Integrating kinematic and thermal models to quantify magnitude and variability in rates of faulting through the eastern Himalaya
Abstract
Displacement fields of continental deformation derived from global positional system (GPS) measurements afford an unprecedented view of the last 10-15 years of deformation occurring at plate boundaries. However, this is a geologically instantaneous snapshot of plate motion, and displacement histories over thousands to millions of years are required to address how the lithosphere responds to changes in plate kinematics, or to determine what portion of the modern strain recorded by GPS networks results in permanent deformation. Estimates of fault displacement magnitudes combined with precise age constraints for fault motion can serve as a proxy over geological time scales for GPS measurements. The limiting factor is our ability to precisely document both magnitude and age of faulting. Because of this difficulty in determining the exact age of displacement on individual thrust faults, documented rates of thrusting through the Himalaya are 1) long-term shortening rates, determined by dividing the total amount of shortening by the age of initial motion on the Main Central Thrust (~20×2.0 mm/yr), 2) modern rates calculated via geodesy (19×2.5 mm/yr) or, 3) Quaternary rates (21.5×2 mm/yr). Due to similarities between these long- and short-term shortening rates, a commonly held assumption is that Himalayan shortening rates have remained constant though time. Older than expected cooling ages from the Bhutan Himalaya (3-7 Ma apatite fission track (AFT) ages, and 6-11 Ma zircon helium (ZHe) ages) suggest slower rates of shortening over the last ~10 Myr. To test the assumption of constant shortening rates, we present a coupled kinematic-thermal model that allows us to test the effect of geometry and rates on cooling ages. We propose that the cooling history is a function of the kinematic path that rocks take as they approach the surface, and that the kinematics of deformation are best predicted by a balanced cross-section, which links the surface geometry of structures to a geometry at depth. If correct, the cooling history recorded by a suite of thermochronometers must match that predicted by the kinematics of a balanced cross-section for the cross-section to be valid. A series of displacement fields from a sequentially restored cross section are turned into velocity fields by assigning a window of time to each displacement amount. We evaluate the cooling history associated with a constant rate of shortening as well as cooling associated with rates that are 2.0, 1.5, 0.75 and 0.5x the constant rate. In addition we test rates that vary with time to determine which best matches the measured cooling ages. The combination of relatively old AFT and ZHe ages combined with younger (15-9 Ma) 40Ar/39Ar ages from white mica is best matched with faster than constant rates from 15 to 10 Ma and slower than constant rates from 10 Ma to present.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.T34B..05M
- Keywords:
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- 8175 TECTONOPHYSICS Tectonics and landscape evolution;
- 8011 STRUCTURAL GEOLOGY Kinematics of crustal and mantle deformation;
- 1209 GEODESY AND GRAVITY Tectonic deformation;
- 8108 TECTONOPHYSICS Continental tectonics: compressional