Was the Pamir salient built along a Late Paleozoic embayment on the southern Asian margin?
Introduction
Different geometric features along continental orogenic belts, such as salients and recesses, represent variations in map-view of along-strike complexities in the evolution of orogens (Macedo and Marshak, 1999; Marshak, 2004; Weil and Sussman, 2004). The Pamir, a ∼300 by 600 km orogenic salient located at the western end of the Himalaya-Tibetan orogen, stands out within the Apline-Zagros-Himalayan orogenic system due to its prominent northward convex shape and deflection of tectonic terranes (Fig. 1). Knowledge of how and when the Pamir salient was formed is not only important in understanding the tectonic evolution of the Pamir and western Tibetan-Himalaya orogenic belt (Burtman and Molnar, 1993), but also in understanding the development of large-scale orogenic curvature in general. Unlike salients developed in typical fold-thrust-belts, the Pamir is composed of a series of terranes and suture zones formed over different geologic periods. The complexity of large-scale orogenic curvature like Pamir salient raises questions about the primary kinematic mechanisms accommodating development of large-scale convex geometries, possible inheritance of pre-existing convex passive or active margins prior to accretion and whether different tectonic units in curved orogenic belts experience the same kinematic history.
Pioneering studies of Tibetan-Himalayan geology interpreted the convex Pamir to have formed during the Cenozoic India-Asia collision by displacement of a continuous, broadly linear sequence of pre-Cenozoic terranes and suture zones along the southern Asian margin from Afghanistan to Tibet (e.g. Tapponnier et al., 1981; Burtman and Molnar, 1993). This interpretation was supported by several geological observations and interpretations (Burtman and Molnar, 1993): 1) Paleozoic and Mesozoic terranes and suture zones in the Pamir are deflected to the north by ∼300 km; 2) Cretaceous to Paleogene facies belts in the Tarim and Tajik basins are displaced to the north by ∼300 km along the northern outer edge of the Pamir; 3) Declinations in Early Cretaceous and Paleogene sedimentary rocks in the western foreland of the Pamir record counterclockwise rotations interpreted to reflect Cenozoic bending of the Pamir; 4) Possible significant Cenozoic crustal shortening in the Tajik basin interpreted to result from the northward translation of the Pamir; and 5) a south-dipping lithospheric slab underneath the Pamir characterized by a well-developed Wadi-Beniov zone, interpreted to be ∼300 km of deeply subducted Asian continental lithosphere to compensate the Cenozoic northward translation of the Pamir (Burtman and Molnar, 1993; Thomas et al., 1996; Sobel and Dumitru, 1997; Negredo et al., 2007; Cowgill, 2010; Sippl et al., 2013a, Sippl et al., 2013b; Sobel et al., 2013; Schneider et al., 2013; Kufner et al., 2016; Perry et al., 2019; Molnar and Bendick, 2019).
While there has been a general consensus regarding the Cenozoic origin of the Pamir salient over the past 40 years (Tapponnier et al., 1981; Burtman and Molnar, 1993; Sobel and Dumitru, 1997; Cowgill et al., 2003; Cowgill, 2010; Schmidt et al., 2011; Sobel et al., 2011, 2013; Bosboom et al., 2014; Blayney et al., 2016, 2019; Molnar and Bendick, 2019), studies in the surrounding foreland basins have generally found low amounts of shortening calling into question the interpretation of large magnitudes of Cenozoic translation (Coutand et al., 2002; Chapman et al., 2017; Laborde et al., 2019; Li et al., 2019). However, the geology of the Pamir itself, in particular the Northern Pamir which forms the outer portion of the salient, has not been assessed in relation to predictions made by kinematic models of salient formation (i.e. radial thrusting and transfer faulting, Cowgill, 2010). Further, studies that suggest the Pamir salient may be a pre-Cenozoic feature have not addressed when and how the northward convex geometry was formed.
In this study, we first address the tectonic evolution of the Northern Pamir to demonstrate that regional geologic and structural relationships are inconsistent with kinematic predictions required by ∼300 km of northward translation of the Pamir, either during Cenozoic or Mesozoic. We then present evidence integrating new detrital zircon analyses with existing data that indicate fundamentally different signatures between the eastern and western Northern Pamir, and suggest that the current northward convex Northern Pamir-Western Kunlun belt may reflect the original geometry of the Late Permian through Triassic southern margin of Asia. Finally, we reinterpreted the regional Mesozoic and Cenozoic tectonics of the Pamir within the framework of limited Cenozoic northward translation and assess possible paleogeographic implications.
