Garnet–chloritoid equilibria in eclogitic pelitic rocks from the Sesia zone (Western Alps): their bearing on phase relations in high pressure metapelites

Abstract A detailed study of garnet–chloritoid micaschists fom the Sesia zone (Western Alps) is used to constrain phase relations in high pressure (HP) metapelitic rocks. In addition to quartz, phengite, paragonite and rutile, the micaschists display two distinct parageneses, namely garnet + chloritoid + chlorite and garnet + chloritoid + kyanite. Talc has never been observed. Garnet and chloritoid are more magnesian when chlorite is present instead of kyanite. The distinction of the two equilibria results from different bulk rock chemistries, not from P–T conditions or redox state. Estimated P–T conditions for the eclogitic metamorphism are 550–600°C, 15–18 kbar.
The presence of primary chlorite in association with garnet and chloritoid leads us to construct two possible AFM topologies for the Sesia metapelites. The paper describes a KFMASH multisystem for HP pelitic rocks, which extends the grid of Harte & Hudson (1979) towards higher pressures and adds the phase talc. Observed parageneses in HP metapelites are consistent with predicted phase relations. Critical associations are Gt–Ctd–Chl and Gt–Ctd–Ky at relatively low temperatures and Gl–Chl–Ky and Gt–Tc–Ky at relatively high temperatures.     

Matrix‐dependent garnet growth in polymetamorphic rocks of the Sesia zone,Italian Alps

Mushroom‐ and atoll‐shaped garnet crystals were found in high‐pressure quartz‐rich pelitic layers from the Monte Mucrone area (Western Alps, Italy). These garnet crystals are characterized by a peninsula‐shaped core surrounded by a partially crystallized, dodecahedral external rim. Textural observations and thermodynamic modelling point towards growth of the atoll garnet from the Monte Mucrone area during two distinct orogenic cycles. The core region and the inner part of the ring forming the edge of the atoll grew under Barrovian metamorphic conditions of likely Hercynian age, while the outer rim of the atoll structure developed under Alpine high‐pressure conditions. Electron backscatter diffraction analyses indicate that the atoll‐shaped structure has one single crystallographic orientation, despite its complex compositional zoning. Thermodynamic modelling reveals marked changes in equilibrium assemblage leading to changes in stoichiometry of the garnet‐forming reaction, which in turn explain the characteristic garnet morphology. Small amounts of quartz are consumed during the earlier stage of garnet growth history, whereas the production of garnet requires a much larger amount of quartz during the final stage of its growth. This leads to a change from initial poikiloblastic to non‐poikiloblastic textures. This change is responsible for the formation of atoll‐shaped garnet. Finally, garnet in intercalated mica‐rich layers forms idiomorphic crystals, continuous from the centre to rim. This study highlights the importance of the difference between the local matrix composition and the aggregate composition of the reactants needed for the garnet‐forming reaction. Finally, we show that interaction between matrix and reaction stoichiometry can lead to porphyroblast precipitation inside the already grown porphyroblast.     

Application of K-feldspar–jadeite–quartz barometry to eclogite facies metagranites and metapelites in the Sesia Lanzo Zone (Western Alps, Italy)*

The eclogite facies assemblage K-feldspar–jadeite–quartz in metagranites and metapelites from the Sesia-Lanzo Zone (Western Alps, Italy) records the equilibration pressure by dilution of the reaction jadeite+quartz=albite. The metapelites show partial transformation from a pre-Alpine assemblage of garnet (Alm₆₃Prp₂₆Grs₁₀)–K-feldspar–plagioclase–biotite±sillimanite to the Eo-Alpine high-pressure assemblage garnet (Alm₅₀Prp₁₄Grs₃₅)–jadeite (Jd_80–97Di_0–4Hd_0–8Acm_0–7)–zoisite–phengite. Plagioclase is replaced by jadeite–zoisite–kyanite–K-feldspar–quartz, and biotite is replaced by garnet–phengite or omphacite–kyanite–phengite. Equilibrium was attained only in local domains in the metapelites and therefore the K-feldspar–jadeite–quartz (KJQ) barometer was applied only to the plagioclase pseudomorphs and K-feldspar domains. The albite content of K-feldspar ranges from 4 to 11 mol% in less equilibrated assemblages from Val Savenca and from 4 to 7 mol% in the partially equilibrated samples from Monte Mucrone and the equilibrated samples from Montestrutto and Tavagnasco. Thermodynamic calculations on the stability of the assemblage K-feldspar–jadeite–quartz using available mixing data for K-feldspar and pyroxene indicate pressures of 15–21 kbar (±1.6–1.9 kbar) at 550±50 °C. This barometer yields direct pressure estimates in high-pressure rocks where pressures are seldom otherwise fixed, although it is sensitive to analytical precision and the choice of thermodynamic mixing model for K-feldspar. Moreover, the KJQ barometer is independent of the ratio PH2O/P_T. The inferred limiting a(H₂O) for the assemblage jadeite–kyanite in the metapelites from Val Savenca is low and varies from 0.2 to 0.6.     

