Journal of Geography and Geology; Vol. 8, No. 4; 2016 ISSN 1916-9779 E-ISSN 1916-9787 Published by Canadian Center of Science and Education
Thermal Structure of the Outer Northern Apennines along the CROP-03 Profile Stefano Santini1, Stefano Mazzoli2, Antonella Megna3 & Stefania Candela1 1
Dipartimento di Scienze di Base e Fondamenti (DiSBeF), University of Urbino, Via Santa Chiara 27, 61029 Urbino, Italy 2
Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse (DiSTAR), Università di Napoli Federico II, Largo San Marcellino 10, 80138 Napoli, Italy 3
Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione di Sismologia e Tettonofisica, Via di Vigna Murata 605, 00143 Roma, Italy
Correspondence: Stefano Santini, Dipartimento di Scienze di Base e Fondamenti (DiSBeF), University of Urbino, Via Santa Chiara 27, 61029 Urbino, Italy. Tel: 39-0722-303-390, Fax: 39-0722-303-399. E-mail:
[email protected] Received: September 24, 2016
Accepted: November 18, 2016
Online Published: November 18, 2016
doi:10.5539/jgg.v8n4p1
URL: http://dx.doi.org/10.5539/jgg.v8n4p1
Abstract In this study, we elaborated a 2D model that reproduces the thermal structure of the central-northern Adriatic offshore and adjacent onshore area of the Italian peninsula. Based on the crustal structure along the trace of the CROP-03 deep section, the geotherms offshore Gabicce (northern Marche region) were obtained by an analytical procedure taking into account the role of thrusting within the sedimentary cover. Basement involvement at depth beneath the neighbouring Mondaino area to the SW, where a crustal thrust ramp dips towards the hinterland, required the use of a different analytical procedure. The results obtained in this study allowed us to define a satisfactory description of the thermal state of the northern Marche coastal area and adjacent Adriatic offshore. These results, integrated with those obtained by previous studies, confirm that the isotherms of 250°C and 400°C are placed in the stable Adriatic lithosphere at depths of about 11 km and 22 km, respectively. Furthermore, the 400°C isotherm is deeper in the onshore area, reaching a depth of about 30 km in the zone comprised between Gabicce and Mondaino, whereas the 250°C isotherm deepens towards the SW along the Adriatic Sea sector, to reach a maximum depth of 13 km in coastal area, rising again at a depth of 11 km in the innermost sector of the studied section. Keywords: central Adriatic offshore, northern Marche onshore, surface heat flow, thermal modeling, temperature profiles 1. Introduction The deep structure and active tectonic setting of the outer northern Apennines are widely debated, particularly concerning two main issues: (i) the degree of basement involvement in shortening (i.e. ‘detachment–dominated’, or thin-skinned, vs. ‘crustal-ramp–dominated’, or thick-skinned, styles of thrusting; e.g. Bally et al., 1986; Barchi et al., 1998; Coward et al., 1999; Mazzoli et al., 2001, 2005; Speranza & Chiappini, 2002), and (ii) the role and extent of thrust activity (i.e. thrusting– vs. strike-slip–dominated active stress field; e.g. Di Bucci et al., 2003; Vannoli et al., 2004; Basili & Barba, 2007; Santini et al., 2011; Kastelic et al., 2013; Mazzoli et al., 2014, 2015). Improving our understanding of the rheology of this sector of the Apennine orogen is fundamental in order to carry out meaningful seismotectonic analysis and modeling. Taking into account that rheology depends on the thermal model that is assumed, this paper aims at defining the thermal structure of the outer northern Apennines. To this purpose, geothermal modeling is carried out along a cross-section integrating the interpreted CROP 03 deep seismic reflection profile (Barchi et al., 1998) and the structure of the adjacent Adriatic offshore sector (Morgante et al., 1998; Coward et al., 1999; Mazzoli et al., 2001). 2. Geological setting 2.1 Stratigraphy Deep parts of the Apennine stratigraphy are known from deep wells. These, on top of crystalline basement units, 1
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penetratedd Permo-Triasssic continentall siliciclastic ddeposits (i.e. thhe Verrucano Group). The ooverlying Umbria– Marche suuccession conssists mainly of carbonates ((Upper Triassiic–Eocene), diisplaying signiificant verticall and lateral varriations of bothh facies and thhickness of thhe formations (Figure 1a) annd recording JJurassic rifting g and subsequennt developmennt of a passiive continentaal margin. B Both rift-relateed ‘complete’’ and ‘conden nsed’ successionns are stratigrraphically overlain by uppeermost Jurassiic to Oligocenne limestones and marls of the Maiolica aand Marne a Fucoidi F Fms., and by the S Scaglia Group. The stratigraaphically overrlying hemipellagic, turbiditic aand also evapooritic units of M Miocene–Plioccene age incluude the markerr horizons of thhe Bisciaro (Lower Miocene) and Gessoso––Solfifera (Meessinian) Fms.. In the Marcche foothills aand Adriatic ooffshore areass, the deformed M Mesozoic–Palleogene sequennce is mostly bburied beneathh these Miocenne–Pliocene strrata (Figure 1b b).
