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Relative velocities of the Pacific, North America and Cocos plates in the middle America region
EARTH AND PLANETARY SCIENCE LETTERS 7 (1970) 425-428. NORTH-HOLLAND PUBLISHING COMP., AMSTERDAM
RELATIVE VELOCITIES OF THE PACIFIC, NORTH AMERICA AND COCOS PLATES IN THE MIDDLE AMERICA REGION* Roger L. LARSON** and Clement G. CHASE*** University of California, San Diego, Scripps Institution of Oceanography, La Jolla, California 92037, USA Received 22 December 1969 Relative velocities of crustal plates are used for the first time to test the conventional assumption that such plates are rigid. Spreading rates derived from magnetic anomaly data, fracture zone trends, and earthquake mechanisms at the Pacific-North America and Pacific-Cocos plate boundaries are used to predict the direction of thrusting of the Cocos plate under North America to be 031 °. As this direction is in near accord with previously determined earthquake mechanisms at that plate boundary, the assumption that the three plates are rigid appears to be largely justified. The calculated rate of underthrusting at the Cocos-North America plate boundary is about 8 era/yr.
1. Introduction The tectonics of the Middle America region have been recently interpreted by Molnar and Sykes [1 ]. Using seismicity and focal mechanisms they described the Pacific, North America, Cocos and Caribbean plates in that region and their directions o f relative motion (fig. 1). Spreading rates computed from magnetic anomaly profiles across the East Pacific Rise crest west of Mexico are combined here with focal mechanism results and bathymetry for the following purposes: (1) to redeterminate the Pacific-Cocos rational pole; (2) to test the rigid plate assumption for the three plates; and (3) to determine the rate of thrusting of the Cocos plate under the North America plate at the Middle America Trench.
2. Magnetic prof'de interpretations Larson et al. [2] computed the average spreading half-rate of 3.0 cm/yr on the East Pacific Rise at the mouth of the Gulf of California just north of the Rivera Fracture Zone. Their data show a spreading * Contribution of the Scripps Institution of Oceanography, n e w series.
** Marine Physical Laboratory. *** Department of Scripps Institution of Oceanography.
half-rate of 2.9 cm/yr north of the Tamayo Fracture Zone inside the Gulf. This rate and the trend of the Tamayo Fracture Zone, which is parallel to the San Andreas fault direction, define the relative motion between the North America and Pacific plates. Several lines o f evidence imply that the spreading rates immediately to the south and the trend of the Rivera Fracture Zone may not be representative of the North America-Pacific relative motion. Existence of a bathymetric depression beyond the continental margin [3], a possible difference in trends of the Tamayo and Rivera Fracture Zones and detailed examination of magnetic profiles across the rise ail suggest a small amount of underthrusting of the sea floor under North America at the Middle America Trench between the Rivera and Tamayo Fracture Zones. Chase [3] has mapped the East Pacific Rise and its related fracture zones between the Gulf of California and the equator (fig. 1). Magnetic anomaly profiles taken by various Scripps Institution of Oceanography expeditions across the rise between the Orozco and Siquieros Fracture Zones are shown in fig. 2 [4]. We interpret the anomalies to display the pattern of linearity, symmetry and parallelism to the rise crest that typifies the sea-floor spreading process as described by Vine and Matthews [5] and Vine [6]. The anomalies can be correlated on both sides
426
R.L.LARSON and C.G.CHASE
'"I IY' 9"108"
I IO*
I00"
90*
80*
Fig. 1. Relative motions and plate boundaries discussed in the text. Divergingarrows indicate crustal formation; converging arrows indicate crustal consumption. Rates are shown by the length of the solid arrows. Dashed arrows indicate directions of earthquake mechanisms. Names of plates are shown in large letters; fracture zones in small letters. Established plate boundaries are solid lines; uncertain plate boundaries are dashed lines. Location of fig. 2 shown by insert. Earthquake mechanism directions and some plate boundaries are from Moinar and Sykes [ 11]. The figure is a standard Mercator projection.
