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Subduction rollback, slab breakoff, and induced strain in the
uppermost mantle beneath Italy
Francesco Pio Lucente
Lucia Margheriti
Istituto Nazionale di Geofi sica e Vulcanologia, Centro Nazionale Terremoti (CNT), Rome 00143, Italy
ABSTRACT
Differences in seismic anisotropy revealed by split SKS waves that traverse the upper mantle
beneath the Italian region reveal four areas of internally coherent anisotropic strength, provid-
ing evidence for mantle strain partitioning. When compared to uppermost mantle structure
imaged by tomography, the sequence of these areas displays a straightforward parallelism.
Under proper assumptions, the correspondence between areas of coherent splitting degree
and areas of coherent velocity perturbations offers new insights into the late evolution of sub-
duction in Italy and their effect on mantle strain.
the incoming rays (see Margheriti et al., 2003),
and their time delays (δt) were interpolated
using a nearest neighbor algorithm (Fig. 1). The
pattern obtained by interpolating the splitting
time delays reveals a sharp partitioning of δt
along the Italian region: δt values are relatively
high in the northern Apennines and in the south-
ern Tyrrhenian area, and are markedly lower
in the south-central Apennines. High δt values
are also observable in Sardinia. A straight cor-
respondence exists between areas of higher δt
and areas where subducting lithosphere in the
uppermost mantle beneath Italy was imaged
by
P-
wave tomography (Lucente et al., 1999;
Piromallo and Morelli, 2003) (Fig. 2). These
two sets of observations are almost uncorre-
lated, because teleseismic
P
waves typically
arrive with steep incidence, and anisotropy with
a horizontal symmetry, seen by SKS, would not
affect traveltimes greatly.
We divided our study area into subregions
based on the presence or absence of subduct-
ing lithosphere inferred from the mean
Vp
perturbation values in the uppermost 250 km
of the mantle calculated from the model of
Lucente et al. (1999) (Fig. 2). Three of the
subregions are along the Apennines (northern
Apennines, south-central Apennines, and the
southern Tyrrhenian area), which is the accre-
tionary wedge exposed during the last episodes
of the subduction process in Italy (Lucente
et al., 2006, and references therein). The fourth
subregion, the Sardinia area, was the locus of
this subduction between ca. 15 and 10 Ma,
during its retreat toward its current position
(e.g., Faccenna et al., 2001). This partitioning
was plotted on the splitting time delay map of
Figure 1, and mean quantitative attributes of
the splitting indicators in each of the subareas
were computed (Table 1 and Fig. 1).
The anisotropy fast axes, ϕ, are uniformly
oriented with an E-W trend only in the Sardinia
(box SI in Fig. 1) area, while along the Apen-
nines (boxes NA, SCA, and ST in Fig. 1), the
directions of ϕ cover almost the whole azimuth
range. The azimuthal spread of ϕ in the Apen-
nines likely refl ects the complexity of dynam-
ics (and strain) in the underlying mantle as a
response to the hinged kinematics of subduction
in its most recent stages (Lucente at al., 2006).
However, despite the “uniform variability” of
ϕ through the Apennines, the measured strength
of anisotropy, as indicated by the δt proxy, dif-
fers considerably between subareas (Fig. 1).
Keywords:
anisotropy, tomography, subduction, upper mantle, Italy.
INTRODUCTION
A powerful tool to quantify the strain in the
deep Earth is the measurement of anisotropy
from seismic wave propagation. In the upper
mantle, seismic anisotropy primarily results from
elastic anisotropy of olivine, which is its main and
most anisotropic mineral (Ben Ismail and Main-
price, 1998). Solid-state deformation of poly-
crystalline aggregates causes crystallographic
a
-axes [100] of olivine to align in the direction of
maximum extension (Zhang and Karato, 1995).