The Alpine geodynamic evolution of Penninic nappes in the eastern Central Alps

The eastern Central Alps consist of several Pennine nappes with different tectonometamorphic histories. The tectonically uppermost units (oceanic Avers Bündnerschiefer, continental Suretta and Tambo nappes, oceanic Vals Bündnerschiefer) show Cretaceous/early Tertiary W-directed thrusting with associated blueschist facies metamorphism related to subduction of the Pennine units beneath the Austroalpine continental crust. This event caused eclogite facies metamorphism in the underlying continental Adula nappe. The gross effect was crustal thickening. The tectonically lower, continental Simano nappe is devoid of any imprint from this event. In the course of continent-continent collision, high- T metamorphism and N-directed movements occurred. Both affected the whole nappe pile more or less continuously from amphibolite to greenschist facies conditions. Crustal thinning commenced during the regional temperature peak. A final phase is related to differential uplift under retrograde P–T conditions. Further thinning of the crust was accommodated by E- to NE-directed extensional deformation.     

Contrasting thermomechanical evolutions in the Southalpine metamorphic basement of the Orobic Alps (Central Alps, Italy)

In the Southern Alps a progressive metamorphic zonation, with an increase in the geothermal gradient from NE to SW, has been widely proposed. However, recent investigations have shown that the greenschist metamorphic imprint of the low-grade zone corresponds to a metamorphic retrogression following amphibolite facies conditions. On the other hand, in the medium-grade zone, a later low-pressure, high-temperature (LPHT) metamorphic event has also been proposed. In an attempt to resolve these different interpretations, new petrological and partly new structural data have been obtained for two sectors of the Orobic Alps, traditionally attributed to different metamorphic zones. Thermobarometric determinations, supported by microstructural analysis, indicate the following different pressure-retrograde paths in each sector: (1) in the Val Vedello basement (VVB) rocks, a first metamorphic imprint characterized by P = 7–9 kbar and T = 570–610°C was followed by a greenschist retrogression ( P ≤ 4 kbar and T ≤ 500° C); (2) in the Lario basement (LB) rocks, the first detectable metamorphic stage, characterized by mineral assemblages indicating P = 7–9 kbar and T = 550–630° C, was followed by a LPHT event, synkinematic with F2 extensional deformation. A greenschist retrogression marks the final uplift of these rocks.
Reinterpretation of the available geochronological data indicates a diachronism for the two thermomechanical evolutions. In the light of these data, we interpret the retrograde P–T–t path of the VVB rocks as a pre-Permian post-thickening uplift and the retrograde P–T–t evolution of the LB rocks as a Permo-Mesozoic uplift related to the extensional tectonic regime of the Tethyan rifting.     

Quantitative basin characterisation to refine debris-flow triggering criteria and processes: an example from the Italian Western Alps

This paper investigates rapid channelized debris flow related to rainfalls in small alpine basins. Its goal is to evaluate and correlate different geological and technical aspects with predisposing and triggering factors that can control these phenomena. The study area is the upper part of the Susa Valley where 12 small basins were selected. For each of them, lithological, geomorphological, climatic and technical information were mapped and analysed. Debris-flow triggering conditions, flow and depositional processes were related to physical characteristics of the basin that can be easily measured and quantified. At least three different groups of basins were found: G1) basins with one event each 4–6 years, characterised by massive or blocky calcareous rocks, G2) basins with more than one event per year that show an abundance of layered or sheared fine-grained rocks and G3) basins with recurrence levels exceeding 10 years, activated only by heavy and prolonged rainfalls, marked by massive or blocky coarse-grained igneous rocks. Furthermore, important morphometric differences were found. These considerations are useful in terms of hazard zonation and risk mitigation.     

