Tectonic Evolution of the FAMOUS Area of the Mid-Atlantic Ridge, Lat 35°50' to 37°20'N

Tectonic evolution of the FAMOUS area of the Mid-Atlantic Ridge, lat 35°50' to 37°20'N

IVAR B. RAMBERG* Department of Geology, Stanford University, Stanford, California 94305 DALE F. GRAY Department of Geophysics, Stanford University, Stanford, California 94305 ROBERT G. H. RAYNOLDS Department of Applied Earth Sciences, Stanford University, Stanford, California 94305

This article is one of a series appearing in the April and May issues of the Geological Society of America Bulletin on the scientific results of Project FAMOUS. These studies were undertaken in the axial area of the Mid-Atlantic Ridge between approximately 36°30' and 37°N latitudes.

ABSTRACT During the period 1972 to 1974 the FAMOUS area was surveyed by nearly all available techniques, including some that have rarely The FAMOUS area, which straddles the Mid-Atlantic Ridge on before been applied to scientific investigations of this type, such as the southwest extension of the Azores Plateau in the Atlantic a multiple narrow-beam array acoustic survey; large-area photo- Ocean, has been studied by means of morphotectonic and magnetic graphy (LIBEC); and long-range side-scan sonar (GLORIA). anomaly analysis. Major morphologic elements are (1) the rift val- Through the generosity of several investigators, in particular J. D. leys and associated fault blocks, (2) the transform faults, and (3) Phillips of Woods Hole Oceanographic Institution and H. S. Flem- diagonal linear trends intersecting the azimuths of the rift valleys ing of the Naval Research Laboratory, contour maps of the region and transform faults. Within the past 6 m.y. the area has been for several parameters — such as regional bathymetry at a 100-fm characterized by a reorientation of the spreading axis, from an ob- contour interval, detailed bathymetry at a 5-fm contour interval, lique trend toward an orthogonal pattern relative to the transform magnetic anomalies contoured at 100 y and a sediment isopach faults. The long and nearly continuous early spreading axis, with a map based on seismic reflection data — were made available to us. strike of N50 °E, broke up into smaller rift segments, which prog- We did not have available to us the original geophysical profiles ressively rotated to their present strike (about N23 CE) and now along ships' traverses. It is on this data, as well as on published in- appear to be recombining through asymmetric spreading to form formation, that the following interpretation of the structure and the continuous axis it now has. The process involved complex mi- tectonic evolution of the FAMOUS area is based. The same data grations of the larger rift segments through asymmetric spreading, has been the subject of independent interpretations with different as well as jumping of shorter rift segments and migration of trans- emphasis by other investigators (Laughton and Rusby, 1975; Phil- form faults, the latter mechanism leading to the diagonal troughs. lips and Fleming, in prep.; Phillips and others, in prep.). The presence of parallel spreading axes and the propagation of rift axes may be due to coupling across transform plate boundaries. MORPHOLOGY OF THE FAMOUS AREA These boundaries are complex shear zones with tension cracks and en echelon fault scarps deviating from the overall trend of the zone; The FAMOUS area forms the southwestern part of a rift system this implies that the fracture zones might be leaky. The reorienta- that splits the Azores Plateau in two (Fig. 1). In this area, the rift tion of the spreading axis in the FAMOUS area seems not to be a system consists of a series of rift valley segments 20 to 45 km in unique event but a phenomenon that has been repeated periodi- s s- s cally. It has tentatively been correlated with recurrent plume activ- |36°W I [34 I [32 ! [30° I [28 I IS> ity below the Azores Plateau.

INTRODUCTION

Segments of the Mid-Atlantic Ridge south of the Azores (Fig. 1) have been intensely investigated as part of the French-American Mid-Ocean Undersea Study (Project FAMOUS). The FAMOUS area, and in particular one of its rift valley segments, has con- sequently become the best-known part of the oceanic rift system (ARCYANA, 1975; Ballard and others, 1975; Bellaiche and others, 1974; Detrick and others, 1973; Laughton and Whitmarsh, 1974; Laughton and Rusby, 1975; Macdonald and others, 1975; Moore and others, 1974; Needham and Francheteau, 1974; Phil- lips and Fleming, in prep.; Phillips and others, in prep.; Renard and others, 1975; Reid and Macdonald, 1973; Spindel and others, 1974). The region of the Atlantic surrounding the FAMOUS area has been considered in several papers (Francheteau, 1973; Laughton and Whitmarsh, 1974; Pitman and Talwani, 1972; Schil- ling, 1975). Figure 1. Index map of the Azores triple junction area. The FAMOUS area lies on the southwestern tip of the plateau. Depths above 2,000 m are * Present address: Department of Geology, Oslo University, Blindem, Oslo 3, Nor- shaded. Fracture zone and rift valley system shown in detail only within the 'way. FAMOUS area.

Geological Society of America Bulletin, v. 88, p. 609-620, 13 figs., May 1977, Doc. no. 70501.

609

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/5/609/3418333/i0016-7606-88-5-609.pdf by guest on 27 September 2021 Figure 2. Bathymetry of the FAMOUS area: contour interval of 100 fm (183 m) not corrected for changes in sound velocity in sea water. Thick double lines represent spreading axes; dashed lines indicate fracture zones. Depths below 1,300 fm (2,379 m) are shaded dark; above 800 fm (1,464 m) are shaded light. Roman numerals and dotted lines identify areas discussed in text. Contours after Phillips and Fleming (in prep.) are based on narrow-beam and conventional acoustic surveys. Scale in kilometres.

