Crustal Doubling Research Articles

Andean evolution, orogenic deformation and uplift of the Western Cordillera and Altiplano of southern Peru, northern Bolivia and Chile: Eocene-Oligocene lithospheric delamination

Reappraisal of Andean evolution in southern Peru, northern Bolivia and northern Chile provides support for models of orogenic cyclicity in which Andean crustal shortening in the Western Cordillera led to Eocene-Oligocene lithospheric delamination which triggered a renewed pulse of Andean uplift. The sedimentary fill of the Arequipa "rift" back-arc basin (Rhaetian-Neocomian) was controlled by two major graben-bounding fault systems: the Condoroma-Caylloma and Cincha-Lluta-Incapuquio; as well as the Cusco-Lagunillas fault system separating the back-arc region from the Cusco-Puno high. These thinned cortical inherited structures, were the most favourable places to undergo a positive tectonic inversion at the beginning of Andean compression (Albian), and then during Late Cretaceous-Paleocene shortening, they continued to develop as east-vergent crustal decollements which allowed crustal doubling and Western Cordillera thickening. The crustal thickening of >45 km took place along the Condoroma-Caylloma inverted graben where the lower crust, which was probably enriched in basalts, was subjected to high-pressure metamorphism leading to eclogitization, and became denser than the underlying mantle lithosphere and therefore delamination occurred. The subsequent asthenospheric upwelling led to lithospheric thinning, asthenospheric mantle magmatism of the first stage of Andahuaylas-Yauri batholith emplacement (∼48-43 Ma), which crystallized under temperatures of 1200–700 °C and pressures of 1.0–0.5 GPa (>40-30 km deep). This process was followed by the rapid surface uplift of the Western Cordillera that reached 3.1 km and fed the Kayra and Lower Moquegua basins, located on the Altiplano and in the forearc respectively. The delamination process was still active during 43–30 Ma, with second-stage magmatism of the Andahuaylas-Yauri batholith, emplaced continuously from deep mafic intrusions (40-30 km) to superficial porphyries (3 km). This indicates an important uplift of the Western Cordillera to 3.3 km, supported by mantle upwelling and the continuous flat-slab compression that prevented normal faults from developing, and rather produced the shortening of the delaminated crust. The Anta-Soncco foreland basins (43-30 Ma) of the Altiplano and Moquegua basin of the forearc, received the products of Cordilleran denudation at a rate of up to 700 m/My. The alkaline potassic magmatism (29–27 Ma) could represent episodes of post-delamination magmatism and the late stage of flat subduction.

No mafic layer in 80 km thick Tibetan crust

All models of the magmatic and plate tectonic processes that create continental crust predict the presence of a mafic lower crust. Earlier proposed crustal doubling in Tibet and the Himalayas by underthrusting of the Indian plate requires the presence of a mafic layer with high seismic P-wave velocity (Vp > 7.0 km/s) above the Moho. Our new seismic data demonstrates that some of the thickest crust on Earth in the middle Lhasa Terrane has exceptionally low velocity (Vp < 6.7 km/s) throughout the whole 80 km thick crust. Observed deep crustal earthquakes throughout the crustal column and thick lithosphere from seismic tomography imply low temperature crust. Therefore, the whole crust must consist of felsic rocks as any mafic layer would have high velocity unless the temperature of the crust were high. Our results form basis for alternative models for the formation of extremely thick juvenile crust with predominantly felsic composition in continental collision zones.

Crustal structure beneath North-West Iberia imaged using receiver functions

In the last years, the deep crustal structure of the North-Western part of the Iberian Peninsula has been explored using teleseismic receiver function (RF) analysis of P to S conversions at main crustal interfaces. This area has been previously investigated by seismic reflection and refraction experiments and therefore provides an excellent opportunity to compare the results of both approaches. The region shows the imprint of two orogenic events, the Variscan and Alpine ones that exhibit different, reverse intensities from west to east. In a first stage, a N–S transect across the Cantabrian Mountains was instrumented to study the area affected by the Alpine compressional tectonics. Later on, the limit between the undisturbed Variscan units and the reworked Alpine zones was explored by N–S and E–W transects. Finally, an array was deployed in the NW edge of Iberia, over the Variscan hinterland zone. The receiver functions are calculated by inverse filtering deconvolution of the L component from the Q component, and the resulting RF are processed using a simple form of migration to obtain images in depth of the lithosphere that can be compared to the 2-D velocity–depth models from active seismic experiments. The deep crustal structure constrained by both techniques is remarkably consistent, and provides further evidence on the crustal doubling and wedging between the Iberian and European crusts throughout the northern part of the Iberian Peninsula affected by the Alpine tectonics. The undisturbed Variscan domains, characterized by a clear subhorizontal Moho and few intracrustal convertors, show up at the southern edge of the N–S transects (southward of the Alpine deformation front) and over the Variscan hinterland in the E–W transect. The crust resulting from Alpine reworking presents a complex structure, with short and frequently dipping convertors, which in some cases seem to image preserved Variscan structures.

