p.1

THE PALEORIDGE SEGMENT STRUCTURE AND THE BONINITE-MORB
DIKE SWARMS IN THE FIZH MASSIF
Susumu Umino (Kanazawa University, JAMSTEC)

Magma plumbing system of V1 and V2 volcanism in the Fizh massif, northern Oman Ophiolite
The ophiolite nappe in the northernmost Oman

orientations of the sheeted dike complex in the Fizh massif

Mountains consists of several blocks gently dipping to theshow the subsurface structure of paleoridge segments

east, with the eastern blocks thrusting onto the western

divided by a <5km wide interval of either gradual change inblocks (Fig. 1) (Umino et al., 1990; Béchennec et al., 1993).

strike up to 20° or mutually intrusive dikes, comparable to a

The largest block in the Fizh-Hilti massif is the structurally

third-order boundary (Macdonald et al., 1991).

second one from the top, which includes the entire volcanic

In the north of Wadi Fizh, five swarms ofsequence from V1 MORB-like extrusive rocks through V2

subparallel dikes 1–5 km in width extend WNW-ESE

arc tholeiitic rocks and boninite to V3 alkalic basalt flows in(Umino et al., 2019). Each swarm has a core of 100 %ascending order. The MORB-like V1 flows and pyroclastic

sheeted dikes 1–2 km in width and emanated from therocks are underlain by the intense swarms of sheeted dikes

dunite-wherlite-clinopyroxenite-gabbronorite-diorite-tonalite

dominantly striking N-S, which indicates the N-S orientation

complexes below and intruded through V1 and into V2of the paleospreading axis in the northern ophiolite. The V2

extrusive rocks. Most E-W-striking dikes comprise lessextrusive rocks were fed through low-angle sheets and NW-

altered boninitic and altered MORB-like dikes. SomeSE- to WNW-ESE-striking dikes, which intruded through

MORB dikes are mutually intrusive to N-S-striking MORB

the V1 extrusive rocks and into the V2 extrusive rocks.

dikes, indicating their emplacement during the transition

A second-order paleoridge segment is preservedin the Fizh-Hilti massif with the segment center at Wadi
Thuqbah and segment ends at Wadi Fizh and Wuqbah-

period from the V1 spreading stage with MORB activity to
the V2 arc stage with boninite magmatism.
We will visit the upper crustal sequence in Wadi

Haylayn (Fig. 1; Nicolas, 1989; Nicolas and Boudier, 1995;

Fizh and the southernmost E-W dike swarm along Wadi

Miyashita et al., 2003; Umino et al., 2003). The overlapped

Dhabeen to see the duplicated structure of the upper crust at

spreading centers in the Fizh area resulted in the unusually

the paleoridge segment and the occurrence of the boninite

thick, two-storied upper crust with the doubled V1 extrusive

and MORB dikes during the tectonic conversion fromlayers and sheeted dikes. Folding- and tilting-corrected dikespreading to subduction.

p.2

Fig. 1. Geological outline of the northern Oman Ophiolite, on the north of Haylayn—Nakhl division (broken like), modified
after Béchennec et al. (1993). Segment centers and boundaries are after Nicolas (1989), Nicolas and Boudier (1995) andMiyashita et al. (2003). Rectangles show the area of Figs. 3 and 6.

Paleoridge segment structure of the V1 upper crust
Structures, geochemistry and mineralogy of themagma chamber below the dying ridge segment (Fig. 3;

extrusive rocks, the sheeted dike complex and the

Umino et al., 2003). Dikes in the north of Wadi Khabiyat

underlying layered gabbros of V1 spreading stage indicate astrike NW-NNW (285°-355°) but those between wadis

paleoridge segment center at Wadi Thuqbah and segment

Khabiyat and Bani Umar al Gharbi strike dominantly N-S.

