Active stress along the ne external margin of the Apennines: the Ferrara arc, northern Italy

\ PERGAMON

Geodynamics 17 "0888# 140Ð154

Active stress along the NE external margin of the Apennines] the Ferrara arc\ northern Italy Paola Montone\ M[ Teresa Mariucci Istituto Nazionale di Geo_sica\ Via di Vigna Murata\ 594 99032 Rome\ Italy Received 19 February 0887^ received in revised form 5 November 0887^ accepted 5 November 0887

Abstract We have analysed borehole breakout data from 01 deep wells in order to constrain the direction of the minimum and maximum horizontal stress in a part of the Po Plain\ northern Italy\ characterised by a ½NÐ S prevailing compressional stress regime\ and in order to shed light on the regional state of stress and on the correlation between the active stress _eld and the orientation of tectonic structures[ The results have been compared with seismological data relating to 0877Ð0884 crustal seismicity "1[4³Md³3[7# and to the 0872 Parma "Ms4[9# and the 0885 Reggio Emilia "Ms4[0# events[ Plio! Pleistocene mesostructural data are also described in order to better de_ne the present!day stress _eld and to understand the active tectonic processes in particular stress provinces[ The borehole breakout analysis\ in accordance with the seismicity and mesostructural data\ shows the presence of a predominant compression area\ characterised by approximately NÐS maximum horizontal stress\ along the outer thrust of the Ferrara arc[ Particularly\ the breakout analysis indicates a minimum horizontal stress\ N70W211> relative to a total of eleven analysed wells\ with 2635 m cumulative total length of breakout zones[ Among these\ nine wells are located in the same tectonic structure\ consisting of an arc of asymmetric folds overthrust towards the NE[ The breakout results for these wells are quite similar in terms of minimum horizontal stress direction "½EÐW oriented#[ The other two wells are located in the outside sector of the arc and one of them shows a di}erent minimum horizontal stress direction\ probably distinctive of another tectonic unit[ On the basis of these new reliable stress indicators\ the active compressive front in this area is located along the termination of the external northern Apenninic arc[ Þ 0888 Elsevier Science Ltd[ All rights reserved[

0[ Introduction The tectonic setting of the Apennines is due to the convergence between the African and European plates which has occurred since the Late Cretaceous "Robertson and Grasso\ 0884#[ The  Corresponding author[ Fax] ¦9928!95!4930070^ e!mail] montoneÝing649[ingrm[it

9153!2696:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[ PII] S 9 1 5 3 ! 2 6 9 6 " 8 7 # 9 9 9 3 0 ! 5

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Fig[ 0[ Tectonic setting of northern Italy[ The investigated area includes the Emilia arc and the FerraraÐRomagna arc[ Modi_ed in accordance with Ambrosetti et al[\ 0876a^ Bigi et al[\ 0878[

northern Apennines fold and thrust belt is the result of this collision developed prevalently in Neogene times "Figure 0#[ The tectonic evolution of the northern Apennines belt is linked to the migration of deformation\ from west to east toward the Adriatic foreland domain\ as a consequence of the NWÐW dipping retreating subduction of the Adria lithosphere "Malinverno and Ryan\ 0875^ Jolivet et al[\ 0887#[ In this context\ extension in the western part "Tuscany# and compression more to the east have been contemporaneously active and have migrated toward the foreland from the late Miocene to Recent "Elter et al[\ 0864^ Carmignani and Klig_eld\ 0889^ Carmignani et al[\ 0884^ Jolivet et al[\ 0887#[ Therefore\ the back!arc extension in the Tyrrhenian sea has been synchronous with the thrust emplacement[ In the Italian peninsula\ the zones of active compression are generally located along the outer thrust fronts of the northern Apennines and along the external area of the Calabrian arc "Patacca and Scandone\ 0878#\ while active extension with normal faulting stress regime a}ects the Apenninic belt "Anderson and Jackson\ 0876 among many others#[ The state of active stress along the external margin of the northern Apennines is not well de_ned[ Geological data indicate few compressional structures in Recent units along the Pede!Apenninic thrust front\ while seismic pro_les show compressive tectonic structures sealed by upper PlioceneÐ Quaternary deposits "Pieri and Groppi\ 0870#[ The lack of other reliable active stress indicators is evident if we look at the di}erent interpretations given on the location of the active front[ Some authors "e[g[ Boccaletti et al[\ 0874# consider the area of active compression to be located between the Pede!Apenninic thrust fault and the buried arcuate structures "Fig[ 0#^ Vai "0878# suggests that the active compression is limited to the Pede!Apenninic margin as a consequence of out!of! sequence processes[ Conversely\ a recent paper "Bertotti et al[\ 0886# shows that the external margin\ close to Bologna\ has undergone extension at least since the middle Pleistocene[ According

