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| Titel |
Thermal and rheological properties of the NW sector of the Adria microplate between Alps and Apennines (Northern Italy) |
| VerfasserIn |
Alfio Vigano', Bruno Della Vedova, Giorgio Ranalli, Silvana Martin, Davide Scafidi |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250036644
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| Zusammenfassung |
| The present structural setting of the NW sector of the Adria microplate, overridden by the
advancing fronts of the Alpine and Apennine chains from nearly opposite directions, is the
result of polyphase tectonic deformation beginning in the Late Cretaceous. The deformation
was accomodated at different times by S-verging (Alpine front) and N-verging (Appennine
front) thrust systems, in which fault patterns are strongly affected by inherited Mesozoic
extensional N-S oriented faults. We study the thermal and rheological properties of
the central part of the present Po Plain (approximately 44.5-45.7 Ë N lat, 9.4-11.2
Ë E long), which records the convergence of the Alpine and Apennine orogenic
fronts.
The present thermal regime of the crust is constrained by geological and geophysical
results from oil exploration. A set of 38 deep boreholes (Eni Exploration & Production)
provides lithology and temperature data (bottom hole, drill stem, and production test
temperatures) down to 6-7 km of depth. Bottom hole data were processed to estimate
undisturbed formation temperatures. The thermal conductivity was estimated from lithology
and logging data. The thermal resistance method was applied to verify the appropriateness of
purely conductive and steady-state heat transfer conditions. Temperature-depth
plots show two clearly distinguishable average geotherms, corresponding to the
Western (W) and Eastern (E) areas of the NW Adria microplate sector. The two zones
show significantly different crustal structures. The internal consistency of data in
each zone and the difference between the two groups of data, which is larger than
measurement uncertainties, confirm the validity of spatial zonation as a first-order working
hypothesis.
Temperature measurements in the upper ~7 km of the crust are used to constrain 1D
thermal models at the crustal scale. Although the Adria microplate in this area is
expected to exhibit lateral heterogeneities due to its complex 3D structure, we derive
simple averaged 1D geometries for each zone as a first approximation. The crustal
sections are simplified into four layers: molasse sediments, carbonatic series, upper
crystalline crust, and lower crust. In a first series of models, sensitivity analyses are
carried out for the variations of thermophysical parameters and lower boundary
conditions for selected 1D crustal sections. In a second series of models, the effect of
variable geometries for constant thermophysical parameters and boundary conditions
is examined. The best fitting geotherms for the two areas give estimated Moho
temperature ~100 Ë C higher in zone W than in zone E (620±80 Ë C and 520±70 Ë C,
respectively).
Using the estimated crustal structure and composition and the best-fitting geotherms,
strength envelopes are constructed, taking into account three deformation mechanisms
(frictional sliding, high-pressure failure, and power-law creep). The results, obtained for
hydrostatic and supra-hydrostatic pore pressures, show significant differences between zone
W and zone E in terms of depth to brittle-ductile transition, lower crust rheology, and total
crustal strength. The latter is always |
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