Article Geochemistry 3 Volume 6, Number 7 Geophysics 12 July 2005 Q07L10, doi:10.1029/2004GC000705 GeosystemsG G ISSN: 1525-2027 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Seismic stratigraphy of Detroit , Hawaiian- Emperor seamount chain: Post-hot-spot shield-building volcanism and deposition of the Meiji drift

Bryan C. Kerr, David W. Scholl, and Simon L. Klemperer Department of Geophysics, Stanford University, Mitchell Building 353, Stanford, California 94305-2215, USA ([email protected])

[1] Detroit Seamount, one of the northernmost of the Hawaiian-Emperor seamount chain, was formed at ca. 76 Ma. New seismic data suggest renewed volcanism as late as 25 m.y. after initial seamount formation. We use high-resolution single-channel seismic (SCS) data acquired over the summit of Detroit Seamount in 2001 on Ocean Drilling Program (ODP) Leg 197, supplemented by older SCS data acquired as part of the GLORIA mapping program of the U.S. Geological Survey, to characterize the seismic stratigraphy of Detroit Seamount. Volcanic edifices occur on the summit of the seamount and are older than the oldest beds of the Meiji drift (early Oligocene: ca. 34 Ma). On the basis of ash layers in ODP drill holes, we suggest the edifices were active throughout much of the Eocene (ca. 52–34 Ma), with activity possibly extending into the early Oligocene (<34 Ma). Hence the age difference between the shield- building and the postshield cones on Detroit is far greater than the shield/postshield age differences observed on the Hawaiian Islands, suggesting that renewed volcanic activity and tectonic collapse may be possible on any of the Hawaiian Islands. We confirm earlier assertions that the thick sediment cap, Oligocene and younger in age, was deposited by an ocean-bottom current with a southeastward flow direction, along the northeast facing flank of the Emperor Seamount chain. This sediment cap, the Meiji drift, was deposited by a lower-velocity current than many other sediment drifts. A low-angle normal fault, dipping 19°, suggests topographic collapse of Detroit seamount sometime during the Eocene or late Cretaceous.

Components: 9315 words, 10 figures. Keywords: drift deposits; ocean-bottom currents; rejuvenated volcanism. Index Terms: 3099 Marine Geology and Geophysics: General or miscellaneous; 3022 Marine Geology and Geophysics: Marine sediments: processes and transport; 8499 Volcanology: General or miscellaneous. Received 27 January 2004; Revised 25 October 2004; Accepted 18 November 2004; Published 12 July 2005.

Kerr, B. C., D. W. Scholl, and S. L. Klemperer (2005), Seismic stratigraphy of Detroit Seamount, Hawaiian-Emperor seamount chain: Post-hot-spot shield-building volcanism and deposition of the Meiji drift, Geochem. Geophys. Geosyst., 6, Q07L10, doi:10.1029/2004GC000705.

———————————— Theme: Movement, Dynamics, and Geochemical Evolution of the Hawaiian Hot Spot Guest Editors: R. Duncan, J. A. Tarduno, D. Scholl, and T. Davies

1. Introduction mount chain (Figure 1). It is a broad volcanic plateau approximately 10,000 km2 in area [2] Detroit Seamount is one of the northernmost [Lonsdale et al., 1993], with Campanian crestal surviving volcanoes in the Hawaiian-Emperor Sea- flows (ca. 76 Ma) [Duncan and Keller, 2004],

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Figure 1. Regional map of the northwest Pacific showing location of Detroit Seamount relative to the rest of the Hawaiian-Emperor chain (modified from Tarduno et al. [2003]) and Leg 197 drill sites (red dots). White dots mark the locations of undated seamounts. that rises from abyssal depths to 1500 m deep of the cones crop out above the Meiji drift (Figure 2). Lonsdale et al. [1993] refers to the [Lonsdale et al., 1993]. shallowest cone as Detroit Seamount and calls the [3] In July 2001, the JOIDES Resolution obtained broader plateau Detroit Plateau. However, in this basalt cores from two holes drilled near the summit paper we refer to the broader plateau as Detroit of Detroit Seamount as part of ODP Leg 197’s goal Seamount, following the usage of Rea et al. to constrain the paleolatitude of the Hawaiian hot [1993]. A sequence of sediment as thick as spot [Tarduno et al., 2003]. Drill locations were 800–900 m blankets most of the edifice [Rea et placed away from faults and eruptive edifices to al., 1993; Shipboard Scientific Party, 2002]. The ensure that the emplacement orientation of the Meiji drift, constructed of Oligocene to Quaternary basalt flow units had not been disturbed, which pelagic and hemipelagic mud deposited by ocean- would degrade the accuracy of paleolatitude mea- bottom currents, forms the bulk of the sediment surements. To locate and characterize suitable cap [Creager et al., 1973; Rea et al., 1995a, drilling sites we used a Seismograph Services 1995b; Shipboard Scientific Party, 2002]. Post- Inc. (SSI) 80-in3 watergun to collect high- hot-spot shield-building volcanic cones rise resolution single-channel seismic profiles. above the summit platform on Detroit Seamount [Lonsdale et al., 1993]. Some of the cones peak [4] In addition to the digital seismic data gathered 1–2 km above the summit platform, and many by ODP Leg 197, previous survey lines recorded 2of18 Geochemistry 3 kerr et al.: detroit seamount Geophysics 10.1029/2004GC000705 Geosystems G

Figure 2. Map of Detroit Seamount with seismic lines from Leg 197 and lines F41 and F43 of R/V Farnella cruise F-2-87-AA. Detroit, Windsor, and Wayne denote cones imaged by Lonsdale et al. [1993] (black dots); 882, 883, and 884 are ODP Leg 145 site locations, and 1203 and 1204 are ODP Leg 197 site locations. Seismic lines are marked in 10 km increments. Contour map from Rea et al. [1993]. Color map from Smith and Sandwell [1997]. Contour interval is 100 m, and contour labels are in meters. over Detroit Seamount were integrated into our Childs ([email protected]) for seismic availability study. The U.S. Geological Survey (USGS) col- and information, or see http://walrus.wr.usgs.gov/ lected digital seismic data, as well as gravity, infobank/f/f287aa/html/f-2-87-aa.meta.html). magnetic, and bathymetry recordings over Detroit Lonsdale et al. [1993] collected lines of SeaBeam, Seamount as part of the GLORIA Mapping Pro- magnetic, and analog seismic data in addition to gram [U.S. Geological Survey, 1987] (contact Jon rock dredging over Detroit Seamount.

