Rifting And Opening Of The North Atlantic Ocean: Geological Setting And Discussion

tectonics.

Topic: Select one of the following topics:

(1) The Zagros Foreland fold-thrust belt of southwest Iran

(2) The Pan-African Orogeny

(3) Rifting and opening of the North Atlantic Ocean

(4) Tectonic development of the Adelaide Rift Complex

Other topics can be chosen with regard to the interests and background of the student in consultation

with the coordinator (Chris Fergusson).

Geological Setting

Subsequent to the Pangea breakup at c. fifty-five Ma, the North Atlantic has witnessed widespread volcanism. As 1 of the enormous world igneous province, this zone remained influenced by Iceland hotspot. Such a hotspot is usually regarded to be surface manifestation of the convection plume of the anomalously hot substance uplifting from deep mantle. Nevertheless, Icelandic plume hypothesis has lately been put to task and additional mechanisms like small scale convection/fertile mantle melting have been already proposed. Continental rifting alongside sea floor spreading have further remained integral to this basin evolution and hence temporal as well as spatial connections between magmatic and tectonic events avail significant clues to the Iceland hotspot origin.   

The sea floor compartment north east of Iceland, circumscribed by Iceland Jan Mayen and Faeroes fracture regions remains complex. The oceanic crust here accreted along the presently extinct Aegir Ridge whereas continental stretching remained taking place in East Greenland. The course rifted off the continental JMR, and oceanic spreading began along Kolbeinsey Ridge at c. twenty-five Ma, where Aegir Ridge became extinct. There has been a great enhancement in the understanding of such a compartment in the recent past hence the argument that the Iceland-Jan Mayen region holding several of the model for Tertiary evolution of North Atlantic.

The paper undertakes to review various works in this area prominently anchored on the wider perspective, ocean bottom seismic (OBS) data alongside constraining the outcome by newfangled information of the prospective field data. In addition, the paper will utilize the information from OBS data for constraining composition of lithosphere as well as determining its mechanical strength. Eventually, the paper will avail the clear and comprehensive information for effective understanding of the rifting and opening of the North Atlantic Ocean (Chenin et al. 2015).

The gravity anomaly map below indicates the location of the conjugate southeast Greenland as well as Hatton Bank margin profiles. The black areas mark the North Atlantic igneous province: flood basalts of East Greenland Traps as well as the British Tertiary volcanic province (BTVP) as shown in a. The temperature as well as the mantle depletion with melt fraction, contoured as shown in the figure one below whereby the at the centre of extension prior to breakup from the preferred model that regards the Hatton Basin opening. The location of the Hatton Basin is marked below by the white triangle alongside that of southeast Greenland or Hatton Bank breakup is denoted by black triangle. In the model below, there is a fifty kilometer thick thermal anomaly at 200 degrees centigrade beneath lithosphere. As shown in c and d, the predicted igneous crustal thickness and predicted average lower-crustal seismic velocity respectively over period. Uncertainties in observations remain representative values taken from references twenty-five (Ellis and Stoker 2014).

Before the beginning of the seafloor spreading in the Northeast Atlantic fifty-five million years down the line, the hydrocarbon basis of the Jameson land, onshore East Greenland as well as basins offshore western Norway, Shetland as well as in the North Sea remained located in close proximity to each other. Right from that period, they have slowly moved apart due to the plate tectonic movement which created Iceland as well as the surrounding ocean basins. Besides such development, the Jan Mayen Ridge remained slowly separated from the continental shelf of the Norway as well as Greenland thereby becoming isolated far from the shore as well as the surrounded by newly established ocean floor. This took place in parallel with the slow emergence of Iceland alongside its surrounding insular shelf (Casey, Krueger and Norton 2016).

Discussion

The topography alongside the tectonic structures of Jan Mayen Ridge (JMR) are triggered as well as shaped by the Mesozoic rifting originally, and of course prominently by the aforementioned opening of the Northeast Atlantic Ocean. The opening of the Atlantic numerous phases that remain of significance to the MJR. The original opening of the Norwegian-Greenland Sea that occurred east of the Jan Mayen Ridge in the course of Late Paleocene to Early Eocene which was linked to intensive as well as widespread volcanism alongside marked the entire of the North Atlantic region. The records such are observed as the Plateau basalts alongside enormous intrusions onshore along the East Greenland coast as well as the offshore on seismic records along East Greenland as well as Norwegian shelfs (Biari et al. 2017).

