Late Cretaceous exhumation history of an extensional extruding wedge (Graz Paleozoic Nappe Complex, Austria)

Late Cretaceous structures within the eastern Graz Paleozoic Nappe Complex define an extruding wedge with north-eastward directed thrusting in eastern portions and strike-slip shear along the margins. Stacking structures are overprinted by south-westward directed extension with low-grade metamorphic rocks in the hangingwall and high-grade basement rocks in the footwall. Pressure–temperature and structural data are obtained from successively opening quartz veins that record various stages of progressive deformation and metamorphism. Fluid inclusion data and related structures show that during extension isothermal decompression from ca. 550°C and 8 kbar down to ca. 450°C and 2 kbar was related to exhumation of rocks from deep crustal levels. The data point to a high geothermal gradient and explain condensed paleo-isotherms due to ductile normal faulting in the eastern areas of the Graz Paleozoic Nappe Complex. The investigated Late Cretaceous structural elements suggest that the Graz Paleozoic Nappe Complex decoupled from the surrounding basement units and operated as a large-scale extension–extrusion corridor that evolved prior to Miocene extrusion tectonics in the Eastern Alps.     

Tectonometamorphic evolution of the Koralm Complex (Eastern Alps): constraints from microstructures and textures of the ‘Plattengneis’ shear zone

Lattice preferred orientations (LPO) of quartz have been investigated along a south–north oriented section across the Plattengneis of the Koralm Complex (Eastern Alps). The Plattengneis forms an important shear zone within the Austroalpine nappe complex of the Eastern Alps, which has developed during the Cretaceous evolution of the Alpine orogen. The quartz c-axes form small circle distributions in the southernmost parts of the Koralm Complex, which represent the uppermost structural level of the Plattengneis. Further to the North two maxima between the Y and Z directions of the finite strain can be interpreted in terms of preferred slip on the rhomb planes. These fabrics continuously grade into (type I and type II crossed) girdle distributions in a northward direction. A strong maximum near the Y-axis with the tendency to be distributed along a single girdle, with three corresponding maxima of a-axes near the margin of the pole figure, can be observed in the central and northern parts. Such LPO are characteristic for both high grade metamorphic conditions and high finite strain. The microstructures display that the deformation within the Plattengneis shear zone was synmetamorphic. A continuous increase of peak temperatures (and pressure) from approximately 550 °C to approximately 750 °C from the South to the central parts can be inferred from geothermometric calculations. The temperatures then decrease to approximately 650 °C from the central parts to the North. The related pressures increase from 8 to 16 kbar, and then decrease to 10 kbar. The LPO changes that have been observed in the study area are best interpreted in terms of temperature dependence of the activation of glide systems within quartz aggregates. The temperature and pressure evolution may indicate that the central parts of the Koralm Complex have been exhumed by larger amounts than the northern and southern parts. This is also documented by the LPO evolution. Therefore, we assume that the Plattengneis shear zone formed during the exhumation of the Koralm Complex, and is related to the exhumation of high-pressure units in the footwall of this shear zone. Accordingly, the kinematics of the Plattengneis shear zone is rather extensional than thrust-related. The implications for the structural evolution of the Eastern Alps are shortly discussed.     

Structural and metamorphic evolution of the phengite‐bearing schists of the northern Adula Nappe (Central Alps,Switzerland)

Phengite‐bearing schists of the northern Adula Nappe experienced a polymetamorphic and polycyclic evolution that was associated with five deformation episodes. Evidence of a pre‐Alpine metamorphic event is preserved within garnet cores of some amphibole‐bearing schists. The D1 and D2 deformation episodes are recorded by S1 and S2 foliations preserved only within metre‐scale domains of low‐D3 strain. S1 is a relict foliation. Blueschist‐facies conditions at 565 ± 10°C and 11.5 ± 1.5 kbar were attained during D2 and were associated with the development of isoclinal folding and an S2 foliation. The D3 episode took place at 665 ± 50°C and 11.5 ± 2.1 kbar and was responsible for the development of a transpositive S3 foliation. The D4 episode took place at T < 550 ± 10°C and was associated with the development of a discrete S4 foliation and S‐C structures. The D5 episode is recorded by sub‐vertical metre‐scale open folds or centimetre‐scale kinks. The structural and metamorphic evolution described here indicates that the northern and central parts of the Adula Nappe were distinct continental crustal fragments and were brought together under amphibolite‐facies conditions. Copyright © 2007 John Wiley & Sons, Ltd.     