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[5] Following a description of the geographic, the Hawaiian Islands, a large subaerial edifice physiographic, and geologic setting of Detroit did form above the cresting summit of Detroit Seamount, including new and older seismic, and Seamount. relevant drilling data, we explore two principal topics in this paper, the age of postshield volcanism [8] Plate reconstructions predict that the Hawaiian at the summit of Detroit Seamount, and the depo- hot spot was near a spreading ridge at ca. 80 Ma sitional processes involved in forming its thick [Rea and Dixon, 1983; Mammerickx and Sharman, mantle of sedimentary deposits. We confirm the 1988], so Detroit Seamount formed on young conclusion of Lonsdale et al. [1993] that the age (10 m.y. old), therefore relatively warm and thin difference between the shield-building lavas and oceanic lithosphere. In contrast, the island of the postshield cones on Detroit is far greater than is built on much older oceanic lithosphere the shield/postshield age differences observed (ca. 90–100 Ma) [Mu¨ller et al., 1997]. The result- on the Hawaiian Islands. We extend the time of ing differences in subsidence rate have implica- postshield volcanism to at least the end of the tions that are discussed later in the paper. Eocene. Second we corroborate the views of other authors [Scholl et al., 2003] that Detroit’s sediment [9] Other undated seamounts exist 100 km cap is largely an Oligocene and younger drift northeast of the Emperor Seamount chain deposit: the Meiji drift. The geometry and size of (Figure 1), but it is not clear if they are related bedforms surprisingly imply that the depositing to the Hawaiian hot spot. Lonsdale et al. [1993] drift current was not particularly swift and that its suggests that these undated seamounts are likely vigor has decreased with time. no older than Cenozoic (early Tertiary) in age, on the basis of SeaBeam data showing that the summit of one of these seamounts was never 1.1. Geologic Setting of Detroit Seamount wave-base leveled, and is less than 2 km below [6] Detroit Seamount lies at the northwestern end sea level. of the nearly 6,000-km-long Hawaiian-Emperor volcanic chain (Figure 1). The chain records the 1.2. New Seismic Data relative movement of the Pacific plate over the Hawaiian hot spot. The age of the volcanoes [10] Leg 197 data were collected using a single- increases north and west of the island of Hawaii, channel streamer to provide high vertical and the current locus of eruptive activity. lateral resolution to assist detailed analysis of Detroit’s stratigraphic rock and sedimentary se- 3 [7] Detroit Seamount, a large plateau roughly the quence. The 80-in SSI watergun used by the size of the Big Island of Hawaii, is centered at Resolution provides higher frequencies (spectral approximately 51.25°N, 167.5°E (Figure 2). The peak 40–45 Hz) than conventional airguns. Slow plateau has a broadly smooth surface disrupted ship speeds during the surveys enabled relatively by (1) troughs cut by ocean-bottom currents; closely spaced shot intervals that ranged between (2) fault scarps; and (3) cones or domes that 15–20 m. The F-2-87-AA data recorded an 80-in3 were subaqueously emplaced and never leveled conventional airgun source using two single- at wave-base [Lonsdale et al., 1993] (Figure 2). channel streamers at shot intervals between 40– Some of these cones, not covered by sediment, 50 m. The streamers were towed from opposite have been dredged [Lonsdale et al., 1993]. The sides of the ship, and the active section of both summit structure and stratigraphy of Detroit streamers was offset 400 m behind the airgun. Leg Seamount generally consist of a relatively flat 197 data were collected in two separate surveys, basement surface deeply buried (up to 840 m) by referred to as Survey 1203 (about ODP Site 1203) a channelized sediment drift deposit, the Meiji and Survey 1204 (about ODP Sites 883/1204). drift [Rea et al., 1993; Shipboard Scientific Lines 1203-1, 1203-3, and 1203-6 all intersect at Party, 2002]. The Shipboard Scientific Party Site 1203; lines 1204-1 and 1204-3 intersect at [2002] confirm that Detroit lavas erupted inter- ODP Site 883, 500 m northwest of Site 1204. mittently in shallow marine and nonmarine envi- The two USGS or F-2-87-AA lines (Lines F41 and ronments. No significant time hiatus has been F43) are subparallel, separated by 25 km. found between the youngest shield lava flows Line F41 intersects Lines 1204-1 and 1204-3, and the overlying Late Cretaceous sediment at and Line F43 passes within 1 km of Sites 883 and 1204 [Rea et al., 1993; Shipboard Lines 1203-3 and 1203-4, allowing limited Scientific Party, 2002], suggesting that, unlike correlation with these profiles also (Figure 2).

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Figure 3. Seismic section and location map from Line 1204-1 with vertical exaggeration (V. E.) 8:1 at the seafloor. The thick black line on the location map shows the location of the seismic section on the track line with respect to the seismic lines shown in Figure 2. (a) Uninterpreted seismic section. Inset shows Meiji drift sediment onlapping onto volcanic cones. (b) Interpreted seismic section with seismic velocities logged at Site 883 [Rea et al., 1993]. Velocities shown below 4.2 s are Site 1204 Physical Properties measurements [Shipboard Scientific Party, 2002]. Lithologic boundaries are from Rea et al. [1993], and age boundaries are from Rea et al. [1995b]. Letters A–F denote Horizons A–F. Horizons are dashed where uncertain.

