Large Igneous Provinces (LIPs) are massive magma emplacements and intrusions. While their formation process is still under heavy discussion, their environmental impact and relevance is not debated. Understanding the chronological, petrological, and geodynamic development of LIPs is hence very important.
LIPs can be found both on continents and in the ocean and include continental flood basalt provinces (CFBP), volcanic passive margins, oceanic plateaus, submarine ridges, and ocean-basin flood basalts. The magmatism associated with LIP formation is currently estimated to represent about 10 % of the mass and energy transfer from the Earth’s deep interior to the surface. This transfer is distinctly episodic in geological time periods, i.e. in millions of years. LIPs are found to have had a significant effect on the environment. Subaerial basaltic eruption release enormous amounts of heat and of volatiles such as CO2, S, Cl, and F. Depending on latitude, those volatiles can easily reach the stratosphere (12-50 km of the atmosphere), where they have a longer residence time and greater global dispersal and therefore greater impact on climate. The emplacement of LIPs have further been identified to have caused a massive release of methane via melting of gas hydrates.
It is assumed that oceanic plateaus and CFBP are constructed by eruptions from a vent system or a set of fissures. This way different magma-supplying locations acting almost simultaneously can form a single LIP as we can observe e.g. for Iceland, which is a conglomerate of a number of volcanoes.
Looking up the ages of the globally observed LIPs it is interesting to note that most were formed during the period 150-50 million years. Much fewer LIPs were emplaced before 150 million years or after 50 million years. Seafloor spreading rates are also found to be high during a large part of this period. The episodicity in LIP emplacement appears to reflect variations in rates of mantle circulation with the period 150-50 million years representing a very active time, a kind of ‘mantle heartburn’. At the same time (~145 million years to ~50 million years) the global oceans were characterised by strong chemical variations, high temperatures, high relative sealevel, episodic deposition of black shales, high production of hydrocarbons, and mass extinction of marine organisms. A causal relationship of these environmental changes with the emplacement of LIPs is suspected.
To quantify and really grasp the environmental impact the construction of a specific LIPs has had, detailed seismic studies in combination with sampling of not only the surface but the deeper structure (by drilling) are needed. This then should be supplemented by the numerical simulation of the spreading of volatiles and heat in both atmosphere and the ocean and a modelling of the effect on flora and fauna.
South African Super LIP
What was the contribution of the South African Super-LIP's formation to modifications of palaeoclimate and palaeoenvironment during the Cretaceous?
During the Early Cretaceous the African-Southern Ocean (A-SO) gateway was closed and the South American, African, and Antarctic continents were connected. Breakup of this part of Gondwana commenced ~146 Ma and was complicated by the formation of two oceanic plateaux, the Mozambique Ridge (MozR) (~136-120 Ma) and the Agulhas Plateau (AP) (105-95 Ma). Those seafloor elevations constituted barriers for deep, intermediate, and bottom-water masses preventing a free exchange of watermasses between the Tethys/Indian Ocean and the evolving Southern Ocean/Atlantic Ocean. These complications notwithstanding, a deep water connection between the South Atlantic and the Indian Ocean opened with the separation of the Falkland Plateau from Africa 120-100 Ma.
Both origin and development of structural units in the South African gateway (Agulhas Plateau; Mozambique Ridge; Maud Rise MR; Astrid Ridge AR; Madagascar Ridge MadR; Gunnerus Ridge GR) are under heavy debate. Seismic studies and plate tectonic reconstructions of the Agulhas Plateau suggest that the plateau is a Large Igneous Province (LIP) that formed no earlier than 105 Ma in combination with the Northeast Georgia Rise (NEGR) and the Maud Rise. On the other hand, Gohl et al. suggested the existence of a southeast African super-LIP forming from 140 Ma to 95 Ma as a result of strongly varying magmatism and consisting of the Northeast Georgia Rise, Agulhas Plateau, Maud Rise, Astrid Ridge, Mozambique Ridge, and the Transkei Rise. This SE African super-LIP is the same size and age as the Kerguelen-Heard LIP. It is unclear whether this LIP embraces continental fragments.
Seismic P-wave velocity–depth model of profile AWI-20050200. Grey triangles mark OBS positions and numbers over triangles indicate station numbers. White triangles mark OBS which did not record any data. Black lines represent model layer boundaries with thick white lines marking positions of reflected phases at these boundaries. Thinwhite lines are velocity isolines. Dark shaded areas are not covered by rays. Abbreviations are AFFZ, Agulhas-Falkland Fracture Zone; AP, Agulhas Plateau; APA, Agulhas Passage.
