RUSSIAN JOURNAL OF EARTH SCIENCES VOL. 10, ES4001, doi:10.2205/2007ES000218, 2008
Geodynamic zonation of the Atlantic Ocean lithosphere: Application of cluster analysis procedure and zoning inferred from geophysical dataS. Yu. SokolovGeological Institute of the Russian Academy of Sciences, Moscow, Russia N. S. Sokolov Geological department of the M. V. Lomonosov State University, Moscow, Russia L. V. Dmitriev V. I. Vernadsky Institute of Geochemistry and Analytical Chemistry of the Russian Academy of Sciences, Moscow, Russia Contents
Abstract[1] Ten geological-geophysical parameters used in geodynamics directly or indirectly reflecting geometry of the Atlantic lithosphere inner boundaries, mass distribution within the lithosphere, and energy release made it possible to calculate 15 stable combinations of parameters whose manifestation areas are interpreted as geodynamically different districts. The Atlantic lithosphere zonation allows a new segmentation of a mid-oceanic ridge zone presenting the alternation of "cold'' and "hot'' blocks, marked by discrete conditions of basalt melts formation. Additional phenomena superimposed on the standard oceanic lithosphere are discussed. The phenomena present zones pseudosymmetric with respect to a mid-oceanic ridge marked by highly productive plume magmatism, these zones are more extensive than it has been considered earlier, and sublatitudinal zones exhibiting some features of fore-arc zones. 1. Introduction. Status and Approach to Solution of the Problem
[3] The list is far from being complete but the above facts imply a gap between facts and working geodynamic model meant to explain them. The latter is based on the mantle convection when ridge push by extension along its axis, slab pull and dredging of the lithosphere by astenospheric current resulted from convection are considered to be the main forces responsible for surface dynamics of the lithosphere masses. The forces mentioned and mechanism of their energy supply cannot explain their and other factors appearance by deficiency of motion horizontal component nonorthogonally to the MAR and because a discrete pattern of parameters of magmatic processes operating along the MAR do not agree with the notion of continuous ascending substance flow along the divergent zone of the convective cell. In other words: lithospheric masses move over the Earth's surface in a more complex way than that stipulated by the geodynamic model. [4] Noteworthy, that discrete conditions of basalt magma formation also poorly correlate with the usual notion of the convective cell system. This contradiction cannot be resolved even when we proceed from the assumption of a possible southward migration of the Atlantic superplumes along the Mid-Atlantic Ridge axis [Dmitriev et al., 2001]. [5] The questions stated cannot be resolved in the context of a single paper. The authors only generalize new covering original data equally cover the offshore area with the classification of geodynamic environments. Our investigation will make possible the solution of fact - theory discrepancy by compilation of geodynamical maps and better understanding of physical meaning of types recognized. [6] Thus, problems are the following:
2. Statement and Formalization of a Problem2.1. Problem of Geodynamic Zonation and Previous Studies[7] Qualitative approach to the solution of geodynamic problem can be based only on coherent definition of "geodynamics''. Comparison of experts' views with those of their opponents [Belousov, 1975; Pavlenkova, 1987; Zonenshein and Kuzmin, l993] shows the presence of a common pivot proceeds from the definition of dynamics used in physics. "Mechanics studies the simplest form of matter motion i.e. mechanical movement to change mutual arrangement of bodies and their parts in space and time. Bodies are macroscopic systems consisting of a very large number of molecules and atoms, so sizes of the systems many times higher than intermolecular distances. Kinematics studies mechanical motion of bodies without the connection defining the interaction between bodies. Dynamics studies the way in which force produces motion [Yavorsky and Detlaf, 1974]. Interaction means the analysis of forces and energy sources. Therefore, "geodynamics'' is considered as science that studies the interaction between geological object with time. The development of approaches to parametrization of complex properties of geological bodies to further use them in the quantative analysis is important for solution of geodynamic problems. A very similar to the above definition of "geodynamics'' was given in the work by Khain and Lomize [1995]. Objects in terms of geodynamic turn to be much more complicated than those in classical physics which makes this subject quite unique. Complexity of objects affects greatly the presence of adequate and efficient quantitative models describing processes of geodynamics. [8] The above definition assumes that parameters describing a geodynamic object are to be delivered into three main groups: [9] 1. Description of geometry and physical properties of the object. [10] 2. Description of forces and energy release within the object. [11] 3. Description of object motion behavior resulted from the action of forces on the object and energy release in it. [12] Thus, the objective of the geodynamic zonation is a search for different stable combinations of parameters describing a geodynamic object and analysis of their distribution in space.
[13] The first works concerning the qualitative approach to solution of geodynamical problems
using several parameters are those by
Reisner and Reisner [1987, 1990].
They made the analysis of endogenous regimes for most of Europe, Caucasus, and the Carpathians.
