and longitude 
can be described by the
gradient of the harmonic potential 
as:
John A. Tarduno*
Department of Earth and Environmental Sciences, University of Rochester,
Rochester, NY
Rory D. Cottrell
Department of Earth and Environmental Sciences, University of Rochester,
Rochester, NY
Bernhard Steinberger
Inst. f. Meteorology and Geophysics, University Frankfurt, Fedbergstr.
47, 60323 Frankfurt, Germany
| Abstract | 1 |
| 1.0 Overview | 1 |
| 2.0 New Paleolatitude Data for the Emperor Seamounts | 5 |
| 3.0 Hotspot Motion and True Polar Wander | 7 |
| 4.0 Introduction to Drilling Plan | 7 |
| 5.0 Geomagnetic Objectives | 9 |
| 5.1 The time-average geomagnetic field | 9 |
| 5.2 Geomagnetic paleointensity | 10 |
| 6.0 Implications for Studies of Paleoclimate | 12 |
| 7.0 Summary of Linkages to ODP LRP | 12 |
| 8.0 Issues Commonly Raised | 14 |
| 8.1 Bias in sedimentary inclinations | 14 |
| 8.2 Averaging secular variation | 18 |
| 8.3 Other objectives and program priorities | 18 |
| 9.0 Logging Plan | 18 |
| 10.0 Drilling Plan | 19 |
| 11.0 References | 22 |
| 12.0 Site Forms | 26 |
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 primarily record differences in motion of the Hawaiian hotspot relative to the Pacific lithosphere. Four lines of inquiry support the latter view: 1. global plate motions predicted using relative plate motion data 2. spreading rate data from the North Pacific basin 3. mantle flow modeling utilizing geoid and seismic tomography constraints and 4. new paleomagnetic data from the Emperor chain. Although the rate of motion is difficult to constrain because previous drilling has been limited, the best available paleomagnetic data suggest Pacific hotspots may have moved at rates comparable to those of lithospheric plates in Late Cretaceous 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. This proposal seeks to test the hypothesis of southward motion of the Hawaiian hotspot by drilling 6-8 basement sites in the Emperor seamount trend. The principal drilling objectives are to obtain moderate penetration basement sites (150-250 m) suitable for radiometric age and paleomagnetic paleolatitude determinations. A comparison of these dated paleolatitude values versus fixed- and moving-hotspot predictions form the basis of the proposed test.
Our sampling strategy will also allow us to address important geomagnetic
questions which require paleomagnetic data from the Pacific plate, including
the history of the time-average field and its paleointensity. The data
obtained will place fundamental constraints on the Late Cretaceous to early
Tertiary motion of the Pacific plate. An improved picture of this motion
history is needed if proxy climatic data from previous and future drill
sites are to be used to define past latitudinal gradients.
1.0 Overview
Many of our ideas on where mantle plumes originate, how they interact with the convecting mantle and how plates have moved in the past rest on interpretations of the Hawaiian-Emperor hotspot track. One reason the track has attained this conceptual stature lies in its prominent bend at 43 Ma. The bend, which separates the westward trending Hawaiian islands from the northward-trending Emperor seamounts (Figure 1) has no equal among the Earth's hotspot tracks; it is the clearest physical manifestation of a change in plate motion in a fixed hotspot reference frame. Because the bend is so distinct it can be used to estimate plume diameters and to place bounds on the convecting mantle wind that may deflect plumes (Duncan and Richards, 1991). However shortly after hotspots were used as a frame of reference (Morgan, 1971), apparent discrepancies involving the Hawaiian-Emperor track arose (Molnar and Atwater, 1973). Attempts to model past plate motions failed to predict the bend; instead, a more westerly track was derived (Solomon et al., 1977). Tests of the fixed hotspot hypothesis suggested large relative motions between Hawaii and other hotspots (Molnar and Atwater, 1973; Molnar and Stock, 1987), but uncertainties in the plate circuits employed in these tests limited their resolving power (Acton and Gordon, 1994).
