Surface wave tomography of the Makran subduction zone

Document Type : Research Article

Authors

1 Ph.D. student, Department of Earth Sciences, Institute for Advanced Studies in Basic Sciences, Zanjan, Iran

2 Associate Professor, Department of Earth Sciences, Institute for Advanced Studies in Basic Sciences, Zanjan, Iran

Abstract

This study presents a 3-D model of the shear wave velocity (Vs) for SE Iran, containing the Makran subduction zone and termination of the Zagros collision zone. The model is developed using surface wave tomography and the associated local dispersion curves that were inverted to the Vs velocity maps. Our findings provide insights into the tectonics of the SE Iran region and the geometry of the subducting lithosphere. To obtain Rayleigh waveforms, we extracted data from 458 teleseismic earthquakes recorded by 58 seismic stations in the Makran region between June 2016 and May 2019. Events with magnitudes greater than 5.5, epicentral distances between 30º and 120º, and depths shallower than 50 km were considered for two-plane wave tomography. This resulted in phase velocity maps at nine periods ranging between 25 s and 125 s. Our results suggest the presence of a high-velocity anomaly at the northern coast of the Gulf of Oman extending ~100 km northward up to the Qasr-e Qand fault and the southern edge of the Jaz Murian depression as it was observed at shorter periods of 25-40 s. Further west of Makran and north of the Strait of Hormuz, there is also a high-velocity anomaly at the southwest of the Zendan-Minab-Palami fault at periods of 25-40 s. At periods of 50-125 s, this anomaly is observed at the northern latitudes beneath the Sanandaj-Sirjan zone at the termination of the Zagros collision zone. Our analysis suggests that this anomaly reveals underthrusting of the Arabian lithosphere under Sanandaj-Sirjan zone and Urumieh-Dokhtar magmatic arc. Additionally, we detected a low-velocity anomaly at periods of 25-33 s showing a crustal root potentially generated by underthrusting of the Arabian lithosphere in this region. At the second inversion step, we employed a nonlinear inversion of the local dispersion curves to construct a 3-D Vs model. Our results indicate that there is a flat
high-velocity anomaly in the middle of the study region (in the western Makran and under the depression of Jaz Murian), indicating a horizontal oceanic lithosphere, potentially remaining from a truncated oceanic lithosphere between the collision and subduction zones. Furthermore, our findings suggest the presence of a near-horizontal oceanic lithosphere in the eastern part of the study area, extending at a low angle underneath the northern edge of the Jaz Murian depression and subsequently subducting beneath the volcanic arc with a steep angle. Finally, we identified a low Vs anomaly at the crustal-scale in the Sistan suture zone, which stretches towards the north-northwest and is limited by the large N-S directed strike-slip faults.
 

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References

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Iranian Journal of Geophysics
Volume 17, Issue 5
January and February 2024
Pages 117-133

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