Plate-tectonic reconstructions predict part of the Hawaiian hotspot track to be preserved in the Bering Sea Bernhard Steinberger* Center for Geodynamics, Geological Survey of Norway, Leiv Eirikssons vei 39, Carmen Gaina N-7491 Trondheim, Norway ABSTRACT (Baranov et al., 1991). An 40Ar/39Ar (plagioclase) We use plate reconstructions to show that parts of the Hawaiian hotspot track of ca. 80– age 27.8 ± 1.1 Ma was determined for an andesite 90 Ma age could be preserved in the Bering Sea. Based on these reconstructions, the Hawaiian (Cooper et al., 1987a). No oceanic-island basalts hotspot was beneath the Izanagi plate before ca. 83 Ma. Around that time, the part of the plate are known to have been recovered from these carrying the hotspot track was transferred to the Kula plate. After 75–80 Ma the Hawaiian ridges (D. Scholl, 2000, personal commun.). hotspot underlay the Pacifi c plate. Circa 40–55 Ma, subduction initiated in the Aleutian These fi ndings and interpretations do not exclude Trench. Part of the Kula plate was attached to the North American plate and is preserved as the possibility that the ridges were fabricated out the oceanic part of the Bering Sea. We show that for a number of different plate reconstruc- of pre-existing structures of a different nature. We tions and a variety of assumptions covering hotspot motion, part of the hotspot track should speculate that a hotspot track localized the later be preserved in the Bering Sea. The predicted age of the track depends on the age of Aleutian Bowers and Shirshov Ridges. subduction initiation. We speculate that Bowers and Shirshov Ridges were formed by paleo- Hawaiian hotspot magmatism. PACIFIC PLATE-TECTONIC RECONSTRUCTIONS Keywords: Hawaii, hotspots, plate motion, Kula plate, Bering Sea, Bowers Ridge. Reconstructions of relative plate motions (Table DR1 in the GSA Data Repository1) and INTRODUCTION therefore commence with a brief review of the geometries in the Pacifi c Ocean Basin are based Age-progressive, intraplate volcanism along tectonic setting and conclude on a somewhat on marine magnetic anomalies. In order to fi nd the Hawaiian-Emperor Chain (Pacifi c plate) led speculative note how what is proposed here may the location of the plates within the Pacifi c Wilson (1963) to fi rst suggest a causal relation- be reconciled with geologic evidence. Ocean Basin relative to the plates surround- ship with an upwelling from deep inside Earth ing it, and to plot them on a map with latitudes (later called “mantle plume”) that is overridden REGIONAL TECTONIC SETTING and longitudes, these reconstructions must be by a moving plate. However, the Hawaiian- OF SHIRSHOV AND BOWERS RIDGES embedded in a suitable absolute reference frame. Emperor Chain ends at the Aleutian subduction IN THE BERING SEA A fi xed-hotspot reference frame has frequently zone. Its northernmost part, north of, and pos- Based on the age of oldest volcanic activity, been used (e.g., Duncan and Clague, 1985). sibly including, the Detroit seamount, aged 76– the Aleutian Arc is believed to have formed at However, there are a number of indications that 81 Ma (Keller et al., 1995; Duncan and Keller, ca. 40–55 Ma (Scholl et al., 1987; Jicha et al., the Hawaiian hotspot has moved and was farther 2004), and oceanic basalts from accretionary 2006). The ocean fl oor to the north is prob- north in the geologic past. These include analy- complexes in eastern Kamchatka (Portnyagin ably a piece of captured Kula plate; most of ses of plate circuits (e.g., Raymond et al., 2000), et al., 2006), may have formed through chan- this plate subducted beneath continental crust sedimentological evidence (Parés and Moore, neling of plume material to the ridge (Tarduno from Kamchatka to the Bering Shelf (Scholl 2005), numerical models (e.g., Steinberger et al., et al., 2003), hence a corresponding track may et al., 1975, 1986). Cooper et al. (1987b) sug- 2004), and paleomagnetic data (e.g., Tarduno have formed on the Izanagi and Kula plates. It is gested that large structural depressions fi lled and Cottrell, 1997; Tarduno et al., 2003). The not clear whether parts of the hotspot track are with deformed sedimentary prisms beneath latter indicate that the Hawaiian hotspot was preserved beyond the subduction zone. the continental slopes are remnants of ancient at ~30–35° N at 75–80 Ma and had moved to The ocean basin in the Bering Sea north of trenches. Probably Cenozoic crust formed due close to its present latitude at the time of the the Aleutian Trench is usually interpreted as a to backarc extension in the Komandorsky and Hawaiian-Emperor bend. True polar wander captured remnant of the Kula plate, which has possibly