Recently, several works have readdressed these questions. Norton (1995) has suggested that the Hawaiian-Emperor bend records when the hotspot became fixed in the mantle. Prior to 43 Ma, the hotspot would have moved southward, creating the Emperor seamount chain. This work is difficult to assess because of the lack of formal error analyses, but the interpretation reiterates findings of updated plate circuit studies which consider rotation pole errors (Cande et al, 1995). In addition, there is no obvious change in spreading rate at 43 Ma in the well-studied marine magnetic anomaly record of the North Pacific (Atwater, 1989). Many feel the lack of such a response by overlying plates to a change of absolute plate motion as large as that indicated by the Hawaiian-Emperor trend is reason enough to question hotspot fixity. New modeling efforts, utilizing a viscosity structure based on geoid constraints, mantle flow fields consistent with tomographic data and plate motion estimates, also predict motion of hotspot groups (Steinberger and O'Connell, 1997). For the Emperor trend, the predicted motion is 10 mm yr-1 (Steinberger, 1996) (Figure 2).
While these recent studies have raised the tone of discussions regarding hotspot fixity (see also Christensen, 1998; Wessel and Kroenke, 1998), they face some fundamental data limitations. Fortunately, the hypothesis of hotspot motion can be tested independently using paleomagnetism. If the hotspot has remained fixed, the paleolatitudes of extinct volcanic edifices comprising the Emperor chain should equal that of present-day Hawaii. These tests, however, are difficult in practice. For example, paleomagnetic data from some deep sea sediments show a bias caused by compaction-induced flattening (Tarduno, 1990). Such problems can be avoided through the study of well-dated basalt drill cores. But until recently, only a few sites had sufficient depth penetration. This situation has improved with the latest Pacific drilling (ODP Legs 143, 144 and 145). Data from Legs 143 and 144 indicate significant motions between hotspot groups in the Atlantic and Pacific Oceans during the mid-Cretaceous (128-95 Ma) (Tarduno and Gee, 1995). The motion is rapid, at speeds typical of lithospheric plates (30 mm yr-1).
These findings indicating an older episode of hotspot motion, coupled
with the inferences based on relative plate motions suggest to us that
Hawaiian hotspot motion is a viable hypothesis which should be tested further.
New data obtained from the Emperor chain drilled during Leg 145 (Figure
1) have allowed a preliminary test. Below we summarize these analyses
(Tarduno and Cottrell, 1997), as they provide support for the hypothesis
and have guided our proposed sampling plan. In addition, we outline how
sites chosen to address the question of hotspot fixity can provide crucial
data required for understanding the past geomagnetic field and for paleoclimate
studies which rely on reconstructions of the Pacific plate.
2.0 New Paleolatitude Data for the Emperor Seamounts
During Leg 145, 87 m of lava flows were penetrated on Detroit Seamount (Figure 1) (Rea et al., 1992). 40Ar/39Ar radiometric analyses yield an age (81.2 ± 1.3 Ma (Keller et al., 1995)) older than that assumed in hotspot-based plate motion models (65-75 Ma) (Duncan and Clague, 1985). Characteristic magnetizations derived from basalt samples mainly have negative inclinations indicating reversed polarity (Figure 1). This polarity assignment is consistent with the radiometric age data, suggesting eruption of the basalts during chron 33R (Tarduno and Cottrell, 1997).
A potential problem in obtaining reliable paleomagnetic data from any basalt drill hole is the uncertain timescale between eruptions. If most flows reflect rapid eruptions, one could easily obtain a biased paleolatitude estimate by giving equal weight to each flow unit. To address this concern the inclination-only averages derived from each flow unit (McFadden and Reid, 1982) must be checked for serial correlation (Cox, 1970; Kono, 1980; Tarduno and Sager, 1995). These analyses lead to inclination-group models (Figure 3). The directional angular dispersion, estimated from the inclination-model data and transformed into pole-space (Cox, 1970; Tarduno and Sager, 1995), is indistinguishable from the predicted virtual geomagnetic pole scatter from global data sets (McFadden et al., 1991) (Figure 3). As discussed below, there is only one other paleomagnetic data set from the Emperor trend that satisfies these geomagnetic-sampling requirements.
