Imaging the mantle transition zone beneath the Transantarctic Mountains, Antarctica: There is no plume! Mouse Marie Larson PSU Geodynamics seminar 30 November 2006

Acknowledgements Thanks to: Andy Nyblade Maggie Benoit Tim Watson Paul 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? No

East and West Antarctica are geologically distinct Modified from Anderson (1999)

Geologic Overview-I The West Antarctic Rift System (WARS) –Extensional tectonism since the Jurassic (Dalziel and Lawver, 2001). –Previous studies estimate crustal thickness between km.

Geologic Overview-II The East Antarctic Craton (EAC) –Stable Precambrian shield –Unusually topographically high, >1km (Cogley, 1984) –Thicker crust, 35-40km

Geologic 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

Tectonic Models Isostatic uplift induced by crustal thickening and hot mantle (Fitzgerald et al, 1986)

Tectonic 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 Ma (ten Brink et al, 1997)

Tectonic Models Flexural uplift of a continuous plate. Crustal thickening during Ross Orogeny and then erosion induced 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 transformation at ~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 ( ) included 41 portable broadband seismometers TAMSEIS

Three arrays: 1.Coastal array 2.North array 3.East array

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. ocs/rftn01.html

Geographical binning reduces bias based on azimuth longitude latitude

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 km cross-sectional slice 660 km cross-sectional slice

The EW line

Some results TZT=260km

Small vs big bins

Going Coastal

? ? ? NorthSouth

? ? ?

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 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?