1-s2.0-S0012821X05006874-mainfds

www.elsevier.com/locate/epsl

Earth and Planetary Science L

Flexural uplift of a lithospheric slab near the Vema transform

(Central Atlantic): Timing and mechanisms

Enrico Bonatti a,b,c, Daniele Brunelli a,d, W. Roger Buck b, Anna Cipriani a,b,

Paola Fabretti a, Valentina Ferrante a, Luca Gasperini a, Marco Ligi a,*

a Istituto di Scienze Marine, Geologia Marina, CNR, Via Gobetti 101, 40129, Bologna, Italyb Lamont Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA

c Dipartimento di Scienze della Terra, Universita bLa SapienzaQ, P.le Aldo Moro 5, 00187, Rome, Italyd Laboratoire Pierre Sue, C.E. Saclay/Bat. 637, F-91191, Gif sur Yvette Cedex, France

Received 22 May 2005; received in revised form 2 October 2005; accepted 10 October 2005

Available online 8 November 2005

Editor: V. Courtillot

Abstract

The Vema Transverse Ridge (VTR) is a prominent, long and narrow topographic anomaly that runs for over 300 km along a sea

floor spreading flow line south of the Vema transform at 118 N in the Atlantic. It rises abruptly about 140 km from the axis of the

Mid-Atlantic Ridge (MAR) in ~10 Myr old crust and runs continuously up to ~25 Myr old crust. It reaches over 3 km above the

predicted lithospheric thermal contraction level. It is absent in crust younger than 10 Myr; thus, the uplift of the VTR must have

ended roughly 10 Ma. The VTR is interpreted as the exposed edge of a flexured and uplifted slab of oceanic lithosphere that was

generated at an 80 km long MAR segment. Based on satellite gravimetry imagery this MAR segment was born roughly 50 Ma and

increased its length at an average rate of 1.6 mm/yr. Multibeam data show that the MAR-parallel sea floor fabric south of the VTR

shifts its orientation by 58 to 108 clockwise in ~11–12 Myr old crust, indicating a change at that time of the orientation of the MAR

axis and of the position of the Euler rotation pole. This change caused extension normal to the transform, followed between 12 and

10 Ma by flexure of the edge of the lithospheric slab, uplift of the VTR at a rate of 2 to 4 mm/yr, and exposure of a lithospheric

section (Vema Lithospheric Section or VLS) at the northern edge of the slab, parallel to the Vema transform. Ages of pelagic

carbonates encrusting ultramafic rocks sampled at the base of the VLS at different distances from the MAR axis suggest that the

entire VTR rose vertically as a single block within the active transform offset. A 50 km long portion of the crest of the VTR rose

above sea level, subsided, was truncated at sea level and covered by a carbonate platform. Subaerial and submarine erosion has

gradually removed material from the top of the VTR and has modified its slopes. Spreading half rate of the crust south of the

transform decreased from 17.2 mm/yr between 26 and 19 Ma to ~16.9 mm/yr between 19 and ~10 Ma, to ~13.6 mm/yr from 10 Ma

to present. The slowing down of spreading occurred close in time to the change in ridge/transform geometry, suggesting that the

two events are related. A numerical model relates lithospheric flexure to extension normal to the transform, suggesting that the

extent of the uplift depends on the thickness of the brittle layer, consistent with the observed greater uplift of the older lithosphere

along the VTR.

D 2005 Elsevier B.V. All rights reserved.

Keywords: oceanic transform; mid-ocean ridge; oceanic lithosphere; lithospheric flexure

0012-821X/$ - s

doi:10.1016/j.ep

* Correspondin

E-mail addre

etters 240 (2005) 642–655

ee front matter D 2005 Elsevier B.V. All rights reserved.

sl.2005.10.010

g author.

ss: [email protected] (M. Ligi).

E. Bonatti et al. / Earth and Planetary Science Letters 240 (2005) 642–655 643

1. Introduction

The Vema Transverse Ridge (VTR) is a major

topographic anomaly that rises in the Central Atlantic

south of (and parallel to) the Vema Fracture Zone

(VFZ) at 118 N (Figs. 1 and 2). It has been studied

intermittently for over 30 yr, starting with Heezen et

al. [1] and Van Andel et al. [2]. Rock sampling [3],

submersible dives [4], as well as multibeam and seis-

mic reflection surveys [5–8] have led to the suggestion

that this feature is the edge of a slab of oceanic

lithosphere uplifted due to transform-related tectonics.

Interest in the origin and evolution of the VTR feature

has been renewed recently because it exposes an ~300

km long section of relatively undisturbed oceanic

lithosphere (Vema Lithospheric Section or VLS), pro-

viding a unique opportunity to study an ~20 Myr long

record of creation of lithosphere at a ridge segment

[7]. This slab of lithosphere has been generated at the

80 km long eastern Mid-Atlantic Ridge (EMAR) seg-

ment that meets the transform from the south (Figs. 1

and 2).

We report in this paper new data bearing on the

events that led to the uplift and exposure of the VTR

lithospheric edge; we offer a time scale for these

events, and we propose a model that explains them

and helps understand vertical tectonics in oceanic

transforms.

