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Journal ArticleDOI

Active surface deformation and sub-lithospheric processes in the western Mediterranean constrained by numerical models

01 Sep 2010-Geology (Geological Society of America)-Vol. 38, Iss: 9, pp 823-826

AbstractWe present the results of dynamic modeling of the western Mediterranean that accounts for observed global positioning system (GPS) surface deformation of the Alboran Sea and surrounding Rif and Betic Mountains as the result of the combined effects of relative motion of the Eurasian and Nubian plates, low strength in the Alboran Sea region and sub-lithospheric processes occurring beneath the External Rif domain. Assuming that the lithosphere behaves elastically over the short time period of the GPS observations, an elastic plate model is considered in our study, including an area of weak lithosphere (factor of 10) centered on the Alboran Sea and in which lateral boundary conditions consist of the Nubia-Eurasia oblique convergence. Sub-crustal processes are modeled by application of a horizontal traction on a small area (patch) at the base of the elastic plate. Our modeling studies demonstrate the need for sub-crustal or sub-lithospheric, southwestward-directed forcing to account for observed southwestward motion of the Rif and Betic domains. Based on the location, orientation, and small area of the traction patch, we hypothesize that forcing is associated with delamination and rollback of the subducted African continental lithospheric mantle beneath the External Rif zone, due to the pull of the oceanic part of the Western Mediterranean slab, a dynamic process that may be similar to that where the over-riding plate is driven toward the subduction zone during slab rollback.

Topics: Subduction (54%), Lithosphere (53%), Slab (53%)

Summary (1 min read)

INTRODUCTION

  • The authors modeling studies demonstrate the need for sub-crustal or sub-lithospheric, southwestward-directed forcing to account for observed southwestward motion of the Rif and Betic domains.
  • The authors then discuss the implications of these model results in light of previous geodynamic models of the plate boundary zone.

TECTONIC SETTING OF THE WESTERN MEDITERRANEAN

  • In the western Mediterranean, the Alboran Sea is a thinned continental domain (15 km thickness; Lonergan and White 1997) surrounded by the Internal Rif and Internal Betics , which are the westernmost limit of the Alpine mountain belt (Fig. 1).
  • Three factors are likely to infl uence the spatial distribution of the interseismic strain 1) lateral plate driving forces due to long term Nubia-Eurasia oblique convergence, 2) low rigidity of the diffuse plate boundary zone, and 3) deep traction beneath the plate boundary due to upper mantle drag or slab traction.
  • The RMSs for the entire zone and for the Rif-Betics region are summarized in Table 1.
  • Depending on the thermal regime, the effective elastic thickness of continental plates varies from 3 to 80 km (Watts and Burov, 2003).

DISCUSSION AND GEODYNAMIC IMPLICATIONS

  • The authors modeling experiments include no a priori information on sub-lithospheric geometry and are designed to determine whether sublithospheric processes are needed to account for observed deformation of the western Mediterranean region.
  • Geodynamic models of the zone involving still active westward rollback of the western Mediterra- nean narrow slab (Gutscher et al., 2002) cannot generate such a small coupling zone confi ned to the External Rif. Spakman and Wortel (2004) suggested that the western Mediterranean slab is detached under the Betics.
  • The authors further suggest that the horizontal traction patch could represent the remaining coupling zone between the slab and the overlying continental lithosphere.
  • According to their delamination model (Fig. 3) the traction zone is expected to move to the south-southwest following propagation of the delamination front.
  • During the Pliocene-Quaternary, eastward subduction has died out, as suggested by the accretionary wedge sealed by undeformed sediments (Zitellini et al., 2009).

ACKNOWLEDGMENTS

  • The authors thank G. Bokelmann, S. Lallemand and J.L. Bodinier for their fruitful discussions, and to C. Faccenna and fi ve anonymous reviewers for their constructive comments on this manuscript.
  • Reilinger benefi ted from a Visiting Researcher Fellowship from the Observatoire de Recherche Méditerranéen en Environnement of Montpellier while engaged in this study.