Imaging the transition from Aleutian subduction to Yakutat collision in central Alaska, with local earthquakes and active source data

In southern and central Alaska the subduction and active volcanism of the Aleutian subduction zone give way to a broad plate boundary zone with mountain building and strike‐slip faulting, where the Yakutat terrane joins the subducting Pacific plate. The interplay of these tectonic elements can be best understood by considering the entire region in three dimensions. We image three‐dimensional seismic velocity using abundant local earthquakes, supplemented by active source data. Crustal low‐velocity correlates with basins. The Denali fault zone is a dominant feature with a change in crustal thickness across the fault. A relatively high‐velocity subducted slab and a low‐velocity mantle wedge are observed, and high Vp/Vs beneath the active volcanic systems, which indicates focusing of partial melt. North of Cook Inlet, the subducted Yakutat slab is characterized by a thick low‐velocity, high‐Vp/Vs crust. High‐velocity material above the Yakutat slab may represent a residual older slab, which inhibits vertical flow of Yakutat subduction fluids. Alternate lateral flow allows Yakutat subduction fluids to contribute to Cook Inlet volcanism and the Wrangell volcanic field. The apparent northeast edge of the subducted Yakutat slab is southwest of the Wrangell volcanics, which have adakitic composition consistent with melting of this Yakutat slab edge. In the mantle, the Yakutat slab is subducting with the Pacific plate, while at shallower depths the Yakutat slab overthrusts the shallow Pacific plate along the Transition fault. This region of crustal doubling within the shallow slab is associated with extremely strong plate coupling and the primary asperity of the Mw 9.2 great 1964 earthquake.

Paleozoic and Mesozoic Tectonic Evolution of Central Asia: From Continental Assembly to Intracontinental Deformation

Thirty years ago, the interior of Asia was inaccessible to most Earth scientists, as was much of the relevant literature. Students of continental evolution were frustrated. We knew that Asia, nearly alone among the Earth's continents, had been assembled in relatively recent geological times, but we could only speculate about the processes that led to its assembly. In the past 27 years, Asian and international researchers have improved our understanding. Ideas about continental evolution as diverse as modeling the deforming lithosphere as a thin, viscous sheet; tectonic escape; Altaid style assembly; crustal doubling; orogenic collapse; delamination; the role of ultra‐high pressure metamorphism; the role of localized erosion and active deformation on local and continental scales are all being tested in Asia. There is no consensus yet, but the Asian natural laboratory is being used well. We may soon come to know which ideas we should abandon.

