Diverging Margins – Review of the Rift Concept

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Fig27 Edwarded

Strike-slip fault deformation pattern in the Pleistocene sediments of Lake Edward. Line drawing after a seismic profile published in the Journal of Sedimentary Research, McGlue et al., 2006. Green dotted lines are showing the original fault interpretation, red lines are evidencing our interpretation.

The East African Rift

The Eastern African Rift System is given in many places as the best example for continental rifting; hence it is worth considering its structural elements from a kinematic point of view.

The Turkana Corridor is found in eastern Africa, at the intersection of two different fault sets, and forms a geomorphological corridor between the Ethiopian and Kenyan highlands. The axis of the Turkana Corridor corresponds to the Y-shear of the Cretaceous Panglobal Fracture System.

The Turkana Rift (Fig. 30) is a minor N-S trending rift, which is situated in the continuation of the Kenya and Main Ethiopian Rifts, inside the Turkana Corridor. It is actually a younger rift that is superposing an older NE-SW trending rift, the Anza Rift, which is a Neocomian transtensional basin, a lateral ramification of the Cretaceous Panglobal Fracture System, born as a major tension fracture. During Late Cretaceous, the Anza Basin has undergone a phase of major fault activity, when older basins were partly cannibalized (Bosworth and Morley, 1994). Towards the Chalbi Desert area, the oldest known deposits are Cenomanian lacustrine carbonates (Morley et al., 1999a). On the western side of Lake Turkana Oligocene half-grabens were also outlined (Vétel, 2004).

Tectonic inversion has affected several basins in eastern Africa; it is present in the Rukwa and Anza Basins, but the strongest inversion among all of them occurred in the Turkana Basin, twice, in the Paleogene and the Plio-Pleistocene (Morley et al., 1999b).

In the case of the North Malawi Basin (Fig. 32), Mortimer links basin-orientation to the underlying Pan-African foliation, however he notes that opinions are differing about the timing and the effect of strike-slip kinematics on the basin development of the Malawi Rift (Mortimer, 2007). Based on multifold seismic profiles Wheeler states that some aspects of the Livingstone Basin (North Malawi or Nyasa Rift) resemble pull-apart, extensional duplex, and extensional imbricate fan fault geometries, as expected in an incipient strike-slip basin (Wheeler and Rosendahl, 1994). Fault striations are evidencing both normal and strike-slip displacement (Delvaux et al., 1992).

McGlue has studied the depositional systems of Lake Edward because, unlike the other larger rift basins, it shows only one undisturbed depositional system from a seismic stratigraphic point of view. However, it seems that rifting preconceptions may have biased at least the structural interpretation of seismic profiles. It is more likely the basin was deformed by a strike-slip fault system instead of classic normal faults (McGlue, 2006).

In our interpretation, the East African Rift is originating in the internal simple shear stress field of the African Plate, which has been generated by the velocity contrast of the different plate segments during their eastward movement. The main (Y) shear direction is roughly parallel to the Equator, and the principal displacement was materialized in the Cretaceous Panglobal Fracture Zone.

Other important principal displacement zones of Africa, showing similar Y-shear directions, can be identified at the Congo – Tanganyika, Tanganyika– Malawi, Malawi – Okavango microplate boundaries.

Cyclicity of the observed extensional and inversion periods can be translated into periods of transtension and transpression involving large separation regional strike-slip faults.

Congo Basin

The Congo Basin is one of the largest intracratonic basins with an almost complete Neoproterozoic to Recent sedimentary sequence. It is underlain by a ~200km thick lithosphere showing current seismic activity and tectonic dislocations (Kadima et al., 2011). Various mechanisms such as a downward dynamic force linked to a high-density lithospheric object (Downey and Gurnis, 2009) or downwelling mantle plume (Hartley and Allen, 1994) are invoked to explain subsidence observed in the Congo Basin.

According to Fig. 33, the last major simple shear stress related structural event of the Busira Subbasin happened in the early Paleozoic involving the Neoproterozoic – earliest Paleozoic deposits, as suggested by the onlapping sedimentary sequence (above the green unconformity). Strike-slip faults are reaching to the surface, indicating that the Paleozoic fault system has experienced some minor reactivations recently.

In our view, the structural style of the Congo Basin is perfectly fitting into the GSST model, because it is mapping the current stress field and it is compatible with that observed in the East African Rift area, despite the more reduced degree of deformation. Hence, subsidence history of the Congo Microplate should be interpreted in the common, codependent history of the Central African kinematic chain.

In conclusion, in any of the reviewed African ‘rift basins’ strike-slip tectonics is unequivocally present, and the basin opening and later deformations are related to the movement of the equatorial kinematic chains. Due to the irregularity of microplates local stress fields are variable along microplate boundaries. Thus, rift zones in the context of GSST are assimilated with simple shear tension fractures, and consequently, the East African basin evolution is guided by the movement of microplates and by their inherent low hierarchy plate fragments. The takeaway for the GSST approach it is that we should avoid applying the rift term for transtensional basins and incipient tension fractures, unless oceanic sea floor spreading can be proved.

Published in: Kovács, J.Sz., 2015 (in press), Elements of Global Strike-Slip Tectonics: a Quasi-Neotectonic Analysis, Journal of Global Strike-Slip Tectonics, v1., Szekler Academic Press, Sfintu Gheorghe.

Consumption of Oceanic Plates – Review of the Subduction Concept

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Fig24MarianaSubduction first of all it means recycling, i.e. recycling of crustal fragments into the asthenosphere. When oceanic crustal fragments are rising above baselevel, we speak about obduction processes, whilst in case crustal fragments are indeed melted into the asthenosphere we may speak about subduction processes.

