Presiding: J Park, Yale University; L Royden, Massachusetts Institute
of Technology
T51E-01 INVITED 08:00h
CAT/SCAN, the Calabria-Apennine-Tyrrhenian/Subduction
-Accretion-Collision Network
* Steckler, M S
(steckler@ldeo.columbia.edu) , Lamont-Doherty Earth Observatory of Columbia
University, 61 Route 9W, Palisades, NY 10964
Amato, A , Istituto
Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, Roma, 00143
Italy
Guerra, I , Dipartimento di Fisica, Universitˆ della Calabria Via
P. Bucci, Cosenza, 87040 Italy
Di Luccio, F , Istituto Nazionale di
Geofisica e Vulcanologia, Via di Vigna Murata, 605, Roma, 00143
Italy
Lerner-Lam, A , Lamont-Doherty Earth Observatory of Columbia
University, 61 Route 9W, Palisades, NY 10964
Persaud, P , Lamont-Doherty
Earth Observatory of Columbia University, 61 Route 9W, Palisades, NY 10964
Seeber,
L , Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W,
Palisades, NY 10964
Armbruster, J , Lamont-Doherty Earth Observatory of
Columbia University, 61 Route 9W, Palisades, NY 10964
Tolstoy, M ,
Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W,
Palisades, NY 10964
Cimini, G B , Istituto Nazionale di Geofisica e
Vulcanologia, Via di Vigna Murata, 605, Roma, 00143 Italy
Piromallo, C ,
Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, Roma,
00143 Italy
Di Maro, R , Istituto Nazionale di Geofisica e Vulcanologia,
Via di Vigna Murata, 605, Roma, 00143 Italy
D'Anna, G , Istituto
Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata, 605, Roma, 00143
Italy
Gervasi, A , Dipartimento di Fisica, Universitˆ della Calabria Via
P. Bucci, Cosenza, 87040 Italy
De Rose, C , Dipartimento di Fisica,
Universitˆ della Calabria Via P. Bucci, Cosenza, 87040 Italy
Calabria, the toe of Italy, is bordered on the south
by the Calabrian Arc with northward subduction of the Ionian Sea. Seismicity of
the subducting slab extends to 400 km depth, but is restricted to a compact arc
with well-defined edges. Calabria is an exotic block that rifted off from
Sardinia and Corsica opening the Tyrrhenian Sea backarc basin in its wake. The
rapid SE advance of this island arc with subduction ahead and extension behind
is believed to be driven by rollback of the old Mesozoic seafloor of the Ionian
Sea. Progressive collision of the arc with Adria formed the Apennines, the mountainous
backbone of Italy. Collision of the arc with Africa formed the Maghrebides in
Sicily. The Calabrian Arc is the last remaining piece of oceanic subduction.
However, the rapid advance of the arc has slowed, or possibly even halted, by
these collisions. Calabria and the surrounding region, meanwhile, have been
undergoing rapid uplift as a result of the evolving tectonics. Whether rollback
of the oceanic lower plate of the Ionian Sea continues and whether the upper
plate of Calabria continues to move as an independent plate are both uncertain.
The answer will provide insight into the relative forces involved in rollback
and collision. The progressive collisions on either side of Calabria have been
steadily decreasing the negative buoyancy of the subducting slab while
increasing collision resistance. CAT/SCAN, the
Calabria-Apennine-Tyrrhenian/Subduction-Accretion-Collision Network, is a
multidisciplinary effort to understand the tectonics and dynamics of the
region. The first phase is a passive seismic experiment designed to image the
structure of southern Italy. CAT/SCAN I consists of an array of 39 broadband
seismometers onshore, deployed in the winter of 2003/4 and 12 broadband Ocean
Bottom Seismometer deployed in Oct. 2004. All instruments will remain deployed
until Sept. 2005. Across strike, the experiment will determine the structure of
the entire Calabrian subduction and southern Apennine collision systems from
the incoming plate to the arc, slab and backarc spreading system. Along strike,
the experiment will map the structure of the transition from oceanic subduction
in Calabria to continental collision in the southern Apennines. The experiment
is also ideally positioned to detect proposed patterns of mantle flow related
to possible slab gaps and Tyrrhenian extension. We will discuss the goals of
the project, present and future experiments and initial results.