Multistage metamorphism and deformation in high‐pressure metabasites of the northern Adula Nappe Complex (Central Alps,Switzerland)

Metabasites from the northern Adula Nappe Complex (ANC) display a complex microstructural evolution recording episodes of deformation and metamorphic re‐equilibration that were obliterated in the surrounding phengite‐bearing schists. Pre‐D1 and D1 deformation episodes are preserved as mineral inclusions within garnet cores of some amphibole‐bearing eclogites and record high‐temperature greenschist‐/amphibolite‐facies conditions. D2 produced an eclogite‐facies foliation which developed at 580 ± 70°C and 19 ± 3 kbar. D3 was a composite deformation episode which can be divided into three sub‐episodes D3m, D3a and D3b which occurred as the metamorphism evolved from post‐eclogitic high‐pressure and low‐temperature conditions through to amphibolite‐facies conditions at 590 ± 30°C and 11.7 ± 1.3 kbar. The D3 deformation episode was responsible for the development of the S3 regional‐scale foliation in the surrounding schists, whilst D4 caused the development of an S4 greenschist foliation. The composite nature of the D3 episode indicates that rocks of the northern ANC experienced a protracted post‐eclogitic structural reworking and that the current structure of this part of the Alps is a late‐Alpine feature. Copyright © 2010 John Wiley & Sons, Ltd.     

Kinematics and geochronology of Early Cretaceous thrusting in the northwestern paleozoic of Graz (Eastern Alps)

Abstract

The multiply deformed Upper Austro-Alpine nappe pile of the Graz area is built up of low-grade metamorphosed Paleozoic rocks which are discordantly overlain by sediments of Santonian (Late Cretaceous) age (“Gosau” formation). Slices of Permo-Mesozoic rocks are absent. Analyses of structures, microfabrics, strain and shear directions were used to decipher the kinematic history; geochronological investigations to date the age of thrusting. K/Ar and Rb/Sr ages of synkinematically grown mica suggest an eo-Alpine (Early Cretaceous) age for the major deformation D1. D1 is characterized by non-coaxial rock flow which caused SW- to W directed nappe imbrication. Incremental strain measurements indicate the progressive superposition of D2 over Dl. In the higher nappe (Rannach Nappe) nappe imbrication continued during D2 changing the direction of nappe transport from SW to NW. Enhanced flattening strain in the deeper nappe (Schöckel Nappe) led to recumbent folds in all scales during D2. This study emphasized two interpretations : (1) The Alpine deformation in the Upper Austro-Alpine nappe pile of the Paleozoic of Graz started in the Earliest Cretaceous (about 125 Ma.). (2) The emplacement of nappes followed a curved translation path in the studied area.     

Crystallization and very rapid exhumation of the youngest Alpine eclogites (Tauern Window,Eastern Alps) from Rb/Sr mineral assemblage analysis

