the migration history of the nazca ridge along the peruvian active margin.pdf

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The migration history of the Nazca Ridge along the Peruvianactive margin: a re-evaluation

Andrea Hampel

GEOMAR Research Center for Marine Geosciences, Wischhofstr. 1^3, 24148 Kiel, Germany

Received 21 March 2002; received in revised form 9 July 2002; accepted 23 July 2002

Abstract

The collision zone of the 200 km wide and 1.5 km high Nazca Ridge and the Peruvian segment of the convergent

South American margin between 14S and 17S is characterized by deformation of the upper plate and several

hundred meters of uplift of the forearc. This is evident by a narrowing of the shelf, a westward shift of the coastline

and the presence of marine terraces. As the Nazca Ridge is oblique with respect to both trench and convergence

direction of the Nazca Plate, it migrates southward along the active plate boundary. For reconstructing the migration

history of the Nazca Ridge, this study uses updated plate motion data, resulting from a revision of the geomagnetic

time scale. The new model suggests that the ridge crest moved laterally parallel to the margin at a decreasing velocity

of V75 mm/a (before 10.8 Ma), V61 mm/a (10.8^4.9 Ma), and V43 mm/a (4.9 Ma to present). Intra-plate

deformation associated with mountain building in the Peruvian Andes since the Miocene reduces the relative

convergence rate between Nazca Plate and Peruvian forearc. Taking an intra-plate deformation at a rate ofV10 mm/a,

estimated from space-geodetic and geological data, into account, does not significantly reduce these lateral migration

velocities. Constraining the length of the original Nazca Ridge by its conjugate feature on the Pacific Plate yields a

length of 900 km for the subducted portion of the ridge. Using this constraint, ridge subduction began V11.2 Ma ago at

11S. Therefore, the Nazca Ridge did not affect the northern sites of Ocean Drilling Program (ODP) Leg 112 located at

9S. This is supported by benthic foraminiferal assemblages in ODP Leg 112 cores, indicating more than 1000 m of

subsidence since at least Middle Miocene time, and by continuous shale deposition on the shelf from 18 to 7 Ma,

recorded in the Ballena industrial well. At 11.5S, the model predicts the passage of the ridge crest V9.5 Ma ago. This

agrees with the sedimentary facies and benthic foraminiferal stratigraphy of ODP Leg 112 cores, which argue for

deposition on the shelf in the Middle and Late Miocene with subsequent subsidence of a minimum of several hundred

meters. Onshore at 12S, the sedimentary record shows at least 500 m uplift prior to the end of the Miocene, also in

agreement with the model.

2002 Elsevier Science B.V. All rights reserved.

Keywords: Nazca Ridge; oblique subduction; plate reconstruction; forearc; Peru

1. Introduction

Seamount chains, submarine ridges and other

bathymetric highs on oceanic plates entering sub-

duction zones will, in general, laterally migrate

along the active margin, unless they are parallel

0012-821X / 02 / $ ^ see front matter 2002 Elsevier Science B.V. All rights reserved.PII: S 0 01 2 - 8 2 1X ( 02 ) 0 0 8 59 - 2

* Present address: GeoForschungsZentrum Potsdam, Tele-

grafenberg, 14473 Potsdam, Germany.

Tel.: +49-331-288-1376; Fax: +49-331-288-1370.

E-mail address: [email protected] (A. Hampel).

Earth and Planetary Science Letters 203 (2002) 665^679

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to the convergence direction (e.g. [1,2]), and may

aect the sedimentological and tectonic evolution

of the forearc system signicantly. The lateral mo-

tion of such features can lead to a temporal se-

quence of uplift and subsidence of the forearc,frequently accompanied by enhanced surface and

tectonic erosion as well as steepening of the inner

trench wall and faulting in the upper plate (e.g.

[3^8]). These eects are generally recorded in the

morphology and sedimentary facies of the forearc

and in uplifted coastal shorelines. As a conse-

quence, models resolving the history of forearc

and arc systems must account for these three-di-

mensional eects and their development through

time.

The velocity at which a bathymetric high moves

along an active margin is controlled by three pa-

rameters: the convergence velocity vc and the two

angles a and P, dened by the orientation of the

bathymetric high relative to convergence direction

and trench, respectively (Fig. 1). The lateral ve-

locity vlat of a bathymetric high parallel to the

plate boundary is then:

vlat vcsina

sinP

Even if the convergence velocity is constant, acurvature of the trench line, i.e. a variable angle P,

would result in a variable lateral migration veloc-

ity.

