! Force balance around the Andes. N m -1. Andean load ~ N m -1 It must be balanced on both sides

Transcription

1 ! Force balance around the Andes N m -1. Andean load ~ N m -1 It must be balanced on both sides

2 ! Force balance around the Andes Mountain belts balance tectonic forces and reciprocally To the west: Nazca resists the Andes slab anchoring (c.f. Faccenna) + Pacific shear To the east? Resisted by the forces that drive South America to the West : * Atlantic ridge push: not much * basal drag : must be equal to the remaining andes ridge push basal drag

3 ! Can basal drag balance the Andes? mantle flow underneath the Atlantic (Clint Conrad) The cell is excited by the mantle upwelling

4 Mantle upwellings above the Large Low Shear wave Velocity Provinces (LLSVP)? (Bull et al., 09) (Garnero et al., 2006)

5 Mantle upwellings above the Large Low Shear wave Velocity Provinces (LLSVP)? (Bull et al., 09) (Bull et al.) (Garnero et al., 2006)

6 African superplume (Bull et al., 09, from Ritsema s S20RTS) could the African (Pangean) superplume drive plates?

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8 Net$rota*on$of$the$lithosphere$ Ou en inversant les SKS

9 Pleistocene (Scotese) 66 Ma 0 Ma No mountain belts Mountain belts everywhere as obvious as they may be, mountain belts give a straightforward view on the stress regime in the lithosphere

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11 Net$rota*on$of$the$lithosphere$ Ou en inversant les SKS

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13 (Cadek and Ricard, 1992)

14 The eastern side noaa

15 The eastern side (T. Becker)

16 The eastern side (Becker and Faccenna, 12)

17 noaa (T. Becker) (Becker & Faccenna, subm.)

18 The subtle erosional forcing (special request from Manon)

19 The eastern side (Iaffaldano et al, 11)

20 (T. Becker)

21 (Faccenna et al., 2013) c.f. seminar by C. Faccenna: Mountain building and mantle convection (T. Becker)

22 The earth is small. Not many plates over not many cells. which move coherently. (T. Becker)

23 The earth is small. Not many plates over not many cells. which move coherently. The self-consistency of the convecting mantle not only includes upwellings and downwellings but also plates at the surface (same, but older: Alvarez, 1982)

24 The earth is small. Not many plates over not many cells. which move coherently. The self-consistency of the convecting mantle not only includes upwellings and downwellings but also plates at the surface (same, but much older: Holmes, 1919)

25 The earth is small. Not many plates over not many cells. which move coherently. The self-consistency of the convecting mantle not only includes upwellings and downwellings but also plates at the surface OK, but how about small plates and small convection systems? > go ask Claire

26 6. Going vertical: Dynamic topography

27 Deepest topography on Earth besides trenches : back-arc basins Banda sea : m to be compared to mid-oceanic ridges : m

28 Topography$ Isosta$c$component$ Dynamic$component$

29 elevation? h= What if isostasy was a wrong approximation?

30 (J. Braun) Stokes$sinker$and$dynamic$topo$(courtesy$J.$Braun)$

31 (from Y. Ricard)

32 The didactic oceanic example sea level relative to Earth's geoid

33 The didactic oceanic example sea level relative to Earth's geoid Maximal values where grad u is max (not where u is max)

34 In the oceans, dynamic topography is the surface response to oceanic flows (no flow, no dynamic topography) In the solid Earth, dynamic topography is the surface response to mantle flow In the oceans, one can measure dynamic topo: the residual topography is difference between actual sea level and geoid In the solid Earth, the residual topography is more elusive

35 Can$we$see$dynamic$topography?$ (from O. Cadek) Theory Real Earth

36 Residual topography Moho model from Spada et al 13 Residual topography

37 Dynamic topography Shallow slab anomalies better show up P zz = 3Fh3 2 r 5

38 Modeling$dynamic$topo$in$the$upper$mantle$ (Zhong and Gurnis, 1992)

39 Modeling$dynamic$topo$in$the$upper$mantle$ (Zhong and Gurnis, 1992) A tedious debate on the definition of dynamic topo