Oxygen and hydrogen isotope compositions of eclogites and associated rocks from the Eastern Sesia zone (Western Alps,Italy)

Oxygen and hydrogen isotope analyses have been made of mineral separates from eclogites, glaucophanites and glaucophane schists from the eastern Sesia zone (Italian Western Alps). Regularities in (1) hydrogen isotope compositions, (2) order of ¹⁸O enrichment among coexisting minerals, and (3) ¹⁸O (quartz-rutile) and ¹⁸O (quartz-phengite) imply attainment of a high degree of isotopic equilibrium. However, some scattering of ¹⁸O values of individual minerals indicates that the eclogitic assemblage did not form in the presence of a thoroughly pervasive fluid. Minerals from an eclogitic lens enclosed in marble have ¹⁸O values distinctly different from those measured in the other rocks. The ¹⁸O values are high in comparison with other type C eclogites of the world, and it is proposed that the fluid present during the high pressure metamorphism has to a large extent been inherited from the precursor rocks of amphibolite facies.An average formation temperature of 540 ° C is inferred from the oxygen isotope fractionations between quartz and rutile and between quartz and white mica. This temperature is in accordance with petrologic considerations and implies subduction of the precursor rocks into the upper mantle to achieve the high pressures required.     

Interdependence of deformation, fluid infiltration and reaction progress recorded in eclogitic metagranitoids (Sesia Zone, Western Alps)

The effects of high-strain deformation and fluid infiltration during Alpine eclogite facies metamorphism have been studied across ductile shear zones in relatively undeformed metagranitoids at Monte Mucrone (Sesia Zone, Western Alps, Italy). Microfabrics together with bulk rock and stable isotope data indicate that the mineralogical and chemical variations are related to the degree of deformation, rather than to changes in P-T conditions or tectonic position. Transformation of meta-quartz diorite to recrystallized eclogitic mylonites involved the breakdown of biotite and plagioclase and required the influx of H₂O. Bulk-rock geochemical data show that ductile deformation to form eclogitic mylonites involved an increase in volume with a weight percent gain in H₂O and Si and variable loss of K, Na, Ca and Al. δ¹⁸O changes systematically across ductile shear zones into the undeformed country rocks. Constant values in shear zone centres indicate advection parallel to the shear zone and within 10 cm of the mylonites. A dominant component of diffusive oxygen exchange perpendicular to the shear zones produced isotopic fronts, evident from a gradual increase in δ¹⁸O values to the reference values of the country rocks. The degree of isotopic shift within the shear zones reflects increasing deformation and degree of reaction progress. Multiple phases of Alpine deformation and mineral growth are recognized in the Monte Mucrone metagranitoids, and in some cases, eclogite facies shear zones were reactivated under greenschist facies conditions. The results of this study suggest that high-strain deformation provided pathways for both synkinematic and post-kinematic metamorphic fluids which were necessary for complete reactions. Relict igneous fabrics, as well as the presence of corona textures around biotite and pseudomorphs after primary igneous plagioclase in the least deformed rocks, indicate a paucity of hydrous fluids and support the conclusion that fluid movement was channelled rather than pervasive.     

Horizontal contraction or horizontal extension? Heterogeneous Late Eocene and early Oligocene general shearing during blueschist and greenschist facies metamorphism at the Pennine-Austroalpine boundary zone in the Western Alps

