关键词: 伊朗高原, 重力反演, 球坐标系, 莫霍面深度, 地震活动

Abstract:

The multi-stage tectonic evolution of the Iranian Plateau, as recorded in its deep lithospheric structure, provides a comprehensive geological archive of the complete transition from oceanic subduction to continental collision. This unique geological setting constitutes an ideal natural laboratory for investigating the geodynamic processes associated with incipient continental collision and plateau uplift mechanisms. The long-term convergence of the Arabian-Eurasian plate has led to the development of intricate tectonic deformation features, accompanied by significant seismic activity. Consequently, the deep crustal structure of this region, particularly the morphology of the Moho discontinuity, provides critical constraints for understanding the dynamics of continental collision, the seismogenic environment, and the processes of lithospheric evolution. Although various geophysical methods have been employed to investigate the deep structure of the Iranian Plateau in recent years, significant uncertainties remain due to the sparse distribution of seismic stations and the oversimplification of model assumptions.

To analyze the crustal structure and characteristics of strong seismic activity of the Iranian Plateau, this study calculated the Moho depth distribution using a fast nonlinear gravity inversion method in a spherical coordinate system, based on the GOCO06 static gravity field model, the CRUST1.0 model, as well as topographic and seismic data. The results demonstrate that the variable-density nonlinear gravity inversion method in a spherical coordinate system exhibits strong adaptability and effectiveness within the complex tectonic setting of the Iranian Plateau. The Moho depth distribution derived from this method shows a strong spatial correspondence with geomorphological features, tectonic structures, and seismic responses. Compared to the traditional Parker-Oldenburg frequency domain method, this method operates in the spatial domain, thereby avoiding boundary effects and truncation errors associated with the fast Fourier transform and improving the stability and physical interpretation of gravity inversion in large areas.

In addition, the Moho depth of the Iranian Plateau exhibits significant regional heterogeneity and lateral variability. The Zagros Fold-and-Thrust Belt and its adjacent regions exhibit the most profound Moho depth, generally exceeding 55km and locally reaching 60~65km, highlighting pronounced crustal thickening. In contrast, the blocks within central Iran show relatively gentle Moho depths(35~50km) and a more homogeneous crustal structure. The Makran subduction zone and the coastal areas along the Gulf of Oman have shallower Moho depths, mostly below 35km and locally less than 30km, suggesting a thinner crust. The South Caspian Basin exhibits Moho depths of 30~35km, demonstrating characteristic features of a typical thin-crust basin.

Furthermore, seismic activity analysis indicates that earthquakes with a magnitude of more than 5.0 are predominantly concentrated in regions characterized by steep Moho depth gradients and positive isostatic gravity anomalies. Areas with intense tectonic stress and insufficient isostatic compensation, such as the Zagros Fold-and-Thrust Belt, the Sanandaj-Sirjan Zone, and the Kopet Dag Mountains, are high-seismicity zones, where crustal stress is concentrated and seismic sources are densely clustered. Overall, the central region of Iran exhibits higher lithospheric rigidity, a stable crustal structure, and relatively low seismic activity levels.

This study demonstrates the effectiveness and applicability of the spherical-coordinate-based variable-density nonlinear gravity inversion method for investigating deep structures in geologically complex regions. The inversion results reveal a clear spatial correlation between the crustal structure and strong seismic activity on the Iranian Plateau, providing new geophysical evidence for understanding the region's deep tectonic framework and seismogenic environment. In addition, the results of this study indicate that this method not only effectively characterizes deep structural features under complex geological settings but also offers valuable insights into the interpretation of seismotectonic frameworks. Consequently, it provides a scientific basis for assessing regional crustal stability and informing strategies for earthquake hazard prevention and mitigation.

Key words: Iranian Plateau, gravity inversion, spherical coordinate system, Moho depth, seismic activity