1.2.1. Data Processing Leg 197 and F-2-87-AA data sets (nonminimum [11] To maximize our ability to correlate sediment phase watergun versus minimum phase airgun, horizons from line to line, we attempted to process respectively), and different frequency contents both the Leg 197 and the F-2-87-AA data sets (Leg 197 peaks between 40–45 Hz and the F-2- equivalently. The different source signatures of the 87-AA data peaks at 22 and 44 Hz with a 30 Hz

5of18 Geochemistry 3 kerr et al.: detroit seamount Geophysics 10.1029/2004GC000705 Geosystems G notch), limited the degree to which the processing 5 m.y. the documented volcanic history of each sequences could be matched. We attempted a island in the Hawaiian chain is completed. source-signature deconvolution on both Leg 197 surveys (in order to convert the data to mini- [13] We interpret the two peaks or highs in the mum phase) prior to predictive deconvolution. basement topography imaged north of Site 883/ The lack of a true far-field source-signature 1204 (Figure 3) to be volcanic cones akin to those recording of the watergun source and a signif- documented by Lonsdale et al. [1993]. Rising icant loss of signal-to-noise ratio due to source- approximately 500 m above the surrounding sub- signature deconvolution artifacts, rendered horizontal basement, the edifices in Figure 3 are source-signature deconvolution impractical for about one-fourth the height of the tallest cones Survey 1204 data. A persistent receiver ghost, mapped by Lonsdale et al. [1993]. Line 1204-1 prominent just below the seafloor in the Survey probably does not cross directly over the top of the 1203 data, required a signature deconvolution two buried edifices in Figure 3, so the edifices for which we simulated the far-field source- might actually peak more than 500 m above the receiver signature using the water-bottom reflec- surrounding basement. The apparent slope of the tion by stacking 40 traces from a thick (800 ms north flank of the northern edifice (approximately twtt) part of the sediment cap. On all of the km 22 on Line 1204-1) is approximately 30°, and data sets, we performed normal moveout (NMO) the apparent dip of the slopes of the smaller edifice corrections, predictive deconvolution, bandpass (approximately km 18 on Line 1204-1) is about filtering, true amplitude recovery, and Stolt 10°. F-K migration. We used a constant velocity of 1,500 m/s for the NMO and the true amplitude [14] On the basis of the relation of sediment corrections, as well as the migration. Additional horizons with basement relief, these volcanic edi- processing details and data examples are pre- fices formed before significant Meiji sediment sented by Kerr et al. [2005]. accumulation began. At first glance, it appears that the edifices in Figure 3 are younger than the 2. Analysis and Interpretation middle, lower, and pre-Meiji sequences as the surface of the edifices (Horizon A, Figure 3b) appears to truncate sediment horizons. If the edi- 2.1. Volcanic Edifices fices were older than the sediment, one would [12] The shield stage of Hawaiian volcano evolu- expect the sediment horizons, in response to tion (e.g., and Kilauea on Hawaii), particle rain, to drape or be concordant with during which the bulk of each island edifice is built the profile of the basement surface as the sedi- by tholeiitic basalt flows, lasts for <1 m.y. with ment rains down. However, as we establish later eruption intervals on the order of 1–10 years in this paper, an ocean-bottom current influences [Moore and Clague, 1992; DePaolo and Stolper, the deposition of the Meiji drift: Figures 7, 8, 1996; Clague, 1987]. After shield-building ends, and 10 show that Meiji drift deposition is alkalic basalt, hawaiite and trachyte lavas erupt on influenced by bottom-current interaction with the shield summit over a period as long as existing topography, as sediment horizons onlap 0.2 m.y. with eruption intervals of 102–103 years onto existing topography. As can be seen in (e.g., on Hawaii) [Moore and Clague, Figure 3a (inset), sediment horizons onlap onto 1992; DePaolo and Stolper, 1996; Clague, 1987]. the edifices (Horizon A), suggesting the cones After a hiatus of 0.25–2.5 m.y., a rejuvenated are older than the Meiji sedimentation, which stage of alkalic volcanism erupts melilite, began at ca. 34 Ma. The lack of evidence of sills nephelinite, basanite, and alkalic basalt over a in the sediment also implies the edifices are older duration as long as 3 m.y., with eruption intervals than the Meiji drift. of 104–105 years (e.g., on ) [Moore and Clague, 1992; DePaolo and [15] Using thermal subsidence curves from Detrick Stolper,1996;Clague,1987;Langenheim and et al. [1977], Lonsdale et al. [1993] theorized that Clague, 1987]. Rejuvenated-stage lavas appear to the volcanic cones that dot the summit of Detroit be typical along the Hawaiian chain as they have formed as late as 60 Ma. Lonsdale et al. [1993] been sampled from the older, more northwestern presumed that the summit of Detroit was never islands and seamounts (e.g., , Ladd significantly above wave-base, a presumption sup- Bank, and Colahan Seamount) [Clague and ported by the lack of a significant time hiatus Dalrymple,1987].Insummary,withinabout between the youngest shield lava flows and the

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late-stage Hawaiian lavas, which therefore might be anticipated to have erupted much earlier than at 60 Ma [Lonsdale et al., 1993]. Along the Hawaiian chain, alkalic postshield stage lavas are typically erupted approximately 1 m.y. after shield building has ceased (up to 4 m.y. for the Honolulu vol- canics) [Langenheim and Clague,1987].Thus, according to the Hawaiian model of eruption histories, the postshield cones on Detroit Seamount should have formed at ca. 75–71 Ma. However, we suggest below that the summit cones could be even younger than 60 Ma, so that the chemical similarity to late-stage Hawaiian lavas is perplexing, perhaps coincidental. Long time gaps between hot spot- related shield-building volcanism and non-hot- spot-related rejuvenescent volcanism have been reported elsewhere. Dredge samples from Hodg- kins Seamount (northwest Pacific) exhibited age differences of 11.5 m.y. [Turner et al., 1980]. Keller et al. [1997] reported drilled basalt cores from the Patton-Murray seamount platform in the Gulf of Alaska with age differences as much as 16 m.y. Keller et al. [1997] attributed the origin of the youngest basalt cores to extension at a location of hot spot-induced plate weakness.

