Australia: The Land Where Time Began
The Greenland-Iceland-Faroe Complex (GIFRC) has been forming since the opening of the northeast Atlantic Ocean, <55 Ma, standing out as a prominent feature on all geoscientific datasets. The interpretations of Hjartarson et al. have revealed several new abandoned rift centres, which are mapped as syncline and anticline structures. It was suggested that the synclines are manifestations of former rift axes that were abandoned by rift jumps. It appears that these are more common inside the GIFRC region than they are in the adjacent ocean basins, and can be confirmed by observations of cumulative crustal accretion data over time. It was proposed that a major unconformity dating to post 40 Ma across the East Iceland Shelf, which formed a distinct hiatus that was 16-20 Myr long that is covered by a thick, young sedimentary section.
There are several seamounts that were identified on multibeam datasets at a depth in the water of 1,200 m in the Vesturdjúp Basin, which is located just south of the Greenland-Iceland Ridge. These seamounts appear to have formed later than the surrounding ocean floor, which possibly indicates the presence of an intraplate volcanic zone that is still active. The area is characterised by young tectonic features, such as faults, grabens and transverse ridges, presenting a good example of the complexity of the GIFRC, compared to the adjacent abyssal plain.
The aim of this study was to review the structural segmentation and links to chronostratigraphic processes that affect the areas of the Greenland-Iceland Ridge (GIR), the Iceland Plateau and the Iceland-Faroe Ridge (IFR). In this paper these ridges and plateau are summarised as the Greenland-Iceland-Faroe Ridge Complex (GIFRC) and part of the North Atlantic Igneous Province (NAIP), which is one of the largest igneous provinces in the world (Saunders et al., 1997). This review addresses the initiation of the GIFRC, the extent of it, defines the areas of rift jump within the complex, as well as addressing the Iceland-type central volcanoes and in their offshore regions, the seamounts. Also, variations in the thickness of the crust are compared to structural and geochronological variations within the GIFRC that are based on potential Field data.
Previously, abandoned rift systems have been mapped for the onshore region of Iceland for the last 16 Myr (Hjartarson, 2003; Jόhannesson & Sӕmundsson, 2009), and the offshore areas around Iceland by Vogt (1971), Talwani & Eldholm, (1977), Harðarson et al. (1997), Vogt & Jung (2009) and Erlendsson & Blischke (2013). A distinct dip of subaerial lava flow formation sequences from both sides towards the old rift centre (Böðvarsson & Walker, 1964), which is observable in geological maps (Jόhannesson & Sӕmundsson, 2009), and on seismic reflection data for offshore areas as synclines and anticlines.
Previously, field and refraction data studies have defined the term Icelandic-type crust, this has been used to describe the crust beneath the Greenland-Iceland-Faroe Ridge, as it differs fundamentally from oceanic as well as continental crust (Foulger et al., 2003). Oceanic crust is divided in to 3 distinct layers:
· Layer 1 referred to as sediments;
· Layer 2 Composed of extrusive volcanic eruptive rock, mostly pillow lava;
· Layer 3 equivalent to gabbro and cumulates ultramafic rocks.
Icelandic-type crust can be divided in a similar way, though pillow lavas in Layer 2 are replaced by subaerial lava flows.
Generally, seismic velocities of Icelandic-type crust are similar to those of normal oceanic crust, though they have a Layer 3 that is more variable, of overall much thicker crust (Brandsdόttir & Menke, 2008). This atypical crust buildup of thick crust is of interest as interpretations of multichannel seismic (MCS) reflection data might shed light on the internal structures up to depths of 15 km that cannot be imaged by seismic refraction or field data.
The GIFRC has been regarded for a long time as the track of a hotspot, which had formed mainly by subaerial igneous activity (Benjarnason, 2008). The structure of the GIFRC has, however, not been well understood as a result of a lack of geological profiles, rocks from the offshore areas that had been dated, and interpretation difficulties that are consistent with magnetic chron data across the region (Gaina et al., 2017).
