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

ISSN 0001�4370, Oceanology, 2011, Vol. 51, No. 1, pp. 170–182. © Pleiades Publishing, Inc., 2011.Original Russian Text © A.A. Schreider, Al.A. Schreider, A.N. Boiko, J. Galindo�Zaldivar, A. Maldonado, E.I. Evsenko, 2011, published in Okeanologiya, 2011, Vol. 51, No. 1, pp. 175–187.

170

INTRODUCTION

The process of the Scotia Sea formation played avital role in the destruction of the continental bridgebetween South America and Antarctica. With no deepwater drilling holes available in the region of the ScotiaSea, the complex geological–geophysical interpreta�tion of the anomalous magnetic field from the view�point of the plate tectonics conception becomes ofmain value in the study of the principal stages of thisprocess.

Resulting from the geological–geophysical investi�gations carried out in the western part of the Scotia Seabetween 54°–60°S and 41°–67°W, the currently non�active West Scotia mid�oceanic ridge was discovered[1, 3, 4, 6, 7, 10, 11, 13, 36]. In the bottom relief, theridge coincides with the horst�like rise of more than500 km width with intensively eroded relief. Theheight of the crest is up to 2 km above the surface of theadjacent abyssal plains of the Yaghan Basin in thenorthwest and the Ona Basin in the southeast. Thedepth of the basins is about 3.5 km. The ridge is bor�dered by the Shackleton Fracture Zone in the west andby the southern peripherals of the North Scotia Ridgein the east.

An intensive magnetic anomaly of 200–500 nT andup to 50 km in width is observed above the ridge’s axis.In the southeast and northwest, it borders the linearmagnetic anomalies of 100–300 nT with wavelengthsof 20–40 km shifted along several transform faults byseveral tens of kilometers. Additionally, the research�

ers mentioned the extreme scarcity of data, which didnot enable them to identify the young and ancientpaleomagnetic anomalies reliably.

The linear magnetic anomalies and transformfaults in the Scotia Sea are still undetermined pre�cisely, and, being based on a limited amount of geolog�ical–geophysical data, they vary in their outlines indifferent publications. The complex geological–geo�physical study of the available materials on the anom�alous magnetic field (including the original data ofinvestigations carried out onboard the Spanish R/VGesperidas) and the reconstruction of the bottom geo�chronology in the area of the ridge and the adjacentbasins as well can significantly help in the study of thepaleogeodynamical problems. The present work isdevoted to both these objectives.

PARAMETERS OF THE ACOUSTIC BASEMENT

There are two types of the Scotia Sea bottom reliefmaps known, which are distinguished by the principlesthey are based on. The first type comprises theGEBCO maps generalizing all the known soundingprofiles along the routes of specialized research vessels(e.g., [1] et al.). The second type includes the topo�graphical maps of the bottom based on the integrationof sounding measurements and satellite altimetry data(e.g., [34] et al.).

Peculiarities of the Kinematics of the West Scotia Mid�Oceanic Ridge

A. A. Schreidera, Al. A. Schreiderb, A. N. Boikoa, J. Galindo�Zaldivarc, A. Maldonadod, and E. I. Evsenkoa

a Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russiae�mail: [email protected]

b NIIgazekonomika Limited Company, Moscow, Russiac University of Granada, Granada, Spain

d Andalusian Institute for Earth Sciences, Granada, SpainReceived December 4, 2008; in final form, April 13, 2009

Abstract—The first map of the acoustic basement and a new map of the C4–C12 chrons are made for the area ofthe paleospreading West Scotia Ridge. The analysis of the constructed maps and the calculations showed that thebottom growth in the ridge axes began in the southeast in the interval of chron C12r (31.116–33.266 Ma B.P.). Inthe period of chron C11r (30.217–30.627 Ma), a hundred�kilometer northwestward jump of the spreadingaxis 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 propagat�ing of the axes, and the propagating proper had an impulse character.

