Relative sea-level changes in Baltoscandia in the Cumbrian and early Ordovician: the predominance of tectonic factors and the absence of large scale eustatic fluctuations

Fluctuations in sea depth within a magnitude 20–100 m and a duration of 1–10 m.y. are often explained by rapid eustatic changes — so called ‘third-order eustatic events’. Considerable influence of regional tectonics on relative sea-level changes has been demonstrated by many authors, but because of uncertainties in the timing of short events in widely separated regions, the problem of separating tectonic and eustatic factors still remains unsolved. In this paper, a new and simple approach is used to reveal the presence or absence of eustatic events. We consider the St. Petersburg area and North Estonia in the north-eastern region of the East Baltic. From the late Early Cambrian until the middle of the Tremadoc (early Ordovician), deposition was extremely slow and the sea bed remained for a long time in a well defined peritidal zone in a water depth ≤10 m. Under such environmental conditions, a sea-level rise of ≥10 m would result in marked changes in the character of faunas and sedimentation. In the time interval considered here, significant sea deepening in the north-eastern Baltic region occurred only twice, and its magnitude did not exceed 10–20 m. A fall of sea level by ≥10 m would result in complete regression in the peritidal zone. This situation also occurred in region. However, the preservation of a sequence of unconsolidated sands, which is only a few tens of metres thick and includes all the main stratigraphic subdivisions on a regional scale, indicates that the crustal surface reached a very low altitude ≤10–20 m above sea level. These data show that in the late Late Cambrian to the middle of the Tremadoc, over a period of 40 m.y. long, eustatic sea-level changes did not exceed ±10–20 m. This limits the magnitude of several third-order cycles — eustatic events with duration of a few million years, which have been proposed previously for the epoch of the transition from the Cambrian to the Ordovician. In the late Early Cambrian to the Late Cambrian, transgressions and regressions with a magnitude of 50–150 m took place in southern Sweden and Lithuania. Since these phenomena occurred when there were no comparable eustatic sea-level changes, they must be associated with regional tectonic movements. Some were rapid and could be easily misinterpreted as indications of third-order eustatic changes. It is probable that some of the other eustatic events that have been proposed for the Phanerozoic were actually not of eustatic but of tectonic origin. Such rapid tectonic movements with magnitude of 50–100 m in cratonic areas can be caused by changes in the forces in the lithospheric layer with a laterally variable thickness, and by phase transitions in the mafic lower crust. Depending on the spatial distribution of vertical crustal movements, both these mechanisms could have been operating in the East Baltic and southern Sweden in the Cambrian.     

The Occurrence of a Lower Viscosity Layer in the Crust of Old Cratons as a Cause of the Strongly Differentiated Character of Postglacial Uplift

Doklady Earth Sciences - Rapid glacio-isostatic rebound in Fennoscandia and Canada that is nonuniform in time and space indicates that there is a layer with strongly decreased viscosity at shallow...     

Non-Uniform Crustal Subsidence due to the Infiltration of Mantle Fluids: A Case Study of the Moscow Syneclise during the Middle Devonian

Doklady Earth Sciences - The sedimentary basin of the Moscow syneclise is located in the central part of the East European craton. It is characterized by detailed drilling coverage and an extremely...     

On the mechanism of formation of the tyrrhenian sea

We investigated some of the existing hypotheses on the formation of the Tyrrhenian Sea basin, and we found that none of them appears satisfactory. By quantitative considerations we show that proposed mechanisms, such as those based on oceanization or back-arc spreading, cannot explain the origin of this basin. We suggest an alternative mechanism, consisting on the gabbro-eclogite transition, which occurs in the basaltic layer of the continental crust when anomalous mantle contacts the crustal basement. This leads to the formation of eclogite, which tears off the crust, sinks into the astenosphere, and accumulates at its base. As a consequence of the destruction of the basaltic layer the crust is strongly reduced and a deep basin is formed. We also suggest that the processes of crustal thinning and subsidence developed as proposed here at different times in different regions.     

Formation of the superdeep South Caspian basin: subsidence driven by phase change in continental crust

The large hydrocarbon basin of South Caspian is filled with sediments reaching a thickness of 20–25 km. The sediments overlie a 10–18 km thick high-velocity basement which is often interpreted as oceanic crust. This interpretation is, however, inconsistent with rapid major subsidence in Pliocene-Pleistocene time and deposition of 10 km of sediments because the subsidence of crust produced in spreading ridges normally occurs at decreasing rates. Furthermore, filling a basin upon a 10–18 km thick oceanic crust would require twice less sediments. Subsidence as in the South Caspian, of ≥20 km, can be provided by phase change of gabbro to dense eclogite in a 25–30 km thick lower crust. Eclogites which are denser than the mantle and have nearly mantle P velocities but a chemistry of continental crust may occur beneath the Moho in the South Caspian where consolidated crust totals a thickness of 40–50 km. The high subsidence rates in the Pliocene-Pleistocene may be attributed to the effect of active fluids infiltrated from the asthenosphere to catalyze the gabbro-eclogite transition. Subsidence of this kind is typical of large petroleum provinces. According to some interpretations, historic seismicity with 30–70 km focal depths in a 100 km wide zone (beneath the Apsheron-Balkhan sill and north of it) has been associated with the initiation of subduction under the Middle Caspian. The consolidated lithosphere of deep continental sedimentary basins being denser than the asthenosphere, can, in principle, subduct into the latter, while the overlying sediments can be delaminated and folded. Yet, subduction in the South Caspian basin is incompatible with the only 5–10 km shortening of sediments in the Apsheron-Balkhan sill and south of it and with the patterns of earthquake foci that show no alignment like in a Benioff zone and have mostly extension mechanisms.     

Steep bending of continental lithosphere without its stretching or plate collision: an indication for lithospheric failure and phase transitions

The Black Sea and Peri-Caspian basin include steep slopes which are 4–10 km high and only 20–30 km wide. They formed without extension-related normal faulting beneath the slopes and convergence in the adjacent regions. In the Peri-Caspian basin a steep bending of the lithosphere was associated with rapid crustal subsidence which evolved over a few million years. The large intensity and high rate of the deformations indicate a temporary loss of high rigidity in the lithospheric layer which occurred independently of plate-boundary-related processes. The origin of this phenomenon presents a special problem. A tentative explanation is that a lithospheric failure in cool intraplate regions can result from infiltration of volatiles from the asthenosphere and rapid metamorphism in mafic rocks in the lower crust.     

Lomonosov ridge and the Eastern Arctic Shelf as elements of an integrated lithospheric plate: Comparative analysis of wrench faults

The notions of deformations in the juncture area of the Eastern Arctic Shelf and Lomonosov Ridge are highly contradictory. It has been suggested that these geostructures were divided by a large right-lateral wrench fault of the transform type, which is known as the Khatanga–Lomonosov Fault. Data obtained by interpretation of the A7 profile have been compared with seismic sections crossing large-sized wrench faults in other sedimentary basins. The investigations have shown that on the A7 profile there are no structures typical of large-sized wrench faults. The Eastern Arctic Shelf and Lomonosov Ridge, which are located on the same lithospheric plate, form an integrated structure where the ridge is a natural continuation of the shelf.     

The Deep Submerged Continental Crust: The Central Arctic and Zealandia in the Southwest Pacific

Doklady Earth Sciences - A vast area of up to five million square kilometers is located in the Pacific Ocean to the east of Australia. The Earth’s crust up to 10–23 km thick is...