Whether or not the Greenland-Iceland-Faroe Ridge, and Iceland itself, belong to the NAIP can be debated. An area that has undergone extremely large accumulation of igneous rocks over a short period of time is the definition of a large igneous province (LIP). The formation of the NAIP, in that sense, ended by the opening of the ocean, a point at which breakup volcanism changed into drift volcanism and concentrated around the Iceland hotspot and the Ægir, Reykjanes and Kolbeinsey mid-ocean ridges (MORs). However, most work that has been published includes the GIFRC and the Iceland shelf itself in that province (e.g. Saunders et al., 1977). In that sense, the NAIP is still expanding and the duration of its formation is more than 60 Myr.
At 63-60 Ma the NAIP began forming during the pre-breakup phase of the North East Atlantic, at the time at which a deep-seated mantle plume reached the lower crust below central Greenland (Morgan, 1971; Brooks, 1973; White & McKenzie, 1989). This caused volcanism in East and West Greenland, and northern Canada, as well as Faroe and British islands (Ganerød et al., 2000). The breakup phase of the NE Atlantic took place in the Late Palaeocene, 57-55 Ma, forming margins that were magma-rich across the area prior to the final opening of the North Atlantic rift system along areas that were structurally weak within the crust (Larsen & Saunders, 1998; Gaina et al., 2009; Blischke et al., 2016). Extensive lava flows on adjacent elevated margins covered the central NE Atlantic, which are also referred to as plateau basalts (Larsen & Watt, 1985; Larsen et al., 1989; Søager & Holm, 2009). In the Early Eocene, 55-54 Ma, the lithosphere finally ruptured, to initiate the post-breakup phase and marking the onset of spreading of the seafloor in the NE Atlantic. There were 3 MOR segments that formed initially: The Ægir, Mohn’s, and Reykjanes ridges. The initiation sequences are present in seismic reflection data as distinct reflectors formations (SDR) along the breakup margins (e.g. Talwani & Eldholm, 1977; Hinz, 1981; Mutter at al., 1982; Larsen & Jakobsdόttir, 1988; Larsen & Saunders, 1988; Elliott & Parson, 2008; Blischke et al., 2016; Geissler et al., 2016). The opening of the North Atlantic split up the plateau basalts and SDR sequences, that date to the Late Palaeocene and Early Eocene, that comprise the bulk of the NAIP, and are now distributed widely and exposed along both margins of the Atlantic Ocean.
An area of 480,000 km2 of a thick volcanic crust that stretches 1,150 km across the central NE Atlantic Ocean between central East Greenland and the northwestern European margins is covered by the GIFRC. It incorporates the Iceland Plateau, the aseismic GIF and the IFR. The GIFRC Appears to as a prominent phenomenon with respect to bathymetry, morphology of the ocean basin, gravity, palaeomagnetism, the thickness of the crust, geochemistry and petrology characteristics, with a direct influence from the mantle plume (e.g. Jakobsson, 1972; Fitton et al., 1987; Thirlwall et al., 2004; Kokfelt et al., 2006; Thordarson & Larsen, 2007; Parnell-Tumer et al., 2014). It has been confirmed by seismic refraction studies that there is a crustal thickness variation of from 20 to 40 km within the area, accounting for crust that is at least 3-4 times thicker than observed for the average oceanic crust (Funck et al., 2014.
The western border of the GIFRC corresponds to the continent-ocean boundary (COB) of central East Greenland, and the eastern border of the GIFRC corresponds to the continent-ocean boundary west of the Faroe Islands (Hopper et al, 2014). In southeast Iceland the GIFRC reaches up to 2,100 m above sea level, whereas the bathymetrically deepest points of the complex are situated at 600 m below sea level (bsl) between Iceland and Greenland, within the Denmark Strait, and between Iceland and the Faroe Islands, approximately 500 m below sea level. To the north and south of the ridge the ocean basins are more than 2,000 m deep. It is believed that the submarine areas of the GIFRC formed subaerially, though due to erosion and cooling of the crust, have been subsiding below sea level (Lundin & Doré, 2004; Denk et al., 2011). A difference of about 1,500 m in elevation along the GIFRC crest has resulted from the subsidence process.