2. Methods

Multibeam coverage was achieved during cruise

EW9305 of the R/V Ewing, using a Hydrosweep in-

Fig. 1. (a) Predicted topography of the Central Atlantic (from Sandwell and S

imagery of the inset area. The Vema transform valley, the transverse ridge, the

converges towards the Vema FZ; the two join in crust of roughly 50 Myr,

strument [5] and two cruises of the R/V Strakhov, using

a Simrad EM-12S, 81 beam instrument [6,7]. Multi-

channel seismic reflection data were acquired in two

different cruises by the R/V OGS Explora [8] and by R/

V Strakhov [6,7]. On board the R/V Explora the sound

source was a tuned array of 28 airguns (total volume 60

l, working pressure 2000 psi) towed 6 m below the

surface; the receiver was a 3150 m long streamer, with

120 channels spaced 25 m apart, towed 12 m below the

surface; the shot distance was 50 m, allowing a cover-

age of 3000%. The R/V Strakhov data were obtained

using as seismic source an array of 2 Sodera GI guns

(with a capacity of 105 in3 for both the generator and

injector chamber) working with a pressure of 2000–

3000 psi. The receiving streamer employed 24 channels

spaced 25 m apart. The shot interval was 50 m allowing

a coverage of 600%.

3. The Vema Transverse Ridge

Spreading half rates of the lithosphere south of the

Vema FZ, estimated by plate motion reconstructions of

Cande et al. [9], varied significantly during the last 20

Myr. Half spreading rates of 13.6 mm/yr prevailed

between 0 and about 10 Myr; a faster rate of 16.9

mm/yr between 10 and 19 Myr, and an even faster rate

of 17.2 mm/yr between 19 and 26 Myr. Crustal ages

cited further on in this paper are based on these rates,

assuming present-day ridge/transform/ridge geometry.

Multibeam topography indicates that the VTR rises

rather abruptly about 140 km west of the RTI, i.e., in

crust of roughly 10 Myr old (Figs. 2 and 3). Topogra-

phy from the near-zero age RTI to 10 Myr old crust

mith [28]) with the Vema transform offset at 118 N. (b) Satellite gravityLema FZ and the EMAR segment are visible. Note how the Lema FZ

implying a gradual increase in the length of the EMAR segment.

Fig. 2. Multibeam bathymetry of the Vema FZ region. Note how the transverse ridge rises abruptly about 140 km from the EMAR axis, and the

ridge-parallel fabric of crust generated at the EMAR segment changes slightly its orientation roughly 11 Ma.

E. Bonatti et al. / Earth and Planetary Science Letters 240 (2005) 642–655644

shows a nearly normal deepening with age due to

thermal contraction (Fig. 3). The VTR is a 300 km

long, very narrow (a few kilometers near the crest)

topographic anomaly that rises up to over 3 km above

the predicted thermal contraction level. The VTR runs

parallel to the transform on its southern side; its ex-

posed northern scarp (i.e., the southern wall of the

transform valley) reaches in height up to 4 km above

the valley floor, where it plunges beneath the turbiditic

deposits that fill the valley (Fig. 4). The floor of the

Vema transform valley is about 5 km deep, but contains

over 1 km of flat-lying turbidites [2,10–12] implying

that the igneous basement lies N6 km below sea level

(Figs. 4–6). A sharp vertical disruption in the horizontal

strata, noted already by Eittreim and Ewing [11] and

Rowlett and Forsyth [13], marks probably the trace of

the strike-slip motion and separates the African and the

South American plates.

The summit of the VTR tends to become gradually

shallower moving west, i.e., toward older crustal ages,

Fig. 3. Topographic profiles along sea floor spreading flow lines south of th

EMAR segment (light gray shaded profile) deepens with increasing distance

In contrast, the topographic profile along the VTR crest (dark gray shaded) f

Crust older than 10 Myr reaches up to 4 km above the predicted thermal su

up to about 458 W (crustal age of about 25 Myr), where

the VTR disappears rather abruptly (Fig. 2). The along-

strike continuity of the VTR crest is interrupted across

the western RTI by an ~23 km long interval of strong

topographic roughness (i.e. the bbadlandsQ of Kastens etal. [5]). This zone of irregular topography separates two

major segments with different characteristics. The VTR

is oriented 2828 N, from 45805V W to 43852V W, and

shows an asymmetric N–S profile with the northern

flank steeper than the southern one. Between 44835VW and 43855V W, the summit lies about 450 m below

sea level. Samples and seismic reflection profiles col-

lected along this portion of the VTR gave evidence of

an ~500 m thick shallow-water carbonate platform that

caps an ~50 km long stretch of the VTR [8]. Between

the two MAR axes, from 43840V W and 42810V W, the

VTR runs parallel to the transform valley; N–S topo-

graphic profiles show a flat bench, or terrace, on the

northern flank. This terrace deepens and tapers from

west to east and finally pinches out at about 42830V W.

e Vema transform. The oceanic crust along the midpoint trace of the

from the ridge axis, i.e. with increasing age, due to thermal subsidence.

ollows the predicted thermal contraction only up to ~10 Myr old crust.

bsidence age curve (thick black solid line).

Fig. 4. Multichannel seismic reflection profiles perpendicular to the transform. (a) Shaded relief image of the VFZ and location of seismic reflection

profiles; (b) VEMA-03; (c) VEMA-07; (d) VEMA-09 and VEMA-08; (e) VEMA-11 and VEMA-06. Boxes mark portions of the profiles shown in

Fig. 6.


Direct observations and sampling by the submersible

Nautile [4] revealed that the northern side of the trans-

verse ridge at about 42842V W exposes a relatively

complete and undeformed upper lithospheric section

(Vema Lithospheric Section or VLS) consisting of an

~1 km thick mantle peridotite basal unit overlain by a

gabbroic unit, by a several hundred meters thick dike

complex and topped by pillow basalt. In contrast, the

southern slope of the transverse ridge exposes only

upper crustal basalt (Figs. 4e, 5e, and 6a). The litho-

logical asymmetry of the transverse ridge goes together

with the topographic asymmetry. Moreover, gravity and

seismic reflection data indicate that the crust is thinner

on the northern side of the VTR relative to the southern

side [14–16]; i.e. crustal thickness decreases approach-

ing the transform.