Figure 1.. (a) Schematicc stratigraphy oof the study arrea (from Mazzzoli et al., 20005). (b) Geologgical cross-secttion (modiffied after Barchhi et al., 1998;; Morgante et aal., 1998; locatted in Figure 22) 2.2 Tectonics The outer northern Apennnines (Figuree 2) are charactterised by an aarcuate shape aand a main northeast vergence of asymmetriic, mostly faullted anticlines involving Meesozoic-Tertiaryy sedimentaryy successions ((e.g. Mazzoli et e al., 2005, and references theerein). The Uppper Triassic evvaporites form m an important structural deccoupling horizo on so that the unnderlying stratta remain buriied. Other levels within thee Mesozoic and Cenozoic suuccessions are also thought tto have acteed as thrust flats locallyy, particularlyy shales andd marls withhin the otherrwise 2
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carbonate--dominated unnits (e.g. Barchhi et al., 1998)). At depth, the siliciclastic deposits of thee Verrucano Group G overlie cryystalline basem ment units thatt are shown inn Figure 1 as being also invvolved in thickk-skinned thru usting (Coward eet al., 1999; Mazzoli M et al., 2001), as connfirmed by thee interpretationn of the CRO OP 03 deep seiismic reflection profile (Barchhi et al., 1998). This is at varriance with preevious interpreetations of thinn-skinned thru usting that were mainly basedd on magnetic data modellinng providing a uniformly aand gently W--dipping ‘mag gnetic basement’ (e.g. Bally ett al., 1986). A As a matter off fact, new maagnetic data aand modelling indicate basement involvemeent in thrustingg (Speranza & Chiappini, 20002; Tozer et al., 2006). In the Maarche foothill area and adjaacent Adriatic offshore, thee deformed M Mesozoic-Paleoogene multilay yer is mostly burried beneath suuch recent straata.