This property affects the propagation of seismic
waves, causing differential velocity, scattering,
and birefringence. Thus, quantifying these phe-
nomena serves to detail upper mantle anisotropy
structure. Measurement of anisotropy in the upper
mantle is commonly done by using the split-
ting of core-refracted shear waves SKS (Savage,
1999), which are nearly vertically incident and,
in the absence of anisotropy, would be radially
polarized (i.e., vibrating in the plane containing
the seismic ray). Anisotropy causes these waves
to split into two orthogonally polarized waves,
one traveling faster than the other. This splitting
effect produces two measurable quantities (Silver
and Chan, 1991): the azimuth, ϕ, of the fast wave
polarization plane, which is aligned with the
horizontal component of the average
a
-axis ori-
entation, and the time delay, δt, between fast and
slow polarized waves, which basically depends
on the quantity of anisotropy encountered and on
its orientation with respect to the ray path. Both
these observables have a direct relationship with
the crystallographic fabric of mantle rocks, and
hence with the mantle deformation; nevertheless,
retrieving mantle strain from splitting indicators
can be problematic: a trade-off exists between the
thickness of the anisotropic volume traversed and
the strength of intrinsic anisotropy, the location
of the anisotropy along the ray path is undefi ned,
and olivine fabrics with average
a
-axes oriented
along the Earth radius are almost invisible to
SKS splitting (i.e., vertical strain = no measurable
strain). Further complexities may arise because
of alternative causes of seismic anisotropy, such
as the infl uence of water on olivine deformation
(Jung and Karato, 2001), the effects of dynamic
recrystallization (Kaminski and Ribe, 2001), or
the presence of a signifi cant melt fraction (Holtz-
man et al., 2003). All these complications may
occur in subduction environments, where the
radial component of deformation is signifi cant,
and where chemical, physical, and kinematic
complexities are refl ected in an equally complex
variability in SKS splitting observations (i.e.,
Park and Levin, 2002).
Thus the equation,
splitting parameters =
anisotropy = mantle strain,
always requires a set
of a priori assumptions, and interpretations of
splitting observations in terms of mantle strain
mainly rely on ϕ, arguably the less ambigu-
ous of the two anisotropy indicators. This has
been true for previous studies of anisotropy in
the subduction zone extending along the Italian
peninsula (Margheriti et al., 2003; Civello and
Margheriti, 2004; Lucente at al., 2006). Here,
we reconsider the well-established set of SKS
splitting data in the Italian region, focusing on
splitting delay times, δt. This approach reveals
details of the distribution and partitioning of
anisotropy intensity in the area, and, given a set
of plausible assumptions, provides new clues
for interpreting the strain confi guration in the
upper mantle beneath Italy.
LOOKING AT SPLITTING DATA FROM
A
δ
t-CENTRIC POINT OF VIEW
The splitting data set we consider derives
from previous studies (Margheriti et al., 2003;
Civello and Margheriti, 2004; Lucente at al.,
2006), and includes more than 300 good-quality,
non-null, single-event measurements resulting
from analysis of SKS phases at 54 seismic sta-
tions located in Italy and Corsica.
Individual splitting measurements were pro-
jected to the 150-km-depth piercing point along
© 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
Geology
, May 2008; v. 36; no. 5; p. 375–378; doi: 10.1130/G24529A.1; 3 fi gures; 1 table.
375
GEOLOGY, May 2008
To assess the statistical signifi cance of the dif-
ferences between the populations of δt, we
derive the respective 95% confi dence inter-
vals (Table 1). At this confi dence level, the δt
population in the south-central Apennines box
has values signifi cantly lower than in the other
boxes, because its 95% confi dence interval
does not overlap the others (Table 1). As pre-
viously observed, the pattern of δt displays a
straightforward relationship with the confi gura-
tion of subduction in the upper 250 km of the
mantle beneath the Apennines (Fig. 2).
Next we examine the cause of the observed
anisotropy strength partitioning in terms of
strain induced in the mantle by the kinematics
of subduction.
the SKS splitting necessarily require an inter-
pretative framework. This is made by formulat-
ing a series of plausible a priori assumptions:
(1) Kinematics of the subduction zone beneath
the Italian region evolved during slab rollback.
The progressive eastward migration of the trench
(Fig. 2) causes the opening of the Tyrrhenian
basin starting at ca. 10 Ma (e.g., Malinverno
and Ryan, 1986). According to paleogeographic
reconstructions (Faccenna et al., 2001), the rate
of retreat increased from the northern to the
southern boundary of the subduction system
(upper left inset in Fig. 2), as a consequence of
changes in the type of lithosphere arriving at the
trench. This has obvious effects on the degree
of extension, which is higher in the southern
Tyrrhenian area (ST in Figs. 1 and 2).