of the rise out to five, and perhaps seven, million years old. The model anomaly profile shown was computed from a fiat block model at 3 . 3 - 5 . 0 km below sea level with susceptibility -+ 0.01 emu/cm 3 Oe except for the central block which is + 0.02 emu/cm 3 0 e . The direction and intensity of remanent magnetization results from the simplification that the Earth's magnetic field is essentially a dipole, and the assumption that when averaged over > 105 years the dipole is aligned with the Earth's rotational axis. The vertical block boundaries were determined by combining the geomagnetic time scale of Heirtzler et al. [7] with a 5.4 cm/yr spreading half-rate. This appears to be the spreading
106"
104=
102Q
iOO°
Fig. 2. Magnetic anomaly profiles across the Cocos-Pacific plate boundary (East Pacific Rise crest) between the Orozco and Siquieros Fracture Zones. Correlations are shown by fine lines; dashed lines indicate poor correlations or tack of data. The total magnetic field measurements were reduced to magnetic anomaly form by removal of the I.G.R.F. [4]. The model anomaly profile is shown just above the time scale. Its trend is 076 ° which is normal to the strike of the structure. The figure is a standard Mercator projection.
half-rate at the location of the model profile. The observed half-rate increases from 4.8 cm/yr just south of the Orozco Fracture Zone to 6.9 cm/yr just south of the Siquieros Fracture Zone. Further south correlations can be made that indicate a 7.5 cm/yr half-rate near the equator, but at that low latitude the anomaly amplitudes are very small.
3. Relative velocity calculations McKenzie and Parker [8] and Morgan [9] demonstrated that it was useful to represent the relative motion of two rigid plates on the surface of the Earth by a single instantaneous angular velocity vector. The three relative angular velocities associated with any three plates must sum to zero. Spreading
RELATIVE VELOCITIESOF THE PACIFIC, NORTH AMERICA AND COCOS PLATES rates and fracture zone directions are used to determine the relative rotational poles and angular speeds for the Pacific-North America and the PacificCocos plates. These angular velocities are used to derive a Cocos-North America pole and predict the direction of underthrusting of the Cocos plate beneath the North America plate at the Middle America Trench. This result can t'e compared with the measured slip direction [1 ] at that plate boundary to test the assumption that the three plates in question are rigid. Molnar and Sykes [1 ] determined a right-lateral, strike-slip earthquake mechanism for event 168 on the Siquieros Fracture Zone. The strike of the fault plane is 076 °. This slip vector is a poorly determined one. It may be in error by "-+ 10 °, or maybe 20 °'' (Molnar, personal communication). However, it does align well with the Siquieros Fracture Zone at this location, whose trend is well established (H.W.Menard, personal communication). Molnar and Sykes [1] used the strikes of the Orozco and Siquieros Fracture Zones as contoured by Chase [3] to determine the CocosPacific rotational pole as lying "between 20°N, 108°W and 27°N, 114°W ''. R.L.Fisher (personal communication) has recontoured these fracture zones as more parallel figures than those shown by Chase [3]. Their younger portions trend about 080 ° . This redetermination of the fracture zone trends and inspection of the magnetic anomaly data in fig. 2 both indicate the pole is considerably farther north and somewhat to the east of the area indicated by Molnar and Sykes [1 ]. As a rise crest plate boundary approaches its rotational pole, the magnetic anomalies should converge, indicating the decrease in the rate of motion as the pole is approached. This spreading rate decrease from south to north is noted between the Siquieros and Orozco Fracture Zones and is used with the redetermined fracture zone trends to recompute the pole. The Cocos-Pacific pole is located at 40°N, 110°W and has an angular rate of 19.6 × 10-7 deg/yr. The Pacific-North America pole position of LePichon [10] is 53°N, 47°W. The 2.9 cm/yr spreading half-rate north of the Tamayo Fracture Zone [2] yields an angular rotation rate of 6.4 × 10 -7 deg/yr for this pole. The vector sum of the two poles above is the Cocos-North America pole. It is located at 29°N, 125°W and has an angular speed of 15.6 × 10-7 deg/yr.
