JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 87, NO. B8, PAGES 6751-6759, AUGUST 10, 1982 Northern East PacificRise' Evolution From 25 m.y.B.P. to the Present JACQUELINE MAMMERICKX Geological Research Division, Scripps Institution of Oceanography La Jolla, California 92093 KIM D. KLITGORD U.S. Geological Survey, Woods Hole, Massachusetts 02543 The northeast Pacific topography and magnetic lineations (25 m.y.B.P. to the Present)record the traces of three major spreading reorganizations. Only one spreading centeris observed today, but there is evidence for severalephemeral episodes of twin spreading accompanying the evolution from an extensive Pacific-Guadalupe plate boundary to a much shortened Pacific-Cocos and Pacific-Rivera plateboundary. The 25 m.y.B.P. platereorganization culminated with the formation of the Guadalupe plate, bound by the Murray fracture zone to the north and the Cocos-Nazca spreading ridge to the south. Between25 and 12.5 m.y.B.P. spreading continued while the plate retained the same general outline. The 12.5-11 m.y.B.P. reorganizationresulted in the creation of a much shortened Pacific-Cocos plate boundarylocatedin its early stages over the Mathematician seamounts and a much reduced Cocos-plate. The last reorganizaton (6.5-3.5 m.y.B.P.) resulted in the abandonment of the Mathematician spreading ridge as a Pacific-Cocos plate boundary in favor of the East Pacific Rise. INTRODUCTION The tectonic history of the northeastern Pacific for the last 25 m.y. has included at least three major plate motion reorganizations. The magnetic anomaly and bathymetric data between the equator and 30øN latitude which have recorded this history are discussed in detail by K!itgordand Mammerickx [this issue]. Each of these reorganizations was of short duration followed by long periods of rela- tively uniform seafloor spreading. Discontinuitiesor dis- turbances in the magnetic anomaly and topographic features are associated with these short events. In this paper we examine in more detail these reorganizationsand construct a history for the evolution of the East Pacific Rise (EPR) during the last 25 m.y. This historycomple- ments the history of the eastern Pacific to the north [Menard, 1978] and to the south [Handschumacher, 1976]. A graphic display of this history is summarized in the tec- tonic map for the northeastern Pacific (Figure 1). The region contains several segmentsof abandoned spreading centers, an abandoned trench, other old plate boundaries caused by plate reorganizations, and the presently active spreading center, subductionzone, and transform faults. A major plate reorganizationin the Pacificwas triggered by the intersectionof the Pacific-Farallon spreading center with the subduction zone bordering North America just south of the Mendocino fracture zone (Atwater [1970] and Figure 2a). The ages of the youngest recognizable anomalieson the Pacifcplate decreases progressively from a high value of 29 m.y.B.P. just south of the Mendocino fracture zone to 25 m.y.B.P. just north of the Murray fracture zone. Progressive consumption of the Pacific- Farallon spreading center by the subduction zone and the Copyright ¸ 1982 by the American Geophysical Union. Paper number 1B1869. 0148-0227/82/001B-1869505.00 replacement of the spreading center and subduction zone as plate boundaries by a lengthening transform fault provide an explanation for this age progression as well as the fault zone that was developing at this time [Atwater, 1970]. Handschumacher [1976] suggested that the direct coupling of the America and Pacific plates resulted in a radical change in plate spreading kinematics. While the timing may be coincidentwith this change from a conver- gent to tranform boundary(not coupling) along western North America, the actual changesin plate kinematics are dominated by the breakup of plates enabling them to move more independently. In our reconstruction model based on the reevaluation of magnetic and bathymetric data we retain the part of Atwater's model that proposes a progressive consumption of the Pacific-Farallonridge from the Mendocino fracture zone (29 m.y.B.P.) to the Murray fracturezone (25 m.y. B.P.). The evolution for youngercrust appears thereafter to be marked by episodesof rapid and major reorganiza- tion of plate boundaries alternating with episodesof rela- tive stability of these same boundaries. Several segments of East Pacific Rise form the spread- ing center that presently separatesthe Pacific plate from the Rivera, Cocos, and Nazca plates. The Late Cenozoic tectonic evolution of this accreting plate boundary has been dominated by the breakup of larger plates as seg- ments of the spreading center approached the subduction zone. We shall examine in detail the data pertaining to the evolution of one of the new plates: the Guadalupe plate [Menard, 1978] which extended from the Mendocino and then the Murray fracture zone (off southern California) to the Grijalva fracturezone (near the equator). In late Oli- gocene (29 m.y.B.P.), smaller platesbroke off from the Farallon plate north of the Mendocino fracture zone [Atwater, 1970; Herron, 1972; Handschumacher, 1976; Menard, 1978]. Just after magnetic anomaly 7 (25 m.y. 6751