Interpretation of heat-flow density in the Central Andes

This study focuses on the interpretation of surface heat-flow density data in the central Andes to examine possible lithospheric thermal structures. Along a profile at about 21°S from the Peru–Chile trench in the west to the Andean foreland in the east, surface heat-flow density is used to constrain quantitative models that are employed to investigate the influence of various effects on the thermal field. Negligible changes in surface heat-flow density along strike of the Andean orogen, in contrast to major variations across the orogen, seem to justify the application of two-dimensional (2-D) cross-section models. The region of the Andean orogen between the trench and the volcanic front is described by 2-D subduction models, the back-arc region, characterised by crustal doubling, is analysed by 2-D models describing simple underthrust phenomena and the Andean foreland is investigated by one-dimensional temperature estimates. The influence of various geometrical and petrophysical parameters on the thermal structure is investigated. Model calculations show that the low heat-flow density values in the fore-arc region suggest low shear stresses on the order of 15 MPa along the plate contact. Variations of the subduction angle influence mainly the W–E extent of the surface heat-flow density anomaly. The distribution of the crustal radiogenic heat-production rate influences the surface heat-flow density significantly, but has only a minor influence on crustal temperatures. Modelling results show that temperatures at the maximum depth of seismic coupling between the oceanic Nazca plate and the overriding South American continental plate (∼45 km) are on the order of 250 to 300°C, whereas at the maximum depth of the plate contact (∼60 km) temperatures are on the order of 300 to 350°C. The lithospheric temperature structure in the active magmatic arc region is strongly sensitive to temperature changes caused by the occurrence of an assumed asthenospheric mantle wedge at shallow depth (∼70 km). To match conditions for melting at subcrustal levels, the minimum extent of an asthenospheric wedge towards the west has to coincide with the position of the volcanic front. The thermally thinned lithosphere results in a surface heat-flow density on the order of 60–70 mW m −2. Locally higher heat-flow density in the volcanic arc is interpreted as the effects of shallow magma chambers. The doubling of the crust in the back-arc region affects the lithospheric thermal structure considerably, but models do not show reverse temperature–depth distributions in the continental crust. In the back-arc region the transition from a thermally thinned lithosphere to a `normal' lithospheric thickness for continental shields may occur. The observed surface heat-flow density in the Andean foreland suggests lithospheric thermal conditions as typical for shield areas.

Recent deeper geophysical results better account for the tectonics in the Italian area

Results from extended DSS profiles (1956-1986) in Italy and surrounding land and sea areas offer good constraints for other geophysical and geological data. Integrated interpretations outline the main tectonic features. Collisional tectonics is predominant in the Alps, for which the Adriatic plate acted as hinterland against the European plate foreland. Main results: W-wards, NW- and N-wards oriented overthrusting on the European crust, bending of the lower European crust, European Moho to 70 km depth with the Adriatic mantle indented above, crustal doubling (Adriatic over the European one). In the Apennines, on the contrary, the Adriatic plate acted as a foreland, against the overthrusts generated by the Tuscanian and Tyrrhenian mantellic bodies, heated, elevated and migrated NE-wards and SE-wards, respectively. Also the Adriatic plate bends under this load-centripetally towards the Tyrrhenian sea, so that the Adriatic Moho from 35 km depth is presumed to descend through a flexure till 40-50 km below the Tuscanian and Tyrrhenian land areas. The external peri-Apenninic area is still in compression and includes thick sedimentary basins, from the Po-plain to Sicily. The internal area is in extension, overlapped by thin, stretched crusts of Ligurian and Tyrrhenian origin, whose remnants occupy most of both seas areas, with two areas of oceanic crust in the SE-Tyrrhenian. Rifting and opening is in action also in the Ligurian Sea and Sicily Strait.

Processing of crust in the root of the Caledonian continental collision zone: the role of eclogitization

Segments of the root to the Caledonian collision zone are exposed along the west coast of Norway, allowing the study of processes and petrophysical conditions at the deepest crustal levels. P-T conditions prevailing in these crustal root zones correspond to eclogite facies. Eclogitization is associated with marked changes in petrophysical properties, notably a density (10–15%) and Vp increase which may give a mantle signature to an eclogitized crust. The observed ductility enhancement may be caused by transformation plasticity, reduction in grain size, formation of new mineral assemblages and presence of fluid. In the Western Gneiss Region and the Bergen Arcs, eclogites form along shear zones, interpreted as fluid pathways with sharp boundaries to their metastable protolith. This makes eclogite shear zones potential deep crustal reflectors. Dry crust subducted into roots of continental collision zones will undergo eclogitization only if hydrous fluids are available. Timing of metamorphic reactions does not depend on crossing of equilibrium boundaries in the P-T space, but rather on the introduction of fluids into the system. Eclogites are not only passive recorders of P-T-t paths illustrating very deep subduction of crust (> 100 km); rapid changes in petrophysical properties make fluid-induced eclogitization a dynamic process that influences geodynamics. Stresses set up by the volume changes or by tectonic forces are released by ductile deformation along shear zones and by seismic faulting as evidenced by syn-eclogitic pseudotachylytes in the metastable protolith. Deformation, including seismicity, enhances fluid infiltration and further eclogitization. Dense and rheologically weak eclogites may delaminate and sink into the mantle. In the absence of fluids, crustal doubling may occur without metamorphic re-equilibration. The evolution of a collision zone depends on the fluid regime which should be considered in geodynamic modelling in addition to P and T.

