The paleomagnetism of eastern Nazca plate seamounts

Tectonophysics,

170 (1989) 279-287

Elsevier Science Publishers

279

B.V.. Amsterdam

- Printed

in The Netherlands

The paleomagnetism of eastern Nazca plate seamounts JOHN A. HILDEBRAND Institute of Marine Resources, A-005, Scripps Institution

of Oceanography,

University of California,

San Diego, LA Jo114

CA 92093 (U.S.A.) (Received

August

8,1988;

revised version

accepted

April 18, 1989)

Abstract Hildebrand,

J.A., 1989. The paleomagnetism

Paleomagnetism Cenozoic

of eastern

times. Magnetic

Basin and

for Piquero-2

Piquero-1

seamount

seamount

indicates

than predicted

and basalt

plate

and bathymetric

they are used to calculate

paleopole motion

Nazca

of eastern

Nazca

seamounts surveys

paleomagnetic

is coincident

plate seamounts.

defines

are presented poles

Nazca

with uniform

with the earth’s

hotspot

reference

Farallon

for two eastern and

pole, suggesting

that the Nazca plate moved 23O northward

by a Pacific

and

Tectonophysics,

during

Nazca

absolute

plate

motion

plate seamounts

nonuniform a young

170: 279-287.

magnetic

seamount.

during

in the Chile modeling.

The paleopole

The for

O-50 ma. This is 13O more latitudinal

frame and 20 o more motion

than predicted

by DSDP

sediment

paleomagnetism.

Introduction

1978)

The Farallon plate, which once occupied the eastern Pacific Basin, fragmented during Cenozoic

have included a N-S component (Engebretson et al., 1985; Gordon and Jurdy, 1986). Analysis of seafloor magnetic anomalies on the

times when it interacted with the west coast of the Americas (Menard, 1978). Knowing the absolute motion of the Farallon plate is important for

Nazca and Pacific plates provides the relative motion between these two plates (Herron, 1972; Mammerickx et al., 1975; Handschumacher, 1976;

understanding interactions between the Americas and the subducting oceanic crust (Atwater, 1970; Lonsdale, 1978; Pilger, 1981, 1984; Jordan et al., 1983; Wortel, 1984). The absolute motion of this plate is poorly understood; no hotspot chain has been mapped or dated to give a direct measure of its absolute motion. Most of the plate has been subducted, and the only remaining fragments are now part of the Cocos and Nazca plates. The

Pilger, 1978; Mammerickx et al., 1980; Rea, 1981). With Nazca and Pacific plate relative motion, Nazca plate absolute motion can be calculated from the absolute motion of the Pacific plate, which is known from a hotspot reference frame (e.g. Gordon and Jurdy, 1986) or from a paleomagnetic reference frame (e.g., Gordon, 1982; Sager, 1987). Paleomagnetic cores from deep sea drilling are azimuthally unoriented and therefore yield only a magnetization inclination and paleo-

Nazca plate contains the oldest remaining piece of the Farallon plate and so may reveal the greatest time span of the Farallon plate absolute motion. Forces driving modem Nazca plate motion are probably different from those which acted upon the Farallon plate. Present-day Nazca plate motion is predominantly E-W (Minster and Jordan, OCUO-1951/89/$03.50

0 1989 Elsevier Science Publishers

B.V.

whereas earlier Farallon plate motion may

latitude. Seamounts are a better source of oceanic paleomagnetic data since they yield magnetic inclination and declination. Knowledge of a seamount’s paleomagnetism is particularly useful when combined with isotopic or fossil dating (Harrison et al., 1975). In this paper, I present

280

J.A.HILDEBRAND

paleomagnetic pole determinations for two seamounts on the eastern portion of the Nazca plate, and I discuss their implications for Faralion and Nazca absolute plate motion. The Nazca plate is situated in the east-central Pacific, surrounded by the Pacific plate to the west, the Cocos plate to the north, the South American plate to the east, and the Antarctic plate to the south (Fig. 1). The oceanic crust comprising the Nazca plate was created at two different spreading centers: the East Pacific Rise (EPR) at the Pacific-Nazca plate boundary and the