The preferred inclination group model, where groups are distinct at >95% confidence (n=10) (Kono, 1980), suggests a paleolatitude of 36.2o+6.9o-7.2o, clearly discordant from the present-day latitude of Hawaiian (~19o) (Figure 3). This discrepancy is too large to be explained by tectonic tilt. Tilts of 1-3o have been reported previously for some of the northern Emperor seamounts (Lonsdale et al., 1993). Because these tilts are small and the angle between the remanent magnetization vector and down-dip azimuth of tilt is large (>60o), the effect on the paleolatitude is negligible. Measurements made at unit contacts also fail to indicate significant dips.
The new paleomagnetic result directly questions the validity of the
Late Cretaceous Pacific apparent polar wander path (Figure
3). But how could these prior results be so errant? Previous Late Cretaceous
poles are heavily or solely based on the inversion of magnetic surveys
over seamounts (Gordon, 1983; Sager and Pringle, 1988). Reviews of the
methods used to fit these poles suggest they are far more uncertain than
commonly supposed (Parker, 1991). Viscous and induced magnetizations can
also bias the resulting pole positions (Gee et al., 1989). Interestingly,
high latitude poles similar to the new colatitude result (Figure
3) have been reported from preliminary analyses of marine magnetic
anomaly skewness data of comparable age (Vasas et al., 1994).
3.0 Hotspot Motion and True Polar Wander
The other paleolatitude value from the Emperor trend that adequately averages secular variation was derived from Suiko Seamount (65 Ma) (Kono, 1980) (Figure 1). The 8o discrepancy between the Suiko Seamount paleolatitude and the present-day latitude of the Hawaiian has been attributed previously to early Cenozoic true polar wander (Gordon and Cape, 1981; Sager and Bleil, 1987), a rotation of the entire solid Earth in response to shifting mass heterogeneities in the mantle (Goldreich and Toomre, 1969). True polar wander predictions based on global paleomagnetic data from the continents (Besse and Courtillot, 1991), however, do not agree with the new Detroit Seamount data (Tarduno and Cottrell, 1997).
Having excluded Late Cretaceous true polar wander, we must now consider hotspot motion as an explanation for the new paleomagnetic data. We can isolate the latitudinal history of the Emperor seamounts from that of the Hawaiian chain by subtracting the difference between the present-day latitudes of the 43 Ma bend and Hawaii from the present-day latitudes of each of the Emperor seamounts. In effect, we slide the Emperor trend down the Hawaiian chain to the present-day latitude of Hawaii (Figure 4). In so doing, we produce a plot predicting the paleolatitude of Emperor seamounts if they were formed by a moving hotspot beneath a stationary plate. The new Detroit result together with the Suiko Seamount data parallel this predicted trend and provide support for the hotspot motion hypothesis. Differences between the data and predicted values also allow for some northward plate motion.
It is difficult to place error bounds on the rate of motion because
there are only two estimates of paleolatitude available. Nevertheless,
the data suggest that the Hawaiian hotspot could have moved continuously
southward from 81 Ma to 43 Ma (Norton, 1995), at a rate of 30-50 mm yr-1,
while the Pacific plate moved slowly northward, in a paleomagnetic (spin
axis) frame of reference (Figure 3).
4.0 Introduction to Drilling Plan
Interpretations of the Hawaiian-Emperor bend have had a tremendous impact on our understanding of the history and dynamics of plate motions. Diverse new data sets suggest these interpretations may be wrong, or at best largely incomplete. Below (Section 9) we outline a drilling plan to test the hypothesis of Hawaiian hotspot motion. We will target moderate penetration of basaltic sections on several of the Emperor trend guyots and seamounts, with the aim of obtaining an average of secular variation (15 independent paleomagnetic inclination units) at each site. Because of this approach, we can also address other geomagnetic and paleoclimate issues of the Late Cretaceous to early Tertiary. These issues are outlined below.