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GEOLOGY, September 2010 823
INTRODUCTION
Deformations around transpressive plate
boundaries are thought to be due to the mechan-
ical interaction of plates (i.e., elastic strain and
associated faulting, e.g Savage and Burford,
1973), sub-crustal processes (e.g., subduction
and slab rollback, mantle delamination, Roy-
den,1993), and/or stresses induced by gravita-
tional potential energy differences (e.g., England
and McKenzie 1982). South-southwest–ori-
ented crustal motions in the western Mediter-
ranean (Fig; 1; Vernant et al., 2010) appear to
be incompatible with simple plates interaction
as they involve motion normal to the direction
of Nubia-Eurasia relative motion in a region
dominated by strike-slip and extensional tecton-
ics (for further discussion see Fadil et al., 2006
and Vernant et al., 2010), or to gravitational
potential energy differences as motion of the
Rif Mountains is directed away from the low-
lying, thinner lithosphere of the Alboran Sea.
Although the importance of subduction related
processes for the tectonic evolution of the west-
ern Mediterranean has been discussed (e.g.,
Royden, 1993; Lonergan and White 1997; Fac-
cenna et al. 2004; Spakman and Wortel 2004),
the rather localized, present-day motions of the
Betic–Alboran Sea–Rif domain have not been
subject to quantitative modeling studies. Here,
we use numerical models constrained by global
positioning system (GPS) observations and
Geology, September 2010; v. 38; no. 9; p. 823–826; doi: 10.1130/G30963.1; 3 fi gures; 1 table.
© 2010 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
Active surface deformation and sub-lithospheric processes in the
western Mediterranean constrained by numerical models
Eugénie Pérouse
1
, Philippe Vernant
1
, Jean Chéry
1
, Robert Reilinger
2
, and Simon McClusky
2
1
Laboratoire Géosciences Montpellier CNRS (Centre National de la Recherche Scientifi que)-Université Montpellier 2, 34095
Montpellier, France
2
Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts
02139, USA
ABSTRACT
We present the results of dynamic modeling of the western Mediterranean that accounts for
observed global positioning system (GPS) surface deformation of the Alboran Sea and sur-
rounding Rif and Betic Mountains as the result of the combined effects of relative motion of
the Eurasian and Nubian plates, low strength in the Alboran Sea region and sub-lithospheric
processes occurring beneath the External Rif domain. Assuming that the lithosphere behaves
elastically over the short time period of the GPS observations, an elastic plate model is con-
sidered in our study, including an area of weak lithosphere (factor of 10) centered on the
Alboran Sea and in which lateral boundary conditions consist of the Nubia-Eurasia oblique
convergence. Sub-crustal processes are modeled by application of a horizontal traction on a
small area (patch) at the base of the elastic plate. Our modeling studies demonstrate the need
for sub-crustal or sub-lithospheric, southwestward-directed forcing to account for observed
southwestward motion of the Rif and Betic domains. Based on the location, orientation, and
small area of the traction patch, we hypothesize that forcing is associated with delamination
and rollback of the subducted African continental lithospheric mantle beneath the External
Rif zone, due to the pull of the oceanic part of the Western Mediterranean slab, a dynamic
process that may be similar to that where the over-riding plate is driven toward the subduc-
tion zone during slab rollback.
350° E 352°E 354°E 356°E 358°E
30°N
32°N
34°N
36°N
38°N
40°N
Alb
EURASIAN
PLATE
4 mm/yr
0 30 60 90 120 660
Depth (km)
Betics
PLATE
Observed
velocities
Boundary conditions
of the model
Mw
Internal
zone
External
zone
AH
eq
NUBIAN
Rif
Major
thrust
faults
456
Spain
Med. sea
Mo
Figure 1. Global positioning system velocities
(Vernant et al. 2010) in Nubia fi xed reference
frame and 95% confi dence ellipses. Gray ar-
rows indicate velocities consistent with Iberia
or Nubia plate motion; black arrows indicate
velocities inconsistent with either Iberia or Nu-
bia. Colored circles depict earthquakes with
Mw > 3.5 (National Earthquake Information
Center–U.S. Geological Survey, 1973–2009;
http://earthquake.usgs.gov/regional/neic/).
Major geological structures are simplifi ed
from Jolivet et. al (2008) and Zitellini et al.
(2009); red fault—Boussekkour fault; AH
eq—M = 6 Al Hoceima earthquake of 1994
(from www.globalcmt.org); Alb—Alboran
Sea. Boundary conditions of the elastic
plate model are shown in red. Inset: Tectonic
sketch of the western Mediterranean. Black
box outlines study area; Mo—Morocco; Med.
sea—Mediterranean sea; hachured areas—
Alpine orogenic belts.