The fate of bathymetric highs during subduc-

tion to greater depth has long been subject to

controversy. While some authors note the tempo-rally irregular occurrence and reduced number of

large earthquakes in the vicinity of such features

(e.g. [10]), others argue that subducting sea-

mounts and ridges form asperities, at which earth-

quakes may nucleate [11] and increase seismic

coupling [12]. In addition, the buoyancy of sub-

ducted bathymetric highs may decrease the dip of

the subducting slab and thus may terminate the

magmatic activity in the overriding plate[7,10,13^

15].

An outstanding example of a subducting bathy-

metric high migrating along an active plate

boundary is the Nazca Ridge, which has aected

the Peruvian portion of the long-lived Andean

subduction zone. Due to southward migration of

the ridge, the Peruvian margin displays, from

south to north, dierent stages of its tectonic evo-

lution during and after ridge passage. Various

features in the oshore and onshore geology of

the Peruvian margin, such as uplift and subsi-

dence of forearc basins, tectonic erosion of the

lower continental slope and uplift of marine ter-

races have been attributed to ridge subduction[16^21]. Moreover, the coastal area above the

subducting ridge was ruptured by two shallow

thrust earthquakes with magnitudes of Mw= 8.1

and Mw= 7.7 in 1942 and 1996, respectively [22].

The downward continuation of the ridge has been

related to a zone of reduced intermediate depth

seismicity and to the southern boundary of the

low-angle subduction segment beneath Southern

Peru [23^25], which coincides with the terminus

of the Quaternary volcanic arc [14,26]. To corre-

late these dierent observations with the subduc-tion of the Nazca Ridge, it is crucial to constrain

both the rate of its lateral movement along the

margin and the original length of this feature.

The rst part of this study calculates the migra-

tion velocity of the Nazca Ridge and yields a sig-

nicantly slower lateral motion than previously

inferred[16,18^21,25,27,28], with the consequence

that ages at which the ridge passed specic sites

increase signicantly. The second part species

Fig. 1. Geometric relations between the lateral migration ve-

locity vlat of a bathymetric high parallel to an active plate

boundary, the plate convergence velocity vc, and the orienta-

tion of the bathymetric high relative to convergence direction

and trench[9].

A. Hampel / Earth and Planetary Science Letters 203 (2002) 665^679666


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the onset of ridge subduction, assuming that the

original length of the Nazca Ridge approximates

that of its conjugate feature on the Pacic Plate

[18,25,27,28].

2. Geodynamic setting

The Nazca Ridge is a more than 1000 km long

and 200 km wide aseismic submarine ridge, which

formed at the Pacic^Farallon/Nazca spreading

center in the early Cenozoic [25,29,30] (Fig. 2).

The linear crest of the ridge is elevated 1500 m

above the surrounding sea oor and trends

N42E. The average crustal thickness of the ridge

derived from the analysis of Rayleigh waves is

18 3 km[30]. Where the ridge descends beneath

the South American Plate, the trench does not

show a pronounced deviation from its linear

trend, but the water depth along the trench line

shoals from 6500 m south of the ridge to 4000 m

at the ridge crest. In bathymetry and side-scan

sonar images, features indicating ongoing surface

erosion and faulting have been identied on the

continental slope [32,33]. Landwards, the recent

collision zone is expressed by a narrowing of the

shelf, a seaward shift of the coastline and the

presence of raised marine terraces at the coastbetween 13.5S and 15.6S [19,20]. Above the

northern ank of the subducted ridge, the recent

subsidence of the marine terraces, which had been

uplifted earlier by the ridge until the passage of its

crest, illustrates its southward movement [19,20].

Further inland, the Abancay Deection (Fig. 2),

which marks the northern boundary of the zone

of active arc volcanism and separates segments of

continental crust diering in geochemical compo-

sition, has been related to the continuation of the

Nazca Ridge [34].North of the collision zone, a small accretion-

ary wedge may have begun to grow in the wake of

the ridge [18]. Further north, o Central and

North Peru, the absence of a large accretionary

prism and tectonic erosion as the dominant mass

transfer regime have been recognized [35,36].

Along this part of the margin, long-term tectonic

erosion since at least the Middle Miocene has led

to rapid subsidence of the forearc and to an east-

ward shift of the trench and the magmatic arc

[27]. However, interpretations of seismic data

and ODP cores, in particular in the Lima Basin

at 11.5S, indicate that during some periods, the

forearc subsided at a lower rate than during timesof prevailing long-term tectonic erosion or has

even been uplifted [17,37].