2.1.1. Indications of Local Volcanic Activity

[16] Volcanic material recovered during ODP Leg 197 drilling suggest that Detroit’s summit cones (including the edifices imaged on Line 1204-1) might be even younger than Lonsdale et al.’s [1993] 60 Ma estimate. Figure 5 shows the down- Figure 4. Cartoon of how Lonsdale et al. [1993] hole locations of ash layers (as thick as 0.5 m) derives age of cones on top of Detroit using Detrick et reported by Rea et al. [1993, 1995a, 1995b] and al.’s [1977] thermal subsidence rates for oceanic the Shipboard Scientific Party [2002]. These obser- plateaus. At 78 Ma the summit of Detroit Seamount is vations were made on the summit of Detroit (ODP near sea level. At 61 Ma, Detroit Seamount has subsided Sites 883/1204) as well as on the flanks to the about 1500 m. Between 61 and 60 Ma, renewed northeast of the summit platform (ODP Site 884) volcanism has built volcanic cones that reach or nearly [Rea et al., 1993, 1995a, 1995b; Shipboard reach the surface. At present, the shallowest of the volcanic cones peaks at nearly 1400 m below sea level, Scientific Party, 2002]. Ash layers occur through- and much of Detroit Seamount is blanketed by drift out most of the Eocene section. The earliest Eocene deposits. ash layer was deposited at ca. 52 Ma (Site 1204). Glass intraclasts, ca. 48 Ma in age, were found at Site 883 [Rea et al., 1993]. Rea et al. [1993] overlying Cretaceous sediment at Sites 883 and describe these intraclasts as brown and angular 1204 [Rea et al., 1993; Shipboard Scientific Party, suggesting they are basaltic and were not trans- 2002]. Lonsdale et al. [1993] also assumed the ported over a long enough distance to round the cones never grew above wave-base, and that the clasts. On the summit at Site 883, black ash layers subsidence rate of Detroit Seamount was not reset occur as late as 32 Ma [Rea et al., 1993]. At Site by this later volcanism (Figure 4). However, the 884 the ash-rich layers are several meters thick, cones could have formed later, in a deep marine occurring throughout the Eocene [Rea et al., 1993]. environment. Rocks that Lonsdale et al. [1993] Site 884 is 50 km downslope from Sites 883 and dredged from these cones geochemically resemble 1204.

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Figure 5. Downhole location of ashes and disrupted sediment structures in cores from Sites 883, 884, and 1204. Site 1203, devoid of mafic ashes and disrupted sediment, is not shown. Only downhole sections below 600 meters below seafloor (mbsf) are shown here to avoid showing shallower ash layers that are likely from Kamchatka, and hence from a different eruptive source than the deeper ashes. Site 884, 50 km northwest and downslope from Sites 883 and 1204, shows the thickest ashes and most examples of displaced and deformed sediment. The letters A and B mark Horizons A and B shown in Figures 3, 7, and 8.

[17] Sites 883 and 1204 (500 m apart) are within throughout the Eocene (Figure 5), suggest slope 7–11 km of the two summit cones observed on instability induced by rapid deposition of volcanic Line 1204-1 of our Leg 197 data. Ash layers material, as well as the uplift of that material recovered from Sites 883 and 1204 that are vitric caused by igneous intrusion associated with the in texture are also dark in color, suggesting a mafic volcanism. Deformed or sediment-mobilized struc- composition [Rea et al., 1993, 1995a, 1995b; tures include convoluted and contorted bedding, Shipboard Scientific Party, 2002]. Other ash layers slump blocks and folds, microfaults, load casts, and have been altered to palagonite and are brown in scour surfaces [Rea et al., 1993]. Disrupted sedi- color, perhaps indicating submarine eruption. In ment is observed at Site 883 at ca. 45, 47–48, and addition, the Eocene vitric ash layers observed at 51 Ma, some of which are coincident with the glass Sites 883 and 884, by Rea et al. [1993, 1995a, intraclasts mentioned above [Rea et al., 1993]. 1995b] and at Site 1204 by the Shipboard Scientific Party [2002] are absent at Site 1203 (30 km to the [19] Geochemical analysis by Duncan and Keller south) [Shipboard Scientific Party, 2002] implying [2004] reveals alteration of feldspars to sanidine in that they were derived locally. Hence it seems Site 1204 basalt beginning at ca. 58 Ma and ending likely that these and other summit cones on Detroit at ca. 33 Ma. Duncan and Keller [2004] attribute are the source of the Eocene ashes cored at Sites this age resetting to circulating K-rich fluids pro- 883, 884, and 1204. duced during postemplacement volcanic activity [see also Dalrymple et al., 1980]. We propose that [18] The numerous observations of displaced and the Eocene ashes, reworked sediment, and chemi- deformed sediment at Sites 883, 884, and 1204 cally altered basement rocks observed at Sites 883, 8of18 Geochemistry 3 kerr et al.: detroit seamount Geophysics 10.1029/2004GC000705 Geosystems G

Figure 6. Cartoon of mudwave migration caused by bottom (i.e., countourite) or turbidity current flow modified from Flood [1988]. Measurements showing horizontal migration and vertical aggradation of wave crests are those from Line 1203-1 mentioned in the text. The arrow next to ‘‘Fault Scarp’’ points toward the direction where a feature similar to Summit Fault would have to be located relative to ‘‘Spillover Flow Direction’’ to create mudwave crests that migrate toward the right. Vertical exaggeration 5:1.