Rheology and seismotectonic regime in the northern central Mediterranean

The connection between thermal field and mechanical properties is analysed in the northern central Mediterranean region, extending from the Ligurian-Provençal basin to the Adriatic foredeep. As the thermal regime is still far from equilibrium in most of the tectonic units, transient thermal models are used. The temperature-depth distribution is estimated in four areas affected by the volcanic activity, which from the Neogene to the Present shifted from Corsica to the Apenninic arc. In the Adriatic foredeep, the thermal effects of the recent thrust-faulting phase in the buried sectors of the northern Apennines are taken into account. The general context consists of convergence involving westward subduction of the Adriatic plate. This process caused anti-clockwise rotation of Corsica and Sardinia, which led to formation of the Ligurian marginal basin, and also resulted in crustal doubling and overthrusting in the northern Apennines and rifting in the northern Tyrrhenian. Seismic activity is focused in the internal and external zones of the Apenninic arc, where low surface heat flux is observed, and in the western margin of the Ligurian-Provençal basin. This is a consequence not only of lateral variations in the thermal field but also of the different tectonic settings. Regional extensional patterns in the shallow crust, with minimum principal stress axes trending N60°E and E-W, are observed in the northern and in the southern sectors of the Apenninic arc, respectively. A compressional regime at depths greater than 30 km is observed below the northern sector of the arc, while to the south a change in the structure of the lithosphere is marked by a decrease in deeper seismic activity. Thrust faults and strike-slip faults with a thrust component support a compressional regime along the western margin of the Ligurian basin with maximum principal stress axis oriented N120°E. Two lithospheric cross-sections across the study region are constructed, based on structural, thermal, gravity, rheological and seismic data. There is clear evidence of the presence of the subducting slab of the Adriatic plate, corresponding to a thickening of the uppermost brittle layer. The crustal seismicity cut-off corresponds to temperatures of 320–390°C. A brittle layer of considerable thickness is present in the uppermost mantle beneath Variscan Corsica and the Adriatic foredeep, with estimated seismic cut-off temperature of about 550 ± 50°C.

PT conditions of metamorphism in the Wami River granulite complex, central coastal Tanzania: implications for Pan-African geotectonics in the Mozambique Belt of eastern Africa

Solid-solution equilibria for gamet-clinopyroxene pairs in the early Pan-African Wami River granulite complex of central coastal Tanzania indicate metamorphic recrystallization at a temperature of about 700°C and a pressure of 8–9 kb, corresponding to metamorphism at a depth of 30–40 km. This suggests that granulite formation was preceded by an anomalous regional crustal thickening, similar to the crustal doubling that accompanies Phanerozoic continent-continent collisions of the Himalaya type. The analogy prompts the interpretation of the Wami River granulite complex, and possibly the rest of the granulite complexes in the Mozambique Belt, as slices of the underthrusted plate, which were accreted to the present day African plate following a continent-continent collision during early Pan-African time.

Large-scale three-dimensional seismic tomography of the Zagros and Pamir-Hindukush regions

The nature of the deep seismic structure of the Zagros and Pamir-Hindukush regions is inferred from the three-dimensional P-wave velocity images obtained using the Aki, Christofferson and Husebye (ACH) inversion technique. We use teleseismic (25 ⩽ Δ ⩽ 90°) P-wave arrival times, recorded at 24 seismic stations during the period 1977–1986. A study of the azimuthal variation of residuals reveals that despite the crustal doubling, the Pamir-Hindukush region is characterized by faster arrivals (≈ − 1.0 s) of teleseismic P-waves. The Iranian plateau, on the other hand, exhibits delayed arrivals (0.4–1.0 s) from teleseisms. The results of the ACH inversion reveal contrasting P-wave velocity characteristics in the upper mantle of these deformed zones. In the depth interval 0–200 km, the Pamir-Hindukush region is characterized by 1–4% faster velocities relative to the Iranian plateau. Accounting for the anomalous crustal thickening in the Hindukush results in the subcrustal mantle (up to 200 km) exhibiting a significant higher velocity (≈ 2%), comparable to that of the Indian plate. However, in the depth interval 200–450 km, the Pamir-Hindukush region has 1–3% lower velocities relative to the Indian plate, while the upper mantle beneath the central Iranian plateau is 2–4% slower relative to the surrounding region. The high-velocity upper mantle till a depth of 200 km beneath the Hindukush region, reflects the possible presence of both the oceanic lithosphere and the crust and uppermost mantle of the Indian plate, which subducted beneath Eurasia. The low velocity observed beneath central Iran could be a reflection of the upper mantle thermal anomalies related to the Pliocene-Quaternary volcanism in this region.