Mylonitic structures related to two orogenic events are described from the upper and lower contacts of the Combin zone and the immediately overlying upper Austroalpine Dent Blanche nappe/Mont Mary klippe and the directly underlying lower Austroalpine Etirol-Levaz slice. The first event, Late Eocene in age, commenced during blueschist facies P-T conditions, but pre-dated the peak of subsequent greenschist facies overprint. The second event, Early Oligocene in age, took place during retrograde greenschist facies conditions. Most sense of shear indicators associated with the retrograde mylonites indicate top SE shearing, but subordinate top NW displacing shear sense indicators have also been mapped. Mylonitic top SE shearing appears to be restricted to the Combin zone and its upper and lower contacts. Within the Dent Blanche nappe and Mont Mary klippe and at the base of the Etirol-Levaz slice, structures were observed which developed during blueschist/greenschist facies conditions and are, in conjunction with the P-T-t history of these rocks, inferred to be older. Associated kinematic data indicate a top NW shear sense. Comparable blueschist/greenschist facies shear sense indicators have not been observed in the Combin zone. Nonetheless, the foliation in the Combin zone shows a progressive evolution from blueschist facies to greenschist facies to retrograde greenschist facies conditions. This indicates that the Combin zone and the immediately over- and underlying Austroalpine units shared a common tectono-metamorphic evolution since the Late Eocene. Finite strain data reveal oblate strain fabrics, which are thought to result from a true flattening strain geometry. Flow path modelling reveals a general non-coaxial deformation régime and corroborates significant departures from a simple shear deformation. In the study area, mylonitic top SE shearing in the Combin zone is attributed to Early Oligocene backfolding and backthrusting of the Mischabel phase. Temperature-time curves suggest slight reheating in the Monte Rosa nappe underneath and cooling in the Dent Blanche nappe above the Combin zone, hence confirming a thrust interpretation for this event. The top NW displacing structures are thought to result from Late Eocene emplacement of the Dent Blanche nappe and the Combin zone onto the Middle Pennine Barrhorn series along the Combin fault. As related structures initiated during mildly blueschist facies conditions in the Dent Blanche nappe and the underlying Combin zone and both were emplaced together onto the greenschist facial Barrhorn series, it is concluded that the structures developed as the nappes moved upward relative to the earth's surface. Thus the Combin fault is regarded as a thrust. The geometry of this structure indicates that the Combin fault is an out of sequence thrust that locally cut down section. Hence, top NW out of sequence thrusting caused local thinning of the metamorphic/structural section in association with horizontal shortening. Out of sequence thrusts cutting down section, and back-thrusts, offer the possibility of explaining the pronounced break in the grade of metamorphism across the Combin fault, i.e. the contact between the eclogite facial Zermatt-Saas zone and the overlying lower grade Combin zone, by contractional deformation.     

Eclogitic metamorphism in carbonate rocks: the example of impure marbles from the Sesia-Lanzo Zone,Italian Western Alps1

The impure marbles of the internal Sesia-Lanzo Zone underwent a multi-stage metamorphic evolution of Alpine age and retain early-Alpine eclogitic assemblages, partially recrystallized under blueschist to greenschist facies conditions. These high-P assemblages consist of carbonates, phengite, quartz, omphacite, grossular-rich (locally spessartinic) garnet, zoisite and Al-rich titanite. Retrogressive stages are characterized by the growth of glaucophane, paragonite, phlogopite, tremolite and albite. Halogen-rich biotite and amphibole are also present. P-T estimates of the early-Alpine metamophism have been calculated from these unique high-P assemblages, in order to test the applicability of some calibrations to impure carbonate systems. In particular, some Gt-Cpx calibrations and the phengite geobarometer give results (T= 575 ± 45° C at 15 kbar for the eclogitic climax and T≤ 500° C at PH₂O ≤ 9 kbar for early-Alpine retrogressive stages) which are within the range obtained from the surrounding lithologies. Phase relationships in P-T-XCO₂ space indicate that mineral assemblages in the impure marbles coexisted with H₂O-rich fluids (XCO₂ <0.03) during their entire Alpine evolution.     

Geomorphological hazards related to deep dissolution phenomena in the Western Italian Alps: Distribution, assessment and interaction with human activities

Deep dissolution affects great part of soluble rocks (e.g. gypsum and anhydrite) of the Western Italian Alps. The related superficial phenomena (sinkholes, gravity-induced processes and a local worsening of geomechanical rock properties) are not limited to typical karsts landscape and cause slope instability also affecting populated sites and infrastructures. The paper aims to describe general characteristic of dissolution phenomena, to interpret their conditioning factors and evolutionary stages and to assess possible hazards due to their superficial effects.The search for evidences of deep dissolution leads to the selection of representative sites in the central part of the Western Italian Alps (Piemonte and Valle d'Aosta Region). Detailed geological and geomorphological studies have been used to classify the selected sites by type, size and variable state of activity. Very different evolutionary stages of dissolution phenomena have been interpreted by comparison of case-studies: some are early “embryonic”; others are more evolved, up to typical sinkholes, or even remodelled by other phenomena. Some cases show an extreme complexity in the interactions between corrosion phenomena and other geomorphic processes: slope deformations, from one side, and karst, fluvial and glacial phenomena, to the other. A wide range of movement rates on slope instabilities induced by deep dissolution have been estimated by topographic and geomorphic data. Geochemical data on removed rocks by dissolution indicate 0.4 mm/year values for local subsidence. Historical and technical data indicate low frequency of major dissolution-induced collapses, but highlight widespread damages to tunnels, roads and buildings, especially along slopes.     

