Crustal structure of the transpressional Variscan orogen of SW Iberia: SW Iberia deep seismic...

Crustal structure of the transpressional Variscan orogen of SW

Iberia: SW Iberia deep seismic reflection profile (IBERSEIS)

J. F. Simancas,1 R. Carbonell,2 F. Gonzalez Lodeiro,1 A. Perez Estaun,2 C. Juhlin,3

P. Ayarza,4 A. Kashubin,3 A. Azor,1 D. Martınez Poyatos,1 G. R. Almodovar,5 E. Pascual,5

R. Saez,5 and I. Exposito6

Received 14 November 2002; revised 27 April 2003; accepted 2 June 2003; published 5 November 2003.

[1] IBERSEIS, a 303 km long (20 s) deep seismicreflection profile, was acquired across the Variscanbelt in SW Iberian Peninsula. The acquisitionparameters were designed to obtain a high-resolutioncrustal-scale image of this orogen. The seismic profilesamples three major tectonic terranes: the SouthPortuguese Zone, the Ossa-Morena Zone, and theCentral Iberian Zone, which were accreted in LatePaleozoic times. These terranes show a distinctiveseismic signature, as do the sutures separating them.Late strike-slip movements through crustal wedges areapparent in the seismic image and have stronglymodified the geometry of sutures. The upper crustappears to be decoupled from the lower crust all alongthe seismic line, but some deformation has beenaccommodated at deeper levels. A sill-like structure isimaged in the middle crust as a 1–2 s thick and 175 kmlong high-amplitude conspicuous reflective band. It isinterpreted as a great intrusion of mafic magma in amidcrustal decollement. Taking into account surfacegeological data and the revealed crustal architecture, atectonic evolution is proposed for SW Iberia whichincludes transpressional collision interacting duringEarly Carboniferous with a mantle plume. The Mohocan be identified along the entire transect assubhorizontal and located at 10 to 11 s, indicating a30–35 km average crustal thickness. Its seismicsignature changes laterally, being very reflectivebeneath the South Portuguese Zone and the CentralIberian Zone, but discontinuous and diffuse below theOssa Morena Zone. INDEX TERMS: 5475 Planetology:

Solid Surface Planets: Tectonics (8149); 0935 Exploration

Geophysics: Seismic methods (3025); 0905 Exploration

Geophysics: Continental structures (8109, 8110); 8102

Tectonophysics: Continental contractional orogenic belts; 8109

Tectonophysics: Continental tectonics—extensional (0905);

KEYWORDS: seismic reflection, crustal structure, Variscan

orogen, SW Iberia. Citation: Simancas, J. F., et al., Crustal

structure of the transpressional Variscan orogen of SW Iberia: SW

Iberia deep seismic reflection profile (IBERSEIS), Tectonics,

22(6), 1062, doi:10.1029/2002TC001479, 2003.

1. Introduction

[2] The IBERSEIS deep seismic reflection profile pro-vides, for the first time, a complete section at crustal scale ofthe Variscan Belt in SW Iberia. Changes in structure through-out the orogen are documented by the profile, providing newdata to understand the unusual tectonic evolution and litho-spheric processes which have affected this orogen. The SWIberian transect is the missing link in deep imaging of theVariscan Belt in Europe (Figure 1), complementing previousstudies performed in the northern Iberian Massif (ESCI[Perez-Estaun et al., 1994, 1995; Pulgar et al., 1995;Martınez Catalan et al., 1995; Ayarza et al., 1998], in centralEurope [British Institutions Reflection Profiling Syndicate(BIRPS ) and Etude Continentale et Oceanique par Reflexionet Refraction (ECORS), 1986; Deutches KontinentalesReflektionsseismisches Programm (DEKORP) ResearchGroup, 1985, 1988;Onken et al., 2000], and across the Urals[Echtler et al., 1996; Carbonell et al., 1996, 2000; Juhlin etal., 1998; Tryggvason et al., 2001; Friberg et al., 2002]).[3] The Variscan Belt in southwest Europe was produced

by a continent-continent collision between an Ibero-Armor-ican indentor and a northern continent, Laurussia, resultingin the Ibero-Armorican arc (Figure 1a) [Matte and Ribeiro,1975; Brun and Burg, 1982; Matte, 1986]. An obliquecompressional regime operated along both the northernand southern branches of the arc, closing the Rheic Oceanby subduction/obduction.[4] The Iberian Massif is the best exposed fragment of

European Variscan basement. In SW Iberia, the VariscanOrogen consists of a series of zones representing differenttectonostratigraphic units exposed at a variety of structurallevels (Figure 1b). Variable components of shortening andleft-lateral strike-slip deformation have been recognizedfrom field studies and geodynamic models. Thus, theregion offers an opportunity to image and study a trans-pressional orogen, and to examine the partitioning of

TECTONICS, VOL. 22, NO. 6, 1062, doi:10.1029/2002TC001479, 2003

1Departmento de Geodinamica, Facultad de Ciencias, Universidad deGranada, Granada, Spain.

2Instituto de Ciencias de la Tierra ‘‘Jaume Almera,’’ Consejo Superiorde Investigaciones Cientıficas, Barcelona, Spain.

3Department of Earth Sciences, Uppsala University, Uppsala, Sweden.4Departamento de Geologıa, Universidad de Salamanca, Salamanca,

Spain.5Departamento de Geologıa, Facultad de Ciencias Experimentales,

Universidad de Huelva, Huelva, Spain.6Departamento de Ciencias Ambientales, Universidad Pablo de Olavide,

Sevilla, Spain.

Copyright 2003 by the American Geophysical Union.0278-7407/03/2002TC001479

1 - 1

deformation in three dimensions and in time. For thesereasons, the region has attracted broad scientific interest,resulting in the SW Iberia interdisciplinary scientific pro-gram promoted by EUROPROBE [Ribeiro et al., 1996].Further interest in IBERSEIS is due to the well knownIberian Pyrite Belt massive sulphide deposits located in theSouth Portuguese Zone. This paper presents the acquisi-tion, processing and interpretation of the deep seismicreflection profile IBERSEIS, acquired across the SWIberian Orogen (Figure 1b).

2. Geological Setting

[5] The Variscan Belt is the product of the collision of anumber of continental blocks formed by the Early Paleozoicfragmentation of a Late Proterozoic megacontinent [Murphyand Nance, 1991]. Although important aspects of thisfragmentation and subsequent wandering are still controver-sial, a general picture is emerging [Dalziel, 1997; Tait et al.,1997; Crowley et al., 2000; McKerrow et al., 2000]. One ofthe continental fragments, the Avalonian plate, accreted inOrdovician-Silurian times to the Laurentian continent (Cale-donian Orogeny). Then, in Devonian-Carboniferous times,the Armorican fragment (in the sense of Matte [2001]) andthe Gondwanan continent collided with the Avalonian bor-der of Laurentia, resulting in the Variscan Orogenic Belt andthe Late Paleozoic Pangea [Matte, 1986, 2001].

[6] The Iberian Massif constitutes a major part of theIberian Peninsula (Figure 1), containing a nearly completecross section of the Variscan Belt. The Iberian transect hasbeen divided into a number of zones, in a similar way as theVariscan Belt in central Europe (Figure 1b): CantabrianZone, West Asturian-Leonese Zone, Central Iberian Zone,Galicia Tras-Os-Montes Zone, Ossa-Morena Zone andSouth Portuguese Zone.[7] The Cantabrian Zone to the north, and the South

Portuguese Zone (SPZ) to the south, represent oppositeforeland fold and thrust belts of the orogen (Figure 1b). Inthe northwestern part of the Iberian Peninsula, the GaliciaTras-Os-Montes Zone consists of a pile of allochthonoustectonic units with high-pressure metamorphism and rocksincluding ophiolites [Arenas et al., 1986, 1997; Ribeiro etal., 1990a]. The suture is placed beneath the mafic andultramafic complexes of this zone, and is rooted toward theAtlantic margin [Matte, 1986; Martınez Catalan et al.,1997]. The broad architecture of the orogen indicates thatthis suture must outcrop in southwest Iberia; in fact, twocontacts of probable suture character have been recognizedhere, corresponding to both the northern and southernboundaries of the Ossa-Morena Zone (OMZ).[8] The boundary between the OMZ and the SPZ

(Figures 1b and 2a) can be considered as a suture basedon the existence of a strip of oceanic amphibolites (the Beja-Acebuches ophiolite [Bard, 1977; Andrade, 1983; Munha et

Figure 1. (a) Outcrops and main zonal division of the European Variscides. (b) Zones of the IberianMassif and location of the IBERSEIS seismic line. The IBERSEIS line crosscuts the South PortugueseZone (SPZ), the Ossa-Morena Zone (OMZ) and the Central Iberian Zone (CIZ) in the southwestern partof the Iberian Variscides. The line was designed to be perpendicular to the main geological structures,mainly the two sutures separating these three zones.

