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Hydromagmatic eruption during the buildup of a Triassic (Oman Exotics): Eruptive style and associated deformations

Christophe Basile, Franc¸ois Chauvet

PII: S0377-0273(09)00145-0 DOI: doi: 10.1016/j.jvolgeores.2009.03.009 Reference: VOLGEO 4286

To appear in: Journal of Volcanology and Geothermal Research

Received date: 20 October 2008 Accepted date: 11 March 2009

Please cite this article as: Basile, Christophe, Chauvet, Fran¸cois, Hydromagmatic erup- tion during the buildup of a Triassic carbonate platform (Oman Exotics): Eruptive style and associated deformations, Journal of Volcanology and Geothermal Research (2009), doi: 10.1016/j.jvolgeores.2009.03.009

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Hydromagmatic eruption during the buildup of a Triassic carbonate platform (Oman Exotics): eruptive style and associated deformations

Christophe Basile (corresponding author), François Chauvet (1) Laboratoire de Géodynamique des Chaînes Alpines, CNRS UMR 5025, Observatoire des Sciences de l’Univers de Grenoble, Université Joseph Fourier Maison des Géosciences, 1381 rue de la Piscine 38400 St Martin d’Hères, France tel. 334 765 140 69 fax 334 765 140 58 email [email protected] ; [email protected] (1) present adress: Laboratoire de Planétologie et Géodynamique de Nantes, CNRS UMR 6112, Université des Sciences de Nantes, 2 rue de la Houssinière, BP92208, 44322 Nantes Cedex 3, France

Abstract Oman Exotics represent remnants of a Triassic carbonate platform, also named Misfah Formation. Several basaltic intrusions or flows were coeval with the carbonate platform build-up. We present field observations related to one of these volcanic events, including associated deformation of the host rocks and interaction of volcanism with the sedimentation. These observations allow us to reconstruct the structure of the volcanic conduit and vent and the successive stages of the evolution of the eruption. The initial ascent of probably occurred along on a normal fault related to gravity-driven sliding towards the platform edge. Magma was first emplaced in a saucer-shaped sill a few tens of meters below the sea-floor. This intrusion provided a decollement layer, that may have sped up the gravity-driven sliding, opening fractures that brought sea-water into contact with the magma, hence triggering a hydromagmatic eruption. Eruption was followed by the collapse of the host limestones that formed a megabreccia infilling the eruptive vent and prohibiting further contact between sea water and magma. Most of the magma was emplaced at depth in two superposed small magmatic bodies (300 x 100 m and 200 x 50 m in sections, respectively) that replaced the host sediments and uplifted the overlying vent. Despite the shallow submarine setting, this eruption produced a vent quite similar to subaerial maars developped in soft substrate. This emphasizes that the rheology of the substrate may be more important than the amount of water to control magma-water interaction in hydromagmatic eruptions. In an active carbonate platform like Misfah Formation during Triassic times, the early diagenesis of lime mud determined the mechanical comportment of the sediment at the time of eruption, and especially the way it could be mixed with the magma and/or slide into the vent.

Keywords ; sill; magma stock; megabreccia; carbonate platform ; diatreme

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1. Introduction with water (Sheridan and Wohlez, - shallow-marine Surstey-type Since the birth of Surtsey 1983) or water-rich sediments (White, eruptions (e.g. Kokelaar, 1983), where (Thorarinsson, 1967; Kokelaar, 1983) 1996), and with the local conditions magma enters within a large volume of and Capelinhos (Machado et al., 1962; that bring magma and water into surface water. The interaction of Cole et al., 2001) volcanic islands, the contact. Many examples of magma with water in excess produces interaction of magma and external hydromagmatic eruptions have been numerous explosions that build water appeared as one of the most described all over the world, and over cones and/or rings by fallout and important factors influencing the various types of substratum. The two pyroclastic surges (Cole et al., 2001; volcanic eruption processes (Sheridan main types are: Solgevik et al., 2007). and Wohletz, 1983; Zimanowski et al., - subaerial maars (e.g. Lorenz, Carbonate platforms represent a 1991). This interaction results from an 1974), where magma encounters large proportion of shallow-marine efficient heat transfer from the magma ground or underground water. The environments, but we found only a few to the water, generating large amounts evolution of this type of volcano published observations of volcanic of steam. When lava and water come depends on the rheology of the eruptions during carbonate platform together in an underground closed substratum (soft- and hard-substrate build-up. In most cases, carbonate space, the increasing volume of steam maars: Lorenz, 2003; Sohn and Park, platforms developed above or on the may induce overpressure, triggering an 2005; Martin and Nemeth, 2005; Auer side of inactive volcanoes, either in explosive reaction and hydromagmatic et al., 2007), and on the way water and intraplate settings (e.g. Hess, 1946; eruption. The morphology, structure magma circulate to produce a series of Matthews et al., 1974), or in and lithology of the volcanic edifices explosions within the diatreme (Lorenz subduction-related volcanic arcs (e.g. built by hydromagmatic eruptions vary and Kurzlaukis, 2007). Fulthorpe and Schlanger, 1989; Soja, with the ratio of magma interacting 1996). When volcanism and carbonate

