Was the Samail ophiolite emplaced by
obduction or subduction?
Did the Samail ophiolite originate as a mid-ocean ridge
or intra-oceanic arc (back-arc)?
Where did oceanic thrusting begin?
How old was the ophiolite when thrusting began?
How do enormous (>10,000 km2 ), dense ophiolites
from ocean basins get onto low-density continents?
Why did obduction stop?
What direction did the ophiolite come from?
Map view (from Hacker et al. 1996,
Figure 1)
Dimensions - 400 km long, 150 km wide
, 60,000 km2
It is still intact because there have been no
continental collisions that have taken place since its emplacement.
X-section (from Michard et al., 1991,
Figure 3)
Stratigraphic section (from Hacker et
al. 1996, Figure 2)
ophiolitic section (this has been
described in previous discussions)
metamorphic sole (where all the action took place)
peridotite banded unit - peridotite
mylonite ; where most of the shear deformation was accommodated
partially-melted granulite- and amphibolite-facies
rocks (oceanic crustal protolith) up to 300 m-thick. Thought to represent
oceanic crust that was overridden during early stages of overthrusting
of the nearby ridge.
greenschist-facies rocks (sedimentary and volcanic
protoliths) up to 200-300 m thick. Thought to be basalt, clastics
and chert that were overthrust later when the overlying rocks were cooler.
continental margin and shelf
rocks
virtually unmetamorphosed pellites,
carbonates, and clastics
Tethyan - at a passive margin
characterized by large thrust sheets
upon a continental substrate
most common emplacement method
examples - Oman (Samail), Tibet, New Caledonia, Papua-New
Guinea
Cordilleran - at an active margin
characterized as mélange
or dismembered portions of ophiolites that have been incorporated into
accretionary prisms
examples - Josephine, Franciscan, Alaska
Although there is general consensus as to the chronology of emplacement of the ophiolite, the mechanism of emplacement is still controversial. I have included two figures that are representative of differences in emplacement style.
Obduction without subduction zone at margin (from Michard et al., 1991, Figure 8)
Obduction with subduction zone at margin (from Lippard et al., 1986, Figure 4.11)
Africa-Arabic plate moves to NE
while spreading in Tethyan Sea occurring
Spreading stops and intraoceanic
shallow thrusting begins (E over W)
Amphibolite-facies (High T) due to young, hot overriding
decoupled oceanic lithosphere over oceanic crust (seds and volcanics).
The relatively dense oceanic lithosphere
approaches and the continental margin begins to subside
Continental margin continues to
subside
upper plate of thrust approaches margin
Subsidence depth along with pressure of overriding
oceanic lithosphere causes high-P blueschist metamorphism in margin basin
protoliths
Hot oceanic lithosphere overrides
continent causing biotite granites and aplites to intrude metamorphic sole
and ophiolite
Greenschist metamorphism throughout metamorphic sole
due to overburden pressure and frictional heat
Obduction ceases
Foreland begins to rebound as erosion takes place.
Retro-metamorphism of blueschist to greenschist occurs
Ophiolite uplift results in unstable, high Potential
Energy terrain
Downward flexure of the western adjacent margin
Ophiolite slides to west due to
gravitational collapse
Failure occurs at weakest part of ophiolite, paleoridge
Samail nappe forms, thrusting in front, normal faulting
in back, ccw rotation of mass
Subduction to the NE within the Tethyan Sea (present
Gulf of Oman) commences with north-dipping subduction zone
Continued continental uplift and
sliding ophiolite. Positive feedback loop.
Portion of continent east of collapsed ophiolite continues
to uplift.
Q. Was the Samail ophiolite emplaced by obduction or subduction?
A. The jury's still out, however, the
preponderance of information supports obduction. The difference between
the two may be very subtle. Obduction requires depression of the continental
lithosphere due to the load of the oceanic lithosphere as it encroaches
upon the continental margin. Subduction requires the continental lithosphere
to actual subduct beneath the denser oceanic lithosphere.
Obduction - the ophiolite
thrust over a depressed continental crust (Coleman, 1981; Michard et al.,
1991; Shelton and Egan, 1991)
Evidence for -
hot, buoyant lithosphere can be
elevated above surrounding, cold lithosphere
arc-like volcanic rocks can be explained by thrusting
HT lithospheric base (metamorphic sole) directly over hydrated oceanic
crust along a shallowly-dipping interface to produce hydrous anatexis.