T51E-03 INVITED 08:30h
What is the link between slab retreat and
synconvergent extension in the Apennines, Italy?
* Brandon, M T
(mark.brandon@yale.edu) , Yale University, Department of Geology and
Geophysics, New Haven, CT 06520-8109 United States
Willett, S D
(swillett@u.washington.edu) , University of Washington, Department of Earth and
Space Sciences, Seattle, WA 98195-1310 United States
The Apennines Mountains in Italy represent a classic case
where plate convergence is accompanied by large-scale orogen-normal extension.
Intuition suggests that the observed extension must somehow be related to slab
retreat, but geodynamic models have generally failed to show the linkage
between these two phenomena, especially in cases, such as the Apennines, where
convergence is associated with accretion. The key issue in the Apennines is how
to account for the closely-spaced pattern of surface deformation, with
horizontal contraction on the Adriatic-Po side of the range and horizontal
extension on the Tyrrhenian side of the range. We approach this problem by
asking what velocity field is needed in the upper mantle to account for the
paired contractional and extensional belts observed at the surface of the
range. In this way, we consider the observed surface velocity field to be a
highly filtered image of the velocity field in the upper mantle, with the
crustal deformation in the Apennine wedge serving as the filter. We use a
standard finite-element model with both frictional and thermally-activated
viscous rheologies and coupled heat transport to represent the crustal
deformation in the Apennines wedge. We have investigated a range of deformation
scenarios, as represented by different basal velocity fields applied to an
upper plate stretched by slab retreat. In order to produce surface extension
that is localized and large, as observed in the Apennines, we find that
horizontal deformation in the upper mantle must be localized. Distributed
stretching at depth is not sufficient to induce lithospheric failure. To
produce extension closely juxtaposed with contraction, the extension must be
localized at a distance of about one crustal thickness from the S point, where
the slab subducts beneath the upper plate Moho. We speculate that this deep
localization of extension is controlled by two opposing corner flows, which is
the circulation predicted for asthenospheric mantle above a retreating slab.
The divergence in the flow at the top of these two corner flows may be
responsible for localizing extensional failure in the upper mantle.
T51E-04 08:45h
Buoyancy-Driven Deformations and Contemporary
Tectonic Stress in the Lithosphere Beneath North-Central Italy
Aoudia, K (aoudia@dst.units.it)
, SAND Group, the Abdus Salam International Centre for Theoretical Physics,
Strada Costiera 11, Trieste, 34100 Italy
Aoudia, K (aoudia@dst.units.it)
, Dipartimento di Scienze della Terra, Università degli Studi di
Trieste, via E. Weiss 1, Trieste, 34127 Italy
* Ismail-Zadeh, A
(aismail@mitp.ru) , SAND Group, the Abdus Salam International Centre for
Theoretical Physics, Strada Costiera 11, Trieste, 34100 Italy
*
Ismail-Zadeh, A (aismail@mitp.ru) , International Institute of Earthquake
Prediction Theory and Mathematical Geophysics, Russian Academy of Sciences,
Warshavskoye sh. 79-2, Moscow, 117556 Russian Federation
* Ismail-Zadeh,
A (aismail@mitp.ru) , Geophysikalisches Institut, Universitaet Karlsruhe,
Hertzstr. 16, Karlsruhe, 76187 Germany
Panza, G (panza@dst.units.it) ,
SAND Group, the Abdus Salam International Centre for Theoretical Physics,
Strada Costiera 11, Trieste, 34100 Italy
Panza, G (panza@dst.units.it) ,
Dipartimento di Scienze della Terra, Università degli Studi di
Trieste, via E. Weiss 1, Trieste, 34127 Italy
The juxtaposed contraction and extension, a
long-standing geological enigma recognized in the Italian Peninsula, has for a
long time attracted the attention of geoscientists. Several models, invoking
mainly external forces, have been put forward to explain the close association
of these two end-member deformation mechanisms clearly observed by geophysical,
geodetic and geological investigations. These models appeal to interactions
along plate margins or at the base of the lithosphere such as back-arc
extension or shear tractions from mantle flow or to subduction processes such
as slab roll back, retreat or pull and detachment. We combined seismic data
from Central Italy to image the crust and upper mantle and thus to understand
the complex lithospheric deformations observed in the Apennines. The well
developed low velocity zone in the uppermost mantle between the crust and the
underlying continental lithosphere supports the lithospheric delamination
beneath the peninsula and provides a new unifying background for geophysical,
petrological and geochemical studies of recent magmatism and volcanism. The
rate of the modeled lithospheric movement is in agreement with GPS data. The
pattern of the movement can explain the heat flux and the regional geology and
provide a new background for the genesis and age of the recent Tuscan
magmatism. The modeled stress in the lithosphere is spatially correlated with
gravitational potential energy patterns and show that internal buoyancy forces,
solely, can explain the coexisting regional contraction and extension and the
unusual intermediate depth seismicity.