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a continental slope (Sumeini Formation: Glennie et al., 1974), and basinal environments (Hawasina Formations: Glennie et al., 1974; Béchennec, 1988). Sumeini and Hawasina formations were thrusted on the arabian platform during the upper obduction of the Sumail ophiolites (Béchennec et al., 1988) (Fig. 1). It is noteworthy that the paleopositions of the sedimentary units on the continental margin were mainly derived from their positions in the tectonic pile, assuming an outward thrust sequence (Glennie et al., 1973, 1974; Béchennec, 1988; Robertson and Searle, 1990). There is an ongoing debate on the nature of the basement below the Hawasina basin, supposed to be either oceanic (Glennie et al., 1973, 1974; Stampfli et al., 1991; Pillevuit et al., 1997), continental (Béchennec, 1988; Béchennec et al., 1990, 1991) or intermediate (Graham, 1980; Searle Figure 1: Location of the studied area (Jabal Misfah, boxed) in a simplified and Graham, 1982). As a consequence, geological map of central and eastern Oman Mountains (modified after the Permian (e.g. Pillevuit et al., 1997) Béchennec et al., 1990). Inset: simplified geological map of northern Oman, or Triassic (e.g. Béchennec, 1988) age modified after Glennie et al., 1974. for the onset of the oceanic accretion is also a matter of debate. During Triassic times, a carbonate platform sedimentation are coeval, they However, this study based on drilling platform developed within the are mainly restricted to cores does not provide information at Hawasina basin. Because they are interstratification of sediments and lava the vent scale nor for the successive mainly found in tectonic slices, flows or volcanic fallouts (e.g. Buigues stages of the eruption. remnants of this platform were named et al., 1992; Soja, 1996; Beltramo, In this paper, we describe the Oman Exotics by Glennie et al. (1974). 2003). interaction of a volcanic eruption with They were referred to the Kwar Group We found only a few examples of the formation of a Triassic carbonate by Béchennec (1988). In the Hawasina volcanism contemporaneous with a platform in Oman. Since a full section nappes, the Kwar Group crops out growing carbonate platform, such as of this volcanic edifice outcrops, it mainly south of the western the Triassic dikes and emplaced provides outstanding observations of termination of Jabal Akhdar anticline in the Dolomites (Southern Alps : the geometry of the vent, of the in several mountains dominated by Blendinger, 1985), or an successive stages of the eruption, and high carbonate cliffs: Jabal Misht, basaltic intrusion and associated allows one for the first time to evaluate Jabal Misfah, Jabal Kwar, Jabal Ghul volcaniclastic sediments in Spain the respective evolutions of the feeding (Fig. 1). This Group has been divided (Fernández-Mendiola and García- system, water-magma contact zone and by Béchennec (1988) into four Mondéjar, 2003). Actually, the best subsea volcanic edifice, emphasizing Formations, later-on subdivided into described example of shallow-marine associated small- and large-scale six Formations by Pillevuit (1993). eruption through a growing carbonate deformations. Stratigraphy has been defined on the platform is the MIT in the northern and eastern slopes of Jabal Pacific ocean (Shipboard Scientific 2. Geological setting Misfah (Fig. 2). Party, 1993; Martin et al., 2004). As part of the southern margin of (i) At the base, a volcanic unit is There, a 200 m-thick unit is the Neo-Tethys ocean, the Oman composed of massive basaltic pillow interpreted as resulting from a continental margin formed during lavas, hyaloclastites and fine-grained through a semi- Permian-Triassic times (Béchennec, volcanogenic sediments (tuffites), and solidified early carbonate 1988; Robertson and Searle, 1990; dated Ladinian-Carnian from platform. Tephra consists of Sengör et al., 1993). Palinspatic found in few intercalated palagonitized basaltic clasts, carbonate reconstructions suggest the limestones (Pillevuit, 1993). This unit mud- to grainstone clasts and armoured development of contrasted sedimentary thrusts the Late Permian to Liasic lapili (Martin et al., 2004). They were environments since Middle Permian bathyal sedimentary rocks of Al Jil emplaced by mass flows and gravity- times, with a continental platform (limestones and cherts) and Matbat driven sliding inside the volcanic vent. (Saiq Formation: Glennie et al., 1974),

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Figure 2: Stratigraphy of th e Kwar Group. Lithologic log is detailed only in the lowest part Figure 3: (A) Schematic geological map of the studied area. Topography of Misfah Formation (see text (meters) from Beurrier et al., 1986. Thick stippled lines are recent faults, for details). Ages according to thin stippled line represents the unobserved limit between Misfah and Pillevuit et al. (1997) and Matbat formations. Figures 5 to 7 and 9 to 11 a re located by ellipses. (B) Beurrier et al. (1986). The Same as Figure 3A, with location of the upper tuffs, megabreccia and studied magmatic event is magmatic stocks, and rose diagrams of structural observations. Rose related to the upper tuffs diagrams (a) to (k), see comments in the text. Same scale for all rose (boxed). diagrams, maximum se ven values on rose c. All structural measurements are related to the same magmatic event, with the exception of N-S structures at sites (i) and (k) which are related to a more recent eastward normal shear. (limestones, sandstones and siltstones) (1993). Krystyn (in Baud et al., 2001) from the Jurassic to Cretaceous Fatah Formations (Beurrier et al., 1986). proposed a revised age of Middle-Late (Pillevuit, 1993), Nadan and Safil (ii) Above the volcanic unit, the Norian for the lower member, and Formations. Misfah Formation is made of thinly topmost Norian-Rhaetien for the upper bedded marly limestones at the base member. Few intercalations of coarse- 3. Field observations (Subayb Formation, Pillevuit, 1993), to fine-grained volcaniclastic 3.1. Stratigraphy overlain by thick and massive sediments occur in the lower part of The studied volcano-sedimentary platform limestones. These two Misfah Formation, also intruded by section belongs to the lowest part of members were dated respectively basaltic dikes and sills. Misfah Formation, northeast of Jabal Ladinian-Carnian, and Ladinian- (iii) The upper part of the Kwar Misfah, near Subayb hamlet. It may Carnian to Rhaetian by Pillevuit group consists of pelagic limestones belong to the Subayb Formation as

