Deformation of lacustrine shorelines in central Tibet: Implications for lake level history, fault kinematics, and crustal rheology

Open Access
Author:
Shi, Xuhua
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
April 23, 2014
Committee Members:
  • Eric Kirby, Dissertation Advisor
  • Kevin Patrick Furlong, Dissertation Advisor
  • Donald Myron Fisher, Committee Member
  • Robert George Crane, Committee Member
Keywords:
  • Deformation
  • Shorelines
  • Central Tibet
  • Siling Co
  • Lake Level
  • Elastic thickness
  • Viscosity
  • Rheology
  • Gyaring Co
  • Slip rate
Abstract:
Quantification of the rheology of Tibetan lithosphere is critical to understanding the deformational processes that drive the growth of the Tibetan plateau, but remains poorly understood. Here I constrain effective elastic thickness (Te) of Tibetan crust by measuring surface deformation of the highstand shorelines (~ 64 m above the lake level in 1976) around Siling Co - the largest lake in central Tibet - that occurred in response to climatically induced changes in lake levels. Optically-stimulated luminescence (OSL) dating of numerous shorelines around Siling Co indicate this lake was reached at the highstand at 6 – 4 ka and subsequently fell to present-day levels. This Middle Holocene lake highstand was reached or exceeded several times during each period of 100 – 200 ka and 40 – 10 ka. The highstand shorelines are deflected by 3 – 5 m over wavelengths of 10s of km. Comparison of measured shoreline deflections with a 3D elastic model provides a Te of ~ 12 – 14 km in central Tibet. When combined with geophysical constraints on crustal structure, composition, and heat flow, the strain rates implied by flexural rebound require a relatively low viscosity middle crust (effective viscosity of ~ 10^18 - 10^20 Pa s) at ~ 20 – 40 km depth, consistent with the view of relatively weak deep crust beneath Tibet and the notion of channelized crustal flow in this region. Fault kinematics of the Tibetan plateau provides alternative means to examine the crustal deformation. Here I document 12 ± 1 m of right-lateral displacement of lacustrine shorelines across the Gyaring Co fault, one of the primary active strike-slip faults in central Tibet. OSL ages of the shorelines are tightly clustered between 4.1 and 4.4 ka. These data require an average slip rate of 2.1 – 3.2 mm/yr along the central Gyaring Co fault during the latter half of the Holocene. These results suggest that active slip along the Gyaring Co fault is similar to other strike-slip faults in interior Tibet, supporting the notion that active deformation in this region is distributed among numerous, slowly moving faults.