Bowers Basins ( Cooper et al., 1987a, (e.g., Besse and Courtillot, 2002) appears not for the most part been subducted (Scholl et al., 1992; Baranov et al., 1991). to have contributed more than a few degrees of 1975, 1986). Hence, it may have preserved older Only undated arc-type volcanic rocks have latitude change (Tarduno and Smirnov, 2001), parts of the Hawaiian hotspot track. Their exis- been dredged from Bowers Ridge (Cooper et al., regardless of whether it is computed in a fi xed- tence and identifi cation could give important 1987a). Thus the ages of formation of Shirshov hotspot reference frame or a mantle reference insights about the age and earlier history of the and Bowers Ridges are unknown. Bowers Ridge frame that considers hotspot motion (Torsvik Hawaiian hotspot, thus further constraining the is bordered on its convex side by a sediment-fi lled et al., 2006). We determine the best-fi tting nature of mantle plumes. trench (Ludwig et al., 1971). Seismic, magnetic, Pacifi c plate motion assuming a hotspot motion In this paper, we show that plate-tectonic and gravity data support its interpretation as a that is broadly consistent with numerical mod- reconstructions (Fig. 1) yield a predicted hot- volcanic arc at a fossil subduction zone (Kienle, els for Hawaiian and Louisville hotspot motion spot track through the oceanic part of the Bering 1971). Trench sediments were deposited and sub- (Koppers et al., 2004) and paleomagnetic data. Sea. It is possible that two ridges in this basin, sequently deformed probably during the Ceno- Shirshov and Bowers (Fig. 2), were originally zoic (Marlow et al., 1990). Shirshov Ridge is 1GSA Data Repository item 2007098, informa- formed by the Hawaiian hotspot. However, a characterized by thick sediments along its eastern tion on the construction of past plate boundaries, hotspot track origin contrasts with other inter- fl ank and steep scarps on its western side (Rabino- Table DR1 (relevant fi nite plate rotation parameters) pretations for the formation of the ridges (e.g., witz and Cooper, 1977). Various concepts of its and Figure DR1 (magnetic anomalies in the Bering Sea region), is available online at www.geosociety. Cooper et al., 1992; Baranov et al., 1991). We uncertain origin are reviewed by Baranov et al. org/pubs/ft2007.htm, or on request from editing@ (1991). Rock dredgings on Shirshov Ridge recov- geosociety .org or Documents Secretary, GSA, P.O. *E-mail: [email protected]. ered basalts, gabbros, and other datable rocks Box 9140, Boulder, CO 80301, USA. © 2007 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. GEOLOGY,Geology, May May 2007; 2007 v. 35; no. 5; p. 407–410; doi: 10.1130/G23383A.1; 2 fi gures; Data Repository item 2007098. 407 We assume the Hawaiian hotspot moved 13° Assumed location of Hawaiian hotspot Computed hotspot track: → → southward and 3° eastward between 90 and Plate boundaries at time indicated IZA KUL at 83 Ma, KUL NAM at 47 Ma IZA→KUL at 93 Ma, KUL→NAM at 47 Ma 47 Ma, and 2° southward and 2° eastward since ~ 4 Myr earlier → → ~ 4 Myr later IZA KUL at 73 Ma, KUL NAM at 47 Ma → → 47 Ma, and the Louisville hotspot has moved inferred from marine magnetic anomalies IZA KUL at 83 Ma, KUL NAM at 54 Ma IZA→KUL at 83 Ma, KUL→NAM at 40 Ma 10° eastward and 4° southward since 120 Ma, Other intra-Pacific plate boundaries all at constant speed. Optimization procedure Selected other plate boundaries Isochrons at 120 Ma and age data from both hotspot tracks are the 10 cm/yr (at 45º N) Isochrons at 84 Ma same as in Koppers et al. (2004), who showed that new radiometric age data are consistent 104 Ma 120 88 Ma 140 with relative hotspot motion as assumed here. Results are included in Table DR1 (see foot- 60˚ N note 1). Note that the Pacifi c plate motion is 110 130 Izanagi Kula / Izanagi thus determined independent of the global 50˚ N plate circuit and Indo-Atlantic hotspot tracks. 100 120 Pacifi c plate rotation rates before 83 Ma are 40˚ N 110 from Duncan and Clague (1985), i.e., fi nite 90 Farallon rotations at 100 and 150 Ma were corrected 30˚ N for inferred hotspot motion since 83 Ma. Con- Farallon struction of plate boundaries is detailed in the 20˚ N Pacific Pacific GSA Data Repository (see footnote 1). 10˚ N Figure 1 shows reconstructions for this case: 110 110 The Hawaiian hotspot fi rst (top left panel) 72 Ma 56 Ma 100 occupied an intraplate location on the Izanagi 100 110 90 plate, which moved northwestward at a speed 100 60˚ N 90 90 of >10 cm/yr. After ca. 100 Ma, Pacifi c plate 100 Kula Kula motion also had a northward component: The 90 50˚ N 80 Izanagi-Pacifi c boundary moved northward, 90 80 90 approaching the Hawaiian hotspot at ~8 cm/yr.