Evidence for Plio‐Pleistocene north‐south extension at the southern margin of the Tibetan Plateau, Nyalam region

Abstract

[1] The southern Tibetan Plateau margin between~83 E and 86.5 E is defined by an abrupt change from the low-relief Tibetan Plateau to the rugged topography and deep gorges of the Himalaya. This physiographic transition lies well to the north of active thrusting, and thus, the mechanism responsible for the distinct topographic break remains the focus of much debate. While numerous studies have utilized thermochronology to examine the exhumation history of the Himalaya, few have done so with respect to variations across the Himalaya-Tibetan Plateau transition. In this work, we examine the nature of the transition where it is accessible and well-defined in the Nyalam valley of south-central Tibet. We employ several new and previously published thermochronologic datasets (with a closure temperature range of~70C-300 C) in conjunction with river incision patterns inferred by the longitudinal profile of the Bhote Kosi River. The results reveal a sharp change in cooling rate at~3.5 Ma at a location corresponding to a pronounced river knickpoint representing a sharp increase in river gradient and presumably incision rate (a proxy for rock uplift). Margin retreat models for the physiographic transition are inconsistent with the cooling pattern revealed by low-temperature thermochronologic data. Models invoking passive uplift of the upper crust over a midcrustal ramp and associated duplex to account for the physiographic transition do not explain the observed break in cooling rate there, although they may explain a suggesting in the thermochronologic data of progressively increasing exhumation rates south of the transition. The simplest model consistent with all observations is that passive uplift is augmented by contemporaneous differential uplift across a young (Pliocene-Quaternary) normal fault at the physiographic transition. Drawing on observations elsewhere, we hypothesize that similar structural relationships may be characteristic of the Tibetan Plateau-Himalaya transition from~83 E -86.5 E.

Evidence for Pleistocene Low-Angle Normal Faulting in the Annapurna-Dhaulagiri Region, Nepal

“…Recent work in the Nyalam region of southcentral Tibet documented evidence for PlioPleistocene, top-to-the-north displacement on a low-angle detachment at approximately the same structural level as the detachments described here (McDermott et al 2013). As is the case in the Kali Gandaki and Myagdi Khola areas, the structure near Nyalam also disrupts the local pattern of cooling ages for (U-Th)/He thermochronometers.…”

“…A number of previous studies of Greater Himalayan Sequence samples from Nepal have yielded Pliocene-Quaternary fission-track and (UTh)/He dates (e.g., Blythe et al 2007;Robert et al 2009;Herman et al 2010;Nadin and Martin 2012;Streule et al 2012;McDermott et al 2013), and our samples from the Annapurna-Dhumpu footwall in figure 2. Note: vertical exaggeration is not the same for both cross sections.…”

“…For samples from farther south and at deeper structural levels, the same assumption is likely to be incorrect because of evidence in the region for accelerated exhumation since the Pliocene McDermott et al 2013), and we instead assumed onset for the most recent exhumation phase at 4 Ma (just slightly older than the oldest ZrnHe date).…”

“…Quaternary displacement on E-W-to NW-SE-striking, shallowly to moderately N-dipping faults with variable components of northward displacement has been documented near the range crest in several regions spanning a distance of more than 600 km across the central Himalaya. Some of these faults have been interpreted previously as part of the STFS (Wu et al 1998;Hurtado et al 2001), while others transect the Miocene structural grain of the Himalaya and have been regarded as distinctive (Nakata 1989;McDermott et al 2013;Murphy et al 2014). The tectonic significance of this young extensional faulting has not been established with confidence because of uncertainties in the regional extent of individual fault strands, a general lack of direct constraints on the timing of faulting, and the absence of estimates of slip magnitude on many strands.…”

Evolution and variability of the Asian monsoon and its potential linkage with uplift of the Himalaya and Tibetan Plateau

“…There is some evidence of accelerated erosion in the Himalayan region during the PlioPleistocene (Huntington et al 2006). For example, McDermott et al (2013) suggest an increase in the erosion rate in the area south of STDS at~3.5 Ma in the Nyalam region in the central Himalaya.…”