New contraints on Rivera-Pacific relative motion from multibeam bathymetric data along the MSS and Rivera Transform

To better constrain the Euler pole describing the recent motion of the Pacific plate relative to the Rivera plate, total- field magnetic data, multi-beam bathymetric data and sidescan sonar images were collected during the BART and FAMEX campaigns of the N/O L'Atalante conducted in April and May of 2002 in the area surrounding the Moctezuma Spreading Segment of the East Pacific Rise, located offshore of Manzanillo Mexico at 106°16'W and between 17.8°N and 18.5°N, and the adjacent Rivera Transform. Better constraining Rivera-Pacific relative motion is crucial for understanding the geologic forces and associated hazards present in western Mexico. Among the main results are: (1) the principle transform displacement zone of the Rivera Transform is very narrow and well defined east of 107° 15'W and these azimuths should be used preferentially when determining plate motions in this area, and (2) spreading rates along the Moctezuma Spreading center should not be used in plate motion studies as either seafloor spreading has been accommodated at more than one location since the initiation of seafloor spreading in the area of the Moctezuma Spreading Segment, or this spreading center is not a Rivera-Pacific plate boundary as has been previously assumed. Comparison of observed transform azimuths with those predicted by six previous models of Rivera- Pacific relative motion indicate the need to redefine the current model of Rivera-Pacific relative motion. Thus, using these results, a new plate motion model, RP2005-2, is presented which better describes Rivera-Pacific relative motion occurring during the past several hundred thousand years.