427
The direction of underthrusting at the CocosNorth America plate boundary that results from this derived pole is 031 o along this entire plate boundary. The direction of this motion was previously known from 15 earthquake mechanism solutions to lie between 035 ° and 045 ° [1 ]. Our independent calculation is within 10 ° of these earthquake slip directions. Thus it appears that the rigid plate assumption is here largely correct. It is not clear that the 10 ° discrepancy is significant. Some degree of non-rigidity is possible in the North American plate at the Trans-Mexico Volcanic Belt. The rate of thrusting of the Cocos plate under the North America plate in western Mexico was determined by several qualitative means to be between 1.5 and 10 cm/yr [1 ]. The 10 cm/yr rate was computed from the Cocos-Pacific and Cocos-North America pole positions of Molnar and Sykes. As they point out, the pole locations were not well known and may lead to excessively large rates. A lower rate was computed by relating seismic moment to the earthquake magnitude, and slip rate in turn to the moment [11 ]. This rate (3.2 cm/yr for western Mexico) was compatible with the rate of 1.5 cm/yr determined from the down-dip length of the seismic zone [12]. Molnar and Sykes [1 ] point out that the seismic-moment method may give results too low by a factor of two [11 ]. The method using the down-dip length of the seismic zone assumes a steady state situation has persisted for the past 10 million years. Our computation indicates the rate of underthrusting is 7.9 cm/yr at 17°N, 98°W. It increases to 9.0 cm/yr at 15°N, 94°W (fig. 1). If these rates are in fact correct, then the 1.5 cm/yr rate computed by the other method indicates the present system may not be 10 million years old or that the time constant of 10 million years may be too large. Magnetic anomalies south of the Orozco Fracture Zone have not been correlated beyond seven million years, however, the East Pacific Rise north of the Rivera Fracture Zone was an active plate boundary between the Pacific plate and a northern extension of the Cocos plate 20 million years ago [13]. There is considerable evidence that plate boundaries have changed and spreading directions have thus been altered, at least in this northern region, from 5 to 15 million years ago [13, 14]. This and possible similar changes to the south may account for the low 1.5 cm/yr value.
428
R.L.LARSON and C.G.CHASE
Acknowledgements We are grateful to the following people for their help. R.L.Fisher made available his unpublished bathymetric contours of the area. R.L.Parker explained many principles of geometry on a spherical earth. J.D.Mudie, H.W.Menard, L.R.Sykes, J.R.Curray and R.L.Parker critically read and discussed the manuscript with us. This research was supported by the Office of Naval Research and the National Science Foundation.
[5] [6] [7]
[8]
[9]
References [1] P.Moinar and L.R.Sykes, Tectonics of the Caribbean and Middle America regions from focal mechanisms and seismicity, Geol. Soc. Amer. Bull. 80 (1969) 1639. [2] R.L.Larson, H.W.Menard and S.M.Smith, Gulf of California: A result of ocean-floor spreading and transform faulting, Science 161 (1968) 781. [3] T.A.Chase, Sea-floor topography of the central eastern Pacific Ocean: United States Dept. of Int., U.S.Fish and Wildlife Service, Bureau of Commercial Fisheries, Circular 291. [4] S.R.C.Malin, Synthesis of International Geomagnetic Reference Field values, Nature (in press).
[10] [11]
[12] [13]
[14]
IAGA Commission 2 Working Group 4, Analysis of the geomagnetic field, International Geomagnetic Reference Field 1965, J. Geophys. Res. 74 (1969) 4407. F.J.Vine and D.H.Matthews, Magnetic anomalies over oceanic ridges, Nature 199 (1963) 947. F.J.Vine, Spreading of the ocean floor: New evidence, Science 154 (1966) 1405. J.R.Heirtzler, G.O.Dickson, E.M.Herron, W.C.Pitman III and X.Le Pichon, Marine magnetic anomalies, geomagnetic field reversals and motions of the ocean floor and continents, J. Geophys. Res. 73 (1968) 2119. D.P.McKenzie and R.L.Parker, The north Pacific: An example of tectonics on a sphere, Nature 216 (1967) 1276. W.J.Morgan, Rises, trenches, great faults and crustal blocks, J. Geophys. Res. 73 (1968) 1959. X.Le Pichon, Sea-floor spreading and continental drift, J. Geophys. Res. 73 (i968) 3661. J.N.Brune, Seismic moment, seismicity and rate of slip along major fault zones, J. Geophys. Res. 73 (1968) 777. B.Isacks, J.Oliver and L.R.Sykes, Seismology and the new global tectonics, J. Geophys. Res. 73 (1968) 5855. C.G.Chase, H.W.Menard, R.L.Larson, G.F.Sharman III and S.M. Smith, History of sea-floor spreading west of Baja California, Geol. Soc. Amer. BuU. (in press). R.L.Larson, Near-bottom studies of the East Pacific Rise crest and tectonics of the mouth of the Gulf of California, unpub. Ph.D. dissertation, University of California, San Diego (1970) 153 p.