Volcanic basalts depositions several kilometers thick are discovered in East Greenland close to palaeo-geographic locations of Faroe Islands, however, tend to remain not as enormous further Northwards. The complex faults at the western as well as eastern boundaries of the Jan Mayen Ridge remained initiated in the course of the period of the continental drift as well as spreading ridge formation in the course of the forming as well as spreading of the Aegir Ridge to the East of JMR between the Early Eocene alongside the Early Oligocene (Gouiza, Hall and Welford 2017).

The Kolbeinsey Ridge formed to Westwards of JMR from the Early Miocene to current, leaving a time duration, beginning during the end of the Middle Eocene as well as particularly between the Early Oligocene as well as the Early Miocene, to explicate for the enormous volcanic activities right around JMR as well as enormous scale uplift alongside erosion of the high blocks in the ridge individually whereas the JMR moved over the Iceland Hotspot. In the course of that time fault alongside block separation surged in southern parts of ridge, forming the Southern Ridge Complex, initially due to feasibly two attempted propagation of mid oceanic rift into MJR, originally in the course of Middle Eocene alongside in the course of Early Oligocene besides eventually feasibly breaking through at the old weak zone along western flank of MJR, forming Kolbeinsey Ridge as it is presently known. Such processes ushered in elaborate stretching as well as structural complexity to MJR (Brune et al. 2016).

Right from beginning of the seafloor spreading along Kolbeinsey Ridge as well as moving away from the hotspot, Jan Mayen Ridge region rotated anti-clockwise, triggering slight reverse faulting alongside has been cooling down, provoking the region to subside. Such a process led to an alteration from the locally based erosion as well as deposition around the diverse blocks, to a basin depositional environment in the course of the ridges subsidence.

The deep seismic data from Hatton-Rockall area, the mid-Norway margin as well as the SW Barents Sea offers images of the crustal structure that allows the estimation of the relative amount of the crustal thinning for the Late Jurassic-Cretaceous as well as Maastrichtian-Paleocene North East Atlantic rift episodes, Besides, plate reconstructions showcase the relative movements between the Greenland ad Eurasia back to Mid-Jurassic period. The NE Atlantic rift systems developed due to a series of rift episodes from Caledonian orogeny to early Tertiary period. The Late Paleozoic rifting remains poorly constrained, especially with regards to timing. Nevertheless, rifted basin geometrics, incidental to be of such age, are witnessed at the depth of the seismic data on flanks of younger rift structures. The intra-continental rifting in Late Jurassic-Cretaceous periods triggered c. 50-70 kilometers of the crustal extension alongside the following Cretaceous basin subsidence from Rockall Through-North Sea regions in the south, to the South west Barents Sea in the north.

During the late Early to early Late Cretaceous periods, newfangled rifting happened in Rockall Through as well as Labrador Sea linked to northward propagation of North Atlantic sea floor spreading. Where the sea floor spreading remained approached in Labrador Sea Rockall rift seemingly remained extinct. The eventual NE Atlantic rift episode remained initiated near the Campanian-Maastrichtian boundary thereby lasting till the continental separation near Paleocene-Eocene transition, alongside triggered c.140 kilometer extension. The late syn-rift alongside earliest sea floor spreading times remained affected by the widespread igneous activities crossways a c. 300 kilometer  wide region along rifted plate boundary. The deep seismic data give lower crustal structural geometries which denote boundary conditions for the better mapping alongside understanding of extensional thinning of crust. The crustal geometries question extension approximates initially made from the basin subsidence examination, alongside aid in definition of bodies of magmatic under plating below outer volcanic margins.           

As the idea of the sea floor spreading gained increasing acceptance in the late sixties, the aftermaths for geology slowly started dawning. One of the initial people to acknowledge how the plate tectonics might be applied to geological records was Wilson Tuzo J. In case continents rift away from one another thereby forming ocean basins, additional oceans have to close. Such a rift might be repeated through Earth history (Koopmann et al. 2014). For instance, the IAPETUS Ocean between England and Scotland in the Lower Paleozoic closed in Caledonian and subsequent opening of Atlantic, almost in identical location. The cycle is referred to as the Wilson Cycle: (i) continent rifting by mantle diapirism (ii) continental rift, sea floor spreading alongside ocean basin formation, (iii) progressive ocean basin closure by ocean lithosphere subduction and (IV) continental collision alongside eventual ocean basin closure.The illustration of certain simple ancient concepts of the continental rifting such as the Gondwana continent at the beginning of the Wilson Cycle.