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to this hypothesis\ the data demonstrating a compressive regime would be related to local per! turbations produced by the ~exural bending of the underlying plate "Bertotti et al[\ 0886#[ New geophysical data can presently be considered to better de_ne the tectonic evolution of this area[ The purpose of this paper is to verify the state of stress in this region and to determine the relationship between the active stress _eld and the tectonic structures orientation[ In the _rst part\ we describe the available results relative to a structural analysis performed on Plio!Quaternary deposits and the main characteristics of the crustal seismicity which occurred in the study area[ We then discuss the results inferred from breakout analysis with respect to the Recent tectonic evolution of the region\ trying to restrict the position of the active front and the stress regime\ and to detect the presence of local stress perturbations of the regional stress _eld[ The available Italian data set of stress orientations "Rebai et al[\ 0881^ Muller et al[\ 0881^ Montone et al[\ 0886# shows that broad regions are subjected to rather uniform regional stress[ In northern Italy\ along the outer front of the Adriatic margin\ a NW!minimum horizontal stress "Shmin# direction parallel to the main fold and thrust structures could indicate a present!day compression toward the foreland while\ more to the south\ the active stress _eld shows a NEÐShmin direction\ probably related to an active extension\ as also evidenced in the southern Apennines "Amato et al[\ 0884^ Amato and Montone\ 0886^ Mariucci et al[\ 0887#[

1[ Geology and seismicity of the study area The knowledge of the geology and tectonics of the Po Plain derives from the intense oil exploration of this area that AGIP "now ENI S[p[A[# has been performing for thirty years\ with many seismic lines and several deep wells[ The Po Plain is covered by Recent alluvial deposits and by a thick cover of terrigenous Plio! Pleistocene sediments "up to 7 km# originating from the Alpine and Apenninic belts "Figure 1#[ This initially wide area of the Po Plain has been reduced by compressive tectonics from the Miocene to Quaternary times[ The compressional tectonic phases have produced asymmetric folds\ overthrust towards the NE or NNE\ involving both the terrigenous sedimentary cover and the carbonate Mesozoic sequence "Ghelardoni\ 0854#[ In the Po Plain\ we can distinguish "Fig[ 0# the South alpine folds in the northern part and the Pedealpine homocline in the central part[ In the southern part\ three main folded arcs can be found\ namely\ from west to east] the Monferrato arc\ the Emilia arc and the FerraraÐRomagna arc[ These three arcs represent\ at the broad scale\ the termination of the external northern Apenninic arc[ The area where the breakout analysis was performed includes the FerraraÐRomagna arc\ the Emilia arc and the weakly deformed foredeep "Figures 0Ð1#[ The Emilia arc is thrust onto the Pedealpine homocline and\ on its eastern part\ onto the FerraraÐRomagna folded arc[ The FerraraÐRomagna arc is delimited externally by reverse faults which separate it from the Pedealpine homocline "Fig[ 0#[ It is possible to subdivide the structure of FerraraÐRomagna into three relatively minor groups] Ferrara folds\ Romagna folds and\ more to the east\ Adriatic folds "Fig[ 1#[ The Ferrara folds are buried beneath a reduced thickness of Plio! Pleistocene terrigenous deposits "about 199 m# and consist of carbonate Mesozoic sequence strongly involved in Neogene and Quaternary tectonic phases[ The Romagna folds are the more internal

