Seeking a Rift - 15/12/2014
Confused by Fracture Zones
In 1953, Hans Pettersson published Westward Ho with the Albatross, a popular account of his around-the-world scientific cruise on the Swedish sailing ship Albatross. In it he stated, “… geological evidence found in the last thirty years indicates that the Ridge is probably built up by extensive submarine volcanic action, i.e. by molten magma from deeper layers in the crust being extruded through an enormously long fissure in the bed of the Atlantic.” Although Pettersson and earlier authors certainly came close to describing the concept of a rift, it was not until the late 1950s that a rift and rift valley were widely accepted by the geologic community. This was largely based on the 1956 pronouncement of a worldwide rift by Maurice Ewing followed by the 1957 publication of the Bruce Heezen and Marie Tharp Physiographic Diagram of the North Atlantic Ocean and the publication of Geological Society of America Special Paper 65, The Floors of the Ocean: Text to Accompany the Physiographic Diagram of the North Atlantic in 1959 by Heezen, Tharp, and Ewing.
Because of the iconic nature of the physiographic diagram and its role in acceptance of the reality of a rift zone, a critical look will be taken at this map and the associated publications. The map was included in the January 1957 edition of The Bell System Technical Journal. This volume included the article ‘Oceanographic Information for Engineering Submarine Cable Systems’ by C. H. Elmendorf, a cable engineer, and Heezen that discussed the physical and geological oceanography of the North Atlantic and its relation to cable engineering. The accompanying physiographic diagram was quite large, extending from 50N to 17N Latitude and between 0 and 82W Longitude. The rift valley shows as a somewhat sinuous line bisecting the centre of the diagram with no indication of fracture zones. However, south of 24N the rift is not shown as the legend has been placed over the rift zone. As Heezen and Tharp had data to 15N and a number of ridge crossings between 24N and 15N, the question arises as to why the physiographic diagram ended at 17N and hid the region between 24N and 17N.
To understand the map and how it evolved requires study of Heezen’s 1957 and 1959 publications and their accompanying graphics. These works included three basic types of associated graphics: 1) trackline maps; 2) profiles of the seafloor that displayed its nature including depictions of the rift; and 3) earthquake epicentre maps that were meant to demonstrate the correlation of earthquakes with the median rift valley. The 1959 publication also included a map showing ‘Distribution of Deep Sea Sands in Relation to Physiographic Provinces’, a map labelling physiographic provinces, and a ‘Control Chart’ that showed all trackline used in the construction of the large-scale physiographic diagram. The 1957 publication included a separate map that showed physiographic provinces including the rift valley extending from 60N to 30N and a map showing trackline extending to approximately 12N.
After reviewing these publications, Gunter Dietrich’s influence on Heezen’s work as related to the Mid-Atlantic Ridge becomes apparent. Recalling Heezen’s discovery of a ‘skimpy’ paper related to Meteor work in the late 1930s, Dietrich’s paper that described six ‘striking depressions’ along the MAR was found in the bibliography of the 1959 publication. Heezen referred to this article once in The Floors of the Oceans and only in the context of a source of soundings and not as a source of concepts. There were only two discrete soundings with associated position information in this article and four profiles. The two soundings noted the depth and location of what Dietrich called Great Meteor Seamount and the depth and location of the ‘most impressive’ of Dietrich’s ‘striking depressions’.
The trackline graphics from 1957 and 1959 show Dietrich’s influence between 30N and 15N. The ‘Control Chart’ published in 1959 shows the tracklines used in producing the physiographic diagram down to 17N. Figure 4 of Heezen’s 1957 publications shows track extending to 12N. Barely noticeable on the 1959 control chart are a small number of tracks identified as coming from Meteor expeditions. Centred near 25 N 45 W are the three segments of line run by Gunter Dietrich on the Meteor in 1938 that cut the ridge and accounted for three of Dietrich’s six ‘striking depressions’. Between 30N and 15N all but one Heezen line cut the ridge in close proximity to Dietrich crossings. With the exception of a line run by the Lamont ship Vema after 1952 that brackets Dietrich’s ‘most impressive’ depression, all of these lines pre-dated the claim of Marie Tharp’s ‘discovery’ of the rift valley. It is difficult to avoid the conclusion that Heezen was looking for Dietrich’s ‘striking depressions’.
There were numerous sets of profiles produced for publication in the Floors of the Oceans. From the standpoint of defining the MAR and the rift valley there were two significant sets. The first is the famous set of six profiles Marie Tharp produced and from which she claimed priority of discovery of the rift valley of the MAR. As noted in the last issue of Hydro International, there are a number of variations of this story. Regardless of which or any version being true, the first three profiles were touted as the ones on which the rift valley was most prominent. Out of these three, two never actually crossed the ridge at the location of the notch as they showed profiles of fracture zones: Profile I crosses the Oceanographer Fracture Zone and Profile III crosses the Atlantis Fracture Zone.