Figure 2. Tectonic sketcch map of CRO OP-03 profile (after Barchi eet al., 2003, moodified) showiing location off the crustal crross-section X X-X' Late Quaternary thrust activity a in thiss frontal part of the belt is widely debateed (DISS Worrking Group, 2010; 2 Mazzoli et al., 2015, annd references therein). Timiing of the defformation has been obtainedd by Coward et al. (1999) andd Di Bucci et al. a (2003) baseed on the detaailed analysis oof syntectonic growth strata imaged on seiismic profiles caalibrated with well w logs. Theese studies inddicate 1.8 My-llong activity ((between 2.5 aand 0.7 Ma) fo or the thrust splaay underlying the Mondainoo site in Figurre 2, and 1.1 M My-long activvity (from 1.8 to 0.7 Ma) fo or the thrust splaay carrying the Frontal Anticcline. Howeverr, it is worth nooting that the w whole thrust syystem includin ng the two splayss analysed heree is elsewhere considered as presently activve and seismogenic (e.g. DIS SS Working Group, G 2010). 3. Method ds The analyytical procedurre applied in this study takkes into accouunt both the temperature vaariation due to o the re-equilibrrated conductivve state after tthrusting and ffrictional heatiing. Input paraameters include heat flow de ensity data and a series of geollogically derivved constraintss. The map of the heat flow density in Itally by Scrocca et al. (2003) is uused to constraain surface heaat flow in the study area. In order to perfoorm the temperature calculattions, the crustall structure of the study areea has been siimplified as ccomposed of ttwo units: (i) an upper unitt that includes alll sedimentaryy layers from thhe Pleistocenee to the Permo--Triassic; and (ii) a lower unnit that include es the upper crusst (orogenic baasement) and thhe lower crust (Figure 3). The fundaamental heat soources to be coonsidered in thhe computationns include bothh the heat risinng from the mantle m 3
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and the raddiogenic heat due d to the pressence of radioaactive elements in the lower unit. In fold annd thrust belts such as the Apeennines, a furtther heat sourcce is representted by frictionnal heating asssociated with tthrust faulting.. The computingg procedure foollows the proocedure develooped by Molnnar et al. (19883), which deefines the temporal evolution oof the thermal state as the suum of two term ms: ,
,
(1)
the new eqquilibrium finaal state and a tiime dependentt term: ∑
,
,
(2)
,
(3)
defined byy the perturbedd initial state: ,0 with
2
1
⁄2 .
The two tterms T and θ depend on the heat sourcce type, the thhrust depth (z ), the crustall thickness (h)), the thermal caapacity (k) andd diffusivity ( ).
Schematic crusstal model for geotherm calcculation along the CROP-03 profile (locateed in Figure 2). The Figure 3. S parameteers are: densityy of upper (ρs) and lower uniit (ρb), radiogeenic heat (A0) aand its length sscale (hr), therm mal conductivvity (k and diiffusivity ( ), crustal thickenning ∆z , surrface heat flow w ), surfacee temperature (T a a depth at a site of ggeotherm calcuulation, a crusstal thickeningg ∆ is taken n into For a thruust occurring at account inn the computaations by carryying out a sim mple restoratioon based on ooffset stratigraaphy, as show wn in Candela ett al. (2015) annd Megna et all. (2014). The procedure is ddifferent in casse crustal thickkening occurs only by shortenning of the seddimentary covver, or in casee it involves aalso the basem ment along the vertical profiile of geotherm ccalculation. Inn the former caase, the flow off radiogenic heeat is unchangged (so ), w whereas the the ermal state due to unitm mantel heat flow w is modified with a temperrature jump in correspondennce of the thrusst. At the initial time of thruusting (t = 0), the temperaature jump coorresponds to the differencee between n the temperaturre at thrust deppth and thhat at the depthh (M Megna et al., 20014). For 0, the temperrature evolves unntil reaching thhe new equilibbrium state forr → ∞. At thhese two heat flows must bee added the flo ow of frictional hheat . The thermal state ffor the two uniits is defined bby the followinng equations: , ,
where is the surface temperature, the bottom m of the sedimeentary cover.
,
, ,
,
,
is the thickkness of the uppper unit, and
4
, ,
(4) (5)
iss the temperatu ure at
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In the casee of a thrust faault involving the basement along the verrtical profile of geotherm caalculation, thru usting causes a perturbation to thermal state due to mantle--derived heat fflow and to raadiogenic heat flow. Furtherm more, for the callculation of thhe thermal statte of upper unnit ( the inncrease of raddiogenic heat aassociated with h the increased bbasement thickkness (∆ ) muust be taken innto account (Caandela et al., 22015). Also inn this case, the flow of frictionaal heat m be added tto the other two heat flow coomponents. Thhe temperature for the two un must nits is obtained bby the followinng equations: , ,
, ,
, ,
,
,
,
(6)
.