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Figure 4: Section across the eruptive point, reconstructed at the time of deposition of the upper limestones. Figures 5, 7, 10 and 11, and rose diagrams (a) to (k) (Figure 3) are located by ellipses or arrows. White dots and lines in the magmatic stocks represent sedimentary blocks and layers, respectively. defined by Pillevuit (1993), but we do carbonates are laterally extensive, bedded marly limestones, two meters not support this stratigraphic effusive and intrusive magmatic of marls, and fifteen meters of thirty terminology as both published formations occur only locally. centimeters-thick grey limestones reference sections appeared to be Above submarine basaltic volcanic strata (Fig. 2). These carbonates are limited upward by tectonic contacts. rocks (pillow lavas and hyaloclastites), locally truncated by a twelve meter- The description of volcanic and the sedimentary section begins with a thick volcano-sedimentary unit sedimentary section here (Fig. 2) few beds of volcanic pebbles in a including, from base to top, tuffites, synthesizes observations from the carbonate matrix. Then a first volcanic breccia, submarine lava flows northeastern part of Jabal Misfah, carbonate unit comprises from the and elongated globular volcanic clasts omitting the numerous and more recent base to the top ten meters of mixed with sediments. Directly above magmatic intrusives and fault offsets. decimeter-thick beds of yellow marly begins a second carbonate unit, It is important to note that while limestones, forty meters of wavy- approximately one hundred and fifty meters-thick, made of thirty centimeter-thick beds of black limestones with a few white stromatolithic layers. Two meter- to a few meters-thick volcano-sedimentary layers are intercalated within this carbonate unit. They consist mainly of tuffites, with volcanic and sedimentary clasts, separated by a few limestone beds exhibiting numerous sliding structures. According to Pillevuit (1993), this second carbonate unit was deposited at shallow depths, mainly in the tidal zone. In the following description of the studied magmatic event, the various parts of this second carbonate unit are referred to as (from base to top) lower limestones, lower tuffs, intermediate limestones, upper tuffs, and upper limestones (Fig. 2). 3.2. Magmatic event 3.2.1. Vent and ejecta The stratigraphic units appear continuously along Jabal Misfah cliffs. However, important variations occur in a 500 meters-wide area southwest of Figure 5: View towards southwest of the megabreccia. Indicative scale Subayb hamlet (Figs. 3A and 4). Here (beware the perspective). Note the tilted blocks within the megabreccia, its the upper tuffs do not lie as usual on stepped and erosive basal surface, and the normal fault sealed by the upper intermediate limestones, but on a tuffs. The upper part o f Misfah Formation forms the cliffs in the background. megabreccia lying itself on an Structural observations made in this area are shown in Figure 3B in the rose erosional surface cutting the diagram (b). Location Figures 3 and 4. intermediate limestones, the lower tuffs

4 In press, Journal of Volcanology and Geothermal Research and the lower limestones (Fig. 4). The megabreccia consists of meter- to decameter-scale limestones beds tilted in an intraformational breccia composed of angular blocks of the eroded limestones mixed in a grey limestone matrix (Fig. 5). It is noteworthy that some basaltic gravels occur within the limestone breccia, but that there is no volcanic nor volcano- detritic layer between the megabreccia and the erosional surface. The megabreccia is laterally continuous with the intermediate limestones, where numerous tension gashes develop. These tension gashes are connected to and filled by interlayed marly limestones, suggesting in situ brecciation. The megabreccia and its tilted blocks are sealed by few meters of platy limestones, then by the upper tuffs, thickening up to 15 meters (Fig. 4). In this area, the upper tuffs consist Figure 6: Syn-diagenetic normal faults in the intermediate limestones, of intercalations of thin tuffite beds indicating sliding towards N30°. Wulff net (lower hemisphere): small crosses with massive breccia made of are poles for bedding, great circles are normal faults, stippled great circle and limestones and a few basaltic blocks in star are the best great circle for bedding and its pole (i.e. the axis of rotation), a calcareous matrix. In the overlying respectively. Notebook for scale (17.5 cm-high, 11.5 cm-wide). Structural upper carbonates, there are no lateral measurements are also shown Figure 3B in rose diagram (d). Approximate variations of facies nor thicknesses. location Figure 3. Below the megabreccia, the erosional surface deepens toward a calcareous, although others are derived the time of tuff emplacement, and that central point where there are no more from volcanic lithic fragments or glass. the products of erosion were mixed lower limestones, but a mix of The edges of lapilli cores are very with the tuffs in their lowermost part. and sediments, overlying massive irregular, corroded and underlined by Above, thinly layered tuffs exhibit (Fig. 4). The erosional surface oxidation (Fig. 8D). Lapilli rims also basaltic and limestone clasts, as big as affects a 150 meter-wide zone include palagonitized shards in a one meter in diameter. The asymmetric southwest of this central point, and a cryptocristallyne matrix showing impact sags (Fisher, 1977) show that wider (300 m) zone northeastward, concentric structures (Fig. 8). the blocks were ejected from the where erosion reaches deeper levels At the base of the upper tuffs, southwest and fell on previously (Fig. 4). numerous shells are reworked from an deposited tuffs (Fig. 9). The same The upper tuffs contain numerous underlying lumachelle, and mixed with displacement of ejecta towards N10° is lapilli in a cryptocristalline lime mud limestone clasts in the tuff matrix. This indicated by oblique bedding in small matrix (Fig. 8), where glassy or indicates that the substratum of the (twenty centimeters-high) dunes (Fig. palagonitized shards are commonly upper tuffs has been locally eroded at 10). observed. Most lapilli cores are