824 GEOLOGY, September 2010
seismic, and other geophysical data to address
this issue and identify where these sub-crustal
processes may occur. Because the confi gura-
tion of the Nubian-Eurasian plate boundary in
the western Mediterranean and the geometry of
the subducted plate are still debated (Calvert et
al., 2000; Gutscher et al., 2002; Faccenna et al.,
2004; Spakman and Wortel, 2004), we use an
elastic plate model approach in order to evalu-
ate the role of basal traction below the crust for
active deformation, without using strong a priori
constraints on the geometry of the lithosphere in
the region. We then discuss the implications of
these model results in light of previous geody-
namic models of the plate boundary zone.
TECTONIC SETTING OF THE
WESTERN MEDITERRANEAN
In the western Mediterranean, the Alboran
Sea is a thinned continental domain (15 km
thickness; Lonergan and White 1997) sur-
rounded by the Internal Rif (Morocco) and
Internal Betics (Spain), which are the western-
most limit of the Alpine mountain belt (Fig. 1).
The tectonic evolution of the western Mediter-
ranean and the Alboran Sea has been dominated
by the long history of Nubia-Eurasia plate con-
vergence associated with the subduction of the
Neotethys Ocean (e.g., Faccenna et al., 2004).
Ideas to explain the striking topographical
symmetry of the region as well as the appar-
ently synchronous extension of the Alboran Sea
and shortening of the Betic and Rif Mountain
belts during the Neogene and Quaternary are
still debated. Current tectonic models for the
Alboran domain include four broad categories
of hypotheses: (1) backarc extension driven by
the westward rollback of an eastward subducting
slab (Royden, 1993; Lonergan and White, 1997;
Gutscher et al., 2002; Faccenna et al., 2004),
which passively falls in the mantle driven by its
own weight (subduction without convergence,
Jolivet et al. 2008) ; (2) break-off of a subducting
lithospheric slab (Blanco and Spakman, 1993);
(3) crustal extrusion due to forces transmitted
across the Eurasia-Africa plate boundary (Rebai
et al., 1992); and (4) delamination and convec-
tive removal of the lithospheric mantle root
beneath the collisional orogen (Platt and Vissers,
1989; Seber et al., 1996; Calvert et al., 2000).
Tomographic studies (Calvert et al. 2000;
Gutscher et al., 2002; Faccenna et al., 2004;
Spakman and Wortel, 2004) reveal a narrow east
dipping slab (100–200 km wide) located in the
Gibraltar Arc. Furthermore, seismic ray disper-
sion shows that the slab corresponds to oceanic
lithosphere material (Bokelmann and Maufroy,
2007). This slab seems to be detached under the
Betics (Spakman and Wortel 2004). According
to geochemical studies of igneous rocks of the
zone, beneath the Alboran Basin the slab is a
remnant part of the Tethys oceanic lithosphere,
but beneath the External Betics and Rif it con-
sists of delaminated (or subducted) continental
lithosphere (Duggen et al. 2008). The mecha-
nism responsible for the continental and oceanic
nature of the slab (delamination or continental
subduction) is still debated (Faccenna et al.
2004; Duggen et al. 2008).
Present-day tectonic processes occur within
the context of ongoing, WNW-ESE convergence
between Africa and Iberia in the Strait of Gibral-
tar (4.3 ± 0.5 mm/yr along the N116°E ± 6°
direction, McClusky et al., 2003, Fig. 1). How-
ever, the location or even existence of a discrete
Africa-Eurasia plate boundary in the Rif-Betics-
Alboran region is equivocal (Fadil et al. 2006).
The GPS velocities show signifi cant motions in
the Rif and Betics relative to Nubia and Eurasia
respectively (black arrows, Fig. 1). This south-
southwest motion of the Rif (5.4 ± 1.5 mm/yr)
in a Nubia fi xed reference frame, and to a lesser
extent the southern velocity component for sites
in the Betics with respect to Eurasia, depict more
complicated dynamics than can be ascribed to
simple crustal block interactions; or to stresses
induced by gravitational potential energy differ-
ences, since the Rif motion is not consistent with
the highest topographic gradient north of the Rif.
NUMERICAL MODEL OF THE RIF-
BETICS VELOCITY FIELD
According to Vernant et al. (2010) the GPS
velocities accurately depict the secular strain
eld and are not signifi cantly affected by either
co- or post-seismic deformation. Although the
lithosphere behaves elastically over the short time
period of the GPS observations, away from the
immediate vicinity of active faults, GPS observa-
tions are consistent with longer time period dis-
placements (from 10
3
to 3.10
6
yr and more, e.g.,
Reilinger et al., 2006). Three factors are likely to
infl uence the spatial distribution of the interseis-
mic strain 1) lateral plate driving forces due to
long term Nubia-Eurasia oblique convergence, 2)
low rigidity of the diffuse plate boundary zone,
and 3) deep traction beneath the plate boundary
due to upper mantle drag or slab traction.
In order to quantify the impact of these three
factors on the geodetic motion, we start with a
simple fi nite element model made of a 30-km-
thick homogeneous elastic plate driven by the
horizontal motion of the plates (Fig. 1). We
used the three-dimensional (3-D) noncommer-
cial fi nite element model software, ADELI 3D
(see http://www.dstu.univ-montp2.fr/PERSO/
chery/Adeli_web for details). Nubia plate sides
are thus locked (velocity = 0 mm/yr) and the
rigid rotation of Eurasia relative to Nubia is
applied on the Eurasian plate faces. We impose
the boundary conditions as a linear velocity
gradient on the western and eastern sides of
the model far from the study area, crossing the
diffuse seismicity between 36°N and 37°N that
defi nes the eastward prolongation of the Gloria
fault system and plate boundary zone (Fig. 1).
We estimate the fi t of the model by computing
the root mean square (RMS) residuals. This fi rst
experiment shows that crustal plate interaction
due to northwest-southeast Eurasia – Nubia
transpression explains quite well the Iberian
intra-plate motion (stations located up to the
lat. 38.2°N; Fig. 2A) and the motion of the sta-
tions located west of the Gibraltar Arc along
the Atlantic shoreline. The model velocities do
not fi t the observations in the Betics. East of the
Gibraltar Arc, signifi cant residuals of 2–6 mm/
yr are observed in the Rif (Fig. 2A). The RMSs
for the entire zone and for the Rif-Betics region
are summarized in Table 1.
In our second set of experiments, we investi-
gated the impact of a partial strength reduction
of the Alboran Sea and north Morocco sug-
gested by elevated heat fl ow (of 80–100 mW/
m
2
, e.g., Fernandez-Ibañez and Soto, 2008) on
the computed strain. Depending on the thermal
regime, the effective elastic thickness of conti-
nental plates varies from 3 to 80 km (Watts and
Burov, 2003). As average heat fl ow on continen-
tal areas is ~60 mW/m
2
, values of 80–100 mW/
m
2
may correspond to a thin mechanical litho-
sphere with strength limited to the seismogenic
zone (i.e., 6–12 km for the Alboran domain,
Fernandez-Ibañez and Soto, 2008). Therefore,
we can expect a ratio of ~10 between the Albo-
ran domain and the surrounding plates’ effective
elastic thickness. This effective elastic thick-
ness difference is equivalent to an elastic plate
of constant thickness with a factor of 10 differ-
ence in rigidity contrast (Chéry, 2008). One of
the best results is obtained with a low strength
zone limited to the region of high heat fl ow (> 80
mW/m
2
) in the Alboran Sea and adjacent areas
of the Rif (Fig. 2B and Table 1, experiment 2).
The model improves the RMS in the Betics, but
the misfi t in the Rif with respect to experiment
1 remains signifi cant, with residuals remaining
higher than the velocity uncertainties.
We ran a third set of experiments in which we
added to the previous model a horizontal trac-
tion beneath the elastic plate using a patch that
simulates the coupling between the upper plate
and the mantle at depth. We vary the patch size,
TABLE 1. SUMMARY OF THE NUMERICAL
EXPERIMENTS AND THEIR DATA FIT
Experiment
set
Weak plate
boundary
zone
Deep
traction
Best RMS*
(average)
Best
RMS
(zone)
1 NO NO 1.82 2.23
2 YES NO 1,63 1.98
3 YES YES 0.89 0.99
*RMS—root mean square.

