Testing the effects of topography, geometry, and kinematics on modeled thermochronometer cooling ages in the eastern Bhutan Himalaya

Abstract

Abstract. In this study, reconstructions of a balanced geologic cross section in the Himalayan fold-thrust belt of eastern Bhutan are used in flexural-kinematic and thermokinematic models to understand the sensitivity of predicted cooling ages to changes in fault kinematics, geometry, topography, and radiogenic heat production. The kinematics for each scenario are created by sequentially deforming the cross section with ∼ 10 km deformation steps while applying flexural loading and erosional unloading at each step to develop a high-resolution evolution of deformation, erosion, and burial over time. By assigning ages to each increment of displacement, we create a suite of modeled scenarios that are input into a 2-D thermokinematic model to predict cooling ages. Comparison of model-predicted cooling ages to published thermochronometer data reveals that cooling ages are most sensitive to (1) the location and size of fault ramps, (2) the variable shortening rates between 68 and 6.4 mm yr −1 , and (3) the timing and magnitude of out-of-sequence faulting. The predicted ages are less sensitive to (4) radiogenic heat production and (5) estimates of topographic evolution. We used the observed misfit of predicted to measured cooling ages to revise the cross section geometry and separate one large ramp previously proposed for the modern décollement into two smaller ramps. The revised geometry results in an improved fit to observed ages, particularly young AFT ages (2-6 Ma) located north of the Main Central Thrust. This study presents a successful approach for using thermochronometer data to test the viability of a proposed cross section geometry and kinematics and describes a viable approach to estimating the first-order topographic evolution of a compressional orogen.

Building a Young Mountain Range: Insight Into the Growth of the Greater Caucasus Mountains From Detrital Zircon (U‐Th)/He Thermochronology and 10Be Erosion Rates

“…For the detrital samples, there is not a single time-temperature path, so we first calculate a mean cooling rate for the given time intervals for both the top and bottom of the catchment and then calculate the mean cooling rate as the average of these two. In reality, the relationship between cooling rate and exhumation rate is non-trivial because of the advection of heat and the evolving geothermal gradient in response to both rock uplift and erosion (e.g., Moore & England, 2001;Willett & Brandon, 2013), especially in thrust belts due to the potential importance of lateral motion (e.g., Batt & Brandon, 2002;Gilmore et al, 2018;Lock & Willett, 2008).…”

Causes of Variable Shortening and Tectonic Subsidence During Changes in Subduction: Insights From Flexural Thermokinematic Modeling of the Neogene Southern Central Andes (28–30°S)

“…Finally, this study builds on development and recent applications of integrated flexural and thermokinematic modeling (e.g., Braza & McQuarrie, 2022a,;Ghoshal et al, 2020;Gilmore et al, 2018;McQuarrie & Ehlers, 2015;McQuarrie et al, 2019;Parks & McQuarrie, 2019;Rak et al, 2017) and multiple flexural, kinematic, and thermochronological investigations of retroarc evolution in the southern central Andes (∼27-32°S; Dávila et al, 2007Dávila et al, , 2010Fosdick et al, 2017;Mardonez et al, 2020;Nassif et al, 2019;Stevens Goddard & Carrapa, 2018). The framework for estimating erosion and tectonic/sedimentary load dimensions varies among these examples, often resulting in discrepancies in predicted subsidence and exhumation (e.g., Mardonez et al, 2020;Nassif et al, 2019).…”

“…Each deformation step creates a tectonic load, defined by the two-dimensional difference between the deformed topographic surface and the previous topographic profile, for which flexural subsidence is calculated (deflection of a continuous elastic sheet; Turcotte & Schubert, 2002). Erosion is estimated by assigning a new topographic surface that increases at an assigned taper angle (α) from the deformation front where structural uplift has occurred but otherwise matches previous, flexurally loaded topography (Dahlen, 1990;Gilmore et al, 2018;McQuarrie & Ehlers, 2015). The new topographic profile and α are maintained while material above the erosion surface is instantaneously removed and compensated though flexural unloading.…”

“…Extensive structural, chronostratigraphic, provenance, and thermochronometric data have been published for the region; we contribute five new detrital zircon U-Pb and 19 apatite (U-Th)/He sample analyses to improve coverage where the Precordillera and Sierras Pampeanas converge at ∼28.5-29°S. We then demonstrate a flexural thermokinematic forward modeling workflow that calculates flexural subsidence in response to an evolving tectonic load derived from balanced cross section and kinematic interpretations, and approximates erosion based on critical wedge growth (e.g., Braza & McQuarrie, 2022a;Gilmore et al, 2018;McQuarrie & Ehlers, 2015;Rak et al, 2017). This modeling procedure predicts incremental subsidence, topographic, and exhumation histories that are evaluated against observed foreland basin accumulation, modern topography, and thermochronology data along each profile.…”

Examining the tectono-stratigraphic architecture, structural geometry, and kinematic evolution of the Himalayan fold-thrust belt, Kumaun, northwest India

“…A younger muscovite 40 Ar/ 39 Ar cooling age in Garhwal could imply an age for motion on the Main Central thrust as young as 6.3 Ma. However, young cooling ages north of the Main Central thrust are common along the Himalayan arc due to uplift and exhumation from a ramp in the Main Himalayan thrust Ehlers, 2015, 2017;Gilmore et al, 2018). In far western Nepal, the eastern extension of the Almora klippe is the Dadeldhura klippe.…”