Mechanical Models for East-West Extension within the Pamir Orogen: Insights from of the Holocene slip rate of the Kongur Normal Fault

The Pamir orogen, laying at the northwestern syntaxis of the Indo-Asian collision zone and one of the most tectonically active regions of central Asia, experiences significant extension in its interior. Although internal extension is common in mature orogens, the Pamir is special because (i) its extension is primarily concentrated on the Kongur Extensional System (KES) rather than distributed on multiple normal faults; (ii) the KES is localized at the eastern region of the Pamir rather than centralized, suggesting asymmetric extension; and (iii) extension along the KES decreases southward, instead of decreasing from central portion to its northern and southern ends. Because of these unique characteristics, the causes of internal extension of the Pamir and formation of the KES have inspired numerous investigations, which in turn have led to the proposal of various mechanical models. Defining multi-timescale deformation rates along the KES, especially for late Quaternary and modern slip rate, is prerequisite for better understanding the nature of extension in the Pamir.

In this study, we focus on one of the most debated structure within the Pamir: the nearly NNW-trending Kongur Normal Fault (KNF), the most primary and striking part of the KES. This fault is characterized by dramatically increased topography (elevation up to > 7,500 m) which is expressed as lofty Kongur and Muztaghata massif in its footwall. Thermochronology ages suggest that the initiation, largest magnitude of extension and highest long-term exhumation rates along the KES are in the vicinity of the Kongur massif. Although some studies have focused on determining the tectonic activity of the KES since the late Cenozoic, almost no late Quaternary rates estimate yet exists on active fault (KNF) bounding the Kongur massif. Moreover, Glaciers have oscillated considerably throughout the Quaternary at Kongur and Muztaghata massif, offering a unique opportunity to expand our understanding of the role of glaciers in shaping the topography.

At Bulunkou, the KNF is branched into two segment. The surface trace of the both segments of KNF is clearly visible as a straight line feature, and characterized by offsets of different geomorphic surfaces. We determined the Holocene slip rates of both segment of KNF through geomorphic mapping on high-resolution DEMs and cosmogenic ¹⁰Be exposure dating of boulders on displaced geomorphic surfaces. Finally, we observed a very high Holocene slip rates (even probably can reach to 10-13 mm/a) of KNF at Bulunkou. Correlating our new observations of KNF with kinematics and slip rates along the whole KES, we clarify the role of the KES in accommodating internal extension of the Pamir.