Alps portal
45-593: The Molasse basin (or North Alpine foreland basin ) is a foreland basin north of the Alps which formed during the Oligocene and Miocene epochs . The basin formed as a result of the flexure of the European plate under the weight of the orogenic wedge of the Alps that was forming to the south. In geology , the name "molasse basin" is sometimes also used in a general sense for
90-508: A eustatic drop of the sea level combined by tectonic uplift, the basin was now above sea level. This second formation therefore consists of fluviatile sands and clays and huge alluvial fan systems (conglomerates and breccias) originating from the Alps to the south. By 22 million years ago, the Alpine foreland was flooded again due to tectonic subsidence. A shallow ( intertidal ) marine environment formed from Lyon to Vienna . In this environment
135-447: A fold-thrust belt , the décollement is the lowest detachment (see Fig 1.) and forms in the foreland basin of a subduction zone. A fold-thrust belt may contain other detachments above the décollement—an imbricate fan of thrust faults and duplexes as well as other detachment horizons. In compressional settings, the layer directly above the décollement will develop more intense deformation than other layers, and weaker deformation below
180-407: A subaerial wedge is flanked with terrestrial or shallow marine foreland basins". The temperature underneath the orogen is much higher and weakens the lithosphere. Thus, the thrust belt is mobile and the foreland basin system becomes deformed over time. Syntectonic unconformities demonstrate simultaneous subsidence and tectonic activity. Foreland basins are filled with sediments which erode from
225-404: A brittle manner above the décollement surface, with intense ductile deformation below the décollement surface. Décollement horizons may be at depths as great as 10 km and form due to high compressibility between differing rock bodies or along planes of high pore pressures. Typically, the basal detachment of the foreland part of a fold-thrust belt lies in a weak shale or evaporite at or near
270-446: A décollement. Concentric folding is identified by uniform bed thickness throughout the fold, and is necessarily accompanied by detachment or a décollement as part of the deformation that occurs with a thrust fault. Disharmonic folding does not have uniform bed thickness throughout the fold. Décollements in extensional settings are accompanied by tectonic denudation and high cooling rates. They can form by several methods: Located in
315-786: A narrow band, just 10 km wide. After that it widens again to the east. North of the Danube River it connects with the Vienna basin , and is thus connected with the Pannonian Basin further east. The region where the molasse crops out is divided into two: the Subalpine Molasse zone along the Alps and the Foreland Molasse zone further into the foreland. In the Foreland Molasse zone the molasse sediments are relatively undisturbed; in
360-607: A regime's tectonic origin and development as well as the lithospheric mechanics. Migrating fluids originate from the sediments of the foreland basin and migrate in response to deformation. As a result, brine can migrate over great distances. Evidence of long-range migration includes: 1) correlation of petroleum to distant source rocks , 2) ore bodies deposited from metal-bearing brines, 3) anomalous thermal histories for shallow sediments, 4) regional potassium metasomatism and 5) epigenetic dolomite cements in ore bodies and deep aquifers. Fluids carrying heat, minerals, and petroleum, have
405-399: A synorogenic (formed contemporaneously with the orogen ) foreland basin of the type north of the Alps. The basin is the type locality of molasse , a sedimentary sequence of conglomerates and sandstones , material that was removed from the developing mountain chain by erosion and denudation , that is typical for foreland basins. The Molasse basin stretches over 1000 kilometers along
450-454: A vast impact on the tectonic regime within the foreland basin. Before deformation, sediment layers are porous and full of fluids, such as water and hydrated minerals. Once these sediments are buried and compacted, the pores become smaller and some of the fluids, about 1 / 3 , leave the pores. This fluid has to go somewhere. Within the foreland basin, these fluids potentially can heat and mineralize materials, as well as mix with
495-406: Is a structural basin that develops adjacent and parallel to a mountain belt . Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithosphere to bend, by a process known as lithospheric flexure . The width and depth of the foreland basin is determined by the flexural rigidity of the underlying lithosphere, and
SECTION 10
#1733084462280540-507: Is found closer to the orogen and oil is found further away. Decollement Décollement (from French décoller 'to detach from') is a gliding plane between two rock masses, also known as a basal detachment fault. Décollements are a deformational structure, resulting in independent styles of deformation in the rocks above and below the fault. They are associated with both compressional settings (involving folding and overthrusting ) and extensional settings. The term
