Lithosphere structure in the north Zagros collision revealed by joint inversion of P receiver function and surface wave dispersion

Document Type : Research Article

Authors

1 Institute of Advanced Studies in Basic Sciences

2 Institute for Advanced Studies in Basic Sciences

Abstract

The Zagros mountain belt, situated on the northern margin of the Arabian plate, is one of the youngest continental collision belts. This belt was formed by a collision between the Arabian plate and the Central Iranian micro-continent. In this study, we used data from 38 temporary seismological stations installed on a 400 km long profile from May to November 2003.The trend of the profile is N58°E across northern Zagros and part of the Central Iran. The stations are part of Zagros03 profile (Paul et al., 2010) operated by the International Institute of Earthquake Engineering and Seismology (IIEES) of Iran in collaboration with CNRS - Université Joseph Fourier, France. We examine the structure of the lithosphere, across the profile by analysis of P-wave receiver functions and Rayleigh wave fundamental mode phase velocity dispersion curves. Joint inversion of Rayleigh wave phase velocity dispersion and receiver functions have been used to estimate the velocity structure beneath 28 seismic stations. Receiver functions are time-series computed from the three-component body-wave seismograms and are sensitive to the earth structure near the receiver station. They are composed of P- to S-wave conversions in discontinuities under the stations. These converted waves are isolated by deconvolving the vertical component of a teleseismic P-wave record from its radial component. For each event, a 120s time-window centered at the direct P arrival is selected and used for the calculation of the receiver function. The deconvolution used is the iterative deconvolution method of Ligorria and Ammon (1999). Surface waves arise from the presence (boundary conditions) of the stress-free surface of the Earth, and in the presence of layering, they are dispersed. They provide valuable information on the absolute S-wave velocity, but they are relatively insensitive to sharp velocity contrasts. On the other hand, receiver functions are sensitive to S-wave velocity contrasts, which give rise to converted phases, but allow for a substantial trade-off between the depth and velocity above an impedance change. Combining them in a joint inversion process bridges the resolution gaps associated with each data set. We jointly inverted the stacked receiver function and surface wave dispersion data. We employ the program joint96 which is available in the software package “Computer Program in Seismology” (Herrmann and Ammon, 2003). In this study, we try to calculate the Moho depth and velocity structure in the north Zagros collision zone using the joint inversion of receiver function and surface wave dispersions. Receiver functions are calculated using teleseismic events of magnitude greater than 5.1, located between 30◦ and 95◦ epicentral distances. The fundamental mode Rayleigh-wave group velocities are extracted from thetomographic study conducted by Rahimi et al. (2014). The 1D velocity models resolved by joint inversion are juxtaposed, and a 2D velocity model is obtained. Results obtained from the 2D model reveal that the thickness of the sediments beneath the Zagros is 12 km, the Moho depth beneath this region of Zagros is 43-57 km, which increases towards Sanandaj–Sirjan zone and Urumieh–Dokhtar magmatic arc and reaches an expanse of 62 km and then decreases in the central Iran with a depth of 42 km. The velocity model confirms the presence of a crustal root and a thick high-velocity lithosphere beneath and north of the suture. These evidences imply that the Arabian plate continues to underthrust beneath the Central Iran.
The Zagros mountain belt, situated on the northern margin of the Arabian plate, is one of the youngest continental collision belts. This belt was formed by a collision between the Arabian plate and the Central Iranian micro-continent. In this study, we used data from 38 temporary seismological stations installed on a 400 km long profile from May to November 2003.The trend of the profile is N58°E across northern Zagros and part of the Central Iran. The stations are part of Zagros03 profile (Paul et al., 2010) operated by the International Institute of Earthquake Engineering and Seismology (IIEES) of Iran in collaboration with CNRS - Université Joseph Fourier, France. We examine the structure of the lithosphere, across the profile by analysis of P-wave receiver functions and Rayleigh wave fundamental mode phase velocity dispersion curves. Joint inversion of Rayleigh wave phase velocity dispersion and receiver functions have been used to estimate the velocity structure beneath 28 seismic stations. Receiver functions are time-series computed from the three-component body-wave seismograms and are sensitive to the earth structure near the receiver station. They are composed of P- to S-wave conversions in discontinuities under the stations. These converted waves are isolated by deconvolving the vertical component of a teleseismic P-wave record from its radial component. For each event, a 120s time-window centered at the direct P arrival is selected and used for the calculation of the receiver function. The deconvolution used is the iterative deconvolution method of Ligorria and Ammon (1999). Surface waves arise from the presence (boundary conditions) of the stress-free surface of the Earth, and in the presence of layering, they are dispersed. They provide valuable information on the absolute S-wave velocity, but they are relatively insensitive to sharp velocity contrasts. On the other hand, receiver functions are sensitive to S-wave velocity contrasts, which give rise to converted phases, but allow for a substantial trade-off between the depth and velocity above an impedance change. Combining them in a joint inversion process bridges the resolution gaps associated with each data set. We jointly inverted the stacked receiver function and surface wave dispersion data. We employ the program joint96 which is available in the software package “Computer Program in Seismology” (Herrmann and Ammon, 2003). In this study, we try to calculate the Moho depth and velocity structure in the north Zagros collision zone using the joint inversion of receiver function and surface wave dispersions. Receiver functions are calculated using teleseismic events of magnitude greater than 5.1, located between 30◦ and 95◦ epicentral distances. The fundamental mode Rayleigh-wave group velocities are extracted from thetomographic study conducted by Rahimi et al. (2014). The 1D velocity models resolved by joint inversion are juxtaposed, and a 2D velocity model is obtained. Results obtained from the 2D model reveal that the thickness of the sediments beneath the Zagros is 12 km, the Moho depth beneath this region of Zagros is 43-57 km, which increases towards Sanandaj–Sirjan zone and Urumieh–Dokhtar magmatic arc and reaches an expanse of 62 km and then decreases in the central Iran with a depth of 42 km. The velocity model confirms the presence of a crustal root and a thick high-velocity lithosphere beneath and north of the suture. These evidences imply that the Arabian plate continues to underthrust beneath the Central Iran.
 
Keywords: receiver functions, surface wave dispersion, joint inversion, velocity structure, north Zagroscollision

Keywords

References

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