imaging the mantle transition zone beneath the transantarctic mountains, antarctica: there is no...
TRANSCRIPT
Imaging the mantle transition zone beneath the Transantarctic
Mountains, Antarctica:
There is no plume!
Mouse Marie LarsonPSU Geodynamics seminar
30 November 2006
Acknowledgements
Thanks to:
Andy NybladeMaggie BenoitTim WatsonPaul Winberry
1. What’s the point/Backgroud
2. How
3. Results
4. Weird Results?!?
5. What does it mean?
Talk outline
This study uses receiver functions to place depth constraints on any thermal anomaly beneath the TAMs
The main question addressed here:
1. Is there evidence for thinning of the transition zone between the 410 and 660?
http://home.freeuk.com/gtlloyd/tam/main.htm
No
East and West Antarctica are geologically distinct
Modified from Anderson (1999)
Geologic Overview-IGeologic Overview-I• The West Antarctic Rift System (WARS)
– Extensional tectonism since the Jurassic (Dalziel and Lawver, 2001).– Previous studies estimate crustal thickness between 18-25 km.
Geologic Overview-IIGeologic Overview-II• The East Antarctic Craton (EAC)
– Stable Precambrian shield– Unusually topographically high, >1km (Cogley, 1984)– Thicker crust, 35-40km
Geologic Overview-IIIGeologic Overview-III• The Transantarctic Mountains (TAM)
– Geologic boundary between the EAC and the WARS– The mountains extend ~3500km and reach heights of 4500m– Lack evidence of compressional tectonics.
Many different mechanisms have been proposed for creating the tectonic features of Antarctica
http://mitglied.lycos.de/mapu2001/dryvalleys.html
Tectonic Tectonic ModelsModels
• Isostatic uplift induced by crustal thickening and hot mantle (Fitzgerald et al, 1986)
Tectonic ModelsTectonic Models
• Flexural uplift of a broken plate supported by a thermal load (Stern and ten Brink, 1989; ten Brink and Stern, 1992)
• Decoupling between EA and WARS lithosphere in response to transtensional plate motion @ 61-53 Ma (ten Brink et al, 1997)
Tectonic ModelsTectonic Models
• Flexural uplift of a continuous plate. Crustal thickening during Ross Orogeny and then erosion induced uplift @ c. 55 Ma triggered by climate change. (Karner et al, 2005 and Studinger et al, 2004)
Previous studies show a negative anomaly beneath the Ross Sea
Seiminski et al, 2003
at 300 km
Watson 2005
Ok, so what am I going to do?
From Brian White’s TAMSEIS photos
Figure courtesy of Lars Stixrude
Phase transformation at ~ 410 km
Phase transformationat ~660 km
Bina and Helffrich, 1994
Clapeyron Slopes
The thermal anomalies affect the depth of the major phase transformations
Lebedev et al, 2002
The Transantarctic Mountains Seismic Experiment (2000 - 2003) included 41 portable broadband seismometers
TAMSEIS
http://epsc.wustl.edu/admin/whatsnew/tamseis/
Three arrays:
1. Coastal array
2. North array
3. East array
http://epsc.wustl.edu/admin/whatsnew/tamseis/
Photos from Brian White’s TAMSEIS website
Sometimes there can be problems…
Receiver functions generated using Ammon’s water level-deconvolution code and stacked using Owen’s stacking codes.
http://epsc.wustl.edu/admin/whatsnew/tamseis/
http://eqseis.geosc.psu.edu/~cammon/HTML/RftnDocs/rftn01.html
Geographical binning reduces bias based on azimuth
longitude
latit
ude
Each bin table is populated by the names of station/event pairs for which the wave passed through that node as well as the time that this occurs
Station1-eventA
Station1-eventG
Station3-eventA,
etc
t*
Maps of the points show the lateral extent of the ray sampling from teleseismic events
410 kmcross-sectional slice
660 kmcross-sectional slice
The EW line
Some results
TZT=260km
Small vs big bins
Going Coastal
??
?
North South
??
?
The mysterious NS line
The NS line
The average global thickness of the mantle transition zone
is 242 +/- 2km - Lawrence and Shearer 2006
Preliminary Results:
AND
http://home.freeuk.com/gtlloyd/tam/main.htm
The ice layer may be causing the double peak…more study is needed.
The average transition zone thickness is greater than 250 km (the global average) -- currently no evidence for a plume or thermal anomaly in the transition zone
Any questions?