GEOLOGY, September 2010 825
its location, and its velocity. The best model
corresponds to a traction applied to a small
surface (100 km × 50 km) located beneath the
External Rif zone at the limit of the weak plate
area (Fig. 2C). This model accounts for both the
motion toward the south-southwest observed in
the Rif and the clockwise rotation of velocity
vectors around the Gibraltar Arc. Moreover, the
traction area adjacent to the low strength region
of the Alboran Sea domain provides a signifi cant
change of the velocity fi eld in southeast Spain,
leading to close agreement of the modeled and
observed velocities in the Internal Betics.
DISCUSSION AND GEODYNAMIC
IMPLICATIONS
Our modeling experiments include no a pri-
ori information on sub-lithospheric geometry
and are designed to determine whether sub-
lithospheric processes are needed to account
for observed deformation of the western Medi-
terranean region. According to our best-fi tting
model, a traction patch applied beneath the elas-
tic plate is required to generate the unexpected
strain pattern indicated by the GPS velocity
eld (Fig. 2C), supporting the notion that either
sub-crustal or sub-lithospheric processes are
expressed in the surface displacements.
The small area of the coupling zone in our
model appears inconsistent with large-scale
mantle fl ow, and is more plausibly related to
phenomenon occurring not far from the Moho
(30 km depth in our model). Slab rollback,
that has already been proposed for the overall
formation of the Alboran Sea (e.g., Jolivet et
al., 2008), may be involved. However, geody-
namic models of the zone involving still active
westward rollback of the western Mediterra-
nean narrow slab (Gutscher et al., 2002) cannot
generate such a small coupling zone confi ned to
the External Rif. Spakman and Wortel (2004)
suggested that the western Mediterranean slab
is detached under the Betics. We further suggest
that the horizontal traction patch could represent
the remaining coupling zone between the slab
and the overlying continental lithosphere. This
region is located at the southern extremity of the
north-south–oriented lineament of intermediate
depth seismicity (60–120 km, Fig. 1) related to a
bend in the oceanic part of the slab (Fig. 3). This
old slab may be rolling back toward the south-
west, pulling on its upper extremities and leading
to the delamination of the lithospheric mantle in
the region of our patch. The high heat fl ow of
the region prevents the continental lithospheric
mantle from exhibiting brittle, seismic behavior.
Therefore, the patch would refl ect the interac-
tion between the delaminated African continen-
tal lithospheric mantle and the overlying plate
(Fig. 3). By analogy to subduction zones where
34°N
36°N
38°N
RMS zone = 1.98 mm/yr
352°E 354°E 356°E 358°E
RMS zone = 2.23 mm/yr
4 mm/yr
Modeled
Observed
Surface velocities :
354°E 356°E 358°E
ABC
352°E
34°N
36°N
38°N
3.6 mm/yr
RMS zone = 0.99 mm/yr
Patch velocity :
352°E 354°E 356°E 358°E
660 km
Base of the
overlying plate
Figure 3. Three-dimensional schematic diagram of the geodynamic model proposed in this
study based on tomographic studies (Calvert et al. 2000; Gutscher et al., 2002; Faccenna et
al., 2004 ; Spakman and Wortel, 2004). Base of the overlying lithosphere is schematically
represented as a plane. C.L—continental lithospheric mantle (green domain); O.L—oceanic
lithosphere (white domain). The shaded arrow represents the sinking of detached western
Mediterranean slab. Black arrow is the pull of the oceanic part of the slab at depth. Red
hachured area and red arrow are modeled traction patch (Fig. 2C) simulating the remain-
ing coupling interaction between the slab and the overlying lithosphere. Blue circles are
60–120 km depth seismicity (see Fig. 1).
Figure 2. Results of numerical modeling experiments. Boundary conditions are plotted in red in Figure 1. Shorelines are in black. RMS zone
is the root mean square calculated for global positioning system sites located in zone bound by the two dashed black lines. A: Northwest-
southeast Eurasia-Nubia transpression. Homogeneous rheology Young’s modulus E = 10
11
Pa (white domain) and Poisson’s ratio ν = 0.25.
B: Northwest-southeast Eurasia-Nubia transpression including Alboran Sea weak zone. Shaded zone: E = 10
10
Pa and ν = 0.25. C: Northwest-
southeast Eurasia-Nubia transpression including Alboran Sea weak zone and a horizontal basal traction. Rheology is the same as in B. Black
hatched area is the horizontal traction patch with a velocity of 3.6 mm/yr, 214°N directed (open black arrow), applied beneath the elastic plate
at 30 km depth.

