The magma poor passive margin

transition zone there is no straight forward discrimination between the ultramafic and the mafic section, but in the field the border is often seen as a mappable line (Nicolas and Prinzhofer, 1983).

The origin of the transition zone is still unclear and debated. One theory states that this is cummuls olivine coming from melts rich in MgO. The second theory states that the dunite is mantle peridotite that has been relocated by melts and transformed into its present day origin.

In the Oman ophiolite there has been proposed that both these theories can be correct (Abily and Ceuleneer, 2013).

The peridotites found in the Caledonides have previously been and in some cases still are interpreted to be the lowermost part of ophiolites. The peridotites found North of Otta have recently been described to be of ophiolitc affinity (Nilsson and Roberts, 2014) while the ones found South of Otta in the melange unit have been described as remnants of hyperextension (Andersen et al., 2012, Nilsson and Roberts, 2014). The ophiolite model is however

problematic since the sheeted dike complex, the pillow basalt and the rest of the ophiolite pseudostratigraphy are missing.

1.4.2 The magma poor passive margin

Magma poor passive margins have been suggested to consist of four main domains, created by the rifting processes that take place at such margins. The different domains and their position relative to each other can be seen in

Figure 5. Below follows a short description of each following the terminology of Manatschal and coworkers;

The proximal domain: Consists of the landward part of the margin and is formed in basins, where normal faults can be seen (Manatschal et al.,, 2010). The faults are dipping towards the centre of the basins, resulting in a near symmetrical graben structure in the centre and tilted blocks at the borders (Manatschal, 2004). The faults apparently penetrates down to middle crustal levels (Manatschal et al., 2010).

The necking zone: This zone is defined as the area where the thickness of the crust is reduced to less than 10 km. The zone separates the landward proximal domain and the seaward distal domain (Péron-Pinvidic and Manatschal, 2009).

15 The distal domain: Is the oceanward part of passive margins. It is characterized by

detachment faults reaching down in to the mantle. These, low angle normal faults, are believed to be an important factor in the exhumation of ultramafic mantle rocks. The

exhumation of mantle gives the lithosphere in the distal margins distinctive properties, since its characteristics neither are continental or oceanic (Manatschal et al.,, 2007). The crust in this domain is hyperextended commonly to less than 10 km thickness (Manatschal et al., 2010). In the Iberia-Newfoundland margin the detachment faults are seen to separate continental crust from serpentinized mantle (Pérez-Gussinyé, 2013).

The ocean continent transition zone: It consist of serpentinized mantle rocks together with breccia’s of ophicarbonated rocks as witnessed on the Iberia Newfoundland margin (Manatschal et al., 2010).

The classical locality for description of modern passive margin architecture is the Iberia Newfoundland margin since this margin has been dredged and drilled (Boillot et al.,, 1980) together with geophysical surveys. This gives it the status as the best studied present day passive margin in the world. This is however, not the only magma poor passive margin available. Below follows some other examples;

 Brazil-Angola margin e.g. (e.g. Aslanian et al.,, 2009)

 North Western Australia margin (e.g. Direen et al.,, 2007)

 The Norwegian Sea margin (e.g. Osmundsen and Ebbing, 2008)

 South China Sea (Zhou et al.,, 1995, Hayes and Nissen, 2005, Yan et al.,, 2006, Zhou and Yao, 2009).

16 Figure 5. 2D model of margin architecture at magma poor passive margins. Modified after (Péron-Pinvidic and Manatschal, 2009)

The recent recognition of hyperextended passive margins around the world points towards its important role in thinning of continental lithosphere and its important role in the kick-off stage in the Wilson cycle.

The process of hyperextension

A commonly used, but rather simplistic model to describe the thinning of continental crust is the pure shear model (e.g. McKenzie, 1978). This model proposes a rapid and symmetrical stretching of the crust, resulting in thinning and upwelling of hot asthenosphere followed by faulting of blocks and thermal subsidence.

The second model was based on field observations. Wernicke and Burchfield (1982) observed faults that were separating middle and lower crustal metamorphic rocks from upper crustal rocks. This kind of faults implied that extension was dominated by simple shear. These faults were named detachments faults.

Exactly how hyperextension do occur at magma poor passive margins are not well understood, and therefore several models for the process exist. As mentioned above

detachment faults have been imaged in seismic profiles at passive margins (e.g. Osmundsen and Ebbing, 2008) and they are thought to play an important part in the hyperextension process. The detachment faults are believed to be active at low angels and a polyphase rifting model (Figure 6) has been proposed (e.g. Péron‐Pinvidic et al.,, 2007). In this model the crust is first stretched by high angel brittle faults. The second part involves detachment fault

17 forming along the lithosphere, which thins the crust. This may lead to mantle exhumation and crustal rupture (Manatschal, 2004). The model presented reproduce the deformation

sequences that can be witnessed on the Iberia-Newfoundland margin and the Alpine Tethys margin as well (Péron‐Pinvidic et al., 2007).

Figure 6. Simplified 2D model of how hyperextension is thought to happen at magma poor passive margin through a polyphased rifting model and the following exhumation of serpentinized peridotite.

