AN: V51A-11TI: The Position of the Kula-Farallon Ridge Against North America During    the Late CretaceousAU: Jose C. Guerrero-GarciaAF: Instituto de Geologia, UNAM, Cd. Universitaria, 04510 Mexico City,    MexicoEM: josec@servidor.unam.mxAU: Emilio Herrero-BerveraAF: SOEST-HIGP, 2525 Correa Rd., Honolulu,Hawaii 96822EM: herrero@akolea.soest.hawaii.eduAB: The initial break-up of the Pacific plate about 85 Ma ago into the    Izanagi, Pacific, Kula and Farallon plates proposed by Woods and    Davies (1982), has been documented by Engebretson et al. (1985) based    on the absolute movement of the plates with respect to fixed Hot Spots    (Hawaii and Yellowstone). Due to the lack of data from the oceanic    crust that has already been subducted at the trench, in their    corresponding reconstruction at 80 Ma ago, the uncertainty exists as    to whether the crest was abutting against North America in southern    Guatemala or north of Vancouver, an uncertainty of about 4500 km.    Isotopic ages of crystalline rocks along the pacific coast of Mexico    from the state of Jalisco to the state of Oaxaca, a stripe 1,100 km    long (Guerrero-Garcia and Herrero-Bervera,1993), indicate a systematic    decrease in Rb/Sr mineral ages, from 80 Ma in Jalisco to 11 Ma in    Oaxaca,apparently related to the migration of the Chortis block    (Honduras and Nicaragua) being detached from Mexico and transported by    the Farallon plate toward the southeast. At Puerto Vallarta, Jalisco    the biotite mineral age obtained is 80 Ma, soon after the    fragmentation of the Pacific plate proposed by Woods and Davies. This    change in tectonic regime from a subduction zone to a transform margin    is documented in that part of Mexico by a magmatic hiatus lasting    roughly 25 Ma that initiates about 80 Ma ago, as well (Pantoja-Alor,    1983). These data suggests that the Kula-Farallon ridge had to be    located somewhere north of Puerto Vallarta, at the time of    fragmentation of the Pacific plate 85 Ma ago. On the other hand, the    analysis of paleomagnetic data and field geology from British Columbia    (Umhoefer, 1987) indicates a large displacement of that region along    the pacific margin of North America, of the order of 2,400 km, from    the present-day position of the Baja California Peninsula to its    current location, transported by the Kula plate to the north along a    transform margin, between 85 Ma and 66 Ma ago. This time interval is    characterized as well by the absence of igneous activity in the Sierra    Nevada. From the synchronous nature of the displacement of British    Columbia to the north and Chortis to the south it is inferred that the    triple point Kula-Farallon-North America was located at the latitude    of Puerto Vallarta at the time of break-up of the Pacific plate during    the early Campanian, 85 Ma ago.SC: VDE: 3040DE: 8157DE: 9614MN: 1997 Fall Meeting