T51E-05 09:00h
Variation of Seismic Coupling Along the Western
Hellenic Subduction with slab detachment and upper plate structure
* Laigle, M
(laigle@ipgp.jussieu.fr) , Sismologie Experimentale, Institut de Physique du
Globe de Paris, case 89, 4 Place Jussieu, Paris cedex 05, 75252 France
Sachpazi,
M (m.sachp@egelados.gein.noa.gr), Geodynamics Laboratory, National Observatory
of Athens, Lofos Nymfon, Athens, 00000 Greece
Hirn, A
(hirn@ipgp.jussieu.fr) , Sismologie Experimentale, Institut de Physique du
Globe de Paris, case 89, 4 Place Jussieu, Paris cedex 05, 75252 France
The highest seismic activity in Europe occurs
currently in the region of the Western Hellenic Arc. However, it has been
commonly considered as a largely aseismic subduction zone, from the comparison
of a small rate of shortening derived from the seismic moment release, with a
large rate of convergence inferred from geology. In the northern part, in the
Ionian Islands, we have recently showed instead that full seismic coupling
could be achieved accordingly to the model of control by plate tectonics
forces, with a trenchward motion of the upper plate, and the pull of the
broken-off slab that is here missing in the balance. We suggested that the
moderate moment rate can be related to a shallow downdip limit that is
documented for the seismogenic part of the subduction interplate using seismic
reflection profiling and local earthquakes tomography,in agreement with the
shallow depth (10-15 km) of the major historical subduction earthquakes. This
peculiarity may be attributed to the ductility of the lower crust of the upper
plate resulting from orogenic thickening, which also allows a decollement of
the upper crust and its overriding of the lower plate. This new view on the
seismic coupling of this region leads us to extend the discussion to the part
further South, from West of Peloponnesus to Western Crete, where major
Mediterranean earthquake occured in the past. There, we suggest that despite a
similar seismic moment release rate and trenchward upper plate velocity,
seismic coupling might be incomplete on the subduction interplate that would
have there a wider seismogenic part. This interpretation is based on analyses
of the structural conditions, plate tectonics forces as well as spatial- and
magnitude-distributions of interplate seismicity, that strongly change along
the Hellenic Arc. A cause may be the lateral addition of extra weight to the
slab pull force in this area, due to free-fall of the slab part that has broken
off from the surface farther north, and that seems to have remained attached to
the slab part beneath Crete. This increased slab pull would reduce the
compressive normal stress across the seismogenic part of the subduction
interplate and thus causes partial seismic coupling in its shallower part, that
may however participate to the megathrust earthquake rupture. Hints at
anomalies in structure and seismicity, which need to be resolved, may relate to
the present location of the edge of the tear in the slab.
T51E-06 09:15h
Subduction Zone Dynamics and the Recent Evolution
of the Hellenic Subduction System
* Royden, L H
(lhroyden@mit.edu) , Department of Earth, Atmospheric and Planetary Sciences,
M.I.T., MIT 54-826, Cambridge, MA 02139 United States
Papanikolaou, D
(lhroyden@mit.edu), Department of Geology/ University of Athens,
Panepistimioupoli Zografou, Athens, non 157 84 Greece
The Hellenic subduction system of western Greece and
Albania accommodates northeastward subduction of both oceanic and continental
lithosphere and offers an excellent opportunity to study the relationship among
subduction rate, trench retreat and density of subducted lithosphere. The
Hellenic system can be divided into a northern and a southern segment across a
~100 km dextral discontinuity corresponding to the west-southwest trending
Kephalonia Transform. To the north, continental crust ~25 km thick is subducted
at 5-10 mm/yr; to the south, oceanic crust is subducted at 35-40 mm/yr. A broad
zone of dextral shear (the Central Hellenic Shear Zone) divides the over-riding
lithosphere into northern and southern segments. Geological reconstructions
show that the Hellenic system formed a continuous thrust belt until late
Miocene time and shallow water, presumably continental, lithosphere was
subducted beneath the belt from ~30-10 Ma, at an average rate of 5-10 mm/yr.