Figure 7: Syn-sedimentary deformation in the platy limestones, indicating sliding towards N190° to N220°. Same legend as for Figure 6 for the stereo net; the thick great circle represents the sedimentary dike. Same notebook as for Figure 6. Structural observations made in and around this outcrop are shown Figure 3B in rose diagram (c). Location Figures 3 and 4.

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striking N130°, that separates the lowest part of the lower limestones from the base of the megabreccia (Fig. 3B-a, Fig. 4). Some conjugated normal faults striking N155° occur on both sides of the vent (Fig. 3B-e and f). They were all sealed by the megabreccia. Northward the significant throws are located on north-dipping normal faults, while southward the southernmost fault dips south. Those faults farther from the center of the vent shift the erosive base of the megabreccia (Figs. 4 and 5), indicating they were slightly younger than the conjugated faults closer to the center of the vent. These younger normal faults define a small horst centered on the eruptive point. It is noteworthy that the strike of the normal faults significantly changes from N160° southwestward to N110° northeastward, while the NNE- SSW stretching direction indicated by the deformation of the upper tuff layers does not significantly change (Fig. 3B- a to g). In the megabreccia, as in the in-situ brecciated intermediate limestones, numerous sliding structures were observed. The sliding blocks consist at least partly of dolomite beds, probably more lithified than the limestones at the time of the sliding. Sliding planes and syn-sedimentary tilt axes in the megabreccia and intermediate limestones predominantly strike either N110° or N-S (Fig. 3B-b to d, Fig. 6). They are sliding towards the center of the vent, with the exception of a structure observed as far as 1.8 km northeastward (Fig. 3B-d, Fig. 6). Similarly, we observed several syn- sedimentary listric faults in the platy limestones, indicating NNE-SSW extension and sliding towards the Figure 8: Texture of the upper tuffs. A: scanned thin section. B: lapilli rim center of the vent (Fig. 3B-c, Fig. 7). with yellowish glassy shards in a cryptocristalline matrix. C: glassy 3.2.3. Intrusions shards palagonized and oxided in the tuffs matrix. D: armoured lapillis Within the vent, folded beds of with altered volcanic cores. Note the corroded edges of the volcanic limestones are mixed together with a fragments. Figures 8B and 8D are located in Figure 8A; Figure 8C is a basaltic breccia containing numerous detail from an other thin section. calcareous clasts and blocks. Folds are roughly cylindrical, with sub- We interpret this erosional zone as a 3.2.2. Faulting in the host horizontal axial surfaces and axes volcanic vent (White, 1991; Lorenz, limestones trending N130° to N150° (Fig. 3B-a). 2003). The infilling megabreccia During the magmatic event, some Right below the eruptive point, scree resulted from the collapse of its walls, normal faults affected the underlying hides the deep structures, which can and the upper tuffs represent the ejecta limestones (Fig. 4). These faults were only be seen northward below the deposited during the eruption. all sealed by the upper tuffs. At the top lower limestones. There, a basaltic of the massive basalts, the southwest body intrudes the wavy-bedded edge of the vent is a normal fault limestones (Figs. 3A and 4). In this