Multibeam bathymetry of the entire transverse ridge,

(Fig. 2) as well as extensive rock sampling by dredging

[7], suggest that the VLS is exposed continuously for at

least 300 km, corresponding to a time interval of

Fig. 5. Interpreted line drawings of the seismic reflection profiles of Fig. 4. (a) Location map. (b) VEMA-03; profile located outside of the transform

zone. Reflector B may mark the contact between a carbonate platform and the eroded top of the oceanic crust. The fossil transform valley contains a

relatively thin sediment pile. (c) VEMA-07; the PTDZ is imaged within the transform valley as a pressure ridge. Recent strike-slip tectonic

deformation affects only the northern sector of the transform valley sedimentary infilling. (d) VEMA-09 and VEMA-08; the PTDZ is marked by a

furrow at the sea floor with recent deformation of the sedimentary pile filling the valley affecting a narrow area on both sides of the fault. (e)

VEMA-11 and VEMA-06; the PTDZ is marked here by a furrow with recent deformation affecting the sedimentary sequence in a wide area on the

southern side of the fault. This profile is located above the Nautile submersible dives [4]. Lithology is based on data obtained from submersible

observation and sampling. Note that the VTR topography, imaged along the profiles located within the active portion of the transform valley, shows

increasing amount of uplift and topographic asymmetry; and increasing separation between the summit of VTR and the PTDZ with increasing

crustal age.


roughly 20 Myr, if we assume that this lithosphere was

formed at the EMAR segment that impacts the trans-

form from the south. This segment is about 80 km long

from its northern edge at the intersection with the Vema

transform to an oblique discontinuity that if projected

along a flow line, joins a linear topographic feature, i.e.,

the Lema FZ. The Lema FZ [5], marked by a major

topographic step, has a slightly different orientation (by

about 58) from that of the Vema FZ. It runs westward

up to about 50 Myr old crust, where it merges with the

Fig. 6. Close-up of portions of the seismic reflection profiles of Fig. 4. (a) Summit of the VTR along profile VEMA-06. (b), (c), (d) and (e) Fracture

zone valley as imaged along profiles VEMA-06, VEMA-08, VEMA-07 and VEMA-03, respectively.


trace of the Vema FZ (Fig. 1b). The implication is that

the EMAR segment was born about 50 Ma, and in-

creased its length gradually since then at an average rate

of ~1.6 mm/yr.

4. Seismic reflection profiles

We describe a set of seismic reflection profiles

obtained in the Vema FZ area, starting with those across


the transform, moving from east to west, i.e., from

younger to older crustal ages on the VTR (Figs. 4 and 5).

4.1. Profiles VEMA-06 and VEMA-11

VEMA-06 runs N–S across the transform at about

42842V W, above the sections explored by the submers-

ible Nautile (Figs. 4e and 5e). Age of the crust on the

southern side of the transform (S. American plate) is

roughly 13.3 Myr. The sediments filling the transform

valley reach a maximum thickness of about 1.5 s (two

way travel time, TWT). They were imaged as parallel,

horizontal reflectors onlapping the valley walls (Fig.

6b). A disruption in the horizontal strata similar to those

interpreted by Eittreim and Ewing [11] and Rowlett and

Forsyth [13] as the trace of the strike-slip motion, was

observed near shot 750. The Principal Transform Dis-

placement Zone (PTDZ) is marked here by a furrow,

probably the trace of a sub-vertical fault affecting the

entire sedimentary sequence.

Although the PTDZ appears very narrow at the sea

floor (~0.5 km), we observe a wider deformation zone

(~2.5 km) within the sediments south of the PTDZ. It

consists of a gentle asymmetric folding draping a topo-

graphic high of the acoustic basement, suggesting a

vertical or horizontal overstep related to local transpres-

sion; thus the active deformation is concentrated in this

profile south of the PTDZ.

The summit of the VTR is here about 3.2 s, i.e. 2.4

km, below sea level (Fig. 6a), the topographic profile is

asymmetric, with the southern slope smoother and less

steep (~188) than the northern one (~208) in the lower

part of the scarp and about 358 in the upper part). A flat

bench or terrace interrupts the northern slope at about

5.0 s (TWT) depth. Lithology is also asymmetric on the

two sides: the Nautile dives revealed that all the pre-

dicted units of the upper oceanic lithosphere are ex-

posed on the northern side: mantle ultramafic rocks at

the lower part of the slope; gabbros on the flatter bench;

a sheeted dyke complex and basalts on the less steep

upper slope (Fig. 5e). However, basalt only was sam-

pled by dredging at several sites on the southern side

[7]. A continuous and strong reflector can be followed

parallel to the southern flank about 0.5 s (TWT) below

the sea floor (Fig. 6a). This reflector intersects the

northern flank near the contact between the dyke com-

plex and the upper basalt unit, as observed by submers-

ible. We interpret this reflector as marking this contact,

i.e. the base of layer 2A. The asymmetric lithology of

the VTR is consistent with the VLS being the exposed

edge of a flexured slab of lithosphere, as discussed

further down.