Regarding the temporal evolution of the colli-

sion zone between the Nazca Ridge and the Pe-

ruvian margin, current models dier in the lateral

migration velocities, in the ages of ridge passage

assigned to dierent latitudes and in the predicted

length of the original Nazca Ridge. The following

reconstructions cover the migration history of the

Nazca Ridge along the entire Peruvian margin:

Pilger ([25] ; his gure 4) shows that the ridge rst

came in contact with the Peruvian trench at 5S in

the Middle Miocene and later passed 10S at V9

Ma. Other studies [16,18,27], based on plate re-

constructions [28] and the NUVEL-1A conver-

gence rate[38], inferred that the Nazca Ridge be-

gan to subduct 8 Ma ago at 8S and was located

at 9S and 11.5S at 6^7 Ma and 4^5 Ma, respec-

tively. Three other reconstructions concentrate on

the migration of the ridge from the end of the

Miocene to the present: Based on the plate mo-

tion data by Pardo-Casas and Molnar [39], Hsu

[19] infers a lateral migration velocity of V71mm/a. Machare and Ortlieb [20] use the plate

motion data by Pardo-Casas and Molnar [39] to

deduce a passage of the ridge crest at 13S at

4 Ma, i.e. a lateral velocity ofV64 mm/a. Le Roux

et al. [21] suggest that the ridge crest was located

at V13.5S at 5.3 Ma and thus o Lima (12S)

before the end of the Miocene, i.e. laterally mov-

ing at a velocity ofV42 mm/a, derived from con-

vergence rates given by Stein et al. [40]. These

dierences in the inferred migration rates of the

Nazca Ridge underline the importance of the re-evaluation presented here.

3. Reconstruction of the migration history

3.1. Lateral migration velocity

Unraveling the migration history of subducting

ridges, seamount chains and other submarine

A. Hampel / Earth and Planetary Science Letters 203 (2002) 665^679 667


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Fig. 2. Map [31] showing the location of the Nazca Ridge, the spatial distribution of seismicity and active volcanoes (black trian-

gles; from the Smithsonian Global Volcanism Program). ODP Leg 112 sites and two industrial wells (Ballena, Deln) are marked

by white circles. The Peruvian low-angle subduction segment is located between 5S and 14S. Note the gap in the intermediate

depth seismicity (70^300 km) (dotted line) and the presence of deep seismic events (500^650 km) beneath Brazil (dashed line).

(Earthquake data from 1973 to 2002; US Geological Survey^National Earthquake Information Center.)



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bathymetric highs requires knowledge of past

plate motions, which can be obtained by two

types of data sets. Plate motions averaged over

the last 3 Ma are provided by the NUVEL-1A

model, based on evaluation of spreading rates,transform fault azimuths and earthquake slip vec-

tors [38]. On longer time scales, paleo-plate posi-

tions and motions can be reconstructed by ana-

lyzing the magnetic anomalies of the oceanic

crust. This method yields average velocity vectors

for dierent time intervals (e.g. [28,41]).

This study uses updated Nazca (Farallon)^

South American relative motions [42] which take

into account a revision of the global geomagnetic

time scale [43]. This data set provides constant

convergence velocities and directions for dierent

time intervals for the last 40 Ma at dierent lat-

itudes, of which the values given at 12S are ap-

plied (Table 1). The convergence rate of 75 mm/a

for the last 5 Ma [42] agrees well with the NU-

VEL-1A prediction[38]. Both estimates are higher

than the current convergence rate determined by

space-geodetic measurements, i.e. 61 3 mm/a at

12S [44,45]. Since the convergence rate may be

slowing with time, the space-geodetic values are

less relevant for this reconstruction.

Using the average convergence velocities and

directions for the three latest time intervals, threedisplacement vectors and respective paleo-posi-

tions of the Nazca Ridge relative to a xed South

American Plate are constructed (Table 1andFig.

3a). The resulting time path allows one to deter-

mine when the ridge crest passed a specic point

on the trench line, assuming a linear continuation

of the ridge towards the trench, as suggested by

the shape of the present ridge, and a paleo-trench

position similar to the present trench line [18^

21,25,27,28] (Fig. 3b).

Uncertainties in the convergence velocities are

not specied[42],but may be of the order of 10%

[44]. Since the errors are likely to be smaller in the

latest time interval, as suggested by the errors of

the NUVEL-1A convergence rates [38], and may

be larger in the earliest time interval, this studyassigns uncertainties of 5%, 10% and 15% to the

convergence velocities of the 0^4.9 Ma, 4.9^10.8

Ma and 10.8^16 Ma time intervals, respectively

(Table 1). Using these error limits, the uncertain-

ties in the ages of ridge passage with respect to the

convergence rates of the three time intervals are

given in Table 1. Potential errors of the geomag-

netic time scale and of the convergence azimuths

for the dierent time intervals have not been tak-

en into account.