The uprising plume triggers doming of crust with the magma chamber developing below. As the extension progress, the ocean basin forms, and thick sedimentary sequences emerge at the continental margins as the rivers dump sediments within the deep water. Nevertheless, in practice might be a bit increasingly complex. The typical rifted passive margin tectonic development advances from the four phases (rift valley, youthful, mature and fracture) in continental rifting (rrr alongside RRR triple junctions).

It is essential to understand the initiation of the rifting. Over the past years, there has been substantial deliberations on the initiation of the rifting. Scholars have ascribed it to up-doming of the crust over the hotspot; absolutely portions of the East African rift system are highly elevated in comparison to the remaining sectors, depicting that the doming denotes the underlying hot low density mantle plume. In additional instances, the geophysical models indicate the asthenospheric mantle is increasing to higher degrees below the rift. Nevertheless, it is further obvious that rifting can occur even in the absence of extensive uplift in which case it could be the convective processes in underlying asthenosphere that are triggering the extension. The rifts associated with numerous feasible thermal domes are essential in rifting a continent apart to allow them link together. It has been suggested that as the continents slowly drift over the hotspots such hotspots weaken the plate analogous to a blowtorch impinging on base and such weakened regions become the continental rifting sites.

It has been as well suggested that in the aftermaths of the hypothesis of doming within the domal areas, three rift could emerge thereby forming the rrr triple junction. Despite its feasible that all the 3 rifts could develop into the ocean, RRR, it remains increasingly likely that 2 of such rifts might develop into the ocean RRr thereby leaving the 3^rd rift as the failed arm. It remained speculated that on various continents, it stood feasible to acknowledge such RRr junctions. Such failed arm rift might finally subside as thermal anomaly decayed as well as become the sites of the main depositional basin or the key river channel as well as delta. The Benue Trough in Nigeria remained recognized as the instance of such a failed arm subsequent to the opening of the South Atlantic. Where the oceans finally close it remains feasible to acknowledge such failed arms as the depositional basins oriented perpendicular to collision mountain belt (a great proportion of basins tend to be aligned parallel to mountain belts). Such are dubbed aulacogens (Jones et al. 2016).

Source: https://www.le.ac.uk/gl/art/gl209/lecture3/lecture3.html 

From the above figure, a doming via a mantle plum is illustrated as associated with volcanicity in A. On the other hand, Rifting/rrr junction is initiated in B with C showcasing additional development resulting in 2 of the rifts thereby developing into the ocean, the 3^rd describes a failed arm. The less probably is that all 3 arms develop into oceans is represented by D and E indicates common circumstance that the failed arm develops into a key river system feeding the continental margin (Lundin et al. 2017). The expansion of the oceans on the finite earth is never feasible is illustrated by F that requires a plate subduction, somewhere, sometime. The closure of the oceans leads to island arc development overhead the subduction region is denoted by G with H showcasing the continued closure leading to collision with the main fold alongside the thrust belts. However, often the failed arm remains still preserved.

The development of the continental rifts can as well be understood. The earlier concepts on the rifts development remain conceptualized in the figure below:

Source: https://www.le.ac.uk/gl/art/gl209/lecture3/lecture3.html 

The above diagram is anchored on the African rift system in which there remains substantial rift magmatism. There remains a noticeable extension indicated by the diagram block widening by at minimum fifty kilometer. At the similar time there remains uplift/ascent of a more ductile mantle, particularly the asthenosphere. The crust alongside especially the upper crust remains assumed to act in the brittle fashion. Figure 5a indicates progressive formation of the rift valley via extension of lithosphere as well as continental crust by around fifty kilometer. It should be acknowledged that uprise alongside decompression of underlying asthenosphere leads to the formation of magma. The crust reacts by brittle fracture. The Early rift sediments are down-faulted into developing rift (graben). Erosion occurs on slides of rift valley.  