(2) The anisotropy we observe (Fig. 1) is
caused by lattice-preferred orientation of olivine
crystals (LPO), induced by present and past
fl ow in the asthenosphere. The dominant source
of anisotropy is located in the uppermost mantle
(the anisotropic mantle as defi ned by Gaherty
and Jordan [1995]), and contributions from the
Apennine lithosphere and the Adriatic-Ionian
subducting plate are subordinate.
(3) The anisotropic fabric acquired by the
upper mantle at a given time is preserved as long
as no tectonic and/or thermal episode occurred
subsequently (Vauchez et al., 1999).
(4) The nature, composition, and thickness of
the anisotropic mantle do not vary signifi cantly
along the subduction system; therefore, coef-
fi cients in the equation
splitting parameters =
anisotropy = mantle strain,
remain constant
through the study area.
(5) Finally, we do not enter into the olivine
fabric types argument (e.g., Kneller at al.,
2007), because we do not have grounds to
discuss this topic.
FORMULATING A SET OF PLAUSIBLE
A PRIORI ASSUMPTIONS
Due to the intrinsic ambiguity of SKS split-
ting quantities as strain indicators, any infer-
ences concerning mantle deformation based on
POSSIBLE CAUSES OF
δ
t
PARTITIONING: QUALITATIVE
INTERPRETATIONS IN TERMS
OF MANTLE STRAIN
Hypothetical causes of the observed aniso-
tropic strength partitioning imply assigning a
geodynamic signifi cance to the combination of
splitting delay times (Fig. 1) and
Vp
anomalies
(Fig. 2) in each of the study subregions, and
reconciling them to a picture consistent with
the supposed geodynamic evolution of this part
of the Mediterranean. We consider the splitting
observed in the Sardinia area (SI in Fig. 1) as
representative of the horizontal strain induced in
the backarc mantle by subduction in its “steady-
state” phase. We assume the subduction to be
in a “steady-state” before the slab fragmentation
episodes documented by tomography (Fig. 2)
occurred as a response to the heterogeneous
nature (continental or oceanic) of the lithosphere
arriving at the trench along the Apenninic mar-
gin (Faccenna et al., 2004). To establish this
link, we rely on the azimuthal property of the
splitting. In particular, the close match between
the uniformly E-W–oriented ϕ in Sardinia (SI)
and the prevailing direction of extension in
the Tyrrhenian basin indicate the importance
of the subduction rollback-type kinematics for
the state of strain in the mantle. Beginning in
Tortonian time (ca. 10 Ma), the Tyrrhenian
basin opened with a northward decreasing rate
of extension (Faccenna et al., 2001) as a con-
sequence of the counterclockwise retreat of
the trench, whose strike changed from ~N-S to
NW-SE at present (upper left inset in Fig. 2).
Given our assumptions, these kinematics should
have affected the state of deformation of the sur-
rounding mantle proportionally, i.e., generating
a progressively lower amount of the horizontal
strain (and hence δt) going from the southern
boundary (southern Tyrrhenian) of the subduc-
tion system to the northern boundary (north-
ern Apennines). Splitting δt values are indeed
N
12°
14°
16°
3.2
3.0
2.8
Tyrrhenian
sea
2.6
2.4
2.2
2.0
1.8
1.6
NA
1.4
1.2
SCA
1.0
0.8
0.6
ST
0.4
SI
0.2
0.0
Figure 1. Map of the SKS splitting time delays (
δ
t) in the Italy region. Individual splitting time
delays (white dots) were interpolated using a nearest neighbor algorithm which averages,
over a 10
′
(latitude) × 10
′
(longitude) grid, measurements within a 1° radius, if there is at
least one value inside each of the three 120° azimuthal sectors. Boxes NA (northern Apen-
nines), SCA (south-central Apennines), ST (southern Tyrrhenian), and SI (Sardinia area) are
imported from Figure 2. Rose diagrams show SKS fast directions in each of the four boxes:
the length of each petal is proportional to the average
δ
t in the corresponding direction.
Measurements included in rose diagrams and in Table 1 are circled.
376
GEOLOGY, May 2008
highest in the southern Tyrrhenian area, where
the mean value equals the mean δt in Sardinia,
and decrease northward (northern Apennines
and south-central Apennines; see Table 1). How-
ever, splitting delays in the south-central Apen-
nines are lower than in the northern Apennines.