1 - 2 SIMANCAS ET AL.: CRUSTAL STRUCTURE OF SW IBERIA

al., 1986; Quesada et al., 1994]) and an accretionary prismwith slices of oceanic metabasalts (the Pulo do Lobo Unit[Silva et al., 1990]). Small ophiolitic klippen imbricatedwith high-pressure continental rocks, which outcrops insouthwest OMZ [Fonseca et al., 1999], should be rootedperhaps in the SPZ/OMZ boundary. On its own, theboundary between the OMZ and the Central Iberian Zone(CIZ) is marked by a complex tectonic unit called theBadajoz-Cordoba Shear zone or Central Unit [Burg et al.,1981; Azor et al., 1994; Simancas et al., 2001], whichincludes some retroeclogites [Abalos et al., 1991; Azor,1994; Lopez Sanchez-Vizcaıno et al., 2003] and amphibo-lites with oceanic chemical signature [Gomez Pugnaire etal., 2003]. There are two different interpretations of theOMZ/CIZ boundary, both considering it as a major tectoniccontact: (1) for some authors, this boundary is a Cadomiansuture reactivated as an intraplate shear zone during theVariscan orogeny [Ribeiro et al., 1990b; Quesada, 1991;Abalos et al., 1991]; (2) for others, this boundary is a trueVariscan suture [Brun and Burg, 1982; Matte, 1986; Azor etal., 1994; Simancas et al., 2001, 2002]. The radiometricages available for the high-pressure rocks outcropping alongthis boundary, although not fairly consistent between them,are all Variscan [Schafer et al., 1991; Ordonez Casado,1998], thus giving support to the latter interpretation. In thisview, the OMZ/CIZ boundary is considered to have been inconnection with the root of the allochthonous tectonicunits of the Galicia Tras-Os-Montes Zone, in NW Iberia(Figure 1).[9] In summary, the Iberian Massif is the result of the

amalgamation of three continental blocks: the SPZ, theOMZ, and the ensemble of the CIZ, West Asturian-Leoneseand Cantabrian zones (Figure 1b). The three latter onesbelonged to the Gondwana continental margin while theSPZ seems to be the Avalonian border of the oppositecontinent; in between, the OMZ would be a terrane whosedegree of separation from Gondwana is still controversial.The IBERSEIS deep reflection seismic profile runs acrossthese continental pieces.[10] In SW Iberia, outcropping deformed rocks range in

age from Upper Precambrian (the Serie Negra) to UpperCarboniferous. Nearly all the mapped structures are Varis-can in age, i.e., formed in Devonian and Carboniferoustimes (Figure 2) [Simancas et al., 2001]. Shear zonesdeveloped at medium- to high-metamorphic grade outcropin very restricted areas. Some of these medium- and high-grade domains formed during the Variscan collision [Bard,1969; Crespo Blanc, 1992; Azor et al., 1994; Azor andBallevre, 1997] and others have recently been interpreted asformed in Cambrian-Early Ordovician times, during therifting stage that was related to the fragmentation of theLate Proterozoic megacontinent [Exposito et al., 2003].Nevertheless, Upper Precambrian rocks (Serie Negra) showsome evidence of Late Precambrian (Cadomian) deforma-tion [Blatrix and Burg, 1981; Dallmeyer and Quesada,1992], and uppermost Precambrian to lowermost Cambriancalc-alkaline igneous rocks indicate subduction at that time[Sanchez Carretero et al., 1990; Martınez Poyatos, 1997;Pin et al., 1999a].

[11] At present, SW Iberia is bounded by two main post-Variscan realms: to the west and south the Atlantic Ocean andto the southeast the Alpine Betic Orogen (Figure 1).Although fairly close to the IBERSEIS profile location, theopening of the Atlantic appears not to have affected thesurveyed area. As it was already known from previousrefraction work [Matias, 1996; Gonzalez et al., 1998], thecrust starts to thin further to the south of the IBERSEISprofile. On the other hand, Alpine deformation produced theBetic Orogen in southeastern Iberia, but little evidence forAlpine deformation has been found in SW Iberia. Geomor-phic and structural studies indicate minor Alpine reworkingof some existing faults, and moderate regional uplift [Stapel,1999]). In summary, the upper crust in SW Iberia consistsmainly of rocks ranging in age from Upper Precambrian toUpper Carboniferous, and the vast majority of structureswere formed during the Variscan Orogeny.

3. Seismic Data Acquisition and Processing

[12] The 303 km long deep seismic reflection profileIBERSEIS crosses the SPZ, OMZ and CIZ and theirtectonic boundaries (Figure 2). The profile was designedto be perpendicular to the main geological contacts resultingin an arcuate shape due mostly to the fan-like strike of thestructures in the OMZ. The transect was recorded in 56 days(from May to July 2001) using a 400 channel SERCEL 388and five 22 Ton Vibroseis trucks (Table 1). The asymmetricsplit spread configuration changed along the profile accord-ing to the dips of outcropping structures. To achieve a highresolution at shallow levels and to image steep dips, a 35 mstation spacing and a 70 m Vibration Point (VP) intervalwas used with a high fold (60 in average). A relatively longsweep (20 s) was chosen to increase the source energyrather than increasing the number of sweeps per VP. Thesweeps were diversely stacked before correlation, and thencorrelated in the field. Except for the station and VPspacing, the IBERSEIS profile was acquired with parame-ters (Table 1) that are typical of recent deep seismicreflection surveys (URSEIS [Echtler et al., 1996]; Lithop-robe [Cook et al., 1999]; TransAlp [Gebrande et al., 2001].The geometry data, x, y and z coordinates were calculatedusing a high-precision GPS system. The seismic data wererecorded in SEGB format and then transferred to DAT tapesin SEGY format with all the geometry information, stationand source location (station numbers and GMT coordinates,including topography) in the field. The signal-to-noise ratiois generally high that little trace editing and attenuation ofcultural noise was required. The processing flow wasdesigned to preserve relative true amplitudes (Table 2).

4. Description of the IBERSEIS Seismic Image

4.1. General Overview

[13] The seismic image (Figure 3) is characterized byrelative high-quality data and features numerous reflectionevents which, in the upper crust, can be traced to the surface.TheMoho is marked by a sharp decrease in reflectivity, and itis placed at approximately 10.5 s along the entire profile. As a

SIMANCAS ET AL.: CRUSTAL STRUCTURE OF SW IBERIA 1 - 3

Figure

2.

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whole, the seismic image reveals several specific domains.The upper crust can be divided into three domains, each ofwhich would roughly correspond to the upper crust of each ofthe different tectonic zones recognized in this orogen: theSPZ in the south, the OMZ located in the center and the CIZto the northeast. In general, there is a sharp change in theorientation of the reflectivity from the upper crust to the lowercrust, being subhorizontal and less common in the latter andstrongly dipping and more abundant in the former. However,the lower crust of the SPZ is very reflective and different fromthe one of the OMZ, which lacks localized reflections, andfrom the one of CIZ, whose lower crust features highamplitude reflectivity. The most outstanding feature in theseismic image is the broad band of high amplitude reflectivitywhich extends for more than 170 km and is located atmidcrustal level between 4 and 6 s in the OMZ and part ofthe CIZ, marking the boundary between upper and lowercrust.

4.2. South Portuguese Zone

[14] The reflectivity of the SPZ crust (Figures 3 and 4)can be divided into upper and lower zones. The boundarybetween them is marked by a 0.3 s thick undulatingsubhorizontal and poorly reflective band (UB in Figure 4)placed at 5 s.[15] Between CDPs 100 and 3600, the upper crust

shows several packages of continuous high-amplitudereflections dipping 30�–50� toward the north. Thesereflections (i.e., labeled 2, 3 and 5 in Figure 4) tend tomerge into the poorly reflective band UB. From CDP 3950to 5260 there are a series of steeply dipping reflections,fan-like in shape, merging at 5 s (PT and AF in Figure 4).This latter set of events is less marked than the former, andfeatures lower amplitudes; it straddles the boundarybetween the SPZ and the OMZ. In addition, above 5 s,there are also discontinuous and less-inclined events cross-cut by the previous mentioned steeply inclined ones. These

events are generally weak, but the one labeled as 1 inFigure 4 is distinctive.[16] Subhorizontal and slightly SW dipping reflection

packages characterize the reflection fabric of the lower crust(Figure 3). These SW dipping reflections (6, 7 in Figure 4),merge at the Moho (10.5 s) and extend up to the base of theunreflective band (UB) previously mentioned. These reflec-tions crosscut the horizontal reflectivity and deform the UB.The Moho is imaged as a 0.2 s thin, sharp and high-amplitude horizontal event.