RELATIVE VELOCITIES OF THE PACIFIC, NORTH AMERICA AND COCOS PLATES IN THE MIDDLE AMERICA REGION* Roger L. LARSON** and Clement G. CHASE*** University of California, San Diego, Scripps Institution of Oceanography, La Jolla, California 92037, USA Received 22 December 1969 Relative velocities of crustal plates are used for the first time to test the conventional assumption that such plates are rigid. Spreading rates derived from magnetic anomaly data, fracture zone trends, and earthquake mechanisms at the Pacific-North America and Pacific-Cocos plate boundaries are used to predict the direction of thrusting of the Cocos plate under North America to be 031 °. As this direction is in near accord with previously determined earthquake mechanisms at that plate boundary, the assumption that the three plates are rigid appears to be largely justified. The calculated rate of underthrusting at the Cocos-North America plate boundary is about 8 era/yr.
1. Introduction The tectonics of the Middle America region have been recently interpreted by Molnar and Sykes [1 ]. Using seismicity and focal mechanisms they described the Pacific, North America, Cocos and Caribbean plates in that region and their directions o f relative motion (fig. 1). Spreading rates computed from magnetic anomaly profiles across the East Pacific Rise crest west of Mexico are combined here with focal mechanism results and bathymetry for the following purposes: (1) to redeterminate the Pacific-Cocos rational pole; (2) to test the rigid plate assumption for the three plates; and (3) to determine the rate of thrusting of the Cocos plate under the North America plate at the Middle America Trench.
2. Magnetic prof'de interpretations Larson et al. [2] computed the average spreading half-rate of 3.0 cm/yr on the East Pacific Rise at the mouth of the Gulf of California just north of the Rivera Fracture Zone. Their data show a spreading * Contribution of the Scripps Institution of Oceanography, n e w series.
** Marine Physical Laboratory. *** Department of Scripps Institution of Oceanography.
half-rate of 2.9 cm/yr north of the Tamayo Fracture Zone inside the Gulf. This rate and the trend of the Tamayo Fracture Zone, which is parallel to the San Andreas fault direction, define the relative motion between the North America and Pacific plates. Several lines o f evidence imply that the spreading rates immediately to the south and the trend of the Rivera Fracture Zone may not be representative of the North America-Pacific relative motion. Existence of a bathymetric depression beyond the continental margin [3], a possible difference in trends of the Tamayo and Rivera Fracture Zones and detailed examination of magnetic profiles across the rise ail suggest a small amount of underthrusting of the sea floor under North America at the Middle America Trench between the Rivera and Tamayo Fracture Zones. Chase [3] has mapped the East Pacific Rise and its related fracture zones between the Gulf of California and the equator (fig. 1). Magnetic anomaly profiles taken by various Scripps Institution of Oceanography expeditions across the rise between the Orozco and Siquieros Fracture Zones are shown in fig. 2 [4]. We interpret the anomalies to display the pattern of linearity, symmetry and parallelism to the rise crest that typifies the sea-floor spreading process as described by Vine and Matthews [5] and Vine [6]. The anomalies can be correlated on both sides
426
R.L.LARSON and C.G.CHASE
'"I IY' 9"108"
I IO*
I00"
90*
80*
Fig. 1. Relative motions and plate boundaries discussed in the text. Divergingarrows indicate crustal formation; converging arrows indicate crustal consumption. Rates are shown by the length of the solid arrows. Dashed arrows indicate directions of earthquake mechanisms. Names of plates are shown in large letters; fracture zones in small letters. Established plate boundaries are solid lines; uncertain plate boundaries are dashed lines. Location of fig. 2 shown by insert. Earthquake mechanism directions and some plate boundaries are from Moinar and Sykes [ 11]. The figure is a standard Mercator projection.
