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Paleomagnetic During Evidence for Motion of the Hawaiian Hotspot

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Paleomagnetic During Evidence for Motion of the Hawaiian Hotspot EPSL ELSEVIER Earth and Planetary Science Letters 153 (1997) 171-180 Paleomagnetic evidence for motion of the Hawaiian hotspot during formation of the Emperor seamounts John A. Tarduno *, Rory D. Cottrell Department of Earth and Environmental Sciences, University of Rochester, Rochester, NY 14627, USA Received 13 June 1997; revised 23 September 1997; accepted 23 September 1997 Abstract The bend in the Hawaiian-Emperor chain is the best example of a change in plate motion recorded in a fixed-hotspot frame of reference. Alternatively, the bend might record primarily differences in motion of the Hawaiian hotspot relative to the Pacific lithosphere. New paleomagnetic data from the Emperor chain support the latter view. Although the rate of motion is difficult to constrain because of uncertainties posed by true polar wander and limited sampling of the chain, the best available paleomagnetic data suggest Pacific hotspots may have moved at rates comparable to those of lithospheric plates ( > 30 mm yr- ’ ) in late Cret aceous to early Tertiary times (81-43 Ma). If correct, this requires a major change in how we view mantle dynamics and the history of plate motions. In the early to mid-Cretaceous (128-95 Ma), hotspots in the Atlantic moved at similar rates. These episodes during which groups of hotspots appear to move rapidly are separated by times of much slower motion, such as the past 5 m.y. 0 1997 Elsevier Science B.V. Keyvords: Hawaii; Emperor Seamounts; hotspots; plate tectonics; movement: paleomagnetism 1. Introduction tion of a change in plate motion in a fixed hotspot reference frame. Because the bend is so distinct it Many of our ideas on where mantle plumes origi- can be used to estimate plume diameters and to place nate, how they interact with the convecting mantle bounds on the convecting mantle wind that may and how plates have moved in the past rest on deflect plumes [l]. interpretations of the Hawaiian-Emperor hotspot However, shortly after hotspots were used as a track. One reason the track has attained this concep- frame of reference [2], apparent discrepancies involv- tual stature lies in its prominent bend at 43 Ma. The ing the Hawaiian-Emperor track arose [3]. Attempts bend, which separates the westward-trending Hawai- to model past plate motions failed to predict the ian islands from the northward-trending Emperor bend; instead, a more westerly track was derived [4]. seamounts (Fig. l), has no equal among the Earth’s Tests of the fixed hotspot hypothesis suggested large hotspot tracks; it is the clearest physical manifesta- relative motions between Hawaii and other hotspots [3,5], but uncertainties in the plate circuits employed in these tests limited their resolving power [6]. * Corresponding author. Recently Norton [7] has suggested that the bend 0012-821X/97/$17.00 0 1997 Elsevier Science B.V. All rights reserved. PII SOO12-821X(97)00169-6 172 J.A. Tarduno, R.D. Cottrell/ Earth and Planetav Science Letters 153 (1997) 171-180 records when the Hawaiian hotspot became fixed in 2. Detroit Seamount the mantle, rather than a change in plate motion. Prior to 43 Ma, the Hawaiian hotspot would have During Ocean Drilling Program Leg 145, 87m of moved southward, creating the Emperor seamount massive and pillowed lava flows were penetrated on chain. This proposal is testable by paleomagnetism. Detroit Seamount (Hole 884E: 51”27.034’N, If the hotspot has remained fixed, the paleolatitudes 168”20.216’E). The basalt sequence can be separated of extinct volcanic edifices comprising the Emperor into 13 lithologic units based on chilled margins and chain should equal that of present-day Hawaii. New phenocryst content (Fig. 1) [8]. “‘Ar/ 39Ar radiomet- data obtained from Detroit Seamount, part of Em- ric data yield a plateau age of 8 1.2 + 1.3 Ma for a peror chain near the Aleutian-Kuril trench (Fig. 1) plagioclase free component and an isochron age date allow us to conduct such a test. of 80.0 k 0.9 Ma [9]. This age is older than the 160’ 160’ 200’ 220’ @St/l&o Nintoku _ i7 _ B B C _ _ _ C c IT D D E _5 _E _T F F G G z H _ _ H H I ; J L ; 8 80 -60 -40 -20 0 Flow Inclination Inclination (O) Units Groups Fig. 1. Basalt stratigraphy [8] and characteristic remanent magnetization (ChRM) inclinations vs. depth in meters below seafloor (mbsf) from Detroit Seamount. Open symbols represent positive inclinations (flow unit 4) that have been inverted. Inset is a Mercator projection of the North Pacific Basin showing the Hawaiian-Emperor Seamount chain with locations of Detroit (triangle) and Suiko seamounts (square). Inclination groupings are based on lithology and inclination-only averaging [ 15- 171. In the 1 l-inclination group model, adjacent inclination averages are distinct at the 90% confidence level; in the lo-inclination group model (preferred), averages are distinct at > 95% confidence. J.A. Tarduno, R.D. Conrell/Earth