Constraining crustal rheology and lower crustal flow in the Tibetan plateau Update from CIDER 2011: Dynamics of Mountain Building Marianne Karplus1,2, Warren Caldwell1, Flora Bajolet3, Whitney Behr4, JiajunChong5,6 1 Stanford University, 2 University of Southampton, 3 Università Roma TRE, 4 University of Texas, 5 ESS, USTC, Hefei, China, 6Berkeley Seismological Lab, Berkeley, CA, United States.
Outline: CIDER 2011 crustal flow project • Motivation • Observations bearing on crustal flow • Methods: literature review & flow law modelling • Results & discussion • Future work…
How is Tibet deforming in response to the collision? Terrane motion along strike-slip faults (e.g., Tapponnier et al., 2001) Crustal flow outwards from the plateau (e.g., Clark & Royden, 2000) = motion into the screen = motion out of the screen
Proposed locations & directions of crustal flow:Southern Tibet (to Banggong-Jiali system): south-directed crustal flow driven by GPE, orographic exhumation and lithospheric underthrusting; Northern Tibet: east-directed mixed crustal & mantle flow driven by north-south compression and east-west extension
Observations bearing on channel flow Inferences bearing on channel flow Geological observations • xenoliths • magma composition Seismological observations • reflectivity (e.g., bright spots) • attenuation • tomography • anisotropy Other geophysical observations • gravity • heat flow • thermal gradient • strength • composition • % H2O • viscosity • ductility • cumulative strain/ flow Consistent with channel flow or not??
Focus areas within Tibet Qaidam North Central East South
Channel flow model (Clark et al., 2005) Suggested best fit channel flow model to explain magnitude of dynamic topography at the Eastern Plateau margin: Flow rate divided by channel thickness gives us spatial gradients in velocity, which is strain rate. • channel viscosity of ~1018 Pa s • channel thickness of ~ 15 km • channel flow rate of 80 mm/yr • 80 mm/yr / 15 km = 2x10-13/s strain rate These estimates of strain rate and viscosity allow us to test different experimental flow laws to see if we can place simple constraints on where in the middle or lower crust channel flow may be occurring
Flow laws used and related assumptions • Wet quartzite, assuming maximum water fugacity at all depths • If seismic anisotropy is observed, we use Hirth et al. (2001) quartzite flow law for dislocation creep • If anisotropy is weak or absent, we use Rutter & Brodie (2004) quartzite flow law for diffusion creep Middle crust Lower crust • Both wet and dry anorthite, assuming maximum water fugacity at all depths • If seismic anisotropy is observed, we use Rybacki & Dresen (2006) anorthite flow law for dislocation creep • If anisotropy is weak or absent, we use Rybacki & Dresen (2006) anorthite flow law for diffusion creep Influence of melt • Assumed to scale exponentially and depends on melt fraction and dihedral angle • Dihedral angle assumed to be 18 for quartz and 25 for anorthite (from Holness, 2006)
Bulk resistivity vs. melt fraction Bulk resistivity as a function of melt fraction obtained from Archie’s law for melt resistivities of 0.1 and 0.3m. The shaded areas indicate the range of melt fractions required to explain the magnetotelluric data in the northern Lhasa block (left) and the southern Lhasa block and Qiangtangterrane (right) Rippe & Unsworth, 2010
Summary of results In most of Tibet, models show 1018 Pa*s could be achieved for narrow depth intervals in lower crust. • Central: 55-60 km • East: 53-58 km • South: 48-58 km • North: 53-56 km • Qaidam Basin: 30-37 km, 43-47 km Flow channel may be deeper in central Tibet compared to the margins (?) Viscosity heavily dependent on: temperature, depth of top ‘lower crust’, crustal composition, strain rate (Future) 3-D cartoon of Tibet showing composition and intervals of possible flow in various regions of plateau
Challenges • Structural/ compositional disagreements & ambiguities in literature • Sparse data in Tibet • Constraining viscosity reasonable for channel flow
Ambient noise tomographyVs perturbation maps Yang et al., 2012
North-South cross sections East-West cross sections Yang et al., 2012
Ideas for future work • Better constraints on crustal composition (literature) • Better constraints on viscosity required for channel flow (literature, topographic modelling for more regions of the plateau) • Improve temperature modelling (i.e., non-linear geotherm) • Compare results from flow laws used for Tibetan crust in the past with those we use • Measuring water content in xenoliths (new proposal)
Better constraints on composition • Measuring water content in xenoliths • Keying profiles to viscosity estimates from regional topography • See if Marin still thinks the numbers are valid (for “best fit” viscosity, channel thickness, etc.) • Compare to MT and Yang’s flow paper • 3-D figure!!!!! Showing where flow is… comparison to Yang or MT papers about where flow is.