Calculations using the cluster analysis algorithm as a version of popular multi-dimensional
statistic classification of objects. This method is of practical importance because a man cannot
make a reliable visual correlation of parameters if their number is above four. Such parameters as
thickness and average seismic velocity of the Earth's crust, elevation, depth to the consolidated
basement were discussed in the works mentioned. Heat flow is taken as a parameter describing
energy release. Isostatic gravity anomalies and velocity of recent vertical movements were used
to describe the resultant motion of geomedia. A complete set of parameters was presented for the
land and grouped into average values
20 [14] The paper by Ioganson and Boltyshev [2000] presented the cluster analysis for the eastern Eurasia. This study differs from the previous works by showing changes in cluster classification as the number of clusters increases and in revealing of stable (homogenous) areas. This does not result in differentiation of a separate region into subclasses of smaller area. Qualitatively, they used linear and dispersed heterogeneity of territories. The former assumes division of territory into contrasting classes much smaller in area (size) than homogeneous areas. At the same time, it made possible to retain a stable mosaic pattern as the classification number increases. Dispersed heterogeneity was meant as "comminution'' of territory into small and comparable with size of a zone cell with different combination of main parameters chaotically covering territory. Prior to the processing stage the study area was homogeneous (linearly heterogeneous) on steps having small classification number. [15] This paper discusses the Atlantic ocean structure in terms of linear heterogeneity. However, situation will be considered to be optimal when the number of classes allowing to divide the study territory (see Section 5) does not result in critical comminution of the recognized stable zones into much smaller zones whose size is comparable with that of a cell. The authors believe that dispersed (scattered) heterogeneity mentioned in the paper by Ioganson and Boltyshev [2000] is directly associated with the scatter of parameters used for their analysis. Its qualitative measure is the scatter of parameters values within a zone of one or another cluster. 2.2. Approach to Selection of Parameters and Their Coordinates[16] The parameters selected for the analysis should be homogeneously defined within the study area and should have similar detailness. Similar detailness is necessary for adequately assess different parts of the territory because the detail level incorporated into a general data set of low density will make algorithm respond to a difference thus affecting classification results due to detection of false differences. A similar detail level will allow assign to each cell (into which territory is divided) a full vector of parameters used in the analysis. Noteworthy, that the rule can be broken in some cases if parameter should be added to the calculation. Heat flow is such a parameter. It is not equally known in the Atlantic area, so sites lacking interpolation measurement should be covered by calculation of grid (parameter values regularly spaced) and only then fit a high frequency grid component to the level of other parameters.[17] Parameters to be selected must describe three groups of properties mentioned in Section 2.1. Parameters describing structural features of the lithosphere (group 1) are very easy to select (see Section 3). [18] Parameters defining energy release (group 2) can be easily selected except for problems related to irregular heat flow measurements. [19] Most difficult is to describe the resultant motion (group 3). Vertical movements inferred from repeated geodetic measurements from GPS data can be used for the land. There is no such measurements for the sea floor and a regular observation network (measurement grid) can hardly be obtained in future. Therefore, to include data of 3-type group into calculations one should use the so-called "surrogate'' parameters reflecting indirectly values no measured or partially measured in the ocean or they present a combination of many effects including those to be processed. In this case, such an approach is the only way to show the necessary information in absence of detailed data. [20] All the parameters used in this paper are values inferred from instrumental measurements reflecting only the present state of all three groups of parameters. Paleogeodynamic reconstructions of the Atlantic ocean are always based on incomplete information with many assumptions for values remaining unknown. In paleogeodynamics not measurements but interpretations of geological-geophysical data that should reflect paleostate of the lithosphere become important. The ambiguity of interpretation will always make the result debatable. [21] The paper discusses only the recent state of the lithosphere. Besides, the authors do not use the age of the oceanic lithosphere inferred from magnetometric data as a parameter for calculation [Mueller et al., 1993] because the authors of a data source do not define general linear anomaly position in most complex and important parts of the Atlantic (transition from northern to southern segment), important for geodynamic. The latter follows from the data [Cande et al., 1993] used for the construction of the known age map. Nevertheless, the map in fact completely covers the Atlantic. It means that all the estimates for which we use it in most important areas will reflect the pecularities of interpolation algorithms and not value a parameter. Therefore, the information on the age of the lithosphere can at best be used as a coordinate parameter to present the analysis results along with latitude and longitude. [22] In this study one arc degree is proposed as the best size for a cell within which the parameters used show similar detail level. In other words, the latter of all the parameters is not worse than this value. The dimensions of the cell are comparable with an average thickness of the lithosphere. The more detailed parameters are to be fit to a chosen threshold by frequency filtration or by moving average. The real size of an area created by a chosen cell decreases at high latitudes which creates less statistical importance of parameters. Nonetheless, as this concerns all the parameters at once the authors do not use estimates in the projection space having equal size. However, the average value of parameters within cell is evaluated well enough. 2.3. Approach to Selection of Data Processing Methods[23] Three methods of large data sets multivariant statistical classification could be pointed out - discriminant, factor, and cluster analysis. They efficiently use geological-geophysical data and differ in specific features.[24] The discriminant analysis is aimed at classification of objects by selection of its parameters and comparing their values with "learning'' standards. Requiring the presence of known a priori stable type, this method is not applicable because prior to the analysis we can not be sure in the result. [25] The factor analysis assumes that the available object data set consists of combination of two or more processes actions, each of them contributing to values of all parameters. In other words, there are independent geodynamic phenomena that form superposition of measurement values (we subdivide them by means of factor analysis). At present, it is fairly difficult to construct a model for operation of two or more global processes whose contribution to all the parameters will be of statistical importance, however, this can be done in future. [26] The cluster analysis assumes recognition of stable combinations of parameters not discernible by visual analysis of maps and seems to be the most adequate at this stage of the study as has been shown earlier for other regions (see Section 2.1.). The factor analysis might be used in case of geodynamic model construction exhibiting one (or more) mechanisms affecting surface tectogenesis. 3. Data Applied
[27] Geodynamic zonation of the Atlantic Ocean lithosphere encloses deep-water areas, the
Mid-Atlantic Ridge area, passive continental margins, and offshore area (Figure 1). The analysis
does not include arc and backarc zones of the Caribbean and Scotia, seas different in
geodynamics in environments characteristic of the entire Atlantic. Thus, the class of phenomena
studied does not contain collision zones. It is the 82o N at the transition of the Mid-Atlantic
Ridge zone to Gakkel Ridge. To the south, it is confined to Bouvet triple junction where a drastic
change in the structural pattern of most geophysical anomalies takes place farther south.
