The crustal and upper-mantle structure beneath NW Iran: an integrated analysis of surface waves and gravity data

Author(s)

Shirzad, Taghi ; VahidRavesh, Shaghayegh ; Mortezanejad, Gholamreza ; Abdollahi, Somayeh ; Kakhki, Mohsen Kazemnia ; Naghavi, Mojtaba ; Rahimi, Habib ; Hatami, Mohammad Reza

• Document type: Journal article
• Source: Geophysical Journal International; v. 239, n. 2, p. 18-pg., 2024-09-16.

Abstract

Understanding the crustal seismic characteristics of tectonically active regions is crucial for seismic hazard assessment. The study conducted in NW Iran utilized surface wave tomography, radial anisotropy and density information to analyse the complex crustal structure of the region, which is outstanding because of diverse tectonic features, sedimentary basins and volcanic formations. By selecting a data set of 1243 events out of over 3500 earthquakes with M > 4, and employing strict data selection criteria (such as SNR, M and Delta), the researchers calculated Rayleigh and Love wave group velocity dispersion curves using Gaussian multiple filters and phase-matched filtering. The tomographic procedure was initiated by excluding data with residuals > 2 sigma for enhanced stability. Individual inversions were then carried out for local Rayleigh and Love wave dispersion measurements to obtain 1-D V-SV and V-SH models. Radial anisotropy and V-S(iso) were determined through a discrepancy and averaging of the obtained V-SH and V-SV, respectively. Gravity modelling was also employed alongside surface wave analysis to understand the region's complex geology, revealing insights into upper-middle-lower crust boundaries, subsurface structures and Moho depths. The study's velocity maps reveal significant findings related to geological units and tectonic features in various regions based on the provided results. Low velocities in the South Caspian Basin (SCB) and Kura Depression (KD) regions are attributed to substantial sedimentary layers, while low velocities, and depth of V-S in NW Iran and Eastern Anatolian Accretionary Complex (EAAC) regions suggest the presence of partially molten materials in the upper and middle crust. The Sanandaj-Sirjan Zone (SSZ) region shows a low-velocity anomaly in longer periods and greater depths of V-S, surrounded by normal to high velocities, indicating a thick middle crust. Analysing radial anisotropy and V-S(iso) profiles offers insights into upper-middle-lower crust boundaries, subsurface structures and Moho depths, highlighting middle crust thickening and lower crust thinning beneath the SSZ. The study confirms the gentle subduction of the SCB oceanic-like lower crust beneath NW Iran in the Talesh (TAL) region, with a rigid middle crust. Additionally, cross-sections reveal igneous laccoliths underplate with a V-S(iso) of 3.7 km s(-1) in the volcanic region. The difference observed by subtracting the velocity models at two adjacent depths, combined with parametric test results, indicates that the Sahand volcanic system is clearly identifiable, while the influence of subtle subduction on the Sabalan volcano at depths up to 30 km remains less distinct. The magma chamber beneath Sahand is situated at depths ranging from 18 to 25 km. (AU)

• FAPESP's process: 16/20952-4 - Environmental noise tomography in the Pantanal and Paraná basins
• Grantee: Taghi Shirzad Iraj
• Support Opportunities: Scholarships in Brazil - Post-Doctoral
• FAPESP's process: 13/24215-6 - Pantanal-Chaco-Paraná basins (PCPB): crust and upper mantle seismic structure and evolution
• Grantee: Marcelo Sousa de Assumpção
• Support Opportunities: Research Projects - Thematic Grants