Geometry, kinematics, and magnitude of extension across the Thakkhola Graben, Central Nepal Himalaya

Abstract

The Thakkhola Graben has been a region of geologic inquiry for many decades. Although it is widely viewed to be in a class of structures that are important in accommodating the three-dimensional strain within the Himalayan thrust wedge, we still lack a detailed understanding of the total finite strain accommodated by graben-bounding faults, as well as their shape and cross-cutting relationships with structures deeper in the thrust wedge. Using geologic mapping and structural analysis, we show that a suite of pre-extensional shortening structures is offset by normal-oblique faults bounding the Thakkhola Graben that we use to define a piercing line. We calculate these faults to have accommodated 8.7 kilometers of vertical thinning, 7.2 kilometers of arc-perpendicular shear, and only 2.2 kilometers of arc-parallel extension. The magnitude of arc-parallel extension is quite low compared to extensional structures to the west in the Gurla Mandhata-Humla region. The cross-cutting relationships established in this study and timing constraints determined by previous works are consistent with a structural history of crustal thickening leading to foreland propagation of the locus of arc-perpendicular shortening contemporaneous with hinterland extension.

Prolonged Slip on the South Tibetan Detachment Constrains Tectonic Models for Synorogenic Extension in the Central Himalaya

“…This similarity may be coincidental, but it also could be interpreted as supporting the argument of Baltz et al. (2021) that the Upper Mustang massif exposes regionally metamorphosed STDS footwall rocks rather than contact metamorphosed Tibetan Sedimentary sequence rocks. More importantly, published evidence for Dangardzong fault activity at ca.…”

“…We show the Dangardzong fault as entirely restricted to the hanging wall of the Annapurna detachment at that time with the detachment acting as a decoupling horizon such that the upper crust was extending parallel to the Himalayan arc, while crust below continued to accommodate N-S convergence. (We are aware of no data supporting the contention of Baltz et al (2021) that the Dangardzong fault "does not sole into the STDS beneath the Thakkhola graben").…”

“…Panel (d 1 ) illustrates the configuration we envision if the host rocks for the Mustang and Mugu leucogranites are contact metamorphosed units of the Tibetan Sedimentary sequence as interpreted by Lihter et al (2020). Panel (d 2 ) depicts an alternative configuration if we accept the existence of the Mustang detachment inferred-but not directly studied-by Baltz et al (2021). Those authors correlated their Mustang detachment with the basal STDS exposed farther south and inferred that the two equivalent structures are today exposed on either flank of a large, upright synform.…”

Megathrust Heterogeneity, Crustal Accretion, and a Topographic Embayment in the Western Nepal Himalaya: Insights From the Inversion of Thermochronological Data

“…(2020) are very consistent with our new data from the northern Himalaya antiform and the hinge of the Dolpo syncline, far away from the fault. The graben‐bounding fault is steep and has only accommodated 2.2 km of wedge‐parallel extension (Baltz et al., 2021). Therefore, the graben structure is unlikely to affect the thermal structure of our sampling area.…”

“…In the western Nepal Himalaya, the Northern Himalayan antiform is characterized by an east‐west‐trending belt of dome‐shaped outcrops of GHS rocks from 81°E to 92°E (Figure 1). The Thakkhola graben is bounded by several steeply dipping north‐south striking normal faults (Baltz et al., 2021; Hurtado et al., 2001), and the Gurla Mandhata area is a metamorphic core complex bounded by the Gurla Mandhata‐Humla fault system (Murphy & Copeland, 2005; Murphy et al., 2002). They both developed mainly starting from middle‐late Miocene and accommodated orogen‐parallel extension (Brubacher et al., 2020; Coleman & Hodges, 1995; McCallister et al., 2014; Murphy & Copeland, 2005; Murphy et al., 2002).…”