Multimineral Rb/Sr internal isochrons from eclogite facies rocks of the Eclogite Zone (Tauern Window, Eastern Alps) consistently yield an Early Oligocene age of 31.5±0.7 Ma. This age has been obtained both for late-prograde, dehydration-related eclogitic veins, and for rocks variably deformed and recrystallized under eclogite facies conditions (2.0–2.5 GPa, 600°C). Initial Sr-isotopic equilibria among all phases indicate absence of significant post-eclogitic isotope redistribution processes, therefore the ages date eclogite facies assemblage crystallization. Equilibria also prove that no prolonged pre-eclogite facies history is recorded in the rocks. Instead, subduction, prograde mineral reactions, and eclogitization proceeded rapidly. Fast exhumation immediately after eclogitization, with minimum rates >36 mm/a is inferred from a 31.5±0.5 Ma internal mineral isochron age of a post-eclogitic greenschist facies vein assemblage. Such rates equal typical subduction rates. Late Eocene to Early Oligocene subduction of the European continental margin, with subsequent rapid exhumation of high-pressure nappe complexes has previously been recognized only in the Western Alps. The new data signify synchronous continental collision all along the Alpine belt. Our results demonstrate the unique potential of Rb/Sr assemblage system analysis for precise dating of both eclogite facies and post-eclogitic events, thus for precisely constraining exhumation rates of deep-seated rocks, and for straightforward linkage of petrologic evidence with isotopic ages.     

The Neogene Fohnsdorf Basin: basin formation and basin inversion during lateral extrusion in the Eastern Alps (Austria)

The evolution of the early/middle Miocene Fohnsdorf Basin has been studied using borehole data, reflection seismic lines, and vitrinite reflectance. The basin is located along the sinistral Mur-Mürz fault system and probably formed as an asymmetric pull-apart basin, which was subsequently modified by halfgraben tectonics, as a consequence of eastward lateral extrusion. Sedimentation started with the deposition of fluvio-deltaic sediments. Thick coal accumulated in the northwestern basin. Thereafter subsidence rates increased dramatically with the formation of a lake several hundred meters deep. The lake was filled mainly from the north with more than 1500?m of sediments showing a coarsening-upward trend due to southward prograding deltaic lobes. A sequence of more than 1000?m of boulder gravels (Blockschotter) in the southeastern part of the basin are interpreted as the upper part of a coarse-grained fan delta succession, which accumulated along a normal fault along the southern basin margin. Fan deltas reached the central basin only during the early stages of sedimentation and during the late stages of basin formation. Miocene heat flow was approximately 65–70?mW/m², which is significantly lower than in other basins along the Mur-Mürz fault system. The present-day southwestern basin margin is a recent feature, which is related to transpression along the dextral Pöls-Lavanttal fault system. It is formed by reverse faults constituting the northeastern part of a flower structure. Miocene sediments in the Feeberg valley are preserved along its southwestern part. Uplift of the central part of the flower structure was at least 2.4?km. North–south compression resulted in the deformation of the basin fill, uplift of the E/W-trending basement ridge separating the Fohnsdorf and Seckau basins, and in the erosion of 1750?m of sediments along the northern basin margin.     

Thermal and tectonic evolution of the southern Alps (northern Italy) rifting: Coupled organic matter maturity analysis and thermokinematic modeling

The Pearl River Mouth Basin (PRMB) is one of the most petroliferous basins on the northern margin of the South China Sea. Knowledge of the thermal history of the PRMB is significant for understanding its tectonic evolution and for unraveling its poorly studied source-rock maturation history. Our investigations in this study are based on apatite fission-track (AFT) thermochronology analysis of 12 cutting samples from 4 boreholes. Both AFT ages and length data suggested that the PRMB has experienced quite complicated thermal evolution. Thermal history modeling results unraveled four successive events of heating separated by three stages of cooling since the early Middle Eocene. The cooling events occurred approximately in the Late Eocene, early Oligocene, and the Late Miocene, possibly attributed to the Zhuqiong II Event, Nanhai Event, and Dongsha Event, respectively. The erosion amount during the first cooling stage is roughly estimated to be about 455–712 m, with an erosion rate of 0.08–0.12 mm/a. The second erosion-driven cooling is stronger than the first one, with an erosion amount of about 747–814 m and an erosion rate between about 0.13–0.21 mm/a. The erosion amount calculated related to the third cooling event varies from 800 m to 3419 m, which is speculative due to the possible influence of the magmatic activity.     