and Planetar?; Science Letters 153 (1997) 171-180 173 65-75 Ma age assumed in hotspot-based plate mo- ence of hematite (Fig. 2). If this hematite carries a tion models [lo]. coherent field direction, there should be a consistent Tarduno and Gee [I l] derived a paleolatitude of difference between its declination and the declination 32.6” from preliminary paleomagnetic data collected isolated at lower temperatures, for samples of the by the Shipboard Scientific Party [8]. This nominal same lithologic unit. Such consistency was not ob- value does not agree with the current position of the served. The inclination of the high unblocking tem- Hawaiian hotspot or any predictions based on other perature component is also inconsistent between paleomagnetic data [ 111. The dispersion character- lithologic units, leading us to conclude that hematite istics of the preliminary data suggest that a reliable carries no useful geomagnetic signal in these rocks. paleolatitude might be obtainable with a thorough Characteristic remanent magnetizations (ChRMs) land-based study [ 111. calculated from the thermal demagnetization data and those derived from the alternating field data are very similar (AF values: I = 57.9Y:!:30, k = 20, n = 3. Rock magnetism and paleomagnetism 10). But because hematite can bias alternating field Azimuthally unoriented samples (n = 94) were results, we consider only the thermal demagnetiza- collected from the recovered basalt cores and ana- tion data below. lyzed in the Paleomagnetic Laboratory at the Univer- Nearly all the ChRM’s have negative inclinations, sity of Rochester. Koenigsberger ratios for the sam- the only exception being samples from lithologic ples average 9.89, suggesting high stability of rema- unit 4 (n = 6). The coring record suggests that it is nence. Magnetic hysteresis curves show character- unlikely these positive inclinations are artifacts istics ranging from multi- to single- domain, but over caused by the accidental inversion of samples during half the data have parameters attributable to single core recovery or storage [S]. Assuming a northern domain behavior. Together the hysteresis parameter hemisphere origin, the negative inclinations denote data lie along a trend that mimics that displayed by reversed polarity. This polarity assignment is consis- magnetite and low-titanium titanomagnetites [12]. tent with the “Ar/ j9Ar radiometric age data that This similarity is also seen in unblocking tempera- suggests eruption of the basahs during chron 33R of ture characteristics. the Campanian [13]. Some prior work in the Pacific Each sample was subjected to detailed thermal has noted a possible geomagnetic excursion within demagnetization (25°C steps with a temperature range sediments recording chron 33R [14]; the positive of 50-675°C). A subsample from each unit was also inclinations observed from lithologic unit 4 might demagnetized using stepwise alternating field treat- record this excursion. Because excursions could have ment in increments of 5-10 mT (5-100 mT). Upon a cause different from that of normal secular varia- thermal and alternating field demagnetization, most tion, we have excluded data from unit 4 from our samples showed a univectorial decay after the re- subsequent inclination analysis. moval of a small viscous magnetization (Fig. 2), These positive inclinations, however, provide allowing calculation of a characteristic direction with valuable information on the fidelity of the magneti- principal component analysis (n = 79). zation isolated. A common source of bias in pateo- Some exceptions to this ideal behavior were noted. magnetic data derived from oceanic core material is In a few samples, a stronger and coherent low-tem- a nearly vertical drilling-induced remanence. The perature component was observed, attributable to the positive inclinations argue against the presence of modem field at the site. For ten samples, the demag- such an overprint because they are nearly opposite netization decay was less regular and a Fisher aver- the mean of the negative inclinations (see below). age was used to obtain the final direction and mag- netic alteration caused by thermal treatment forced 4. Inclination group models and secular variation us to reject results from five samples. Approximately 10% of samples analyzed showed Another potential problem in obtaining paleomag- an additional component having unblocking tempera- netic data from a basalt drill hole is the uncertain tures greater than 580°C which indicates the pres- timescale between eruptions. If most flows reflect 74 J.A. Tarduno, R.D. Cottrell/ Earth and Planetary Science Letters 153 (19971 171-180 a. b. No h,Up North,Up 1 25 West East West East South. Down Sodh,Down 4 North,Up C. North,Up I 325 t 1 West West ast Sodth,Down South.Down J.A. Tarduno, RD. Cottrell/Earth and Planetap Science Letters 153 (1997) 171-180 175 a. , 1 -80 ? F-60 0 ‘E g -40 = 2 -20 - Predicted I from Pacific APWP -a0 - e 9 60 - ‘iiz 40 - 2 i 1 I 2 20 r Present day latitude of Hawaii 10 11 12 inclination Groups Fig.
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