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Paleogene History of the Kula Plate: Offshore Evidence and Onshore Implications

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Paleogene History of the Kula Plate: Offshore Evidence and Onshore Implications Paleogene history of the Kula plate: Offshore evidence and onshore implications PETER LONSDALE Scripps Institution of Oceanography, University of California, San Diego, La Jolia, California 92093 ABSTRACT INTRODUCTION 45-35 mm/yr until the time of Anomaly 25 (late Paleocene, 59 Ma, according to the DNAG Paleocene to middle Eocene magnetic Soon after the present pattern of lithospheric time scale of Berggren and others, 1985, that is anomalies were mapped over oceanic crust plates had been delineated by mapping the char- used throughout this paper). The "half-rate" is that accreted at the Kula-Pacific spreading acteristic structures and seismicity of their the rate of accretion to the Pacific plate; by as- center and is now obliquely entering the boundaries, Pitman and Hayes (1968) recog- suming that spreading had been symmetric, western Aleutian Trench between 179°E and nized that east-west-striking, southward-aging, Engebretson and others (1984,1985) were able 168°E. The strike of anomalies and the pat- magnetic anomalies south of the Aleutian to use these data for quantitative modeling of the tern of abyssal hills and fracture zones Trench implied the former existence of a now- 82-59 Ma motion of the Kula plate relative to changed abruptly during 56-55 Ma, when vanished plate, part of the North Pacific floor the Pacific plate and its hot spots, and thereby north-south spreading veered to northwest- that had moved northward faster than had the relative to the other plates that had bounded it southeast (310°-130°). Kula-Pacific spread- Pacific plate. Grow and Atwater (1970), believ- (Farallon, North American, Eurasia). It is clear ing ceased in 43 Ma. A 75-km-long section of ing that the hypothetical plate had been entirely from these models, and from similar earlier re- the fossil Kula Rift axis has avoided subduc- consumed at subduction zones, named it Kula (= constructions (for example, Jackson and others, tion, although it now intersects the trench all gone); it was the first recognized of several 1978), that the Kula plate did not "die" by oblit- axis (almost orthogonally) near 171.5°E. A such plates (Izanagi, Aluk, and others) now eration when its trailing edge entered the trench narrow remnant of the former Kula plate, known to have been involved in accretion of the and spreading was stifled, as Pitman and Hayes northwest of this fossil spreading center, is present Pacific floor and in the creation of plate- (1968), Grow and Atwater (1970), and deLong bounded by a fossil Kula-Pacific transform boundary structures that survive around its rim. and others (1978) had inferred. Instead it lost its with a high transverse ridge alongside a The magnetic anomalies that require the ex- separate identity sometime in the Eocene by fus- sediment-filled transform valley. Anomalies istence of the Kula plate also constrain the time ing with the Pacific plate while most of the on this remnant show that Eocene Kula- of its birth and its subsequent motion. The plate Kula-Pacific boundary was still far from any Pacific spreading was highly asymmetric originated by splitting from the Farallon plate trench, leaving a fossil Kula Ridge to be subse- (2:1). during Late Cretaceous rifting (Rea and Dixon, quently subducted. The 56-55 Ma change in Kula plate rota- 1983), but almost all of the crust accreted by No direct information on when Kula-Pacific tion inferred from the change in spreading Kula-Farallon spreading was later subducted spreading ceased has been obtained from the direction coincided with birth of the Aleutian beneath North America, and only Kula-Pacific well-mapped truncated rise flank south of the subduction zone, and was probably a conse- motion has left a usable magnetic record. Chi- eastern Aleutian Trench, because no fossil quence of the resulting change in slab-pull nook Trough (Fig. 1) was one site of initial spreading axes have been recognized, and no stresses on the oceanic lithosphere. The Kula-Pacific rifting (Erickson and others, 1969; anomalies younger than Anomaly 25 have change in direction of Kula-North American Woods and Davies, 1982), and the Upper Cre- avoided subduction, except for a tentatively motion is a plausible explanation for the de- taceous and Paleocene crust between this rift identified Anomaly 24 now in the trench east of tachment of continental terranes from the valley and the Aleutian Trench represents the 160°W (Peter and others, 1970). Byrne (1979) Pacific Northwest and their migration around truncated southern flank of a Kula-Pacific rise, inferred a 56 Ma date for cessation of Kula the Gulf of Alaska, and for the early Eocene the Kula Ridge of Grow and Atwater (1970). A Ridge spreading, based