of the rise out to five, and perhaps seven, million years old. The model anomaly profile shown was computed from a fiat block model at 3 . 3 - 5 . 0 km below sea level with susceptibility -+ 0.01 emu/cm 3 Oe except for the central block which is + 0.02 emu/cm 3 0 e . The direction and intensity of remanent magnetization results from the simplification that the Earth's magnetic field is essentially a dipole, and the assumption that when averaged over > 105 years the dipole is aligned with the Earth's rotational axis. The vertical block boundaries were determined by combining the geomagnetic time scale of Heirtzler et al. [7] with a 5.4 cm/yr spreading half-rate. This appears to be the spreading
106"
104=
102Q
iOO°
Fig. 2. Magnetic anomaly profiles across the Cocos-Pacific plate boundary (East Pacific Rise crest) between the Orozco and Siquieros Fracture Zones. Correlations are shown by fine lines; dashed lines indicate poor correlations or tack of data. The total magnetic field measurements were reduced to magnetic anomaly form by removal of the I.G.R.F. [4]. The model anomaly profile is shown just above the time scale. Its trend is 076 ° which is normal to the strike of the structure. The figure is a standard Mercator projection.
half-rate at the location of the model profile. The observed half-rate increases from 4.8 cm/yr just south of the Orozco Fracture Zone to 6.9 cm/yr just south of the Siquieros Fracture Zone. Further south correlations can be made that indicate a 7.5 cm/yr half-rate near the equator, but at that low latitude the anomaly amplitudes are very small.
3. Relative velocity calculations McKenzie and Parker [8] and Morgan [9] demonstrated that it was useful to represent the relative motion of two rigid plates on the surface of the Earth by a single instantaneous angular velocity vector. The three relative angular velocities associated with any three plates must sum to zero. Spreading
RELATIVE VELOCITIESOF THE PACIFIC, NORTH AMERICA AND COCOS PLATES rates and fracture zone directions are used to determine the relative rotational poles and angular speeds for the Pacific-North America and the PacificCocos plates. These angular velocities are used to derive a Cocos-North America pole and predict the direction of underthrusting of the Cocos plate beneath the North America plate at the Middle America Trench. This result can t'e compared with the measured slip direction [1 ] at that plate boundary to test the assumption that the three plates in question are rigid. Molnar and Sykes [1 ] determined a right-lateral, strike-slip earthquake mechanism for event 168 on the Siquieros Fracture Zone. The strike of the fault plane is 076 °. This slip vector is a poorly determined one. It may be in error by "-+ 10 °, or maybe 20 °'' (Molnar, personal communication). However, it does align well with the Siquieros Fracture Zone at this location, whose trend is well established (H.W.Menard, personal communication). Molnar and Sykes [1] used the strikes of the Orozco and Siquieros Fracture Zones as contoured by Chase [3] to determine the CocosPacific rotational pole as lying "between 20°N, 108°W and 27°N, 114°W ''. R.L.Fisher (personal communication) has recontoured these fracture zones as more parallel figures than those shown by Chase [3]. Their younger portions trend about 080 ° . This redetermination of the fracture zone trends and inspection of the magnetic anomaly data in fig. 2 both indicate the pole is considerably farther north and somewhat to the east of the area indicated by Molnar and Sykes [1 ]. As a rise crest plate boundary approaches its rotational pole, the magnetic anomalies should converge, indicating the decrease in the rate of motion as the pole is approached. This spreading rate decrease from south to north is noted between the Siquieros and Orozco Fracture Zones and is used with the redetermined fracture zone trends to recompute the pole. The Cocos-Pacific pole is located at 40°N, 110°W and has an angular rate of 19.6 × 10-7 deg/yr. The Pacific-North America pole position of LePichon [10] is 53°N, 47°W. The 2.9 cm/yr spreading half-rate north of the Tamayo Fracture Zone [2] yields an angular rotation rate of 6.4 × 10 -7 deg/yr for this pole. The vector sum of the two poles above is the Cocos-North America pole. It is located at 29°N, 125°W and has an angular speed of 15.6 × 10-7 deg/yr.