P-T grids for silica-undersaturated granulites in the systems MAS (n+ 4) and FMAS (n+ 3)-tools for the derivation of P-T paths of metamorphism

Abstract Considering the minerals cordierite (Cd), sapphirine (Sa), hypersthene (Hy), garnet (Ga), spinel (Sp), sillimanite (Si) and corundum (Co) in the system FeO-MgO-Al₂O₃-SiO₂ (FMAS), the stable invariant points are [Co], [Ga], [Cd] and [Sa]. Constraints imposed by experimental data for the system MAS indicate that under low P _H2o conditions the invariant points occur at high temperature (> 900° C) and intermediate pressure (7-10 kbar). This temperature is higher than that commonly advocated for granulite facies metamorphism. In granulites Fe-Mg exchange geothermometers may yield temperatures of 100–150° C below peak metamorphic conditions and evidence for peak temperatures is best preserved by relict high-temperature assemblages and by Al-rich cores in orthopyroxene. Application of the FMAS grid to some well-documented granulite occurrences introduces important constraints on their P-T histories. Rocks of different bulk compositions, occurring in close proximity in the field, may record distinct segments of their P-T paths. This applies particularly to rocks with evidence for reaction in the form of coronas, symplectites and zoned minerals. Consideration of curved reaction boundaries and _XMs isopleths may explain apparently contradictory results for the stability of cordierite obtained from low-temperature experiments and thermochemical calculations on the one hand and hightemperature experimental data on the other.     

P–T conditions for the eclogitic re-equilibration of the metaophiolites from Val d'Ala di Lanzo (internal Piemontese zone, Western Alps)

Abstract A study has been made of the high-pressure early-Alpine re-equilibration in the eclogites and metasedimentary cover of the Val d'Ala di Lanzo ophiolite. All of the main high-pressure minerals have been analysed and their compositions used to determine re-equilibration temperatures. The minimum conditions proposed ( P = 1.3 GPa, T = 450–460°C) are also indicated by the presence of a jadeite+quartz-bearing metagranite.
The temperatures are compared with those reported for similar eclogites from the Voltri Group, the Aosta Valley and the Valais. Comparison of recalculated temperatures shows that the temperature (and probably the pressure) of the eclogitic re-equilibration increased in the Aosta Valley and the Valais, in keeping with what has been observed in the internal Penninic basement of the Gran Paradiso and Monte Rosa crystalline massifs.     

Ductile-brittle transition in pre-Alpine amphibolite facies mylonites during evolution from water-present to water-deficient conditions (Mont Mary nappe, Italian Western Alps)

In the Austroalpine Mont Mary nappe (Italian Western Alps) discrete zones of mylonites–ultramylonites developed from coarse-grained, upper amphibolite facies metapelites of pre-Alpine age. The syn–mylonitic mineral assemblage is quartz–biotite–muscovite–plagioclase–garnet–sillimanite–ilmenite–graphite, and formed via the model hydration reaction: Grt₁+Kfs+H₂O=Bt₂+Ilm₂+Qtz+Ms± Sil .Grain-size reduction of about three orders of magnitude was accompanied by extensive recrystallization of all minerals except sillimanite, and by compositional changes of garnet and biotite. Deformation took place at temperatures of 510–580  °C under low-pressure conditions (0.25–0.45 GPa) and corresponds to the latest stages of pre-Alpine metamorphic evolution. The pre-Alpine mylonitization conditions were close to the brittle-ductile transition, as indicated by syn–mylonitic generation of pseudotachylytes and high differential stress inferred from quartz grain-size piezometry. The brittle-ductile behaviour at a relatively high temperature, and the absence of annealing textures in quartz aggregates, are suggestive of water-deficient conditions during mylonitization. These were accomplished through progressive consumption of water by syn–kinematic hydration reaction and by adsorption onto the greatly increased grain boundary area resulting from dynamic recrystallization.     