Tectonic vs. gravitational morphostructures in the central Eastern Alps (Italy): Constraints on the recent evolution of the mountain range

Deep-seated gravitational slope deformations (DSGSDs) influence landscape development in tectonically active mountain ranges. Nevertheless, the relationships among tectonics, DSGSDs, and topography are poorly known. In this paper, the distribution of DSGSDs and their relationships with tectonic structures and active processes, surface processes, and topography were investigated at different scales. Over 100 DSGSDs were mapped in a 5000 km² sector of the central Eastern Alps between the Valtellina, Engadine and Venosta valleys. Detailed lineament mapping was carried out by photo-interpretation in a smaller area (about 750 km²) including the upper Valtellina and Val Venosta. Fault populations were also analysed in the field and their mechanisms unravelled, allowing to identify different structural stages, the youngest being consistent with the regional pattern of the ongoing crustal deformation. Finally, four DSGSD examples have been investigated in detail by geological and 2D geomechanical modelling.DSGSDs affect more than 10% of the study area, and mainly cluster in areas where anisotropic fractured rock mass and high local relief occur. Their onset and development is subjected to a strong passive control by mesoscopic and major tectonic features, including regional nappe boundaries as well as NW–SE, N–S and NE–SW trending recent brittle structures. The kinematic consistency between these structures and the pattern of seismicity suggests that active tectonics may force DSGSDs, although field evidence and numerical models indicate slope debuttressing related to deglaciation as a primary triggering mechanism.     

Understanding the pre-Variscan and Variscan basement components of the central Tauern Window, Eastern Alps (Austria): constraints from single zircon U-Pb geochronology

New single-grain and within-grain U-Pb zircon ages from the central Tauern Window help sorting out the time dimension among the various Variscan and pre-Variscan basement components that were strongly overprinted by Alpine orogeny. Single-grain isotope dilution (ID-TIMS) U-Pb zircon geochronology of three Basisamphibolit samples yield protolith formation ages of 351±2, 349±1 and 343±1 Ma. Laser ablation ICP-MS and ID-TIMS U-Pb detrital zircon dating of the Biotitporphyroblastenschiefer constrained the maximum time of sedimentation to between 362±6 Ma and 368±17 Ma. Paragneisses from the Zwölferzug yield maximum sedimentation ages from 345±5 Ma (ion microprobe data) to 358±10 Ma. Zircons from gabbroic clasts and detrital zircons from a meta-agglomerate from the Habach Phyllite give an upper intercept age of 536±8 Ma and a near-concordant age of 506±9 Ma, respectively. Hence, apart from the Habach Phyllite, the maximum sedimentation ages of the metasediments investigated range from Upper Devonian to Lower Carboniferous. Consequently, the Basisamphibolit, the Biotitporphyroblastenschiefer, and the paragneisses of the Zwölferzug form parts of the Variscan basement series. The Basisamphibolit (351-343 Ma) is distinct both in space and time of formation from the Zwölferzug garnet amphibolite (c. 486 Ma), which forms part of the pre-Variscan basement.     

Sm-Nd isotope systematics in garnet from different lithologies (Eastern Alps): age results, and an evaluation of potential problems for garnet Sm-Nd chronometry [Chem. Geol. 185 (2002) 255-281]