Analysis area is shown on Figure 1. The brief description of chosen parameters for each
1o 3.1. Bottom Topography[28] Bottom topography in the first and the major parameter describing the top of the earth's crust and the lithosphere (see Figure 1). It was inferred from ETOPO5 (1995), lowpass filtered frequency filtration and recalculated to one degree cell. Its shape gathers together the effect of many processes: magmatism, ocean floor deformation, sedimentation, etc. In our classification it is assigned to group 1 of parameters describing the object geometry being so a direct measure of the required characteristics. Qualitatively, the bottom topography assumes the result of the crustal block movement under the effect of contacting forces (parameter of group 3). An accurate observation over the movement similar to ground-based GPS measurements for the ocean floor are absent, off processing of the bottom topography data indirectly, though not precisely, is accounting these movements.3.2. The Thickness of Sedimentary Cover
3.3. Tomography Inferred From Surface Love Waves
3.4. Bouguer Anomaly
3.5. Isostatic Anomalies
![]() where H is a depth of the compensational surface, T - level of reduction, s c - crustal density, s w - water density, s m - mantle density; this will allow us to calculate correction for Bouguer anomaly. They eliminate the effect of a hypothetical surface topography obtained as in the case of bottom topography. The residual field represents isostatic anomalies when their positive values imply an excess of masses above the compensation surface unlike the negative values pointing to their deficiency. The excess of masses might well result in submergence of the crustal block in a given site, while emergence together with the mantle part owes it to deficiency. If the action (e.g., thrust) is not completed then we'll get both the excess of masses (positive isostatic anomalies) and positive vertical movements of the crust. The interpretation of isostatic anomalies being ambiguous, its resolution calls for further investigation of the general tectonics of the region. In terms of geodynamics this parameter concerns directly the variation of crust density properties intensity of energy release in the crust, and generation of stresses (modulus of isostasy gradient) caused by the transition from disturbed state into equilibrium. This parameter is also a "surrogate'' in description of the resultant vertical movement of the crustal blocks subjected to energy release. The isostatic anomaly field presents the above properties as, in fact, an indivisible combination. 3.6. Heat Flow
3.7. Tomography by S-Waves
![]() [35] So, this parameter is an almost indivisible combination of effects of energy release (heated state) and medium geometry (zone of prolific magma production and greater thickness of the crust). This parameter is "surrogate'' for both groups reflecting indirectly and not directly properties of the groups. 3.8. Tomography by P-Waves
![]() ![]() 3.9. Total Seismic Moment[37] This parameter is used for calculation of the total energy released during earthquakes. It was a global query (ANSS, February 2004, http://quake.geo.berkeley.edu/anss/) for events with a Richter magnitude of above 4.5 for a layer of 0 to 100 km. The approach published by Boldyrev [1998] was used in our estimates. Summation of released energy for events within a degree cell was calculated from the formula![]() ![]()
![]() 3.10. Lithospheric Component of the Earth's Magnetic Field
4. Method Description4.1. Cluster Analysis
[39] Cluster analysis is a method of multi-dimensional statistic classification, based on a
compact measurement groups selection (stable parameters composition in multi-dimensional
space) and outlining geometry of the groups to access distances between their centers and
showing the limit dividing the space according to the assignment to one or another group. As the
result of analysis the original points aggregate in multi-dimensional space (which depends on the
number of parameters, applied for the classification - 10-dimensional in our case) is divided into
clusters or groups of similar objects. The object is meant as elementary
1o [40] But it clearly defines its properties and tasks being in fact comprehensive. [41] In current study calculations were performed in STATISTICA after the loading of the prepared data. It means the authors didn't go into detail of the algorithms, implemented in STATISTICA. The authors knew only a general procedure of classification, confined to parameters available in the user's menu of the program. The number of clusters N into which all the objects are to be divided presents the main parameter. The selection of an optimal number of clusters will be discussed below in Section 4.2. [42] Standardized parameters (see Section 3) for each lithospheric cell (see Section 6.1.), represented in form of the table, where columns show values of one of 10 parameters for each line, corresponded with cells, are the original data for the calculations. Then the matrix of distances between each pair of objects is calculated within the multi-dimensional space. Algorithm at a given number of required clusters N divides the entire set of objects into N clusters. The general idea of the procedure is the following. At first it is given such a measure (radius), which is greater than total space objects occupation, and using this radius any object could be reached from any of her object. Then the algorithm decreases it until the appearance of separation dense groups from the general "cloud'', when the mutual access between groups at the current radius becomes impossible. The method of groups densities and areas weights estimation won't be discussed here. Such procedure could also be carried out in opposite direction: minimum measure (the shortest distance between objects) increases until the aggregation of objects from given number of clusters (equal to number of objects) into N groups. [43] The above presents the main idea of clusterization using the physically simplest k -means clustering method. This method, realized in STATISTICA, fits our problem the best. STATISTICA suggests a variety of parameters and clusterization algorithms details, but their description is not essential for our work. 4.2. Approach to Criteria Identification for Attainment of Results[44] A brief description of the method showed that our task is aimed at breaking all the objects into stable and distinctly isolated N -number statistic groups with N number as large as possible. Each group contains a certain combination of all the parameters. Apparently, groups with distinct extreme for any parameter are the first to be singled out. Division using least pronounced variations starts only after the appearance of groups formed due to maximum values or values spanning the main variability range of each parameter. At this stage it is essential to find a moment when separation by statistically different mean values in areas outlined is replaced by "forced'' separation, i.e. extraction of clusters slightly differing in value, comparable to dispersion or parameter instrumental error within zone selected. This moment corresponds to the condition when the analysis procedure terminates estimation of environment linear heterogeneity (see Section 2.1.) and starts to analyze scattered heterogeneity. In this case, geodynamical interpretation of separate clusters seems to be useless and the analysis should be stopped at the current N value. A diversity assigned to scattered heterogeneity should be statistically estimated using characteristics of high-order moment type uniform for the entire area. The availability of physical validity and geological meaning for different parameters of each cluster will also be a criterion of the result accessibility. A set of values for characteristics given for each parameter for each of clusters is the solution of the stated geodynamical zonation problem.5. Algorithm of Geodynamical Classification[45] Data preparation for cluster classification includes the formation of spatially identical matrixes for all the parameters used (see Section 3) and their values standardization required for this algorithm accounted for calculation of distances (required uniform parameter dimension). Then the tabulated data is loaded into program environment. [46] The next step should be the classification test by minor N values. Here algorithm should step by step accomplish the classification of space analyzed into clusters, geologically valid. Starting with N =2 algorithm divides the area analyzed into oceanic and continental (shelf areas). At the next step ( N =3) the oceanic area is divided into basins and most elevated parts of MAR. During the following steps (up to N =5) successive isolation of MAR zone, including flanks and division into "cold'' and "hot'' parts takes place. [47] Starting from step N > 5 trivial solutions are followed by situations not visually discernible. For example, differentiation of basins, MAR flanks and continent-ocean transition zones appears. At steps from N =8 to N =10 flank MAR zones obliquely oriented and locally deeply incised into basins along with isolation of MAR zone north of Iceland and pseudosymmetric superimposed effects start to appear. Final stable differentiation of MAR zone as well as most of basins and continental margins into clusters with physically clearly specification takes place on steps N =11 to N =13. Steps from N =14 to N =15 show final extraction of nontrivial clusters superimposed on main oceanic structural elements. The parameters of these elements differ by a value above parameter scatter within isolated zones belonging to one of the clusters.