4.3. Ossa-Morena Zone

[17] The most prominent feature in this part of theseismic profile (Figures 3 and 5) is the 1.5–2 s continuousthick band of high amplitude reflectivity, located at 6 sbeneath CDP 6400 and shallowing toward the northeast,where it is located at 3.5 s beneath CDP 15000 (well intothe CIZ). We call it the Iberseis Reflective Body (IRB). Thisbody, that features lateral changes in thickness, is composedof lens-shaped interfingering reflection packages with adistinct seismic fabric. The events within each individualpackage feature consistent dip. The IRB, located at mid-crustal levels, separates two contrasting seismic fabricscorresponding to upper crust and lower crust.[18] From southwest to northeast, the first prominent

seismic event of the OMZ upper crust is located at CDP5260 (AF in Figure 5). This is the northernmost reflection ofthe previously referred fan at the SPZ/OMZ boundary, andit is steeper than similar reflections to the south. It can beseen to continue down to 5 s, where it flattens betweenCDPs 6200 and 6400. Farther to the northeast, there existsgently dipping reflectivity (a in Figure 5). Southwestdipping events are identified between CDPs 6800 and7200 at 2–3 s (b in Figure 5). Between CDPs 8000 and8700, a seismic fabric dipping 40�–45� to the northeast isdelineated by two parallel events that extend from 1 s to4.5–5 s (c in Figure 5). Between CDPs 8750 and 10000, theseismic section features a thin 2 cycle event concaveupward (d in Figure 5), a conspicuous reflection thatcontrasts with the surrounding multicyclic and dippingevents. This event appears to coalesce with a broad bandof high-amplitude reflectivity (CDP interval 10000–11000),dipping approximately 30�–35� to the northeast (N1 in

Table 1. Acquisition Parameters Used in the IBERSEIS Deep

Seismic Reflection Profile

Acquisition Parameters Description

Recording instrument SERCEL 388Number of channels (400 total) 240 active minimumStation spacing 35 mStation configuration 12 Geophones per string in lineNatural frequency of geophones 10 HzFilters OutSource type 4, 22 TM VibratorsSweep frequencies 8–80 HzSweep type nonlinearSweeps per VP 6Sweep length 20 sRecording length 40 s listening timeRecord length after correlation 20 sSource spacing 70 mSample rate 0.002 sSpread type asymmetric split spreadNominal fold 60 (minimum)Field processing sweep correlationNoise suppression diversity stacking the sweeps

Table 2. Processing Flow and Parameters

Processing Modules Description

Field processing Read SEGB tapes, include geometryinformation UTM coordinates, write outSEGY tapes

Laboratory processing Read SEGY tapes, anti-alias filtering, timeresample (0.002 s)

Quality control Trace editing, noise attenuationAmplitude corrections trace balancing, time scaling, geometrical

spreadingStatic corrections First arrival time picking, elevation statics,

refraction statics, CDP sorting, velocityanalysis, normal move out correction,residual statics, stacks, trace balancing, timemigration (Stolt)


Figure 5). Prominent reflections in this band crosscut lessinclined, arcuate and subhorizontal weaker events. Thestraight band of reflections dipping to the northeast fromCDP 10900 (CU in Figure 5) coincides with the boundarybetween the OMZ and the CIZ.[19] The lower crust of the OMZ is mostly transparent.

Below CDP 6200 there is an abrupt truncation of the 2 sthick package of high amplitude reflectivity (IRB) (W inFigure 5). Between CDPs 7400–8500, the reflectivity of thelower crust is marked by reflections dipping to the northeastthat extend from the IRB and reach the Moho (S inFigure 5). Weak reflectivity with opposite dip is cut bythese reflections.

4.4. Central Iberian Zone

[20] The upper crust of the CIZ features less prominentreflections than the OMZ and SPZ, except for a NE dippingseries of events located between 1 and 4 s (Figure 3).Although the seismic image of the uppermost crust hasrelatively poor reflectivity, between CDPs 11500 and 12000a weak fabric is identified in the stacked section, dipping30� to the southwest (e in Figure 5). The dip of this fabriccontrasts with the one in the adjacent OMZ. Together theydelineate a bivergent structure. Farther to the northeast,between CDPs 12000 and 14750, northeast dipping seismicevents appear in the upper crust at 2 to 4 s (labeled N2, N3,N4, N5 in Figure 5). At the end of the profile, within the

range of CDPs 14750–15100, a cloud of reflectivity with ahorizontal fabric is imaged at 1.5 s.[21] The lower crust under the OMZ between 7 and 11 s

and CDPs 9000–13500 is nearly transparent. On thecontrary, from CDP 13000 to the end of the profile thereflectivity of the lower crust is very intense, characterizedby subhorizontal to slightly dipping (10�–20�) continuous,high-amplitude events (W1, W2 in Figure 5). These reflec-tions dip in opposite senses, defining a wedge-like structure.The lack of a prominent Moho reflection in the intervalCDP 9000–13000 contrasts with the 0.5 to 1.0 s thick bandof relatively high reflectivity associated with the Moho tothe northeast, in the CIZ, and to the southwest, in the SPZ.

5. Interpretation of the IBERSEIS

Deep Seismic Reflection Profile

[22] The interpretation of the reflection patterns along theIBERSEIS profile is driven by the detailed geologic andstructural knowledge of the region (Figure 2). Upper crustreflectivity shows good correlation with geological struc-tures at surface.

5.1. South Portuguese Zone

[23] From surface geology, it is known that the SPZextends from the beginning of the IBERSEIS profile to

Figure 3. (a) Topography (black line) and Gravity (red line) profiles along the transect IBERSEIS, thestations numbers and the corresponding CDP’s are also indicated. (b) Stacked image, (c) time migratedstack image and (d) line drawing of the IBERSEIS deep seismic reflection profile. The location of themain tectonic units along the seismic profile is indicated: SPZ, South Portuguese Zone; OMZ, Ossa-Morena Zone; CIZ, Central Iberian Zone; Suture, accretionary complex between SPZ and OMZ; CU,accretionary wedge between OMZ and CIZ, named Central Unit. The gray body indicates the locationand geometry of the Iberian Reflective Body. See color version of this figure at back of this issue.


CDP 3950, where Devonian-Carboniferous rocks of theIberian Pyrite Belt are in contact with the Pulo do LoboComplex [Silva et al., 1990] along the Pulo do Lobo Thrust(PT). The upper crust of the SPZ shows a southwest vergentimbricate thrust system with numerous thrusts featuringsmall displacement (Figure 4). The result of a detailedprocessing of high-amplitude reflective bands is shown inFigure 6. It gives support to the geological interpretation ofthe high-amplitude NE dipping events as thrusts and the lessinclined weaker reflections as bedding. Nevertheless, thereflectivity of some packages may have been enhanced byigneous bodies intruded along faults (e.g., 2 and 5 inFigure 4) or along bedding (9, Figure 4). The thrusts ofthe imbricate system merge in the unreflective band (UB),interpreted as a basal decollement placed at 12–15 km. Thisdetachment level is subhorizontal and, surprisingly, lacksrelevant reflectivity, which is probably due to its complex

internal structure and/or nature. From surface geology, it isimpossible to know the stratigraphic level of this decolle-ment, because in the SPZ rocks older than Upper Devoniando not outcrop (Figure 2). The Precambrian-Paleozoicboundary is, in our opinion, a reasonable candidate for theposition of this level. The SPZ upper crust is affected bymore thrusts than shown on geological maps, since some ofthem are blind thrusts represented at surface by faultpropagation folds developed at the top of fault tips(Figure 4). The age of the thrusts and folds in the SPZ isUpper Visean to Namurian, delayed until Westphalian A insouthwesternmost outcrops in Portugal [Oliveira, 1990].[24] The northernmost part of the SPZ has a more

complex structure and more intense strain than the southernpart. The structures developed in a forward propagationsequence, migrating from northeast to southwest, but thereare some out-of-sequence thrusts in the northeast evidenced