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restricted when compared with the thickness of the magmatic body. It seems that the magmatic body was emplaced by fracturing the host rock and assimilating at least partially the limestone blocks. The basalts are quite homogeneous in the magmatic body, with the exception of an increasing alteration upward. Alteration probably results from higher water content at the time of the magmatic event. No trace of a magmatic body has been found south of the eruptive point, but a basaltic sill underlines the base of the lower limestones (Fig. 4). While presently horizontal, this sill is offset by a normal fault sealed by the upper tuffs. Below the magmatic body, several tens of meters of pelagic silicified limestones and sandstones attributed to the Matbat Formation were deformed by two successive ductile events: Figure 9: Ballistically-emplaced limestone block. The grey limestone normal shearing towards N50° blocks were projected in the tuffs. The white l imestones blocks are screes characterized the older one; N-S from the above cliff. Same notebook as for Figure 6. Structural trending folds represent a younger observations made around this outcrop are shown Figure 3B in rose eastward normal shear (Fig. 3B-i). diagram (g). Location Figure 3. There is no significant shear zone

between the Matbat limestones and the overlying basalts, but both are interlayered and affected by the two ductile deformations, suggesting that the magma was ductile at the time of deformation. Below these deformed sediments, similar limestone beds and blocks from the Matbat Formation are included inside a deeper magmatic body, where radiolarites were also found as xenoliths. As in the upper magmatic body, sedimentary beds are often folded, with fold axes trending N90° to N140° (Fig. 3B-j, Fig. 11). Shear zones indicate top-to-N50° displacements (Fig. 3B-k). The sedimentary blocks or layers are clearly metamorphosed in this lower magmatic body, while they are not, at macroscopic scale, in the upper stock.

4. Interpretation Figure 10: Dune structure in the upper tuffs. Same notebook and same 4.1. Structure legend for the stereo net as for Figure 6. Location Figures 3 and 4. The vent and both upper and lower magmatic bodies appear aligned in a magmatic body, blocks or beds of the main magmatic body, that pinches out WNW-ESE direction parallel to the enclosing or overlying limestones are laterally and accommodates the main normal faults (Fig. 3B). This mixed with basalts. Some limestone incipient fall of a limestone block at feature can not be fitted by a circular beds are longer than 40 m, and folded the side of the magmatic body (Fig. 4). crater, i.e. a structure centered on the with a N150°-trending axis (Fig. 3B- We made no observation indicating eruptive point. As this linear h). The enclosing limestones are cross- that the magma may have pushed away disposition is parallel or sub-parallel cut by N130°-trending dikes, and the host rock laterally. Upward push is with the normal fault bounding the locally by a sill connected with the probable, but would have been very

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northeastwards (Fig. 3B). Moreover, N110°-striking normal faults are in some places associated with N-S- striking normal faults or sliding structures (Fig. 3B-b and c). These N110° and N-S structures may have been formed as conjugated faults in a transtensional regime, indicating a N60° lengthening direction, perpendicular to N160° normal faults. Both changes in orientation and transtension can be explained by an homogeneous displacement on an arcuate structure (Fig. 12): in the central part, displacement is perpendicular to the structure, and results in N110°-striking normal faults in an extensional regime; on the sides, displacement becomes oblique, and can be partitioned in divergence perpendicular to the structure, and strike-slip parallel to the structure. The Figure 11: Folded sedimentary bed within the lower magmatic stock (view resulting faults are either controlled by towards WSW). Same legend for the stereo net as for Figure 6; the black the transtensional tectonic regime dot and the star represent the mea sured and computed fold axis, (conjugated N110°- and N-S-striking respectively. The fold axis is also shown Figure 3B in rose diagram (j). faults), or by the local topography Location Figures 3 and 4. imposed by the main structure (N160°- striking normal faults, parallel to the main structure). As subsurface deformation is probably linked to the magma-feeding fissure, this interpretation suggests that the feeding fissure may be curved and several kilometers-long. Moreover, several observations indicate an asymmetry in the deformation: the lower limestones are more thinned and the normal faults more numerous north of the vent than southward, the main normal faults are dipping northeast as the fault bounding the vent, the deep shear zones also indicate displacements toward NE. This suggests that the carbonate platform was stable southwest of the feeding fissure, while the northeastern part slid northeastward on the intrusive magma and a deep shear zone, possibly towards the edge of the carbonate platform. Figure 12: Map view of the inferred eruptive fissure. Changes in fault strike 4.2. Hydromagmatic eruption are interpreted as an effect of displacement p artitioning along an arcuate In the upper tuffs, the volcanic eruptive fissure. See text for comments. shards are palagonitized, and the surfaces of volcanic cores in lapili are eruptive point, with the other major rose to the subsurface along a commonly corroded and oxidized (Fig. normal faults, and with the shear zones WNW-ESE- trending fissure. 8). This suggests that these tuffs at depth, it suggests that both magmatic We also observed slight changes in resulted from an explosive mixing of and tectonic processes were associated the strike of the main normal faults, magma and steam, where volcanic in a NE-SW extensional tectonic from N160° in the southwestern part of shards were hydrated and corroded. regime, and that the magma probably the outcrop, to N110° eastwards or Together with other ejected objects as

