*Douwe J. J. van Hinsbergen1, Kalijn Peters1, Marco Maffione1, Wim Spakman1, Carl Guilmette2, Cedric Thieulot1, Oliver Plümper1, Derya Gürer1, Fraukje M. Brouwer3, Ercan Aldanmaz4, and Nuretdin Kaymakcı5

Conclusions

Problem Statement

Slab Flattening and Exhumation of the Metamorphic SoleSupra-subduction zone (SSZ) ophiolites are widely recognized to be the direct product of in-traoceanic subduction initiation. Using structural, petrological, geochemical, and plate kine-matic constraints on their kinematic evolution we show that SSZ crust forms at forearc spreading centers at the expense of a mantle wedge, thereby flattening the nascent slab. This leads to the typical inverted pressure gradients found in metamorphic soles that form at the subduction plate contact below and during SSZ crust crystallization. Former spreading centers are preserved in forearcs when subduction initiates along transform faults or offridge oceanic detachments. We show how these are reactivated when subduction initiates in the absolute plate motion direction of the inverting weakness zone. Upon inception of slab pull due to, e.g., eclogitization, the sole is separated from the slab, remains welded to the thinned overriding plate lithosphere, and can become intruded by mafic dikes upon asthenospheric influx into the mantle wedge. We propound that most ophiolites thus formed under special geodynamic circumstances and may not be representative of normal oceanic crust. Our study highlights how far-field geodynamic processes and absolute plate motions may force in-traoceanic subduction initiation as key toward advancing our understanding of the entire plate tectonic cycle.

Absolute Plate Motion and Upper Plate Extension

Dynamics of intra-oceanic subduction initiation: supra-subduction zone ophiolite formation and metamorphic sole exhumation in context of absolute plate motions

* corresponding author: d.j.j.vanhinsbergen@uu.nl1Department of Earth Sciences, University of Utrecht, Utrecht, Netherlands; 2Centre for Earth Evolution and Dynamics, Oslo, Norway; 3Department of Geology and Geological Engineering,

Laval University, Quebec City, Canada; 4Faculty of Earth and Life Sciences, VU University, Amsterdam Netherlands; 5Department of Geology, University of Kocaeli, Izmit, Turkey; 6Department of Geological Engineering, Middle East Technical University, Ankara, Turkey

Bay of Island ophiolites, Canada

Semail ophiolite, Oman

Anatolian ophiolites

Hellennic/Dinaridic ophiolites

Blueschist

Eclogite

Amphibolite

Granulite

SanidinitePyroxene Hornfels

HornblendeHornfelsAb-Ep

Hornfels

Prehnite- Pumpellyite

100 200 300 400 500 600 700 800 900 1000

2

4

6

8

10

12

14

16

10

20

30

40

50

Temperature (˚C)

)rabk( erusserP

)mk( ht pe

D

H

A

A

A

A

A

A

A

C

H

D

D

E

E, F

B

G

top

base

Greenschist

Zeolite

40 80 120 160 200

40

80

120

160

200

0Crystallization age ophiolitic crust

cool

ing

age

met

amor

phic

sol

e

age sole = age crust

Dinaridic (K)Hellennic (L)

Semail, Oman (N)Anatolian (M)

Muslim Bagh (J)

23456

subduction initiatedalong oceanic detachment

1123456SSZ

MORBMORB

nascent slab acting

as mantle anchor

ongoing overriding platemotion accommodated

by forearc spreading centerboninitic

volcanism

mantle wedgePlate A Plate B

fossil detachment

Plate C

STAGE 2: Slow-spreading ridge (or transform) inversion; overriding plate spreading above nascent slab mantle anchor

1234

Activespreading ridge

oceanic detachment oceanic detachment

1234

~50 km

virtual hotspot track

Plate A Plate BSlow-spreadingplate boundary zone

STAGE 1: Slowing of ocean spreading rates preceding ridge inversion

hotspot sourcestable relative to

upper mantle

virtual hotspot track

arc volcanism 1011121314 9SSZ MORBMORB

Plate A Plate BPlate C

Ridge migrated towards back-arc

metamorphic sole welded to SSZ lithosphere;mantle-derived dykes intruding

Slab steepeninginception slab pull;roll-back

++

+

+

+

continental margin

SSZ MORBMORB

Plate A Plate BPlate C

Continuing (a-magmatic) spreading;ophiolite attenuation

45678 3

ongoing subductionboninitic

volcanism

Future ophiolite

Potential subduction erosion

mantle lithosphere

crust

greenschist

amphibolite

granulite

metamorphic sole

mk04~

metamorphic sole

moving trench

AB, tUM

A BUM

tPlate A

Plate B

B. Stage 2a: Inversion stageSlowing of the spreading rate before ridge inversion produces oceanic detachment faults

virtualhotspot track Plate A

Absolute plate motion vector

Plate A

Plate B

tran

sfor

m

virtualhotspot track Plate B

oceanic detachment faultmid-ocean ridge

1

23

4

1234

~50

km

A. Stage 1: (Slow) spreading stageSlowing of the spreading rate before ridge inversion produces oceanic detachment faults