Modified after Mohn et al., (2010)

Another model that aims to explain hyperextension was proposed by Ranero and Pérez-Gussinyé (2010) and Pérez-Pérez-Gussinyé (2013) and explain hyperextension of faults not being active at low angles. The faults are active at high angles, resulting in rotation of previous faults and they then appear as being active at low angles. This way of describing

hyperextension explains it within an Andersonian framework (normal faults active at high angles 60^o and revers faults being active at low angles 30^o (Anderson, 1951)). The model has two stages and explains hyperextension as starting in basins;

18 The basin stage: the rifting inside basins often start with a number of unconnected small faults that are dipping both outward and inward. During faulting they may rotate a few degrees; this leading to the faults getting bigger and they connect. The strain now focuses on the faults dipping inwards toward the basin centre and the ones dipping outward become inactive. At the end of this stage, the strain focuses only on one major fault that defines the centre of the basin (

Figure 7a-c.) (Ranero and Pérez-Gussinyé, 2010).

The margin stage: Stress still causes the plates to move apart, thus the crust in the hanging wall of the last active fault in the basin stage behaves brittle. New faults will then start to form in the hanging wall, slipping and further thinning the crust. Successively more faults will be created in the same way (

Figure 7 and Figure 8) (Ranero and Pérez-Gussinyé, 2010). Exhumation of mantle rocks will follow when the crust has been thinned sufficiently (Pérez-Gussinyé, 2013).

The transformation from planar to listric to detachment faults are another characteristic imaged in seismic interpretation at passive margin. This can be explained by the model described above. When crust gets thinned, new faults form at smaller spaces; the inactive faults will start rotating and get a similar geometry of the previous inactive fault at depth. The no longer active faults will rotate to a lower angle and produce the classical listric geometry (Pérez-Gussinyé, 2013).

As mentioned earlier the detachments faults are believed to play an important part in the exhumation of mantle rocks. It has also, however, been proposed that the mantle rocks may be ascending due to their own buoyancy when they become serpentinized (Pérez-Gussinyé, 2013).

In the recent year several places containing serpentinized peridotites have been seen to be former passive margins. Below follows a few examples;

The Caledonides as already mentioned, but also the lherzolites in the Pyrenees has been interpreted as hyperextended exhumed mantle rocks. The exhumation in this case is not the result of opening of an ocean as was the case in the Caledonides, since no samples of oceanic lithosphere can be found at this location. The exhumation of subcontinental mantle happened in pull apart basins (Fig. 9) during the separation of Iberia and European plates (Lagabrielle and Bodinier, 2008, Lagabrielle et al.,, 2010)

19 Figure 7. Detailed model for hyperextension based on angles observed at magma poor passive

margins. The grey part of the model is pre-rift sediments deposited under shallow water level. The red lines mark active faults, while the pink lines mark inactive faults. The thin red lines indicate initial fault geometries before the faults get rotated. At the bottom of the model is a blue line, this line represent moho.

20 Figure 8. Continued from

Figure 7 the blue and green dots marks the position of structures and Moho respectively before and after faulting. The orange shaded areas represent the lower part of the crust.

21 Figure 9. Model of the hyperextension process in the Pyrenees. a: show how and when the pull a part basins formed and their location within the Pyrenees. b: model of the basins and how the

hyperextension happens in pull a part basins. C: profile of one of the hyperextended basins in the Pyrenees. Figure from (Lagabrielle and Bodinier, 2008).

Exhumed mantle rocks

The Iberia Newfoundland margin is the only place where a complete OCT has been drilled at a magma poor passive margin. This margin has also extensively been analysed by geophysical methods, but detailed information on OCT rock types and their relationship are not found from the drilling or the seismic. For this information we need to turn to ancient magma poor margins and look at the remnants of the OCT rocks. The ancient magma poor rifted margins can be found in mountain belts as the Caledonides and the Appalahines (Andersen et al., 2012, Chew and van Staal, 2014), the Alps (e.g. Beltrando et al., 2014) and the Pyrenees (Lagabrielle and Bodinier, 2008, Lagabrielle et al., 2010). The Alpine Tethys ophiolites have been of interest, since they contain OCT rocks of Jurassic Thethyan ophiolites (150 Ma).

It should be mentioned that the Alpine Tethys are not true 3 layered ophiolites (Manatschal and Müntener, 2009), the Alpine ophiolite do not show the characteristics of slow spreading ridges or transform environments, but show characteristics that are commonly seen at the OCT at the magma poor passive margins.

22 In some Alpine ophiolites, detachment faults and low angle normal faults can be observed (Froitzheim and Rubatto, 1998). The low angle normal faults are characterized by

cataclastites and gouges that are often ophicarbonated. The ophicarbonates rocks have been taken as evidence of detachment faults, they are often found in clast supported breccia’s consisting with exhumed serpentinized peridotites. The faults are overlain by tectono-sedimentary breccia’s that continue up in the post-rift tectono-sedimentary layers (Manatschal and Müntener, 2009). Below the detachment fault, serpentinized peridotites together with gabbros can be observed. Primary olivine, cpx and opx are rarely found inside the peridotite since the serpentinization process in Alpine peridotites commonly is almost complete. In the hanging wall of the detachment fault, veining with chlorite and serpentine can be observed. The intensity of the brittle deformation increases upwards into a fault core zone of serpentinite gouges. In some cases, there have been found clasts of dolerite inside the fault core, which imply that the detachment fault was followed by magmatic activity (Manatschal and Müntener, 2009).

At the passive continental margin in the Alpine Thethys, the exhumed mantle rocks show compositional differences with respect to their location inside the OCT. The exhumed rocks in close proximity to the continent are serpentinized spinel peridotites with abundant

pyroxene layering. The exhumed mantle rocks located further away from the continent do not show any pyroxene layers at all (Schaltegger et al.,, 2002).