A balanced model of the southern Central Andes

To explain the actual crustal thickness of the southern Central Andes a balanced cross‐section at 21°S latitude was modeled. A forward modeling procedure was used on a crustal scale to explain the gross crustal structure obtained by seismic refraction investigations. In the model, 320 km of shortening since late Cretaceous, based on balanced geological cross sections, are assumed for the whole Andes from the trench to the foreland, and the initial crustal thickness varies between 30 and 40 km. It results that about 20% of the actual crustal volume must be explained by other sources than crustal shortening. Structures of high‐velocity material in the upper crust of the Eastern Cordillera can be explained by detached and overthrust material from the crustal base thus producing a crustal doubling in the back arc. Underplating of continental material that was removed by subduction erosion and magmatic addition of derivates of the subducted Nazca plate may contribute to the lower crust of the forearc and arc regions. Crustal thickening of the Eastern Cordillera and parts of the Altiplano was undoubtedly produced by tectonic thickening, whereas the sources of thickening in the Western Cordillera and forearc region are uncertain.

Tectonics and heat sources for granulite metamorphism of supracrustal-bearing terranes

There are generally three important processes in the granulite-facies metamorphism of supracrustal rocks: (1) transport from the surface to 15–30+ km, (2) heating to 700–800+ °C, and (3) transport back to the surface and re-exposure at the top of a crust of “normal” thickness (35–40 km). Mantle-derived magmatism, and/or magmatic and/or metasomatic fluid flow, can transport the necessary heat for granulite metamorphism, but many granulite terranes do not contain sufficient volumes of mafic intrusive rocks, or evidence of extensive fluid flow, to readily explain heating by this mechanism. One possible process for the formation of large granulite terranes involves continental collision and crustal doubling by simple-shear thrusting. Heating of the underthrust supracrustal rocks would occur by thermal relaxation, but, if granulite-facies temperatures are to be attained at mid-crustal levels, this mechanism necessarily produces extensive melting of the lower crustal plate, suggesting that granulite terranes formed in this way should contain substantial syn- or post-metamorphic crustally-derived intrusions. In this situation, subsequent uplift and re-exposure at the surface is a natural consequence of erosion and isostatic readjustment. An alternative process which may allow the heating of supracrustal rocks to granulite-facies temperatures is imbricate thrusting in which thin slivers or “flakes” of supracrustal rocks are thrust down to the lowermost crust. This mechanism requires a separate tectonic event, such as a second thrusting event, to transport the granulite-facies rocks back to the surface. Other possible heat sources for supracrustal granulite terranes include preheating of the overthrust crust by arc or pre-orogenic magmatism, and shear heating on thrust planes. Models indicate that pre-heating of the overthrust plate has only minor thermal effects upon the thermal history of the underthrust crust, although shear heating may be significant. Heat sources for granulite metamorphism of the Archean Limpopo Belt of South Africa are considered in terms of possible tectonic mechanisms. The metamorphic grade and presence of abundant supracrustal rocks in this terrane demonstrate that it was once at the Earth's surface, buried to ∼ 8.5 kbar (∼ 28 km) and heated to ∼ 850°C, then re-exposed at the surface. There are no recognizable mantle-derived igneous rocks of suitable age, and no evidence for extensive fluid flow that could have provided a magmatic heat source for the metamorphism. Shear heating is unlikely because the pattern of metamorphic isograds is not related to faults or shear zones. Imbricate thrusting of thin silvers of supracrustal rocks into the lowermost crust to allow heating without magmatism would require a separate tectonic event to transport the granulites to the surface; there is no evidence for such a later event. The most satisfactory and self-consistent model for Limpopo metamorphism involves thermal relaxation of a perturbed temperature profile produced by Tibetan-style continental collision of the Zimbabwe and Kaapvaal Cratons, with consequent crustal doubling to at least 65 km thick, and the generation of crustal melts which advected heat into the zone of granulite metamorphism. There is no evidence for significant pre-collisional heating of either crustal plate; both probably had typical “shield-type” pre-collisional geothermal gradients. Crustally-derived magmas, therefore, must have been produced during thermal and structural equilibration of the Limpopo collision. Abundant granitoid rocks of similar age to the metamorphism occur both as migmatites and as discrete plutons.

