*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 Sole
Supra-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.nl
1Department 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
Sanidinite Pyroxene
Hornfels Hornblende
Hornfels Ab-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( htpeD
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
0 Crystallization age ophiolitic crust
cooling age metamorphic sole
age sole = age crust Dinaridic (K)
Hellennic (L)
Semail, Oman (N) Anatolian (M)
Muslim Bagh (J)
2 3
5 4 6
subduction initiated along oceanic detachment
1 1
2 3 4
5
6 SSZ
MORB MORB
nasce nt sla
b acting as mantle
anchor
ongoing overriding plate motion accommodated by forearc spreading center boninitic
volcanism
mantle wedge
Plate A Plate B
fossil detachment
Plate C
STAGE 2: Slow-spreading ridge (or transform) inversion; overriding plate spreading above nascent slab mantle anchor
1 2
3 4
Active
spreading ridge
oceanic detachment oceanic detachment
1 2 3 4
~50 km virtual hotspot track
Plate A Slow-spreading Plate B
plate boundary zone
STAGE 1: Slowing of ocean spreading rates preceding ridge inversion
hotspot source stable relative to
upper mantle
virtual hotspot track
arc volcanism 14 13 12 11 10 9
SSZ MORB
MORB
Plate A Plate C Plate B
Ridge migrated towards back-arc
metamorphic sole welded to SSZ lithosphere;
mantle-derived dykes intruding Slab steepening
inception slab pull;
roll-back
+ + + +
+
continental margin
SSZ MORB
MORB
Plate A Plate C Plate B
Continuing (a-magmatic) spreading;
ophiolite attenuation
4 5
7 6
8 3
ongoing subduction boninitic volcanism
Future ophiolite
Potential subduction erosion
mantle lithosphere crust
greenschist
amphibolite
granulite metamorp
hic sole
mk04~
metamorphic sole
moving trench
A B, t UM
A B
UM Plate A t
Plate B
B. Stage 2a: Inversion stage
Slowing of the spreading rate before ridge inversion produces oceanic detachment faults
virtual hotspot track Plate A Absolute plate
motion vector
Plate A
Plate B
transform
virtual hotspot track Plate B oceanic detachment fault
mid-ocean ridge 1 2 3 4
12 34
~50 km
A. Stage 1: (Slow) spreading stage
Slowing of the spreading rate before ridge inversion produces oceanic detachment faults
A B
UM
r
da db
moving ridge
A r B
UM
1 2 3 4 5
Plate C1 6
Plate C2
12 34
5 6
Plate A
Plate B
C. Stage 2b: Anchoring stage
Nascent slab anchors in shallow mantle; absolute overriding plate motion generates third plate; reactivates ridge
A B
UM
t
C r
A B
UM C, t
{
trench
advance forearc spreading
{
r
da db
r
1 2 3 4 5
1 34 5
2
t t
t r
MORB SSZ 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.
References
van 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
(Maffione 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.