585-441: Is generated by lithospheric extension. Foreland basins can be divided into two categories: DeCelles & Giles (1996) provide a thorough definition of the foreland basin system. Foreland basin systems comprise three characteristic properties: The wedge-top sits on top of the moving thrust sheets and contains all the sediments charging from the active tectonic thrust wedge. This is where piggyback basins form. The foredeep
630-456: Is more geologically accurate within a specific region. Seismicity determines where active zones of seismic activity occur as well as measure the total fault displacements and the timing of the onset of deformation. Foreland basins form because as the mountain belt grows, it exerts a significant mass on the Earth's crust, which causes it to bend, or flex, downwards. This occurs so that the weight of
675-404: Is the thickest sedimentary zone and thickens toward the orogen. Sediments are deposited via distal fluvial, lacustrine, deltaic, and marine depositional systems. The forebulge and backbulge are the thinnest and most distal zones and are not always present. When present, they are defined by regional unconformities as well as aeolian and shallow-marine deposits. Sedimentation is most rapid near
720-725: Is treated as a group divided into four formations . This division is made on whether the sedimentary facies is continental or marine. The lowermost formation is the Lower Marine Molasse (in German : Untere Meeresmolasse ). Its age is Rupelian (early Oligocene, 34 to 28 million years old) and it consists of shallow marine sand, clay and marl. On top of this is the Lower Freshwater Molasse (German: Untere Süsswassermolasse ) of Chattian and Aquitanian age (late Oligocene-early Miocene, 28 to 22 million years old). Due to
765-569: The Helvetic nappes , which caused the deformation in the Subalpine Molasse zone. The Jura mountains , a fold and thrust belt along the present Swiss-French border, also originated from this tectonic phase. In some places in the Jura mountains, molasse deposits were folded together with older Mesozoic limestones . In present-day central Switzerland, however, the molasse formed a thick competent mass that
810-462: The Jura Mountains , north of the Alps, it was originally thought to be a folded décollement nappe. The thin-skinned nappe was sheared off on 1000 meter-thick deposits of Triassic evaporites . The frontal basal detachment of the Jura fold-and-thrust belt forms the most external limit of the Alpine orogenic wedge with the youngest fold-and-thrust activity. The Mesozoic and Cenozoic cover of
855-401: The basement . Rocks above the décollement are allochthonous , rocks below are autochthonous . If material is transported along a décollement greater than 2 km, it may be considered a nappe . The faulting and folding that occurs with a regional basal detachment may be referred to as "thin-skinned tectonics", but décollements occur in 'thick-skinned' deformational regimes as well. In
900-617: The Jura Mountains never developed. Due to the last phase of tectonic uplift around 5 million years ago, the molasse in the Swiss Plateau , the South Bavarian plain and Eastern Austria is now 350 to 400 meters above sea level at its northern rim, slowly rises southwards and can reach more than 1,000 m at its contact with the Alps. The molasse deposits in the basin can be up to 6 kilometers thick. Lithostratigraphically , this molasse
945-564: The Subalpine Molasse zone the beds are often tilted, folded and thrust over each other. Before the development of the Molasse basin, in the Mesozoic era , the region was covered by a shallow sea that was the northern margin of the Alpine part of the Tethys Ocean ( Valais Ocean in the west, Tethys Ocean proper in the east). The Molasse basin was formed when the orogenic wedge of the forming Alps
SECTION 20
#1733084462280990-504: The actual term was not used until Buxtorf's 1907 publication. Décollements are caused by surface forces, which 'push' at converging plate boundaries , facilitated by body forces (gravity sliding). Mechanically weak layers in strata allow the development of stepped thrusts (either over- or underthrusts), which originate at subduction zones and emerge deep in the foreland . Rock bodies with differing lithologies have different characteristics of tectonic deformation. They can behave in
1035-404: The adjacent mountain belt. In the early stages, the foreland basin is said to be underfilled . During this stage, deep water and commonly marine sediments, known as flysch , are deposited. Eventually, the basin becomes completely filled. At this point, the basin enters the overfilled stage and deposition of terrestrial clastic sediments occurs. These are known as molasse . Sediment fill within