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length trending about N23 CE ± 6°. They are offset in a right-lateral Sleep (1977) has interpreted the extension of rift valleys across sense by short fracture zones 15 to 25 km long that trend N84 °W fracture zones as a consequence of the hydrodynamic forces creat- ± 5°. This present rift-transform system deviates by about 15° to ing the rift valley. He has further argued that, in the case of oblique 20° from an ideal, low-energy (Lachenbruch, 1973; Vogt and spreading, the asymmetric distribution of tension fields relative to others, 1969) orthogonal configuration. On the basis of a position the transform faults would tend to align the spreading direction of the pole of rotation for Africa-North America at about lat more perfectly and would create short transform faults. The 58 °N, long 36 °W (Morgan, 1968), the transform faults deviate theoretical dimensions of the extensions discussed by Sleep are from the predicted east-west direction by about 5°. More recent within the range of those observed in the FAMOUS area. pole determinations by Minster and others (1974) reduce this de- Along strike, rift valleys and fracture zones in the FAMOUS area viation to less than 4°. are convex upward, and the deepest depressions occur at their in- A dominant morphologic feature is formed by the high crestal tersections. Thus, the ridge system is composed of a series of domed mountains (Fig. 2) that slope from the edges of the rift valleys to the segments with the maximum heights always at greatest distance subdued, low terrain at about 2,300 m or deeper on both sides from the two bounding fracture zones. This domed aspect is not within a distance of 50 to 80 km. Superimposed on this first-order restricted to the inner rift valleys but applies to the crestal portions feature are three major linear elements: (1) the rift valleys, (2) the as well. As a result, the central rifts show a characteristic outline, transform faults, and (3) diagonal linear features trending N 65° with a narrowest and shallowest central portion (Fig. 2). to 70 °E. These trends are discussed below. In addition, shallow In cross section, the inner floors, terraces, and walls are all com- plateaus above 1,400 m (800 fm) flank the northern part of the posed of blocklike linear steps parallel or subparallel to the rift area in the northwest and east. axis. The scarps facing the axis tend to be steep, commonly around 30° or more; dive data show (Ballard and others, 1975) that they Rift Valleys and Fault Blocks are composed of fault faces with dips of as much as 60° to 80° (see also Bellaiche and others, 1974; Macdonald and others, 1975). The The approximately N23 °E rift trend can be recognized as a pre- upper surfaces of these blocks dip outward at angles of 3° to 10°. dominant linear feature extending beyond the transform limits of The regional and local scarps and blocks can be interpreted as the the rift valley segments. Ridges, scarps, and troughs slice the crestal result of normal faulting related to the uplift of the walls of the rift mountains into subparallel blocks, implying the presence of an ex- valleys, but hills and ridges with slopes of as much as 30° that are tended region of block faulting far outside the present rift valleys. superimposed upon them are probably the result of constructive At a distance of 45 to 55 km from the rift axis the present trend volcanism, as are the ridges and hills of the inner rift floor, includ- changes into a more northeasterly to southwesterly direction ap- ing the central ridges. These tectonic and volcanic interpretations proximately paralleling the southwestern extension of the Azores rest primarily upon detailed topographic, deep-towed geophysical, Plateau (Fig. 1). This change in trend toward the periphery of the and submersible studies of the inner valleys of rift valleys 2 and 3 FAMOUS area is strongly reflected in the magnetic lineaments, as (ARCYANA, 1975; Ballard and others, 1975; Ballard and van An- discussed below. del, 1977; Bellaiche and others, 1974; Bryan and Moore, 1977; The FAMOUS area contains four rift valley segments designated, Laughton and Rusby, 1975; Macdonald and others, 1975; Need- from north to south, 1 through 4 (Fig. 2). The segments display a ham and Francheteau, 1974; Ramberg and van Andel, 1977). rather similar overall topographic character. Using the categories of Needham and Francheteau (1974; Macdonald and others, Transform Faults and Fracture Zones 1975), the following physiographic provinces can be recognized in all of them: (1) the inner floor, usually below 2,400 m (~ 1,300 fm), The three small fracture zones (A, B, and C; Fig. 2) that offset the with a central ridge, (2) the inner walls, steep inward-sloping scarps rift valley segments form deep but indistinct depressions 20 to 30 that separate the inner floor from (3) the terraces, undulating, rela- km wide. Their topographic identity is lost at rather small distances tively flat, elevated areas 5 to 15 km wide at a depth of about 1,800 from the rift-to-rift segments, the maximum extent beyond a rift ± 300 m, and (4) the outer walls, opposing scarps at about 1,300 valley being about 30 to 35 km in the eastern portion of fracture ± 200 m, which form the limits of the rift valley and the inner edge of (5) the rift shoulders or crestal mountains. From the crestal mountains the ridge flanks slope outward to the deep Atlantic. So defined, the widths of rift valleys 1 through 4 from rift shoulder to rift shoulder are 32, 31, 25, and 26 km respectively. Within the valleys, however, the widths and heights of the various units Fracture zone vary more widely and unsystematically. Within the FAMOUS area, the eastern outer wall of each segment tends to be aligned with the western inner wall of the next segment to the north (Fig. 2), thus forming scissor faults (faults in which adjacent blocks plunge in opposite directions) symmetrical about the fracture zones (Fig. 3). Although this may be coincidental, it implies an imperfect decoupling across the transform plate boundaries, and the amount of transform offset may be closely related to the morphotectonic characteristics of the rift valleys. Imperfect decoupling is also suggested by the fact that the de- Rift Valley pressions of the inner-floor provinces tend to continue past one Rift another across fracture zones into opposing plates. This is best Valley exemplified by rift valleys 2 and 3 at fracture zone B (Figs. 2, 4) but Figure 3. Sketch illustrat- can be observed in other fracture zones as well. These extended de- ing apparent coupling of ad- pressions cut far into the crestal mountains of the adjacent rift seg- jacent rift valley segments Figure 4. Sketch illustrating extension ments. This topographic evidence suggests that continued propaga- where outer and inner walls of rift valleys across fracture zones into tion of the depressions might eventually lead to a splitting apart of of adjacent segments form opposite plates and rift-to-rift transition the adjacent plate and a consequent jump of the spreading axis. scissor faults. accomplished by en echelon small rifts.

