Abstract

This paper examines the stratigraphy, structure, and seismicity of the Peter the First Range, an actively deforming foreland basin in Soviet Tadjikistan. The range represents a highly deformed segment of a large intracontinental basin, the Tadjik Depression, which is being shortened in response to the Cenozoic convergence between two orogenic massifs, the Pamir and the Tien Shan Ranges. The Late Jurassic to Paleogene stratigraphy of the Peter the First Range includes more than 2,000 m of shallow marine to nonmarine sediments deposited under relatively quiescent conditions, close to sea level. In contrast, the correlative units of the northernmost Pamir, directly south of the Peter the First Range, show evidence of proximity to a major, uplifted source area to the south. These Mesozoic-early Cenozoic strata are overlain by thick sequences of alluvial sandstone and conglomeratic units that coarsen upward through the Neogene and record the progressive uplift of the Pamir. Shortening within the Peter the First Range is recorded by kilometer-scale isoclinal, upright-to-over-turned folds, and imbricate, north-verging thrusts that emerge near the northern edge of the range and mark the active tectonic boundary with the Tien Shan. Most of the deformation appears to postdate the accumulation of the Mio-Pliocene molasse. The stratigraphy and structure of the region are best explained by a model of the Mesozoic Tadjik Depression as an intracontinental, foreland basin, developed in a back-arc position with respect to the active Andean or collisional orogens located in northern Afghanistan and the Pamir. Seismicity in the region is dominated by earthquakes extending from the surface to ∼12-km depth, apparently occurring within the deformed sediments of the Peter the First Range. A subhorizontal zone of relatively low activity at this depth may mark the location of a basal detachment that underlies a structurally thickened sedimentary section. This aseismic zone is in turn underlain by a south-dipping belt of seismicity that extends from about 17- to 35-km depth, and which can be traced updip to the seismogenic crystalline basement of the Tien Shan range to the north. We interpret this south-dipping structure as a zone of intracontinental subduction that may be an updip continuation of the enigmatic, intermediate-depth Wadati-Benioff Zone beneath the Pamir Range to the south.

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