GIFRC rift centres and rift relocations
According to Hjartarson et al. it appears that the complexity of the GIFRC is connected closely to frequent rift jumps. There are several rift jumps that are known of and have been documented. The Ægir Ridge is a specific example that formed during the initial opening of the northeast Atlantic, the initial breakup phase of which was between 55 and 53 Ma, and by about 50 Ma was fully established (Gaina et al., 2009; Gernigon et al., 2015). Spanning the distance between the IFR and the Jan Mayen Transformation Zone (Blischke et al., 2016), it propagated from north to south. On the Iceland Plateau, rifting took place simultaneously with that on the Ægir Ridge between 49 Ma and 25 Ma along the Iceland-Faroe Fracture Zone, prior to the complete transfer of the rift to the Kolbeinsey Ridge system, connecting the Reykjanes Ridge directly and separating the Jan Mayen microcontinent from the central coast of Greenland (Brandsdόttir et al., 2015; Blischke et al., 2016).
In the Early Oligocene, Cessation of seafloor spreading and extinction of the Ægir MOR system occurred, about 24-21 Ma, which coincided with the activation of the Kolbeinsey Ridge around anomaly C6b, or 22-21 Ma (Gernigon et al., 2105), and Iceland becoming an insular shelf probably due to the plume-ridge activity. The formation of the Iceland Shelf as a volcanic region within the GIFRC was the result of intensive volcanism and a high production of lava accompanying the initiation of the Kolbeinsey MOR.
It is believed the northwest Rift Zone formed about 24 Ma to the west of the NW peninsula of Iceland (Harðarson et al., 1997). It was suggested by Hjartarson et al. it was most likely to have been a direct continuation of the Kolbeinsey Ridge, forming the oldest and most southern part of this ridge. Up until about 15 Ma it was active for about 8-10 Myr.
As a result of its exact location being speculative, its manifestations have never been clear in geophysical potential field data. In seismic reflection data profiles, however, syncline structures can be seen. The site is just to the north of the GIR and is approximately and is parallel to the 15 Ma time line, according to the geochron model of Gaina (2014). It is suggested by Hjartarson et al. that this hypothetical NW Rift Zone, which is believed to be somewhere in the insular shelf off the NW peninsula of Iceland (Harðarson et al., 1997), which correlates with syncline ‘e’, therefore it confirms this ancient spreading axis by geophysical data. The formations of the NW Rift Zone that dip seawards are submerged almost totally bellow the seafloor, with the exception of their eastern most extensions, which are exposed along the outermost coasts of the Icelandic Westfjords, where they form the anticline structure ‘f’ in Figs. 2, 5c and 9b. These formations are overlain by a lignite horizon, which represents a 1-1.5 Myr hiatus prior to the next rift jump and before the onset of the Snæfellsnes-Húnalaflόi Zone took place (Riishuus et al., 2013).
Snæfellsnes-Húnalaflόi Rift Zone formed about 14-15 Ma by relocation of an eastwards spreading centre from the NW Rift Zone, that was active for 8-10 Myr (Harðarson et al., 2008). This rift zone is present as regional dipping formations onshore West Iceland that forms a distinct syncline centre line of that rift zone. The rift zone is comprised of 2 segments located in Snæfellenes and Húnaflόi, respectively, which may have been connected by a transform fault system. This rift zone formed the majority of Icelandic subaerial volcanic strata that dates to the Miocene.
Relocation of active spreading from the Snæfellsnes-Húnalaflόi Rift Zone to its present location formed the rift zones of the present about 6 Ma, forming the Western Volcanic Zone (WVZ) and the Northern Volcanic Zone (Sӕmundsson, 1974, 1979; Jόhannesson, 1980). The rift zone remained active, however, until about 5Ma (Pringle et al., 1997). Separately, the most recent rift relocation to the Skagafjörður Rift Zone took place in North Iceland, becoming activated about 1 Ma and forming a temporary rift axis for about 1 Myr (Hjartarson, 2003). The East Iceland Volcanic Zone appears to be an evolving spreading system (Sӕmundsson, 1979). This zone was initiated about 2-3 Ma, and is now propagating to the southeast from the WVZ towards the EVZ, forming a dual-zone rift system (Einarsson, 2008).