ends at Wadi Fizh and Wuqbah-Haylayn (Fig. 1; Nicolas,

Further south, dikes trend easterly (mostly 10°–20°). Such

1989; Nicolas and Boudier, 1995; Miyashita et al., 2003;

variation in the dike trends may allow us to divide the

Umino et al., 2003). The upper crust in Wadi Fizh shows

sheeted dike complex into three domains. However, thetwo-storied structures of doubled V1 extrusive layers andsouthern half of the Fizh massif is easterly tilted and folded

sheeted dikes (Fig. 2). The boundary of extrusive rocks andwith south-plunging fold axes and a wavelength of 5 km.

sheeted dikes is displaced dextrally along Wadi Fizh, where

After correcting the tilting and folding, the restored original

extrusive rocks exposed in the wadi are intruded by theorientations of the sheeted dike complex show N-S strikes insheeted dikes in the north, which in turn is overlain by thethe north of Wadi Bani Umar al Gharbi and NNE-SSW

extrusive layers to the east. This structure suggests that the

strikes in the south, which gradually change in strike to N-S

paleoridge segment to the north of Wadi Fizh propagatedfurther in the south along Wadi Hayl. Thus, the sheeted dikesouthward into the upper crust formed on the southern ridge

complex can be divided into two domains in the south ofsegment, now exposed in the south of Wadi Fizh. Because of

Wadi Bani Umar al Gharbi, where the dikes show either

this dual structure, the upper crust is much thicker in Wadi

gradual change in strike up to 20° or mutually intrusive

Fizh than any other area (Fig. 2).

relationships. No shear zones or intense alteration is present

The V1 sheeted dike complex in the south of Wadi

between the two domains. The dike domain boundary is

Fizh well preserves the original paleoridge segment

considered to represent a subsurface structure of astructures, which formed during a waning stage of the axial

paleoridge segment boundary consisting of two adjacentp.3
overlapping spreading centers with a small offset, whichinflections at 90–150 cm and 500 cm that divide themost likely corresponds to a third-order boundary

thickness plot into three regions. The dike thickness does not(Macdonald et al., 1991).

show any systematic variation with depth through theAmong 2505 dikes measured for thickness

sheeted dike complex. On the contrary, it is clearly shown

analysis, 1526 dikes have chilled margins on both sides

that thicker dikes concentrate in the northern end of the dike(‘‘two-sided’’ dikes), while 979 dikes lack one or bothcomplex along Wadi Fizh (Fig. 5). Although the number ofmargins by later intrusions of neighboring dikes. Because of

two-sided dikes in each 2 km wide zone is small (37–218

this, thickness data is limited to only two-sided dikes and thedikes, averaging 56), the frequency of dike thickness has atrue population of dike thickness is uncertain. Two-sided

similar pattern to the whole dike thickness. Large standard

dikes have a wide range of thickness from less than 1 cm todeviations arise from large ranges in thickness, but increase

more than 13 m. Among these, 73% of 1511 simple dikes arein the average thickness is obvious specifically in the northless than one meter thick and 36% are less than 20 cm thick,domain, inheriting from a higher abundance of thicker dikes

while those >5 m are only 0.3% (Fig. 4a). The average dike

>1 m. A small peak at average dike thickness appears in a

thickness is 71.3 cm and the median is 40.0 cm. Thezone at Northing 2694–2696 km, which corresponds to theaccumulated number of dike thicknesses shows two

southern end of the north domain.

Fig. 2. The upper crustal structures of the Oman Ophiolite (a, e) and the oceanic crust spread at intermediate (b), fast (c) andultrafast (d) rates. Note the upper crustal structure of the Oman segment center is similar to that of the superfast-spread crust inHole 1256D. With decreasing spreading rate, the oceanic upper crust becomes thinner, and so at the segment end in the Oman
Ophiolite.