DOI: 10.1134/S0001437011010176

MARINE GEOLOGY

OCEANOLOGY Vol. 51 No. 1 2011

PECULIARITIES OF THE KINEMATICS OF THE WEST SCOTIA MID�OCEANIC 171

Satellite tracks cover the studied area in a relativelyeven manner, and this fact enables one to preparemaps with the resolution between the adjacent obser�vation tracks being 0.5 miles. The most modern ver�sion of such a map is the variant based on half�minuteaveraging of the results of the mathematical model forthe bottom relief [35, 38, 40]. However, the compari�son between the GEBCO bottom relief maps and themaps based on the satellite altimetry for the Scotia Seademonstrates their noncoincidence.

To choose the type of map for further use in thepresent work, we compared the data of the modernelectronic versions of both these types of maps at thepoints of the real sounding measurements carried outalong the profiles (Fig. 1) of the R/V Gesperidas in thewestern part of the Scotia Sea with the use of a SIM�RAD EM 12 digital sounding device. The oceano�graphic corrections for the primary observational dataderived from the sounding device were performed bythe Spanish side using specialized software developedat New Brunswick University (Great Britain) espe�cially for the specified conditions of the Scotia Sea.

The visual comparison evidences the fact that thedata of the satellite altimetry are more close to the dataof the real sounding measurements. The statisticalprocessing of the discrepancy values between thesounding data, on the one hand, and the electronicvalues of the satellite altimetry and the electronic val�ues of the GEBCO maps, on the other hand, at thepoints of the sounding measurements evidences thesame. The mean deviation for 1032 points is 54 m bythe modulus for the altimetry data at the maximal dis�crepancy of 540 m, while the mean deviation for theGEBCO map exceeds 157 m at the maximal discrep�ancy of 1486 m. The value of the distribution medianis 30 m in the first case and more than 100 m in the sec�ond one. Hence, the data of the electronic bottommap (Fig. 2) based on the integration of the results ofthe sounding measurements and satellite altimetry(the data of the predict�relief map) reflect the bottomrelief features of the Scotia Sea more completely thanthe GEBCO�derived data do. It should be noted thatthe problem about the creation of a synthetical data�base uniting the best advances of both approaches inthe representation of the sea�bottom bathymetry isunder discussion by UNESCO. It is intended that thenew database would cover the World Ocean regionsfrom 80°N until 80°S.

The digital database for the complete sedimentthickness of the Scotia Sea [39] is part of the digitaldatabase for the World Ocean and shelf seas collectedby NOAA’s National Geophysical Data Center by theresults of studies along the profiles of uninterruptedseismic profiling archived by NOAA and by the seismicdata and isopachic maps available as part of the Inter�national Geological–Geophysical Atlases [1, 2, 16,19–21, 26].

Contour maps for the Pacific Ocean isopachs weretransferred into the digital form by Gregory Cole (Los

Alamos National Laboratory); for the Indian Ocean,by Carol A. Stein (Northwestern University); and forthe South Ocean, by Dennis Hayes (the Lamont–Doherty Earth Observatory). Transferred into the dig�ital form for NOAA, these data were then laid on a gridwith an interval of 5 × 5 minutes of arc with the conse�quent using of a spline smoothening algorithm inaccordance with [35]. The map of the sediment distri�bution in the Scotia Sea reflects only the most signifi�cant features of the sedimentary body and gives nodetails. However, it provides a certain idea concerningthe thickness of the sediments in the water area. It fol�lows from the map given in Fig. 3 that the thickness ofthe sediments is maximal in the southeast of theregion, reaching 700–800 m. Going northwestwards,the thickness reduces to about 200 m.