According to the GSST theory, kinematic plate fragments are in constant motion, showing certain relative plate velocity contrasts. Under constraints of the Coriolis force, hence, due to their accumulated momentum, microplates tend to rather develop transpressional orogenic build-ups on their longer edges than forming subduction zones during their constant eastward drift. As a consequence, oceanic plate fragments instead of being recycled are rather preserved, being obducted into orogenic complexes, where they are forming suture zones. In other words, the presence of ophiolites in orogenic complexes is the proof of obduction processes, and only indirectly can be inferred the presence of subduction zones from mass balance considerations. Hence, it is not surprising that the peri-Tethys area is full of obducted ophiolite units; starting with the Vardar, Transylvanides, Dinarides, dozens of Asian (e.g. Anatolian) obducted ophiolites in the geological record are all product of transpressional strike-slip tectonics.

Because of global kinematic constraints, subduction may happen only perpendicularly to the principal plate drifting vector or more frequently, as a function of the principal drifting vector of kinematic chains. This means, that real subduction may only form roughly in N-S direction. Basically there are only two such kind of convergent plate boundary lineaments on the Earth, where plate recycling could happen: 1) at the western Pacific plate margin, 2) and at the eastern Pacific plate margin.

In section 3, we have already reviewed the main geological, geophysical, geodetic observations made regarding the evolution of the Japan Island. All these data are proving the significance of shear deformation in northeast Japan, where the intensity of fault activities is very pronounced, earthquake epicenters indicating the presence of E-W directed fault planes. Clustering of earthquake epicenters in stress nodes is generally present all along the Western Pacific subduction zones, from the Sumatra Arc to the Philippine subduction zone. Pacific plate velocity measurements might change with time as new GPS stations will be installed in the oceanic domain, and if the motion of smaller crustal units is also going to be integrated. The origin of the Mariana Trench and the Bonin Ridge is linked to the same strike-slip displacement, which was made along the irregular Hawaii North Mp/ Bonin Nanoplate boundary, in S to N direction. This irregular plate boundary in turn is the result of another plate velocity contrast, which is characterizing the Bonin Np–Hawaii North Mp and the North Philippine–Hawaii South Mp kinematic chains.

Therefore, structural features of the Japan Island and the whole southeastern Asia island-belt, suggest that instead of real subduction we are facing various transpressional space management processes, best described with the term of plate-overriding, i.e a mix of obduction, lateral escape, plate buckling mechanisms.

From a kinematic perspective, real subduction, where recycling of crustal fragments occurs it could only happen in the western margin of the American continents. Here, all the kinematic prerequisites of subduction are present: 1) the motor of subduction is present, and it can be identified in the Coriolis force, which is transferred and exploited by the momentum of plates, 2) there is an obvious density, hence a momentum contrast, between the oceanic and continental crustal fragments.

Momentum contrasts between plate fragments can play a significant role in the initiation of both oceanic spreading and subduction processes, in case the Earth suffers angular acceleration. Whether the angular acceleration of planets is a common planetary phenomenon or not, it is out of our knowledge, but certainly accidental larger meteoric impacts may affect the spinning velocity of the Earth.

Published in: Kovács, J.Sz., 2015 (in press), Elements of Global Strike-Slip Tectonics: a Quasi-Neotectonic Analysis, Journal of Global Strike-Slip Tectonics, v1., Szekler Academic Press, Sfintu Gheorghe.

Definition of Micro-, Nano-, Pikoplates and Kinematic Chains

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kchain

Marquesas Mp-Ganges Mp kinematic chain is found south of the Cretaceous Panglobal Fracture Zone, and incorporates microplates of the Pacific Ocean, South America, Atlantic Ocean, Africa, India and the Indian Ocean

Being involved into various shear zones during continental drift, world tectonic plates area broken into smaller and smaller tectonic units with time. Contact zones of microplate are marked by pushup ranges and evolved orogenes, deep sedimentary basins, and transform fault scars on the oceanic floor. Most probably, a great percentage of fault-scars preserved on the surface of the Earth are somewhat older than Holocene, and ocean-floor fault scars are even older. The ‘quasi-neotectonic’ term points to this timing uncertainty.

Size of these crustal fragments is varying within a wide range, thus a plate hierarchy can be established according to their size ranges. In GSST we propose the simultaneous usage of conventional ‘plate‘ name to refer to the classic large plate tectonics units. For mid-size plate fragments we also keep the ‘microplate‘ term. In order to study the movement of the even smaller, but well individualized crustal fragments, we propose the use of the ‘nanoplate‘ term, without any direct reference to the absolute plate size range. Nanoplates usually appear in areas of long lasting contractional stress, and are made up of incipient or well developed plate duplex horses.

‘Pikoplates’ are the internal components of nanoplates, smaller with about one order of magnitude. These are the most important elements of structural hydrocarbon plays and are driving the regional distribution of hydrocarbon and ore accumulations. What we know as hydrocarbon trends, for example, are related usually to the presence of pikoplates.

Example: The Marquesas Mp-Ganges Mp kinematic chain is found south of the Cretaceous Panglobal Fracture Zone, and incorporates microplates of the Pacific Ocean, South America, Atlantic Ocean, Africa, India and the Indian Ocean.

Published in: Kovács, J.Sz., 2015 (in press), Elements of Global Strike-Slip Tectonics: a Quasi-Neotectonic Analysis, Journal of Global Strike-Slip Tectonics, v1., Szekler Academic Press, Sfintu Gheorghe.