Figure 4. (a) Seismic structure of the South Portuguese Zone (SPZ); see also Figure 3. (b) Geologicalinterpretation of the SPZ crustal seismic structure. A number of reflection events are indicated, in order tofacilitate the comparison between Figures 4a and 4b. The Suture between the SPZ and the Ossa-MorenaZone (OMZ) is made up by an accretionary complex which includes the Pulo do Lobo Unit (PL) and theBeja-Acebuches oceanic amphibolites (BAA). In the SPZ there outcrop only Upper Devonian detriticrocks (brown color), Tournaisian - Lower Visean volcanics (red) and Upper Visean - Namurian flysch(stippled). See color version of this figure at back of this issue.


by the seismic reflectivity pattern (Figure 4). The firstthrusts, placed in the uppermost part of the structure, weredeformed by thrusts of the second system. This entirestructure is truncated to the northeast by the Pulo do LoboFault (PT).[25] Within the lower crust of the SPZ, the southward

inclined reflections (6 and 7 in Figure 4) cut a previouslydeveloped reflectivity (8 in Figure 4), whose age could beeither Precambrian or formed during the Early Paleozoicrifting episode. The southward inclined reflections slightlydeform the base of the main decollement at 12–15 km depth(UB), resulting in the wavy shape of this band. Relying onthis geometric argument, the inclined reflections are inter-preted as Variscan thrusts formed after the development ofthe main midcrustal decollement.[26] The very sharp and distinct Moho beneath the SPZ

up to CDP 4000, coupled with the almost constant erosionlevel observed at surface (where only very-low metamor-phic grade, Upper Devonian to Lower Carboniferous rocksoutcrop), favors its interpretation here as an old pre-Varis-can feature. Nevertheless, the Moho has an asymptoticgeometrical relationship with the lower crustal dipping

events, which indicates some Variscan reworking of thispre-orogenic Moho.

5.2. Accretionary Complex and the BoundaryBetween South Portuguese and Ossa-MorenaZones: Image of a Suture?

[27] The Pulo do Lobo Unit, which outcrops between SPZand OMZ (CDPs 3950–4800), contains slices of metabasaltsof oceanic affinity [Quesada et al., 1994], and it is inter-preted as a Devonian accretionary prism formed over oce-anic floor [Eden and Andrews, 1990; Silva et al., 1990]. Tothe north of the Pulo de Lobo, a thin band of basic rocksoutcrops, namely the Beja-Acebuches amphibolites (BAA inFigures 4 and 5) [Bard, 1977]; this band is interpreted as anophiolite [Munha et al., 1986; Quesada et al., 1994; Castroet al., 1996]. The Pulo do Lobo Unit plus the Beja-Ace-buches amphibolites are here referred to as the AccretionaryComplex. Further to the north, high-grade continental meta-morphic rocks on the border of the OMZ (HM in Figure 5)geometrically overlie the ophiolite [Bard, 1969, 1977;Crespo Blanc, 1989, 1992; Castro et al., 1999].

Figure 5. (a) Seismic structure of the Ossa-Morena (OMZ) and the Central Iberian (CIZ) zones; see alsoFigure 3. (b) Geological interpretation of the crustal seismic structure. A number of reflection events areindicated, in order to facilitate the comparison between both figures. In common with Figure 4, it isshown the Accretionary Complex at the suture between the SPZ and the OMZ, which includes the Pulodo Lobo Unit (PL) and the Beja-Acebuches oceanic amphibolites (BAA). A nearly complete successionfrom Late Proterozoic to Early Carboniferous outcrops in the Ossa Morena Zone: gray, UpperProterozoic; blue line, Lower Cambrian carbonates; green line, Middle Cambrian basalts; red line, top ofthe Ordovician; stippled, unconformable Lower Carboniferous. In the southern Central Iberian Zone, theLower Ordovician (green strip) lies unconformable over Upper Proterozoic (gray). See color version ofthis figure at back of this issue.


[28] The southern boundary of the Accretionary Complexis the Pulo do Lobo thrust (PT, Figures 4 and 5). Thenorthern boundary is a high-grade shear zone between theophiolite and the continental rocks of the OMZ, but neitherthis shear zone nor the thin ophiolite are clearly imaged inthe profile. The internal reflectivity of all these units is weakand the base of the Accretionary Complex cannot beconsidered well defined from the seismic image. On thebasis of weak gently dipping reflectivity (Figure 3) andsurface geological data (Figure 2b), the base of the Accre-tionary Complex is suggested to be at about 2 s and have aflat geometry (Figures 4 and 5).[29] The Accretionary Complex plus the high-grade

continental rocks are smeared out by a fan-like ensembleof relatively late faults (between CDPs 3950 and 5200;Figure 3). The faults of this fan merge together at 5 s, theirboundaries being the PT and the Aroche Fault (AF in

Figures 4 and 5). They have a left-lateral component ofdisplacement, which is much more important in thenorthern fault (AF) than in the southern one (PT). Theleft-lateral displacements are perhaps responsible forthe lack of some units characteristic of classical collisionzones, such as the magmatic subduction complex andexhumed high-pressure rocks, both of which are missingalong the section. Small, dismembered ophiolitic klippenimbricated with high-pressure continental rocks in thesouthwest of OMZ [Fonseca et al., 1999], west of theIBERSEIS transect, may be a small representation of thesemissing units. In this sense, it could be said that thesection does not preserve the complete image of the suturebetween the OMZ and the SPZ (Figure 7).[30] The boundary between SPZ and OMZ is not well

defined in the lower crust. A relatively sharp steep boundarycan be traced (W in Figure 5) marked by the abrupt truncation

Figure 6. Detailed analysis of dipping and horizontal reflections. The dipping reflections represented byA and C; B is an example of a horizontal reflection. The top panel shows the location of the events in thestacked section (notice that this analysis was done for reflection events located in the SPZ). The bottompanels show shot gathers (a) centered in each location A, B and C, which image the event analyzed ineach case; (b) is a detailed view of the event, and (c) illustrates the shape, or waveform of the reflection.The shape or waveform of the reflection (c) is compared with the shape of the first arrival (d) for eachshot gather. Note that reflection B shows a change in polarity of the wavelength when compared with thefirst arrival.


of the midcrustal reflectivity of the OMZ (Figure 3). Thismarked lateral change in seismic fabric is probably a result ofout-of-the-plane movements. Beneath 8 s within the lowercrust of the OMZ, the seismic line images two north dippingreflections (S in Figure 5) which may represent the seismicexpression at this level of the suture between the SPZ and theOMZ (Figure 7).

5.3. Upper Crust of the Ossa-Morena Zone

[31] The OMZ extends from CDP 4800 to CDP 10900(Figures 3 and 5). Its upper crustal reflectivity shows theexistence of large synforms and antiforms, and steeplydipping reflection bands. The seismic pattern correlates wellwith the geological cross section obtained from surfacedata (Figure 2b), the steeply dipping reflections mostlycorresponding to faults. Dips of the reflections are somewhatunderestimated due to the slightly oblique orientation of theprofile with respect to the strike of the geologic units.[32] The imaged folds (i.e., the Terena synform, CDPs

5500–7000, and the Monesterio antiform, CDPs 7000–9000) belong to a late folding event. Kilometer-scale recum-bent tight folds determined from geological studies [Vauchez,1975; Exposito et al., 2002] are not seismically imaged due totheir small interlimb angle (Figure 5b). In the southern part ofthe OMZ, the steeply dipping reflections correspond mostlyto thrusts, the most important and best imaged being theMonesterio thrust [Eguiluz, 1987; Exposito, 2000] (c inFigure 5). Another prominent reflection here interpreted asa thrust (in this case, gently dipping) is the one labeled as a;truncation of reflections at b is interpreted as a back-thrust. Inthe northern part of the OMZ, most of the steep reflections arenormal faults, themost important being the one labeled as N1,which bounds the Santos de Maimona Carboniferous basin.

Thrusts and normal faults merge into the midcrustal broadband of strong reflectivity, the IRB. This band coincides withthe detachment level of the upper crustal structures, but itsnature is clearly more complex and will be interpreted in aseparate section. The downward projection of the surfacegeology following themain trends of the reflectivity results inthe interpretation shown in Figure 5b. The tight high ampli-tude reflection d is difficult to interpret and it could be an out-of-the-plane reflection or more likely a dike structure.