An implicit assumption of this reconstruction is

that the decreasing relative convergence rate be-

tween the Nazca Plate and stable South America

over the last 15^20 Ma, as derived from plate

reconstructions, equals the amount of relative mo-

tion between the Nazca Plate and the Peruvian

forearc. This assumption has also been the basis

for all previous reconstructions of the Nazca

Ridge motion [16,18^21,25,27,28]. However, the

presence of the Andean mountain belt east of

the forearc demonstrates that, strictly speaking,

this assumption is not correct, since some of the

relative plate motion is taken up by intra-platedeformation within the South American Plate.

Obviously, this intra-plate deformation tends to

reduce the relative motion between the Nazca

Ridge and the Peruvian forearc system. At

present, a rigorous assessment of the amount

and the direction of shortening accommodated

in the Peruvian Andes is dicult due to the lack

of sucient geological data. Nevertheless, space-

geodetic measurements[44]and geological proles

([46^48] and references therein) across the Andes

can be used to estimate the present-day and past

Table 1

Relative plate motion between Nazca and South American plates at 12S [42]

Time interval Convergence velocitya Convergence direction Length of displacement vector Age uncertainty

[Ma] [km/Ma] [km] [Ma]

0^4.9 (chrons 0^3) 75 4 77 368 20 0.3

4.9^10.8 (chrons 3^5) 106 11 82 625 65 0.6

10.8^16 (chrons 5^5C) 123 18 84 640 94 0.8

a Errors are assumed to be 5%, 10% and 15% for the latest, intermediate, and earliest time interval, respectively.



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shortening rates across the Eastern Cordillera and

the Subandean belt [49]. These data show that

geologic and space-geodetic displacement rates

are generally consistent and that directions of

shortening in the Eastern Andes are approxi-

mately parallel to the Nazca^South America con-

vergence vector. The data have been interpreted in

terms of a two-stage model with rates of short-

ening across the Eastern Andes of 5^8 mm/a for

the last 25^10 Ma and of 10^15 mm/a for the last

10 Ma [49]. In order to account for the Andean

intra-plate deformation, the lateral migration ve-

locity of the Nazca Ridge is also presented for a

scenario in which an average Andean shortening

rate of 10 mm/a for the last 16 Ma is subtracted

from the relative convergence velocity between

Fig. 3. (a) Three paleo-positions of the Nazca Ridge and displacement vectors for the present intersection point of ridge and

trench. Gray lines represent the assumed linear continuation of the ridge. Inset (b) shows diagram in which the latitudinal posi-

tion of the linearly continuing ridge crest on the trench line and the migration velocity of the ridge parallel to the plate boundary

are plotted versus time. The two black lines are derived by using the relative plate motion data as given in [42]. The black arrow

marks the onset of ridge subduction inferred by this study (Section 3.2). The two gray lines refer to a scenario in which a small

amount of intra-plate deformation (10 mm/a) accommodated in the Peruvian Andes is subtracted from the convergence rates of

[42].



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Nazca and South American plates (gray lines in

Fig. 3b). Considering the intra-plate deformation

tends to slightly increase the ages of ridge passage

assigned to specic latitudes, in other words, the

lateral migration velocity of the ridge slightly de-creases. However, the geological implications of

the model (see below) remain valid, even if the

intra-plate deformation is taken into account.

To allow a straightforward comparison of the

model with previous reconstructions of the Nazca

Ridge motion, the following discussion uses the

model-curve neglecting intra-plate deformation

(black lines inFig. 3b). Once more detailed infor-

mation on Andean shortening rates and directions

in Peru becomes available, it should be incorpo-

rated into the model.

In summary, the rst part of the reconstruction

demonstrates that the ridge moved signicantly

slower parallel to the margin than inferred by

previous studies[16,18^21,25,27,28]. In particular,

a ridge of sucient length would have passed the

ODP Leg 112 sites in the Trujillo/Yaquina (9S)

and Lima basins (11.5S) at V14.5 Ma and at

V9.5 Ma, respectively. Apart from this migration

history, deducing the onset of ridge subduction

requires an estimate of the length of the original

ridge.

3.2. Original length of the Nazca Ridge and onset

of ridge subduction

The preservation of oceanic ridges and plateaus

in the Southeastern Pacic oers the possibility to

constrain the shape of already subducted parts of

bathymetric highs on the Nazca Plate by their

mirror images on the Pacic Plate (Fig. 4). As

these pairs of conjugate highs have formed simul-

taneously at the Pacic^Farallon/Nazca spreading

center (e.g. [29,52]), they are thought to have asimilar length and shape assuming symmetric

spreading[25,52].