PACIFIC

Mendoza-Roggeveen-Selkirk Rise (MRSR) at the Pacific-Farallon plate boundary (Herron, 1972). The younger EPR created the crust in the western portion of the Nazca plate with age O-20 Ma; 20 Ma; the older MRSR created the crust in the eastern portion of the Nazca plate with an age of 20 Ma or older. The modern EPR is spreading predominantly E-W whereas the MRSR spread in a northeast and southwest direction at an angle of about 45” to the EPR. The northern Nazca plate tectonic history is further complicated because the fossil Galapagos Rise occupies the original EPR

PLATE 00

NAZCA

PLATE SOL’TH AMERICA

-

-+.._ --__

:: GJ

..- BOUNDARY CRUST ,... CREATED AT E PR

ANTARCTIC

PLATE

J

i

1 Fig. 1. Ta?tonic setting of the Nazca plate. Indicated features are the East Pacific Rise and the Chile Rise (solid lines), the fossil Galapagos Rise-Mendoza

Rise-Roggeveen

Rise-Se&irk

Rise (hatched lines), basement age isochrons (X,30,

Sites 320 and 321 (crossed circles). and Piquero seamounts (stars).

35,#,

45 Ma), DSDP

PALEOMAGNETISM

I

OF EASTERN

I

NAZCA

PLATE

SFAMOUNTS

J.A. HILDEBRAND

282

position

which later jumped

rent position plate detached Ma when created.

westward

(M~rne~c~

from the Nazca

the Cocos-Nazca

tachment

plate at about

spreading

A change in absolute

is thought

to have occurred

plate including

age isochrons

25

ridge was

Seamount bathymetric uniform

with de-

1978; Pilger

configuration

of

the EPR, MRSR,

and

for the eastern

portion

of the

Parker,

SEAMOUNT

’ /i,dj

from

location,

as-

dipole for the earth’s

field. In this paper 2

and

is calculated

and the seamount

y \

either

et al., 1975) or

(Hildebrand

an axially geocentric

-------r

assuming

(Harrison

magnetization

PIQUERO

BATHYMETRY

!

with

surveys where the aver-

1987). The paleopole

the magnetization suming

can be studied

is calculated

magnetization

nonuniform

magnetic 2

and magnetic

coincident

plate. PIQUERO

data

paleomagnetism

age magnetization

of the Cocos plate (Chase,

the Nazca

Seamount paleomaguetic

Nazca plate motion

1978). Figure 1 shows the current crustal

to its cur-

et al., 1980). The Cocos

SEAMOUNT

i.~‘~_~

the seminorm

mini-

MAGNETICS

4T RESIDUAL

-__---

c

285

2

285.4

285

6

Fig. 3. Piquero-2 seamount. a. Bathymetry. b. Observed magnetic seminorm minimization approach. d. Residual magnetic

field anomaly. c. Calculated magnetic field anomaly field. Dotted lines in (a) denote ship track coverage.