5.4. Central Unit: Another Reworked Suture Zone

[33] Between CDPs 10900 and 11450, a wedge ofnortheast dipping reflectivity strongly contrasts and cutsacross the reflectivity on both sides of it (Figure 3). Atsurface, this wedge corresponds to the fault-bounded out-crop of metamorphic rocks of the Central Unit (the Badajoz-Cordoba Shear Zone of other authors). The seismic contrastof this band has its counterpart in the lithologic andmetamorphic singularity of this unit with respect to itssurroundings. The Central Unit includes amphibolites,orthogneisses, paragneisses, schists and extremely rareoccurrences of peridotite; some of the amphibolites havean oceanic signature [Gomez Pugnaire et al., 2003]. Themetamorphic evolution of the Central Unit is characterizedby an initial high-pressure/high-temperature event with peakconditions of at least 15 kbar and 700�C [Abalos et al.,1991; Azor, 1994; Lopez Sanchez-Vizcaıno et al., 2003].This metamorphism has been considered Cadomian bysome authors [e.g., Ribeiro et al., 1990b], but radiometricages support a Variscan origin [Schafer et al., 1991;Ordonez Casado, 1998]. According to these ages, the high-pressure metamorphism is related to the Variscan under-thrusting of the Central Unit beneath the southern border of

Figure 7. Present geometry of the suture between the South Portuguese Zone (SPZ) and the Ossa-Morena Zone (OMZ). The presumed crustal blocks of the SPZ and the OMZ are indicated with differentgray intensities. The Accretionary Complex at the suture is made up by the Pulo do Lobo accretionaryprism of sediments (PL), which includes oceanic basalts (PLB), and the Beja-Acebuches oceanicamphibolites (BAA). Thick lines mean true suture contacts, whereas thinner lines as AF and W are latecollisional faults reworking the original suture. See color version of this figure at back of this issue.


the CIZ, in an early compressional stage whose structuralrecord has become obliterated by later shearing that pene-trates the whole unit. This shearing had a marked left-lateralcomponent and produced the retrogression of the high-pressure assemblages first to amphibolite and then togreenschist facies assemblages, as long as rocks were beingexhumed [Azor et al., 1994; Simancas et al., 2001].[34] The reflectivity in the Central Unit shows a lower

part characterized by high-amplitude energy almost parallelto the southern boundary fault (the Azuaga fault). This bandof reflectivity (CU in Figure 5) corresponds to gneisses andamphibolites, which are dominant in the lower part of theCentral Unit, whereas the less reflective upper part withwavy reflections corresponds to schists. Truncation ofreflections along the northern border of the Central Unitdefines the Matachel fault (N2 in Figure 5).[35] The relatively late Azuaga fault, present-day south-

ern boundary of the Central Unit, has strike-slip displace-ment and a straight trace on geological maps, suggesting asteep dip that has not been confirmed by the IBERSEISimage (observed dip of 45�–50�). The fault bounding thisunit to the north, the Matachel Fault (N2), features a dipwhich is similar in the seismic image and the previousgeological interpretation (compare Figures 5 and 2b). Theoblique (left-lateral and normal) movement of the Matachelfault, established from field geology, is considered to be thelast significant contribution to the exhumation of the CentralUnit [Martınez Poyatos, 1997; Simancas et al., 2001].[36] The Matachel fault cuts the internal reflectivity of the

Central Unit, which progressively thins downward. At 5 s,the extremely thinned Central Unit merges with the midcrustal reflection band and no similar dipping structure isimaged in the lower crust. Instead, the wedge-shaped body at5 s under CDPs 12500–13000, directly overlying the IRB(Figure 5), may be a displaced portion of the Central Unit.[37] The Central Unit contains some of the elements that

characterize a suture and it has been interpreted as such[Azor et al., 1994; Simancas et al., 2001]. Nevertheless, out-of-the-plane movements obliterate the classical image of asuture zone, in the same way as in the boundary between theSPZ and the OMZ. The wrenching displacements haveblurred the geometric picture of superposed continentalblocks so that, at a first glance, seismic fabrics on eachside seem to be face to face in the suture zone. However,reflections at the northern edge of the OMZ dip to thenortheast under the Central Unit, indicating that this regionrepresents the footwall of the suture between the OMZ andthe CIZ. The blurred geometry of the superposition sug-gests, together with the wedge shape of units on thegeological map, that some suture units have been displacedout of the cross section.

5.5. Upper Crust of the Central Iberian Zone

[38] The CIZ upper crust extends from CDP 11450 to theend of the profile (Figure 3). Its main seismic events aresteeply to moderately northward dipping reflections, mainlydeveloped between 2 to 4 s (N3, N4, and N5 in Figure 5).These reflections crosscut gently dipping reflectivity in theupper crust, and have been interpreted as normal faults

according to their characteristics and surface geologicaldata. Geological studies have established the existence ofnormal faults that cut early recumbent folds and are foldedby late upright folds (Figure 2b) [Martınez Poyatos et al.,1995; Simancas et al., 2001], thus allowing us to correlatethese faults with the northward dipping events, and foldedlayering with the gently and variably dipping reflectivity.The architecture of the set of 2–4 s normal faults (N3, N4and N5) suggests a domino-like extensional system, whichwould include the previously described N1 and N2 faults.The sedimentation in the Santos de Maimona Carboniferousbasin is controlled by one of these faults (N1), thusindicating a Visean age for this extensional event.

5.6. Lower Crust of the Ossa-Morenaand Central Iberian Zone

[39] It is not possible to separate in the seismic imagethe lower crust of the OMZ from that originally belongingto the CIZ block. The lower crust that originally belongedto the CIZ may have been displaced to the north duringcontinental convergence. As the IBERSEIS seismic profiledoes not penetrate a long distance into the CIZ, it is notclear whether or not original CIZ lower crust has beenimaged. The lower crust under the present day OMZ andCIZ can be divided into three zones based on differencesin the reflectivity signature: CDPs 6400–9000, 9000–13000 and 13000–15500 (Figure 3). The first and thirdare more reflective than the central one, which featuresonly scattered events. The lack of reflectivity in the centralpart is probably geological in origin and not due toamplitude losses due to the IRB. If the latter were areason, then the lower crust in the adjacent zones shouldalso be poorly reflective. The reflectivity beneath thesouthwestern part of the OMZ is interpreted as related tothe SPZ/OMZ suture zone, as explained previously. Theintense reflection events at the northeastern end of theprofile show crosscutting relationships, suggesting a lowercrustal wedge built up by thrusts (Figure 5). This wedgingresults in a stack that can be related to the generation ofthe Extremadura dome, a broad antiform structure out-cropping just northeast of the IBERSEIS profile.

5.7. IBERSEIS Reflective Body

[40] The most outstanding feature of the entire deepseismic reflection image is the IRB, a conspicuous reflec-tion sequence 175 km long, with an average thickness of1.5 s, located between 6 s beneath CDP 6000 and 3.5 sbeneath CDP 15000 (Figures 3 and 5). As previouslydiscussed, the IRB is characterized by lateral changes inthickness, and consists of an amalgamation of lenses withinternal seismic fabrics. An analysis of the amplitude decayof the seismic data in true amplitude processed shot gathersshows a 20% increase in its amplitude with respect to anyother reflectivity above and/or below except for the Moho.An estimate of the reflection coefficient for this structurecan be calculated by using an average value for the Mohoreflection coefficient. At Moho depth, the P wave seismicvelocity changes from values typical for lower crustal rocks(6.8 to 7.2 km/s) to lithospheric mantle values (above