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zone B. Thus, only the active segments between rift axes are well ridges, but the principal expression of these transverse structures defined. occurs in the magnetic anomaly patterns. Whereas the general topographic trend of the fracture zones is about N84 °W, the presence of zones of microearthquake activity Diagonal Linear Trends in fracture zones A and B (Reid and Macdonald, 1973; Spindel and others, 1974) indicates active faulting along a direction of about A trend of approximately N65° to 70 °E can be recognized as N70 °W. Deep-towed geophysical studies (Detrick and others, persistent lineaments as much as 50 to 60 km long (Fig. 2). They 1973) and side-scan sonar data (Laughton and Rusby, 1975) show are very apparent on the east (African) side, whereas on the west a third trend marked by smaller scarps oriented approximately they are absent except in a mountain range on the northwestern east-west or N88 °W. Detailed bathymétrie charts based on border of the FAMOUS area. This mountain and its eastern coun- narrow-beam surveys (Phillips and Fleming, in prep.; Renard and terpart form the southernmost extension of the Azores Plateau others, 1975) show these topographic features clearly (AR- (Fig. 1). CYANA, 1975). Fracture zone A has a single east-trending scarp The diagonal lineaments on the east side are troughs that merge on its axis in the western part and two en echelon scarps in the east. with the structure parallel to the rift axes at some distance from the These define an overall transform azimuth of N84 °W. North- and axis, defining slightly curved patterns. The troughs consistently ap- northeast-trending scarps cross the fracture valley floor to within pear to branch off from the present fracture zones. They are com- 0.5 km of the transform axis. Almost right-angle scarps are commonly zig-zag in shape and characteristically contain the thickest mon where the walls of the rift and fracture valleys meet (Laughton sediments (as much as 300 to 600 m) in the region (Fig. 6), thus and Rusby, 1975). implying that they are at present inactive or at least fairly old Fracture zone B is a broad depression (Fig. 5) with lineations that tectonic features. Discounting the sedimentary fill, the depths of alternately parallel transform and rift trends. Again, there is evi- these troughs equal those of the fracture zones and inner rift val- dence for parallel and en echelon fault scarps. Scarps and troughs leys. A complementary northwest trend can be discerned west of with an approximately east trend occur interspaced with north- the ridge axis, but its topography is obscured because the ridges trending ridges over a 6- to 8-km-wide zone. The southern part of and depressions making up the flanks of the Mid-Atlantic Ridge are the zone connects the southern tip of rift valley 2 with a marked perpendicular to this trend. kink in rift valley 3 at a point where the trend of rift valley 3 changes from N18 °E to a slightly more easterly direction. Inner Floors and Central Ridges Fracture zone C at the southern end of rift valley 3 is anomalous. Instead of a reasonably well defined fracture valley, the transition Rift valley 2 has a well-defined, single, central volcanic ridge from rift valley 3 to 4 is accomplished by two small (15 km) inter- consisting of several volcanic hills (Ballard and others, 1975; mediate steps consisting of troughs parallel to rift valley 3 and Laughton and Rusby, 1975; Moore and others, 1974; Needham separated from it and from one another by ridges that are as much and Francheteau, 1974). Bottom photographs and observations as 500 m high. In this zone the inner floor (18 km wide) is more from submersibles (Ballard and others, 1975; Ballard and van An- than twice the width of the floor farther north. There is no detailed del, 1977; Bellaiche and others, 1974; Phillips and others, in prep.) bathymétrie data for the southernmost part of this transition zone have revealed that this ridge consists of the youngest lava flows on near rift valley 4, but as far south as lat 36°14'N, there is no evi- the inner floor and that the flows are not deformed by faulting, dence for identifiable transform trends (Laughton and Rusby, although they are intensely fissured. Away from the central axis, 1975). sediment cover, manganese coatings, and palagonite crusts increase Outside the active transform zones the approximately east trend in thickness (Bryan and Moore, 1976), and uplift and truncation by of the fractures is poorly represented. Close inspection reveals that faults assume significant proportions. As a result, the central ridge short lineaments parallel to this trend exist in the topography of the is irregularly lobate in outline, whereas the volcanic hills near the foot of the inner walls are truncated to straighter outlines parallel to the rift axis. In contrast, rift valley 3 is much broader (8 to 9 km instead of 3 to 4 km) and contains five or six discontinuous subparallel ridges, which, like the central ridge in rift valley 2, consist of individual hills 0.5 to 1 km wide and 2 to 4 km long. The long axis is parallel to the rift axis, and the hills rise 200 to 300 m above the inner floor. One of the ridges has the irregularly lobate outline typical of the central ridge of rift valley 2, whereas the others are truncated by fault scarps. By analogy with rift valley 2, the ridges of the inner floor of rift valley 3 have been interpreted as originally having been formed by constructive volcanism at the rift axis and subsequently fractured, faulted, and somewhat uplifted during transfer to the inner walls (Ramberg and van Andel, 1977). Similar ranges of volcanic hills can be observed on the steps of the inner walls and the terraces beyond, although their recognition becomes more difficult with increasing distance from the axis because of faulting and uplift. The most irregular and lobate of the ridges in rift valley 3 (Fig. 7) is likely to represent the active rift (Ramberg and van Andel, 1977). Its volcanic hills do not lie in a simple straight line but are scattered over a 2- to 3-km-wide zone of the inner floor. Shape and distribution suggest an irregular and discontinuous occurrence of the shal- Figure 5. Details of bathymetry of fracture zone B [50-fm (91-m) con- lowest part of the underlying magma chamber. The good preserva- tour interval]. Area below 1,150 fm (2,100 m) shaded. Simplified after tion of individual hills, with fault scarps located generally on their Fleming and Phillips (in prep.). lower slopes, confirms that, as postulated by Ballard and van Andel