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Fig[ 1[ Top] Location of the 01 wells considered "black circles#\ as compared to the main tectonic structures\ and to the base of Pliocene isobaths "grey lines#[ The rectangular area is the boundary of Fig[ 5[ Bottom] two orthogonal sections showing the deep tectonic structures of the area[ Simpli_ed from Pieri and Groppi\ 0870[

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structure of the arc "Fig[ 1#\ characterized by a suite of folds of the clastic Tertiary formations\ completely independent from the underlying Mesozoic limestones "Pieri and Groppi\ 0870#[ The arcuate shape observable along the Padana folded arcs could be due to hypothesized transversal NEÐSW oriented lineaments that accompanied the emplacement of the arcs "Pieri and Groppi\ 0870#[ The presence of the arcs is also interpreted as being due to di}erent shortening values\ increasing from west to east "Pieri and Groppi\ 0870^ Gasperi et al[\ 0874^ Royden et al[\ 0876#[ Another hypothesis about the generation of the arc structures invokes the presence of rigid obstacles\ such as intrusive volcanic bodies or the elevation of crystalline basement "Perotti\ 0880^ Castellarin and Vai\ 0875#[ The arcuate shape is also interpreted from paleomagnetic data "referred to the late MioceneÐearly Pliocene# as an orocline\ formed from the bending of a previous straight belt and\ consequently\ to the thrust emplacement "Speranza et al[\ 0886#[ No paleomagnetic data on the post early Pliocene are available[ The progressive shortening observed in this sector of the Po Plain is due to the subduction mechanism of the AdriaticÐIonian lithosphere beneath the northern Apennines[ The retreating subduction\ caused by gravitational sinking\ has controlled the extensionÐcompression pair deforming the Meso!Cenozoic Apenninic belt "Castellarin and Vai\ 0875^ Malinverno and Ryan\ 0875^ Royden et al[\ 0876^ Patacca and Scandone\ 0878#[ The presence of remnant slab beneath the northern Apennines is well constrained by subcrustal earthquakes and tomography images that evidence the subducted lithosphere down to about 299 km "Selvaggi and Amato\ 0881^ Amato et al[\ 0882#[ Data on active stress concerning structural analysis and earthquake focal solutions in the Po Plain show compressional directions from the NE to the NNW\ due to the presence of structural arcs "Perotti\ 0880^ Perotti and Vercesi\ 0886#[ In particular\ structural analysis of Plio!Pleistocene marine deposits shows strikeÐslip faults and\ subordinately\ thrust faults with the maximum compression NNWÐSSE oriented "Perotti\ 0880 and references therein#[ The age of the last compressive tectonic phase\ determined on continental Pleistocene deposits\ is post middle Pleistocene "9[1 Ma#\ according to the paleo!stress _eld and to the active stress _eld "Bernini and Clerici\ 0872#[ Marabini et al[ "0875# provide evidence of recent compressive tectonics along a recently reactivated WNW!fault in post!Wurm deposits located SE of Bologna[ From a seismological point of view\ the study area is characterized by a low level of background seismicity\ with a maximum concentration of events along the Apenninic belt "Gasparini et al[\ 0874^ Cocco et al[\ 0882^ Frepoli and Amato\ 0886#\ but also scattered in the outer thrust fronts of the Po Plain "Figure 2#[ The focal mechanisms of moderate 0877Ð0884 crustal earthquakes show strikeÐslip and thrust focal plane solutions with horizontal P!axes ½NWÐSE oriented and\ subordinately\ NEÐSW oriented "Frepoli and Amato\ 0886#[ The most remarkable recent events are the 0872 Parma earthquake "Ms4[9# and the 0885 Reggio Emilia earthquake "Ms4[0# "Fig[ 2#[ The focal mechanism of the Parma event\ computed with the Centroid Moment Tensor "CMT Harvard Catalog\ Dziewonski et al[\ 0872# technique\ shows a thrust solution with a P!axis oriented N25W\ plunging 05> towards the NW[ The Reggio Emilia CMT fault plane solution indicates a thrust regime with a small strikeÐslip component and P!axes N13W oriented\ plunging 0> towards the NW[ The results of stress inversion from 45 best aftershocks of the Reggio Emilia event reveal an horizontal N06W oriented s0 "Azzara et al[\ 0886#[

