Tectonic Evolution of the Pamir Recorded in the Western Tarim Basin (China): Sedimentologic and Magnetostratigraphic Analyses of the Aertashi Section

Tamsin Blayney,Niels Meijer,Guillaume Dupont‐Nivet,Zhaojie Guo,Yani Najman,Ian Millar,Edward R Sobel,Jean‐Noël Proust,Pierrick Roperch

doi:10.1029/2018tc005146

Abstract

AbstractThe northward indentation of the Pamir salient into the Tarim basin at the western syntaxis of the India‐Asia collision zone is the focus of controversial models linking lithospheric to surface and atmospheric processes. Here we report on tectonic events recorded in the most complete and best‐dated sedimentary sequences from the western Tarim basin flanking the eastern Pamir (the Aertashi section), based on sedimentologic, provenance, and magnetostratigraphic analyses. Increased tectonic subsidence and a shift from marine to continental fluvio‐deltaic deposition at 41 Ma indicate that far‐field deformation from the south started to affect the Tarim region. A sediment accumulation hiatus from 24.3 to 21.6 Ma followed by deposition of proximal conglomerates is linked to fault propagation into the Tarim basin. From 21.6 to 15.0 Ma, increasing accumulation rates of fining upward clastics is interpreted as the expression of a major dextral transtensional system linking the Kunlun to the Tian Shan ahead of the northward Pamir indentation. At 15.0 Ma, the appearance of North Pamir‐sourced conglomerates followed at 11 Ma by Central Pamir‐sourced volcanics coincides with a shift to E‐W compression, clockwise vertical‐axis rotations and the onset of growth strata associated with the activation of the local east vergent Qimugen thrust wedge. Together, this enables us to interpret that Pamir indentation into Tarim had started by 24.3 Ma, reached the study location by 15.0 Ma and had passed it by 11 Ma, providing kinematic constraints on proposed tectonic models involving intracontinental subduction and delamination.

Highlights

The uplift and formation of the Pamir orogeny (Figure 1) in response to the archetypal India‐Asia continental collision have been linked to deep lithospheric processes of slab break off and roll back, continental subduction, and/or delamination (e.g., Chapman et al, 2017; Küfner et al, 2016; Sobel et al, 2013), as well as to surface processes of the Pamir plateau, and to Asian atmospheric circulation and related desertification (e.g., Caves et al, 2015; Thiede et al, 2013)
The sampling strategy involved digging trenches to preferentially sample the finest grain size in order to prioritize the single‐ domain magnetic particles that are more stable at longer timescales (Butler, 1992) and because the smallest grains are the most likely to align with the field during deposition
Sedimentologic, and provenance results from the Aertashi section of western Tarim provide a sedimentary record of Pamir deformation, revealing the following sequence of tectonic events. (1) 41–24.3 Ma; a marine to continental facies transition and increased clastic sediment accumulation are related to increased tectonic activity

Summary

Introduction

The uplift and formation of the Pamir orogeny (Figure 1) in response to the archetypal India‐Asia continental collision have been linked to deep lithospheric processes of slab break off and roll back, continental subduction, and/or delamination (e.g., Chapman et al, 2017; Küfner et al, 2016; Sobel et al, 2013), as well as to surface processes of the Pamir plateau, and to Asian atmospheric circulation and related desertification (e.g., Caves et al, 2015; Thiede et al, 2013). Tectonic models diverge in explaining the formation of the Pamir mountain range and its indentation into the Tajik and Tarim basins (the Pamir salient; Figure 1), in relation to the tomographically imaged southward dipping and 300‐km long Alai slab (e.g., Negredo et al, 2007). The subduction model (Burtman & Molnar, 1993; Sobel et al, 2013) suggests that the Pamir terranes penetrated ~300 km northward into Tarim as a result of continental subduction‐induced processes linked to roll back of the Alai slab constituted of Asian lithosphere. In the alternative delamination model (Bird, 1979), the Pamir formed from northward underthrusting of the Indian plate, which forced the delamination and rollback of the Alai slab constituted either of Asian

Results

Discussion

Conclusion