Central Volcanoes and seamounts on the GIRFC
An important role in the buildup and structure of the Icelandic volcanic strata was played by the central volcanoes, and they have been studied intensively (e.g. Sӕmundsson, 1979; Harðarson et al., 2008). Not much is known of the existence and role of the central volcanoes and seamounts within the submarine areas of the GIFRC. According to Hjartarson et al. the central volcanoes of Iceland can be divided into rift zone and off-rift central volcanoes. High, prominent volcanoes are often formed by the off-rift central volcanoes (e.g. Snӕfellsjökull & Eyjafjallajökull), in contrast to central volcanoes of the rift zones that are lower and of a more irregular shape, and many of them have formed calderas. It has been found that the life time of an individual central volcano, until it cools down, varies between 300 years to more than 1 million years (Sӕmundsson, 1979; Harðarson et al., 2008). The only known example of a submarine central volcano that is active is the Njörður Volcano located on the Reykjanes Ridge that is close to the Icelandic shelf (Höskuldsson et al., 2013).
In the Neogene formations of Iceland there are more than 40 former rift zone central volcanoes that are known of that are now inactive. Often they are deeply eroded, and represented by acid and intermediate rocks, local cone sheet swarms of regional dykes and faults (Sӕmundsson, 1979).
There are also central volcanoes on the shelf area all around Iceland. There are several central volcanoes that have been inferred from potential field data east and west of Iceland, and in some cases they are confirmed by dredging (Kristjánsson, 1976; Jόnsson & Kristjánsson, 1997). It is believed they formed subaerially, though as their emplacement area cooled down they submerged, while drifting away from the spreading axis.
The definition of seamounts is isolated topographical features of volcanic origin that rise from the ocean floor, though they didn’t rise high enough to extends above the surface of the ocean and become islands. They are of various heights from hundreds of metres to 4,000 m. Their growth, activity and cessation follows a distinctive pattern, and in general, they are formed near mid-ocean ridges, above upwelling mantle plumes (hotspots) in convergent settings of island arcs (Staudigel & Clague, 2010). Therefore, they can provide important clues as to where old rift systems might have been located in magmatically inactive areas.
The features of seamounts have been mapped across the GIFRC area, and north and south of Iceland (Funck et al., 2014; Gaina et al., 2016). They are situated mostly on the floor of the deep ocean on both sides of the Reykjanes and Kolbeinsey ridges. Very few are on the ridge complex itself, though some are near to the GIFRC. The igneous complexes of the GIFRC were possibly formed aerially, that was partially eroded after cessation and submerged due to thermal cooling. Shapely seamounts have, however, been found close to the ridge complex, specifically on the flanks of east and west Iceland’s offshore areas. A group of small seamounts has been identified in the Vesturdjúp Basin, at a depth of about 1,200 m, west of Iceland and just south of the GIR. These volcanoes, or mud volcanoes, were described and discussed by Helgadόttir (2012). Most of them are cone shaped ridges, though there are also table-like mountains that have been eroded. It appears that conventional volcanism is the most likely cause of these features, because of these various types of igneous-complex-like structures. The largest of the cones is about 500 m above the surrounding ocean floor, and has a diameter of 5,000 m. It appears that these seamounts are much less eroded and younger than the neighbouring ocean floor, and possibly indicate a flank igneous system or intraplate volcanism, which is accompanied by young tectonism with faults, graben and transverse ridges that characterise the area.
This section addresses individual key stages that affected the GIFRC since the breakup of the North Atlantic, in order to assess the development of the GIFRC as an igneous complex within the NAIP.
The formation of the GIFRC began along with the breakup of the continent between Greenland and Eurasia, and the initiation of the spreading of the sea floor, 55-53 Ma and 36 Ma, north and south of the GIFRC (Gaina et al., 2009; Gernigon et al., 2015), though not affecting the GIFRC to a great extent. The Eurasian and North American continental margins were located very close to each other during the Eocene, and the plume that was situated below Greenland that sustained a subaerial connection between the 2 continents, and forming a land bridge between Greenland and the Faroes and onwards to the European continent. Geoseismic investigations (Parnell-Turner et al., 2014), as well as palaeobotanical evidence (Denk e al., 2011), support this connection.
Active rifting north and south of the GIFRC
The first phase of rifting 53.36-49 Ma, of the northeast Atlantic after the breakup heavily affected the GIFRC area, with the emplacement of large volumes of extrusive and intrusive magmatic material building up the oldest part of the complex. Overlapping rift systems were active during this phase that overlaid older crustal segments which led to a very thick crustal formation from east Iceland to the IFR region.