5.0 Geomagnetic Objectives
To fully understand the nature of the geomagnetic field, global data
are required. Progress in our understanding of the geomagnetic field over
the past 150 million years is hindered by the lack of sufficient high resolution
data from the Pacific plate. By targeting sites where a secular variation
record can be obtained in basalt, significant advances can be made in our
understanding of the time-averaged geomagnetic field and its intensity
for Late Cretaceous to early Tertiary times.
5.1 The Time-averaged geomagnetic field
The need for high resolution paleomagnetic data to constrain this history reaches far beyond the paleomagnetic community. Recent advances in modeling, that have produced realistic simulations of the geodynamo (e.g. Glatzmaier and Roberts, 1995), highlight one need for paleomagnetic constraints on model parameters. The nature and history of the time-averaged geomagnetic field is a major topic of interest for many scientists interested in studies of the Earth's deep interior (SEDI). A full description the past field requires data from the Pacific Ocean basin, because of potential longitudinal components.
The geomagnetic field at a radius r, colatitude
and longitude can be described by the
gradient of the harmonic potential
as:
The Gauss coefficients gml and hml
describe the size of spatially varying fields. For the present field, and
models of the Late Cretaceous to early Tertiary field, the axial dipole
term (gol) is overwhelmingly dominant. Therefore,
other terms will not affect greatly the accuracy of data used to test hotspot
motion hypothesis. However, the data obtained can be used to better constrain
the Gauss coefficients of the past field. For non-zonal (m
0;
i.e. those terms varying with longitude) terms, data from the Pacific basin
are essential because of its sheer size; no global description of the field
can be considered complete without data from the region.
While the general importance and need for Pacific data are generally appreciated, the methods used to summary past data prior to modeling (spherical harmonic analysis) have been given less consideration. For the early Tertiary and Late Cretaceous, plate motion can not be neglected, as they can for analyses of data over the past 5 m.y. (Constable, 1992), but instead the data must be first rotated into a common reference. The few analyses that have tried to incorporate data from the Pacific (principally older seamount results), have relied on a fixed hotspot frame of reference; hence previous estimates of Gauss coefficients may contain considerable errors if the hotspot motion hypothesis is correct.
Interesting, these analyses show a dramatic change in the Gauss coefficients
(a
change in sign) during the critical Late Cretaceous to early Tertiary interval
we have targeted for study (Livermore et al., 1984) (Figure
5a). Therefore, the data collected at the sites proposed for study
can simultaneously address the hypothesis of hotspot motion, and the reality
of this change in sign of the spatially varying Late Cretaceous-early Tertiary
geomagnetic field.
5.2 Geomagnetic paleointensity
When compared to the considerable success of studies that ultize directional data derived from paleomagnetic measurements, work devoted to understanding the past intensity of the geomagnetic field has advanced more slowly. However, the long-term variations of paleointensity are essential for a complete description of the field, as well as for understanding the long-term magnetic signature of ocean crust.
One reason progress has been slow is related to selection criteria needed to ensure reliable paleointensity determination. The preferred method of paleointensities measurement, Thellier-Thellier double heating experiments of basalts (Thellier and Thellier, 1959; as modified by Coe, 1967), often encounter problems due to chemical alteration during heating. Significant recent progress has been made in by studying basaltic glass (Pick and Tauxe, 1993) which shows ideal magnetic properties. The available DSDP and ODP sites having basaltic glass have now been analyzed (Juarez et al., 1998) so further progress requires additional drilling (Figure 5b).
The drilling we propose has the potential to yield several reference
sites for Late Cretaceous-early Tertiary paleointensity. Because we propose
to sample a significant number of flow units at each site drilled, the
chances of obtaining a time-averaged paleointensity value at our site are
greatly increased. Even if basaltic glass is not recovered, recent advances
in paleointensity measurements measured on single plagioclase crystals
(Cottrell and Tarduno, 1997) may allow considerable new paleointensity
data to be recovered for the Late Cretaceous to early Tertiary interval.
