Al. A. Schreider

Variations of the Geomagnetic Dipole Magnitude over the Past 400 My

The international bank of the virtual dipole moment data supplemented by the values from more recent publications is used as the basis for an analysis of the behavior of the virtual dipole moment values over the last 400 My. The results obtained revealed a positive linear trend from 4.1 × 1022 to 5.7 × 1022 A m2 during the last 400 My. Against the background of the linear increase, fluctuations with a periodicity of about 40 My were observed. In the Phanerozoic time, minimums within the intervals of 340–370, 290–300, 240–270, 190–210, 165-140 (chrons M17-M43), 130-120 (chrons M2-M10), 100–110 (chron 34), 75–85 (chron C33 and the beginning of chron C34), 70-60 (chrons C31-C27), and 40-15 (chrons C18-C5AD) My B.P. are found. The distribution of the virtual dipole moment is strictly related to the distribution of the ancient geomagnetic field and may be taken into consideration when modeling the magnetization of the inversive magnetic layer of the ocean.

Geochronology of the American-Antarctic Ridge

A new map of chrons for the American-Antarctic Ridge area has been compiled. Its analysis and the calculations performed showed that the seafloor spreading with respect to its axis started before 85 My B.P. The spreading directions were 115° (chrons C34-C29), 145° (chrons C29-C21), 110° (chrons C21-C5C), and 85° (chrons C5C-C1). The maximum rates of about 4 cm/year were reached earlier than 52 My B.P.; subsequently, a progressive general decrease in the spreading rate has been observed. According to our forecast, the spreading may cease in the following 3.5 My.

Variations of the Earth’s magnetic field in the Phanerozoic

The global VDM database, which was later supplemented by new determinations published in the world literature (a total of 3194 determinations), is used as the basis for addressing the VDM behavior in the Phanerozoic (0–542 Ma) and up to 580 Ma. The results revealed a positive linear trend to higher VDM values from 3.5 × l022 Am2 to 5.7 × l022 Am2. Against this background, fluctuations of the mean VDMs occur with a periodicity of about 40 Myr. In the Phanerozoic, prominent minima of the intensity are found in the time intervals of 510–520, 420–460, 340–370, 290–300, 240–270, 190–210, 165–140 Ma (chrons M17–M43), 130–120 Ma (chrons M2–M10), 100–110 Ma (chron C34), 75–85 Ma (chrons C33 and the beginning of chron C34), 70–60 Ma (chrons C31–C27), and 40–15 Ma (chrons C18–C5AD). The distribution of the VDMs appears to reflect the paleomagnetic field behavior and may be taken into account in a magnetization model for the reversely magnetized oceanic crust.

Thermal evolution of the lithosphere of buried structures of the deep-water basin of the Black Sea

The GALO system for basin modeling is applied for numerical reconstruction of the thermal history of the lithosphere of the Western Basin and the Shatsky and Andrusov rises in the Black Sea. The modeling showed that the variant of the thermal evolution of the lithosphere of the region that was used by us for the Eastern Basin in our previous study is also applicable to the thermal evolution of the lithosphere of various tectonically different structures of the deep-water part of the Black Sea. These structures include both the Western and Eastern basins of the sea characterized by a granite-free crust formed in the course of the back-arc spreading and the Shatsky and Andrusov rises with the continental type of crust. The proposed version of the lithosphere evolution in the deep-water part of the Black Sea implies the initial stage of quasi-rift heating in the Upper Cretaceous and the three-staged thermal activation of the plate in the Cenozoic accompanied by three successive stages of crustal thinning. The latter resulted in the gradual deepening of the sea down to the present-day depth of 2.2 km.