Interpretation of Sm-Nd garnet ages is frequently impaired by one of the following restrictions: (a) high-LREE inclusions, (b) isotopic disequilibrium, and (c) the uncertainty about the closure temperature. These issues are addressed by way of an evaluation of garnet Sm-Nd data from different rock types of the Austroalpine basement units, Eastern Alps, including metabasic eclogites, mica schist and paragneiss, metapegmatite and metagranite.Nd concentration in handpicked garnet varies between 0.021 and 23.1 ppm in metabasites, 0.49 and 17.4 ppm in metapelites and between 0.024 and 4.6 ppm in metapegmatites and metagranites. The overall range of ¹⁴⁷Sm/¹⁴⁴Nd is 0.15-2.5 in garnet from metabasites, 0.12-3.03 in metapelite garnet and 0.66-7.21 in Mn-rich garnet from metapegmatites and metagranites. A clear negative correlation between Nd concentration and Sm/Nd is observed in garnets from all these lithologies. Therefrom, it is concluded that even optically “clean” garnet separates may contain high-LREE microinclusions, such as epidote-allanite, zoisite, apatite, sphene, monazite or zircon. However, very low Nd concentrations correlated with low Sm/Nd as well as high Nd concentrations (>5 ppm) correlated with fairly high Sm/Nd ratios (0.8) have also been observed. Apart from replicate analyses within as well as between samples with a common PT-history, leaching experiments are a useful technique to elucidate any distorting influence of unequilibrated inclusions on the garnet age, especially if the observed Sm/Nd ratio is low (<0.5). Leaching of garnet separates with HCl (2.5, 5.8 M) produces no obvious element fractionation, but may improve Sm/Nd, and hence age precision, considerably. Isotopic disequilibrium between garnet and other matrix minerals is observed preferentially in basic eclogites, derived from gabbroic precursors.Sm-Nd garnet analysis allows the recognition of several distinct garnet-forming events in the Eastern Alps.(a) A Variscan high-P event is documented in metabasites from the northern-central Ötztal basement around 360-350 Ma, whereas garnet from sillimanite-bearing gneisses dates the Variscan thermal peak in the western part of the same subunit around 345-330 Ma.(b) A long-lived, Permian to Triassic event (285-225 Ma), correlated with crustal extension and low-P metamorphism, is documented by spessartine-rich garnet from metapegmatites as well as almandine-rich garnet cores from mica schist.(c) Age data of garnet from eo-Alpine (Cretaceous) deeply subducted rocks of the southern/eastern Austroalpine units are related to near-peak PT, eclogite- to amphibolite-facies metamorphic conditions (peak: 2 GPa/685 °C), and/or incipient isothermal decompression, due to fast, tectonically driven exhumation (110/100-85 Ma). At cooling rates of 20-30 °C/Ma (exhumation rates: 3-5 km/Ma), the Sm-Nd closure temperature (T_c) for mm-sized garnet in these rocks is estimated at 650-680 °C.     

Garnet Sm–Nd data from the Saualpe and the Koralpe (Eastern Alps, Austria): chronological and P–T constraints on the thermal and tectonic history

Garnets from recrystallized, staurolite- and kyanite-bearing mica schists from the central Saualpe basement, representing the host rocks of the type-locality eclogites, give concordant Sm–Nd garnet–whole-rock isochron ages between 88.5±1.7 and 90.9±0.7 Ma. The millimetre-sized, mostly inclusion-free grains show fairly homogeneous element profiles with pyrope contents of 25–27%. Narrow rims with an increase in Fe and Mn and a decrease in Mg document minor local re-equilibration during cooling. According to phengite geothermobarometry, peak metamorphic conditions at 90 Ma were close to 20  kbar and 680  °C and similar to those recorded by the eclogites. The garnet rims record about 575  °C/7  kbar for the final stages of metamorphism. A phengitic garnet–mica schist, sampled at the immediate contact with the Gertrusk eclogite, gave a garnet–whole-rock Sm–Nd age of 94.0±2.7 Ma.
Garnet porphyroclasts separated from a pegmatite–mylonite of the Koralpe plattengneiss near Stainz are unzoned and show spessartine contents of 15%. Composition and Sm–Nd ages of close to 260 Ma point to a magmatic origin for these garnets.
The garnet data from the Saualpe document an intense Alpine metamorphism for this part of the Austroalpine basement. The mica schists recrystallized during decompression and rapid exhumation, at the final stages of and immediately following a high- P event. The Koralpe data show that high Alpine temperatures did not reopen the Sm–Nd isotope system, implying a closure temperature in excess of c . 600  °C for this isotopic system in garnet.     

