault zone condition during seismicity, and earthquake source of the region. One of the most famous and strong earthquake in area was the Sagaing Earthquake of 1956 and caused severely damaged at both Sagaing and Mingun ridges. Keyword : Sagaing Fault, structure deformations, active tectonics
Figure 1. N-S trending Sagaing fault zone between Burma plate and Sunda plate, latitudes 21ºN to 22º N area. Win Swe (1972) proposed the name the “Sagaing fault”.
Figure 6. Showing the orientation of NW fold axis in folded sedimentary rocks, west of Yega village (A) Looking toward W. (B) Looking toward NW.
Figure 5. Rose diagram plot of foliation trends and total foliation planes
Methods of Study The geological and geomorphological methods are applied for this study. Field studies are focused on detail geological mapping along the Sagaing fault zone at a scale of UTM 1:50,000 and fault zone structures. Shear senses are determined by kinematic indicators. Thin sections were prepared under the petrographic microscope. The structural data analysis is based on the structural data from field observation. Further data collection is required to determine the geologic evidences of late Quaternary rupture history of the segments.
Fault Zone Geology The Sagaing fault is essential for the study of earthquake geology and is located several kilometers from north of Myanmar border to south Andaman sea. There is a large number of studies describing the role of Sagaing fault (e.g., Win Swe, 1981; Myint Thein et al., 1991; Soe Thura Tun, 2005; Wang Yu, 2013; Saw Ngwe Khaing, 2014). In the present area, the fault zone is about 8 km wide with the major NW-SE foliation trend. The metamorphic grade of Sagaing Metamorphic rock is high, amphibolite-granulite facies at the Sagaing ridge. In contrast, Minwun Metamorphic rock has low grade, greenschist-amphibolite facies at the Minwun ridge (Myint Thein, 2011). The main fault core can be traced by its prominent morphology, breccias, small scale displacement and typical comprise of phyllosilicate such as serpentines. The damage zone of the area is characterized by fault related structures such as small faults, veins and fractures of both brittle and ductile deformation body of Sagaing and Mingun domain.
Figure 4. Mountain Front segments and sinuosities of the Sagaing, Minwun, and Mingun ridges for regional Smf index assessment. The west side of Mingun ridge indicates the straight mountain front along the Sagaing fault zone. Rapid uplift along a range-bounding fault maintains the linear nature of the front.
Morphotectonics The highest tectonic intensity of the area is recognized at the Mingun ridge. Results of morphotectonic analysis suggest that the study area has very high – high tectonic activity with the strike slip faulting of Sagaing fault.
Alluvial Fan deposit
Figure 2. Geological map of the field study area (Kaung Si Thu, 2017)
Figure 3. Geomorphic features occurring in the study area: (A) Sagaing fault Valley, NS trending Sagaing and Minwun ridges are separated by this narrow fault valley, view from Minwun ridge, looking toward SE. (B) Pull apart basin, about 2 km north of Yega Inn, looking toward SW (C) Meandering –Braided river transition pattern of Ayeyarwady River, view from Swanooponenyashin pagoda, looking toward NE (D) Triangular facets along the Sagaing fault, looking toward E. (E) Yega Inn (Sag pond), prominent surface depression of Sagaing fault, tectonic and earthquake deformation evidence in the past, looking toward SW.
Sagaing Fault Zone Structures Foliations trends show most of the foliation plane have a dominant NW-SE and NNW-SSE trends. Field observations across the Sagaing-Mingun area suggest the area is large scale thrust-related anticline structure. Thrust fault evidences are rarely observed. Depend on the rock resistance, high angle minor reverse faults are only observed in Pliocene to Quaternary Irrawaddy sandstones, near Mingun ridge. One possibility is the major thrust fault does not reach the surface, close to the Sagaing Fault
Lateral Motion Evidences
Figure 12. Evidences for strike slip fault deformation: (A) Quaternary Fanglomerates, highly brecciated, looking toward NE. (B) High angle joints, looking toward N. (C) N-S trending brittle fault cutting veins, right lateral motion evidence, displacement is about 34 cm. (D) Right lateral vein displacement, looking toward: down.