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Figure 13: Reconstitution of the successive stages of eruption. Stage E corresponds to Figure 4. Dotted lines and question marks indicate inferred - but not observed - structures. See text for comments. spinel crystals or sedimentary impact sags (Fig. 9) (Fisher, 1977; solid rafts in the megabreccia or as fragments, they were wrapped within Pardo et al., 1998), and base surge clasts in the tuffs, marly limestones the eruption cloud by a rim of heated (Fisher, 1977) suggested by in situ were still soft, deformed ductilely in mud, and fall-out to form the upper erosion and resedimentation of shells at the megabreccia, and were injected in tuffs. All these elements favour the base of the tuffs. tension gashes. Lime mud was hydromagmatic processes, where the Finally, it is obvious from its probably the more recent sediment at eruption results from the production of location that this phreatomagmatic sea-floor, and was incorporated in the over-pressured steam by underground eruption occurred within a carbonate eruption to form the matrix of the tuffs. mixing of water and magma (Wohletz, platform. Moreover, the lime mud 4.3. Evolution 1986; White, 1996). nature of the tuff matrix, the numerous It’s possible to reconstruct in detail Several observations indicate that limestone clasts in the tuffs, the how the volcanic event proceeded (Fig. the tuff layer is related to a sub-aerial limestone boulders associated with 13): eruption. These observations include impact sags, and the mechanical - During a first stage, the magma well-bedded and locally dune-bedded behavior of the megabreccia point out rose to the base of the lower poorly sorted sediments (Fig. 10) (e.g. the variable diagenesis in the host limestones, probably along an incipient Gençalioglu-Kuscu et al., 2007; Pardo limestones at the time of eruption: normal fault (Fig. 13-A). The magma et al., 2008), isolated cobbles and while limestone and dolomitic beds intruded as sub-horizontal sills in the boulders of limestones associated with were well indurated and behave as main sedimentary discontinuities,

9 In press, Journal of Volcanology and Geothermal Research especially at the base of the lower cut the saucer-shaped sill suggests that - Finally, the calcareous limestones, which was probably the they formed when the magma was still sedimentation proceeded again and surface with the highest rheological fluid in the sill, and consequently that sealed the whole structure. At that time constrast. Once the displacements the eruption was sub-contemporaneous the vent was probably a several meter- induced by more recent faults have with the intrusion. deep trough, where platy limestones been removed, the present-day - Back-stripping of the present day were deposited. Differential geometry indicates that this magmatic section indicates that the sealed normal compaction locally increased and intrusion was saucer-shaped faults do not accommodate the maintained the subsidence along the (Chevallier and Woodford, 1999), and subsidence of the northeast block, but vent (Lorenz, 2007), and explains the less than one hundred meters below the instead the uplift of a narrow horst gravity-driven deformations that sea-floor (Fig. 4). It was probably fed centered on the vent, contemporaneous occurred during the platy limestones’ by a central magmatic body, as in the with the eruption, but prior to the deposition. Then the calcareous model proposed by Thomson and redeposition of the megabreccia. sedimentation proceeded Hutton (2004). We call it a proto- Normal faults farther from the center homogeneously from the base of the magmatic body (Fig. 13A), as it of the vent shift the bottom of the upper limestones, with no significant represents an early stage of the upper megabreccia, but are sealed at its top lateral variations of facies nor magmatic stock. Although there is no (Fig. 13-E). They define a wider horst thicknesses. Subsequent deformations direct evidence for it, it is assumed that formed during the stabilization of the and magmatic events deformed this the two magmatic bodies initiated at megabreccia. The southernmost normal structure later-on, as shown by the the same time. At this stage, there is no fault cuts the saucer-shaped sill, more recent N-S-striking dikes and evidence of explosion associated with indicating that megabreccia veins cross-cutting the upper magmatic sill intrusion, suggesting the absence of stabilization was long enough to allow body (Chauvet, 2007). an aquifer below the lower limestones. magma cooling and solidification in - The increasing size of the the sill. Enlarging horst formation can 5. Discussion magmatic intrusion may have allowed be understood as resulting from the 5.1. Comparison with the MIT the overlying carbonates to slide on the inflation of the upper magmatic body, guyot eruption proto-magmatic body (Fig. 13-B). This probably due to a very rapid magmatic As stated in the introduction, there destabilization may have opened ascent compensating the unloading is only one other published example of tension cracks in the lower limestones, produced by the explosion. The a volcanic eruption through an active that brought sea-water into contact magmatic body clearly replaced the carbonate platform, the MIT guyot with the magma. The resulting enclosing limestones, with some (Shipboard Scientific Party, 1993; hydromagmatic explosion locally remnants that sank and were deformed Martin et al., 2004). Both MIT guyot removed the overlying sediments on as blocks or layers in the magma. and Misfah platform present a tephra the normal fault hanging wall (Fig. 13- - After the stabilization of the unit, that consists of a mix of C). This eruption occurred in a marine megabreccia, minor hydromagmatic palagonitized basaltic clasts and environment, but displays sub-aerial explosions still produced ejecta, that armoured lapilli in a carbonate mud. attributes, indicating a very shallow fed the overlying tuffs and breccia. It Both eruptions were hydromagmatic, sea, consistent with the carbonate indicates that the magma still locally triggered by the interaction between platform environment (Pillevuit, 1993; met the sea water or soft sediments, magma and water-satured sediment in Bernecker, 2007). probably in the mixed lava and a semi-solidified carbonate platform. - At the surface, this explosion may sediment layer, located at the base of However, the tephra unit (MIT guyot) have produce a crater, possibly the vent between the magmatic body and upper tuff (Misfah) present elongated along a WNW-ESE direction and the megabreccia. However, no significative differences : the tephra similar to the underlying normal fault. eruptive structures such as vents or unit is 200 m-thick, with numerous The walls of the crater then collapsed, diatremes were observed across the sliding structures, while the upper tuffs and the surface and sub-surface megabreccia. These structures and the are 10 meters-thick, and seal the sediments may have slid towards this associated eruptions were probably deformed underlying megabreccia. crater, inducing in-situ brecciation by smaller by several order of magnitude One may propose that the Misfah’s breaking and sliding of the more than the initial one. equivalent of the MIT guyot tephra lithified beds, mixed in a lime mud - During these first phases, the unit can be the thick megabreccia, matrix (Fig. 13-D). This megabreccia carbonate platform slid above ductile which has been also emplaced by lies on an erosional surface that magma near the vent. Rapidly, the gravity-driven sliding, and which represents the lower limit of the magma closest to the surface cooled contains few volcanic clasts. The unstable sediments and that cross-cut and became solid, but the platform difference between those two sites may the intermediate limestones, the lower continued to slide northeastward on the be related to the rheology of the host tuffs, the upper part of the lower underlying Matbat sediments and on sediments where the magma intruded. limestones, and the normal faults the lower magma body that was In the MIT guyot, a soft substrate may which are closest to the eruption cooling more slowly than the upper have been totally destabilized during center. The fact that the conjugated one (Fig. 13E). the eruption, mixed with the magma, normal faults south of the vent do not and subsequently displaced by grain