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Fig[ 2[ Northern and Central Italy 0877Ð0884 crustal seismicity "Frepoli and Amato\ 0886#[ In the _gure\ two focal plane solutions relating to the 0872 Parma and 0885 Reggio Emilia events "CMT data\ Harvard Catalog# are also indicated[ The rectangular area represents the boundary of Fig[ 1[ Tectonics from Bigi et al[\ 0878[

2[ Borehole breakout analysis Since 0881\ borehole breakout data\ from deep wells drilled for oil and geothermal exploration by Agip and Enel "the National Oil and Electricity Authorities\ respectively#\ have been used to determine minimum and maximum horizontal stress directions in Italy "Amato et al[\ 0884^ Mariucci et al[\ 0887^ Montone et al[\ 0881\ 0884\ 0886#[ Borehole breakouts are shear failures that occurr on the opposite walls of a well drilled in an anisotropic stress _eld[ They produce an enlargement of the well section along the direction of the minimum horizontal stress "Shmin#[ A complete description of this phenomenon was given by Bell and Gough "0872#\ Plumb and Hickmann "0874#\ and Zoback "0881#\ among many others[ In our analyses\ we always investigate well log depths greater than 499 m to avoid the super_cial e}ects of interference in the measurements^ moreover\ we consider only the well deviation range between 9[4> and 04>[ To attribute a quality value to each well\ we adopted the quality ranking system proposed by Zoback "0881# for the World Stress map] from {A| "best data# to {E| "discarded wells#\ depending on the length of breakout interval\ and standard deviation of the mean direction[ For example\ the {A| quality restrains well data with a length interval of more than 299 m and standard deviation of less than 01>^ the {B| quality for a length range of 099Ð299 m and standard deviation of less than 19>[ It must be remembered that this {quality| estimate is a measure of the

146

P[ Montone\ M[ T[ Mariucci : Geodynamics 17 "0888# 140Ð154 Table 0 Breakout analysis results Well Age No[

Breakout length "m#

Depth interval of breakouts "m#

Breakout azimuth 2s[d[

Data quality

0 1 2 3 4 5 6 7 8 09 00 01

078 040 159 058 595 153 629 209 323 259 51 162

415Ð2184 4919Ð4217 3480Ð4993 499Ð0266 2169Ð3552 0459Ð3056 1464Ð4247 1424Ð3037 2099Ð3581 0114Ð1224 499Ð549 571Ð3146

N72W209> N42W204> N60W203> N02W26> N74W209> N65E213> N45W215> N65E219> N69W219> N61W212> N39E26> N46E209>

B "B# B "B# B "B# D "B# B "A# D "C# D "D# B "B# B "B# C "B# E "C# B "A#

middleÐl[ Miocene e[ Pliocene e[ CretaceousÐl[ Miocene PlioceneÐPleistocene EoceneÐl[ Miocene e[ CretaceousÐPliocene TriassicÐEocene e[ JurassicÐCretaceous e[ JurassicÐl[ Miocene EoceneÐl[ Miocene PlioceneÐPleistocene l[ CretaceousÐl[ Miocene