Section snippets
Overview
Geographically, the Pamir form the high plateau from the Alai Valley in Kyrgyzstan in the north to the Wakhan Corridor along the Tajikistan-Afghanistan border in the south. Structurally, the Pamir define a northward convex orogenic salient bounded by the Tarim craton in the northeast, the Karakum craton in the northwest, and the Tajik Tian Shan in the north (Fig. 1). The modern northern structural boundary of the Pamir salient is the Main Pamir Thrust (MPT) which links with the Kashgar-Yecheng
Kinematic models and predictions
Several different strategies have been used to classify the endmember kinematic models of salient formation (e.g. Carey, 1955; Macedo and Marshak, 1999; Weil and Sussman, 2004). Here we follow the classification of Macedo and Marshak (1999) and Marshak (2004) which proposed seven endmember controls for the formation of salients: 1) lower plate basin; 2) lateral variation of detachment properties; 3) hinterland indenter with radial thrusting; 4) hinterland indenter with strike-slip faults; 5)
A Late Paleozoic embayment between the Tarim and Karakum craton?
As regional geologic relationships do not support a Cenozoic hinterland indenter model for the origin of the Pamir salient, this raises the question as to when and how this prominent feature formed? Here we explore the possibility that the northward convex geometry of the Pamir was controlled by a curved Late Paleozoic-Triassic continental margin, and that the northward convex Northern Pamir is a reflection of its original geometry.
The Late Paleozoic-Triassic geometry of the southern Asian
Mesozoic accretion along a curved continental margin
Over the past 40 years, the Pamir Salient has been interpreted to be the result of ∼300 km of Cenozoic northward translation of the Pamir, resulting in northward deflection of terranes and suture zones from an initial linear trend between the Tibetan Plateau and Afghanistan (e.g. Tapponnier et al., 1982; Burtman and Molnar, 1993). Our interpreted northward convex embayment between the Karakum and Tarim cratons and original geometry of the Paleozoic-Triassic Northern Pamir (Fig. 5, Fig. 7)
Conclusion
While previous interpretations involving formation of the Pamir salient by ∼300 km of Cenozoic northward translation and deflection of suture zones and terranes from an initial linear trend has been widely accepted over past 40 years, several lines of evidence argue against large-scale translation of the Pamir salient, and suggest the current northward convex geometry was inherited from a Late Paleozoic embayment along the southern margin of Asia:
- 1)
Hinterland indenter models of the Pamir salient
CRediT authorship contribution statement
Yipeng Li: Conceptualization, Investigation, Methodology, Analysis, Writing - original draft, Writing - review & edit. Alexander Robinson: Conceptualization, Investigation, Methodology, Writing - review & edit, Supervision, Project administration, Funding acquisition. Mustafo Gadoev: Investigation, Resources. Ilhomjon Oimuhammadzoda: Investigation, Resources.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was funded by National Science Foundation (NSF) grant EAR-1450899 to Alexander Robinson. Dr. Thomas Lapen, Dr. Minako Righter, and Dr. Yongjun Gao at the University of Houston are thanked for their assistance on LA-ICP-MS. Dr. Dustin Villarreal at the University of Houston, Mr. Umedzhon Sharifov and Prof. Negmat Rajabov at the Tajik Academy of Sciences, and Mr. Saif in Dushanbe are thanked for their assistance in the field. Dr. An Yin is thanked for his editorial handling and
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2022, Journal of Structural GeologyCitation Excerpt :GPS results reported here confirm proximately cumulative shortening of 300 km to north of Kashi and ∼300–900 km of crustal shortening during the Cenozoic (Cao et al., 2013; Burtman and Molnar, 1993; Schmidt et al., 2011; Sippl et al., 2013). The age of initial thrusting and onset of the Cenozoic reactivation is 20–25 Myr as indicated by apatite fission-track ages and sediment records (Yin et al., 1998; Li et al., 2020). The controlling factors of FTB deformation considered in the present experimental study of Pamir are discussed below separately.