The Nazca Ridge has a common origin with the

Tuamotu Plateau at the Pacic^Farallon/Nazca

spreading center [25,29,30] and the pre-condition

of symmetric spreading seems to be met, since the

respective segments of the Nazca and Pacic

plates between chrons 13 and 23 have similar

widths (see Figs. 4b and 5).

The N70W trending, elongated Tuamotu Pla-

teau is a composite feature consisting of island

chains and oceanic plateaus with volcanic edices

that once were subaerial and today form atolls

[54], whereas the Nazca Ridge is characterizedby smaller, but similar submarine volcanic fea-

tures[32].Despite these dierences in their topog-

raphy, both ridges have an overall linear trend.

Therefore, the 4000 m water depth contour line

of the Tuamotu Plateau has been used to approx-

imate the outline and total length of the original

Nazca Ridge[18,25,27,28]. To estimate the length

of the subducted part of the Nazca Ridge, how-

ever, it has to be taken into account that the

northwesternmost part of the Tuamotu Plateau

formed on 10^20 Ma old oceanic crust of the

Pacic Plate, indicating an origin 600 km o the

spreading center [54]. The hotspot that generated

the northwesternmost part of the Tuamotu Pla-

teau [55] most likely had no eect on the Nazca

Plate [54]. For this reason, the northwestern end

of the plateau probably does not have a counter-

part on the Nazca Plate. Another assumption

made to specify the onset of ridge subduction is

the use of the present trench line as the paleo-

trench position [18^21,25,27,28].

To estimate the length of the original Nazca

Ridge, a mirror image of the Tuamotu Plateauis created using its 4000 m contour line. To nd

the correct position of the mirror image on the

Nazca Ridge, magnetic anomaly lineations of

the surrounding sea oor are tted, using a global

data set[51,52]together with specic data for the

Tuamotu Plateau region [53]. Chrons 15^20 are

the oldest magnetic anomalies common to the

sea oor close to both features (Fig. 4b).

To t these chrons north and south of the Tua-

motu Plateau to the ones on the Nazca Plate, no

scaling of the mirror image is needed, which in-dicates symmetric sea oor spreading. On the

Nazca Plate, the trends of the chrons are better

constrained north than south of the ridge and

appear to be roughly parallel to each other (Fig.

4b). In contrast, the same magnetic lineations are

at an angle with each other north and south of the

Tuamotu Plateau. As a consequence, tting the

chrons leads to two endmember positions (Fig.

5). Matching chrons 19 and 20, located south of



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the ridges, with chrons 18^21 being parallel, leads

to an abrupt bend of the original Nazca Ridge

which results in a N16W trend and a length of

about 1000 km corresponding to an onset of sub-

duction V10.0 Ma ago at 8.5S (Fig. 5). Adjust-

ing chrons 16 and 18, located north of the bathy-metric highs, with chrons 15^20 being parallel,

leads to the position of the mirror image preferred

by this study, because in that case the Nazca

Ridge continues linearly beneath South America

for 1100 km, suggesting that the rst contact of

ridge and trench occurred V12.5 Ma ago at a

latitude of 10S (Fig. 5). Regarding the location

of chron 18, it should be noted that the spatial

extent of its magnetic signal allows dierent

phases to be picked as chron 18. Since the details

of the picking procedures are not available for all

publications ([51^53] and references therein), this

study uses the locations of chron 18 as shown in

the published maps. Given that the preferred re-

construction is additionally constrained by chrons15 and 16, the possible non-unique identication

of chron 18 by dierent authors is considered to

have only a minor eect on the reconstructed

length of the Nazca Ridge.

The values of 1000 km and 1100 km for the

original length of the Nazca Ridge, as inferred

above, are maximum values. Taking into account

that the V200 km long northwesternmost part of

the Tuamotu Plateau most likely does not have a

Fig. 4. (a) Bathymetric map [31] of the South Pacic showing the Pacic^Nazca spreading center and the conjugate features Naz-

ca Ridge and Tuamotu Plateau. (b) Outlines of the Nazca Ridge and the Tuamotu Plateau are shown by their 4000 m water

depth contour lines. The global age grid [50] of the oceanic crust interpolated from magnetic anomalies is shown by color code.

Selected magnetic anomaly lineations are represented by black [51], blue [52] and red [53] lines.



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counterpart on the Nazca Plate yields the pre-ferred scenario of this study, in which the original

ridge continues for V900 km beneath South

America and entered the trench V11.2 Ma ago

at 11S (Fig. 5).