This breaks the symmetry between the rollback
amount (and corresponding extension in the
Tyrrhenian basin; upper left panel in Fig. 2) and
horizontal mantle strain, since we would expect
a mean δt in the range between 1.84 s (southern
Tyrrhenian) and 1.60 s (northern Apennines)
for the south-central Apennines, where it is
much lower (1.35 s; see Table 1).
This apparent inconsistency is counter-
balanced by the close correspondence between
areas with relatively high δt values and areas
where subduction is continuous up to the sur-
face (cf. Figs. 1 and 2). Together, these incon-
sistencies and correspondences furnish a key
to explain our observations. Based on the last
evolutionary phases of the subduction sys-
tem as inferred from the tomographic images
(Fig. 2), and organizing our observations in a
logical sequence, we propose, as preferred, the
following scenario:
(1) The Tyrrhenian opens as a backarc basin
of the subduction zone, which at present is
located below the Italian peninsula (Fig. 2).
(2) The eastward retreat of the subduction
system, in a regime of slab rollback, causes the
horizontal straining of the uppermost mantle
in a corresponding direction, which is repre-
sented by the splitting parameters in Sardinia
(SI in Fig. 1).
(3) The migration of the subduction system
follows a counterclockwise trajectory (upper
left inset in Fig. 2), implying a differential hori-
zontal straining of the uppermost mantle, which
is refl ected in the relatively higher δt values in
the southern portion of the subduction system
(ST in Fig. 1) with respect to the northernmost
ones (NA and SCA in Fig. 1).
(4) The arrival of continental material at the
trench in the south-central Apennines leads to the
segmentation of the subducting plate (Lucente
and Speranza, 2001), and while the northern
Apennine and southern Tyrrhenian narrow seg-
ments continue their backward migration at dif-
ferent speeds, in the south-central Apennines,
slab breaks off and starts to sink passively into
the mantle (Fig. 3).
(5) The further retreat of the northern Apen-
nine and southern Tyrrhenian slab fragments
maintains the deformation in their neighboring
mantle prevalently horizontally, while the gravi-
tational descent of the south-central Apennines
N
250–550 km depth range
10°
1
2°
1
4°
1
6°
NA
SCA
4
3
1
2
ST
1
SI
0
%
–1
–2
35–250 km depth range
–3
–4
Figure 2. Tomographic map of the mean
Vp
perturbation values in the uppermost 250 km
of the mantle beneath Italy calculated from the model of Lucente et al. (1999). Boxes divide
the mapped area in subregions based on the presence or absence of subducting litho-
sphere. In the upper right inset, the mean
Vp
perturbation values, in the 250- to 550-km-depth
range, reveal a continuous subduction zone running along the Apennines. In the upper left
inset, the notched black line represents the present-day location of the trench, while the
dashed gray lines approximately indicate its past positions at two stages of the retreating
motion (from Faccenna et al., 2001). Red arrows represent the direction and the amount of
extension—or trench retreat—in the Tyrrhenian basin, increasing from the northern to the
southern bounds of the subduction system. NA—northern Apennines; SCA—south-central
Apennines; ST—southern Tyrrhenian; SI—Sardinia area.
NA
SCA
S
T
NA
SCA
STSZ
TABLE 1. STATISTICAL ATTRIBUTES OF THE SPLITTING INDICATOR
δ
t IN THE STUDY AREA
0.0
1.0
2.0
3.0
δ
t (s)
Number of
Mean time
Standard
95% confi dence
Data set
measurements (n)
delay (
δ
t) (s)
deviation (
α
) (s)
interval
Figure 3. Simplifi ed sketch illustrating the
correlation between the subduction confi gu-
ration beneath Italy, as deduced by tomog-
raphy, and the partitioned distribution of the
SKS splitting time delays (color scale as in
Fig. 1). Arrows represent, in our interpre-
tation, the directions wherein subduction
kinematics strain the upper mantle (red—
horizontal; blue—vertical).
Whole data set
309
1.59
0.55
1.53–1.65
NA
46
1.60
0.54
1.44–1.76
SCA
137
1.35
0.51
1.27–1.43
ST
65
1.84
0.49
1.72–1.96
SI
61
1.84
0.50
1.72–1.96
(
)
(
)
is the standard
error of the mean (SE), and 1.96 is the 0.975 quantile of the normal distribution. NA—northern Apennines;
SCA—south-central Apennines; ST—southern Tyrrhenian; SI—Sardinia area.
