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Rises, Trenches, Great Faults, and Crustal Blocks by W. Jason Morgan

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Anthony Sautter

on 25 September 2012

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Transcript of Rises, Trenches, Great Faults, and Crustal Blocks by W. Jason Morgan

Rises, Trenches, Great Faults, and Crustal Blocks The Author W. James Morgan, Ph.D. Graduated Georgia Tech as a physics major in 1957. Earned his Ph.D. in physics from Princeton in 1964. Became an assistant professor at Princeton in 1967. Collaborated with Fred Vine, a great scientist in plate tectonics who studied the magnetic anomalies, while at Princeton. Served in the Navy for two years. Received the National Medal of Science in 2002. The Road So Far J. Tuzo Wilson Described the network of mid-oceanic ridges, transform faults, and subduction zones as boundaries separating an evolving mosaic of enormous plates.

He also proposed the idea of the opening and closing of oceans and the eventual production of an orogenic belt by the collision of
two continents. Alfred Wegener He WAS the first to recognize significant fossil, paleo-topographical, and climato-
logical evidence that supported his theory Made serious arguments for the idea of continental drift in the early 1900s. Wegener could not explain the force that drove continental drift, and his vindication did not come until after his death in 1930. Altho he was not the first, he noted how the east coast of South America and the west coast of Africa looked as if they fitted together. The Goal This article is an attempt to describe present day continental drift.
It is an extension of Wilson’s transform fault concept on a spherical surface. In 1967, Morgan wrote down his two cents on plate tectonics in his paper entitled, "Rises, Trenches, Great Faults, and Crustal Blocks." The Theory The Earth is divided into about twenty units, or blocks There are three types of boundaries between blocks: where new crustal material is being formed (divergent plate boundary) Rise Type where crustal surface is being destroyed (convergent plate boundary) Trench Type where crustal surface is neither created nor destroyed. (transform fault -echoes Wilson 1965) Fault Type A Assumptions 3 1. Distances of places

that reside all on the same

plate should not change over

time. 2. Distances of

places that reside on

different plates should

change over time. 3. The relative motion between two blocks

may be represented by an angular velocity

vector. To understand this concept we must first consider blocks on a plane. > From the rise alone, we cannot tell the direction of the motion of one block relative to the other.

> We can, however, tell the direction from looking at the transform faults.

> The magnetic anomaly pattern which will be parallel to the ridge crest may now be projected and the velocity of one block relative to the other may be determined from the spacing of the anomalies. Assumption #3:
The relative motion between two blocks may be represented by an angular velocity vector. The strike of the transform fault is parallel to the difference of the velocities of the two sides. Development of the rise is an average of the two velocities on either side. But what about a sphere ? Due to rules of geometry, we now must think as the crust as an orange peel. All the transform faults common to these two blocks must lie on concentric circles about the pole of rotation The velocity is greater at the equator than at the pole of rotation, like swinging a rope. To determine pole of rotation, we must look at the transform faults and draw perpendicular lines through the concentric circles. These perpendicular lines result in great circles. The area where these great circles overlap is considered to be a pole of rotation. Great circles are analogous to meridians and the fault lines are analogous to lines of latitude about the pole of rotation. Morgan says that there must be some sort of “self-adjusting mechanism in the rifting process that gives rise to a symmetric magnetic anomaly pattern.” He allows for the possibility of unequal rates on the two sides of a ridge. Still, Morgan recognized this anomaly as an essential part of tectonic rate determination. Crustal Boundaries African and South American Pacific and North American Antarctic and Pacific Antarctica and African Constructing the Model Spreading rate formula:
Vperpendicular = Vmax sin (theta) cosine (STRIKE-(alpha)) To use the preceding formulas for spreading rate, knowledge of the latitude, longitude, and strike of the ridge is needed at each point along the ridge. Morgan got this information from several sources. Spreading rates determined from magnetic anomaly profiles are compared with the values calculated with the model. Solid line shows the predicted rate perpendicular to the strike of the ridge Dashed line show the rate parallel to the direction of spreading. Points are actual rates Results: The present motion of Africa relative to South America point to a pole at 62N, 36W. > The average motion since drifting began.
> Points to a pole at 44.0N, 30.6W The total length of the transform faults in this region suggests that only about half of the motion of these two continents has been about the present pole of motion. Spreading rate was not found due to lack of exploration of magnetic anomalies at this region. Henry W. Menard, a Ph.D. in marine geology, wrote an article in 1967 inferring that these ancient fracture zones shown here do not create great circles. This is what Morgan says spurred his present investigation of crustal blocks. Morgan ran the same model as for the South american and african plates excluding the anomalous Juan de Fuca plate. He excluded the Juan de Fuca plate in this analysis for a couple reasons:
1. The faulting in this area suggest many small blocks moving independently of one another
2. The activity of this region is probably in its last stages (the Cascades are probably the volcanic counterpart of this now extinct system) Results: 53N, 53W To assign a rate to the motion of the Pacific block relative to the North American block, Morgan assumed that the motion along the San Andreas fault is 6.0 +/- 1.0 cm/yr, and is good data to use to estimate the spreading rate.

Converting this value to half velocity, Morgan said that Vmax(of just the Pacific block) = 4.0 +/- 0.6 cm/yr. This was done because there Morgan said that there was very little magnetic anomaly profiling analysis done for this area. > A pole at 71S, 118E was found.

> A half velocity of 5.7 +/- 0.3 cm/yr was found.

> Six large fracture zones have previously been found along this plate boundary, and they all come within 2 degrees of the proposed pole. In order to accomplish this last part of the study, Morgan sums all the angular velocity vectors found in this study to estimate the motion of these two blocks. Africa and Antarctica are separating about a pole in the South Atlantic Ocean with a maximum half-rate of about 1.5 cm/yr. Morgan checked his value with two strikes (Malagasy and Prince Edward fracture zones) and found a pole at 15S, 15W is compatible with the great circles formed. Unfortunately no magnetic profiles to check the predicted spreading rate. Results: Conclusion > Continental units are rigid

> Oceanic units are rigid

> “A strong tectosphere sliding over a weak asthenosphere (crust and upper mantle over the asthenosphere).

> The crustal block model can possibly explain the median position of most oceanic rises and the symmetry of their magnetic pattern.

> Conveyor belt Conclusion > Continental units are rigid

> Oceanic units are rigid

> “A strong tectosphere sliding over a weak asthenosphere (crust and upper mantle over the asthenosphere)

> The crustal block model can possibly explain the median position of most oceanic rises and the symmetry of their magnetic pattern.

> Conveyor belt W. Jason Morgan A possible explanation of this high half-velocity is that Morgan used the magnetic pattern producted during the past 5 million years to determine the rate, hence that rate is an average rate for 5 million years.
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