8.0 km/s). These values and the density contrast provideestimates for the reflection coefficient at the Moho interface,that range from 0.07 to 0.09. Using these values, thereflection coefficient estimated for the IRB ranges from0.06 to 0.08, suggesting that the IRB features relatively highvelocities and densities when compared with the back-ground velocity field (Figure 8). There are three possibleinterpretations for this high amplitude reflective body withinternal structure: (1) a pure tectonic one, as a detachment;(2) a pure magmatic one, as a layered intrusion; and (3) ahybrid one, as a layered intrusion in a detachment level.[41] The seismic image suggests that the northeastern

dipping reflections in the upper crust of the OMZ and theCIZ, that have been interpreted as thrusts and normal faults,merge approximately at the depth level of the IRB (Figure 5).This observation indicates that the IRB coincides with adecollement level, active as such during the shortening andthe extensional episodes of deformation in the region. Nev-ertheless, the seismic image of the IRB does not seem tocorrespond to a decollement level because of its irregularwavy shape and internal structure.[42] Deemer and Hurich [1994] demonstrated, using lab-

oratory measurements of physical properties and syntheticseismic studies, that relatively high reflection coefficients canbe achieved by mafic layered intrusions alone. Thus the highreflectivity estimates for the IRB are well explained by basicand ultrabasic rocks in a layered intrusion. The existence ofan intrusive body at this level of the crust could also explainthe internal structure and irregular thickness of the IRB.However, its precise location and continuity (175 km) isnot clearly explained without some structural control. There-fore we propose a mixed origin for the IRB, consisting ofintrusion of mafic magmatic bodies of mantle origin into astructurally controlled level, probably a main decollement.[43] Surface geological data also favor this interpretation

for the IRB. The OMZ is characterized by the existence ofabundant mafic intrusions of Lower Carboniferous age(diorites, gabbros, gabbros with cumulates), with calc-alkaline geochemical signature [Bard and Fabries, 1970;Capdevila et al., 1973; Aparicio et al., 1977; SanchezCarretero et al., 1990]. A simultaneous manifestation ofmafic magmatism is the volcanism in Lower Carboniferousbasins [Sanchez Carretero et al., 1990]. Ni-Cu ore depositshave been mentioned in the OMZ in relation to mantle-derived gabbroic intrusions. Radiometric ages for thismagmatism cluster around 340 Ma [Dallmeyer et al.,1993, 1995; Casquet et al., 1998; Pin et al., 1999b;Montero et al., 2000], in accordance with geologicallyinferred ages. On this ground, we propose that the IRB isan Early Carboniferous mafic to ultramafic intrusion withina main midcrustal detachment.

6. Discussion and Implications

6.1. Imaging Transpression Tectonics

[44] Surface geological studies in SW Iberia and theIBERSEIS deep seismic reflection profile reveal largeamount of evidence for left-lateral displacements, the most

obvious being the Late Carboniferous fault systems devel-oped at both boundaries of the OMZ [Simancas, 1983;Quesada, 1991]. There are also oblique-slip synmetamorphicshear zones [Crespo Blanc and Orozco, 1988; Crespo Blanc,1992; Azor et al., 1994] of Early Carboniferous age[Dallmeyer et al., 1993; Ordonez Casado, 1998], preferen-tially located again in the OMZ boundaries. Furthermore,regional-scale mapping reveals that the strike of the geolog-ical units is oblique to the OMZ boundaries, with the unitshaving tapering and sigmoidal shape. All this evidence fortranspression applies to the Carboniferous evolution, but thetectonic regime at Devonian times (the early collisional stage)is not well constrained. Paleomagnetic data at global Variscanscale suggest that at the starting of the Variscan Orogeny, thecollision could have been frontal or right-lateral [Badham,1982;Dalziel, 1997; Tait et al., 1997], in accordance with thetectonic evolution reported for central Europe [Franke,2000]. The left-lateral transpressional tectonics of SW Iberiais a direct consequence of its location in the southern branchof the Ibero-Armorican Arc. The indentor models for theIbero-Armorican arc [Matte and Ribeiro, 1975; Brun andBurg, 1982] explain the arcuate shape of the Variscan Orogenin southern Europe (Figure 1) and the broad kinematic picturerevealed by geological studies. This megastructure progres-sively tightened during collision [Perez-Estaun et al., 1988],inducing variable components of left-lateral displacements inSW Iberia [Simancas et al., 2002].[45] The seismic expression of transpression in the

IBERSEIS deep seismic reflection profile is mainlyobserved at the boundaries between major zones. At theSPZ/OMZ boundary, reflections PT, AF and others con-verge at a midcrustal level delineating a fan-like structure(Figures 4 and 5). These reflections correspond to faultswith variable lateral strike-slip components revealing theunequal partitioning of displacements. The deep fan-likeimage of the faults completes the three-dimensional (3-D)wedge geometry of the tapered geological units observedon geological maps. In addition to these 3-D fault wedgegeometries, the most outstanding feature related with trans-pression is the contrasting reflection fabrics on both sides ofthe suture, which indicate wrenching (Figures 3 and 7). Theface-to-face arrangement of the seismic fabrics stronglycontrasts with the images of suture zones in many mountainbelts where packages of reflectivity at one side or both aresubparallel to the suture.[46] A general decoupling between upper and lower crust

is evident from the asymptotic geometry of all faults towardthe middle of the crust in the SPZ (Figures 4 and 5). In thisway, the seismic image demonstrates that a large amount ofthe transpressional movements (shortening on-the-plane andlateral displacement out-of-the-plane) is resolved indepen-dently in the upper crust, giving way to irregular uppercrustal wedges. Small amounts of deformation have beenaccommodated in the lower crust as suggested by the abrupttruncation of the OMZ seismic fabric by reflection W, andthe back-thrusts within the lower crust of the SPZ (Figure 4).Some decollement between the lower crust of the SPZ andits underlying mantle is inferred from the asymptoticgeometry of the lower crustal thrusts toward the Moho.


Figure 8. High-amplitude reflectivity of the Iberseis Reflection Body. (a) Raw shot gathers 2020–2026showing the high-amplitude reflectivity of the IRB between 5.5 and 7 s. (b) Detailed view of the stackimaged between CDPs 7400–9200, showing the high-amplitude reflectivity of the IRB, thesuperimposed box indicates the traces that have been stacked to obtain the amplitude decay curvedisplayed in Figure 8c. (c) Illustration of the reflection amplitude difference between the IRB and theMoho.


[47] Similar features are observed in the OMZ-CIZboundary, where the Central Unit has been imaged as awedge-shaped structure at depth (Figure 5). Together withits tapered geometry shown on geological maps, the seismicimage establishes the 3-D wedge geometry of this unit,characteristic of out-of-the-plane displacements.

6.2. Iberseis Reflective Body

[48] The long, thick, irregular, strongly reflective band inthe middle of the OMZ crust (the Iberseis Reflective Body,IRB) is a major feature of the IBERSEIS seismic profileand, indeed, a very unusual seismic structure. As previouslydescribed, the seismic image of the IRB contains smalldomains of layered reflectivity inter-fingering at differentdips, which have been interpreted as rock layering of adifferentiated mafic and ultramafic igneous body that wouldinclude surrounding rocks with different degrees of assim-ilation. Our interpretation for the IRB gives a coherentframe to a number of important geological data that havebeen extensively reported in SW Iberia but remained poorlyunderstood.[49] 1. Among the singularities of the OMZ, the abun-

dance of Lower Carboniferous mafic igneous rocks hasbeen recognized since a long time [Bard and Fabries, 1970;Capdevila et al., 1973].[50] 2. In addition to these relatively old observations,

modern geochemical and isotopic analysis suggest high heatflow and contamination of Lower Carboniferous magmas bypelitic crustal material [Tornos et al., 2001; Casquet et al.,2001; Salman, 2002]. Thus, in close relation with the maficmagmatism, there are recently investigated Ni-Cu minerali-zations (Aguablanca deposit), for which an evolution includ-ing contamination of a mafic magma by pyritic black slates(the Precambrian ‘‘Serie Negra’’) in a midcrustal magmachamber has been proposed, based on detailed geochemicaland isotopic data [Casquet et al., 2001; Tornos et al., 2001].[51] 3. Just at the southern border of the OMZ (HM in

Figure 5), amphibolite and granulite facies rocks outcrop.They are mainly migmatitic granodiorite, migmatiticgneisses, calc-silicate rocks, amphibolites and basic granu-lites, whose protholiths correspond to the Upper Precam-brian Serie Negra and, perhaps, the overlying LowerCambrian. This metamorphism results from a very unusualcombination of high temperature and low pressure [Bard,1969], implying an intense and concentrated heat source thathas led to proposals such as the subduction of an oceanicridge [Castro et al., 1996]. Instead, we propose that themagma of the IRB represents the heat source for thismetamorphism. Gabbros and amphibolites dated at 340–350 Ma [Dallmeyer et al., 1993; Pin et al., 1999b] are foundtogether with high-grade metamorphic rocks of similar age[Castro et al., 1999], favoring this interpretation.[52] 4. On the Aeromagnetic Map of Spain [Ardizone et

al., 1989], theOMZ shows a great number of local anomalies,most of them related to outcropping mafic rocks, and a large-scale (regional) positive magnetic anomaly that can beexplained by the presence at depth of a relatively planarmafic intrusion of large dimensions.