10 In press, Journal of Volcanology and Geothermal Research flows and turbidity currents (Martin et platform, where both porosity and 5.3 Vent and diatreme shapes al., 2004). In Misfah, the substrate has permeability are controlled partly by When compared with continental been rigidified by early diagenesis of the changes of sedimentary facies, and maar-diatremes, the studied vent carbonates, and especially of mainly by the diagenetic evolution exhibits peculiar features. It has a dolomites; this substrate slided after (Whitaker et al., 1997). After broad and shallow bowl-shaped the initial explosion, but was too rigid diagenesis, the carbonates behave as structure, similar in shape to the soft- to be significantly mixed with the impermeable layers. Karstification substrate maar volcanoes, contrasting magma during this explosion. during platform emersion and with the deep and steep-sided 5.2. Eruptive behavior fracturation are the main processes that diatremes observed for hard-substrate Hydromagmatic eruptions occur in increase notably the carbonate maar volcanoes (Lorenz, 2003; Auer et various environnements, and result in permeability. In Misfah carbonate al., 2007), including those that various morphologies, structures and platform, fracturation of the shallower commonly occur in systems with lithologies. This reflects eruptive part of the platform may have been the carbonated rocks (Nemeth et al., 2001). behaviors mainly controlled by the initial mechanism that brought water in While most limestones and dolomitic local conditions that bring magma and contact with a shallow-seated sill. layers were solidified by early water into contact. The vaporization of As in soft-substrate maar volcanoes diagenesis by the time of eruption, the either phreatic or sea water in contact (Auer et al., 2007), the widening and mechanical behavior (slope failure and with the magma leads to violent infilling of the vent were mainly the angle of repose: Barnett, 2008) of the explosions, to volcanic structures well- result of the collapse of the vent walls host rocks depended on the proportion expressed in the topography (crater after an hydromagmatic eruption. and mixing of soft layers (marls, lime above a diatreme, tuff ring or scoria However, as emphasized in the mud) and hard layers (limestones, cone), and to characteristic volcanic previous section, the collapse is dolostones) in the affected sedimentary formations or rock types (basal surge controlled by the rheology of the section. The resulting collapse of the deposits, ejecta, hyaloclastites, sedimentary section. Soft sediments sides of the vent at the time of eruption ). Is there a specific eruptive collapse and are mixed with magma appears mainly controlled by the soft behavior associated with the carbonate during the eruptions, as in the example layers. platform sedimentary environment ? of the MIT guyot (Martin et al., 2004) The second process that These platforms are below sea-level or Jeju island unconsolidated significantly controls the shape of a and at shallow depth (a few meters to sediments (Sohn and Park, 2005). In maar-diatreme volcano is the depth of ten meters), but unlike Surtsey-type Misfah, early diagenesis allowed initial interaction between magma and eruptions, no tuff- or scoria-cones were sedimentary beds or sets of beds to water, and the repetition of this built during the eruption, because the slide like rafts or tilted blocks within a interaction at deeper levels in the lava did not reach the sea-floor and muddy calcareous matrix to form a diatreme, inducing the downward was not directly in contact with sea megabreccia. The displacements of penetration of the explosion locus and water. There are no lava flows and these solid rafts may strongly control the steepening of the cone-shaped only a very restricted proportion of the evolution of the eruption, creating diatreme (Lorenz, 1986). As proposed hyaloclastite or palagonite in the an impermeable lid that isolated the in other settings (Houghton et al., volcanic ejecta, while a significant underlying magma from the overlying 1999; Nemeth and Marin, 2007), the magmatic volume was emplaced only sea water, and consequently both the initial water/magma interaction few tens of meters below the sea floor. access of most of the magma to the occurred at a shallow depth, less than The eruption seems to have been surface, and the access of sea water to 100 m. In the studied case water almost purely phreatic, with a very the magmatic stock. In this case, the refilling may be favored by a restricted proportion for the juvenile eruptive behavior is first controlled by subaquatic setting, but appears to have volcanic component of tephra, but explosive processes (magma-water been prevented by the lack of there is no morphologic indication of a interaction) for the initial eruption, permeability of the megabreccia filling tuff ring that should have been well- then by gravity-driven processes the diatreme. recorded in a carbonate platform (destabilization of the sedimentary 5.4. Volcanism-sedimentation environment. There is no more cover) that preclude any other interaction sedimentary evidence of erosion and significant eruption and the The initial hydromagmatic re-working of tuff materials, as should development of a diatreme. The explosion locally removed the topmost be expected for a tuff ring or cone interaction of the underlying magma formations. It induced a pressure fall at leveled by wave action (Cas et al., with the water contained in the depth (Lorenz, 1974; Lorenz and 1989; Sohn et al., 2008). megabreccia (especially in the Kurszlaukis, 2007), that allowed the Furthermore, there were no carbonate mud matrix) may have ascent of a significant volume of repeated phreatic eruptions at depth produced only small and secondary magma and its emplacement below the and no associated brecciated pipe, hydromagmatic explosions restricted to lower limestones. This magma suggesting the sea water could not the mixed lava and sediment layer, at represented a decollement layer, where refill an aquifer in contact with the the top of the upper magmatic body. the limestones could slide away from magma. This is probably related to the the vent. Sliding increased the specific hydrology of carbonated unloading, allowing more magma to be