The different studies show clearly that formation age and process of this super-LIP as well as the later development until the separation into Mozambique Ridge, Astrid Ridge, Northeast Georgia Rise, Agulhas Plateau, and Maud Rise remain unresolved. In order to better understand the impact of the formation of this supposed SE African super-LIP directly (emission of heat and volatiles into hydrosphere and atmosphere) and indirectly (construction of obstacles for oceanic circulation) on both palaeoclimate and palaeoenvironment Agulhas Plateau and Mozambique Ridge have been extensively studied using both seismic reflection and refraction methods as well as magnetic data as well as petrological sampling.
An anomalously thick lower crust (≤ 15 km) with unusually high seismic P-wave velocities of 7-7.6 km/s, the occurrence of extrusion centres and lava flow sequences, and an overlap of the complex the Northeast Georgia Rise-Agulhas Plateau-Maus Rise with continental crust during plate tectonic reconstructions have led to the interpretation of the Agulhas Plateau as a LIP. Recently collected seismic refraction data show a similar structure for the sourthern Mozambique Ridge as has been identified for the Agulhas Plateau. A magnetic survey of the Mozambique Ridgealso points towards a magmatic origin.
(a) Subdivision of the Agulhas Plateau in zone a with average crustal thickness of 21 km and zone b with average thickness of 16 km. (b) General structure of an oceanic plateau LIP (Eldholm & Coffin 2000) consisting of an extrusive cover (yellow), an intruded middle part (orange) and lower crustal body (red). (c) Crustal structure of the Agulhas Plateau along profile AWI-20050200 showing the typical structure of a LIP.
Seismic reflection data collected recently will provide a detailed image of the basement structure of the Mozambique Ridge and provide information on the development of this LIP. Additionally, petrological samples (expedition SLIP) will enable a chronological classification of the formation sequence of the ridge.
How has the formation of the Manihiki Plateau influenced climate and circulation since the Cretaceous?
In the Pacific Ocean, flood basalt provinces, such as the Ontong-Java, Hikurangi and Manihiki Plateaus, formed during the late Jurassic-early Cretaceous and the early Cretaceous respectively. Several tectonic reconstruction scenarios of associated lithospheric plates and LIP fragments exist; however, a comprehensive emplacement model remains unclear.
For the Manihiki Plateau, which is located in the central Pacific, different emplacement hypotheses have been put forward. These e.g. discuss continental fragmentation, impact related volcanism, formation at the Phoenix-Farallon-Pacific Plates triple-junction and its combination with rising plumeheads. The most agreed upon formation scenario suggests an emplacement of the Manihiki Plateau together with the Ontong-Java and Hikurangi Plateaus in a large single plateau and a direct break-off of those fragments around ~115 Ma during the Cretaceous magnetic quiet period. This giant LIP covered more than 1% of Earth’s surface.
The Hikurangi Plateau is supposed to have been located at the south to south-west margin of the High Plateau (HP), which forms the main fragment of the Manihiki Plateau. It moved southward due to rifting at the Osbourn spreading center. In the north-east and in the east, missing fragments of the composite LIP likely rifted to the north-east on the Farallon Plate and the Phoenix Plate, respectively, and subducted. Tectonic plate reconstruction studies have suggested that the Ontong-Java Plateau was connected to the Manihiki Plateau at the Western Plateaus and drifted towards the west.
Overview map of the Pacific Ocean in the vicinity of the Manihiki Plateau.
Bathymetric map of the Manihiki Plateau with collected seismic lines in black. Yellow triangles show the locations of ocean bottom seismometers. Red dots show the locations of DSDP Leg 33 Site 317 and red stars locations proposed for IODP proposal 630.
Tectono-magmatic development of the Manihiki Plateau
We try to unravel the relationship between the two largest sub-provinces of the Manihiki Plateau, the Western Plateaus and the High Plateau, which are separated by the Danger Islands Troughs. Did both sub-provinces experience the same magmatic history? The fragmentation of the Manihiki Plateau poses the question, whether distinct phases of magmatic or tectonic processes led to the deformation of the Manihiki Plateau and which role the Danger Islands Troughs played in this scenario. By processing, modeling and interpreting recently acquired seismic refraction/wide-angle reflection data; the deep crustal structure of both sub-provinces is revealed and interpreted for the first time.
In this paper, we use the term initial or first magmatic phase for the initial emplacement of the Manihiki Plateau (>120 Ma). The secondary magmatic stages comprise multiple episodic magmatic activities younger than 120 Ma, which occurred during and after the break-up.
