Transcript Document
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 Talk outline 1. What’s the point/Backgroud 2. How 3. Results 4. Weird Results?!? 5. What does it mean? 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 http://home.freeuk.com/gtlloyd/tam/main.htm 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 18-25 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 http://mitglied.lycos.de/mapu2001/dryvalleys.html 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 motion @ 61-53 Ma (ten Brink et al, 1997) Tectonic 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 at 300 km Seiminski et al, 2003 Watson 2005 Ok, so what am I going to do? From Brian White’s TAMSEIS photos Figure courtesy of Lars Stixrude Clapeyron Slopes Phase transformation at ~ 410 km Phase transformation at ~660 km Bina and Helffrich, 1994 The thermal anomalies affect the depth of the major phase transformations Lebedev et al, 2002 TAMSEIS The Transantarctic Mountains Seismic Experiment (2000 - 2003) included 41 portable broadband seismometers 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 leveldeconvolution code and stacked using Owen’s stacking codes. http://eqseis.geosc.psu.edu/~cammon/HTML/RftnD ocs/rftn01.html http://epsc.wustl.edu/admin/whatsnew/tamseis/ Geographical binning reduces bias based on azimuth longitude 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 TZT=260km Some results 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: 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 AND The ice layer may be causing the double peak…more study is needed. http://home.freeuk.com/gtlloyd/tam/main.htm Any questions?