Page 6 - Tavarnelli_et_al2003

324 E. TAVERNC:LLI ET ALI!
Extens ion a l C o ntract io n al
displacement (-) displacement (+)
1111(
a ~' 2 inferred stt·ain -hardening behaviour, we believe that some
2 :::l ;:o;: l ~ '
l post-orogenic extension simply because t11ey were not
1 1 ~:
~ 2 3\ \4 Km
' In summaty, our study shows that self-similarity is
: l ll l
b ()_.~ 2 0,5 ' 2m
0,5 o 1,5
Fig. 7 - Nonnalised plot of contractional and extensional displace-
ments (bere indicated as positiYe and negative) accommodated
by thrust and nonna! faults within the stn1ctur es illustmted in fig. 5.
C and E indicate the ctunulatiYe contractional and extensional
disp lacements.
- Rappresenta=ione normali==ara dei 1·igetti delle faglie inverse e diret- ACKNOWLEDGEMENTS
te, indicati rispettivamente con valo1·i positivi e negativi, all'interno
delle strutture di fig. 5. C ed E indicano i rigetti cwnulativi compres-
sivo e distensivo.
Sa-b). The distribution of cumulative contractional and
extensional displacement for faults at different scales also
shows a remal'kable consistency (fig. 7). This systematic
distribution, along with the geometrica! similarities out-
lined in fig. 5, indicate a self-similar behaviour for faults
of different scales within the composite structures inves-
tigated (TAVARNELLI et a/ii, 2003).
The recognition of similarities among structures at
different scales may bave important implications, in that
some critica! elements directly observed at the mesos-
copic scale could be used to defme better the shape of the
host larger structures, whose deep geometry is often
poorly constrained (e.g. see PEACOCK, 1995). For exam-
ple, the mesoscopic composite stn1cture at Punta Far·ag-
lione is entirely exposed and easily accessib le to direct
observation, whereas the deepest portion of its host mac-
roscopic structure (i. e. the deepest part of the Favignana
Thn1st) can only be inferred from indirect evidence.
Because of the inferred self-similar·ity, the details seen on
the kinematically related, smaller stuctures at Punta
Faraglione (figs. 2b, 4d) could help improve the interpre-
t.ation of the deep geometty of the regional fault system
of tlle Favignana Tluust (fig . lb) .
Documentation of pre-existing thrusts t11.mcated by
younger nonnal faults poses the question of why the for-
mer st11.1ctures were not reactivated dming negative inver-
sion. One primary controlling factor may depend on the
local attitude of the tl111.1st faults . The continental litho-
sphere is mechanically and rheologically heterogeneous,
and its defonnation involves mechanisms that may pro-
mote either strain-hardening or strai.n-weakening (HOLDS-
WORTH et aW, 200 1, and references therein). In general,
faults whose evolution was driven by stt·ain -weakening
mechanisms (e.g. crystal p lasticicty) are particularly prone
to reactivation (e.g. the Sahnon River suture zone ofl daho:
TIKOFF et a/ii, 2001), whereas faults that involve dilatancy,
frictional flow and hydrofracture veining are mechanically
more resistent to reactivation (HOLDSWORTH et alii, 200 1).
The th11.1sts examined in this study developed under brittle
to semi-brittle conditions, and were accompanied by pres-
sure-solution mechanisms. The related syn-tectonic h ydro-
fracture veining probably induced a stt·ain -hardening beha-
viour of the evolving thn1st sm·faces. Therefore, we argue
that the tluust faults were n ot reactivated because they
became mechanically strong during th11.1st emplacement,
and this prevented t11eir further remobilisation as nonnal
faults during negative tectonic inversion. Moreover , the
reactivation process appears controlled by the local atti-
tude of pre-existing faults. Therefore, in addition to the
tl111.1st faults were not reactivated as nonnal faults during
favourably oriented during negative inversion .
not unique to stn1ctures of different scales that were pro-
duced under one single defonnation regime (i.e. exten-
sional, contractional or strike-slip), as it is widely
reported (e.g. see ARBOLEYA & ENGELDER, 1995), but
rather can also be recognised for composite structures
resulting from superimposed deformations. These results
may ultimately provide useful analogues for quantitative
analysis of composite st11.1ctures produced during oro-
genie contraction and p ost-orogenic extension in the
Apermine-Maghrebide chain and in other collapsed oro-
gens, as well as for other superimposed defonnations
resulting from p ositive or negative tectonic inversion.
This research was suppot·ted by the University of Siena (PAR-
SERV. fund to E . TaYarnelli and COFIN 2000 fund to F.A. Decandia)
and by M IUR (60% grant toP. Renda) . We would like to thank Cri-
stina and Rino Grimaldi for providing accommodat ion and logist ics
during fieldwork. The constn•cti,·e reviews by Eugenio Turco and an
anonyntous referee are a lso thankfully acknowledged.
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