Catherine A. Rychert University of Bristol

Transcription

1 Catherine A. Rychert University of Bristol Thanks to: James O. S. Hammond, J. Michael Kendall, Nicholas Harmon, Derek Keir, Cindy Ebinger, Atalay Ayele, Ian Bastow, Graham Stuart, Manahloh Belachew

2 What is the lithosphere-asthenosphere boundary? We would like to find the boundary between the rigid lithosphere and the convecting asthenosphere.

3 Seismic waves resolve LAB globally. Regionally constrain character of velocity gradient, & defining mechanism. Receiver Functions [Nettles & Dziewonski, 2008] Rayleigh Waves [Rychert et al. JGR, 2007] Rychert et al., Nature, 2005] Depth km] [Li et al., 2003]

4 Globally Receiver functions lateral coverage limited to seismic station locations [Rychert et al., 2010] [Fischer et al., 2010] SS bounces in locations where station coverage is sparse (Li et al., 2000; Li et al., 2004; Collins et al., 2002; Wolbern et al., 06; Heit et al., 2007; Li et al., 2007; Rychert et al., 2005; Rychert et al., 2007; Snyder, 2008; Kumar et al., 2005; Sodoudi et al., 2006; Ozacar et al., 2008; Angus et al., 2006; Mohsen et al., 2006; Hansen et al., 07; Kumar et al., 2007; Wittlinger and Farra, 2007; Hansen et al., 2009; Sodoudi et al., 2009; Kumar et al., 2005; Oreshin et al., 2002; Kumar et al., 2006; Sodoudi et al., 2006; Chen et al., 2006; Chen et al., 2008; Chen, 2009; Kawakatsu et al., 2009) [Rychert & Shearer, 2011]

5 [Nettles & Dziewonski, 2008] [Ford et al., 2010] [Yuan & Romanowicz, 2010] [Bostock, 1999]

6 Afar triple junc/on Detailed resolu/on of the lithosphere asthenosphere boundary at transi/on of tectonic environments

7 Afar triple junc/on Gulf of Aden [Beutel et al., 2010]

8 Previous seismic results SKS & surface waves aligned melting in upper 75 km. Body wave velocity anomalies beneath rift [Kendall et al., 2005; Bastow et al,. 2010] P-to-S: Moho shallows, Vp/Vs high beneath rift [Hammond et al., 2011]

9 Method Station 1) Rotate recorded waveform to P and S components. 2a) Bin data by conversion point, simultaneously deconvolve and migrate to depth in 1- D. 2b) Extended multi-taper receiver function technique and 3-D migration. Crust Lithosphere S-to-P conversion Ppps Psps Ppss Sp Ps P-to-S conversion Asthenosphere

10 Bin by conversion point. Bin radius = ¾ degree, 75 km depth.

11 Afar triple junc/on, 75 km depth Strong varia/on in waveform character from flank to ria.

12 Afar triple junc/on, 75 km depth Velocity decreases with depth beneath the flank. Velocity increases beneath the rift. [Rychert et al., submi2ed]

13 Flank cross sec/on Results from the migrated extended mul/taper method Strong LAB beneath flank, shallows beneath flood basalts Strong velocity decrease likely requires a mechanism such as mel/ng in the asthenosphere.

14 Flank to ria cross sec/on Results from the migrated extended mul/taper method Strong LAB beneath flank. No LAB beneath ria. Sharp transi/on implies rigidity of the lid. Moho LAB No LAB [Rychert et al., submioed]

15 Good agreement with previous seismic results. 30 km or possibly 100 km Sp receiver functions [Hansen et al., 2009] 80 km Joint Ps receiver function surface waves km thick lid vs. no lid beneath rift [Dugda et al., 2007]. Surface waves [Fishwick et al., 2010].

16 Synthetic Waveform Modeling km Strong velocity decrease likely requires a mechanism such as mel/ng in the asthenosphere km 7 62 km Strong velocity increase likely requires a mechanism such as a sharp decrease in melt concentra/on.

17 More Synthetic Waveform Modeling Examples km km 5 %, 41 km 7 %, 62 km 6%,64 km 5%,68 km

18 Geodynamic rift modeling Strong sharp velocity increase with depth at 75 km consistent with onset of decompression melting, Tp = 1350 C.

19 Afar looks like the 70 km depth [Kustowski et al., 2008] No significant plume influence is required!

20 No plume visible beneath Afar in joint body wave surface wave tomography. Heat flow similarly low [Buck, 1991]. Depth of melting consistent with geochemical estimates (70 90 km) [Furman, 2007]. Sub-crustal lithospheric accommodation consistent with plate stretching [Eagles, 2002]. [Chang & van der Lee, 2011] Afar Plume required to weaken the lithosphere and initiate melting, but no significant signature today. Volcanism continues today, perhaps driven by melt buoyancy effects, or channelized flow.

21 Conclusions Receiver functions image a sharp lithosphere-asthenosphere boundary in many locations globally that requires a mechanism besides temperature, such as dehydration/depletion or melting. SSLIP inversions for a lithosphere-asthenosphere boundary find a well-resolved discontinuity at km depth over much of the Pacific, correlated with distance along flow lines. A sharp rigid lid is imaged on the flank of the Afar rift at ~75 km depth. The transition from flank to rift is abrupt. The sub-crustal lithosphere beneath the rift has been destroyed. A discontinuity consistent with the onset of decompression melting is imaged beneath the rift. Its depth is shallow, indicating no significant plume influence today.