The stages of the development of the basin of the Bransfield Strait

The study of the sedimentary body of the Bransfield Strait has made it possible to identify several sedimentary complexes, to construct the first electronic charts for the acoustic basement, and to establish four stages of the evolution of its floor, which updates the previous knowledge about the formation of the strait. At the first stage, there was an increase in tension stresses that were accompanied by the local splits of the continental crust at the periphery of the Antarctic Peninsula. At the second stage, a graben-like structure filled with the Lower stratigraphic complex was formed northward of the Antarctic Peninsula. At the third stage, the continuing processes of extension led to intensive explosive activity of the growing volcanic structures and filling of the graben with sediments of the Middle seismostratigraphic complex. The fourth stage, which has continued until recently, is characterized by quasi-linear localization of the major centers of volcanic activity in the band closer to the South-Shetland Islands and the formation of the Upper seismostratigraphic sedimentary complex. The evolution of the floor of the Bransfield Strait reflects the process of penetration of the American-Antarctic ridge to the continental lithosphere of the Antarctic Peninsula for the last million years.

Virtual dipole moment variations through the Proterozoic-Phanerozoic eons

The international bank of virtual dipole moment (VDM) data, combined with materials from recent publications (3384 values in total), served as a basis for the analysis of the VDM’s distribution through the Proterozoic and Phanerozoic eons (0–2.6 Ga). The VDM distribution obtained by the method of a moving average exhibits a positive linear trend from 3.7 × 1022 Am2 2.6 Ga ago to 5.8 × 1022 Am2 at present. Against the background of this linear growth, fluctuations with a periodicity of approximately 390 Ma are defined. The obtained data substantially specify the available data on the behavior of the magnetic field during the Proterozoic and Phanerozoic eons and should be taken into consideration for modeling the physical processes in the development of the Earth in the geological past and predicting its ecological and energetic evolution in the future.

Initial phase of the West Scotia mid-oceanic ridge opening

The first map illustrating the position of anomaly C12 extending in the NE-SW direction (25°) in the eastern peripheral part of the West Scotia Ridge is presented. Calculations of the paleomagnetic anomalies show that the spreading initiated in the period corresponding to Chron C12r (31.116–33.266 Ma ago). Chron C11r (30.217–30.627 Ma ago) was marked by a 200 km northwestward jump of the spreading axis. The calculation of the Euler poles and the rotation angles made it possible to reconstruct the zone of the initial breakup between South America and the Antarctic Peninsula along the western periphery of the continental Terror Rise.

Peculiarities of the East Scotia Ridge's Geochronology

The first map of the acoustic basement and the new map of chrons C1-C5E for the region of the East Scotia mid-ocean ridge have been made. The analysis of the maps and the calculations have indicated that the sea-floor spreading at the ridge’s flanks started in its southeast in the interval of chrons C5Er-6An (18.52–20.17 Ma BP). The maximal spreading rate (5.3 cm/year) was in the interval of chrons C5Bn-C5Br (14.78–15.97 Ma BP). Then, the spreading rate was slow and increased again from 3–6 Ma BP until the present. The spreading in the last 1–2 Ma was accompanied by the propagating of the axes southwards in the E1, E2, and E4 segments and northwards in the E8 and E9 segments.

Peculiarities of the kinematics of the west scotia mid-oceanic ridge

The first map of the acoustic basement and a new map of the C4-C12 chrons are made for the area of the paleospreading West Scotia Ridge. The analysis of the constructed maps and the calculations showed that the bottom growth in the ridge axes began in the southeast in the interval of chron C12r (31.116–33.266 Ma B.P.). In the period of chron C11r (30.217–30.627 Ma), a hundred-kilometer northwestward jump of the spreading axis occurred. The maximal values of the bottom growth (about 6.3 cm/yr) were in the interval of chrons C6–C6B (18.748–22.546 Ma); then, the spreading began to fade. In the time of chron C3n.1r (3.300–4.493 Ma B.P.), the axis of the paleo-mid-oceanic ridge died. The spreading was accompanied with northeastward propagating of the axes, and the propagating proper had an impulse character.

Transtension of the Romanche transform fault

Areas of transtension are discovered in the western part of the Romanche transform fault. Geodynamical parameters of the transtension are calculated on the basis of three-dimensional modeling of the inversive magnetic layer using survey data. The interval spreading rates calculated on this basis appeared to be smaller (1.65 cm/y for paleoanomalies C1-C5E on the northern side of the fault and 1.56 cm/y for paleoanomalies C6–C24 on the southern side) than those derived from the theoretical concepts based on the NUVEL-1 and NUVEL-1A models.