6.0 Implications for Studies of Paleoclimate
Sea surface temperatures (SST) determined from isotopic data provide fundamental constraints on past climatic gradients, and are an essential tool in studies of past climatic extremes (Huber et al., 1995). To be of use, however, these isotopic data must be placed in a reliable latitudinal framework.
Two general classes of reconstruction programs have been employed. In one, the spatial distribution of climatic data (e.g. climatically sensitive sedimentary beds, precipitation, orography, temperature sensitive flora and fauna) are used to constrain paleolatitudes; although the distinction is often abused, these reconstruction do not provide a independent reference frame to consideration addition paleoclimate questions. The second, and more common approach for oceanic plates, is the use of a fixed hotspot reference frame. As discussed throughout this proposal, substantial errors could be introduced. Sometimes commercial rotation programs have been employed in the literature; for these cases the exact nature of the reference frame is unclear.
To illustrate the problem, we consider the limited isotopic data for the Campanian-Santonian interval (Cottrell and Tarduno, 1998) (Figure 6). Because of the large, recent revisions in the Pacific apparent polar wander path motivated by the new data from Detroit seamount, large changes in the reconstructed latitudes of Pacific sites, compared to previous reconstruction, are indicated. We are unaware of any prior work that has recognized that the past reconstructed latitudes of sites yielding important isotopic data are large (20-30o).
If substantial progress is to be made on studies of past climate extremes
of the Paleogene and Late Cretaceous, Pacific isotopic data from the tropics
is essential (Zachos et al., 1994; D'Hondt and Arthur, 1996). However,
these studies must rely on an accurate latitudinal data base such as that
which can be provided by the drilling of the Late Cretaceous to early Cenozoic
Emperor trend seamounts.
7.0 Summary of Linkages to ODP Long Range Plan
The goals of the proposed drilling direct relate to the ODP Long Range Plan (LRP) which emphasizes mantle dynamics. These are also key issues for the SEDI international community and were the subject of discussion at the AGU Chapman Conference on the History and Dynamics of Global Plate Motions (1997); members of the modeling community are now involved with this proposed drilling program. Geomagnetic goals go beyond items explicitly mentioned in the ODP LRP, but include several items recognized in the recent COMPOST II report. Because of the importance of Pacific plate reconstruction to paleoclimate issues, the proposed work is directly related to the ODP LRP and several topics identified by the JOIDES planning structure as of particular importance. These include topics represented by Program Planning Groups addressing ``Climate-Tectonic Links" and ``Extreme Climate and Environments of the Paleogene and Cretaceous". A detailed knowledge of plate interactions for essential for assessing the tectonic role in Late Cretaceous to early Tertiary climate change and for evaluating the cause of climatic extremes during this time interval.
8.0 Issues Commonly Raised
This proposal was developed following recommendations by ISSEP (1997).
In their review of our preproposal, ISSEP felt the work was ``... well
justified with a clearly stated objective that requires drilling. The scientific
goal is highly relevant to the mandate if the SSEP-Interior in particular
and to the ODP Long Range Plan in general." The panel recommended further
that we consider several issues in developing a full proposal including:
the potential use of paleomagnetic analyses on sediment cores; petrologic
studies on the volcanic rocks; and additional discussion of the averaging
of secular variation. These issues are discussed below (Sections 8.1-8.3).
Requests for additional sites closer to the Hawaiian-Emperor bend, and
additional discussion of logging, are addressed in the logging and drilling
plans (Sections 9.0-10.0).
8.1 Paleomagnetic Bias in Sedimentary Inclinations
One potential way to address the question of hotspot fixity is to obtain paleomagnetic data from deep sea sediments. The advantages of this approach are clear: at a given site well-dated sediments might allow a nearly continuous latitudinal record of plate motion. By combining data from several distant sites paleomagnetic poles can be constructed, yielding an apparent polar wander path.