6. Geodynamic Interpretation of Results6.1. Clusters of Geophysical Parameters[49] A model of statistic cluster zoning for the Atlantic consisting of 15 stable combinations of parameters used (see Table 1) resulted from calculations using the procedure (see Section 4) and solution selection algorithm (see Section 5) based on parameters (see Section 3). The procedure being based on distance assessment in multi-dimensional space, calculations were made for standardized parameter (parameters of identical dimension with zero mean and unit variation). It is not quite clear if normalizing statistic moments should be calculated only for the study area or for the Earth as a whole. The authors preferred the latter version, because otherwise it would be difficult to quantitatively compare the results from different regions as parameter norm vary from area to area. All the data used in the present work are presented in matrixes for the entire Earth and parameter standardization was carried out for the above area. All the parameters in the study area have extreme values close to absolute minima and maxima except total seismic moment. Its mean values obtained from clusters by an order of magnitude 4-5 less maximum reported from the Pacific island arc zones not falling into the region studied. Nevertheless, this parameter has also been normalized by global value. It resulted in higher deviation of the parameter by clusters in the Atlantic (see Table 1). In such a case central values by a given parameter for clusters obtained become informative.
[50] Figure 11 shows cluster profiles for parameter central values in dimensionless standardized
coordinates according to calculated parameters. These profiles imply that each parameter value is
involved in cluster combinations reflecting so nearly all the main
[52] Geodynamical zonation of the Atlantic lithosphere resulted in obtaining of 15 stable clusters that could be conventionally divided into 4 unequal groups according to the main ocean structure zones: [53] group 1 for mid-oceanic ridge (7 clusters - 2, 5, 7, 10, 11, 8, 14); [54] group 2 for deep sea depressions (2 clusters - 6, 12); [55] group 3 for continental margins (4 clusters - 4, 9, 13, 15); [56] group 4 for superimposed effects (2 clusters - 1, 3). [57] Table 2 shows calculation for areas, occupied by each cluster, accounted for change in size of degree cell at high latitudes. [58] 74.75 mln km 2 was totally defined in the region. Cluster 6 (basin group) occupies the largest area of 16.06 mln km 2 (21.5%). These include basins with maximal water depth, sediment thickness, Bougue and Love anomalies, tomography by S-waves. Cluster 5 (ridge group) has a minimal area of 0.89 mln km 2 (1.2%). These are parts of high heat flow along the Mid-Atlantic Ridge; cluster 14 (0.84 mln km 2, 1.1%) and cluster 15 (0.57 mln km 2, 0.8%) present the region north of Iceland with high extreme values of P-waves tomography, superimposed on the ridge zone. [59] Comparison between the above zonation and segmentation of the Atlantic which can be made separately for each of the parameters (see Sections 3.1.-3.10.) shows that this zonation can't be carried out by means of classification using of or several parameters. Nevertheless there are parameters e.g. bottom topography or heat flow whose effect is stronger than that of the others. However, the visual analysis based only on bottom topography is not sufficient for reliable subdivision of the area analyzed: the ridge will exhibit less distinct structure, if only topography data are used, than if it is statistically compared. This may be attributed to the overloading of the topography as a geometric parameter with other parameters associated with energy release and geometry of the lithosphere inner boundaries. As a result we get a basis for geodynamic classification. Noteworthy that visual correlation leads to comparison of parameters by distinct extreme values of one or another parameter, whereas numerical correlation makes it possible to compare different background (average for separate regions) values in fact not visually discernible, but important in case of parameters to be obtained for extensive areas. 6.2. Description of Groups Obtained in Terms of Geodynamics[60] Now we turn to description and geodynamic interpretation of obtained cluster groups. For physical meaning of parameters the readers are referred to Section 3.6.2.1. Group of mid-oceanic ridges.