826 GEOLOGY, September 2010
the slab is retreating, the coupling zone of our
patch would correspond to the forearc moving
toward the trench. Our modeling experiments
do not allow us to constrain the depth of the
delamination processes, which could be inside
the continental lithospheric mantle (Duggen et
al., 2008), at the Moho interface, or confi ned to
the crust at the brittle-ductile transition of the
lower crust (Gueydan et al., 2003). Since there
may not be full coupling between the depth of
the delamination and the surface, the 3.6 mm/
yr of patch displacement is the lower bound of
the delamination-induced horizontal velocities
at depth.
According to our delamination model (Fig. 3)
the traction zone is expected to move to the
south-southwest following propagation of the
delamination front. Identifying the faults that are
likely to accommodate this deep displacement is
not trivial. The surface boundaries are probably
located to the east on the northeast-southwest
trending Boussekkour fault associated with the
Al Hoceima earthquakes (Fig. 1), and to the
south on the southern thrust of the Rif, but to the
west, the locations remain unknown. During the
Pliocene-Quaternary, eastward subduction has
died out, as suggested by the accretionary wedge
sealed by undeformed sediments (Zitellini et
al., 2009). This reorganization of the tectonic
processes and the presumed south-southwest
motion of our traction patch may obscure any
surface of the western boundary. Further neotec-
tonic studies may help to clarify this point.
In conclusion, we suggest that the unexpected
velocity fi eld in the Rif-Betic zone is the result
of the combined effects of (1) long-term north-
west-southeast oblique convergence between
the Eurasian and the Nubian plates; (2) low
rigidity of the Alboran Sea domain; and (3) a
south-southwest–directed horizontal traction
applied beneath the External Rif (Fig. 2). We
suggest that this horizontal traction is due to the
rollback of the delaminated African continental
lithospheric mantle pulled by the sinking oce-
anic Western Mediterranean slab (Fig. 3).
ACKNOWLEDGMENTS
We thank G. Bokelmann, S. Lallemand and J.L.
Bodinier for their fruitful discussions, and to C. Fac-
cenna and fi ve anonymous reviewers for their con-
structive comments on this manuscript. Reilinger ben-
efi ted from a Visiting Researcher Fellowship from the
Observatoire de Recherche Méditerranéen en Envi-
ronnement of Montpellier while engaged in this study.
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Manuscript received 14 December 2009
Revised manuscript received 20 April 2010
Manuscript accepted 27 April 2010
Printed in USA
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Citations
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Journal ArticleDOI
Abstract: I combine recently published GPS results to derive a geodetic horizontal velocity field consistent at the scale of the Mediterranean and the surrounding Alpine belts. The velocity field is then used to discuss the boundary conditions around each major deforming area in the Mediterranean, to describe the main patterns of motion and deformation, to critically review the existing kinematics models and to finally point out the main unresolved kinematics questions. Strain rate in Europe north of the Alpines belt is dominated by the signature of the Glacial Isostatic Adjustment and tectonic strain remains below the current accuracy of GPS results. In the western Mediterranean, deformation is restricted to the Betics, the Alboran and the Morrocan Rif, with west-to-southwestward motion with respect to Iberia, which is part of stable Europe. Shortening, consistent with the relative Nubia/Eurasia plate motion, is found throughout the Maghrebides, but the distribution of deformation in northern Africa remains largely unknown. The central Mediterranean is dominated by the counter-clockwise rotation of the Adriatic. The junction between the southern Adriatic domain and Nubia has yet to be firmly established. The deformation over a wide area, east of the Maghrebides, in Tunisia and the gulf of Sirte in Libya still remains to be quantified. In the eastern Mediterranean, the velocity field is dominated by a general anti-clockwise rotation and a general trend towards the Hellenic trench, with velocity magnitude increasing with decreasing distances from the trench. This trend is observed not only in the Aegean and Anatolia, but also in the southern Balkans. Geodetic results emphasize that the convergence of the Nubia and Arabia plates towards Eurasia directly controls the deformation across only very few segments along the plate boundary zone. Additional processes are therefore required to explain the observed velocity field and deformation pattern.