Crustal structure of the Southern Alps beneath the intersection with the European Geotraverse

Newly digitized and amplitude controlled record sections from the 1977 Southern Alps refraction campaign permitted a reinterpretation of the crustal structure in the area between western Lombardy and the Giudicaria fault. The resulting model exhibits considerable lateral heterogeneity: in the west, below 7.5 km of sediments of the Lombardy Basin, the crust reaches a depth of only 31 km, whereas it thickens towards the more mountainous area in the east, reaching a depth of 46 km below the Adamello Massif. Although the signal character of the corresponding reflections is somewhat erratic, the data are satisfied best by models with a low-velocity zone in the upper crust. An additional small velocity discontinuity from 6.2 to 6.4 km/s was found in the middle crust at around 20 km. Earlier interpretations, based on travel-times alone, included a layer with a velocity of about 7 km/s at this depth. This high-velocity layer was then interpreted as lower-crustal material of the Adriatic — African plate, which had been overthrust onto the European plate during the Alpine orogeny, thus explaining the uplift of the Southern Alps. However, this model of crustal doubling is questionable, because such a mid-crustal high-velocity layer is not in agreement with the amplitude data. The relatively thin crystalline part of the crust under the Lombardy Basin is interpreted, in accordance with geological evidence, as a relic of a Late Hercynian rifting event.

Kibaran events in the southernmost Mozambique belt

An area of ∼ 20 000 km 2 in northern Mozambique has been mapped at 1 : 50 000 scale. The dominant, ∼ 1000 Ma old Lurian event (Kibaran orogeny) produced the “Namama belt” — a SE vergent satellite of the Lurio belt described further to the north by Bureau de Recherches Géologiques et Minières (BRGM) in the 1970's. To the west of Namama belt, the Lurian event resulted in an extensive nappe of granulitic rocks. This is perhaps rooted some 200 km to the WNW near the Malawi border. This crustal doubling has led to local increases in metamorphic grade beneath the nappe. The Lurian event involved both pre-Kibaran basement and “Lurian” cover. Its development was preceded by subsidence and crustal thinning that influenced the deposition of the cover. At the NW margin of the Namama belt is a zone of intense deformation and thrusting, where siliceous metasediments are associated with basic and some ultrabasic rocks of ophiolitic aspect. The development of the Kibaran event was accompanied by the emplacement of batholitic granites. The whole-rock Rb/Sr isochrons give Kibaran isotopic ages and low initial ratios even for the supposedly oldest parts of the basement, suggesting pervasive homogenisation. The subsequent Pan-African event (∼ 500 Ma) resulted in the emplacement of circumscribed, crust-derived granites, and radiometric rejuvenation of the minerals, but no Pan-African orogenic process is recognised. The c. 1000 Ma old events of Mozambique fit well in the frame of a major, Lurian-Irumide (Kibaran) orogeny involving considerable crustal shortening.

Implications of the seismic structure for the orogenic evolution of the Pyrenean Range

The dynamic evolution of the Pyrenees is discussed in the light of geophysical data. Recent deep seismic sounding have revealed the crustal structure of the Pyrenees which is used to test the different evolutionary models proposed until now. The crustal thickness of the Paleozoic Axial Zone (PAZ) and the North Pyrenean Zone (NPZ) differ by more than 10 km, ranging from about 30 km in the NPZ to 40–50 km in the PAZ. The transition from PAZ to the NPZ, identified at the surface as the North Pyrenean Fault (NPF), is sharp at depth and marked by a vertical step, at least in the eastern half of the range. The NPZ is characterized by additional throws and dips of the Moho in the east whereas in the west a heterogeneous middle to lower crust is encountered, with high velocity anomalies. The seismic results suggest that the PAZ and the NPZ belong to different plates, the NPF being the plate boundary. These results are inconsistent with evolutionary models involving lithospheric subduction or crustal doubling and intracratonic rifting with the main tectonic lineations following NNE-SSW directions. They rather suggest that after a period of extension, two main orogenic events took place: a phase involving shearing and thinning which affected mainly the present-day NPZ and a later compressive phase which explains the building up of the chain, the thickening of the crust and the enhancement of a pre-existing difference in crustal thickness between the European and Iberian plates.