Note
: The 95% confi dence interval is defi ned as: mean (
δ
t) ±
σ
n
∗
. , where
σ
n
GEOLOGY, May 2008
377
196
slab fragment strains the mantle in the vertical
direction (Fig. 3), thus inducing a reorganization
of the olivine fabric accordingly.
(6) Given the assumption that the thickness of
the anisotropic layer does not vary, the process
of reorientation of the olivine fast axes in the
vertical direction erases part of the previously
horizontal fabrics (which are those seen by
SKS waves), producing the relatively lower δt
observed in the south-central Apennines sub-
region (Fig. 3). Assuming a mean δt value of
~1.7 s for the south-central Apennines (inter-
mediate between mean δt values in the northern
Apennines and the southern Tyrrhenian area;
cf. Table 1) before the slab fragment sinking
process starts to infl uence the mantle deforma-
tion, and since the relationship between S-wave
anisotropy and olivine fabric strength is almost
linear in the range of anisotropy percentages gen-
erally assumed for the upper mantle (up to 7%;
Ben Ismail and Mainprice, 1998; Savage, 1999),
we can derive a rough estimate of the percentage
of fast olivine axes reoriented in a vertical direc-
tion that is approximately the same amount as
the δt defi cit (~20%). It is noteworthy that, in this
hypothesis, the vertical orientation of fast axes
would affect the result of the isotropic inversion
(Fig. 2) by speeding up the teleseismic rays at
the south-central Apennines with respect to the
neighboring subareas. As an outcome, the differ-
ence in the isotropic elastic properties between
the subducting lithosphere (in the northern Apen-
nines and the southern Tyrrhenian area) and the
ambient mantle (in the south-central Apennines)
in Italy is possibly larger than that inferred by the
P-
wave velocity tomography.
Within the framework settled by our a priori
assumptions, alternative hypotheses are: 1) The
retreat of the slab fragment in the south-central
Apennines stopped farther west of the area
sampled by the SKS observations reported in
Figure 1; therefore, differences in splitting delays
are simply related to the longer retreat experi-
enced both in space and in time, by the northern
Apennines and southern Tyrrhenian areas. 2) The
slab fragments imaged in the northern Apennines
and the southern Tyrrhenian were once joined,
forming a continuous subducting body. At some
point, this body underwent a lateral tear process,
which led to the current confi guration through
the progressive rollback of two separated slab
fragments. Also, in this case, the lower δt values
in the south-central Apennines result from the
absence, in this area, of the process responsible
for the horizontal deformation in the mantle, i.e.,
the retreat of the slab. Lateral tears in the slab
would also account for the lengthening of the
Apennine trench during its eastward migration
(upper left inset in Fig. 2). Although we cannot
exclude that alternative mechanisms concurred
with the anisotropy signature we observe, these
hypotheses are diffi cult to reconcile with both
surface and deep expressions of the subduction
beneath the Apennines: the presence of an almost
continuous subduction-related magmatic arc
along the Tyrrhenian margin of Italy (Serri et al.,
1993) implies that slab reached and possibly
passed beyond this margin also in the south-central
Apennines; the slab at depth does not show off-
sets along its strike, running almost parallel to the
Tyrrhenian coast for its entire length (upper right
inset in Fig. 2); also, the total length of the slab at
depth is greater than the combined length of the
northern Apennine and southern Tyrrhenian slab
fragments (upper right inset in Fig. 2).
Therefore, we believe that our preferred sce-
nario is more consistent than the other hypoth-
eses discussed. It can provide a reliable, if only
qualitative, explanation for the mantle strain
partitioning implied by the SKS splitting time
delays in the Italian region, and can be used
as a basis for more sophisticated, quantitative
analyses, as, for instance, on the time scales
for the evolution of anisotropic fabric in the
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ACKNOWLEDGMENTS
We are greatly indebted to Ray Russo and
Martha Savage for extremely constructive reviews and
comments. We also thank Silvia Pondrelli, Antonio
Piersanti, and Alberto Basili for useful discussions.
We are grateful to Prof. Maurizio Parotto.
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Printed in USA
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(1590 KB)
Taiwan.pdf
(3825 KB)
Taiwan SE accret.pdf
(4916 KB)
Taiwan deformat.pdf
(4083 KB)
Taiwan collision.pdf
(2913 KB)
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Literatura dodatkowa
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