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Master of Science (MS)
Engebretson, David C.
Burmester, Russell F.
Talbot, James L.
A new method is presented and used to determine estimates of instantaneous relative motion vectors for the Pacific plate relative to the Farallon and Kula plates between magnetic chrons 34 and 24 (84 to 56 Ma). A weighted chi-square minimization technique was employed that has as input sequential magnetic anomaly picks taken from original ship-track profiles, and fracture-zone azimuths estimated from bathymetric control and offsets in magnetic lineations. Assignment of errors to the input data allows the use of critical chi-square limits to obtain confidence intervals on pole locations and their angular rates. Eleven distinctive magnetic anomalies chosen as control points within the 28 million year time span result in ten possible instantaneous relative motion poles. Within the uncertainties, 3 distinct pole positions have been modeled for Pacific-Kula spreading spanning the time between chrons 34 to 33 (84 to 77 Ma), 33 to 25 (77 to 59 Ma), and 25 to 24 (59 to 56 Ma). Six significant rate changes are seen within the interval between chrons 33 and 25. These results are consistent with but more detailed than earlier hypotheses, which suggested that Pacific-Kula relative motions can be described by a single pole location between chron 32 and 25, accompanied by three changes in angular rates. Previous workers have determined that Pacific-Farallon relative motion can be described by two pole locations; this study used these pole positions to update Pacific-Farallon angular rates for the ten time intervals.
Updated relative motions were used in combination with the trace of the Pacific- Farallon-Kula (PFK) triple junction to investigate the possibility of asymmetric spreading. Except for two time intervals, the updated relative motions agree well with the observed migration of the PFK triple junction (as seen at the apexes of the Great Magnetic Bight) which implies, at least on average, consistency with the assumption of symmetric spreading.
Results of this study differ from those of previous studies in the greater number of relative motion changes found, presumably in response to changes along Kula and Farallon subduction zones. These updated models should provide increased insight into the cause of changing tectonic environments along western North America during the late Cretaceous and early Tertiary.
Western Washington University
North Pacific Ocean
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Cashman, Harold J. (Harold John), "A New Determination of Pacific-Kula Relative Motions" (1990). WWU Graduate School Collection. 657.