Legend] agethe chronological interval where breakouts have been detected^ s[d[standard devi! ation value[ In brackets\ we indicate the quality according to the WSM criteria "M[L[ Zoback\ personal communication#[

statistical distribution of the breakout data\ not of the single measurements[ In Table 0\ the qualities assigned to each well are compared to those "in brackets# obtained by taking into consideration only the World Stress Map criteria "M[L[ Zoback\ personal communication#[ In our classi_cation\ we generally downgrade the quality of a well if the criteria are not ful_lled "see Table 0\ well nos[ 3 and 00# because we also consider the crossed lithology and the depth interval where the breakout is detected "Fig[ 3#[ The study of 01 deep wells for the borehole breakout analysis in an area 49×29 km1 wide allowed us to recognize local patterns of crustal stress[ The breakout analysis is restricted to between 499 and 4399 m depth\ crossing Trias!Miocene calcareousÐsiliceous and anhydrites sequences and\ subordinately\ Plio!Pleistocene clayÐarenaceous units "Fig[ 3#[ Ten of the twelve selected wells are located in the same structure "Fig[ 1# along the Ferrara folds "wells from no[ 2 to no[ 01#[ This gives us the possibility to observe interferences between pre!existing tectonic structures and present! day stress directions[ Two wells are located out of this structure] one in the Emilia folds "well no[ 0# and the other in the weakly deformed foredeep "well no[ 1\ see also pro_le A of Fig[ 1# outside the thrust zone[ The results are presented in Fig[ 4 as separate rose diagrams of the breakout directions for each well\ where the length of each peak is proportional to the frequency and the width to the variance of its gaussian curve[ No well obtained quality A "the best in the ranking system#\ seven of the twelve examined wells gave a good result "quality B#\ one has quality C\ and three have quality D[ Only one borehole "no[ 00# has been excluded from the evaluation of the stress _eld[ This well is drilled into Plio!Pleistocene units "sand\ clay and silt#\ the total length of its breakout zones is short "especially if compared to the breakouts in the other wells#\ the breakout interval is not too deep and the hole|s deviation from the vertical is high " from 0> to more than 19>#[ Consequently\

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Fig[ 3[ Stratigraphic sections of the analysed wells "courtesy of AGIP#[ On the left side of the column\ the breakout intervals recognized are also indicated[ Top of the column] breakout rose diagrams for each well[ Di}erent greys indicate the assigned quality value[

even if the standard deviation of the data is low\ we believe that the measures obtained are not reliable stress indicators "Fig[ 4#[ In general\ without taking into account the discarded well\ the breakout intervals in the boreholes are quite deep^ most of them crossed the Meso!Cenozoic calcareousÐsiliceous sequences down to the Triassic dolomites\ except for wells no[ 1 and 3 "Fig[ 3#[ The total breakout length for each well is high\ from 040 to 629 m[ The resulting average Shmin directions for each borehole have small values of standard deviation "Table 0#^ the greatest ones are due to the solutions with a double peak "wells no[ 5 and 6^ Fig[ 4#[ The existence of two prevailing orientations in the same borehole does not seem to be connected to well!de_ned zones\ like di}erent tectonic or stratigraphic units[ The breakout average directions show rapid variation in space\ probably due to the di}erent tectonic orientations of the buried thrusts "Figs[ 4Ð5#[ In most cases\ Shmin directions "well nos[ 0\ 5Ð09# seem to follow the orientation of the arc structures^ otherwise\ the breakout directions "well nos[ 2\ 4 and 01# seem to be nearly perpendicular to the main plate convergence\ NNWÐSSE directed "De Mets et al[\ 0889#[ Some wells have di}erent directions that can depend on incoherent lithology\ or on some

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Fig[ 4[ Breakout rose diagrams[ For each well\ the Shmin and Shmax azimuth are indicated showing the standard deviation value\ the total length of the detected breakout interval and the quality value] black for B quality\ grey for C quality\ light grey for D quality and white for E quality "discarded#[

