The Tajik Basin: A composite record of sedimentary basin evolution in response to tectonics in the Pamir

Abstract

Investigation of a >6-km-thick succession of Cretaceous to Cenozoic sedimentary rocks in the Tajik Basin reveals that this depocentre consists of three stacked basin systems that are interpreted to reflect different mechanisms of subsidence associated with tectonics in the Pamir Mountains: a Lower to mid-Cretaceous succession, an Upper Cretaceous-Lower Eocene succession and an Eocene-Neogene succession. The Lower to mid-Cretaceous succession consists of fluvial deposits that were primarily derived from the Triassic Karakul-Mazar subduction-accretion complex in the northern Pamir. This succession is characterized by a convex-up (accelerating) subsidence curve, thickens towards the Pamir and is interpreted as a retroarc foreland basin system associated with northward subduction of Tethyan oceanic lithosphere. The Upper Cretaceous to early Eocene succession consists of fine-grained, marginal marine and sabkha deposits. The succession is characterized by a concave-up subsidence curve. Regionally extensive limestone beds in the succession are consistent with late stage thermal relaxation and relative sea-level rise following lithospheric extension, potentially in response to Tethyan slab rollback/foundering. The Upper Cretaceous-early Eocene succession is capped by a middle Eocene to early Oligocene (ca. 50-30 Ma) disconformity, which is interpreted to record the passage of a flexural forebulge. The disconformity is represented by a depositional hiatus, which is 10-30 Myr younger than estimates for the initiation of India-Asia collision and overlaps in age with the start of prograde metamorphism recorded in the Pamir gneiss domes. Overlying the disconformity, a >4-km-thick upper Eocene-Neogene succession displays a classic, coarsening upward unroofing sequence characterized by accelerating subsidence, which is interpreted as a retro-foreland basin associated with crustal thickening of the Pamir during India-Asia collision. Thus, the Tajik Basin provides an example of a long-lived composite basin in a retrowedge position that displays a sensitivity to plate margin processes. Subsidence, sediment accumulation and basin-forming mechanisms are influenced by subduction dynamics, including periods of slab-shallowing and retreat. K E Y W O R D S basin subsidence, foreland basins, geodynamics, stratigraphy, subduction-related basins, tectonics and sedimentation 526 | EAGE CHAPMAN et Al.

Parathethys Last Gasp in Central Asia and Late Oligocene Accelerated Uplift of the Pamirs

“…Field investigations show abrupt changes in lithology from the top of the Kamolin member and the base of the Childara member ( Figure S5), suggesting a regional depositional hiatus. This upper Eocene to lower Oligocene sedimentary hiatus is also found in the Cenozoic sedimentary sequences throughout the western Tarim Basin (Bosboom et al, 2014;Yang et al, 2015) and the eastern Tajik Basin (Chapman et al, 2019).…”

“…At~37.4 Ma, the Paratethys retreated from central Asia permanently. Because the ages for the last two regressions are significantly older than major global sea level falls at the Eocene-Oligocene transition but correlate with active tectonics in the Pamir and foreland basin deposition (Carrapa et al, 2015;Chapman et al, 2019), we suggest that regional tectonism played a more important role than global sea level changes in driving the final retreat of the Paratethys from central Asia.…”

“…B., Henriquez, S., Wang, M., DeCelles, P. G., et al (2019) The Tajik Basin was covered by the Paratethys sea during the early Cenozoic and is a flexural foreland basin related to the uplift of the Pamir-Tian Shan orogen ( Figure 1; Carrapa et al, 2015;Chapman et al, 2017). The Pamir formed as a result of accretion of allochthonous terranes to Eurasia during the Mesozoic and experienced significant crustal thickening and uplift since the late Eocene (Burtman & Molnar, 1993;Carrapa et al, 2015;Chapman et al, 2019). The Tian Shan is part of a Paleozoic orogenic system that was reactivated during the late Cenozoic (Sobel & Dumitru, 1997;Worthington et al, 2017).…”

Tajik Basin and Southwestern Tian Shan, Northwestern India‐Asia Collision Zone: 2. Timing of Basin Inversion, Tian Shan Mountain Building, and Relation to Pamir‐Plateau Advance and Deep India‐Asia Indentation

“…The preorogenic strata comprise Lower‐Middle Jurassic coal‐bearing clastic rocks followed by shallow‐marine limestones, Upper Jurassic evaporites, Lower Cretaceous (Cr 1 ) red beds, and variegated Upper Cretaceous (Cr 2 ) to Oligocene shallow‐marine strata. Upper Oligocene‐Miocene to Recent, synorogenic, mostly coarse‐grained clastic rocks comprise more than half (4–9 km) of the sedimentary section (e.g., GRI, 1961–1964; see Text S1 in the supporting information; Carrapa et al, 2015; Chapman et al, 2019; Klocke et al, 2017; Vlasov et al, 1991).…”

“…The Tajik basin of Central Asia is a retro‐foreland basin with ~7–10 km, southward and eastward thickening, Mesozoic‐Cenozoic strata at the northwestern tip of the India‐Asia collision zone (Figure 1a; e.g., Burtman & Molnar, 1993; Carrapa et al, 2015; Chapman et al, 2019; Klocke et al, 2017; Nikolaev, 2002). Jurassic‐Oligocene pretectonic, shallow‐marine to continental strata evolved into Neogene synorogenic, continental clastic deposits derived from the Tian Shan in the north and the Pamir and Hindu Kush in the east and south.…”