4. Discussion

The new model presented for the kinematic

evolution of the Nazca Ridge predicts, for bothendmember positions described above (Fig. 5), a

lateral migration history that diers signicantly

from previous studies [16,18^21,25,27,28]. With

respect to the two possible positions of the mirror

image of the Tuamotu Plateau, this study prefers

tting the magnetic anomaly lineations 16 and 18

north of the plateau instead of chrons 19 and 20

south of it for the following reasons: First, with

this t, the straight Nazca Ridge continues with-

Fig. 5. Migration history of the Nazca Ridge on the assumption that the mirrored Tuamotu Plateau resembles the subducted

part of the Nazca Ridge. The magnetic anomalies on the Nazca Plate are marked in black and blue. The magnetic lineations

north and south of the Tuamotu Plateau have red and green colors, respectively (see inset). At the present collision zone, two

endmember models for the continuation of the Nazca Ridge are shown: Adjusting chrons 15^20, located north of both features,

yields the red mirror image of the plateau. Fitting chrons 18^21, located south of both ridges, leads to a position of the mirrored

Tuamotu Plateau shown as the green mirror image. Both mirror images are plotted without consideration of the variable dip of

the subducting plate. For both mirror images, the lighter colors at their northeasternmost ends mark the V200 km long part of

the Tuamotu Plateau which most likely does not have a counterpart on the Nazca Plate (see text for details). Thus, the red mir-

ror image with a linear continuation of V900 km is the preferred scenario of this study. Note the coincidence of the preferred

red mirror image with the reduced intermediate depth seismicity (dotted line) and with the presence of deep seismic events be-

neath Brazil (dashed line). For the onset of ridge subduction, three dierent scenarios are presented: Using the preferred congu-

ration, the original Nazca Ridge entered the trench V11.2 Ma ago at 11S (red). If the original Nazca Ridge continues for 1100

km, its subduction began V12.5 Ma ago at 10S (light red). The northernmost possible contact of ridge and trench at 8.5S cor-

responds to a mirror image adjusted to chrons 19^21 (green).



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11/15

out a bend. Second, the outline of the northern

arm of the Tuamotu Plateau resembles the mod-

ern Nazca Ridge, in agreement with their prob-

able alignment during their common origin [30].

Apart from that, the uncertainties in the directionof the magnetic anomalies 16 and 18 are consid-

erably smaller than those of the shorter chrons 19

and 20, which, like in the Tuamotu Plateau re-

gion, might not be parallel to chrons 16 and 18.

Dierent orientations of chrons 18 and 19 are

also suggested by the magnetic anomalies of the

Nazca Plate south of the Nazca Fracture Zone

(Fig. 4). Another argument is that a linear con-

tinuation of the Nazca Ridge coincides well with

the zone of reduced intermediate depth seismicity

and the southern boundary of the segment of low-

angle subduction beneath South Peru (Figs. 2, 5).

The predicted northeastern end of the ridge cor-

relates with the cluster of deep seismic events

beneath Brazil between V8.5S and V10.5S

(compare Figs. 2 and 5). This agrees well with

interpretations of the deep seismicity that propose

an association of the southern earthquake cluster

with the subducted part of the Nazca Ridge

[56,57]. Moreover, a VN42E trending ridge co-

incides with the northern boundary of active vol-

canism and the Abancay Deection[34,15]. Pilger

[25], however, argued for the position tted tochron 19, because the ridge then extends farther

to the north and thus can explain the at slab

beneath Northern Peru. This northern at slab,

however, may be caused by the subducted part

of the Carnegie Ridge o North Peru/Ecuador

or by another, completely subducted oceanic

plateau [58]. Taken together, these arguments

strongly support a linear continuation of the ridge

of V900 km and an onset of ridge subduction

V11.2 Ma ago at 11S.

If the Nazca Ridge, continuing with a lineartrend, had entered the trench at 8S [16,18,27,

28], its subduction would have begun V16 Ma

ago and the original ridge would have to be at

least 1500 km long. Such a length is not sup-

ported by the conjugate feature of the Nazca

Ridge on the Pacic Plate, since the entire Tua-

motu Plateau is at most 1100 km longer than the

modern Nazca Ridge. Thus, although the trench

has probably been shifted eastward for at least 20Ma due to tectonic erosion [27], a linearly trend-

ing original Nazca Ridge could not have reached

the trench north of 10S.

