What Makes the Lithosphere Strong and the Asthenosphere Weak?

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What makes the lithosphere strong and the asthenosphere weak? Karen M. Fischer, Brown University Thanks to: David Abt, Heather Ford, Scott French, Kate Rychert, Stéphane Rondenay Zarane k (2006) Crust Mantle Lithosphere dry, depleted? melt-free Asthenosphere LAB hydrated? partial melt? Temperature Viscosity Shear-wave velocity LAB Temperature only Temperature + water in asthenosphere Temperature + water & melt in asthenosphere Nettles and Dziewonski (2008) What can we learn from scattered/converted waves? What is the depth of the lithosphere-asthenosphere boundary beneath continents? How sharp is the lithosphere-asthenosphere velocity gradient? Why is th e asth enosph ere weak - tempera ture, wat er, melt? Zaranek (2006) Sp or Ps “receiver functions” 421 Events 0.2 0.15 HRV Sp: 421 events st Moho uu 010.1 Cr 0.05 0 osphere hh LAB Lit 0 100 200 300 Depth (km) 1) Decompose the wavefield into its incident P and S components using a free- surface transform (Kennett, 1991) 2) Deconvolve S from P (Sp) or P from S (Ps) using a simultaneous frequency- domain approach (Bostock , 1998) or an iterative time-domain approach (Ligorria and Ammon, 1999) and migrate to depth in 1D with corrections for lateral heterogeneity Combined inversions of Ps and Sp 606.0-96%9.6% < 5-11 km 5.3-7.4% Rychert et al. (2005 & 2007) Combined inversions of Ps and Sp Zaranek (2006) 606.0-96%9.6% < 5-11 km 5.3-7.4% Rychert et al. (2005 & 2007) North America: Sp Abt et al. (2009) • Strong LAB phase beneath western U.S. andid portions o f Appalachians • No LAB phase beneath cratons • Discontinuity within cratonic lithosphere North America: Sp Abt et al. (2009) • Sharp LAB beneath younger continent - < 30 km - water/melt in asthenosphere • Gradual/weak LAB beneath cratons - > 50 km - temperature alone Australia: Sp (Ford et al., in prep) Distance (km) AA’0 500 1000 1500 2000 2500 3000 (a) (b) TOO YNGARMA EIDS CTAO COEN 0 • Sharp LAB beneath 50 Phanerozoic 100 E. Australia 150 < 30 km - 200 wat/ter/melt lti in 250 asthenosphere Distance (km) Distance (km) BB’0 500 1000 1500 2000 2500 3000 CC’0 500 1000 1500 2000 2500 3000 (c) (d) NWAOKMBL FORT BBOO STKA YNG MBWA FITZ WRAB CTAO • Gradual/weak 0 0 LAB beneath 50 50 craton - 100 100 > 50-90 km - 150 150 temperature alone 200 200 250 250 Sp conversion point depths: LAB in Southern California French et al. Conclusions Phanerozoic North America and Australia • LABat50kmto110km • LAB velocity gradient sharp - water/melt in asthenosphere Craton • No “LAB” phase at depths comparable to base of fast lid • LAB velocity gradient small or gradual - consistent with purely thermal boundary • Velocity drop internal to lithosphere at 60 to 115 km Southern California • Lithosp here is: • < 60 km beneath southern CA Borderlands • locally thinned (10-15 km) beneath San Jacinto Fault .

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