[62] Cluster 5. This cluster is characterized by maximal heat flow, maximal seismic moment,
tomography by S-waves close to minimal, and fairly high elevation. This cluster shows up (see
Figure 12, Table 1) in areas where deep plumes are in superposition to the Mid-Atlantic Ridge
structures (Iceland, Azores archipelago area, isolate shows north of the fault at 15o20
[63] Cluster 10. This cluster is characterized by high heat flow, high seismic moment, low tomography value by S-waves, and low gradient of the magnetic field. In general, cluster 10 has the same parameters as in cluster 5 but they are less pronounced. They differ in deeper elevation, and magnetic gradient, close to minimal. The cluster (see Figure 12, Table 1) shows itself in the same areas as cluster 5, in plan it "frames'' the distribution of extreme values physically and geodynamically characteristic parameters of cluster 10 are the same as those of cluster 5. The low magnetic field suggests, most likely, a low concentration of magnetically susceptible matter in the heated zone and fast disintegration of magnetic properties going from zones occupied by cluster 5. However, main parameters of the cluster correlate well (see Figure 13) with petrologic data. [64] Cluster 7. This cluster is characterized by minimal heat flow, fairly high seismic moment, low tomography value by S-waves, and minimal gradient of the magnetic field. In plan, cluster 7 covers in fact the entire area along the Mid-Atlantic Ridge (see Figure 12, Table 1) not occupied by clusters 5 and 10 (except for the site north of Iceland, occupied by clusters 8 and 14). Minimal heat flow in the Mid-Atlantic Ridge area not intersecting with deep plumes cropping out at the surface is easily explicable. According to the data by Podgornykh and Khytorskhoy [1997] heat flow values on the surface reflect in general the geodynamic state of the Earth interior at the measurement site (conductive component correlating with tomography data). However, along the Mid-Atlantic Ridge heat flow varies greatly due to irregular "convective'' component whose value is caused by water washing out of heat along the strong fractures in the nearby ridge areas, and due to heat discharge by degassing of the magmatic substrate [Letyshkov et al., 1997]. A high seismic activity occurs along the entire Mid-Atlantic Ridge as a result of processes responsible for emplacement of the young oceanic crust due to accretion of products of magmatic activity. However, in the main part of the Mid-Atlantic Ridge seismic activity is lower than in zones presented in clusters 5 and 10. Seismic events within cluster 7 area are not so often and of higher magnitude [Dmitriev et al., 1999], but in general manifestation the seismic moment is higher. Low value of tomography by S-waves is characteristic of the entire Mid-Atlantic Ridge due to the mantle uprise and formation of melts, but within cluster 7 it is lower than in case of "plume'' clusters. Based on the data by Dmitriev and Sokolov [2003], this area is marked by low production rate of basalt magmatism of spreading association caused by the adiabatic rise of the depleted mantle from a depth of below 200 km and its weak partial melting at depths of 15-30 km at a temperature of about I200o C. According to the classification of Wilson [1989] these basalts are assigned to N-MORB. Weak, in this case, minimal value of the magnetic field gradient is a characteristic feature of areas adjacent to the Mid-Atlantic Ridge (see Figure 10). This can be attributed to the fact that the field reduced to an altitude of 100 km reflects an integral characteristics of the layer commensurable to an altitude of field sources. In this case, a high frequency spatial magnetic field will be strongly smoothed. However, the main factor responsible for low value of this parameter we ascribe to a low total concentration of the matter in the heated zone with high partial melting in the upper mantle. [65] Geodynamically, cluster 7 is similar to clusters 5 and 10: environment with high energy capacity along with the rise of the mantle matter, basalt magmatism of different productivity rate, and high seismicity, specific variation of Bouguer anomaly (see Figures 4, 13), and tomography by S-waves. They differ fundamentally in discrete conditions of magma formation and P-T values. In this case change in space from one type to another may take place along the Mid-Atlantic Ridge within the range of 70-100 km. Figure 13 shows a close correlation of zones along the Mid-Atlantic Ridge with D1 > 255.5 (spreading basalt association) and geophysical parameters reflecting respectively the low crustal thickness, low elevation value, and high Bouguer anomaly. [66] Clusters 2 and 11. These clusters present flanks of the Mid-Atlantic Ridge and differ slightly in heat flow and seismic moment. They show a transitional zone from purely ridge clusters 5, 7 and 10 to the ocean floor clusters inheriting some features of the Mid-Atlantic Ridge clusters. In plan, this clusters (see Figure 12, Table 1) occupy a band of about 500 km on either side of the Mid-Atlantic Ridge, within the equatorial Atlantic they "strangle'' the Mid-Atlantic Ridge represented by clusters with high heat flow and form "a cold belt of the equatorial Atlantic'' [Bonatti et al., 1993]. The latter in all other parameters analyzed is similar to an intermediate type with respect to that of basins. All the above clusters of the group are, in general terms, isostatically compensated. Clusters 2 and 11 in the southern Atlantic are deeply incised into the area of basins and join clusters of the superimposed events (Martin Vaz, Rio Grande, Walvis Ridge) as will be discussed later. Cluster 2 having higher heat flow, than that of cluster 11, is more close to clusters of these superimposed events. Cluster 2 also has higher (though low in absolute expression) seismic moment, than cluster 11, implying that it encloses zones of intraplate earthquakes. Noteworthy, that zones of cluster 2 in general are oriented northwestwards, this agrees with published data on the intraplate earthquakes and north-western faults [Mazarovich and Sokolov, 2002]. Of interest also, that these zones branch off the Mid-Atlantic Ridge on sites of plume clusters 5 and 10. Thus, these clusters having resulted from the action of forces, whose superposition gives rise to a system of north-western spurs, start to show themselves from weakened and energetically active plume zones on the Mid-Atlantic Ridge and involve flanks and partly basins in the field of their activity. Of interest also is projection of zones of these clusters into the north-west striking Labrador Basin. [67] Based on other parameters, clusters 2 and 11 are presented as follows. Bouguer anomalies acquire values close to maximal of those in basins. Tomography by Love waves shows stable high values characteristic of the oceanic zones. Tomography by P-waves in fact does not differ over most of the region except its northern part. The sedimentary cover within margins (like that on the Mid-Atlantic Ridge) has a minimal thickness of about first hundreds of meters. Tomography by S-waves has low negative values characteristic of marginal zone. Elevation is close to the weighted average for the ocean. Magnetic field gradient is close to minimum. [68] Cluster 8. This cluster is marked by the lowest for the Mid-Atlantic Ridge Bouguer anomaly, heat flow above the average value, low tomography by Love waves implying greater thickness of the crust in the region. Besides, this cluster presents high seismic moment, minimal tomography by S-waves, and high elevation value. The cluster (see Figure 12, Table 1) takes up Iceland area. According to the above set of specific parameters this region should be assigned to cluster of type 5 or 10, but it was defined as a separate type. The thing is, that this region is superimposed by a zone of strong negative values, based on tomography by P-waves characteristic of collision areas of the Earth (see Figure 8). Besides, this cluster has high gradient of the magnetic field. The comparison between the main parameters for the region of this cluster and petrological data for the Mid-Atlantic Ridge (see Figure l3) does not doubt the plume nature of cluster 8. Moreover, a deep-seated nature of the heated zone in the Iceland