Mantle Flow and Melting at! Back

Mantle Flow and Melting at!
Back-Arc Spreading Centers!
Paul Hall !
Department of Earth Sciences, Boston University!
Collaborators!
Terry Plank (BU/LDEO), Lauren Cooper (BU/LDEO/ETH) !
CIDER Workshop!
29 June 2010!
Backarc Spreading Centers (BASCs)!
Syracuse and Abers (2006)!
Tonga Arc and the Lau Basin!
Zellmer and Taylor (2001)!
Tonga Arc and the Lau Basin!
FRSC
MTJ
Convergence
Rate
Spreading
Rate
Arc –BASC
Distance
fast
fast
large
slow
slow
small
after Zellmer and Taylor (2001)!
Along-Strike Trends in BASC Magmatism in the Lau Basin!
Martinez and Taylor (2002)!
Evolution of the Mantle Beneath a BASC!
I.!
IV.!
II.!
V.!
III.!
Taylor and Martinez (2003)!
Along-Strike Trends in Arc Magmatism!
Samoa
North Tonga
boninites
16
°S
Tafahi
MTJ
Niuatoputapu
FSC
Lau Basin
18
F (V)
CLSC
Late (IV/V)
Metis Shoal (V)
ILSC
Tofua (V)
20
Cooper et al. (in press, JGR)!
A (V)
ELSC
1 (V)
?? (IV?)
22
Ata (III)
VFR
7 (III)
Magmatic Heritage
basalt
I.E.-depleted basalt
boninite
SLR
24
18 (I)
19 (I)
I.E.-depleted basalt / boninite
Evolution of the Mantle Beneath the Arc!
Stage I:
pre-back-arc
Stage IV:
back-arc spreading:
Arc: basalt
!xCF = !xi
Arc:
I.E.-dep.bas.
UCF
Stage II:
back-arc initiation
!xi
Arc: basalt
Stage V:
back-arc spreading:
!xCF
> !xi
Arc: boninite
USR
F2
F1
Stage III:
back-arc spreading:
!xCF
< !xi
Arc: basalt
Stage VI:
fertile mantle incursion
Arc: basalt
!xCF
Cooper et al. (in press, JGR)!
fertile mantle (F1 = 0)
refractory mantle (low F1)
refractory mantle (high F1)
Model Setup and Initial Conditions!
"# u = 0
"# $ % "p = 0
&T
k
= %( u# ")T +
" 2T
&t
'C p
Continuity!
Momentum!
) E ,
( = Aexp+ .
* RT -
Diffusion creep!
Heat!
Model Setup!
Prior to spreading at BASC
(establishes initial conditions)
After onset of spreading
Mantle Flow!
Mantle Depletion!
BASC (initial)!
Arc!
t = 0 Myr!
Mantle Depletion!
BASC (initial)!
Arc!
t = 1 Myr!
Mantle Depletion!
BASC (initial)!
Arc!
t = 2 Myr!
Mantle Depletion!
BASC (initial)!
Arc!
t = 3 Myr!
Mantle Depletion!
BASC (initial)!
Arc!
t = 4 Myr!
Depletion of Sub-Arc Mantle!
Usub = 100 mm/yr
Uspread = 50 mm/yr
Usub = 100 mm/yr
Uspread = 25 mm/yr
Depletion of Sub-Arc Mantle!
Usub = 100 mm/yr
Uspread = 50 mm/yr
Usub = 50 mm/yr
Uspread = 50 mm/yr
Conclusions!
1. Time-dependent kinematic forcing due to a migrating BASC leads to!
!
complex mantle flow (instantaneous streamlines are misleading)!
2.!
!
!
!
Increase in extent of depletion of the sub-arc mantle, along with!
shoaling of location of maximum depletion, is consistent!
with the conceptual model of Cooper et al.!
!
- inconsistent with model of Caulfield et al. (2008), which requires minimal!
!
!
depletion (<1%) of arc source material at the BASC!
3.! Time lag between onset of rifting and appearance maximum!
!
depletion beneath arc strongly controlled by subduction rate!
4.! Flow pattern and depletion not consistent with closed circulation of!
!
Taylor and Martinez (2003) model!
!
!
- horizontal gradients in viscosity due to water?!
!
!
- buoyancy due to melt depletion?!