Hindu Kush earthquakes: Relict seismicity in the zone of a closed Cretaceous-Paleogene basin

Abstract

We consider different formation models of deep focus earthquakes at the boundary between the Pamir and Hindu Kush mountains including the Hindu Kush megasource. Reconstruction of the Late Cretaceous‐ Paleogene basin between the Northern and Central Pamirs allows us to connect anomalously high seismicity with the deformation of the subduction plate (slab) of the oceanic lithosphere in this basin during its submersion into the mantle. In this sense the Vrancea zone in the Carpathians is an analog of the Hindu Kush megasource. The concentration of mantle earthquakes of the intermediate type with hypocenter depths up to 300 km and magnitudes up to 8 and greater [1] within a small megasource region at the boundary of tectonic zones in the Hindu Kush, Central, and Southwestern Pamirs has long attracted the interest of researchers (Fig. 1). Here, 90% of strong earthquakes in the Pamir‐Hindu Kush zone and 95% of released energy are concentrated on a territory of 200 × 120 km [2]. In the western region, mantle earthquakes are sharply interrupted at 69 ° 30 � E. The zone of mantle earthquakes approximately 350 km long with hypocenter depths up to 200 km is extended to the northeast of the megasource already on the territory of the Pamirs. Thus, the belt of mantle seismicity with a total length up to 500 km is clearly pronounced. The Hindu Kush source is confined to its southwestern part. L.I. Lobkovskii suggested a model based on the concept of two-level plate tectonics to explain the nature of the Hindu Kush earthquakes [3‐5]. Calculations demonstrated that dissipative heating of the upper parts of the mantle up to 700‐750 ° C occurs during the underthrust of the continental lithosphere in the collision zones. At such temperatures, the mechanism of brittle deformation (earthquake) becomes impossible. At the same time, the mantle part of the lithosphere separated by the asteno-layer from the crust can submerge into the upper mantle. This process is generally aseismic. Deep-focus mantle earthquakes appear only in the anomalous regions either as a result of insufficient heating, or due to the penetration of water or other fluids. In the latter case, the rocks become brittle and capable of seismogenic deformations. According to the other version [2], mantle seismicity in the Hindu Kush is related to the submersion of rocks of the ancient oceanic crust (Paleozoic or Early Mesozoic ophiolites) to the depths of 40‐70 km under the weight of the tectonic nappes. In this case, the basalt-eclogite phase transition occurs and their submersion to the upper mantle of lower density. This is an earthquake-related process. We eventually admit that the local source of mantle seismicity can be related to submersion, deformation, and separation of a slab of oceanic lithosphere in the basin that had closed recently. For example, seismicity in the Vrancea zone (East Carpathians) is related to the residual phenomena in the subduction zone of a deep basin that completely closed in the Middle‐Late Miocene (see Fig. 4.11 in [5] and references in this publication). Such an explanation was not applied to the Hindu Kush earthquakes because the existence of a young basin and related subduction were not assumed in this region.