plume area is well known and was widely discussed. The fundamental interpretation of the region is mainly out of question. Superposition of such a outstanding plume event as that in Iceland area, manifested as a specific extremum according to tomography by S-waves (see Figure 7) with a similar extremum according to tomography by P-waves (see Figure 8) seems quite unique for the Earth. This superposition allows to arrive at two different conclusions. First, due to outstanding nature of the Iceland plume event, this zone is the only place on the Earth's surface showing correlation of strong minima both by P- and S-wave tomography and having similar thermal nature. Second, these extreme still differ in nature. It means that the Eurasian collision zone, singled out by minimum of P-wave tomography, incorporates the Iceland area owing to its extensive front. The following facts favor for the second interpretation. The Azores plume, similar in degree of contrast in tomography, has no analogy to the above one, and two types of tomography show no correlation. The crustal thickness in Iceland area inferred from the DSS data reaches 30 km [Udintsev, 1990]; this igneous formation mass is fairly large to affect greatly the tomographic image by the presence of forces responsible for collision of the type existing in Eurasia. At present, the authors cannot give the ultimate answer to this question. A high magnetic field gradient in cluster 8 may be explained in the following way. The integral effect of the field at an altitude of 100 km does not result in complete averaging of the high frequency component because the magnetic anomaly pattern in Iceland area exhibits the presence of extensive zones with a homogeneous field stripped of linear magnetic anomalies [Udintsev, 1990]. Besides, an extreme deep type of basalt magmatism (TOP-Fe) was recorded in Iceland area [Dmitriev et al., 1999], which probably contributes to concentration of magnetically active matter, though the other factors within this cluster should decrease the value of the magnetic field. [69] Cluster 14. This cluster is marked by low Bouguer anomaly, high heat flow, maximal values of tomography by P-waves, large seismic moment, great average thickness of the sedimentary cover, positive value of tomography by S-waves, and high gradient of the magnetic field. In plan (see Figure 12, Table 1) this cluster occupies the northern Atlantic north of 71o N and nowhere else. Its main feature is the maximal value of tomography by P-waves occurring north of the minimal value of the same parameters in cluster 8 along with the combination of minima and maxima of P-waves in northern Europe (from the North sea to Scandinavia, see Figure 8). Besides, tomography by S-waves shows for this region a uniform chain of minima, that is in keeping with the Mid-Atlantic Ridge, but here there are positive values characteristic of cold oceanic and continental areas. Even within the cold equatorial segment of the Mid-Atlantic Ridge, tomography by S-waves has not been disturbed in such a way. Bouguer anomaly values are low there approaching to continental ones and extend for the entire width of the northern Atlantic, from Greenland to Scandinavia not showing a distinct minimum of the Mid-Atlantic Ridge (see Figure 4). A high heat flow and marked increase of seismic moment there, nevertheless point to an active phase of rifting. A great thickness of sediments suggests a proximity and activity of the source areas. A high magnetic field gradient has a pattern similar to that of the adjacent land. Such a set of main parameters in cluster 14 area implies that it is unique. The geodynamic interpretation of the region seems to be fairly difficult, and a possible solution may be an assumption of the original continental nature of this block, going now through the early phase of rifting. 6.2.2. Group of deep ocean basins.[70] Clusters 6 and 12 were assigned to this group (see Figure 12, Table 1). The interpretation of clusters is given below.[71] Cluster 6. This cluster is characterized by low Bouguer anomaly, average for the Earth heat flow, isostatic equilibrium, maximal value by Love waves, sediment thickness of about 700 m, positive tomography by S-waves, maximally deep topography, and low magnetic field gradient. In plan (see Figure 12) the cluster takes up areas of ocean basins as reflected in deep and almost flat bottom topography, but cluster occupies less square compared to generally accepted because it does not include basin areas occupied by tails of the Mid-Atlantic Ridge flank clusters, as well as projection of the continental rise, assigned to a different class of clusters. Minimal values of Bouguer anomaly are indicative of cold and dense state of the lithosphere. Besides, along with the minimal seismic moment and average heat flow point to geodynamic "state of rest'' in which these areas mainly reside. Maximal and positive values respectively by S-waves and Love waves show that this area is typically oceanic and not affected by deep energy transferred across the plume system. It lies very deep (averaging -5012 m) and is overlain by pelagic sediments averaging 685 m in thickness. Disturbances of "the state of rest'', caused by vertical movements, are isostatic in nature; north-west strike slips; delivery of energy of plumes, and other superimposed events, are formalized in other types of clusters. [72] Cluster 12. This cluster is a modified cluster 6 presenting the transition from ocean basins to superimposed (see 6.2.4.) or plume areas in the absence of clusters 2 and 11 in plan (see Figure 12) essentially represented as a pair symmetrical to the Mid-Atlantic Ridge (see also Figure 10) suggesting an impulse appearance of this part of the lithosphere in the Mid-Atlantic Ridge area and its further division due to spreading. It occurs in symmetrical formations such as the Agulhas plateau: north of South Georgia islands - Walvis Ridge (northern part) - Rio Grande Rise, areas south of the Blake plateau and south of Guinea plateau, parts of Newfoundland and south of Josephine bank, in Labrador basin area and southern spurs of Rockall plateau, Western flanks of Greenland and Norwegian basin. The main features there are a high magnetic field gradient reaching that of the continental field and Iceland area (see Figure 10). In addition to the above pairs, occurring in the study area, there are also pairs covering mainly the continent. It means that the lithosphere may owe its origin to magmatism similar to that of Iceland, i.e. deep ferric varieties of basalts with emplacement of covering sediments exhibiting extensive distribution and similarly magnetically oriented, therefore they haven't been averaged by the field record from satellite. In this case, products of deep plume magmatism were laying both on the young oceanic lithosphere and on continental lithosphere subjected to rifting. However, magmatism of this type has not "healed'' the entire rifting zone. The ascent of deep mantle plumes to the surface was spartially a fairly rare event, nowadays basalts of plume association present an episodic event, whereas spreading association basalts are common all over the Mid-Atlantic Ridge. [73] The other parameters of cluster 12 are confined to its primary magmatic nature. There were high elevation caused by high rate of magmatic activity, high, as compared to that of basins, seismic activity, because of the proximity of cluster 12 areas of active superimposed events; not very high values by Love waves, low value of Bouguer anomaly, and a greater thickness of sediments because cluster 12 areas are located around the periphery of the ocean. Geodynamically, this cluster presents relict effect of paleoplumes on the Atlantic lithosphere. 6.2.3. Continental margin group.[74] This group includes clusters 4, 9, 13, and 15 (see Figure 12, Table 1). The interpretation of these clusters is the least conjectural.