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Fig. 3. Trend of dikes in the sheeted dike complex between Wadi Fizh and Wadi Hayl-Jizi. Location of the rectangle is shownin Fig. 1. The shaded area represents the sheeted dike complex; NSZ, North shear zone; SSZ, South shear zone. The number ofdikes is counted within ±5-degrees from a direction 0° to 350° spaced at a 10-degree interval and indicated by the length ofbars shown below. Southern end of each bar points the locality. Center and right figures are stereographic projections of dikepoles with mean vectors (large circles) and rose diagrams for dike trends in each block. Uncorrected dike trends (a) andcorrected dike trends by restoring folding and tilting (b). Green arrows in b indicate the position of the domain boundary.

p.5

Fig. 4. Cumulative numbers of thickness of the V1 sheeted dikes (a) and E-W swarm dikes (b) (Umino et al., 2019). The
number of total dikes measured, average and median thicknesses of V1 and V2 are 1511 and 328, 71.3 cm and 70.3 cm, and40.0 and 36.0 cm, respectively. Both dikes show similar thickness distributions, which are deficient in the number of thicker
dikes than 560 cm and 580 cm, respectively.

Fig. 5. Variation in thickness of V1 MORB dikes in the sheeted dike complex (left), thickness of the sheeted dike complex +layered gabbros (center) and average thicknesses of dikes in bins 2 km wide N-S (right) between Wadi Fizh and Wadi HaylJizi. Note the presence of thicker dikes in the paleorige segment end in Wadi Fizh and the minimum thickness of the sheeted
dike complex + layered gabbros at the sheeted dikes domain boundary.

p.6

The E-W dike swarms during the transition from V1 to V2 stage and in V2 arc stage
WNW-ESE-striking extensive swarms of dikes(extensional shear dikes) are not uncommon (Fig. 7f).

distributed over 25 km N-S between wadis Asher and Fizh

Highly variable dike orientations, mutually intrusive dikes(Fig. 6; Umino et al., 2019). The dike swarms are wellcrossing at high angles and extensional shear dikes are

exposed in mountainous areas and are easily recognizable byvirtually absent in the sheeted dikes of the V1 spreading

Google Earth and Bing satellite images. They form WNW-

stage (Umino et al., 2003), suggesting distinctly different

ESE-extending swarms of dikes 1–5 km in width, spaced

stress conditions for the E-W dike swarms and the V1every 5 km N-S, and are denoted as Swarm-1 to -5 fromsheeted dikes.

north to south (Yamazaki, 2013). Each swarm has a core

Among the ca. 900 dikes examined, 47 % haveconsisting of 100 % sheeted dikes 1–2 km in width, whichchilled margins on both sides, out of which 328 dikeextends for 4–10 km from the dunite-wherlite-

thicknesses were measured. Other dikes have lost chilled

clinopyroxenite-gabbronorite-diorite-tonalite plutonic

margin on one side or both intruded by the adjacent dikes.

complexes below through V1 and into V2 extrusive rocks

Two-sided dikes have a wide range of thickness from lessabove (Fig. 6).

than 1 cm to more than 11 m. Thicker dikes more than 10 m

Although the overall trend of the E-W dikeare exclusively of quartz dacite. Thirty eight percent of theswarms runs WNW-ESE, individual dikes vary from NW-SE

328 dikes are less than 20 cm thick, while those more than 2

to E-W and rarely N-S, which are highly varied compared tom thick are only 7 % (Fig. 4b). The average dike thickness isthe V1 sheeted dikes between wadis Fizh and Jizi (Fig. 3;

70.3 cm and the median is 36.0 cm. The accumulated

Umino et al., 2003). In the sheeted dikes, both subparallel

number of dikes plotted against thickness shows a sudden

and mutually intrusive dikes are common. However, somedecrease at 140 cm and reaches almost zero at 580 cm. This

dikes are crosscut by others at a high angle, and some show

thickness distribution strikingly resembles that of the V1

an abrupt change in dike orientation (Fig. 7d, 7e).

sheeted dikes (Fig. 4a).

Furthermore, dikes that opened oblique to their strikes

Fig. 6. The E-W dike swarms in the northern Oman Ophiolite
distributed across the boundary between the extrusive rocks
and the V2-stage late intrusive complex of dunite-wherlitegabbronorite-diorite-tonalite below that intruded into the V1stage layered and upper gabbros (modified after Yamazaki,2013). Dips and strikes of dikes are shown. Intensity of dikes
are defined as the total percentage of dikes per unit length ofthe outcrop and was determined based on the data where dikethicknesses were measured and on photographs of the outcrops(red dots) and Google Earth satellite images in other places.
The contour lines encircle areas with 5 %, 50 % and 100 %dikes, which show five swarms of dikes (SWM-1 to -5) 1–3
km in width and 3–5 km apart. Symbols are; 1, V2 extrusive
rocks; 2, E-W sheeted dikes; 3, V1 extrusive rocks; 4, V1
sheeted dikes intruded by V2 dikes; 5, late dunite-wherlitegabbronorite-diorite-tonalite complex, 6. upper gabbros; 7,
layered gabbros; 8, mantle peridotite.