The initial phase assumes that graben-like faults start to form in brittle crust. The second phases indicates concurrent necking of lithosphere with uprise of the asthenosphere diapir. Decompression linked to the latter triggers of melting of mantle to provide alkaline basaltic magmas. The pre-existing sediments remain down faulted into graben. The 3^rd phase remains accompanied by substantial extension as well as by more uprise of asthenosphere. The latter triggers doming of crust that remain evidential along East African rift system, however, remains variably developed. Newfangled sediments remain deposited in graben due to erosion of uplifting sides of graben. Accordingly, both pre-and syn-rift sediments within developing rift valley, nonetheless on flanks remain progressively eroded away. It must be noted that complex normal faulting within the rift valley individually.  The 4^th phase as show in figure 5b below demonstrates the really rifting-apart of continent and hence the asthenosphere uplifts towards surface triggering decompression as well as the extensive melting.

The newfangled basaltic oceanic crust is subsequently formed. Eventually, the sea floor spreading takes over as the widening of the ocean basin continue. The sequence of the rift sedimentary becomes buried below younger marine sediments. It must be noted that on the above diagram, the sediments at continental margin remain showcased as not increasingly thick. This is due to model being anchored on East African Rift System that does not have a great deal of the subsidence linked to rifting. Nevertheless, additional rifted continental margin sequence remain highly distinct, with thick sedimentary sequences (Granot and Dyment 2015).   

The continental shelf sediments is also essential to be understood. A real condition at passive continental margins is illustrated in the figure below:

 Source: https://www.le.ac.uk/gl/art/gl209/lecture3/lecture3.html 

The above diagram indicates a typical of the number of the crustal cross-sections crossway continental shelf of eastern Atlantic seaboard of North America that is projected down thirty kilometers-anchored greatly on gravity as well as magnetic proof, plus seismic profiles, and certain extrapolation from the land geology anchored on deep drill holes. The critical point remains the enormous thicknesses of the Mesozoic alongside Tertiary sediments as shown about fifteen kilometers, but in additional cross-sections such could be even thicker. It has to be acknowledged that at the bottom of the pile lie volcanic alongside volcanogenic sediments alongside evaporates that most probably remain shallow water (Koopmann et al. 2016). Further, the enormous carbonate reef structures that has to be shallow water, however, have to be demonstrate progressive subsidence gradual enough that shallow water sedimentation could keep stride with it. In various sections of continental shelf off such eastern seaboard of United States of America there is a main coast-parallel magnetic structure, feasibly a major intrusion. Nevertheless, its age remains known. The above diagram shows the profile of the deep structure of the continental shelf off the Atlantic coast of the eastern North America that remains typical of passive continental margins which is anchored on gravity, magnetics. The critical points concerning the above profile include large thickness of post-rift sediments of Mesozoic-Tertiary age, up to fifteen kilometers and that a great proportion of such sediments remain shallow-water kind. It must be noted that volcanic as well as evaporates besides reef exist (Nirrengarten et al. 2017).   

Understanding the rifts and mineralization is also significant. The rifting structure usually better sites for mineralization based on various reasons. The first reason is that they are able to be sites of thick clastic sedimentation that hold huge amount of inter-granular salt water or brines. Such brines could be in contact with decreasing sediments like carbonaceous shales, further a ready supply of Sulphate or Sulphur (Olafsson 2017). The brines remained expelled as the sediments become compact and are able to move laterally for the huge distances till they move up rift faults. Having been deeply buried, such brines become hot and are able to be highly corrosive. Accordingly, en route the brines are able to dissolve substantial quantities of metals. Nevertheless, where they uplift up the rift faults and are cooled, such mental shall be precipitated out. This is able to be improved since oxidizing meteoric water could further penetrate down such faults, hence metals shall be precipitated out where the two intersect.

The rift structures are further thermally anomalous hot regions which is as a result of frequently underlain by the igneous intrusion or granite/gabbro plutons. Such magmatic heat propels the hydrothermal systems. Significantly, such hydrothermal system could last for several million years, and hence the hot fluids within such hydrothermal systems might leach away at rocks in rift system as well as precipitate leached metals nearer surface. Since the rift structures stay topographically low structures for several tens of million years, such metals concentrations might remain preserved eroded for extended times. Rift regions could be effective sites of different rocks especially basaltic lavas that are able to release respective metals on the hydrothermal change. Nevertheless, as a result of the rift fault being able to stretch extremely deep, there might be a constituent of deep fluids alongside metals within the system of hydrothermal (Nirrengarten et al. 2016).