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crust, the anomalies are longer than in the interior, indicating that IP the old spreading axis was essentially continuous. Subsequently, it ^ FAULT broke into smaller segments and changed strike from the initial direction of N50 °E to about N23 °E. In section IV (Fig. 8) this change was completed before 5 m.y. B.P., and in section II, farther to the north, at about 3.5 m.y. B.P. The new orientation established a less oblique relation with respect to the transform faults. A total change of about 27° has occurred during the past 6 m.y., with most of it taking place during the period from 5 to 3 m.y. B.P. Additional reorientation seems to be taking place at present, according to the evidence presented below.

Identification of Magnetic Anomalies

In order to identify the magnetic anomalies of Figure 8 and corre- late them with the magnetic polarity reversal time scale, simulated anomaly profiles based on a spreading rate of 1.3 cm/yr and a magnetic layer thickness of 500 m (Marshall and Cox, 1971) were prepared with a computer program for two-dimensional magnetic bodies (Talwani and Heirtzler, 1964). The profiles were corrected for the effect of topography, using data from Figure 2. In the diagonal and transform troughs, distinct anomaly patterns are absent, possibly because of numerous faulted, normal and reversed magnetized blocks (which interact to smooth out the field), since we found that the maximum water depth in these areas is too small to erase the magnetic patterns. In the FAMOUS area, the interac- tion between the magnetic fields of the crustal blocks and the present regional field causes small shifts of the anomalies toward the southwest. This shift was measured and compensated for in as- signing ages at the reversal points. Most anomalies were readily identifiable, but occasionally this was not the case, probably as a result of a complex local history of strike change and spreading- axis jumps. In certain subregions the computed profiles gave a poor fit, for example, in the western part of section III (Fig. 8). These subregions were excluded from the calculations of the spreading rates. Figure 9 summarizes our interpretation of the spreading history. morphology based on detailed bathymétrie chart with 5-fm (9-m) contour interval (Phillips and Fleming, in prep.). After Ramberg and van Andel (1977). Spreading Rates

(1976) for rift valley 2, rifting occurs on one or the other side of the Distances from spreading axes parallel to the observed N84 °W completed volcanic body and that the volcanoes are transferred trend of the transforms were plotted against reversal ages (Cox, and uplifted as units onto the walls and terraces. Hence the rifting 1969) for a number of traverses where anomalies had been iden- axis flips back and forth within the central valley. It is also evident tified with the greatest confidence (Fig. 10). Small changes in the that the trend of the central ridge complex is more closely north- azimuths of these traverses (±5°) have little effect on the spreading south (N15 °E) than the inner walls, which follow the average rift rates. Slopes of the lines of best fit to each data set give the spread- trend. ing rate with an estimated precision of ±0.1 cm/yr. In the northern part of the FAMOUS area (Fig. 10, traverses W2, MAGNETIC ANOMALY PATTERNS E2a, and E2b), spreading rates are strongly asymmetric: 1.3 cm/yr. to the east and 1.0 cm/yr to the west. This compares well with rates The magnetic anomaly data consist of a processed contour map estimated by Needham and Francheteau (1974; 1.3 cm/yr to the with a 100-y contour interval after removal of the regional field, east, 0.9 cm/yr to the west) and by Greenewalt and Taylor (1974; constructed-from surface magnetometer tows. Track locations and 1.3 and 1.0 cm/yr to the east and west, respectively, both based on details on the methods of data processing are reported by Phillips the Brunhes-Matuyama boundary). Macdonald (1975) gave simi- and Fleming (in prep.). Unprocessed data and magnetic anomaly lar values for rift valley 2: 0.7 cm/yr westward and 1.3 cm/yr to the profiles along ships' tracks were not available for this study. east. In the central part of the FAMOUS area, anomalies cannot be The magnetic anomalies within the FAMOUS area are restricted identified with confidence west of the ridge axis, but to the east the to crust less than about 6 m.y. old. They vary in length from 15 to estimated spreading rate is 1.2 cm/yr (Fig. 10, traverse E3). Farther more than 100 km (Fig. 8). Some of them are offset or kinked along south, the spreading asymmetry is reversed, with a westward rate the trace of the present transform faults (see fracture zone A at lat of 1.3 cm/yr and an eastward rate of 1.1 cm/yr (Fig. 10, traverses 36°55'N and fracture zone B at lat 36°35'N). Other offsets reflect W4, E4a, and E4b). Thus, the opening rates (sum of spreading transient transforms, for instance at lat 36°05'N, long 33°50'W rates) are similar in the north and south, being 2.3 and 2.4 cm/yr, and lat 36°45'N, long 32°55'W. This implies that an originally col- respectively. This difference is within the limits of error of the esti- linear section of the spreading axis broke into separate and offset mate, but a spreading-rate increase of this order of magnitude can segments that have subsequently healed, with concomitant re- be expected from the increase in distance (across the FAMOUS alignment of spreading along a single axis. area) from the American-African pole of rotation. At the edges of the FAMOUS area, on approximately 6-m.y.-old The asymmetric spreading rates in section II indicate that the

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/88/5/609/3418333/i0016-7606-88-5-609.pdf by guest on 27 September 2021 Figure 8. Simplified magnetic anomaly map with 200-y contour interval. Positive (normally magnetized) anomalies above the -100-v contour shaded. After Phillips and Fleming (in prep.).