on the mapped pattern demise of Alaska Range arc volcanism. The dense coverage of bathymetric and magnetic of Pacific-Farallon anomalies in the Gulf of cessation of Kula-Pacific spreading coincides data, including results from the systematic Alaska. Engebretson and others (1984) specu- with a major change in Pacific-plate rotation, SEAMAP surveys (Erickson and Grim, 1969; lated that spreading continued until the Pacific and the subsequent direction of convergence Peter and others, 1970), allowed Grim and plate changed direction at 43 Ma; in which case, of the Pacific plate with the Aleutian arc was Erickson (1969) and Hayes and Heirtzler (1968) the Kula plate had an undetermined 15-m.y. similar to the 55-43 Ma direction of Kula- to estimate that the rise had spread almost ex- Eocene history. plate convergence. actly north-south with an average "half-rate" of In 1986 we explored the Pacific floor south of Geological Society of America Bulletin, v. 733-754,15 figs., May 1988. 733 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/5/733/3380331/i0016-7606-100-5-733.pdf by guest on 28 September 2021 Figure 1. Successive interpretations of crustal patterns south of the western Aleutian Trench, based on increasing amounts of data. Favored interpretation £ relies on a few additional post-1970 magnetic and bathymétrie profiles, as well as the new data along the Seabeam track (dashed line). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/100/5/733/3380331/i0016-7606-100-5-733.pdf by guest on 28 September 2021 PALEOGENE HISTORY OF KULA PLATE 735 the western Aleutian Ridge, where there were the extent of early Tertiary crust in the north- of old abyssal-hill faults. West of 178°E, how- two reasons for suspecting that more of the later west Pacific. ever, where the pre-existing faults strike into the history of Kula-Pacific spreading might be pre- trench at a high angle (parallel to the magnetic served, even though the extent of the Tertiary THE BOUNDARIES OF stripes in Fig. 6), new fractures are formed, and crust was poorly known. First, the western part TERTIARY CRUST IN THE the abyssal hill relief is obliterated by deep gra- of the trench has consumed only a small area of NORTHWEST PACIFIC bens subparallel to the trench axis (for example, ocean crust since 43 Ma, because of its high northeast end of profile C, Fig. 5). obliquity to Pacific-North American motion The Cretaceous/Tertiary crustal boundary on (McKenzie and Parker, 1967). Second, Erickson the south flank of Kula Ridge is marked by the STRUCTURAL PATTERNS OF and others (1970) mapped a large left-lateral east-west line of Chron 30, 300-500 km south PALEOGENE CRUST SOUTH OF offset of magnetic anomalies near 178°E, which of the trench axis (Figs. IE, 4). No east-west THE WESTERN ALEUTIAN RIDGE brought the late Paleocene crust west of their Tertiary anomalies have been identified beyond "Rat fracture zone" more than 300 km south of 174.8°E, even on the well-placed north-south Paleocene Crust East of I78°E the trench axis (Fig. 1C). The pioneering study magnetic profiles of Erickson and others (1970) of Hayes and Heirtzler (1968), however, as- (Fig. 1C). The implication is that the Paleocene My maps of crustal patterns east of Rat frac- signed a right-lateral offset to the large fracture Kula Ridge was bounded on the west by a ture zone (Figs. IE, 6) differ from previous in- zone, leaving less room for Paleogene crust major transform fault whose trace (labeled terpretations in showing a set of oblique, south of the western part of the trench (Fig. 1 A), "Stalemate fracture zone" on Profile C of northwest-trending fracture zones, none of and this interpretation was followed by Rea and Fig. 5) is now near 174.5°E. A set of magnetic which intersects our Seabeam swath between Dixon (1983). Our most novel results, including lineations west of this meridian, tentatively iden- the normal Rat and Amlia fracture zones, al- definition of the western boundary of Tertiary tified with Chron 32 (for example, on the Con- though they cause repetition of anomalies on crust and discovery of a fossil Kula-Pacific rad 1108 profile Figs. 1 A, 4), could be evidence north-south tracks. Hayes and Heirtzler (1968, spreading center attached to a sizeable remnant that Kula-Pacific spreading extended farther p. 4642) could not explain this phenomenon of Kula plate, come from west of Rat fracture west during the Early Cretaceous. If so, a fossil with any "reasonable orientation" of fracture zone. New data farther east are also important Kula-Pacific spreading center may exist in the zones, but oblique fracture zones were subse- for documenting the response of the Kula- poorly surveyed region east of the northern Em- quently recognized as common features else- Pacific spreading center to changes in relative peror Trough, which is itself interpreted as a where in the North Pacific (Bassinger and plate motions.
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