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Fig. 7. The occurrences of the E-W dike swarms. (A) Intense swarms of E-W-striking dikes in Wadi Dhabeen. (B) Sheeted
dikes of tholeiitic basalt and boninite in Wadi Dhabeen. (C) Thick sheeted dikes of dacite to the south of Wadi Rajmi. Width ofthe photograph is ~8 m. (D) A N-S-striking dike truncated by a NW-SE-striking dike. Numbers denote dip azimuths/angles.
(E) Abrupt change in dike orientation from WNW-ESE to E-W. (F) Extensional shear dike opened oblique (indicated by thebroken line connecting arrows) to its orientation. Note that there is neither fault nor cracks that displaced the dike.

p.8

Petrography and geochemistry
Microscopic observations and whole-rockhigh-Ca boninite (UV2) from the Oman Ophiolite (Ishikawa

chemical analyses below confirm that 22 out of 56 samples

et al., 2002, 2005; Kusano et al., 2014a, 2017). Tholeiitic

examined are tholeiitic and the rest 34 samples are boninitic.

rocks are more depleted in LILEs, Th, Ta and Nb compared

Tholeiitic and boninitic dikes are generally aphyric and

to REEs that show whale-humped patterns characteristic toporphyritic, respectively, and are discriminated by the

mid-ocean ridge basalts (MROBs). These trace element

phenocryst assemblages as shown in Fig. 8. In addition topatterns resemble the V1 flows and dikes (Kusano et al.,basalt and boninite, both tholeiitic and boninitic dikes

2012, 2017) but are totally different from the arc tholeiitic

include more differentiated varieties ranging from andesite

lavas (LV2 of Kusano et al., 2014, 2017) that have less andto dacite.

flat REE patterns and positive LILE anomalies.
Boninitic rocks are discriminated from tholeiitic

Boninitic dikes generally strike WNW-ESE,rocks by their higher MgO and CaO and lower TiO2, Na2O,

however, strikes are fairly varied between NW-SE and E-W

P2O5, FeO* (total Fe as FeO)/SiO2 ratio at a given SiO2 (Fig.

(Fig. 10). Likewise, MORB-like tholeiitic dikes vary in

9). Boninitic rocks have distinctly lower abundances ofstrikes from NW-SE to E-W and some dikes even strike N-

HFSEs and REEs than tholeiitic rocks (Fig. 9g). The spoon-

S. It should be remarked that both E-W-striking and N-S-

shaped trace element patterns with positive spikes of Sr, andstriking tholeiitic dikes are mutually intrusive.

low Zr/Sm and Zr/Ti ratios are typical to the low-Si and

Fig. 8. Microphotographs of representative tholeiitic basalt (A, under plane-polarized light) and boninite (B, under crosspolarized light). (A) Besides some Fe-Ti oxides, almost all primary igneous minerals in the tholeiitic basalt are altered andreplaced by secondary minerals such as chlorite after clinopyroxene and albite after plagioclase. (B) Boninite dikes are lessaltered than tholeiitic dikes and primary clinopyroxene is generally preserved.

p.9

Fig. 9. Whole rock compositions of the E-W swarm dikes.