Rift orientation and Ægir Ridge transition
In the Reykjanes MOR system to the south continuous spreading was active, though rift transfer began to form the Ægir Ridge system along the Iceland Plateau Rift (IPR) corridor south of the JMMC between about 49 Ma and 40 Ma. The crustal accretion of the GIFRC reflects this, with increased magmatic activity between the IPR system and the Iceland-Faroe Fracture Zone (IFFZ) (Blischke et al., 2016). It is indicated by recent reconstruction work of the region that the edge of the East Iceland Shelf is parallel to the proto-Reykjanes Ridge location at anomaly C19n (40.32 Ma) (Gaina, 2014; Blischke et al., 2016), cutting into the older crust of the IFR area.
Spreading rate decrease along the Ægir MOR system
The Greenland-Eurasian plate system was situated below the Greenland margin 35-30 Ma, moved northwest relative to mantle plume. Along the Reykjanes and Ægir MORs, ocean spreading was active, though was gradually slowing down to spreading ultra-slowly past 30 Ma within the Ægir MOR system (Gernigon et al., 2015). This also affects directly the GIRFC, where much lower accretion volume can be observed over time in connection with the slowing down of the rift systems. It is possible that rift jumps took place in a westwards direction that can be seen in the seismic reflection record in the form of synclines and anticlines along the Greenland-Iceland and Iceland-Faroe ridges.
Ægir Ridge cessation – Kolbeinsey Ridge insular shelf
Along the Ægir Ridge spreading activity ceased about 22 Ma (Gernigon et al., 2015) and spreading of the seafloor concentrated only along the Kolbeinsey Ridge from 24 Ma onwards. The extension of the margin of Greenland situated immediately north of the Reykjanes Ridge, which led to the final detachment of the Jan Mayen microcontinent (JMMC) from the central margin of East Greenland (Blischke et al., 2016) resulted from the process of establishing a new plate boundary and ultimately the Kolbeinsey Ridge. An increase in magmatic activity along the GIFRC area, as well as further north along the western to southwestern margin of the JMMC, was a result of the process of establishing a new plate boundary and ultimately the Kolbeinsey Ridge, as well as the interaction between the Iceland plume and the MOR system that was newly formed.
It was indicated by recent age models that were based on interpretations of Palaeomagnetic chrons of the floor of the ocean around Iceland, there was a major hiatus crossing the insular shelf near the eastern to southeastern coast (Gaina et al., 2017) that may possibly be identified on interpretations of seismic reflection data. It was suggested by Hjartarson et al. that the hiatus is related to increased magmatic accretion of the central Iceland region, with extrusive rock overlying discordantly older igneous formation and crust, which formed a much thicker crust in the area. The subcrop of this unconformity boundary, which is buried beneath thick layers of sediments, 8-10 km inside the bathymetric shelf break, according to Jόnsson & Kristjánsson (1997), though it can be seen near the anticline ‘m’ in Figs. 2 and 5d. The rocks at the edge of the East Greenland Ice Shelf are believed to be from about 20-24 Ma, which would correlate with the time of original opening of the Kolbeinsey Ridge system during its initiation. For that section the age of the underlying volcanic basement might be about 40 Ma according to the age model of Gaina et al. (2017), which corresponds to a hiatus time span of about 16-20 Myr.
This magmatic activity increase, and probably also thermal uplift of the GIFRC area, led to the formation of proto-Iceland; therefore inducing a major hiatus and related unconformity between the young formations of the Kolbeinsey Ridge and the basement rock that was older of the IFR and GIR. At the present, the early stage of the subaerial insular shelf region is mostly submerged, though it forms large areas of the insular shelf in the east, west and north of Iceland.
Miocene – Rift jumps on Iceland during the Pliocene
The axis of the mid-Atlantic ridge approached and crossed the location of the Iceland plume in the Early Miocene (Harðarson et al., 2008). In Iceland the spreading ridge systems have remained linked to the plume since then. The rift centres are periodically recaptured by the plume by rift-jumping as the spreading axis moves away from the location of the central plume. It has been proposed that for Iceland a complete rift cycle lasts for at least 12 Myr, from propagation initiation to extinction (Harðarson et al., 1997, 2008). The rift-jumping control is clearly related to the interaction of the mantle plume, which is static, with the overlying plate of Eurasia that is migrating to the northwest (Gaina et al., 2017). According to Hjartarson et al. relocation towards the plume of active magmatism may simply be a response to this migration.