Profile VEMA-11 extends line VEMA-06 southward

to the Lema FZ, thus imaging the stretch of lithosphere

produced ~13.3 Ma by the entire EMAR segment. The

Lema FZ is marked by an asymmetric, ~500 m high

topographic scarp with a slope of about 128 (Figs. 4e

and 5e). The north-facing scarp bounds a small basin

filled by undisturbed sediments onlapping the fault

scarp. The sea floor between Lema and Vema shows

a broad, gentle bulge with the shallowest part 4200 m

below sea level. The buckling of the oceanic litho-

sphere between the two fracture zones might be related

to the flexural process.

4.2. Profiles VEMA-08 and VEMA-09

VEMA-08 is located at 43810V W, about 50 km west

of VEMA-06 (Figs. 4d and 5d). The transform valley

contains here only 1 s (TWT) thick sediment column.

The PTDZ (Fig. 6c) is marked by a furrow at the sea

floor, while the recent deformation below the sea floor

affects a narrow area (about 1.5 km) on both sides of

the fault suggesting a near vertical strike-slip fault. The

VTR is located in crust of about 16 Myr, and is shal-

lower (1875 m b.s.l.) than in the section further east.

The N–S profile is strongly asymmetric but internal

reflectors are poorly developed. The strong reflector

interpreted in VEMA-06 as the base of layer 2A is

here discontinuous and terminates across the southern

flank, near shot 800, at about 3.7 s (TWT). This reflec-

tor limits a triangular crustal sector, probably made of

basalts, as suggested by observations and sampling

made along profile VEMA-06. The absence of a well

developed, continuous basaltic layer and the sea floor

roughness in the vicinity of this profile suggest that

gravity failures or slumpings occurred along the south-

ern flank of the VTR in this area.

Profile VEMA-09 is the southward continuation of

VEMA-08 across the Lema FZ, expressed here by an

800 m N-facing scarp. The sedimentary infill shows the

same characteristics of the section further east. A topo-

graphic bulge is observed in the oceanic lithosphere

between the two fracture zones, superimposed over

short-scale rough topography.

4.3. Profile VEMA-07

It is located at 43830V W, about 37 km west of

VEMA-08, in crust (south of the transform) about 18

Myr in age (Figs. 4c and 5c). The PTDZ is visible

within the transform valley as a pressure ridge (Fig.

6d). A wide disrupted area (about 4 km), observed

throughout the sediment cover north of the PTDZ, is


probably due to transpressional deformation. The VTR

is shallower than further east and displays again strong

topographic asymmetry. The lithospheric unit bounded

by the reflector interpreted as the base of layer 2A is

reduced here to a narrow, triangular section (shots

1465–1510) with a maximum thickness of 0.5 s

(TWT), as in profile VEMA-08. Rugged topography

at the base of the southern flank of the VTR suggests

mass wasting. Profile VEMA-07 extends southwards

across the Lema FZ.


It is located at about 44830V W, 110 km west of

VEMA-07; thus outside the active transform zone, in

24.4 Myr old crust (Figs. 4b and 5b). This profile is not

oriented strictly N–S, but 128 N, perpendicular to the

western portion of the VTR. The acoustic basement

below the non-active transform valley is shallower than

in the other sections. The graben geometry of the valley

suggests the presence of a master N-dipping fault (the

northern side of the VTR) and of southward tilted blocks

bounding three small basins (Fig. 6e). The undisturbed

sedimentary sequence indicates the absence of recent

deformation in the valley. The topographic asymmetry

of the VTR is here less evident than in profiles further

east due to the lack of the flat bench on the northern side

that, however, is still steeper (308) than the southern one(248). The VTR summit shoals here up to about 600 m

below sea level; we observed a strong reflector about

0.45 s (TWT) below the top. This reflector corresponds

to reflector B of profile VEMA-02 (Fig. 7), that runs

along the crest of the VTR, interpreted as marking the

base of a carbonate platform resting on a horizontal

erosion surface of oceanic crust [8], as discussed next.

Fig. 7. Portion of the seismic reflection profile VEMA-02 running along t

velocity profile used to perform the time to depth conversion is indicated

(reflector B), interpreted as the eroded top of the basaltic crust, overlain by a c

shallow-water carbonate sedimentary sequence. Location of this profile is s


It runs E–W for about 50 km along the crest of the

western part of the VTR, covering a crustal age interval

ranging roughly from 21 to 25 Myr. It differs from the

profiles described above because it is a time to depth

converted section, i.e., the vertical axis refers to depths in

meters instead of two way travel time (Fig. 7). A strong,

continuous, horizontal reflector (reflector B) lies about

1100 m below sea level, separating two major units, i.e.,

a semi-transparent, layered, ~500 m thick unit above,

and a non-transparent unit below. A second horizontal

reflector (reflector A) lies within the upper unit, about

700 m below sea level. Acoustic velocities estimated

from refracted returns as well as rock samples suggest

that the upper seismostratigraphic unit represents a

shallow-water carbonate platform capping a wave-trun-

cated erosional surface (i.e. reflector B), marking the

top of the oceanic crust. Acoustic velocities suggest

also that the basalt unit is reduced or missing. These

results suggest that the summit of the western portion of

the VTR rose above sea level and then subsided [8,17].

The absence of the basaltic layer may be caused by

gravity failure and erosion above and at sea level.

4.6. Partial conclusions

The seismic reflection data are consistent with flexur-

ing of the lithospheric slab that extends from the Lema to

the Vema Fracture Zones. They further suggest that: (a)

deformation along the VTR is moderate and the primary

layering of the oceanic lithosphere is preserved; (b) the

height of the VTR and the distance between the crest of

the VTR and the PDTZ trace increase with age of the

crust; (c) the thickness and lateral extent of basaltic layer

he crest of the VTR (time to depth converted section). The P-wave

in the middle of the section. Note a prominent horizontal reflector

arbonate platform. Reflector A marks a major unconformity within the

hown in Fig. 4.


