ArcParallel Flow in the Mantle Wedge Beneath Costa Rica

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ArcParallel Flow in the Mantle Wedge Beneath Costa Rica

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Title: ArcParallel Flow in the Mantle Wedge Beneath Costa Rica

1
Arc-Parallel Flow in the Mantle Wedge Beneath
Costa Rica

Kaj Hoernle, David L Abt, Karen M Fischer, Holly
Nichols, Folkmar Hauff, Geofery A. Abers, Paul
van den Bogaard, Ken Heydolph, Guillermo
Alvarado, Marino Protti Wilfried Stacuh
Nature Feb 2008
Presented by Ben Luetkemeyer

2
A Brief Outline

Introduction
Discussion of Classical Model
Recent Developments
Tectonic Setting
Discussion of Geochemical Evidence
Discussion of Seismic Evidence
Mechanisms for arc-parallel flow
Conclusions

3
Introduction

Importance
Resolving flow in the mantle wedge allows us to
understand
Thermal structure
Chemical structure
Plate dehydration
Melt generation
In subduction zones

4
Introduction Continued

This paper discusses two lines of evidence that
contradict the classical models of flow in the
mantle wedge.
(1) Geochemical Evidence
(2) Seismological Evidence

5
Classical Model

Subducting slab dragging mantle down in a
circular unidirectional flow on both sides of the
slab.
This is a 2-D model

6
Developments

Flow in the wedge corner flow will produce a
preferential alignment of minerals (eg. Olivine)
that is arc normal.
This should cause seismic waves to travel faster
in the arc normal direction.
Global survey of seismic signals do not show any
systematic patterns to support this conclusion.

7
Developments Cont.

Arc-parallel flow suggested as early as 1994 by
Russo and Silver (Science of 25 February 1994)
Trench migration can induce arc-parallel flow in
the mantle wedge.

8
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9
Using Isotopes as Tracers

232Th?208Pb as a ratio to 204Pb
147Sm?143Nd as a ratio to 144Nd
Use radiogenic/non-radiogenic because the ratios
of the same isotope do not change due geologic
events.

10
Evolution of Crust (Pb/Nd)
CRUST (OIB)
208Pb/204Pb
CHUR
iC/jC
MANTLE
143Nd/144Nd
2.7 Ga
4.6 Ga
time
11
Evolution of Crust (Pb/Nd)
CRUST (OIB)
143Nd/144Nd
CHUR
iC/jC
MANTLE
208Pb/204Pb
2.7 Ga
4.6 Ga
time
12
Why use Nd? (Philpotts)

Continental crustal rocks have a wider range of
isotopic composition than do mantle derived
rocks.
Crustal differentiation leaves 143Nd/144Nd ratios
unaffected.
Nd content in seawater
lt3x10-5ppm.
The addition of seawater will cause a very slight
shift in isotopic composition.

13
Why use Pb?

Radiogenic isotopes ratios are not fractionated
by
Partial melting
Magma differentiation
Radiogenic 208/206Pb
OIB is enriched
MORB is depleted
Crust is intermediate

14
Volcanic Rocks of Costa Rica have OIB signature

OIB mantle Enriched Mantle
Three possible origins
(1) residual mantle from the formation of the
Carribean Large Igneous Province (CLIP)
(2)flow of OIB mantle from NW margin of South
America
(3)slab window beneath Costa Rica

15
Slab Window
16
Geochemical SignatureOIB to MORB

The data suggest OIB signature is derived
subducting Galapagos hot spot track
Mixing involves a classic two end member mixing
array
End member include
Seamounts (OIB)
Subducting Slab (MORB)
NW Nicaragua (depleted Pb)
Slab fluids mixing with depleted mantle wedge.
SW Nicaragua to NW Costa Rica
Subducting Cocos/Coiba Ridges and eroded igneous
fore-arc sediments.
Central Costa Rica (enriched Pb)
Subducting Seamount province

17
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18
Ruling out Possibilities

Upper Plate variation?
CLIP terrane distinct from Seamount province.
ODP Leg 170 does not have Pb composition of
Seamount Province, but plots within Cocos Ridge
field.
The seamount subducting beneath central Nicaragua
has a composition similar to the depleted
end-member.

19
So what?

Micocene Costa Rica show depleted signature
similar to Nicaragua NWVF.
South and Central Component become seamount in
signature.
Seamount component constrained to 6my ago.
Subducting rates of 85mm/yr
Arc-parallel flow must be on the order of 63 to
190mm/yr
Lateral flow in the wedge can compete with flow
entrained by subduction.

20
The Smoking Gun?

Seismic anisotropy in the mantle wedge beneath
Nicaragua and Costa Rica provides corroborating
evidence for arc-parallel flow.

21
Methodology Shear Wave Splitting

TUCAN Broadband Seismic Experiment
Consists of 48 stations
In the fore-arc and back-arc
Two cross-arc lines with spacing of 10-50km.

22
Subsets show greater coherence

NW Nicaragua

Legend

23
Subsets show greater coherence

Overall View of Nicaragua

Legend

24
3-D Models

Vector shows orientation of a-axis of olivine.
Length indicates strength of anisotropy.
Vector thickness corresponds to resolution.

25
3-D Models (cont.)

TUCAN array on surface
Top two layers lie within the upper plate
The wedge begins 50km
a-axis become arc-normal in NW where Galapagos
component is absent.

26
MechanismsGenerating Arc-Parallel Flow

Westward migration of volcanic arc is greatest in
Nicaragua, Honduras, and El Salvador.
Slab dips become steeper to the NW
Suggests differential roll back of the slab
drawing wedge material from the south.

27
MechanismsGenerating Arc-Parallel Flow

Collision of the thickened Cocos ridge crust
could also help drive the wedge material
northward.

28
Conclusion

When modeling the thermal and chemical structure
of subduction zones we need to take into account
the events occurring along the strike of the
trench/arc complex.
In this paper we see how the subduction of the
Galapagos Seamounts effect the chemical signature
of the arc volcanoes 500km away.

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