The marginal eastern and western parts of the land bridge cooled, partially eroded and gradually submerged (Denk et al., 2011), as the North Atlantic Ocean widened, as can be seen as a base sediment horizon from the Cainozoic in Figs. 5d & 9. This first occurred along the eastern area of the IFR, then by the western area because of its proximity to the mantle plume. It is indicated by palaeobotanical observations that the latest evidence for plant migration on land between Europe and Iceland dates to about 9 Ma and between Greenland and Iceland about 6 Ma (Denk et al., 2011). The age of the GIR and the IFR as submarine areas is therefore less than 10 Myr, and the age of Iceland as an isolated island is about 6 Myr.
The Greenland-Iceland-Faroe Ridge Complex (GIFRC) has been “under construction” since the initiation of the opening of the northeast Atlantic about 55 Ma. In all geological and geophysical datasets it appears as a prominent feature. According to the various data sources available it can be drawn in ways that are slightly different. In spite of a small areal outline compromise between the different datasets, including bathymetry, gravity, magnetic and crustal thickness maps, as well as seismic profiles over the region, it is shown by all of these as an anomalous feature within the oceanic crustal fabric of the northeast Atlantic.
Synclines and anticlines that have been published have been summarised (Table 3), and several new synclines and anticlines that have been revealed by seismic reflection data across the GIFRC east, west and north of Iceland. The offshore anticlines and synclines specifically, may be related to old rift systems before the formation of Iceland as an insular shelf region >24 Ma. It has been suggested that synclines are manifestations of former rift axes that were abandoned by rift jumps. It appears these rift jumps are more common inside the GIFRC region than in ocean basins north and south of the area, and can also be confirmed by the observation of cumulative crustal accretion over time.
The GIFRC represents therefore a complex region of crustal accretion in 3 dimensions as a result of overlapping rift systems, complex interlinked rift and transform zones, as well as several unconformities that suggest a history of variable uplift and subsidence for the ridge complex. Seismic reflection data extending along the southwest slope of the Iceland-Faroe Ridge (IFR) is an excellent example to visualise such processes of vertical crustal accretion and rift jumps. They display clearly the internal structure of basement blocks, which are separated by a syncline and younger rift system, and the formation of an anticline across the basement rocks that are buried deeply that are overlain by SDR structures.
Hjartarson et al. suggest there is a major hiatus, from 40 Ma to 24-20 Ma and a related unconformity at the boundary at the edge of the volcanic insular shelf of east Iceland and the Faroe Ridge, which are buried beneath thick sediment layers, 8-10 km inside the bathymetric shelf break.
They also suggest the hypothetical NW Rift Zone, which is believed to be somewhere on the insular shelf off the northwest peninsula of Iceland (Harðarson et al., 1997), correlates with syncline ‘e’ just north of the Greenland-Iceland Ridge (GIR), parallel to the age line 15 Ma of the geochron model (Gaina et al, 2017), and therefore confirms by geophysical data this ancient spreading axis.
Several seamounts have been observed on multibeam datasets from the Vesturdjúp Basin west of Iceland, just south of the GIR at a depth of about 1,200 m. Most are cone-shaped, though there are also ridges and table mountains that were found. It appears that these seamounts are much less eroded and younger than the neighbouring ocean floor, which may indicate a flank that is still active or an intraplate volcanic zone. The area is also characterised by young tectonism with faults, graben and transverse ridges, and most of the volcanic cones are located along fault plans and/or within the graben of the Vesturdjúp, which gives a good example of the complexity of the GIFRC compared to simple areas of the ocean floor.
Hjartarson, Árni, Erlendsson, Ögmundur & Blitschke, Anett, April 2017, The Greenland–Iceland–Faroe Ridge Complex, Geological Society of London, Special Publications, DOI: 10.1144/SP447.14
|Author: M.H.Monroe Email: firstname.lastname@example.org Sources & Further reading|