The Influence of Foreland Structures on Hinterland Cooling: Evaluating the Drivers of Exhumation in the Eastern Bhutan Himalaya

Abstract

Understanding, and ideally quantifying, the relative roles of climatic and tectonic processes during orogenic exhumation is critical to resolving the dynamics of mountain building. However, vastly differing opinions regarding proposed drivers often complicate how thermochronometric ages are interpreted, particularly from the hinterland portions of thrust belts. Here we integrate three possible cross‐section geometries and kinematics along a transect through the eastern Bhutan Himalaya with a thermal model (Pecube‐D) to calculate the resulting thermal field and predict potential ages. We compare predicted ages to a suite of new and published cooling ages. Our results argue for ramp‐focused exhumation of the Main Central thrust from 16 to 14 Ma at shortening rates of 40–55 mm/year, followed by slower rates (25 mm/year) during the last 50 km of Main Central thrust displacement and growth of the Lesser Himalayan duplex from 14 to 11 Ma. Emplacement of frontal Lesser Himalayan thrust sheets occurred rapidly (55–70 mm/year) between ~11 and 9 Ma, followed by a decrease in shortening rates to ~10 mm/year during motion on the Main Boundary thrust. Modern shortening rates (17 mm/year) and out‐of‐sequence motion on the Main Boundary thrust from 0.5 Ma to present reproduce the young cooling ages near the Main Boundary thrust. We show that the dominant control on exhumation patterns in a fold‐thrust belt results from the evolution of ramps and emphasize that the geometry and kinematics of structures driving hinterland exhumation need to be evaluated with their linked foreland structures to ensure the viability of the proposed geometry, kinematics, and thus cooling history.

Deformational Temperatures Across the Lesser Himalayan Sequence in Eastern Bhutan and Their Implications for the Deformation History of the Main Central Thrust

“…(a) McQuarrie et al (2019) model a. (b) McQuarrie et al (2019) model b. (c) McQuarrie et al (2019) model c. Line of section by McQuarrie et al (2019) is indicated as B in Figure 1.…”

“…The MCT mylonitic belt extends structurally downward for at least 3 km, as documented by pervasive ductile deformation of quartzites with progressive changes in dynamic recrystallization mechanisms (Grujic et al, 1996; Long, McQuarrie, Tobgay, & Hawthorne, 2011; Long et al, 2016; Starnes et al, 2020; this work). In addition, several bodies of mid‐Proterozoic orthogneiss have been mapped within the Daling‐Shumar Group (Daniel et al, 2003; Gansser, 1983; McQuarrie et al, 2019). Their contacts with metasedimentary rocks are highly strained, characterized by mylonites and ultramylonites (McQuarrie et al, 2008; our field observations).…”

“…The 16 new and published muscovite 40 Ar/ 39 Ar thermochronometer ages are combined with 4 published apatite and 11 zircon (U‐Th)/He and 37 apatite fission track ages from Coutand et al (2014) (Figure 7 and Table S6 in the supporting information) to perform formal inversions and assess the sensitivity of thermochronometric data to tectonic scenarios involving changing displacement rates on the MHT and various duplex‐driven uplift processes. Although many more data are available for the region (Coutand et al, 2014; Long et al, 2012), the data were compiled along a relatively narrow, 25 km wide swath (Figure 7) to avoid the influence of lateral structural variations in both the structures in the LHS (e.g., Long, McQuarrie, Tobgay, & Grujic, 2011; Long et al, 2012; McQuarrie et al, 2019) and the MHT itself (e.g., Singer et al, 2017). The inversions use the neighborhood algorithm (Rickwood & Sambridge, 2006; Sambridge, 1999a, b) to select input values for the three‐dimensional (3‐D) thermokinematic model Pecube (Braun, 2003; Braun et al, 2012) and determine permissible ranges of geological parameters such as fault geometry, fault slip rate, crustal thermal properties, and the width, location and rate of duplex‐induced accretion.…”

“…The modern topography is shifted at the start of the simulations such that lateral advection of the topography resulting from fault movement translates the model topography to its present‐day location at 0 Ma (see Appendix F of Coutand et al, 2014, for details). We note that some recent works have utilized an alternative approach to modeling topographic development in thermokinematic numerical models in Bhutan (McQuarrie et al, 2019; McQuarrie & Ehlers, 2015) where topography develops in response to fault‐driven uplift with a defined elevation limit based on the taper angle of the topography across the Bhutanese Himalaya. While this approach has the benefit of simulating natural topographic growth, we have opted to utilize lateral translation of the modern topography because it is the simplest topographic treatment that ends with the present‐day topography and includes its influence on the thermochronometer age data.…”

“…For comparison indicated are the published MHT geometries in dashed lines: (l) Long et al (2011) section AA′ in Figure 1. (a) McQuarrie et al (2019) model a. (b) McQuarrie et al (2019) model b.…”