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valid on a regional and global scale, they do not satisfactorily explain the detailed tectonic features as exemplified by the Vema Fracture Zone (van Andel and others, 1971), so that additional complications such as compression and extension on fracture zones must be assumed. The detail available for the FAMOUS area indicates the presence of other complications that need to be taken into account. One such complication follows from the discrepancies between the transform direction predicted from the pole of rotation, the general morphologic trend of the fracture zones, and the direction of active strike-slip faulting derived from microearthquake studies (Reid and Macdonald, 1973; Spindel and others, 1974). These trends and a possible interpretation of them are shown in Figure 11. The general fractue zone trend (Fig. 2) appears to be the result of a series of approximately east-trending scarps and troughs. If these scarps and troughs are arranged in an en echelon configuration, the envelope would be the observed general trend of N84 °W. The distribution of earthquakes within fracture zone B, if represen- tative of the activity over a longer period of time, also implies the existence of more than one zone of strike-slip movement arranged in an oblique fashion. These active fault zones trend about N70°W, whereas the visible scarps are oriented approximately east- west. Thus, movements cannot take place along a single fault plane but must be accommodated by a series of faults within a broad shear zone. The existence of such a broad shear zone, in which individual faults are not simultaneously active, is confirmed by dive observations in fracture zones A and B (P. Tapponier and Tj. H. van Andel, 1975, oral commun.), which show deformation by shear of variable ages of igneous and sedimentary strata over a zone several kilometres wide perpendicular to the strike of the fracture valley. It is also confirmed by the existence of short individual, northeast-trending scarps and fissures in fracture zone A (Detrick Figure 9. Interpretation of magnetic anomaly map (Fig. 8). Positive and others, 1973). Given the left-lateral sense of displacement in anomalies are labeled 1, Brunhes; 2, Olduvai; 2', Gauss; and 3 and 3', Gil- the transform zone, these scarps parallel the short axis of the strain bert. Pairs of straight lines define inflection points at anomaly reversal ellipsoid (Fig. 11) and probably represent tension cracks. boundaries. Lines parallel to transform faults labeled E and W with qual- Thus, the local direction of plate movement may be approxi- ifiers indicate profiles used in Figure 10 for spreading-rate calculations. mately east-west, as indicated by the morphologic scarps and by the shear couple (Fig. 11). An alternative interpretation is that the spreading axis must be moving westward at about 0.3 cm/yr. In transform direction is parallel to the overall trend of the short frac- section IV, the reverse is true: the axis is moving east at 0.1 cm/yr. ture zones of approximately N84 °W, this is in accord with the In the center part of the area the westward rate is not known, but magnetic anomaly pattern that also gives a trend slightly south of given the nearly identical opening rates in the north and south, the east. This interpretation is compatible with the inferred regional di- same total rate may be assumed for rift valley 3. This, then, would rection (Francheteau, 1973; Laughton and Whitmarsh, 1974). It yield a westward rate of about 1.2 cm/yr and approximately sym- would result in a small amount of divergence in the fracture zones metrical spreading. The net result is a straightening of the overall (Fig. 12) with a rate of about 0.1 cm/yr. of the same order as that axial trend by migration of the axes through asymmetrical spread- inferred for the Vema Fracture Zone (van Andel and others, 1971). ing. The westward migration of rift valley 2 since the inception of The small troughs observed in fracture zone A (Detrick and others, asymmetric spreading 3.5 m.y. ago has reduced the offset on frac- 1973) should then be interpreted as tension fissures, and the fracture zone B by about 10 km to a present 23 km. ture zones would be leaky. In some cases (for example, Fig. 10a), the line of best fit does not Whereas the short fracture zones appear to define the present pass through the origin, thus implying an axis jump. There are transform direction, the microearthquake distribution shows that other examples of such jumps, but they rarely seem to affect any- actual displacement is taking place along several east-west fault thing but very short rift segments with lengths on the order of 10 planes within a broad zone (Fig. 11; Reid and Macdonald, 1973; km. More complex spreading histories also occur — for instance, Spindel and others, 1974). Evidently, this sense of movement can- that shown in Figure 10c, which will be discussed below. There is not last long without causing significant deformation within the no evidence for changes in spreading rate with time as reported by fracture zone. Macdonald (1975), which would be expressed as changes in slope of the lines of best fit. If they did occur, they must have been small Coupling across Plate Boundaries (a few tenths of a centimetre per year). Another complication results from the nature of the short frac- DISCUSSION ture zones. In general, friction in fracture zones will act against plate movement. The resistance will attain a minimum value in the Fracture Zones and Spreading Direction ideal case of a single transform plane exactly parallel to the rotation vectors. In a more complex case, deformation occurs over a Fracture zones are customarily interpreted as present, or past, wider area, the total effect being similar to that of a finite, "soft," transform faults (Wilson, 1965), which ideally parallel exactly the viscoelastic zone sheared between rigid plates. This is the case in plate rotation vectors. While these rigid geometric concepts seem the FAMOUS area.

