User:Peytong67/The Pannonian Basin

= Pannonian Basin = The Pannonian Basin is a sedimentary basin located in the southwest region of Eastern Europe, centered in the country of Hungary and surrounding states. The sediment-filled basin occupies what was once the Pannonian Sea, whose area underwent episodes of uplift and extension following the Alpine orogeny in the late Paleogene across most of geographical Europe.

Composed of many distinct geological units, the largest accommodation space is situated across most of the modern-day Great Hungarian Plain. Flanking this are the Dinaric Alps to the west, and the Carpathian Mountains which straddle the north and east. The modern geologic makeup is more precisely classified as an intracratonic back-arc basin formed from initial crustal extension and eventual rollback of slabs from the greater Eurasian area following the overall Cenozoic tectonic events of the Mediterranean region as a whole.

The geography of the modern Pannonian Basin is characterized by a vast covering of fertile soils and arable land deposited in the recent Cenozoic, with rich deposits of coal, oil, and natural gas.

Modern Geologic Setting
The modern Pannonian Basin is described best as a classical back-arc basin. The overall Mediterranean region experienced convergence between the plates of Eurasia and Africa since the Early Cretaceous, leading to the rather large Alpine Orogeny. By the Paleocene, this orogenic event had uplifted rock as far away as the North Downs chalk formation in England. As the Adriatic microplate and Eurasian plate formed complex subduction zones, trench rollback was followed by rapid extension in the Pannonian from the Paleocene to the Oligocene. The subsequent delamination of both the Adriatic and Eurasian plates signaled the end of extension in the Late Oligocene after the consumption of all subductable lithosphere. The complex geometry left behind by the subducting plates has led to modern circulating mantle flow. This flow in turn rotates the Pannonian and has been responsible for Cenozoic folding and subsidence.

Stratigraphic units throughout the basin are characterized by low velocities. Underlined by weak mantle strength and non-uniform stretching, the thickness of the crust and mantle lithosphere around the basin are reduced by an average stretching factor, β, of 1.6 indicating a relatively instantaneous time frame of stretching according to the McKenzie model. The discrepancy between a low β and extremely thin lithosphere is explained by thermal attenuation in addition to tectonic stretching of the crust.

Geography
This Basin lies in the heart of Central Europe. Centered in Hungary and the surrounding states, the Pannonian in the modern era consists of rich fields of fertile loess partitioned between sets of mountains brought up during the Alpine Orogeny. The Alps lie to the west, with the Dinarides straddling the southwest and the Carpathians wrapping around clockwise from north to south. Smaller mountain ranges dissect the basin in some areas, and peripheral basins exist throughout the entire Pannonian “system.”

Geological History
the Late Jurassic, the Paratethys Sea was connected to the Tethys Ocean. By the time of the Cretaceous, convergence and the onset of mutual subduction between Eurasia and Africa marked the beginnings of the Alpine orogeny. The boundary between the Eocene and Oligocene is characterized by a large drop in eustatic sea level and sudden cooling of the climate. Here the Pannonian Sea became isolated from the rest of the Paratethys Sea. The Alpine orogeny peaked, bringing up the Alps, the Dinarides, and the Carpathians. Rapid extension took place from the Late Paleocene to the Oligocene, a rather short amount of time, and only halted after the complete subduction of all lithosphere and delamination of plates at the end of the Oligocene. By the Pliocene, the Pannonian Sea started becoming progressively shallower, only to dry up completely a few million years later by the Pleistocene. Complex rotation and folding has been in effect since the delamination and end of extension in the Oligocene, with uplift from the Alpine orogeny continuing in the mountainous flanks and subsidence seen throughout the rest of the basin system.

Depositional Environment Evolution
An early Late Miocene unconformity divides accommodated strata into two megasequences: Syn-rift and Post-rift. The entire Pannonian basin is characterized by extremely thick post-rift strata making it unique to its surrounding peripheral basins where large pre-rift and syn-rift deposits are common.

A rapid transition from syn-rift to post-rift phase is marked by a quick reversion to transgressive facies in the Middle Miocene. This period included rapid limestone deposits with intermittent salt and calc-alkaline volcanics. [1] The onset of post-rift deposition at the Middle-Late Miocene boundary 11.6Ma marked the first phase of basin inversion. This period saw the removal of parts of underlying upper Miocene succession, as well as the development of an unconformity as peak collision of the Carpathians until thrusting ceased at 9Ma. [2] The formation of a basin-wide angular unconformity took place at the Miocene-Pliocene boundary, representing both the onset of the secondary inversion phase as well as the local Messinian Salinity Crisis. Large deltaic sediments were deposited during this time, likely due to a large river system similar in composition with the modern Danube and Tisza rivers which bisect the basin. [3] An alluvial succession started in the Late Miocene with an initial transgression followed by shelf margin and slope progradation phases. On average, seven kilometers of this alluvial succession exists at the surface. In more recent times, this unit has started reverting to a transgressive phase as subsidence has begun to increase. [4]

Tectonic Evolution
Following the onset of the Alpine Orogeny, the area of the Pannonian started to generate its primary tectonic structures. From the Mesozoic to the Oligocene, the older orogenic ALCAPA and Tisza-Dacia supergroups underwent simple folding associated with a subduction zone. Paleocene rollback of oceanic  lithosphere facilitated extrusion of metamorphics and basement rock to the surface by means of an asymmetric simple-shear extensional mechanism. Abrupt breakoff of subvertical slabs at the Oligocene-Miocene boundary brought about a halt to extension. The older units of the Pannonian entered its post-rift phase, characterized by large-scale rotation and inversion periods. (the first being in the early Late Miocene in the heart of the basin, and the second being an inversion and rotation event from that Late Miocene to the Holocene which mainly effected the flanks of the basin). Extensional disintegration of older Eocene orogenic terrains following back-arc inversion led to deep half-grabens, high basement,  and exposed substrata in the Miocene. A late-stage inversion event during the Pliocene to Quaternary led to major high-angle reverse faulting across the basin. Lithospheric-scale folding continues today along with continual buildup of surrounding mountains, and continual deposition of eroded sediment into newly formed accommodation space from subsidence.

Heat Flow
A shallow asthenosphere dome and attenuated crust influence high heat flow around the Pannonian. Thermal modelling suggests a stronger mantle lithosphere, but direct measurements show it to be weak. On average, heat flow distribution ranges anywhere from 50-130 mW/m2. Near the center of the basin the geothermal gradient is typically measured to be 45-50°C/km.

Hydrocarbon Exploration
Despite the excessive heat generated around the region, there exists pockets with relatively cool flow owing to local factors such as rapidly subducting sediment and/or deeper mantle depths. In many areas this pushes organic-rich sediment into the hydrocarbon window. There are currently six total petroleum systems (TPS) across the Pannonian. The source rock in this basin is almost always the organic-rich Miocene with a porosity anywhere from 7.8-10.2%. Traps include compactional anticlines and high-angle reverse faults, with stratigraphic traps holding hydrocarbons in the thicker flanks of the basin.