Paleomagnetic data from sediments are available from numerous Pacific DSDP and ODP sites; these data can be examined to test whether they define an internally consistent picture of plate motion (Figure 7). Unfortunately, the data are systematically shallower than coeval inclination values derived from igneous sources (e.g. Gordon, 1990). The differences are consistent with compaction-induced inclination shallowing (Tarduno, 1990; Butler, 1992).
Sediments can acquire a detrital magnetization when magnetic grains orient in the presence of the Earth's geomagnetic field. A flattening of inclination, however, is well-documented for certain sediment types, especially glacial varves. The error is described as:
where Io is the observed inclination, Ie is the expected inclination, and f is an empirically determined flattening factor. For glacial varves, f=0.40 have been reported (King, 1955).
It was once thought the deep sea sediments might escape the effects of inclination-error because their magnetization was thought to be a post-depositional remanent magnetization (pDRM), which was locked in not at the sediment-water interface, but slightly deeper, when magnetic grains were no longer free to rotate in pore spaces (Verosub, 1977; Demenocal et al., 1990).Nevertheless, observations from all the world's oceans appeared to define shallow inclinations in some deep sea sediments (e.g. Celaya and Clement, 1988; Arason and Levi, 1990; Larson et al., 1992) and subsequent experimental results on synthetic samples have confirmed that a pDRM can be shallowed by compaction (Kodama and Sun, 1992; Sun and Kodama, 1992).
The Pacific sedimentary inclination data studies by Tarduno (1990), which are mainly Cretaceous in age, define a f=0.52. Considerable scatter in this value is not surprising: sediments ranging from carbonates to volcaniclastics were combined and sediment-type clearly must play a role in compaction. This calculation also relies on expected inclinations calculated from the sparse, circa 1990 igneous data set (e.g. basalt colatitudes). Results from subsequent drilling have tended to confirm previous notions of mid-Cretaceous pole positions (Tarduno and Sager, 1995; Tarduno and Gee, 1995). In addition, while errors in the reference data (poles) can effect the degree of flattening, the overall distribution remains internally inconsistent and requires inclination shallowing. Any remaining doubt on the mechanism was put to rest by the magnetic anisotropy work by Hodych and Bijaksana (1993) on the same sites studied in Tarduno (1990). This work documented that the development of a magnetic fabric in Pacific deep sea sediments was proportional to the flattening values predicted in Tarduno (1990).
The potential problems caused by inclination shallowing are particularly severe in our proposed study for several reasons. The amount of error varies with expected latitude. Because the expected latitude values, in the moving hotspot hypothesis, range between 40o and 60o, the errors will be at their maximum potential values for flattening values similar to those reported in natural sediments (Figure 8a). These errors are of the same magnitude as the total inclination offset we seek to test (Figure 8b).
While substantial advances have been made in using magnetic anisotropy to correct for inclination shallowing (e.g. Jackson et al., 1991), the potential errors are, in our opinion, too large for this to yield an unambiguous, high resolution test of the fixed vs. moving hotspot models.
We note, however, that a few sites in the Pacific, particularly shallow-water
sediments (Tarduno and Gee, 1995) have yielded data which do not appear
to have suffered significant inclination shallowing. This may be due to
relatively early cementation and similar sediments could be encountered
during drilling. However, because it can be difficult to obtain high-resolution
age data on some shallow-water sediments, and recovery is problematic,
they do not provide a viable alternative to basalt drilling.
8.2 Secular Variation in Previous Deep Sea Drilling Studies
A key question concerning paleomagnetic tests such as those proposed here is the penetration needed to average adequately secular variation. It has been suggested that recent drilling on Hawaii should be taken as a guide. In our view, the best estimate of the depth penetration needed is provided by previous drilling in the Emperor trend. This drilling record provides a better temporal gauge of the waning stages of basalt extrusion on seamounts 50-90 million years ago. Drilling on Detroit Seamount indicates that as little as 85 m of basalt penetration may be needed at some sites to obtain an average of secular variation. When basalt penetration was greater than 120 m during previous drilling of Cretaceous plateaus, seamounts and guyots in the Pacific, enough independent time units were recovered to average secular variation (Tarduno and Sager, 1995; Tarduno and Gee, 1995). This value does not differ greater from the depth range over which secular variation is averages (100-200 m) in analyses (Holt et al., 1996) of basalt cores obtained by drilling on Hawaii.