from the

PALEOMAGNETISM

OF EASTERN

NAZCA

283

PLATE SEAMOUNTS

mization approach is used to model nonuniform seamount magnetization (Parker et al., 1987) and the statistical approach to model the confidence limits associated with each paleopole (Parker, 1988). The two seamounts examined are located on the eastern Nazca plate adjacent to South America on crust of 45 Ma age or older (Fig. 1). These seamounts, located in the Chile Basin, were surveyed during the Piquero Expedition (1969) conducted by the R/V “Thomas Washington” of Scripps Institution of Oceanography (Anderson et al., 1976). These are all called the Piquero-1 and the Piquero-2 seamounts. A proton-precession magnetometer collected total magnetic field measurements; the magnetic anomaly field was obtained by subtracting the definitive geomagnetic reference field (DGRF-70). Bathymetric and magnetic maps were provided by V. Vacquier (pers. commun., 1988). Piquero-1 and Piquero-2 bathymetry and magnetic field anomalies are shown in Figs. 2 and 3. Piquero-l’s bathymetry has two distinct peaks aligned NE-SW: its magnetic anomaly is a simple pair of high and low values indicating normal polarity magnetization (high value to the north). Piquero-2’s bathymetry also has two peaks aligned NE-SW: its magnetic anomaly, however, is complex with several distinct local highs and lows. The strongest high to the south and the strongest low to the north indicate a dominantly reversed polarity. For paleopole inversion the bathymetric and magnetic maps were digitized on a regular grid of

between 100 and 300 points (l-3 km spacing) using a higher density of points to sample the seamount summits. Results

For each seamount the least squares and seminorm minimizing models were calculated. The results are summarized in Table 1. For both seamounts the goodness-of-fit parameter (GFR), which is the ratio of the observed to the residual anomaly, indicated a poor fit for the least squares models. The GFR was 1.5 for Piquero-1 and was 1.1 for Piquero-2. The least squares models, therefore, explain no more than one-third of the observed anomaly for either seamount. The seminorm minimizing models were chosen to have 30 nT misfit, the expected value of noise due to variations in crustal magnetization (Parker et al., 1987). The resulting GFR was 2.2 for Piquero-1 and 2.4 for Piquero-2, although these values could be made arbitrarily large by increasing the contribution of the nonuniform components to the models. The seminorm model calculated and residual fields are shown in Figs. 2 and 3. For Piquero-1 a short wavelength residual field occurs over the seamount summit, indicating unmodeled shallow magnetization. For Piquero-2 the residuals are largest at the edges of the observation area, possibly indicating unmodeled crustal magnetization. A model was calculated for the interior magnetization of each seamount using both the uniform and nonuniform components of the seminorm

TABLE 1 Nazca seamount pakopoles Seamount

Dec.

Inc.

Uniform

Non-uniform

Pole position

(OE)

down

intensity

RMS intensity

Iat, N

(“)

(A/m)

(A/m)

GFRa

long QE

RMS residual (nT)

995

major

minor

azimuth b

(“)

(“)

(“)

Piquero-1 least squares

22.1

- 58.2

5.2

_

64.3

65.3

1.5

43

-

Piquero-1 seminorm

24.9

-63.2

4.8

0.9

59.1

72.1

2.2

30

27.7

15.7

Piquero-2 least squares

161.2

53.2

2.3

_

70.7 c

159.5 c

1.1

66

-

-

Piquero-2 seminorm

176.9

42.2

2.5

3.5

87.1 = 179.7 =

2.4

30

27.9

11.1

* South Pole. a Goodness of fit parameter (Harrison, 1971). b 95% confidence ellipse axes in degrees and azimuth of major axis measured east of north. ’ South Pole.

43 163

284

J.A. HILDEBBAND

Fig. 4. Fiquero-1seamountmagnetizationmodel.The contour

levels. Vectors

amplitude

are drawn

of total magnetization the seamount

to represent

model is displayed

the magnetization

and the shape of the vector head denotes

has slightly smaller magnetization

amphtude.

minimizing model. Three horizontal planes were chosen within the body of the seamount (vertically spaced at 375 m intervals for Piquero-1 and 700 m intervals for Piquero-2) and the magnetization was calculated for each plane at the ho~zonta~ location of all input bathymetry points falling within the contour line. For Piquero-1 (Fig. 4) the model has uniform normal polarity, although slightly diminished amplitude is observed for the eastern peak. For Piquero-2 (Fig. 5) the model has reversed polarity, but it contains significant nonuniformity in two areas. Near the summit increased inclination and amplitude variations are observed, and in the southwestern peak an area of increased inclination may indicate normal magnetization polarity. Figure 6 plots a Lambert equal-area projection of the paleopoles calculated by least squares and

for three horizontal

at a point

within

the direction

Maximum

sections

the body;

of magnetization.