1080-587: The basin was in a continental facies again. The fluviatile sands and clays and fan conglomerates of this time form the Upper Freshwater Molasse (German: Obere Süsswassermolasse ), the topmost molasse formation. Around 5 million years ago a phase of uplift occurred in the Alps. During this phase the Molasse basin ceased to be an area of net sedimentation. 47°46′08″N 10°08′25″E / 47.76889°N 10.14028°E / 47.76889; 10.14028 Foreland basin A foreland basin
1125-453: The characteristics of the mountain belt. The foreland basin receives sediment that is eroded off the adjacent mountain belt, filling with thick sedimentary successions that thin away from the mountain belt. Foreland basins represent an endmember basin type, the other being rift basins . Space for sediments (accommodation space) is provided by loading and downflexure to form foreland basins, in contrast to rift basins, where accommodation space
1170-403: The décollement. Décollements are responsible for duplex formation , the geometry of which greatly influences the dynamics of the thrust wedge . The amount of friction along the décollement affects the shape of the wedge; a low-angle slope reflects a low-friction décollement, whereas a higher-angle slope reflects a higher-friction basal detachment. Two different types of folding may occur at
1215-569: The fold-and-thrust belt and the adjacent Molasse Basin have been deformed over the weak basal décollement and displaced by some 20 km and more toward the northwest. The Appalachian - Ouachita orogen along the southeastern margin of the North American craton includes a late Paleozoic fold-thrust belt with a thin-skinned flat-and-ramp geometry, related to lateral and vertical variations in rock lithologies. The décollement surface varies along and across strike . Promontories and embayments in
1260-421: The foredeep acts as an additional load on the continental lithosphere. Although the degree to which the lithosphere relaxes over time is still controversial, most workers accept an elastic or visco-elastic rheology to describe the lithospheric deformation of the foreland basin. Allen & Allen (2005) describe a moving load system, one in which the deflection moves as a wave through the foreland plate before
1305-464: The forming mountain chain filled the basin and made it shallower. During the Oligocene and Miocene epochs (more exactly, between 10 and 30 million years ago), shallow marine to continental molasse was deposited in the basin. Around 10 to 5 million years ago, tectonic uplift had raised the basin so high that net sedimentation stopped. From the south, the molasse deposits were overthrust about 10 kilometers by
1350-679: The lithosphere beneath the mountain range becomes ductile almost entirely, except a thin (about 6 km in the center) brittle layer near the surface and perhaps a thin brittle layer in the uppermost mantle." This lithospheric weakening underneath the orogenic belt may in part cause the regional lithospheric flexure behavior. Foreland basins are considered to be hypothermal basins (cooler than normal), with low geothermal gradient and heat flow . Heat flow values average between 1 and 2 HFU (40–90 mWm . Rapid subsidence may be responsible for these low values. Over time sedimentary layers become buried and lose porosity. This can be due to sediment compaction or
1395-424: The load system. The deflection shape is commonly described as an asymmetrical low close to the load along the foreland and a broader uplifted deflection along the forebulge. The transport rate or flux of erosion, as well as sedimentation, is a function of topographic relief. For the loading model, the lithosphere is initially stiff, with the basin broad and shallow. Relaxation of the lithosphere allows subsidence near
Molasse basin - Misplaced Pages Continue
1440-457: The local hydrostatic head. Orogen topography is the major driving force of fluid migration. The heat from the lower crust moves via conduction and groundwater advection . Local hydrothermal areas occur when deep fluid flow moves very quickly. This can also explain very high temperatures at shallow depths. Other minor constraints include tectonic compression, thrusting, and sediment compaction. These are considered minor because they are limited by
1485-559: The long axis of the Alps, in France , Switzerland , Germany and Austria . The west end is at Lake Geneva , where the basin's outcrop is just 20 km wide. Further to the northeast the basin becomes wider. It forms the subsurface of the Swiss Mittelland and reaches its widest (circa 130 km) in the Bavarian foreland of the Alps. From Amstetten to Sankt Pölten the basin forms
1530-436: The mountain belt can be compensated by isostasy at the upflex of the forebulge. The plate tectonic evolution of a peripheral foreland basin involves three general stages. First, the passive margin stage with orogenic loading of previously stretched continental margin during the early stages of convergence. Second, the "early convergence stage defined by deep water conditions", and lastly a "later convergent stage during which