It is important to note, that most of the publications have focused on the High Plateau along with the Danger Islands Troughs and the Manihiki Scarp. Other areas of the Manihiki Plateau, such as the basement of the Western Plateaus, are poorly sampled and therefore the evolution of the different sub-provinces and their magmatic and tectonic relationship to the other parts of the plateau is still poorly understood. Although satellite-derived gravity anomaly maps indicate different crustal structures of the sub-provinces, plate tectonic reconstructions of the Cretaceous western Pacific treat the Manihiki Plateau as a single tectonic block and disregard its different sub-provinces.
We present the first seismic refraction/wide-angle reflection P-wave and S-wave models of the two main sub-provinces – the Western Plateaus and the High Plateau – of the Manihiki Plateau. From the newly gained information on the crustal structure of the sub-provinces, the tectonic and magmatic evolution of this oceanic LIP can be further illuminated. Additionally we gain insight into the relationship between the sub-provinces, which have been treated as a single crustal block in previous studies.
The High Plateau shows key features of a LIP such as a HVZ in the lower crust and basaltic flow units in the upper crust. The crustal thickness presents a constant 20 km throughout the southern High Plateau. Eruptive centers of secondary magmatic phases are visible in the upper crust and major magmatic pathways can be traced in the middle and lower crust of the LIP throughout the High Plateau. The Western Plateaus, on the other hand show a gradual decrease in crustal thickness from 17.3 km in the east to 9.2 km in the west. The presence of the HVZ within the lower crust indicates a joined emplacement with the High Plateau during an initial magmatic stage. The upper crust of the Western Plateaus does not consist of major basaltic flow units and massive amounts of volcaniclastics such as the upper crust of the High Plateau. Therefore we propose an individual development of the two main sub-provinces of the Manihiki Plateau after the initial emplacement of the LIP. The two sub-provinces show a clear distinction between later magmatic stages, which has not been reported for any other oceanic LIP so far.
Several magmatic phases led to the formation of the Manihiki Plateau!
A detailed magmatic evolution history of the Manihiki Plateau is still lacking. Two volcanic periods have been distinguished. The main emplacement period occurred during the early Cretaceous (~ 125-110 Ma) and consisted of massive eruptions of basaltic, which resulted in an anomalously thick oceanic crust up to 20 km [Hochmuth et al., in revision]. Based on dredge samples and borehole data of DSDP Leg 33 Site 317, those are considered to have formed in a shallow water environment. Around ~90-65 Ma, a second smaller volcanic period (secondary volcanism) emplaced alkalic volcaniclastics. This second period is discussed to have been triggered by the tectonic break-up processes.
We have further investigated the volcanic emplacement history of the High Plateau, part of the Manihiki Plateau using new high resolution seismic reflection data gathered during cruise So224. We could extend the existing seismostratigraphic model by incorporating new reflections R5a, R6a and IB1, which correspond to the onset of sedimentation after the secondary volcanic period (~65 Ma, R5a), represent the onset of the alkali late stage volcanic period (~90 Ma, R6a), and likely represents an earlier volcanic flow period prior to R7 (>125 Ma, intrabasement reflection sequence IB1). The Intrabasement reflection sequence forms a high in the southern central part of the High Plateau and is interpreted to represent a shield volcano. We conclude that it represents the first volcanic emplacement phase within oceanic crust prior to the main emplacement phase.
The end of the major initial emplacement (Unit 4) is marked by the top of basalt (reflection R7) and characterized by a multitude of extrusion centers. We interpret those as a result of initial emplacement volcanism in the central part of the High Plateau (R7) and volcanism induced by tectonics at the eastern, western and southern flanks. For the late stage volcanic period, ending ~65 Ma the spatial distribution of the identified extrusion centers shows a relocation from the center of the High Plateau towards its eastern, southern and western flanks. We suggest two different source mechanisms have to be considered: sources either related to the initial emplacement of the HP and tectonic induced volcanism at the margins. This is in agreement with petrological data taken along the Suvarov and Danger Island Troughs.
At the eastern margin, the abrupt termination of the intrabasement reflection sequence IB1 across the Manihiki Scarp supports the hypothesis that a missing piece of the Manihiki Plateau rifted to the east before the formation of reflection R7. The south-western margin is characterized by a normal fault structure and a smooth appearance of reflection R7 and R6. This appearance supports the hypothesis of a stretched and rifted margin, which is associated with the break-up of the Hikurangi Plateau at the Osbourn Trough. The Western Plateaus in comparison to the High Plateau are characterized by a deficit of two distinguishable volcanic phases. Combining refraction and reflection data [Hochmuth et al., in revision], we conclude that this fragment was separated along the Danger Islands troughs at the end of the volcanic main period. The graben structure along the Danger Islands troughs and fault structures gives evidence that the High Plateau has been tectonically active after the alkali late stage volcanic period (<65 Ma).