289 citations


Journal ArticleDOI
Abstract: We present a geodynamic reconstruction of the Central–Western Mediterranean and neighboring areas during the last 50 Myr, including magmatological and tectonic observations. This area was interested by different styles of evolution and polarity of subduction zones influenced by the fragmented Mesozoic and Early Cenozoic paleogeography between Africa and Eurasia. Both oceanic and continental lithospheric plates were diachronously consumed along plate boundaries. The hinge of subducting slabs converged toward the upper plate in the double-vergent thick-skinned Alps–Betics and Dinarides, characterized by two slowly-subsiding foredeeps. The hinge diverged from the upper plate in the single-vergent thin-skinned Apennines–Maghrebides and Carpathians orogens, characterized by a single fast-subsiding foredeep. The retreating lithosphere deficit was compensated by asthenosphere upwelling and by the opening of several back-arc basins (the Ligurian–Provencal, Valencia Trough, Northern Algerian, Tyrrhenian and Pannonian basins). In our reconstruction, the W-directed Apennines–Maghrebides and Carpathians subductions nucleated along the retro-belt of the Alps and the Dinarides, respectively. The wide chemical composition of the igneous rocks emplaced during this tectonic evolution confirms a strong heterogeneity of the Mediterranean upper mantle and of the subducting plates. In the Apennine–Maghrebide and Carpathian systems the subduction-related igneous activity (mostly medium- to high-K calcalkaline melts) is commonly followed in time by mildly sodic alkaline and tholeiitic melts. The magmatic evolution of the Mediterranean area cannot be easily reconciled with simple magmatological models proposed for the Pacific subductions. This is most probably due to synchronous occurrence of several subduction zones that strongly perturbed the chemical composition of the upper mantle in the Mediterranean region and, above all, to the presence of ancient modifications related to past orogeneses. The classical approach of using the geochemical composition of igneous rocks to infer the coeval tectonic setting characteristics cannot be used in geologically complex systems like the Mediterranean area.

287 citations


01 Aug 2012
Abstract: We present a geodynamic reconstruction of the Central–Western Mediterranean and neighboring areas during the last 50 Myr, including magmatological and tectonic observations. This area was interested by different styles of evolution and polarity of subduction zones influenced by the fragmented Mesozoic and Early Cenozoic paleogeography between Africa and Eurasia. Both oceanic and continental lithospheric plates were diachronously consumed along plate boundaries. The hinge of subducting slabs converged toward the upper plate in the double-vergent thick-skinned Alps–Betics and Dinarides, characterized by two slowly-subsiding foredeeps. The hinge diverged from the upper plate in the single-vergent thin-skinned Apennines–Maghrebides and Carpathians orogens, characterized by a single fast-subsiding foredeep. The retreating lithosphere deficit was compensated by asthenosphere upwelling and by the opening of several back-arc basins (the Ligurian–Provencal, Valencia Trough, Northern Algerian, Tyrrhenian and Pannonian basins). In our reconstruction, the W-directed Apennines–Maghrebides and Carpathians subductions nucleated along the retro-belt of the Alps and the Dinarides, respectively. The wide chemical composition of the igneous rocks emplaced during this tectonic evolution confirms a strong heterogeneity of the Mediterranean upper mantle and of the subducting plates. In the Apennine–Maghrebide and Carpathian systems the subduction-related igneous activity (mostly medium- to high-K calcalkaline melts) is commonly followed in time by mildly sodic alkaline and tholeiitic melts. The magmatic evolution of the Mediterranean area cannot be easily reconciled with simple magmatological models proposed for the Pacific subductions. This is most probably due to synchronous occurrence of several subduction zones that strongly perturbed the chemical composition of the upper mantle in the Mediterranean region and, above all, to the presence of ancient modifications related to past orogeneses. The classical approach of using the geochemical composition of igneous rocks to infer the coeval tectonic setting characteristics cannot be used in geologically complex systems like the Mediterranean area.

281 citations


Cites background from "Active surface deformation and sub-..."

  • ...There are GPS and paleomagnetic evidences of active radial motion of the Betic and Riff nappes (Cifelli et al., 2008; Pérouse et al., 2010) and the tomography would also suggest an E-ward dipping slab (Gutscher et al., 2002; Spakman and Wortel, 2004)....

    [...]

  • ...There are GPS and paleomagnetic evidences of active radial motion of the Betic and Riff nappes (Cifelli et al., 2008; Pérouse et al., 2010) and the tomography would also suggest an E-ward dipping slab (Gutscher et al....

    [...]


01 Apr 2009
Abstract: The missing link in the plate boundary between Eurasia and Africa in the central Atlantic is presented and discussed. A set of almost linear and sub parallel dextral strike–slip faults, the SWIM1 Faults, that form a narrow band of deformation over a length of 600 km coincident with a small circle centred on the pole of rotation of Africa with respect to Eurasia, was mapped using a new swath bathymetry compilation available in the area offshore SW Portugal. These faults connect the Gloria Fault to the Rif–Tell Fault Zone, two segments of the plate boundary between Africa and Eurasia. The SWIM faults cut across the Gulf of Cadiz, in the Atlantic Ocean, where the 1755 Great Lisbon earthquake, M ~ 8.5–8.7, and tsunami were generated, providing a new insight on its source location.