2A decrease with crustal age, consistently with either a

decrease in production of crust with age, or with a

gradual erosion of the upper part of the crust, or both;

(d) the narrow PTDZ is punctuated by local transtensive/

transpressive deformation.

5. Erosion of the transverse ridge

The following observations support the idea that

erosion and down slope mass wasting have affected

the VTR: (a) abundant talus breccias have been ob-

served with the submersible Nautile at the base of the

southern wall of the transform valley close to the RTI

where the exposed lithospheric section is relatively

young [18]; (b) multibeam topography of the VTR

(Fig. 2) shows a number of canyon and slump/detach-

ment-like features, particularly prominent in the N~15

Myr portion of the VTR northern slope. A broad, 20 km

diameter bamphitheaterQ interrupts the northern slope of

the VTR at about 44839V W. It is probably the scar of

either a single massive slump or of multiple slumps.

The narrow continuous crest of the VTR is interrupted

between 43852V W and 43841V W by a 23 km interval

where it is replaced by a deeper, broader summit, with a

chaotic surface that resembles a collapsed feature, i.e.

the bbadlandsQ of Kastens et al. [5]. (c) Multichannel

data (Profile VEMA-02 described above), obtained

along the VTR crest above the ~50 km long carbonate

platform (Fig. 7), allowed an estimate of acoustic ve-

locities in the crustal section below the platform. Ve-

locities of ~5.4 km/s were found about ~200 m below

the horizontal erosional surface (base of the carbonate

platform), suggesting that either lower crustal (gabbro-

ic) or partly serpentinized mantle lies close to the top of

this stretch of the VLS. A possible implication is that

most of the basaltic unit has been eroded away in this

stretch of the VLS. Erosion rates in the VTR must have

been particularly high during the time interval when its

crest was above and near sea level during subsidence,

consistently with the flat erosional horizon shown by

the reflection data.

Fig. 8. Cartoon illustrating flexure of lith

Removal of material from the VTR due to erosion

might have contributed to its vertical motions. Basile

and Allemand [19] discussed how erosional unloading

can cause flexure at continental and oceanic transform

edges, including Vema.

6. Flexure, uplift and subsidence of the lithospheric

slab

Taken together all observations indicate that the

northern side of the VTR exposes the edge of a flex-

ured and uplifted slab of oceanic lithosphere. We dis-

cuss now when, how and why this slab of lithosphere

was uplifted, and its northern edge became exposed at

the sea floor (Fig. 8). Two key observations enabled us

to date uplift and subsidence of the transverse ridge,

and to infer mechanisms and timing of its vertical

movements.

Observation one: the eastern edge of the VTR is in ~10

Myr old crust, where we observe an

abrupt topographic transition from

the transverse ridge to the west, to

younger non-flexured bnormalQ lith-osphere to the east (Fig. 2). We as-

sume, therefore, that flexure and

uplift terminated roughly 10 Ma.

Observation two: multibeam data show an abrupt

change in the orientation of ridge-

parallel sea floor structural fabric

south of the Vema FZ at about

42812V W: fabric is parallel to the

present-day MAR axis from 0 to

about 11–12 Myr old crust, but its

orientation is shifted about 58 to 108clockwise in crust older than ~11–12

Myr (Fig. 2). This implies a counter-

clockwise change in orientation of

the ridge axis and of the spreading

direction of the lithosphere adjacent

to the transform; and, a change in the

osphere near the Vema transform.


position of the Eulerian pole of rota-

tion that defines the motion of this

lithosphere. Close examination of the

zone of abrupt transition from youn-

ger, MAR-parallel fabric to older,

oblique fabric, suggests that the

shift of MAR orientation (i.e., the

change of pole of rotation) took

less than ~1 Myr. This reorientation

must have induced a transtensional

regime along the Vema transform

~11–12 Ma (Fig. 9). We assume

that reorientation of the bhotQ ridge

axis may be rather rapid, but the

transtensional response along the

transform is slower, since it involves

older, thicker lithosphere. An exten-

sional component perpendicular to

the transform would cause normal

faulting, and trigger lithospheric

Fig. 9. Cartoon illustrating the evolution of the Vema FZ area: (a)

ridge–transform–ridge geometry prior to ~12 Ma; (b) change in ridge–

transform geometry related to a change in Euler pole of rotation

occurring ~11 Ma, causing extension normal to the transform and

flexure of the lithospheric slab between Vema and Lema; (c) geometry

at the end of flexure and uplift of the VTR; (d) present-day geometry.

White and gray alternating zones indicate 1 Myr crustal stripes.

flexure and vertical tectonics, facili-

tated by the fact that oceanic trans-

forms are weak [20], as discussed in

the next section of this paper.

Given that: (a) uplift initiated ~11–12 Ma; (b) ter-

minated ~10 Ma, and (c) involved vertical motion of

roughly 3–4 km, the average uplift rate was roughly 2–

4 mm/yr. These estimated ages of uplift are consistent

with ages of semi-consolidated pelagic carbonate sedi-

ments encrusting ultramafic rocks sampled at various

sites along the lower part of the exposed lithospheric

section [17]. We assume these carbonates were depos-

ited on the ultramafic rocks after they became exposed

on the sea floor due to the uplift. Therefore, their ages

are minimum ages for the uplift. Their ages, determined

from calcareous microfossils, range around 12 Ma [17].

The hypothesis that the transverse ridge was uplifted

as a single block is supported by the observation that a

second minor horizontal wave-truncated surface,

capped by an ~60 m thick carbonate platform, lies on

the transverse ridge crest about 100 km east of the

major limestone platform (Fig. 7) at the same level

(~1100 m) of the major truncated western surface [5].