The presence of data on the sedimentary sequencehas enabled us to built the first electronic scheme forthe surface of the acoustic basement in the westernpart of the Scotia Sea; this scheme has the character ofa first approximation and reflects only the most signif�icant features of the basement. It is seen from the mapexhibited in Fig. 4 that the depth of the basement inthe axial part of the paleospreading West Scotia Ridgeis about 2 km. Going far from the axis to the Yaghanand Ona Basins, the depth reaches 4 km in the westand more than 4.5 km in the east of the area. In theareas of transform shears, single depressions of thebasement are observed at depths of up to 5 km andmore. It is important to note that the derived scheme,in several cases, demonstrates the peculiarities of thetransform faults more accurately, which enables one tosupplement the data on the position and configurationof the faults earlier known only by the bottom reliefdata.

PARAMETERS OF THE BOTTOM EXTENSION

The first identifications of the linear magneticanomalies in the Drake Passage eastwards of theShackleton Fracture Zone were proposed in [6, 10].According to them, the most ancient magnetic anom�aly there is the C8 anomaly; hence, the spreadingbegan about 26 Ma B.P. and ended about 6 Ma B.P.These data were repeatedly corrected. Above the WestScotia paleo�mid�oceanic ridge and in the adjacentbasins, linear magnetic anomalies C3–C8 have beenidentified [1, 11, 13, 36]. Additionally, two alternativemodels were proposed in [27]. The first one describedthe observed undulations of the anomalous magneticfield by model anomalies C6C–C12, and the second,by anomalies C4–C6. Finally, the authors preferredthe second model, according to which the spreadingepisode was referred to the period of 9–20 Ma B.P.

The analysis of the data from numerous studies ofthe anomalous magnetic field in the basin evidencesdiscrepancies in both the spatial position of the paleo�magnetic anomalies and their identification; e.g., inthe West Scotia Ridge, between the Endurance and

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Quest transform faults [1], the axes of paleoanomaliesC5B and C5C are located in the interval of C5A–C5Band C5B–C5E, respectively (from [22]). The axes ofpaleoanomalies C6B–C8 (from [1, 24]) are in theinterval of the C5–C5C anomalies in [22].

In [1, 3, 29, 30, 31 et al.], the most ancient reliablydetected magnetic anomaly between the Quest andShackleton transform faults is paleoanomaly C10,while, in [22], anomaly C8 is believed to be so. Thereare no known paleogeodynamical reconstructions(including determinations of the Euler rotation polesand turning angles) earlier than the time of chron C8in the literature. The trace of the transform failurerestored by the paleogeodynamical reconstructions in[22] obliquely cuts the Tehuelche Fault clearlydetected by the previous researchers. The configurationsof the transform faults in [1, 3, 22, 31] do not coincidewith each other. The Berwood Transform Fault in [22] islocated farther to the north than in [1, 3].

The investigations of the recent years [8, 9, 12, 15,16, 18, 22, 24, 28, 29, 31, 33, 37] have enabled us toobtain new data on the anomalous magnetic field ofthe area. The comparison between the observed andtheoretical magnetic anomalies in the model of thebottom spreading (Figs. 5a–5d) has enabled us tospecify the position of the earlier known paleomag�netic anomalies and reidentify their numbers whenneeded. This has enabled us to renew (Figs. 6a and 6b)the previous schemes of the bottom’s age [1, 8, 9, 22,31 et al.]. The new scheme of the chrons for the EastScotia Ridge, in accordance with the comparative analy�sis of the observed and theoretical magnetic anomalies inthe model of the bottom spreading (Figs. 5a–5d), isgiven in Fig. 7.

The existence of anomalies up to C10 southeast ofthe West Scotia mid�oceanic ridge in the Ona andYaghan Basins was shown in [1, 24, 30]. The additionaldata on the magnetic field obtained during the Spanishexpedition SCAN 2004 (Fig. 5) and modeled by theauthors of the present work [33] have enabled us todetect paleoanomaly C11 and to distinguish two moreundulations of the magnetic field in a small enclosedbasin further along the profile. If these undulations areof spreading nature, then they can be considered astwo symmetric anomalies C12 (Fig. 5c, Fig. 7).