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Fig[ 5[ Breakout results compared to the tectonic structures of the investigated area "tectonics from Ambrosetti et al[\ 0876b#[ 0*Po Plain sector^ 1*Apenninic belt sector[ Tectonics] 2*Major thrust "above# and normal faults^ 3*Major folds axes "anticline and syncline#^ 4*~exures "squares on lowered areas#^ grey lines indicate a PlioÐQuaternary age and black lines the middle PleistoceneÐHolocene age[ Shmin directions] 5*B quality^ 6*C quality^ 7*D quality[

unknown problem that arose during the data acquisition "Fig[ 5#[ Well no[ 3\ drilled in the Plio! Pleistocenic clay and sand "Fig[ 3#\ shows the Shmin direction N02W oriented\ in disagreement with the other breakout results^ due to the breakout length and to the shallow depth in which the breakouts are detected\ we have assigned them D quality and\ therefore\ this well is not considered as a reliable stress indicator "Table 0#[ Well no[ 1 belongs to the weakly deformed foredeep "see section A in Fig[ 1# and its breakout orientation seems quite well related to the presence of an ½EÐW oriented thrust[ Well no[ 0 is located in another structure\ the Emilia folds\ and the breakouts are coherent with a maximum horizontal stress "Shmax# ½NS\ nearly perpendicular to the thrust direction at that point[ The Shmin directions resulting from the total data analysis point to a roughly EÐW trend\ even though local variations have been observed[ The rose diagrams of Fig[ 6"a# represent the computed Shmin value considering the 00 Shmin directions weighted with respect to their quality\ where the B wells were counted three times\ the C twice\ and D wells only once[ Considering all the wells\ the Shmin orientation obtained is N70W211>^ selecting only the B and C quality wells\ the direction of Shmin is N71W219> ðFig[ 6"b#Ł[ The di}erent directions that we observe among the wells\ as mentioned above\ are also due to the crossed stratigraphic interval[ An analysis of the Plio!Pleistocene sequence and\ separately\ of

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Fig[ 6[ Cumulative Shmin direction rose diagrams] "a# Shmin considering B\ C and D quality "00 wells#^ "b# Shmin considering B and C quality "6 wells#^ "c# Shmin relating to PlioÐPleistocene sequence "1 wells#^ "d# Shmin relating to MesoÐCenozoic sequence "8 wells#[ The Shmin directions have been computed using the mean Shmin azimuth for each well as reported in Table 0[

the Meso!Cenozoic sequence\ clearly shows two di}erent directions in the Shmin orientation[ We have N33W206> and N75W207> for the Plio!Pleistocene and the Meso!Cenozoic sequence\ respectively ðFig[ 6"c\d#Ł[ 3[ Discussion and conclusion With the aim of constraining the active stress _eld orientation in the Po Plain along the Ferrara arc\ we have performed a breakout analysis in 01 deep wells\ located very close to the epicenter of the 04th October 0885 Reggio Emilia earthquake "Ms4[0#[ From the breakout analysis of 00 wells\ we observed a nearly EÐW Shmin direction\ as was also shown by the analysis of only B and C quality data] N70W211> and N71W219>\ respectively ðFig[ 6"a\b#Ł[ Few wells show discordant orientations] among them are well nos[ 1\ 3 and 01\ with N42W\ N02W and N46E Shmin directions\ respectively[ If we separately analyse the {Plio!Pleistocene breakouts| "well nos[ 1 and 3# and the {Meso! Cenozoic breakouts| "the other 8 wells#\ we obtain very di}erent Shmin values[ Analysis of the Plio! Pleistocene sequence indicates a Shmin of N33W206>\ while an analysis of the Meso!Cenozoic

