Rises, trenches, great faults, and crustal blocks

The transform fault concept is extended to a spherical surface. The earth's surface is considered to be made of a number of rigid crustal blocks. It is assumed that each block is bounded by rises (where new surface is formed), trenches or young fold mountains (where surface is being destroyed), and great faults, and that there is no stretching, folding, or distortion of any kind within a given block. On a spherical surface, the motion of one block (over the mantle) relative to another block may then be described by a rotation of one block relative to the other block. This rotation requires three parameters, two to locate the pole of relative rotation and one to specify the magnitude of the angular velocity. If two adjacent blocks have as common boundaries a number of great faults, all of these faults must lie on ‘circles of latitude’ about the pole of relative rotation. The velocity of one block relative to the other must vary along their common boundary; this velocity would have a maximum at the ‘equator’ and would vanish at a pole of relative rotation. The motion of Africa relative to South America is a case for which enough data are available to critically test this hypothesis. The many offsets on the mid-Atlantic ridge appear to be compatible with a pole of relative rotation at 62°N (±5°), 36°W (±2°). The velocity pattern predicted by this choice of pole roughly agrees with the spreading velocities determined from magnetic anomalies. The motion of the Pacific block relative to North America is also examined. The strike of faults from the Gulf of California to Alaska and the angles inferred from earthquake mechanism solutions both imply a pole of relative rotation at 53°N (±3°), 53°W (±5°). The spreading of the Pacific-Antarctic ridge shows the best agreement with this hypothesis. The Antarctic block is found to be moving relative to the Pacific block about a pole at 71°S (±2°), 118°E (±5°) with a maximum spreading rate of 5.7 (±0.2) cm/yr. An estimate of the motion of the Antarctic block relative to Africa is made by assuming closure of the Africa-America-Pacific-Antarctica-Africa circuit and summing the three angular velocity vectors for the cases above.

[1]  Xavier Le Pichon,et al.  Sea‐floor spreading and continental drift , 1968 .

[2]  J. E. Everett,et al.  The fit of the continents around the Atlantic , 1965, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[3]  F. P. Shepard,et al.  Bathymetry and Faults of Gulf of California , 1964 .

[4]  Xavier Le Pichon,et al.  Magnetic anomalies over the Reykjanes Ridge , 1966 .

[5]  P. St.-Amand.  Geological and geophysical synthesis of the tectonics of portions of British Columbia, the Yukon Territory, and Alaska , 1957 .

[6]  D. Tocher.  The Alaska earthquake of July 10, 1958: Introduction , 1960 .

[7]  L. Sykes.  Seismicity of the South Pacific ocean , 1963 .

[8]  Arthur D Raff,et al.  Magnetic survey off the west coast of North America , 1961 .

[9]  L. Sykes.  Mechanism of earthquakes and nature of faulting on the mid‐oceanic ridges , 1967 .

[10]  V. Vacquier,et al.  Evidence for connection between heat flow and the mid‐atlantic ridge magnetic anomaly , 1964 .

[11]  W. Stauder.  The Alaska earthquake of July 10, 1958: Seismic studies , 1960 .

[12]  J. Ryther.  Photosynthesis and fish production in the sea. , 1969, Science.

[13]  C. R. Allen,et al.  Relationship between seismicity and geologic structure in the Southern California region , 1965 .

[14]  M. Tharp,et al.  Chain and romanche fracture zones , 1964 .

[15]  T. Davis,et al.  Alaska earthquake of July 10, 1958: Intensity distribution and field investigation of northern epicentral region , 1960 .

[16]  D. Tocher.  The Alaska earthquake of July 10, 1958: Movement on the Fairweather fault and field investigation of southern epicentral region , 1960 .

[17]  Xavier Le Pichon,et al.  Crustal structure of the mid‐ocean ridges: 3. Magnetic anomalies over the mid‐Atlantic ridge , 1965 .

[18]  J. Phillips.  Magnetic Anomalies over the Mid-Atlantic Ridge near 27�N , 1967, Science.

[19]  W. C. Pitman,et al.  Magnetic anomalies in the Pacific and sea floor spreading , 1968 .

[20]  H. W. Menard.  Extension of Northeastern-Pacific Fracture Zones , 1967, Science.

[21]  M. Tharp,et al.  The Vema fracture zone in the equatorial Atlantic , 1964 .

[22]  M. Tharp,et al.  Tectonic fabric of the Atlantic and Indian oceans and continental drift , 1965, Philosophical Transactions of the Royal Society of London. Series A, Mathematical and Physical Sciences.

[23]  W. Milne,et al.  Direction of faulting in certain earthquakes of the North Pacific , 1951 .

[24]  J. Wilson,et al.  A New Class of Faults and their Bearing on Continental Drift , 1965, Nature.

[25]  W. Bradley Myers,et al.  Cenozoic tectonics of the western United States , 1966 .

[26]  F. J. Vine,et al.  Spreading of the Ocean Floor: New Evidence , 1966, Science.