Products of this process would be picritic magmas. Anatexis of the
metamorphic sole could produce K-bearing granitic intrusions (Lippard et
al., 1986; Michard et al., 1991)
Evidence against -
it is a complicated process
Subduction - the continental crust
is subducted beneath the oceanic crust (Andrews-Speed and Brookfield, 1983)
Evidence for -
arc-type volcanic rocks overlying
the ophiolite suggest melt off subducting slab
Evidence against -
the Samail ophiolite is remarkably
intact, an unlikely result for a Cordilleran-style ophiolite
Map and cross-section across Oman, Gulf of Oman and
Iran (from Coleman, 1981, Figure 1 and 4)
it is unlikely that continental
lithosphere (density - 2.7-2.8 g/cm3) would be subducted very far beneath
oceanic lithosphere (3.0 - 3.3 g/cm3)
the metamorphic P-T paths describe
initial HT and LP followed by HP. The subduction model would be more
consistent with higher P initially and not the HT in the shallow portion
of the section
Pressure-Temperature path (from Michard
et al., 1991, Figure 9)
Q. Did the Samail ophiolite originate as a mid-ocean ridge or intra-oceanic arc (back-arc)?
A. Not definite. The answer depends largely on the answer to the first question. If there was no subduction then there was no arc.
Calc-alkaline volcanic rocks that made their way through and overly the oceanic crust are consistent with arc-related volcanics and melting of a hydrated slab.
Subduction zone cross-section (from Alabaster,
1982, Figure 9)
However, if there was a very hot hanging wall (e.g., a mid-ocean ridge) that overrode saturated pelagic sediments along a low angle thrust, a similar suite of volcanics might be produced
Mid-ocean ridge cross-section (from Boudier
et al., 1988, Figure 5.5)
Q. Where did oceanic thrusting begin and how old was the oceanic lithosphere where thrusting began?
A. The answer lies in the crystallization
ages for the oceanic crust, time of formation of the metamorphic sole,
and the metamorphic history of the sole. Oceanic crustal crystallization
occurred at between 97-94 Ma. This age range based on:
U/Pb zircon ages for plagiogranites
(considered to be late-stage differentiates of earlier gabbros) at about
94.8 ± 0.1 Ma,
40Ar/39Ar ages from plagiogranites, gabbros, and veins
in peridotite that have mean age of 94.4 ± 0.3 Ma.
Cretaceous radiolaria in lower V1 volcanics and foraminifera
in middle V2 volcs are, 98.9-93.5 Ma and 93.5-89.0 Ma, respectively
The metamorphic sole formed about 93.5 ± 0.1 Ma based on 40Ar/39Ar
hornblende dates. So its formation postdates the oceanic crust crystallization
by only 0-4 My. The upper part of the sole is characterized by high
temperature amphibolite facies metamorphism. Garnet-clinopyroxene
thermometry suggests that the highest temperatures attained were about
775-875°C.
The most likely site for thrusting to initiate that is consistent with the age and thermal data would be at about the 1000°C isotherm which coincides with the base of the lithosphere in oceanic lithosphere less than 4 My (closer to 2 My) and close to the weak M.O.R.
Mid-ocean ridge cross-section (from Boudier
et al., 1988, Figure 5.5)
Q. How do enormous (>10,000 km2 ), dense ophiolites from ocean basins get onto low-density continents?
density of ophiolitic material
= 3.0 - 3.3 g/cm3
initial elevation of oceanic crust >-2.5 km
density of continental material = 2.7 - 2.8 g/cm3
elevation of obducted ophiolite > 1 km
A. Oceanic lithosphere younger than ~20-40 Ma is more buoyant than mean mantle material and will not sink, even when overlain by aesthenosphere (Davies, 1992; Hacker and Gnos, 1997; Hacker et al., 1996). The oceanic lithosphere took less than 20 My to encounter the continental margin. It was traveling along a very low angle thrust plane and depressed the continental margin to such a degree that it was able to traverse the margin. Once the obduction ceased the subsided continent rebounded isostatically and the ophiolite slid westward due to gravity. This scenario has been supported by numerical modeling (Shelton and Egan, 1991)
Q. Why did obduction stop?
A. Speculative answer (mine). Once the thermally buoyant and weak ridge had passed over the continental margin it was trailed by progressively colder, stronger and more dense oceanic lithosphere. About 40 My to the northeast was oceanic lithosphere that was near the threshold of spontaneous subduction. The obduction process might have been the catalyst to initiate the subduction to the northeast (which continues today). Once that subduction commenced there was no further need to obduct.