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sequence shows a Shmin of N75W207>\ very similar to what we obtained for the whole dataset ðFig[ 6"c\d#Ł[ We have not detected any clear relationship between the structural arc and the breakout directions\ despite the fact that some local rotations of Shmin seem to follow the shape of the tectonic structure[ In particular\ breakout directions are nearly parallel to the thrust front only where the latter is favourably oriented with respect to the regional stress _eld[ As demonstrated by our breakout data and con_rmed by background seismicity and by the location of two major earthquakes "Parma 0872 and Reggio Emilia 0885#\ the present deformation front seems to be active in this area[ We have observed some di}erences in the position with respect to the location of the north Apennine active thrust front proposed by Vai "0878#] this can be due either to a new thrust front migration during the past thousand years\ or to a lack of reliable active stress indicators to constrain the active front[ In this area\ from the structural and seismological data\ we can associate the maximum horizontal stress "about NÐS directed# with the direction of s0[ Both structural data " faults and fractures on Recent units^ Perotti\ 0880# and seismological data "inversion data from smallÐmedium earth! quakes fault plane solutions^ Frepoli and Amato\ 0886# indicate\ within the uncertainties\ a compression direction about NÐS[ The results of the inversion method for small earthquakes\ related to the 0885 Reggio Emilia sequence\ show a horizontal s0 oriented N06W "Azzara et al[\ 0886#[ The CMT fault plane solutions "Harvard Catalog# related to the two main events "Ms×4# show a nearly N29W oriented P!axes[ The slight di}erences that we observe in the values of s0 directions\ inferred from the various stress indicators\ are comparable if we take into account the errors that a}ect each dataset[ However\ we might explain this discrepancy as being due to the di}erent ranges of depth considered[ In point of fact\ stress could change with depth from the surface\ where structural data are detected\ to the deeper part of the crust where breakouts "9Ð5 km# and earthquakes "4Ð19 km# are generally located[ Otherwise\ we could interpret these di}erences as being due to the presence of two or more decoupled structures with non uniform stress conditions[ The recent evolution of northern Italy has mainly been controlled by] "a# the interaction of three main plates "Africa\ Adria microplate and Europe#^ "b# the westward subduction of the Adriatic lithosphere^ and "c# the related opening of back!arc basins[ The driving forces responsible for the active compression can be related to the convergence between Africa and Europe\ as shown by global plate!motion studies "Argus et al[\ 0878^ De Mets et al[\ 0889#[ We can include the Adria microplate in this framework since it is characterized by very low seismicity "Anderson and Jackson\ 0876# and by a prevailing northward motion\ as derived from SRL and GPS data "Noomen et al[\ 0885#[ Locally\ the indentation of Adria under the Alps generates a fan!shaped pattern of Shmax directions\ from NS in the Friuli area\ NNW in the Central Alps "Muller et al[\ 0881# to ½EW in the south!western Alps "Zoback\ 0881#[ The active stress pattern is also in~uenced by the existence of active subduction of the Adriatic lithosphere beneath the northern Apennines\ as suggested by subcrustal earthquakes "Selvaggi and Amato\ 0881# and moreover by the high velocity zone beneath this region "Amato et al[\ 0882#[ The inferred Shmin "N70W# can be considered as locally representative of the active stress in the FerraraÐRomagna folded arc[ At this stage\ we cannot extrapolate the results to the entire northern Apennines thrust fronts because\ in this region\ it seems that the main tectonic structures control the stress direction[ This conclusion is therefore di}erent from the results obtained in the southern

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Apennines\ where NE!oriented breakouts are spread in a broad region from the Tyrrhenian to the Adriatic margin "Amato et al[\ 0884#[ Along the southern Apenninic belt\ a comparison with crustal seismicity "09Ð04 km depth# testi_es that the entire brittle crust is undergoing NE extension "Amato and Montone\ 0886#[

Acknowledgements We gratefully thank Prof[ Boschi "I[N[G[# and Ing[ Belotti " from AGIP!ENI S[p[A[#\ who made this work possible[ We would also like to thank Dr[ M[ Cesaro " from AGIP# for providing the borehole data[ This paper has bene_ted from constructive discussions and a critical review by Dr[ A[ Amato[ Thanks are also due to Prof[ G[ Ranalli and to an anonymous referee for reviewing and improving the manuscript[ We thank Dr[ M[L[ Zoback for the helpful suggestions concerning the data quality ranking system[

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