The North Atlantic Ocean opening describes the geological event that took place over several million years in the past, in the course which the supercontinent Pangea got broken up. As the contemporary-day Europe also called Eurasia plate alongside North American hereby referred as North American Plate separated in the course of the eventual breakup of Pangea during the early Cenozoic Era, they formulated the North Atlantic Ocean. Geologist hold that the breakup happened either as a result of primary processes of Iceland plume or the secondary courses of the lithospheric extension from the various plate tectonics (Nicolas and Montpellier 2014).

The rocks from North Atlantic Ingenious Province have already been discovered in Greenland, the Faroe Islands, the Irminger Basin, among many other places. The supercontinent called Pangea existed in the course of the late Paleozoic as well as early Mesozoic periods and started rifting about two hundred million years down the line. The Pangea had 3 main stages of breakup. The initial stage started in Early-Middle Jurassic, occurring between North America and Africa. The 2^nd key stage of breakup commenced in Early Cretaceous. The South Atlantic Ocean subsequently opened about one-forty million years down the line as Africa separated from the South America, and around the identical period, India got separated from Antarctica alongside Australia thereby leading to the formation of the central Indian Ocean. The eventual main stage of breakup happened in the early Cenozoic, as the Laurentia departed from Eurasia. As the 2 plates departed free from one another, the Atlantic Ocean endured to expand.

The Iceland plume theory describes the Iceland plume as the mantle plume under the Iceland which hosts the hot substance from deep within the mantle of Earth upwards to crust. The increasing hot leave the lithosphere highly weakened thereby making separation of the plates easier. The hot plume substance flow creates the volcanism under the continental lithosphere. The Iceland extends crossways the Mid-Atlantic Ridge (Stica, Zalán and Ferrari 2014). The Mid-Atlantic Ridge remains the divergent plate border, and it departs the Eurasian and the North American plates (Boyle et al. 2017). The earliest volcanic rocks’ plates from such plume lie within the late Paleocene, and each side of Atlantic Ocean entails such rocks. Because such rocks have already been dated to late Paleocene that lines up with the period of the breakup of North Atlantic continent thereby some people think it might have been the contributing factor. The plate tectonics theory perceived volcanism as the resultant of lithosphere courses than heat from rising up mantle. Rather than heat arising from deep within the mantle, volcanic anomalies arise from the shallow source. The volcanism, therefore, takes place when the crust remains easier to break up as a result of being already stretched by the extension of the lithosphere thereby permitting melt to hit the surface (Nairn 2013).

The volcanic anomalies remain established by plate tectonics including spreading plate borders or subduction regions. The position of volcanism becomes governed by stress field within plate and amount of melt being governed by mantle beneath fusibility. The plate tectonics are able to explicate a great proportion Earth volcanism.   The active as well as the passive plates are also playing key role in this case. Active rifting like the one formed by Iceland plume is propelled by hotspot besides mantle plume activities. From the deep in the Earth, the hot mantle uplifts to propel doming of crust. This triggers the thinning of crust alongside lithosphere and subsequently melting alongside under plating take place (Manspeizer 2015). Eventually, there remains rifting at crest of domed crust as well as volcanism takes place. On the other hand, the passive rifting, propelled by the plate tectonics, crust alongside lithosphere extend due to plate border forces like slab pull. The far field subsequently stresses thin the crust and lithosphere mantle while the hot asthenospheric mantle enters thinned region passively. The asthenosphere upwelling is never engaged in the real rifting processes. The asthenosphere upward flow culminates in decompression melting, magmatic under plating as well as certain volcanism which might take place in the area of the rift (Artemieva and Thybo 2013).     

Conclusions

The rifting as well as magmatism remain fundamental geological processes which shape the surface of the planet. The relationship between the 2 vastly recognized but its clear nature has eluded geoscientist as well as stood controversial. The North American alongside Eurasian plates are moving apart from one another along the line of Mid Atlantic Ridge. Such a ridge extends into South Atlantic Ocean between South American as well as African Plates. The ocean ridge uplifts to between two and three kilometers overhead the ocean floor, and has the rift valley at its crest thereby making the location at which the 2 plates are moving away from one another.

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