Presentation on theme: "Constraining crustal rheology and lower crustal flow in the Tibetan plateau Update from CIDER 2011: Dynamics of Mountain Building Marianne Karplus 1,2,"— Presentation transcript:
Constraining crustal rheology and lower crustal flow in the Tibetan plateau Update from CIDER 2011: Dynamics of Mountain Building Marianne Karplus 1,2, Warren Caldwell 1, Flora Bajolet 3, Whitney Behr 4, Jiajun Chong 5,6 1 Stanford University, 2 University of Southampton, 3 Università Roma TRE, 4 University of Texas, 5 ESS, USTC, Hefei, China, 6 Berkeley 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? Crustal flow outwards from the plateau (e.g., Clark & Royden, 2000) 3 Terrane motion along strike-slip faults (e.g., Tapponnier et al., 2001) = 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 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 Inferences bearing on channel flow Consistent with channel flow or not??
Focus areas within Tibet South Central North Qaidam East
Channel flow model (Clark et al., 2005) Suggested best fit channel flow model to explain magnitude of dynamic topography at the Eastern Plateau margin: channel viscosity of ~10 18 Pa s channel thickness of ~ 15 km channel flow rate of 80 mm/yr Flow rate divided by channel thickness gives us spatial gradients in velocity, which is strain rate. 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 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 Lower crust 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 Rippe & Unsworth, 2010 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 Qiangtang terrane (right)
Summary of results In most of Tibet, models show 10 18 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
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.
length scales of 10 2 - 10 3 km; vertical scales of a few to a few tens of km; developed on time-scales of a few to a few tens of Ma Topographic ooze Clark & Royden, Geology, 2000 Topographic studies imply low-strength crustal layers
The mechanical behaviour of the system is a function of the ratio h2/ μ ; therefore we cannot independently determine both h and μ. For example, the same dynamic topography profile would result from a thin channel with a very low viscosity as from a much thicker channel with a higher viscosity.