The ‘Walled Basin’ Model of the Transylvanian Basin: Geodynamic Implications

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Fogaras

In Central Asia several ‘walled basins’  (Carroll et al., 2010) exist which are recording thick lacustrine to alluvial deposits through geological times, and are actually showcasing the various types of strike-slip basins, in nature. Sedimentation is starting in some basins, like in the Junggar Basin, already in the Late Permian (Hendrix et al., 1992), in others during the Mesozoic (Fig. 28, 29). In western China strike-slip basins are contractional in origin, while in eastern China basins are younger pull-apart basins; the interior of these basins practically had stayed undeformed, only basin margins evolved into high to very high orogenic build-ups (Carroll et al., 2010). According to Carroll, many terms have been used to describe these basins ‘broken foreland’, ‘cornered foreland’,  ‘Chinese-type basin’, ‘collisional successor basin’.

If we recall, such walled basins can be seen in the Mediterranean area, as well, like in the case of the South Adriatic basin.

The Neogene Transylvanian Basin it is also a good candidate, in terms of basin mechanism, quality of sedimentary infill. It shows  very similar ‘walled basin’ margin character in all directions, a lacustrine-alluvial sedimentary infill, and it was called for a long time as a back-arc basin, assuming the Roydenian tectonic model. Herein, we propose to adopt the ‘walled basin’ model for the Neogene Transylvanian basin because many of the characteristics traditionally considered as mysterious  are getting a robust explanation, like 1) basin geometry, 2) sediment infill, 3) subsidence rates, 4) thickening of the lithosphere, 5) low heat flux, 6) uplift mechanism of the Southern Carpathians, 7) Peri-Carpathian basin geometry and many others that are going to be presented  in a separate paper.

We need to note, that the geodynamic evolution of the Southern Carpathians was generally neglected from the geodynamic models, because did not really fit into the back-arc model of the Pannonian-basin.  Secondly, it is obvious that the highest mountains of the Earth should share something in common regarding their origin, and geodynamic history. The Tian Shan Mts. are showing peaks of 7000m, the Caucasus has peaks above 5000m, the Atlas is rising above 4000m, the Alps have peaks of 3000m, and finally the highest peaks of Romania are found in the Southern Carpathians (above 2500m). All these very  high and relatively narrow mountain ranges are bounded by steep transpressional fault systems, just as in the case of the Southern Carpathian Mts.

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.

Consistency Check I: The Mediterranean Area and the Vøring Basin

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Fig15Herodotus

Strike-slip fault system of the South Mediterranean area (Matruh and Herodotus Basin), extracted from composite seismic profile published by Tari et al., 2012. Two different fault systems can be identified; the primary one is the regional strike-slip system. The secondary deformation events, materialized as listric faults and thrustings, are also triggered by strike-slip tectonics: 1) gravitational sliding in the transtensional stress field sector, 2) and contractional vertical and lateral duplexing in the transpressional stress field sector.

The thinning of the Mediterranean crust below the Sirte Basin and the Ionian Sea  has been evidenced recently by gravity inversion (Cowie and Kusznir, 2012). Cowie and Kusznir have also noted that the nature of the crust could be either oceanic or thinned continental. On their second profile, which transects the Herodotus Basin the transform nature of the basin margin i.e the sudden increase of crustal thickness is more prominent, as a consequence most probably the masterfault of basin opening was localized on the southern basin margin of the Mediterranean Sea.

In our interpretation, gravity profile from Fig.20 is showing additional arguments for the transtensional origin of the Mediterranean crust. Here, contrary to the southern margin, crustal thinning was enabled by overstepping strike-slip faults. The crustal thinning mechanism cannot be understood solely from the published profiles, but certainly must be sought in those of transtensional strike-slip basins.

Southern transform margin of the Mediterranean Basin (Matruh-Herodotus Basin)

An ENE-ESE opening of the East Mediterranean area related to a transform margin was documented by several authors (Longacre et al., 2007; Walley, 1998, 2001) in (Tari et al., 2012), and indeed strike-slip tectonics suggested by gravity profiles (Cowie and Kusznir, 2012) is supported by seismic profiles as well. Reinterpreting a composite profile across the Matruh-Herodotus Basin margin, we have concluded that  it is likely, that an important amount of the structural traps identified in the Matruh Basin were generated by the local Neogene transpressional stress regime. Actually, down-dip contractional horses interpreted by (Tari et al., 2012) might originate partly in strike-slip deformation than solely in gravitational sliding on a Cretaceous shale detachment, as indicated by the presence of numerous antithetic faults. The NNE-SSW Matruh Canyon itself, described by Tari as an aborted syn-rift basin, it is overlaying a Jurassic(?)-Cretaceous transtensional shear zone, most probably initiated as a plate-scale tension fracture, which is depicting the whole coeval North African stress regimes. Interestingly, the main strike of the basin is parallel with those of the NOSA zone in Tunis, which also belongs to the Sirte Microplate in our plate tectonic subdivision.

Eratosthenes Seamount

The Eratosthenes Sea Mount of the Levantine basin was reimaged recently with modern processing techniques (Peace et al., 2012). Around the Eratosthenes seamount, two sets of strike-slip faults can be differentiated highlighting the transpressional origin of the seamount. This crustal fragment initially it constituted a footwall segment of the Levantine Basin, related to a major transtensional fault, which later got inverted  with the evolution of the local stress-regime along the main displacement zone. Stratigraphy given in Fig. 22 is very uncertain; layering is rather reflecting seismic packages than established stratigraphic units.

Northern margin of the Mediterranean Basin

The Corinth Trough (Fig. 26) can be described as a 1-2 Ma old, ~100X30km high-strain band, which shows 5-15mm/year N-S extension (Bell et al., 2009), and segmented, overstepping boundary faults in plain view. Bell is referring to three prevailing theories which are used to explain basin extension: 1) back-arc extension related to the Hellenic Trench, 2) westward propagation of the North Anatolian fault (Dewey and Şengör, 1979), 3) gravitational collapse of the Hellenide orogeny lithosphere (Jolivet, 2001).

In the frame of GSST, the Corinth Trough is interpreted as an internal shear zone of the South Anatolian Nanoplate, which has developed internally an overstepping fault network in the principal displacement zone (PDZ), instead of a continuous masterfault. Given the significant space created in the releasing bend of the PDZ, gravitational collapse proposed by Jolivet (2001) is regarded as a complimentary effect of the regional strike-slip tectonic deformation.

The Vøring Basin, offshore Norway

Within the Vøring Basin, several compressional (Cenomanian-Turonian, Maastrichtian-Paleocene, Middle Miocene) and extensional collapse phases (Paleozoic, Late Jurassic, Early Cretaceous hyperextension, Late Paleocene, Early Eocene) have been documented (Lundin et al., 2013). Overviewing Lundin’s data, we have noted that the ‘early’ (Permian–Jurassic) tension fractures in many places have evolved into overstepping strike-slip faults and several isolated subbasins formed. It should be also noted, that main (SSW-NNE trending) sedimentary trenches, are cut by  further W-E striking strike-slip systems in the continuation of mid-oceanic transforms, giving birth to complex structural patterns. During  periods of inversion, i.e. when structural blocks have arrived into restraining phase, some fault blocks evolved into push-up ranges, like the Vema Dome (Fig. 24), which has been formed in the Middle Miocene.

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.