Fig. 10. Lower hemisphere stereographic projections

Major element oxides (A to D) and FeO*/MgO ratio (E)showing the poles of the E-W swarm dikes, Swarm-1 (A, B),

plotted against SiO2. (F) Trace element abundance-3 (C, D), -4 (E) and -5 (F). Plotted are estimated maximum

normalized against primitive mantle after Sun and
McDonough (1989). Ranges of V1 extrusive rocks and(σ1), intermediate (σ2) and minimum (σ3) stress axes with 95% confidence ellipses. V2 dike Swarm-1 intruded into V1

sheeted dikes, LV2 and UV2 extrusive rocks are shown for

extrusive rocks (A), sheeted dikes of Swarm-1 (B), dikes incomparison after Kusano et al. (2012, 2014b). A broken line

Swarm-3 intruded into the V2 arc stage gabbros (C), sheeted

divides tholeiitic (TH) and calc-alkaline (CA) series rocks indikes of Swarm-3 (D), all dikes in Swarm-4 (E) and -5 (F).

(E) after Miyashiro (1974).

(G) Scale bar for dikes including two clusters (A, E-F)
represents probability for the dike belonging to either cluster
1 (dark colors) or cluster 2 (light colors) by grading color.
(H) Triangle represents the probability for the dikebelonging to either cluster 1 (red), 2 (green) or 3 (blue), bymixed color gradation. See the Supplementary materials andYamaji and Sato (2011) and Yamaji (2016) for the detail.

p.10

Paleostress conditions of E-W dike swarms
Paleostress analyses of the E-W dike swarms were

4 and 5 have highly scattered dike clusters that intruded

done using the software package GArcmB (Yamaji et al.,under high magma pressures. High magma pressures2010; Yamaji and Sato, 2011; Yamaji, 2016a, b), whichenabled dikes to intrude oblique to the minimum andallowed us to estimate the orientations and ratios ofmaximum stress axes, resulting in extensional shear dikes.

paleostresses and the relative magmatic pressure that formed

Assuming the average density of the upper crustdike swarms. By knowing absolute values of at least two

to be 2760 kg/m3 (Umino et al., 2008), the vertical stress,

principal stresses, we can determine the absolute values ofwhich is equal to σ1, is estimated as 111–192 MPa in thedepth of the top of V2 plutonic rocks ranging from 4100 m

the third principal stress and the magmatic pressure.
Most dikes have E-W to NW-SE strikes andin Wadi Faydh (Swarm-1) to 7100 m in Wadi Rajmi

intruded under the stress field of normal fault regime, while(Swarm-4) with an average of 5550 m (Kusano et al., 2017).

some dikes formed under a strike-slip fault regime (Fig. 10).

The minimum principal stress is given by σ3 = 0.56 σ1 as

However, moderate to high stress ratios (σ1 ≈ σ2) of the most

62–108 MPa. Then, the differential stress, σ1 − σ3, isestimated as 49–85 MPa, which gives the excess anddikes yield large uncertainties in σ1 and σ2 orientations,which is rather different from typical normal fault or strike-absolute pressures of magma of 1.2–36 MPa (averaging 17

fault regime. Especially some dike clusters in Swarm-1, 4MPa) and 63–133 MPa (averaging 98 MPa), respectively.

and 5 with high stress ratios of 0.64–1.0 formed under aThese excess pressures of magma are within the range ofstress condition of “axial extension”. Furthermore, Swarm-1,

those estimated for a Hawaiian rift zone (Rubin, 1999).

Conversion tectonics from V1 spreading center to V2 paleoarc
The short-lived (<3 Myr) Oman arc magmatism

suggesting their older ages. This indicates that thethat resulted from the absence of convection within the V2

paleoridge segment of the microplate was still active or the

mantle wedge, the localized V2 magmatism in the northern

MORB-like magma existed beneath the microplate and fed

and central ophiolites (Godard et al., 2003; Kusano et al.,some E-W-striking tholeiitic dikes under the rotated stress

2014, 2017), and the conversion of stress field fromfield. The less common N-S-striking tholeiitic dikes that areextensional V1-spreading stage to inextensional or

mutually intrusive to the E-W-striking dikes are most likely

compressive V2-arc stage are all incompatible with the

intruded along preexisting faults and fractures formed during

IBM-type subduction initiation accompanied by the trench

the V1 spreading stage before the clockwise rotation. With

retreat (Umino et al., 2015, 2018a, 2019; Umino, 2019).