884 and 1204 are due to a renewed phase of centered over the active hot spot as a cause of volcanism occurring on Detroit Seamount through- volcanism because by 52 Ma, the Hawaiian hot out the Eocene, and possibly into the early Oligo- spot was 1500 away, between Nintoku and Koko cene. Such a young age is consistent with the Seamounts (Figure 1) [Shipboard Scientific Party, onlapping of Unit III onto the volcanic edifice, 2002]. Perhaps significantly, volcanism occurred at and its eventual overtopping by Unit II, seen on an increased rate during the Eocene throughout the Line 1204-1 (Figure 3b). Pacific Basin [Kennett et al., 1977]. Seismic activ- ity, associated with increased rates of volcanism, [20] Even though we believe the nature of the ash might explain the widespread recording of sedi- layers implies a local source for these volcanics, ment deformation and displacement [e.g., the apparent Eocene volcanism on Detroit Sea- Schlanger and Premoli Silva, 1981; Thiede, mount might have been part of a Pacific Basin- 1981a, 1981b; Lancelot et al., 1990; Storms et wide event. During the early and middle Eocene, al., 1991; Rea et al., 1995a, 1995b]. the present crust of the northwestern Pacific Basin was affected by plate reorganization tectonism as [21] More work could be done to determine if the the Kula-Pacific plate boundary immediately east summit cones documented by Lonsdale et al. of Detroit rotated by nearly 45° [Lonsdale, 1988]. [1993] and imaged along Line 1204-1 on Leg The Aleutian island arc also formed at this time 197 are related to the Eocene ashes, displaced [Scholl et al., 1987]. It is possible that Detroit and deformed sediment, and altered basement rock. Seamount’s proximity to the rotating Pacific-Kula Geochemical analysis needs to be performed on the plate boundary changed the regional stress regime ashes from Sites 883, 884, and 1204 to constrain such that melt was able to work its way to the their origin. Specifically, matching the geochemical surface. We can rule out extension linked to signatures of the Eocene ashes with signatures formation of the flexural arch around the islands measured from Lonsdale et al.’s [1993] rock

9of18 Geochemistry 3 kerr et al.: detroit seamount Geophysics 10.1029/2004GC000705 Geosystems G dredges on Detroit Seamount would help confirm wave model (Figure 6), in which mudwaves hor- our suggestion that the Eocene ashes record local- izontally migrate upcurrent and upslope when ized volcanic activity on its summit. bottom-current flow (assumed to be orthogonal to the wave front) velocities are approximately 9– 2.2. Meiji Drift 30 cm/s (0.3–1.1 km/hr). At slower bottom-current velocities, sediment tends to accumulate in the 2.2.1. Sediment Drifts wave troughs [Flood, 1988]. If the mean bottom- current velocity is greater than 16 cm/s (0.6 km/hr), [22] Sediment drift deposits record important in- formation about global ocean currents such as erosion will begin to occur on the downcurrent location, speed, and direction [Dorn and Werner, flank of the mudwave. As the mean bottom-current 1993; Flood, 1994; Manley and Caress,1994; velocity increases, the zone of erosion increases Cunningham and Barker, 1996; Howe, 1996; along the downcurrent flank of the mudwave until McCave and Carter, 1997]. The Meiji sediment sediment accumulates only on the upcurrent flank, drift, up to 1800 m thick, over 1000 km long, and so that migration occurs without aggradation [Flood, 1988]. In other words, the wave crests will 350 km wide, has been accumulating since the early Oligocene beneath a southeasterly flowing horizontally migrate, but they will not become bottom current in the northwest Pacific [Scholl et taller, even though the sediment supply remains al., 1977, 2003; Mammerickx, 1985; Rea et al., constant. When the mean bottom-current velocity 1995a, 1995b]. Thermohaline circulation (THC) exceeds approximately 30 cm/s (1.1 km/hr), drives the current responsible for the Meiji sedi- erosion occurs across the entire mudwave. The ment drift [Scholl et al., 2003]. Prior to the late minimum velocity necessary for migration of mud- Cenozoic, it is unclear whether the THC responsi- waves increases with wavelength (due to drag) and ble for the Meiji sediment drift was generated latitude (due to Coriolis effects). The velocity at locally (Bering Sea), or distantly (Pacific Bottom which erosion begins to occur on the downcurrent Water), or is a combination of the two source flank of the mudwave increases as wave height and regions [Scholl et al., 2003]. Surface water salinity length decrease. The velocity at which complete is currently too low to drive THC in the Bering Sea erosion occurs (the maximum velocity at which Basin, hence bottom flow (which has not been wave migration is possible) is dependent only on instrumentally confirmed) over the Meiji drift is mudwave wavelength [Flood, 1988]. presumably generated distantly by THC in the [25] The structure of sediment bedforms proximal north Atlantic and peripheral to Antarctica. The to channels is a key paleocurrent indicator. The rare, but consistent, presence of Arctic-boreal dia- long travel distances, on the order 104 km, of major toms indicate that at least some of the Meiji drift ocean-bottom currents make them subject to Cori- was sourced from a Bering Sea connection [Barron olis effects, which in the northern hemisphere and Gladenkov, 1995]. Changes in Meiji drift deflect current flow to the right of the flow direc- sedimentation rate may correlate with changes in tion. For example, a northern-hemisphere bottom delivery rate and sources of deep waters in re- current flowing along a southeast-trending channel sponse to large-scale shifts in abyssal circulation will have a spillover component above the top of caused by tectonism and changes in the global the southwest side of the channel [Normark et al., climatic regime. Hence our seismic profiles that 2002]. Mudwaves formed on the southwestern image the total thickness of the Meiji drift have the bank will migrate against the spillover current potential to date episodes of climate change. toward the channel (Figure 6) [Flood, 1988; [23] Sediment drift deposits are primarily charac- Normark et al., 2002]. terized by wave-like bedforms often referred to as mudwaves, and the Meiji drift exhibits these in 2.2.2. Meiji Drift Lithostratigraphy on abundance. Wynn and Stow [2002] provide a com- Detroit Seamount prehensive summary of the characteristics (wave- length, wave height, wave symmetry, etc.) of [26] The lithostratigraphy of Detroit Seamount was mudwaves. characterized at Site 883 (Figure 2) by Rea et al. [1993]. The 840-m-thick sediment cap here is [24] Horizontal migration of fine-grained mudwave divided by Rea et al. [1993] into 5 units on the crests and troughs is controlled by the velocity of basis of physical properties measurements, diatom the bottom current [Wynn and Stow, 2002]. Flood abundance, distinct color changes, and presence of [1988] explains this wave migration using a lee- sedimentary structures (e.g., scoured surfaces, lam-