magnetization

at 3375, 3750, and 4125 m

the size of the vector The northeast

vector drawn

denotes portion

‘1 \

:

Fig. 5. Piquero-2

seminorm minimization along with their 95% confidence ellipses. The seamount paleopole confidence ellipses are characterized by their major and minor axes and their azimuthal orientation (Table 1). These measures are directly analogous to the polar error (dp, dm), used in conventional paleomagnetism, except that the seamount confidence ellipses may have arbitrary azimuth (Parker, 1988). For both seamounts the 95% confidence ellipses had major axes of appro~mately 28” and minor aces of about one-half this width. For Piquero-1 the least squares and seminorm pole agree to within the limit of the confidence ellipse, as one would expect from the uniformity of the seminorm model. For Piquero-2 the least squares and seminorm paleopoles are significantly different, consistent with the significant nonuniformity in the seminorm model.

seamount

‘1

magnetization

occurs in the summit. The southwest

model. The model is displayed peak has reduced

of

is 5.8 A/m.

,,--\ /

the

magnetization

for 2200, 2900, and 3600 m. Magnetization amplitude.

Maximum

nonuniformity

vector drawn is 3.7 A/m.

PALEOMAGNETISM

OF EASTERN

NAZCA

PLATE

SEAMOUNTS

A predicted apparent polar wander path for the Nazca plate may be calculated from the Nazca plate’s pole of rotation with respect to the hotspot reference frame. The Nazca-hotspot rotation pole is obtained from the Pacific-hotspot rotation pole and the Pacific-Nazca relative rotation pole (Gordon and Jurdy, 1986). The Nazca-hotspot rotation pole is used in Fig. 7 to predict an apparent polar wander path for the Nazca plate at 5 m.y. increments between O-50 Ma. The sharp bend in the polar wander path at 25 Ma corresponds to the detachment of the Cocos plate from the Nazca plate. For times O-25 Ma, Nazca plate motion is predominantly west-to-east; for times 25-50 Ma, plate motion has a northward component. The paleopoles from the two Nazca plate seamounts may be compared to predicted Nazca polar wander path (Fig. 7). For Piquero-2 seamount the calculated paleopole is located near the earth’s pole and is nearly coincident with the predicted 5 Ma apparent polar wander. The major axis of the Piquero-2 confidence ellipse has an azimuth similar to the apparent polar wander di-

Fig. 6. Piquero displaying

form modeling statistical

seamount

the seminorm (asterisks),

modeling,

uniform modeling dence

ellipse

paleopoles. minimizing

the 95% confidence

and the least squares

(triangles).

includes

Equal-area paleopoles

The Piquero-2

the earth’s pole

center of the Piquero-1 confidence

projection

from nonuniellipses

from

paleopoles

from

seamount

confi-

(cross)

whereas

ellipse is 31“ different

latitute (Table 1).

the in

J

Fig. 7. The apparent calculated

polar wander path for the Nazca

from Nazca and Pacific relative motion and assum-

ing that Pacific hotspots

are fixed (Gordon

and Jurdy, 1986).

Open circles along the path represent 5 m.y. increments. Piquero-2

plate

seamount

confidence

ellipse

includes

the

The polar

wander path between O-30 Ma, whereas the Piquero-1 confidence ellipse does not overlap with the polar wander path.