Late Cretaceous to Early Tertiary palaeogeography of the Western Carpathians (Slovakia) and the Eastern Alps (Austria): implications from heavy mineral data

The Late Cretaceous Brezová and Myjava Groups of the Western Carpathians in Slovakia and formations of the Gosau Group of the Northern Calcareous Alps in Lower Austria comprise similar successions of alluvial/shallow marine deposits overlain by deep water hemipelagic sediments and turbidites. In both areas the heavy mineral spectra of Late Cretaceous sediments contain significant amounts of detrital chrome spinel. In the Early Tertiary the amount of garnet increases. Cluster analysis and correspondence analysis of Coniacian/Santonian and Campanian/Early Maastrichtian heavy mineral data indicate strong similarities between the Gosau deposits of the Lunz Nappe of the north-eastern part of the Northern Calcareous Alps and the Brezova Group of the Western Carpathians. Similar source areas and a similar palaeogeographical position at the northern active margin of the Adriatic/Austroalpine plate are therefore suggested for the two tectonic units.Basin subsidence mechanisms within the Late Cretaceous of the Northern Calcareous Alps are correlated with the Western Carpathians. Subsidence during the Campanian-Maastrichtian is interpreted as a consequence of subduction tectonic erosion along the active northern margin of the Adriatic/Austroalpine plate. Analogous facies and heavy mineral associations from deep water sandstones of the Manin Unit and the Klape Unit indicate accretion of parts of the Pieniny Klippen Belt during the Late Cretaceous along the Adriatic/Austroalpine margin.     

Late Neogene–Quaternary evolution of the intermontane Clusone Basin (Southern Alps,Italy): integration of seismic and geological data

In the Clusone Basin (a large intermontane basin filled by thick late Neogene–Quaternary sediments in the Middle Val Seriana, Southern Alps), two high‐resolution seismic profiles have been acquired in order to reconstruct the geometries of the sediments that fill the depression, with a maximum thickness of more than 200 m as documented by available well data, and to define their relationships with the bedrock, consisting of Late Triassic carbonates. In addition to standard seismic reflection processing, a seismic refraction inversion technique has been applied. The integration of geological (both surface and well data) and seismic data indicates a complex history of the drainage patterns of the Clusone Basin, documenting a shift of the Serio River from a palaeodrainage toward the southeast (Val Borlezza) to the present situation, toward the south (Val Seriana): between the older and the present‐day drainages an important depositional stage occurred, as documented by the thick sediments that fill the Clusone Basin, controlling the capture of the Serio River along the Val Seriana. Copyright © 2004 John Wiley & Sons, Ltd.     

The structural evolution of the southern Apuan Alps: new constraints on the tectonic evolution of the Northern Apennines (Italy)Évolution structurale des Apuanes méridionales : nouvelles contraintes sur l'évolution tectonique des Apennins septentrionaux (Italie)

Structural analysis performed in the southern sector of the Apuan Alps Metamorphic Complex (AAMC) and on the overlaying Tuscan Nappe (TN) pointed out a structural evolution much more complex than that outlined till now. The comparison between the structural evolutions of the two tectonic units could provide new important constraints on the tectonic evolution of the whole belt. The two tectonic units recorded different tectonic evolution during the first stages of compression, while they shared the same deformation history later after the Tuscan Nappe overlapped the AAMC. The coupled tectonic units have been then deformed by two systems of folds, in a compressive tectonic regime. Extensional tectonics affected the units later, at upper crustal levels. To cite this article: R. Carosi et al., C. R. Geoscience 334 (2002) 339–346.     

The Gibraltar Arc seismogenic zone (part 1): Constraints on a shallow east dipping fault plane source for the 1755 Lisbon earthquake provided by seismic data, gravity and thermal modeling