The calculation of the interval rates of spreadingwas performed for the distinguished sequences ofchrons, which enabled us to analyze the peculiaritiesof the process of the new oceanic crust growth usingstatistical methods for processing the interval rates ofspreading and the interval rates of opening with the useof data on the asymmetry of the opening process [5].The scale of the linear magnetic (paleomagnetic)anomalies was taken from [25].

The intervals of the rate calculation were distin�guished between the most reliably [1, 22, 23] deter�mined magnetic anomalies, namely, C5, C5C, C6,C6B, C8, and C10. It was taken into consideration inthe calculations that the spreading axis died during

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chron C3n.1r [23]. According to the calculations, thebottom spreading was not monotonous, the intervalrates the spreading changed from 0.9 to 2.9 cm/yr inthe northwestern flank of the ridge and from 0.8 to3.5 cm/yr in the southeastern flank. The highest ratesare noted for the chron interval of C6–C6B.

The distribution of the spreading asymmetry valuesin time demonstrates that the growth rates of thenorthwestern flank of the ridge are greater than rates ofthe southeastern one in the intervals before 22–26 andafter 16 Ma B.P., while, in the intervals of 16–22 andbefore 26 Ma B.P., the southeastern flank grew moreintensively than the northwestern one. The spread ofthe calculation values is within the limits of ±20% fromof 1 corresponding to the symmetric growing charac�ter of the new oceanic crust material.

The summation of the flank spreading rate valuesin every single spreading cell has enabled us to derivethe values of the bottom opening rates and to analyzethe distribution of the relative interval spreading rates.

According to the calculations, the values of theopening rates varied from 1.8 to 6.4 cm/yr. The rategrew beginning from the time of chron C11rC11r(30.217–30.627 Ma B.P.) and later and reached itsmaximum in the interval of chrons C6–C6B (18.748–22.564 Ma B.P.). Then, the general deceleration of theprocess of the new oceanic crust growing had begun;spreading rate had been progressively decreasing, andit had reached values of 1.8 cm/yr in the interval ofchron C5–C3An.1n (such rates are typical forultraslow spreading ridges). Soon after this, thespreading axis of the West Scotia Ridge dies in theinterval of chron C3n.1r (3.300–4.493 Ma B.P.).

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Fig. 5. Magnetic anomalies in the model of the bottom spreading observed (solid line) in various areas (a–d) of the West ScotiaRidge [22, 33] and the theoretical anomalies (dashed line). The profile numbers of the anomalous magnetic field are the same asin the corresponding insets. The location of the Magtrans 4 profile is given in the inset of Fig. 1. The upper boundary of the inver�sional magnetoactive layer coincides with the relief of the acoustic basement. The lower surface of the layer conforms to the reliefof the acoustic basement with the thickness of the layer being 0.5 km, while the magnetization is assumed to be 5 A/m. The angu�lar parameters of the magnetization vector are taken in accordance with the parameters of the field of the axisymmetric terrestrialdipole, and the parameters of the modern magnetic field correspond to those in the DGRF field of the surveyed epoch. The namesof the identified polarity chrons are given.



It was said above that two undulations of the mag�netic field were distinguished in the marginal south�east of the area in a local depression, and we suppos�edly consider these anomalies as two symmetricanomalies C12 [14, 15, 23, 33]. If we assume this, thenthe episode of the bottom growth in this local depres�sion took place in the interval of chrons C12r–C11r(30.2–33.2 Ma B.P.) and the growing rate was about1 cm/yr.

DISCUSSION

The analysis of the results concerning the bottomrelief and the thickness of the sedimentary sequence

has enabled us to compile the first scheme of theacoustic basement relief in the area of the West Scotiapaleo�mid�oceanic ridge. The prolonged relief formsof the basement corresponding to the transform faultslocated here are clearly seen in the scheme. The anal�ysis of the data on the anomalous magnetic field andthe linear magnetic anomalies in the west of the ScotiaSea has enabled us to construct a new map of thechrons for the studied area. The comparison betweenthe schemes of the acoustic basement and the bottomgeochronology enabled us to prove and to correct insome cases the location and configuration of theknown transform failures of the bottom of the ScotiaSea’s western part.