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Figure 7

11 of 18 Geochemistry 3 kerr et al.: detroit seamount Geophysics 10.1029/2004GC000705 Geosystems G inations, and bored hardgrounds) (see insets to are concordant. Horizon E (within Unit II) is Figures 3b, 7b, and 8b as well as the annotated another strong reflector (3.5–3.8 Ma) on which well log inserted in seismic section of Figure 3b). overlying sediment horizons downlap at the north- Unit I (87 m thick at Site 883, late Pliocene to eastern ends of Lines 1203-1 and 1203-6 (inset B Quaternary in age) consists of clay with quartz and in Figure 7a, inset in Figure 8a). Horizon F is diatoms. Unit II (371 m thick, late Miocene to another strong reflector (ca. 1 Ma) near the top of late Pliocene in age) consists of diatom ooze. Vitric the Meiji drift. ash layers are present in both Unit I and Unit II. Unit III (194 m thick, early Miocene to late 2.2.3. Meiji Drift Imaged on Lines 1203 Miocene in age), devoid of distinct volcanic ash and 1204 layers, is composed of calcareous diatomaceous ooze with interbeds of calcareous chalk. Unit IV [29] From Figure 7a, it is clear that Horizon B (162 m thick, early Eocene to late Oligocene in marks a significant erosional event. Pre-Meiji (Pa- age) consists of nannofossil chalk with increasing leocene to Eocene) horizons terminate upward into quantities of altered ash downhole. Altered ash and Horizon B between km 34 and km 38. Nannofossil claystone comprise the bulk of Unit V (25 m stratigraphy indicates a lower Oligocene to upper thick, late Cretaceous to Paleocene in age). Basalt Eocene hiatus at Site 883 that likely coincides with underlies Unit V. Horizon B at Site 1203 [Barron et al., 1995]. The pre-Meiji sediment thickens southward away from [27] We have picked 6 prominent and laterally Site 1203 (Figures 7b and 8b). continuous seismic reflection horizons (Horizons A–F) to help characterize temporal evolution of [30] In the vicinity of Site 1203, sediment thins the seismic stratigraphy along Surveys 1203 and (between Horizons D and E, and Horizons E and F) 1204 (insets to Figures 3b, 7b, and 8b). We and horizons downlap (onto Horizons D and E) to estimate the ages of these horizons using the sonic the northeast (inset B in Figure 7a, inset in Figure logging data from Site 883 (Leg 145) (Figure 3b) 8a). However, 5 km southwest of Site 1203 (km 38 to convert two-way travel time to a horizon to that on Figure 7b, and km 135 Figure 8b), sediment horizon’s depth of recovered core [Rea et al., thickness is relatively constant and downlaps are 1993]. We use the depths of the biostratigraphic absent. Thinning and downlapping sediment hori- datum levels from Barron et al. [1995] to get the zons near Site 1203 indicate lower rates of sedi- age of each seismic horizon. Horizon A marks the ment deposition relative to 5 km southwest of Site contact between volcanic basement and the over- 1203, indicating greater bottom-current velocities lying sediment (base of Unit V). Horizon B (within near Site 1203 than 5 km to the southwest. Unit IV) is the contact between sediment that is late Eocene and older in age, and the Meiji drift [31] Approximately 15 km southwest of Site 1203 sediment of Oligocene and younger age. Horizon on lines 1203-1 (between km 24 and km 31, C marks the top of highly reflective late Miocene Figure 7) and 1203-6 (between km 122 and and older Meiji sediment (approx. top of Unit III). km 127, Figure 8), the early Meiji sediment Horizon C is ca. 6.2 Ma, and is prominent across between Horizons B and C onlaps onto the pre- the summit of Detroit Seamount on the Leg 197 Meiji sediment and basement (Figures 7b and 8b). data (Figures 3, 7, and 8). This is evidence of Meiji sediment horizons prograding upslope against the spillover current [28] Horizon D (5.1–5.3 Ma) is overlain by sedi- from the channel-levee-like system created by a ment horizons that downlap on to it at the north- normal fault discussed later (at km 32 on 1203-1 eastern ends of Lines 1203-1 and 1203-6 (inset B and km 127 on 1203-6). in Figure 7a, inset in Figure 8a). Horizon D’s high amplitude enables identification of this horizon on [32] Figures 7 and 8 show two profiles from Survey Line 1204-1 (Figure 3) where overlying horizons 1203 that cross a prominent channel or trough

Figure 7. Seismic section and location map from Line 1203-1 with vertical exaggeration (V. E.) 8:1 at the seafloor. The thick black line on the location map shows the location of the seismic section on the track line with respect to the seismic lines shown on Figure 2. (a) Uninterpreted seismic section. (b) Interpreted seismic section with seismic velocities logged at Site 1203. Lithologic boundaries are from the Shipboard Scientific Party [2002], and age boundaries are from Siesser [2004] and Tremolada and Siesser [2005]. Letters A–F denote Horizons A–F. Horizons are dashed where uncertain. (c) Enlarged plot of Figure 7b shown with no V. E. at the seafloor to show shallow dip of Summit Fault. 12 of 18 Geochemistry 3 kerr et al.: detroit seamount Geophysics 10.1029/2004GC000705 Geosystems G