279 citations


Journal ArticleDOI
Abstract: We use velocities from 65 continuous stations and 31 survey-mode GPS sites as well as kinematic modeling to investigate present day deformation along the Africa–Iberia plate boundary zone in the western Mediterranean region. The GPS velocity field shows southwestward motion of the central part of the Rif Mountains in northern Morocco with respect to Africa varying between 3.5 and 4.0 mm/yr, consistent with prior published results. Stations in the southwestern part of the Betic Mountains of southern Spain move west–southwest with respect to Eurasia (∼ 2–3 mm/yr). The western component of Betics motion is consistent with partial transfer of Nubia–Eurasia plate motion into the southern Betics. The southward component of Betics motion with respect to Iberia is kinematically consistent with south to southwest motion of the Rif Mountains with respect to Africa. We use block modeling, constrained by mapped surface faults and seismicity to estimate the geometry and rates of strain accumulation on plate boundary structures. Our preferred plate boundary geometry includes one block between Iberia and Africa including the SW Betics, Alboran Sea, and central Rif. This geometry provides a good fit to the observed motions, suggesting a wide transpressive boundary in the westernmost Mediterranean, with deformation mainly accommodated by the Gloria–Azores fault system to the West and the Rif–Tell lineament to the East. Block boundaries encompass aspects of earlier interpretations suggesting three main deformation styles: (i) extension along the NE–SW trending Trans-Alboran shear zone, (ii) dextral strike-slip in the Betics corresponding to a well defined E–W seismic lineament, and (iii) right lateral strike-slip motion extending West to the Azores and right-lateral motion with compression extending East along the Algerian Tell. We interpret differential motion in the Rif–Alboran–Betic system to be driven both by surface processes related the Africa–Eurasia oblique convergence and sub-crustal dynamic processes associated with the long history of subduction of the Neotethys ocean lithosphere. The dextral slip identified in the Betic Mountains in Southern Spain may be related to the offshore fault that produced the Great 1755 Lisbon Earthquake, and as such may represent a significant seismic hazard for the West Mediterranean region.

147 citations


Cites background from "Active surface deformation and sub-..."

  • ...…et al., 2006; Tahayt et al., 2008; Vernant et al., 2010)) emphasized the importance of dynamic processes below the crust for driving surface deformation, including delamination and southward rollback of the subducted African lithosphere beneath the Alboran/Rif domains (Perouse et al., 2010)....

    [...]

  • ..., 2010)) emphasized the importance of dynamic processes below the crust for driving surface deformation, including delamination and southward rollback of the subducted African lithosphere beneath the Alboran/Rif domains (Perouse et al., 2010)....

    [...]


References
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Journal ArticleDOI
Abstract: [1] The GPS-derived velocity field (1988–2005) for the zone of interaction of the Arabian, African (Nubian, Somalian), and Eurasian plates indicates counterclockwise rotation of a broad area of the Earth's surface including the Arabian plate, adjacent parts of the Zagros and central Iran, Turkey, and the Aegean/Peloponnesus relative to Eurasia at rates in the range of 20–30 mm/yr. This relatively rapid motion occurs within the framework of the slow-moving (∼5 mm/yr relative motions) Eurasian, Nubian, and Somalian plates. The circulatory pattern of motion increases in rate toward the Hellenic trench system. We develop an elastic block model to constrain present-day plate motions (relative Euler vectors), regional deformation within the interplate zone, and slip rates for major faults. Substantial areas of continental lithosphere within the region of plate interaction show coherent motion with internal deformations below ∼1–2 mm/yr, including central and eastern Anatolia (Turkey), the southwestern Aegean/Peloponnesus, the Lesser Caucasus, and Central Iran. Geodetic slip rates for major block-bounding structures are mostly comparable to geologic rates estimated for the most recent geological period (∼3–5 Myr). We find that the convergence of Arabia with Eurasia is accommodated in large part by lateral transport within the interior part of the collision zone and lithospheric shortening along the Caucasus and Zagros mountain belts around the periphery of the collision zone. In addition, we find that the principal boundary between the westerly moving Anatolian plate and Arabia (East Anatolian fault) is presently characterized by pure left-lateral strike slip with no fault-normal convergence. This implies that “extrusion” is not presently inducing westward motion of Anatolia. On the basis of the observed kinematics, we hypothesize that deformation in the Africa-Arabia-Eurasia collision zone is driven in large part by rollback of the subducting African lithosphere beneath the Hellenic and Cyprus trenches aided by slab pull on the southeastern side of the subducting Arabian plate along the Makran subduction zone. We further suggest that the separation of Arabia from Africa is a response to plate motions induced by active subduction.

1,427 citations


Journal ArticleDOI
Abstract: [1] The western Mediterranean subduction zone (WMSZ) extends from the northern Apennine to southern Spain and turns around forming the narrow and tight Calabrian and Gibraltar Arcs. The evolution of the WMSZ is characterized by a first phase of orogenic wedging followed, from 30 Ma on, by trench retreat and back-arc extension. Combining new and previous geological data, new tomographic images of the western Mediterranean mantle, and plate kinematics, we describe the evolution of the WMSZ during the last 35 Myr. Our reconstruction shows that the two arcs form by fragmentation of the 1500 km long WMSZ in small, narrow slabs. Once formed, these two narrow slabs retreat outward, producing back-arc extension and large scale rotation of the flanks, shaping the arcs. The Gibraltar Arc first formed during the middle Miocene, while the Calabrian Arc formed later, during the late Miocene-Pliocene. Despite the different paleogeographic settings, the mechanism of rupture and backward migration of the narrow slabs presents similarities on both sides of the western Mediterranean, suggesting that the slab deformation is also driven by lateral mantle flow that is particularly efficient in a restricted (upper mantle) style of mantle convection.

812 citations


"Active surface deformation and sub-..." refers background or methods in this paper

  • ...…plate boundary in the western Mediterranean and the geometry of the subducted plate are still debated (Calvert et al., 2000; Gutscher et al., 2002; Faccenna et al., 2004; Spakman and Wortel, 2004), we use an elastic plate model approach in order to evaluate the role of basal traction below the…...