The implication is that the two sites, although ~100 km

apart on the crest of the transverse ridge, were at sea

level at about the same time.

The average subsidence rate of the transverse ridge

was estimated assuming that it subsided as a single

block and that subsidence started soon after the end

of the uplift phase, that is, 10 Ma. The wave-truncated

horizontal surface of oceanic crust at the base of the

carbonate platform in the western portion of the VTR

lies presently 1100 m below sea level (Fig. 7), suggest-

ing an average subsidence rate slightly over 0.1 mm/yr.

Subsidence could have been faster if we consider that

erosion above and close to sea level could have grad-

ually lowered the surface during subsidence.

The VTR (i.e., the northern edge of the flexured slab)

extends westward to ~25 Myr old crust, well outside the

active transform. The length of the VTR, about 300 km,

is similar to the length of the present Vema active

transform offset, suggesting that the initial uplift of the

transverse ridge took place 11–12 Ma along the entire

active offset zone (Fig. 9). The southern edge of the

flexured slab abuts against the ~1 km high Lema scarp, a

major dip-slip fault that reactivated the Lema FZ, mark-

ing an old transform offset of the MAR axis. The Lema

FZ decouples the flexured slab from the bnormalQ lith-osphere to the south (Fig. 8). Thus, the tilted and flex-

ured lithospheric slab is about 300 km long in an E–W

direction and ~80 km in a N–S direction (Fig. 2). The


VLS consists of a younger (0 to 10 Myr), eastern

unflexured portion, with a thinner exposed lithospheric

section, and an older (~N10 Myr), flexured portion, with

a thicker exposed section (Fig. 8).

7. Model of extensional flexure and uplift of

transverse ridges

Extension of brittle lithosphere can produce topo-

graphic relief when strain is localized on a fault that

moves in a normal sense (e.g. Vening-Meinez) [21].

Forsyth [22] suggested that offset of a normal fault

changes the stress field around the fault making con-

tinued offset more difficult. Eventually, a new fault

might form while the initial fault would cease moving.

The weaker the initial fault, the farther it can be offset

before being replaced by another fault. Buck [23] ap-

plied this idea to a normal fault cutting an elastic–

plastic layer, using the thin layer approximation to

simplify the estimation of the change in stresses caused

by offset of a single normal fault. Buck [23] showed

how lithospheric thickness enters the problem in two

ways. First, the thicker the lithosphere the greater the

resistance to fault offset. Therefore, in order to achieve

a given fault slip (or horizontal offset) for a thick layer

requires a weaker fault than to achieve the same slip for

a thin layer. Second, the fault topographic relief in-

creased with fault offset up to a maximum value. The

maximum relief scales with the lithospheric thickness.

Pockalny et al. [24] suggested that normal fault slip

might produce some transform ridges. They used the

thin plate model of Buck [23] to show that the shape of

normal fault related topographic relief is similar to the

relief observed across transform ridges. The question

we wish to address is whether the large magnitude relief

mapped along the Vema transform could have been

produced by such normal fault slip. We assume that

the transform fault was initially a strike-slip fault zone

that accommodated some dip-slip motion during a

phase of transform normal extension.

We describe two-dimensional numerical models of a

normal fault offsetting a brittle layer. These models

should give more realistic results than the thin plate

models and allow us to better compare the model

topographic relief to the observed relief across the

Vema transform. We have two further goals in the

models we describe here. First, we will estimate how

weak faults have to be in order to allow the magnitude

of normal slip implied by the transform ridges with the

largest relief. If a fault is not sufficiently weak then we

expect multiple faults to form and decrease the topo-

graphic step across an initial single fault. Second, we

will evaluate the effect of expected variations in brittle

lithospheric thickness along the transform on the uplift

of the footwall side of the fault. The age of the footwall

lithosphere at the time of the uplift varies from 0 to

about 20 Myr. Thus, the lithospheric thickness should

have varied from perhaps 5 km for the zero age litho-

sphere to about 30 km for the older plate. We compare

here models for 10 and 20 km thick brittle layers.

We use a numerical approach based on the Lagrang-

ian FLAC technique of Cundall [25] modified by Polia-

kov and Buck [26]. The model set-up treats extension

of an elasto-plastic brittle layer floating on an inviscid

substrate, following Lavier et al. [27]. We consider

extension of Mohr–Coulomb layers with strength con-

trolled by a friction angle (set to a value of 408 in areas

of no strain) and a cohesion (set to 44 MPa in areas of

no strain). Both friction and cohesion are reduced by

strain weakening.

There are two major differences between our simu-

lations and those of Lavier et al. [27]. First, we allow a

very large reduction of the layer strength. Second, we

begin the extension of the layer with a pre-existing zone

of weakness, meant to represent the weak transform

fault. The fault initially dips at 728, but the offset of thefault causes its rotation to somewhat lower dips. 40

square grid elements span the layer vertically and 500

laterally. The layer is 12.5 times as wide as it is thick.

Fig. 10 shows the two model cases where topograph-

ic uplift on the footwall side of the fault reached a

maximum. Note that the uplift of the 20 km thick

layer is much larger than for the 10 km thick layer.

The distribution of plastic strain, which controls the

fault weakening, shows that the faults have rotated to

a dip of about 458 near the surface, even though they

began with a dip of 728. Fig. 11 shows the evolution of

height of the highest point on the footwall as a function

of horizontal offset of the lithosphere.