50- a (northeast) b (east-central)

30-1

Figure 10. Graphs of distance from spreading 1 axis versus anomaly reversal age along traverses shown in Figure 9. Distances shown for both co O 1.16 ±0.1 cm/yr anomaly peaks and reversal (inflection) bound- transform fault; b shows spreading east of rift ¿50- C (southeast) d (W2: northwest: valley 3; c shows creation of a transient trans- Î W4: southwest) form and lateral axis migration or jump between ?40- 3 2.4 and 1.9 m.y. B.P.; and d shows rates west of W2; l.0± 0.1 cm/yr ridge axis; anomalies are poorly defined. 30-

20-

10- 1.13 t 0.1 cm/yr W4: 1.25 ±0.1 cm/yr

0- n I I I I I I 12 3 4 AGE B.P (my)

Our evidence indicates that stresses are transmitted across this walls. Although the pattern is complicated, the general tendency soft plate boundary. The observed continuity of rift depressions toward a progressively more orthogonal relation to the transform into the opposite plate across fracture zones and the linking be- direction remains clear. Finally, in the southern part of the region tween outer and inner walls (Figs. 3, 4) indicate coupling between (section IV), the reorientation, if any took place, occurred before 5 structural events in each pair of adjacent rift valleys. Although m.y. B.P., because the anomaly trends show little change with time. most of the shear stress will be released by movements in the frac- Bird and Phillips (1975), in an aeromagnetic study from fracture ture zones, it is possible that the process of coupling across trans- zone B south to the Oceanographer Fracture Zone, found oblique form plate boundaries in this area may be responsible for the prop- spreading stopping earlier (at about 9 m.y. B.P. in sections III and agation of parallel spreading axes (Fig. 4) and for migrating rift IV). The discrepancy in section III may lie in the poorer resolution axes and faults, thus adding to the instability of the rift zone offered by the 10-km line spacing in the Bird and Phillips study. already discussed on a small scale. As the estimates of spreading rates show, lateral migration of plate edges through asymmetric spreading has occurred in addition Spreading History to rotation of axis segments. The distribution of asymmetric spreading is such that its effect also amounts to regional rotation in Within the past 6 m.y. there has been a major reorientation of the same sense, which reinforces the rotations discussed above. In the strike of the spreading axis in the FAMOUS area, from an ob- fact, one might postulate that the two processes are, to a large ex- lique trend to an orthogonal configuration relative to the transform tent or entirely, one and the same. faults. During this period, the long and nearly continuous early Several lateral jumps of short rift segments can also be identified, spreading axis broke up into smaller rift segments which prog- and others may remain obscured in the complex anomaly patterns. ressively rotated to their present strike (Fig. 9) and which now ap- A good example is the pattern in section IV (Fig. 9). On the east pear to be recombining through asymmetric spreading to form a set side, the lineations from the Olduval event are distinctly offset at of long transform faults and large spreading axes retaining the cur- lat 36 °N, long 33°55'W, whereas the nearby Gauss lineations form rent axial direction. The process involved complex migrations of a straight line, thus indicating a small ridge jump 2 to 2.5 m.y. ago. the larger rift segments through asymmetric spreading, as well as Another probable axial jump occurs between rift valleys 2 and 3. jumping of shorter rift segments and migration of transform faults. We have interpreted morphologic data as evidence that the south- Our interpretation of this history — based mainly on magnetic ernmost 10 km of rift valley 2 were recently abandoned as a result anomaly patterns, spreading rates, and topography — is given in of a jump to the northern extension of rift valley 3. Simultaneously, Figure 13. a probable transform fault at a topographic low at the south end of In the northern part of the region, a 40-km-long initial axis ro- rift valley 2 was abandoned, also in favor of the present position of tated 27° counterclockwise between 5 and 3 m.y. B.P. to the fracture zone B, as identified by microseismic activity and fault present strike of rift valley 2. The initial trend of this axis is scarps. The postulated southern position would have linked the reflected clearly in the linear topographic features at distances of 45 southern termination of rift valley 2 via some prominent scarps and to 55 km from the ridge axis; these features trend considerably troughs in the fracture zone with rift valley 3 at the position of a more easterly than do the younger topographic units. In the central pronounced kink and change in direction of both of its inner walls part of the region (section III), the reorientation followed a slightly (Fig. 7; Ramberg and van Andel, 1977). The displacement of the different sequence, although starting from an initial axis with the transform fault may have been accomplished in one or more steps. same trend as the one farther north. In the course of the last 5 m.y., In either case, the implication is that the southernmost 5 to 10 km this axis broke up into a series of short segments, 10 to 15 km in of rift valley 2 represent an abandoned rift valley and that rift val- length, that have continued to rotate at varying rates to the present ley 3 was recently extended northward over a similar distance. Un- time. The most recent phase of this counterclockwise rotation is fortunately, magnetic evidence for this shift and its timing has been reflected in the inner floor of rift valley 3, where the central ridge lost in the fracture trough area, which has low magnetic intensities trends more northerly than the somewhat older ridges near the and truncates the anomalies. However, if the interpretation is cor-

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Rift valley trend Figure 12. Spreading direction parallel to fracture Predicted" zone leads to extension and transform slow spreading within frac- trend ture zone perpendicular to en echelon fractures. See Overall morph. text for discussion. Spreading direction trend of FZ. [i N 84°W) S •