A BUM

rda db

moving ridge

A BrUM

1

23

456

Plate C1

Plate C2

1234

5

6

Plate A

Plate B

C. Stage 2b: Anchoring stageNascent slab anchors in shallow mantle; absolute overriding plate motion generates third plate; reactivates ridge

A BUM

tC

r

ABUM C, t

{trench advance {forearc

spreading

r

da

db

r

1

23

45

1

34

5

2

t

t

t

r

MORBSSZ

MORBSSZ

inactive oceanic detachment fault

Formation of SSZ ophiolites and associated metamorphic soles requires (i) upper plate extension in the initial phases of subduction initation, (ii) and subduction starting below or near a hot spreading ridge (Figure 1). How can these two conditions be satisfied? Are these processes synchronous? How is the metamorphic sole exhumed and welded below the ophiolite soon after subduction initiation?

Figure 1. (Above) Two proposed modes of subduction initiation, along (a) a transform-fracture zone, and (b) an oceanic detachment fault, in which a former spreading ridge is preserved in a forearc position.

Figure 2. (Above) Conceptual pressure-temperature paths of different levels of metamorphic soles based on four well-studied sole systems. The structurally higher part of the sole underwent a longer, deeper, and hotter evolution than the structurally lower part, which was welded to the sole during decompression and cooling of the upper part. Top left inlet show how the ages of the SSZ crust and metamorphic sole are comparable.

STAGE 1. Immediately before ridge in-version, slow spreading is accompanied by oceanic detachment fault formation. Both plate A and B are moving (to the right) relative to the mantle, as it is the spread-ing ridge.

STAGE 2. Detachment inversion aids subduction initiation upon forced conver-gence. Extension in the upper plate causes reactivation of the paleoridge, and the for-mation of SSZ oceanic crust at the ex-pense of the underlying mantle wedge. Metamorphism of the uppermost level of the slab occurs.

STAGE 3. Mantle melt extraction and ultradepletion, possibly associated with forearc hyperextension (Maffione et al., 2015b) reduced the volume of the manle wedge. This is accommodated by slab shal-lowing. At the plate contact, rocks from the top of the nascent slab are welded to the base of the hot mantle section to form a metamorphic sole.

STAGE 4. Upon eclogitization in the nascent slab creating negative buoyancy, slab pull starts and the slab decouples from the sole and steepens. This leads to asthenospheric inflow into the mantle wedge, reflected in the intrusion of mafic dykes into the sole and the overlying lith-ospheric mantle. Ophiolites are formed upon arrival of a buoyant collider, e.g., a passive margin, in the subduction zone, and the consequent uplift of the fore-arc.

Figure 4. (Above) Schematic plate kinematic scenario of in-tra-oceanic subduction initiation in a mantle reference frame. (Left) Plates and plate boundaries, with absolute plate motions illustrated through virtual hot spot tracks. (Top right) Plates in cross-sectional view. (Bottom right) Plates and plate boundaries on a velocity line (see Cox and Hart, 1986), in which a position of plate B to the right of plate A on the velocity line shows that plate B is moving to the right relative to plate A at a rate pro-portional to the distance between the points.

Figure 3. (Above) Proposed evolution of intraoceanic subduction initiation, SSZ ophiolite formation, and metmorphic sole formation and exhumation, in a mantle reference frame.

Stage 2b. Inversion of a slow spreading ridge that moves relative to the mantle will generate an ad-vancing trench if plate B becomes the overriding plate. An advancing trench will experience resistance of the mantle that will lead to an-choring of the slab and stagnation of the trench. If the velocity of plate B does not change, or chang-es less, a third plate will form by reactivation of ridge r, which then forms a fore-arc spreading center that can generate a SSZ ophiolite.

Stage 1. The slow spreading phase prior to subduction initia-tion cause the formation of ocean-ic detachment faults at/near the ridge.

Stage 2a. Inversion of detach-ment faults and transforms upon forced, oblique convergence. Sub-duction will be favored in the di-rection of absolute plate motion.

Referencesvan Hinsbergen, D. J. J., et al. (2015), Dynamics of intraoceanic subduction initiation: 2. Suprasubduction zone ophiolite forma-tion and metamorphic sole exhumation in context of absolute plate motions, Geochem. Geophys. Geosyst., 16, doi:10.1002/2015GC005745.

Subduction initiation at transform(Dewey & Casey, 2011)

Subduction initiation at oceanic detachment(Ma�one et al, 2015a)

~30 km

future SSZ ophiolite

future SSZ ophiolite

Marco Maffione, et al. (2015), Dynamics of intra-oceanic subduction initiation: 1. Oceanic detachment fault inversion and the formation of forearc ophiolites. Geochemistry, Geophysics, Geosystems, 16, doi:10.1002/2015GC005746..

Maffione, M., et al. (2015). Forearc hyperextension dismembered the South Tibetan ophiolites. Geology, 43, 475–478

Cox, A., and R. B. Hart (1986), Plate Tectonics, How It Works, John Wiley, U. K.