Large-scale three-dimensional seismic tomography of the Zagros and Pamir-Hindukush regions

Abstract

The nature of the deep seismic structure of the Zagros and Pamir-Hindukush regions is inferred from the three-dimensional P-wave velocity images obtained using the Aki, Christofferson and Husebye (ACH) inversion technique. We use teleseismic (25 ⩽ Δ ⩽ 90°) P-wave arrival times, recorded at 24 seismic stations during the period 1977–1986. A study of the azimuthal variation of residuals reveals that despite the crustal doubling, the Pamir-Hindukush region is characterized by faster arrivals (≈ − 1.0 s) of teleseismic P-waves. The Iranian plateau, on the other hand, exhibits delayed arrivals (0.4–1.0 s) from teleseisms. The results of the ACH inversion reveal contrasting P-wave velocity characteristics in the upper mantle of these deformed zones. In the depth interval 0–200 km, the Pamir-Hindukush region is characterized by 1–4% faster velocities relative to the Iranian plateau. Accounting for the anomalous crustal thickening in the Hindukush results in the subcrustal mantle (up to 200 km) exhibiting a significant higher velocity (≈ 2%), comparable to that of the Indian plate. However, in the depth interval 200–450 km, the Pamir-Hindukush region has 1–3% lower velocities relative to the Indian plate, while the upper mantle beneath the central Iranian plateau is 2–4% slower relative to the surrounding region. The high-velocity upper mantle till a depth of 200 km beneath the Hindukush region, reflects the possible presence of both the oceanic lithosphere and the crust and uppermost mantle of the Indian plate, which subducted beneath Eurasia. The low velocity observed beneath central Iran could be a reflection of the upper mantle thermal anomalies related to the Pliocene-Quaternary volcanism in this region.