Apparent trend The migrating or jumping rift axes discussed above as occurring of micro- within the FAMOUS area have been associated with the develop- earthquakes ment of transient transform faults. Where the outer Olduvai boundary (1.9 m.y.) was displaced relative to the inner margin of the Gauss lineations (2.4 m.y.) at lat 36°42'N, long 32°50'W, the fissures resulting transient transform can be viewed as a mechanism for slight counterclockwise rotation of the spreading axis. This is the INTERPRETATION same sense of reorientation that took place to a larger degree during the period 5 to 3 m.y. B.P. and that still seems to operate in rift Approx. en echelon valley 3. This transform apparently grew gradually, then disap- {arrangement of peared rapidly between 2.4 and 1.9 m.y. B.P., when the southern faults section of the axis migrated about 5 km to the east and rejoined the northern segment of the main axis. Several active In overview, the evolution of spreading in the FAMOUS area is fault zones characterized by the breakup of a nearly continuous axis with a co-existing strike of N50 °E into small rift segments that individually reoriented to a strike of N23 °E through a complex process of axial Figure 11. Observed and inferred transform trends and their possible jumps (Fig. 13) and asymmetric spreading. The sudden jumps, if interpretation. See text for discussion. they occurred, were small, perhaps around 5 to 10 km. The tec- rect, the rather large extension of the northern part of rift valley 3 tonic evolution illustrated in Figure 13 fits the general anomaly across the fracture zone may indicate a continued lengthening of pattern well when a transform trend of N84 °W is assumed, and the this rift segment at the expense of rift valley 2 and hence a further major anomalies match well within this frame of reference. On a straightening of the plate edge. Shih and Molnar (1975) have in- small scale, the matches are less perfect, and alternative interpreta- voked a similar process, which is somewhat analogous to the prop- tions are possible, including some that postulate changes in spread- agation of a crack through a plate, to account for certain magnetic ing direction of as much as 15°. Bird and Phillips (1975) indicated anomaly patterns in the eastern Pacific. that oblique spreading may be a transient response to a new spread- The same mechanism might explain the origin of the diagonal ing direction. We feel that anomaly matches do not necessitate topographic trends discussed above. These consist of a set of large spreading-direction changes associated with oblique spread- northeast- or N65° to 70°E—trending troughs on the east flank of ing occurring at 25 and 9 m.y. B.P. (from Bird and Phillips) and at 4 the Mid-Atlantic Ridge and their mirrored counterparts, a set of m.y. B.P. (from our study). Such frequent and major direction more diffuse northwest-trending troughs on the west flank (Fig. 2). changes would involve the entire American and African plates and The troughs occur in areas of complex magnetic anomalies and many complex side effects. Our study of transform trend topog- may have formed as a series of short fracture zones that were suc- raphy and detailed magnetics leads us to prefer a basic constancy of cessively abandoned and reformed at more southerly locations, the spreading direction. leaving behind the snowplow-shaped diagonal troughs. The dif- ferences in morphology between the eastern and western limbs can REGIONAL IMPLICATIONS be explained as the result of interference between the troughs, which trend at complementary angles on the two sides of the The continuous shifting and reorientation of spreading axes on Mid-Atlantic Ridge, and the rift-parallel morphologic trends. On the intermediate scale represented in Figure 13 is matched by a the east side, the northeast trend becomes progressively more paral- similar instability on an even smaller scale within individual rift lel to the rift-parallel trend, which swings around to the northeast valleys (Ballard and van Andel, 1977; Ramberg and van Andel, as the eastern edge of the area is approached. Hence, the two line- 1977). They place significant constraints upon models for the un- aments will enhance each other and form slightly curved troughs derlying magma source and for the dynamics at the edges of spread- trending generally northeast. On the west side, the northwest trend ing plates. The shifting positions of the axis over short time inter- intersects the rift-parallel trend nearly at right angles and creates an vals imply that conditions in the upper mantle must be such as to interference pattern, with the individual depressions only in general allow considerable latitude in the location of the axis. As the plates following the northwesterly diagonal direction. move apart, the magma passively fills the space made available by Another aspect of the diagonal trends was noted by Sleep (1974), tensional gaps that occur randomly within a rather wide zone over who contended that excess volcanism in "plume" regions is the re- the long-term plate boundary. The width of this zone increases as sult of more efficient segregation of melt in the lower regions of the variability is considered over longer periods and larger areas; it conduit, which is narrower for disorganized and poorly aligned ranges from 1 to 2 km over time spans of a few hundred thousand ridges. Thus, we are implying that features previously called plume years to 5 to 10 km on a scale of 1 m.y. or more. When rifting shifts tracks may result from the migration of small transform faults and to a new axis, the old zone remains manifest as a relict feature, other irregularities in the ridge axis. which is transported with the plate to one or the other side. In this