(7)
is the ffinal temperatu In both casses, each term depending onn time has the ggeneric equatioon (1), where ure of the new eqquilibrium stattus ( → ∞ aand , is calculated byy equations (2)) e (3). and , show s differencess due to therm mal source typee and dependinng on thrustingg involving thee sedimentary ccover (Megna et al., 2014) or thhe basement allong the verticcal profile of geeotherm compputation (Candela et al., 20155). 4. Results ms for the studdy area are callculated along three vertical profiles throuugh the crust of o the Representaative geotherm outer nortthern Apenninnes. Two geottherms are callculated for thhe onshore (M Mondaino) andd offshore (Frrontal Anticline) sectors of thee frontal part oof the fold andd thrust belt (F Figure 2). To ccomplete the ttreatment, a fu urther geotherm is calculated in the forelaand basin (Addriatic Offshorre) at about 330 km from the coastline. The interpolateed isotherms are a then extendded to the geoothermal profille calculated bby Mongelli ett al. (2006) fu urther offshore (A Adriatic Platfoorm). The map bby Scrocca et al. (2003) wass used to constrain surface hheat flow in thhe study area. The two innerrmost sites of geeotherm compuutation (i.e. M Mondaino and F Frontal Anticliine) fall close to the contourrs of 40 mW/m m3 in the map, w whereas the siite Adriatic Offfshore sits in an area of heat flow > 40 m mW/m3. Baseed on their possition with respeect to the heatt flow contourrs, we chose a value of 400 mW/m3 for both the Monndaino and Frrontal Anticline ggeotherms, andd a
of 45 m mW/m3 for the Adriatic offsshore geotherm m.
m (Mongelli ett al., 2006). For the Figure 4. Calculated geootherms and teemperature proofile for the Addriatic Platform F Anticlinne (green), geootherms for the two activity periods of thruusting are show wn, Mondainno (blue) and Frontal together with their resppective temperrature differencce profiles ∆T T) 3.5 times ennlarged in the temperature sc cale 5
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For the Moondaino site, the t geotherm hhas been calculated taking innto account a ddip separation of 9 km (meassured at top-baseement level) along a the thrusst fault, produucing a crustall thickening ∆ ) of 1.4 km m along the vertical profile of geotherm com mputation (Figuure 3). A crusttal thickening ∆ ) of 1.2 kkm is producedd along the vertical profile of ggeotherm com mputation for thhe Frontal Antticline, the undderlying thrustt having a dip separation of 8 km (measuredd at top-Oligoccene level). Taaking into accoount the continnuing debate aabout thrust acctivity/inactiviity in the outer pportion of the northern Apennnines in the sstudy area, twoo different geootherms have bbeen calculated for each of thhe two sites loocated in the fr frontal part of the thrust beltt. For the Monndaino site, teemperature pro ofiles have been obtained for a timing of acttivity ∆t) of 11.8 Ma (i.e. beetween 2.5 andd 0.7 Ma, as suuggested by seiismic stratigraphhic evidence), and for ∆t = 2.5 Ma (i.e. aassuming its ppresent-day acttivity). Similarrly, for the Frrontal Anticline ssite, temperatuure profiles havve been obtainned for a ∆t off 1.1 Ma (i.e. bbetween 1.8 annd 0.7 Ma), an nd ∆t = 1.8 Ma (i.e. assuminng it is presenntly active). Shear heating has been obtaained, based oon Sibson's (1 1974) formulatioon for frictionnal heating, uusing the calcculation adoptted in Candella et al. (2014) with a friction coefficientt equal to 0.6. All A further parrameters requirred to define eeach geotherm are shown in F Figure 3. 5. Discusssion and conclluding remark ks Our resultts show that, for the upper unit of the ccrust, the geottherms obtaineed for the Froontal Anticline e and Adriatic O Offshore sites differ essentially around thhe depth (4 kkm) of the thrrust fault encoountered along g the vertical prrofile of the foormer, mainly due to the coontribution of frictional heaating. For the M Mondaino site e, the contributioon of both fricttional heating and radiogeniic heat associatted with basem ment-involved thrusting is clearly recorded bby the temperrature profile aat the depth oof the thrust (ca. 9 km). Onn the other haand, the geoth herms obtained bby assuming a different tim ming of activityy of the thrusst system diffeer only