[53] The IRB is interpreted as the visualization of a greatmagmatic mafic reservoir at midcrustal level in the OMZ. Aspreviously discussed, the emplacement of the IRB may havebeen controlled by a mechanical discontinuity of regionalextent, i.e., a midcrustal decollement. We suggest that thediscontinuous character of the Moho reflection beneath theOMZ, as well as the relatively transparent OMZ lower crust(both facts contrasting with the adjacent SPZ and CIZ) isprobably the result of the intrusion of magmas. This intru-sion reworked the old lower crust destroying its internalstructure. On the other hand, some bright reflectors withpositive polarity in the upper crust of the OMZ mayrepresent magmatic conduits (i.e., d in Figure 5). This mayalso be the case of the domino-like normal faults observed inthe northeastern part of the IBERSEIS profile, and of somethrusts in the OMZ (Figure 5). Most probably, some ofthese faults acted as conduits for the midcrustal magma.Finally, it is interesting to note that present day seismicityin the OMZ [Hernaiz et al., 1996] is located preferentiallywithin the 12–20 km depth, in close coincidence with thelocation of the IRB.[54] A mantle plume that developed under SW Iberia in

the Early Carboniferous is the most plausible geodynamicscenario for all the discussed facts. The mantle plume wouldhave been most active during the period 355–335 Ma, theage of most Variscan mafic rocks. The mantle plumehypothesis accounts for the formation of the IRB and therelated plutons and volcanics in the OMZ, but it would alsoapply to the volcanism of the same age in the SPZ [Oliveira,1990; Nesbitt et al., 1999; Nieto et al., 2000a], with its veryconspicuous mineralizations that form the Iberian PyriteBelt [Leistel et al., 1998; Carvalho et al., 1999; Saez et al.,1999; Relvas et al., 2002]. It is important to note that anabnormally high heat supply has been advocated in order toaccount for such huge hydrothermally driven metal concen-tration [Saez et al., 1999; Relvas et al., 2001]. Going further,it has been also suggested that basic magmas could havecontributed to the chemical and isotopic signatures of themassive sulphide mineralizations of the Iberian Pyrite Belt[Nieto et al., 2000b]. The IBERSEIS seismic image leads usto suggest that the main difference between the contempo-raneous magmatism of the SPZ [Munha, 1983; Mitjavilla etal., 1997; Thieblemont et al., 1998] and the OMZ [Casquetet al., 2001; Salman, 2002] may be the formation in thelatter of a great mafic magma chamber in the middle of thecrust (the IRB), which is not the case in the former. Thiscould also be the main reason for the distinctive metal-logenic evolution of each zone (e.g., compare Almodovar etal. [1998] with Tornos et al. [2001]). On the other hand, theEarly Carboniferous mantle plume hypothesis also helps toexplain the development of a transient extensional stageduring collision [Simancas, 1993; Simancas et al., 2002],well documented from the evolution of the SPZ [Oliveira,1990] and from the development of Lower Carboniferousbasins in the OMZ and the southernmost CIZ [Quesada etal., 1990; Giese et al., 1994].[55] On a larger scale, we suggest that the syn-to-late-

convergent high-temperature metamorphism and volumi-nous magmatism in the Variscides [Zwart, 1967; Henk et


al., 2000] are a consequence of that mantle plume. Anexplanation for these features based on tectonic scenariosdominated by processes such as lithospheric delaminationor slab breakoff seem much less suitable for SW Iberia,because the foreland location of the SPZ would haveprevented this region from the consequences of thosemechanisms. The peculiar high-thermal evolution of thisforeland region is a key element to be considered in anytectonic interpretation.[56] The Cambro-Ordovician period in the OMZ and

other regions of the Variscides records a rifting that hasbeen related to the activity of a mantle plume at that time[Crowley et al., 2000]. We do not support, however, that asingle long-lived (Cambrian to Carboniferous) mantleplume could have existed, because Lower Paleozoic andLower Carboniferous riftings are separated in SW Iberia byan apparently normal collisional stage, at Devonian times.

6.3. Mohorovicic Discontinuity Beneath SW Iberia

[57] The seismic character of the Moho changes laterallyin its frequency content, amplitude, waveform and cyclicity(Figure 3). The Moho reflection beneath the SPZ is sharpand high amplitude, with an almost constant thickness. Atsurface, the erosion level of most of the SPZ is shallow andapproximately constant. This observation, coupled with theseismic character of the Moho suggests that: (1) in the SPZthere was not significant crustal thickening during collision;(2) the Moho imaged in this part of the IBERSEIS profile ismainly an old pre-Variscan Moho, scarcely modified duringthe Variscan collision time. In effect, the asymptotic rela-tionship between the lower crustal reflections in the SPZand the Moho reflection (Figures 3 and 4) indicates someVariscan reworking of this Moho.[58] Under the OMZ, the Moho is discontinuous and,

when observed, increases in thickness up to 0.5–1.0 s(Figures 3 and 5). The emplacement of the IRB is probablythe main reason for the lack of a well-defined Moho in thecentral part of the OMZ. The intrusion of mafic magmasthrough a weakened lower crust would have generatedvertical pathways, which may have been responsible forthe lateral discontinuity of the Moho reflection.[59] Further to the northeast, under the CIZ (Figures 3

and 5), arcuate reflections within the lower crust connectasymptotically with a sharp multicyclic reflection, indicat-ing a Moho of mostly tectonic origin. The Moho beneaththis area is probably a decoupling zone between the mantleand the crust.

6.4. Geodynamic Evolution

[60] The IBERSEIS seismic image provides a practicallyfrozen snapshot of the crust in southwest Iberia, as it was atthe end of the Variscan collision (Figures 3 and 9a). In thelower crust, some seismic fabrics must be due to the poorlyknown Precambrian evolution. Nevertheless, the imagedstructure results mainly from cumulative displacementsalong a history of continental collision lasting from 390 to300 Ma. The main points of this Variscan history, fromgeological data and the IBERSEIS seismic image, are asfollows (Figure 9b):

[61] 1. Collision started in Early to Middle Devonian, atthe OMZ/CIZ boundary. From Middle to Late Devonian(about 385–365 Ma) recumbent SW vergent folds andthrust developed in the upper crust of the OMZ (e.g., MTin Figures 5 and 9a), together with NE vergent folds(back-folding) in the southernmost CIZ [Simancas et al.,2001]. At the same time, deformation in the Pulo do Loboaccretionary prism (PL in Figures 4, 5 and 9a) andobduction of the Beja-Acebuches ophiolite occurred atthe SPZ/OMZ boundary [Fonseca and Ribeiro, 1993].Deformation appears not to have penetrated into the SPZcrust (Figure 9b).[62] 2. Earliest Carboniferous (about 360 to 350 Ma)

was a transient period characterized by extension andmagmatism (Figure 9b). Normal faults responsible forthe thinning of the northernmost OMZ and CIZ uppercrust are mapped at surface and observed in the seismicimage (N1–5 in Figure 5); they developed also in thesouthernmost OMZ and in the SPZ, but are now thrustsdue to later shortening. The tectonic regime would havebeen transtensional (left-lateral), as indicated by the kine-matics of shearing at the boundary between the OMZand the CIZ [Azor et al., 1994; Simancas et al., 2001].Exhumation of the high-pressure rocks outcropping at thatboundary (CU in Figure 5) would have been completed atthis time. Bimodal volcanism is abundant in the SPZ(Iberian Pyrite Belt), as well as in the OMZ and the CIZLower Carboniferous basins. Gabbros and diorites intrudeinto the OMZ upper crust toward the end of this period,probably coming from the IRB magmatic chamber. Dur-ing this period, a mantle plume is considered to haveplayed the major role in the thermal and stress state of theSW Iberian crust.[63] 3. A transpressional period is documented since

Visean times, lasting until the blockade of the continentalcollision (345–300 Ma). Plutonism is initially mostly mafic,but then becomes granitic. The contractive deformationaffecting the SPZ was entirely developed in this period, aswell as a second stage of shortening in the OMZ and theCIZ (Figure 9b). Left-lateral displacements due to thetranspressional tectonic regime are mainly concentrated atthe boundaries of the OMZ, first as oblique syn-metamor-phic shear zones [Crespo Blanc and Orozco, 1988] and thenas semibrittle to brittle strike-slip fault systems; thesestructures appear well defined in the seismic image (e.g.,PT and AF in Figures 4 and 5). Basaltic dikes and a fewhalf-graben basins (e.g., the Viar basin), filled with unde-formed molassic sediments and basalts [Simancas, 1983;Sierra and Moreno, 1998], bear witness to the latestextensional readjustments of the Variscan lithosphere(300–280 Ma). We interpret that the flat, subhorizontalMoho in SW Iberia is a frozen image of the Moho at thistime.