11 In press, Journal of Volcanology and Geothermal Research emplaced at shallow depth. Rising 6. Conclusion edge. We also suggest that these magma produced a tectonic instability, We presented a detailed description gravity-driven displacements may be that in return favoured the magma of an hydromagmatic eruption both an important control factor in the ascent in a feedback mechanism. within, and coeval with, an active eruptive behavior, as they localized the Comparable feedback has been carbonate platform. While this eruption rising of magma and prevented proposed for the emplacement of occurred below sea level, we did not repeated hydromagmatic explosions. magmatic domes such as Mt St. Helens observe tuff- or scoria-cones at surface Finally, it is important to note that a (Lipman and Mullineaux, 1981). In this nor brecciated pipe underground, that very small amount of the magmatic pelean eruption, the emplacement of a may have indicated a repeated contact rocks reached the surface during this crypto-dome at depth triggered a between magma and sea water. We eruption, and that it may induce an tectonic instability (gravitational found two superposed magmatic underestimation of this type of sliding on the volcano flanks), that bodies that intruded the sedimentary volcanism coeval with the building of induced the blast by decompression of cover a few tens of meters below the carbonate platforms. the unroofed crypto-dome. sea floor. At the top of the shallower The eruption induced very limited magmatic stock, the sediments were Acknowledgements changes in the carbonate locally removed by an hydromagmatic This paper is dedicated to our sedimentation. At the time of the eruption, and the host sediments colleague Henriette Lapierre, deceased eruption, carbonate sedimentation collapsed from the walls of the vent to in January 2006, who initiated our probably locally ceased because of form a megabreccia. The megabreccia work in Oman. The field work has water turbidity, and only previously filled the vent and prevented any been partially funded by the formed sediments were reworked. One further contact between sea water and Groupement de Recherche 'Marges'. may expect vertical displacements and magma. This allowed the inflation of a The Centre National de Recherche carbonate facies changes associated shallower magmatic body, with at its Scientifique and Université Joseph with the volcanic event, but they top, a 20 meters-thick layer where Fourier also provided financial support, appear to be restricted. The sub-surface magma and sediments have been together with various funds initially intrusions are mainly compensated by mixed together. The small quantity of scheduled for other scientific works. the removal of the overlying water available in the sediment allowed We thank F. Béchennec and N. Arndt sediments, either by their projection only small explosions from this mixed for very fruitful discussions at various during the eruption or by sliding in the layer. This volcanic structure is quite stages of this work, and Dr Hilal megabreccia. Local uplift above the similar to subaerial maars erupted in Mohammed Sultan Al Azri from vent can be estimated from the slip on soft sediments. It underlines that the Omani Ministry of Commerce and normal faults: it represents no more main control factor on hydromagmatic Industry for his kind welcome and than 20% of the observed thickness of eruptions is not the amount of available support in Oman. Rose diagrams and the upper magmatic stock, and this water, but the mechanisms that bring stereo nets were drawn using A. vertical displacement is almost totally into contact water and magma. In the Pecher’s Stem software. The absorbed within the megabreccia. The studied case the rheology of lime mud manuscript benefited from thorough only significant effect on and limestones appears to be a key reviews by K. Nemeth and an sedimentation is a local increase of parameter, and was probably mainly anonymous reviewer, and was kindly depth and subsidence in the vent area controlled by early diagenesis. checked by A. Bargery. By reference (Lorentz, 2007), allowing deposition of Structural observations from the to the goats grazing in the magmatic platy limestones. This change does not sedimentary section and the syn- stock, we shall appreciate if this seem to be strictly related to the volcanic deposits suggest that the volcanic event and the associated eruption, but mainly to the compaction eruption did not produce a circular structures will be named Goat's of the megabreccia that filled the vent. vent, but an arcuate fissure, probably Volcano. related to the gravity-driven sliding of the carbonate platform towards its ______

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