1575-453: The moving thrust sheet. Sediment transport within the foredeep is generally parallel to the strike of the thrust fault and basin axis. The motion of the adjacent plates of the foreland basin can be determined by studying the active deformation zone with which it is connected. Today GPS measurements provide the rate at which one plate is moving relative to another. It is also important to consider that present day kinematics are unlikely to be
1620-511: The physical or chemical changes, such as pressure or cementation . Thermal maturation of sediments is a factor of temperature and time and occurs at shallower depths due to past heat redistribution of migrating brines. Vitrinite reflectance, which typically demonstrates an exponential evolution of organic matter as a function of time, is the best organic indicator for thermal maturation. Studies have shown that present day thermal measurements of heat flow and geothermal gradients closely correspond to
1665-424: The rheological structure of the lithosphere underneath the foreland and the orogen are very different. The foreland basin typically shows a thermal and rheological structure similar to a rifted continental margin with three brittle layers above three ductile layers. The temperature underneath the orogen is much higher and thus greatly weakens the lithosphere. According to Zhou et al. (2003), "under compressional stress
1710-430: The same as when deformation began. Thus, it is crucial to consider non-GPS models to determine the long-term evolution of continental collisions and in how it helped develop the adjacent foreland basins. Comparing both modern GPS (Sella et al. 2002) and non-GPS models allows deformation rates to be calculated. Comparing these numbers to the geologic regime helps constrain the number of probable models as well as which model
1755-401: The slow rates of tectonic deformation, lithology and depositional rates, on the order of 0–10 cm yr , but more likely closer to 1 or less than 1 cm yr . Overpressured zones might allow for faster migration, when 1 kilometer or more of shaley sediments accumulate per 1 million years. Bethke & Marshak (1990) state that "groundwater that recharges at high elevation migrates through
1800-441: The subsurface in response to its high potential energy toward areas where the water table is lower." Bethke & Marshak (1990) explain that petroleum migrates not only in response to the hydrodynamic forces that drive groundwater flow, but to the buoyancy and capillary effects of the petroleum moving through microscopic pores. Migration patterns flow away from the orogenic belt and into the cratonic interior. Frequently, natural gas
1845-548: The third formation, the Upper Marine Molasse (German: Obere Meeresmolasse ), was formed. It consists of marine sands, clays and marls and new fan conglomerates and is of Burdigalian to Langhian age (early Miocene, 22 to 16 million years old). More tectonic uplift caused the sea to retreat one final time and during the Serravallian , Tortonian and Messinian / Pontian ages (late Miocene, 16 to 5 million years ago),
Molasse basin - Misplaced Pages Continue
1890-444: The thrust, narrowing of basin, forebulge toward thrust. During times of thrusting, the lithosphere is stiff and the forebulge broadens. The timing of the thrust deformation is opposite that of the relaxing of the lithosphere. The bending of the lithosphere under the orogenic load controls the drainage pattern of the foreland basin. The flexural tilting of the basin and the sediment supply from the orogen. Strength envelopes indicate that
1935-470: Was first used by geologists studying the structure of the Swiss Jura Mountains , coined in 1907 by A. Buxtorf, who released a paper that theorized that the Jura is the frontal part of a décollement at the base of a nappe , rooted in the faraway Swiss Alps . Marcel Alexandre Bertrand published a paper in 1884 that dealt with Alpine nappism . Thin-skinned tectonics was implied in that paper but
1980-651: Was pushed north over the European continental margin , due to the convergent movement of the European and Apulian plates during the Paleogene period . The weight of the orogenic wedge made the European plate bend downward, forming a deep marine foredeep. In the Eocene epoch (55 to 34 million years ago) the basin became deeper until north of the developing orogen it formed a small oceanic trench , in which flysch sediments were deposited. The huge amounts of sediments eroded from
2025-581: Was thrust northward in one piece over a decollement horizon at the base of the Mesozoic, in Triassic evaporites . Deformation instead localized further north, thus forming the relatively flat Swiss Mittelland between the Alps and the Jura Mountains. The Swiss part of the Molasse basin is now between the Alps and the Jura mountains, as a large piggyback basin . In the Eastern Alps an external mountain range such as
#279720