However, it is not possible to determine prior to drilling the time
sequence represented by the basalt lavas at a given site. As described
in Section 2, one needs to evaluate the angular dispersion of independent
flow (inclination units) and compare this versus global paleomagnetic data
to confirm whether secular variation has been adequately sampled at a given
site. It is possible to collect paleomagnetic data at sea, and to makes
these calculations during drilling to insure the resulting record will
provide an adequate average (see Section 10).
8.3 Other Objectives and Program Priorities
Our program focuses on objectives that have been recognized as of priority
in the ODP LRP (mantle dynamics). Progress can also be made on paleoclimate
issues recognized in the LRP by constraints the data will place on plate
reconstructions. We recognize that geochemical objectives, including the
long term evolution of the Hawaiian hotspot source might also be met at
the sites we propose. The JOIDES planning structure has reviewed previously
a Letter of Intent (LOI-29, ``Evolution of the Hawaiian Hot Spot", by Spencer
and Mahoney, 1994) focusing on such objectives. The Lithosphere Panel concluded
that these objectives were of relatively low priority and the drilling
effort was not encouraged. In light of this prior review history, we feel
that geochemical objectives are best met by interested geochemists joining
the scientific party and proposing shipboard and shorebased studies on
the sections which we hope to recover.
9.0 Logging Plan
An essential part of our proposed drilling program will be use of logging
for potential core orientation. Because the principal offset we choose
to examine is in latitude, it is not necessary to obtain fully oriented
core samples to test the basic hypothesis of hotspot motion. However, declination
data will allow higher resolution tests and will provide tighter constraints
on Pacific plate reconstructions. We propose to use the FMS to identify
fractures which we hope to match with the recovered core; such matches
can allow the core pieces to be oriented. These data can also assist in
tests of tectonic tilting. In addition to FMS, we propose use of the magnetic
susceptibility tool and the standard (``triple-combo") logging package
in the basaltic sections. The magnetic tools can assist in the determination
of magnetic stratigraphy, helping in the overall stratigraphy of the lowermost
sediments and the basaltic sections.
10.0 Drilling Plan
We propose to drill 6-8 basement sites. We have added sites closer to the Hawaiian Emperor bend in response to a request by ISSEP during their review of our preproposal. We feel all the drilling proposed can no longer be completed during a single standard drilling leg. According, we outline three priorities groups as follows: Group 1 will be on a seamount younger than Detroit, Group 2 on Detroit Seamount itself and Group 3 on the oldest portion of the Emperor chain.
We propose basement penetration to moderate depth (150-250 m). Our preferred strategy will be to employ minicones for reentry. However, we will also consider deployment of full reentry cones if problems with hole stability arise. For each site proposed, previous drilling has touched basement, or penetrated the sediment cover, providing information on the nature of the sediment cover and basement depth, as well as drilling times. Because these sediment columns have been cored previously, the problem of bias in paleomagnetic inclinations derived from sediments (Section 8.1), and the need for time to penetrate a substantial basaltic section at each site, we propose washing down through the sediments to basement. However, if ESSEP identifies important objectives in the sediment column at any of our sites, we will seek to modify this approach. Site survey data used in the approval of these DSDP/ODP locations have been used to guide our proposed drilling. We have placed nominal depths for the basement penetration based on our experience drilling on other Pacific seamount and plateau sites (Tarduno and Gee, 1995) (see section 8.2). While these estimates are needed for the planning process, we envision an interactive process based on the recovery. We ideally hope to recovery at least 15 flow units (distinct lava flows, not pillows) at each hole for detailed paleomagnetic and radiometric age (40Ar/39Ar) analysis. If this is obtained in a given hole, we would prefer to drill additional sites on a seamount (or additional seamounts) to improve the accuracy of paleolatitude determinations and assist in the overall test. Below we include a brief description and rationale for each of the drilling sites.
