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?