For our 20 km lithosphere case, the transverse ridge

model uplift is as great as the maximum seen along the

Vema transform and could expose the top of the ridge

above sea level. For thinner lithosphere we expect less

uplift no matter how great the fault offset. For the

model 10 km thick lithosphere the fault relief would

be insufficient to have the uplifted transform ridge

reached above the sea surface.

We speculate that in the real case of extension across

a transform fault, the dip of the fault might be affected

by the strike-slip component of the transform motion.

The strike-slip motion should favor a high dip angle on

the fault. Less work is done for a steeper dipping

transform, and in purely strike-slip motion the most

favorable dip would be 908.

Fig. 10. Two model cases of topographic uplift of the footwall side of the fault for a 12 km horizontal offset: (a) 10 km thick layer; and (b) 20 km

thick layer.


If the combined normal and strike-slip motions re-

sult in a fault that remains at a steep dip (say N708),then the amount of fault offset necessary to produce

large transform relief may be less than in our two-

Fig. 11. Height of the footwall versus horizontal offset of the

lithosphere.

dimensional model calculation. To get a total vertical

relief across the faults of 6 km (about 3 km of uplift of

the footwall) would require just 2 km of horizontal

offset, while in our two-dimensional model the hori-

zontal offset required for such uplift is several times

larger. Fig. 12 shows how we might expect a transform

system undergoing extension to operate, and to relate to

observations. Since the active part of the fault remains

steep, the fault scarp would be similarly steep. Howev-

er, mass wasting of that scarp would produce a slope

that is closer to the observed slope on the inner side of

the transform ridge (about 30–408).

8. Conclusions

(1) Flexure and uplift of the edge of a lithospheric

slab on the southern side of the Vema transform

took place within a time interval of about 1–2

Myr from ~11–12 to ~10 Myr before present.

Average uplift rate was about 2 to 4 mm/yr.

(2) Uplift of the lithospheric slab took place roughly

when the direction of spreading of the plates

Fig. 12. Cartoon illustrating a model of extension and lithospheric

uplift at a transform boundary.


adjacent to the Vema transform underwent a

counter-clockwise shift of several degrees, i.e., a

change in the position of the Euler rotation pole.

The two events are probably related, in so far as

the change in the position of the rotation pole

caused extension along the transform, with con-

sequent flexure and uplift of the lithospheric sliv-

er between Vema and Lema, decoupled from the

lithosphere to the north by the weak Vema trans-

form zone.

(3) The lithospheric slab that underwent flexuring,

bound to the north by the Vema transform, was

decoupled from bnormalQ lithosphere to the south

by the Lema FZ. Thus, the dimensions of the slab

that underwent tilting and flexure are roughly 300

km (along a transform/parallel direction) by 80

km (along a MAR/parallel direction).

(4) The Vema FZ and the Lema FZ are not parallel,

and gradually converge moving west towards

older crustal ages; satellite gravimetry indicates

that they merge in about 50 Myr old crust. This is

the age of birth of the EMAR segment that in-

creased its length gradually to its present ~80 km.

(5) The average spreading rate of the plate on the

southern side of the Vema transform appears to

have undergone a significant deceleration about

10 Ma, from ~17 mm/yr before 10 Ma to ~13.6

mm/yr after. This deceleration may be related to

the change of direction and geometry of spread-

ing that occurred about 11 Ma.

(6) Numerical models duplicate topographic relief as

a result of extension across a transform zone and

are consistent with the observed features of the

VTR. In particular, they predict a higher relief

when a thicker brittle layer is involved, in agree-

ment with the observation that the uplift of the

VTR was highest in the older, thicker portion of

the Vema lithospheric slab.

(7) The events described above provide an example

of how transform-related vertical tectonic move-

ments may build major ocean floor relief (trans-

verse ridge) in relatively short time intervals.

Acknowledgments

We thank Kim Kastens co-chief scientist on cruise

EW9305. We are grateful to the Captain, officers, crew

and technicians of R/V Ewing, R/V OGS Explora and R/

V Strakhov. Field work on cruises S-19 and S-22 by the

R/V Strakhov was part of an Italian–Russian project on

mid-ocean ridges. Research was supported by the Ita

lian Consiglio Nazionale delle Ricerche and the U. S.

National Science Foundation (Grant OCE-9911753 and

OCE-0328217). Geologia Marina Contribution 1447

and Lamont-Doherty Earth Observatory Contribution

6833.

References

[1] B.C. Heezen, R.D. Gerard, M. Tharp, The Vema Fracture Zone

in the equatorial Atlantic, J. Geophys. Res. 69 (1964) 733–739.

[2] T.H. Van Andel, R.P. Von Herzen, J.D. Phillips, The Vema

Fracture Zone and the tectonics of transverse shear zones in

oceanic crustal plates, Mar. Geophys. Res. 1 (1971) 261–283.

[3] E. Bonatti, J. Honnorez, Sections of the Earth’s crust in the

equatorial Atlantic, J. Geophys. Res. 81 (1976) 4104–4116.

[4] J.M. Auzende, D. Bideau, E. Bonatti, M. Cannat, J. Honnorez,

Y. Lagabrielle, J. Malavieille, V. Mamaloukas-Frangoulis, C.

Mevel, Direct observation of a section through slow-spreading

oceanic crust, Nature 337 (1989) 726–729.

[5] K. Kastens, E. Bonatti, D. Caress, G. Carrara, O. Dauteuil, G.

Frueh-Green, M. Ligi, P. Tartarotti, The Vema transverse ridge

(central Atlantic), Mar. Geophys. Res. 20 (1998) 533–556.