slightlyy for a single site, reaching m maximum valuues of 6 °C (aat ca. 10 km ddepth) and 10 °C (ca. 15 km m) for the Froontal Anticline e and Mondaino sites, respecttively (Figuree 4). This is bbecause frictioonal heating is proportionaal to slip velo ocity; therefore, for a constantt total fault dissplacement, thhe effect of friictional heat prroduction over longer periods of time is couunterbalanced by higher slipp rates in casee of shorter tim mes of fault acctivity. As a reesult, the isoth herms produced bby the interpoolation of the vvertical tempeerature profiless define a therrmal structure for the study area (Figure 5) that is almost independent of thrust activvity or quiescence during thhe late Quaternnary (i.e. since 0.7 Ma). Our modeelling results show s that the thermal struccture of the sttudy area is cconsistent withh a ‘crustal-ra amp– dominatedd’ (Butler & Mazzoli) M style of thrusting. The modelledd geotherms, obtained by inncluding basement thrusts in tthe computatioons, fit satisfacctorily the avaiilable constrainnts in terms off measured heaat flow in the study s area. On tthe other handd, our results indicate that the occurrencce – or lack – of active thhrusting in the e late Quaternaryy is negligiblee in terms of tthermal effectss. Therefore, tthermal modellling cannot prrovide indepen ndent constraintss on the ongoinng debate on thhe active deforrmation and seeismotectonicss of the outer nnorthern Apenn nines. However, it is well knoown that defining the thermaal setting is fuundamental inn rheology studdies and nume erical modeling oof tectonic proocesses (e.g. C Candela et al., 22015). Therefoore, the thermaal structure obtained in this study s can be useed in any modeelling studies aimed at imprroving our undderstanding of active deform mation in the frrontal part of thee northern Apeennines. Moree in general, anny quantitativee modeling off geodynamic pprocesses affecting the study aarea, and requuiring the definnition of rheological parameeters, can be caarried out by ttaking into acc count the thermaal structure obbtained in this study, irrespeective of tectonnic interpretatiions involvingg the occurrence or lack of acttive thrusting in i the Adriatic sector of the ffrontal part of tthe northern A Apennines.
Figure 5. Thermal structture for the stuudy area as deffined by isotheerms producedd by the interpoolation among four verticall temperature pprofiles (whitee isotherms aree for active thru rusting; see texxt) 6
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Mongelli, F., Minelli, G., & Loddo, M. (2006). Geodynamics, thermal structure and depth of the brittle and semi-brittle layers in the Northern Apennine (Italy). Boll. Geofis. Teor. App., 47, 375–399. Morgante, A., Barchi, M. R., D’Offizi S., Minelli, G., & Pialli G. (1998). The contribution of seismic modeling of the interpretation of the CROP-03 line. Mem. Soc. Geol. It., 52, 441–455. Santini, S., Saggese, F., Megna, A., & Mazzoli, S. (2011). A note on central-northern Marche seismicity: new focal mechanisms for events recorded in years 2003-2009. Boll. Geofisica Teorica ed Applicata, 52, 639-649. http://dx.doi.org/10.4430/bgta0025 Scrocca, D., Doglioni, C., & Innocenti, F. (2003). Constraints for an interpretation of the Italian geodynamics: A review. Mem. Descr. Carta Geol. d’It., 62, 15-46. Sibson, R. H. (1974). Frictional constraints on thrust, wrench and normal faults. Nature, 249, 542–544. http://dx.doi.org/10.1038/249542a0 Tozer, R. S. J., Butler, R. W. H., Chiappini, M., Corrado, S., Mazzoli, S., & Speranza, F. (2006). Testing thrust tectonic models at mountain fronts: where has the displacement gone? J. Geol. Soc. London, 162, 1–14. http://dx.doi.org/10.1144/0016-764904-140 Vannoli, P., Basili, R., & Valensise, G. (2004). New geomorphic evidence for anticlinal growth driven by blind faulting along the northern Marche coastal belt (Central Italy). J. Seismol., 8, 297-312. http://dx.doi.org/10.1023/B:JOSE.0000038456.00574.e3 Copyrights Copyright for this article is retained by the author(s), with first publication rights granted to the journal. This is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license (http://creativecommons.org/licenses/by/4.0/).
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