427
The direction of underthrusting at the CocosNorth America plate boundary that results from this derived pole is 031 o along this entire plate boundary. The direction of this motion was previously known from 15 earthquake mechanism solutions to lie between 035 ° and 045 ° [1 ]. Our independent calculation is within 10 ° of these earthquake slip directions. Thus it appears that the rigid plate assumption is here largely correct. It is not clear that the 10 ° discrepancy is significant. Some degree of non-rigidity is possible in the North American plate at the Trans-Mexico Volcanic Belt. The rate of thrusting of the Cocos plate under the North America plate in western Mexico was determined by several qualitative means to be between 1.5 and 10 cm/yr [1 ]. The 10 cm/yr rate was computed from the Cocos-Pacific and Cocos-North America pole positions of Molnar and Sykes. As they point out, the pole locations were not well known and may lead to excessively large rates. A lower rate was computed by relating seismic moment to the earthquake magnitude, and slip rate in turn to the moment [11 ]. This rate (3.2 cm/yr for western Mexico) was compatible with the rate of 1.5 cm/yr determined from the down-dip length of the seismic zone [12]. Molnar and Sykes [1 ] point out that the seismic-moment method may give results too low by a factor of two [11 ]. The method using the down-dip length of the seismic zone assumes a steady state situation has persisted for the past 10 million years. Our computation indicates the rate of underthrusting is 7.9 cm/yr at 17°N, 98°W. It increases to 9.0 cm/yr at 15°N, 94°W (fig. 1). If these rates are in fact correct, then the 1.5 cm/yr rate computed by the other method indicates the present system may not be 10 million years old or that the time constant of 10 million years may be too large. Magnetic anomalies south of the Orozco Fracture Zone have not been correlated beyond seven million years, however, the East Pacific Rise north of the Rivera Fracture Zone was an active plate boundary between the Pacific plate and a northern extension of the Cocos plate 20 million years ago [13]. There is considerable evidence that plate boundaries have changed and spreading directions have thus been altered, at least in this northern region, from 5 to 15 million years ago [13, 14]. This and possible similar changes to the south may account for the low 1.5 cm/yr value.
428
R.L.LARSON and C.G.CHASE
Acknowledgements We are grateful to the following people for their help. R.L.Fisher made available his unpublished bathymetric contours of the area. R.L.Parker explained many principles of geometry on a spherical earth. J.D.Mudie, H.W.Menard, L.R.Sykes, J.R.Curray and R.L.Parker critically read and discussed the manuscript with us. This research was supported by the Office of Naval Research and the National Science Foundation.
[5] [6] [7]
[8]
[9]
References [1] P.Moinar and L.R.Sykes, Tectonics of the Caribbean and Middle America regions from focal mechanisms and seismicity, Geol. Soc. Amer. Bull. 80 (1969) 1639. [2] R.L.Larson, H.W.Menard and S.M.Smith, Gulf of California: A result of ocean-floor spreading and transform faulting, Science 161 (1968) 781. [3] T.A.Chase, Sea-floor topography of the central eastern Pacific Ocean: United States Dept. of Int., U.S.Fish and Wildlife Service, Bureau of Commercial Fisheries, Circular 291. [4] S.R.C.Malin, Synthesis of International Geomagnetic Reference Field values, Nature (in press).
[10] [11]
[12] [13]
[14]
IAGA Commission 2 Working Group 4, Analysis of the geomagnetic field, International Geomagnetic Reference Field 1965, J. Geophys. Res. 74 (1969) 4407. F.J.Vine and D.H.Matthews, Magnetic anomalies over oceanic ridges, Nature 199 (1963) 947. F.J.Vine, Spreading of the ocean floor: New evidence, Science 154 (1966) 1405. J.R.Heirtzler, G.O.Dickson, E.M.Herron, W.C.Pitman III and X.Le Pichon, Marine magnetic anomalies, geomagnetic field reversals and motions of the ocean floor and continents, J. Geophys. Res. 73 (1968) 2119. D.P.McKenzie and R.L.Parker, The north Pacific: An example of tectonics on a sphere, Nature 216 (1967) 1276. W.J.Morgan, Rises, trenches, great faults and crustal blocks, J. Geophys. Res. 73 (1968) 1959. X.Le Pichon, Sea-floor spreading and continental drift, J. Geophys. Res. 73 (i968) 3661. J.N.Brune, Seismic moment, seismicity and rate of slip along major fault zones, J. Geophys. Res. 73 (1968) 777. B.Isacks, J.Oliver and L.R.Sykes, Seismology and the new global tectonics, J. Geophys. Res. 73 (1968) 5855. C.G.Chase, H.W.Menard, R.L.Larson, G.F.Sharman III and S.M. Smith, History of sea-floor spreading west of Baja California, Geol. Soc. Amer. BuU. (in press). R.L.Larson, Near-bottom studies of the East Pacific Rise crest and tectonics of the mouth of the Gulf of California, unpub. Ph.D. dissertation, University of California, San Diego (1970) 153 p.