Figure 13. Fracture-filled in initial sedimentation of sub recent sediments (Meso-and micro scale structures). This rock was deformed under brittle conditions and suggests the recent active tectonic process (at lease two processes).
Conclusion This study has identified the deformation structures of Sagaing fault zone. The evidences of NW-SE trending major foliation suggest that the principle stress direction is characterized by NE-SW compression and NW-SE extension. The overall shear sense indicators suggest that dextral shear. Microphotographs of fault rocks suggest that at least two recent tectonic process and shallow earthquake activity. Mountain front sinuosity index provides potential hazards to paleoseismologists. Future research is required to investigate the shallow earthquake faulting in the Quaternary time and earthquake cycle of Sagaing fault segments.
Acknowledgement This work is a part of MRes study and it is partly supported by Asian Disaster Preparedness Center (ADPC) research grant. We would like to thanks Dr Htay Lwin, Professor from Geology Department, University of Yangon for critically reading of this manuscript and constructive comments.
References Kaung Si Thu, 2017. Earthquake Geology of Active Sagaing Fault Zone, Sagaing-Mingun Segment, Myanmar. M.Res Thesis (unpublished),
Department of Geology, Pathein University , 55p. Myint Thein, Kyaw Tint, and Aye Ko Aung 1991. On the lateral displacement of Sagaing fault. Georeports, 1, 1, 23-34. Myint Thein, 2011. Age, Petrology and Deformation of the Laminated Limestone Unit on the West Side of Sagaing Fault in Sagaing
Division. Journal of the Myanmar Geosciences Society 4, 2, 11-32. Saw Ngwe Khaing, 2014. Geomorphology, Structural Geology and Paleoseismology of the Sagaing Fault between Singu and Wundwin,
Central Myanmar. Ph.D. Thesis (unpublished), Department of Geology, University of Yangon, 113p. Soe Thura Tun, 2005. The Sagaing Fault, A desk study report on seismotectonic implications in Myanmar, Myanmar Earthquake Committee,
133p. Wang Yu, 2013. Earthquake Geology of Myanmar. Ph.D. Thesis, California Institute of Technology, 300p. Win Swe, 1972. Strike-slip fault in central belt of Burma (abs). In: Haile, N.S. (Ed.), Regional Conference on the Geology of Southeast Asia,
Kuala Lumpor, Malaysia Annex to Newsletter No.34, 59p. Win Swe, 1981. A major strike-slip fault in Burma. Contributions to Burmese Geology, Department of Geological Survey and Exploration,
Ministry of Mines, Myanmar, Myanmar, 63-72.
Figure 10. Duplex type thrust system in highly deformed strike slip Fault zone, at the Minwun transpressive ridge, looking toward E, (B & C) closed –up view.
Idealized Fault Zone Model Shear Sense Indicators
Z shaped Drag fold
Layer Parallel shearing
Figure 8. Reverse fault evidence in the Sandstone of Pliocene to Quaternary Irrawaddy Formation
Figure 7. Geomorphic features of Mingun ridge may be associated with blind thrust fault
Figure 11. Idealized fault zone model of the area based on the fault rock distribution and structures
Evidence of Shallow Earthquake Related Fault Rocks
Continuing the Search for 1956 Sagaing Earthquake (Mw 7.1)
Figure 14. Interviewing to local peoples about their experiences of 1956 Sagaing earthquake. Many aftershocks are continued to shake the whole ridge about 2 day. Daw Khin Yi, 88 years old, Mingun U Win Maung, 81 years old, Sagaing
Figure 9. The contact between the different metamorphic rock exposures: sudden changes of rocks across the shear zone, looking toward E.
Hinthada University www.hinthadauniversity.edu.mm