Figure 8. Seismic section and location map from Line 1203-6 with vertical exaggeration (V. E.) 8:1 at the seafloor. The thick black line on the location map shows the location of the seismic section on the track line with respect to the seismic lines shown on Figure 2. (a) Uninterpreted seismic section. (b) Interpreted seismic section with seismic velocities logged at Site 1203. Lithologic boundaries are from the Shipboard Scientific Party [2002], and age boundaries are from Siesser [2004] and Tremolada and Siesser [2005]. Letters A–F denote Horizons A–F. Horizons are dashed where uncertain. controlled by a normal fault (here named northwest-southeast, and are crossed by Lines Summit Fault; described in the next section) in 1203-1, -6 and -4. Mudwaves are prominent on the basement 12 km south of Site 1203. Figure 2 the southwest side of the channel (upthrown Sum- shows that the channel and Summit Fault trend mit Fault block) in Figure 7 along Line 1203-1 13 of 18 Geochemistry 3 kerr et al.: detroit seamount Geophysics 10.1029/2004GC000705 Geosystems G between km 23 and km 31, and between 3.4 s and 4.1 s two-way travel time (twtt). The crests of the mudwaves migrate upslope (to the northeast) to- ward the channel. Assuming a constant seismic velocity of 1600 m/s, the amplitudes of mudwave horizons decrease over time. At Horizon C, mudwave amplitudes (half of the peak-to-trough heights) are 35 m. However, in Figure 7b, mudwave amplitudes at Horizon D are 22 m, and 10 m at Horizon E, suggesting a decrease in bottom-current flow energy over time [Normark et al., 1980; Carter et al., 1990]. The likely increase in seismic velocity due to compaction and dew- atering is much less than a factor of two (1600 m/s to 3200 m/s), yet the apparent mudwave ampli- tudes decrease by a factor of 3.5 (40 ms to 12 ms), implying that the real amplitudes also decrease (35 m to <20 m). The section between the horizons C and D is 100 m thick (assuming a Vp = 1600 m/s), and represents approximately 1 m.y. of deposition. Within this section, wave crests horizontally prograde approximately 250 m upslope, giving a horizontal migration rate of 0.25 m/ka (250 m/m.y.). This rate is the same order of magnitude as rates recorded in the north Atlantic on the Feni Ridge drift body (0.25 m/ka) [Lonsdale and Hollister, 1979], and in nearby Rockall Trough Figure 9. Conceptual diagram illustrating bottom (0.4–0.9 m/ka) [Masson et al., 2002]. Along current interaction with Detroit Seamount and Summit Line 1203-1 on the summit of Detroit Seamount Fault. Regional flow represents the main bottom current mudwave wavelengths are approximately 2.7 km, responsible for the Meiji drift. Summit flow is the and wavelengths appear to be relatively constant bottom current flow directed over the summit of Detroit during the late Miocene (despite the 35% decrease Seamount due to Coriolis forces. Channel flow in wave amplitude). represents a component of summit flow entrapped by Summit Fault’s scarp. Spillover flow results from the Coriolis effect forcing channel flow over Summit Fault’s [33] The velocity of the bottom current responsible scarp. Location of the mudwaves observed in Figures 7 for the mudwaves between Horizons C and D and 8 is shown relative to Summit Fault. Climbing (ca. 6.2–5.2 Ma) was quite slow. Using Flood’s bedforms are seen in Figure 10. [1988] lee-wave model, the 2.7 km wavelength of the mudwaves between Horizons C and D predicts Argentine Basin with wavelengths between 5–6 km the mudwave crests would migrate in response to and amplitudes between 50–70 m. bottom-current velocities between 4–21 cm/s (0.14–0.76 km/hr). Given the range of mudwave [34] On the southwestern flank of the channel amplitudes between Horizons C and D (35 m and (upthrown side of Summit Fault), marker horizons 22 m, respectively), erosion of the downcurrent show clear mudwaves. In contrast, on the north- flank of the mudwaves would occur at velocities of eastern or downthrown side of the channel, hori- 16 cm/s (0.58 km/hr) and 17 cm/s (0.61 km/hr), zons are generally flat except on Line 1203-1 respectively. Because sediment accumulation is where the northeast bank slumps into the channel greater in the mudwave troughs than the crests (Figure 7). This relation implies much lower flow between 6.2 and 5.2 Ma (shown by the decrease in velocity across the northeastern flank of the chan- mudwave amplitude from 35 m to 22 m), the nel (parallel to Lines 1203-1 and 1203-6), as we velocity of the current across these mudwaves would expect to observe mudwaves recording any was approximately 4 cm/s (0.14 km/hr). For component of southwest/northeast bottom-current comparison, Blumsack and Weatherly [1989] flow. Thus we interpret that the mudwaves on the recorded a bottom-current velocity of 10 cm/s southwestern side of the channel result from (0.36 km/hr) orthogonal to mudwaves in the central (1) southeasterly channel flow spilling over a

14 of 18 Geochemistry 3 kerr et al.: detroit seamount Geophysics 10.1029/2004GC000705 Geosystems G

