Subduction

Question 1

Shift from proto-subduction to modern
reference
Early Earth dominated by?
WHY? timescale of subduction? Geochemical data model shows? Could be? Criticisms of paper

Answer 1

Moyen and van Hunan, 2012
Subduction in early Earth was dominated by short term subduction style of slab break off. Slabs were hotter therefore weaker meaning subduction could start and stop over millions of years. Model shows switches between arc and non arc vulcanise at similar timescale. Could be link between pre-plate tectonics and modern
Potential criticisms are geochemical signatures may be false, or apparent cyclicality only appears as poor preservation

Question 2

Shallow - steep subduction - why adakitic magma produced?
reference
adakitic magma produced by? Contradictory of? Why? Most of known Pliocene- quaternary are? Why should not be possible? Switch from shallow to steep explains this, what are three stages?

Answer 2

Gutscher et al, 2000

adakitic magma produced by slab melting, contradictory of models restricting partial melting to v. young crust (

Question 3

Cessation of subduction
References
Possible that subduction ceases when?
Results in? 
Example is?
What has caused detachment of root?
remnant crust?
Induced? Due to?
Effect on seismic images?

Answer 3

Zandt et al, 2004
Convergent margin transported elsewhere by transform faults
Results in downwelling of dense batholithic root from old subduction zone to mantle
Sierra Nevada
Near horizontal shearing detached ultramafic root from batholith.
Remnant root has begun to Downwell into mantle
Induced localised crystal thickening due to viscous drag from drip creating welt beneath western flank of range
Moho is absent from seismic images as crust dripped into mantle

Question 4

Subduction cycle is neutral
Reference
Subduction zones and plumes? How does the cycle work? What is lost? Cycle explained by? Magma and crust…
Created how? How does Arc crust felsic magma form?

Answer 4

Tatsumi, 2005
Operate together as a cycle, transporting material from crust to mantle and back again, only losing heat and volatile, cycle explained by calf-alkaline andesite as dominant in subduction magma and the crust
Created by mixing of two end member magma; mantle derived basalt and arc crust derived felsic magma
Felsic formed by fractionation and waste magic material is transported down into mantle to return later in plumes

Question 5

Crustal balance, two end members
Reference 
Increase or decrease in crust?
Two end members
How does accretionary form, how does erosional form?

Answer 5

Lallemand, 1995
Unclear whether nets +/- of continental crust
accretionary builds crust, build up of accretionary wedge at margin, 25% underpaying and ofscrapin, 50% subducted, erosional is destructive, removes crust, all crust subducted inc. Some continental

Question 6

Composition of arc magmatism - during flat subduction
Reference
High flux of magma associated with? Expected Composition?
Actual? Where not? Derived from? Mass balance calcs show? Why?

Answer 6

Ducea and Barton, 2007
High flux of magma associate with arc vulcanise, composed of mantle derived material, actually composed of reworked oceanic plate in North Cordillera, mass balance calcs show

Question 7

What causes flare ups?
Reference
Linked to? 
Why unlikely? 
What does isotope data show? 
Episodic magmatism will? 
Causes of flare up magmatism related to? As?

Answer 7

Ducea and Barton, 2007
Linked to crystal thickening
Unlikely as can only cause nets gain to continent if simultaneous removal of more edense, magic material during subduction,
isotope data shows removal of magic consequence not cause, episodic magma might thicken crust, but needs to be removal of magic during subduction
Related to tectonic shortening, erosion from trench side and retroarc shortening from forehand side
Increase in subducted material and volatile triggering melting and magmatism

Question 8

Deep mantle convection - force driving tectonics
Reference
Where is it driving force? Why? How does it occur? What does an intent in south America act as? Flow curves where? What is flow related to?

Answer 8

Russo and SIlver, 1996
Driving force in Andes, as combined force of slab pull and push from opening of Atlantic is not sufficient
Occurs as trench parallel flow of mantle is detected moving away from central stagnation point
Intent seen as stagnation point
Flow curves around northern and southern edges, curved flow related to smaller neighbouring plates moving eastwards