Multistage tectono-stratigraphic evolution of the Canavese Intracontinental Suture Zone: New constraints on the tectonics of the Inner Western Alps

The Canavese Intracontinental Suture Zone (CISZ) within the Inner Western Alps represents the remnant of a long-lived minor subduction zone involving a narrow, thinned continental crust/oceanic lithosphere seaway between two continental domains of the Adria microplate (i.e., the Sesia Zone and the Ivrea-Verbano Zone). As opposed to many suture zones, the CISZ mostly escaped pervasive tectonic deformation and metamorphism, thus preserving the original stratigraphy and allowing the relationships between tectonics and sedimentation to be defined. Through detailed geological mapping (1:5000 scale), structural analysis, stratigraphic and petrographic observations, we document evidences for the late Paleozoic to late Cenozoic tectonic evolution of the CISZ, showing that it played a significant role in the context of the tectonic evolution of the Inner Western Alps region from the early to late Permian Pangea segmentation, to the Jurassic Tethyan rifting, and up to the subduction and collisional stages, forming the Western Alps. The site of localization/formation of the CISZ was not accidental but associated with the re-use of structures inherited from regional-scale wrench tectonics related to the segmentation of Pangea, and from the subsequent extensional tectonics related to the Mesozoic rifting, as documented by crosscutting relationships between stratigraphic unconformities and tectonic features. Our findings document that evidences derived from stratigraphy, facies indicators, and relationships between tectonics and sedimentation in the shallow crustal portions of suture zones, such in the CISZ, are important to better constrain the tectonic history of those metamorphic orogenic belts around the world in which evolutionary details are commonly complicated by high-strain deformation and metamorphic transformations.     

A 40Ar/39Ar laser probe study of micas from the Sesia Zone, Italian Alps: implications for metamorphic and deformation histories

Two samples from the Eclogite Micaschist Complex (EMC) and the Seconda Zona Diorito–Kinzigitica (IIDK) of the Sesia Zone have been studied using a high-spatial resolution laser probe ⁴⁰Ar/³⁹Ar technique with the aim of investigating the complexities of argon behaviour in metamorphic rocks and comparing their thermal histories. Data from a single large phengite grain from the EMC show a range of ages from mid-Jurassic to Upper Cretaceous. These ‘apparent age’ variations are spatially related to both location within the grain and to intragrain microstructure. Modelling of the data shows that the profile formed by the diffusion of an excess argon component into the grain, parallel to the mica cleavage. Profile asymmetry is explained by temporal variations in microstructural development enabling excess argon to enter the grain at different times in different places. The temperatures of the initiation of deformation and the possible time-scales for the deformation can be calculated as a function of cooling rate. All estimates suggest deformation at greenschist facies, in accord with the observed retrograde mineral assemblage. Absolute temperature estimates for deformation vary by less than 22 °C for different cooling rates of 10 and 30 °C Ma^?1 but vary by 80 °C with different estimates of diffusion parameters. The duration of deformation was for at least 2 Ma at 10 °C Ma^?1 or 0.7 Ma at 30 °C Ma^?1. Biotites from the IIDK sample record a Permian to Upper Cretaceous age range that correlates with grain size, the smallest grain sizes yielding the youngest ages. This relationship is best explained by a partial resetting of biotites during an Alpine thermal event initiated not more than 70 Ma ago. Modelling of these data suggest that the sample never exceeded 300 °C during the Alpine. The profoundly different thermal histories of the two units—the EMC recrystallized at 550 °C whilst the IIDK remained below 300 °C—suggests that they may not have been juxtaposed until much later than the eclogite facies metamorphism.     

The geodynamic evolution of the Internal Zone of the Betic Cordilleras (south-east Spain): a model based on structural analysis and geothermobarometry

ABSTRACT The Internal Zone of the Betic Cordilleras consists of several superimposed major thrust sheets with different P-T-t evolutions. On the basis of an integrated field, microscopic and laboratory study, the tectono-metamorphic history of the Mulhacen Complex and Almanzora Unit has been reconstructed in detail. The Mulhacen Complex has been affected by at least five phases of penetrative deformation, which have been labelled D_x-1, D_x, D_x+1, D_x+2 and D_x+3. D_x-1, and D_x are related to continent-continent collision, which is indicated by high pressure-low temperature (HP/LT) and subsequent intermediate P/T metamorphic conditions. D_x+1 is related to crustal thinning and heterogeneous extension. During this event the Almanzora Unit was juxtaposed against the Mulhacen Complex. This phase was succeeded by the establishment of low pressure-high temperature (LP/HT) conditions and at least two phases of folding and overthrusting. The Almanzora Unit shows a comparable tectono-metamorphic evolution post D_x+1. However, the P/T conditions prior to D_x+1 indicate a higher crustal position with respect to the Mulhacen Complex during the collisional event.