A second set of twenty-six profiles, and the most convincing concerning the rift valley, is found in Figure 45 (p. 93) of The Floors of the Oceans. The majority of these crossings have a significant central depression. Profiles R1 through R25 extend from 48N to 15N although the Physiographic Diagram cuts off at 17N. Surprisingly, Heezen and Tharp scaled off depths from the prominent profile Dietrich had published in 1939 and used it as Profile R23. The associated track segments for profiles R1 through R25 are found on Plate 23 (plates are included as foldouts at end). When plotting the rift valley location on the trackline segments of Plate 23, a much different view of the median valley emerges from that shown on the Physiographic Diagram. There are offsets with the correct orientation at what are now known as the Oceanographer (35N), Atlantis (30N), Kane (24N), and 15 20 (15N) Fracture Zones. The location of the rift valley when superimposed on the trackline of Plate 23 compares favourably to modern bathymetric maps of the ridge and rift. Because the physiographic map was drawn to show the ridge and rift with no offsets, there are errors approaching 100 nautical miles in the location of the ridge and rift as shown on this famous map. Heezen and Tharp apparently chose to hide their confusion concerning fracture zones by a variety of tactics. The two most apparent being: 1) the placement of the legend just below 24 N which hid the Kane Fracture Zone; and 2) terminating the map at 17N instead of continuing it to 15N thereby cutting off the 15 20 Fracture Zone.
Heezen and Tharp reinforced their claim to having discovered the rift valley by correlating earthquake epicentres with the rift valley. Prior to the development of the World-Wide Standardized Seismological Network (WWSSN) in the early 1960s, the best that the location of epicentres could be determined in the vicinity of the MAR was approximately 100 kilometres. It is noted that a number of investigators had discovered the correlation of a broad zone centred on the ridge with numerous earthquake epicentres prior to declaration of the rift. Given that the width of the median valley is of the order of 20 kilometres, it would seem impossible to find the valley based on pre-WWSSN earthquake epicentre locations. Furthermore, many of the earthquakes associated with the ridge occur on ridge-ridge transform faults. The confusion that this caused is best shown on two 1959 Heezen-Tharp graphics – Plate 28 which shows a distinct and correct bend to the east in the ‘Rift Mountains’ at 24N and Plate 29 which shows the rift valley as a relatively straight feature bending to the west at 22N (Figure 2). To be logically consistent, the rift valley should roughly follow the centre of the rift mountains. Heezen and Tharp hid their confusion by placing the legend in this area.
By the early 1960s, Heezen and Tharp began to understand the nature of fracture zones and reported on Atlantic fracture zones at a conference in 1961 and then published a paper on the equatorial Chain and Romanche Fracture Zones in 1964. In 1968, Heezen was made chief scientist of the brand new USNS Kane and was told he could take it anywhere in the Atlantic he wished on its maiden voyage. He went straight to the feature he called ‘Scarp D’, which happened to be the location of Dietrich’s ‘most impressive striking depression’ at 24N. Curiously, the track Heezen ran during this cruise, when combined with an earlier 1950s Vema trackline, circumscribed a polygon around Dietrich’s depression. This depression is at the western ridge-transform fault intersection of what Heezen subsequently named the Kane Fracture Zone. Perhaps Heezen’s apparent sailing in Dietrich’s wake ended with this cruise. By this time many more crossings of the ridge had been made, most of the major fracture zones in the Altantic had been mapped, and the rift valley was a well-established fact.
In looking back it becomes apparent that Gunter Dietrich was the first to discover the median valley in the Mid-Atlantic Ridge and influenced Bruce Heezen, Marie Tharp, and probably Maurice Ewing in much of their work on the MAR. It seems an inescapable conclusion that Heezen was following Dietrich’s lead from as early as 1948 in attempting to define the median valley and that he, Maurice Ewing, and Marie Tharp were cognisant of Dietrich’s work and never gave him credit for his contribution. However, to Heezen, Tharp, and Ewing’s credit, they were the first to describe the physiography of an entire oceanic region and the Heezen-Tharp follow-on maps of the oceanic basins painted by Heinrich Berann served to educate a generation as to the nature of our planet. Perhaps the moral of the story of discovery on the MAR is that the mapping of the seafloor and discovery of its true nature is not the work of one person or even one group of people. The discovery and comprehension of the nature of the Mid-Atlantic Ridge began over a century and a half ago with the work of Lieutenant Matthew Fontaine Maury and Lieutenant Otway Berryman and is still continuing today.Last updated: 25/10/2020