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In their sum, the obtained data has allowed us tosignificantly renew the existing ideas [9, 13–15, 17,22, 23, 28, 29] concerning the bottom kinematics inthe area of the West Scotia paleo�mid�oceanic ridge

and in the adjacent areas. According to them, one canbelieve that the spreading in the Drake Passage areabegan in the time of chron C12r (31.1–33.2 Ma) andlasted until the time of chron C11r (30.2–30.6 Ma).

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Then, a northwestward jump of the spreading axisoccurred. Additionally, since the time of chron C11r, thebottom growth rate increased and reached its maximumin the interval of chrons C6B–C6 (19.7–23.1 Ma B.P.).After this, the spreading process became less intensiveand its rate attenuated.

The implemented analysis of the bottom geochro�nology scheme evidences the fact that the paleoanom�alies older than C8 can be distinguished in the south�western and central parts of the studied area; the pale�

oanomalies older than C5C cannot be detected in thenortheast of the area. Most probably, this fact isexplained by the impulse character of the northeast�ward propagating of the spreading axes.

In the time of C11r (30.2–30.6 Ma B.P.), thespreading occupied the space to the northeast of theShackleton Fracture Zone up to the Birdwood trans�form fault. During chron C6B (21.7–22.5 Ma B.P.),the spreading axes reached the northern part of theTehuelche Fracture. Finally, in the time of chron C5C

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(15.9–17.2 Ma B.P.), the spreading axes moved fur�ther northeastwards to the area of the North ScotiaRidge, and, soon after the time of chron C3n.1r(3.300–4.493 Ma B.P.), the axis of the paleo�mid�oceanic ridge died.

CONCLUSIONS

Thus, resulting from the carried out study is thefirst map for the surface of the acoustic basement inthe western part of the Scotia Sea. It helped to specifythe configuration of the transform faults in the newlymade map of the chrons for the area of the West Scotiamid�oceanic ridge. The spreading in the Drake Pas�sage area began in the time of chron C12r (31.1–33.2 Ma)and lasted until the time of chron C11r (30.2–30.6 Ma).Then, a northwestward jump of the spreading axisoccurred. Additionally, since the time of chron C11r,the bottom growth rate increased and reached its max�imum in the interval of chrons C6B–C6 (19.7–23.1 Ma B.P.). After this, the spreading processbecame less intensive and its rate attenuated.

The bottom’s growth was accompanied by thepropagating of the spreading axes, while the propagat�ing proper had an impulse character. In the time ofC11r (30.2–30.6 Ma B.P.), the spreading occupied thespace to the northeast of the Shackleton FractureZone up to areas located one hundred kilometersnorth to the Endurance Fault. In the time of chron C8(25.2–30.6 Ma B.P.), the spreading axes moved north�wards to the Birdwood transform fault.

During chron C6B (21.7–22.5 Ma B.P.), the spread�ing axes reached the northern part of the Tehuelche Frac�ture. In the time of chron C5C (15.9–17.2 Ma B.P.), thespreading axes moved further northeastwards to thearea of the North Scotia Ridge. In the time of chronC3n.1r (3.300–4.493 Ma B.P.), the axis of the paleo�mid�oceanic ridge died. The growth rates of the north�western flank of the ridge were greater than the rates ofthe southeastern one in the intervals before 22–26 andafter 16 Ma B.P., while, in the intervals of 16–22 andbefore 26 Ma B.P., the southeastern flank grew moreintensively than the northwestern one.

ACKNOWLEDGMENTS

This work was supported by the Russian Founda�tion for Basic Research (project no. 08�05�00138).

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