7. Conclusions

[64] The IBERSEIS deep seismic reflection transect,acquired using the Vibroseis technique, provides the mostcomplete continuous crustal section of the Variscan Belt of


Europe. The image documents the changes in structurethroughout the orogen and reveals the partitioning ofdeformation in three dimensions and in time. The seismicimage provides the main constraints for the reconstructionof the geometry and history of this orogen, produced byoblique collision of lithospheric plates.[65] The high-resolution image obtained shows a thrust

pattern within the SPZ, which is characteristic of forelandthrust and fold belts. The thrusts dip to the northeast andmerge in the middle crust, indicating a major decouplingzone at this level. An unusual feature of the SPZ is theexistence of lower crustal southwest dipping reflections,interpreted as back-thrusts. Folds and thrusts in the uppercrust of the OMZ are also accommodated in the middlecrust, as well as a set of northeast dipping reflectorsinterpreted as normal faults which dominate the structural

grain of the CIZ. Under the CIZ, a wedge-like structure isprominent in the lower crust.[66] The boundary zones SPZ/OMZ and OMZ/CIZ dis-

play dipping reflectors with a fan-like geometry, interpretedas faults with variable strike-slip component of displacement.This imaged faults, and the tapering geometry on geologicalmaps of outcropping units, define the 3-Dwedge geometry ofthe upper crust due to transpressional collision. Out-of-the-plane displacements produced by this collision can be themain reason why the boundaries SPZ/OMZ and OMZ/CIZlack or have a poor representation of some units characteristicof sutures in classical collision zones, such as the magmaticsubduction complex and the exhumed high-pressure rocks.[67] A 175 km long thick band of relatively high ampli-

tude reflectivity beneath the OMZ and the CIZ (the IRB) isinterpreted as a structurally layered mafic/ultramafic body

Figure 9. (a) Crustal architecture of Southwest Iberia, after our interpretation of the IBERSEIS deepreflection seismic profile. (b) Interpretation of the Variscan evolution of Southwest Iberia. Subduction inthe boundary SPZ/OMZ (with the development of an accretionary prism and ophiolitic obduction)happened at the same time that collision started in the OMZ/CIZ boundary. A transtensional stage duringthe Tournaisian is related with the extrusion of volcanics, mainly in the SPZ, and the intrusion of a greatvolume of magma at a midcrustal level of the OMZ (the Iberian Reflective Body in the IBERSEISseismic profile). Since Visean, transpressional shortening dominated until the end of collision, giving thefinal architecture of the southern Iberia crust. See color version of this figure at back of this issue.


that intruded along a midcrustal decollement. This bodyplayed an important role in the petrological evolution andthe origin of mineralizations in the OMZ. A disturbance inthe mantle, perhaps a mantle plume, interfering with anoblique collision is the global scenario we envisage toaccount for the Early Carboniferous evolution of SWIberia, which is characterized by basin formation, abundantbimodal magmatism and, in the SPZ, an exceptionaldevelopment of mineralizations. In fact, evidences for adeep thermal disturbance in Carboniferous time are foundin most of the Variscides and it is tempting to invoke acommon general cause.[68] The crust is 30–35 km thick all along the IBERSEIS

profile, as suggested by the horizontal Moho reflectionlocated at approximately 10.5 s.

[69] Acknowledgments. Funding for the field acquisition of theIBERSEIS deep seismic reflection profile has been provided by CICYT-FEDER (1FD1997-2179/RYEN1), Junta de Andalucıa, ENRESA, SwedishResearch Council and the Instituto Geologico y Minero de Espana. Thisresearch was supported also by the Spanish Ministery of Science andTechnology (grants BTE2000-0583-C02-01, BTE2000-3035-E, andBTE2000-1490-C02-01). We gratefully acknowledge the support given byJ. A.Almarza andM.Donaire, from the Junta deAndalucıa, andR.Rodrıguezand J. L. Plata from the Instituto Geologico y Minero de Espana. We alsothank the field crew of our contractor CGG for their assistance during theacquisition. Special thanks are given to the university fieldwork team, led byH. Palm and constituted by F. Alonso, J. Alvarez Marron, F. Bohoyo,T. Benıtez, S. Castillo, T. Donaire, J. R. Figueira, I. Flecha, E. Galadı,J. Galindo Zaldıvar, F. Gonzalez, P. Gonzalez Cuadra, I. Macıas, D. Martı,A. Martın, L. M. Martın Parra, J. R. Martınez Catalan, J. M. Nieto, J. M.Nogales, P. Ruano, M. Ruiz, M. Sanchez Gomez, and M. Toscano. Theirenthusiastic work in hard weather conditions has been essential for theaccomplishment of this project. Constructive comments by A. Ribeiro,A. Mateus, and an anonymous reviewer are sincerely acknowledged.

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SIMANCAS ET AL.: CRUSTAL STRUCTURE OF SW IBERIA

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Figure 3. (a) Topography (black line) and Gravity (red line) profiles along the transect IBERSEIS, thestations numbers and the corresponding CDP’s are also indicated. (b) Stacked image, (c) time migratedstack image and (d) line drawing of the IBERSEIS deep seismic reflection profile. The location of themain tectonic units along the seismic profile is indicated: SPZ, South Portuguese Zone; OMZ, Ossa-Morena Zone; CIZ, Central Iberian Zone; Suture, accretionary complex between SPZ and OMZ; CU,accretionary wedge between OMZ and CIZ, named Central Unit. The gray body indicates the locationand geometry of the Iberian Reflective Body.


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Figure 4. (a) Seismic structure of the South Portuguese Zone (SPZ); see also Figure 3. (b) Geologicalinterpretation of the SPZ crustal seismic structure. A number of reflection events are indicated, in order tofacilitate the comparison between Figures 4a and 4b. The Suture between the SPZ and the Ossa-MorenaZone (OMZ) is made up by an accretionary complex which includes the Pulo do Lobo Unit (PL) and theBeja-Acebuches oceanic amphibolites (BAA). In the SPZ there outcrop only Upper Devonian detriticrocks (brown color), Tournaisian - Lower Visean volcanics (red) and Upper Visean - Namurian flysch(stippled).


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Figure 5. (a) Seismic structure of the Ossa-Morena (OMZ) and the Central Iberian (CIZ) zones; see alsoFigure 3. (b) Geological interpretation of the crustal seismic structure. A number of reflection events areindicated, in order to facilitate the comparison between both figures. In common with Figure 4, it isshown the Accretionary Complex at the suture between the SPZ and the OMZ, which includes the Pulodo Lobo Unit (PL) and the Beja-Acebuches oceanic amphibolites (BAA). A nearly complete successionfrom Late Proterozoic to Early Carboniferous outcrops in the Ossa Morena Zone: gray, UpperProterozoic; blue line, Lower Cambrian carbonates; green line, Middle Cambrian basalts; red line, top ofthe Ordovician; stippled, unconformable Lower Carboniferous. In the southern Central Iberian Zone, theLower Ordovician (green strip) lies unconformable over Upper Proterozoic (gray).


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Figure 7. Present geometry of the suture between the South Portuguese Zone (SPZ) and the Ossa-Morena Zone (OMZ). The presumed crustal blocks of the SPZ and the OMZ are indicated with differentgray intensities. The Accretionary Complex at the suture is made up by the Pulo do Lobo accretionaryprism of sediments (PL), which includes oceanic basalts (PLB), and the Beja-Acebuches oceanicamphibolites (BAA). Thick lines mean true suture contacts, whereas thinner lines as AF and W are latecollisional faults reworking the original suture.


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Figure 9. (a) Crustal architecture of Southwest Iberia, after our interpretation of the IBERSEIS deepreflection seismic profile. (b) Interpretation of the Variscan evolution of Southwest Iberia. Subduction inthe boundary SPZ/OMZ (with the development of an accretionary prism and ophiolitic obduction)happened at the same time that collision started in the OMZ/CIZ boundary. A transtensional stage duringthe Tournaisian is related with the extrusion of volcanics, mainly in the SPZ, and the intrusion of a greatvolume of magma at a midcrustal level of the OMZ (the Iberian Reflective Body in the IBERSEISseismic profile). Since Visean, transpressional shortening dominated until the end of collision, giving thefinal architecture of the southern Iberia crust.


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Crustal structure of the transpressional Variscan orogen of SW Iberia: SW Iberia deep seismic...

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