Influence of thrust belt geometry and shortening rate on thermochronometer cooling ages: Insights from thermokinematic and erosion modeling of the Bhutan Himalaya

• Published in: Tectonics (Washington, D.C.) D.C.), 2015-06, Vol.34 (6), p.1055-1079
• Main Authors: McQuarrie, Nadine, Ehlers, Todd A.
• Format: Article
• Language: English
• Subjects: Basins; Cooling; fold-thrust belts; Geometry; Heat transport; Himalayas; Kinematics; Rocks; shortening rates; thermal modeling; thermochronometry
• ISSN: 0278-7407; 1944-9194
• DOI: 10.1002/2014TC003783

Advancements in thermochronology and numerical modeling offer the potential to associate the age of thermochronometric samples to both exhumational and deformational processes. However, understanding how these components are related in compressional systems requires linking the geometry and magnitude of fault slip to the distribution and amount of erosion. To address this, we apply a 2‐D thermokinematic model to a forward modeled balanced cross section to quantify the cooling history in fold‐thrust belt settings. The restored cross section provides a kinematic path of rocks and structures necessary to reproduce the surface geology. By assigning ages to displacement amounts, we produced a range of potential velocity vectors used to calculate heat transport, erosion, and rock cooling. We test the predicted ages against a suite of previously published thermochronometric data from the Bhutan Himalaya to explore the utility of the data to constrain the timing, rate, and geometry of fault motion as well as variations in the exhumation rate. We evaluate the cooling history associated with a constant rate of shortening of 18 mm/yr, rates that are 2.0, 1.5, 0.75, and 0.5 times the constant rate, and rates that vary with time to determine which kinematic history best matches the measured cooling ages. The combination of relatively old apatite fission track and zircon (U‐Th)/He measured ages and younger (15–9 Ma) 40Ar/39Ar ages from white mica is best matched with faster rates (relative to constant rates) between 11.5 and 8 Ma and slower than constant rates from 17 to 11.5 Ma and 8 Ma to present. Key Points A thermokinematic model is linked to forward modeled balanced cross section Thrust belt geometry imparts a first‐order control on predicted cooling ages Variable shortening rates match cooling ages in the Bhutan Himalaya