[6] P. Fabretti, E. Bonatti, A. Peyve, D. Brunelli, A. Cipriani, X.

Dobrolubova, V. Efimov, S. Erofeev, L. Gasperini, J.A. Hanley,

M. Ligi, A. Perfiliev, V. Rastorguyen, Y. Raznitsin, G. Savelieva,

V. Semjenov, S. Sokolov, S. Skolotnev, S. Susini, I. Vikentyev,

First results of cruise S19 (PRIMAR Project); petrological and

structural investigations of the Vema transverse ridge (Equatorial

Atlantic), G. Geol. 60 (1998) 3–16.

[7] E. Bonatti, M. Ligi, D. Brunelli, A. Cipriani, P. Fabretti, V.

Ferrante, L. Gasperini, L. Ottolini, Mantle thermal pulses

below the Mid-Atlantic Ridge and temporal variations in the

formation of oceanic lithosphere, Nature 423 (2003) 499–505.

[8] E. Bonatti, M. Ligi, L. Gasperini, G. Carrara, E. Vera, Imaging

crustal uplift, emersion, and subsidence at the Vema fracture

zone, EOS Trans. AGU 75 (1994) 371–372.

[9] S.C. Cande, J.L. LaBreque, W.F. Haxby, Plate kinematics of the

south Atlantic, Chron C34 to the present, J. Geophys. Res. 93

(1988) 13479–13492.


[10] R.G. Bader, R.D. Gerard, W.E. Benson, H.M. Bolli, W.W. Hay,

W.T. Rothwell Jr., M.H. Ruef, W.R. Riedel, F.L. Sayler, Site 26,

Initial Rep. D.S.D.P. 4 (1970) 77–91.

[11] S. Eittreim, J. Ewing, Vema fracture zone transform fault, Ge-

ology 3 (1975) 555–558.

[12] K. Perch-Nielsen, P.R. Supko, A. Boersma, E. Bonatti, R.L.

Carlson, F. McCoy, Y.P. Neprochnov, H.B. Zimmerman, Site

353; Vema fracture zone, Initial Rep. D.S.D.P. 39 (1977) 27–44.

[13] H. Rowlett, D.W. Forsyth, Recent faulting and microearthquakes

at the intersection of the Vema fracture zone and the Mid-

Atlantic Ridge, J. Geophys. Res. 89 (1984) 6079–6094.

[14] K.E. Louden, R.S. White, C.G. Potts, D.W. Forsyth, Structure

and seismotectonics of the Vema fracture zone, Atlantic Ocean,

J. Geol. Soc. (Lond.) 143 (1986) 795–805.

[15] C.G. Potts, R.S. White, K.E. Louden, Crustal structure of At-

lantic fracture zones; II, the Vema fracture zone and transverse

ridge, Geophys. J. R. Astron. Soc. 86 (1986) 491–513.

[16] R.A. Prince, D.W. Forsyth, Horizontal extent of anomalously

thin crust near the Vema fracture zone from the three-dimen-

sional analysis of gravity anomalies, J. Geophys. Res. 93 (1988)

8051–8063.

[17] L. Gasperini, E. Bonatti, A.M. Borsetti, D. Brunelli, L. Capo-

tondi, G. Carrara, A. Cipriani, P. Fabretti, M. Ligi, K. Kastens,

Time constraints on the emplacement of an uplifted sliver

of lithosphere at the Vema transverse ridge (central Atlantic),

J. Conf. Abstr. 4 (1999) 757.

[18] Y. Lagabrielle, V. Mamaloukas-Frangoulis, M. Cannat, J.M.

Auzende, J. Honnorez, C. Mevel, E. Bonatti, Vema Fracture

Zone (Central Atlantic): tectonic and magmatic evolution of

the Median Ridge and the eastern ridge transform intersection,

J. Geophys. Res. 97 (B12) (1992) 17331–17351.

[19] C. Basile, P. Allemand, Erosion and flexure uplift along trans-

form faults, Geophys. J. Int. 151 (2002) 646–653.

[20] M.D. Behn, J. Lin, M.T. Zuber, Evidence for weak oceanic

transform faults, Geophys. Res. Lett. 29 (2002) 2207.

[21] F.A. Vening Meinesz, Les graben africains resultat de compres-

sion ou de tension dans la croute terrestre, Inst. R. Colon. Belg.

Bull. Sceances 21 (1950) 539–552.

[22] D.W. Forsyth, Finite extension and low-angle normal faulting,

Geology 20 (1992) 27–30.

[23] W.R. Buck, Effect of lithospheric thickness on the formation of

high- and low-angle normal faults, Geology 21 (1993) 933–936.

[24] R.A. Pockalny, P. Gente, W.R. Buck, Oceanic transverse ridges;

a flexural response to fracture-zone-normal extension, Geology

24 (1996) 71–74.

[25] P.A. Cundall, Numerical experiments on localization in frictional

materials, Ing. Arch. 58 (1989) 148–159.

[26] A.N.B. Poliakov, W.R. Buck, Mechanics of stretching elastic–

plastic–viscous layers: applications to slow-spreading mid-ocean

ridges, in faulting and magmatism at mid-ocean ridges, Geo-

phys. Monogr. Ser. 106 (1998) 305–325.

[27] L.L. Lavier, W.R. Buck, A.N.B. Poliakov, Factors controlling

normal fault offset in an ideal brittle layer, J. Geophys. Res. 105

(2000) 23431–23442.

[28] D.T. Sandwell, W.H.F. Smith, Marine gravity anomaly from

Geosat and ERS 1 satellite altimetry, J. Geophys. Res. 102

(1997) 10039–10054.

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