HR: 15:15h
AN: U53A-06    [Abstracts]
TI: An Updated Interpretation to the Position of the Kula-Farallon Ridge Against North America During the Late Cretaceous: Isotopic Evidence.
AU: * Guerrero-Garcia, J C
EM: josec@servidor.unam.mx
AF: Instituto de Geologia, UNAM, Cd Universitaria, Mexico, DF 04510, Mexico
AU: Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: SOEST-HIGP, Paleomagnetics and Petrofabrics Laboratory, University of Hawaii at Manoa, 1680 East West Rd, Honolulu, HI 96822, United States
AB: 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 and the state of Oaxaca, a stripe 1,100 Km long, 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. These results agree with previous published results. 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 [1982]. 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 suggest 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. The counterclockwise paleomagnetic rotations reported from both the Pacific coast of Mexico and Honduras support these conclusions. On the other hand, the analyses 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 and66 Ma ago. This time interval is characterized as well by the absence of igneous activity in the Sierra Nevada. More recently [Umhoefer, 2003] has diminished the length of displacement of Baja to 1,650 Km assigning the position of the triple junction to a point in the neighborhood of the USA-Mexican border on the Pacific coast. From the apparently synchronous nature of the displacement of British Columbia to the north and Chortis to the south is inferred that the triple point Kula-Farallon-North America was located somewhere near the latitude of Puerto Vallarta at the time of break-up of the Pacific plate during the early Campanian, 85 Ma ago.
DE: 1100 GEOCHRONOLOGY
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 7205 Continental crust (1219)
DE: 8100 TECTONOPHYSICS
SC: Union [U]
MN: 2007 Joint Assembly