Atfter ~10 Ma, oceanic lithosphere entered the southern Hellenic trench while
continental lithosphere continued to be subducted in the north. From ~10-0 Ma,
the subduction rate increased to ~40 mm/yr in the southern Hellenides but
remained largely unchanged in the north. Over the same period, the southern
trench migrated 100 km southwest relative to the northern segment of the
Hellenides, while the Kephalonia transform and the Central Hellenic Shear Zone
experienced ~100 km of dextral shear, continuing today at ~25 mm/yr. Dynamic
modeling shows that the increase in rate and magnitude of trench retreat in the
southern Hellenides is consistent with entry of dense oceanic lithosphere into
the southern trench at ~10 Ma, with the increased rate of retreat being due to
the increasing density of the subducted slab. Thus the Hellenic system exhibits
exhibits a spatial and a temporal variability in subduction that correlates
closely with the density of subducting lithosphere and strongly indicates the
major role of slab density in driving the subduction process and upper plate
deformation.
T51E-07 09:30h
Tectonic Accretion Channel (TAC): Its Role in the
Exhumation of HP Metamorphic Rocks in the Central Alps
* Brouwer, F M
(brouwer@geo.unibe.ch) , Institute of Geological Sciences, University of Bern,
Baltzerstrasse 1, Bern, CH-3012 Switzerland
Engi, M (engi@geo.unibe.ch) ,
Institute of Geological Sciences, University of Bern, Baltzerstrasse 1, Bern,
CH-3012 Switzerland
Berger, A (berger@geo.unibe.ch) , Institute of
Geological Sciences, University of Bern, Baltzerstrasse 1, Bern, CH-3012
Switzerland
In the Central Alps high-pressure metamorphic rocks
are confined to but a few tectonic units. In the Adula nappe pressures range
from about 1.25 GPa in the north, to 2.5 GPa in the south [1]. The Southern
Steep Belt (SSB) is essentially composed of a high-strain zone surfacing at the
contact between rocks deriving from Apulia (S) and Eurasia (N). The SSB
contains a tectonic composite of ortho- and paragneisses, with widespread
fragments of mafic and ultramafic composition. Many of the mafic lenses are garnet-amphibolites
or eclogites, with a highly variable degree of retrogression. Our petrological
studies indicate that the HP rocks in the SSB show extensive variation in
metamorphic pressure. In mafic fragments, pressures retained by assemblages
predating the amphibolite facies overprint range from 0.8 to 2.1 GPa, while
pressure estimates for some peridotites are $>$3 GPa. Some fragments show
evidence of substantial heating during decompression. Lu-Hf geochronology, in
conjunction with previously published data, indicates a spread in ages obtained
from the high-pressure metamorphic assemblage (63 to 36 Ma). Thermal models
based on simplified kinematics produce computed PTt histories that resemble
those documented in individual HP fragments [2]. The SSB, the Adula nappe, and
associated tectonic slices with HP-relics are interpreted to represent an
exhumed part of a Tectonic Accretion Channel (TAC, cf. [3]). This TAC is
assembled of numerous, relatively small fragments, which reflect a variety of
PTt-paths. The different residence times and exhumation rates reflect a
protracted history of subduction and extrusion, with substantial movement of
fragments with respect to their current neighbours. In the Central Alps all
HP-relics known to be Tertiary in age are contained in tectonic slices now
interpreted to be TAC fragments. The mechanisms responsible for the rapid
extrusion of deep portions of the TAC to mid-crustal levels and their
incorporation in the nappe stack remain controversial. However, it seems clear that
the decom-pression history of eclogites and garnet peridotites in the Central
Alps was initiated by disruption of the TAC. [1] Dale & Holland (2003) J.
Metam. Geol. 21: 813. [2] Roselle et al. (2002) Amer. J. Sci. 302: 381. [3]
Engi et al. (2001) Geology 29: 1143.
Author(s) (2004), Title, Eos Trans. AGU, 85(47),
Fall Meet. Suppl., Abstract #####-##.