Was the Pamir salient built along a Late Paleozoic embayment on the southern Asian margin?

Highlights

• Geology of the Northern Pamir does not support ∼300 km of N-ward translation.

• A Late Paleozoic S-Asian embayment controlled to form the convex Pamir salient.

• N-ward deflection of the Pamir terranes-suture zones occurred during accretion.

Abstract

While the northward convex Pamir salient has been interpreted to be a Cenozoic feature, resulting from terranes of the Pamir experiencing ∼300 km of northward translation relative to Tibet and Afghanistan during the India-Asia collision, structural evidence for large magnitudes of translation is generally lacking. We focus on the geology of the Northern Pamir, which forms the outer margin of the salient, to argue that the Pamir salient has not experienced significant northward translation as has been suggested by several recent studies. We then present evidence based on new detrital zircon results that the Pamir salient may be an inherited feature from the Late Paleozoic southern margin of Asia. Kinematic endmember models of northward translation of the Pamir predict either ∼300 km of arc-parallel extension (radial thrusting model) or truncation of geologic terranes at the margins of the salient by strike-slip faults (transfer faulting model). However, both models are inconsistent with the regional geology and structural evolution, which shows minimal extension along outer arc of the Pamir (∼50 km) and no clear truncation or thinning of geologic terranes that make up the Northern Pamir. Regarding the origin of the Pamir salient, a pronounced change in detrital zircon age signature of the Northern Pamir from west to east suggests the regions were not directly connected (i.e. part of a continuous linear belt), and were sourced from distinct cratons. Further, an absence of Triassic arc/forearc flysch deposits and arc plutons along WNW portion of the Northern Pamir suggests the possibility of a transform boundary segment along the Triassic Paleo-Tethys oceanic subduction zone. Based on these observations, we suggest an embayment existed between Tarim and Karakum Cratons after their amalgamation to the southern margin of Asia in the Early Permian. During the Late Triassic, colliding Cimmerian Gondwanan terranes (Central-Southern Pamir) filled in the embayment, resulting in the northward deflection in the trace of the Paleotethyan suture zone and Gondwanan terranes and the current arcuate geometry of the Pamir. Subsequent Cenozoic northward translation of the Pamir is interpreted to be only ∼50 km, no more than ∼80-100 km, and may be coupled with northward translation of northwestern Tibet and northeastern Afghanistan.

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.