    [...]

  • ...Tomographic studies (Calvert et al. 2000; Gutscher et al., 2002; Faccenna et al., 2004; Spakman and Wortel, 2004) reveal a narrow east dipping slab (100–200 km wide) located in the Gibraltar Arc....

    [...]

  • ...The tectonic evolution of the western Mediterranean and the Alboran Sea has been dominated by the long history of Nubia-Eurasia plate convergence associated with the subduction of the Neotethys Ocean (e.g., Faccenna et al., 2004)....

    [...]

  • ...The mechanism responsible for the continental and oceanic nature of the slab (delamination or continental subduction) is still debated (Faccenna et al. 2004; Duggen et al. 2008)....

    [...]

  • ...…(1) backarc extension driven by the westward rollback of an eastward subducting slab (Royden, 1993; Lonergan and White, 1997; Gutscher et al., 2002; Faccenna et al., 2004), which passively falls in the mantle driven by its own weight (subduction without convergence, Jolivet et al. 2008) ; (2)…...

    [...]


Journal ArticleDOI
01 Jun 1989-Geology
Abstract: Several features of the Alboran Sea suggest that it may have been a high collisional ridge in Paleogene time that subsequently underwent extensional-collapse, driving radial thrusting around the Gibraltar arc. (1) The basin is underlain by thin (13-20 km) continental crust, has an east-west-trending horst and graben morphology, was the locus of Neogene volcanism, and has subsided 2-4 km since the middle Miocene. (2) Extension and subsidence in the basin coincided in time with outwardly directed thrusting in the surrounding mountain chains. (3) Africa and Europe were converging slowly during this period, so extension must have been driven by internally generated forces. (4) Onshore, rocks metamorphosed at 40 km depth are exposed beneath major low-angle normal faults that separate them from low-grade rocks above. (5) Emplacement of solid bodies of Iherzolite at asthenospheric temperature into the base of the collisional edifice in late Oligocene time suggests detachment of the lithospheric root beneath the collision zone. This would have increased the surface elevation and the potential energy of the system and would have favored extensional collapse of the ridge.

789 citations


"Active surface deformation and sub-..." refers background in this paper

  • ...…1993); (3) crustal extrusion due to forces transmitted across the Eurasia-Africa plate boundary (Rebai et al., 1992); and (4) delamination and convective removal of the lithospheric mantle root beneath the collisional orogen (Platt and Vissers, 1989; Seber et al., 1996; Calvert et al., 2000)....

    [...]


Journal ArticleDOI
Abstract: Summary. For the purposes of describing its large-scale and long-term deformation, the continental lithosphere is regarded as a continuum, obeying a Newtonian or a power law rheology. The flow of a thin sheet of power law material overlying an inviscid substrate is studied under the assumption that vertical gradients of the horizontal velocity are negligible. A numerical model is used to investigate the deformation of such a sheet under conditions approximating those of continent-continent collision. The material flows in response to forces applied to its boundaries (for example, the indenting of one continent by another) and to forces in its interior arising from gradients in crustal thickness. The horizontal divergence of the flow produces changes in the crustal thickness and hence a time-dependent form to the flow itself. For a given set of boundary conditions, the flow depends on the stress exponent in the power law rheology, n, and on the Argand number& which is a measure of the ratio between the stress arising from crustal thickness contrasts and the stress required to deform the material at the ambient strain rates. When the effective viscosity of the medium is very high (Ar-+O), crustal thickness variations do not influence the flow. If the material is Newtonian (n = l), the deformation associated with an influx of material (approximating an indenter) is of much greater lateral dimension than the width of the indenter, whereas when material has a power law rheology (n = 3, 5 are used), the deformation is confined to a region of lateral extent comparable to that of the indenter. As Ar increases, the forces arising from crustal thickness contrasts exert more influence on the flow, and the maximum crustal thickness that can be sustained by a given influx of material is related by a simple expression to the effective viscosity of the medium at the ambient strain rates. In the limit of a very weak medium (Ar> 10) the lithosphere is unable to sustain appreciable crustal elevation contrasts. The results of these numerical experiments show that systems in which the effective viscosities are such that the maximum deviatoric stresses are between 1 kbar

787 citations


"Active surface deformation and sub-..." refers background in this paper

  • ...…interaction of plates (i.e., elastic strain and associated faulting, e.g Savage and Burford, 1973), sub-crustal processes (e.g., subduction and slab rollback, mantle delamination, Royden,1993), and/or stresses induced by gravitational potential energy differences (e.g., England and McKenzie 1982)....

    [...]


Journal ArticleDOI
Abstract: Geodetic data along the San Andreas fault between Parkfield and San Francisco, California (latitudes 36°N and 38°N, respectively), have been re-examined to estimate the current relative movement between the American and Pacific plates across the San Andreas fault system. The average relative right lateral motion is estimated to be 32 ± 5 mm/yr for the period 1907–1971. Between 36°N and 37°N it appears that most, if not all, of the plate motion is accommodated by fault creep. Although strain is presumably accumulating north of 37°N (San Francisco Bay area), the geodetic evidence for accumulation is not conclusive.

668 citations