rection and it includes the polar wander path for the time span O-50 Ma. The paleopole for Piquero-2, therefore, may indicate a younger age for the seamount than for the underlying crust. The intrusion of young seamounts into the Nazca plate has been documented for the Sala y Gomez Ridge (Clark and Dymond, 1977) and this has been explained as a “hot line” in earth’s mantle (Bonatti and Harrison, 1976; Bonatti et al., 1977). The Piquero-2 seamount lies along the line of the Sala y Gomez Ridge, on a separate ridge in the Chile Basin; it may originate from a related magmatic source. For Piquero-1 seamount the calculated paleopole is significantly different than the earth’s pole or the predicted polar wander path for O-50 Ma (Fig. 7). It is also significantly different than the Piquero-2 paleopole since their 95% confidence ellipses do not intersect. The Piquero-1 paleolatitude, calculated from the inclination of its magnetization, is 44.7 “S which differs by 23” from its current location at 21.5 OS. The Piquero-1 paleopole, therefore, implies 23” of northward displacement of the Nazca plate during the period O-50 Ma. This is 13O more northward motion than

J.A

286

predicted

by the 50 Ma apparent

polar

wander

path for the Nazca plate. Results tism

may

basalts

obtained

be compared

and sediments.

clination

latitude.

Drilling collected portion

seamount to results

Drill samples information

During

The with

frame for O-50 Ma. Close proximity

between

Piquero-2

pole argues

drilled

preserve

in-

orienta-

only on paleo-

plate (Ade-Hall

components. is consistent

from

Sea

rocks were

at two sites (320 and 321) in the eastern of the Nazca

paleopole

polar wander

azimuthal

and basement

nonuniform

seamount

paleomagne-

Leg 34 of the Deep

Project sediment

significant

Piquero-2

but do not preserve

tion so they provide pole

from

shows

HII.DEBiZANI~

and John-

son, 1976a). Site 320 was situated just north of the Mendana Fracture Zone in 30 Ma crust, and site 321 was situated just south of the Mendana Fracture Zone in 40 Ma crust (Fig. 1). Paleomagnetic study of Leg 34 sediments and basalts suggests that N-S motion of the Nazca plate over the past 30-40 Ma has been negligible (3 _t 2O ) (Ade-Hall and Johnson, 1976b, c). These results are inconsistent with the paleopole for Piquero-1 seamount and with the hotspot predicted motion. An alternative hypothesis accounting for the Piquero-1 paleopole’s departure from its expected position is that the seamount has been uniformly tilted. A 25” tilt is needed to bring the present seamount inclination ( - 63 o ) in accord with the inclination expected at 21.5” S (- 38”). A tilt could result, for example, by uplift of the seamount’s northern flank. If the northeast portion of Piquero-1 was erupted after the main body of the seamount, magmatic intrusion might have resulted in a tilt (see Fig. 2). Significant deformation of Piquero-1, however, seems unlikely given the uniformity of its magnetic model. Another possibility is that Piquero-1 was erupted over a short period of time so that secular variation contributed to its paleopole position. Although secular variation is thought to average to an axial dipole over the time required for seamount formation, this may not always be the case.

paleopole

for a young northward

and

motion

the earth’s

(5 Ma). During

seamount

paleopole

of the Nazca

13” more than that predicted reference DSDP

frame and 20”

plate motion

is E-W

reference the

O-50 Ma,

predicts plate,

a 23”

which

is

by a Pacific hotspot

more than predicted

Leg 34 paleomagnetism.

Since

during

by

the Nazca

the past 25 Ma, the

23” latitute change is inferred to have occurred during the period 25550 Ma. The large northward Nazca plate motion suggested by the Piquero-1 seamount paleopole argues for the need to obtain accurate dates and a larger number of magnetically surveyed seamounts from the eastern portion of the Nazca plate. These data would improve our understanding of the Nazca polar wander path and provide a way to study the motion of the Farallon plate during its fragmentation along the west coast of the Americas. Acknowledgments I thank Victor Vacquier for providing the Piquero seamount data, Peter Lonsdale and Lisa Tauxe for helpful comments, and Melissa Hagstrum for editorial assistance. Research support was from the Office of Naval Research contract N-00014-87-K-0010 and from the San Diego Supercomputer

Center.

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