Map and cross-section across Oman, Gulf of Oman and Iran (from Coleman, 1981, Figure 1 and 4)
Sense of shear indicators in the metamorphic sole suggest that south-directed thrusting occurred throughout the majority of the Samail ophiolite followed by west-directed thrusting which is preserved in the south. Boudier (1988) and Hacker (1996) suggest that ridge-parallel thrusting occurred from about 95.7 - 93.5 My (S-directed) and ridge-normal thrusting occurred later from about 94.9 - 92.6 My.
Map view (from Hacker et al. 1996,
Figure 1)
Hacker (1996) argues that the initial intraoceanic thrust must have place young oceanic lithosphere over young oceanic lithosphere based on the HT amphibolite metamorphism and timing constraints. However, fossil ages from chert and limestone directly beneath the metamorphic sole suggest that oceanic lithosphere was 50 My at the time of overthrusting.
He resolves the problem by initiating the thrust at a transform fault
(orthogonal to ridge). Ridge parallel transport places hot lithos
over hot lithos for about 1-2 My during which time the amphibolite portion
of the metamorphic sole is formed. This is followed by ridge normal
thrusting of hot lithosphere over 50 My oceanic crust for about 25-33 My
and emplacement onto the Arabian continent.
Map view (from Hacker et al. 1996)
This change in direction may be evident in the paleomag record (Thomas
et al., 1988). The cause may be due to buoyancy contrasts or to small
plate rotations induced by far-field tectonic forces.
Plan view of rotations (from Thomas,
1988, Figure 20)
indicates assigned reading
Alabaster, T., Pearce, J. A., and Malpas, J., 1982, The volcanic stratigraphy and petrogenesis of the Oman ophiolite complex: Contributions to Mineralogy and Petrology, v. 81, p. 161-183.
Andrews-Speed, C. P., and Brookfield, M. E.,
1983, Tectonic setting for ophiolite obduction in Oman; discussion:
Journal of Geophysical Research, v. 88, no. 1, p. 609-611.
Boudier, F., Ceuleneer, G., and Nicolas, A., 1988, Shear zones, thrusts and related magmatism in the Oman ophiolite: Initiation of thrusting on an oceanic ridge: Tectonophysics, v. 151, p. 275-296.
Coleman, R. G., 1981, Tectonic setting for ophiolite obduction in Oman: Journal of Geophysical Research, v. 86, no. B4, p. 2497-2508.
Cooper, D. J. W., 1988, Structure and sequence of thrusting in deep-water sediments during ophiolite emplacement in the south-central Oman Mountains: Journal of Structural Geology, v. 10, no. 5, p. 473-485.
Davies, G. F., 1992, On the emergence of plate tectonics: Geology, v. 20, p. 963-966.
Hacker, B. R., 1994, Rapid emplacement of young
oceanic lithosphere: Argon geochronology of the Oman ophiolite: Science,
v. 265, p. 1563-1565.
Hacker, B. R., and Gnos, E., 1997, The conundrum of Samail; explaining the metamorphic history: Tectonophysics, v. 279, no. 1-4, p. 215-226
Hacker, B. R., Mosenfelder, J. L., and Gnos, E., 1996, Rapid emplacement of the Oman ophiolite: Thermal and geochronologic constraints: Tectonics, v. 15, no. 6, p. 1230-1247.
Lippard, S. J., Shelton, A. W., and Gass, I. G., 1986, Ophiolite detachment, emplacement and subsequent deformation, The ophiolite of northern Oman: London, The Geological Society, p. 178.
Michard, A., Boudier, F., and Goffe, B., 1991,
Obduction versus subduction and collision in the Oman case and other Tethyan
settings, in Peters, T., Nicolas, A., and Coleman, R. G., eds., Ophiolite
Genesis and Evolution of the Oceanic Lithosphere: Dordrecht, Ministry of
Petroleum and Minerals, Sultante of Oman, p. 447-467.
Nicolas, A., 1995, The Mid-Oceanic Ridges: Berlin, Springer-Verlag, 200 p.
Shelton, A. W., and Egan, S. S., 1991, The obduction of the northern Oman ophiolite - crustal loading and flexure, in Peters, T., Nicolas, A., and Coleman, R. J., eds., Ophiolite genesis and evolution of the oceanic lithosphere, Kluwer Academic Publishers, p. 469-483.
Thomas, V., Pozzi, J. P., and Nicolas, A., 1988, Paleomagnetic results from Oman ophiolites related to their emplacement: Tectonophysics, v. 151, p. 297-321.