The Great Lisbon earthquake of 1755 with an estimated magnitude of 8.5–9.0 is the most destructive earthquake in European history, yet the source region remains enigmatic. Recent geophysical data provide compelling evidence for an active east dipping subduction zone beneath the nearby Gibraltar Arc. Marine seismic data in the Gulf of Cadiz image active thrust faults in an accretionary wedge, above an east dipping decollement and an eastward dipping basement. Tomographic and other data support subduction and rollback of a narrow slab of oceanic lithosphere beneath the westward advancing Gibraltar block.Although, no instrumentally recorded seismicity has been documented for the subduction interface, we propose the hypothesis that this shallow east dipping fault plane is locked and capable of generating great earthquakes (like the Nankai or Cascadia seismogenic zones). We further propose this east dipping fault plane to be a candidate source for the Great Lisbon earthquake of 1755. In this paper we use all available geophysical data on the deep structure of the Gulf of Cadiz–Gibraltar region for the purpose of constraining the 3-D geometry of this potentially seismogenic fault plane. To this end, we use new depth processed seismic data, have interpreted all available published and unpublished time sections, examine the distribution of hypocenters and perform 2-D gravity modeling. Finally, a finite-element model of the forearc thermal structure is constructed to determine the temperature distribution along the fault interface and thus the thermally predicted updip and downdip limits of the seismogenic zone.     

Petrology and metamorphic evolution of ultramafic rocks and dolerite dykes of the Betic Ophiolitic Association (Mulhacén Complex, SE Spain): evidence of eo-Alpine subduction following an ocean-floor metasomatic process

The Betic Ophiolitic Association, cropping out within the Mulhacén Complex (Betic Cordilleras), is made up of numerous metre- to kilometre-sized lenses of mafic and/or ultramafic and meta-sedimentary rocks. Pre-Alpine oceanic metasomatism and metamorphism caused the first stage of serpentinization in the ultramafic sequence of this association, which is characterized by local clinopyroxene (Cpx) breakdown and Ca-depletion, and complementary rodingitization of the basic dykes intruded in them. Subsequent eo-Alpine orogenic metamorphism developed eclogite facies assemblages in ultramafic and basic lithotypes, which were partly retrograded in Ab-Ep-amphibolite facies conditions during a meso-Alpine event. The heterogeneous development of the oceanic metasomatism in the ultramafic rock-types led to the patchy development of highly serpentinized Ca-depleted domains, without gradual transition to the host, and less serpentinized, Cpx-bearing ultramafites, mainly lherzolitic in composition. The high-pressure eo-Alpine recrystallization of these ultramafites in subduction conditions originated secondary harzburgites in the Ca-depleted domains, consisting of a spinifex-like textured olivine+orthopyroxene paragenesis, and a diopside+Ti-clinohumite paragenesis in the enclosing lherzolitic rocks. During the meso-Alpine event, secondary harzburgites were partly transformed into talc+antigorite serpentinites, whereas the diopside and clinohumite-bearing residual meta-lherzolites were mainly transformed into Cpx-bearing serpentinites. Relics of mantle-derived colourless olivine may be present in the more or less serpentinized secondary harzburgites. These relics are overgrown by the eo-Alpine brown pseudo-spinifex olivine, which contains submicroscopic inclusions of chromite, ilmenite and occasional halite and sylvite, inherited from its parental oceanic serpentine. The same type of mantle-derived olivine relics is also preserved within the Cpx-bearing serpentinites, although it has been partly replaced by the eo-Alpine Ti-clinohumite. The dolerite dykes included in the ultramafites were partly rodingitized in an oceanic environment. They were then transformed during the eo-Alpine event into meta-rodingites in their border zones and into eclogites towards the innermost, less-rodingitized portions. Estimated P–T conditions for the high-pressure assemblages in ultramafic and basic lithotypes range from 650 to 750°C and 16–25 kb.     

The thermal history of the eastern Officer Basin (South Australia): evidence from apatite fission track analysis and organic maturity data