[75] Cluster 4. In plan this cluster (see Figure 12) is located on the continental shelf very close to the land or to the source area. The distinctive features of the cluster are: anomaly Bouguer of 56 mgal implying its continental nature, because its value of 175 mgal is the most acceptable value for of separation of the continent from the ocean. Love wave value is below zero and along with positive values by S-waves they argue for continental nature. Heat flow is close to the average. Isostatic anomaly is slightly higher because it reflects loading of sediments on the crustal and mantle substrate not fully compensated in the course of downwarping. The thickness of sediments averages 2041 m at an average water depth of 561 m. The magnetic field gradient is high implying the effect of the continental basement rocks. The geodynamic meaning of the cluster is the operation of the process responsible for isostatic geometric smoothing of the crustal block underlain by viscous substrate resulted from increase of sediment load. [76] Cluster 13. This cluster essentially inherits all the features of cluster 4 except value for elevation and sediments. Sediment thickness averages at maximum 6491 m at an average elevation of -1317 m. It places (see Figure 12) cluster 13 on the shelf edge and into the upper part of the continental slope. Whereas compensation downwarping in the cluster area gave rise to a great submergence of the substrate, the intensity of isostatic process being the same as that of cluster 4. [77] Cluster 9. This cluster inherits features of cluster 13 and is located offshore from cluster 4 and cluster 13, respectively. Main parameter values are as follows there. Bouguer anomaly is high as compared to that of clusters 4 and 13 (26l mgal), it means that sediments rest there on the peripheral oceanic crust which cold and more dense, than the continental crust. These factors do not result in down-warping reflected in isostatic anomaly. The magnetic field gradient also has features similar to those of the ocean basins. The average thickness of sediments equals 3273 m at an average elevation of -3747 m. So, this cluster in plan (see Figure 12) occupies areas of the continental rise and adjacent part of ocean basins filled in sediments. [78] Cluster 15. This cluster is similar in geodynamics with that of clusters 4 and 13; but based on the following reasons, is set aside as a separate type. In plan (see Figure 12) it occurs only in the northern Atlantic along the periphery of also a unique cluster 14, whose peculiar features affect those of the continental margin cluster showing a great thickness of sediments averaging 6040 m. Bouguer anomaly in cluster 15 is typical of continents and equals 57 mgal. Isostatic anomalies within the cluster area (see Figure 12) are marked by difference in direction resulting in neutral average value for the cluster as a whole. Strong positive anomaly inferred P-wave tomography extends to cluster 15 area as well. Elevation averages 542 m. The magnetic field gradient was found to have high value characteristic of the northern clusters in the Iceland plume area. In terms of geodynamics this cluster is meant to combine features characteristic of the continental margin clusters with those of the anomalous northern block as a substratum for intense sedimentation. 6.2.4. Group of superimposed events.[79] The group includes clusters 1 and 3 (see Figure 12, Table 1). Their interpretation seems to be the most nontrivial.[80] Cluster 1. This cluster is marked by low, as compared to those of basins, Bouguer anomaly, maximal isostatic anomaly for the Atlantic, and high seismic moment. The remaining parameters have similar average values. In plan (see Figure 12) this cluster presents areas superposed on the basin and flank zones of the Mid-Atlantic Ridge. Locally, these areas form separate extensive groups (Walvis Ridge, Rio Grande, Cape Verde Islands, Cameroon line), but essentially always they occur as. pairs pseudosymmetrical about the Mid-Atlantic Ridge. They might well be tracks of the plume magmatic events that took place in the Mid-Atlantic Ridge area and occurring in the course of spreading on the opposite sides from the divergent zone of the ocean. Strictly speaking, these formations may be called microscopic "bull's eyes'' or be considered as traces hotspots [Courtillot et al., 2002] that functioned over a period of time under the Mid-Atlantic Ridge system, hence their asymmetry on the opposite sides of the ocean. Zones occupied by cluster 1 show up as volcanic edifices, some of them are still active. The edifices have high rate of magmatic activity that gives rise to excessively massive formations above the compensation surface within the ocean floor. This provides an explanation of extreme positive value of isostatic anomaly in the volcanic edifice area, as well as low Bouguer value as compared to background value for basins. High seismic moment may be attributed to the intense recent volcanic activity. The geodynamic meaning of this cluster is interpreted in a way similar to that of clusters 5 and 10 (and to a certain extent cluster 12) as resulted from deep plume energy release. However, in this case it is not aligned with the Mid-Atlantic Ridge system whose heat flow is slightly higher than an average for the Earth and differs in magma composition. Based on the data by Sobolev and Nikogosyan [1994], Sobolev [1997], parental melts of oceanic intraplate igneous products are resulted from the mantle uprise from a depth of about 1000 km and its melting at depths of 100-130 km, at a temperature of l400-l650o C. [81] Cluster 3. This cluster (see Figure 12) represents mainly a sublatitudinal structure superimposed on ocean basins and the Mid-Atlantic Ridge Zone, it differs in two features from the cluster area on which it is superposed. It has extreme maximal values of Bouguer anomaly and extreme minimal value of isostatic anomaly, such a combination of the above parameters has been recorded only in the fore-arc zones, e.g. of the Pacific Ocean, when extensive thrusting in the island arc area, when ahead of their front there are formed zones with isostatic mass deficit and high Bouguer anomaly. Noteworthy, that based on data of Silantiev [2003] igneous rocks, having no equivalents in recent magmatism of the northern Mid-Atlantic Ridge, match these anomalous sublatitudinal zones at 15o N and 25o N. According to this author, products of magmatic activity in these zones might well be formed in the presence of the subcontinental mantle substrate or due to active mixing of melting products and the lithospheric matter as exemplified by the subduction zones. Such a coincidence of geophysical and petrologic characters suggesting not a quite ordinary environment for the Atlantic is hardly to be considered casual. The authors of the present paper think possible to make a tactful statement concerning the change in the Altantic of the horizontal movement vector from sublatitudinal to sublongitudinal during the recent epoch. The GPS and VLBI data reported from the adjacent continents make the above statement more reliable. 6.3. Comparison of the Results Obtained With Main Notions About the Atlantic Ocean Geodynamics[82] As was mentioned in the introduction, the accumulation of geological-geophysical data on the structure of the Atlantic ocean floor made possible constant recognition of facts not easily explicable in terms of classical geodynamic model for the ocean. The model showing the interaction between the lithosphere formed in the divergent zone and main forces, used in the classical model, which are responsible for the present day tectonics of the ocean. The present paper is not aimed at the discussion of the alternative mechanisms, therefore we have proposed a version to systematize geodynamic environments of the Atlantic with account of new data available. It was done unbiased by means of digital methods and have proposed a geodynamic interpretation of the obtained clusters following the principle of "maximum likelihood'' by data interpretation. However, alternative interpretations are not ruled out. First, it should be noted that the authors have obtained a set of clusters reflecting geodynamic features of the ocean structural elements easily explicable in terms of the plate tectonic model. Their description is given in Section 6.2. and presents a small part of the results of the investigation. The authors also obtained a number of clusters that can be explained only by involving geodynamic mechanisms and by means of considerable modification of the existing mechanism. But this goes beyond the scope of this paper. We shall just emphasize the nontrivial results of cluster analysis.