Group 1: Meiji Seamount (~86Ma)
Given the new age data from Leg 145, the age progression of the Emperor trend north of Detroit Seamount is now uncertain. There is also a bend in the northern Emperor trend that, if better dated, could be used to examine independently some of the issues of plate and hotspot motion discussed here. We propose two holes on Meiji Seamount (Figures 1, 9) near DSDP Site 192. If the Emperor trend seamounts reflect continuous hotspot motion, we expect to find a paleolatitude of ~40o from the recovered basalts. HE-1A is proposed at the location of DSDP Site 192. Prior drilling indicates a sediment cover of 1044 m, composed of ooze, chalk and clays, above subaerial basalt. An alternate site has also been identified (HE-1B), 6 km SW of DSDP Site 192. We expect this site to have a thinner sediment cover than that penetrated at Site 192.
Group 2: Detroit Seamount (81 Ma)
Drilling on Detroit Seamount is of highest priority. We propose single holes at two sites; the summit (HE-3; Figures 1, i9; near ODP Site 883) and the south flank of the seamount (HE-2; near ODP Site 882). We feel these sites will enable us to confirm or refute the new paleomagnetic results currently available from just a single site (Site 884). HE-2 is located 7 km NW of ODP Site 882, where the sediment column is thinner (approximately 800 m of oozes, chalks and clays). HE-3 is proposed at Site 883 where the sediment column above basaltic basement is 840 m thick.
Group 3: Nintoku (56 Ma) and Ojin Seamounts (<56 Ma) and Koko Guyot (~48 Ma)
Present data from basalt cores are insufficient to investigate how the 8o paleolatitude discrepancy between Suiko Seamount and present-day Hawaii accumulated and whether true polar wander or hotspot motion is the cause. We propose drilling Nintoku and Ojin Seamounts and Koko Guyot (Figures 1-9) to investigate these issues. Paleolatitude results can also be compared with global data to test for true polar wander.
We have added addition sites closer to the Hawaiian Emperor bend in response to review comments by ISSEP (1997). We note that the tests at with these holes are close to the practical resolution limit of paleomagnetic data and may require penetration greater than that at the more northerly sites. The decreased angular dispersion of paleomagnetic directions at the lower latitudes of this sites, however, acts to allow a more precise paleolatitude estimate for a given number of independent flow units.
If the Emperor trend represents southward hotspot motion of the Hawaiian hotspot, we should obtain a paleolatitude of 25-27ofor sites on Nintoku Seamount. Our proposed site HE-4A is positioned on Nintoku Seamount at DSDP Site 432(A), near the northwest edge of the seamount on a flat-lying, stratified sediments. Previous drilling indicates the sediments are 42 m thick, above basaltic basalt. Prior drilling indicates the uppermost flows are separated by soil horizons, indicating significant time between flow units. An alternate site has also been identified (HE-4B), offset 28 km to the NW on the guyot summit.
HE-5A is positioned on Ojin Seamount at DSDP Site 430 (Figures 1, 9). Approximately 60 m of sediments (ooze, sand and volcanic ash) overlie basalts at this site. An alternate site (HE-5B) has also been chosen, 18 km to the NE of DSDP Site 430. We note that available radiometric (K-Ar) ages for Nintoku and Ojin Seamounts are similar (Duncan and Clague, 1985). We feel this indicates that the uncertainties in the age data are far greater than the quoted K-Ar precisions. We hope to address this issue through the application of 40Ar/39Ar age dating of all sites drilled.
HE-6A is positioned at DSDP Site 308 (Figures 1,
9) on Koko Guyot. Previous drilling penetrated
approximately 70 m of clays and volcaniclastic sandstone. Biostratigraphic
data indicate an Eocene age for the base of the section, suggesting the
hole was terminated (due to excursions of the ship from the beacon location)
near basement. An alternate site is provided by DSDP Site 309 (HE-6B) on
the same guyot. This hole was also terminated prior to penetrating basement.
Both sites are located in sedimented areas where drilling can be easily
started.
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