Figure 10. Farnella Line 41 off the northeast flank of Detroit (see location map); northeast is to the right. Onlapping bedforms on Horizon A, the basement slope, indicate the presence of a bottom current. Vertically aggrading bedforms suggest low flow velocity. V. E. = 8:1 at the seafloor. levee, similar to those described by Normark et al. of Detroit Seamount basement in Figure 10 is [2002] and by Lewis and Pantin [2002], formed consistent with a southerly-flowing bottom current above the upthrown block of a normal fault (Summit as sediment is transported up and over the plateau. Fault), and (2) southwesterly flow over the summit Otherwise, the lack of a bottom current would of Detroit Seamount. The velocity of spillover flow result in sediment horizons concordant with the over the mudwaves was much greater than the basement. Near-vertical aggradation of the climb- velocity of the summit flow. We infer the relatively ing bedforms suggests low current velocities. Sum- slow summit flow velocity because of the lack of mit Fault’s scarp diverts a component of summit mudwaves northeast of Summit Fault. The com- flow into southeasterly channel flow along the bined flow along the mudwaves is likely low veloc- scarp, and the Coriolis effect on the channel flow ity relative to other mudwave-producing bottom creates spillover flow over the scarp (Figure 9). currents as horizontal migration (250 m) of the mudwaves is roughly 2.5 times vertical accretion [36] In summary, the mudwaves observed on our profiles are entirely characteristic of deep-water (100 m). In other words, for every meter of vertical accretion, the wave crests have migrated only 2.5 m drifts. The velocity of the constructive current of in the horizontal direction, as shown in Figure 6. the Meiji drift (e.g., the summit flow) is not swift This is less than the factor of 5 that Normark et al. enough to generate mudwaves unless the flow [2002] uses to distinguish between vertical wave interacts with seafloor topography. Hence we are aggradation (<5) and horizontal migration (>5) of unable to measure the velocity of the regional flow mudwaves in turbidite fan systems. over the summit of Detroit Seamount. So we are predominantly characterizing spillover flows rather [35] Figure 9 schematically illustrates bottom- than primary flows. However, we can infer that the current flow in the vicinity of Detroit Seamount. regional summit flow is significantly slower than The Coriolis effect on the regional bottom current the 4 cm/s velocity measured between Horizons produces flow over the summit of Detroit Sea- C and D (ca. 5.7 Ma). The velocity of the Meiji mount. Sediment onlapping on the northeast flank current and its evolution with time would best be 15 of 18 Geochemistry 3 kerr et al.: detroit seamount Geophysics 10.1029/2004GC000705 Geosystems G

determined by new, dedicated seismic profiles [39] We interpret Summit Fault as recording north- recorded parallel to the current in the deep ocean, east-southwest extension of the southeastern sector away from perturbing seamounts, and extending of the summit plateau, possibly a manifestation of for 10–100 km. In principle, properly oriented and slope failure. The fault can not be traced across spaced profiles could recognize current velocity Lines F41 or F43 from cruise F-2-87-AA, so it is changes potentially linked to changes in sources likely a plateau-edge fracture, not a result of and vigor of bottom water flow with north Pacific regional tectonism. Nonetheless, as with the vol- or global paleoclimatic implications. canic cones, Summit Fault suggest a record of geological instability long after the Detroit shield 2.2.4. Summit Fault volcano would normally have been assumed to be quiescent. [37] The normal fault imaged during Survey 1203 (here named Summit Fault) is located 12 km southwest of Site 1203. The fault trends about 300° 3. Conclusions (see Figure 2) and dips northeast with apparent dips ranging from 22° in the southeast (Line 1203- [40] It appears Detroit Seamount was the site of 4) to 16° in the northwest (Line 1203-6). We episodes of postshield volcanism that occurred calculate apparent fault dip, heave, and throw using much later than is typical for the Hawaiian Islands. top-basement (Horizon A) offsets on all three lines, Volcanic activity occurring on Detroit Seamount and converting time to depth using Vsediment = 24–42 m.y. after the end of the main shield- building phase is considerably later than the late- 1,600 m/s and Vwater = 1,500 m/s. The true dip of the fault, 25°, is likely nearly the same as the stage volcanism occurring only a few million years apparent dip because all three survey lines are after shield-forming ceased on the Hawaiian oriented approximately normal to the trend of the Islands. Renewed volcanism on Detroit might be fault (Figure 2) (see true-scale image Figure 7c). related to changes in the motions of the Kula plate The apparent vertical offset (throw) of the base- during the Eocene. It is interesting and important ment increases from 300 m in the southeast (Line that the geochemistry of the late-stage Detroit 1203-4) to 500 m in the northwest (Line 1203-6). Seamount volcanism matches that of the much On 1201-1, the throws are: 450 m at Horizon A; shorter-hiatus alkalic rejuvenated stage of Hawai- 300 m at Horizon B; and <50 m on Horizons C, D, ian volcanism. E, and F. Smaller normal faults occur northeast [41] By analyzing the relation of strata and mud- of Summit Fault in the downthrown block wave geometries with preexisting bathymetric re- (Figure 7b). These smaller faults likely formed lief, we provide confirming evidence that the Meiji coincidentally with Summit Fault. Because sediment body is a drift accumulation deposited by Summit Fault is nonplanar (fault dip varies an ocean-bottom current flowing southeast along between the Survey 1203 seismic lines), the down- the northeast-facing flank of the Emperor Sea- thrown block will likely deform as it moves down- mount chain. The estimated bottom current veloc- dip [e.g., White et al., 1986]. ity of 4 cm/s (0.14 km/hr) represents the sum of [38] Summit Fault was most active sometime be- (1) the relatively slow regional flow over summit tween 76 and 34 Ma. The older age limit assumes and (2) the considerably faster Summit Fault scarp that the fault is not older than the age of the shield. spillover current. We infer that the regional bottom The younger age limit is the onset of the Meiji current velocity over the summit of Detroit Sea- bottom current. In order to control bottom-current mount is significantly smaller than some other flow, and hence mudwave formation (as previously inferred bottom currents. Future seismic profiling discussed), Summit Fault’s scarp must have existed efforts in this region might usefully target time before deposition of the Meiji drift. Because of the variation of the regional current velocity as a proxy apparent erosion of pre-Meiji sediment by bottom for bottom-water composition and hence paleocli- currents, and because of the rapidly varying thick- matic events. ness of Meiji deposition, we cannot use the appar- ent throw of Horizons A to F to more accurately Acknowledgments constrain the time interval of Summit Fault’s ac- tivity. However, because all sedimentary sequences [42] This research was supported by NSF and participating exhibit a change in thickness across Summit Fault, countries under management of the Joint Oceanographic we recognize that much its vertical offset could Institutions. We thank the scientific and operational partici- have formed in the latest Cretaceous. pants of Leg 197 for their efforts and expertise at sea; Jon 16 of 18 Geochemistry 3 kerr et al.: detroit seamount Geophysics 10.1029/2004GC000705 Geosystems G

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