[84] The cluster classification of the Atlantic geodynamic lithosphere resulted also in establishing of zone oblique to major structural elements (flank clusters, see 6.2.1.). These zones have been repeatedly mentioned as inferred from detailed geophysical survey in the Atlantic [Mazarovich and Sokolov, 2002]. Their presence have found confirmation in macroscopic description of the lithospheric geodynamic parameters implying fundamental cause-and-effect relationship between parameters of different scale. The north-western orientation correlates with that of many small structural elements inferred from multi-beam survey of the ocean floor and from seismic profiling of sediment deformations. Geodynamically, it means that the actual tectonosphere has a system of forces tangential to the earth's surface and not orthogonal to the Mid-Atlantic Ridge, responsible for this phenomenon. The working geodynamic model cannot provide an explanation for this phenomenon.
[85] A combination of cluster 3 parameters (see 6.2.4.) characteristic of the fore-arc zones is
considered to be nontrivial. The sublatitudinal orientation of this cluster zones really suggests the
presence of the sublongitudinal movement component that is not consistent with the working
geodynamic hypothesis. An agreement with exceptional geochemical data ( [Silantiev, 2003]
is hardly to be accidental. Besides, in this cluster area near the fault at 15o20
[86] Anisotropy of sediments in the Atlantic basins [Mazarovich and Sokolov, 2004] represented by foldings, discernible only on sublongitudinal seismic profiles, also implies the presence of the sublongitudinal component of movement. GPS and VLBI data on the adjacent continents support to such pattern of movement. The above-listed set of factors made us possible to speak with confidence about the phenomena whose presence seems imperative in terms of classical plate tectonics theory. The authors are not going to develop in this paper the ideas about a mechanism responsible for their origin, but it should be pointed out, that the classical theory may be applied if it is granted that the crucial changes in all movement components began during the present epoch. 7. Conclusions[87] 1. Zonation of the Atlantic Ocean lithosphere based on cluster analysis involving 10 geological-geophysical parameters, interpreted geodynamically and classifying the lithospheric structure and energy release allowed us to divide the region into four groups of clusters (all in all 15 cluster combinations of parameters). They cannot be established visually using any parameter or their limited combination. The resultant groups exhibit geologically specified features:
[88] 2. Inhomogeneity along the strike of the ridge is an alternation of "hot'' and "cold'' blocks differing greatly in heat flow, total seismic moment, Bouguer anomaly, as used here, signifies the rate of magmatic activity, and tomography by S-waves, marking a degree of the mantle partial melting along with the presence of isolated plumes going deep into the mantle (up to 700 km). The recognized zones correlates well with published data on discreteness of P-T conditions during the formation of the oceanic ridge basalt melts, whose areas are not widely spaced. The above implies superposition of two independent mechanisms for accretion and further dynamics of the oceanic crust in the Mid-Atlantic Ridge zone. [89] 3. On the Mid-Atlantic Ridge flanks there were recognized zones north-western oriented to major structural elements of the Atlantic, crossing ocean basins, the Mid-Atlantic Ridge and projecting into the continental margins. [90] 4. Superimposed sublatitudinal phenomena outside of the Mid-Atlantic Ridge refers to zones resulted from eruptive impulses of high rate magmatic activity (sometimes ongoing); they are similar to the recent manifestation of plumes under the Mid-Atlantic Ridge, caused by spreading of pseudosymmetric structures on either side of the ridge. Vestiges of these phenomena inferred from geophysical parameters imply that frequency rate of such manifestations in the Atlantic lithosphere remained, as a whole, unchangeable since the time of its opening. [91] 5. Another type of sublatitudinal superimposed phenomena is represented by zones marked by a stable distinct combination of high Bouguer and low isostatic anomalies similar to that of the Pacific fore-arc zones. This cluster type correlates in space with anomalous geochemistry of basalts, convergent zones of passive parts of transform faults, sediment anisotropy of ocean basins, and orientation of the western Atlantic subduction zones. 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Keywords: Geodynamic zonation, cluster analysis, mid-oceanic ridge zone. Index Terms: 3235 Mathematical Geophysics: Persistence, memory, correlations, clustering; 8120 Tectonophysics: Dynamics of lithosphere and mantle: general; 8130 Tectonophysics: Heat generation and transport; 9325 Geographic Location: Atlantic Ocean. ![]() Citation: 2008), Geodynamic zonation of the Atlantic Ocean lithosphere: Application of cluster analysis procedure and zoning inferred from geophysical data, Russ. J. Earth Sci., 10, ES4001, doi:10.2205/2007ES000218. (Copyright 2008 by the Russian Journal of Earth SciencesPowered by TeXWeb (Win32, v.2.0). |