crust of intermediate thickness (Whitmarsh, 1975; Fowler and / Matthews, 1974). An increased crustal or lithospheric thickness would favor more efficient magma segregation and plateau build- ing (Sleep, 1974). ------I Schilling (1975) has shown by means of rare-earth-and major- element analyses of basalts that there is a progressive change from / enrichment to depletion in light-rare-earth elements in basalt6 from the central part of the plateau at lat 40 °N southward to lat 33°40'N. The combined morphologic, geophysical, and chemical data have been considered by Schilling as evidence that mantle up- 5-6 m.y. B.P / 3.4 m.y. B.P welling is taking place beneath the Azores Plateau, analogous to that postulated on similar grounds for Iceland and the Afar triangle 1 i l (Schilling, 1973a, 1973b). Vogt (1971) and Morgan (1973) have 37° N I suggested that horizontal asthenospheric flow could be associated ^t.. with mantle plume activity, and Vogt and Johnson (1975) have n rv21 / postulated that changes in the rate of flow could have caused the / observed late Tertiary transition from orthogonal to oblique spreading south of Iceland by altering the stress field or weakening m 12 / Tre the lithosphere in the flow direction. Oldenburg and Brune (1972) /ftV 3 indicated that oblique spreading could occur when plate shear strength drops or stress on transform faults increases. Plate shear strength could be reduced by composition and temperature changes 36° N jjr associated with periodic mantle upwelling. / In the FAMOUS area, at least for the past 6 m.y., the observed 1.9 m.y. B.P Present 40 changes have been the opposite of those south of Iceland, with a 34°W| 1 |33°W change from oblique spreading to a staircase pattern with numerous short rift and fracture zone segments. Both cases of oblique Figure 13. Evolution of spreading in the FAMOUS area interpreted spreading are spatially associated with atypical portions of the from magnetic anomaly patterns. Axis configuration is shown for 5 to 6, Mid-Atlantic Ridge near presumed mantle plume areas. If this rela- 3.4, 1.9, and 0 m.y. B.P. Rift valleys 2 and 3 and sections I through IV are tionship is other than coincidental, the contrast between the two labeled in lower right-hand corner. Dashed lines indicating limits of section III are retained in all figures for reference. Present spreading rates in cen- patterns of evolution needs explanation. Regional tectonic evidence timetres per year are shown in lower right figure. Transform trends as- (Francheteau, 1973; Pitman and Talwani, 1972; Pitman and sumed constant for past 6 m.y. others, 1974) indicates that a discernible degree of oblique spreading has characterized this section of the Mid-Atlantic Ridge south model, asymmetric spreading and axis migration must be common of the Azores during most of Cenozoic time. Bird and Phillips on the short time scale, since a new rift may appear randomly on (1975) found two distinct periods of oblique spreading, determined either side of the old one, and minor superimposed stresses should from anomalies beyond the edges and south of the FAMOUS area. suffice to give it direction. As the temporal and spatial scales in- This, coupled with our findings, suggests that the reorientation seen crease, the deterministic component should also increase. In this in the FAMOUS area is not a unique event but a phenomenon that model there is no terminal point to the instability. is repeated periodically, perhaps as a consequence of fluctuations in Alternatively, a continuous linear magma source parallel to the plume acitivty. Periods of strong horizontal flow would transform regional trend of the Mid-Atlantic Ridge but at varying angles to the pattern to oblique spreading, whereas relatively quiet periods local trends may be assumed. The angular discrepancies of the in- would allow a return to an orthogonal low energy configuration, as dividual rift valleys would then impose stresses on the small-scale suggested by Vogt and Johnson (1975). Alternatively, the depen- instability, which would lead to a continuous readjustment that dence of magma segregation on factors such as the geometry of the would be self-terminating once the alignment approached perfec- ridge axis (Sleep, 1974) suggests that the plume activity might be tion. The angular discrepancies themselves would be reflected in the effect and not the cause of changes in the ridge pattern. In any the lengths of transform offsets. The larger the angular discre- case, the present situation in the FAMOUS area would reflect a pancy, the shorter will be the rift segments needed to adjust to the period of reduced activity of the presumed Azores mantle plume. It underlying source. Petrological asymmetry matching the spreading appears worthwhile to pursue this hypothesis further by examining asymmetry in rift valley 2 (Bryan and Moore, 1976) may imply possible long-term variations in morphology and basalt composi- such an angular discrepancy or may simply reflect a difference in tion in this region. local cooling rates. It would be interesting to ascertain whether the opposite situation exists in rift valley 4, where the spreading ACKNOWLEDGMENTS asymmetry is reversed. An important question is whether and to what extent the adja- This investigation was inspired by a seminar on the processes of cent Azores Plateau and its history have affected the tectonic evolu- mid-ocean ridges, particularly in the FAMOUS area, held at Stan- tion of the FAMOUS area, which straddles, at least in its northern ford University in the winter and spring of 1975. We are very much and central parts, the extreme southwestern limb of the plateau. indebted to Tj. H. van Andel for constant advice and inspiration Topographic data indicate that the Mid-Atlantic Ridge loses its during this study; to K. C. Macdonald, Robert D. Ballard, and character just north of the FAMOUS area; north of about lat 38°N other members of the FAMOUS project for discussions; to N. H. the rift valley is no longer detectable. The wide, shallow, and par- Sleep for suggesting improvements to the manuscript; and in par- tially exposed Azores Plateau has an anomalously large crustal ticular to J. D. Phillips and H. S. Fleming for the generosity with thickness; the crustal thickness beneath the median valley appears which they made the basic data available. Ramberg was supported to increase from about 6 km at the periphery to about 12 km at the by the University of Oslo and the Norwegian Research Council for center of the plateau (Nafe and Drake, 1969). Recent seismic re- Science and Humanities. The investigation was also supported by fraction studies in the FAMOUS area confirm the presence of a National Science Foundation Grant DES74-19237.

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D., 1974, Preliminary model for extrusion and rifting at the axis of the Mid-Atlantic Ridge, 36°48' MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER 26, 1975 North: Geology, v. 2, p. 437-440. REVISED MANUSCRIPT RECEIVED JULY 1, 1976 Nafe, J. E., and Drake, C. L., 1969, Floor of the North Atlantic, summary MANUSCRIPT ACCEPTED SEPTEMBER 2, 1976

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