The new reconstruction has signicant implica-

tions for models of the tectonic, sedimentological

and geomorphic evolution of the Peruvian forearc

and arc systems. In particular, dierent seismic

data sets (e.g. [59,60]) and ODP Leg 112 cores

in the Trujillo/Yaquina (9S) and Lima basins

(11.5S) have been interpreted [17,18,60,61] in

light of previous reconstructions suggesting that

the ridge crest passed these sites V6 Ma and

V4 Ma ago, respectively[16,18,27,28]. According

to this study, however, the ridge was not su-

ciently long to inuence the region at 9S, while

at 11.5S, it already caused maximum uplift V9.5

Ma ago (Fig. 3). The marine and subaerial sedi-

mentological record of the forearc, the onshore

tectonic history, and the temporal and spatial

evolution of volcanism of the Andean magmatic

arc in Peru support the new model and will be

discussed in some detail.

In marine sediments, uplift of the forearc regioncan, in general, be derived from a trend to coarser

deposits, possibly accompanied by an increase in

the number of unconformities, and from benthic

foraminiferal stratigraphy that gives information

on the water depth at which the sediment was

deposited. At 9S, benthic foraminiferal assem-

blages in ODP Leg 112 cores and dredge samples

indicate that the continental slope and shelf sub-

sided V1500 m between the Middle Eocene to

Middle Miocene and experienced further subsi-

dence of 1300 m since 12^13 Ma [37,62]. Apartfrom that, cores recovered during ODP Leg 112

and two industrial wells are characterized by the

deposition of ne-grained material, while sandy

deposits are missing in the Miocene (Fig. 6a)

6

Fig. 6. (a) 9S: Lithology of ODP Leg 112 sites [60] and of the Ballena industrial well, located on the shelf, with ages of dated

samples (black circles) [63]. (b) 11.5S: Lithology of ODP Leg 112 sites that drilled into Miocene strata [60]. Paleo-bathymetry is

derived from benthic forminiferal assemblages of site 679, located on the outer shelf [37].



12/15

[60]. Especially in the Ballena industrial well,

located above the crest of the outer shelf high,

continuous shale deposition between 18 and

7 Ma [63] argues against the disturbance of the

deposition milieu due to the passage of a ridge(Fig. 6a). A comparison with the recent collision

zone shows that the shelf area is strongly aected

by the Nazca Ridge. Marine deposits of Eocene

to Upper Pliocene age that correlate with equiv-

alent strata in submerged oshore forearc basins

o Central Peru have been raised above sea level

[19].

At 11.5S, deposits at ODP Leg 112 sites be-

come coarser, with a decrease in mud and an in-

crease in silt and sand during the Middle and Late

Miocene (Fig. 6b). At site 679, a layer of con-

glomerates has been deposited before the end of

the Miocene. In cores recovered at ODP site 679,

Middle and Late Miocene benthic foraminiferal

assemblages reect deposition on the inner shelf

in shallow water [37]. Following the hiatus at the

end of the Late Miocene, deposition resumed at

the outer shelf in the early Pliocene. The next

foraminifers-bearing strata are of Quaternary

age, with deposition depth uctuating around

400 m. At site 682, Middle to Late Miocene fora-

miniferal assemblages have been deposited at mid-

dle bathyal depths (500^1500 m), while the LatePliocene paleo-environment was lower bathyal

(2000^4000 m) [37]. Site 688 is barren of Late

Miocene foraminiferal assemblages, however, be-

tween Early Miocene and Quaternary, the paleo-

biotope changed from upper middle bathyal (500^

1500 m) to lower bathyal depth (2000^4000 m)

[37]. In addition, earlier investigations based on

dredge samples indicate more than 2000 m subsi-

dence for 6 Ma, since Late Miocene benthic for-

aminifers, living at V500 m depth, were recov-

ered in the Lima Basin at a water depth ofmore than 2600 m [62]. Based on these initial

ODP Leg 112 results, a phase of uplift and ero-

sion at 11.5S was derived to begin at 11 Ma and

last until 7 Ma, while 6 Ma ago, a transition from

uplift to subsidence occurred [16]. The oshore

geological record of ODP Leg 112 as summarized

above shows uplift of the forearc during Middle

and Late Miocene and subsidence since the end of

the Miocene. This correlates very well with the

age ofV9.5 Ma derived from the new reconstruc-

tion for passage of the ridge crest.

The new model is also compatible with the sed-

imentological record of the R|mac^Chillo n rivers

at 12S, which eroded deep valleys on the Limacoastal plain during the Miocene. The alluvial fan

deposited by these rivers experienced uplift of at

least 500 m, which is attributed to the passage of

the Nazca Ridge [21]. Potential sea level changes

during the Quaternary and Pliocene are smaller

than V125 m and have been considered [21].