40 Modeling$dynamic$topo$ Stokeslets =(F/8 )rsin 2 P zz = 3Fh3 2 r 5 (Batchelor, 1967)

41 Stokeslets (Morgan, 1965; Harper, 1984 )

42 Sco*a$sea$ (Husson et al., 2012)

43 m Sco*a$sea$ m White : slab Color coded : dynamic topo Map view, Stokeslet model Thermal subsidence removed Scotia Sea (Husson, 06)

44 Himalaya (Husson et al., 2014) The creepy real Earth recipe: 1. From seismic wave speed anomalies to thermal anomalies!! 2. From thermal anomalies to density anomalies!! 3. From thermal anomalies to rheology!! 4. From density and rheology to flow field 5. From flow field to dynamic topography

45 The creepy real Earth recipe: 1. From seismic wave speed anomalies to thermal anomalies!! 2. From thermal anomalies to density anomalies!! 3. From thermal anomalies to rheology!! 4. From density and rheology to flow field 5. From flow field to dynamic topography (Becker and Faccenna, 2011)

46 Himalaya$ Residual topo (Becker and Faccenna, 2011)

47 Himalaya$ Dynamic topo (Becker and Faccenna, 2011)

48 Mediterranean$sea$ Residual topo (Faccenna and Becker, 2010)

49 Mediterranean$sea$ Dynamic topo (Faccenna and Becker, 2010)

50 Cretaceous$North$america$ $ Late Cretaceous

51 Cretaceous$North$america$ $ (Liu et al 08, Spasojevic et al 09)

52 Etc $ The$concept$of$dynamic$topo$applies$(very)$well$to$the$ upper$mantle$ $ Could$it$be$more$complicated?$ (Burov and Gerya, 14)

53 The subtle erosional forcing (special request from Manon, bis) - Left: without erosion - Right: with erosion (Braun, 2010)

54 Etc $ The$concept$of$dynamic$topo$applies$(very)$well$to$the$ upper$mantle$ $ How$about$the$lower$mantle?$$ Stokeslets (Ricard et al., 1993)

55 A$trickier$game$that$depends$on$the$assump*ons$made$

56 A$trickier$game$that$depends$on$the$assump*ons$made$

57 A$trickier$game$that$depends$on$the$assump*ons$made$ Shallow slab anomalies better show up Impact of deep anomalies is less predictible P zz = 3Fh3 2 r 5

58 Residual topography Need for a referential (Sandiford) Actual topography z ~ sqrt(t)? Residual topography (Sandiford)

59 Residual topography; the referential issue Need for a referential (Sandiford) Actual topography Residual topography Could ridges be reference levels? * Actual depths varies from 0 m to 6000 m (and not a plain 2500 m) * do ridges owe their bathymetric changes to mantle flow or to partial melting (e.g. Langmuir). Cf. Laurie Reisberg

60 Residual topography; the referential issue Need for a referential (Sandiford) Actual topography Residual topography Could ridges be reference levels? * Actual depths varies from 0 m to 6000 m (and not a plain 2500 m) * do ridges owe their bathymetric changes to mantle flow or to partial melting (e.g. Langmuir). Cf. Laurie Reisberg

61 observa*ons$ Residual topography (Flament et al., 2013)

62 models$ Modeled dynamic topography (Flament et al., 2013)

63 Dynamic topography Residual topography

64 Models: going backwards, how reasonable is that? Reversing mantle flow equations. - * Thermal diffusion not reversible - * Backwards flows lead to stratified structure Dynamic topo (B. Steinberger) Uplift rates (Conrad and Gurnis,2003)

65 Models: going backwards, how reasonable is that? OK, but can t we get insights from the past instead of clumsily speculating on present-day information? * «Guide» the mantle structure in the past, based on geological arguments (Liu and Gurnis, 08)

66 Models: going backwards, how reasonable is that? OK, but can t we get insights from the past instead of clumsily speculating on present-day information? * «Guide» the mantle structure in the past, based on geological arguments (HP Bunge) * Data assimilation? > go ask Marie

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