the progress of thrust-induced subduction beneath the

Alternatively, the switching of σ2 and σ3 stress axes throughthe transition from the N-S-striking sheeted dike complex ofrotating microplate, boninitic magma was generated andthe V1 spreading stage to the E-W-striking dike swarms

plutonic complexes. Less intensive arc magmatism toward

coincides with a clockwise rotation of the northern Oman

the southern ophiolite could be explained by the proximity

Ophiolite by 100°–150° as indicated by paleomagnetic

to the Arabian continent when the detachment of the oceanic

declinations (Perrin et al., 2000; Weiler, 2000). The NE-SW

lithosphere took place in response to collision of the active

trending paleoridge segment is proposed to have rotated as aridge against the continent (Boudier and Nicolas, 2018).

microplate from V1 to V2 stage before obducting onto the

The switching of σ2 and σ3 axes as well as morevaried orientations and higher magmatic pressures for the E-

Arabian Peninsula. The ephemeral V2 arc magmatism is

emplaced as the E-W-striking dike swarms and the lateconsidered to have been caused by the microplate rotation

W dikes than the V1-stage sheeted dike complex are

issued from the intraoceanic thrusting between young

reconciled with the more compressive stress field on theoceanic lithospheres (Boudier et al., 1997; Ishikawa et al.,rotating microplate thrusting over the surrounding plate of

2002, 2005; Godard et al., 2003), which could be found inslightly older or similar age. This tectonic situation

the framework of the tectonic reconstruction of the northerncorresponds to the case of continuous subduction without

and central Oman Ophiolite (Nicolas and Boudier, 2017;

backarc spreading of Leng and Gurnis (2011), where a

Boudier and Nicolas, 2018).

compressive stress acting on the boundary of the rotating

The MORB-like tholeiitic dikes in the E-W dikeswarms are more intensely altered than boninitic dikes,

microplate and the subducting young plate under the ridge
push force prohibited neither trench retreat nor backarc

p.11
spreading. Because of its small density contrast, forced

V2 magmatism that ended after progressive depletion of the

subduction of the slab of similar age formed a thinmantle wedge, as articulated by the change in V2 arc

overriding mantle wedge, where convection is suppressedmagmas from slightly depleted LV2 arc tholeiite to severely

and hence cools in a few million years (Gutscher et al.,depleted UV2 boninite (Kusano et al., 2014, 2017).

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p.13

Table 1. List of STOPs.
STOP

Long

Lat

Note

1

56.34286 24.5245 Duplicated upper crustal sequence in Wadi Fizh

256.3914 24.2846 The southernmost E-W dike swarm

Fig. 11. A satellite image of the N-S V1 sheeted dike complex (V1-SDC) and the E-W dike swarm-5 with contours of number
density of dikes (5%, 50%, 100%). Also shown are approximate boundaries between the V1 extrusive-sheeted dike complexgabbros (GB).

p.14

STOP-1. Duplicated upper crustal sequence in Wadi Fizh
The lowermost V1 extrusive rocks in Wadi Fizh are intruded by N-S-striking dikes which gradually increase in number
downward for a distance of ~500 m along wadi and grade into the sheeted dike complex. 800 m upstream pillow flows
interbedded with pyroclastic rocks appear on the right coast of Wadi Fizh, which are intruded by dikes crossing at high angles.

Fig. 12. (A) The transition zone from the V1 extrusive rocks (downstream) to the sheeted dike complex (upstream). (B) Welded
driblets and scoriae of V1 MORB of the older crust, 800 m upstream of A. (C) Cross-cutting dikes into the older V1 pillow lava.

p.15

STOP-2. Sheeted dikes of the southernmost E-W dike swarm-5 in Wadi Dhabeen

Fig. 14. Sheeted dikes of tholeiitic basalt and boninite in Wadi Dhabeen. (A) Obliquely cross-cutting dikes. (B) A later dike
abruptly changed in dike orientation from WNW to EW and truncated adjacent dikes at a high angle. (C) Aphyric MORB
(labeled “3”) and porphyritic boninite (labeled “2”) dikes. (D) Large clinopyroxene phenocrysts in a boninite dike.