The eastern Officer Basin in South Australia contains a Neoproterozoic to Devonian succession overlain by relatively thin (<500 m) Permian, Mesozoic and Tertiary deposits. Within the basin fill, there are several major unconformities representing uncertain amounts of erosion. Three of these surfaces are associated with regional deformational events. Regional unconformities formed between 560 and 540 Ma (Petermann Ranges Orogeny), approximately 510–490 Ma (Delamerian Orogeny), 370–300 Ma (Alice Springs Orogeny), 260–150 Ma; and 95–40 Ma. AFTA® results from 13 samples of Neoproterozoic, Cambrian and Permian sedimentary rocks in five wells (Giles-1, Manya-2, -5 and -6 and Lake Maurice West-1) show clear evidence for a number of distinct thermal episodes. Results from all samples are consistent with cooling from the most recent thermal episode beginning at some time between 70 and 20 Ma (Maastrichtian–Miocene). AFTA results from Giles-1 indicate at least two pre-Cretaceous thermal episodes with cooling beginning between 350 and 250 Ma (Carboniferous–Permian) and between 210 and 110 Ma (Late Triassic–Albian). Results from Manya-2, -5 and -6 and Lake Maurice West-1 show evidence for at least one earlier higher temperature event, with cooling from elevated paleotemperatures beginning between 270 and 200 Ma (Late Permian to Late Triassic). These episodes can be correlated with other cooling/erosional events outside the study area, and the AFTA-derived paleotemperatures are consistent with kilometre-scale erosion for each of the episodes identified. Integration of the AFTA data with organic thermal maturation indicators (MPI) in the Manya and Giles-1 wells suggests that the Cambrian and Neoproterozoic successions in the northern part of the study area reached peak maturation prior to the Permian, while limited data from Lake Maurice West-1 allows peak maturation to have occurred as young as the Late Permian to Late Triassic thermal episode revealed by AFTA. The approach outlined in this study is relevant to all ancient basins as it emphasises the importance of understanding events associated with neighbouring regions. The thermal history of the Officer Basin, as with most other ancient basins, has been strongly affected by significant tectonic events throughout its history, even though younger deposits are not preserved in the basin itself. The recognition of these younger events, and the implications of these events for the depositional history, is important as it allows identification of the best regions for preservation of early generated hydrocarbons, and in some cases, suggests areas where generation of hydrocarbons could have occurred more recently than previously thought.     

High-pressure micro-inclusions and development of an inverted metamorphic gradient in the Santiago Schists (Ordenes Complex, NW Iberian Massif, Spain): evidence of subduction and syncollisional decompression

Abstract The Santiago Schists are located in the Basal Unit of the Ordenes Complex, one of the allochthonous complexes outcropping in the inner part of the Hercynian Belt in the north-west of the Iberian Peninsula. Their tectonothermal evolution is characterized by the development of an eo-Hercynian metamorphic episode (c. 374 Ma) of high-P, low- to intermediate-T. The mineral assemblage of the high-P episode is preserved as a very thin Si= S1 foliation included in albite porphyroblasts, being composed of: albite + garnet-I + white mica-1 + chlorite-1 + epidote + quartz + rutile ± ilmenite. The equilibrium conditions for this mineral assemblage have been estimated by means of different thermobarometers at 495 ± 10 °C and 14.7 ± 0.7 kbar (probably minimum pressure). The later evolution (syn-D2) of the schists defines a decompressive and slightly prograde P-T path which reached its thermal peak at c. 525 ± 10 °C and 7 kbar. Decompression of the unit occurred contemporaneously with an inversion of the metamorphic gradient, so that the zones of garnet-II, biotite (with an upper subzone with chloritoid) and staurolite developed from bottom to top of the formation. The estimated P-T path for the Santiago Schists suggests that the Basal Unit, probably a fragment of the Gondwana continental margin, was uplifted immediately after its subduction at the beginning of the Hercynian Orogeny. It also suggests that the greater part of the unroofing history of the unit took place in a context of ductile extension, probably related to the continued subduction of the Gondwana continental margin and the contemporaneous development of compensatory extension above it. The inverted metamorphic gradient seems related to conductive heat transferred from a zone of the mantle wedge above the subducted continental margin, when it came into contact with the upper parts of the schists along a detachment, probably of extensional character. The general metamorphic evolution of the Santiago Schists, with the development of high-P assemblages with garnet prior to decompressive and prograde parageneses with biotite, is unusual in the context of the European Hercynian Belt, and shows a close similarity to the tectonothermal evolution of several high-P, low- to intermediate-T circum-Pacific belts.