The uplift maximum at 12S was attained before

the end of the Miocene [21].

Another piece of evidence in support of the

presented model may be inferred from the corre-

lation of the Nazca Ridge with the associated seg-

ment of low-angle subduction and the cessation of

magmatic arc activity. At present, the boundary

between active and ceased volcanism in the south

and in the north, respectively, is located in the

landward continuation of the ridge, but may

have gradually propagated southward due to the

lateral movement of the ridge. Oshore, volcanic

ash layers recovered during ODP Leg 112 have

been interpreted to show higher activity of the

Peruvian volcanic arc in the Late Miocene for

9S than for 12S [64]. Onshore geochronological

data throwing light on a possible southward prop-agating zone, where volcanism has ceased, are,

however, rather limited[65,66]. Pulses of Miocene

volcanic activity [65,66] have been interpreted in

context of the Quechua tectonic phases of the

Andean orogeny in Peru during the Middle to

Late Miocene [64,67]. The Quechua II (V10

Ma) and Quechua III (V5 Ma) tectonic phases,

which seem to be related to changes in the relative

plate motion of the Nazca and South American

plates [65,39], have been correlated with uncon-

formities in ODP Leg 112 cores at 11.5S [16].According to this study, the Nazca Ridge inu-

enced this region V9.5 Ma ago, which seems to

coincide with the Late Miocene Quechua II tec-

tonic phase. Despite this apparent correlation, it

should be noted that the concept of distinct tec-

tonic phases in Peru has been criticized, as the

available temporal constraints argue in favor of

prolonged periods of tectonic activity [68]. Never-

theless, in the Ecuadorian Andes, subduction of



13/15

the Carnegie Ridge since the Middle Miocene

may be responsible for the development of a high-

er topography, a compressional stress regime, and

increased crustal cooling and exhumation rates,

deduced from ssion track data in the collisionzone [69].

While, in summary, no single observation is

conclusive about its relation to the subduction

of the Nazca Ridge, the combination of the argu-

ments raised above strongly suggests that the new

model is more compatible with the existing geo-

logical and geomorphic data.

5. Conclusions

This new reconstruction of the migration history

of the Nazca Ridge along the Peruvian margin

suggests that the lateral motion of the ridge has

decelerated through time. Considering that a small

amount of the relative convergence rate between

the Nazca and South American plates is taken up

by intra-plate deformation in the Andean moun-

tain belt results in slower lateral migration of the

ridge. However, this has no eect on the geolog-

ical implications of the new model. On the as-

sumption that the original Nazca Ridge has a

length similar to its mirror image on the PacicPlate, it continues for V900 km beneath South

America. Therefore, the northeastern end of the

Nazca Ridge entered the trench V11.2 Ma ago

at 11S. As a consequence, the ridge did not

have an impact on the region north of 10S, where

the northern transect of ODP Leg 112 is located.

The region at 11.5S o Lima has been aected by

ridge subduction V9.5 Ma ago. Support for the

model is provided by the sedimentological and pa-

leo-bathymetric record in ODP Leg 112 and indus-

trial well cores. At 9S, cores show mostly ne-grained sediments on the continental slope and,

on the shelf, continuous shale deposition. At

11.5S, the predicted age of the new model corre-

lates well with a Late Miocene period of uplift and

erosion followed by subsidence since V6 Ma.

In light of this study, seismic and drilling data

sets acquired along the Peruvian margin in the

last decades oer the possibility to compare re-

gions that have not been aected by the ridge

passage with regions that have been inuenced

by the ridge, but otherwise share similar boundary

conditions. Such a comparison may enable a bet-

ter quantication of the geodynamic inuence of

the Nazca Ridge on the Peruvian margin in futurestudies. The case of the Nazca Ridge emphasizes

that models regarding the geodynamic evolution

of active margins have to take into account the

migration history and three-dimensional eects

associated with laterally migrating bathymetric

highs.

Acknowledgements

Helpful comments and discussions with Nina

Kukowski, Onno Oncken, Ulrich Riller and

David Hindle are gratefully acknowledged. Udo

Barckhausen and Garrett Ito are thanked for

their help with the magnetic anomaly data and

useful comments. Many thanks to Edmundo

Norabuena for his helpful comments on the plate

motion data. The GMT[70]software was used to

create Figs. 2^5. I thank the reviewers Emile

Okal, Tim Dixon and Steven Cande for construc-

tive comments that helped to improve the manu-

script. Funding was provided by the German

Ministry of Education, Science and Technology(BMBF) within the GEOPECO project (Grant

no. 03G0146A).[AC]

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